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Cutting Edge: IL-7-Dependent Homeostatic Proliferation of CD8+ T Cells in Neonatal Mice Allows the Generation of Long-Lived Natural Memory T Cells
Abstract
Healthy, nonimmunized C57BL/6 (B6) mice contain memory phenotype CD8+ T cells, which are assumed to be generated in response to environmental Ags. Since neonatal mice are functionally lymphopenic within the first days after birth, we investigated the alternative possibility that the memory CD8+ T cells of untreated B6 mice are the result of lymphopenia-induced proliferation during neonatal life. We show here that adoptively transferred CD8+ T cells proliferate in neonatal B6 mice, rapidly produce IFN-gamma, and develop into memory cells which are maintained until adulthood. In contrast to CD4+ T cells, neonatal lymphopenia-induced proliferation of CD8+ T cells was IL-7 dependent. Thus, neonatal lymphopenia seems to allow CD8+ thymic emigrants to undergo lymphopenia-induced proliferation during early neonatal life to equip the immune system with a set of preactivated CD8+ T cells before any infection, which might contribute to the rapid initiation of immune responses in the adult.
... Newborn T cell-competent mice are nearly devoid of peripheral T cells (7). Surprisingly, only a minority of thymic emigrants and adoptively transferred naive T cells undergo LIP in lymphopenic neonates (8)(9)(10), and lymphopeniainduced tissue destruction is not observed. Based on these findings, we hypothesized that T cell-independent mechanisms prevent LIP of neonatal T cells, subsequent autoaggression (3), and the generation of a limited T cell repertoire (11). ...
... The adoptive transfers of CD8 + T cells were performed as described previously (10). Briefly, CD8 + T cells from Thy1.1 + OT-I mice were purified using CD8a-specific microbeads and autoMACS (both Miltenyi Biotec, Bergisch Gladbach, Germany). ...
... During this period, a small fraction of adoptively transferred naive CD44 low polyclonal CD8 + T cells undergo LIP in B6 neonates. Importantly, however, most naive CD8 + T cells do not undergo LIP (figure 3A in Ref. 10). To test whether neonatal non-T cells suppress LIP, naive TCR-transgenic (TCR tg ) CD8 + Thy1.1 + OT-I T cells were labeled with CFSE and transferred into Thy1.2 ...
In adult mice, lymphopenia-induced proliferation (LIP) leads to T cell activation, memory differentiation, tissue destruction, and a loss of TCR diversity. Neonatal mice are lymphopenic within the first week of life. This enables some recent thymic emigrants to undergo LIP and convert into long-lived memory T cells. Surprisingly, however, most neonatal T cells do not undergo LIP. We therefore asked whether neonate-specific mechanisms prevent lymphopenia-driven T cell activation. In this study, we show that IL-7R-dependent innate lymphoid cells (ILCs) block LIP of CD8(+) T cells in neonatal but not adult mice. Importantly, CD8(+) T cell responses against a foreign Ag are not inhibited by neonatal ILCs. This ILC-based inhibition of LIP ensures the generation of a diverse naive T cell pool in lymphopenic neonates that is mandatory for the maintenance of T cell homeostasis and immunological self-tolerance later in life.
... Such homeostatic memory CD8 T cells express numerous phenotypic and functional traits of conventional memory CD8 T cells, including their ability to effectively control infection (1,12,13). Although initially studied in the context of induced lymphopenia (1,(14)(15)(16), additional studies suggested that physiological lymphopenia occurs during the neonatal period in mice, and induces production of homeostatic memory T cells (14,(16)(17)(18)(19)(20). Finally, recent studies from our group and others have identified a third mechanism for generating memory-like CD8 T cells, which involves the response of thymocytes to the cytokine IL-4. ...
... These findings are similar to the changes observed in the bulk CD8 T cell pool, which gradually accumulates in the periphery from birth to 5 wk of age (Fig. 5A), whereas we found that the frequency of bulk memoryphenotype CD8 T cells peaks in younger animals (Fig. 5B), similar to the findings reported by Ichii et al. (18) earlier. Previous studies showed that adoptively transferred naive T cells proliferate in neonatal mice and acquire the phenotype of memory cells (17)(18)(19)(20), suggesting that the neonatal environment is functionally lymphopenic. Hence, taken together, our data are most consistent with VM cells arising in the peripheral pool, in response to neonatal lymphopenia, rather than being generated in the thymus. ...
... Previous studies, using adoptive transfer approaches, suggested that lymphopenia supports homeostatic proliferation of naive CD8 T cells in very young neonatal mice, but that this effect was lost by 2 wk after birth (20). In contrast, we observed continued accumulation of VM cells up until 4 wk after birth (Fig. 4). ...
Memory CD8(+) T cells are an important component of the adaptive immune response against many infections, and understanding how Ag-specific memory CD8(+) T cells are generated and maintained is crucial for the development of vaccines. We recently reported the existence of memory-phenotype, Ag-specific CD8(+) T cells in unimmunized mice (virtual memory or VM cells). However, it was not clear when and where these cells are generated during normal development, nor the factors required for their production and maintenance. This issue is especially pertinent given recent data showing that memory-like CD8 T cells can be generated in the thymus, in a bystander response to IL-4. In this study, we show that the size of the VM population is reduced in IL-4R-deficient animals. However, the VM population appears first in the periphery and not the thymus of normal animals, suggesting this role of IL-4 is manifest following thymic egress. We also show that the VM pool is durable, showing basal proliferation and long-term maintenance in normal animals, and also being retained during responses to unrelated infection.
... More importantly, this response is not toward a single foreign antigen, but is a polyclonal response. This expansion of T cells also allows low-affinity T cells that normally would not have been activated to acquire effector traits: expressing CD44, Ly6c and producing IFNγ [129][130][131][132][133]. During HP, T cells are allowed to expand in response to self-antigen, thus skewing the TCR repertoire toward self-reactivity [134][135][136][137]. ...
... HP is distinct from T cell expansion in response to a foreign antigen, as it allows for expansion of T cells that have a low affinity for selfantigens. These self-reactive T cells become activated and acquire features of memory T cells, including expression CD44 and Ly6c markers and producing IFNϒ, granzyme B, and perforin following short-term PMA stimulation [129][130][131][132][133]. Therefore, HP allows for the proliferation and activation of possible self-reactive T cells creating an environment for the development of autoimmunity. ...
T cell development begins with the migration of early progenitors from the bone marrow to the thymus. Once out of the thymus, na??ve T cells are held in an inactive state until they encounter their cognate antigen/MHC complex to further differentiate into effector T cells. Many studies have shown that Wnt signaling is a crucial pathway that regulates many aspects of T cell biology from development to effector T cell polarization, however, little is known about the role of Wnt in na??ve T cells. In this study, we show that Adenomatous polyposis coli (APC) is critical for the maintenance of na??ve T cells. Wnt signaling has also been linked to cancers, though Wnt???s association with autoimmune disease is less known. Here, we show an association between ectopic dysregulation of Wnt causing a T-cell driven autoimmunity with a Crohn???s disease-like phenotype.
Using an Apc conditional allele that has loxP sites inserted into introns 13 and 14 of the endogenous Apc gene under the control of a T cell specific cre-recombinase, CD4-Cre or Lck-cre, we can delineate the function of Apc in T cells. Flow cytometry, survival curves, and immunohistochemistry were used to study the T cell phenotype and autoimmune disease caused by ectopic activation of Wnt. Creation of bone marrow chimeras allowed us to investigate whether this T cell phenotype was cell-intrinsic and whether the observed autoimmunity was linked to homeostatic proliferation. Using an exon 3 deletion of cMyc, we studied the impact of transcriptional activation of cMyc.
Though an Apc deficiency does not affect thymocyte development, we show that it does causes a massive cell-intrinsic loss of peripheral na??ve T cells. We concluded that loss of Apc not only causes a loss of na??ve T cells due to cMyc??-induced apoptosis, but also results in highly activated T cells that drives the development of a Crohn???s disease-like phenotypes. Our study has further linked ectopic Wnt activation to autoimmunity through a T cell specific deletion of Apc, suggesting a direct role of Wnt in regulating T cell driven etiology of autoimmunity.
... Similarly, conditions of extreme lymphopenia induce the formation of memory phenotype cells through HP induced by cytokines such as IL-7, IL-12, and IL-15 (4). This form of proliferation results in the expression of many, though not all, memory activation markers and the acquisition of an increased degree of immune protective function relative to naive phenotype cells (5)(6)(7)(8)(9)(10)(11). Although HP requires TCR/MHC interactions (5,(12)(13)(14), it does not require or induce overt TCR-mediated stimulation, as evidenced by the differential expression of activation markers such as CD49d (15). ...
... Perhaps the most significant application of our data is the fact that we have identified a physiologically relevant, lymphopeniaindependent form of T cell homeostatic expansion. Although the reality of HP in extreme lymphopenia is well established (5)(6)(7)(8)(9)(10)(11), the physiological relevance of HP outside of bone marrow transplantation is unclear, as is the representation of HP memory T cells within a normal, unmanipulated host. Our data address both of these gaps in the knowledge base first by identifying nominal Ag-specific T cells within the pool of memory phenotype cells in the unprimed host and second by clarifying the mechanism by which these cells come into existence. ...
Various populations of memory phenotype CD8(+) T cells have been described over the last 15-20 y, all of which possess elevated effector functions relative to naive phenotype cells. Using a technique for isolating Ag-specific cells from unprimed hosts, we recently identified a new subset of cells, specific for nominal Ag, but phenotypically and functionally similar to memory cells arising as a result of homeostatic proliferation. We show in this study that these virtual memory (VM) cells are independent of previously identified innate memory cells, arising as a result of their response to IL-15 trans presentation by lymphoid tissue-resident CD8α(+) dendritic cells in the periphery. The absence of IL-15, CD8(+) T cell expression of either CD122 or eomesodermin or of CD8a(+) dendritic cells all lead to the loss of VM cells in the host. Our results show that CD8(+) T cell homeostatic expansion is an active process within the nonlymphopenic environment, is mediated by IL-15, and produces Ag-inexperienced memory cells that retain the capacity to respond to nominal Ag with memory-like function. Preferential engagement of these VM T cells into a vaccine response could dramatically enhance the rate by which immune protection develops.
... Starting at an age of 2 wk and lasting for at least 18 mo, approximately half of all NK cells defined as CD3-NK1.1 + lymphocytes displayed intermediate H2B-eGFP levels (GFP int ), whereas the majority of NK cells derived from 4-d-old mice uniformly showed low H2B-eGFP levels (GFP low ) (Fig. 1A, Supplemental Fig. 1B). This difference in very young mice may be due to loss of H2B-eGFP by homeostatic proliferation of NK cells because very young mice (less than 1 wk) are lymphopenic (34,35). To demonstrate such loss of H2B-eGFP through proliferation directly, purified NK cells from adult mice were cultured in the presence of high levels of IL-2 where they expanded and became lymphokine-activated killer cells. ...
... In very young mice, however, it appears that homeostatic cues drive NK cell proliferation and in so doing induce loss of intermediate H2B-eGFP levels. Similar to what is known for expansion of neonatal CD8 + cytotoxic T cells (34,35), it is likely that mechanisms involving IL-7 and IL-15 underlie the observed augmented proliferation of neonatal NK cells. ...
Mouse CMV (MCMV) infection rapidly induces the proliferation of NK cells, which correlates with immunological protection. Whether NK cells primed during acute response against MCMV are maintained for the long term is not known. In this study, we used TcrdH2BeGFP mice in which maturing NK cells are genetically labeled with a pulse of very stable histone-2B-eGFP. In this system, we found that the reporter protein was diluted out upon NK cell division during acute MCMV infection. At the same time, mature NK cells in uninfected mice showed only very limited turnover in vivo. Three months after primary infection when MCMV latency was established, the majority of peripheral NK cells still displayed a higher record of proliferation than NK cells in mock-infected controls. This observation included both Ly49H(+) and Ly49H(-) NK cells. Conversely, naive NK cells did not show more proliferation after transfer into latently MCMV-infected mice than that after transfer into mock-infected control mice. This indicated that the observed alterations of the NK cell compartment in MCMV latency were "legacy" (i.e., resulting from prior events during the initial immune response). Together, these results suggest that antiviral immune responses induce sustained alterations of innate lymphocyte populations that extend far beyond the first days of acute infection.
... This small proportion of activated CD8 + T cells may represent ''virtual memory'' (T VM ) T cells, which are abundant in neonates. 42,43 Increased activation status of CD8 + T cells at the early time point of 18 h post-infection may further implicate T VM cells, as they are capable of responding to infection faster than naive T cells due to their memory-like capacity. 44,45 An additional outstanding question raised by the present findings is the extent to which gd T cell effector cytokine responses depend directly on bacterial factors such as virulence factors vs. the upstream immune response to such bacterial components. ...
Neonates born prematurely are vulnerable to life-threatening conditions such as bacterial sepsis. Streptococcus agalactiae (GBS) and Escherichia coli are frequent causative pathogens of neonatal sepsis, however, it remains unclear if these pathogens induce differential immune responses. We find that γδ T cells rapidly respond to single-organism GBS and E. coli bloodstream infections in neonatal mice. Furthermore, GBS and E. coli induce distinct cytokine production from IFN-γ and IL-17 producing γδ T cells, respectively. We also find that IL-17 production during E. coli infection is driven by γδTCR signaling, whereas IFN-γ production during GBS infection occurs independently of γδTCR signaling. The divergent effector responses of γδ T cells during GBS and E. coli infections impart distinctive neuroinflammatory phenotypes on the neonatal brain. Thus, the neonatal adaptive immune system differentially responds to distinct bacterial stimuli, resulting in unique neuroinflammatory phenotypes.
... In some ways, the increased percentage of VM cells we found in the neonatal mice was not surprising, as earlier studies have suggested that CD8 + T cells undergo more homeostatic proliferation in neonatal mice because they are lymphopenic. 49,50 Due to this, the dogmatic view was that neonatal CD8 + T cells are exposed to greater amounts of homeostatic cytokines (IL-7 and IL-15) than adult CD8 + T cells and undergo more homeostatic proliferation in an attempt to "fill the space." To test whether it is true that CD8 + T cells proliferate more in early life to fill an empty periphery, we transplanted a ...
Historically, the immune system was believed to develop along a linear axis of maturity from fetal life to adulthood. Now, it is clear that distinct layers of immune cells are generated from unique waves of hematopoietic progenitors during different windows of development. This model, known as the layered immune model, has provided a useful framework for understanding why distinct lineages of B cells and γδ T cells arise in succession and display unique functions in adulthood. However, the layered immune model has not been applied to CD8⁺ T cells, which are still often viewed as a uniform population of cells belonging to the same lineage, with functional differences between cells arising from environmental factors encountered during infection. Recent studies have challenged this idea, demonstrating that not all CD8⁺ T cells are created equally and that the functions of individual CD8⁺ T cells in adults are linked to when they were created in the host. In this review, we discuss the accumulating evidence suggesting there are distinct ontogenetic subpopulations of CD8⁺ T cells and propose that the layered immune model be extended to the CD8⁺ T cell compartment.
... 121 In line with these findings, it is widely reported that neonatal CD8+ T cells are highly proliferative through homeostatic proliferation. 100,122,123 This highly proliferative programme extends into activation. 124 Neonatal CD8+ ...
Immune responses of neonates differ markedly to those of adults, with skewed cytokine phenotypes, reduced inflammatory properties and drastically diminished memory function. Recent research efforts have started to unravel the role of cellular metabolism in determining immune cell fate and function. For studies in humans, much of the work on metabolic mechanisms underpinning innate and adaptive immune responses by different haematopoietic cell types is in adults. Studies investigating the contribution of metabolic adaptation in the unique setting of early life are just emerging, and much more work is needed to elucidate the contribution of metabolism to neonatal immune responses. Here, we discuss our current understanding of neonatal immune responses, examine some of the latest developments in neonatal immunometabolism and consider the possible role of altered metabolism to the distinctive immune phenotype of the neonate. Understanding the role of metabolism in regulating immune function at this critical stage in life has direct benefit for the child by affording opportunities to maximize immediate and long‐term health. Additionally, gaining insight into the diversity of human immune function and naturally evolved immunometabolic strategies that modulate immune function could be harnessed for a wide range of opportunities including new therapeutic approaches.
... The presence of these CD44+ T cells may indicate true, conventional memory T cells that resulted from previous microbial exposures in the murine specific pathogen-free (SPF) colony. Alternatively, such cells could also be unconventional memory cells that possess a memory phenotype despite not having encountered cognate antigen [31][32][33][34][35]. Nevertheless, CD44+ T cells of either origin could participate in early viral control through bystander-mediated activity and thus confer a protective advantage through rapid viral containment before the virusspecific T cell response has been generated [36]. ...
The COVID-19 pandemic has revealed that infection with SARS-CoV-2 can result in a wide range of clinical outcomes in humans. An incomplete understanding of immune correlates of protection represents a major barrier to the design of vaccines and therapeutic approaches to prevent infection or limit disease. This deficit is largely due to the lack of prospectively collected, pre-infection samples from indiviuals that go on to become infected with SARS-CoV-2. Here, we utilized data from genetically diverse Collaborative Cross (CC) mice infected with SARS-CoV to determine whether baseline T cell signatures are associated with a lack of viral control and severe disease upon infection. SARS-CoV infection of CC mice results in a variety of viral load trajectories and disease outcomes. Overall, a dysregulated, pro-inflammatory signature of circulating T cells at baseline was associated with severe disease upon infection. Our study serves as proof of concept that circulating T cell signatures at baseline can predict clinical and virologic outcomes upon SARS-CoV infection. Identification of basal immune predictors in humans could allow for identification of individuals at highest risk of severe clinical and virologic outcomes upon infection, who may thus most benefit from available clinical interventions to restrict infection and disease.
... These CD44 + T cells may represent true memory T cells that have resulted from previous microbial exposures, because these mice were housed in specific pathogen-free (SPF) but not germ-free conditions. Alternatively, these cells could also represent unconventional memory phenotype cells that acquired a memory phenotype without exposure to cognate antigen [29][30][31][32][33]. In either case, however, it is possible that these cells could participate in early viral control through bystander-mediated action and thus potentially confer a survival Based on these data, they were grouped into "No Mortality" or "Mortality in all 3" categories as shown in Table 1 advantage to the host through rapid viral containment before the generation of a virus-specific T-cell response [34]. ...
Background:
Virus infections result in a range of clinical outcomes for the host, from asymptomatic to severe or even lethal disease. Despite global efforts to prevent and treat virus infections to limit morbidity and mortality, the continued emergence and re-emergence of new outbreaks as well as common infections such as influenza persist as a health threat. Challenges to the prevention of severe disease after virus infection include both a paucity of protective vaccines, as well as the early identification of individuals with the highest risk that may require supportive treatment.
Methods:
We completed a screen of mice from the Collaborative Cross (CC) that we infected with influenza, SARS-coronavirus, and West Nile virus.
Results:
CC mice exhibited a range of disease manifestations upon infections, and we used this natural variation to identify strains with mortality following infection and strains exhibiting no mortality. We then used comprehensive pre-infection immunophenotyping to identify global baseline immune correlates of protection from mortality to virus infection.
Conclusions:
These data suggest that immune phenotypes might be leveraged to identify humans at highest risk of adverse clinical outcomes upon infection, who may most benefit from intensive clinical interventions, in addition to providing insight for rational vaccine design.
... In addition to this canonical pathway, it has become evident that unconventional T cells with a memory phenotype can develop without exposure to foreign antigens, but rather through homeostatic proliferation driven by lymphopenia (4)(5)(6)(7)(8). Recently, it was shown that antigen-specific, nonself-reactive CD8+ T cells from unimmunized/unmanipulated hosts are detectable in germ-free and specific pathogen-free mice. ...
Significance
T cell-mediated immune memory provides the host with the capacity to rapidly and efficiently respond to pathogen reencounter, often without symptoms or disease. In addition to conventional memory T cell responses that are pathogen-specific, there also exists a subset of unconventional memory T cells that share many of the characteristics of conventional memory T cells without the initiating requirement of cognate antigen recognition. Here, we demonstrate a mechanism whereby regulatory T cells limit the expansion of these unconventional memory T cells to preserve a host’s capacity to mount a memory CD8 T cell response that remains protective upon pathogen reencounter.
... Conversely, naive mice deficient for IL-4 production or signaling have a reduced frequency of IN memory CD8 T cells (21,22). In physiological conditions, LIP memory cells generation occurs during the neonatal period in naive mice (21,23,24) and Th2 immune responses might also favor the generation of IN memory CD8 T cells (25). ...
The pool of memory-phenotype CD8 T cells is composed of Ag-induced (AI) and cytokine-induced innate (IN) cells. IN cells have been described as having properties similar to those of AI memory cells. However, we found that pathogen-induced AI memory cells can be distinguished in mice from naturally generated IN memory cells by surface expression of NKG2D. Using this marker, we described the increased functionalities of AI and IN memory CD8 T cells compared with naive cells, as shown by comprehensive analysis of cytokine secretion and gene expression. However, AI differed from IN memory CD8 T cells by their capacity to migrate to the lung parenchyma upon inflammation or infection, a process dependent on their expression of ITGA1/CD49a and ITGA4/CD49d integrins.
... IL-7 and IL-15 signaling have both been further shown to be important in the long-term maintenance of CD4 + and CD8 + memory T cells by promoting homeostatic proliferation [71,72,75]. In both humans and mice, T cells derived from fetal tissues or cord blood have an increased tendency to proliferate in response IL-7 and IL-15 signaling relative to adult controls [76,77]. The increased susceptibility of neonatal T cells to apoptosis following activation can be ameliorated by IL-7 and IL-15 in vitro [62,63], suggesting that this pathway could be targeted for enhancing memory T cell generation from neonatal T cells. ...
The immune system in early life is tasked with transitioning from a relatively protected environment to one in which it encounters a wide variety of innocuous antigens and dangerous pathogens. The immaturity of the developing immune system, and particularly the distinct functionality of T lymphocytes in early life, has been implicated in increased susceptibility to infection. Previous work has demonstrated that immune responses in early life are skewed toward limited inflammation and atopy; however, there is mounting evidence that such responses are context- and tissue-dependent. The regulation, differentiation, and maintenance of infant T cell responses, particularly as it relates to tissue compartmentalization, remains poorly understood. How the tissue environment impacts early-life immune responses and whether the development of localized protective immune memory cell subsets are established is an emerging area of research. As infectious diseases affecting the respiratory and digestive tracts are a leading cause of morbidity and mortality worldwide in infants and young children, a deeper understanding of site-specific immunity is essential to addressing these challenges. Here, we review the current paradigms of T cell responses during infancy as they relate to tissue localization and discuss implications for the development of vaccines and therapeutics.
... Our results are consistent with findings that mouse, neonatal CD8 + T cells exhibit increased proliferative expansion when stimulated (Smith et al., 2014), as do human cord-blood T cells (Schönland et al., 2003;Schüler et al., 2004)-features consistent with progression toward a terminal differentiation program. Infection of T-bet +/− infant mice expressing intermediate levels of T-bet, promoted TRM establishment at levels similar to that observed in both WT and T-bet +/− adults, suggesting a key role for that regulator in determining infant T cell fate. ...
Infants suffer disproportionately from respiratory infections and generate reduced vaccine responses compared with adults, although the underlying mechanisms remain unclear. In adult mice, lung-localized, tissue-resident memory T cells (TRMs) mediate optimal protection to respiratory pathogens, and we hypothesized that reduced protection in infancy could be due to impaired establishment of lung TRM. Using an infant mouse model, we demonstrate generation of lung-homing, virus-specific T effectors after influenza infection or live-attenuated vaccination, similar to adults. However, infection during infancy generated markedly fewer lung TRMs, and heterosubtypic protection was reduced compared with adults. Impaired TRM establishment was infant–T cell intrinsic, and infant effectors displayed distinct transcriptional profiles enriched for T-bet–regulated genes. Notably, mouse and human infant T cells exhibited increased T-bet expression after activation, and reduction of T-bet levels in infant mice enhanced lung TRM establishment. Our findings reveal that infant T cells are intrinsically programmed for short-term responses, and targeting key regulators could promote long-term, tissue-targeted protection at this critical life stage.
... Due to the small numbers of immune cells in murine neonates, the peripheral lymphoid environment can be characterized as lymphopenic. Indeed, recent studies 39,40 have shown that both CD4 + and CD8 + adult T cells proliferate spontaneously (i.e., undergo homeostatic proliferation) and acquire Th1/Tc1 effector function when transferred to 1-day-old neonatal mice. This spontaneous cycling was associated with the upregulation of the surface molecule CD44, a marker of previous antigen exposure and proliferation. ...
Murine neonatal CD4+ responses are often biased to Th2 function. There is increasing evidence that this phenomenon may be regulated both at the level of the thymus and the peripheral lymphoid compartment. In particular, residual fetal influence on the neonatal thymus may lead to an imprinting of developing T cells that is maintained in CD4+ cells when they emigrate to peripheral organs. Such imprinting may involve epigenetic modification of the Th2 cytokine gene locus and acquisition of the capacity to undergo rapid cell cycling. These properties, coupled with the homeostatic proliferation occurring in the peripheral tissues of neonates, shape a CD4+ population with the capacity for enhanced Th2 responsiveness. Neonatal T helper cell responses Over the past decade, it has become clear that murine neonates are competent to develop mature CD4-mediated inflammatory responses, leading to adult-level Th1 and CTL function. 1 However, these mature responses are generally only achieved under very potent Th1-driving conditions. What is more often seen in murine neonates is a deviance to Th2 function. 2-4 Notably, mice initially immunized as neonates mount Th2-dominant memory responses when re-exposed to the same antigens as adults. Moreover, the primary antigen-specific responses that develop in vivo during neonatal life are Th2-skewed. 5 This phenomenon is apparently widespread since it has been observed in neonatal mice of both the BALB/c and C57BL/6 strains and with a variety of different antigens under conventional immunization conditions. Furthermore, a bias to Th2 function is also evident in vitro since freshly isolated neonatal, but not adult, T cells rapidly produce high levels of the Th2 cytokine IL-4 upon polyclonal stimulation in culture. 6 Interestingly, the in utero exposure of human fetuses to common environmental antigens also results in universally Th2-biased responses. 7-9 Thus, murine neonatal responses may be considered an excellent model for understanding human Th2 pathologies that begin in early life, such as asthma and allergy. While the observation of a neonatal Th2 bias is well established, the mechanisms underlying the phenomenon are less clear. This is due, in part, to the complex nature of regulatory events during ontogeny. We propose that the robust Th2 responses of murine neonates are governed by processes occurring both within the thymus and post-thymically. Emerging evidence in support of this idea is summarized in the sections to follow.
... VM cells appear in the periphery rapidly following the postnatal period, reaching their peak frequency at approximately 3 weeks of age (161). Because neonatal mice support lymphopenia-induced proliferation (165,166), and because of their many phenotypic similarities to HP memory cells, VM cells are often assumed to be naturally-occurring HP memory cells (159,161,162). and IL-18 (159,167,168). ...
A variety of external factors can affect the activation status, effector function, and memory differentiation of T cells. Such factors include resident microbiota and the cytokine milieu produced in a host. Loss of tolerance to commensal bacteria and excess inflammatory cytokines such as IL-4 have been implicated in the pathogenesis of many diseases, including inflammatory bowel disease (IBD) and allergy. Although these diseases are traditionally associated with CD4+ T cells, it is increasingly appreciated that CD8+ T cells in these settings may either contribute to or reduce pathology. This work explores the influence of various factors on CD8+ T-cell responses in an inflammatory setting, mice deficient in Nedd4-family interacting protein 1 (Ndfip1). Ndfip1 restricts IL-4 production in CD4+ T cells by facilitating degradation of the transcription factor JunB. Ndfip1-deficient CD4+ T cells have increased JunB levels and consequently overproduce IL-4. This excess IL-4 impairs Th17 and iTreg differentiation. Mice lacking Ndfip1 develop severe TH2-mediated inflammation at sites of environmental antigen exposure, including skin, GI tract, and lung, and develop IBD-like symptoms. We first tested the role of bacterial antigens in triggering this phenotype by treating mice lacking Ndfip1 in T cells with a cocktail of antibiotics to deplete intestinal bacteria. We then analyzed T cells for activation markers and examined tissues for signs of inflammation in the GI tract. Next, we analyzed whether exposure to the high levels of IL-4 in Ndfip1KO mice affects CD8+ T cell differentiation. We concluded that IL-4, but not intestinal bacteria, is required for the increase in activated/memory phenotype CD8+ T cells observed in Ndfip1KO mice. These findings add to the growing body of literature describing the importance of extrinsic cytokines to the development of memory-phenotype CD8+ T cells, and raise the possibility that such cells may be clinically relevant in diseases which are characterized by local increases in IL-4.
... Alternatively, the memory-like cells we detected in unimmunized animals may be antigen inexperienced, having been generated from the naive T cell pool through homeostatic mechanisms. Conversion from naive to functional memory phenotype cells occurs in response to induced lymphopenia ( 16,17,36 ) but has also been reported to occur in physiological circumstances of lymphopenia, such as in neonatal mice ( 7,9 ). To begin distinguishing these possibilities, we analyzed the antigen-specifi c T cell pool in gnotobiotic (GF) mice. ...
... Ο πολλαπλασιασμός αυτός επάγεται από το λεμφοπενικό περιβάλλον που επικρατεί κατά περίπτωση σε κάθε όργανο, οφείλεται πιθανόν στην ευαισθησία των Τ-κυττάρων στις κυτταροκίνες που έχουν κοινή γ-αλυσίδα και λαμβάνει χώρα απουσία αντιγονικής διέγερσης. 79,80 σημαντική πρόοδος αναφορικά με την κατανόηση των μηχανισμών που ελέγχουν την ΤΗ-διαφοροποίηση στα νεογνά επιτεύχθηκε την τελευταία δεκαετία από τις μελέτες της ομάδας του Becky Adkins. 81 κομβικής σημασίας σημείο της ερευνητικής τους προσπάθειας ήταν η αναγνώριση του γεγονότος ότι τα έως τώρα αποτελέσματα που Εικόνα 4. Μια από τις βασικότερες επιδημιολογικές μελέτες που υποστήριξαν τη διατύπωση της υπόθεσης της υγιεινής ήταν αυτή που αποκάλυψε τη θεαματική αύξηση της επίπτωσης των αλλεργικών νοσημάτων σε παιδιά από την ανατολική Γερμανία, που γεννήθηκαν μετά από την πτώση του Τείχους του Βερολίνου (1989). ...
Immaturity of the neonatal immune system has been suggested in the past as the factor underlying the high rates of morbidity and mortality from infections in newborn infants and the impairment of the neonatal immune response to most vaccines. A biased TH1/TH2 differentiation favoring TH2- rather than TH1-polarization has been incriminated in the susceptibility of newborns to allergic reactions and microbial infections, respectively. Over the last decade, increasing evidence has been produced which indicates that the neonatal immune system has been shaped to accommodate the abrupt transition of the newborn infant from a sterile intrauterine environment into a world rich in foreign antigens, with the concomitant avoidance of harmful inflammatory responses. At the same time, neonatal immunity appears to have a high degree of plasticity and flexibility, allowing mature responses when the infant is faced with life-threatening infectious agents. Deviations from this equilibrium are responsible for many aspects of neonatal mortality. Improving the understanding of neonatal immune function, provides the possibility of developing targeted therapeutic interventions against infections, allergies and other pathological mechanisms related to intrauterine inflammation. In addition, the documented ability of newborn infants to achieve mature immune functions under certain conditions of antigen exposure has provided incentive for the development of novel approaches to enhancing the efficacy of neonatal vaccination.
... Interestingly, regardless of the intensity of conditioning, hostderived hematopoietic cells, including conventional T cells, Tregs, and DCs, increased in number after transplantation, peaked on day 4, and subsequently decreased. Although host immune cells peaked earlier than donor T cells, this transient cell expansion is considered mainly to have been due to homeostatic proliferation (40,41); however, because the magnitude of the transient increase in the number of host Tregs in RIST on day 4 significantly diminished (to ∼30%) in a syngeneic transplant setting (Fig. 3E), this expansion was related partially to allogeneic response. Whether increased host Tcons (CD4 + or CD8 + T cells) exerted host-versus-graft reaction remains to be determined because it could not be analyzed; however, this may be not the case as the percentage of host effector/memory phenotype (CD44 + CD62L 2 ) T cells was significantly lower in the RIST recipients than in the MAST recipients (Supplemental Fig. 3A, 3B). ...
Host Foxp3+CD4+ regulatory T cells (Tregs) have been shown to suppress graft-versus-host disease (GVHD) in experimental bone marrow transplantation (BMT) models; however, the detailed mechanism is unknown. To address this issue, we established a murine MHC-haploidentical BMT model (BDF1 (H-2b/d) → B6C3F1 (H-2b/k)), in which transplantation following conditioning with high-dose (13 Gy) or low-dose (5 Gy) total body irradiation corresponds to myeloablative stem cell transplantation (MAST) or reduced-intensity stem cell transplantation (RIST) BMT. All MAST recipients died of GVHD within 70 d, whereas RIST recipients developed almost no GVHD and survived for at least 3 mo. In this BMT model, we investigated the kinetics of immune cells in the mesenteric lymph nodes because GVHD was most prominent in the intestines. Host Tregs that survived after total body irradiation could proliferate transiently by day 4. Comparing the kinetics of immune cells among MAST, RIST, and anti-CD25 mAb-treated RIST, we found that the transiently surviving host Tregs were fully functional, closely contacted with host dendritic cells (DCs), and significantly restrained the maturation (CD80 and CD86 expression) of DCs in a dose-dependent manner. There was a positive correlation between the ratio of DCs to host Tregs and the extent of maturation of DCs. Host Tregs suppressed alloresponse mainly by contact inhibition. Host Tregs are already active in lymph nodes before transplantation and restrain the maturation of host DCs, thereby dampening the ability of DCs to activate allogeneic donor T cells and consequently reducing the magnitude of graft-versus-host reaction. Thus, host Tregs are negative regulators of host DCs that act in the peritransplantation period.
... We found that the adoptively transferred CD8a + cells, after homeostatic expansion, also virtually all displayed a memory phenotype in spleen, lymph node, and blood, representing either TCM (CD44-high, CD62L-high) or TEM (CD44-high, CD62L-low) cells (Supplemental Fig. 2B). The conversion of naive CD8 + T cells into CD44-high memory-type CD8 + cells following adoptive transfer into lymphopenic hosts has been noted previously (39) and may have promoted the emergence of keratitogenic CD8 + T cells from normal B10 donors in this experiment. ...
To prevent potentially damaging inflammatory responses, the eye actively promotes local immune tolerance via a variety of mechanisms. Owing to trauma, infection, or other ongoing autoimmunity, these mechanisms sometimes fail, and an autoimmune disorder may develop in the eye. In mice of the C57BL/10 (B10) background, autoimmune keratitis often develops spontaneously, particularly in the females. Its incidence is greatly elevated in the absence of γδ T cells, such that ∼80% of female B10.TCRδ(-/-) mice develop keratitis by 18 wk of age. In this article, we show that CD8(+) αβ T cells are the drivers of this disease, because adoptive transfer of CD8(+), but not CD4(+), T cells to keratitis-resistant B10.TCRβ/δ(-/-) hosts induced a high incidence of keratitis. This finding was unexpected because in other autoimmune diseases, more often CD4(+) αβ T cells, or both CD4(+) and CD8(+) αβ T cells, mediate the disease. Compared with wild-type B10 mice, B10.TCRδ(-/-) mice also show increased percentages of peripheral memory phenotype CD8(+) αβ T cells, along with an elevated frequency of CD8(+) αβ T cells biased to produce inflammatory cytokines. In addition, B10.TCRδ-/- mice have fewer peripheral CD4(+) CD25(+) Foxp3(+) αβ regulatory T cells (Tregs), which express lower levels of receptors needed for Treg development and function. Together, these observations suggest that in B10 background mice, γδ T cells are required to generate adequate numbers of CD4(+) CD25(+) Foxp3(+) Tregs, and that in B10.TCRδ(-/-) mice a Treg deficiency allows dysregulated effector or memory CD8(+) αβ T cells to infiltrate the cornea and provoke an autoimmune attack.
... This is likely due to the fact that the first migrants into the periphery of young mice undergo lymphopenia-induced proliferation in response to the "space" in the peripheral lymphoid organs, a phenomenon that is accompanied by CD44 up-regulation (155). These CD44 hi cells remain to constitute the fraction of previouslyactivated cells typical of peripheral lymphoid organs (156). However, the frequency of these cells gradually diminishes in wildtype spleens, as cells continually migrate from the thymus into the periphery, filling up space and preventing further proliferation of the newly emigrated cells from the thymus [ Figure 8C]. ...
The Tec family kinases Itk and Rlk have been previously shown to have an important role in signaling downstream of the T cell receptor [TCR]. Almost all of the work done in the past on these two kinases looked at their role in conventional αβ T cells, specifically CD4+ T cells. These studies demonstrated functions for Itk [primarily] and Rlk in T cell development, activation, and differentiation. However, despite the wealth of knowledge on conventional CD4+ T cells, prior to the work presented here little to no studies addressed the role of Tec family kinases on CD8+ or innate T cell development. My studies show a clear role for Itk [and in some cases Rlk] in innate T cell development; whether it be deprecating, in the case of innate CD8+ T cells or some subsets of γδ T cells, or beneficial, in the case of NKT cells. I show that Itk has a crucial role in conventional CD8+ T cell development, as absence of Itk [or Itk and Rlk] causes strongly reduced numbers of conventional CD8+ T cells and a vigorous enhancement of an innate-like CD8+ T cell population. In NKT cells, my work demonstrates that Itk [and to a lesser extent Rlk] is required for terminal maturation, survival, and cytokine secretion. Finally, on γδ T cells Itk is important in maintaining the Th1 cytokine secretion profile usually associated with these cells, and regulating the development of CD4+ or NK1.1+ γδ T cells. Taken together, this work clearly illustrates an important role for Tec family kinases in innate T cell development and function.
... Le TCR peut être impliqué mais les molécules coactivatrices ne sont pas requises (CD28, CD40… -Prlic 2001-). Les lymphocytes pourraient acquérir une plus grande sensibilité aux peptides du soi et aux antigènes périphériques(Fry 2001, Plas 2002, Kieper 2004, Almeida 2005 par baisse de leur seuil d'activation (Marleau 2005) maintenant la prolifération de certains clones, même au stade de mémoire(Schuler, 2004). La production de facteurs de croissance semble importante, notamment d'Il-7 pour les lymphocytes naïfs et T CD4+ alors que l'Il-15 serait plus active sur les lymphocytes cytotoxiques (T CD8+, NK et Tγδ -Marrack ;Prlic 2002-). ...
... La reprise de la thymopoïèse a lieu parce que les suspensions de lymphocytes transférées contiennent toujours un petit nombre de progéniteurs T qui colonisent le thymus et génèrent de nouveaux lymphocytes naïfs. La colonisation du thymus est favorisée lorsque les receveurs sont irradiés(Tanchot, Le Campion et al. 2002).La majorité des conclusions apportées par les travaux de transferts adoptifs pourraient donc être remise en cause ou s'avérer être irrelevante après comparaison avec les situations physiologiques où l'amplitude de variation du nombre de LyT est bien moins importante.Cependant de manière assez rassurante des études portant sur des souris néonatales où les organes lymphoïdes sont pratiquement vides en LyT ont montré qu'il y'a bien de la PIL(Min, McHugh et al. 2003;Schuler, Hammerling et al. 2004;Min, Yamane et al. 2005). Au cours de la prolifération, les LyT transférés ou les RTE subissent une modification de phénotype et ils adoptent un phénotype mémoire. ...
T lymphocyte homeostasis is tightly regulated. Cytokines such as interleukin-7 (IL-7) are critically involved in regulating T cell homeostasis. During this thesis work, in order to investigate more precisely the role of IL-7 on lymphopenia-induced proliferation, we have transferred CFSE-labeled cells into novel lymphopenic IL-7 transgenic mouse lines. Results obtained indicate that T lymphocyte subsets do not respond in the same way to IL-7 homeostatic signals. We took advantage of these experiments to dissect pleiotropic effects of IL-7 on T lymphocytes. Moreover, studies of our transgenic animals also provided new insights into effects of IL-7 on early T cell development. Taken together our results indicates that increased IL-7 availability perturbs T cell homeostasis by modulating peripheral T cell proliferation and survival rather than T cell development.
... While work on lymphopenia-induced memory has relevance for situations of lymphopenia that accompany some infections and therapeutic treatments (see Section 4), it was less clear that innate memory cells would arise during normal physiology by this mechanism. However, several groups found that T cells transferred into neonatal mice undergo proliferative responses as if the environment was lymphopenic, and this correlates with the finding that the peripheral T cell compartment takes several weeks to "fill" to the levels seen in adult mice (Akue, Lee, & Jameson, 2012;Le Campion et al., 2002;Min et al., 2003;Schuler, Hammerling, & Arnold, 2004). This raised the possibility that neonatal lymphopenia might spark a physiological form of lymphopenia-induced proliferation and memory differentiation in the first waves of T cells leaving the thymus. ...
Memory T cells are usually considered to be a feature of a successful immune response against a foreign antigen, and such cells can mediate potent immunity. However, in mice, alternative pathways have been described, through which naïve T cells can acquire the characteristics and functions of memory T cells without encountering specific foreign antigen or the typical signals required for conventional T cell differentiation. Such cells reflect a response to the internal rather the external environment, and hence such cells are called innate memory T cells. In this review, we describe how innate memory subsets were identified, the signals that induce their generation and their functional properties and potential role in the normal immune response. The existence of innate memory T cells in mice raises questions about whether parallel populations exist in humans, and we discuss the evidence for such populations during human T cell development and differentiation.
© 2015 Elsevier Inc. All rights reserved.
... Additionally, it was shown that IL-7 was required for the proliferation of naïve cells in response to low affinity Ags during HPE 86,87 , whereas IL-15, IL-4 and other cytokines tested were not required. Importantly, the role for IL-7 in the induction of HPE appears to be consistent regardless of the method by which lymphopenia is induced 86,88 . ...
... Thus, the comment by the Red Queen "It takes all the running you can do to keep in the same place" (in Through the Looking Glass, L. Carroll, 1872) can be applied to the peripheral immune system as well. [27][28][29][30][31][32][33] Since T cells require cognate receptor engagement in their "flight for survival", 28 we adapted the complementarity theorem of Dillon and Root-Bernstein 34,35 for a new T cell model. In this model a dynamic steady state is achieved through low affinity complementary TCR-MHC interactions between T cells and host cells. ...
Current consensus postulates that the class I-antigen processing system is evolved to present microbial antigens to specific T cells. Since such cells are rare and short-lived, they require three to five days to attain fighting strength. During this critical period he innate immune system holds back the briskly multiplying pathogens. Nevertheless, a T cell response is measurable in the lymph nodes draining the infection site within 12 to 18 h. In order to explain this paradox here we suggest a new T cell model. This is based on the observation that T cells require continuous contact of the T cell receptor (TCR) with selecting self-peptide–major histocompatibility complex (MHC) molecules in the periphery for their survival. We postulate that a dynamic steady state, a so-called coupled system is formed through low affinity complementary TCR–MHC interactions between T cells and host cells. Under such condition it is sufficient to recognize what is self in order to attack what is not self. A coupled system is regulated via soluble forms of peptide–MHC and TCR molecules by the law of mass action. In a coupled system one signal is sufficient for T cell activation. The new model implies that a significant fraction of the naive polyclonal T cells are recruited into the first line of defense from the very outset of an infection, so the number of activated T cells is increased by several orders of magnitude compared to conventional models. The one-signal model also predicts that therapeutic administration of soluble agonist or antagonist T cell receptor ligands may be able to fine tune the homeostatic physiological regulatory mechanism and thus improve the treatment of some chronic diseases such as metastatic cancer, HIV/AIDS, and transplantation.
... A second possibility relates to the fact that neonatal mice have few peripheral CD8 + T cells. As a result, neonatal CD8 + T cells attempt to "fill" the available space by undergoing extensive homeostatic proliferation (24,(44)(45)(46)(47). Therefore, it is possible that neonatal CD8 + T cells are more functionally reactive than adult CD8 + T cells because they have already divided numerous times before encountering foreign Ag. ...
Neonates often generate incomplete immunity against intracellular pathogens, although the mechanism of this defect is poorly understood. An important question is whether the impaired development of memory CD8(+) T cells in neonates is due to an immature priming environment or lymphocyte-intrinsic defects. In this article, we show that neonatal and adult CD8(+) T cells adopted different fates when responding to equal amounts of stimulation in the same host. Whereas adult CD8(+) T cells differentiated into a heterogeneous pool of effector and memory cells, neonatal CD8(+) T cells preferentially gave rise to short-lived effector cells and exhibited a distinct gene expression profile. Surprisingly, impaired neonatal memory formation was not due to a lack of responsiveness, but instead because neonatal CD8(+) T cells expanded more rapidly than adult cells and quickly became terminally differentiated. Collectively, these findings demonstrate that neonatal CD8(+) T cells exhibit an imbalance in effector and memory CD8(+) T cell differentiation, which impairs the formation of memory CD8(+) T cells in early life.
... Ο πολλαπλασιασμός αυτός επάγεται από το λεμφοπενικό περιβάλλον που επικρατεί κατά περίπτωση σε κάθε όργανο, οφείλεται πιθανόν στην ευαισθησία των Τ-κυττάρων στις κυτταροκίνες που έχουν κοινή γ-αλυσίδα και λαμβάνει χώρα απουσία αντιγονικής διέγερσης. 79,80 σημαντική πρόοδος αναφορικά με την κατανόηση των μηχανισμών που ελέγχουν την ΤΗ-διαφοροποίηση στα νεογνά επιτεύχθηκε την τελευταία δεκαετία από τις μελέτες της ομάδας του Becky Adkins. 81 κομβικής σημασίας σημείο της ερευνητικής τους προσπάθειας ήταν η αναγνώριση του γεγονότος ότι τα έως τώρα αποτελέσματα που Εικόνα 4. Μια από τις βασικότερες επιδημιολογικές μελέτες που υποστήριξαν τη διατύπωση της υπόθεσης της υγιεινής ήταν αυτή που αποκάλυψε τη θεαματική αύξηση της επίπτωσης των αλλεργικών νοσημάτων σε παιδιά από την ανατολική Γερμανία, που γεννήθηκαν μετά από την πτώση του Τείχους του Βερολίνου (1989). ...
Immaturity of the neonatal immune system has been suggested in the past as the factor underlying the high rates of
morbidity and mortality from infections in newborn infants and the impairment of the neonatal immune response
to most vaccines. A biased TH1/TH2 differentiation favoring TH2- rather than TH1-polarization has been incriminated in the susceptibility of newborns to allergic reactions and microbial infections, respectively. Over the last decade, increasing evidence has been produced which indicates that the neonatal immune system has been shaped to accommodate the abrupt transition of the newborn infant from a sterile intrauterine environment into a world rich in foreign antigens, with the concomitant avoidance of harmful inflammatory responses. At the same time, neonatal immunity appears to have a high degree of plasticity and flexibility, allowing mature responses when the infant is faced with life-threatening infectious agents. Deviations from this equilibrium are responsible for many aspects of neonatal mortality. Improving the understanding of neonatal immune function, provides the possibility of developing targeted therapeutic interventions against infections, allergies and other pathological mechanisms related to intrauterine inflammation. In addition, the documented ability of newborn infants to achieve mature mmune functions under certain conditions of antigen exposure has provided incentive for the development of novel approaches to enhancing the efficacy of neonatal vaccination.
... Several studies have examined TCR diversity of CD4 + CD25 + Foxp3 + Treg cells. 30,31 In neutral conditions, the TCR of CD4 + CD25 + Foxp3 + Treg cells is diverse. 32,33 We found characteristically skewed TCR use in CD8 + CD122 + CD49d high cells, which is different from that in CD4 + CD25 + Foxp3 + Treg cells. ...
We identified CD8(+) CD122(+) regulatory T cells (CD8(+) CD122(+) Tregs) and reported their importance in maintaining immune homeostasis. The absence of CD8(+) CD122(+) Tregs has been shown to lead to severe systemic autoimmunity in several mouse models, including inflammatory bowel diseases (IBD) and experimental autoimmune encephalomyelitis (EAE). The T-cell receptors (TCRs) expressed on CD8(+) CD122(+) Tregs recognize the target cells to be regulated. To aid in the identification of the target antigen(s) recognized by TCRs of CD8(+) CD122(+) Tregs, we compared the TCR diversity of CD8(+) CD122(+) T cells with that of conventional, naïve T cells in mice. We analyzed the use of TCR-Vβ in the interleukin 10 (IL-10)-producing population of CD8(+) CD122(+) T cells marked by high levels of CD49d expression, and found the significantly increased use of Vβ13 in these cells. Immunoscope analysis of the complementarity-determining region 3 (CDR3) of the TCR β-chain revealed remarkable skewing in a pair of Vβ regions, suggesting the existence of clonally expanded cells in CD8(+) CD122(+) T cells. Clonal expansion in Vβ13(+) cells was confirmed by determining the DNA sequences of the CDR3s. The characteristic TCR found in this study is an important building block for further studies to identify the target antigen recognized by CD8(+) CD122(+) Tregs. © 2013 The Authors. Immunology © 2013 Blackwell Publishing Ltd.
... That is, any T cell or TCR from such a repertoire can interact with some self MHC-self peptide, with some medium affinity. Several experiments [29, 30, 31, 32, 33, 34] seem to support that T cells in newborns are of homeostatic type. Also, an important experiment [35] shows the presence of lymphocyte reaction to self antigens in patients with no autoimmune diseases. ...
Our main tenet argues that the primary role of positive thymic selection and
the resulting T cell population is the maintenance of a homeostatic equilibrium
with self MHC-self peptide complexes. The homeostatic T cell repertoire can
recognize infections non-specifically and this is an indirect (negative)
recognition: the whole homeostatic T cell population together "holds a mirror"
to the whole self, and any MHC-peptide complex that is "not reflected in the
mirror" can be perceived by surrounding homeostatic T cells as a signal of the
presence of a foreign entity. On the other hand, infection-specific T cell
clones arise in a different pathway in the periphery, do not enter the thymus,
and form a functionally different population. Here we summarize the basic
assumptions and consequences of a logic-based new model, which differs from
conventional models in many respects.
... antigen primed) memory cells (Surh and Sprent, 2008), and are capable of undergoing a strong proliferative response when stimulated by foreign antigen and mediating protective immunity against infection (Hamilton et al., 2006). Although typically studied in artificial situations of lymphopenia, similar process may occur in normal physiological stages, such as during the lymphopenia associated with neonatal mice (Ichii et al., 2002;Min et al., 2003;Schuler et al., 2004). This raises the possibility that the endogenous T cell pool may contain memory-like cells produced without conventional priming ...
During a typical immune response to an acute pathogen, antigen-specific cells are activated, proliferate vigorously, and expand extensively. This expansion phase yields a large population of effector T cells, most of which will die in the subsequent contraction phase of the response (Figure 1). However, the expansion phase also yields cells that will eventually form the memory cell pool—primed cells maintained long term after immunization. Immune memory indicates a qualitatively and/or quantitatively distinct immune response upon successive (but interrupted) exposures to antigen. This leads to an improved secondary immune response compared to the primary response—“improved” in this context usually means greater in magnitude, faster, more sensitive to low doses of antigen, and more effective in the diversity or complexity of secondary effectors (Kaech and Wherry, 2007, Harty and Badovinac, 2008 and Williams and Bevan, 2007).
The neonatal phase of life is a time during which susceptibility to infection is particularly high, with prematurely born neonates being especially vulnerable to life-threatening conditions such as bacterial sepsis. While Streptococcus agalactiae, also known as group B Streptococcus (GBS) and Escherichia coli are frequent causative pathogens of neonatal sepsis, it is still unclear how the neonatal adaptive immune system responds to these pathogens. In the present study, we find that γδ T cells in neonatal mice rapidly respond to single-organism sepsis infections of GBS and E. coli, and that these infections induce distinct activation and effector functions from IFN-γ and IL-17 producing γδ T cells, respectively. We also report differential reliance on γδTCR signaling to elicit effector cytokine responses during neonatal sepsis, with IL-17 production during E. coli sepsis being associated with TCR signaling, whereas IFN-γ production during GBS sepsis is TCR-independent. Furthermore, we report that the divergent effector responses of γδ during GBS and E. coli sepsis impart distinctive neuroinflammatory phenotypes on the neonatal brain. The present study sheds light on how the neonatal adaptive immune response responds differentially to bacterial stimuli and how these responses impact neonatal sepsis-associated neuroinflammation.
Immunological experiences lead to the development of specific T and B cell memory, which readies the host for a later pathogen rechallenge. Currently, immunological memory is best understood as a linear process whereby memory responses are generated by and directed against the same pathogen. However, numerous studies have identified memory cells that target pathogens in unexposed individuals. How "pre-existing memory" forms and impacts the outcome of infection remains unclear. In this review, we discuss differences in the composition of baseline T cell repertoire in mice and humans, factors that influence pre-existing immune states, and recent literature on their functional significance. We summarize current knowledge on the roles of pre-existing T cells in homeostasis and perturbation and their impacts on health and disease.
The presence of CD8+ T cells with a memory phenotype in nonimmunized mice has been noted for decades, but it was not until about 2 decades ago that they began to be studied in greater depth. Currently called virtual memory CD8+ T cells, they consist of a heterogeneous group of cells with memory characteristics, without any previous contact with their specific antigens. These cells were identified in mice, but a few years ago, a cell type with characteristics equivalent to the murine ones was described in healthy humans. In this review, we address the different aspects of its biology mainly developed in murine models and what is currently known about its cellular equivalent in humans.
The COVID-19 pandemic has revealed that infection with SARS-CoV-2 can result in a wide range of clinical outcomes in humans, from asymptomatic or mild disease to severe disease that can require mechanical ventilation. An incomplete understanding of immune correlates of protection represents a major barrier to the design of vaccines and therapeutic approaches to prevent infection or limit disease. This deficit is largely due to the lack of prospectively collected, pre-infection samples from indiviuals that go on to become infected with SARS-CoV-2. Here, we utilized data from a screen of genetically diverse mice from the Collaborative Cross (CC) infected with SARS-CoV to determine whether circulating baseline T cell signatures are associated with a lack of viral control and severe disease upon infection. SARS-CoV infection of CC mice results in a variety of viral load trajectories and disease outcomes. Further, early control of virus in the lung correlates with an increased abundance of activated CD4 and CD8 T cells and regulatory T cells prior to infections across strains. A basal propensity of T cells to express IFNg and IL17 over TNFa also correlated with early viral control. Overall, a dysregulated, pro-inflammatory signature of circulating T cells at baseline was associated with severe disease upon infection. While future studies of human samples prior to infection with SARS-CoV-2 are required, our studies in mice with SARS-CoV serve as proof of concept that circulating T cell signatures at baseline can predict clinical and virologic outcomes upon SARS-CoV infection. Identification of basal immune predictors in humans could allow for identification of individuals at highest risk of severe clinical and virologic outcomes upon infection, who may thus most benefit from available clinical interventions to restrict infection and disease.
Summary
We used a screen of genetically diverse mice from the Collaborative Cross infected with mouse-adapted SARS-CoV in combination with comprehensive pre-infection immunophenotyping to identify baseline circulating immune correlates of severe virologic and clinical outcomes upon SARS-CoV infection.
We are just beginning to understand the diversity of the peripheral T cell compartment, which arises from the specialization of different T cell subsets and the plasticity of individual naive T cells to adopt different fates. Although the progeny of a single T cell can differentiate into many phenotypes following infection, individual T cells are biased towards particular phenotypes. These biases are typically ascribed to random factors that occur during and after antigenic stimulation. However, the T cell compartment does not remain static with age, and shifting immune challenges during ontogeny give rise to T cells with distinct functional properties. Here, we argue that the developmental history of naive T cells creates a ‘hidden layer’ of diversity that persists into adulthood. Insight into this diversity can provide a new perspective on immunity and immunotherapy across the lifespan. The authors describe how the naive T cell compartment is built across a lifetime. They propose that functional diversity among naive T cells is linked to when they were created. Naive T cells adapt to meet changes in the external environment at different stages of life, persist into adulthood and contribute to the T cell compartment in adults.
The survival of peripheral T cells is dependent on their access to peripheral lymph nodes (pLNs) and stimulation by Interleukin‐7 (IL‐7). In pLNs fibroblastic reticular cells (FRCs) and lymphatic endothelial cells (LECs) produce IL‐7 suggesting their contribution to the IL‐7‐dependent survival of T cells. However, IL‐7 production is detectable in multiple organs and is not restricted to pLNs. This raises the question whether pLN‐derived IL‐7 is required for the maintenance of peripheral T cell homeostasis. Here, we show that numbers of naive T cells (TN) remain unaffected in pLNs and spleen of mice lacking Il7 gene activity in pLN FRCs, LECs or both. In contrast, frequencies of central memory T cells (TCM) are reduced in FRC‐specific IL‐7 knockout mice. Thus, steady state IL‐7 production by pLN FRCs is critical for the maintenance of TCM, but not TN, indicating that both T cell subsets colonize different ecological niches in vivo. This article is protected by copyright. All rights reserved
Interleukin-7 (IL-7) availability determines the size and proliferative state of the resting T cell pool. However, the mechanisms that regulate steady-state IL-7 amounts are unclear. Using experimental lymphopenic mouse models and IL-7-induced homeostatic proliferation to measure IL-7 availability in vivo, we found that radioresistant cells were the source of IL-7 for both CD4⁺ and CD8⁺ T cells. Hematopoietic lineage cells, although irrelevant as a source of IL-7, were primarily responsible for limiting IL-7 availability via their expression of IL-7R. Unexpectedly, innate lymphoid cells were found to have a potent influence on IL-7 amounts in the primary and secondary lymphoid tissues. These results demonstrate that IL-7 homeostasis is achieved through consumption by multiple subsets of innate and adaptive immune cells.
Memory-phenotype CD8(+) T cells exist in substantial numbers within hosts that have not been exposed to either foreign antigen or overt lymphopenia. These antigen-inexperienced memory-phenotype T cells can be divided into two major subsets: 'innate memory' T cells and 'virtual memory' T cells. Although these two subsets are nearly indistinguishable by surface markers alone, notable developmental and functional differences exist between the two subsets, which suggests that they represent distinct populations. In this Opinion article, we review the available literature on each subset, highlighting the key differences between these populations. Furthermore, we suggest a unifying model for the categorization of antigen-inexperienced memory-phenotype CD8(+) T cells.
During the past three decades, the field of neuroimmune interactions grew steadily, and has now evolved into a sophisticated and credible area of biology. A number of terms have been proposed to name this multidisciplinary science. Because this science deals with the physiology and pathophysiology of higher organisms, in 2000 the term neuroimmune biology (NIB) was coined. It is clear that the central nervous system (CNS), the endocrine system (ES), and the immune system (IS) form a regulatory circuit, which integrates, coordinates, and regulates all functions in higher organisms from conception till death. It appears that in utero the IS relies on placental lactogens for development. After birth, pituitary GH, PRL, IGF-1, and the HPA axis are involved in regulating ADIM function. This applies to naïve lymphocytes. After priming with antigen, ADIM cells depend on type I CTKs for their survival and function. This is the case also for memory cells. Type I CTKs also support the HP and survival of naïve and memory T cells, B cells, and NK cells. Clearly, GLH (GH, PRL, and placental lactogens) are involved in the development of the IS, including the bone marrow, thymus, and maintain naïve lymphocytes until they are primed with antigen (e.g., maintenance of immunocompetence). GLH are involved in regeneration of immune function after severe immunosuppressive insults to the body. Compelling experimental and clinical evidence supports the involvement of PRL in autoimmune disease and in cancer.
Cardiac allograft vasculopathy (CAV) is a major complication limiting the long-term survival of cardiac transplants. The role of memory T cells (Tmem) in the pathogenesis of CAV remains elusive. This study investigated the role of Tmem cells in the development of CAV and the therapeutic potential of targeting the OX40/OX40L pathway for heart transplant survival.
Tmem cells were generated in Rag-1(-/-) C57BL/6 (B6) mice by homeostatic proliferation (HP) of CD40L null CD3(+) T cells from B6 mice. Rag-1(-/-) B6 mice (H-2(b)) harboring Tmem cells received cardiac allografts from BALB/c mice (H-2(d)), and were either untreated or treated with anti-OX40L monoclonal antibody (mAb) (0.5 mg/mouse/day) for 10 days.
Six weeks after HP, the majority of transferred CD40L(-/-) T cells in Rag-1(-/-) B6 mice were differentiated to CD44(high) and CD62L(low) Tmem cells. BALB/c heart allografts in Rag-1(-/-) B6 recipient mice in the presence of these Tmem cells developed a typical pathological feature of CAV; intimal thickening, 100 days after transplantation. However, functionally blocking the OX40/OX40L pathway with anti-OX40L mAb significantly prevented CAV development and reduced the Tmem cell population in recipient mice. Anti-OX40L mAb therapy also significantly decreased cellular infiltration and cytokine (IFN-γ, TNF-α and TGF-β) expression in heart allografts.
Tmem cells mediate CAV in heart transplants. Functionally blocking the OX40/OX40L pathway using anti-OX40L mAb therapy prevents Tmem cell-mediated CAV, suggesting therapeutic potential for disrupting OX40-OX40L signaling in order to prevent CAV in heart transplant patients.
Martin Braddock – Associate director Respiratory and Inflammation, AstraZeneca R&D Charnwood, Bakewell Road, Loughborough, LE11 5RH, UK
Ian Adcock – Department of Thoracic Medicine, National Heart and Lung Institute, Imperial College of Science, Technology & Medicine, Dovehouse Street, London, SW3 6LY
In vivo immunological screening promises the potential to dissect the molecular involvement of any cell type of the immune system in immune-mediated disease. Although the technology is still under development, such a screening strategy, may be applied to a drug or combination of drugs in early clinical development. It offers the potential to enable differentiation of a compound in a compound class, mechanism or disease indication such that it may accelerate clinical development and provide additional confidence for development to launch. Professor Garside is a leading researcher in helping our understanding of the regulation of tolerance and immunity and tracking lymphocytes in vivo.
egulatory T cells (Tregs)4 have recently been the subject of intense research due to their critical role in preventing autoimmune disease. However, the molecular factors that regulate the development, homeostasis, and function of Tregs re- main to be fully elucidated. Previous studies have demonstrated that the loss of IL-2, IL-2R, or IL-2R leads to the unexpected development of lethal autoimmune disease (1-3). Subsequent stud- ies by Malek et al. (4) established that the primary defect in IL- 2R-deficient mice was the loss of functional Tregs, as they found that transfer of small numbers of wild-type (WT) Tregs into neo- natal IL-2R-deficient mice completely prevented the onset of au- toimmune disease. These findings suggested that IL-2/IL-2R sig- naling plays an important role in Treg biology. Although IL-2 and IL-2R signaling are critical for the formation or maintenance of a functional Treg population, the exact role that IL-2R signaling plays in Treg biology has not yet been defined. Over the last few years, several studies have reported conflicting evidence about the role of IL-2 or IL-2R subunits in Treg devel- opment, homeostasis, or function. For example, Malek et al. (5) used a transgenic mouse line in which IL-2R expression was restricted to the thymus to address the role of IL-2 in Treg devel- opment. When these thymus-restricted IL-2R transgenic mice were crossed onto an IL-2R-deficient background, CD4CD25 Tregs were restored in both the thymus and spleen, and autoim- munity was prevented. These results suggest that IL-2/IL-2R sig- naling is required only for the development of Tregs in the thymus and that IL-2 is not required to promote their homeostasis and function in the periphery. In contrast, other work suggested a role for IL-2 in maintaining CD4CD25 Treg homeostasis. Specifi- cally, administration of an IL-2-neutralizing Ab to WT mice sig- nificantly reduced the population of CD4CD25 T cells in the periphery (6). These studies suggested that IL-2 signaling is crit- ical for the survival of Tregs. Conversely, two other groups dem- onstrated that IL-2/ mice have either identical (7) or only slightly reduced numbers (8) of Tregs when compared with WT mice, suggesting that IL-2 is not required for Treg development or homeostasis. Finally, several groups have claimed that IL-2 is pri- marily required for the activation and function of Tregs. For ex- ample, studies by Lafaille and colleagues (9) demonstrated that the transfer of CD4 T cells from IL-2/ mice could prevent auto- immune disease in a mouse model that normally develops spon- taneous experimental autoimmune encephalomyelitis. In contrast, transfer of IL-2R/ CD4 T cells did not prevent experimental autoimmune encephalomyelitis in this model, suggesting a require- ment for IL-2 to support suppressive capacity. Likewise, using in vitro suppression assays, two separate groups demonstrated that neutralization of IL-2 inhibits Treg suppressor function in vitro (10, 11). Taken together, the published literature reveals ongoing uncertainty regarding the role of IL-2 and the IL-2R in the devel- opment, homeostasis, and/or function of Tregs.
CD4(+) regulatory T cells (Tregs) control adaptive immune responses and promote self-tolerance. Various humanized mouse models have been developed in efforts to reproduce and study a human immune system. However, in models that require T cell differentiation in the recipient murine thymus, only low numbers of T cells populate the peripheral immune systems. T cells are positively selected by mouse MHC and therefore do not function well in an HLA-restricted manner. In contrast, cotransplantation of human fetal thymus/liver and i.v. injection of CD34(+) cells from the same donor achieves multilineage human lymphohematopoietic reconstitution, including dendritic cells and formation of secondary lymphoid organs, in NOD/SCID mice. Strong Ag-specific immune responses and homeostatic expansion of human T cells that are dependent on peripheral human APCs occur. We now demonstrate that FOXP3(+)Helios(+) "natural" Tregs develop normally in human fetal thymic grafts and are present in peripheral blood, spleen, and lymph nodes of these humanized mice. Humanized mice exhibit normal reversal of CD45 isoform expression in association with thymic egress, postthymic "naive" to "activated" phenotypic conversion, and suppressive function. These studies demonstrate the utility of this humanized mouse model for the study of human Treg ontogeny, immunobiology and therapy.
Homeostatic regulation of cell numbers is an important principle in biology. Mechanisms that function to maintain or re-establish homeostasis in the immune system include interactions among antigen-presenting cells, regulatory T cells and cytokines. The vital role that homeostatic regulation plays in maintaining a functionally intact immune system is illustrated by the perturbation of the peripheral T cell repertoire that occurs after lymphopenic incidents, which frequently provoke either exacerbated immune or autoimmune responses. Recent studies show that transient states of lymphopenia occur in viral infections and in the neonatal state and might be involved in the development of autoimmune diseases. On the positive side, lymphopenia-provoked T cell expansion might enhance weak immune responses and thereby aid the rejection of tumours or the elimination of parasites.
Murine neonatal CD4+ responses are often biased to Th2 function. There is increasing evidence that this phenomenon may be regulated both at the level of the thymus and the peripheral lymphoid compartment. In particular, residual fetal influence on the neonatal thymus may lead to an imprinting of developing T cells that is maintained in CD4+ cells when they emigrate to peripheral organs. Such imprinting may involve epigenetic modification of the Th2 cytokine gene locus and acquisition of the capacity to undergo rapid cell cycling. These properties, coupled with the homeostatic proliferation occurring in the peripheral tissues of neonates, shape a CD4+ population with the capacity for enhanced Th2 responsiveness.
To investigate a new clinically relevant immunoregulatory strategy based on treatment with murine Thymoglobulin mATG Genzyme and CTLA4-Ig in NOD mice to prevent allo- and autoimmune activation using a stringent model of islet transplantation and diabetes reversal.
Using allogeneic islet transplantation models as well as NOD mice with recent onset type 1 diabetes, we addressed the therapeutic efficacy and immunomodulatory mechanisms associated with a new immunoregulatory protocol based on prolonged low-dose mATG plus CTLA4-Ig.
BALB/c islets transplanted into hyperglycemic NOD mice under prolonged mATG+CTLA4-Ig treatment showed a pronounced delay in allograft rejection compared with untreated mice (mean survival time: 54 vs. 8 days, P < 0.0001). Immunologic analysis of mice receiving transplants revealed a complete abrogation of autoimmune responses and severe downregulation of alloimmunity in response to treatment. The striking effect on autoimmunity was confirmed by 100% diabetes reversal in newly hyperglycemic NOD mice and 100% indefinite survival of syngeneic islet transplantation (NOD.SCID into NOD mice).
The capacity to regulate alloimmunity and to abrogate the autoimmune response in NOD mice in different settings confirmed that prolonged mATG+CTLA4-Ig treatment is a clinically relevant strategy to translate to humans with type 1 diabetes.
Acute cellular rejection of organ transplants is executed by donor-reactive T cells, which are dominated by interferon-gamma-producing cells. As interferon-gamma is dispensable for graft destruction, we evaluated the contribution of interleukin-17A (IL-17) to intragraft inflammation in major histocompatibility complex-mismatched heart transplants. A/J (H-2(a)) cardiac allografts placed into wild-type BALB/c (H-2(d)) mice induced intragraft IL-17 production on day 2 after transplant. Allografts placed into BALB/c IL-17(-/-) recipients demonstrated diminished production of the chemokines CXCL1 and CXCL2 and delayed neutrophil and T cell recruitment. However, by day 7 after transplant, allografts from IL-17(-/-) and wild-type recipients had comparable levels of cellular infiltration. The priming of donor-specific T cells was not affected by the absence of IL-17, and the kinetics of cardiac allograft rejection were similar in wild-type and IL-17(-/-) recipients. In contrast, IL-17(-/-) mice depleted of CD8 T cells rejected A/J allografts in a delayed fashion compared with CD8-depleted wild-type recipients. Although donor-reactive CD4 T cells were efficiently activated in both groups, the infiltration of effector T cells into allografts was impaired in IL-17(-/-) recipients. Our data indicate that locally produced IL-17 amplifies intragraft inflammation early after transplantation and promotes tissue injury by facilitating T cell recruitment into the graft. Targeting the IL-17 signaling network in conjunction with other graft-prolonging therapies may decrease this injury and improve the survival of transplanted organs.
Th2-biased immune responses characterizing neonates may influence the later onset of allergic disease. The contribution of regulatory T cell populations in the prevention of Th2-driven pathologies in early life is poorly documented. We investigated the potential of CD8(+) T cells stimulated at birth with alloantigens to modulate the development of allergic airway inflammation. Newborn mice were immunized with semiallogeneic splenocytes or dendritic cells (DCs) and exposed at the adult stage to OVA aeroallergens. DC-immunized animals displayed a strong Th1 and Tc1/Tc2 alloantigen-specific response and were protected against the development of the allergic reaction with reduced airway hyperresponsiveness, mucus production, eosinophilia, allergen-specific IgE and IgG(1), and reduction of lung IL-4, IL-5, IL-10, and IL-13 mRNA levels. By contrast, splenocyte-immunized mice displayed a Th2 and a weak Tc2 alloantigen-specific response and were more sensitive to the development of the allergen-specific inflammation compared with mice unexposed at birth to alloantigens. DC-immunized animals displayed an important increase in the percentage of IFN-gamma-producing CD8(+)CD44(high), CD8(+)CD62L(high), and CD8(+)CD25(+) subsets. Adoptive transfers of CD8(+) T cells from semiallogeneic DC-immunized animals to adult beta(2)m-deficient animals prevented the development of allergic response, in particular IgE, IL-4, and IL-13 mRNA production in an IFN-gamma-dependent manner, whereas transfers of CD8(+) T cells from semiallogeneic splenocyte-immunized mice intensified the lung IL-4 and IL-10 mRNA level and the allergen-specific IgE. These findings demonstrated that neonatal induction of regulatory CD8(+) T cells was able to modulate key parameters of later allergic sensitization in a bystander manner, without recognition of MHC class I molecules.
C57BL/6 mice genetically deficient in interleukin 15 (IL-15−/− mice) were generated by gene targeting. IL-15−/− mice displayed marked reductions in numbers of thymic and peripheral natural killer (NK) T cells, memory phenotype CD8+ T cells, and distinct subpopulations of intestinal intraepithelial lymphocytes (IELs). The reduction but not absence of these
populations in IL-15−/− mice likely reflects an important role for IL-15 for expansion and/or survival of these cells. IL-15−/− mice lacked NK cells, as assessed by both immunophenotyping and functional criteria, indicating an obligate role for IL-15
in the development and functional maturation of NK cells. Specific defects associated with IL-15 deficiency were reversed
by in vivo administration of exogenous IL-15. Despite their immunological defects, IL-15−/− mice remained healthy when maintained under specific pathogen-free conditions. However, IL-15−/− mice are likely to have compromised host defense responses to various pathogens, as they were unable to mount a protective
response to challenge with vaccinia virus. These data reveal critical roles for IL-15 in the development of specific lymphoid
lineages. Moreover, the ability to rescue lymphoid defects in IL-15−/− mice by IL-15 administration represents a powerful means by which to further elucidate the biological roles of this cytokine.
Peripheral T lymphocytes from newborn (4-6-d-old) mice, isolated from the spleen or lymph nodes, show phenotypic features of immature cortical thymocytes, such as high frequencies of proliferating cells and of peanut lectin-binding cells. These are features of peripheral T cells of recent thymic origin, as shown by in situ labeling of thymocytes and subsequent observation of the migrants to the spleen, which were mainly peanut lectin-binding cells. The function of newborn peripheral T cells was compared, on a per T cell basis, with that of thymocytes and of fully mature peripheral T cells of the adult, using preparations of newborn lymph node cells containing approximately 80% of T lymphocytes. They were strikingly (about 10-fold) less competent than adult T cells in their phytohemagglutinin responsiveness, their capacities to induce a graft vs. host reaction, to proliferate in the mixed lymphocyte reaction, and to help B lymphocytes in a humoral response in vivo and in vitro. In contrast, newborn T lymphocytes were comparable to those of adults in their capacity to generate cytotoxic T lymphocytes. No suppressive effect of newborn T lymphocytes could be demonstrated in several of these assays. These results argue for an asynchronous maturation of two T cell subsets during ontogeny and demonstrate that at least some T lymphocytes leave the thymus as immature T cells resembling cortical thymocytes and further mature at the periphery. Investigation of mice submitted to thymectomy of 5 d of age showed that these incompetent post-thymic T lymphocytes are capable of considerable expansion and maturation in the peripheral lymphoid organs in the absence of a thymic influence.
On the basis of their surface markers, T lymphocytes are divided into subsets of "naive" and "memory cells". We have defined the interrelationship and relative life spans of naive and memory T cells by examining the surface markers on murine T cells incorporating bromodeoxyuridine, a DNA precursor, given in the drinking water. Three findings are reported. First, using a new method we show that the release of newly formed naive T cells from the unmanipulated thymus is very low (confirming the findings of others with surgical approaches). Second, in thymectomized mice, T cells with a naive phenotype remain in interphase for prolonged periods; however, some of these cells divide and retain (or regain) their "naive" markers. Third, most T cells with a memory phenotype divide rapidly, but others remain in interphase for many weeks. Collectively, the data indicate that long-lived T cells have multiple phenotypes and contain a mixture of memory cells, naive (virgin) cells, and memory cells masquerading as naive cells.
Peripheral T cell maintenance requires a survival signal delivered upon T cell receptor (TCR)-major histocompatibility complex (MHC) molecule interaction. Since self-peptides play a critical role in the intrathymic positive selection of the mature TCR repertoire, we hypothesized an equally important role in T cell persistence. We used mice with a normal expression of MHC class II molecules but a restricted self-peptide complexity (H-2M alpha-/-) to show that an MHC class II-restricted T cell specificity that displays a deficient positive selection in the H-2M alpha-/- thymus shows an impaired persistence after adoptive transfer in H-2M alpha-/- recipients. Finally, a wild-type CD4+ TCR repertoire is incompletely maintained in H-2M alpha-/- recipients. These observations suggest that, similar to intrathymic positive selection, the maintenance of the mature TCR repertoire relies on the recognition of self-peptide:self-MHC complexes.
We investigated the mechanism by which alpha/beta T cells expand upon transfer to T cell-deficient host mice by injecting carboxyfluorescein diacetate succinimidyl ester-labeled T cells into mice depleted of T cells by sublethal irradiation. We found that CD4+ T cells divided when transferred to irradiated hosts and that the division of more than half of these cells required class II expression. However, division of transferred CD4+ T cells did not occur in irradiated hosts that expressed class II molecules occupied solely by the peptide responsible for thymic selection, indicating that peptides distinct from those involved in thymic selection cause the division of CD4+ T cells in irradiated mice. These data establish that class II-bound peptides control the expansion of CD4+ T cells transferred to T cell-deficient hosts and suggest that the same peptides contribute to the maintenance of T cell numbers in normal mice.
In a depleted lymphoid compartment, naive T cells begin a slow proliferation that is independent of cognate antigen yet requires recognition of major histocompatibility complex-bound self-peptides. We have followed the phenotypic and functional changes that occur when naive CD8(+) T cells undergo this type of expansion in a lymphopenic environment. Naive T cells undergoing homeostasis-driven proliferation convert to a phenotypic and functional state similar to that of memory T cells, yet distinct from antigen-activated effector T cells. Naive T cells dividing in a lymphopenic host upregulate CD44, CD122 (interleukin 2 receptor beta) and Ly6C expression, acquire the ability to rapidly secrete interferon gamma, and become cytotoxic effectors when stimulated with cognate antigen. The conversion of naive T cells to cells masquerading as memory cells in response to a homeostatic signal does not represent an irreversible differentiation. Once the cellularity of the lymphoid compartment is restored and the T cells cease their division, they regain the functional and phenotypic characteristics of naive T cells. Thus, homeostasis-driven proliferation provides a thymus-independent mechanism for restoration of the naive compartment after a loss of T cells.
The developmental requirements for immunological memory, a central feature of adaptive immune responses, is largely obscure. We show that as naive CD8 T cells undergo homeostasis-driven proliferation in lymphopenic mice in the absence of overt antigenic stimulation, they progressively acquire phenotypic and functional characteristics of antigen-induced memory CD8 T cells. Thus, the homeostasis-induced memory CD8 T cells express typical memory cell markers, lyse target cells directly in vitro and in vivo, respond to lower doses of antigen than naive cells, and secrete interferon gamma faster upon restimulation. Like antigen-induced memory T cell differentiation, the homeostasis-driven process requires T cell proliferation and, initially, the presence of appropriate restricting major histocompatibility complexes, but it differs by occurring without effector cell formation and without requiring interleukin 2 or costimulation via CD28. These findings define repetitive cell division plus T cell receptor ligation as the basic requirements for naive to memory T cell differentiation.
Adult naive T cells, which are at rest in normal conditions, proliferate strongly when transferred to lymphopenic hosts. In neonates, the first mature thymocytes to migrate to the periphery reach a compartment devoid of preexisting T cells. We have extensively analyzed the proliferation rate and phenotype of peripheral T cells from normal C57BL/6 and T cell antigen receptor transgenic mice as a function of age. We show that, like adult naive T cells transferred to lymphopenic mice, neonatal naive T cells proliferate strongly. By using bone-marrow transfer and thymic-graft models, we demonstrate that the proliferation of the first thymic emigrants reaching the periphery requires T cell antigen receptor-self-peptide/self-MHC interactions and is regulated by the size of the peripheral T cell pool.
Recent work shows that after stimulation with antigen, CD4+ and CD8+ T cells embark on a programme of proliferation that is closely linked with the acquisition of effector functions and leads ultimately to memory-cell formation. Here, we discuss the signals required for commitment to this programme of development and the factors that might influence its progression. Models of the pathways of effector and memory T-cell differentiation are discussed, and we highlight the implications of this new understanding for the optimization of vaccine strategies.
Naïve T cells proliferate and differentiate into memory cells after antigenic stimulation or in a lymphopenic environment. We showed here transient increases in memory phenotype CD8+ T cell numbers in the lymphopenic environment of spleens of very young mice. The magnitude of the increase correlated with Bcl-6 expression in the T cells. Bcl-6 controlled the generation and maintenance of antigen-specific memory phenotype CD8+ T cells in the spleens of immunized mice. These data suggest that Bcl-6, which is essential for memory B cell development in germinal centers, is a key molecule for the establishment not only of memory T cells but also of the peripheral T cell compartment in infancy.
The overall size and composition of the pool of naive and memory T cells are tightly regulated by homeostatic mechanisms. Recent work has shown that homeostasis of naive T cells is controlled by two factors, self-major histocompatibility complex (MHC)/peptide ligands and a cytokine, interleukin (IL)-7. In particular, contact with these two factors is required for naive CD4+ and CD8+ cells to undergo "homeostatic" proliferation, i.e., proliferation induced as a consequence of severe T cell depletion. In contrast to naive T cells, the factors that drive memory T cells to undergo homeostatic proliferation are poorly understood. To address this issue, purified memory phenotype CD4+ and CD8+ cells from normal mice were adoptively transferred into various gene-knockout mice rendered T cell-deficient by sublethal irradiation. Three findings are reported. First, unlike naive T cells, homeostatic proliferation of memory T cells is largely MHC independent. Second, memory CD8+ cells can utilize either IL-7 or IL-15 to undergo homeostatic proliferation; however, in the absence of both IL-7 and IL-15, homeostatic proliferation fails to occur. Third, unlike memory CD8+ cells, homeostatic proliferation of memory CD4+ cells is independent of IL-7 and IL-15 (also IL-4). Thus, the homeostatic proliferation mechanisms that control memory CD8+ cells and memory CD4+ cells are quite distinct.
Both naive and memory T cells undergo antigen-independent proliferation after transfer into a T cell–depleted environment
(acute homeostatic proliferation), whereas only memory T cells slowly divide in a full T cell compartment (basal proliferation).
We show, first, that naive and memory CD8+ T cells have different cytokine requirements for acute homeostatic proliferation. Interleukin (IL)-7 receptor(R)α–mediated
signals were obligatory for proliferation of naive T cells in lymphopenic hosts, whereas IL-15 did not influence their division.
Memory T cells, on the other hand, could use either IL-7Rα– or IL-15–mediated signals for acute homeostatic proliferation:
their proliferation was delayed when either IL-7Rα was blocked or IL-15 removed, but only when both signals were absent was
proliferation ablated. Second, the cytokine requirements for basal and acute homeostatic proliferation of CD8+ memory T cells differ, as basal division of memory T cells was blocked completely in IL-15–deficient hosts. These data suggest
a possible mechanism for the dearth of memory CD8+ T cells in IL-15– and IL-15Rα–deficient mice is their impaired basal proliferation. Our results show that naive and memory
T lymphocytes differ in their cytokine dependence for acute homeostatic proliferation and that memory T lymphocytes have distinct
requirements for proliferation in full versus empty compartments.
Development of tumor immunotherapies focuses on inducing autoimmune responses against tumor-associated self-antigens primarily encoded by normal, unmutated genes. We hypothesized that such responses could be elicited by T cell homeostatic proliferation in the periphery, involving expansion of T cells recognizing self-MHC/peptide ligands. Herein, we demonstrate that sublethally irradiated lymphopenic mice transfused with autologous or syngeneic T cells showed tumor growth inhibition when challenged with melanoma or colon carcinoma cells. Importantly, the antitumor response depended on homeostatic expansion of a polyclonal T cell population within lymph nodes. This response was effective even for established tumors, was characterized by CD8(+) T cell-mediated tumor-specific cytotoxicity and IFN-gamma production, and was associated with long-term memory. The results indicate that concomitant induction of the physiologic processes of homeostatic T cell proliferation and tumor antigen presentation in lymph nodes triggers a beneficial antitumor autoimmune response.
We have previously hypothesized that maintaining a balanced peripheral immune system may not be the sole responsibility of a specialized subset of T cells dedicated to immune regulation, but also a side effect of normal competition for shared resources within an intact immune system. Here we show that regulatory activity is correlated with high homeostatic expansion potential, reflecting the avidity for self-peptide:MHC complexes. Monoclonal transgenic T cells with high homeostatic expansion potential and lacking characteristics previously associated with regulatory function were able to regulate wasting disease induced by transfer of a small number of naive CD45RB(hi) CD4 T cells into lymphopenic hosts. Self-regulatory function is also found in the naive polyclonal T cell repertoire depleted of CD25(+) T cells. T cells capable of preventing immune pathology, like the transgenic T cells, express higher than average levels of CD5, an indicator of avidity for self:MHC peptide complexes. We therefore propose that dysregulated expansion of potentially pathogenic T cells in a lymphopenic environment can be prevented by members of the naive T cell repertoire, irrespective of their specificity, as a side effect of their response to homeostatic and antigenic stimulation.
Peripheral T lymphocytes from newborn (4-6-d-old) mice, isolated from the spleen or lymph nodes, show phenotypic features of immature cortical thymocytes, such as high frequencies of proliferating cells and of peanut lectin-binding cells. These are features of peripheral T cells of recent thymic origin, as shown by in situ labeling of thymocytes and subsequent observation of the migrants to the spleen, which were mainly peanut lectin-binding cells. The function of newborn peripheral T cells was compared, on a per T cell basis, with that of thymocytes and of fully mature peripheral T cells of the adult, using preparations of newborn lymph node cells containing approximately 80% of T lymphocytes. They were strikingly (about 10-fold) less competent than adult T cells in their phytohemagglutinin responsiveness, their capacities to induce a graft vs. host reaction, to proliferate in the mixed lymphocyte reaction, and to help B lymphocytes in a humoral response in vivo and in vitro. In contrast, newborn T lymphocytes were comparable to those of adults in their capacity to generate cytotoxic T lymphocytes. No suppressive effect of newborn T lymphocytes could be demonstrated in several of these assays. These results argue for an asynchronous maturation of two T cell subsets during ontogeny and demonstrate that at least some T lymphocytes leave the thymus as immature T cells resembling cortical thymocytes and further mature at the periphery. Investigation of mice submitted to thymectomy of 5 d of age showed that these incompetent post-thymic T lymphocytes are capable of considerable expansion and maturation in the peripheral lymphoid organs in the absence of a thymic influence.
The spleen of adult antigen-free mice contains a sizable proportion (5-15%) of activated cells in all lymphocyte sets, as marked by the membrane expression of immunoglobulins, L3T4 and Lyt-2 antigens. The frequency of activated cells is very high in early post-natal life, and reaches adult levels by 6 weeks of age when it is comparable to that observed in healthy unmanipulated mice raised in conventional conditions. The effector B cell compartment is quantitatively similar in antigen-free mice and specific pathogen-free mice, but the former is deficient in isotype diversification, since IgG- and IgA-secreting cells are drastically reduced. The effector T cell compartment is slightly reduced in number, but is equally competent in providing help or suppression of syngeneic B cells. The results indicate the existence of a compartment of the immune system displaying autonomous self-determined activity which is predominant early in life. This compartment, physically localized to the spleen, appears to be distinct from an antigen-dependent compartment which is essential for the development of peripheral lymphoid organs draining sites of “natural” environmental immunization.
Immunological memory is a key characteristic of specific immune responses. Persistence of increased levels of precursor T cells is antigen-independent and is often used as an indicator of T cell memory. This study documents that, depending on the chosen readout, cytotoxic T lymphocyte (CTL) memory against lymphocytic choriomeningitis virus (LCMV) appears long- or short-lived in the absence of persisting antigen. To study T cell memory in the absence of persisting antigen, either short-lived antigens were used for immunization or adoptive transfer methods were used to eliminate possibly persisting antigen. These experiments revealed that increased specific precursor frequencies and CTL-mediated protection against an i.v. infection with LCMV were long-lived. In contrast, CTL-mediated protection against a peripheral infection of the skin with LCMV, or of the ovary with recombinant vaccinia virus, was short-lived. These results show that maintenance of increased specific CTL precursor frequencies and central T cell memory in lymphoid tissue (where preexisting neutralizing antibodies usually provide protection anyway) is long-lived and antigen-independent. In contrast, in protection against peripheral viral infections, where the relative kinetics of virus growth and virus elimination by T cells are of key importance, T cell memory is short-lived in the absence of antigen. This indicates that peripheral T cell memory in antibody-inaccessible tissues is mediated by antigen-activated effector T cells and apparently not by specialized memory T cells.
Whether memory T cells require persisting antigen for their survival has been a matter of debate. One prominent view that memory T cells do not require persisting antigen is based in part on studies in which T cell populations have been transferred into antigen-free mice. To generate "space" recipients were often irradiated; the functional properties of the transfused T cells were then evaluated after prolonged periods. In this report we show that transferring cytotoxic T lymphocytes (CTL) into irradiated or T and B cell-deficient hosts results in their proliferation and a change of their activation state. Moreover, naïve T cell receptor-transgenic CTL specific for the lymphocytic choriomeningitis virus glycoprotein spontaneously developed cytotoxic effector function under such conditions. Therefore, some of the conclusions based on transfer of T cell populations into irradiated recipients to investigate T cell memory may have to be reevaluated.
In the absence of thymic emigration, the peripheral T cell pool is maintained by division of mature lymphocytes. We have examined the molecular interactions required for peripheral CD8+ T cell expansion in lymphopenic mice without conventional antigenic stimulation. Expansion of CD8+ T cells in lymphopenic hosts was found to be peptide specific. An antagonist peptide known to serve as a ligand for positive selection of these T cells promoted expansion; however, a control peptide that binds the same class I molecule did not. Surprisingly, the cells undergoing proliferation in lymphopenic hosts did not mature to cytotoxic effectors and displayed a partially activated surface phenotype. These data suggest that division of T cells in the periphery of lymphopenic hosts requires specific recognition of self-peptide/MHC complexes, similar to the signal for thymocyte maturation.
Positive selection to self-MHC/peptide complexes has long been viewed as a device for skewing the T cell repertoire toward recognition of foreign peptides presented by self-MHC molecules. Here, we provide evidence for an alternative possibility, namely, that the self-peptides controlling positive selection in the thymus serve to maintain the longevity of mature T cells in the periphery. Surprisingly, when total T cell numbers are reduced, these self-ligands become overtly stimulatory and cause naive T cells to proliferate and undergo homeostatic expansion.
Memory T cells maintain their numbers for long periods after antigen exposure. Here we show that CD8+ T cells of memory phenotype divide slowly in animals. This division requires interleukin-15 and is markedly increased by
inhibition of interleukin-2 (IL-2). Therefore, the numbers of CD8+ memory T cells in animals are controlled by a balance between IL-15 and IL-2.
This study shows that naive CD8 T cells can acquire characteristics of memory T cells in the absence of stimulation with specific Ag simply by the process of homeostatic proliferation under lymphopenic conditions. This Ag-independent T cell differentiation pathway did not result in up-regulation of early activation markers (CD69, CD25, CD71), but expression of several memory markers (CD44, CD122, Ly6C) increased progressively with successive divisions. These markers were then stably expressed, and these cells also became more responsive functionally to specific Ag. Thus, all "memory" phenotype T cells in an individual may not be true Ag-experienced cells and may include naive cells masquerading as memory cells. These findings are specially relevant in cases of disease or treatment-induced lymphopenia such as in HIV-infected individuals or transplant recipients. In addition, this study may have implications for autoimmunity because homeostatic proliferation of naive T cells requires interaction with self peptide plus MHC molecules.
The naïve and memory T lymphocyte pools are maintained through poorly understood homeostatic mechanisms that may include signaling via cytokine receptors. We show that interleukin-7 (IL-7) plays multiple roles in regulating homeostasis of CD8+ T cells. We found that IL-7 was required for homeostatic expansion of naïve CD8+ and CD4+ T cells in lymphopenic hosts and for CD8+ T cell survival in normal hosts. In contrast, IL-7 was not necessary for growth of CD8+ T cells in response to a virus infection but was critical for generating T cell memory. Up-regulation of Bcl-2 in the absence of IL-7 signaling was impaired after activation in vivo. Homeostatic proliferation of memory cells was also partially dependent on IL-7. These results point to IL-7 as a pivotal cytokine in T cell homeostasis.
Naive T cells undergo spontaneous slow proliferation on adoptive transfer into syngeneic T cell (T)-deficient hosts. Recent work has shown that such "homeostatic" T cell proliferation is driven by MHC molecules loaded with self-peptides rather than foreign peptides. Because naive T cells in normal T-sufficient hosts remain in interphase despite continuous contact with self-MHC/peptide ligands, T cells apparently inhibit homeostatic proliferation of neighboring T cells. To address this, we have investigated the requirements necessary for "bystander" T cells to inhibit homeostatic proliferation of other T cells. Three key findings are reported. First, homeostatic proliferation of T cells only occurs in specific microenvironments, namely the T cell compartment of the secondary lymphoid tissues. Second, direct entry into T cell compartments is also required for bystander inhibition of homeostatic proliferation. Third, bystander inhibition is mediated largely by naive rather than activated/memory T cells and does not require proliferation or TCR ligation. These findings suggest that homeostasis of naive T cells is unlikely to be regulated through competition for systemic soluble factors or for specific stimulatory self-MHC/peptide ligands. Rather, the data favor mechanisms that involve competition for local non-MHC stimulatory factors or direct cell-to-cell interactions between the T cells themselves within the T cell compartment.
Typically, neonates exhibit decreased or aberrant cellular immune responses when compared to adults, resulting in increased susceptibility to infection. However, it is clear that newborns are able to generate adult-like protective T cell responses under certain conditions. The focus of our research is to understand the deficiencies within the neonatal immune system that lead to improper cellular responses and how priming conditions can be altered to elicit the appropriate T cell response necessary to protect against development of pathogen-induced disease. With these goals in mind, we are exploring the attributes of neonatal T cells and their development, as well as the conditions during priming that influence the resulting response to immune challenge during the neonatal period.
The use of the Cre/loxP system has greatly empowered the field of gene targeting. Here we describe the successful establishment of a novel knock-in EGFP reporter mouse line to monitor Cre-induced recombination in the vast majority of cell types. The value of this reporter mouse line is demonstrated by the use of a novel Tie2Cre transgenic mouse line that facilitates gene targeting in endothelial and hematopoietic cells. High efficiency of recombination was found in all endothelial cells and in the majority of hematopoietic cells but was absent in other tissues. Furthermore, in the second generation, the Tie2Cre mouse can be used to get 100% recombination of one allele, whilst allowing tissue specific in the second, therefore offering excellent efficiency.
In T cell-deficient conditions, naive T cells undergo spontaneous "homeostatic" proliferation in response to contact with self-MHC/peptide ligands. With the aid of an in vitro system, we show here that homeostatic proliferation is also cytokine-dependent. The cytokines IL-4, IL-7, and IL-15 enhanced homeostatic proliferation of naive T cells in vitro. Of these cytokines, only IL-7 was found to be critical; thus, naive T cells underwent homeostatic proliferation in IL-4(-) and IL-15(-) hosts but proliferated minimally in IL-7(-) hosts. In addition to homeostatic proliferation, the prolonged survival of naive T cells requires IL-7. Thus, naïve T cells disappeared gradually over a 1-month period upon adoptive transfer into IL-7(-) hosts. These findings indicate that naive T cells depend on IL-7 for survival and homeostatic proliferation.
T cells expand without intentional antigen stimulation when transferred into adult lymphopenic environments. In this study, we show that the physiologic lymphopenic environment existing in neonatal mice also supports CD4 T cell proliferation. Strikingly, naive CD4 T cells that proliferate within neonates acquire the phenotypic and functional characteristics of memory cells. Such proliferation is inhibited by the presence of both memory and naive CD4 T cells, is enhanced by 3-day thymectomy, is independent of IL-7, and requires a class II MHC-TCR interaction and a CD28-mediated signal. CD44(bright) CD4 T cells in neonates have a wide repertoire as judged by the distribution of Vbeta expression. Thus, lymphopenia-induced T cell proliferation is a physiologic process that occurs during the early postnatal period.
Naive cytotoxic T lymphocytes spontaneously acquire effector function in lymphocytopenic recipients: a pitfall for T cell memory studies? T cell homeostatic proliferation elicits effective antitumor autoimmunity
- S Oehen
- K Brduscha-Riem
- W Dummer
- A G Niethammer
- R Baccala
- B R Lawson
- N Wagner
- R A Reisfeld
Oehen, S., and K. Brduscha-Riem. 1999. Naive cytotoxic T lymphocytes spontaneously acquire effector function in lymphocytopenic recipients: a pitfall for T cell memory studies? Eur. J. Immunol. 29:608. 15. Dummer, W., A. G. Niethammer, R. Baccala, B. R. Lawson, N. Wagner, R. A. Reisfeld, and A. N. Theofilopoulos. 2002. T cell homeostatic proliferation elicits effective antitumor autoimmunity. J. Clin. Invest. 110:185.
Reversible defects in naturalkillerandmemoryCD8Tcelllineagesininterleukin15-deficientmice
- M K Kennedy
- M Glaccum
- S N Brown
- E A Butz
- J L Viney
- M Embers
- N Matsuki
- K Charrier
- L Sedger
- C R Willis
Kennedy, M. K., M. Glaccum, S. N. Brown, E. A. Butz, J. L. Viney, M. Embers, N. Matsuki, K. Charrier, L. Sedger, C. R. Willis, et al. 2000. Reversible defects in naturalkillerandmemoryCD8Tcelllineagesininterleukin15-deficientmice.J.Exp. Med. 191:771
Tcellregulationasasideeffectof homeostasis and competition
- T Barthlott
- G Kassiotis
- Andb Stockinger
Barthlott,T.,G.Kassiotis,andB.Stockinger.2003.Tcellregulationasasideeffectof homeostasis and competition. J. Exp. Med. 197:451. 19The Journal of Immunology by guest on June 13, 2013 http://www.jimmunol.org/ Downloaded from
AutonomousactivationofBandTcellsinantigen-freemice
- P Pereira
- L Forni
- E L Larsson
- M Cooper
- C Heusser
- A Coutinho
Pereira, P., L. Forni, E. L. Larsson, M. Cooper, C. Heusser, and A. Coutinho. 1986. AutonomousactivationofBandTcellsinantigen-freemice.Eur.J.Immunol.16:685
Naive cytotoxic T lymphocytes spontane-ouslyacquireeffectorfunctioninlymphocytopenicrecipients:apitfallforTcellmem-ory studies?
- S Oehen
- K Brduscha
Oehen, S., and K. Brduscha-Riem. 1999. Naive cytotoxic T lymphocytes spontane-ouslyacquireeffectorfunctioninlymphocytopenicrecipients:apitfallforTcellmem-ory studies? Eur. J. Immunol. 29:608
Turnoverofnaive-andmemory-phenotypeTcells
- D F Tough
Tough,D.F.,andJ.Sprent.1994.Turnoverofnaive-andmemory-phenotypeTcells. J. Exp. Med. 179:1127