Bone-Immune Cell Crosstalk: Bone Diseases

Abstract

Bone diseases are associated with great morbidity; thus, the understanding of the mechanisms leading to their development represents a great challenge to improve bone health. Recent reports suggest that a large number of molecules produced by immune cells affect bone cell activity. However, the mechanisms are incompletely understood. This review aims to shed new lights into the mechanisms of bone diseases involving immune cells. In particular, we focused our attention on the major pathogenic mechanism underlying periodontal disease, psoriatic arthritis, postmenopausal osteoporosis, glucocorticoid-induced osteoporosis, metastatic solid tumors, and multiple myeloma.

Full text

Review Article
Bone-Immune Cell Crosstalk: Bone Diseases
Giorgio Mori,1Patrizia D’Amelio,2Roberta Faccio,3and Giacomina Brunetti4
1Department of Clinical and Experimental Medicine, University of Foggia, 71100 Foggia, Italy
2Department of Medical Science, Section of Gerontology and Bone Metabolism Diseases, University of Torino, 10126 Torino, Italy
3Department of Orthopedics, Washington University School of Medicine, St. Louis, MO 63110, USA
4Department of Basic Medical Sciences, Neurosciences andSenseOrgans,SectionofHumanAnatomyandHistology,
University of Bari, 70124 Bari, Italy
Correspondence should be addressed to Giacomina Brunetti; giacomina.brunetti@uniba.it
Received  October ; Revised  January ; Accepted  January 
Academic Editor: Chikao Morimoto
Copyright ©  Giorgio Mori et al. is is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Bone diseases are associated with great morbidity; thus, the understanding of the mechanisms leading to their development
represents a great challenge to improve bone health. Recent reports suggest that a large number of molecules produced by immune
cells aect bone cell activity. However, the mechanisms are incompletely understood. is review aims to shed new lights into the
mechanisms of bone diseases involving immune cells. In particular, we focused our attention on the major pathogenic mechanism
underlying periodontal disease, psoriatic arthritis, postmenopausal osteoporosis, glucocorticoid-induced osteoporosis, metastatic
solid tumors, and multiple myeloma.
1. Introduction
Bone is an active tissue that undergoes continuous remod-
elling by two distinct processes, bone formation and bone
resorption []. ese events are strongly linked and tightly
regulated to maintain skeletal homeostasis []. e bone
cells responsible for the dual processes include the bone
resorbing cells, that is, the osteoclasts (OCs), which are
dierentiated cells derived from hematopoietic cells of the
monocyte-macrophage lineage and bone forming cells, that
is, the osteoblasts (OBs), which are of mesenchymal origin.
Alteration of the dierentiation/activity of OCs as well as
OBs leads to bone diseases. e close relationship between
theboneandtheimmunesystemhasbeenincreasingly
recognized, in particular during pathological conditions in
which activation of both systems occurs []. It is known that
inammation increase leads to an augment in the immune
function, which culminates in an increased production of
tumour necrosis factor (TNF) or receptor activator of NF-
kB ligand (RANKL) by activated T cells, that has been
linked to bone loss associated diseases (inammatory and
autoimmune disease, postmenopausal osteoporosis). Dier-
ent studies have been performed to identify the T cell subset
involved in osteoclastogenesis. In general, T cells could be
classied as eector-cytotoxic T population (CD+ cells) and
helper T cells (CD+ cells). CD+ T cells, upon activation
and expansion, develop into diverse T helper () cell subsets
secreting signature cytokine proles and mediating distinct
eector functions []. Until recently, T cells were divided
into  or  cells, depending on the cytokines they
produced (with  producing IFN-gamma and IL- and
 producing primarily IL-/IL-/IL-). Regulatory T cells
(Tregs, CD+CD+Foxp+) potently inhibit the function of
eector T cells []. A third subset of IL--producing eector
T helper cells, called  cells, has been more recently dis-
covered and characterized.  cells produce IL-, IL-F,
and IL-, thereby inducing a massive tissue reaction owing
to the broad distribution of the IL- and IL- receptors.
 cells support OC formation mostly through the expres-
sion of IL-, which is recognized to induce RANK expression
on OC precursors as well as RANKL production by cells sup-
porting OC formation [,]. IL- also makes possible local
inammation through the recruitment and the activation
of immune cells, leading to the release of proinammatory
molecules, as IL- and TNF𝛼[]. ese proinammatory
molecules increase RANKL expression and synergize with
Hindawi Publishing Corporation
Journal of Immunology Research
Volume 2015, Article ID 108451, 11 pages
http://dx.doi.org/10.1155/2015/108451

Journal of Immunology Research
RANKL signalling to maximize OC formation. A relatively
high expression of RANKL on  cells may also participate
in the enhanced osteoclastogenesis. Collectively,  cells
can be considered an osteoclastogenic  subset; however,
they are not the only ones. In fact, activated T cells, expressing
high RANKL levels, have the ability to directly induce OC
dierentiation by acting on OC precursor cells [].
However, because T cells/immune cells also secrete a
variety of cytokines and express membrane-bound factors
other than RANKL, which could support OC formation,
mainly in pathological condition; this issue might be further
explored, together with the mechanisms that could modulate
their expression.
We describe recent eorts highlighting the prominent
role of immune system in the alteration of bone remod-
elling, thus favouring the development of many bone dis-
eases, such as periodontal disease (PD), psoriatic arthri-
tis (PsA), postmenopausal osteoporosis, glucocorticoid-
induced osteoporosis (GIO), metastatic solid tumors, and
multiple myeloma (MM).
Periodontal Disease. PD is a common complex infection of the
oral cavity that specically aects the gingiva, the periodontal
ligament, and the alveolar bone. It is characterized by an
inammatory response to bacteria present in the gingival
pocket [] and may remain conned to the gingiva or may
progress to extreme periodontal destruction with the loss
of the alveolar bone. PD is the main cause of tooth loss
among adults and is associated with important alteration
in facial aesthetics and defeat of masticatory and phonetics
function []. It is also well recognized that the presence of
only pathogenic bacteria is insucient to PD. Progression
of this disease occurs due to a combination of factors,
including the presence of periodontopathic bacteria, high
levels of proinammatory cytokines (IL-, TNF𝛼,IL-,IL-
, IL-, and IL-), prostaglandin E (PGE2),lowlevelsof
anti-inammatory cytokines including IL-, transforming
growth factor (TGF-𝛽), and retinoic acid []. Genetic factors
increase the susceptibility of some individuals in developing
this inammatory disease. It has been supported by reports
of familial aggregation of severe forms of the disease [], and
twin studies []. Recent candidate gene studies for periodon-
tal disease have focused on genes related to host immunity
and inammatory response such as cytokines, cell-surface
receptors, chemokines, enzymes, and antigen recognition.
Histological examination of periodontitis lesions reveals that
thegranulocytesappeartoplayakeyroleinthemain-
tenance of the periodontal health. ese cells are present
in the junctional epithelium in large numbers and they
isolate tissues from the bacteria action; thus, severe forms of
periodontitis frequently aect patients with diseases such as
leukocyte adhesion deciency and neutropenia. e failure
of granulocytes to transmigrate into the endothelium results
in an increase on the inammatory response and reduces
theprotectiveresponseagainstperiodontalpathogens.Inthe
presence of active disease, the epithelial migration causes
a deep periodontal pocket resulting in bacterial invasion,
inammation, and destruction of the connective tissue, with
subsequent bone loss and possible tooth loss. Langerhans
cells and dendritic cells of bone marrow origin, that are
located within the epithelium, are a connecting link with
acquiredimmunity.eadaptiveimmuneresponseisacti-
vated when the epithelial barrier, with its innate system,
is penetrated. e dendritic cells participate to the innate
inammatory response and moreover they capture and
present antigens to B and T cells of the acquired immune
system []. Activated CD T helper cells produce subsets
of cytokines with dierent immune responses:  and
 cells, respectively, associated with cellular and humoral
immunity []. e recently described  and Treg cells
have antagonistic roles as eector and suppressive cells [].
B cells dierentiate into plasma cells producing specic
antibodies. us, tissues aected by periodontitis become
colonized with both lymphocytes subtypes with B cells being
more represented than T cells. In a not progressive lesion,
IFN-𝛾increases the phagocytic activity of both neutrophils
and macrophages and hence contains the infection. In case of
a reduced innate immune response, a consequent weak 
response may not contain infection. Moreover, activated mast
cells determine a  response, B cell activation, and antibody
production. e antibodies can control the infection or, as in
thecaseofproductionofIgGinlargeamount,thelesionwill
persist. Sustained B cell activation may lead to IL- secretion
and periodontal disease progression.  cells have been
identied in the periodontal tissues. IL- mainly produced
by  has been shown to stimulate epithelial, endothelial,
andbroblasticcellstoproduceIL-,IL-,andPGE
2,thus
sustaining the disease progression. In addition, IL- induces
RANKL production by osteoblasts stimulating bone resorp-
tion. It has been demonstrated that periodontitis bacteria
induce a signicant increase in the production of IL- [].
According to recent studies, IL- signicantly enhances
RANKL and inhibits osteoprotegerin (OPG) expression in
human periodontal ligament cells []. It has been hypoth-
esized that  cells may be involved in  modulation and
enhanced inammatory mediators’ production by gingival
broblasts in periodontal disease. Circulating T cells express
high levels of RANKL and spontaneously promote osteoclas-
togenesis in patients [].  and  cells, as well as B cells,
increase RANKL expression []. Other studies demonstrated
that also B cells produce RANKL in response to periodontal
pathogen stimulation []. Contrarily, Treg cells decrease
RANKL secretion whereas TGF-𝛽stimulates Treg cell dier-
entiation. is process is supported by retinoic acid and coun-
teracted by IL- and IL-. In chronic inammatory disease
as PD, retinoic acid levels are suppressed and Treg activity is
inhibited in favor of  pathogenic eect []. In PD, proin-
ammatory cytokines overcome anti-inammatory ones, and
 cells surmount Treg: this inammatory state determines
the destruction of connective tissue and alveolar bone.
Psoriatic Arthritis. Psoriasis is a chronic inammatory disease
of the skin; a considerable part of patients with psori-
asis develops an inammatory arthritis characterized by
increased bone remodeling with osteolysis called PsA [].
e mechanisms responsible for the development of the
PsAshouldbebetterexplained,buttheimmunesystemplays
the main role in the pathogenesis of this disorder, that has to

Journal of Immunology Research 
be considered as a chronic inammation. erefore, patients
withpsoriasishaveelevatedlevelsofcirculatingneutrophils.
e  cells play an important role; in particular,  and 
are involved in the pathogenesis of the disease. PsA is char-
acterizedbyTandBcellinltratesandneoangiogenesisin
the synovial membrane and by the overexpression of inam-
matory cytokines. PsA synovitis is indicated by hyperplasia
of the synovial lining cells and mononuclear cell inltration.
Moreover, ectopic lymphoid neogenesis appears. Fibroblasts
and T cells in PsA synovial uid induce osteoclastogenesis
and bone resorption, mediated by RANKL, TNF-𝛼,andIL-
[]. Inammatory cytokine set such as TNF-𝛼,IL-𝛽,IL-,
IFN-𝛾, IL-, IL-, IL-, and IL- were highly expressed in
synovial uid of PsA patients, while broblasts isolated from
their skin and joints secreted IL- and IL-; some of these
cytokineshavealsorecognizedosteoclastogenicfeatures.
Both T cell suppression and TNF-𝛼inhibitors are eective
in humans in the treatment of psoriasis. In PsA patients, there
is a great increase in the number of peripheral blood 
cells. us, recent studies indicated that  cells []are
the cells most signicantly involved in psoriasis. Like  and
 cells,  cells appear to be evolved in inducing acquired
immune responses against microorganisms, such as bacteria.
Abnormal  responses are believed to play a signicant
role in the onset of various autoimmune diseases. Moreover,
IL- is indispensable for  eector functions in immune
disorders and maintenance of  cells.
Orphan nuclear receptor ROR𝛾t(retinoid-relatedorphan
receptor gamma t) has been identied as specic 
transcription factor []. ROR𝛾tisinvolvedintheproduction
of IL- receptor (ILR) which is expressed by monocytes,
, , , NK, and dendritic cells. ILR has an
important role in stimulating  cells. IL- receptors
promote IL- transcription and  cell dierentiation via
enforced ROR𝛾t expression. IL- acts on cells that have been
dierentiated into  cells, potentiating ROR𝛾tactivity,and
participates in maintenance and proliferation of  cells.
Clinical trials studying the eects of anti-IL- and anti-
IL- neutralizing antibodies in PsA patients are in progress
[], while the rst results seemed to be not as impressive
as those for TNF-𝛼inhibitor therapy, a recent clinical trial
indicated that Brodalumab, an IL-RA inhibitor, determined
a signicant improvement, when administered for  weeks
to PsA patients []. Moreover, new studies demonstrated
that Ustekinumab, a monoclonal antibody against both IL-
 and IL- cytokines, interfering, respectively, with  and
 activity, improved signicantly PsA symptoms, although
similar ecacy of TNF-𝛼inhibitors needs about  weeks of
treatment to be achieved []. Although immune responses
mediated by IL- and IL- are not as evident as those with
TNF-𝛼,cellsappeartoplayanimportantroleinPsA.
e activation of natural immunity in PsA stimulates
 and  cells, which sustain autoimmune pathology.
ere is an interesting report regarding the relationship of
PsA with microbial infection that is suggestive of PsA patho-
genesis [].
e observation of the lack of symptoms improvement
in PsA patients underwent to HIV infection and thus to
CD+ reduction, suggested that  cells cooperate in the
pathogenesis with CD+ cells []. Probably, CD+T cells
potentiates the production of cytokines in the synovial
membrane, and the cytokines induce broblast proliferation
promoting brosis [–], that probably contribute to joint
stiness and ankylosis [].
Postmenopausal Osteoporosis.Postmenopausalosteoporosis
is a systemic skeletal disorder characterized by reduced bone
mineral density and microarchitectural deterioration of bone
tissue resulting in fragility and susceptibility to fractures
[] and uncoupling of osteoblast-mediated bone formation
and osteoclast-mediated bone resorption. Postmenopausal
osteoporosis stems from the cessation of ovarian function at
menopause and from genetic and nongenetic factors which
heightenandprolongtherapidphaseofbonelosschar-
acteristic of the early postmenopausal period. OC activity
increases aer menopause; these cells may be considered as
cells at the crossroad between immune system and bone as
their precursors circulate within the mononuclear fraction
of peripheral blood [–] and they interact with other
immunecellsasTcells[].
OC precursors increase during estrogen deciency []
and in condition characterized by increased bone turnover as
bone metastases [,] or inammatory diseases [–].
Estrogens act on OC formation and activity both directly
and indirectly, in particular their action is mediated through
theinuenceonimmunesystem[]. In particular, estrogen
loss upregulates OC formation and activity through an
increased production of proosteoclastogenic cytokines by
bone marrow cells [], OBs [], and immune cells [,].
Proinammatory and proosteoclastogenic cytokines as
macrophage colony stimulating factor (M-CSF) and RANKL
are increased during estrogen deciency [,].
Additional inammatory cytokines are responsible for
the upregulation of OC formation observed during estrogen
deciency; some of these molecules have a well-established
role in osteoclastogenesis and bone loss, while others have
not. Among these molecules, the most involved ones in
estrogen deciency bone loss appear to be TNF-𝛼, IL-, IL-
, an d I L-  [ ,–]. A key role of T cell-produced TNF𝛼
has been demonstrated also in bone metastasis [,].
Estrogens are key regulators of immune function as
demonstratedbothinanimalsandinhumans[,]. Des-
pite some inverse reports [,], the main body of literature
rmlysupportstheessentialroleofactivatedTcellsin
regulating bone loss induced by estrogen deciency [,,
,–].
In humans, we have demonstrated a fundamental role for
T cells in postmenopausal bone loss. In particular, we showed
that osteoclastogenesis from peripheral blood precursors
occurs only in the presence of T cells and that T cells are more
active than in healthy post- and premenopausal controls [].
T cells from osteoporotic patients produce more RANKL
and TNF-𝛼, thus inducing OC formation and activity [].
It has also been demonstrated that hormone replacement
therapy decreases osteoclastogenic cytokine production in
postmenopausal women. RANKL expression on lympho-
cytes and marrow stromal cells is signicantly elevated during
estrogen deciency in humans and correlates directly with

Journal of Immunology Research
increases in bone resorption markers and inversely with
serum estrogen levels [].
Estrogen loss promotes T cell activation by increasing
antigen presentation [,]andincreasesthymusoutput
of T cells into peripheral blood []. Estrogen loss expands
theproliferationandlifespanofbonemarrowTcells[,]
increasing expression of class II transactivator (CIITA), a
transcriptional coactivator acting on MHCII promoter [,
,].
Estrogen deciency increases the number of activated
CDL-expressing T cells that promote the expression of M-
CSF and RANKL by stromal cells and downregulates the
production of OPG. e net result is a signicant increase
intherateofosteoclastogenesis[,]. is mechanism
was also described in bone loss due to increased PTH levels
[,]. It is known that the CD/CDL system is crucial
for T cell activation and several functions of the immune sys-
tem. It promotes macrophage activation and dierentiation,
antibody isotype switching, and the adequate organization of
immunological memory in B cells.
Also, the  cells have been implicated in ovariectomy-
induced bone loss; these cells increased aer ovariectomy
and stimulate osteoclastogenesis through IL- production
[]. is eect is reversed by treatment with estradiol. IL-
 increases OB production of proosteoclastogenic cytokines
as TNF𝛼, IL-, and RANKL; these eects are antagonized by
estradiol.
ActivatedTcellshavealsobeensuggestedtoinhibit
osteoclastogenesis by diverting early OC precursors towards
dendritic cells dierentiation []. Indeed T cells have the
capacity to generate both osteoclastogenic cytokines such
as RANKL and TNF-𝛼[], as well as antiosteoclastogenic
factorssuchasIL-.Ithasalsobeensuggestedthattheeects
ofactivatedTcellsonosteoclastogenesisin vitro depend on
the manner in which they are activated []. e net eect
of T cells on OC formation may consequently represent the
prevailing balance of anti- and proosteoclastogenic T cell
cytokine secretion. However, in humans, T cells seem to be
proosteoclastogenic in dierent diseases including estrogen
deciency [,,,–].
Taken together, these observations demonstrate the
causal relation among estrogen deprivation, T cell activation,
increased cytokines production, and bone demineralization.
Also another type of immune cell the B cell has recently
been studied as directly implicated in the regulation of bone
resorption and may be directly involved in the pathogenesis
of postmenopausal osteoporosis. Recent data have shown
that B cells are the dominant producers of OPG in the bone
microenvironment in vivo []. In fact, B cell KO mice
have an osteoporotic phenotype with enhanced osteoclastic
bone resorption and reconstitution with B cells by adoptive
transfer, completely rescued mice from development of osteo-
porosis, and normalizing OPG production [].
In human and animal B cells, OPG production can be sig-
nicantly upregulated by the activation of CD []. In line
with these data, both CD and CDL KO mice displayed an
osteoporotic phenotype and a signicant deciency in bone
marrow OPG concentrations [].
us, the emerging data suggest that the B lineage, rather
than the OB lineage, is likely the major source of OPG in the
bone microenvironment and that T cell signalling to B cells,
through the costimulatory molecules CDL and CD,
plays an important role in regulating basal OC formation and
in regulating bone homeostasis.
On the other hand, it has been recently demonstrated that
activated B cells overexpress RANKL, contributing to bone
resorption [,] and that ovariectomy in mice increases
thenumberofRANKL-expressingBlymphocytesinthebone
marrow [].
A recent paper shows that mice lacking RANKL in B cells
were partially protected from the ovariectomy-induced loss
of cancellous bone []. e role of B-lymphocytes has also
beenevaluatedindiseasecharacterizedbyfocalboneloss
as in periodontal inammation [,,]andrheumatoid
arthritis []. In rheumatoid arthritis, a recent paper sug-
gests that B cells depletion ameliorates the suppressed bone
turnover [].
Taken together, these data suggest that B-lymphocyte
involvement in the adaptive immune response contributes to
bone resorption by the upregulation of RANKL expression
through Toll-like receptor pathways and aligns with the
known ability of T cells to produce RANKL in the presence
of immune stimulus and to increase osteoclastogenesis. e
eect of estrogen deciency on B cell modulation may be one
of the mechanisms through which menopause aects bone
metabolism.
us, the involvement of T and B cells in the control
of bone turnover may provide a novel explanation for the
propensity to osteopenia and osteoporosis development aer
the cessation of ovarian function.
Glucocorticoid-Induced Osteoporosis.GIOisthemostfre-
quent origin of secondary osteoporosis in adults due to the
direct eects of glucocorticoids (GCs) on bone cells [].
GCs primarily aect trabecular bone, whereas the cortical
bone mass is reduced to a lower and slower extent. us,
fractures of the vertebrae are more recurrent. GC exposure
determines a rapid and early phase of bone loss, which is
the consequence of bone resorption exacerbation. is phase
is followed by a more chronic and progressive phase in
which bone mass declines because of impaired OB activity.
GCs augment RANKL expression and reduce OPG levels
in stromal and osteoblastic cells leading to the initial phase
of rapid bone loss. Further, GCs increase MCSF expression
as well as receptor subunits for osteoclastogenic cytokines
of the gp family. However, the main pathophysiological
mechanismofGIOistheimpairedboneformation,dueto
reduced OB formation and activity [,]. GCs impair on
OBs the synthesis of type I collagen, the major protein in
bone matrix. GCs may also inuence osteocyte metabolism
and function, modifying the elastic modulus adjacent to the
osteocyte lacunae leading to reduced mineral to matrix ratios
in the same areas with an enlargement of the lacunar size.
Besides the GC direct actions on bone cells, GC extraskeletal
eects on calcium metabolism have been reported. In par-
ticular, GCs decrease renal tubular calcium reabsorption and

Journal of Immunology Research 
calcium absorption from the gastrointestinal tract is reduced
by mechanisms that oppose vitamin D action [].
GCs also impair bone metabolism during the growth.
In particular, in animal models, GC administration during
growth is the cause of decreased bone formation and resorp-
tion, reductions in the age-dependent increases in trabecular
bone mineral and trabecular thickness, and reductions in lin-
ear growth and accrual of cortical thickness in the femur [].
A decrease of bone mineral density (BMD) has been
reported in numerous pediatric diseases that require
GCs, both as long term replacement therapy, such as -
hydroxylase deciency (-OHD), and as treatment of acute
phase, such as asthma, systemic lupus erythematosus, juve-
nile rheumatoid arthritis, inammatory bowel disease, organ
transplantation, and steroid sensitive nephrotic syndrome
[]. In particular, in -OHD patients on chronic GC ther-
apy, the high osteoclastogenic potential of peripheral blood
mononuclear cells has been reported []. It is supported by
boththepresenceofcirculatingOCprecursorsandRANKL
released by T cells []. Further, high dickkopf- (DKK)
levels, a secreted antagonist of the Wnt/𝛽-catenin pathway,
have been demonstrated in sera and circulating monocytes,
T cells, and neutrophils from -OHD patients [].
Multiple Myeloma. MM is a haematological malignancies,
characterized by the clonal proliferation of plasma cells in
thebonemarrow[]. A major number of mechanisms have
been proposed to explain the increased formation and activ-
ity of the bone resorbing cells, the OCs in MM bone disease,
whereas few mechanisms have been identied to explain the
impairment of the bone forming cells, the OBs. In particular,
MM cells produce dierent cytokines that directly or indi-
rectly aect the bone cell activity, such as IL-, MIP-alpha,
IL-, DKK, and sclerostin [–]. e proposed mecha-
nism is that MM cells adhere to bone marrow stromal cells
(BMSCs) and induce the secretion of numerous proosteo-
clastogenic and antiosteoblastogenic cytokines. e adhesion
involved integrins such as CTLA- and VLA- expressed by
MM cells and VCAM- expressed on BMSCs [].
Moreover, it has previously demonstrated an important
role of T cells in supporting the formation and sur vival of OCs
from peripheral blood mononuclear cells (PBMCs) isolated
from MM patients with osteolysis, through the expression of
high levels of RANKL and decoy receptor  (DcR) [,].
Interestingly, Giuliani et al. showed that malignant human
myeloma cells stimulate RANKL expression in T cells [].
Additionally, other authors demonstrated the high expression
levels of IL- in T cells from MM patients [–]. IL-
 plays a key function in the progression of bone disease
in MM, since the levels of IL- are higher in the more
advanced bone disease. IL- is also able to increase RANKL
expression on BMSCs, thus determining osteoclastogenesis
increase and consequently the development of bone lesions
[]. e amount of  in the bone marrow positively
correlated with the number of osteolytic lesions []as
well as the clinical tumor stage []. Very recently, the
involvement of LIGHT/TNFSF has been reported in MM-
bone disease []. LIGHT is a newly identied member of
the TNF superfamily, expressed by activated leukocytes [].
Recent literature data linked the high serum levels of LIGHT
with the bone loss associated with rheumatoid arthritis [].
Higher expression levels of LIGHT were found in CD+
T cells, monocytes, and neutrophils from osteolytic MM
patients with respect to the same cells from asymptomatic
MM patients as well as monogammopaty of undetermined
signicant (MGUS) and healthy subjects. Further, LIGHT
inhibition signicantly reduces OC formation from PBMCs
of osteolytic MM patients and stimulates OB dierentiation
in cultures derived from MM bone marrow mononuclear
cells, as demonstrated by the increase of colony forming units
of OBs and by the upregulation of osterix transcription factor,
bone sialoprotein, and osteocalcin bone matrix proteins.
Bone Metastatic Tumors. e skeleton is the predominant
metastatic site for many cancers, including breast, prostate,
and lung cancers [,]. Tumor invasion into bone is
associated with dramatic skeletal related events (SRE) such
as fractures, bone pain, hypercalcemia, and spinal cord com-
pression []. e current model for the pathophysiology of
bone metastasis centers on the interaction between tumor
cellsandOCsandisknownasthe“tumor/bonebone
vicious cycle.” Tumor cells secrete a plethora of factors and
cytokines that can directly stimulate OC activation. Once
mature OCs start to resorb the bone, they release bone-
stored factors, such as TGF-𝛽, that further stimulate tumor
cell recruitment and proliferation []. Animal studies have
shown that antiresorptive therapies protect from SRE and
reduce tumor burden. us, antiresorptive agents, such as
zoledronic acid (ZOL) and the anti-RANKL monoclonal
antibody (Ab), denosumab, are widely used in the clinic in
patients with bone metastasis [,,]. Despite reducing
tumor-associated bone complications, recent meta-analysis
studies show controversial results on the antitumor eects of
OC blockade in breast cancer patients with bone metastasis
[,]. A signicant fraction of breast cancer patients with
bone metastases shows progression in their bone disease
while they are on potent antiresorptive agent treatment [–
]. A recent study suggested the existence of a preosteolytic
early phase of bone metastasis that is independent of OC
activation []. Considering the complexity of the bone
microenvironment, serving as home to hematopoietic stem
cells and their progeny, which constitute the immune system,
it is logical to consider the interactions between tumor cells
and immune cells as potentially important regulators of bone
metastasis beyond the OC.
PresenceofactivatedCD+andCD+Tcellshasbeen
observed in the bone marrow of untreated patients with
breast cancer []. CD+ T cells have the capacity to specif-
ically identify and eliminate tumor cells via recognition of
tumor-specic antigens. Activated CD+ T cells can further
facilitate the development of cytotoxic CD+ T cells by
secreting numerous cytokines, including Interferon 𝛾(IFN).
Interferon 𝛾(IFN) exerts antiproliferative [], proapoptotic
[], and angiostatic [] eects resulting in the killing of a
proportion of the tumor. us, presence of CD+ and CD+
T cells at tumor site is a good prognostic indicator. However,
whetherTcellsmodulatebonemetastaticdissemination
and/or tumor growth in the bone microenvironment is not

Journal of Immunology Research
totally clear. In a recent report, Bidwell et al. demonstrated
that silencing of IFN regulatory factor (Irf), a transcription
factor controlling the induction of IFN genes, in breast
cancercellspromotesbonemetastasesthroughescapingfrom
immune control []. Importantly, an association with low
expressionofIrfsignaturesinprimarybreasttumorsand
higher number of bone metastatic events has been observed
[].isndingisastrongindicationthattheimmune
system can modulate metastatic dissemination to bone in
breast cancer patients.
Using animal models with established T cell immune
deciencies, we have also demonstrated that CD+ and
CD+ T cell populations exert antitumor eects in the
context of bone metastases []. We found that depletion of
both CD+ or CD+ T cell subsets can reduce the antitumor
eects ZOL in animals with bone metastases. Importantly,
ZOLtreatmentisstillhighlyeectiveinsuppressingtumor-
induced bone loss []. Conversely, T cell activation induced
by administration of anti-CTLA Ab can signicantly reduce
bone tumor burden []. ese observations have important
clinical implications and suggest that reduced T cell numbers
or impaired T cell activation might be the cause for the failure
of ZOL to reduce tumor burden and increase survival in
breast cancer patients.
Developing neoplasms can also acquire the ability to
escape CD+ T cell cytotoxicity by promoting expansion of
-polarized CD+ T helper and regulatory T cells, as well
as immune suppressor cells of myeloid origin reviewed in
[–]. Monteiro et al. recently found that CD+ T cells
isolated from bone marrow of tumor bearing mice are potent
stimulators of osteoclastogenesis []. is subset of tumor-
specic CD+ T cells has the ability to promote OC activation
and induce osteolytic bone disease even before seeding of
tumor cells in the bone microenvironment. Importantly,
when tumor-specic CD+ T cells are adoptively transferred
into mice orthotopically injected with T tumor cells, tumor
colonization to bone, but not to other metastatic sites, is
increased. Whether this particular population of CD+ T
cells is increasing tumor bone metastases by aecting the
OCs or also by inducing an immune suppressive environment
needs to be established.
e bone microenvironment is particularly enriched in a
highly heterogeneous population of immature myeloid pro-
genitor cells that have the ability to exert immune suppressive
eects in the presence of a tumor. is immature myeloid
population, herein referred to as myeloid derived suppressor
cells (MDSCs), represents –% of the total bone marrow
cells of na¨
ıve mice and is further expanded up to –%
of total marrow cells depending on the tumor type [].
Circulating MDSCs are detected in the blood of patients with
various types of cancer []. In response to factors secreted
by a tumor, MDSCs leave the bone marrow and are found in
highnumbersincirculation,spleen,andtumorsiteswhere
they induce suppression of cytotoxic T cells []. MDSCs
exert their proneoplastic eects through the release of small
soluble oxidizers, by altering T cell/antigen recognition, and
depletion of essential amino acids from the local extracellular
environment, all ultimately leading to T cell suppression
[–]. In addition, MDSCs can induce the expansion
ofregulatoryTcells,asubtypeofTcellsexertingimmune
suppressive functions. Furthermore, direct eects of MDSCs
on tumor proliferation through overproduction of cytokines
and angiogenic factors have also been proposed [].
A correlation between high MDSC numbers, advanced
stageofmalignancy,andpoorprognosishasbeenobserved.
We have recently shown that increased bone metastasis in
PLC𝛾−/−mice is due to suppression of antitumor T cells
responses. Although PLC𝛾 is not expressed by T cells, we
found that PLC𝛾−/−mice have increased MDSC numbers
with more potent immune suppressive eects than WT
[]. Downregulation of PLC𝛾activationalsooccursinthe
MDSCs of patients with advanced pancreatic cancer [].
Recent evidence also indicates that MDSCs participate
in the preparation of premetastatic niches where they create
a favorable environment for subsequent tumor colonization
[,]. Accumulation of MDSCs in bone marrow has been
observed during early stages of MM []. A role for MDSCs
in promoting tumor growth in bone through the OCs has
also been proposed. Zhuang et al. discovered that MDSCs
from mice injected with MM cells have increased osteoclasto-
genic potential. Importantly, coinjection of tumor-challenged
MDSCs together with MM cells leads to increased tumor
burden and osteolytic lesions, an eect that is inhibited
by administration of ZOL []. Similarly, Sawant et al.,
usinganimmunecompetentmodelofbreastcancerbone
metastases, showed that MDSCs isolated from the tumor
bone microenvironment dierentiate into resorbing OCs in
vitro. Remarkably, MDSCs isolated from tumor-free mice
or tumor-bearing animals without bone metastases lack the
ability to undergo OC dierentiation []. is impor-
tant observation suggests that there are intrinsic dierences
between MDSCs, depending on the tumor location. Why
MDSCs from mice bearing bone metastases have the ability
to dierentiate into OCs might depend on the proosteoclas-
togenic rich cytokine milieu that characterizes the tumor
bone microenvironment. However, it is unlikely that the
bone tumor promoting eects of this subset of MDSCs is
primarily dependent on their ability to dierentiate into
OCs. PLC𝛾−/−mice display increased bone metastatic
dissemination and higher MDSC numbers, but deletion of
PLC𝛾−/−also impairs the OC dierentiation process [,
]. us, in the context of PLC𝛾−/−deciency, MDSCs are
more likely to support tumor growth in bone by suppressing
T cell activity. All together, these studies indicate that MDSCs
are central players in the tumor/bone vicious cycle either
through suppression of antitumor T cell responses or through
dierentiation into resorbing OCs.
Unfortunately to date there is no curative treatment
for bone metastasis. Tumor cells that reach the bone envi-
ronment are usually resistant to the current antitumor
therapeutic approaches. e only options for these patients
are palliative treatments to reduce bone pain and prevent
additional bone destruction. More studies are needed to
exploit the importance of antitumor and tumor promoting
immune responses in patients with bone metastases and
whether manipulation of T cell-MDSC interactions could
oer therapeutic advantages to maximize the antitumor
eects of OC blockade.

Journal of Immunology Research 
T : Established and possible novel therapeutic targets in the
dierent bone diseases.
(a)
Established
therapeutic
targets
Pathologies References
TNF-𝛼PsA []
RANKL
PsA, osteoporosis,
MM, and bone
metastatic tumors
[–]
IL- PsA []
IL- PsA [,]
IL-RA PsA []
(b)
Possible
novel
therapeutic
targets
Pathologies References
LIGHT MM []
DKK Bone metastasis and
GIO []
IL- MM [–]
MDSC
targeting Bone metastasis [,]
2. Conclusions
e reviewed mechanisms underlying the bone disease
clearly highlighted the key involvement of the cells with an
immunological role. Further, it is also clear that numerous
pathways are common to the dierent diseases, whereas
others are disease-specic. us, these recent ndings repre-
sent an important issue, leading to the identication of new
therapeutic targets, mainly biological drugs, which in the last
years are in strong development (Tabl e  ).
Conflict of Interests
e authors declare that there is no conict of interests
regarding the publication of this paper.
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