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J. Exp. Med.
The Rockefeller University Press • 0022-1007/2002/06/1533/7 $5.00
Volume 195, Number 12, June 17, 2002 1533–1539
http://www.jem.org/cgi/doi/10.1084/jem.20020067
1533
Overexpression of Interleukin (IL)-7 Leads to
IL-15–independent Generation of Memory Phenotype
CD8
T Cells
William C. Kieper,
1
Joyce T. Tan,
1
Brea Bondi-Boyd,
1
Laurent Gapin,
2
Jonathan Sprent,
1
Rhodri Ceredig,
3
and Charles D. Surh
1
1
Department of Immunology, The Scripps Research Institute, La Jolla, CA 92037
2
Division of Developmental Immunology, The La Jolla Institute for Allergy and Immunology,
San Diego, CA 92121
3
U548 INSERM, CEA-Grenoble, F-38054 Grenoble, France
Abstract
Transgenic (TG) mice expressing a high copy number of interleukin (IL)-7 cDNA under the
control of the major histocomaptability complex (MHC) class II promoter display a 10–20-fold
increase in total T cell numbers. Here, we show that the increase in T cell numbers in IL-7 TG
mice is most apparent at the level of memory phenotype CD44
hi
CD122
hi
CD8
cells. Based
on studies with T cell receptor (TCR) TG mice crossed to IL-7 TG mice, increased levels of
IL-7 may provide costimulation for TCR recognition of self-MHC ligands and thus cause naive
CD8
cells to proliferate and differentiate into memory phenotype cells. In addition, a marked
increase in CD44
hi
CD122
hi
CD8
cells was found in IL-7 TG IL-15
mice. Since these cell
are rare in normal IL-15
mice, the dependency of memory phenotype CD8
cells on IL-15
can be overcome by overexpression of IL-7.
Key words: T lymphocytes • homeostasis • cytokines • mice • transgenic
Introduction
The overall size and composition of the mature T cell pool
are closely regulated by homeostatic mechanisms (1). Re-
cently, it has been shown that homeostasis of naive T cells
is primarily controlled through TCR contact with self-
MHC/peptide ligands and exposure to the cytokine IL-7
(2–5). Thus, in the absence of interaction with either of
these ligands, naive T cells disappear and fail to undergo
“homeostatic” proliferation in T cell–deficient conditions
(2–5). Since the density of self-MHC/peptide ligands is
constant, IL-7 may be the key factor in determining the
overall size of the naive T cell pool. Accordingly, overpro-
duction of IL-7 would presumably lead to a proportional
enlargement of the naive T cell pool. In contrast to naive
cells, homeostasis of memory T cells is known to be regu-
lated independently of contact with MHC molecules (6, 7).
In terms of cytokine requirements, none of the common
chain (
c)
*
family of cytokines (IL-2, -4, -7, -9, -15) appears
essential for memory CD4
cells (8). By contrast, memory
CD8
cells are heavily dependent on IL-15 (9, 10). Thus,
most memory CD8
cells express elevated levels of the
IL-15R CD122, undergo selective proliferation in response
to IL-15, and are markedly depleted in IL-15
–
and IL-15R
mice (9–12).
IL-7 was initially described as a growth factor for B cell
progenitors and was shown to be produced by bone marrow
stromal cells (13). Subsequent studies showed that IL-7 is
also synthesized by other tissues, including thymic and in-
testinal epithelial cells (14, 15). Furthermore, a generation
of IL-7
and IL-7R
mice revealed that IL-7 has a nonre-
dundant role in supporting early development of both B
and T cells (16, 17). In addition to knockout mice, four in-
dependent lines of transgenic (TG) mice overexpressing
IL-7 were generated in the early 1990s (18–21). Although
most of the TG mice were found to possess increased num-
bers of B and T cells, a careful examination of the T cell
pool was not performed.
To study the effect of IL-7 overproduction on T cell ho-
meostasis, we analyzed a B6 TG line that expresses a high
copy number of IL-7 cDNA under the control of the
MHC class II promoter (21). Despite a marked (25–50-
fold) elevation in IL-7 levels, these mice appear healthy
Address correspondence to Charles D. Surh, Dept. of Immunology,
IMM-26, The Scripps Research Institute, 10550 North Torrey Pines Rd.,
La Jolla, CA 92037. Phone: 858-784-2006; Fax: 858-784-8227; E-mail:
csurh@scripps.edu
*
Abbreviations used in this paper:
c, common
chain; LM, littermate;
TG, transgenic.
1534
IL-7 Regulation of Memory CD8
Cell Homeostasis
and, unlike other lines (18, 20), remained free of dermatitis
(21); tumor formation is low and restricted to B cells. Here,
we show that the massive overproduction of T cells in IL-7
TG mice is largely skewed to memory-phenotype CD44
hi
CD122
hi
CD8
cells. Based on the results of crossing IL-7
TG mice to TCR TG and IL-15
mice, IL-7 may play an
important role in guiding both the generation and mainte-
nance of memory CD8
cells.
Materials and Methods
Mice.
IL-7 TG mice (21) were obtained from J. Andersson at
the Basel Institute and maintained by breeding to C57BL/6 (B6)
or B6.Ly 5.1
mice. All mice used in the current study were
hemizygous for the transgene. B6 and B6.PL congenic mice were
purchased from The Scripps Research Institute (TSRI) breeding
facility. Origins of OT-I, HY, IL-15
, and
2m
K
D
mice
were described previously (3, 5).
FACS
®
Analysis.
Suspensions of thymus, LN, and spleen
cells were double- and triple-stained as described previously (2)
using various combinations of PE-anti-CD44, PE-anti-CD122,
Cy5-anti-CD8, FITC-anti-CD8, Cy5-anti-CD4, Cy5-anti-Ly
5.1, and biotinylated T3.70 (all from eBioscience). Biotinylated
anti–Thy-1.1, anti-
c and FITC-V
5, and PE-V
2 antibodies
were purchased from BD PharMingen. Biotinylated mAbs were
detected with Cy5-conjugated streptavidin (Jackson ImmunoRe-
search Laboratories).
For intracellular cytokine staining, spleen cells (4
10
6
cells
per milliliter) in complete medium containing 0.67
ls/ml
Brefeldin A (GolgiStop; BD PharMingen) were incubated for 7 h
in 24-well plates coated with anti-CD3 (2C11; eBioscience)
mAb (0.5
g/ml) and stained as described previously (22).
Briefly, cells were first stained with Cy5-anti-CD8 and PE-anti-
CD44, fixed with paraformaldehyde followed by permeabiliza-
tion with saponin and stained with FITC-anti–IFN-
mAb
(eBioscience).
To assess homeostatic proliferation, small numbers (10
6
cells per
mouse) of FACS
®
-sorted CD44
hi
CD8
cells were labeled with
CFSE (Molecular Probes) and injected into mice exposed to 600
cGy irradiation and analyzed 7 d later as described previously (2).
The rate of background T cell turnover in adult thymecto-
mized mice was performed by adding BrdU (0.8 mg/ml) into the
drinking water for 7 d and staining LN cells as described previ-
ously (23).
Effect of cytokines in vitro was assessed by culturing CD4
and
CD8
cells (2
10
6
cells per milliliter), purified as described pre-
viously (2), for 5 d in complete medium containing IL-7 and/or
IL-15 at 20 ng/ml. Some cultures contained mAbs to IL-7R
(A7R34) (24) and IL-2R
(TM-
1) (25) at 50
g/ml. The in
vivo effect of blocking IL-7R was measured by injecting mice in-
traperitoneally with either 200
g of salt-precipitated A7R34 as-
cites or 200
g of rat IgG every other day over a 7-d period. In
vitro–cultured cells and LN and spleen cells from antibody-
injected mice were analyzed by triple staining with FITC-anti-CD4,
Cy5-anti-CD8 and PE-anti-CD44, or PE-anti-CD122 mAbs.
CDR3 Lengths Analysis of TCR Chains.
Spectratyping of
TCR-V
and -V
repertoire was performed on purified CD4
and CD8
cells as described previously (26). Briefly, after cDNA
synthesis from total RNA, the RNA was denatured and reverse
transcribed. Small aliquots of the cDNA were amplified by PCR
using primers previously described to analyze the repertoire of V
and V
chains (27). The PCR product was then subjected to run-
off reaction with fluorescent-tagged primers and analyzed by cap-
illary electrophoresis in an automated DNA sequencer (Applied
Biosystems) and the distribution of CDR3 lengths determined.
Results and Discussion
Characterization of T Cells in IL-7 TG Mice.
As reported
previously (21), young adult IL-7 TG mice were found to
contain massive numbers of T (and B) cells (10–20 times
above normal) in the secondary lymphoid tissues, despite
the presence of a relatively normal sized thymus (Fig. 1 A).
The increase in peripheral T cell numbers was more pro-
nounced for CD8
cells than CD4
cells and resulted in a
reduction in the CD4/CD8 ratio. The increase in total T
cell numbers and the preferential effect of IL-7 on CD8
cells are likely to be a consequence of IL-7 acting directly
on mature T cells as similar effects were observed in normal
mice given repeated injection of recombinant IL-7 (28–
Figure 1. Characterization of T cells in young adult IL-7 TG mice. (A)
T cell subset distribution and cell numbers in the thymus, LN, and spleen
in IL-7 TG and LMs. Cells were triple-stained for CD4, CD8, CD44, or
CD122 and analyzed by flow cytometry as described in Materials and
Methods. Total numbers of thymocytes and indicated naive and memory
phenotype CD4 and CD8 cells, combined from spleen and pooled
LNs, IL-7 TG, and LMs are shown. Data are representative of 20 TG
and LM mice. (B) Expression of CD44 and CD122 on gated LN CD4
and CD8 cells from IL-7 TG and LMs. (C) Randomly distributed
CDR3 lengths of TCR-V chains expressed on CD8 cells from IL-7
TG mice. CDR3 lengths on most V and all V chains were determined
on purified CD8 cells from IL-7 TG and LM mice as described in Mate-
rials and Methods. Representative data for indicated V chains are shown;
all analyzed chains were randomly distributed in both TG and LM cells.
1535
Kieper et al.
30). Strikingly, the majority (55–70%) of CD8
cells in IL-7
TG mice had a memory (CD44
hi
and CD122
hi
) pheno-
type, whereas most CD4
cells had a naive (CD44
lo
) phe-
notype (Fig. 1 B). For CD8
cells, the increase in total cell
numbers was
50-fold for CD122
hi
cells and
25-fold for
CD44
hi
cells but only
4-fold for naive CD44
lo
cells. For
CD4
cells, CD44
hi
cells were elevated by
5-fold and
CD44
lo
cells by
4-fold (Fig. 1 A). Expression of other
markers, such as CD25, CD62L, CD69, IL-7R
, on TG T
cells was comparable to wild-type T cells (data not shown).
To determine whether the enlargement of the mature T
cell pool is due to an oligoclonal expansion, the TCR rep-
ertoire of purified IL-7 TG CD4
and CD8
cells was ex-
amined. A previous study did not reveal any significant
skewing of the TCR-V
repertoire (21), but it was still
possible that an oligoclonal expansion could be revealed
from analysis of the CDR3 lengths. Examination of all V
chains and the majority of V
chains revealed that the
CDR3 lengths were randomly distributed for all chains
similar to T cells from littermates (LMs) (Fig. 1 C).
The experiments discussed below address the issue of
why IL-7 TG mice show a preferential elevation of mem-
ory phenotype CD8
cells. Why these mice also show a
milder increase in naive phenotype cells, despite having a
normal thymus, will be the subject of another paper.
Functional Capacity of IL-7 TG CD44
hi
CD8
Cells. By
two different parameters, TG CD44hi CD8 cells closely
resembled CD44hi CD8 cells from wild-type mice. First,
as measured by BrdU incorporation in thymectomized
mice, the turnover of TG CD44hi CD8 (and CD44hi
CD4) cells was relatively slow, in fact slower than in LMs;
this finding applied both in adult (8-wk) mice (Fig. 2 A)
and also in neonatal (3-wk) mice (data not shown). Hence
the increase in CD44hi CD8 cells in IL-7 TG mice did
not reflect an increase in their rate of division. However,
total numbers of proliferating T cells in IL-7 TG mice were
7–10-fold higher than in wild-type mice. Second, as mea-
sured by division of CFSE-labeled cells, sorted TG CD44hi
CD8 cells transferred to irradiated B6 hosts behaved simi-
larly to normal B6 CD44hi CD8 cells in their rapid rate of
homeostatic proliferation in these T cell–depleted hosts
(Fig. 2 B). Moreover, like normal CD44hi CD8 cells (Fig.
2 C), but unlike naive CD44lo CD8 cells (6), IL-7 TG
CD44hi CD8 cells underwent efficient homeostatic prolif-
eration in MHC class I (2mDK) mice (Fig. 2 C).
By the above parameters, TG CD44hi CD8 cells closely
resembled normal B6 CD44hi CD8 cells. However, dif-
ferent results were obtained for IFN- synthesis. Thus, af-
ter in vitro stimulation with anti-CD3 mAb, the propor-
tion of IFN- synthesizing cells was much higher for TG
CD44hi CD8 cells (49%) than for normal B6 CD44hi
CD8 cells (9%) (Fig. 3 D). Since IL-7 has costimulatory
activity for T cell stimulation and function (30, 31), this
difference could be a reflection of prior exposure of the TG
cells to high levels of IL-7 in vivo.
Generation of TCR TG CD44hi CD8
Cells in IL-7 TG
Mice. T cells with a memory phenotype usually originate
from naive T cells responding to foreign antigen, but can
also arise from naive T cells undergoing homeostatic prolif-
eration to self-ligands (32). To examine which ligands, self
or foreign, cause production of memory phenotype CD8
cells in IL-7 TG mice, we crossed two TCR TG lines,
OT-I and HY, to IL-7 TG mice.
Figure 2. CD44hi CD8 cells from IL-7 TG mice functionally resem-
ble CD44hi CD8 cells from wild-type mice. (A) Background turnover
over of T cells IL-7 TG mice. Adult thymectomized IL-7 TG and LMs
were given BrdU in the drinking water for 7 d and LN cells were triple
stained for BrdU, CD44, and either CD4 or CD8 as described in Materi-
als and Methods. (B) IL-7 TG CD44hi CD8 cells undergo the same rate
of homeostatic proliferation in T cell–depleted hosts as wild-type CD44hi
CD8 cells. CD44hi CD8 cells from Ly 5.1 IL-7 TG and LM mice pu-
rified by cell sorting were CFSE labeled and injected into irradiated (600
cGy) B6 mice. 7 d after transfer donor cells in host LN were analyzed for
CFSE expression by staining for Ly5.1 and CD8. Shown are gated do-
nor LMs and TG CD8 cells. (C) Homeostatic proliferation of IL-7 TG
CD44hi CD8 cells is MHC independent. Sorted and CFSE-labeled
CD44hi CD8 cells from Ly 5.1 IL-7 TG mice were injected into irra-
diated B6 and MHC class I (2m–DK) mice and analyzed 7 d later as
in B. (D) IL-7 TG CD44hi CD8 cells synthesize IFN- upon activation.
Spleen cells from IL-7 TG and LMs were stimulated with anti-CD3 mAb
in vitro for 7 h in the presence of Brefeldin A and stained for CD8,
CD44, and intracellular IFN- as described in Materials and Methods.
Shown are gated CD8 cells. All data are representative of results from
three experiments.
1536 IL-7 Regulation of Memory CD8 Cell Homeostasis
For the OT-I line, nearly all (95%) of the CD8 cells
in this line are clonotype-positive (V2 V5) cells and
have a naive CD44lo phenotype (data not shown). These
naive cells appear to have significant affinity for self-
MHC/peptide ligands because exposure to these ligands
after transfer to T cell–depleted hosts causes strong ho-
meostatic proliferation of the cells and a switch to a mem-
ory phenotype (3). When OT-I mice were crossed to IL-7
TG mice, thus producing OT-I IL-7 TG mice, total num-
bers of CD8 cells in these mice were increased about
threefold relative to OT-I LMs that lacked the IL-7 trans-
gene (Fig. 3). Significantly, numbers of CD44hi CD8
cells were 20-fold higher in OT-I IL-7 TG mice than in
normal OT-I LMs. In addition, in contrast to OT-I LMs,
many of the “CD44lo” cells in OT-I IL-7 TG mice were
actually CD44int rather than CD44lo (Fig. 3). This finding
is of interest because OT-I cells show a similar CD44int/hi
phenotype after homeostatic proliferation in T cell–depleted
hosts (3). Since nearly all (95%) of the CD8 cells in
OT-I IL-7 TG mice were V2 V5, therefore, it
would seem highly likely that the stimulus for expansion
of CD44int/hi cells in these mice was provided by self-
MHC/peptide ligands rather than foreign antigens. Thus,
the implication is that high levels of IL-7 in OT-I IL-7
TG mice act as a costimulator to drive naive OT-I cells to
undergo proliferation and differentiation in response to
self-MHC/peptide ligands. Despite the high penetrance of
the OT-I transgene, however, one caveat is that the IL-7
TG OT-I mice were not in a background (recombination
activating gene [RAG] or TCR-) that precludes ex-
pression of endogenous TCR chains. Therefore, the con-
tribution of foreign antigens in upregulation of CD44 can-
not be totally excluded.
If self-MHC/peptide ligands are driving production of
CD44hi CD8 cells in IL-7 TG mice, the capacity of high
levels of IL-7 to augment T cell reactivity to self-MHC/
peptide ligands might not apply to T cells with “below av-
erage” affinity to self-ligands. To assess this possibility we
examined the HY TCR TG line. As shown previously, the
TCR clonotype-positive cells in this line (detected by
T3.70 mAb) fail to undergo homeostatic proliferation in T
cell–depleted hosts (2), presumably because the TCR affin-
ity of T3.70 cells for self-ligands is quite low. HY T cells
do undergo homeostatic proliferation but this applies only
to the T3.70 subset (2). As shown in Fig. 3, HY IL-7 TG
mice showed a marked expansion of CD44hi CD8 cells.
Significantly, almost all of these cells were T3.70; there
was no expansion of T3.70 cells and these cells retained a
naive CD44lo phenotype. Thus, expansion of T3.70 but
not T3.70 cells in HY IL-7 TG mice correlated closely
with the capacity of T3.70 but not T3.70 cells to un-
dergo homeostatic proliferation in T cell–depleted hosts.
Collectively, these data on OT-I and HY mice suggest
that a significant proportion of the expanded CD44hi
CD8 cells in IL-7 TG mice are derived from precursor
cells stimulated by self-MHC/peptide ligands. This notion
is consistent with the finding that the TCR repertoire of
TG CD8 cells appears very diverse (Fig. 1 C). In addition,
however, a proportion of the expanded CD44hi CD8 cells
may arise through the action of IL-7 on naive CD8 cells
responding to foreign antigens. In this respect, we have ob-
served that IL-7 has marked costimulatory activity for T
cells responding to foreign antigens in vivo. Thus, injection
of male splenic APC into IL-7 TG HY mice led to much
greater expansion of T3.70 cells followed by establish-
ment of a larger pool of T3.70 memory CD44hi CD8
cells than was observed in HY mice lacking IL-7 transgene
(data not shown). Although the relative contribution made
by foreign versus self-antigens in the production of CD44hi
CD8 cells in IL-7 TG mice is unknown it is notable that
the marked increase in CD44hi CD8 cell numbers in these
mice is apparent even in neonatal (1.5 wk) mice (data not
shown). With high IL-7 levels, CD44hi CD8 cells arise
very early in life and accumulate progressively.
IL-15–independent Generation of CD44hi CD8
Cells in
IL-7 TG Mice. As mentioned earlier (see Introduction),
survival of most CD44hi CD8 cells in T cell–sufficient
mice has recently been shown to be heavily dependent on
IL-15 (9–11). IL-15 dependency is particularly pronounced
for CD122hi CD44hi CD8 cells, as these cells are conspic-
Figure 3. Di
ff
erentia
l
e
ff
ect o
f
IL-7 overpro
d
uction on
OT-I and HY TCR TG CD8 cells. OT-I and HY TCR
TG mice were bred to express the IL-7 transgene; LN and
spleen cells from 4-wk-old mice double TG mice were
stained for CD8, V2, and CD44 for OT-I mice and
CD8, T3.70, and CD44 for HY female mice. Shown on
the left side for OT-I cells are the profiles of CD44 on
gated CD8 V2 cells from normal OT-I TG mice (thin
line) and IL-7 TG OT-I mice (thick line); CD44 levels on
polyclonal CD8 cells (broken line) from IL-7 TG mice
are shown for comparison. For HY cells, shown are CD44
levels on gated T3.70 cells from normal HY TG mice
(thin line), IL-7 TG HY mice (thick line) and T3.70 cells
from IL-7 TG HY mice (broken line). The total numbers
(pooled from LN and spleen) of recovered CD44lo and
CD44hi populations of CD8 cells from the OT-I and HY
mice are shown on the right side. Similar results were
found when the cells were analyzed in terms of CD122 ex-
pression (not shown). Data are representative of results
from four to six individuals for each type of mice.
1537 Kieper et al.
uously absent in IL-15 mice (unpublished data). To deter-
mine whether the generation and/or the maintenance of
CD44hi CD8 cells in IL-7 TG mice is IL-15 dependent,
IL-7 TG mice were crossed with IL-15 mice. Surpris-
ingly, comparison of 4-wk-old IL-15 IL-7 TG mice with
age-matched control IL-15 IL-7 TG LMs revealed that
IL-15 is dispensable for generation of CD122hi CD44hi
CD8 cells in IL-7 TG mice. Thus, in terms of both total
cellularity and phenotype, including both CD44 and
CD122 expression, the peripheral CD8 cell compartment
was comparable in IL-15 IL-7 TG versus IL-15 IL-7 TG
mice (Fig. 4). The proportions of CD122hi CD8 and
CD44hi CD81 cells in cell cycle, as determined by BrdU la-
beling, were also comparable between IL-15 versus IL-
15
IL-7 TG mice (data not shown). It should be men-
tioned that IL-15 deficiency was accompanied by a slight
reduction in the total number of CD8 cells and a minor
increase in the total number of CD4 cells (Fig. 4). The
point to emphasize, however, is that numbers of IL-15 re-
sponsive CD122hi CD8 cells rose from nearly undetect-
able levels in IL-15 mice to very high levels in IL-7 TG
IL-15 mice (Fig. 4).
Since nonTG IL-15 mice are virtually devoid of
CD122hi CD8 cells (reference 11 and Fig. 4), the above
finding with IL-7 TG IL-15 mice indicates that high con-
centrations of IL-7 can compensate for absence of IL-15 in
promoting survival of CD44hi CD122hi CD8 cells. In
support of this idea, a submitogenic dose (20 ng/ml) of ei-
ther IL-7 or IL-15 maintained viability of CD44hi CD8
cells in vitro for 5 d, whereas these cells died in the absence
of cytokines (Fig. 5 A). This finding applied equally to T
cells purified from IL-7 TG or wild-type mice. Whereas
IL-7 was also able to enhance survival of other T cell sub-
sets, the antiapoptotic function of IL-15 was largely re-
stricted to CD44hi CD8 cells, presumably because the IL-
15 receptor, CD122, is expressed at a much higher level on
CD44hi CD8 cells than on other T cell subsets (9).
Figure 4. IL-15 is not required for generation of CD44hi CD8 cells in
IL-7 TG mice. LN cells from young adult IL-7 TG mice bred to an IL-15
background were stained for CD4, CD8, and CD44 or CD122 and ana-
lyzed. Shown are histograms of CD44 and CD122 on gated CD4 and
CD8 cells as compared with the same cells from age-matched control
LMs, IL-7 TG, and IL-15 mice. Total numbers of CD4, CD8, and
CD122hi CD8 cells from spleen and LNs from the mice are shown be-
low. Data are representative of results from six to eight individuals for
each type of mice.
Figure 5. Role of IL-7 in supporting survival of CD44hi CD8 cells in
vitro and in vivo in IL-7 TG mice. (A) IL-7 can maintain survival of
CD44hi CD8 cells in vitro. Aliquots of purified IL-7 TG LN T cells
were incubated with the indicated cytokines at (20 ng/ml) either alone or
in the presence of anti–IL-7R mAb (A7R34), anti-CD122 mAb
(TM-1), or both mAbs at 50 g/ml. The cells were harvested 5 d later
and stained for CD4, CD8, and CD44, and the numbers of viable CD44hi
CD8 cells were calculated. (B) Blocking the IL-7R leads to depletion of
CD44hi CD8 cells in IL-7 TG mice. Groups of two to three LMs, IL-7
TG, and IL-15 IL-7 TG mice were injected three times with either 200
g of anti–IL-7R mAb (A7R34) or rat IgG at 2-d intervals. 2 d after the
last injection, LN and spleen cells were collected and stained for CD4,
CD8, and CD44. The total numbers of CD44hi CD8 cells recovered
from spleens and pooled LN are shown. Another experiment showed
similar results.
1538 IL-7 Regulation of Memory CD8 Cell Homeostasis
Although IL-7 is presumed to mediate its effect through
the IL-7R, it is conceivable that the high levels of IL-7
available in IL-7 TG mice might act through weak cross-
reactive binding to the IL-15R. This possibility seems un-
likely for two reasons. First, the capacity of IL-7 to rescue
CD44hi CD8 cells from spontaneous death in vitro was
strongly inhibited by anti–IL-7R mAb, and the presence
of anti-CD122 mAb did not diminish the effect of IL-7
(Fig. 5 A). This finding also applied when the concentra-
tion of IL-7 was raised to a high level (data not shown).
Second, injecting either IL-7 TG or IL-15 IL-7 TG mice
for 1 wk with anti–IL-7R mAb (200 g every other day)
caused a marked reduction in total numbers of CD44hi
CD8 cells in the mice (Fig. 5 B). A partial reduction in
CD44hi CD8 cells numbers was also observed in control
LM mice (Fig. 5 B). The reduction in cell numbers in these
mice is unlikely to be due to antibody-mediated opsoniza-
tion as depletion of other IL-7Rhi cells, e.g., CD44hi CD4
cells and mature B cells, was minimal (data not shown).
Our observation that the elevation of T cell numbers in
IL-7 TG mice is skewed toward CD8 cells is consistent
with prior evidence on the effects of short-term injection
of IL-7 into normal mice (28–30). Although these latter
data indicated that IL-7 acted largely on peripheral T cells
(rather than thymocytes), the issue of whether IL-7 stimu-
lated naive T cells or memory T cells or both subsets was
not resolved. The data in this paper suggest that, in IL-7
TG mice, elevated levels of IL-7 boost homeostasis of
memory CD8 cells through two different mechanisms.
First, the data on IL-7 TG TCR TG mice suggest that
high levels of IL-7 amplify TCR recognition of self-MHC
ligands by naive CD8 cells. For these cells, TCR recogni-
tion of self-ligands in the presence of background levels of
IL-7 normally induces a covert signal that is sufficient to
keep naive CD8 cells alive, but without inducing prolifer-
ation or a change in surface phenotype. With high levels of
IL-7, however, TCR signaling is enhanced and contact
with self-ligands causes naive CD8 cells to proliferate and
differentiate into memory phenotype cells.
Second, the high levels of IL-7 contribute to survival and
turnover of established memory phenotype CD8 cells. As
discussed earlier, the background turnover and survival of
CD44hi CD8 cells is normally controlled by IL-15; IL-7 is
not important, probably because levels of IL-7 in the pe-
ripheral lymphoid tissues of normal mice are too low to
play a significant role in homeostasis. However, in IL-7 TG
mice one can envisage that IL-7 and IL-15 both contribute
to homeostasis. The presence of large numbers of CD44hi
CD122hi CD8 cells in IL-15
IL-7 TG mice but not nor-
mal IL-15 mice is then readily explained. Here, high lev-
els of IL-7 compensate for the lack of IL-15 and homeosta-
sis of CD44hi CD8 cells remains normal. The finding that
the total numbers of CD44hi CD8 cells in IL-15 IL-7
TG mice dropped sharply after treatment with anti–IL-7R
mAb is consistent with this idea. However, the finding that
anti–IL-7R mAb treatment had a similar effect on IL-15
IL-7 TG mice is surprising because here we expected the
cells to be rescued by IL-15. It is conceivable, however,
that the massive overproduction of CD122hi CD8 cells in
IL-7 TG mice depletes IL-15, thus causing the cells to be
heavily dependent on IL-7.
In addition to memory phenotype CD8 cells, IL-7 TG
mice clearly contain increased numbers of naive T cells, in-
cluding both CD4 and CD8 cells. Likewise, naive T cell
increase in number after injection of IL-7 (28–30). How
IL-7 leads to expansion of naive T cells is unknown, al-
though increases in cell survival, proliferation, and release
of new T cells from the thymus are all likely possibilities.
Resolving this important issue will have to await further
investigation.
We are grateful to Drs. P. Marrack and J. Andersson for sending
A7R34 hybridomas and the IL-7 TG mice, respectively. We also
thank J. Kuhns, M. Chan, and D. Kim for various supports. This is
publication number 14506-IMM from TSRI.
This work was supported by U.S. Public Health Service grants
AI21487, CA38355, and AI46710 (to J. Sprent) and AI41079,
AI45809, and AG20186 (to C.D. Surh). J.T. Tan and W.C. Kieper
are supported by U.S. Public Health Service Institute National Re-
search Service Award HL07196 and AI07244, respectively. C.D.
Surh is a Scholar of the Leukemia and Lymphoma Society.
Submitted: 15 January 2002
Revised: 4 April 2002
Accepted: 9 May 2002
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