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Howard Hughes Medical Institute and Department of Immunology, University of Washington, School of Medicine, Seattle, WA 98195
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Since then, several reports examining the proliferative response of adoptively transferred H-Y- or Ld-specific CD8+ (4, 10, 11, 12), or pigeon cytochrome c-specific CD4+ (11), TCR transgenic T cells in T cell-deficient hosts claimed that homeostatic expansion of naive peripheral T cells is dependent on Ag. Accordingly, homeostatic proliferation of naive polyclonal T cells was attributed to recognition of environmental Ags (10, 11). However, expansion of naive polyclonal wild-type T cells (13, 14), as well as naive T cells from other (but not all) TCR transgenic lines (15, 16, 17, 18, 19), in Ag-free lymphopenic mice has also been reported, demonstrating that homeostatic T cell proliferation is not necessarily a consequence of antigenic stimulation. Instead, due to the importance of self peptide:MHC complexes in regulating thymocyte positive selection, it was proposed that these weakly binding TCR ligands might control T cell survival and homeostatic proliferation in the periphery (15).
In support of this hypothesis, a number of studies have demonstrated impaired survival of mature CD4+ or CD8+ T cells in animals lacking selecting class II or class I MHC molecules in the periphery, respectively (12, 20, 21, 22, 23, 24, 25). However, the role of specific MHC bound peptides in the fate of peripheral T cells was not addressed. Furthermore, interpretation of these results is complicated by the lack of single cell analyses in individual animals under conditions where the lymphoid population being investigated included both dividing and nondividing cells.
Recent data showing impaired proliferation of adoptively transferred CD4+ or CD8+ T cells in irradiated class II or class I MHC-deficient hosts, respectively (14, 15, 17, 18, 19, 26), also support a role for selecting MHC expression in homeostatic T cell proliferation. Moreover, several groups have shown greatly diminished proliferation of naive T cells upon transfer into T cell-depleted hosts bearing a nonselecting self peptide:MHC repertoire (15, 17, 18). Therefore, these findings endorse the notion that TCR engagement by specific self peptide:MHC complexes, positively selecting ligands in particular, is essential for homeostatic T cell proliferation. Such a conclusion is favorable because the paucity or absence of certain positively selecting ligands in the peripheral lymphoid organs may explain why some, but not all, TCR transgenic T cells fail to proliferate in syngeneic T cell-deficient mice. However, this notion was challenged by another study, which found that only self peptides distinct from those involved in thymic positive selection drive homeostatic CD4+ T cell proliferation (26).
To further investigate the contribution of specific self peptide:MHC
complexes to T cell survival vs homeostatic proliferation, we examined
the fate of mature CD4+ T cells after adoptive
transfer into irradiated or unirradiated wild-type mice, class
II-deficient
(Aß0)3
mice, or mice expressing a drastically altered repertoire of class II
bound peptides (H-2 M-knockout (H-2 M
0) mice).
Our analysis of individual quiescent naive CD4+ T
cells suggests only a modest role for class II MHC molecules in
long-term T cell survival and, unlike homeostatic T cell proliferation,
shows that this process is independent of specific self peptide:MHC
interactions. Furthermore, our results indicate that, whereas
homeostatic T cell proliferation occurs more efficiently in the
presence of a selecting peptide repertoire, specific TCR-self
peptide:MHC interactions are not absolutely required for this response.
Examination of activation marker expression also revealed that, during
homeostatic proliferation, T cells retain a quasi naive phenotype
different to that of T cells dividing in response to antigenic
stimulation. Hence, we propose that alternative signals, in addition to
those delivered through the TCR, are capable of regulating homeostatic
T cell proliferation.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Mice used were 6–20 wk of age. H-2 M0,
(C57BL/6 (B6) x B6.PL)F1 and TEa TCR
transgenic mice (27) were bred and maintained at the
University of Washington (Seattle, WA); (B6 x
129/J)F1 mice were purchased from The Jackson
Laboratory (Bar Harbor, ME); B6 mice were purchased from Charles River
Breeding Laboratories (Wilmington, MA); Aß0
mice were purchased from Taconic Farms (Germantown, NY).
Purification of donor T cells
CD4+ and CD8+ T
cells were purified from the lymph nodes and spleens of wild-type or
TEa TCR transgenic mice by treatment with a mixture of anti-HSA
(J11d) and anti-class II (25-9-17s and BP107) Ab supernatants
followed by rabbit complement (C-SIX Diagnostics, Mequon, WI). Cells
were then passed over a 20-ml Sephadex G10 (Pharmacia,
Piscataway, NJ) column to remove adherent cells. In some experiments,
class II+ cells were further depleted by negative
panning on anti-I-Ab Ab (Y3P)-coated plates.
T cell purity was tested by flow cytometry using FITC-conjugated
anti-CD8 (PharMingen, San Diego, CA), PE-conjugated
anti-CD4 (PharMingen), and biotin-conjugated anti-CD44
(PharMingen) followed by streptavidin-TRI-COLOR (Caltag, South San
Francisco, CA), as well as PE-conjugated anti-B220 (Caltag) and
biotin-conjugated anti-I-Ab (Y3P) followed by
streptavidin-TRI-COLOR. Stained cells were analyzed on a Becton
Dickinson FACScalibur flow cytometer using CellQuest software. The
resulting population routinely showed >94% CD4+
plus CD8+, <8% CD44high,
and contained 
1% class II+
(Y3P+) cells, most of which were also
B220+. These donor cells were labeled with
5,6-carboxyfluorescein diacetate succinimidyl ester (CFSE) (Molecular
Probes, Eugene, OR) as described (28) and injected i.v.
into host mice.
Survival studies
For long-term survival studies, 5–6 x 106 CFSE-labeled T cells were injected into sex-matched unirradiated mice. The presence of donor cells in peripheral blood, lymph nodes and spleen was monitored by flow cytometry for up to 12 wk using peridinin chlorophyll protein-conjugated anti-CD4 (PharMingen), APC-conjugated anti-CD8 (PharMingen), and biotin-conjugated anti-CD44 followed by streptavidin-PE (Vector Laboratories, Berlingame, CA).
Homeostatic proliferation studies
For homeostatic proliferation studies, 2–3 x
106 CFSE-labeled T cells were injected into
sex-matched unirradiated mice or mice that had been irradiated for 600
rad 1–2 days before injection. Then, 7, 14, 21, and 28 days later, the
peripheral blood, lymph nodes, and spleen were analyzed by flow
cytometry using peridinin chlorophyll protein-conjugated anti-CD4
with APC-conjugated anti-CD8 (see Figs. 2
and 3), with
PE-conjugated anti-V2 (PharMingen) (see Fig. 4), or with
PE-conjugated anti-Thy-1.1 (PharMingen) (see Fig. 5). In some
studies, the phenotype of the donor T cells was also assessed using
biotin-conjugated anti-CD44, anti-CD69, anti-CD25 (IL-2R
-chain), anti-CD62L, or anti-CD45RB (all PharMingen)
followed by streptavidin-APC (Caltag). To test for the presence of
contaminating class II+ donor APCs in the
Aß0 hosts on day 28, spleen cells were stained
with a combination of PE-conjugated anti-B220 plus
biotin-conjugated anti-I-Ab (Y3P) followed by
streptavidin-TRI-COLOR.
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B6 mice were injected i.v. with 5 x
106
E52–68/I-Ab-specific
TCR transgenic CD4+ T cells purified from the
lymph nodes of TEa mice. The next day, mice were injected i.p. with 100
µg E52–68 peptide emulsified in CFA (Sigma,
St. Louis, MO). Control mice were given PBS emulsified in CFA. Then,
42 h after in vivo priming, the lymph node and spleen cells from
individual mice were analyzed by flow cytometry, and the phenotype of
the donor CD4+ T cells was assessed.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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To investigate the importance of class II MHC molecules for peripheral CD4+ T cell survival, 5–6 x 106 naive CD4+ and CD8+ T cells from wild-type B6 mice were injected i.v. into unirradiated wild-type or Aß0 mice. The CD8+ T cells cotransferred with the CD4+ T cells served as an important internal control population for survival within individual hosts. To assist detection of the donor T cells, the CD4+ and CD8+ T cells were labeled in vitro with the cytoplasmic fluorescent dye CFSE. Proliferation of the transferred T cells, if any, could also be assessed because division of CFSE-labeled cells is associated with a 2-fold reduction in CFSE fluorescence in both daughter cells (28). Survival of the undivided donor CFSE+ CD4+ and CD8+ T cells was determined by flow cytometric analysis of the peripheral blood of the host mice. Thus, this approach allowed us to determine the fate of individual quiescent cells within the donor T cell population.
As shown in Fig. 1a,
comparable survival of undivided donor CD4+ T
cells relative to CD8+ T cells was observed in
wild-type and Aß0 mice for up to 42 days.
However, by day 56, a slight reduction in the relative percentage of
CD4+ T cells was detected in the
Aß0 mice. Impaired survival of
CD4+ T cells in Aß0 mice
was also reflected in the average number of undivided donor
CD4+ T cells present in the spleens of host mice
on day 56: wild-type mice = 1.57 ± 0.52 x
105 (n = 5) and
Aß0 mice = 0.50 ± 0.10 x
105 (n = 4). Very little
proliferation of either CD8+ or
CD4+ T cells was detected over this period, and
the cells retained a CD44low phenotype (data not
shown). Therefore, the data imply a role for class II MHC molecules in
CD4+ T cell maintenance. However, because the
average life span of peripheral naive T cells in wild-type mice has
been reported as 5–8 wk (4, 5), the presence of class II
MHC molecules may only be necessary to prolong survival at a time when
the CD4+ T cells would normally enter cell cycle
or die.
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0 mice on a mixed B6/129/J background. In the
absence of H-2 M, the majority of class II MHC molecules are bound to
CLIP peptide. Despite this, the overall cell surface class II
expression in H-2 M0 mice is equivalent to
that of wild-type mice and MHC class I presentation is normal
(29, 30, 31). Importantly, the TCR repertoires positively
selected in wild-type and H-2 M0 mice are
largely nonoverlapping (27, 32, 33, 34). As shown in Fig. 1
b, the maintenance of undivided donor
CD4+ T cells relative to donor
CD8+ T cells over a 60-day period was similar in
wild-type and H-2 M0 mice. In one experiment,
transferred cells were monitored for up to 12 wk without any difference
in the relative survival of CD4+ T cells in
wild-type vs H-2 M0 hosts (data not shown).
Analysis of splenic T cell populations was again consistent with the
peripheral blood results. In two separate experiments, the average
number of undivided donor CD4+ T cells in the
spleen on day 60 was 2.17 ± 0.20 x
105 (n = 5) and 1.34 ±
0.86 x 105 (n = 2) for
wild-type mice, and 2.04 ± 0.47 x 105
(n = 7) and 1.30 ± 0.85 x
105 (n = 2) for H-2
M0 mice, respectively. Thus, in contrast to
class II MHC molecules, it appears that a selecting peptide repertoire
is not required for peripheral maintenance of naive
CD4+ T cells. Homeostatic proliferation of naive CD4+ T cells can occur independently of selecting ligand or class II MHC expression
The role of a diverse peptide:MHC repertoire in homeostatic T cell
expansion was investigated by injecting 2–3 x
106 CFSE-labeled CD4+ and
CD8+ T cells into host mice whose lymphoid
compartments were acutely ablated by sublethal irradiation (600 rad)
1–2 days earlier (Fig. 2). As expected,
transfer of naive T cells from wild-type (B6 x
126/J)F1 mice into irradiated, syngeneic hosts
resulted in significant cell division of donor
CD4+ and CD8+ T cells in
the lymph nodes (Fig. 2b), spleen, and peripheral blood
(data not shown) by day 7. However, unlike the rapidly induced
proliferative response to Ag (see below), division of donor
CD8+ and CD4+ T cells was
not usually observed until day 3 posttransfer (data not shown).
Homeostatic proliferation in irradiated wild-type hosts was
monitored over 4 wk (data not shown) and, by day 28, the majority of
the transferred CD4+ and
CD8+ T cells had undergone cell division (Fig. 2c). No proliferation was observed following transfer into
unirradiated hosts (Fig. 2, a and d).
When the same CFSE-labeled CD4+ and
CD8+ T cells were transferred into irradiated H-2
M0 hosts, only marginal
CD4+ T cell proliferation was observed on day 7
(Fig. 2e). This diminished proliferation did not result from
an intrinsic inability of T cells to proliferate in the H-2
M0 mice because the control population of
donor CD8+ T cells divided normally (Fig. 2e). In agreement with other recent studies, this result
appeared to support the idea that positively selecting or antagonist
peptides are stringently required to drive homeostatic proliferation
(15, 17, 18). However, when proliferation of adoptively
transferred CD4+ T cells was examined in
irradiated H-2 M0 mice beyond day 7, a
significant amount of CD4+ T cell division was
observed. By day 28, a large proportion of the donor
CD4+ T cells had divided at least once in
irradiated H-2 M0 mice, with many cells having
undergone two or more cell divisions (Fig. 2f). Similar
results were generated using
E52–68:I-Ab-specific
TCR transgenic CD4+ T cells from TEa mice that
are not selected on an H-2 M0 background (data
not shown). This demonstrates that proliferation of the wild-type
polyclonal CD4+ T cells in irradiated H-2
M0 hosts on day 28 (Fig. 2f) was
not simply limited to those cells that may have been positively
selected on CLIP:I-Ab. Therefore, these latter
results suggest that the presence of selecting self peptide:MHC
complexes is not absolutely required to initiate homeostatic
proliferation of CD4+ T cells.
To determine whether homeostatic CD4+ T cell
proliferation can occur in the absence of any class II MHC-derived
signal, similar experiments were performed using irradiated
Aß0 or wild-type hosts. Once again,
proliferation was extensive for both CD4+ and
CD8+ T cell subsets in 600-rad wild-type hosts on
days 7 and 28 (Fig. 3, b and
c). As seen for H-2 M0 mice,
homeostatic expansion of CD8+ T cells occurred
normally in irradiated Aß0 mice on day 7, but
donor CD4+ T cell division was barely detectable
at this time (Fig. 3e). Nevertheless, by day 28, a
substantial number of the adoptively transferred
CD4+ T cells had divided in the irradiated
Aß0 mice (Fig. 3f). Similar results
were generated when
E52–68:I-Ab-specific
TCR transgenic CD4+ T cells were transferred with
or without CD8+ T cells into
Aß0 x RAG-I double-deficient host mice (data
not shown). Hence, this response is neither an artifact of T cell
depletion by sublethal irradiation, nor a bystander effect
created by the presence of proliferating host T cells or
cotransferred CD8+ T cells. Furthermore, the
ability of naive CD4+ T cells to proliferate in
the absence of class II MHC molecules implies that cell division
was not the result of TCR-mediated recognition of minor
histocompatibility or cross-reactive environmental Ags.
In contrast to the critical role of thymic cortical epithelial cells in
positive selection, bone marrow-derived cells have been implicated in
presenting self peptides to CD4+ T cells
undergoing homeostatic proliferation (data not shown). In this regard,
it is important to note that homeostatic proliferation of the donor
CD4+ T cells in H-2 M0
and Aß0 mice was not likely due to the presence
of contaminating class II+ bone marrow-derived
APCs in the transferred T cell population. Flow cytometric analysis of
the donor population routinely showed >94% T cells with 1% class
II+ cells, essentially all of which were B cells.
No expansion of the class II+ cells was detected
in irradiated Aß0 hosts on day 28 (data not
shown). To ensure that homeostatic CD4+ T cell
proliferation in the irradiated Aß0 hosts was
not due to the 1% contaminating class II+ cells,
CD4+ and CD8+ donor T cells
were isolated from the lymph nodes of mice that had been lethally
irradiated (950 rad) and reconstituted with Aß0
bone marrow cells. No class II (Y3P)+ cells were
found in the lymph nodes or peripheral blood of these donor mice 7 wk
after reconstitution, but normal percentages of
CD4+ T cells were detected, presumably as a
result of positive selection on class II+ thymic
cortical epithelial cells (data not shown). T cells from these bone
marrow chimeras were labeled with CFSE and injected i.v. into
irradiated wild-type or Aß0 hosts. Then, 14
days later, one to two rounds of donor CD4+ T
cell division were again observed in the lymph nodes and spleen of
irradiated Aß0 hosts (data not shown).
Altogether, these results demonstrate that homeostatic naive
CD4+ T cell proliferation can occur independently
of class II MHC expression, albeit much less efficiently.
The phenotype of naive CD4+ T cells undergoing homeostatic proliferation is different from that of T cells dividing in response to antigenic stimulation
Division of naive CD4+ T cells in the
absence of class II suggests that homeostatic proliferation can be
induced by a non-TCR-derived stimulus. Therefore, the expression of
cell surface markers normally associated with TCR-mediated activation
was assessed for those CD4+ T cells undergoing
cell division in lymphopenic hosts. These markers included the early T
cell activation marker CD69, the activation/memory cell marker CD44,
IL-2 receptor -chain (IL-2R), the lymph node homing receptor,
CD62L, and the memory cell marker, CD45RB.
Initially, we analyzed expression of these markers on T cells
undergoing Ag-specific proliferation. CFSE-labeled
E52–68:I-Ab-specific
TCR transgenic CD4+ T cells transferred into
unirradiated syngeneic mice were analyzed 2 days after in vivo priming
with antigenic peptide in CFA (Fig. 4a). By this time, many of the
TCR transgenic CD4+ T cells had undergone three
to four rounds of cell division (Fig. 4a, right
panel), whereas cells exposed to PBS in CFA remained undivided
(Fig. 4a, left panel). CD69 was highly expressed
on the undivided primed T cells, illustrating the early up-regulation
of this marker upon TCR engagement, and was progressively lost with
each subsequent cell division. CD44 expression changed from
intermediate to high following the first round of cell division.
IL-2R expression was low on undivided cells but increased slightly
on divided cells. As expected, high levels of CD62L and CD45RB were
found on the undivided population of TCR transgenic
CD4+ T cells, but expression of these markers
progressively decreased with Ag-induced proliferation (Fig. 4a). Similar dynamics of several of these markers in
response to in vivo priming were recently reported for CFSE-labeled
DO11.10 TCR transgenic T cells (35).
In contrast to the T cell phenotype characteristic of TCR-induced
proliferation, no increase in CD69 expression was observed when
CFSE-labeled
E52–68:I-Ab-specific
TCR transgenic CD4+ T cells from the same donor
population were transferred into irradiated B6 mice and analyzed up to
28 days later (Fig. 4b, and data not shown). IL-2R levels
on the CFSE-labeled TCR transgenic CD4+ T cells
from the 600-rad host mice were similar or only very slightly increased
compared with the levels observed on donor T cells from the
unirradiated hosts. Donor T cells from the irradiated mice also
maintained high expression of CD62L and CD45RB as they proliferated
(Fig. 4b). The only change detected during homeostatic T
cell proliferation was an increase in CD44 expression. However, unlike
Ag-driven proliferation, CD44 up-regulation occurred gradually,
requiring at least two to three rounds of cell division before being
observed (Fig. 4b). Thus, unlike T cells undergoing
TCR-induced proliferation, the T cells seen dividing in response to
homeostatic signals appear to retain an essentially naive phenotype.
This confirms that the donor T cells were not dividing in response to
environmental or minor histocompatibility Ags.
A similar quasi naive phenotype of dividing T cells was also observed
for wild-type polyclonal donor CD4+ T cells 7,
14, and 28 days after transfer into irradiated wild-type mice (Fig. 5a; data not shown), as well
as on days 14 and 28 in irradiated Aß0 mice
(Fig. 5b; data not shown). However, unlike the TCR
transgenic donor CD4+ T cells the wild-type
polyclonal donor CD4+ T cells
(Thy-1.1+ x Thy-1.2+)
expressed a Thy-1 Ag different from the host T cells
(Thy-1.2+). This allowed us to distinguish those
donor CD4+ T cells that had lost their CFSE as a
result of many cell divisions from any host CD4+
T cells that were never labeled with this dye. Interestingly, when the
phenotype of the CFSE-negative donor cells in the irradiated hosts was
examined, the majority not only expressed high levels of CD44, but were
low for CD62L and intermediate for CD45RB (Fig. 5). Such a phenotype is
reminiscent of memory CD4+ T cells
(36) and may be related to the finding that, after many
rounds of homeostatic cell division, naive CD8+ T
cells transiently acquire a memory-like phenotype (37). Alternatively
or additionally, T cells in this population may be derived from a small
memory T cell pool, present in the initial donor population, that
divided more rapidly in response to homeostatic stimuli than the naive
CD4+ T cells. Such accelerated homeostatic
proliferation of memory T cells has recently been documented for
CD8+ T cells and has been shown to occur
independently of class I MHC expression (38). The presence
of a CFSE negative donor T cell population in the unirradiated
Aß0 hosts (Fig. 5b), but not in the
unirradiated wild-type hosts (Fig. 5a), also suggests that
this population may represent an expanded memory population. That is,
whereas the presence of naive CD8+ T cells has
been shown to inhibit homeostatic proliferation of naive
CD4+ T cells (17), naive
CD8+ T cells may not be able to inhibit
proliferation of memory CD4+ T cells which are
generally absent in Aß0, but not in
wild-type, mice.
To address the possibility that the CFSE-negative donor T cells were derived from preexisting memory T cells in our donor population, (Thy-1.1+ x Thy-1.2+) donor T cells were sorted for CD44low and CD69low expression and then injected into unirradiated or 600-rad wild-type and Aß0 hosts. Sorted donor T cells that had undergone two cell divisions were detected as early as day 7 in irradiated Aß0 mice and expressed a naive phenotype (CD69low, CD44low, CD25low, CD62Lhigh, CD45RBhigh) (data not shown). Thus, whereas transfer of sorted T cells did reduce the number of CFSE-negative donor T cells in some irradiated Aß0 mice, it did not completely abolish their appearance, especially in the spleen. In fact, this population appeared to develop more rapidly in irradiated Aß0 mice than it did in the irradiated wild-type hosts. Similar results were also obtained using sorted CD69high and CD69low CD4 single-positive thymocytes (data not shown). Hence, it is unlikely that the CFSE-negative population simply appeared because sorting for CD44low CD69low expression on mature T cells did not exclude all memory T cells from our donor CD4+ T cell population.
These findings suggest that the CFSE-negative donor T cells detected previously in the Aß0 mice were not all derived from a small number of contaminating memory T cells, but also represented a subset of naive T cells that divide rapidly and acquire a memory phenotype. In an attempt to view these rapidly dividing T cells at an earlier time point when they had intermediate expression of CFSE, irradiated Aß0 mice injected with unsorted donor T cells were analyzed 3, 5, 7, and 9 days posttransfer. The CFSE-negative, memory-like population appeared as early as day 5 in both irradiated and unirradiated Aß0 mice and increased in number thereafter (data not shown). However, the small number of these T cells together with their rapid cell division made it very difficult to detect populations of CFSE intermediate T cells.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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By carefully examining the fate of undivided naive
CD4+ T cells in unirradiated
Aß0 hosts, we showed that T cell survival in
the absence of TCR-MHC interactions was better than previously
estimated. This difference may arise from the fact that several of the
earlier studies assessed the presence of T cells from populations that
included dividing cells. Hence, the apparent loss of these populations
in MHC-deficient hosts may actually reflect their impaired ability to
undergo homeostatic expansion, rather than the inability of individual
T cells to survive. Nevertheless, our experiments do support previous
findings that class II MHC expression is necessary to prolong the
maintenance of polyclonal naive CD4+ T cells in a
full lymphoid compartment. The fact that survival of naive wild-type
CD4+ T cells was not impaired in unirradiated H-2
M0 hosts suggests that presentation of self
peptides, similar to those involved in thymic positive selection, is
not a necessary function of class II MHC molecules in this process.
Potential interaction of CD4 with class II MHC molecules may account
for the observed difference.
In contrast to CD4+ T cell survival, selective
engagement of TCRs by self peptide:MHC complexes does appear to promote
division of CD4+ T cells under severe lymphopenic
conditions. Similar observations were reported recently by several
groups for both polyclonal and monoclonal CD4+
and CD8+ T cells (14, 15, 17, 18, 19, 26). These results led to the view that homeostatic naive T cell
proliferation is stringently dependent on TCR stimulation by positively
selecting or low affinity peptide:MHC complexes. However, the low level
of T cell division observed in the absence of specific self peptide:MHC
complexes by day 7–9 in some of these studies was not commented on
(15, 17, 19, 26). Here, we show that such division in mice
lacking specific peptide:MHC complexes becomes quite substantial over
time. Interestingly, T cell proliferation in the absence of class II
MHC molecules was usually associated with the appearance of a
CFSE-negative, memory-phenotype population in both irradiated and
unirradiated Aß0 hosts. This population appears
to include a subset of naive T cells that divide rapidly and acquire a
memory-like state. As well, these cells are probably derived from a
small pool of preexisting memory T cells in the donor population. The
latter alternative raises the possibility that the T cells that had
undergone several divisions in the irradiated
Aß0 hosts were not naive T cells, but instead
represented intermediates of the contaminating memory T cell
population. However, the fact that intermediates of the CFSE-negative
population could not be detected at earlier times together with the
fact that these dividing donor T cells in the irradiated
Aß0 mice express a naive phenotype (Fig. 5b), argues against this view.
In conclusion, the data suggest that different MHC-derived signals are required to facilitate naive T cell survival vs homeostatic T cell proliferation. Our results agree with the view that low affinity TCR interactions are necessary to induce efficient homeostatic proliferation. However, based on the data presented above, we also feel that a TCR-derived signal is not the only factor important for initiating naive T cell division in a lymphopenic environment. Other signals critical for regulating homeostatic T cell proliferation may include an inhibitory interaction between T cells that prevents their proliferation in a full lymphoid compartment. When such an inhibitory signal is missing in a severely lymphopenic environment, pathways allowing T cell division may be activated and can be assisted by weak signaling through the TCR by self peptide:MHC complexes. In the absence of such "TCR tickling," cell division pathways may still be initiated, but proliferation is delayed. Additionally, there may be a third mitogenic signal, possibly delivered via the APC, which in the absence of TCR-mediated signaling, can also induce naive T cell division when the inhibitory interaction between T cells is removed. Therefore, a detailed molecular description of this important immunological phenomenon is necessary to identify the other signals regulating T cell homeostasis and will be of significance for the development of clinical treatments for lymphopenic states.
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Acknowledgments |
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Footnotes |
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2 Address correspondence and reprint requests to Dr. Alexander Y. Rudensky, Howard Hughes Medical Institute, University of Washington, Box 357370, Seattle, WA 98195-7370.
3 Abbreviations used in this paper: Aß0, class II-deficient; H-2 M0, H-2 M-knockout; CFSE, 5,6-carboxyfluorescein diacetate succinimidyl ester; B6, C57BL/6.
Received for publication December 27, 1999. Accepted for publication June 20, 2000.
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References |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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/ß T cells in secondary lymphoid organs. J. Exp. Med. 177:891.ß T cells. Int. Immunol. 3:1077.This article has been cited by other articles:
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  S. Li, E. J. Gowans, C. Chougnet, M. Plebanski, and U. Dittmer Natural Regulatory T Cells and Persistent Viral Infection J. Virol., January 1, 2008; 82(1): 21 - 30. [Full Text] [PDF]
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 K. Rezvani, A. S. M. Yong, B. N. Savani, S. Mielke, K. Keyvanfar, E. Gostick, D. A. Price, D. C. Douek, and A. J. Barrett Graft-versus-leukemia effects associated with detectable Wilms tumor-1 specific T lymphocytes after allogeneic stem-cell transplantation for acute lymphoblastic leukemia Blood, September 15, 2007; 110(6): 1924 - 1932. [Abstract] [Full Text] [PDF]
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 A. De Riva, C. Bourgeois, G. Kassiotis, and B. Stockinger Noncognate Interaction with MHC Class II Molecules Is Essential for Maintenance of T Cell Metabolism to Establish Optimal Memory CD4 T Cell Function J. Immunol., May 1, 2007; 178(9): 5488 - 5495. [Abstract] [Full Text] [PDF]
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E. Shklovskaya and B. Fazekas de St. Groth Severely Impaired Clonal Deletion of CD4+ T Cells in Low-Dose Irradiated Mice: Role of T Cell Antigen Receptor and IL-7 Receptor Signals J. Immunol., December 15, 2006; 177(12): 8320 - 8330. [Abstract] [Full Text] [PDF]
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B. Martin, C. Becourt, B. Bienvenu, and B. Lucas Self-recognition is crucial for maintaining the peripheral CD4+ T-cell pool in a nonlymphopenic environment Blood, July 1, 2006; 108(1): 270 - 277. [Abstract] [Full Text] [PDF]
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M. Shimoda, F. Mmanywa, S. K. Joshi, T. Li, K. Miyake, J. Pihkala, J. A. Abbas, and P. A. Koni Conditional Ablation of MHC-II Suggests an Indirect Role for MHC-II in Regulatory CD4 T Cell Maintenance. J. Immunol., June 1, 2006; 176(11): 6503 - 6511. [Abstract] [Full Text] [PDF]
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 J. Hataye, J. J. Moon, A. Khoruts, C. Reilly, and M. K. Jenkins Naive and Memory CD4+ T Cell Survival Controlled by Clonal Abundance Science, April 7, 2006; 312(5770): 114 - 116. [Abstract] [Full Text] [PDF]
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G. Raimondi, I. Zanoni, S. Citterio, P. Ricciardi-Castagnoli, and F. Granucci Induction of Peripheral T Cell Tolerance by Antigen-Presenting B Cells. II. Chronic Antigen Presentation Overrules Antigen-Presenting B Cell Activation J. Immunol., April 1, 2006; 176(7): 4021 - 4028. [Abstract] [Full Text] [PDF]
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A. Jabbari and J. T. Harty Cutting Edge: Differential Self-Peptide/MHC Requirement for Maintaining CD8 T Cell Function versus Homeostatic Proliferation J. Immunol., October 15, 2005; 175(8): 4829 - 4833. [Abstract] [Full Text] [PDF]
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L. Pace, C. Pioli, and G. Doria IL-4 Modulation of CD4+CD25+ T Regulatory Cell-Mediated Suppression J. Immunol., June 15, 2005; 174(12): 7645 - 7653. [Abstract] [Full Text] [PDF]
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T. J. Fry and C. L. Mackall The Many Faces of IL-7: From Lymphopoiesis to Peripheral T Cell Maintenance J. Immunol., June 1, 2005; 174(11): 6571 - 6576. [Abstract] [Full Text] [PDF]
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D. J. Zhang, Q. Wang, J. Wei, G. Baimukanova, F. Buchholz, A. F. Stewart, X. Mao, and N. Killeen Selective Expression of the Cre Recombinase in Late-Stage Thymocytes Using the Distal Promoter of the Lck Gene J. Immunol., June 1, 2005; 174(11): 6725 - 6731. [Abstract] [Full Text] [PDF]
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G. Lombardi, D. Burzyn, J. Mundinano, P. Berguer, P. Bekinschtein, H. Costa, L. F. Castillo, A. Goldman, R. Meiss, I. Piazzon, et al. Cathepsin-L Influences the Expression of Extracellular Matrix in Lymphoid Organs and Plays a Role in the Regulation of Thymic Output and of Peripheral T Cell Number J. Immunol., June 1, 2005; 174(11): 7022 - 7032. [Abstract] [Full Text] [PDF]
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