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Conversion of Naive T Cells to a
Memory-Like Phenotype in Lymphopenic
Hosts Is Not Related to a Homeostatic
Mechanism That Fills the Peripheral Naive T
Cell Pool
Corinne Tanchot, Armelle Le Campion, Bruno Martin,
Sandrine Léaument, Nicole Dautigny and Bruno Lucas
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Copyright © 2002 by The American Association of
Immunologists All rights reserved.
Print ISSN: 0022-1767 Online ISSN: 1550-6606.
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J Immunol 2002; 168:5042-5046; ;
doi: 10.4049/jimmunol.168.10.5042
http://www.jimmunol.org/content/168/10/5042
The Journal of Immunology
Conversion of Naive T Cells to a Memory-Like Phenotype in
Lymphopenic Hosts Is Not Related to a Homeostatic
Mechanism That Fills the Peripheral Naive T Cell Pool1
Corinne Tanchot,* Armelle Le Campion,* Bruno Martin,* Sandrine Léaument,†
Nicole Dautigny,* and Bruno Lucas2*
P
eripheral naive T cells do not cycle under normal conditions (1–5). Nevertheless, using monoclonal naive T cells
from TCR-transgenic mouse strains and purified polyclonal naive T cells from normal mice, it has recently been shown
that most but not all tested naive T cells proliferate when transferred to lymphopenic hosts (5–21). This proliferation does not
involve Ag recognition but nevertheless requires interactions with
self-MHC molecules. From these observations, most authors have
inferred that the proliferation of naive T cells in lymphopenic recipients might reveal the existence of a homeostatic mechanism for
filling the peripheral naive T cell pool. To confirm the existence of
this homeostatic mechanism, one needs to establish 1) whether this
proliferation/expansion completely fills the peripheral naive T cell
pool, and 2) that the phenotype and functional capacities of proliferating naive T cells are not modified, or at least that the cells
revert to normal once they have returned to a resting state.
Recent studies have followed the phenotypic and functional
characteristics of naive T cells over a relatively long period (⬎1
mo) (14 –17, 21). They all showed that, during proliferation, naive
T cells converted to a memory T lymphocyte phenotype. In all but
one case, the phenotypic and functional characteristics of transferred T cells remained stable over time. Indeed, only Goldrath et
al. (14), using irradiated normal mice rather than recombinationactivating gene (rag)3-deficient mice as lymphopenic recipients,
observed that the transferred cells stopped cycling and reverted to
†
*Institut National de la Santé et de la Recherche Médicale, Unité 345, and Laboratoire d’Expérimentation Animale et de Transgénèse, Faculté de Médecine NeckerEnfants Malades, Université René Descartes, Paris, France
Received for publication January 11, 2002. Accepted for publication March 18, 2002.
The costs of publication of this article were defrayed in part by the payment of page
charges. This article must therefore be hereby marked advertisement in accordance
with 18 U.S.C. Section 1734 solely to indicate this fact.
1
This work was supported by the Agence Nationale de Recherches sur le
SIDA. A.L.C. was supported by a Ph.D. fellowship from Ensemble contre le SIDA.
2
Address correspondence and reprint requests to Dr. Bruno Lucas, Institut National
de la Santé et de la Recherche Médicale, Unité 345, Faculté de Médecine NeckerEnfants Malades, 156 rue de Vaugirard, F-75730 Paris Cedex 15, France. E-mail
address: [email protected]
3
Abbreviations used in this paper: rag, recombination-activating gene; Lin, lineage.
Copyright © 2002 by The American Association of Immunologists
a naive phenotype after filling the peripheral T cell pool. Moreover,
they showed that the same monoclonal naive T cells continued to
divide in rag-deficient mice, in which the lymphocyte compartment
was never reconstituted. Thus, they proposed that, in rag-deficient
hosts, the absence of fully developed secondary lymphoid organs or
the absence of B cells might explain the discrepancies between their
data and results published by other groups (15, 16). Our recent results
showing that homeostasis is not restored in CD3⑀-deficient mice rule
out both hypotheses (21).
In this study, to directly examine whether a limited number of
naive T cells transferred to a lymphopenic host can truly fill the
peripheral naive T cell pool, we compared the expansion and phenotype of naive T cells transferred to rag-deficient mice, CD3⑀deficient mice, and irradiated normal mice.
Materials and Methods
Mice
H-2b/b AND TCR-transgenic rag-20/0 mice (21), H-2k/k AND TCR-transgenic rag-20/0 mice (21), H-2k/k CD3⑀-deficient mice (21), B10.A 5CC7
TCR-transgenic rag-20/0 mice (22), B10.A CD3⑀-deficient mice (22), and
B10.A mice were maintained in our animal facilities. B10BR mice,
C57BL/6 mice, C57BL/6 Ba (Thy1.1) mice, H-2b/b CD3⑀-deficient mice,
and C57BL/6 rag-20/0 mice were obtained from Centre de Développement
des Techniques Avancées pour l’ Expérimentation Animale (Orléans,
France).
Adoptive transfer of naive T cells
One million CD4⫹ T cells from lymph nodes or spleen of AND TCRtransgenic rag-20/0 mice were injected i.v. into rag-20/0 mice, CD3⑀-deficient mice, and irradiated normal mice of the same haplotype. Spleens and
lymph nodes were recovered and pooled at various times after transfer.
Three million CD4⫹ T cells from lymph nodes of B10.A 5CC7 TCRtransgenic rag-20/0 mice were injected i.v. into B10.A CD3⑀-deficient mice
and irradiated B10.A mice. Spleens and lymph nodes were recovered and
pooled 14 days after transfer.
Normal mice were sublethally irradiated (650 rad) 2 days before
transfer.
Flow cytometry
Abs were purchased from BD PharMingen (San Diego, CA). The following Ab combinations were used: PE-conjugated anti-CD8, FITC-conjugated anti-V␣11, PerCP-conjugated anti-CD4, and biotinylated anti-V␤3
revealed by allophycocyanin streptavidin (BD PharMingen); PE-conju0022-1767/02/$02.00
Downloaded from http://www.jimmunol.org/ by guest on July 31, 2017
To examine directly whether a limited number of naive T cells transferred to lymphopenic hosts can truly fill the peripheral naive
T cell pool, we compared the expansion and phenotype of naive T cells transferred to three different hosts, namely recombinationactivating gene-deficient mice, CD3⑀-deficient mice, and irradiated normal mice. In all three recipients, the absolute number of
recovered cells was much smaller than in normal mice. In addition, transferred naive T cells acquired a memory-like phenotype
that remained stable with time. Finally, injected cells were rapidly replaced by host thymic migrants in irradiated normal mice.
Only continuous output of naive T cells by the thymus can generate a full compartment of truly naive T cells. Thus, conversion
of naive T cells to a memory-like phenotype in lymphopenic hosts is not related to a homeostatic mechanism that fills the peripheral
naive T cell pool. The Journal of Immunology, 2002, 168: 5042–5046.
The Journal of Immunology
5043
gated anti-V␣11, FITC-conjugated anti-V␤3, PerCP-conjugated anti-CD4,
and biotinylated anti-CD25 or CD44 revealed by allophycocyanin streptavidin; PE-conjugated anti-Thy1.2, FITC-conjugated anti-CD8, PerCP-conjugated anti-CD4, and biotinylated anti-V␤3 revealed by allophycocyanin
streptavidin; PE-conjugated anti-Thy1.2, FITC-conjugated anti-V␣11,
PerCP-conjugated anti-CD4, and biotinylated anti-CD25 or CD44, revealed by allophycocyanin streptavidin; PE-conjugated anti-lineage (Lin)
markers (Lin ⫽ CD19 ⫹ GR.1 ⫹ MAC.1 ⫹ TER119 ⫹ NK1.1), FITCconjugated anti-CD45, PerCP-conjugated anti-CD4, and biotinylated antiV␤3 revealed by allophycocyanin streptavidin.
Calculations
⫹
Absolute numbers of recovered CD4
T cells and recovered
V␤3⫹V␣11⫹CD4⫹ T cells were calculated at various times after transfer of AND CD4⫹ T cells to irradiated C57BL/6 and B10BR mice. In
normal C57BL/6 and B10BR mice, some CD4⫹ T cells coexpress a
TCR V␣11 chain and a TCR V␤3 chain (C57BL/6: p ⫽ 0.24%; B10BR:
p ⫽ 0.55%). To precisely estimate the number of donor (AND)-derived
CD4⫹ T cells among all recovered V␤3⫹V␣11⫹CD4⫹ T cells, the
following calculations were performed: (V␤3⫹V␣11⫹CD4⫹)recovered ⫽
(V␤3⫹V␣11⫹CD4⫹)host ⫹ (V␤3⫹V␣11⫹CD4⫹)AND. (CD4⫹)recovered ⫽
(CD4⫹)host ⫹ (V␤3⫹V␣11⫹CD4⫹)AND, all AND CD4⫹ T cells coexpressing a V␣11 chain and a V␤3 chain.
Moreover, (V␤3⫹V␣11⫹CD4⫹)host ⫽ P ⫻ (CD4⫹)host, where P represents the proportion of host-derived CD4⫹ T cells coexpressing a V␣11
chain and a V␤3 chain. Therefore, (V␤3⫹V␣11⫹CD4⫹)AND ⫽ (V␤3⫹
V␣11⫹CD4⫹)recovered ⫺ P ⫻ (CD4⫹)host, and (V␤3⫹V␣11⫹CD4⫹)AND ⫽
((V␤3⫹V␣11⫹CD4⫹)recovered ⫺ P ⫻ (CD4⫹)recovered)/(1 ⫺ P).
FIGURE 2. H-2k/k AND CD4⫹ T cells converting to a memory phenotype in lymphopenic hosts are not maintained in irradiated hosts. One million lymph node CD4⫹ T cells from H-2k/k AND TCR-transgenic rag-20/0
mice were transferred to H-2k/k CD3⑀-deficient mice and irradiated normal
B10BR mice. A, At various times after transfer, peripheral T cells were recovered, counted, and stained for CD4, CD8, V␣11, and V␤3 surface expression. Absolute numbers of recovered V␤3⫹V␣11⫹CD4⫹ T lymphocytes were
calculated. B, Absolute numbers of recovered V␤3⫹V␣11⫹CD4⫹ T lymphocytes (E) and estimated numbers of host (X) and AND (‚)
V␤3⫹V␣11⫹CD4⫹ T cells at various times after transfer to irradiated B10BR
mice. C, One week after transfer, peripheral T cells were recovered and stained
for CD4, V␣11, V␤3, and CD25 or CD44 surface expression. Shown are
CD25 and CD44 fluorescence histograms of recovered V␤3⫹V␣11⫹CD4⫹ T
cells (thick line) in comparison with CD25 and CD44 expression on naive
H-2k/k AND CD4⫹ T cells before transfer (thin line).
Results and Discussion
Naive CD4⫹ T cells converting to a memory phenotype in
lymphopenic hosts are not maintained in irradiated recipients
One million lymph node CD4⫹ T cells from AND TCR-transgenic
rag-20/0 mice (H-2b/b or H-2k/k) were transferred to rag-2-deficient
mice, CD3⑀-deficient mice, and irradiated normal mice of the same
MHC haplotype.
As previously shown (21), no significant expansion of injected
AND CD4⫹ T cells was found when H-2b/b AND CD4⫹ T cells
Table I. 5CC7 CD4⫹ T cells converting to a memory phenotype in
lymphopenic hosts are not maintained in irradiated hostsa
Recipient
Absolute number of
V␤3⫹V␣11⫹CD4⫹
T cells (⫻ 10⫺6)
CD44high cells in
V␤3⫹V␣11⫹CD4⫹
T cells (%)
B10A
CD3⑀KO
Irradiated
B10A
3.8 ⫾ 0.8
0.6 ⫾ 0.4
62.4 ⫾ 3.2
1.4 ⫾ 0.6
a
Three million CD4⫹ T cells from lymph nodes of B10.A 5CC7 TCR-transgenic
rag-20/0 mice were injected i.v. into B10.A CD3⑀-deficient mice and irradiated B10.A
mice. Fourteen days after transfer, peripheral T cells were recovered and stained for
CD4, V␤3, V␣11, and CD44 surface expression.
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FIGURE 1. H-2b/b AND CD4⫹ T cells converting to a memory phenotype in lymphopenic hosts are not maintained in irradiated hosts. One million lymph node CD4⫹ T cells from H-2b/b AND TCR-transgenic rag-20/0
mice were transferred to rag-2-deficient, CD3⑀-deficient, and irradiated
normal C57BL/6 mice. A, At various times after transfer, peripheral T cells
were recovered, counted, and stained for CD4, CD8, V␣11, and V␤3 surface
expression. Absolute numbers of recovered V␤3⫹V␣11⫹CD4⫹ T lymphocytes are shown. B, Absolute numbers of recovered V␤3⫹V␣11⫹CD4⫹ T
lymphocytes (E) and estimated numbers of host (X) and AND (‚)
V␤3⫹V␣11⫹CD4⫹ T cells at various times after transfer to irradiated
C57BL/6 mice. C, One week after transfer, peripheral T cells were recovered and stained for CD4, V␤3, V␣11, and CD25 or CD44 surface expression. Shown are CD25 and CD44 fluorescence histograms of recovered V␤3⫹V␣11⫹CD4⫹ T cells (thick line) in comparison with
CD25 and CD44 expression on naive H-2b/b AND CD4⫹ T cells before
transfer (thin line).
5044
were transferred to H-2b/b CD3⑀-deficient mice (Fig. 1A): the absolute numbers of recovered AND CD4⫹ T cells remained low,
but constant. Similar results were found after transfer to rag-2-
FIGURE 4. Fifteenfold more hematopoietic precursors are coinjected together with 1 ⫻ 106 spleen AND
CD4⫹ T cells than with 1 ⫻ 106 lymph node AND
CD4⫹ T cells. Lymph nodes and spleen from H-2b/b
AND TCR-transgenic rag-20/0 mice were harvested and
stained for CD4, V␤3, CD45, and Lin marker (Lin ⫽
CD19 ⫹ GR.1 ⫹ MAC.1 ⫹ TER119 ⫹ NK1.1) surface
expression. CD4/V␤3 fluorescence dot plots are presented for all cells and Lin/CD45 fluorescence dot plots
are presented for V␤3⫺ cells. The percentage of
V␤3⫺Lin⫺CD45⫹ cells are given, as well as the absolute number of such cells coinjected together with 1 ⫻
106 AND CD4⫹ T cells.
deficient mice, except that cell recovery was 3-fold lower than in
CD3⑀-deficient mice.
The results obtained after transfer to irradiated normal mice
were totally different. Two weeks after transfer to irradiated H-2b/b
normal mice (C57BL/6), the number of recovered V␤3⫹
V␣11⫹CD4⫹ T lymphocytes was intermediate between values obtained in rag-deficient mice and CD3⑀-deficient mice, and this
number fell strongly with time thereafter (Fig. 1A). The absolute
numbers of host and AND-derived V␤3⫹V␣11⫹CD4⫹ T cells
among all recovered V␤3⫹V␣11⫹CD4⫹ T cells were estimated in
these chimeras (see Materials and Methods) (Fig. 1B). Calculation
showed that virtually no injected AND CD4⫹ T cells could be
recovered 12 wk after transfer to irradiated normal mice. Indeed,
all V␤3⫹V␣11⫹CD4⫹ T cells were of host origin 12 wk posttransfer. These data were further confirmed by transferring AND
CD4⫹ T cells (Thy1.2) to congenic irradiated C57BL/6 Ba mice
(Thy1.1) (see Fig. 3). Expression of activation markers (CD25,
CD44) by AND CD4⫹ T cells was studied 2 wk after transfer, at
which time nearly all V␤3⫹V␣11⫹CD4⫹ T cells were derived
from injected AND CD4⫹ T cells in irradiated normal mice (Fig.
1B). CD25 and CD44 expression was strongly up-regulated after
transfer to rag- and CD3⑀-deficient mice (Fig. 1C). No CD25 expression and slight up-regulation of CD44 expression by
V␤3⫹V␣11⫹CD4⫹ T cells were observed after transfer to irradiated C57BL/6 mice, suggesting that injected AND CD4⫹ T cells
had undergone less marked activation than in the other two
recipients.
Contrary to most naive CD4⫹ and CD8⫹ T cells, H-2b/b AND
CD4⫹ T cells proliferated but did not expand after transfer to
rag-2- and CD3⑀-deficient mice (21) (Fig. 1). We thus performed
similar experiments with H-2k/k AND CD4⫹ T cells, as we have
previously shown that these cells expand strongly when transferred
to H-2k/k CD3⑀-deficient mice (Ref. 21 and Fig. 2A). Surprisingly,
these cells did not expand when transferred to irradiated B10BR
mice (Fig. 2A). By estimating the numbers of AND CD4⫹ T cells
in these transfers, we found that, as after transfer of H-2b/b AND
CD4⫹ T cells, H-2k/k AND CD4⫹ T cells disappeared after transfer to irradiated mice (Fig. 2B). Similarly, 1 wk after transfer,
CD44 up-regulation on H-2k/k AND CD4⫹ T cells was less
marked in irradiated mice than in CD3⑀-deficient mice (Fig. 2C).
Together with the observed lack of expansion, this suggested that
naive AND CD4⫹ T cells were submitted to a far less marked
activation phase in irradiated mice than in CD3⑀-deficient mice.
Therefore, the activation and subsequent expansion of injected naive T cells seemed to be inversely proportional to the absolute
number of preexisting T cells in the host.
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FIGURE 3. Spleen but not lymph node naive AND CD4⫹ T cells reconstitute the peripheral naive T cell pool after injection into irradiated
hosts. One million lymph node (LNs) or spleen CD4⫹ T cells from H-2b/b
AND TCR-transgenic rag-20/0 mice (Thy1.2) were transferred to irradiated
normal C57BL/6 Ba mice (Thy1.1). At various times after transfer, peripheral T cells were recovered, counted, and stained for CD4, CD8,
Thy1.2, and V␤3 surface expression. A, V␤3/Thy1.2 fluorescence dot plots
are presented for gated peripheral CD8⫺CD4⫹ T cells, 2 and 8 wk after
transfer. B, Absolute numbers of recovered Thy1.2⫹V␤3⫹CD8⫺CD4⫹ T
lymphocytes were calculated at each time point. C, Eight weeks after transfer, peripheral T cells were stained for CD4, Thy1.2, V␣11, and CD25 or
CD44 surface expression. Shown are CD25 and CD44 fluorescence histograms of recovered Thy1.2⫹V␣11⫹CD4⫹ T cells derived from injected lymph node cells (thin line) in comparison with CD25 and CD44
expression by recovered Thy1.2⫹V␣11⫹CD4⫹ T cells derived from
injected spleen cells (thick line).
NAIVE T CELL BEHAVIOR IN LYMPHOPENIC HOSTS
The Journal of Immunology
5045
Similar results were obtained after transfer of lymph node CD4⫹
T cells from B10.A 5CC7 TCR-transgenic rag-20/0 mice. Indeed,
when naive 5CC7 CD4⫹ T cells were transferred to syngeneic
CD3⑀-deficient mice they proliferated, resulting in their expansion
and conversion to a memory-like phenotype that remained stable
with time (22), whereas they did not expand or convert to a memory-like phenotype after transfer to irradiated normal B10.A mice
(Table I). Therefore, independently of the hosts used, activation of
naive CD4⫹ T cells did not lead to filling of the peripheral naive
T cell pool. Furthermore, absolute numbers of recovered injected
cells fell rapidly after transfer to irradiated normal mice, probably
reflecting competition for space between injected T cells and
newly produced host T cells (23–25). Contrary to Goldrath et al.
(14), we did not find that naive T cells expanded better in irradiated
mice than in T cell-deficient recipients (rag-deficient or CD3⑀deficient mice). One major methodological difference was that we
injected lymph node naive T cells instead of a pool of spleen and
lymph node cells (14).
One million lymph node or spleen CD4⫹ T cells from H-2b/b AND
TCR-transgenic rag-20/0 mice (Thy1.2) were transferred to irradiated normal C57BL/6 Ba mice (Thy1.1). For 2 wk after transfer,
recovery of AND CD4⫹ T cells (Thy1.2⫹V␤3⫹CD4⫹ cells) was
similar with spleen and lymph node cells (Fig. 3, A and B). Moreover, AND CD4⫹ T cells converted to a memory phenotype in
both cases (data not shown). Four weeks after spleen cell transfer,
the absolute number of recovered AND CD4⫹ T cells started to
increase, reaching a plateau at 8 wk, whereas, as shown above
(Fig. 1), the absolute number of AND CD4⫹ T cells in mice injected with lymph node cells fell rapidly (Fig. 3, A and B). Moreover, 8 wk after transfer, AND CD4⫹ T cells recovered after
spleen cell injection exhibited a true naive phenotype, which contrasted with the stable memory-like phenotype of AND CD4⫹ T
cells recovered after lymph node cell injection (Fig. 3C).
It is unlikely that intrinsic differences between lymph node- and
spleen-derived naive T cells would affect their behavior after transfer to irradiated animals to such an extent. A more logical explanation would be that the hematopoietic precursors contained in the
spleen of adult mice (26, 27) reconstituted the thymuses of irradiated hosts, thereby permitting the generation of large numbers of
naive AND CD4⫹ T cells. Indeed, 15-fold more hematopoietic
precursors (defined by the absence of expression of Lin markers
and the expression of CD45) are coinjected together with 1 ⫻ 106
spleen AND CD4⫹ T cells than with 1 ⫻ 106 lymph node AND
CD4⫹ T cells (Fig. 4).
Reconstitution of the host thymus by spleen hematopoietic
precursors, rather than peripheral homeostasis, explains
peripheral naive T cell pool reconstitution by CD4⫹ splenocytes
At various times after transfer of lymph node or spleen AND
CD4⫹ T cells to irradiated normal mice, thymocytes were recovered, counted, and stained for CD4, CD8, Thy1.2, and V␤3 surface
expression. As observed in the periphery of these mice (see
above), no difference between spleen and lymph node cell transfer
was noted for 2 wk after transfer: at these early time points the
thymus was reconstituted only by host-derived precursors (Fig. 5,
A and B). Later after spleen cell transfer, but not after lymph node
cell transfer, thymocytes comprised large numbers of donor cells.
These results could not be explained by reentry of activated peripheral T cells into the thymus, as all thymic subsets comprised
donor-derived cells (Fig. 5, A and B). Indeed, 4 wk after transfer
and onwards, a large proportion of immature double-positive thy-
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Spleen but not lymph node naive CD4⫹ T cells reconstitute the
peripheral naive T cell pool of irradiated hosts
FIGURE 5. Reconstitution of the host thymus by spleen hematopoietic
precursors, rather than peripheral homeostasis, explains the reconstitution
of the peripheral naive T cell pool by AND CD4⫹ splenocytes. One million
lymph node (LNs) or spleen CD4⫹ T cells from H-2b/b AND TCR-transgenic rag-20/0 (Thy1.2) mice were transferred to irradiated normal
C57BL/6 Ba (Thy1.1) mice (A and B) or to irradiated or nonirradiated
CD3⑀-deficient C57BL/6 mice (C). At various times after transfer, thymocytes were recovered, counted, and stained for CD4, CD8, Thy1.2, and
V␤3 surface expression. A, At various time points after transfer to irradiated C57BL/6 mice, absolute numbers of recovered CD8⫺CD4⫹ (䡺) and
Thy1.2⫹V␤3⫹CD8⫺CD4⫹ (E) thymocytes were calculated. B, At various
time points after transfer to irradiated C57BL/6 mice, absolute numbers of
recovered double-positive (䡺) and Thy1.2⫹V␤3⫹ double-positive (E) thymocytes were calculated. C, CD4/CD8 fluorescence dot plots are presented
for all thymocytes 8 wk after transfer to irradiated or nonirradiated CD3⑀deficient mice.
mocytes were found to derive from injected splenocytes (Fig. 5B).
By contrast, no immature double-positive thymocytes were found
to derive from injected lymph node cells at any time point studied.
5046
Acknowledgments
We thank A. Banz, C. Bourgeois, and F. Lambolez for illuminating discussions, and C. Pénit for critically reading the manuscript.
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Most mature CD4⫹ thymocytes produced 4 wk after spleen cell
transfer were AND CD4⫹ T cells, and this production was stable
with time (Fig. 5A). These results suggested that the hematopoietic
precursors injected together with spleen AND CD4⫹ T cells possessed self-renewal capacities. Thus, our results confirm that
spleen cell transfer to irradiated animals permits thymus reconstitution by donor cells (26, 27).
Therefore, the different behavior of lymph node and spleen
AND CD4⫹ T cells after transfer to irradiated mice would be due
to de novo generation of naive CD4⫹ T cells by the thymus rather
than to homeostatic proliferation restricted to spleen naive CD4⫹
T cells. Our results suggest that the observation of Goldrath et al
(14) that naive T cells transiently acquire a memory-like phenotype after transfer to irradiated hosts would reflect replacement of
memory-like OT-1 CD8⫹ T cells by newly generated OT-1 CD8⫹
thymic migrants rather than a phenotypic and functional reversion
of these memory-like cells to a naive state after peripheral T cell
pool filling.
Finally, to explain the different behavior of spleen naive T cells
in rag-deficient recipients and irradiated normal hosts (14), we
transferred one million lymph node or spleen CD4⫹ T cells from
H-2b/b AND TCR-transgenic rag-20/0 mice to irradiated or nonirradiated CD3⑀-deficient mice. Thymus reconstitution by donor
cells was studied 8 wk after transfer (Fig. 5C). Our results clearly
show that both irradiation and subsequent injection of spleen cells
are required for host thymus reconstitution by donor cells, in
agreement with early papers describing thymic reconstitution after
irradiation and injection of hematopoietic precursors (28 –32).
Thus, these observations, together with the finding that cell recovery was much lower in rag-2-deficient mice than in CD3⑀-deficient
mice (Fig. 1), provide a highly plausible explanation for the different behavior of spleen naive T cells after transfer to rag-deficient recipients and irradiated hosts.
Naive T cells transferred to lymphopenic recipients (rag-deficient mice, CD3⑀-deficient mice, and irradiated normal mice)
failed to fill the peripheral naive T cell pool (15–17, 21). Indeed,
absolute numbers of recovered T cells were far below those in the
full peripheral naive T cell pool of normal mice. Moreover, injected naive T cells acquired a memory-like phenotype that remained stable with time, despite the absence of foreign antigenic
stimulation, and their functional capacities were modified, enhanced, or abolished (15–17, 21). Finally, injected cells were rapidly replaced by host thymic migrants after transfer to irradiated
normal mice. These data argue against the view that the proliferation of naive T cells in lymphopenic mice represents a homeostatic mechanism regenerating the naive T cell pool. Further studies should focus on the mechanisms underlying this proliferation
and the relevance of this process to disease-induced lymphopenia.
Note. During the processing of this manuscript, an article on the
same field reaching similar conclusions was published (33).
NAIVE T CELL BEHAVIOR IN LYMPHOPENIC HOSTS