Blood, 15 December 2002, Vol. 100, No. 13, pp. 4676-4677
CORRESPONDENCE
To the editor:
IL-7 and CD4+ T-cell proliferation
A recent article by Steffens et al1 assessed
whether the IL-7 cytokine promotes productive HIV-1 infection in naive
CD4+ T cells. This is an important biologic issue because,
while most circulating peripheral T lymphocytes are in the
G0 state, the productive infection of both naive and memory
CD4+ T cells with HIV-1, at least outside the context of
lymphoid tissues,2 requires a progression into the
G1b stage of the cell cycle.3 It is assumed
that, in vivo, HIV-1 infection occurs in CD4+ T cells that
are cycling because they have recently been stimulated by their cognate
antigen. One quandary that arises from this hypothesis is that those T
cells stimulated through their T-cell receptor (TCR) attain
characteristics of memory lymphocytes and lose their naive phenotype.
The detection of HIV-1 in naive T cells in vivo, albeit at
significantly lower levels than in the memory T-cell subset,4-7 raises questions regarding the mechanisms
controlling HIV-1 infection in the former population. Thus it is
possible that a cytokine such as IL-7, which does not significantly
modify the naive phenotype of T cells,8-10 may be
involved in this process.
A study of HIV-1 infection in the context of IL-7 stimulation requires
an understanding of the biologic effects of IL-7. But while Steffens et
al reported that naive human CD4+ T cells proliferated in
response to IL-7 in vitro, our 2 groups have independently found that
naive CD4+ T cells do not proliferate under these
conditions.11,12 The recent data of Geginat et
al13 support these observations; they find that in vitro
IL-7 synergizes with IL-15 in driving the proliferation of memory
CD4+ T cells but has no effect on naive T
cells.13 Indeed, we determined that only recent thymic
emigrants, naive CD4+ T cells isolated from umbilical cord
blood, proliferate significantly in response to IL-7 in
vitro.11,12 Moreover, even under conditions where T
lymphocytes are cultured in the presence of both IL-2 and IL-7
cytokines, only recent thymic emigrants, and not naive CD4+
T cells, enter into the S or G2 phases of the cell cycle at
significant levels (Figure 1). Finally, using a technique that permits simultaneous DNA and RNA
quantitation, we previously demonstrated that, although 10% to 20% of
IL-7-stimulated memory CD4+ T cells progress to the
G1b phase of the cell cycle, only 3% to 5% of naive
CD4+ T cells progress past G1a.12
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| Figure 1.
The IL-2 and IL-7 cytokines preferentially induce
cell-cycle entry in recent thymic emigrants.
Naive (CD45RA+; TN) and memory
(CD45RO+; TM) CD4+-resting T
lymphocytes were isolated from umbilical cord blood (UCB) and adult
peripheral blood (APB) by negative selection. Cells were cultured in
vitro in the presence of IL-2 (100 U/mL) and/or IL-7 (10 ng/mL), and
cell-cycle entry was monitored at day 4 by assessing DNA content of
propidium iodide-stained cells on a FACScan cytometer (Becton
Dickinson, San Jose, CA). The percentage of cells in the S and
G2/M phases of the cell cycle are
indicated, and results are representative of data obtained in 1 of 3 representative experiments.
|
|
It should nevertheless be noted that all studies assessing the effects
of recombinant IL-7 on human T-cell subsets, including our own, may be
biased because they have been performed ex vivo. In mice, homeostatic
T-cell proliferation is readily detectable, but only under conditions
of lymphopenia. In a lymphopenic environment, proliferation of naive T
cells in mice is conditional on TCR/selfpeptide/major histocompatibility complex (MHC)-derived
signals14-22 but is also strictly dependent on the
presence of endogenous IL-7.23,24 Proliferation of T cells
in nonlymphopenic mice can be induced by injection of high doses of
recombinant IL-7, resulting in cell-cycle entry of a low percentage of
CD4+ T cells (approximately 4%) and a higher percentage of
CD8+ T cells (10%-11%).25 Thus, in
interpreting the in vitro studies performed with human T cells, it is
important to take into account the limitations resulting from the
absence of in vivo factors that likely cooperate with IL-7 in driving
cell-cycle progression in lymphoid organs.
One technical difference that may explain the apparent discrepancies
between the respective in vitro human T-cell studies performed by our
groups11,12 and that performed by Steffens et al concerns
the manner in which CD4+ T cells were selected. In order to
eliminate confounding effects due to in vivo activated T cells, we
removed T lymphocytes expressing the HLA-DR activation marker with a
monoclonal antibody. HLA-DR+ cells account for 2% to 10%
of the T-cell population, depending on the donor. Under conditions
where these cells are removed, purified CD4+ T cells do not
proliferate in response to either IL-7, the phytohemagglutinin (PHA) lectin, or a mitogenic anti-CD3 mAb.3,11,12
These data are pertinent to HIV-1 infection because Korin and Zack
found that, under conditions where CD4+ T cells are
stimulated with an 
-CD3 mAb, the presence of HLA-DR+
cells modulates HIV infection: productive HIV infection is observed in
the presence of HLA-DR+ cells, whereas in their absence,
infection is blocked.3 Thus the HIV-1 infection observed
by Steffens et al in IL-7-treated naive CD4+ T cells may
be specific to their experimental conditions. Furthermore, Steffens et
al found that only naive T cells from 4 of 6 donors were responsive to
IL-7-mediated HIV infection, and as such it is possible that this
discrepancy results from differences in the percentages of
activated T cells present in these individuals at a given time. This
hypothesis is attractive since IL-7 is not likely to exert its effect
at the level of enhanced viral entry; while IL-7 up-regulated
expression of the HIV gp120 coreceptor CXCR4, this phenomenon was also
observed in T cells from donors who were resistant to IL-7-mediated
infection.1
In many of the experiments presented by Steffens et al,
CD8+ T cells were maintained in the culture, and it is
therefore possible that the capacity of naive CD4+ T cells
to be infected by HIV-1 following IL-7 treatment is modulated by the
presence of CD8+ T cells and/or activated T cells. Indeed,
both in vitro and in vivo, CD8+ T cells appear to undergo
significantly more division in response to IL-7.1,25
Further studies will be necessary to distinguish the effects of IL-7 on
naive CD4+ T cells in the context of distinct hematopoietic
cell populations. Finally, it will be important to elucidate how the
cellular environment in lymph nodes and the peripheral circulation
differentially regulates the responsiveness of naive T cells to IL-7
and HIV-1 infection. Specifically, the in vivo data obtained in mice
suggest that the effects of IL-7 may differ in conventional in vitro
cultures and in the context of the 3-dimensional architecture and
cellular environment present in lymphoid tissues.24,25 The
data obtained from the ensemble of these experiments will allow us to
determine whether, and under what conditions, recombinant IL-7 may be
beneficial as an immune modulator in HIV-infected individuals.
Sara Jaleco, Sandrina Kinet, Jaythoon Hassan, Valérie Dardalhon, Louise Swainson, Denis Reen, and Naomi Taylor
Correspondence: Naomi Taylor, Institut de
Génétique Moléculaire de Montpellier, UMR 5535/IFR
24, Montpellier, France F34293;
e-mail:
taylor{at}igm.cnr-mop.fr
References
1.
Steffens CM, Managlia EZ, Landay A, Al-Harthi L.
Interleukin-7-treated naive T cells can be productively infected by T-cell-adapted and primary isolates of human immunodeficiency virus 1.
Blood.
2002;99:3310-3318[Abstract/Free Full Text].
2.
Eckstein DA, Penn ML, Korin YD, et al.
HIV-1 actively replicates in naive CD4(+) T cells residing within human lymphoid tissues.
Immunity.
2001;15:671-682[CrossRef][Medline]
[Order article via Infotrieve].
3.
Korin YD, Zack JA.
Progression to the G1b phase of the cell cycle is required for completion of human immunodeficiency virus type 1 reverse transcription in T cells.
J Virol.
1998;72:3161-3168[Abstract/Free Full Text].
4.
Schnittman SM, Lane HC, Greenhouse J, Justement JS, Baseler M, Fauci AS.
Preferential infection of CD4+ memory T cells by human immunodeficiency virus type 1: evidence for a role in the selective T-cell functional defects observed in infected individuals.
Proc Natl Acad Sci U S A.
1990;87:6058-6062[Abstract/Free Full Text].
5.
Ostrowski MA, Chun TW, Justement SJ, et al.
Both memory and CD45RA+/CD62L+ naive CD4(+) T cells are infected in human immunodeficiency virus type 1-infected individuals.
J Virol.
1999;73:6430-6435[Abstract/Free Full Text].
6.
Cayota A, Vuillier F, Scott-Algara D, Feuillie V, Dighiero G.
Differential requirements for HIV-1 replication in naive and memory CD4 T cells from asymptomatic HIV-1 seropositive carriers and AIDS patients.
Clin Exp Immunol.
1993;91:241-248[Medline]
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7.
Cayota A, Vuillier F, Scott-Algara D, Dighiero G.
Preferential replication of HIV-1 in memory CD4+ subpopulation [letter].
Lancet.
1990;336:941[CrossRef][Medline]
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8.
Hassan J, Reen DJ.
IL-7 promotes the survival and maturation but not differentiation of human post-thymic CD4+ T cells.
Eur J Immunol.
1998;28:3057-3065[CrossRef][Medline]
[Order article via Infotrieve].
9.
Soares MV, Borthwick NJ, Maini MK, Janossy G, Salmon M, Akbar AN.
IL-7-dependent extrathymic expansion of CD45RA+ T cells enables preservation of a naive repertoire.
J Immunol.
1998;161:5909-5917[Abstract/Free Full Text].
10.
Unutmaz D, KewalRamani VN, Marmon S, Littman DR.
Cytokine signals are sufficient for HIV-1 infection of resting human T lymphocytes.
J Exp Med.
1999;189:1735-1746[Abstract/Free Full Text].
11.
Hassan J, Reen DJ.
Human recent thymic emigrants-identification, expansion, and survival characteristics.
J Immunol.
2001;167:1970-1976[Abstract/Free Full Text].
12.
Dardalhon V, Jaleco S, Kinet S, et al.
IL-7 differentially regulates cell cycle progression and HIV-1-based vector infection in neonatal and adult CD4+ T cells.
Proc Natl Acad Sci U S A.
2001;98:9277-9282[Abstract/Free Full Text].
13.
Geginat J, Sallusto F, Lanzavecchia A.
Cytokine-driven proliferation and differentiation of human naive, central memory, and effector memory CD4(+) T cells.
J Exp Med.
2001;194:1711-1719[Abstract/Free Full Text].
14.
Clarke SR, Rudensky AY.
Survival and homeostatic proliferation of naive peripheral CD4+ T cells in the absence of self peptide:MHC complexes.
J Immunol.
2000;165:2458-2464[Abstract/Free Full Text].
15.
Beutner U, MacDonald HR.
TCR-MHC class II interaction is required for peripheral expansion of CD4 cells in a T cell-deficient host.
Int Immunol.
1998;10:305-310[Abstract/Free Full Text].
16.
Viret C, Wong FS, Janeway CA Jr.
Designing and maintaining the mature TCR repertoire: the continuum of self-peptide:self-MHC complex recognition.
Immunity.
1999;10:559-568[CrossRef][Medline]
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17.
Bender J, Mitchell T, Kappler J, Marrack P.
CD4+ T cell division in irradiated mice requires peptides distinct from those responsible for thymic selection.
J Exp Med.
1999;190:367-374[Abstract/Free Full Text].
18.
Ernst B, Lee DS, Chang JM, Sprent J, Surh CD.
The peptide ligands mediating positive selection in the thymus control T cell survival and homeostatic proliferation in the periphery.
Immunity.
1999;11:173-181[CrossRef][Medline]
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19.
Goldrath AW, Bevan MJ.
Low-affinity ligands for the TCR drive proliferation of mature CD8+ T cells in lymphopenic hosts.
Immunity.
1999;11:183-190[CrossRef][Medline]
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20.
Kieper WC, Jameson SC.
Homeostatic expansion and phenotypic conversion of naive T cells in response to self peptide/MHC ligands.
Proc Natl Acad Sci U S A.
1999;96:13306-13311[Abstract/Free Full Text].
21.
Muranski P, Chmielowski B, Ignatowicz L.
Mature CD4+ T cells perceive a positively selecting class II MHC/peptide complex in the periphery.
J Immunol.
2000;164:3087-3094[Abstract/Free Full Text].
22.
Murali-Krishna K, Lau LL, Sambhara S, Lemonnier F, Altman J, Ahmed R.
Persistence of memory CD8 T cells in MHC class I-deficient mice.
Science.
1999;286:1377-1381[Abstract/Free Full Text].
23.
Schluns KS, Kieper WC, Jameson SC, Lefrancois L.
Interleukin-7 mediates the homeostasis of naive and memory CD8 T cells in vivo.
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2000;1:426-432[CrossRef][Medline]
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24.
Tan JT, Dudl E, LeRoy E, Murray R, Sprent J, Weinberg KI, Surh CD.
IL-7 is critical for homeostatic proliferation and survival of naive T cells.
Proc Natl Acad Sci U S A.
2001;98:8732-8737[Abstract/Free Full Text].
25.
Geiselhart LA, Humphries CA, Gregorio TA, Mou S, Subleski J, Komschlies KL.
IL-7 administration alters the CD4:CD8 ratio, increases T cell numbers, and increases T cell function in the absence of activation.
J Immunol.
2001;166:3019-3027[Abstract/Free Full Text].
Response:
IL-7 modulation of HIV-1 infection of naive T
cells
In our published report, we provided data that IL-7 induces the
productive infection of HIV-1 in the naive CD4+ T-cell
compartment.1 Jaleco et al, based on a report by Korin and
Zack,2 state that productive infection requires that the cell enter the G1b phase of the cell cycle. While we have
not directly evaluated the stage of the cell cycle that these
HIV+ naive T cells are in, others have recently reported
that resting naive T cells, albeit in lymphoid tissue explant model,
can still support HIV infection without progressing past the
G1b phase of the cell cycle.3 This productive
infection, however, was dependent on the environmental milieu of these
lymphoid tissues.3 We believe that IL-7 is probably a key
cytokine in promoting this productive infection. Kinter et al have also
reported that the microenvironment of thymic ex vivo cultures mediates
HIV infection of resting T cells.4
Another key point is that data from Jaleco and colleagues demonstrate
that IL-7 does not induce the proliferation of naive T cells beyond
G1a. This finding is not in conflict with our own published
data. We have found that treatment of naive CD4+ T cells
with IL-7 does lead to a small subpopulation of naive T cells,
approximately 4%, that entered the cell cycle, as determined by
intracellular Ki-67 staining. It is important to note that Ki-67 is a
nuclear protein present in cells once they have passed G0
and entered the cell cycle, but will not identify the exact stage of
the cell cycle. Therefore, it is possible that this subpopulation may
not have progressed past the G1a stage, as Jaleco et al
report in their letter. We are in the process of determining
whether this small population reflects the infected fraction of
the naive CD4+ T cells. Nonetheless, this 4% cycling naive
subpopulation in response to IL-7 has not acquired CD45RO expression
(unpublished data). Preliminary phenotypic analysis from our
laboratory of IL-7-treated naive T cells also indicates that they do
not up-regulate CD69 or HLA-DR expression.
Jaleco et al suggest that the infected population may be that which
initially expressed HLA-DR, which is an attractive hypothesis. But,
though in the report published we have not depleted for HLA-DR expression, phenotypic analysis of the purified naive T-cell population reflected less than 2% of CD45RO-depleted cells to express HLA-DR. Others have depleted for HLA-DR expression, to isolate resting cells,
and have been able to detect intracellular p24 expression within
resting/quiescent naive T cells.3,4 Jaleco et al also suggested that maintaining CD8+ T cells in these
experiments may impact the ability of IL-7 to modulate HIV infection of
CD4+ naive T cells. But even in experiments where
CD8+ T cells were depleted, we were still able to
demonstrate IL-7 enhancement of HIV replication in the naive
CD4+ T-cell compartment, as defined by
CD4+CD45RA+CD45RO
expression
(unpublished data). Additionally, we were able to also demonstrate that
IL-7 can induce HIV replication in CD4+ memory
(CD4+CD45RACD45RO+) and even
total peripheral blood mononuclear cells (PBMCs).
IL-7 is an attractive immune-modulator. But prior to clinical studies
utilizing this cytokine, a clear understanding of its role in
modulating HIV replication in resting/naive T cells and even on other
cellular fractions needs to be elucidated.
Lena Al-Harthi
Correspondence: Lena Al-Harthi,
Rush-Presbyterian-St Luke's Medical Center, Rush Medical College,
Department of Immunology/Microbiology, 1653 West Congress
Parkway, Rm 1577 JSC, Chicago, IL 60612;
e-mail:
lalharthi{at}rush.edu
References
1.
Steffens CM, Managlia EZ, Landay A, Al-Harthi L.
Interleukin-7-treated naive T cells can be productively infected by T-cell-adapted and primary isolates of human immunodeficiency virus 1.
Blood.
2002;99:3310-3318[Abstract/Free Full Text].
2.
Korin YD, Zack JA.
Progression to the G1b phase of the cell cycle is required for completion of human immunodeficiency virus type 1 reverse transcription in T cells.
J Virol.
1998;72:3161-3168[Abstract/Free Full Text].
3.
Eckstein DA, Penn ML, Korin YD, et al.
HIV-1 actively replicates in naive CD4(+) T cells residing within human lymphoid tissues.
Immunity.
2001;15:671-682[CrossRef][Medline]
[Order article via Infotrieve].
4. Kinter A, Moorthy A, et al. Productive HIV infection of
resting CD4+ T cells: the role of lymphoid tissue microenvironment
[abstract 35]. J Human Virol.
2001.
