Thymic myoid cells protect thymocytes from apoptosis and modulate their differentiation: implication of the ERK and Akt signaling pathways

Thymic myoid cells correspond to a muscle-like cell population present in the thymic medulla. They are well conserved throughout species evolution, but their biological role is not known. We demonstrated that myoid cells protected thymocytes from apoptosis as evidenced by a strong decrease of annexin-V-FITC positive thymocytes. This effect was (1) specific of myoid cells compared to thymic epithelial cells; (2) dependent on direct cell-to-cell contacts and (3) triggered rapidly after 2 h in cocultures. This protective phenomenon was due to the activation of prosurvival mechanisms. Indeed, myoid cells activated extracellular-regulated kinases (ERK1/2) and Akt in thymocytes. Myoid cells also influenced thymocyte maturation. We observed an increase in CD4(+) and a decrease in CD8(+) single positive (SP) thymocytes when cocultured with myoid cells, independently of a CD8(+)SP increased death or a CD4(+)SP overproliferation. Consequently, thymic myoid cells protect thymocytes from apoptosis and could also modulate their differentiation process.     

Expression profiling of a transformed thymocyte cell line undergoing maturation in vitro identifies multiple genes involved in positive selection

Biochemical and genetic studies of thymocyte maturation would be facilitated by the development of cultured cell lines that reflect stages of positive selection. We have derived a CD4(+)CD8(+)TCR(+) T-lymphoid cell line (M20) from a murine thymic tumor induced by a retrovirus carrying the v-myc oncogene (M-MuLV(myc)). M20 subclones undergo several aspects of positive selection in response to co-culture with a thymic stromal cell line (St3), including down-regulation of CD4 and CD8, and up-regulation of CD5 and TCR. M20 possesses a functional TCR complex, and ligation of this complex produces changes similar to co-culture with St3 stroma. Expression profiling of M20 cells in this system identified 23 genes previously shown to be important in thymocyte maturation, as well as several novel candidate genes. This system provides a new model to elucidate the molecular mechanisms of thymocyte maturation and TCR-mediated cell signaling in double-positive thymocytes.     

Expression of VLA-4 on thymocytes. Maturation stage-associated transition and its correlation with their capacity to adhere to thymic stromal cells.

The present study investigates the expression of VLA-4 on thymocytes at various stages of maturation and their capacity to adhere to thymic stromal cells. Whole thymocytes were stained with anti-CD4 and anti-CD8, as well as anti-VLA-4 antibodies. Flow microfluorometric analyses revealed that a) most of CD4-8- (double negative DN) and CD4-8intermediate thymocyte populations expressed large amounts of VLA-4, b) the levels of VLA-4 were considerably and markedly reduced on CD4+8+ (double positive DP) and single positive (SP) (CD4+8- or CD4-8+) populations, respectively. This contrasted with an increase in the levels of LFA-1 along with thymocyte maturation. DN, DP, and SP subsets were isolated and examined for their capacity to express VLA-4 and to adhere to fibronectin (FN) molecules as well as thymic stromal cells expressing FN. DN, DP, and SP subsets were confirmed to express the respective high, low, and very low levels of VLA-4, respectively. Approximately 70% of DN thymocytes became bound to FN-precoated culture plates, whereas 30 to 40% of DP and only 10 to 20% of SP cells adhered to FN. Similar patterns of adhesion were observed between these thymocyte subsets and thymic stromal monolayers. The binding of the DN subset to FN-plates or thymic stromal monolayers was inhibited only marginally by the RGDS peptide, but was efficiently inhibited by V10 peptide (cell-binding sequence that is located in the V region on FN and reacts with the VLA-4 integrin) or anti-VLA-4 antibody. Anti-VLA-4 antibody plus RGDS peptide strongly inhibited DN cell binding to FN-coated plates and thymic stromal monolayers. These results indicate that i) VLA-4 expressed on DN thymocytes functions as an important integrin for interacting with thymic stromal cells; ii) the expression level of this integrin decreases with the progress of thymocyte maturation, and iii) most of the mature thymocytes (SP) are rendered less adhesive to thymic stromal cells by reducing the level of VLA-4 expression.     

The thymic stromal cell line MTSC4 induced thymocyte apoptosis in a non-MHC-restricted manner

Mouse thymic stromal cell line 4 (MTSC4) is one of the stromal cell lines established in our laboratory. While losing the characteristics of epithelial cells, they express some surface markers shared with thymic dendritic cells (TDCs). To further study the biological functions of these cells, we compared the capability of MTSC4 with TDCs in the induction of thymocyte apoptosis, using thymic reaggregation culture system. Apoptosis of thymocytes induced by MTSC4 and TDCs was measured by Annexin V and PI staining and analyzed by flow cytometry. We found that MTSC4 selectively augmented the apoptosis of CD4^ 8^ (DP) thymocytes. This effect was Fas/FasL independent and could not be blocked by antibodies to MHC class I and class II molecules. In addition, MTSC4 enhanced the apoptosis of DP thymocytes from different strains of mice, which implies that MTSC4-induced thymocyte apoptosis is not mediated by the TCR recognition of self peptide/MHC molecules. In contrast to MTSC4, thymocyte apoptosis induced by TDCs was MHC-restricted. Thus, MHC-independent fashion of stromal-DP thymocyte interaction may be one of the ways to induce thymocyte apoptosis in thymus. Our study has also shown that the interaction of MTSC4 stromal cells and thymocytes is required for the induction of thymocyte apoptosis.     

Maturation versus death of developing double-positive thymocytes reflects competing effects on Bcl-2 expression and can be regulated by the intensity of CD28 costimulation

Immature double-positive (DP) thymocytes mature into CD4(+)CD8(-) cells in response to coengagement of TCR with any of a variety of cell surface "coinducer" receptors, including CD2. In contrast, DP thymocytes are signaled to undergo apoptosis by coengagement of TCR with CD28 costimulatory receptors, but the molecular basis for DP thymocyte apoptosis by TCR plus CD28 coengagement is not known. In the present study, we report that TCR plus CD28 coengagement does not invariably induce DP thymocyte apoptosis but, depending on the intensity of CD28 costimulation, can induce DP thymocyte maturation. We demonstrate that distinct but interacting signal transduction pathways mediate DP thymocyte maturation signals and DP thymocyte apoptotic signals. Specifically, DP maturation signals are transduced by the extracellular signal-related kinase (ERK)/mitogen-activated protein kinase (MAPK) pathway and up-regulate expression of the antiapoptotic protein Bcl-2. In contrast, the apoptotic response stimulated by CD28 costimulatory signals is mediated by ERK/MAPK-independent pathways. Importantly, when TCR-activated thymocytes are simultaneously coengaged by both CD28 and CD2 receptors, CD28 signals can inhibit ERK/MAPK-dependent Bcl-2 protein up-regulation. Thus, there is cross-talk between the signal transduction pathways that transduce apoptotic and maturation responses, enabling CD28-initiated signal transduction pathways to both stimulate DP thymocyte apoptosis and also negatively regulate maturation responses initiated by TCR plus CD2 coengagement.     

The Eph/ephrinB signal balance determines the pattern of T-cell maturation in the thymus

In order to carry out an in-depth study of the roles of EphB receptors in T-cell development and to determine the specific relevance of forward and reverse signals in the process, we established severe combined immunodeficient (SCID) mice chimeras with wild-type (WT) or EphB-deficient bone marrow cells. The obtained results demonstrate that EphB2 contributes more significantly than EphB3 in the control of CD4(-)CD8(-) (DN)-CD4(+)CD8(+) (DP) progression, and that reverse signals generated in SCID mice receiving EphB2LacZ precursors, which express the EphB2 extracellular domain, partially rescue the blockade of DN cell maturation observed in EphB2-null chimeras. In addition, increased apoptotic DP thymocytes occurring in EphB2 and/or EphB3 SCID chimeras also contribute to the reduced proportions of DP cells. However, EphB2LacZ chimeras do not show any changes in the proportions of apoptotic DP cells, thus suggesting that there is a role for ephrinB reverse signaling in thymocyte survival. The maturation of DP to CD4(+)CD8(-) or CD4(-)CD8(+) seems to need EphB2 forward signaling and EphB3; a fact that was confirmed in reaggregates formed with either EphB2- or EphB3-deficient DP thymocytes and WT thymic epithelial cells (TECs). The DP thymocyte-TEC conjugate formation was also affected by the absence of EphB receptors. Finally, EphB-deficient SCID chimeras show profoundly altered thymic epithelial organization that confirms a significant role for EphB2 and EphB3 receptors in the thymocyte-TEC crosstalk.     

The transient expression of C-C chemokine receptor 8 in thymus identifies a thymocyte subset committed to become CD4+ single-positive T cells

Developing T cells journey through the different thymic microenvironments while receiving signals that eventually will allow some of them to become mature naive T cells exported to the periphery. This maturation can be visualized by the phenotype of the developing cells. CCR8 is a ss-chemokine receptor preferentially expressed in the thymus. We have developed 8F4, an anti-mouse CCR8 mAb that is able to neutralize the ligand-induced activation of CCR8, and used it to characterize the CCR8 protein expression in the different thymocyte subsets. Taking into account the intrathymic lineage relationships, our data showed that CCR8 expression in thymus followed two transient waves along T cell maturation. The first one took place in CD4(-) CD8(-) double-negative thymocytes, which showed a low CCR8 expression, and the second wave occurred after TCR activation by the Ag-dependent positive selection in CD4(+) CD8(+) double-positive cells. From that maturation stage, CCR8 expression gradually increased as the CD4(+) cell differentiation proceeded, reaching a maximum at the CD4(+) CD8(-) single-positive stage. These CD4(+) cells expressing CCR8 were also CD69(high) CD62L(low) thymocytes, suggesting that they still needed to undergo some differentiation step before becoming functionally competent naive T cells ready to be exported from the thymus. Interestingly, no significant amounts of CCR8 protein were detectable in CD4(-) CD8(+) thymocytes. Our data showing a clear regulation of the CCR8 protein in thymus suggest a relevant role for CCR8 in this lymphoid organ, and identify CCR8 as a possible marker of thymocyte subsets recently committed to the CD4(+) lineage.     

The role of the Ets2 transcription factor in the proliferation,maturation, and survival of mouse thymocytes

In this study, we investigated the effects of Ets2 expression on the proliferation, maturation, and survival of thymocytes by establishing transgenic mice that specifically express Ets2 or a dominant negative form of Ets2, Deltaets2, in the thymus. We show that, in young animals, there are fewer T cells in Deltaets2 transgenic thymi and that the maturation of these T cells is affected at the CD4(-)CD8(-) double-negative to CD4(+)CD8(+) double-positive transition compared with wild-type littermate mice. Partial recovery in the number of thymocytes and full T cell maturation are restored with increasing age of Deltaets2 transgenic animals. However, thymocytes from adult Deltaets2 transgenic mice cultured ex vivo are more sensitive to cell death and to glucocorticoid-induced apoptosis than are T cells from control littermate mice. We also show that T cells from adult ets2 transgenic mice proliferate faster than their wild-type littermates. The proliferation and survival of these T cells are clearly affected upon apoptotic signals: glucocorticoid-induced apoptosis induces T cells from ets2 transgenic mice to continue to proliferate in vivo and to survive better ex vivo than T cells from control littermates. It has been shown that c-Myc expression is required for thymic proliferation and improves thymocyte survival of dexamethasone-treated animals. We show that the expression of c-Myc, an Ets2 target, is elevated in T cells freshly isolated from thymi of ets2 transgenic mice pretreated with dexamethasone. Together, these results show that Ets2 plays a role in the proliferation and survival of thymocytes, implicating a Myc-dependent pathway.     

Thymic stromal cell clone with nursing activity supports the growth and differentiation of murine CD4+8+ thymocytes in vitro

Thymic stromal cell clone, TNC-R3.1 cell, was established from spontaneous AKR/J mouse thymoma. TNC-R3.1 cell, which has the similar properties to thymic nurse cells, formed a unique complex with normal thymocyte subpopulations. Flow cytometry analysis demonstrated that CD4+8+ and CD4-8- immature thymocytes preferentially interacted with TNC-R3.1 stromal cell clone. CD4+8+ thymocytes, which interacted with TNC-R3.1 stromal cell clone, contained a higher proportion of large size and cycling T cells than did noninteracting CD4+8+ thymocytes. As is generally accepted, CD4+8+ thymocytes did not respond to any stimulation such as IL-2, anti-CD3 mAb (2C11), or IL-2 plus 2C11. However, culture of isolated CD4+8+ thymocytes on TNC-R3.1 stromal cell monolayer in the presence of suboptimal dose of IL-2 induced a significant cell growth. Moreover, the addition of 2C11 and IL-2 into this coculture system resulted in a dramatic increase of the proliferative response of thymocytes. Flow cytometry analysis showed the proliferating cells on TNC-R3.1, which originated from CD4+8+ thymocytes, were mostly TCR-alpha beta+ CD3+CD4-8+ T cells. These results provide in vitro evidence that CD4+8+ thymocytes are at an intermediate stage of T cell maturation and TNC-R3.1 stromal cell clone induces the growth and differentiation of CD4+8+ thymocytes into CD4-8+ T cells.     

Caspases in T-cell receptor-induced thymocyte apoptosis.

Apoptosis eliminates inappropriate or autoreactive T lymphocytes during thymic development. Intracellular mediators involved in T-cell receptor (TCR)-mediated apoptosis in developing thymocytes during negative selection are therefore of great interest. Caspases, cysteine proteases that mediate mature T-cell apoptosis, have been implicated in thymocyte cell death, but their regulation is not understood. We examined caspase activities in distinct thymocyte subpopulations that represent different stages of T-cell development. We found caspase activity in CD4+CD8+ double positive (DP) thymocytes, where selection involving apoptosis occurs. Earlier and later thymocyte stages exhibited no caspase activity. Only certain caspases, such as caspase-3 and caspase-8-like proteases, but not caspase-1, are active in DP thymocytes in vivo and can be activated when DP thymocytes are induced to undergo apoptosis in vitro by TCR-crosslinking. Thus, specific caspases appear to be developmentally regulated in thymocytes.     

Synergistic effect of IL-2, IL-12, and IL-18 on thymocyte apoptosis and Th1/Th2 cytokine expression

In the periphery, IL-18 synergistically induces the expression of the Th1 cytokine IFN-gamma in the presence of IL-12 and the Th2 cytokines IL-5 and IL-13 in the presence of IL-2. Although the expression of these cytokines has been described in the thymus, their role in thymic development and function remains uncertain. We report here that freshly isolated thymocytes from C57BL/6 and BALB/c mice stimulated in vitro with IL-2-plus-IL-18 or IL-12-plus-IL-18 produce large amounts of IFN-gamma and IL-13. Analysis of the thymic subsets, CD4(-)CD8(-) (DN), CD4(+)CD8(+), CD4(+)CD8(-), and CD4(-)CD8(+) revealed that IL-18 in combination with IL-2 or IL-12 induces IFN-gamma and IL-13 preferentially from DN cells. Moreover, DN2 and DN3 thymocytes contained more IFN-gamma(+) cells than cells in the later stage of maturation. Additionally, IL-18 in combination with IL-2 induces CCR4 (Th2-associated) and CCR5 (Th1-associated) gene expression. In contrast, IL-18-plus-IL-12 specifically induced CCR5 expression. The IL-2-plus-IL-18 or IL-12-plus-IL-18 effect on IFN-gamma and IL-13 expression is dependent on Stat4 and NF-kappaB but independent of Stat6, T-bet, or NFAT. Furthermore, IL-12-plus-IL-18 induces significant thymocyte apoptosis when expressed in vivo or in vitro, and this effect is exacerbated in the absence of IFN-gamma. IL-12-plus-IL-18-stimulated thymocytes can also induce IA-IE expression on cortical and medullary thymic epithelial cells in an IFN-gamma-dependent manner. Thus, the combination of IL-2, IL-12, and IL-18 can induce phenotypic and functional changes in thymocytes that may alter migration, differentiation, and cell death of immature T cells inside the thymus and potentially affect the Th1/Th2 bias in peripheral immune compartments.     

Disordered T-Cell Development and T-Cell Malignancies in SCL LMO1 Double-Transgenic Mice: Parallels with E2A-Deficient Mice

The gene most commonly activated by chromosomal rearrangements in patients with T-cell acute lymphoblastic leukemia (T-ALL) is SCL/tal. In collaboration with LMO1 or LMO2, the thymic expression of SCL/tal leads to T-ALL at a young age with a high degree of penetrance in transgenic mice. We now show that SCL LMO1 double-transgenic mice display thymocyte developmental abnormalities in terms of proliferation, apoptosis, clonality, and immunophenotype prior to the onset of a frank malignancy. At 4 weeks of age, thymocytes from SCL LMO1 mice show 70% fewer total thymocytes, with increased rates of both proliferation and apoptosis, than control thymocytes. At this age, a clonal population of thymocytes begins to populate the thymus, as evidenced by oligoclonal T-cell-receptor gene rearrangements. Also, there is a dramatic increase in immature CD44(+) CD25(-) cells, a decrease in the more mature CD4(+) CD8(+) cells, and development of an abnormal CD44(+) CD8(+) population. An identical pattern of premalignant changes is seen with either a full-length SCL protein or an amino-terminal truncated protein which lacks the SCL transactivation domain, demonstrating that the amino-terminal portion of SCL is not important for leukemogenesis. Lastly, we show that the T-ALL which develop in the SCL LMO1 mice are strikingly similar to those which develop in E2A null mice, supporting the hypothesis that SCL exerts its oncogenic action through a functional inactivation of E proteins.     

MHC non-restricted, CD95-independent apoptosis of immature thymocytes induced by thymic epithelial cells

The interaction of thymocytes with thymic epithelial cells in the absence of an exogenous antigen was studied in vitro. Thymic, but not splenic epithelial cells induced apoptosis of thymocytes. A thymic epithelial cell line (TEC) induced apoptosis of thymocytes but not of splenic T-cells. The target population for TEC-induced death were immature CD4(+)8(+) (double positive), but not mature single positive thymocytes. TEC also induced DNA fragmentation in day 18 foetal thymocytes, most of which are CD4(+)8(+) cells. Radiation leukemia virus (RadLV)-transformed thymic lymphoma clones expressing various phenotypes reflected this sensitivity, in that a CD4(+)8(+)3(+) clone apoptosed by thymic epithelial cells or TEC. Other, single positive or double negative clones were resistant. Thymocytes from C3H (H-2(k)), C57BL/6 (H-2(b)) and Balb/C (H-2(d)) mice apoptosed equally in response to either C57BL/6 thymic epithelial cells or TEC (H-2(b) x H-2(d)). Likewise, thymocytes from MRLIpr((-/-)) and B6Ipr((-/-)) mice, which do not express CD95 were also apoptosed by TEC.The data suggest that thymic epithelial cells induce MHC non-restricted, Fas-independent apoptosis of immature thymocytes. This response may reflect a mechanism through which thymocytes expressing TcR with no affinity to self MHC/peptide complexes are eliminated.     

Selection of CD4+CD8+ thymocytes by complex formation with medulla-derived epithelial cells

The role of lymphostromal complexes in T-cell differentiation is far from elucidated, mainly because a clear association of a particular stromal cell type with a distinct thymocyte subset has never been identified. Using an in vitro system, detecting the adherence of thymocytes to a thymic medullary epithelial cell line (E-5), we showed that the phenotype of these thymocytes was that of cortical type: Thy-1hi, LFA-1+, PNAhi, CD4+CD8+, MEL-14-/lo, IL-2R-, CD3-/lo, and TcR V beta 8-/lo. They were enriched in cells in G2/M at the time of complex formation, showed a higher basal proliferation in culture, and did not respond to PHA, IL-2 and only marginally to Con A. These data show that complex formation with mouse thymic medullary epithelium selects for CD4+CD8+ thymocytes, as shown by the marked decrease in CD4+CD8-/CD4-CD8+ thymocytes, and the incapacity of CD4-CD8- thymocytes to adhere.     

EphA receptors inhibit anti-CD3-induced apoptosis in thymocytes

The EphA receptor tyrosine kinases interact with membrane-bound ligands of the ephrin-A subfamily. Interaction induces EphA receptor oligomerization, tyrosine phosphorylation, and, as a result, EphA receptor signaling. EphA receptors have been shown to regulate cell survival, migration, and cell-cell and cell-matrix interactions. However, their functions in lymphoid cells are only beginning to be described. We show in this study that functional EphA receptors are expressed by murine thymocytes, including CD4(+)CD8(+), CD4(+)CD8(-), and CD4(-)CD8(+) subpopulations. We demonstrate that activation of EphA receptors by the ephrin-A1 ligand inhibits the anti-CD3-induced apoptosis of CD4(+)CD8(+) double-positive thymocytes. Furthermore, ephrin-A1 costimulation suppresses up-regulation of both the IL-2R alpha-chain (CD25) and early activation Ag CD69 and can block IL-2 production by CD4(+) single-positive cells. In agreement, EphA receptor activation in thymocytes also inhibits TCR-induced activation of the Ras-MAPK pathway. Our findings suggest that EphA receptor activation is antithetical to TCR signaling in thymocytes, and that the level of engagement by ephrin-A proteins on thymic APCs regulates thymocyte selection.     

Bone morphogenetic protein 2/4 signaling regulates early thymocyte differentiation

Bone morphogenetic protein (BMP)2 and BMP4 are involved in the development of many tissues. In this study, we show that BMP2/4 signaling is involved in thymocyte development. Our data suggest that termination of BMP2/4 signaling is necessary for differentiation of CD44(+)CD25(-)CD4(-)CD8(-) double negative (DN) cells along the T cell lineage. BMP2 and BMP4 are produced by the thymic stroma and the requisite BMP receptor molecules (BMPR-1A, BMPR-1B, BMPR-II), and signal transduction molecules (Smad-1, -5, -8, and -4) are expressed by DN thymocytes. BMP4 inhibits thymocyte proliferation, enhances thymocyte survival, and arrests thymocyte differentiation at the CD44(+)CD25(-) DN stage, before T cell lineage commitment. Neutralization of endogenous BMP2 and BMP4 by treatment with the antagonist Noggin promotes and accelerates thymocyte differentiation, increasing the expression of CD2 and the proportion of CD44(-)CD25(-) DN cells and CD4(+)CD8(+) double-positive cells. Our study suggests that the BMP2/4 pathway may function in thymic homeostasis by regulating T cell lineage commitment and differentiation.     

Moloney murine leukemia virus-induced tumors show altered levels of proapoptotic and antiapoptotic proteins

Moloney murine leukemia virus (M-MuLV) is a replication-competent, simple retrovirus that induces T-cell lymphomas when inoculated into neonatal mice. The tumor cells are typically derived from immature T cells. During preleukemic times, a marked decrease in thymic size is apparent in M-MuLV-inoculated mice. We previously demonstrated that this thymic regression is correlated with enhanced levels of thymocyte apoptosis (C. Bonzon and H. Fan, J. Virol. 73:2434-2441, 1999). In this study, we investigated the apoptotic state of M-MuLV-induced tumors. M-MuLV-induced tumors were screened for expression of the apoptotic proteins Fas and Bcl-2 by three-color flow cytometric analysis. Single-positive (SP; CD4(+) CD8(-) and CD4(-) CD8(+)) tumor cells generally displayed lower cell surface expression of Fas than SP thymocytes from uninoculated control mice. Double-positive (DP; CD4(+) CD8(+)) M-MuLV-induced tumor cells fell into two categories: those with normal high levels of Fas and those with low levels of Fas. Additionally, the vast majority of DP tumors showed elevated Bcl-2 levels. The DP tumor cells retaining normal/high Fas expression were capable of transducing an apoptotic signal upon anti-Fas engagement. In addition, DP and CD4(+) SP tumor populations displayed higher levels of Fas ligand than normal thymocytes with the same phenotypes. In contrast, CD8(+) SP and CD4(-) CD8(-) tumors did not show elevated Fas ligand expression. There was no significant correlation between Fas and Fas ligand expression in the DP tumors, suggesting that Fas Ligand expression was not the driving force behind Fas down-regulation. These results suggest that both the Fas death receptor and mitochondrial pathways of apoptotic death are active in M-MuLV-induced tumors and that they must be modulated to permit cell survival and tumor outgrowth.     

Effects of IL-7 on early human thymocyte progenitor cells in vitro and in SCID-hu Thy/Liv mice

IL-7 is a critical component of thymopoiesis in animals and has recently been shown to play an important role in T cell homeostasis. Although there is increasing interest in the use of IL-7 for the treatment of lymphopenia caused by the HIV type 1, evidence that IL-7 may accelerate HIV replication has raised concerns regarding its use in this setting. We sought to identify the effects of IL-7 on human thymocyte survival and to determine the impact of IL-7 administration on in vivo HIV infection of the human thymus. Using in vitro analysis, we show that IL-7 provides potent anti-apoptotic and proliferative signals to early thymocyte progenitors. Analysis of CD34(+) subpopulations demonstrates that surface IL-7 receptor is expressed on most CD34(high)CD5(+)CD1a(-) thymocytes and that this subpopulation appears to be one of the earliest maturation stages responsive to the effects of IL-7. Thus, IL-7 provides survival signals to human thymocytes before surface expression of CD1a. CD4(+)CD8(+) thymocytes are relatively unresponsive to IL-7, although IL-7 protects these cells from dexamethasone-induced apoptosis. IL-7 has a predominantly proliferative effect on mature CD4(+)CD3(+)CD8(-) and CD8(+)CD3(+)CD4(-) thymocytes. In contrast to the in vitro findings, we observe that in vivo administration of IL-7 to SCID-hu Thy/Liv mice does not appear to enhance thymocyte survival nor does it appear to accelerate HIV infection. Given the growing interest in the use of IL-7 for the treatment of human immunodeficiency, these findings support additional investigation into its in vivo effects on thymopoiesis and HIV infection.