Changes in inducibility of IL-2 receptor alpha-chain and T cell-receptor expression during thymocyte differentiation in the mouse

Within the thymus, developing T cells must acquire the competence to respond to appropriate signals by inducing the expression of genes required for immunologic function; one such gene encodes the 55-kDa-chain of the IL-2R (IL-2R alpha). Previously, we showed that most cortical-type thymocytes lack the competence to make this particular response, while most medullary-type cells respond like mature T lymphocytes. The noninducibility of cortical-type cells was striking, because most of their presumed precursors were inducible. To test the relationship between this apparent loss of competence and the positive and negative selection processes that may occur in the thymic cortex, we have assayed the inducibility of thymocyte populations, staged carefully with respect to their expression of TCR. Using size fractionation to enrich for dividing cells, we concentrated and thereby revealed defined developmental intermediates. We report that, although CD4+CD8- thymocytes behave as mature T cells, a significant fraction of CD4-CD8+ cells are noninducible. These noninducible thymocytes are dividing cells, which appear to be in a major developmental continuum between CD4-CD8- blasts and CD4+CD8+ blasts. Furthermore, the noninducible blasts as yet lack surface TCR expression. We also demonstrate the functional similarity of these CD4-CD8+ cells to a major subset of dividing CD4-CD8- precursor cells, which appear to have lost IL-2R alpha expression. These results suggest that precursors of cortical thymocytes lose competence to be induced to express IL-2R alpha several stages before their acquisition of cell-surface TCR complexes. The implications of this characterization are discussed in terms of the possible relationships between IL-2R alpha gene regulation and intrathymic fate determination.     

An essential role for the IL-7 receptor during intrathymic expansion of the positively selected neonatal T cell repertoire

Intrathymic T cell development is a multistage process involving discrete phases of proliferation as well as differentiation. From studies on IL-7 or IL-7Ralpha-deficient mice, it is clear that the IL-7 receptor (IL-7R) plays a critical role during the initial stages of intrathymic CD4-8- precursor development. In contrast, the role of IL-7R in later stages of thymocyte development are unclear. Here, we have used various approaches to investigate directly the role of the IL-7R in thymocyte positive selection and the recently described phase of postselection proliferation. First, we show that positive selection involves selective up-regulation of IL-7Ralpha- and IL-7Rgamma-chains, with the majority of CD4+ and CD8+ cells being IL-7R+. Second, MHC class II+ thymic epithelium-which drives postselection proliferation-expresses IL-7 mRNA. Finally, analysis of positive selection and postselection proliferation in thymocytes from IL-7Ralpha-/- neonates shows that positive selection occurs normally, whereas postselection expansion is drastically reduced. Thus, our data provide the first evidence that, as well as playing a role during early phases of thymic development, IL-7R mediates intrathymic expansion of positively selected thymocytes, which may aid in establishment of the neonatal peripheral T cell pool.     

Role of IL-12 in intrathymic negative selection.

Cytokines are central regulatory elements in peripheral lymphocyte differentiation, but their role in T cell ontogeny is poorly defined. In the present study, we evaluated the role of IL-12 in thymocyte selection more directly by determining its role in two models of in vivo negative selection. In initial studies we demonstrated that abundant intrathymic IL-12 synthesis occurs during OVA peptide-induced negative selection of thymocytes in neonatal OVA-TCR transgenic mice, and such synthesis is associated with increased IL-12R beta2-chain expression as well as STAT4 intracellular signaling. In further studies, we showed that this form of negative selection was occurring at the alphabetaTCRlowCD4lowCD8low stage and was prevented by the coadministration of anti-IL-12. In addition, the IL-12-dependent thymocyte depletion was occurring through an intrathymic apoptosis mechanism, also prevented by administration of anti-IL-12. Finally, we showed that IL-12 p40-/- mice displayed aberrant negative selection of double positive CD4+CD8+ thymocytes when injected with anti-CD3 mAb. These studies suggest that intact intrathymic IL-12 production is necessary for the negative selection of thymocytes occurring in relation to a high "self" Ag load, possible through its ability to induce the thymocyte maturation and cytokine production necessary for such selection.     

Mitogen-induced IL-2 production and proliferation at defined stages of T helper cell development.

Th cell development inside the thymus can be defined on the basis of qualitative and quantitative CD4 and CD8 marker expression and follows the pathway of CD4-8- cells----CD4+8+ cells----CD4+8low cells----CD4+8- cells, which presumably emigrate to seed the periphery and serve as functionally mature Th cells. The various cell subpopulations at defined developmental stages were isolated by electronic cell sorting and examined for mitogen induced IL-2 production and cell proliferation responses. For TCR-alpha beta-bearing CD4+8+ and CD4+8low thymocytes that are actively engaged in positive and negative selection processes, negligible to low levels of IL-2 production and cell proliferation were observed in response to TCR:CD3 triggering or to the combined activation of protein kinase C and calcium mobilization mediated by PMA and ionomycin, respectively. For CD4-8- TCR-alpha beta early thymocytes that have not yet entered the selection process, PMA + ionomycin induced significant cell proliferation but little IL-2 production, in the absence of added IL-1. However, addition of IL-1 caused a powerful induction of IL-2 production that was accompanied by increased cell proliferation. Triggering of the TCR:CD3 complex had no effect on CD4-8-TCR(-)-alpha beta thymocytes as they do not express detectable levels of TCR-alpha beta. For thymus CD4+8- Th cells, the first cells that have completed TCR repertoire selection, vigorous proliferation was observed in response to TCR:CD3 triggering in the presence of added IL-2. However, the development of IL-2 responsiveness was not accompanied by high level IL-2 inducibility as TCR:CD3 triggering caused only marginal IL-2 production. In contrast, spleen CD4+8- T cells, the most "mature" representatives of Th cells, expressed high levels of IL-2 production as well as IL-2 responsiveness in response to TCR:CD3-mediated stimulation. The lack of anti-TCR-induced IL-2 production by thymus CD4+8- T cells was not due to an intrinsic defect as high levels of IL-2 production was induced by PMA + ionomycin. Possible reasons for the temporal acquisition and differential control of IL-2 inducibility and IL-2 responsiveness are discussed in the context of established Th cell development pathway.     

Thymocytes between the beta-selection and positive selection checkpoints are nonresponsive to IL-7 as assessed by STAT-5 phosphorylation

Interleukin-7 is widely accepted as a major homeostatic factor involved in T cell development. To assess the IL-7 responsiveness of thymocytes involved in selection processes, we used a new sensitive flow cytometry-based assay to detect intracellular phosphorylation of STAT-5 induced by IL-7 in defined mouse thymocyte subsets. Using this method, we found the earliest thymocyte subset (CD4(-)CD8(-)CD25(-)CD44(+)) to contain both IL-7-responsive and nonresponsive cells. Transition through the next stages of development (CD4(-)CD8(-)CD25(+)CD44(+ and -)) was associated with responsiveness of all thymocytes within these populations. Passage of thymocytes through beta-selection resulted in a significant reduction in IL-7 sensitivity. In the next phases of development (TCR(-) and TCR(low)CD69(-)), thymocytes were completely insensitive to the effects of IL-7. STAT-5 phosphorylation in response to IL-7 was again observed, however, in thymocytes involved in the positive selection process (TCR(low)CD69(+) and TCR(intermediate)). As expected, CD4 and CD8 single-positive thymocytes were responsive to IL-7. These findings delineate an IL-7-insensitive population between the beta-selection and positive selection checkpoints encompassing thymocytes predicted to die by neglect due to failure of positive selection. This pattern of sensitivity suggests a two-signal mechanism by which survival of thymocytes at these checkpoints is governed.     

Growth-promoting activity of IL-1 alpha, IL-6, and tumor necrosis factor-alpha in combination with IL-2, IL-4, or IL-7 on murine thymocytes. Differential effects on CD4/CD8 subsets and on CD3+/CD3- double-negative thymocytes

Many cytokines (including IL-1, IL-2, IL-4, IL-6, and TNF-alpha) have been shown to induce thymocyte proliferation in the presence of PHA. In this report, we demonstrate that certain cytokine combinations induce thymocyte proliferation in the absence of artificial comitogens. IL-1 alpha, IL-6, and TNF-alpha enhanced the proliferation of whole unseparated thymocytes in the presence of IL-2, whereas none of them induced thymocyte proliferation alone. In contrast, of these three enhancing cytokines, only IL-6 enhanced IL-4-induced proliferation. We also separated thymocytes into four groups based on their expression of CD4 and CD8, and investigated their responses to various cytokines. The results indicate that each cytokine combination affects different thymocyte subsets; thus, IL-1 alpha enhanced the proliferation of CD4-CD8- double negative (DN) thymocytes more efficiently than IL-6 in the presence of IL-2, whereas IL-6 enhanced the responses of CD4+CD8- and CD4-CD8+ single positive (SP) thymocytes to IL-2 or IL-4 better than IL-1 alpha. TNF-alpha enhanced the proliferation of both DN and both SP subsets in the presence of IL-2 and/or IL-7. None of these combinations induced the proliferation of CD4+CD8+ double positive thymocytes. Finally, DN were separated into CD3+ and CD3- populations and their responsiveness was investigated, because recent reports strongly suggest that CD3+ DN thymocytes are a mature subset of different lineage rather than precursors of SP thymocytes. CD3+ DN proliferated in response to IL-7, TNF-alpha + IL-2, and IL-1 + IL-2. CD3- DN did not respond to IL-7 or to IL-1 + IL-2, but did respond to TNF-alpha + IL-2. Finally, we detected TNF-alpha production by a cloned line of thymic macrophages, as well as by DN adult thymocytes. These results suggest that cytokines alone are capable of potent growth stimuli for thymocytes, and indicate that different combinations of these molecules act selectively on thymocytes at different developmental stages.     

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.     

Chagasic thymic atrophy does not affect negative selection but results in the export of activated CD4+CD8+ T cells in severe forms of human disease

Extrathymic CD4+CD8+ double-positive (DP) T cells are increased in some pathophysiological conditions, including infectious diseases. In the murine model of Chagas disease, it has been shown that the protozoan parasite Trypanosoma cruzi is able to target the thymus and induce alterations of the thymic microenvironment and the lymphoid compartment. In the acute phase, this results in a severe atrophy of the organ and early release of DP cells into the periphery. To date, the effect of the changes promoted by the parasite infection on thymic central tolerance has remained elusive. Herein we show that the intrathymic key elements that are necessary to promote the negative selection of thymocytes undergoing maturation during the thymopoiesis remains functional during the acute chagasic thymic atrophy. Intrathymic expression of the autoimmune regulator factor (Aire) and tissue-restricted antigen (TRA) genes is normal. In addition, the expression of the proapoptotic Bim protein in thymocytes was not changed, revealing that the parasite infection-induced thymus atrophy has no effect on these marker genes necessary to promote clonal deletion of T cells. In a chicken egg ovalbumin (OVA)-specific T-cell receptor (TCR) transgenic system, the administration of OVA peptide into infected mice with thymic atrophy promoted OVA-specific thymocyte apoptosis, further indicating normal negative selection process during the infection. Yet, although the intrathymic checkpoints necessary for thymic negative selection are present in the acute phase of Chagas disease, we found that the DP cells released into the periphery acquire an activated phenotype similar to what is described for activated effector or memory single-positive T cells. Most interestingly, we also demonstrate that increased percentages of peripheral blood subset of DP cells exhibiting an activated HLA-DR+ phenotype are associated with severe cardiac forms of human chronic Chagas disease. These cells may contribute to the immunopathological events seen in the Chagas disease.     

Stage specific phosphoinositides turnover capacity of human intrathymic T cells following CD2-triggering

Triggering of distinct CD2 epitopes on human T lymphocytes increases their phosphatidylinositol (PI) cycle-related metabolism. In this work, we investigated the relationship between this signal transduction pathway following surface CD2 antigen triggering and intrathymic T cell development. Therefore, various thymocyte subsets were incubated with co-mitogenic CD2I+III mAb. The cells were then tested for their various phosphoinositides levels as well as their ability to proliferate in response to recombinant interleukin-2 (rIL-2). Our results indicate that immature CD4- CD8- cells have high PI metabolism while more mature CD4+CD8+ and unfractionated thymocytes display significantly lower PI-turnover. Mature CD4+CD8- and CD4-CD8+ thymocytes regain this transduction capacity. Thus, PI-turnover following CD2- triggering is linked to the developmental fate of thymocyte subclasses.     

TCR signaling for initiation and completion of thymocyte positive selection has distinct requirements for ligand quality and presenting cell type

Thymocyte selection involves signaling by TCR engaging diverse self-peptide:MHC molecule ligands on various cell types in the cortex and medulla. Here we separately analyze early and late stages of selection to better understand how presenting cell type, ligand quality, and the timing of TCR signaling contribute to intrathymic differentiation. TCR transgenic CD4+CD8+ thymocytes (double positive (DP)) from MHC-deficient mice were stimulated using various presenting cells and ligands. The resulting CD69high cells were isolated and evaluated for maturation in reaggregate cultures with wild-type or MHC molecule-deficient thymic stroma with or without added hemopoietic dendritic cells (DC). Production of CD4+ T cells required TCR signaling in the reaggregates, indicating that transient recognition of self-ligands by DP is inadequate for full differentiation. DC bearing a potent agonist ligand could initiate positive selection, producing activated thymocytes that matured into agonist-responsive T cells in reaggregates lacking the same ligand. DC could also support the TCR signaling necessary for late maturation. These results argue that despite the negative role assigned to DC in past studies, neither the peptide:MHC molecule complexes present on DC nor any other signals provided by these cells stimulate only thymocyte death. These findings also indicate that unique epithelial ligands are not necessary for positive selection. They provide additional insight into the role of ligand quality in selection events and support the concept that following initiation of maturation from the DP state, persistent TCR signaling is characteristic of and perhaps required by T cells.     

CD1d-independent developmental acquisition of prompt IL-4 gene inducibility in thymus CD161(NK1)-CD44lowCD4+CD8- T cells is associated with complementarity determining region 3-diverse and biased Vbeta2/Vbeta7/Vbeta8/Valpha3.2 T cell receptor usage

Among Ag-inexperienced naive T cells, the CD1d-restricted NKT cell that uses invariant TCR-alpha-chain is the most widely studied cell capable of prompt IL-4 inducibility. We show in this study that thymus CD161-CD44lowCD4+CD8- T cells promptly produce IL-4 upon TCR stimulation, a response that displays biased Vbeta(2/7/8) and Valpha3.2 TCR usage. The association of Vbeta family bias and IL-4 inducibility in thymus CD161-CD44lowCD4+CD8- T cells is found for B6, B10, BALB/c, CBA, B10.A(4R), and ICR mouse strains. Despite reduced IL-4 inducibility, there is a similarly biased Vbeta(2/7/8) TCR usage by IL-4 inducibility+ spleen CD161-CD44lowCD4+CD8- T cells. Removal of alpha-galacotosylceramide/CD1d-binding cells from CD161-CD44lowCD4+CD8- thymocytes does not significantly affect their IL-4 inducibility. The development of thymus CD161-CD44lowCD4+CD8- T cells endowed with IL-4 inducibility and their associated use of Vbeta(2/7/8) are beta2-microglobulin-, CD1d-, and p59fyn-independent. Thymus CD161-CD44lowCD4+CD8- T cells produce low and no IFN-gamma inducibility in response to TCR stimulation and to IL-12 + IL-18, respectively, and they express diverse complementarity determining region 3 sequences for both TCR-alpha- and -beta-chains. Taken together, these results demonstrate the existence of a NKT cell distinct, TCR-repertoire diverse naive CD4+ T cell subset capable of prompt IL-4 inducibility. This subset has the potential to participate in immune response to a relatively large number of Ags. The more prevalent nature of this unique T cell subset in the thymus than the periphery implies roles it might play in intrathymic T cell development and may provide a framework upon which mechanisms of developmentally regulated IL-4 gene inducibility can be studied.     

Soluble factor-mediated differentiation of CD4+CD8+ thymocytes to single positives in vitro

Although the thymic microenvironment provides the necessary elements for T-cell differentiation, the precise role of individual components remains to be determined. In this paper, attempts were made to address the possibility that CD4 or CD8 single-positive (SP) thymocytes could be developed from immature CD4+CD8+ (double-positive; DP) thymocytes in a suspension culture in the presence of soluble factors. We observed that IL-4 and IFN-gamma weakly induced DP cells to differentiate to CD4 cells, but not to CD8. In contrast, IL-2 weakly induced differentiation to CD8. Interestingly, Con A sup strongly induced differentiation to CD8 SP from the purified DP thymocytes prepared from C57BL/6 or LCMV TCRtg mice. In particular, it was found that thymocyte culture with Con A sup generated CD69+DP cells, and the CD69+DP differentiated to CD8 SP under the suspension culture with soluble factors. Thus, Con A sup or combinations of IL-2, IL-4 and IL-7 strongly induced differentiation of CD69+DP to CD8 SP, whereas individual cytokines did not. These results suggest that soluble factors like cytokines play an important role in the generation of SP thymocytes in the absence of thymic stromal cells, at least from a distinctive subpopulation like CD69+DP thymocytes, and perhaps from those of broader range when in conjunction with TCR/MHC interaction.     

Functional maturation of mouse CD4~+CD8~- thymocytes induced by medullary-type thymus epithelial cells

Murine CD4+CD8- (CD4SP) thymocyte subset is a heterogeneous population, in which the Qa-2- cells are less functional, whereas the Qa-2+ cells are fully functional. Evidence is provided here that the transition from Qa-2- to Qa-2+ CD4SP thymocytes is an intrathymic process of differentiation induced by thymic medullary-type epithelial cells. The separated Qa-2-CD4SP could be induced to express Qa-2 molecules up to 84%- 89% of the total viable celb after cocultured for 3d with MTEC1 cells, a murine thymic medullary type epithelial cell line established in our laboratory. Kinetic study showed that both the percentage of Qa-2+ cells and the density of the expressed Qa-2 molecules on CD4SP thymocytes induced by MTEC1 were progressively increasing in 72-h cultures. The MTECl-induced Qa-2+CD4SP thymocytes were fully functional, which exhibited capabilities of proliferation and cytokine secretion in response to Con A stimulation as high as those of freshly isolated Qa-2+CD4SP thymocytes. The profile of cytokine     

Preferential expression of fibronectin receptors on immature thymocytes

Fibronectin-adherent (FNR+) thymocytes are enriched for immature (CD4-8-) and large (CD4+8+) cells, and depleted of mature (CD4-8+ and CD4+8-) and nonmature small (CD4+8+) cells. Among purified CD4-8- thymocytes, cells with the surface marker J11d and the IL-2 receptor, which can give rise to all other thymocyte subsets, showed selective attachment to fibronectin. Analysis of FNR+ thymocytes showed that such cells are greatly enriched for cells in cycle. Additionally, FNR+ cells expressed low levels of T cell receptor. These results suggest a role for the fibronectin receptor during the early, proliferative phase of thymocyte differentiation. The data suggest that loss of the fibronectin receptor is a hallmark of cells that have become committed either to functional maturation or to programmed cell death.     

Cytokine production by mature and immature thymocytes.

We have studied the ability of subpopulations of activated thymocytes to produce four cytokines (IL-2, IL-4, IFN-gamma and TNF-alpha) which are believed to play roles in T cell development. Supernatants from various thymocyte subsets activated with calcium ionophore and PMA were tested for these cytokines. All CD3hi thymocyte subsets (CD4+8-, CD4-8- and CD4-8+) produced high titers of these four cytokines except CD3+4-8+ thymocytes, which did not produce IL-4. In contrast, CD4+8+ thymocytes did not produce any detectable cytokines. CD3-4-8- thymocytes produced IL-2, IFN-gamma, and TNF-alpha (but not IL-4) when activated by calcium ionophore + PMA and IL-1. We then separated CD3-4-8- thymocytes into IL-2R+ and IL-2R-. CD3-4-8-IL-2R+ thymocytes only produced small amounts of IL-2 when activated with calcium ionophore + PMA + IL-1, whereas CD3-4-8-IL-2R- thymocytes did not require IL-1 to produce IL-2, IFN-gamma, and TNF-alpha. Finally, CD4-8+3- thymocytes (an immature population believed to be an intermediate between CD3-4-8- and CD4+8+ thymocytes) only produced marginally detectable levels of IL-2 upon stimulation with calcium ionophore, PMA, and the addition of IL-1 did not result in increased levels of cytokine production. These observations indicate discrete patterns of cytokine production by the subsets studied and suggest specific controls of cytokine gene expression during T cell development.     

Thymus and autoimmunity: production of CD25+CD4+ naturally anergic and suppressive T cells as a key function of the thymus in maintaining immunologic self-tolerance

This study shows that the normal thymus produces immunoregulatory CD25+4+8- thymocytes capable of controlling self-reactive T cells. Transfer of thymocyte suspensions depleted of CD25+4+8- thymocytes, which constitute approximately 5% of steroid-resistant mature CD4+8- thymocytes in normal naive mice, produces various autoimmune diseases in syngeneic athymic nude mice. These CD25+4+8- thymocytes are nonproliferative (anergic) to TCR stimulation in vitro, but potently suppress the proliferation of other CD4+8- or CD4-8+ thymocytes; breakage of their anergic state in vitro by high doses of IL-2 or anti-CD28 Ab simultaneously abrogates their suppressive activity; and transfer of such suppression-abrogated thymocyte suspensions produces autoimmune disease in nude mice. These immunoregulatory CD25+4+8- thymocytes/T cells are functionally distinct from activated CD25+4+ T cells derived from CD25-4+ thymocytes/T cells in that the latter scarcely exhibits suppressive activity in vitro, although both CD25+4+ populations express a similar profile of cell surface markers. Furthermore, the CD25+4+8- thymocytes appear to acquire their anergic and suppressive property through the thymic selection process, since TCR transgenic mice develop similar anergic/suppressive CD25+4+8- thymocytes and CD25+4+ T cells that predominantly express TCRs utilizing endogenous alpha-chains, but RAG-2-deficient TCR transgenic mice do not. These results taken together indicate that anergic/suppressive CD25+4+8- thymocytes and peripheral T cells in normal naive mice may constitute a common T cell lineage functionally and developmentally distinct from other T cells, and that production of this unique immunoregulatory T cell population can be another key function of the thymus in maintaining immunologic self-tolerance.     

Quantitative and functional expression of somatostatin receptor subtypes in human thymocytes

We recently demonstrated the expression of somatostatin (SS) and SS receptor (SSR) subtype 1 (sst1), sst2A, and sst3 in normal human thymic tissue and of sst1 and sst2A on isolated thymic epithelial cells (TEC). We also found an inhibitory effect of SS and octreotide on TEC proliferation. In the present study, we further investigated the presence and function of SSR in freshly purified human thymocytes at various stages of development. Thymocytes represent a heterogeneous population of lymphoid cells displaying different levels of maturation and characterized by specific cell surface markers. In this study, we first demonstrated specific high-affinity 125I-Tyr(11)-labeled SS-14 binding on thymocyte membrane homogenates. Subsequently, by RT-PCR, sst2A and sst3 mRNA expression was detected in the whole thymocyte population. After separation of thymocytes into subpopulations, we found by quantitative RT-PCR that sst2A and sst3 are differentially expressed in intermediate/mature and immature thymocytes. The expression of sst3 mRNA was higher in the intermediate/mature CD3+ fraction compared with the immature CD2+CD3- one, whereas sst2A mRNA was less abundant in the intermediate/mature CD3+ thymocytes. In 7-day-cultured thymocytes, SSR subtype mRNA expression was lost. SS-14 significantly inhibited [3H]thymidine incorporation in all thymocyte cultures, indicating the presence of functional receptors. Conversely, octreotide significantly inhibited [3H]thymidine incorporation only in the cultures of immature CD2+CD3- thymocytes. Subtype sst3 is expressed mainly on the intermediate/mature thymocyte fraction, and most of these cells generally die by apoptosis. Because SS-14, but not octreotide, induced a significant increase in the percentage of apoptotic thymocytes, it might be that sst3 is involved in this process. Moreover, sst3 has recently been demonstrated on peripheral human T lymphocytes, which derive directly from mature thymocytes, and SS analogs may induce apoptosis in these cells. Interestingly, CD14+ thymic cells, which are cells belonging to the monocyte-macrophage lineage, selectively expressed sst2A mRNA. Finally, SSR expression in human thymocytes seems to follow a developmental pathway. The heterogeneous expression of SSR within the human thymus on specific cell subsets and the endogenous production of SS as well as SS-like peptides emphasize their role in the bidirectional interactions between the main cell components of the thymus involved in intrathymic T cell maturation.