Role of the common cytokine receptor gamma chain (gammac) in thymocyte selection

Morphologic and Immunohistochemical Characterization of Spontaneous Lymphoma/Leukemia in NSG Mice

The NOD.Cg-Prkdc^scid Il2rg^tm1Wjl/SzJ strain (NOD scid gamma, NSG) is a severely immunodeficient inbred laboratory mouse used for preclinical studies because it is amenable to engraftment with human cells. Combining scid and Il2rg^null mutations results in severe immunodeficiency by impairing the maturation, survival, and functionality of interleukin 2-dependent immune cells, including T, B, and natural killer lymphocytes. While NSG mice are reportedly resistant to developing spontaneous lymphomas/leukemias, there are reports of hematopoietic cancers developing. In this study, we characterized the immunophenotype of spontaneous lymphoma/leukemia in 12 NSG mice (20 to 38 weeks old). The mice had a combination of grossly enlarged thymus, spleen, or lymph nodes and variable histologic involvement of the bone marrow and other tissues. All 12 lymphomas were diffusely CD3, TDT, and CD4 positive, and 11 of 12 were also positive for CD8, which together was consistent with precursor T-cell lymphoblastic lymphoma/leukemia (pre-T-LBL). A subset of NSG tissues from all mice and neoplastic lymphocytes from 8 of 12 cases had strong immunoreactivity for retroviral p30 core protein, suggesting an association with a viral infection. These data highlight that NSG mice may develop T-cell lymphoma at low frequency, necessitating the recognition of this spontaneously arising disease when interpreting studies.

The common γ-chain cytokine receptor: tricks-and-treats for T cells

Originally identified as the third subunit of the high-affinity IL-2 receptor complex, the common γ-chain (γc) also acts as a non-redundant receptor subunit for a series of other cytokines, collectively known as γc family cytokines. γc plays essential roles in T cell development and differentiation, so that understanding the molecular basis of its signaling and regulation is a critical issue in T cell immunology. Unlike most other cytokine receptors, γc is thought to be constitutively expressed and limited in its function to the assembly of high-affinity cytokine receptors. Surprisingly, recent studies reported a series of findings that unseat γc as a simple housekeeping gene, and unveiled γc as a new regulatory molecule in T cell activation and differentiation. Cytokine-independent binding of γc to other cytokine receptor subunits suggested a pre-association model of γc with proprietary cytokine receptors. Also, identification of a γc splice isoform revealed expression of soluble γc proteins (sγc). sγc directly interacted with surface IL-2Rβ to suppress IL-2 signaling and to promote pro-inflammatory Th17 cell differentiation. As a result, endogenously produced sγc exacerbated autoimmune inflammatory disease, while the removal of endogenous sγc significantly ameliorated disease outcome. These data provide new insights into the role of both membrane and soluble γc in cytokine signaling, and open new venues to interfere and modulate γc signaling during immune activation. These unexpected discoveries further underscore the perspective that γc biology remains largely uncharted territory that invites further exploration.

Bacterial clearance reverses a skewed T-cell repertoire induced by Salmonella infection

Salmonella typhimurium invades the spleen, liver, and peripheral lymph nodes and has recently been detected in the bone marrow and thymus, resulting in a reduced thymic size and a decline in the total number of thymic cells. A specific deletion of the double-positive cell subset has been characterized, yet the export of mature T cells to the periphery remains normal. We analyzed Salmonella pathogenesis regarding thymic structure and the T-cell maturation process. We demonstrate that, despite alterations in the thymic structure, T-cell development is maintained during Salmonella infection, allowing the selection of single-positive T-cell clones expressing particular T-cell receptor beta chains (TCR-Vβ). Moreover, the treatment of infected mice with an antibiotic restored the normal thymic architecture and thymocyte subset distribution. Additionally, the frequency of TCR-Vβ usage after treatment was comparable to that in non-infected mice. However, bacteria were still recovered from the thymus after 1 month of treatment. Our data reveal that a skewed T-cell developmental process is present in the Salmonella-infected thymus that alters the TCR-Vβ usage frequency. Likewise, the post-treatment persistence of Salmonella reveals a novel function of the thymus as a potential reservoir for this infectious agent.

Regulation of mu opioid receptor expression in developing T cells

We have previously reported that functionally active μ-opioid receptors (MOR) are constitutively expressed at relatively low levels by developing T cells in the thymus. However, very little is known about the regulation of MOR expression by immature T cells. In this report, we first attempted to determine the effect of T cell receptor-induced T cell activation on the expression of MOR. We activated T cells with either the combination of anti-CD3 and CD28, or with superantigen, and observed a substantial increase in MOR transcript expression. We also chose to examine the effect of cytokine-mediated T cell activation on the expression of this opioid receptor. We selected certain cytokines that play a role in T cell development and are known to be present at functional levels in the thymus gland. Our results show that interferon γ (IFNγ), IL-1β, and IL-2, and in particular transforming growth factor-β (TGFβ), all induced significant increases in MOR transcript expression. On the other hand, both TNFα and IL-7 exhibited much weaker effects on MOR expression. These results show that MOR expression by developing T cells is strongly regulated by several cytokines involved in T cell development in the thymus gland.

Ten-color flow cytometry reveals distinct patterns of expression of CD124 and CD126 by developing thymocytes

Background

We have developed a 12-parameter/10-color flow cytometric staining method for the simultaneous detection and characterization of 21 mouse thymocyte subpopulations that represent discreet stages of T cell development. To demonstrate the utility of this method, we assessed cytokine receptor expression on mouse thymocyte subsets. These experiments revealed distinct patterns of surface expression of receptors for the cytokines IL-4 and IL-6.

Results

The IL-4 receptor α chain (CD124) was highly expressed on the earliest thymocyte subsets, then downregulated prior to T cell receptor β-selection and finally upregulated in the CD4/CD8 double positive cells prior to positive selection. The IL-6 receptor α chain (CD126) showed a different pattern of expression. It was expressed on the most mature subsets within the CD4 and CD8 single positive (SP) compartments and was absent on all other thymocytes with the exception of a very small cKit^-CD4^-CD8^- population. Intracellular staining of SP thymocytes for phosphorylated STAT-1 demonstrated that IL-6 signaling was confined to the most mature SP subsets.

Conclusions

This 12-parameter staining methodology uses only commercially available fluorochrome-coupled monoclonal antibodies and therefore could be employed by any investigator with access to a 4-laser flow cytometer. This novel staining scheme allowed us to easily phenotype thymocyte subpopulations that span across development, from the early thymic progenitors (ETPs) to the most mature subsets of the CD4 and CD8 single positive populations.

Expansion of human cytomegalovirus-specific T lymphocytes from unfractionated peripheral blood mononuclear cells with artificial antigen-presenting cells

BACKGROUND

The aim of this study was to find a simple and feasible method for ex vivo expansion of human cytomegalovirus (CMV)-specific cytotoxic T cells from unfractionated peripheral blood mononuclear cells (PBMNCs).

STUDY DESIGN AND METHODS

Unfractionated PBMNCs from three HLA-A*0201-CMV-seropositive donors were stimulated with CMVpp65(495-503) peptide-loaded HLA-A*0201-immunoglobulin fusion protein (HLA-A2-Ig) based artificial antigen-presenting cells (aAPCs) on Day 1. Once a week the CMV-specific T cells were harvested and restimulated with fresh aAPCs. T-cell cultures were maintained for 28 days and then analyzed.

RESULTS

With aAPCs and starting with 1x10(7) freshly isolated PBMNCs that were less than 0.1 percent CMV-specific, more than 1x10(7) T cells with a CMV-specific frequency greater than 93 percent in all donors tested were generated. Expanded CD8+ cytotoxic T lymphocytes were functionally active and showed antigen-specific secretion of interferon-gamma and cytotoxic activity. No alloreactivity against unpulsed HLA-A*0201-positive cells was detected.

CONCLUSION

Herein is reported the successful in vitro expansion of CMV-specific cytotoxic CD8+ T cells from unfractionated PBMNCs of healthy CMV-seropositive blood donors by the use of HLA-A2-Ig-based aAPCs. This study demonstrates that more than 1x10(7) CMV-specific T cells can be generated from approximately 1x10(7) unfractionated PBMNCs within 1 month under highly reproducible conditions.

Reduced CD4+ T-cell-specific gene expression in human type 1 diabetes mellitus

Type 1 diabetes mellitus (T1DM) in humans is characterized by the T-cell-dependent destruction of the insulin producing pancreatic beta cells; however, the precise pathogenesis of the disease, especially the initiation of pathologic immune response, is still largely unknown. We hypothesized that the function of human CD4+ T cells is altered in T1DM and analyzed unstimulated human peripheral blood CD4+ T-cell gene expression. We used a novel three-way comparison of DNA microarray data of CD4+ T cells isolated from patients with new onset T1DM, patients with long-term Type 2 diabetes (T2DM), and from healthy control subjects in order to eliminate any possible influence of glucose homeostasis on our findings. We analyzed the T1DM specific gene-expression changes and their functional relevance to T1DM autoimmunity. Our genetic and functional data show that T1DM CD4+ T cells are down-regulated specifically affecting key immune functions and cell cycle. Histone deacetylase gene expression, a key regulator of epigenetic modification is also reduced. The CD4+ T cells showed impaired function, including an abnormal immune response, which may be a key element that leads to the breakdown of self-tolerance.

Cooperation between CD4+ and CD8+ T cells: when, where, and how

Concepts of cell-cell interactions in adaptive immunity have alternated between the simple and the complex. The notion that one population of small, circulating lymphocytes is responsible for adaptive immunity was sequentially supplanted by the concept of separate T and B lymphocyte populations that cooperate to produce IgG antibody responses, by a three-cell model in which a myeloid APC initiates these cooperative lymphoid responses, by the recognition of T cell subsets, and by the idea that CD8+ T cell subset responses to graft antigens depend on CD4+ T cell subset activity. Simplicity was reintroduced with the revelation that CD8+ T cells can act independently of CD4+ T cells against acute viral infections. The pendulum has swung again toward complexity with recognition of the distinct and conjoint contributions of innate stimuli, APCs, NK and NKT cells, Tregs, and CD4+ helper T cells to CD8+ T cell behavior during acute and chronic infections or as memory cells. The renewed appreciation that multiple, sometimes rare cell types must communicate during cell-mediated immune responses has led to questions about how such interactions are orchestrated within organized lymphoid tissues. We review recent advances in deciphering the specific contribution of CD4+ T cells to physiologically useful CD8+ T cell responses, the signals involved in producing acute effectors versus long-lived memory cells, and the mechanisms underlying the cell-cell associations involved in delivery of such signals. We propose a model based on these new findings that may serve as a general paradigm for cellular interactions that occur in an inflamed lymph node during the initiation of immune responses.

Differential Effects of IL-15 and IL-21 in Myeloid (CD11b+) and Lymphoid (CD11b−) Bone Marrow Cells

IL-15 has been found to activate NF-kappaB in various types of cells. However, the role of this transcription factor in IL-15- and IL-21-stimulated murine bone marrow (BM) cells is unclear. In this study, we demonstrated that both IL-15 and IL-21 are capable of delaying BM cell factor deprivation-induced apoptosis, but only IL-15 induced their proliferation. Following separation of BM cells into myeloid (CD11b(+)) and lymphoid (CD11b(-)) cell populations, we found that IL-15, but not IL-21, significantly induced proliferation in both cell populations. Both cytokines significantly delayed apoptosis, but only in CD11b(-) BM cells. IL-15Ralpha, CD122 (IL-2/15Rbeta), and common gamma-chains (CD132) were expressed in both populations, whereas IL-21Ralpha was expressed only in CD11b(-) BM cells. In addition, we demonstrated that IL-15-induced BM cell proliferation was significantly inhibited in NF-kappaBp50(-/-) mice when compared with littermate controls. The ability of IL-15 and IL-21 to delay BM cell apoptosis was slightly inhibited in NF-kappaBp50(-/-) mice, whereas the antiapoptotic effect of LPS was markedly reversed. We conclude that IL-15, but not IL-21, induces BM cell proliferation and that both cytokines delay BM cell apoptosis. These biological activities were preferentially observed in CD11b(-) BM cells. Using NF-kappaBp50(-/-) mice, we demonstrated for the first time that NF-kappaB plays a greater role in IL-15-induced cell proliferation than in IL-15- and IL-21-induced suppression of apoptosis.

Diverse functions of IL-2, IL-15, and IL-7 in lymphoid homeostasis

IL-2, IL-15, and IL-7 are cytokines that are critical for regulating lymphoid homeostasis. These cytokines stimulate similar responses from lymphocytes in vitro, but play markedly divergent roles in lymphoid biology in vivo. Their distinct physiological functions can be ascribed to distinct signaling pathways initiated by proprietary cytokine receptor chains, differential expression patterns of the cytokines or their receptor chains, and/or signals occurring in distinct physiological contexts. Recently, the discovery of a novel mechanism of cytokine signaling, trans-presentation, has provided further insights into the different ways these cytokines function. Trans-presentation also raises several novel cell biological and cellular implications concerning how cytokines support lymphoid homeostasis.

Global analysis of IL-2 target genes: identification of chromosomal clusters of expressed genes

T lymphocytes play a central role in controlling adaptive immune responses. IL-2 critically regulates both T cell growth and death and is involved in maintaining peripheral tolerance, but the molecules involved in these and other IL-2 actions are only partially known. We now provide a comprehensive compendium of the genes expressed in T cells and of those regulated by IL-2 based on a combination of DNA microarrays and serial analysis of gene expression (SAGE). The newly identified IL-2 target genes include many genes previously linked to apoptosis in other cellular systems that may contribute to IL-2-dependent survival functions. We also studied the mRNA expression of known regulators of signaling pathways for their induction in response to IL-2 in order to identify potential novel positive and/or negative feedback regulators of IL-2 signaling. We show that IL-2 regulates only a limited number of these genes. These include suppressors of cytokine signaling (SOCS) 1, SOCS2, dual-specificity phosphatase (DUSP) 5, DUSP6 and non-receptor type phosphatase-7 (PTPN7). Additionally, we provide evidence that many genes expressed in T cells locate in chromosomal clusters, and that select IL-2-regulated genes are located in at least two clusters, one at 5q31, a known cytokine gene cluster, and the other at 6p21.3, a region that contains genes encoding the tumor necrosis factor (TNF) superfamily members TNF, LT-alpha and LT-beta.

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.

Antigenic profile of human thymus in concurrence with "Clusters of Thymic Epithelial Staining" classification

We examined the expression of various CD coded or not yet defined antigens in human thymus samples using indirect immunoperoxidase and immunoflourescent techniques. Data obtained are presented in concurrence with Clusters of Thymic Epithelial Staining (CTES) classification for various monoclonal antibodies recognizing CD antigens (CD1, CD1a, CD6, CD9, CD14, CD16, CD29, CD30, CD32, CD44, CD45RB, CD47, CD48, CD49a, CD49b, CD49c, CD49d, CD49e, CD49f, CD51, CD53, CD54, CD56, CD57, CD63, CD85, CD95, CD98, CD102, CD103, CD106, CD109, CD146, CD147, CD148, CD151, CD152, CD158a, CD158b, CD164, CD165, CD166) and for monoclonal antibodies 1B10, 5G7, A4, BD46, BLTZ, HP1C5, IND.64, M72, WU947 whose specifities are not yet defined. Some of the mAbs such as CD49f, IND.64 and BD46 are detected as good markers for specific cell types or compartments. Significance of the presence of these antigens on thymic epithelial cells at certain locations is briefly discussed.

Analysis of gamma c-family cytokine target genes. Identification of dual-specificity phosphatase 5 (DUSP5) as a regulator of mitogen-activated protein kinase activity in interleukin-2 signaling

Interleukin (IL)-2, IL-4, IL-7, IL-9, IL-15, and IL-21 form a family of cytokines based on their sharing the common cytokine receptor gamma chain, gamma(c), which is mutated in X-linked severe combined immunodeficiency (SCID). As a step toward further elucidating the mechanism of action of these cytokines in T-cell biology, we compared the gene expression profiles of IL-2, IL-4, IL-7, and IL-15 in T cells using cDNA microarrays. IL-2, IL-7, and IL-15 each induced a highly similar set of genes, whereas IL-4 induced distinct genes correlating with differential STAT protein activation by this cytokine. One gene induced by IL-2, IL-7, and IL-15 but not IL-4 was dual-specificity phosphatase 5 (DUSP5). In IL-2-dependent CTLL-2 cells, we show that IL-2-induced ERK-1/2 activity was inhibited by wild type DUSP5 but markedly increased by an inactive form of DUSP5, suggesting a negative feedback role for DUSP5 in IL-2 signaling. Our findings provide insights into the shared versus distinctive actions by different members of the gamma(c) family of cytokines. Moreover, we have identified a DUSP5-dependent negative regulatory pathway for MAPK activity in T cells.

Homeostatic control of lymphocyte survival: potential origins and implications

Lymphocytes depend on extracellular ligands to maintain their viability. Structurally diverse lymphocyte receptors transmit survival signals through separate signal transduction cascades, which all share the ability to sustain viability by maintaining the sequestration of apoptogenic factors within mitochondria. Receptors can induce cellular survival either by promoting the expression and/or function of anti-apoptotic Bcl-2 family proteins or by activating the phosphatidylinositol-3 kinase Akt pathway. Either of these events represses the function of the pro-apoptotic proteins Bax and Bak, which are required for mitochondrial release of cytochrome c. As we discuss here, the apparently redundant functions of Bax and Bak may have evolved to prevent lymphocyte mitochondria from adapting to loss of receptor mediated signal transduction and thus keep lymphocytes from accumulating in a cell-autonomous manner.

Genesis of progressive T-cell deficiency owing to a single missense mutation in the common gamma chain gene

Patients with a moderate X-linked combined immunodeficiency (XCID) owing to a single missense mutation in the common gamma chain (gammac) gene (L-->Q271) were found to have a progressive T-cell deficiency. Blood T cells from four older subjects with XCIDL-->Q271 were studied to ascertain the basis of that progression. Few CD4+ T cells displayed the phenotype (CD45RA+ CD62L+) or deletion circles from T-cell receptor (TCR) Vbeta-gene rearrangements found in recent thymic emigrants. These deficiencies were more severe in older males with XCIDL-->Q271. Relative frequencies of fresh CD4+ and CD8+ T cells that bound annexin V, an early indicator of programmed cell death, or propidium iodide, an indicator of cell necrosis, were greater in XCIDL-->Q271 T cells than in normal fresh T cells. The binding of annexin V and propidium iodide to XCIDL-Q271 T cells increased marginally after stimulation with anti-CD3, but binding by fresh or stimulated XCIDL-Q271 T cells exceeded that found in normal stimulated T cells. Also, telomeres from XCIDL-->Q271 CD4+ T cells were shortened in these patients compared to normal young adults. It therefore appears that the thymus is dysfunctional and that mature T cells are not effectively rescued from apoptosis or replication senescence via gamma-mediated pathways in XCIDL-->Q271.

Signaling domains of the interleukin 2 receptor

Interleukin (IL-)2 and its receptor (IL-2R) constitute one of the most extensively studied cytokine receptor systems. IL-2 is produced primarily by activated T cells and is involved in early T cell activation as well as in maintaining homeostatic immune responses that prevent autoimmunity. This review focuses on molecular signaling pathways triggered by the IL-2/IL-2R complex, with an emphasis on how the IL-2R physically translates its interaction with IL-2 into a coherent biological outcome. The IL-2R is composed of three subunits, IL-2Ralpha, IL-2Rbeta and gammac. Although IL-2Ralpha is an important affinity modulator that is essential for proper responses in vivo, it does not contribute to signaling due a short cytoplasmic tail. In contrast, IL-2Rbeta and gammac together are necessary and sufficient for effective signal transduction, and they serve physically to connect the receptor complex to cytoplasmic signaling intermediates. Despite an absolute requirement for gammac in signaling, the majority of known pathways physically link to the receptor via IL-2Rbeta, generally through phosphorylated cytoplasmic tyrosine residues. This review highlights work performed both in cultured cells and in vivo that defines the functional contributions of specific receptor subdomains-and, by inference, the specific signaling pathways that they activate-to IL-2-dependent biological activities.

A requirement for IL-2/IL-2 receptor signaling in intrathymic negative selection

The nature of the signals that influence thymocyte selection and determine the fate of CD4(+)8(+) (double positive) thymocytes remains unclear. Cytokines produced locally in the thymus may modulate signals delivered by TCR-MHC/peptide interactions and thereby influence the fate of double-positive thymocytes. Because the IL-2/IL-2R signaling pathway has been implicated in thymocyte and peripheral T cell survival, we investigated the possibility that IL-2/IL-2R interactions contribute to the deletion of self-reactive, Ag-specific thymocytes. By using nontransgenic and transgenic IL-2-sufficient and -deficient animal model systems, we have shown that during TCR-mediated thymocyte apoptosis, IL-2 protein is expressed in situ in the thymus, and apoptotic thymocytes up-regulate expression of IL-2RS: IL-2R(+) double-positive and CD4 single-positive thymocytes undergoing activation-induced cell death bind and internalize IL-2. IL-2-deficient thymocytes are resistant to TCR/CD3-mediated apoptotic death, which is overcome by providing exogenous IL-2 to IL-2(-/-) mice. Furthermore, disruption or blockade of IL-2/IL-2R interactions in vivo during Ag-mediated selection rescues some MHC class II-restricted thymocytes from apoptosis. Collectively, these findings provide evidence for the direct involvement of the IL-2/IL-2R signaling pathway in the deletion of Ag-specific thymocyte populations and suggest that CD4 T cell hyperplasia and autoimmunity in IL-2(-/-) mice is a consequence of ineffective deletion of self-reactive T cells.

Cloning and expression of a putative common cytokine receptor gamma chain (gammaC) gene in rainbow trout (Oncorhynchus mykiss)

A full length cDNA of a putative common cytokine receptor gamma chain (gammaC) gene of rainbow trout (Oncorhynchus mykiss) has been cloned and sequenced. The contiguous cDNA contained 2291 nucleotides, consisting of an ORF of 1029 bp, with a 72 bp 5' UTR and a 1190 bp 3' UTR. The coding region showed 44-46% identity to mammalian gammaC genes. The ORF translated into a 343 amino acid protein, with some 28-30% amino acid identity to the coding region of mammalian sequences. A predicted signal peptide and transmembrane domain were identified, giving a 206 amino acid extracellular domain and a 98 amino acid intracellular domain in the trout molecule. Five potential glycosylation sites were present in the extracellular domain, as were six conserved cysteine residues and the W-S-X-W-S motif typical of haemopoietin receptors. One of the most interesting differences between the trout and mammalian sequences was the lack of tyrosines in the trout intracellular domain. RT-PCR studies revealed a wide tissue distribution of gammaC expression, with detectable transcript in blood, spleen, gill, kidney, brain and liver. Low levels of gammaC transcript were detectable in unstimulated macrophage cultures and expression was increased by stimulation of the cells with recombinant trout interleukin-1beta (IL-1beta) or LPS. Similarly, in the RTG cell line which exhibited even lower level constitutive expression, stimulation with IL-1beta increased gammaC transcript levels but LPS had no effect.

Thymic and intestinal intraepithelial T lymphocyte development are each regulated by the gammac-dependent cytokines IL-2, IL-7, and IL-15

Both thymic and extrathymic T lineage development are characterized by cytokine-dependent regulation of complex proliferative, differentiative, and anti-apoptotic processes. The role of the gammac-dependent cytokines in this program has been interpreted as limited to the activity of IL-7. However, through the analysis of double knock-out mice, which lack signaling through the IL-7R and other gammac-dependent cytokines, we revealed a role for IL-15 in the production of early thymic pro-T cells. Although IL-2 does not function in the production of thymocytes, thymic restoration of IL-2R expression prevented fatal autoimmunity associated with IL-2- or IL-2R-deficient mice, suggesting that IL-2R functions non-redundantly at the level of the thymus to regulate self-reactivity. Moreover, IL-2, IL-7, and IL-15 also extend their developmental effects beyond the thymus to other sites of T lymphocyte production, including the gut. Here, their redundant and non-redundant activities are directly correlated to the development of phenotypically diverse subsets of intestinal intraepithelial lymphocytes.