Preassociation of IL-15 with IL-15R alpha-IgG1-Fc enhances its activity on proliferation of NK and CD8+/CD44high T cells and its antitumor action

In the induction of an immune response, IL-15Ralpha on APCs transpresents IL-15 to NK and CD8(+)/CD44(high) T cells that express the IL-2/15Rbeta and gammac subunits only. In this study, we show data mimicking this transpresentation by using IL-15 preassociated with a chimeric protein that is comprised of the extracellular domain of murine IL-15Ralpha and the Fc portion of human IgG1. When tested in vitro, IL-15Ralpha-IgG1-Fc strongly increased the IL-15-mediated proliferation of murine NK and CD8(+)/CD44(high) T cells. The effect of IL-15Ralpha-IgG1-Fc was dependent on the presence of both IgG1-Fc and IL-15Ralpha. When injected into mice, IL-15Ralpha-IgG1-Fc enhanced the capacity of IL-15 to expand the number of NK and CD8(+)/CD44(high) T cells. The effect on cell numbers in vivo also depended on Fc receptor binding because reduced expansion was observed in FcRgamma(-/-) mice. NK cells cultured in IL-15/IL-15Ralpha-IgG1-Fc complex gained cytotoxic activity toward a number of NK-sensitive targets. When mice bearing the NK-sensitive syngeneic tumor B16 were treated, the presence of IL-15Ralpha-IgG1-Fc increased the antitumor activity of IL-15. Thus, a preassociation with IL-15Ralpha-IgG1-Fc enhances the activities of IL-15 in vivo and in vitro that may be useful in the treatment of tumors.     

Evidence of a novel IL-2/15R beta-targeted cytokine involved in homeostatic proliferation of memory CD8+ T cells

The homeostasis of memory CD8+ T cells is regulated by cytokines. IL-15 is shown to promote the proliferation of memory CD8+ T cells, while IL-2 suppresses their division in vivo. This inhibitory effect of IL-2 appears to occur indirectly, through other cell populations including CD25+CD4+ T cells; however, the details of this mechanism remain unclear. In this study, we show that 1) both Ag-experienced and memory phenotype CD8+ T cells divided after the depletion of IL-2 in vivo; 2) this division occurred normally and CD44(high)IL-2/15Rbeta(high) CD8+ T cells generated after IL-2 depletion in IL-15 knockout (KO) and in IL-7-depleted IL-15 KO mice; 3) surprisingly, the blockade of IL-2/15Rbeta signaling in IL-2-depleted IL-15 KO mice completely abolished the division of memory CD8+ T cells, although the only cytokines known to act through IL-2/15Rbeta are IL-2 and IL-15; and 4) the expression of IL-2/15Rbeta molecules on memory CD8+ T cells was required for their division induced by IL-2 depletion. These results demonstrate that the depletion of IL-2 in vivo induced memory CD8+ T cell division by an IL-15-independent but by an IL-2/15Rbeta-dependent mechanism, suggesting the existence of a novel IL-2/15Rbeta-utilizing cytokine that acts directly on memory CD8+ T cells to promote cell division.     

Cutting edge: requirement for IL-15 in the generation of primary and memory antigen-specific CD8 T cells

IL-15 and IL-15Ralpha are required for generation of memory-phenotype CD8 T cells in unimmunized mice. However, the role of IL-15 in primary expansion and generation of Ag-specific memory CD8 T cells in vivo has not been investigated. We characterized the CD8 T cell response against vesicular stomatitis virus (VSV) in IL-15(-/-) and IL-15Ralpha(-/-) mice. Surprisingly, IL-15 was required for primary expansion of VSV-specific CD8 T cells. The generation of VSV-specific memory CD8 T cells was also impaired without IL-15 signaling, and this defect correlated with a decrease in memory CD8 T cell turnover. Despite minimal proliferation without IL-15, a subset of memory cells survived long-term. IL-15Ralpha expression was low on naive CD8 T cells, up-regulated on Ag-specific effector cells, and sustained on memory cells. Thus, IL-15 was important for the generation and the subsequent maintenance of antiviral memory CD8 T cells.     

The contrasting roles of IL-2 and IL-15 in the life and death of lymphocytes: implications for the immunotherapy of rheumatological diseases

Interleukin-15 (IL-15) is a 14-15-kDa member of the 4alpha helix bundle family of cytokines that stimulate T and NK (natural killer) cells. IL-15 and IL-2 utilize heterotrimeric receptors that include the cytokine-specific private receptors IL-2Ralpha and IL-15Ralpha, as well as two receptor elements that they share, IL-2Rbeta and gammac. Although IL-2 and IL-15 share two receptor subunits and many functions, at times they provide contrasting contributions to T-cell-mediated immune responses. IL-2, through its pivotal role in activation-induced cell death (AICD), is involved in peripheral tolerance through the elimination of self-reactive T cells. In contrast, IL-15 in general manifests anti-apoptotic actions and inhibits IL-2-mediated AICD. IL-15 stimulates the persistence of memory phenotype CD8+ T cells, whereas IL-2 inhibits their expression. Abnormalities of IL-15 expression have been described in patients with rheumatoid arthritis or inflammatory bowel disease and in diseases associated with the retrovirus HTLV-I (human T-cell lymphotropic virus I). Humanized monoclonal antibodies that recognize IL-2Ralpha, the private receptor for IL-2, are being employed to inhibit allograft rejection and to treat T-cell leukemia/lymphoma. New approaches directed toward inhibiting the actions of the inflammatory cytokine, IL-15, are proposed for an array of autoimmune disorders including rheumatoid arthritis as well as diseases associated with the retrovirus HTLV-I.     

IL-2Rbeta/IL-7Ralpha doubly deficient mice recapitulate the thymic and intraepithelial lymphocyte (IEL) developmental defects of gammac-/- mice: roles for both IL-2 and IL-15 in CD8alphaalpha IEL development

IL-7Ralpha-chain-deficient (IL-7Ralpha-/-) and common gamma chain-deficient (gammac-/-) mice both exhibit abnormal thymic and intestinal intraepithelial lymphocyte (IEL) development, but the developmental inhibition is not equivalent. In this report, we assessed whether the defects in T cell development associated with gammac-/- mice were due to currently defined gammac-dependent cytokines by cross-breeding IL-7Ralpha-/- mice to mice lacking either IL-2, IL-4, or IL-2Rbeta. IL-2/IL-7Ralpha and IL-4/IL-7Ralpha double knockout (DKO) mice demonstrated equivalent thymic development to IL-7Ralpha-/- mice, whereas IL-2Rbeta/IL-7Ralpha DKO mice, which lack IL-2, IL-7, and IL-15 signaling, displayed thymic T cell defects identical to gammac-/- mice. Collectively, these data indicate that of the gammac-dependent cytokines, only IL-7 and IL-15 contribute to the progression and production of thymic T cells. In the IEL, IL-7Ralpha-/- mice selectively lack CD8alphaalpha TCRgammadelta cells, whereas IL-2Rbeta-/- mice show a significant reduction in all CD8alphaalpha cells. IL-2-/- and IL-2/IL-7Ralpha DKO mice demonstrated a reduction in CD8alphaalpha IELs to nearly the same extent as IL-2Rbeta-/- mice, indicating that IL-2 functions in CD8alphaalpha IEL development. Moreover, IL-2Rbeta/IL-7Ralpha DKO mice lacked nearly all TCR-bearing IEL, again recapitulating the phenotype of gammac-/- mice. Thus, these data point to the importance of IL-2, IL-7, and IL-15 as the gammac-dependent cytokines essential for IEL development.     

Cutting edge: murine dendritic cells require IL-15R alpha to prime NK cells

NK cells protect hosts against viral pathogens and transformed cells, and dendritic cells (DCs) play important roles in activating NK cells. We now find that murine IL-15Ralpha-deficient DCs fail to support NK cell cytolytic activity and elaboration of IFN-gamma, despite the fact that these DCs express normal levels of costimulatory molecules and IL-12. By contrast, IL-15Ralpha expression on NK cells is entirely dispensable for their activation by DCs. In addition, blockade with anti-IL-15Ralpha and anti-IL-2Rbeta but not anti-IL-2Ralpha-specific Abs prevents NK cell activation by wild-type DCs. Finally, presentation of IL-15 by purified IL-15Ralpha/Fc in trans synergizes with IL-12 to support NK cell priming. These findings suggest that murine DCs require IL-15Ralpha to present IL-15 in trans to NK cells during NK cell priming.     

Cutting edge: transpresentation of IL-15 by bone marrow-derived cells necessitates expression of IL-15 and IL-15R alpha by the same cells

IL-15 is critical for generation of multiple lymphoid subsets. Recent data have demonstrated a unique aspect of responses to IL-15, in that cells bearing the IL-15Ralpha chain can bind soluble IL-15 and "transpresent" the cytokine to other cells, allowing the latter to respond to IL-15. However, it is unclear whether IL-15 is normally secreted and then becomes bound to surface IL-15Ralpha on bystander cells, or whether transpresentation is mediated by the same cells which synthesize IL-15. Using mixed bone marrow chimeric mice, we present evidence for the latter model, showing that development of NK cells and memory phenotype CD8 T cells necessitates that both IL-15 and IL-15Ralpha be expressed by the same population of cells. These data argue that soluble forms of IL-15 are irrelevant for physiological responses to this cytokine, and the implications of this finding are discussed.     

Distinctions in lymphocyte responses to IL-2 and IL-15 reflect differential ligand binding interactions with the IL-2Rbeta chain and suggest differential roles for the IL-2Ralpha and IL-15Ralpha subunits

More interleukin 15 (IL-15) than IL-2 was needed to generate comparable proliferative responses by phytohaemagglutinin (PHA) blasts and Tf-1beta cells expressing high affinity and intermediate affinity IL-2 receptor (IL-2R) complexes, respectively. The focus of these experiments was to determine the contribution of the shared IL-2 and IL-15 receptor components to these dose-response differences. Some of this difference can be attributed to the role of the IL-2Rbeta chain, in that HuMikbeta1, a monoclonal antibody recognizing the IL-2Rbeta chain, blocks 92.2+/-2.5% (mean+/-SE) of the IL-2 proliferative response by Tf-1beta cells but only inhibits 57.9+/-3.7% of the IL-15 response, indicating that IL-2 and IL-15 may physically utilize the IL-2Rbeta chain differently. Monoclonal antibody 341, which recognizes IL-2Rbeta but does not inhibit IL-2 binding to the IL-2Rbeta chain, blocks 35.4+/-2.3% of IL-15-stimulated proliferation of PHA blasts, while not affecting the IL-2-stimulated proliferation. Finally, although HuMikbeta1 does not inhibit IL-2 responses by PHA blasts bearing high affinity IL-2 receptors, HuMikbeta1 does block IL-15-stimulated proliferation by these same cells bearing high affinity IL-15 receptors (88.5+/-1.6% inhibition). This indicates that the role of IL-15Ralpha in the high affinity IL-15R complex is distinct from that of IL-2Ralpha in the high affinity IL-2R complex. Overall, these studies show that the physical interactions of the IL-2Rbetagammac complex with IL-2 are different than the interactions with IL-15.     

Elucidation of the interleukin-15 binding site on its alpha receptor by NMR

The cytokine interleukin-15 (IL-15) signals through the formation of a quaternary receptor complex composed of an IL-15-specific alpha receptor, together with beta and gammac receptors that are shared with interleukin-2 (IL-2). The initiating step in the formation of this signaling complex is the interaction between IL-15 and IL-15Ralpha, which is a single sushi domain bearing strong structural homology to one of the two sushi domains of IL-2Ralpha. The crystal structure of the IL2-Ralpha/IL-2 complex has been determined, however little is known about the analogous IL-15Ralpha/IL-15 binding interaction. Here we show that recombinant IL-15 can be overexpressed as a stable complex in the presence of its high affinity receptor, IL-15Ralpha. We find that this complex is 10-fold more active than IL-15 alone in stimulating proliferation and survival of memory phenotype CD8 T cells. To probe the ligand/receptor interface, we used solution NMR to map chemical shifts on 15N-labeled IL-15Ralpha in complex with unlabeled IL-15. Our results predict that the binding surface on IL-15Ralpha involves strands C and D, similar to IL-2Ralpha. The interface, as predicted here, leaves open the possibility of trans-presentation of IL-15 by IL-15Ralpha on an opposing cell.     

An absolute requirement for STAT4 and a role for IFN-gamma as an amplifying factor in IL-12 induction of the functional IL-18 receptor complex

IL-12 and IL-18 are both proinflammatory cytokines that contribute to promoting Th1 development and IFN-gamma expression. However, neither IL-12R nor IL-18R is expressed as a functional complex on most resting T cells. This study investigated the molecular mechanisms underlying the induction of an IL-18R complex in T cells. Resting T cells expressed IL-18Ralpha chains but did not exhibit IL-18 binding sites as detected by incubation with rIL-18 followed by anti-IL-18 Ab, suggesting a lack of IL-18Rbeta expression in resting T cells. Although they also failed to express IL-12R, stimulation with anti-CD3 plus anti-CD28 generated IL-12R. Exposure of these cells to IL-12 led not only to up-regulation of IL-18Ralpha expression but also to induction of IL-18R binding sites on both CD4(+) and CD8(+) T cells concomitant with IL-18Rbeta mRNA expression. The IL-18 binding site represented a functional IL-18R complex capable of exhibiting IL-18 responsiveness. IL-12 induction of an IL-18R complex and IL-18Rbeta mRNA expression was not observed in STAT4-deficient (STAT4(-/-)) T cells and was substantially decreased in IFN-gamma(-/-) T cells. However, the failure of STAT4(-/-) T cells to induce an IL-18R complex was not corrected by IFN-gamma. These results indicate that STAT4 and IFN-gamma play an indispensable role and a role as an amplifying factor, respectively, in IL-12 induction of the functional IL-18R complex.     

Memory-type CD8+ T cells protect IL-2 receptor alpha-deficient mice from systemic infection with herpes simplex virus type 2

IL-2Ralpha-deficient (IL-2Ralpha(-/-)) mice exhibit an impaired activation-induced cell death for T cells and develop abnormal T cell activation with age. In our study, we found that IL-2Ralpha(-/-) mice at the age of 5 wk contained an increased number of CD44(+)CD69(-)CD8(+) T cells in lymph nodes, which expressed a high intensity of IL-2Rbeta and vigorously proliferated in response to a high dose of IL-15 or IL-2. The T cells produced a large amount of IFN-gamma in response to IL-15 plus IL-12 in a TCR-independent bystander manner. When IL-2Ralpha(-/-) mice were inoculated i.p. with HSV type 2 (HSV-2) 186 strain, they showed resistance to the infection accompanied by an increased level of serum IL-15. The depletion of CD8(+) T cells by in vivo administration of anti-CD8 mAb rendered IL-2Ralpha(-/-) mice susceptible to HSV-2-induced lethality. These results suggest that memory-type CD8(+) T cells play a novel role in the protection against HSV-2 infection in IL-2Ralpha(-/-) mice.     

IL-2R beta agonist P1-30 acts in synergy with IL-2, IL-4, IL-9, and IL-15: biological and molecular effects

From the sequence of human IL-2 we have recently characterized a peptide (p1-30), which is the first IL-2 mimetic described. P1-30 covers the entire alpha helix A of IL-2 and spontaneously folds into a alpha helical homotetramer mimicking the quaternary structure of a hemopoietin. This neocytokine interacts with a previously undescribed dimeric form of the human IL-2 receptor beta-chain likely to form the p1-30 receptor (p1-30R). P1-30 acts as a specific IL-2Rbeta agonist, selectively inducing activation of CD8 and NK lymphocytes. From human PBMC we have also shown that p1-30 induces the activation of lymphokine-activated killer cells and the production of IFN-gamma. Here we demonstrate the ability of p1-30 to act in synergy with IL-2, -4, -9, and -15. These synergistic effects were analyzed at the functional level by using TS1beta, a murine T cell line endogenously expressing the common cytokine gamma gene and transfected with the human IL-2Rbeta gene. At the receptor level, we show that expression of human IL-2Rbeta is absolutely required to obtain synergistic effects, whereas IL-2Ralpha specifically impedes the synergistic effects obtained with IL-2. The results suggest that overexpression of IL-2Ralpha inhibits p1-30R formation in the presence of IL-2. Finally, concerning the molecular effects, although p1-30 alone induces the antiapoptotic molecule bcl-2, we show that it does not influence mRNA expression of c-myc, c-jun, and c-fos oncogenes. In contrast, p1-30 enhances IL-2-driven expression of these oncogenes. Our data suggest that p1-30R (IL-2Rbeta)(2) and intermediate affinity IL-2R (IL-2Rbetagamma), when simultaneously expressed at the cell surface, may induce complementary signal transduction pathways and act in synergy.     

IL-2, -7, and -15, but not thymic stromal lymphopoeitin, redundantly govern CD4+Foxp3+ regulatory T cell development

Common gamma chain (gammac)-receptor dependent cytokines are required for regulatory T cell (Treg) development as gammac(-/-) mice lack Tregs. However, it is unclear which gammac-dependent cytokines are involved in this process. Furthermore, thymic stromal lymphopoietin (TSLP) has also been suggested to play a role in Treg development. In this study, we demonstrate that developing CD4(+)Foxp3(+) Tregs in the thymus express the IL-2Rbeta, IL-4Ralpha, IL-7Ralpha, IL-15Ralpha, and IL-21Ralpha chains, but not the IL9Ralpha or TSLPRalpha chains. Moreover, only IL-2, and to a much lesser degree IL-7 and IL-15, were capable of transducing signals in CD4(+)Foxp3(+) Tregs as determined by monitoring STAT5 phosphorylation. Likewise, IL-2, IL-7, and IL-15, but not TSLP, were capable of inducing the conversion of CD4(+)CD25(+)Foxp3(-) thymic Treg progenitors into CD4(+)Foxp3(+) mature Tregs in vitro. To examine this issue in more detail, we generated IL-2Rbeta(-/-) x IL-7Ralpha(-/-) and IL-2Rbeta(-/-) x IL-4Ralpha(-/-) mice. We found that IL-2Rbeta(-/-) x IL-7Ralpha(-/-) mice were devoid of Tregs thereby recapitulating the phenotype observed in gammac(-/-) mice; in contrast, the phenotype observed in IL-2Rbeta(-/-) x IL-4Ralpha(-/-) mice was comparable to that seen in IL-2Rbeta(-/-) mice. Finally, we observed that Tregs from both IL-2(-/-) and IL-2Rbeta(-/-) mice show elevated expression of IL-7Ralpha and IL-15Ralpha chains. Addition of IL-2 to Tregs from IL-2(-/-) mice led to rapid down-regulation of these receptors. Taken together, our results demonstrate that IL-2 plays the predominant role in Treg development, but that in its absence the IL-7Ralpha and IL-15Ralpha chains are up-regulated and allow for IL-7 and IL-15 to partially compensate for loss of IL-2.     

Constitutive expression of IL-7 receptor alpha does not support increased expansion or prevent contraction of antigen-specific CD4 or CD8 T cells following Listeria monocytogenes infection

Expression of IL-7Ralpha (CD127) has been suggested as a major determinant in the survival of memory T cell precursors. We investigated whether constitutive expression of IL-7Ralpha on T cells increased expansion and/or decreased contraction of endogenous Ag-specific CD4 and CD8 T cells following infection with Listeria monocytogenes. The results indicate that constitutive expression of IL-7Ralpha alone was not enough to impart an expansion or survival advantage to CD8 T cells responding to infection, and did not increase memory CD8 T cell numbers over those observed in wild-type controls. Constitutive expression of IL-7Ralpha did allow for slightly prolonged expansion of Ag-specific CD4 T cells; however, it did not alter the contraction phase or protect against the waning of memory T cell numbers at later times after infection. Memory CD4 and CD8 T cells generated in IL-7Ralpha transgenic mice expanded similarly to wild-type T cells after secondary infection, and immunized IL-7Ralpha transgenic mice were fully protected against lethal bacterial challenge demonstrating that constitutive expression of IL-7Ralpha does not impair, or markedly improve memory/secondary effector T cell function. These results indicate that expression of IL-7Ralpha alone does not support increased survival of effector Ag-specific CD4 or CD8 T cells into the memory phase following bacterial infection.     

Increased CD8+ T cell memory to concurrent infection at the expense of increased erosion of pre-existing memory: the paradoxical role of IL-15

The use of cytokines during vaccination, particularly IL-15, is being considered due to the unique ability of IL-15 to enhance the proliferation of memory CD8(+) T cells. However, as homeostatic mechanisms limit excessive lymphocyte expansion, we addressed the consequences of this enhancement of T cell memory by IL-15. Infection of mice with either recombinant Mycobacterium bovis (BCG) expressing IL-15 (BCG-IL-15) or BCG and purified IL-15 resulted in an increased CD44, IL-2Rbeta expression and increased frequency of IFN-gamma-secreting CD8(+) T cells. Surprisingly, the enhancement of memory to concurrent infection by IL-15 exacerbated the attrition of pre-existing memory. Infection of mice with Listeria monocytogenes expressing OVA resulted in potent OVA(257-264)-specific CD8(+) T cell memory, and a challenge of these mice with either BCG-IL-15 or BCG and purified IL-15 resulted in an increased erosion of OVA(257-264)-specific CD8(+) T cell memory, relative to BCG. Enhancement in the erosion of OVA-specific CD8(+) T cell memory by BCG-IL-15 resulted in a consequently greater impairment in protection against a challenge with OVA-expressing tumor cells. We thus raise important questions regarding vaccinations that are aimed at maximizing T cell memory without considering the impact on pre-existing T cell memory.     

STAT6-dependent differentiation and production of IL-5 and IL-13 in murine NK2 cells

NK cells differentiate into either NK1 or NK2 cells that produce IFN-gamma or IL-5 and IL-13, respectively. Little is known, however, about the molecular mechanisms that control NK1 and NK2 cell differentiation. To address these questions, we established an in vitro mouse NK1/NK2 cell differentiation culture system. For NK1/NK2 cell differentiation, initial stimulation with PMA and ionomycin was required. The in vitro differentiated NK2 cells produced IL-5 and IL-13, but the levels were 20 times lower than those of Th2 or T cytotoxic (Tc)2 cells. No detectable IL-4 was produced. Freshly prepared NK cells express IL-2Rbeta, IL-2RgammaC, and IL-4Ralpha. After stimulation with PMA and ionomycin, NK cells expressed IL-2Ralpha. NK1 cells displayed higher cytotoxic activity against Yac-1 target cells. The levels of GATA3 protein in developing NK2 cells were approximately one-sixth of those in Th2 cells. Both NK1 and NK2 cells expressed large amounts of repressor of GATA, the levels of which were equivalent to CD8 Tc1 and Tc2 cells and significantly higher than those in Th2 cells. The levels of histone hyperacetylation of the IL-4 and IL-13 gene loci in NK2 cells were very low and equivalent to those in naive CD4 T cells. The production of IL-5 and IL-13 in NK2 cells was found to be STAT6 dependent. Thus, similar to Th2 cells, NK2 cell development is dependent on STAT6, and the low level expression of GATA3 and the high level expression of repressor of GATA may influence the unique type 2 cytokine production profiles of NK2 cells.     

Efficient induction of primary and secondary T cell-dependent immune responses in vivo in the absence of functional IL-2 and IL-15 receptors

IL-2 and IL-15 are thought to be important cytokines for T cell-dependent immune responses. Mice deficient in IL-2, IL-2Ralpha, and IL-2Rbeta are each characterized by a rapid lethal autoimmune lymphoproliferative disorder that complicates their use in studies aimed at investigating the role of these cytokines and receptors for immune responses in vivo. We have previously characterized a novel transgenic (Tg) mouse on the IL-2Rbeta-/- genetic background (Tg-/- mice) that lacks autoimmune disease but still contains peripheral T cells that are nonresponsive to IL-2 and IL-15. In the present study, these mice were used to investigate the extent by which IL-2 and IL-15 are essential for T cell immunity in vivo. Tg-/- mice generated near normal primary and secondary Ab responses to OVA, readily mounted first and second set allogeneic skin graft rejection responses, and developed primary and recall CD8 T cell responses to vaccinia virus. However, Tg-/- mice generated a slightly lower level of IgG2a Abs to OVA, exhibited a somewhat delayed first set skin graft rejection response with lower allo-specific CTL, and developed a significantly lower number of IFN-gamma-producing vaccinia-specific CD8+ T cells. Thus, although T effector function is somewhat impaired, T cell immunity is largely functional in the absence of IL-2- and IL-15-induced signaling through IL-2Rbeta.     

Suppressor of cytokine signaling 1 regulates IL-15 receptor signaling in CD8+CD44high memory T lymphocytes

T lymphocyte survival, proliferation, and death in the periphery are dependent on several cytokines. Many of these cytokines induce the expression of suppressor of cytokine signaling-1 (SOCS1), a feedback inhibitor of JAK kinases. However, it is unclear whether the cytokines that regulate T lymphocyte homeostasis are critically regulated by SOCS1 in vivo. Using SOCS1(-/-)IFN-gamma(-/-) mice we show that SOCS1 deficiency causes a lymphoproliferative disorder characterized by decreased CD4/CD8 ratio due to chronic accumulation of CD8+CD44(high) memory phenotype T cells. SOCS1-deficient CD8+ T cells express elevated levels of IL-2Rbeta, show increased proliferative response to IL-15 and IL-2 in vitro, and undergo increased bystander proliferation and vigorous homeostatic expansion in vivo. Sorted CD8+CD44(high) T cells from SOCS1(-/-)IFN-gamma(-/-) mice respond 5 times more strongly than control cells, indicating that SOCS1 is a critical regulator of IL-15R signaling. Consistent with this idea, IL-15 stimulates sustained STAT5 phosphorylation in SOCS1-deficient CD8+ T cells. IL-15 strongly induces TNF-alpha production in SOCS1-deficient CD8+ T cells, indicating that SOCS1 is also a critical regulator of CD8+ T cell activation by IL-15. However, IL-15 and IL-2 induce comparable levels of Bcl-2 and Bcl-x(L) in SOCS1-deficient and SOCS1-sufficient CD8+ T cells, suggesting that cytokine receptor signals required for inducing proliferation and cell survival signals are not identical. These results show that SOCS1 differentially regulates common gamma-chain cytokine signaling in CD8+ T cells and suggest that CD8+ T cell homeostasis is maintained by distinct mechanisms that control cytokine-mediated survival and proliferation signals.