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.     

Ligand-independent homomeric and heteromeric complexes between interleukin-2 or -9 receptor subunits and the gamma chain

Signaling via interleukin-2 (IL-2) and interleukin-9 receptors (IL-2R and IL-9R) involves heteromeric interactions between specific interleukin receptor subunits, which bind Janus kinase 1 (JAK1) and the JAK3 binding common gamma chain (gamma c). The potential existence and roles of homomeric and heteromeric complexes before ligand binding and their modulation by ligand and JAK3 are unclear. Using computerized antibody-mediated immunofluorescence co-patching of epitope-tagged receptors at the surface of live cells, we demonstrate that IL-2Rbeta, IL-9Ralpha, and gamma c each display a significant fraction of ligand-independent homomeric complexes (24-28% co-patching), whereas control co-patching levels with unrelated receptors are very low (7%). Heteromeric complex formation of IL2-Rbeta or IL-9Ralpha with gamma c is also observed in the absence of ligand (15-30%). Ligand binding increases this hetero-oligomerization 2-fold but does not affect homo-oligomerization. Co-expression of IL-2Ralpha does not affect the hetero-oligomerization of IL-2Rbeta and gamma c. Recruitment of gamma c into heterocomplexes is partly at the expense of its homo-oligomerization, suggesting that a functional role of the latter may be to keep the receptors inactive in the absence of ligand. At the same time, the preformed complexes between gamma c and IL-2Rbeta or IL-9Ralpha promote signaling by the JAK3 A572V mutant without ligand, supporting a pathophysiological role for the constitutive oligomerization in triggering ligand-independent activation of JAK3 (and perhaps other JAK mutants) mutants identified in several human cancers.     

Oligomerization of IL-2Ralpha

Interleukin (IL) 2 receptor subunit alpha (IL-2Ralpha) increases the affinity of the IL-2 receptor complex while hetero-association of IL-2Rbeta and gamma(c) chains initiates a proliferative signal. We show here that IL-2Ralpha is necessary for receptor clustering required for augmentation of IL-2 signalling. Cells expressing chimeras incorporating the extracellular domain of IL-2Ralpha demonstrated IL-2 independent homo-association of the IL-2Ralpha chimera. Singly or co-transfected IL-2Rbeta and gamma(c) chimeras showed no spontaneous or IL-2-inducible oligomerization. Co-transfection of IL-2Ralpha and IL-2Rbeta (+/- gamma(c)) chimeras diminished spontaneous IL-2Ralpha chimera oligomerization and permitted IL-2-inducible hetero-oligomerization of receptor components. Homo-association of IL-2Ralpha was also demonstrated by fluorescence resonance energy transfer (FRET). The spontaneous homo-oligomerization property of IL-2Ralpha required the membrane proximal region of the receptor (exon 6) by deletion analysis; the IL-2 inducible oligomerization property of IL-2Ralpha required the second "sushi" domain (exon 4). This work provides insight into the mechanics of this complex receptor system and to other receptor complexes in the immune system that send signals by clustering receptor subunits.     

Combined IL-15/IL-15Ralpha immunotherapy maximizes IL-15 activity in vivo

IL-15 has substantial potential as an immunotherapeutic agent for augmenting immune responses. However, the activity of IL-15 is mediated by a unique mechanism in which the cytokine is transpresented by cell-bound high-affinity IL-15Ralpha to target cells expressing the IL-15Rbeta and the common gamma-chain. Thus, the efficacy of administered IL-15 alone may be limited by the availability of free IL-15Ralpha. We now show that administration of soluble IL-15/IL-15Ralpha complexes greatly enhanced IL-15 half-life and bioavailability in vivo. Treatment of mice with this complex, but not with IL-15 alone, resulted in robust proliferation of memory CD8 T cells, NK cells, and NK T cells. The activity of the complex required IL-15Rbeta, but not IL-15Ralpha, expression by the responding cells and was IL-7-independent. Interestingly, IL-15/IL-15Ralpha immunotherapy also caused naive CD8 T cell activation and development into effector cells and long-term memory T cells. Lastly, complexed IL-15, as compared with IL-15 alone, dramatically reduced tumor burden in a model of B16 melanoma. These findings hold significant importance for the use of IL-15 as a potential adjuvant/therapeutic and inducer of homeostatic proliferation, without the necessity for prior immunodepletion.     

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.     

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.     

A function for IL-7R for CD4+CD25+Foxp3+ T regulatory cells

The IL-2/IL-2R interaction is important for development and peripheral homeostasis of T regulatory (Treg) cells. IL-2- and IL-2R-deficient mice are not completely devoid of Foxp3+ cells, but rather lack population of mature CD4+CD25+Foxp3high Treg cells and contain few immature CD4+CD25-Foxp3low T cells. Interestingly, common gamma chain (gammac) knockout mice have been shown to have a near complete absence of Foxp3+ Treg cells, including the immature CD25-Foxp3low subset. Therefore, other gammac-cytokine(s) must be critically important during thymic development of CD4+CD25+Foxp3+ Treg cells apart from the IL-2. The present study was undertaken to determine whether the gammac-cytokines IL-7 or IL-15 normally contribute to expression of Foxp3 and Treg cell production. These studies revealed that mice double deficient in IL-2Rbeta and IL-7Ralpha contained a striking lack in the CD4+Foxp3+ population and the Treg cell defect recapitulated the gammac knockout mice. In the absence of IL-7R signaling, IL-15/IL-15R interaction is dispensable for the production of CD4+CD25+Foxp3+ Treg cells, indicating that normal thymic Treg cell production likely depends on signaling through both IL-2 and IL-7 receptors. Selective thymic reconstitution of IL-2Rbeta in mice double deficient in IL-2Rbeta and IL-7Ralpha established that IL-2Rbeta is dominant and sufficient to restore production of Treg cells. Furthermore, the survival of peripheral CD4+Foxp3low cells in IL-2Rbeta-/- mice appears to depend upon IL-7R signaling. Collectively, these data indicate that IL-7R signaling contributes to Treg cell development and peripheral homeostasis.     

Role of IFN-gamma in Th1 differentiation: IFN-gamma regulates IL-18R alpha expression by preventing the negative effects of IL-4 and by inducing/maintaining IL-12 receptor beta 2 expression

Two key events occur during the differentiation of IFN-gamma-secreting Th1 cells: up-regulation of IL-12Rbeta2 and IL-12-driven up-regulation of IL-18Ralpha. We previously demonstrated that IL-12-driven up-regulation of IL-18Ralpha expression is severely impaired in IFN-gamma(-/-) mice. However, it was unclear from these studies how IFN-gamma influenced IL-18Ralpha since IFN-gamma alone had no direct effect on IL-18Ralpha expression. In the absence of IL-4, IL-12-dependent up-regulation of IL-18Ralpha/IL-12Rbeta2 was independent of IFN-gamma. However, in the presence of IL-4, IFN-gamma functions to limit the negative effects of IL-4 on both IL-18Ralpha and IL-12Rbeta2. Neutralization of IL-4 restored IL-12-driven up-regulation of IL-18Ralpha/IL-12Rbeta2 in an IFN-gamma-independent fashion. In the absence of both IL-12 and IL-4, IFN-gamma up-regulates IL-12beta2 expression and primes IFN-gamma-producing Th1 cells. When T cells were primed in the presence of IL-4, no correlation was found between the levels of expression of the IL-18Ralpha or the IL-12Rbeta2 and the capacity of these cells to produce IFN-gamma, suggesting that IL-4 may also negatively affect IL-12-mediated signal transduction and thus Th1 differentiation. These data clarify the role of IFN-gamma in regulation of IL-18Ralpha/IL-12Rbeta2 during both IL-12-dependent and IL-12-independent Th1 differentiation.     

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-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.     

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.     

Monoclonal antibodies specific for the IL-18 receptor

Interleukin-18, a pleiotropic cytokine is a member of the IL-1 family and has multiple immunoregulatory functions. IL-18 action leads to IFNgamma production by NK or T cells, induces Th1 differentiation and suppresses IgE synthesis by B cells when acting on responding cells in association with IL-12. At present two subunits of the IL-18R have been characterized: IL-18 Ralpha and IL-18 Rbeta. Both receptors belong to the IL-1R family. IL-18 Ralpha has been described as the ligand-binding chain and IL-18 Rbeta as the signal-transduction chain. Three monoclonal antibodies (mAbs) submitted to the HLDA8 workshop, designated H44 (80438), B-B46 (80228), and B-E43 (80232) were evaluated. The mAb specificity was determined by ELISA using coated recombinant IL-18 Ralpha or IL-18 Rbeta. Cell staining was analyzed by flow cytometry. A positive staining with the mAb B-E43 or H44 demonstrated that IL-18 Ralpha is expressed on several myeloid cell lines. No positive cell staining was observed with the anti IL-18 Rbeta mAb B-B46. The mAb biological activity was studied using the cell line KG1. A downmodulation of IFNgamma production was observed with the mAbs B-B46 (80228) and B-E43 (80232).     

Crystal Structure of the interleukin-15.interleukin-15 receptor alpha complex: insights into trans and cis presentation

Interleukin (IL)-15 is a pleiotropic cytokine that plays a pivotal role in both innate and adaptive immunity. IL-15 is unique among cytokines due to its participation in a trans signaling mechanism in which IL-15 receptor alpha (IL-15Ralpha) from one subset of cells presents IL-15 to neighboring IL-2Rbeta/gammac-expressing cells. Here we present the crystal structure of IL-15 in complex with the sushi domain of IL-15Ralpha. The structure reveals that the alpha receptor-binding epitope of IL-15 adopts a unique conformation, which, together with amino acid substitutions, permits specific interactions with IL-15Ralpha that account for the exceptionally high affinity of the IL-15.IL-15Ralpha complex. Interestingly, analysis of the topology of IL-15 and IL-15Ralpha at the IL-15.IL-15Ralpha interface suggests that IL-15 should be capable of participating in a cis signaling mechanism similar to that of the related cytokine IL-2. Indeed, we present biochemical data demonstrating that IL-15 is capable of efficiently signaling in cis through IL-15Ralpha and IL-2Rbeta/gammac expressed on the surface of a single cell. Based on our data we propose that cis presentation of IL-15 may be important in certain biological contexts and that flexibility of IL-15Ralpha permits IL-15 and its three receptor components to be assembled identically at the ligand-receptor interface whether IL-15 is presented in cis or trans. Finally, we have gained insights into IL-15.IL-15Ralpha.IL-2Rbeta.gammac quaternary complex assembly through the use of molecular modeling.     

Differential roles for IL-15R alpha-chain in NK cell development and Ly-49 induction

IL-15Ralpha-deficient (IL-15Ralpha(-/-)) mice lack NK cells. However, when bone marrow (BM) progenitors from IL-15Ralpha(-/-) mice were cultured with IL-7, stem cell factor and flt3 ligand, followed by IL-15, they were able to differentiate into functional NK cells, indicating that IL-15Ralpha is not critical for NK cell development. Whereas NK cells generated in vitro from IL-15Ralpha(-/-) BM progenitors expressed CD94/NKG2, they failed to express Ly-49 receptors. In keeping with this, when IL-15Ralpha(-/-) BM cells were transferred into wild type recipients, they gave rise to NK cells in vivo, but with greatly reduced expression of Ly-49 receptors. Furthermore, the small numbers of NK cells found in IL-15(-/-) as well as IL-15Ralpha(-/-) but not flt3 ligand(-/-) mice expressed much lower levels of Ly-49 receptors than those from wild type mice. These results indicate a novel role for IL-15Ralpha-chain in Ly-49 induction on developing NK cells.     

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.     

Expression of alpha(4)beta(7) integrin defines a distinct pathway of lymphoid progenitors committed to T cells, fetal intestinal lymphotoxin producer, NK, and dendritic cells

During embryogenesis, the Peyer's patch anlagen are induced by a cell population that produces lymphotoxin (LT) alpha(1)beta(2) following stimulation of IL-7Ralpha. In this study, we show that the LT-producing cell is localized within the IL-7Ralpha(+) and integrin alpha(4)beta(7) (alpha(4)beta(7))(+) population in the embryonic intestine. Lineage commitment to the LT producer phenotype in the fetal liver coincides with expression of alpha(4)beta(7). Before expression of alpha(4)beta(7), the potential of IL-7Ralpha(+) population to generate B cells is lost. However, the progenitors for T cells and LT producer cells reside in the IL-7Ralpha(+)alpha(4)beta(7)(+) cells, but during subsequent differentiation, the potential to give rise to T cells is lost. This IL-7Ralpha(+)alpha(4)beta(7)(+) population migrates to the intestine, where it induces the Peyer's patch anlagen. When stimulated with IL-15 or IL-3 and TNF, the intestinal IL-7Ralpha(+)alpha(4)beta(7)(+) population can differentiate into fully competent NK1.1(+) NK cells or CD11c(+) APCs. Expression of alpha(4)beta(7) is lost during differentiation of both lineages; IL-7Ralpha expression is lost during NK1.1(+) cells differentiation. A newly discovered lineage(-)IL-7Ralpha(+)c-Kit(+)alpha(4)beta(7)(+) population in the fetal liver is committed to T, NK, dendritic, and fetal intestinal LT producer lineage, the latter being an intermediate stage during differentiation of NK and dendritic cells.     

Down-regulation of IL-2 receptor α (CD25) characterizes human γδ-T cells rendered resistant to apoptosis after CD2 engagement in the presence of IL-12

We recently identified a CD2-mediated, IL-12-dependent signaling pathway that inhibits apoptosis in mitogen-stimulated human gammadelta-T cells. Here we show that gammadelta-T cells which acquire resistance to mitogen-induced apoptosis upregulate IL-12 receptor beta 1 subunit (IL-12Rbeta1); in contrast, gammadelta-T cells which remain sensitive to mitogen-induced apoptosis fail to express IL-12Rbeta1. Next we show that gammadelta-T cells which are rendered resistant to mitogen-induced apoptosis attenuate their expression of the IL-2 receptor alpha chain (IL-2Ralpha/CD25), this in part accounting for their acquired resistance to IL-2-induced death. In contrast, apoptosis-sensitive gammadelta-T cells are shown to persist in their expression of IL-2Ralpha/CD25, thus remaining sensitive to IL-2-induced death. Moreover, we show that apoptosis-resistant, but not apoptosis-sensitive, gammadelta-T cells display an enhanced responsiveness to IL-15, a finding in keeping with the known function of IL-15 as a growth and survival factor. Finally, we present evidence to suggest that this differential responsiveness to IL-15 occurs in part by the increased expression of the IL-15Ralpha chain on apoptosis-resistant gammadelta-T cells, compared to apoptosis-sensitive gammadelta-T cells. The biological and clinical implications of these findings are discussed.