Roles for common cytokine receptor gamma-chain-dependent cytokines in the generation, differentiation, and maturation of NK cell precursors and peripheral NK cells in vivo

Mature natural killer cells with phenotypic and functional alterations accumulate upon sustained stimulation with IL-15/IL-15Ralpha complexes

Cytotoxic lymphocytes such as natural killer (NK) and CD8 T cells play important roles in immunosurveillance by killing virally infected or malignant cells. The homeostatic cytokine, IL-15, promotes the development, function, and survival of NK and CD8 T cells. IL-15 is normally presented in trans as a surface complex with IL-15 receptor-alpha-chain (IL-15Rα) by dendritic cells (DCs) and monocytes. Signaling by IL-15 occurs via the IL-2/IL-15 receptor β-chain (CD122) which is expressed primarily by NK1.1(+) cells and CD8 T cells. The use of preformed complexes of IL-15 with soluble IL-15Rα complexes to boost the effector function of CD122(+) cytolytic lymphocytes such as NK and CD8 T cells has recently gained considerable attention. Here we describe the impact of transient and prolonged in vivo stimulation by IL-15/IL-15Rα complexes on NK and CD8 T cells. Whereas transitory stimulation increased the number of activated NK cells and significantly enhanced their effector function, prolonged stimulation by IL-15/IL-15Rα complexes led to a marked accumulation of mature NK cells with considerably impaired activation, cytotoxicity, and proliferative activity, and an altered balance of activating and inhibitory receptors. In contrast to NK cells, CD8 T cells exhibited an activated phenotype and robust T cell receptor stimulation and effector function upon chronic stimulation with IL-15/IL-15Rα complexes. Thus, prolonged stimulation with the strong activating signal leads to a preferential accrual of mature NK cells with altered activation and diminished functional capacity. These findings point to a negative feedback mechanism to preferentially counterbalance excessive NK cell activity and may have important implications for cytokine immunotherapy.

A novel Ncr1-Cre mouse reveals the essential role of STAT5 for NK-cell survival and development

We generated a transgenic mouse line that expresses the Cre recombinase under the control of the Ncr1 (p46) promoter. Cre-mediated recombination was tightly restricted to natural killer (NK) cells, as revealed by crossing Ncr1-iCreTg mice to the eGFP-LSLTg reporter strain. Ncr1-iCreTg mice were further used to study NK cell-specific functions of Stat5 (signal transducers and activators of transcription 5) by generating Stat5(f/f) Ncr1-iCreTg animals. Stat5(f/f) Ncr1-iCreTg mice were largely devoid of NK cells in peripheral lymphoid organs. In the bone marrow, NK-cell maturation was abrogated at the NK cell-precursor stage. Moreover, we found that in vitro deletion of Stat5 in interleukin 2-expanded NK cells was incompatible with NK-cell viability. In vivo assays confirmed the complete abrogation of NK cell-mediated tumor control against B16F10-melanoma cells. In contrast, T cell-mediated tumor surveillance against MC38-adenocarcinoma cells was undisturbed. In summary, the results of our study show that STAT5 has a cell-intrinsic role in NK-cell development and that Ncr1-iCreTg mice are a powerful novel tool with which to study NK-cell development, biology, and function.

Tentative and transient natural killer cell polarization balances the requirements for discriminatory recognition and cytolytic efficacy

Natural killer (NK) cells are immune cells that lyse virally infected and tumor cells. Initially, their cytolytic capability is induced by cytokines. Subsequently, in their decision whether to kill a potential target cell, NK cells have to distinguish between small differences in the expression of ligands that report on the viral infection or transformation of the target. NK killing requires tight coupling to the target cell and extensive NK cell polarization. Here we discuss, often in contrast to the second cytolytic immune cell type, cytotoxic T cells, how NK cell polarization is shaped by three constraints of their activation. First, NK cell have to respond to cytokines: Different priming cytokines yield dramatically divergent NK cell polarization. Second, NK cells have to distinguish small differences in ligand expression: NK cell polarization is tentative, likely to allow discriminatory recognition close to the NK cell activation threshold. A critical contributor to the tentative nature of NK cell polarization may be poorly developed spatiotemporal organization of NK cell signaling. Third, NK cells have to kill effectively: NK cell polarization is transient, allowing for efficient killing by sequential interactions of a single NK cell with numerous target cells.

Biopolymer encapsulated live influenza virus as a universal CD8+ T cell vaccine against influenza virus

Current influenza virus vaccines primarily elicit antibodies and can be rendered ineffective by antigenic drift and shift. Vaccines that elicit CD8+ T cell responses targeting less variable proteins may function as universal vaccines that have broad reactivity against different influenza virus strains. To generate such a universal vaccine, we encapsulated live influenza virus in a biopolymer and delivered it to mice subcutaneously. This vaccine was safe, induced potent CD8+ T cell immunity and protected mice against heterosubtypic lethal challenge. Safety of subcutaneous (SQ) vaccination was tested in Rag-/-γc-/- double knockout mice which we show cannot control intranasal infection. Biopolymer encapsulation of live influenza virus could be used to develop universal CD8+ T cell vaccines against heterosubtypic and pandemic strains.

CXCL12-CXCR4 chemokine signaling is essential for NK-cell development in adult mice

Natural killer (NK) cells are granular lymphocytes that are generated from hematopoietic stem cells and play vital roles in the innate immune response against tumors and viral infection. Generation of NK cells is known to require several cytokines, including interleukin-15 (IL-15) and Fms-like tyrosine kinase 3 ligand, but not IL-2 or IL-7. Here we investigated the in vivo role of CXC chemokine ligand-12 (CXCL12) and its primary receptor CXCR4 in NK-cell development. The numbers of NK cells appeared normal in embryos lacking CXCL12 or CXCR4; however, the numbers of functional NK cells were severely reduced in the bone marrow, spleen, and peripheral blood from adult CXCR4 conditionally deficient mice compared with control animals, probably resulting from cell-intrinsic CXCR4 deficiency. In culture, CXCL12 enhanced the generation of NK cells from lymphoid-primed multipotent progenitors and immature NK cells. In the bone marrow, expression of IL-15 mRNA was considerably higher in CXCL12-abundant reticular (CAR) cells than in other marrow cells, and most NK cells were in contact with the processes of CAR cells. Thus, CXCL12-CXCR4 chemokine signaling is essential for NK-cell development in adults, and CAR cells might function as a niche for NK cells in bone marrow.

IL-12 initiates tumor rejection via lymphoid tissue-inducer cells bearing the natural cytotoxicity receptor NKp46

The potent tumoricidal activity of interleukin 12 (IL-12) is thought to be mediated by the activation and polarization of natural killer (NK) cells and T helper type 1 (T(H)1) cells, respectively. By systematic analysis of the IL-12-induced immune response to subcutaneous melanoma (B16), we found that tumor suppression was mediated independently of T lymphocytes or NK cells. IL-12 initiated local antitumor immunity by stimulating a subset of NKp46(+) lymphoid tissue-inducer (LTi) cells dependent on the transcription factor RORγt. The presence of these NKp46(+) LTi cells induced upregulation of adhesion molecules in the tumor vasculature and resulted in more leukocyte invasion. Thus, this innate cell type is responsive to IL-12 and is a powerful mediator of tumor suppression.

Cutting edge: Thymic NK cells develop independently from T cell precursors

Although NK cells in the mouse are thought to develop in the bone marrow, a small population of NK cells in the thymus has been shown to derive from a GATA3-dependent pathway. Characteristically, thymic NK cells express CD127 and few Ly49 molecules and lack CD11b. Because these NK cells develop in the thymus, the question of their relationship to the T cell lineage has been raised. Using several different mouse models, we find that unlike T cells, thymic NK cells are not the progeny of Rorc-expressing progenitors and do not express Rag2 or rearrange the TCRγ locus. We further demonstrate that thymic NK cells develop independently of the Notch signaling pathway, supporting the idea that thymic NK cells represent bona fide NK cells that can develop independently of all T cell precursors.

Unidirectional signaling triggered through 2B4 (CD244), not CD48, in murine NK cells

Engagement of 2B4 (CD244) with CD48 results in activation, costimulation, or inhibition of NK cell activities, depending on the cell types and the stage of differentiation. In vivo, 2B4+ NK cells can interact with CD48+ NK cells and also with surrounding CD48+ hematopoietic cells. Similarly, CD48+ NK cells may be triggered by adjacent 2B4+ NK cells or other hematopoietic cells expressing 2B4, e.g., monocytes, basophils, γδ T cells, etc. As CD48 was also shown to function as an activating receptor, 2B4/CD48 binding in the settings of NK-to-NK or NK-to-non-NK cell interactions may generate bidirectional signals. To address this question, we examined the consequence of CD48 or 2B4 ligation using two experimental settings: one with target (syngeneic EL4 and allogeneic P815) cells, ectopically expressing surface 2B4 or CD48, and the other with direct cross-linking with plate-bound mAb. Here, we report that ligation of CD48 with 2B4+ EL4 or 2B4+ P815 targets, in the absence of other receptor engagement, did not alter NK cell cytotoxicity or proliferation significantly. Similarly, cross-linking of NK cells with plate-bound anti-CD48 mAb in the absence or presence of a suboptimal dose of IL-2 did not modulate NK proliferation, cytotoxicity, or cytokine production. Nonetheless, 2B4 cross-linking promoted NK cell proliferation and effector functions consistently in both settings. Therefore, our results demonstrate unequivocally that CD48 on surrounding NK or non-NK cells serves primarily as a ligand to stimulate 2B4 on the adjacent NK cells in mice.

Interleukin-1beta selectively expands and sustains interleukin-22+ immature human natural killer cells in secondary lymphoid tissue

Among human natural killer (NK) cell intermediates in secondary lymphoid tissue (SLT), stage 3 CD34(-)CD117(+)CD161(+)CD94(-) immature NK (iNK) cells uniquely express aryl hydrocarbon receptor (AHR) and interleukin-22 (IL-22), supporting a role in mucosal immunity. The mechanisms controlling proliferation and differentiation of these cells are unknown. Here we demonstrate that the IL-1 receptor IL-1R1 was selectively expressed by a subpopulation of iNK cells that localized proximal to IL-1beta-producing conventional dendritic cells (cDCs) within SLT. IL-1R1(hi) iNK cells required continuous exposure to IL-1beta to retain AHR and IL-22 expression, and they proliferate in direct response to cDC-derived IL-15 and IL-1beta. In the absence of IL-1beta, a substantially greater fraction of IL-1R1(hi) iNK cells differentiated to stage 4 NK cells and acquired the ability to kill and secrete IFN-gamma. Thus, cDC-derived IL-1beta preserves and expands IL-1R1(hi)IL-22(+)AHR(+) iNK cells, potentially influencing human mucosal innate immunity during infection.

Transience in polarization of cytolytic effectors is required for efficient killing and controlled by Cdc42

Cytolytic effectors polarize toward target cells for effective killing and IFN-gamma secretion. The spatiotemporal features of this polarization and their importance for cytolysis have not been resolved. In cytotoxic T cells and natural killer (NK) cells, transient polarization was consistently associated with effective killing. Polarization was regulated by Cdc42, a small Rho family GTPase universally critical for cytoskeletal dynamics. Transient accumulation of active Cdc42 at the cytolytic effector/target cell interface and focus of such accumulation on the interface center were closely related to cytolysis. Surprisingly, however, the intensity of Cdc42 activation was not. We interfered with Cdc42 activation in NK cells such that sustained polarization in long lasting nonkilling cell couples was selectively blocked. Thus the proportion of the NK cell population displaying transient polarization was increased. As a consequence, cytolytic responder frequency and IFN-gamma production were enhanced upon such interference with Cdc42 activation. These data support the notion that transience in polarization is critical for cytolytic effector function, likely by preventing cytolytic effectors from becoming trapped in nonproductive target cell interactions.

NK cells delay allograft rejection in lymphopenic hosts by downregulating the homeostatic proliferation of CD8+ T cells

T cells present in lymphopenic environments undergo spontaneous (homeostatic) proliferation resulting in expansion of the memory T cell pool. Homeostatically generated memory T cells protect the host against infection but can cause autoimmunity and allograft rejection. Therefore, understanding the mechanisms that regulate homeostatic T cell proliferation is germane to clinical settings in which lymphodepletion is used. In this study, we asked whether NK cells, which regulate immune responses in lymphocyte-replete hosts, also regulate homeostatic T cell proliferation under lymphopenic conditions. We found that T cells transferred into genetically lymphocyte-deficient RAG-/- mice proliferate faster and generate more CD8+ memory T cells if NK cells were absent. CD8+ T cells that underwent homeostatic proliferation in the presence of NK cells generated mostly effector memory (CD44highCD62Llow) lymphocytes, whereas those that divided in the absence of NK cells were skewed toward central memory (CD44highCD62Lhigh). The latter originated predominantly from proliferation of the "natural" central memory CD8+ T cell pool. Regulation of homeostatic proliferation by NK cells occurred independent of perforin but was reversed by excess IL-15. Importantly, NK depletion enhanced CD8+ T cell recovery in T cell-depleted wild-type mice and accelerated rejection of skin allografts, indicating that regulation of homeostatic proliferation by NK cells is not restricted to genetically lymphocyte-deficient animals. These results demonstrate that NK cells downregulate homeostatic CD8+ T cell proliferation in lymphopenic environments by competing for IL-15. Concomitant NK and T cell depletion may be undesirable in transplant recipients because of enhanced expansion of memory CD8+ T cells that increase the risk of rejection.

Characterization and IL-15 dependence of NK cells in humanized mice

Natural Killer cells can distinguish between healthy and malignant cells and have the unique ability to lyse tumour cells without prior sensitization. Differences between murine and human NK cells complicate the translation of this knowledge into useful therapeutics. Humanized mouse models that support the development of human leukocytes are a promising avenue of research that aims to address this problem. Here we provide an in-depth phenotypic analysis of human NK cells in Balb/c Rag2(-/-)γ(c)(-/-) mice reconstituted with human hematopoietic stem cells. We have examined the development of NK cells in bone marrow, thymous, spleen, lymph node (LN) and liver. Interestingly, in naive reconstituted mice, NK cells were found in thymus and LN, but not in bone marrow. These NK cells expressed several inhibitory and activating receptors needed for malignant cell detection. Furthermore, we confirm that administration of recombinant human interleukin-15 (rhIL-15) or Ad-vector expressing hIL-15 is able to significantly enhance NK cell development and maturation, particularly in bone marrow and liver, in this model. Our results suggest that human NK cells developed in mice may have phenotypes and tissue distributions similar to those seen in human.

Identification of an NK/T cell-restricted progenitor in adult bone marrow contributing to bone marrow- and thymic-dependent NK cells

Although bone marrow (BM) is the main site of natural killer (NK)-cell development in adult mice, recent studies have identified a distinct thymic-dependent NK pathway, implicating a possible close link between NK- and T-cell development in adult hematopoiesis. To investigate whether a potential NK-/T-lineage restriction of multipotent progenitors might take place already in the BM, we tested the full lineage potentials of NK-cell progenitors in adult BM. Notably, although Lin(-)CD122(+)NK1.1(-)DX5(-) NK-cell progenitors failed to commit to the B and myeloid lineages, they sustained a combined NK- and T-cell potential in vivo and in vitro at the single-cell level. Whereas T-cell development from NK/T progenitors is Notch-dependent, their contribution to thymic and BM NK cells remains Notch-independent. These findings demonstrate the existence of bipotent NK-/T-cell progenitors in adult BM.

Multiple hats for natural killers

Natural killer (NK) cells are a subset of lymphocytes that kill virus-infected or cancerous cells and influence adaptive immune responses via production of inflammatory cytokines. Unlike B and T lymphocytes, no transcription factors have been identified that are essential for the emergence of NK cell progenitors from their multipotent precursors. We argue that this dearth of essential factors is because of the expression of redundant transcription factors that function at the earliest stages of development. However, multiple essential transcription factors have been identified at later stages of development. Recent studies have revealed novel subsets of NK cells with differing potential for target cell lysis and cytokine production. How these subsets arise from the conventional pathway of NK cell development and identification of the transcriptional networks that control their development are major challenges for future studies.

Natural killer receptors: the burden of a name

A population of cells that expresses the NK cell receptor NKp46 and produces interleukin (IL)-22 have recently attracted considerable attention. The identity of these cells is still the subject of speculation, being variably defined as a novel NK cell subset or as a population containing conventional NK (cNK) cell precursors. In this issue, two studies shed light on this conundrum, demonstrating that NKp46(+) IL-22(+) cells and cNK cells belong to distinct lineages.

IL-7 and IL-15 independently program the differentiation of intestinal CD3-NKp46+ cell subsets from Id2-dependent precursors

The natural cytotoxicity receptor NKp46 (encoded by Ncr1) was recently shown to identify a subset of noncytotoxic, Rag-independent gut lymphocytes that express the transcription factor Rorc, produce interleukin (IL)-22, and provide innate immune protection at the intestinal mucosa. Intestinal CD3(-)NKp46(+) cells are phenotypically heterogeneous, comprising a minority subset that resembles classical mature splenic natural killer (NK) cells (NK1.1(+), Ly49(+)) but also a large CD127(+)NK1.1(-) subset of lymphoid tissue inducer (LTi)-like Rorc(+) cells that has been proposed to include NK cell precursors. We investigated the developmental relationships between these intestinal CD3(-)NKp46(+) subsets. Gut CD3(-)NKp46(+) cells were related to LTi and NK cells in requiring the transcriptional inhibitor Id2 for normal development. Overexpression of IL-15 in intestinal epithelial cells expanded NK1.1(+) cells within the gut but had no effect on absolute numbers of the CD127(+)NK1.1(-)Rorc(+) subset of CD3(-)NKp46(+) cells. In contrast, IL-7 deficiency strongly reduced the overall numbers of CD3(-)NKp46(+)NK1.1(-) cells that express Rorc and produce IL-22 but failed to restrict homeostasis of classical intestinal NK1.1(+) cells. Finally, in vivo fate-mapping experiments demonstrated that intestinal NK1.1(+)CD127(-) cells are not the progeny of Rorc-expressing progenitors, indicating that CD127(+)NK1.1(-)Rorc(+) cells are not canonical NK cell precursors. These studies highlight the independent cytokine regulation of functionally diverse intestinal NKp46(+) cell subsets.

Immune modulation with interleukin-21

Interleukin 21 (IL-21) is produced by activated CD4(+) T cells. The IL-21R shares the common receptor gamma-chain with IL-2, IL-4, IL-7, IL-9, and IL-15, is widely expressed on immune cells, and mediates a variety of effects on the immune system. IL-21 enhances the proliferation, antigen-induced activation, clonal expansion, IFN-gamma production, and cytotoxicity of NK cells and T cells. The antitumor actions of IL-21 have been variously attributed to NK cell and CD8(+) T cell cytotoxicity, CD4(+) T cell help, NKT cells, and the antiangiogenic properties induced by IFN-gamma secretion. In clinical trials IL-21 has been well tolerated and induces a unique pattern of immune activation. IL-21 is therefore an excellent candidate for use in immune therapy.

Altered phenotype of NK cells from obese rats can be normalized by transfer into lean animals

In diet-induced obese rats, leptin-mediated natural killer (NK) cell activation has been demonstrated to be impaired by abrogated intracellular JAK2-STAT3 signaling. The contribution of the obese microenvironment to this NK cell dysfunction and its reversibility remains elusive. In this study, the functions of NK cells from diet-induced obese rats after adoptive transfer into lean littermates were investigated using in vivo and in vitro approaches. Endogenous NK cells of normal-weight and diet-induced obese F344 rats were depleted in vivo. Then, NK cells from either normal-weight or obese donors were transferred. The numbers of peripheral blood NK cells were analyzed by fluorescence-activated cell sorting (FACS) and the distribution pattern of NK cells in lung and spleen by immunohistochemistry. Ob-R expression was evaluated by immunohistology and activation of intracellular target proteins of Ob-R by immunoblotting. The numbers of NK cells in blood and lung were significantly higher in obese animals compared to lean ones after transfer of NK cells from obese F344 rats. This was correlated with increased postreceptor signaling (JAK-2p, PKBpT308, ERK-2p) without altered Ob-R expression in those NK cells transferred to lean (ob-->nw) vs. obese (ob-->ob) animals. These results show for the first time that the altered phenotype of NK cells from obese rats can be normalized by generation of a physiological (metabolic) environment of lean rats.

Dendritic cells support the in vivo development and maintenance of NK cells via IL-15 trans-presentation

IL-15 is a key component that regulates the development and homeostasis of NK cells and is delivered through a mechanism termed trans-presentation. During development, multiple events must proceed to generate a functional mature population of NK cells that are vital for tumor and viral immunity. Nevertheless, how IL-15 regulates these various events and more importantly what cells provide IL-15 to NK cells to drive these events is unclear. It is known dendritic cells (DC) can activate NK cells via IL-15 trans-presentation; however, the ability of DC to use IL-15 trans-presentation to promote the development and homeostatic maintenance of NK cell has not been established. In this current study, we show that IL-15 trans-presentation solely by CD11c(+) cells assists the in vivo development and maintenance of NK cells. More specifically, DC-mediated IL-15 trans-presentation drove the differentiation of NK cells, which included the up-regulation of the activating and inhibitory Ly49 receptors. Although these cells did not harbor a mature CD11b(high) phenotype, they were capable of degranulating and producing IFN-gamma upon stimulation similar to wild-type NK cells. In addition, DC facilitated the survival of mature NK cells via IL-15 trans-presentation in the periphery. Thus, an additional role for NK-DC interactions has been identified whereby DC support the developmental and homeostatic niche of NK cells.

Loss of the pro-apoptotic BH3-only Bcl-2 family member Bim sustains B lymphopoiesis in the absence of IL-7

IL-7 is pivotal for B cell development. Proteins of the Bcl-2 family are essential regulators of lymphocyte survival. Particularly, the pro-apoptotic BH3-only members Bim and Puma mediate lymphocyte apoptosis provoked by cytokine deprivation. Herein, we addressed whether the absence of Bim or Puma within the hematopoietic compartment could bypass the requirement for IL-7-driven B cell development in adult mice. We found that deficiency of Bim, but not Puma, partially rescued B cell development in the absence of IL-7. The numbers of both sIgM(-) and sIgM(+) B cells were markedly increased in the bone marrow of recipients lacking IL-7 upon reconstitution with Bim-deficient hematopoietic progenitors, compared with their control or Puma-deficient counterparts. The augmentation of B cell lymphopoiesis in the absence of Bim was reflected in the mature peripheral compartment by an increase in both the number of immature and mature B cells in the spleen and in the circulating IgM levels. Bim-deficient B cells were also increased in IL-7-sufficient recipients suggesting that peripheral B cells homeostasis is governed by a Bim-dependent and IL-7-independent mechanism. Our data highlight the role of Bim as a key regulator of cell survival during B lymphocyte development in vivo.

Distinct and overlapping patterns of cytokine regulation of thymic and bone marrow-derived NK cell development

Although bone marrow (BM) represents the main site for postnatal NK cell development, recently a distinct thymic-dependent NK cell pathway was identified. These studies were designed to investigate the role of cytokines in regulation of thymic NK cells and to compare with established regulatory pathways of BM-dependent NK cell compartment. The common cytokine receptor gamma-chain (Il2rg) essential for IL-15-induced signaling, and FMS-like tyrosine kinase 3 (FLT3) receptor ligand (Flt3l) were previously identified as important regulatory pathways of the BM NK cell compartment based on lack of function studies in mice, however their complementary action remains unknown. By investigating mice double-deficient in Il2rg and Flt3l (Flt3l(-/-) Il2rg(-/-)), we demonstrate that FLT3L is important for IL2Rg-independent maintenance of both immature BM as well as peripheral NK cells. In contrast to IL-7, which is dispensable for BM but important for thymic NK cells, IL-15 has a direct and important role in both thymic and BM NK cell compartments. Although thymic NK cells were not affected in Flt3l(-/-) mice, Flt3l(-/-)Il2rg(-/-) mice lacked detectable thymic NK cells, suggesting that FLT3L is also important for IL-2Rg-independent maintenance of thymic NK cells. Thus, IL-2Rg cytokines and FLT3L play complementary roles and are indispensable for homeostasis of both BM and thymic dependent NK cell development, suggesting that the cytokine pathways crucial for these two distinct NK cell pathways are largely overlapping.

NK cells in the CD19- B220+ bone marrow fraction are increased in senescence and reduce E2A and surrogate light chain proteins in B cell precursors

E2A encoded proteins, key transcriptional regulators in B lineage specification and commitment, have been shown to decrease in B cell precursors in old age. E2A regulates genes encoding the surrogate light chain proteins lambda5 and VpreB. In old age, B cell precursors express less surrogate light chain and this results in compromised pre-B cell receptor function and diminished expansion of new pre-B cells in senescence. Herein, we show that aged bone marrow has increased Hardy Fraction A (CD19(-) B220(+)) cells, including NK cells, that can inhibit both E47 (E2A) protein and surrogate light chain protein expression in B cell precursors. In vitro, NK-associated inhibition of E47 protein is contact-independent and partially reversed by neutralization of TNFalpha. In vivo, depletion of NK cells in aged mice by treatment with anti-asialo GM1 antibody led to restoration of surrogate light chain protein levels to that typical of young B cell precursors. These studies suggest that NK cells, within the CD19(-) B220(+) bone marrow cell fraction, may contribute to a bone marrow microenvironment that has the potential to negatively regulate E47 (E2A) as well as surrogate light chain levels in B cell precursors in old age.

Microbial flora drives interleukin 22 production in intestinal NKp46+ cells that provide innate mucosal immune defense

Natural killer (NK) cells are innate lymphocytes with spontaneous antitumor activity, and they produce interferon-gamma (IFN-gamma) that primes immune responses. Whereas T helper cell subsets differentiate from naive T cells via specific transcription factors, evidence for NK cell diversification is limited. In this report, we characterized intestinal lymphocytes expressing the NK cell natural cytotoxicity receptor NKp46. Gut NKp46+ cells were distinguished from classical NK cells by limited IFN-gamma production and absence of perforin, whereas several subsets expressed the nuclear hormone receptor retinoic acid receptor-related orphan receptor t (RORgammat) and interleukin-22 (IL-22). Intestinal NKp46+IL-22+ cells were generated via a local process that was conditioned by commensal bacteria and required RORgammat. Mice lacking IL-22-producing NKp46+ cells showed heightened susceptibility to the pathogen Citrobacter rodentium, consistent with a role for intestinal NKp46+ cells in immune protection. RORgammat-driven diversification of intestinal NKp46+ cells thereby specifies an innate cellular defense mechanism that operates at mucosal surfaces.

Exogenous IL-15 in combination with IL-15R alpha rescues natural killer cells from apoptosis induced by chronic alcohol consumption

BACKGROUND

Chronic alcohol consumption reduces the percentage and number of peripheral natural killer (NK) cells in mice and in humans. The underlying mechanism for these changes is only partly known. We recently found that chronic alcohol consumption inhibits NK cell release from the bone marrow (BM) and that this is associated with a decrease in splenic NK cells. The number of peripheral NK cells is tightly controlled by homeostatic proliferation. It is not known whether this mechanism is initiated in response to the reduction in splenic NK cells, or if so, why the steady state levels of NK cells are not restored.

METHODS

To examine this mechanism, female C57BL/6 mice were given 20% w/v alcohol in the drinking water for 3 months. NK cell proliferation and apoptosis were determined before and after treatment with IL-15 alone or combined with its alpha receptor.

RESULTS

Chronic alcohol consumption invoked homeostatic proliferation of splenic NK cells in an attempt to return NK cells to normal levels; however, this did not happen due to enhanced apoptosis of NK cells relative to proliferation. Chronic alcohol consumption decreased IL-15 producing cells in the spleen but not in the BM. The numbers of NK cells in the alcohol-consuming mice returned to normal levels in the spleen and were higher than normal in the BM after 2 daily injections of IL-15; however, the enhanced rate of apoptosis due to alcohol consumption was not decreased in the spleen or BM. Combined IL-15 and IL-15R alpha treatment decreased apoptosis of NK cells from alcohol-consuming mice to levels similar to untreated water-drinking mice and greatly increased the percentage and number of NK cells in both the spleen and BM.

CONCLUSION

Chronic alcohol consumption causes a self-unrecoverable loss of NK cells in the spleen by compromising NK cell release from the BM and enhancing splenic NK cell apoptosis that can be reversed with IL-15/IL-15R alpha treatment.

Clinical and immunologic consequences of a somatic reversion in a patient with X-linked severe combined immunodeficiency

X-linked severe combined immunodeficiency is a life-threatening disorder caused by mutations in the gene encoding the interleukin-2 receptor gamma chain (IL2RG). Hypomorphic mutations and reversion of mutations in subpopulations of cells can result in variant clinical phenotypes, making diagnosis and treatment difficult. We describe a 5-year-old boy with mild susceptibility to infection who was investigated for a mutation in IL2RG due to persistent natural killer (NK)– and T-cell lymphopenia. A functionally relevant novel T466C point mutation was found in B, NK, and epithelial cells, whereas α/β and γ/δ T cells showed the normal gene sequence, suggesting reversion of the mutation in a common T-cell precursor. This genetic correction in T cells resulted in a diverse T-cell repertoire and significant immunity despite failure to produce specific antibodies linked to an intrinsic defect of mutant B cells. These observations confirm the potential of revertant T-cell precursors to reconstitute immune function, but questions remain on the longevity of revertant cells implicating the need for careful follow up and early consideration of hematopoietic stem cell transplantation (HSCT).

The p110 delta of PI3K plays a critical role in NK cell terminal maturation and cytokine/chemokine generation

Phosphatidylinositol 3-kinases (PI3Ks) play a critical role in regulating B cell receptor– and T cell receptor–mediated signaling. However, their role in natural killer (NK) cell development and functions is not well understood. Using mice expressing p110δ^D910A, a catalytically inactive p110δ, we show that these mice had reduced NK cellularity, defective Ly49C and Ly49I NK subset maturation, and decreased CD27^High NK numbers. p110δ inactivation marginally impaired NK-mediated cytotoxicity against tumor cells in vitro and in vivo. However, NKG2D, Ly49D, and NK1.1 receptor–mediated cytokine and chemokine generation by NK cells was severely affected in these mice. Further, p110δ^D910A/D910A NK cell–mediated antiviral responses through natural cytotoxicity receptor 1 were reduced. Analysis of signaling events demonstrates that p110δ^D910A/D910A NK cells had a reduced c-Jun N-terminal kinase 1/2 phosphorylation in response to NKG2D-mediated activation. These results reveal a previously unrecognized role of PI3K-p110δ in NK cell development and effector functions.

RNA silencing and HIV: a hypothesis for the etiology of the severe combined immunodeficiency induced by the virus

A novel intrinsic HIV-1 antisense gene was previously described with RNA initiating from the region of an HIV-1 antisense initiator promoter element (HIVaINR). The antisense RNA is exactly complementary to HIV-1 sense RNA and capable of forming ~400 base-pair (bp) duplex RNA in the region of the long terminal repeat (LTR) spanning the beginning portion of TAR in the repeat (R) region and extending through the U3 region. Duplex or double-stranded RNA of several hundred nucleotides in length is a key initiating element of RNA interference (RNAi) in several species. This HIVaINR antisense RNA is also capable of forming multiple stem-loop or hairpin-like secondary structures by M-fold analysis, with at least one that perfectly fits the criteria for a microRNA (miRNA) precursor. MicroRNAs (miRNAs) interact in a sequence-specific manner with target messenger RNAs (mRNAs) to induce either cleavage of the message or impede translation. Human mRNA targets of the predicted HIVaINR antisense RNA (HAA) microRNAs include mRNA for the human interleukin-2 receptor gamma chain (IL-2RG), also called the common gamma (γc) receptor chain, because it is an integral part of 6 receptors mediating interleukin signalling (IL-2R, IL-4R, IL-7R, IL-9R, IL-15R and IL-21R). Other potential human mRNA targets include interleukin-15 (IL-15) mRNA, the fragile × mental retardation protein (FMRP) mRNA, and the IL-1 receptor-associated kinase 1 (IRAK1) mRNA, amongst others. Thus the proposed intrinsic HIVaINR antisense RNA microRNAs (HAAmiRNAs) of the human immunodeficiency virus form complementary targets with mRNAs of a key human gene in adaptive immunity, the IL-2Rγc, in which genetic defects are known to cause an X-linked severe combined immunodeficiency syndrome (X-SCID), as well as mRNAs of genes important in innate immunity. A new model of intrinsic RNA silencing induced by the HIVaINR antisense RNA in the absence of Tat is proposed, with elements suggestive of both small interfering RNA (siRNA) and miRNA.

Effect of cyclosporin A on interleukin-15-activated umbilical cord blood natural killer cell function

BACKGROUND

Interleukin (IL)-15-activated natural killer (NK) cells may provide a graft-versus-leukemia (GvL) effect post-umbilical cord blood (CB) transplantation. The effect of cyclosporin A (CsA), a calcineurin-inhibitor used for prophylaxis of graft-versus-host disease (GvHD), on IL-15-mediated activation, cytotoxic function and target-induced apoptosis of CB NK cells, was examined in comparison with adult peripheral blood (APB) NK cells.

METHODS

CsA was added to anti-CD3+/-IL-15-stimulated CB and APB mononuclear cells (MNC) for a 5-day incubation. CD3- CD56+ NK cell recovery was determined by flow cytometric analysis. Magnetic bead-purified CB and APB NK cells were stimulated with IL-15 for 18 h under the influence of CsA. NK activation (CD69), K562 cytotoxicity and NK-K562 interactions (CD54, perforin and annexin-V expression 4 h following contact with K562 cells) were assessed by flow cytometry.

RESULTS

CsA decreased CD3- CD56+ NK cell recovery in anti-CD3-stimulated CB MNC 5-day cultures, an effect that could be counteracted by IL-15; comparable effects were observed with APB. Short-term (18-h) experiments revealed that CsA down-regulated K562 cytotoxicity of IL-15-activated (P=0.018) but not resting (P=0.268) purified CB NK cells. IL-15-induced CB NK CD69 expression showed increased CsA sensitivity over APB (P=0.012). CsA down-regulated K562 cell-induced CD54 (P=0.028) but not perforin (P=0.416) expression of IL-15-activated CB NK cells. Target-induced apoptosis of IL-15-activated CB (P=0.043) but not APB (P=0.144) NK cells was decreased by CsA.

DISCUSSION

We have demonstrated differential CsA sensitivity of IL-15-activated CB and APB NK cells. These results may be used to improve the design of IL-15-activated NK cell adoptive immunotherapy in cancer patients receiving CsA post-CB transplantation.

Cutting edge: Priming of NK cells by IL-18

Recent evidence suggests that NK cells require priming to display full effector activity. In this study, we demonstrate that IL-18 contributed to this phenomenon. IL-18 signaling-deficient NK cells were found to be unable to secrete IFN-gamma in response to ex vivo stimulation with IL-12. This was not due to a costimulatory role of IL-18, because blocking IL-18 signaling during the ex vivo stimulation with IL-12 did not alter IFN-gamma production by wild-type NK cells. Rather, we demonstrate that IL-18 primes NK cells in vivo to produce IFN-gamma upon subsequent stimulation with IL-12. Importantly, IL-12-induced IFN-gamma transcription by NK cells was comparable in IL-18 signaling-deficient and -sufficient NK cells. This suggests that priming by IL-18 leads to an improved translation of IFN-gamma mRNA. These results reveal a novel type of cooperation between IL-12 and IL-18 that requires the sequential action of these cytokines.

TGF-beta signaling in dendritic cells is a prerequisite for the control of autoimmune encephalomyelitis

One unresolved issue in immune tolerance is what prevents self-reactive T cells from activation. In this study, we used a transgenic mouse model of targeted functional inactivation of TGF-betaR signaling in CD11c(+) cells (CD11c(dnR) mice) and showed a direct impact on the development of experimental autoimmune encephalomyelitis (EAE). We found that MOG(35-55) immunization of CD11c(dnR) mice results in strong inflammation of CNS, high frequency of T cells in CNS, increased levels of T helper 1 (T(H)1) and T(H)17 cytokines in the periphery, and lack of remission from EAE. Once crossed with mice prone to autoimmunity, double-transgenic CD11c(dnR)Mog(TCR) mice revealed a spontaneous EAE-like disease characterized by early infiltration of activated myelin-specific T cells into CNS, activation of microglial cells, inflammation of CNS, dysfunction of locomotion, and premature death. We constructed chimeric mice and demonstrated that inactivation of TGF-betaR signaling in dendritic cells (DCs) results in augmented EAE-associated T cell responses. Our data provide direct evidence that TGF-beta can control autoimmunity via actions on DCs.

Extrinsic and intrinsic regulation of early natural killer cell development

Natural killer (NK) cells are lymphocytes that play a critical role in both adaptive and innate immune responses. These cells develop from multipotent progenitors in the embryonic thymus and neonatal or adult bone marrow and recent evidence suggests that a subset of these cells may develop in the thymus. Thymus- and bone marrow-derived NK cells have unique phenotypes and functional abilities supporting the hypothesis that the microenvironment dictates the outcome of NK cell development. A detailed understanding of the mechanisms controlling this developmental program will be required to determine how alterations in NK cell development lead to disease and to determine how to harness this developmental program for therapeutic purposes. In this review, we discuss some of the known extrinsic stromal-cell derived factors and cell intrinsic transcription factors that function in guiding NK cell development.

Expanded donor natural killer cell and IL-2, IL-15 treatment efficacy in allogeneic hematopoietic stem cell transplantation

Graft-versus-host disease (GVHD), leukemia relapse, and immune deficiency remain the major limitations of allogeneic hematopoietic stem cell transplantation (allo-HSCT). Donor natural killer cells (NK) and cytokines have good potential in GVHD prevention and immune reconstitution enhancement. Improved survival after allo-HSCT therefore requires effective prophylaxis to reduce GVHD and strategy to mediate graft-versus-leukemia (GVL) effect. We studied the administration of expanded donor NK cell infusion and interleukin-2 (IL-2) and IL-15 mixture treatment in a murine allo-HSCT model for its effects on GVHD, immune reconstitution and leukemia relapse. In the GVHD model, recipient mice were reconstituted with bone marrow (BM) cells and splenocytes via vein. In the leukemia model, recipient mice were inoculated with EL9611 leukemia cells via vein 8 d prior to transplant. NK cell infusion mice group had lower clinical GVHD scores and suffered less severe GVHD-associated weight loss than control mice group. 90% of control mice died of leukemia relapse within 52 d post-transplant. NK cell infusion and IL-2 and IL-15 treatment recipient mice had improved survival compared with control mice. NK cell infusion and IL-2 and IL-15 treatment recipient mice had also accelerated lymphoid immune reconstitution compared with control mice group. Expanded donor NK cell infusion and IL-2 and IL-15 treatment could promote lymphoid immune reconstitution, mitigate GVHD, and reduce leukemia relapse in allo-HSCT recipients. The findings may have important significance for complication prevention in clinical allo-HSCT.

Interleukin-21: basic biology and implications for cancer and autoimmunity

Interleukin-21 (IL-21), a potent immunomodulatory four-alpha-helical-bundle type I cytokine, is produced by NKT and CD4(+) T cells and has pleiotropic effects on both innate and adaptive immune responses. These actions include positive effects such as enhanced proliferation of lymphoid cells, increased cytotoxicity of CD8(+) T cells and natural killer (NK) cells, and differentiation of B cells into plasma cells. Conversely, IL-21 also has direct inhibitory effects on the antigen-presenting function of dendritic cells and can be proapoptotic for B cells and NK cells. IL-21 is also produced by Th17 cells and is a critical regulator of Th17 development. The regulatory activity of IL-21 is modulated by the differentiation state of its target cells as well as by other cytokines or costimulatory molecules. IL-21 has potent antitumor activity but is also associated with the development of autoimmune disease. IL-21 transcription is dependent on a calcium signal and NFAT sites, and IL-21 requires Stat3 for its signaling. The key to harnessing the power of IL-21 will depend on better understanding its range of biological actions, its mechanism of action, and the molecular basis of regulation of expression of IL-21 and its receptor.

Deletion of PI3K-p85alpha gene impairs lineage commitment, terminal maturation, cytokine generation and cytotoxicity of NK cells

Class IA phosphotidylinositol-3-kinases (PI3Ks) are a family of p85/p110 heterodimeric lipid kinases that are important in regulating signaling events in B and T cells. However, their role in natural killer (NK) cells is not understood. Here, using mice that lack the regulatory p85alpha subunit and its alternatively spliced variants p55alpha/p50alpha (collectively termed as p85alpha(-/-)), we defined the role of PI3K in NK cell development and function. p85alpha(-/-) mice had impaired lineage commitment leading to reduced NK cellularity in the bone marrow and liver. p85alpha(-/-) NK cells showed a defective Ly49 subset specification and a decreased expression of CD43. Lack of p85alpha severely reduced the NK-mediated cytotoxicity against tumor cells representing 'induced-self' and 'missing-self'. More importantly, NKG2D and NK1.1 receptor-mediated cytokine and chemokine generation was significantly compromised in p85alpha(-/-) NK cells. These results reveal a previously unrecognized role of p85alpha in the development, terminal maturation, cytokine/chemokine generation and tumor clearance of NK cells.

Death by a thousand cuts: granzyme pathways of programmed cell death

The granzymes are cell death-inducing enzymes, stored in the cytotoxic granules of cytotoxic T lymphocytes and natural killer cells, that are released during granule exocytosis when a specific virus-infected or transformed target cell is marked for elimination. Recent work suggests that this homologous family of serine esterases can activate at least three distinct pathways of cell death. This redundancy likely evolved to provide protection against pathogens and tumors with diverse strategies for evading cell death. This review discusses what is known about granzyme-mediated pathways of cell death as well as recent studies that implicate granzymes in immune regulation and extracellular proteolytic functions in inflammation.

Interleukin 21: a cytokine/cytokine receptor system that has come of age

Interleukin-21 (IL-21) and its receptor represent the sixth cytokine system whose actions were recognized to require the common cytokine receptor gamma chain. IL-21 is produced by activated CD4+ T cells, natural killer T cells, and follicular T helper cells and has actions on a range of lymphohematopoietic lineages. Among its many effects, IL-21 serves a critical role for immunoglobulin production and terminal B cell differentiation, acts as a T cell comitogen and can drive the expansion of CD8+ T cells, can negatively regulate dendritic cell function and plays an essential role in the differentiation of Th17 cells. Importantly, IL-21 is implicated in the pathogenesis of autoimmunity and exhibits potent actions as an antitumor agent. The ability to regulate and manipulate the actions of IL-21, therefore, has important implications for immunoregulation and the therapy of human disease.

In vivo dynamics of intraepidermal CD8+ T cells and CD4+ T cells during the evolution of fixed drug eruption

BACKGROUND

Although a severe form of fixed drug eruption (FDE) clinically and histologically mimics toxic epidermal necrolysis (TEN), subsequent evolution of the two conditions is quite different. It remains unknown, however, which factors determine whether these lesions resolve spontaneously or subsequently progress to TEN.

OBJECTIVES

Because epidermal injury in TEN can be locally reproduced in the evolving FDE lesions, we sought to investigate how epidermal damage can be induced in the evolving FDE lesions and how disease progression to TEN can be prevented, by analysing the FDE lesions induced by clinical challenge with the causative drug.

METHODS

We immunohistochemically investigated in vivo dynamics of T-cell trafficking and activation that occur in the evolving FDE lesions using sequential biopsy specimens obtained at multiple time points from the FDE lesions.

RESULTS

Intraepidermal CD8+ T cells, which are resident in the lesional epidermis as a stable homogeneous population of memory T cells, transiently acquire a natural killer-like phenotype and express cytotoxic granules upon activation. The influx into the epidermis of CD4+ T cells including Foxp3+ regulatory T cells (Tregs) during the evolution serves to ameliorate epidermal damage induced by activation of the intraepidermal CD8+ T cells. Interleukin-15 derived from the lesional epidermis could maintain the survival of the intraepidermal CD8+ T cells even in the absence of antigenic stimulus over a prolonged period of time (> 4 years).

CONCLUSIONS

Whether Tregs could migrate to the lesions upon activation of intraepidermal CD8+ T cells would determine whether the inflammation becomes resolved spontaneously or progresses to TEN.

The ontogeny and fate of NK cells marked by permanent DNA rearrangements

A subset of NK cells bears incomplete V(D)J rearrangements, but neither the consequence to cell activities nor the precise developmental stages in which recombination occurs is known. These are important issues, as recombination errors cause cancers of the B and T lineages. Using transgenic recombination reporter mice to examine NK cell dynamics in vivo, we show that recombination(+) NK cells have distinct developmental patterns in the BM, including reduced homeostatic proliferation and diminished Stat5 phosphorylation. In the periphery, both recombination(+) and recombination(-) NK cells mediate robust functional responses including IFN-gamma production, cytolysis, and tumor homing, suggesting that NK cells with distinct developmental histories can be found together in the periphery. We also show that V(D)J rearrangement marks both human cytolytic (CD56(dim)) and immunoregulatory (CD56(bright)) populations, demonstrating the distribution of permanent DNA rearrangements across major NK cell subsets in man. Finally, direct quantification of rag transcripts throughout NK cell differentiation in both mouse and man establishes the specific developmental stages that are susceptible to V(D)J rearrangement. Together, these data demonstrate that multipotent progenitors rather than lineage-specified NK progenitors are targets of V(D)J recombination and that NK cells bearing the relics of earlier V(D)J rearrangements have different developmental dynamics but robust biological capabilities in vivo.

Th2 lymphoproliferative disorder of LatY136F mutant mice unfolds independently of TCR-MHC engagement and is insensitive to the action of Foxp3+ regulatory T cells

Mutant mice where tyrosine 136 of linker for activation of T cells (LAT) was replaced with a phenylalanine (Lat(Y136F) mice) develop a fast-onset lymphoproliferative disorder involving polyclonal CD4 T cells that produce massive amounts of Th2 cytokines and trigger severe inflammation and autoantibodies. We analyzed whether the Lat(Y136F) pathology constitutes a bona fide autoimmune disorder dependent on TCR specificity. Using adoptive transfer experiments, we demonstrated that the expansion and uncontrolled Th2-effector function of Lat(Y136F) CD4 cells are not triggered by an MHC class II-driven, autoreactive process. Using Foxp3EGFP reporter mice, we further showed that nonfunctional Foxp3(+) regulatory T cells are present in Lat(Y136F) mice and that pathogenic Lat(Y136F) CD4 T cells were capable of escaping the control of infused wild-type Foxp3(+) regulatory T cells. These results argue against a scenario where the Lat(Y136F) pathology is primarily due to a lack of functional Foxp3(+) regulatory T cells and suggest that a defect intrinsic to Lat(Y136F) CD4 T cells leads to a state of TCR-independent hyperactivity. This abnormal status confers Lat(Y136F) CD4 T cells with the ability to trigger the production of Abs and of autoantibodies in a TCR-independent, quasi-mitogenic fashion. Therefore, despite the presence of autoantibodies causative of severe systemic disease, the pathological conditions observed in Lat(Y136F) mice unfold in an Ag-independent manner and thus do not qualify as a genuine autoimmune disorder.

Regulation of interferon-gamma during innate and adaptive immune responses

Interferon-gamma (IFN-gamma) is crucial for immunity against intracellular pathogens and for tumor control. However, aberrant IFN-gamma expression has been associated with a number of autoinflammatory and autoimmune diseases. This cytokine is produced predominantly by natural killer (NK) and natural killer T (NKT) cells as part of the innate immune response, and by Th1 CD4 and CD8 cytotoxic T lymphocyte (CTL) effector T cells once antigen-specific immunity develops. Herein, we briefly review the functions of IFN-gamma, the cells that produce it, the cell extrinsic signals that induce its production and influence the differentiation of naïve T cells into IFN-gamma-producing effector T cells, and the signaling pathways and transcription factors that facilitate, induce, or repress production of this cytokine. We then review and discuss recent insights regarding the molecular regulation of IFN-gamma, focusing on work that has led to the identification and characterization of distal regulatory elements and epigenetic modifications with the IFN-gamma locus (Ifng) that govern its expression. The epigenetic modifications and three-dimensional structure of the Ifng locus in naive CD4 T cells, and the modifications they undergo as these cells differentiate into effector T cells, suggest a model whereby the chromatin architecture of Ifng is poised to facilitate either rapid opening or silencing during Th1 or Th2 differentiation, respectively.

Developmental pathways that generate natural-killer-cell diversity in mice and humans

Natural killer (NK) cells are large granular lymphocytes capable of producing inflammatory cytokines and spontaneously killing malignant, infected or 'stressed' cells. These NK-cell functions are controlled by cell-surface receptors that titrate stimulatory and inhibitory signals. However, we remain puzzled about where and when NK cells develop and differentiate, and this has fuelled the debate over the diversification of the peripheral NK-cell pool: are NK cells functionally homogeneous or are there subsets with specialized effector functions? In this Review, we consider the developmental relationships and biological significance of the diverse NK-cell subsets in mice and humans, and discuss how new humanized mouse models may help to characterize them further.

gamma(c) cytokines provide multiple homeostatic signals to naive CD4(+) T cells

Cytokines signaling through receptors sharing the common gamma chain (gamma(c)), including IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21, are critical for the generation and peripheral homeostasis of B, T and NK cells. To identify unique or redundant roles for gamma(c) cytokines in naive CD4(+) T cells, we compared monoclonal populations of CD4(+) T cells from TCR-Tg mice that were gamma(c) (+), gamma(c) (-), CD127(-/-) or CD122(-/-). We found that gamma(c) (-) naive CD4(+) T cells failed to accumulate in the peripheral lymphoid organs and the few remaining cells were characterized by small size, decreased expression of MHC class I and enhanced apoptosis. By over-expressing human Bcl-2, peripheral naive CD4(+) T cells that lack gamma(c) could be rescued. Bcl-2(+) gamma(c) (-) CD4(+) T cells demonstrated enhanced survival characteristics in vivo and in vitro, and could proliferate normally in vitro in response to antigen. Nevertheless, Bcl-2(+) gamma(c) (-) CD4(+) T cells remained small in size, and this phenotype was not corrected by enforced expression of an activated protein kinase B. We conclude that gamma(c) cytokines (primarily but not exclusively IL-7) provide Bcl-2-dependent as well as Bcl-2-independent signals to maintain the phenotype and homeostasis of the peripheral naive CD4(+) T cell pool.