Differential Roles for IL-15R α-Chain in NK Cell Development and Ly-49 Induction

Role of NK Cells in Skin Wound Healing of Mice

NK cells are best known for their killing of virus-infected cells and tumor cells via release of cytotoxic factors. However, NK cells can also produce growth factors and cytokines, and thus have the potential to influence physiological processes such as wound healing. In this study, we test the hypothesis that NK cells play a physiological role in skin wound healing of C57BL/6J mice. Immunohistochemical and flow cytometry assays showed that NK cells accumulate in excisional skin wounds, peaking on day 5 postinjury. We also found that NK cells proliferate locally in wounds, and blocking IL-15 activity locally reduces NK cell proliferation and accumulation in wounds. Wound NK cells exhibit primarily a mature CD11b+CD27- and NKG2A+NKG2D- phenotype and express LY49I and proinflammatory cytokines such as IFN-γ, Tnf-a, and Il-1β. Systemic depletion of NK cells resulted in enhanced re-epithelization and collagen deposition, suggesting a negative role for these cells in skin wound healing. Depletion of NK cells did not influence accumulation of neutrophils or monocytes/macrophages in wounds but did reduce expression of IFN-γ, Tnf-a, and Il-1β, indicating that NK cells contribute to proinflammatory cytokine expression in wounds. In short, NK cells may impede physiological wound healing via production of proinflammatory cytokines.

IL15 and Anti-PD-1 Augment the Efficacy of Agonistic Intratumoral Anti-CD40 in a Mouse Model with Multiple TRAMP-C2 Tumors

PURPOSE

IL15 promotes activation and maintenance of natural killer (NK) and CD8+ T effector memory cells making it a potential immunotherapeutic agent for the treatment of cancer. However, monotherapy with IL15 was ineffective in patients with cancer, indicating that it would have to be used in combination with other anticancer agents. The administration of high doses of common gamma chain cytokines, such as IL15, is associated with the generation of "helpless" antigen-nonspecific CD8 T cells. The generation of the tumor-specific cytotoxic T cells can be mediated by CD40 signaling via agonistic anti-CD40 antibodies. Nevertheless, parenteral administration of anti-CD40 antibodies is associated with unacceptable side effects, such as thrombocytopenia and hepatic toxicity, which can be avoided by intratumoral administration.

EXPERIMENTAL DESIGN

We investigated the combination of IL15 with an intratumoral anti-CD40 monoclonal antibody (mAb) in a dual tumor TRAMP-C2 murine prostate cancer model and expanded the regimen to include an anti-PD-1 mAb.

RESULTS

Here we demonstrated that anti-CD40 given intratumorally not only showed significant antitumor activity in treated tumors, but also noninjected contralateral tumors, indicative of abscopal efficacy. The combination of IL15 with intratumoral anti-CD40 showed an additive immune response with an increase in the number of tumor-specific tetramer-positive CD8 T cells. Furthermore, the addition of anti-PD-1 further improved efficacy mediated by the anti-CD40/IL15 combination.

CONCLUSIONS

These studies support the initiation of a clinical trial in patients with cancer using IL15 in association with the checkpoint inhibitor, anti-PD-1, and intratumoral optimized anti-CD40.

Implications of a 'Third Signal' in NK Cells

Innate and adaptive immune systems are evolutionarily divergent. Primary signaling in T and B cells depends on somatically rearranged clonotypic receptors. In contrast, NK cells use germline-encoded non-clonotypic receptors such as NCRs, NKG2D, and Ly49H. Proliferation and effector functions of T and B cells are dictated by unique peptide epitopes presented on MHC or soluble humoral antigens. However, in NK cells, the primary signals are mediated by self or viral proteins. Secondary signaling mediated by various cytokines is involved in metabolic reprogramming, proliferation, terminal maturation, or memory formation in both innate and adaptive lymphocytes. The family of common gamma (γc) cytokine receptors, including IL-2Rα/β/γ, IL-7Rα/γ, IL-15Rα/β/γ, and IL-21Rα/γ are the prime examples of these secondary signals. A distinct set of cytokine receptors mediate a ‘third’ set of signaling. These include IL-12Rβ1/β2, IL-18Rα/β, IL-23R, IL-27R (WSX-1/gp130), IL-35R (IL-12Rβ2/gp130), and IL-39R (IL-23Rα/gp130) that can prime, activate, and mediate effector functions in lymphocytes. The existence of the ‘third’ signal is known in both innate and adaptive lymphocytes. However, the necessity, context, and functional relevance of this ‘third signal’ in NK cells are elusive. Here, we define the current paradigm of the ‘third’ signal in NK cells and enumerate its clinical implications.

Logical modeling of thymus and natural killer lymphocyte differentiation

Thymus (T) and natural killer (NK) lymphocytes are important barriers against diseases. Therefore, it is necessary to understand regulatory mechanisms related to the cell fate decisions involved in the production of these cells. Although some individual information related to T and NK lymphocyte cell fate decisions have been revealed, the related network and its dynamical characteristics still have not been well understood. By integrating individual information and comparing with experimental data, we construct a comprehensive regulatory network and a logical model related to T and NK lymphocyte differentiation. We aim to explore possible mechanisms of how each lineage differentiation is realized by systematically screening individual perturbations. When determining the perturbation strategies, the state transition can be used to identify the roles of specific genes in cell type selection and reprogramming. In agreement with experimental observations, the dynamics of the model correctly restates the cell differentiation processes from common lymphoid progenitors to CD4+ T cells, CD8+ T cells, and NK cells. Our analysis reveals that some specific perturbations can give rise to directional cell differentiation or reprogramming. We test our in silico results by using known experimental observations. The integrated network and the logical model presented here might be a good candidate for providing qualitative mechanisms of cell fate specification involved in T and NK lymphocyte cell fate decisions.

Transcription Factors Associated With IL-15 Cytokine Signaling During NK Cell Development

Natural killer (NK) cells are lymphocytes primarily involved in innate immunity and possess important functional properties in anti-viral and anti-tumor responses; thus, these cells have broad potential for clinical utilization. NK cells originate from hematopoietic stem cells (HSCs) through the following two independent and continuous processes: early commitment from HSCs to IL-15-responsive NK cell progenitors (NKPs) and subsequent differentiation into mature NK cells in response to IL-15. IL-15 is the most important cytokine for NK cell development, is produced by both hematopoietic and nonhematopoietic cells, and functions through a distinct delivery process termed transpresentation. Upon being transpresented to NK cells, IL-15 contributes to NK cell development via the activation of several downstream signaling pathways, including the Ras-MEK-MAPK, JAK-STAT5, and PI3K-ATK-mTOR pathways. Nonetheless, the exact role of IL-15 in NK cell development has not been discussed in a consecutive and comprehensive manner. Here, we review current knowledge about the indispensable role of IL-15 in NK cell development and address which cells produce IL-15 to support NK cell development and when IL-15 exerts its function during multiple developmental stages. Specifically, we highlight how IL-15 supports NK cell development by elucidating the distinct transpresentation of IL-15 to NK cells and revealing the downstream target of IL-15 signaling during NK cell development.

Gab3 is required for IL-2- and IL-15-induced NK cell expansion and limits trophoblast invasion during pregnancy

The scaffolding protein Grb2-associated binding protein 3 (Gab3) is a member of the Gab family, whose functions have remained elusive. Here, we identify Gab3 as a key determinant of peripheral NK cell expansion. Loss of Gab3 resulted in impaired IL-2 and IL-15-induced NK cell priming and expansion due to a selective impairment in MAPK signaling but not STAT5 signaling. In vivo, we found that Gab3 is required for recognition and elimination of "missing-self" and tumor targets. Unexpectedly, our studies also revealed that Gab3 plays an important role during pregnancy. Gab3-deficient mice exhibited impaired uterine NK cell expansion associated with abnormal spiral artery remodeling and increased trophoblast invasion in the decidua basalis. This coincided with stillbirth, retained placenta, maternal hemorrhage, and undelivered fetoplacental units at term. Thus, Gab3 is a key component required for cytokine-mediated NK cell priming and expansion that is essential for antitumor responses and limits trophoblast cell invasion during pregnancy.

Trans-endocytosis of intact IL-15Rα-IL-15 complex from presenting cells into NK cells favors signaling for proliferation

Interleukin 15 (IL-15) is an essential cytokine for the survival and proliferation of natural killer (NK) cells. IL-15 activates signaling by the β and common γ (γ_c) chain heterodimer of the IL-2 receptor through trans-presentation by cells expressing IL-15 bound to the α chain of the IL-15 receptor (IL-15Rα). We show here that membrane-associated IL-15Rα-IL-15 complexes are transferred from presenting cells to NK cells through trans-endocytosis and contribute to the phosphorylation of ribosomal protein S6 and NK cell proliferation. NK cell interaction with soluble or surface-bound IL-15Rα-IL-15 complex resulted in Stat5 phosphorylation and NK cell survival at a concentration or density of the complex much lower than required to stimulate S6 phosphorylation. Despite this efficient response, Stat5 phosphorylation was reduced after inhibition of metalloprotease-induced IL-15Rα-IL-15 shedding from trans-presenting cells, whereas S6 phosphorylation was unaffected. Conversely, inhibition of trans-endocytosis by silencing of the small GTPase TC21 or expression of a dominant-negative TC21 reduced S6 phosphorylation but not Stat5 phosphorylation. Thus, trans-endocytosis of membrane-associated IL-15Rα-IL-15 provides a mode of regulating NK cells that is not afforded to IL-2 and is distinct from activation by soluble IL-15. These results may explain the strict IL-15 dependence of NK cells and illustrate how the cellular compartment in which receptor-ligand interaction occurs can influence functional outcome.

IL-15 is a component of the inflammatory milieu in the tumor microenvironment promoting antitumor responses

Previous studies have provided evidence that IL-15 expression within human tumors is crucial for optimal antitumor responses; however, the regulation of IL-15 within the tumor microenvironment (TME) is unclear. We report herein, in analyses of mice implanted with various tumor cell lines, soluble IL-15/IL-15Rα complexes (sIL-15 complexes) are abundant in the interstitial fluid of tumors with expression preceding the infiltration of tumor-infiltrating lymphocytes. Moreover, IL-15 as well as type I IFN, which regulates IL-15, was required for establishing normal numbers of CD8 T cells and natural killer cells in tumors. Depending on tumor type, both the tumor and the stroma are sources of sIL-15 complexes. In analyses of IL-15 reporter mice, most myeloid cells in the TME express IL-15 with CD11b⁺Ly6C^hi cells being the most abundant, indicating there is a large source of IL-15 protein in tumors that lies sequestered within the tumor stroma. Despite the abundance of IL-15-expressing cells, the relative levels of sIL-15 complexes are low in advanced tumors but can be up-regulated by local stimulator of IFN genes (STING) activation. Furthermore, while treatment of tumors with STING agonists leads to tumor regression, optimal STING-mediated immunity and regression of distant secondary tumors required IL-15 expression. Overall, our study reveals the dynamic regulation of IL-15 in the TME and its importance in antitumor immunity. These findings provide insight into an unappreciated attribute of the tumor landscape that contributes to antitumor immunity, which can be manipulated therapeutically to enhance antitumor responses.

Selection, tuning, and adaptation in mouse NK cell education

Natural killer (NK) cells recognize transformed cells with an array of germline-encoded inhibitory and activating receptors. Inhibitory Ly49 receptors bind major histocompatibility complex class I (MHC-I) molecules, providing a mechanism by which NK cells maintain self-tolerance yet eliminate cells expressing reduced levels of MHC-I. Additionally, MHC-I molecules are required for NK cell education, a process in which NK cells acquire responsiveness. In this review, we discuss three facets of MHC class I-dependent education of mouse NK cells: skewing of the inhibitory receptor repertoire, induction of functional responsiveness, and tuning in response to changes in MHC-I expression. We discuss prevailing models for education such as licensing and disarming and propose a model for positive selection of 'useful' NK cell subsets. Furthermore, we argue that both repertoire skewing and functional NK cell education may be altered in mature NK cells subject to changes in MHC-I input and suggest that this process may provide increased dynamics to the NK cell system.

Transcription factor KLF2 regulates homeostatic NK cell proliferation and survival

Natural killer (NK) cells are innate lymphocytes that recognize and lyse virally infected or transformed cells. This latter property is being pursued in clinics to treat leukemia with the hope that further breakthroughs in NK cell biology can extend treatments to other cancers. At issue is the ability to expand transferred NK cells and prolong their functionality within the context of a tumor. In terms of NK cell expansion and survival, we now report that Kruppel-like factor 2 (KLF2) is a key transcription factor that underpins both of these events. Excision of Klf2 using gene-targeted mouse models promotes spontaneous proliferation of immature NK cells in peripheral tissues, a phenotype that is replicated under ex vivo conditions. Moreover, KLF2 imprints a homeostatic migration pattern on mature NK cells that allows these cells to access IL-15-rich microenvironments. KLF2 accomplishes this feat within the mature NK cell lineage via regulation of a subset of homing receptors that respond to homeostatic ligands while leaving constitutively expressed receptors that recognize inflammatory cytokines unperturbed. Under steady-state conditions, KLF2-deficient NK cells alter their expression of homeostatic homing receptors and subsequently undergo apoptosis due to IL-15 starvation. This novel mechanism has implications regarding NK cell contraction following the termination of immune responses including the possibility that retention of an IL-15 transpresenting support system is key to extending NK cell activity in a tumor environment.

Pak2 Controls Acquisition of NKT Cell Fate by Regulating Expression of the Transcription Factors PLZF and Egr2

NKT cells constitute a small population of T cells developed in the thymus that produce large amounts of cytokines and chemokines in response to lipid Ags. Signaling through the Vα14-Jα18 TCR instructs commitment to the NKT cell lineage, but the precise signaling mechanisms that instruct their lineage choice are unclear. In this article, we report that the cytoskeletal remodeling protein, p21-activated kinase 2 (Pak2), was essential for NKT cell development. Loss of Pak2 in T cells reduced stage III NKT cells in the thymus and periphery. Among different NKT cell subsets, Pak2 was necessary for the generation and function of NKT1 and NKT2 cells, but not NKT17 cells. Mechanistically, expression of Egr2 and promyelocytic leukemia zinc finger (PLZF), two key transcription factors for acquiring the NKT cell fate, were markedly diminished in the absence of Pak2. Diminished expression of Egr2 and PLZF were not caused by aberrant TCR signaling, as determined using a Nur77-GFP reporter, but were likely due to impaired induction and maintenance of signaling lymphocyte activation molecule 6 expression, a TCR costimulatory receptor required for NKT cell development. These data suggest that Pak2 controls thymic NKT cell development by providing a signal that links Egr2 to induce PLZF, in part by regulating signaling lymphocyte activation molecule 6 expression.

NK Cell Proliferation Induced by IL-15 Transpresentation Is Negatively Regulated by Inhibitory Receptors

IL-15 bound to the IL-15Rα-chain (IL-15Rα) is presented in trans to cells bearing the IL-2Rβ-chain and common γ-chain. As IL-15 transpresentation occurs in the context of cell-to-cell contacts, it has the potential for regulation by and of other receptor-ligand interactions. In this study, human NK cells were tested for the sensitivity of IL-15 transpresentation to inhibitory receptors. Human cells expressing HLA class I ligands for inhibitory receptors KIR2DL1, KIR2DL2/3, or CD94-NKG2A were transfected with IL-15Rα. Proliferation of primary NK cells in response to transpresented IL-15 was reduced by engagement of either KIR2DL1 or KIR2DL2/3 by cognate HLA-C ligands. Inhibitory KIR-HLA-C interactions did not reduce the proliferation induced by soluble IL-15. Therefore, transpresentation of IL-15 is subject to downregulation by MHC class I-specific inhibitory receptors. Similarly, proliferation of the NKG2A(+) cell line NKL induced by IL-15 transpresentation was inhibited by HLA-E. Coengagement of inhibitory receptors, either KIR2DL1 or CD94-NKG2A, did not inhibit phosphorylation of Stat5 but inhibited selectively phosphorylation of Akt and S6 ribosomal protein. IL-15Rα was not excluded from, but was evenly distributed across, inhibitory synapses. These findings demonstrate a novel mechanism to attenuate IL-15-dependent NK cell proliferation and suggest that inhibitory NK cell receptors contribute to NK cell homeostasis.

Phenotypic changes in the brain of SIV-infected macaques exposed to methamphetamine parallel macrophage activation patterns induced by the common gamma-chain cytokine system

One factor in the development of neuroAIDS is the increase in the migration of pro-inflammatory CD8 T cells across the blood–brain barrier. Typically these cells are involved with keeping the viral load down. However, the persistence of above average numbers of CD8 T cells in the brain, not necessarily specific to viral peptides, is facilitated by the upregulation of IL15 from astrocytes, in the absence of IL2, in the brain environment. Both IL15 and IL2 are common gamma chain (γc) cytokines. Here, using the non-human primate model of neuroAIDS, we have demonstrated that exposure to methamphetamine, a powerful illicit drug that has been associated with HIV exposure and neuroAIDS severity, can cause an increase in molecules of the γc system. Among these molecules, IL15, which is upregulated in astrocytes by methamphetamine, and that induces the proliferation of T cells, may also be involved in driving an inflammatory phenotype in innate immune cells of the brain. Therefore, methamphetamine and IL15 may be critical in the development and aggravation of central nervous system immune-mediated inflammatory pathology in HIV-infected drug abusers.

IL-15 induces alloreactive CD28(-) memory CD8 T cell proliferation and CTLA4-Ig resistant memory CD8 T cell activation

The presence of CD28(-) memory CD8 T cells in the peripheral blood of renal transplant patients is a risk factor for graft rejection and resistance to CTLA-4Ig induction therapy. In vitro analyses have indicated poor alloantigen-induced CD28(-) memory CD8 T cell proliferation, raising questions about mechanisms mediating their clonal expansion in kidney grafts to mediate injury. Candidate proliferative cytokines were tested for synergy with alloantigen in stimulating CD28(-) memory CD8 T cell proliferation. Addition of IL-15, but not IL-2 or IL-7, to co-cultures of CD28(-) or CD28(+) memory CD8 T cells and allogeneic B cells rescued proliferation of the CD28(-) and enhanced CD28(+) memory T cell proliferation. Proliferating CD28(-) memory CD8 T cells produced high amounts of interferon gamma and tumor necrosis factor alpha and expressed higher levels of the cytolytic marker CD107a than CD28(+) memory CD8 T cells. CTLA-4Ig inhibited alloantigen-induced proliferation of CD28(+) memory CD8 T cell proliferation but had no effect on alloantigen plus IL-15-induced proliferation of either CD28(-) or CD28(+) memory CD8 T cells. These results indicate the ability of IL-15, a cytokine produced by renal epithelial during inflammation, to provoke CD28(-) memory CD8 T cell proliferation and to confer memory CD8 T cell resistance to CTLA-4Ig-mediated costimulation blockade.

Differential chemotactic receptor requirements for NK cell subset trafficking into bone marrow

Responsiveness of maturing natural killer (NK) cells to chemotactic molecules directly affect their retention and relocation in selected bone marrow (BM) microenvironment during development, as well as their localization at sites of immune response during inflammatory diseases. BM is the main site of NK cell generation, providing microenvironmental signals required to sustain cell proliferation and differentiation. Drastic changes of expression and function of several chemoattractant receptors can be observed during progression from precursor NK cells to immature and mature NK cells. Indeed, the gradual decrease of CXCR4 expression parallels the increased expression of CXCR3, CCR1, and CX3CR1 and S1P₅ (Sphingosine-1-phosphate receptor 5) on mature DX5⁺ NK cells. The chemokine CXCL12 is produced constitutively in the BM and, acting via CXCR4, is critical for retaining immature and mature NK cell subsets in the BM. During steady-state, the maintenance of NK cells into BM parenchyma depends on the equilibrium of CXCR4 retention and S1P₅ mobilizing functions, as the gradient of S1P coming from the sinusoids facilitates mature NK cell egress into circulation via S1P₅, when CXCR4/CXCL12-mediated retention decreases. Chemoattractants are also key factors for the response to inflammatory or infection conditions that promote mobilization of effector NK cells from storage compartments (including BM) to sites of disease or for NK cell recruitment/response during pathological conditions that affect BM integrity, including hematopoietic malignancies. In this review, we summarize what is known about the requirement for NK cell localization and exit from BM and how chemokine-mediated functions may affect BM NK cell development and immune responses.

Functions of IL-15 in anti-viral immunity: multiplicity and variety

An effective immune response to an invading viral pathogen requires the combined actions of both innate and adaptive immune cells. For example, NK cells and cytotoxic CD8 T cells are capable of the direct engagement of infected cells and the mediation of antiviral responses. Both NK and CD8 T cells depend on common gamma chain (γc) cytokine signals for their development and homeostasis. The γc cytokine IL-15 is very well characterized for its role in promoting the development and homeostasis of NK cells and CD8 T cells, but emerging literature suggests that IL-15 mediates the anti-viral responses of these cell populations during an active immune response. Both NK cells and CD8 T cells must become activated, migrate to sites of infection, survive at those sites, and expand in order to maximally exert effector functions, and IL-15 can modulate each of these processes. This review focuses on the functions of IL-15 in the regulation of multiple aspects of NK and CD8 T cell biology, investigates the mechanisms by which IL-15 may exert such diverse functions, and discusses how these different facets of IL-15 biology may be therapeutically exploited to combat viral diseases.

IL-15-high-responder developing NK cells bearing Ly49 receptors in IL-15-/- mice

In mice lacking IL-15, NK cell development is arrested at immature stages, providing an opportunity to investigate the earliest developing NK cells that would respond to IL-15. We show in this study that immature NK cells were present in the spleen as well as bone marrow (BM) and contained IL-15-high-responder cells. Thus, mature NK cells were generated more efficiently from IL-15(-/-) than from control donor cells in radiation BM chimeras, and the rate of IL-15-induced cell division in vitro was higher in NK cells in the spleen and BM from IL-15(-/-) mice than in those from wild-type mice. Phenotypically, NK cells developed in IL-15(-/-) mice up to the minor but discrete CD11b(-)CD27(+)DX5(hi)CD51(dull)CD127(dull)CD122(hi) stage, which contained the majority of Ly49G2(+) and D(+) NK cells both in the spleen and BM. Even among wild-type splenic NK cells, IL-15-induced proliferation was most prominent in CD11b(-)DX5(hi) cells. Notably, IL-15-mediated preferential expansion (but not conversion from Ly49(-) cells) of Ly49(+) NK cells was observed in vitro only for NK cells in the spleen. These observations indicated the uneven distribution of NK cells of different developing stages with variable IL-15 responsiveness in these lymphoid organs. Immature NK cells in the spleen may contribute, as auxiliaries to those in BM, to the mature NK cell compartment through IL-15-driven extramarrow expansion under steady-state or inflammatory conditions.

Different NK cell developmental events require different levels of IL-15 trans-presentation

NK cell development requires IL-15, which is "trans-presented" to IL-15Rβγ on NK cells by IL-15Rα on other cells. In this study, we report that different levels of IL-15 trans-presentation are required for different NK cell developmental events to reach full maturation status. Because the IL-15Rα intracellular domain has the capacity to recruit signaling molecules, we generated knockin and transgenic (Tg) mice that lack the intracellular domain to assess the role of the IL-15 trans-presentation level independent of the function of this domain. The level of IL-15Rα on various cells of these mice follows the order WT > Tg6 > knockin > Tg1 ≥ knockout. Bone marrow (BM)-derived dendritic cells prepared from these mice induced Stat5 phosphorylation in NK cells. The level of phospho-Stat5 correlated with the level of IL-15Rα on BMDCs, thus offering the opportunity to study quantitative effects of IL-15 trans-presentation on NK cell development in vivo. We found that NK cell homeostasis, mature NK cell differentiation, and acquisition of Ly49 receptor and effector functions require different levels of IL-15 trans-presentation input to achieve full status. All NK cell developmental events examined were quantitatively regulated by the IL-15Rα level of BM-derived and radiation-resistant accessory cells, but not by IL-15Rα of NK cells. We also found that IL-15Rα of radiation-resistant cells was more potent than IL-15Rα of BM-derived accessory cells in support of stage 2 to stage 3 splenic mNK differentiation. In summary, each examined developmental event required a particular level of IL-15 trans-presentation by accessory cells.

IL-15Rα of radiation-resistant cells is necessary and sufficient for thymic invariant NKT cell survival and functional maturation

The development of invariant NKT (iNKT) cells depends on the thymus. After positive selection by CD4(+)CD8(+)CD1d(+) cortical thymocytes, iNKT cells proceed from CD44(low)NK1.1(-) (stage 1) to CD44(high)NK1.1(-) (stage 2), and then to CD44(high)NK1.1(+) (stage 3) cells. The programming of cytokine production occurs along the three differentiation stages, whereas the acquisition of NK receptors occurs at stage 3. Stage 3 thymic iNKT cells are specifically reduced in Il15ra(-/-) mice. The mechanism underlying this homeostatic deficiency and whether the IL-15 system affects other thymic iNKT cell developmental events remain elusive. In this study, we demonstrate that increased cell death contributed to the reduction of stage 3 cells in Il15ra(-/-) mice, as knockout of Bim restored this population. IL-15-dependent upregulation of Bcl-2 in stage 3 cells affected cell survival, as overexpression of hBcl-2 partially restored stage 3 cells in Il15ra(-/-) mice. Moreover, thymic iNKT cells in Il15ra(-/-) mice were impaired in functional maturation, including the acquisition of Ly49 and NKG2 receptors and the programming of cytokine production. Finally, IL-15Rα expressed by radiation-resistant cells is necessary and sufficient to support the survival as well as the examined maturation events of thymic iNKT cells.

Role of common-gamma chain cytokines in NK cell development and function: perspectives for immunotherapy

NK cells are components of the innate immunity system and play an important role as a first-line defense mechanism against viral infections and in tumor immune surveillance. Their development and their functional activities are controlled by several factors among which cytokines sharing the usage of the common cytokine-receptor gamma chain play a pivotal role. In particular, IL-2, IL-7, IL-15, and IL-21 are the members of this family predominantly involved in NK cell biology. In this paper, we will address their role in NK cell ontogeny, regulation of functional activities, development of specialized cell subsets, and acquisition of memory-like functions. Finally, the potential application of these cytokines as recombinant molecules to NK cell-based immunotherapy approaches will be discussed.

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.

Trans-presentation: a novel mechanism regulating IL-15 delivery and responses

Interleukin (IL)-15 is a cytokine that acts on a wide range of cell types but is most crucial for the development, homeostasis, and function of a specific group of immune cells that includes CD8 T cells, NK cells, NKT cells, and CD8 alpha alpha intraepithelial lymphocytes. IL-15 signals are transmitted through the IL-2/15R beta and common gamma (gamma C) chains; however, it is the delivery of IL-15 to these signaling components that is quite unique. As opposed to other cytokines that are secreted, IL-15 primarily exists bound to the high affinity IL-15R alpha. When IL-15/IL-15R alpha complexes are shuttled to the cell surface, they can stimulate opposing cells through the beta/gamma C receptor complex. This novel mechanism of IL-15 delivery has been called trans-presentation. This review discusses how the theory of trans-presentation came to be, evidence that it is the major mechanism of action, the current understanding of the cell types thought to mediate trans-presentation, and possible alternatives for IL-15 delivery.

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.

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.

CCL3 and CXCL12 regulate trafficking of mouse bone marrow NK cell subsets

Herein we have analyzed chemokine involvement in the trafficking of developing and mature mouse natural killer (NK) cells in the bone marrow (BM). We observed drastic changes of CCR1, CXCR3, and CXCR4 expression and function during progression from precursor NK (pNK) cells to immature DX5- NK (iNK) and mature DX5+ NK (mNK) cells. pNK and mNK cells expressed the 3 receptors, while only CXCR4 was detected on iNK cells. Correspondingly, mNK cells migrated to CXCL12, CXCL10, and CCL3, and pNK and iNK cells to CXCL12, whereas pNK cells migrated to CCL3 and CXCL10 only after CXCL12 stimulation. Comparison of BM, peripheral blood, and spleen mNK cell populations revealed that CXCL12, CXCL10, and CCL3 preferentially affected BM mNK cell migration. Administration of the CXCR4 antagonist, AMD-3100, to C57BL/6 mice induced strong reduction of mNK and iNK cells in the BM and increased their number in blood and spleen. Conversely, CCL3 administration selectively mobilized mNK cells from the BM and this effect correlated with its ability to inhibit CXCL12-mediated mNK cell responses in vitro. Our results suggest that the combined action of chemokines selectively regulates localization of NK cell subsets in the BM and direct their maturation and migration to the periphery.

NF-kappa B p50/p65 affects the frequency of Ly49 gene expression by NK cells

In mice, acquisition of Ly49 receptors characterizes one of the developmental stages of NK cells. We previously described a novel Ly49 promoter, Pro1, involved in Ly49 gene regulation in immature NK cells. Pro1 transcriptional activity requires a NF-kappaB binding site; however, only NF-kappaB/p50 binding to this element was observed. Cotransfection of NF-kappaB/p65 with Ly49g Pro1 in LNK cells induced a decrease in the transcriptional activity of the core promoter. Moreover, decreasing NF-kappaB/p65 protein expression by RNA interference increases Pro1 transcriptional activity. A high rate of NF-kappaB/p65 degradation in LNK cells correlates with Pro1 activity, since treatment with the proteasome inhibitor MG132 increased levels of NF-kappaB/p65 protein and decreased Pro1 activity. In addition, analysis of the Ly49 repertoire in NF-kappaB/p50 null mice reveals a decrease in the proportion of NK cells expressing a given Ly49 molecule. The defect in Ly49 expression is observed in the bone marrow and the spleen with a similar altered pattern of developmental stages in each tissue. The frequency of Ly49 expression in NF-kappaB/p52 null mice is slightly increased, indicating the specific role of NF-kappaB/p50 in Ly49 gene activation. These results suggest that NF-kappaB p50/p65 plays a major role in the initiation of Ly49 gene expression in NK cells.

Stromal-cell regulation of natural killer cell differentiation

Natural killer (NK) cells are bone-marrow-derived lymphocytes that play a crucial role in host defense against some viral and bacterial infections, as well as against tumors. Their phenotypic and functional maturation requires intimate interactions between the bone marrow stroma and committed precursors. In parallel to the identification of several phenotypic and functional stages of NK cell development, recent studies have shed new light on the role of stromal cells in driving functional maturation of NK cells. In this review, we provide an overview of the role of bone marrow microenvironment in NK cell differentiation.

CD8(+) T cells resistant to costimulatory blockade are controlled by an antagonist interleukin-15/Fc protein

BACKGROUND

Although permanent engraftment is often achieved with new therapeutics, chronic rejection and graft failure still occur. As the importance of CD8(+) T cells in rejection processes has been underlined in various transplant models, and as interleukin (IL)-15 is involved in the activation of CD8(+) T cells, we hypothesize that CD8(+) T cell "escape" from costimulation blockade might be a IL-15/IL-15R dependent process.

METHODS

In a murine islet allograft model employing a fully major histocompatibility complex-mismatched strain combination of Balb/c donors to CD4 C57BL/6 recipients, a monotherapy with the IL-15 antagonist, IL-15 mutant/Fcgamma2a, or the costimulatory blockade molecule, CTLA4/Fc, was used. In addition to monitoring graft survival, infiltration of alloreactive immune cells was analyzed by histology and immunohistochemistry, and alloimmune response of proliferative CD8(+) T cells was measured in vivo.

RESULTS

Sixty percent of the recipients treated with CTLA4/Fc acutely rejected their islet allograft, comparable to untreated control animals (50% survival). In contrast, the IL-15 antagonist proved to be highly effective, with 100% of recipients accepting their allograft. Immunohistology study demonstrated a remarkable decrease of CD8(+) T-cell intragraft infiltration in IL-15 mutant/Fcgamma2a treated animals with well-preserved islet architecture and a reduced frequency of proliferating alloreactive CD8(+) T cells in comparison with that of untreated and CTLA4/Fc treated groups.

CONCLUSIONS

In this study, we determined the efficacy and potential therapeutic benefit of the IL-15 antagonist on CD4-independent CD8(+) T-cell responses to alloantigens. Targeting the IL-15/IL-15R pathway represents a potent strategy to prevent rejection driven by CD8(+) T cells resistant to costimulation blockade.

Enforced Expression of Bcl-2 Partially Restores Cell Numbers but Not Functions of TCRγδ Intestinal Intraepithelial T Lymphocytes in IL-15-Deficient Mice

IL-15 knockout (KO) mice have severely reduced numbers of TCRgammadelta intestinal intraepithelial T lymphocytes (i-IEL), suggesting requirements of IL-15 signaling in the development or maintenance of i-IEL. To determine an involvement of survival signals via Bcl-2 in IL-15-mediated homeostasis of TCRgammadelta i-IEL, we introduced a bcl-2 transgene into IL-15 KO mice. In situ apoptosis of TCRgammadelta i-IEL was decreased in Bcl-2 transgenic (Tg) x IL-15 KO mice compared with IL-15 KO mice. The enforced expression of Bcl-2 partially restored the numbers of TCRgammadelta i-IEL in IL-15 KO mice. However, effector functions of TCRgammadelta i-IEL, including cytokine production and cytotoxic activity, were not recovered in Bcl-2 Tg x IL-15 KO mice. Importantly, TCRgammadelta i-IEL in Bcl-2 Tg x IL-15 KO mice expressed a reduced level of eomesodermin, a transcription factor critical for effector functions of NK cells and CD8(+) T cells. Similar to the case of TCRgammadelta i-IEL, enforced expression of Bcl-2 restored the numbers but not the functions of NK cells in IL-15 KO mice. These results suggest that Bcl-2-mediated survival signal is involved in the IL-15-mediated homeostasis of TCRgammadelta i-IEL and NK cells, but other signals from IL-15 are critical for inducing transcription factors, such as eomesodermin for their effector functions.

T and B lymphocytes exert distinct effects on the homeostasis of NK cells

There is growing evidence that lymphocytes impact the development and/or function of other lymphocyte populations. Based on such observations we have tested whether the NK cell compartment was phenotypically and functionally altered in the absence of B and/or T cells. Here we show that T cell deficiency significantly accelerates BM NK cell production and the subsequent seeding of splenic and liver NK cell compartments. In contrast, B cell deficiency reduces splenic NK cell survival. In the absence of T and B cells, the size of the NK cell compartments is determined by the combination of these positive and negative effects. Even though NK cell homeostasis is significantly altered, NK cells from T and/or B cell-deficient mice show a normal capacity to kill a susceptible target cell line and to produce IFN. Nevertheless, we noted that the usage of MHC class I-specific Ly49 family receptors was significantly altered in the absence of T and/or B cells. In general, B cell deficiency expanded Ly49 receptor usage, while T cell deficiency exerted both positive and negative effects. These findings show that B and T cells significantly and differentially influence the homeostasis and the phenotype of NK cells.

Natural killer cell developmental pathways: a question of balance

NK cells sit at the crossroads of innate and adaptive immunity and help coordinate tumor immunosurveillance and the immune response against pathogens. Balancing signals to NK cell precursors is crucial for their early development, when transcription factors compete to specify the different lymphocyte subsets. Despite an elaborate schema for NK cell development and differentiation, several major issues remain to be addressed, such as identifying the sites for NK cell maturation and defining the peripheral NK cell niche.

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

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

Tumor growth impedes natural-killer-cell maturation in the bone marrow

Natural-killer (NK)-cell dysfunction and IFN-gamma deficiencies have been associated with increased incidence of both malignancy and infection. The immunologic basis of NK-cell defects in cancer-bearing hosts has not been extensively studied. Here, we demonstrate that multiple lineages of tumors, including thymoma, breast cancer, colon cancer, and melanoma cell lines, interrupt functional maturation during NK-cell development in the bone marrow. The immature NK cells in the periphery of tumor-bearing mice had impaired IFN-gamma production but seemingly normal cytotoxicity. T cells are not involved in this NK maturation arrest, because T-cell depletion did not restore NK-cell development. Moreover, the extent of tumor-cell infiltration into the bone marrow does not correlate with defective NK maturation. Interestingly, the defect was associated with a significant reduction in the IL-15Ralpha+ cells in the non-T, non-NK compartment of bone marrow cells and restored by overexpression of IL-15. Our data demonstrate that tumor growth can impede functional maturation of NK cells, most likely by interrupting the requisite IL-15 signaling pathway.

T-bet concomitantly controls migration, survival, and effector functions during the development of Valpha14i NKT cells

Valpha14i natural killer T (NKT)-cell function has been implicated in a number of disease conditions. The molecular events that drive Valpha14i NKT-cell development remain elusive. We recently showed that T-bet is required for the terminal maturation of these cells. Here we identify some of the genetic targets of T-bet during Valpha14i NKT-cell lineage development. Microarray gene-expression analyses on developing Valpha14i NKT cells were performed and provide a molecular framework to study these maturation events. In vitro ectopic expression of T-bet in immature Valpha14i NKT cells, which do not yet express T-bet, was sufficient to promote Valpha14i NKT-cell maturation, driving the expression of multiple genes, including those that participate in migration, survival, and effector functions. By regulating the expression of T-helper 1 (Th1)-associated cytokines, chemokines, chemokine receptors, and molecules involved in cytolysis, T-bet defines the unique lineage attributes of mature Valpha14i NKT cells and acts to link these attributes to a developmental process.

Ly49 and CD94/NKG2 receptor acquisition by NK cells does not require lymphotoxin-β receptor expression

A crucial step in murine natural killer (NK) cell development, mediated by bone marrow stromal cells, is the induction of Ly49 and CD94/NKG2 receptor expression. The signals that regulate Ly49 receptor expression are still largely undetermined. It has been shown that interaction between lymphotoxin α1β2 (LTα1β2) and LTβ receptor (LTβR), expressed on lymphoid progenitor cells and nonlymphoid bone marrow stromal cells, respectively, is important for both quantitative and functional NK cell development. Therefore, we have investigated the role of LT-LTβR–mediated signaling in Ly49 and CD94/NKG2 receptor acquisition. We show that the NK receptor repertoire of LTβR–/– mice can only be partially analyzed because of the residual 129/Ola mouse genetic background, due to a physical linkage of the LTβR locus and the loci encoding the Ly49 and CD94/NKG2 receptors. Therefore, we transferred wild-type B6 lymphoid-committed progenitor cells into LTβR–/– mice, which differentiated into NK cells with a normal NK cell receptor repertoire. Also, administration of LTβR-immunoglobulin (Ig), which acts as a soluble receptor for LTα1β2, resulted in reduced NK cell percentages but did not influence the Ly49 and CD94/NKG2 receptor acquisition on remaining NK cells. These results indicate that LTβR-mediated signals are not required for Ly49 and CD94/NKG2 receptor acquisition.

Distinguishing features of developing natural killer cells

Although NK cells were initially described as lymphocytes that lacked the characteristic markers of B and T cells, we now appreciate the plethora of activating and inhibitory receptors that define the NK cell surface phenotype. Recent studies have provided new insights into the molecular mechanisms that drive NK cell development, control the NK cell receptor repertoire and the cytokines that define the NK cell niche. These findings form the basis for an updated model of NK cell differentiation.

Developmental program of mouse Valpha14i NKT cells

Natural killer T (NKT) cells are a distinct lymphocyte lineage that regulates immune responses. During their development in the thymus, immature uncommitted double-positive CD4+CD8+ thymocytes that rearrange the semi-invariant T-cell receptor found on mature NKT cells are positively selected by the non-classical MHC class I molecule CD1d, which is expressed at the surface of cortical thymocytes. At this stage, the positively selected cells branch off from the conventional T-cell developmental program and start to acquire activated and/or memory markers and several 'bona fide' NK cell attributes. Recent work has started to reveal the specific developmental requirements for this divergent pathway of differentiation. These include several signal transduction molecules, transcription factors and cytokines, including T-bet, members of the NF-kappaB family, Fyn and IL-15.

IFN Regulatory Factor-2 Deficiency Revealed a Novel Checkpoint Critical for the Generation of Peripheral NK Cells

NK cell development is far less understood compared with that of T and B cells despite the critical importance of NK cells in innate immunity. Mice lacking the transcription factor IFN regulatory factor-2 (IRF-2) are known to exhibit NK cell deficiency. However, the role of IRF-2 in NK cell development has remained unclear. In this study we found that NK cell deficiency in the periphery in IRF-2-deficient mice was due to selective loss of mature NK cells, but not to maturation arrest, and NK cells in these mice exhibited very immature surface phenotypes (CD11b(low)Dx5(low)) with highly compromised NK receptor expression. In contrast, IRF-2-deficient NK cells in bone marrow (BM) showed relatively mature phenotypes (CD11b(low)Dx5(high)) with less compromised NK receptor repertoire. Furthermore, BM NK cells in IRF-2-deficient mice were found to proliferate almost normally, but underwent accelerated apoptosis. These observations indicated that NK cell maturation could advance up to a late, but not the final, stage in the BM, whereas these cells were incapable of contributing to the peripheral NK cell pool due to premature death in the absence of IRF-2. In contrast, NK cell numbers and Ly49 expression were much more severely reduced in BM in IL-15-deficient mice than in IRF-2(-/-) mice. The differential peripheral and central NK cell deficiencies in IRF-2(-/-) mice thus revealed a novel late checkpoint for NK cell maturation, distinct from the early IL-15-dependent expansion stage.

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

NK cells differentiate in adult mice from bone marrow hemopoietic progenitors. Cytokines, including those that signal via receptors using the common cytokine receptor gamma-chain (gamma(c)), have been implicated at various stages of NK cell development. We have previously described committed NK cell precursors (NKPs), which have the capacity to generate NK cells, but not B, T, erythroid, or myeloid cells, after in vitro culture or transfer to a fetal thymic microenvironment. NKPs express the CD122 Ag (beta chain of the receptors for IL-2/IL-15), but lack other mature NK markers, including NK1.1, CD49b (DX5), or members of the Ly49 gene family. In this report, we have analyzed the roles for gamma(c)-dependent cytokines in the generation of bone marrow NKP and in their subsequent differentiation to mature NK cells in vivo. Normal numbers of NKPs are found in gamma(c)-deficient mice, suggesting that NK cell commitment is not dependent on IL-2, IL-4, IL-7, IL-9, IL-15, or IL-21. Although IL-2, IL-4, and IL-7 have been reported to influence NK cell differentiation, we find that mice deficient in any or all of these cytokines have normal NK cell numbers, phenotype, and effector functions. In contrast, IL-15 plays a dominant role in early NK cell differentiation by maintaining normal numbers of immature and mature NK cells in the bone marrow and spleen. Surprisingly, the few residual NK cells generated in absence of IL-15 appear relatively mature, expressing a variety of Ly49 receptors and demonstrating lytic and cytokine production capacity.

Interleukin-2, Interleukin-15, and Their Roles in Human Natural Killer Cells

Natural killer (NK) cells are CD56+CD3- large granular lymphocytes that constitute a key component of the human innate immune response. In addition to their potent cytolytic activity, NK cells elaborate a host of immunoregulatory cytokines and chemokines that play a crucial role in pathogen clearance. Furthermore, interactions between NK and other immune cells are implicated in triggering the adaptive, or antigen-specific, immune response. Interleukin-2 (IL-2) and IL-15 are two distinct cytokines with partially overlapping properties that are implicated in the development, homeostasis, and function of NK cells. This review examines the pervasive effects of IL-2 and IL-15 on NK cell biology, with an emphasis on recent discoveries and lingering challenges in the field.

Modulation of natural killer cell activity by restraint stress during an influenza A/PR8 infection in mice

These experiments were designed to examine the influences of restraint stress (RST) on natural killer (NK) activity and to determine its consequences on influenza A/PR8 (A/PR8) viral replication in mice. The data showed that RST delayed the recruitment of NK1.1+ cells into the lung parenchyma during infection. Quantification of MIP-1alpha and MCP-1 gene expression by real-time PCR revealed that RST suppressed the chemokines responsible for NK cell recruitment into the infected tissue. Additionally, RST suppressed the expression of several macrophage-derived cytokines involved in the effector response of NK cells. IL-15, which is the main cytokine involved in NK cell development and homeostasis, and IL-12, which is important for NK cytotoxicity, were both suppressed. As the NK cell response is an important innate response to control viral replication, we hypothesized that the RST-mediated reduction in NK cell numbers and function would enable viral replication to continue unchecked. In fact, there was enhanced viral replication in the lungs of RST animals. Interestingly, expression of the anti-viral type I interferons (IFN-alpha and IFN-beta) was elevated presumably in response to the elevated viral load in the stressed mice. Together, these data show that RST suppressed expression of the cytokine genes involved in the recruitment and activation of NK cells during an experimental influenza viral infections. The consequence of this effect was diminished NK cell function and enhanced viral replication.

Coordinate expression and trans presentation of interleukin (IL)-15Ralpha and IL-15 supports natural killer cell and memory CD8+ T cell homeostasis

The high affinity interleukin (IL)-15 receptor, IL-15Rα, is essential for supporting lymphoid homeostasis. To assess whether IL-15Rα's role in vivo is to trans present IL-15, we generated mixed bone marrow chimera from IL-15Rα– and IL-2/15Rβ–deficient mice. We find that IL-15Rα–competent, IL-2/15Rβ–deficient cells are able to support IL-15Rα–deficient natural killer (NK) and memory CD8⁺ T cells, thus ruling out secondary signals on these cells and demonstrating that IL-15Rα–mediated presentation of IL-15 in trans is the primary mechanism by which IL-15Rα functions in vivo. Surprisingly, using IL-15– and IL-15Rα–deficient mixed chimera, we also find that IL-15 and IL-15Rα must be expressed by the same cells to present IL-15 in trans, indicating that IL-15Rα is required on a cellular level for the elaboration of IL-15. These studies indicate that IL-15Rα defines homeostatic niches for NK and memory CD8⁺ T cells by controlling both the production and the presentation of IL-15 in trans to NK and CD8⁺ memory T cells.

CD56+Cells Induce Steroid Resistance in B Cells Exposed to IL-15

Interleukin-2 can induce steroid resistance in T cells. IL-15 shares biological activities with IL-2, as both cytokines use IL-2Rgamma for signal transduction. We therefore sought to determine whether IL-15 contributes to induction of PBMC corticosteroid resistance. Surprisingly, we found that incubation of unfractionated PBMC with IL-15 for 48 h resulted in the inhibition of glucocorticoid receptor (GCR) nuclear translocation in response to dexamethasone (DEX) treatment in CD19-positive B cells significantly greater than CD19-negative non-B cells (p < 0.01). However, pure B cells incubated with IL-15 responded normally with nuclear translocation of GCR in response to steroids, but failed to translocate GCR when they were grown in the presence of CD19(-) cells. Coculture of B cells with CD3(+) (T cells), CD14(+) (monocytes), or CD56(+) (NK and NKT cells) in the presence of IL-15 revealed that only CD56(+) cells contributed to the steroid insensitivity of B cells. IL-15 stimulation significantly increased production of IL-4 by CD56(+) cells (p < 0.02). Treatment of purified B cells with combination IL-15/IL-4 resulted in abrogation of glucocorticoid receptor nuclear translocation and the inability of DEX to suppress cytokine production by B cells. In the presence of IL-4-neutralizing Ab, when B cells were cocultured with CD56(+) cells and IL-15, the B cells were found to be steroid sensitive, i.e., DEX induced GCR nuclear translocation. This study demonstrates that B cells develop steroid resistance in the presence of CD56(+) cells after IL-15 stimulation. Furthermore, IL-15 and IL-4 have the capacity to induce B cell insensitivity to steroids.