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

M-CSF directs myeloid and NK cell differentiation to protect from CMV after hematopoietic cell transplantation

Therapies reconstituting autologous antiviral immunocompetence may represent an important prophylaxis and treatment for immunosuppressed individuals. Following hematopoietic cell transplantation (HCT), patients are susceptible to Herpesviridae including cytomegalovirus (CMV). We show in a murine model of HCT that macrophage colony-stimulating factor (M-CSF) promoted rapid antiviral activity and protection from viremia caused by murine CMV. M-CSF given at transplantation stimulated sequential myeloid and natural killer (NK) cell differentiation culminating in increased NK cell numbers, production of granzyme B and interferon-γ. This depended upon M-CSF-induced myelopoiesis leading to IL15Rα-mediated presentation of IL-15 on monocytes, augmented by type I interferons from plasmacytoid dendritic cells. Demonstrating relevance to human HCT, M-CSF induced myelomonocytic IL15Rα expression and numbers of functional NK cells in G-CSF-mobilized hematopoietic stem and progenitor cells. Together, M-CSF-induced myelopoiesis triggered an integrated differentiation of myeloid and NK cells to protect HCT recipients from CMV. Thus, our results identify a rationale for the therapeutic use of M-CSF to rapidly reconstitute antiviral activity in immunocompromised individuals, which may provide a general paradigm to boost innate antiviral immunocompetence using host-directed therapies.

The Roles of IL-7 and IL-15 in Niches for Lymphocyte Progenitors and Immune Cells in Lymphoid Organs

Lymphoid organs consist of immune cells and stromal cells. The stromal cells produce various cytokines that support the development, maintenance, and response of the immune cells. IL-7 and IL-15 are the major cytokines produced by stromal cells and are essential for the development and maintenance of lymphocytes and innate lymphoid cells (ILCs). In addition, IL-7 is indispensable for the organogenesis of lymphoid organs. However, because the amount of these two cytokines is relatively low, it has been difficult to directly detect their expression. Recently, several groups succeeded in establishing IL-7 and IL-15 reporter mouse lines. As expected, IL-7 and IL-15 were detected in mesenchymal stromal cells in the bone marrow and lymph nodes and in epithelial cells in the thymus. Furthermore, IL-7 and IL-15 were differentially expressed in lymphatic endothelial cells and blood endothelial cells, respectively. In addition to their expression, many groups have analyzed the local functions of IL-7 and IL-15 by using cell-type-specific knockout mice. From these experiments, CXCL12-expressing mesenchymal stromal cells were identified as the major niche for early B cell precursors. Single-cell RNA sequencing (scRNA-seq) analysis has revealed different subpopulations of stromal cells in the lymphoid organs, including those that express both IL-7 and IL-15. Future research is still needed to elucidate which stromal cells serve as the niche for the early precursors of ILCs and NK cells in the bone marrow.

Gender Differences in Urothelial Bladder Cancer: Effects of Natural Killer Lymphocyte Immunity

Men are more likely to develop cancer than women. In fact, male predominance is one of the most consistent cancer epidemiology findings. Additionally, men have a poorer prognosis and an increased risk of secondary malignancies compared to women. These differences have been investigated in order to better understand cancer and to better treat both men and women. In this review, we discuss factors that may cause this gender difference, focusing on urothelial bladder cancer (UBC) pathogenesis. We consider physiological factors that may cause higher male cancer rates, including differences in X chromosome gene expression. We discuss how androgens may promote bladder cancer development directly by stimulating bladder urothelium and indirectly by suppressing immunity. We are particularly interested in the role of natural killer (NK) cells in anti-cancer immunity.

TIPE2 is a checkpoint of natural killer cell maturation and antitumor immunity

The maturation process of NK cells determines their functionality during which IL-15 plays a critical role. However, very few checkpoints specifically targeting this process have been discovered. Here, we report that TIPE2 expression gradually increased during NK cell ontogenesis correlating to their maturation stages in both mice and humans. NK-specific TIPE2 deficiency increased mature NK cells in mice, and these TIPE2-deficient NK cells exhibited enhanced activation, cytotoxicity, and IFN-γ production upon stimulation and enhanced response to IL-15 for maturation. Moreover, TIPE2 suppressed IL-15–triggered mTOR activity in both human and murine NK cells. Consequently, blocking mTOR constrained the effect of TIPE2 deficiency on NK cell maturation in response to IL-15. Last, NK-specific TIPE2-deficient mice were resistant to tumor growth in vivo. Our results uncover a potent checkpoint in NK cell maturation and antitumor immunity in both mice and humans, suggesting a promising approach of targeting TIPE2 for NK cell–based immunotherapies.

NKTR-255, a polymer-conjugated IL-15 with unique mechanisms of action on T and natural killer cells

NKTR-255 is a novel polyethylene glycol (PEG)-conjugate of recombinant human IL-15 (rhIL-15) being examined as a potential cancer immunotherapeutic. Since IL-15 responses can be mediated by trans- or cis-presentation via IL-15Rα or soluble IL-15/IL-15Rα complexes, we investigated the role of IL-15Rα in driving NKTR-255 responses using defined naïve and memory ovalbumin-specific CD8 T cells (OT-I) CD8 T and NK cells in mice. NKTR-255 induced a 2.5 and 2.0-fold expansion of CD8 T and NK cells, respectively in WT mice. In adoptive transfer studies, proliferation of naïve and memory Wt OT-I T cells in response to NKTR-255 was not impaired in IL-15Rα-/- mice, suggesting trans-presentation was not utilized by NKTR-255. Interestingly, naïve IL-15Rα-/- OT-I cells had deficient responses to NKTR-255 while memory IL-15Rα-/- OT-I cell responses were partially impaired, suggesting that naive CD8 T cells are more dependent on cis-presentation of NKTR-255 than memory CD8 T cells. In bone marrow chimeras studies, IL-15Rα-/- and WT NK cells present in WT recipients had similar responses to NKTR-255, suggesting that cis-presentation is not utilized by NK cells. NKTR-255 could form soluble complexes with IL-15Rα; binding to murine IL-15Rα generated superagonists that preferentially stimulated NK cells showing that conversion to IL-15Rβ agonist biases the response towards NK cells. These findings highlight the ability of NKTR-255 to utilize IL-15Rα for cis-presentation and act as an IL-15Rαβ agonist on CD8 T cells.

PRDM1 decreases sensitivity of human NK cells to IL2-induced cell expansion by directly repressing CD25 (IL2RA)

IL2 receptor signaling is crucial for human NK cell activation and gain of effector functions. The molecular mechanisms involved in termination of IL2 activation are largely unknown in human NK cells. PR/SET domain 1 was previously reported to decrease cell growth and increase apoptosis in an IL2-dependent manner in malignant NK cell lines, suggesting the possibility of down-regulation of IL2 signaling pathway gene(s) through direct transcriptional repression. Using ChIP-Seq, we identified a PRDM1 binding site on the first intron of CD25 (IL2RA), which codes for the IL2 receptor subunit regulating sensitivity to IL2 signaling, in primary NK cells activated with engineered K562 cells or IL2. Ectopic expression of PRDM1 down-regulated CD25 expression at transcript and protein levels in two PRDM1 nonexpressing NK cell lines. shRNA-mediated knockdown of CD25 in two malignant NK cell lines led to progressive depletion of NK cells in low IL2 concentrations. By contrast, ectopic CD25 expression in primary human NK cells led to progressive increase in cell number in CD25-transduced cells in low IL2 concentrations. Altogether these results reveal a pivotal role of PRDM1 in inhibition of IL2-induced NK cell expansion through direct repression of CD25 in activated human NK cells. These observations provide additional support for the role of PRDM1 in attenuation of NK cell activation and growth, with implications on neoplastic transformation or NK cell function when it is deregulated.

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.

IL-15 and CD155 expression regulate LAT expression in murine DNAM1+ NK cells, enhancing their effectors functions

NK cells are innate immune cells characterized by their ability to spontaneously lyse tumor and virally infected cells. We have recently demonstrated that IL-15-sufficient DC regulate NK cell effector functions in mice. Here, we established that among ITAM-proximal signaling molecules, the expression levels of the scaffold molecule Linker for Activation of T cells (LAT) and its transcription factor ELF-1 were reduced 4 days after in vivo depletion of DC. Addition of IL-15, a cytokine presented by DC to NK cells, regulates LAT expression in NK cells with a significant effect on the DNAM1⁺ subset compared to DNAM1^- cells. We also found that LAT expression is regulated via interaction of the DNAM1 receptor with its ligand CD155 in both immature and mature NK cells, independently of NK cell education. Finally, we found that LAT expression within DNAM1⁺ NK cells might be responsible for enhanced calcium mobilization following the triggering of activating receptors on NK cells. Altogether, we found that LAT expression is tightly regulated in DNAM1⁺ NK cells, via interaction(s) with DC, which express CD155 and IL-15, resulting in rapid activation of the DNAM1⁺ subset during activating receptor triggering.

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.

Rapid isolation and enrichment of mouse NK cells for experimental purposes

Natural killer (NK) cells have shown to play a critical, but as yet poorly defined, role in the process by which the immune system controls tumor progression. Indeed, NK cell-based immunotherapy, particularly NK cell adoptive transfer therapy, has become a very attractive cancer weapon against multiple types of cancers such as metastatic and hematological cancers. Unfortunately, the implementation of these therapies has been challenged by the existence of immunosuppression mechanisms that have prevented NK cell functionality. Additionally, the development of protocols to obtain purified and functional NK cells has faced some difficulties due to the limitations in the numbers of cells that can be obtained and the development of an exhaustion phenotype with impaired proliferative and functional capabilities during lengthy ex vivo NK cell expansion protocols. Thus, the development of new strategies to obtain a rapid expansion of highly functional NK cells without the appearance of exhaustion is still much needed. This is particularly true in the case of mouse NK cells, a surrogate commonly used to evaluate NK cell biology and human NK cell-based immunotherapeutic alternatives. Here, we describe a feasible and rapid protocol to produce strongly activated mouse NK cells in vivo taking advantage of the hydrodynamic delivery of a plasmid that contains interleukin-15, a cytokine known to cause NK cell expansion and activation, fused with the binding domain of the IL-15Rα ("sushi" domain) and apolipoprotein A-I.

Calreticulin arms NK cells against leukemia

Calreticulin (CALR) exposed on the surface of cancer cells succumbing to therapy delivers robust phagocytic signals that support the activation of adaptive anticancer immune responses. Recent data from our group demonstrate that spontaneous CARL exposure on leukemic blasts also supports innate anticancer immunity by natural killer (NK) cells via an indirect mechanism relying on myeloid CD11c+CD14+ cells.

Tissue-resident cytotoxic innate lymphoid cells in tumor immunosurveillance

Innate lymphocytes play an important role in maintaining tissue homeostasis at steady state and during inflammation. The population of innate lymphocytes is incredibly diverse and heterogeneous with the successive identification of new subsets including innate lymphoid cells that arise from progenitors distinct from those of natural killer cells. Although generally considered as T helper-like lymphocytes, innate lymphoid cells with cytotoxic potential can be identified in many tissues. The tissue-resident cytotoxic innate lymphocytes derived from innate lymphoid cell and/or natural killer cell lineages are well positioned in sensing malignant transformation and initiating antitumor immunity. This review provides an overview of innate lymphocyte biology and discuss their roles in tumor immunosurveillance.

Sostdc1 Regulates NK Cell Maturation and Cytotoxicity

NK cells are innate-like lymphocytes that eliminate virally infected and cancerous cells, but the mechanisms that control NK cell development and cytotoxicity are incompletely understood. We identified roles for sclerostin domain-containing-1 (Sostdc1) in NK cell development and function. Sostdc1-knockout (Sostdc1 ^-/-) mice display a progressive accumulation of transitional NK cells (tNKs) (CD27⁺CD11b⁺) with age, indicating a partial developmental block. The NK cell Ly49 repertoire in Sostdc1 ^-/- mice is also changed. Lower frequencies of Sostdc1 ^-/- splenic tNKs express inhibitory Ly49G2 receptors, but higher frequencies express activating Ly49H and Ly49D receptors. However, the frequencies of Ly49I⁺, G2⁺, H⁺, and D⁺ populations were universally decreased at the most mature (CD27^-CD11b⁺) stage. We hypothesized that the Ly49 repertoire in Sostdc1 ^-/- mice would correlate with NK killing ability and observed that Sostdc1^-/- NK cells are hyporesponsive against MHC class I-deficient cell targets in vitro and in vivo, despite higher CD107a surface levels and similar IFN-γ expression to controls. Consistent with Sostdc1's known role in Wnt signaling regulation, Tcf7 and Lef1 levels were higher in Sostdc1 ^-/- NK cells. Expression of the NK development gene Id2 was decreased in Sostdc1^-/- immature NK and tNK cells, but Eomes and Tbx21 expression was unaffected. Reciprocal bone marrow transplant experiments showed that Sostdc1 regulates NK cell maturation and expression of Ly49 receptors in a cell-extrinsic fashion from both nonhematopoietic and hematopoietic sources. Taken together, these data support a role for Sostdc1 in the regulation of NK cell maturation and cytotoxicity, and identify potential NK cell niches.

NK Cells Require Cell-Extrinsic and -Intrinsic TYK2 for Full Functionality in Tumor Surveillance and Antibacterial Immunity

Tyrosine kinase 2 (TYK2) is a widely expressed receptor-associated kinase that is involved in signaling by a variety of cytokines with important immune regulatory activities. Absence of TYK2 in mice results in impaired NK cell maturation and antitumor activity, although underlying mechanisms are largely unknown. Using conditional ablation of TYK2 in NK cells we show that TYK2 is required for IFN-γ production by NK cells in response to IL-12 and for an efficient immune defense against Listeria monocytogenes Deletion of TYK2 in NK cells did not impact NK cell maturation and IFN-γ production upon NK cell activating receptor (actR) stimulation. Similarly, NK cell-mediated tumor surveillance was unimpaired upon deletion of TYK2 in NK cells only. In line with the previously reported maturation-associated Ifng promoter demethylation, the less mature phenotype of Tyk2^-/- NK cells correlated with an increased CpG methylation at the Ifng locus. Treatment with the DNA hypomethylating agent 5-aza-2-deoxycytidine restored the ability of Tyk2^-/- NK cells to produce IFN-γ upon actR but not upon IL-12 stimulation. NK cell maturation was dependent on the presence of TYK2 in dendritic cells and could be rescued in Tyk2-deficient mice by treatment with exogenous IL-15/IL-15Rα complexes. IL-15 treatment also rescued the in vitro cytotoxicity defect and the impaired actR-induced IFN-γ production of Tyk2^-/- NK cells. Collectively, our findings provide the first evidence, to our knowledge, for a key role of TYK2 in the host environment in promoting NK cell maturation and antitumor activity.

Chronic alcohol consumption exacerbates murine cytomegalovirus infection via impairing nonspecific and specific NK activation in mice

Chronic alcohol consumption increases the susceptibility to infectious diseases by compromising immune system. Cytomegalovirus infection is common in human and usually is asymptomatic in immunocompetent people. However, it can induce life-threatening medical complications in immunocompromised individuals such as alcoholics. How chronic alcohol consumption exacerbates cytomegalovirus infection is not known. Herein, we used a mouse cytomegalovirus model to study the underlying cellular and molecular mechanism. We found that alcohol consumption increased viral titers in spleen after 4 days of infection, enhanced body weight loss and inhibited splenomegaly during the acute phase of infection. Blood level of IFN-β, splenic IFN-γ and granzyme B-producing NK cells were lower in alcohol-consuming mice than in water-drinking mice at 12 h after viral infection. Moreover, alcohol consumption decreased IL-15-producing DC after 36 h infection, inhibited NK cell, specifically Ly49H⁺ NK cell maturation and proliferation 3-6 days after viral infection. Surprisingly, alcohol consumption enhanced NK cell and CD8⁺ T cell continuous activation and increased granzyme B-producing cells. However, alcohol consumption decreased the expression of perforin in spleen and liver. Taken together, chronic alcohol consumption exacerbates cytomegalovirus infection via impairing non-specific and specific NK cell activation, specifically IFN-γ and perforin production.

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.

Induction of the BIM Short Splice Variant Sensitizes Proliferating NK Cells to IL-15 Withdrawal

Adoptive transfer of allogeneic NK cells holds great promise for cancer immunotherapy. There is a variety of protocols to expand NK cells in vitro, most of which are based on stimulation with cytokines alone or in combination with feeder cells. Although IL-15 is essential for NK cell homeostasis in vivo, it is commonly used at supraphysiological levels to induce NK cell proliferation in vitro. As a result, adoptive transfer of such IL-15-addicted NK cells is associated with cellular stress because of sudden cytokine withdrawal. In this article, we describe a dose-dependent addiction to IL-15 during in vitro expansion of human NK cells, leading to caspase-3 activation and profound cell death upon IL-15 withdrawal. NK cell addiction to IL-15 was tightly linked to the BCL-2/BIM ratio, which rapidly dropped during IL-15 withdrawal. Furthermore, we observed a proliferation-dependent induction of BIM short, a highly proapoptotic splice variant of BIM in IL-15-activated NK cells. These findings shed new light on the molecular mechanisms involved in NK cell apoptosis following cytokine withdrawal and may guide future NK cell priming strategies in a cell therapy setting.

Chronic alcohol consumption exacerbates murine cytomegalovirus infection via impairing nonspecific and specific NK activation in mice

Chronic alcohol consumption increases the susceptibility to infectious diseases by compromising the immune system. Cytomegalovirus infection is common in humans and usually is asymptomatic in immunocompetent people. However, it can induce life‐threatening medical complications in immunocompromised individuals such as alcoholics. How chronic alcohol consumption exacerbates cytomegalovirus infection is not known. Herein, we used a mouse cytomegalovirus model to study the underlying cellular and molecular mechanism. We found that alcohol consumption increased viral titers in spleen after 4 days of infection, enhanced body weight loss and inhibited splenomegaly during the acute phase of infection. Blood level of IFN‐β, splenic IFN‐γ and granzyme B‐producing NK cells were lower in alcohol‐consuming mice than in water‐drinking mice at 12 hours after viral infection. Moreover, alcohol consumption decreased IL‐15‐producing DC after 36 hours infection, inhibited NK cell, specifically Ly49H⁺ NK cell maturation and proliferation 3‐6 days after viral infection. Surprisingly, alcohol consumption enhanced NK cell and CD8⁺ T‐cell continuous activation and increased granzyme B‐producing cells. However, alcohol consumption decreased the expression of perforin in spleen and liver. Taken together, chronic alcohol consumption exacerbates cytomegalovirus infection via impairing nonspecific and specific NK cell activation, specifically IFN‐γ and perforin production.

Out-sourcing for Trans-presentation: Assessing T Cell Intrinsic and Extrinsic IL-15 Expression with Il15 Gene Reporter Mice

IL-15 is a cytokine of the common γ-chain family that is critical for natural killer (NK), invariant natural killer T (iNKT), and CD8 memory T cell development and homeostasis. The role of IL-15 in regulating effector T cell subsets, however, remains incompletely understood. IL-15 is mostly expressed by stromal cells, myeloid cells, and dendritic cells (DCs). Whether T cells themselves can express IL-15, and if so, whether such T cell-derived IL-15 could play an autocrine role in T cells are interesting questions that were previously addressed but answered with mixed results. Recently, three independent studies described the generation of IL-15 reporter mice which facilitated the identification of IL-15-producing cells and helped to clarify the role of IL-15 both in vitro and in vivo. Here, we review the findings of these studies and place them in context of recent reports that examined T cell-intrinsic IL-15 expression during CD4 effector T cell differentiation.

IL-2 and Beyond in Cancer Immunotherapy

The development of the T- and natural killer (NK) cell growth factor IL-2 has been a sentinel force ushering in the era of immunotherapy in cancer. With the advent of clinical grade recombinant IL-2 in the mid-1980s, oncologists could for the first time directly manipulate lymphocyte populations with systemic therapy. By itself, recombinant IL-2 can induce clinical responses in up to 15% of patients with metastatic cancer or renal cell carcinoma. When administered with adoptively transferred tumor-reactive lymphocytes, IL-2 promotes T cell engraftment and response rates of up to 50% in metastatic melanoma patients. Importantly, these IL-2-driven responses can yield complete and durable responses in a subset of patients. However, the use of IL-2 is limited by toxicity and concern of the expansion of T regulatory cells. To overcome these limitations and improve response rates, other T cell growth factors, including IL-15 and modified forms of IL-2, are in clinical development. Administering T cell growth factors in combination with other agents, such as immune checkpoint pathway inhibitors, may also improve efficacy. In this study, we review the development of T- and NK cell growth factors and highlight current combinatorial approaches based on these reagents.

Inhibition of histone H3K27 demethylases selectively modulates inflammatory phenotypes of natural killer cells

Natural killer (NK) cells are innate lymphocytes, important in immune surveillance and elimination of stressed, transformed, or virus-infected cells. They critically shape the inflammatory cytokine environment to orchestrate interactions of cells of the innate and adaptive immune systems. Some studies have reported that NK cell activation and cytokine secretion are controlled epigenetically but have yielded only limited insight into the mechanisms. Using chemical screening with small-molecule inhibitors of chromatin methylation and acetylation, further validated by knockdown approaches, we here identified Jumonji-type histone H3K27 demethylases as key regulators of cytokine production in human NK cell subsets. The prototypic JMJD3/UTX (Jumonji domain–containing protein 3) H3K27 demethylase inhibitor GSK-J4 increased global levels of the repressive H3K27me3 mark around transcription start sites of effector cytokine genes. Moreover, GSK-J4 reduced IFN-γ, TNFα, granulocyte–macrophage colony-stimulating factor (GM-CSF), and interleukin-10 levels in cytokine-stimulated NK cells while sparing their cytotoxic killing activity against cancer cells. The anti-inflammatory effect of GSK-J4 in NK cell subsets, isolated from peripheral blood or tissue from individuals with rheumatoid arthritis (RA), coupled with an inhibitory effect on formation of bone-resorbing osteoclasts, suggested that histone demethylase inhibition has broad utility for modulating immune and inflammatory responses. Overall, our results indicate that H3K27me3 is a dynamic and important epigenetic modification during NK cell activation and that JMJD3/UTX-driven H3K27 demethylation is critical for NK cell function.

Dose of Retroviral Infection Determines Induction of Antiviral NK Cell Responses

Natural killer (NK) cells are part of the innate immune system and recognize virus-infected cells as well as tumor cells. Conflicting data about the beneficial or even detrimental role of NK cells in different infectious diseases have been described previously. While the type of pathogen strongly influences NK cell functionality, less is known about how the infection dose influences the quality of a NK cell response against retroviruses. In this study, we used the well-established Friend retrovirus (FV) mouse model to investigate the impact of virus dose on the induction of antiviral NK cell functions. High-dose virus inoculation increased initial virus replication compared to that with medium- or low-dose viral challenge and significantly improved NK cell activation. Antiviral NK cell activity, including in vivo cytotoxicity toward infected target cells, was also enhanced by high-dose virus infection. NK cell activation following high-dose viral challenge was likely mediated by activated dendritic cells (DCs) and macrophages and the NK cell-stimulating cytokines interleukin 15 (IL-15) and IL-18. Neutralization of these cytokines decreased NK cell functions and increased viral loads, whereas IL-15 and IL-18 therapy improved NK cell activity. Here we demonstrate that virus dose positively correlates with antiviral NK cell activity and function, which are at least partly driven by IL-15 and IL-18. Our results suggest that NK cell activity may be therapeutically enhanced by administering IL-15 and IL-18 in virus infections that inadequately activate NK cells.

IMPORTANCE In infections with retroviruses, like HIV and FV infection of mice, NK cells clearly mediate antiviral activities, but they are usually not sufficient to prevent severe pathology. Here we show that the initial infection dose impacts the induction of an antiviral NK cell response during an acute retroviral infection, which had not investigated before. High-dose infection resulted in a strong NK cell functionality, whereas no antiviral activities were detected after low- or medium-dose infection. Interestingly, DCs and macrophages were highly activated after high-dose FV challenge, which corresponded with increased levels of NK cell-stimulating cytokines IL-15 and IL-18. IL-15 and IL-18 neutralization decreased NK cell functions, whereas IL-15 and IL-18 therapy improved NK cell activity. Here we show the importance of cytokines for NK cell activation in retroviral infections; our findings suggest that immunotherapy combining the well-tolerated cytokines IL-15 and IL-18 might be an interesting approach for antiretroviral treatment.

Natural killer cells migrate into and control simian immunodeficiency virus replication in lymph node follicles in African green monkeys

Natural killer (NK) cells play an essential role in antiviral immunity, but knowledge of their function in secondary lymphoid organs is incomplete. Lymph node follicles constitute a major viral reservoir during infections with HIV-1 and simian immunodeficiency virus of macaques (SIVmac). In contrast, during nonpathogenic infection with SIV from African green monkeys (SIVagm), follicles remain generally virus free. We show that NK cells in secondary lymphoid organs from chronically SIVagm-infected African green monkeys (AGMs) were frequently CXCR5⁺ and entered and persisted in lymph node follicles throughout the follow-up (240 d post-infection). These follicles were strongly positive for IL-15, which was primarily presented in its membrane-bound form by follicular dendritic cells. NK cell depletion through treatment with anti-IL-15 monoclonal antibody during chronic SIVagm infection resulted in high viral replication rates in follicles and the T cell zone and increased viral DNA in lymph nodes. Our data suggest that, in nonpathogenic SIV infection, NK cells migrate into follicles and play a major role in viral reservoir control in lymph nodes.

An Unbiased Linkage Approach Reveals That the p53 Pathway Is Coupled to NK Cell Maturation

Natural killer cells constitute potent innate lymphoid cells that play a major role in both tumor immunosurveillance and viral clearance via their effector functions. A four-stage model of NK cell functional maturation has been established according to the expression of CD11b and CD27, separating mature NK (mNK) cells into distinct populations that exhibit specific phenotypic and functional properties. To identify genetic factors involved in the regulation of NK cell functional maturation, we performed a linkage analysis on F₂ (B6.Rag1^-/- × NOD.Rag1^-/- intercross) mice. We identified six loci on chromosomes 2, 4, 7, 10, 11, and 18 that were linked to one or more mNK cell subsets. Subsequently, we performed an in silico analysis exploiting mNK cell subset microarray data, highlighting various genes and microRNAs as potential regulators of the functional maturation of NK cells. Together, the combination of our unbiased genetic linkage study and the in silico analysis positions genes known to affect NK cell biology along the specific stages of NK cell functional maturation. Moreover, this approach allowed us to uncover a novel candidate gene in the regulation of NK cell maturation, namely Trp53 Using mice deficient for Trp53, we confirm that this tumor suppressor regulates NK cell functional maturation. Additional candidate genes revealed in this study may eventually serve as targets for the modulation of NK cell functional maturation to potentiate both tumor immunosurveillance and viral clearance.

IL-15 Harnesses Pro-inflammatory Function of TEMRA CD8 in Kidney-Transplant Recipients

The involvement of TEMRA CD8 is evident in a large array of immunological conditions ranging from auto- to allo-immunity. Nevertheless, the factors leading to their accumulation and activation remain ill-defined and, efficient therapeutics to control their inflammatory response is lacking. Here, we show that IL-15-stimulated TEMRA from kidney-transplant (KT) recipients promote inflammation by inducing the expression of CX3CL1 by endothelial cells in an IFN-γ- and TNF-α-dependent manner. The responsiveness of TEMRA to IL-15 is not restricted to chronic stimulation, as TEMRA from healthy volunteers respond earlier and faster when compared to effector memory (EM). IL-15 induces antiapoptotic signals and promotes proliferation dependent of PI3K/Akt, MAPK, and ERK pathways. Without ex vivo stimulation, TEMRA cells are metabolically more active than naive and EM, as shown by their high ATP reservoir and a high expression of genes involved in glycolysis, glutaminolysis, and the Pentose Phosphate Pathway. Upon stimulation, TEMRA adapt their metabolism by sustaining an increased mitochondrial respiration and glycolysis. Finally, we show that the inhibition of glycolysis is highly effective in preventing endothelial inflammation induced by TEMRA from KT recipients. Together, our findings highlight the metabolic fitness that tightly regulates the immune function of TEMRA in physiological and pathogenic situations.

IL-15-deficient mice develop enhanced allergic responses to airway allergen exposure

BACKGROUND

Interleukin-15 is a pleiotropic cytokine that is critical for the development and survival of multiple haematopoietic lineages. Mice lacking IL-15 have selective defects in populations of several pro-allergic immune cells including natural killer (NK) cells, NKT cells, and memory CD8+ T cells. We therefore hypothesized that IL-15-/- mice will have reduced inflammatory responses during the development of allergic airway disease (AAD).

OBJECTIVE

To determine whether IL-15-/- mice have attenuated allergic responses in a mouse model of AAD.

METHODS

C57BL/6 wild-type (WT) and IL-15-/- mice were sensitized and challenged with ovalbumin (OVA), and the development of AAD was ascertained by examining changes in airway inflammatory responses, Th2 responses, and lung histopathology.

RESULTS

Here, we report that IL-15-/- mice developed enhanced allergic responses in an OVA-induced model of AAD. In the absence of IL-15, OVA-challenged mice exhibited enhanced bronchial eosinophilic inflammation, elevated IL-13 production, and severe lung histopathology in comparison with WT mice. In addition, increased numbers of CD4+ T and B cells in the spleens and bronchoalveolar lavage (BAL) were also observed. Examination of OVA-challenged IL-15Rα-/- animals revealed a similar phenotype resulting in enhanced airway eosinophilia compared to WT mice. Adoptive transfer of splenic CD8+ T cells from OVA-sensitized WT mice suppressed the enhancement of eosinophilia in IL-15-/- animals to levels observed in WT mice, but had no further effects.

CONCLUSION AND CLINICAL RELEVANCE

These data demonstrate that mice with an endogenous IL-15 deficiency are susceptible to the development of severe, enhanced Th2-mediated AAD, which can be regulated by CD8+ T cells. Furthermore, the development of disease as well as allergen-specific Th2 responses occurs despite deficiencies in several IL-15-dependent cell types including NK, NKT, and γδ T cells, suggesting that these cells or their subsets are dispensable for the induction of AAD in IL-15-deficient mice.

IL-15 Enables Septic Shock by Maintaining NK Cell Integrity and Function

Interleukin 15 is essential for the development and differentiation of NK and memory CD8⁺ (mCD8⁺) T cells. Our laboratory previously showed that NK and CD8⁺ T lymphocytes facilitate the pathobiology of septic shock. However, factors that regulate NK and CD8⁺ T lymphocyte functions during sepsis are not well characterized. We hypothesized that IL-15 promotes the pathogenesis of sepsis by maintaining NK and mCD8⁺ T cell integrity. To test our hypothesis, the pathogenesis of sepsis was assessed in IL-15-deficient (IL-15 knockout, KO) mice. IL-15 KO mice showed improved survival, attenuated hypothermia, and less proinflammatory cytokine production during septic shock caused by cecal ligation and puncture or endotoxin-induced shock. Treatment with IL-15 superagonist (IL-15 SA, IL-15/IL-15Rα complex) regenerated NK and mCD8⁺ T cells and re-established mortality of IL-15 KO mice during septic shock. Preventing NK cell regeneration attenuated the restoration of mortality caused by IL-15 SA. If given immediately prior to septic challenge, IL-15-neutralizing IgG M96 failed to protect against septic shock. However, M96 caused NK cell depletion if given 4 d prior to septic challenge and conferred protection. IL-15 SA treatment amplified endotoxin shock, which was prevented by NK cell or IFN-γ depletion. IL-15 SA treatment also exacerbated septic shock caused by cecal ligation and puncture when given after the onset of sepsis. In conclusion, endogenous IL-15 does not directly augment the pathogenesis of sepsis but enables the development of septic shock by maintaining NK cell numbers and integrity. Exogenous IL-15 exacerbates the severity of sepsis by activating NK cells and facilitating IFN-γ production.

Independent control of natural killer cell responsiveness and homeostasis at steady-state by CD11c+ dendritic cells

During infection and inflammation, dendritic cells (DC) provide priming signals for natural killer (NK) cells via mechanisms distinct from their antigen processing and presentation functions. The influence of DC on resting NK cells, i.e. at steady-state, is less well studied. We here demonstrate that as early as 1 day after DC depletion, NK cells in naïve mice downregulated the NKG2D receptor and showed decreased constitutive phosphorylation of AKT and mTOR. Subsequently, apoptotic NK cells appeared in the spleen concomitant with reduced NK cell numbers. At 4 days after the onset of DC depletion, increased NK cell proliferation was seen in the spleen resulting in an accumulation of Ly49 receptor-negative NK cells. In parallel, NK cell responsiveness to ITAM-mediated triggering and cytokine stimulation dropped across maturation stages, suggestive of a functional deficiency independent from the homeostatic effect. A role for IL-15 in maintaining NK cell function was supported by a gene signature analysis of NK cell from DC-depleted mice as well as by in vivo DC transfer experiments. We propose that DC, by means of IL-15 transpresentation, are required to maintain not only homeostasis, but also function, at steady-state. These processes appear to be regulated independently from each other.

Chronic alcohol consumption inhibits peripheral NK cell development and maturation by decreasing the availability of IL-15

NK cells are innate immune cells and have important roles in antiviral and antitumor immunity. Based on the transcriptional regulation, organ distribution, and cell function, NK cells have recently been further divided into cytotoxic conventional NK cells (cNK) and noncytotoxic helper-like group 1 innate lymphoid cells (ILC1s). It is well known that chronic alcohol consumption decreases peripheral NK cell number and cytolytic activity; however, the underlying mechanism remains to be elucidated. How chronic alcohol consumption affects ILC1s is, to our knowledge, completely unexplored. Herein, we used a well-established mouse model of chronic alcohol consumption to study the effects of alcohol on transcription factor expression, maturation, and cytokine production of cNK cells and ILC1s in various organs. We found that alcohol consumption significantly decreased Eomes-expressing cNK cells in all the examined organs, except BM, but did not significantly affect ILC1s. Alcohol consumption compromised cNK cell development and maturation. Exogenous IL-15/IL-15Rα treatment caused full recovery of Eomes-expressing cNK cell number and maturation. Taken together, our data indicated that chronic alcohol consumption decreases cNK cell number and cytolytic activity by arresting cNK cell development at the CD27⁺CD11b⁺ stage. This developmental arrest of NK cells results from a lack of IL-15 availability in the microenvironment. IL-15/IL-15Rα treatment can recover alcohol consumption-induced developmental defect in NK cells.

Transpresentation of interleukin-15 by IL-15/IL-15Rα mRNA-engineered human dendritic cells boosts antitumoral natural killer cell activity

In cancer immunotherapy, the use of dendritic cell (DC)-based vaccination strategies can improve overall survival, but until now durable clinical responses remain scarce. To date, DC vaccines are designed primarily to induce effective T-cell responses, ignoring the antitumor activity potential of natural killer (NK) cells. Aiming to further improve current DC vaccination outcome, we engineered monocyte-derived DC to produce interleukin (IL)-15 and/or IL-15 receptor alpha (IL-15Rα) using mRNA electroporation. The addition of IL-15Rα to the protocol, enabling IL-15 transpresentation to neighboring NK cells, resulted in significantly better NK-cell activation compared to IL-15 alone. Next to upregulation of NK-cell membrane activation markers, IL-15 transpresentation resulted in increased NK-cell secretion of IFN-γ, granzyme B and perforin. Moreover, IL-15-transpresenting DC/NK cell cocultures from both healthy donors and acute myeloid leukemia (AML) patients in remission showed markedly enhanced cytotoxic activity against NK cell sensitive and resistant tumor cells. Blocking IL-15 transpresentation abrogated NK cell-mediated cytotoxicity against tumor cells, pointing to a pivotal role of IL-15 transpresentation by IL-15Rα to exert its NK cell-activating effects. In conclusion, we report an attractive approach to improve antitumoral NK-cell activity in DC-based vaccine strategies through the use of IL-15/IL-15Rα mRNA-engineered designer DC.

Fibroblastic reticular cells regulate intestinal inflammation via IL-15-mediated control of group 1 ILCs

Fibroblastic reticular cells influence the function of lymphocytes in secondary lymphoid organs. Ludewig and colleagues demonstrate that they also specifically restrain the activation of group 1 innate lymphoid cells in the presence of microbial stimulation to prevent immunopathology.

Fibroblastic reticular cells (FRCs) of secondary lymphoid organs form distinct niches for interaction with hematopoietic cells. We found here that production of the cytokine IL-15 by FRCs was essential for the maintenance of group 1 innate lymphoid cells (ILCs) in Peyer's patches and mesenteric lymph nodes. Moreover, FRC-specific ablation of the innate immunological sensing adaptor MyD88 unleashed IL-15 production by FRCs during infection with an enteropathogenic virus, which led to hyperactivation of group 1 ILCs and substantially altered the differentiation of helper T cells. Accelerated clearance of virus by group 1 ILCs precipitated severe intestinal inflammatory disease with commensal dysbiosis, loss of intestinal barrier function and diminished resistance to colonization. In sum, FRCs act as an 'on-demand' immunological 'rheostat' by restraining activation of group 1 ILCs and thereby preventing immunopathological damage in the intestine.

Immunotherapy: A promising approach to reverse sepsis-induced immunosuppression

Sepsis is defined as life-threatening organ dysfunction caused by dysregulated host responses to infection (Third International Consensus definition for Sepsis and septic shock). Despite decades of research, sepsis remains the leading cause of death in intensive care units. More than 40 clinical trials, most of which have targeted the sepsis-associated pro-inflammatory response, have failed. Thus, antibiotics and fluid resuscitation remain the mainstays of supportive care and there is intense need to discover and develop novel, targeted therapies to treat sepsis. Both pre-clinical and clinical studies over the past decade demonstrate unequivocally that sepsis not only causes hyper-inflammation, but also leads to simultaneous adaptive immune system dysfunction and impaired antimicrobial immunity. Evidences for immunosuppression include immune cell depletion (T cells most affected), compromised T cell effector functions, T cell exhaustion, impaired antigen presentation, increased susceptibility to opportunistic nosocomial infections, dysregulated cytokine secretion, and reactivation of latent viruses. Therefore, targeting immunosuppression provides a logical approach to treat protracted sepsis. Numerous pre-clinical studies using immunomodulatory agents such as interleukin-7, anti-programmed cell death 1 antibody (anti-PD-1), anti-programmed cell death 1 ligand antibody (anti-PD-L1), and others have demonstrated reversal of T cell dysfunction and improved survival. Therefore, identifying immunosuppressed patients with the help of specific biomarkers and administering specific immunomodulators holds significant potential for sepsis therapy in the future. This review focusses on T cell dysfunction during sepsis and discusses the potential immunotherapeutic agents to boost T cell function during sepsis and improve host resistance to infection.

CCR7 expression alters memory CD8 T-cell homeostasis by regulating occupancy in IL-7- and IL-15-dependent niches

C-C receptor 7 (CCR7) is important to allow T cells and dendritic cells to migrate toward CCL19- and CCL21-producing cells in the T-cell zone of the spleen and lymph nodes. The role of this chemokine receptor in regulating the homeostasis of effector and memory T cells during acute viral infection is poorly defined, however. In this study, we show that CCR7 expression alters memory CD8 T-cell homeostasis following lymphocytic choriomeningitis virus infection. Greater numbers of CCR7-deficient memory T cells were formed and maintained compared with CCR7-sufficient memory T cells, especially in the lung and bone marrow. The CCR7-deficient memory T cells also displayed enhanced rates of homeostatic turnover, which may stem from increased exposure to IL-15 as a consequence of reduced exposure to IL-7, because removal of IL-15, but not of IL-7, normalized the numbers of CCR7-sufficient and CCR7-deficient memory CD8 T cells. This result suggests that IL-15 is the predominant cytokine supporting augmentation of the CCR7(-/-) memory CD8 T-cell pool. Taken together, these data suggest that CCR7 biases memory CD8 T cells toward IL-7-dependent niches over IL-15-dependent niches, which provides insight into the homeostatic regulation of different memory T-cell subsets.

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.

Inflammatory Signals Regulate IL-15 in Response to Lymphodepletion

Induction of lymphopenia has been exploited therapeutically to improve immune responses to cancer therapies and vaccinations. Whereas IL-15 has well-established roles in stimulating lymphocyte responses after lymphodepletion, the mechanisms regulating these IL-15 responses are unclear. We report that cell surface IL-15 expression is upregulated during lymphopenia induced by total body irradiation (TBI), cyclophosphamide, or Thy1 Ab-mediated T cell depletion, as well as in RAG(-/-) mice; interestingly, the cellular profile of surface IL-15 expression is distinct in each model. In contrast, soluble IL-15 (sIL-15) complexes are upregulated only after TBI or αThy1 Ab. Analysis of cell-specific IL-15Rα conditional knockout mice revealed that macrophages and dendritic cells are important sources of sIL-15 complexes after TBI but provide minimal contribution in response to Thy1 Ab treatment. Unlike with TBI, induction of sIL-15 complexes by αThy1 Ab is sustained and only partially dependent on type I IFNs. The stimulator of IFN genes pathway was discovered to be a potent inducer of sIL-15 complexes and was required for optimal production of sIL-15 complexes in response to Ab-mediated T cell depletion and TBI, suggesting products of cell death drive production of sIL-15 complexes after lymphodepletion. Lastly, we provide evidence that IL-15 induced by inflammatory signals in response to lymphodepletion drives lymphocyte responses, as memory CD8 T cells proliferated in an IL-15-dependent manner. Overall, these studies demonstrate that the form in which IL-15 is expressed, its kinetics and cellular sources, and the inflammatory signals involved are differentially dictated by the manner in which lymphopenia is induced.

IL-15 Superagonist Expands mCD8+ T, NK and NKT Cells after Burn Injury but Fails to Improve Outcome during Burn Wound Infection

Background

Severely burned patients are highly susceptible to opportunistic infections and sepsis, owing to the loss of the protective skin barrier and immunological dysfunction. Interleukin-15 (IL-15) belongs to the IL-2 family of common gamma chain cytokines and stimulates the proliferation and activation of T (specifically memory CD8), NK and NKT cells. It has been shown to preserve T cell function and improve survival during cecal ligation and puncture (CLP)-induced sepsis in mice. However, the therapeutic efficacy of IL-15 or IL-15 superagonist (SA) during infection after burn injury has not been evaluated. Moreover, very few, if any, studies have examined, in detail, the effect of burn injury and infection on the adaptive immune system. Thus, we examined the effect of burn and sepsis on adaptive immune cell populations and the effect of IL-15 SA treatment on the host response to infection.

Methods

Mice were subjected to a 35% total body surface area burn, followed by wound infection with Pseudomonas aeruginosa. In some experiments, IL-15 SA was administered after burn injury, but before infection. Leukocytes in spleen, liver and peritoneal cavity were characterized using flow cytometry. Bacterial clearance, organ injury and survival were also assessed.

Results

Burn wound infection led to a significant decline in total white blood cell and lymphocyte counts and induced organ injury and sepsis. Burn injury caused decline in CD4⁺ and CD8⁺ T cells in the spleen, which was worsened by infection. IL-15 treatment inhibited this decline and significantly increased cell numbers and activation, as determined by CD69 expression, of CD4⁺, CD8⁺, B, NK and NKT cells in the spleen and liver after burn injury. However, IL-15 SA treatment failed to prevent burn wound sepsis-induced loss of CD4⁺, CD8⁺, B, NK and NKT cells and failed to improve bacterial clearance and survival.

Conclusion

Cutaneous burn injury and infection cause significant adaptive immune dysfunction. IL-15 SA does not augment host resistance to burn wound sepsis in mice despite inducing proliferation and activation of lymphocyte subsets.

High dose CD11c-driven IL15 is sufficient to drive NK cell maturation and anti-tumor activity in a trans-presentation independent manner

The common gamma (γc)-chain cytokine interleukin 15 (IL15) is a multifunctional immune-modulator which impacts the generation, maturation and activity of many cell types of the innate, as well as the adaptive immune system, including natural killer (NK) and CD8(+) T cells. Using a new series of transgenic mice, we analyzed the in vivo potential of IL15 as an immune-regulator when available at different concentrations or delivery modes, i.e. soluble monomer or complexed to its specific receptor α (Rα)-chain. We have identified distinct effects on selected IL15-responsive populations. While CD8(+) T cells required complexed forms of IL15/IL15Rα for full functionality, mature NK populations were rescued in an IL15/IL15Rα-deficient environment by high levels of CD11c-restricted IL15. These IL15-conditions were sufficient to limit tumor formation in a lung metastasis model indicating that the NK cell populations were fully functional. These data underline the potential of "free" IL15 in the absence of Rα-complex as a powerful and specific immuno-modulator, which may be beneficial where selective immune-activation is desired.

Homeostasis of IL-15 dependent lymphocyte subsets in the liver

IL-15 is a member of the gamma chain family of cytokines (γc - CD132). The IL-15 receptor (IL-15R) complex consists of 3 subunits: the ligand-binding IL-15Rα chain (CD215), the β chain (CD122; also used by IL-2), and the common γ chain. The biological activities of IL-15 are mostly mediated by the IL-15:IL-15Rα complex, produced by the same cell and 'trans-presented' to responder cells expressing the IL-15Rβγc. The peculiar and almost unique requirement for IL-15 to be trans-presented by IL-15Rα suggests that the biological effects of IL-15 signaling are tightly regulated even at the level of availability of IL-15. Tissue-specific deletion of IL-15Rα has shown macrophage-and dendritic cell-derived IL-15Rα mediate the homeostasis of different CD8(+) T cell subsets. Here we show that hepatocyte and macrophage- specific expression of IL-15Rα is required to maintain the homeostasis of NK and NKT cells in the liver. Thus, homeostasis of IL-15-dependent lymphocyte subsets is also regulated by trans-presentation of IL-15 by non-hematopoietic cells in the tissue environment.

Neutrophils Regulate Humoral Autoimmunity by Restricting Interferon-γ Production via the Generation of Reactive Oxygen Species

Here, we examine the mechanism by which plasmacytoid dendritic cells (pDCs) and type I interferons promote humoral autoimmunity. In an amyloid-induced experimental autoimmune model, neutrophil depletion enhanced anti-nuclear antibody development, which correlated with heightened IFN-γ production by natural killer (NK) cells. IFN-α/β produced by pDCs activated NK cells via IL-15 induction. Neutrophils released reactive oxygen species (ROS), which negatively modulated the levels of IL-15, thereby inhibiting IFN-γ production. Mice deficient in NADPH oxidase 2 produced increased amounts of IFN-γ and developed augmented titers of autoantibodies. Both the pDC-IFN-α/β pathway and IFN-γ were indispensable in stimulating humoral autoimmunity. Male NZB/W F1 mice expressed higher levels of superoxide than their female lupus-prone siblings, and depletion of neutrophils resulted in spontaneous NK cell and autoimmune B cell activation. Our findings suggest a regulatory role for neutrophils in vivo and highlight the importance of an NK-IFN-γ axis downstream of the pDC-IFN-α/β pathway in systemic autoimmunity.

IL-15 Superagonist-Mediated Immunotoxicity: Role of NK Cells and IFN-γ

IL-15 is currently undergoing clinical trials to assess its efficacy for treatment of advanced cancers. The combination of IL-15 with soluble IL-15Rα generates a complex termed IL-15 superagonist (IL-15 SA) that possesses greater biological activity than IL-15 alone. IL-15 SA is considered an attractive antitumor and antiviral agent because of its ability to selectively expand NK and memory CD8(+) T (mCD8(+) T) lymphocytes. However, the adverse consequences of IL-15 SA treatment have not been defined. In this study, the effect of IL-15 SA on physiologic and immunologic functions of mice was evaluated. IL-15 SA caused dose- and time-dependent hypothermia, weight loss, liver injury, and mortality. NK (especially the proinflammatory NK subset), NKT, and mCD8(+) T cells were preferentially expanded in spleen and liver upon IL-15 SA treatment. IL-15 SA caused NK cell activation as indicated by increased CD69 expression and IFN-γ, perforin, and granzyme B production, whereas NKT and mCD8(+) T cells showed minimal, if any, activation. Cell depletion and adoptive transfer studies showed that the systemic toxicity of IL-15 SA was mediated by hyperproliferation of activated NK cells. Production of the proinflammatory cytokine IFN-γ, but not TNF-α or perforin, was essential to IL-15 SA-induced immunotoxicity. The toxicity and immunological alterations shown in this study are comparable to those reported in recent clinical trials of IL-15 in patients with refractory cancers and advance current knowledge by providing mechanistic insights into IL-15 SA-mediated immunotoxicity.

Soluble interleukin-15 complexes are generated in vivo by type I interferon dependent and independent pathways

Interleukin (IL)-15 associates with IL-15Rα on the cell surface where it can be cleaved into soluble cytokine/receptor complexes that have the potential to stimulate CD8 T cells and NK cells. Unfortunately, little is known about the in vivo production of soluble IL-15Rα/IL-15 complexes (sIL-15 complexes), particularly regarding the circumstances that induce them and the mechanisms responsible. The main objective of this study was to elucidate the signals leading to the generation of sIL-15 complexes. In this study, we show that sIL-15 complexes are increased in the serum of mice in response to Interferon (IFN)-α. In bone marrow derived dendritic cells (BMDC), IFN-α increased the activity of ADAM17, a metalloproteinase implicated in cleaving IL-15 complexes from the cell surface. Moreover, knocking out ADAM17 in BMDCs prevented the ability of IFN-α to induce sIL-15 complexes demonstrating ADAM17 as a critical protease mediating cleavage of IL-15 complexes in response to type I IFNs. Type I IFN signaling was required for generating sIL-15 complexes as in vivo induction of sIL-15 complexes by Poly I:C stimulation or total body irradiation (TBI) was impaired in IFNAR-/- mice. Interestingly, serum sIL-15 complexes were also induced in mice infected with Vesicular stomatitis virus (VSV) or mice treated with agonistic CD40 antibodies; however, sIL-15 complexes were still induced in IFNAR-/- mice after VSV infection or CD40 stimulation indicating pathways other than type I IFNs induce sIL-15 complexes. Overall, this study has shown that type I IFNs, VSV infection, and CD40 stimulation induce sIL-15 complexes suggesting the generation of sIL-15 complexes is a common event associated with immune activation. These findings reveal an unrealized mechanism for enhanced immune responses occurring during infection, vaccination, inflammation, and autoimmunity.

IL-15: a central regulator of celiac disease immunopathology

Interleukin-15 (IL-15) exerts many biological functions essential for the maintenance and function of multiple cell types. Although its expression is tightly regulated, IL-15 upregulation has been reported in many organ-specific autoimmune disorders. In celiac disease, an intestinal inflammatory disorder driven by gluten exposure, the upregulation of IL-15 expression in the intestinal mucosa has become a hallmark of the disease. Interestingly, because it is overexpressed both in the gut epithelium and in the lamina propria, IL-15 acts on distinct cell types and impacts distinct immune components and pathways to disrupt intestinal immune homeostasis. In this article, we review our current knowledge of the multifaceted roles of IL-15 with regard to the main immunological processes involved in the pathogenesis of celiac disease.

Positive and negative regulation by NK cells in cancer

Our understanding of NK biology has expanded immensely since the initial discovery of natural killer cells in 1975. New studies have uncovered various levels of immune regulation both on and by unique subsets of NK cells, which go well beyond simple receptor-ligand interactions between NK cells and target cancer cells. Distinct suppressor and effector populations of NK cells have been delineated in both viral and tumor models. Interactions between NK cells and dendritic cells, T cells, and B cells also dramatically alter the overall immune response to cancer. To exploit the diverse functional abilities of NK cell subsets for cancer immunotherapies, it is important to understand NK cell biology and NK regulator mechanisms.

Classical Flt3L-dependent dendritic cells control immunity to protein vaccine

Protective immunity to protein vaccines is controlled by Flt3L-dependent classical LN-resident dendritic cells, and dampened by migratory dendritic cells.

DCs are critical for initiating immunity. The current paradigm in vaccine biology is that DCs migrating from peripheral tissue and classical lymphoid-resident DCs (cDCs) cooperate in the draining LNs to initiate priming and proliferation of T cells. Here, we observe subcutaneous immunity is Fms-like tyrosine kinase 3 ligand (Flt3L) dependent. Flt3L is rapidly secreted after immunization; Flt3 deletion reduces T cell responses by 50%. Flt3L enhances global T cell and humoral immunity as well as both the numbers and antigen capture capacity of migratory DCs (migDCs) and LN-resident cDCs. Surprisingly, however, we find immunity is controlled by cDCs and actively tempered in vivo by migDCs. Deletion of Langerin⁺ DC or blockade of DC migration improves immunity. Consistent with an immune-regulatory role, transcriptomic analyses reveals different skin migDC subsets in both mouse and human cluster together, and share immune-suppressing gene expression and regulatory pathways. These data reveal that protective immunity to protein vaccines is controlled by Flt3L-dependent, LN-resident cDCs.

NK cells are primed by ANRS MVA(HIV)-infected DCs, via a mechanism involving NKG2D and membrane-bound IL-15, to control HIV-1 infection in CD4+ T cells

Natural killer (NK) cells are the major antiviral effector cell population of the innate immune system. It has been demonstrated that NK-cell activity can be modulated by the interaction with dendritic cells (DCs). The HIV-1 vaccine candidate Modified Vaccinia Ankara encoding an HIV polypeptide (MVA(HIV)), developed by the French National Agency for Research on AIDS (ANRS), has the ability to prime NK cells to control HIV-1 infection in DCs. However, whether or not MVA(HIV)-primed NK cells are able to better control HIV-1 infection in CD4(+) T cells, and the mechanism underlying the specific priming, remain undetermined. In this study, we show that MVA(HIV)-primed NK cells display a greater capacity to control HIV-1 infection in autologous CD4(+) T cells. We also highlight the importance of NKG2D engagement on NK cells and DC-produced IL-15 to achieve the anti-HIV-1 specific priming, as blockade of either NKG2D or IL-15 during MVA(HIV)-priming lead to a subsequent decreased control of HIV-1 infection in autologous CD4(+) T cells. Furthermore, we show that the decreased control of HIV-1 infection in CD4(+) T cells might be due, at least in part, to the decreased expression of membrane-bound IL-15 (mbIL-15) on DCs when NKG2D is blocked during MVA(HIV)-priming of NK cells.

IL-15: a central regulator of celiac disease immunopathology

Interleukin-15 (IL-15) exerts many biological functions essential for the maintenance and function of multiple cell types. Although its expression is tightly regulated, IL-15 upregulation has been reported in many organ-specific autoimmune disorders. In celiac disease, an intestinal inflammatory disorder driven by gluten exposure, the upregulation of IL-15 expression in the intestinal mucosa has become a hallmark of the disease. Interestingly, because it is overexpressed both in the gut epithelium and in the lamina propria, IL-15 acts on distinct cell types and impacts distinct immune components and pathways to disrupt intestinal immune homeostasis. In this article, we review our current knowledge of the multifaceted roles of IL-15 with regard to the main immunological processes involved in the pathogenesis of celiac disease.

SAP-Dependent and -Independent Regulation of Innate T Cell Development Involving SLAMF Receptors

Signaling lymphocytic activation molecule (SLAM)-associated protein (SAP) plays an essential role in the immune system mediating the function of several members of the SLAM family (SLAMF) of receptors, whose expression is essential for T, NK, and B-cell responses. Additionally, the expression of SAP in double-positive thymocytes is mandatory for natural killer T (NKT) cells and, in mouse, for innate CD8⁺ T cell development. To date, only two members of the SLAMF of receptors, Slamf1 and Slamf6, have been shown to positively cooperate during NKT cell differentiation in mouse. However, it is less clear whether other members of this family may also participate in the development of these innate T cells. Here, we show that Slamf[1 + 6]^−/− and Slamf[1 + 5 + 6]^−/−B6 mice have ~70% reduction of NKT cells compared to wild-type B6 mice. Unexpectedly, the proportion of innate CD8⁺ T cells slightly increased in the Slamf[1 + 5 + 6]^−/−, but not in the Slamf[1 + 6]^−/− strain, suggesting that Slamf5 may function as a negative regulator of innate CD8⁺ T cell development. Accordingly, Slamf5^−/− B6 mice showed an exclusive expansion of innate CD8⁺ T cells, but not NKT cells. Interestingly, the SAP-independent Slamf7^−/− strain showed an expansion of both splenic innate CD8⁺ T cells and thymic NKT cells. On the other hand, and similar to what was recently shown in Slamf3^−/− BALB/c mice, the proportions of thymic promyelocytic leukemia zinc finger (PLZF^hi) NKT cells and innate CD8⁺ T cells significantly increased in the SAP-independent Slamf8^−/− BALB/c strain. In summary, these results show that NKT and innate CD8⁺ T cell development can be regulated in a SAP-dependent and -independent fashion by SLAMF receptors, in which Slamf1, Slamf6, and Slamf8 affect development of NKT cells, and that Slamf5, Slamf7, and Slamf8 affect the development of innate CD8⁺ T cells.

The unconventional expression of IL-15 and its role in NK cell homeostasis

Natural killer (NK) cells are the founding members of the innate lymphoid cell family and contribute to the rapid production of inflammatory mediators upon pathogen detection. The evolution of receptors for self major histocompatibility complex-I and stress-induced ligands also bestows upon NK cells an important effector role in the clearance of virus-infected and transformed cells. NK cells are dependent on the pleiotropic cytokine interleukin (IL)-15 for their development, differentiation and optimal function. Here I review the regulation of IL-15 in vivo, its role in driving NK cell differentiation and discuss the function of NK cell diversification with regard to innate immunity.

Transcription factor Runx3 regulates interleukin-15-dependent natural killer cell activation

Natural killer cells belong to the family of innate lymphoid cells comprising the frontline defense against infected and transformed cells. Development and activation of natural killer cells is highly dependent on interleukin-15 signaling. However, very little is known about the transcription program driving this process. The transcription factor Runx3 is highly expressed in natural killer cells, but its function in these cells is largely unknown. We show that loss of Runx3 impaired interleukin-15-dependent accumulation of mature natural killer cells in vivo and under culture conditions and pregnant Runx3(-/-) mice completely lack the unique population of interleukin-15-dependent uterine natural killer cells. Combined chromatin immunoprecipitation sequencing and differential gene expression analysis of wild-type versus Runx3-deficient in vivo activated splenic natural killer cells revealed that Runx3 cooperates with ETS and T-box transcription factors to drive the interleukin-15-mediated transcription program during activation of these cells. Runx3 functions as a nuclear regulator during interleukin-15-dependent activation of natural killer cells by regulating the expression of genes involved in proliferation, maturation, and migration. Similar studies with additional transcription factors will allow the construction of a more detailed transcriptional network that controls natural killer cell development and function.