Mechanistic and structural insight into the functional dichotomy between IL-2 and IL-15

Divergent Role for STAT5 in the Adaptive Responses of Natural Killer Cells

Natural killer (NK) cells are innate lymphocytes with the capacity to elicit adaptive features, including clonal expansion and immunological memory. Because signal transducer and activator of transcription 5 (STAT5) is essential for NK cell development, the roles of this transcription factor and its upstream cytokines interleukin-2 (IL-2) and IL-15 during infection have not been carefully investigated. In this study, we investigate how STAT5 regulates transcription during viral infection. We demonstrate that STAT5 is induced in NK cells by IL-12 and STAT4 early after infection and that partial STAT5 deficiency results in a defective capacity of NK cells to generate long-lived memory cells. Furthermore, we find a functional dichotomy of IL-2 and IL-15 signaling outputs during viral infection, whereby both cytokines drive clonal expansion, but only IL-15 is required for memory NK cell survival. We thus highlight a role for STAT5 signaling in promoting an optimal anti-viral NK cell response.

Wiedemann et al. demonstrate that Stat5a and Stat5b are induced by IL-12 and STAT4 signaling in NK cells following MCMV infection. They further provide evidence that the cytokines IL-2 and IL-15 upstream of STAT5 differentially promote the early and late stages of the adaptive NK cell response to MCMV infection.

Immunomodulatory Effects of IL-2 and IL-15; Implications for Cancer Immunotherapy

The type I cytokine family members interleukin-2 (IL-2) and IL-15 play important roles in the homeostasis of innate and adaptive immunity. Although IL-2 and IL-15 receptor complexes activate similar signal transduction cascades, triggering of these receptors results in different functional activities in lymphocytes. While IL-2 expands regulatory T cells and CD4+ helper T cells, IL-15 supports the development of central memory T cells and NK cells. Recent data have provided evidence that IL-2 and IL-15 differ in their ability to activate T and NK cells to resist various forms of immune suppression. The diverse roles of these two cytokines have on immune cells lead to critical therapeutic implications for cancer treatment. In this review, we discuss the distinct roles of IL-2 and IL-15 in activating various functions in T and NK cells with a particular focus on the signals that participate in the resistance of tumor-derived immune suppressive factors. Furthermore, we summarize current clinical applications of IL-2 and IL-15 in metastatic malignancies, either as monotherapy or in combination with other agents, and highlight the future trends for research on these cytokine-based immunotherapies.

Ancient Cytokine Interleukin 15-Like (IL-15L) Induces a Type 2 Immune Response

Related interleukin-2, -15, and -15-like (IL-2, -15, and -15L) are ancient cytokines, with all three genes surviving in extant fish and some mammals. The present study is the first to identify IL-15L functions, namely in rainbow trout. In isolated trout splenocytes, and in vivo, purified recombinant IL-15L+IL-15Rα molecules induced expression of IL-4 and IL-13 homologs, which are markers of type 2 immunity. In contrast, trout IL-15 stimulated type 1 immunity markers, thus IL-15 and IL-15L can have opposing functions. Trout IL-15L was more dependent on "in trans" presentation by the receptor chain IL-15Rα than IL-15, and stimulated CD4^-CD8^-(IgM^-) lymphocytes from thymus and spleen. We propose an important role for IL-15L early in the type 2 immunity cytokine cascade. Trout IL-2 and IL-15 exhibited features reminiscent of their mechanistic and functional dichotomy observed in mammals; for example, IL-15 but not IL-2 required a receptor alpha chain (only IL-15Rα in the case of fish) for its stability, and only IL-15 was efficient in stimulating lymphocytes from mucosal tissues. Data suggest that IL-15L and IL-15 may be particularly effective in stimulating innate lymphocyte type 2 cells (ILC2) and natural killer (NK) cells, respectively, but further identification of the cell types is needed. An interesting finding different from in mammals was the efficient stimulation of CD4⁺CD8⁺ thymocytes by IL-2. In short, this study presents fundamental information on the evolution of the IL-2/15/15L cytokine family.

CX3CL1 and IL-15 Promote CD8 T cell chemoattraction in HIV and in atherosclerosis

Atherosclerotic cardiovascular disease (ASCVD) remains an important cause of morbidity in the general population and risk for ASCVD is increased approximately 2-fold in persons living with HIV infection (PLWH). This risk is linked to elevated CD8 T cell counts that are abundant in atherosclerotic plaques and have been implicated in disease pathogenesis yet the mechanisms driving T cell recruitment to and activation within plaques are poorly defined. Here we investigated the role of CD8 T cells in atherosclerosis in a non-human primate model of HIV infection and in the HIV-uninfected elderly; we sought to identify factors that promote the activation, function, and recruitment to endothelium of CX3CR1+ CD8 T cells. We measured elevated expression of CX3CL1 and IL-15, and increased CD8 T cell numbers in the aortas of rhesus macaques infected with SIV or SHIV, and demonstrated similar findings in atherosclerotic vessels of HIV-uninfected humans. We found that recombinant TNF enhanced the production and release of CX3CL1 and bioactive IL-15 from aortic endothelial cells, but not from aortic smooth muscle cells. IL-15 in turn promoted CX3CR1 surface expression on and TNF synthesis by CD8 T cells, and IL-15-treated CD8 T cells exhibited enhanced CX3CL1-dependent chemoattraction toward endothelial cells in vitro. Finally, we show that CD8 T cells in human atherosclerotic plaques have an activated, resident phenotype consistent with in vivo IL-15 and CX3CL1 exposure. In this report, we define a novel model of CD8 T cell involvement in atherosclerosis whereby CX3CL1 and IL-15 operate in tandem within the vascular endothelium to promote infiltration by activated CX3CR1+ memory CD8 T cells that drive further endothelial activation via TNF. We propose that these interactions are prevalent in aging and in PLWH, populations where circulating activated CX3CR1+ CD8 T cell numbers are often expanded.

Comparative Analysis of the Secretome and Interactome of Trypanosoma cruzi and Trypanosoma rangeli Reveals Species Specific Immune Response Modulating Proteins

Chagas disease, a zoonosis caused by the flagellate protozoan Trypanosoma cruzi, is a chronic and systemic parasitic infection that affects ~5–7 million people worldwide, mainly in Latin America. Chagas disease is an emerging public health problem due to the lack of vaccines and effective treatments. According to recent studies, several T. cruzi secreted proteins interact with the human host during cell invasion. Moreover, some comparative studies with T. rangeli, which is non-pathogenic in humans, have been performed to identify proteins directly involved in the pathogenesis of the disease. In this study, we present an integrated analysis of canonical putative secreted proteins (PSPs) from both species. Additionally, we propose an interactome with human host and gene family clusters, and a phylogenetic inference of a selected protein. In total, we identified 322 exclusively PSPs in T. cruzi and 202 in T. rangeli. Among the PSPs identified in T. cruzi, we found several trans-sialidases, mucins, MASPs, proteins with phospholipase 2 domains (PLA2-like), and proteins with Hsp70 domains (Hsp70-like) which have been previously characterized and demonstrated to be related to T. cruzi virulence. PSPs found in T. rangeli were related to protozoan metabolism, specifically carboxylases and phosphatases. Furthermore, we also identified PSPs that may interact with the human immune system, including heat shock and MASP proteins, but in a lower number compared to T. cruzi. Interestingly, we describe a hypothetical hybrid interactome of PSPs which reveals that T. cruzi secreted molecules may be down-regulating IL-17 whilst T. rangeli may enhance the production of IL-15. These results will pave the way for a better understanding of the pathophysiology of Chagas disease and may ultimately lead to the identification of molecular targets, such as key PSPs, that could be used to minimize the health outcomes of Chagas disease by modulating the immune response triggered by T. cruzi infection.

Structural Understanding of Interleukin 6 Family Cytokine Signaling and Targeted Therapies: Focus on Interleukin 11

Cytokines are small signaling proteins that have central roles in inflammation and cell survival. In the half-century since the discovery of the first cytokines, the interferons, over fifty cytokines have been identified. Amongst these is interleukin (IL)-6, the first and prototypical member of the IL-6 family of cytokines, nearly all of which utilize the common signaling receptor, gp130. In the last decade, there have been numerous advances in our understanding of the structural mechanisms of IL-6 family signaling, particularly for IL-6 itself. However, our understanding of the detailed structural mechanisms underlying signaling by most IL-6 family members remains limited. With the emergence of new roles for IL-6 family cytokines in disease and, in particular, roles of IL-11 in cardiovascular disease, lung disease, and cancer, there is an emerging need to develop therapeutics that can progress to clinical use. Here we outline our current knowledge of the structural mechanism of signaling by the IL-6 family of cytokines. We discuss how this knowledge allows us to understand the mechanism of action of currently available inhibitors targeting IL-6 family cytokine signaling, and most importantly how it allows for improved opportunities to pharmacologically disrupt cytokine signaling. We focus specifically on the need to develop and understand inhibitors that disrupt IL-11 signaling.

Complement-activated interferon-γ-primed human endothelium transpresents interleukin-15 to CD8+ T cells

Alloantibodies in presensitized transplant candidates deposit complement membrane attack complexes (MACs) on graft endothelial cells (ECs), increasing risk of CD8+ T cell-mediated acute rejection. We recently showed that human ECs endocytose MACs into Rab5+ endosomes, creating a signaling platform that stabilizes NF-κB-inducing kinase (NIK) protein. Endosomal NIK activates both noncanonical NF-κB signaling to synthesize pro-IL-1β and an NLRP3 inflammasome to process and secrete active IL-1β. IL-1β activates ECs, increasing recruitment and activation of alloreactive effector memory CD4+ T (Tem) cells. Here, we report that IFN-γ priming induced nuclear expression of IL-15/IL-15Rα complexes in cultured human ECs and that MAC-induced IL-1β stimulated translocation of IL-15/IL-15Rα complexes to the EC surface in a canonical NF-κB-dependent process in which IL-15/IL-15Rα transpresentation increased activation and maturation of alloreactive CD8+ Tem cells. Blocking NLRP3 inflammasome assembly, IL-1 receptor, or IL-15 on ECs inhibited the augmented CD8+ Tem cell responses, indicating that this pathway is not redundant. Adoptively transferred alloantibody and mouse complement deposition induced IL-15/IL-15Rα expression by human ECs lining human coronary artery grafts in immunodeficient mice, and enhanced intimal CD8+ T cell infiltration, which was markedly reduced by inflammasome inhibition, linking alloantibody to acute rejection. Inhibiting MAC signaling may similarly limit other complement-mediated pathologies.

convertibleCARs: A chimeric antigen receptor system for flexible control of activity and antigen targeting

We have developed a chimeric antigen receptor (CAR) platform that functions as a modular system to address limitations of traditional CAR therapies. An inert form of the human NKG2D extracellular domain (iNKG2D) was engineered as the ectodomain of the CAR to generate convertibleCAR^TM-T cells. These cells were specifically directed to kill antigen-expressing target cells only in the presence of an activating bispecific adapter comprised of an iNKG2D-exclusive ULBP2-based ligand fused to an antigen-targeting antibody (MicAbody^TM). Efficacy against Raji tumors in NSG mice was dependent upon doses of both a rituximab-based MicAbody and convertibleCAR-T cells. We have also demonstrated that the exclusive ligand-receptor partnering enabled the targeted delivery of a mutant form of IL-2 to selectively promote the expansion of convertibleCAR-T cells in vitro and in vivo. By altering the Fv domains of the MicAbody or the payload fused to the orthogonal ligand, convertibleCAR-T cells can be readily targeted or regulated.

IL-23 and IL-2 activation of STAT5 is required for optimal IL-22 production in ILC3s during colitis

Signal transducer and activator of transcription (STAT) proteins have critical roles in the development and function of immune cells. STAT signaling is often dysregulated in patients with inflammatory bowel disease (IBD), suggesting the importance of STAT regulation during the disease process. Moreover, genetic alterations in STAT3 and STAT5 (e.g., deletions, mutations, and single-nucleotide polymorphisms) are associated with an increased risk for IBD. In this study, we elucidated the precise roles of STAT5 signaling in group 3 innate lymphoid cells (ILC3s), a key subset of immune cells involved in the maintenance of gut barrier integrity. We show that mice lacking either STAT5a or STAT5b are more susceptible to Citrobacter rodentium-mediated colitis and that interleukin-2 (IL-2)- and IL-23-induced STAT5 drives IL-22 production in both mouse and human colonic lamina propria ILC3s. Mechanistically, IL-23 induces a STAT3-STAT5 complex that binds IL-22 promoter DNA elements in ILC3s. Our data suggest that STAT5a/b signaling in ILC3s maintains gut epithelial integrity during pathogen-induced intestinal disease.

Common gamma chain cytokines and CD8 T cells in cancer

Overcoming exhaustion-associated dysfunctions and generating antigen-specific CD8 T cells with the ability to persist in the host and mediate effective long-term anti-tumor immunity is the final aim of cancer immunotherapy. To achieve this goal, immuno-modulatory properties of the common gamma-chain (γ_c) family of cytokines, that includes IL-2, IL-7, IL-15 and IL-21, have been used to fine-tune and/or complement current immunotherapeutic protocols. These agents potentiate CD8 T cell expansion and functions particularly in the context of immune checkpoint (IC) blockade, shape their differentiation, improve their persistence in vivo and alternatively, influence distinct aspects of the T cell exhaustion program. Despite these properties, the intrinsic impact of cytokines on CD8 T cell exhaustion has remained largely unexplored impeding optimal therapeutic use of these agents. In this review, we will discuss current knowledge regarding the influence of relevant γ_c cytokines on CD8 T cell differentiation and function based on clinical data and preclinical studies in murine models of cancer and chronic viral infection. We will restate the place of these agents in current immunotherapeutic regimens such as IC checkpoint blockade and adoptive cell therapy. Finally, we will discuss how γ_c cytokine signaling pathways regulate T cell immunity during cancer and whether targeting these pathways may sustain an effective and durable T cell response in patients.

Mechanisms of Leukemia Immune Evasion and Their Role in Relapse After Haploidentical Hematopoietic Cell Transplantation

Over the last decade, the development of multiple strategies to allow the safe transfer from the donor to the patient of high numbers of partially HLA-incompatible T cells has dramatically reduced the toxicities of haploidentical hematopoietic cell transplantation (haplo-HCT), but this was not accompanied by a similar positive impact on the incidence of post-transplantation relapse. In the present review, we will elaborate on how the unique interplay between HLA-mismatched immune system and malignancy that characterizes haplo-HCT may impact relapse biology, shaping the selection of disease variants that are resistant to the “graft-vs.-leukemia” effect. In particular, we will present current knowledge on genomic loss of HLA, a relapse modality first described in haplo-HCT and accounting for a significant proportion of relapses in this setting, and discuss other more recently identified mechanisms of post-transplantation immune evasion and relapse, including the transcriptional downregulation of HLA class II molecules and the enforcement of inhibitory checkpoints between T cells and leukemia. Ultimately, we will review the available treatment options for patients who relapse after haplo-HCT and discuss on how a deeper insight into relapse immunobiology might inform the rational and personalized selection of therapies to improve the largely unsatisfactory clinical outcome of relapsing patients.

Identification of a γc Receptor Antagonist That Prevents Reprogramming of Human Tissue-resident Cytotoxic T Cells by IL15 and IL21

BACKGROUND & AIMS

Gamma chain (γc) cytokines (interleukin [IL]2, IL4, IL7, IL9, IL15, and IL21) signal via a common γc receptor. IL2 regulates the immune response, whereas IL21 and IL15 contribute to development of autoimmune disorders, including celiac disease. We investigated whether BNZ-2, a peptide designed to inhibit IL15 and IL21, blocks these cytokines selectively and its effects on intraepithelial cytotoxic T cells.

METHODS

We obtained duodenal biopsies from 9 patients with potential celiac disease (positive results from tests for anti-TG2 but no villous atrophy), 30 patients with untreated celiac disease (with villous atrophy), and 5 patients with treated celiac disease (on a gluten-free diet), as well as 43 individuals without celiac disease (controls). We stimulated primary intestinal intraepithelial CD8⁺ T-cell lines, or CD8⁺ T cells directly isolated from intestinal biopsies, with γc cytokines in presence or absence of BNZ-2. Cells were analyzed by immunoblots, flow cytometry, or RNA-sequencing analysis for phosphorylation of signaling molecules, gene expression profiles, proliferation, and levels of granzyme B.

RESULTS

Duodenal tissues from patients with untreated celiac disease had increased levels of messenger RNAs encoding IL15 receptor subunit alpha (IL15RA) and IL21 compared with tissues from patients with potential celiac disease and controls. Activation of intraepithelial cytotoxic T cells with IL15 or IL21 induced separate signaling pathways; incubation of the cells with IL15 and IL21 cooperatively increased their transcriptional activity, proliferation, and cytolytic properties. BNZ-2 specifically inhibited the effects of IL15 and IL21, but not of other γc cytokines.

CONCLUSIONS

We found increased expression of IL15RA and IL21 in duodenal tissues from patients with untreated celiac disease compared with controls. IL15 and IL21 cooperatively activated intestinal intraepithelial cytotoxic T cells. In particular, they increased their transcriptional activity, proliferation, and cytolytic activity. The peptide BNZ-2 blocked these effects, but not those of other γc cytokines, including IL2. BNZ-2 might be used to prevent cytotoxic T-cell-mediated tissue damage in complex immune disorders exhibiting upregulation of IL15 and IL21.

CD16 pre-ligation by defucosylated tumor-targeting mAb sensitizes human NK cells to γc cytokine stimulation via PI3K/mTOR axis

Obinutuzumab is a glycoengineered tumor-targeting anti-CD20 mAb with a modified crystallizable fragment (Fc) domain designed to increase the affinity for the FcγRIIIA/CD16 receptor, which was recently approved for clinical use in CLL and follicular lymphoma. Here we extend our previous observation that, in human NK cells, the sustained CD16 ligation by obinutuzumab-opsonized targets leads to a markedly enhanced IFN-γ production upon a subsequent cytokine re-stimulation. The increased IFN-γ competence in response to IL-2 or IL-15 is attributable to post-transcriptional regulation, as it does not correlate with the upregulation of IFN-γ mRNA levels. Different from the reference molecule rituximab, we observe that the stimulation with obinutuzumab promotes the upregulation of microRNA (miR)-155 expression. A similar trend was also observed in NK cells from untreated CLL patients stimulated with obinutuzumab-opsonized autologous leukemia. miR-155 upregulation associates with reduced levels of SHIP-1 inositol phosphatase, which acts in constraining PI3K-dependent signals, by virtue of its ability to mediate phosphatidylinositol 3,4,5-trisphosphate (PIP3) de-phosphorylation. Downstream of PI3K, the phosphorylation status of mammalian target of rapamycin (mTOR) effector molecule, S6, results in amplified response to IL-2 or IL-15 stimulation in obinutuzumab-experienced cells. Importantly, NK cell treatment with the PI3K or mTOR inhibitors, idelalisib and rapamycin, respectively, prevents the enhanced cytokine responsiveness, thus, highlighting the relevance of the PI3K/mTOR axis in CD16-dependent priming. The enhanced IFN-γ competence may be envisaged to potentiate the immunoregulatory role of NK cells in a therapeutic setting.

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.

Differential Roles of IL-2 Signaling in Developing versus Mature Tregs

Although Foxp3⁺ regulatory T cells (Tregs) require interleukin-2 (IL-2) for their development, it has been unclear whether continuing IL-2 signals are needed to maintain lineage stability, survival, and suppressor function in mature Tregs. We generated mice in which CD25, the main ligand-binding subunit of the IL-2 receptor, can be inducibly deleted from Tregs after thymic development. In contrast to Treg development, we find that IL-2 is dispensable for maintaining lineage stability in mature Tregs. Although continuous IL-2 signaling is needed for long-term Treg survival, CD25-deleted Tregs may persist for several weeks in vivo using IL-7. We also observe defects in glycolytic metabolism and suppressor function following CD25 deletion. Thus, unlike developing Tregs in which the primary role of IL-2 is to initiate Foxp3 expression, mature Tregs require continuous IL-2 signaling to maintain survival and suppressor function, but not to maintain lineage stability.

IL-15Rα membrane anchorage in either cis or trans is required for stabilization of IL-15 and optimal signaling

Interleukin (IL)-15 plays an important role in the communication between immune cells. It delivers its signal through different modes involving three receptor chains: IL-15Rα, IL-2Rβ and IL-2Rγc. The combination of the different chains result in the formation of IL-15Rα/IL-2Rβ/γc trimeric or IL-2Rβ/γc dimeric receptors. In this study, we have investigated the role of the IL-15Rα chain in stabilizing the cytokine in the IL-2Rβ/γc dimeric receptor. By analyzing the key amino acid residues of IL-15 facing IL-2Rβ, we provide evidence of differential interfaces in the presence or in the absence of membrane-anchored IL-15Rα. Moreover, we found that the anchorage of IL-15Rα to the cell surface regardless its mode of presentation - i.e. cis or trans - is crucial for complete signaling. These observations show how the cells can finely modulate the intensity of cytokine signaling through the quality and the level of expression of the receptor chains.

Kinetics of cytokine receptor trafficking determine signaling and functional selectivity

Cytokines activate signaling via assembly of cell surface receptors, but it is unclear whether modulation of cytokine-receptor binding parameters can modify biological outcomes. We have engineered IL-6 variants with different affinities to gp130 to investigate how cytokine receptor binding dwell-times influence functional selectivity. Engineered IL-6 variants showed a range of signaling amplitudes and induced biased signaling, with changes in receptor binding dwell-times affecting more profoundly STAT1 than STAT3 phosphorylation. We show that this differential signaling arises from defective translocation of ligand-gp130 complexes to the endosomal compartment and competitive STAT1/STAT3 binding to phospho-tyrosines in gp130, and results in unique patterns of STAT3 binding to chromatin. This leads to a graded gene expression response and differences in ex vivo differentiation of Th17, Th1 and Treg cells. These results provide a molecular understanding of signaling biased by cytokine receptors, and demonstrate that manipulation of signaling thresholds is a useful strategy to decouple cytokine functional pleiotropy.

The γc Family of Cytokines: Basic Biology to Therapeutic Ramifications

The common cytokine receptor γ chain, γ_c, is a component of the receptors for interleukin-2 (IL-2), IL-4, IL-7, IL-9, IL-15, and IL-21. Mutation of the gene encoding γ_c results in X-linked severe combined immunodeficiency in humans, and γ_c family cytokines collectively regulate development, proliferation, survival, and differentiation of immune cells. Here, we review the basic biology of these cytokines, highlighting mechanisms of signaling and gene regulation that have provided insights for immunodeficiency, autoimmunity, allergic diseases, and cancer. Moreover, we discuss how studies of this family stimulated the development of JAK3 inhibitors and present an overview of current strategies targeting these pathways in the clinic, including novel antibodies, antagonists, and partial agonists. The diverse roles of these cytokines on a range of immune cells have important therapeutic implications.

Mechanistic and Structural Insights on the IL-15 System through Molecular Dynamics Simulations

Interleukin 15 (IL-15), a four-helix bundle cytokine, is involved in a plethora of different cellular functions and, particularly, plays a key role in the development and activation of immune responses. IL-15 forms receptor complexes by binding with IL-2Rβ- and common γ (γc)-signaling subunits, which are shared with other members of the cytokines family (IL-2 for IL-2Rβ- and all other γc- cytokines for γc). The specificity of IL-15 is brought by the non-signaling α-subunit, IL-15Rα. Here we present the results of molecular dynamics simulations carried out on four relevant forms of IL-15: its monomer, IL-15 interacting individually with IL-15Rα (IL-15/IL-15Rα), with IL-2Rβ/γc subunits (IL-15/IL-2Rβ/γc) or with its three receptors simultaneously (IL-15/IL-15Rα/IL-2Rβ/γc). Through the analyses of the various trajectories, new insights on the structural features of the interfaces are highlighted, according to the considered form. The comparison of the results with the experimental data, available from X-ray crystallography, allows, in particular, the rationalization of the importance of IL-15 key residues (e.g., Asp8, Lys10, Glu64). Furthermore, the pivotal role of water molecules in the stabilization of the various protein-protein interfaces and their H-bonds networks are underlined for each of the considered complexes.

Biology and regulation of IL-2: from molecular mechanisms to human therapy

IL-2 was first identified as a growth factor capable of driving the expansion of activated human T cell populations. In the more than 40 years since its discovery, a tremendous amount has been learned regarding the mechanisms that regulate the expression of both IL-2 and its cell surface receptor, its mechanisms of signalling and its range of biological actions. More recently, the mechanisms by which IL-2 regulates CD4⁺ T cell differentiation and function have been elucidated. IL-2 also regulates the effector and memory responses of CD8⁺ T cells, and the loss of IL-2 or responsiveness to IL-2 at least in part explains the exhausted phenotype that occurs during chronic viral infections and in tumour responses. These basic mechanistic studies have led to the therapeutic ability to manipulate the action of IL-2 on regulatory T (T_reg) cells for the treatment of autoimmune disease and on CD8⁺ T cells for immunotherapy of cancer. IL-2 can have either positive or deleterious effects, and we discuss here recent ideas and approaches for manipulating the actions and overall net effects of IL-2 in disease settings, including cancer.

Design and characterisation of a novel interleukin-15 receptor alpha fusion protein and analysis of interleukin-15 complexation

Interleukin-15 (IL15) is one of the most important cytokines currently being considered for cancer therapy applications. It is thought that by administering IL15 in complex with its cognate receptor alpha chain (IL15Rα) its biological activity could be increased manifold. We produced a fusion protein of mouse IL15Rα and the F8 antibody, that targets the alternatively-spliced extra-domain A (EDA) of fibronectin, which is overexpressed in many types of cancer. The fusion protein F8IL15Rα was cloned, expressed and characterized in vitro and its ability to bind to mouse IL15 was assessed with both size exclusion chromatography (SEC) and surface plasmon resonance (SPR) experiments. Furthermore, mouse and human IL15 and their corresponding Fc fused IL15Rα subunits were purchased, characterized and used to compare the capacity of F8IL15Rα to generate complexes. Surprisingly, none of the IL15Rα fusion proteins showed IL15 complexation on SEC. However, on SPR, F8IL15Rα displayed the ability to bind IL15. In a cell-based activity assay none of the IL15Rα fusions were able to increase cellular proliferation in combination with IL15 compared to IL15 alone. A better understanding of the molecular requirements for effective IL15 signalling are likely to be important for the development of IL15-based biopharmaceuticals.

Regulation of T cell proliferation with drug-responsive microRNA switches

As molecular and cellular therapies advance in the clinic, the role of genetic regulation is becoming increasingly important for controlling therapeutic potency and safety. The emerging field of mammalian synthetic biology provides promising tools for the construction of regulatory platforms that can intervene with endogenous pathways and control cell behavior. Recent work has highlighted the development of synthetic biological systems that integrate sensing of molecular signals to regulated therapeutic function in various disease settings. However, the toxicity and limited dosing of currently available molecular inducers have largely inhibited translation to clinical settings. In this work, we developed synthetic microRNA-based genetic systems that are controlled by the pharmaceutical drug leucovorin, which is readily available and safe for prolonged administration in clinical settings. We designed microRNA switches to target endogenous cytokine receptor subunits (IL-2Rβ and γ_c) that mediate various signaling pathways in T cells. We demonstrate the function of these control systems by effectively regulating T cell proliferation with the drug input. Each control system produced unique functional responses, and combinatorial targeting of multiple receptor subunits exhibited greater repression of cell growth. This work highlights the potential use of drug-responsive genetic control systems to improve the management and safety of cellular therapeutics.

Regulation of Diabetogenic Immunity by IL-15-Activated Regulatory CD8 T Cells in Type 1 Diabetes

Unchecked collaboration between islet-reactive T and B lymphocytes drives type 1 diabetes (T1D). In the healthy setting, CD8 T regulatory cells (Tregs) terminate ongoing T-B interactions. We determined that specific CD8 Tregs from NOD mice lack suppressive function, representing a previously unreported regulatory cell deficit in this T1D-prone strain. NOD mice possess 11-fold fewer Ly-49+ CD8 Tregs than nonautoimmune mice, a deficiency that worsens as NOD mice age toward diabetes and leaves them unable to regulate CD4 T follicular helper cells. As IL-15 is required for Ly-49+ CD8 Treg development, we determined that NOD macrophages inadequately trans-present IL-15. Despite reduced IL-15 trans-presentation, NOD Ly-49+ CD8 Tregs can effectively transduce IL-15-mediated survival signals when they are provided. Following stimulation with an IL-15/IL-15Ra superagonist complex, Ly-49+ CD8 Tregs expanded robustly and became activated to suppress the Ag-specific Ab response. IL-15/IL-15Ra superagonist complex-activated CD8+CD122+ T cells also delayed diabetes transfer, indicating the presence of an underactivated CD8 T cell subset with regulatory capacity against late stage T1D. We identify a new cellular contribution to anti-islet autoimmunity and demonstrate the correction of this regulatory cell deficit. Infusion of IL-15-activated CD8 Tregs may serve as an innovative cellular therapy for the treatment of T1D.

Transcription factors IRF8 and PU.1 are required for follicular B cell development and BCL6-driven germinal center responses

The IRF and Ets families of transcription factors regulate the expression of a range of genes involved in immune cell development and function. However, the understanding of the molecular mechanisms of each family member has been limited due to their redundancy and broad effects on multiple lineages of cells. Here, we report that double deletion of floxed Irf8 and Spi1 (encoding PU.1) by Mb1-Cre (designated DKO mice) in the B cell lineage resulted in severe defects in the development of follicular and germinal center (GC) B cells. Class-switch recombination and antibody affinity maturation were also compromised in DKO mice. RNA-seq (sequencing) and ChIP-seq analyses revealed distinct IRF8 and PU.1 target genes in follicular and activated B cells. DKO B cells had diminished expression of target genes vital for maintaining follicular B cell identity and GC development. Moreover, our findings reveal that expression of B-cell lymphoma protein 6 (BCL6), which is critical for development of germinal center B cells, is dependent on IRF8 and PU.1 in vivo, providing a mechanism for the critical role for IRF8 and PU.1 in the development of GC B cells.

Alternative Splicing Rescues Loss of Common Gamma Chain Function and Results in IL-21R-like Deficiency

Inborn errors in interleukin 2 receptor, gamma (IL2RG) perturb signaling of the common gamma chain family cytokines and cause severe combined immunodeficiency (SCID). Here, we report two brothers suffering from chronic cryptosporidiosis, severe diarrhea, and cholangitis. Pan T, B, and NK cell numbers were normal, but immunophenotyping revealed defective B cell differentiation. Using whole exome sequencing, we identified a base pair deletion in the first exon of IL2RG predicted to cause a frameshift and premature stop. However, flow cytometry revealed normal surface expression of the IL-2Rγ chain. While IL-2, IL-7, and IL-15 signaling showed only mild defects of STAT5 phosphorylation in response to the respective cytokines, IL-4- and IL-21-induced phosphorylation of STAT3 and STAT6 was markedly reduced. Examination of RNA isoforms detected alternative splicing downstream of IL2RG exon 1 in both patients resulting in resolution of the predicted frameshift and 16 mutated amino acids. In silico modeling suggested that the IL-2Rγ mutation reduces the stabilization of IL-4 and IL-21 cytokine binding by affecting the N-terminal domain of the IL-2Rγ. Thus, our study shows that IL2RG deficiency can be associated with differential signaling defects. Confounding effects of alternative splicing may partially rescue genetic defects and should be considered in patients with inborn errors of immunity.

The challenges and molecular approaches surrounding interleukin-2-based therapeutics in cancer

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IL2-based cancer therapies are limited by their toxicity and pleiotropy.

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Current engineering approaches target IL2 half-life and cell/receptor specificity.

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IL2 may enhance the efficacy of checkpoint inhibitors and CAR-T-based therapies.

Interleukin-2 has had a long history as a promising cancer therapeutic, being capable of eliciting complete and durable remissions in patients with metastatic renal cell carcinoma and metastatic melanoma. Despite high toxicity and efficacy limited to only certain patient subpopulations and cancer types, the prospective use of novel, engineered IL2 formats in combination with the presently expanding repertoire of immuno-oncological targets remains very encouraging. This is possible due to the significant research efforts in the IL2 field that have yielded critical structural and biological insights that have made IL2 more effective and more broadly applicable in the clinic. In this review, we discuss some of the molecular approaches that have been used to further improve IL2 therapy for cancer.

The tertiary structure of γc cytokines dictates receptor sharing

The γc family of cytokines comprising interleukin-2 (IL-2), IL-4, IL-7, IL-9, IL-15 and IL-2 is an important group of 4-helix bundle cytokines that signals through receptors incorporating the common gamma chain (γc). These cytokines are involved in lymphocyte biology and their specific functions are contingent on binding to cognate receptor chains. Here, we examined the structural relationships between γc cytokines, aiming to understand the basis for receptor chain usage and sharing. To that end, we obtained tertiary structures of human and mouse γc cytokines plus two other related cytokines, IL-13 and TSLP, which share receptors with IL-4 and IL-7, respectively. Subsequently, we compared the cytokine 3D-structures introducing a structural similarity score that grouped γc cytokines in a manner that mirrored the relationships dictated by receptor sharing. Unlike previously thought, we identified that IL-9 is more closely related to IL-2 and IL-15 than to IL-7, which is actually the most distant member of the γc family of cytokines. Moreover, we found that all the members of the γc family of cytokines share the topology of short-chain 4-helix bundle cytokines but IL-7 that with TSLP has the topology of long-chain 4-helix bundle cytokines. We also carried out Maximun-Likehood and Bayesian phylogenetic analyses that supported these results at the amino acid sequence level. Overall, our findings are of paramount relevance to understand receptor sharing among γc cytokines and can lead to the discovery of new cytokine receptor partners.

The Emergence of Natural Killer Cells as a Major Target in Cancer Immunotherapy

Immune 'checkpoint' inhibitors can increase the activity of tumor-resident cytotoxic lymphocytes and have revolutionized cancer treatment. Current therapies block inhibitory pathways in tumor-infiltrating CD8⁺ T cells and recent studies have shown similar programs in other effector populations such as natural killer (NK) cells. NK cells are critical for immunosurveillance, particularly the control of metastatic cells or hematological cancers. However, how NK cells specifically recognize transformed cells and dominant negative feedback pathways, as well as how tumors escape NK cell control, remains undefined. This review summarizes recent advances that have illuminated inhibitory checkpoints in NK cells, some of which are shared with conventional cytotoxic T lymphocytes. It also outlines emerging approaches aimed at unleashing the potential of NK cells in immunotherapy.

Cytokine-Mediated Regulation of CD8 T-Cell Responses During Acute and Chronic Viral Infection

The common γ-chain cytokines, interleukin (IL)-2, IL-7, and IL-15, regulate critical aspects of antiviral CD8 T-cell responses. During acute infections, IL-2 controls expansion and differentiation of antiviral CD8 T cells, whereas IL-7 and IL-15 are key cytokines to maintain memory CD8 T cells long term in an antigen-independent manner. On the other hand, during chronic infections, in which T-cell exhaustion is established, precise roles of these cytokines in regulation of antiviral CD8 T-cell responses are not well defined. Nonetheless, administration of IL-2, IL-7, or IL-15 can increase function of exhausted CD8 T cells, and thus can be an attractive therapeutic approach. A new subset of stem-cell-like CD8 T cells, which provides a proliferative burst after programmed cell death (PD)-1 therapy, has been recently described during chronic viral infection. Further understanding of cytokine-mediated regulation of this CD8 T-cell subset will improve cytokine therapies to treat chronic infections and cancer in combination with immune checkpoint inhibitors.

De novo design of potent and selective mimics of IL-2 and IL-15

We describe a de novo computational approach for designing proteins that recapitulate the binding sites of natural cytokines, but are otherwise unrelated in topology or amino acid sequence. We use this strategy to design mimics of the central immune cytokine interleukin-2 (IL-2) that bind to the IL-2 receptor βγc heterodimer (IL-2Rβγc) but have no binding site for IL-2Rα (also called CD25) or IL-15Rα (also known as CD215). The designs are hyper-stable, bind human and mouse IL-2Rβγc with higher affinity than the natural cytokines, and elicit downstream cell signalling independently of IL-2Rα and IL-15Rα. Crystal structures of the optimized design neoleukin-2/15 (Neo-2/15), both alone and in complex with IL-2Rβγc, are very similar to the designed model. Neo-2/15 has superior therapeutic activity to IL-2 in mouse models of melanoma and colon cancer, with reduced toxicity and undetectable immunogenicity. Our strategy for building hyper-stable de novo mimetics could be applied generally to signalling proteins, enabling the creation of superior therapeutic candidates.

Structural Basis for Signaling Through Shared Common γ Chain Cytokines

The common γ chain (γ_c) family of hematopoietic cytokines consists of six distinct four α-helix bundle soluble ligands that signal through receptors which include the shared γ_c subunit to coordinate a wide range of physiological processes, in particular, those related to innate and adaptive immune function. Since the first crystallographic structure of a γ_c family cytokine/receptor signaling complex (the active Interleukin-2 [IL-2] quaternary complex) was determined in 2005 [1], tremendous progress has been made in the structural characterization of this protein family, transforming our understanding of the molecular mechanisms underlying immune activity. Although many conserved features of γ_c family cytokine complex architecture have emerged, distinguishing details have been observed for individual cytokine complexes that rationalize their unique functional properties. Much work remains to be done in the molecular characterization of γ_c family signaling, particularly with regard to intracellular activation events, and looking forward, new technologies in structural biophysics will offer further insight into the biology of cytokine signaling to inform the design of targeted therapeutics for treatment of immune-linked diseases such as cancer, infection, and autoimmune disorders.

The effects of interleukin-2 on immune response regulation

The immune system has many adaptive and dynamic components that are regulated to ensure appropriate, precise and rapid response to a foreign pathogen. A delayed or inadequate immune response can lead to prolonged disease, while an excessive or under-regulated response can lead to autoimmunity. The cytokine, interleukin-2 (IL-2) and its receptor IL-2R play an important role in maintaining this balance.The IL-2 receptor transduces pSTAT5 signal through both the intermediate and high affinity receptors, which differ from each other by the presence of CD25 chain in IL-2 receptor. We present experimental data on the kinetics of pSTAT5 signalling through both of the receptors and develop a model that captures this kinetics. We then use this model to parameterize key aspects of two additional models in which we propose and study two different mechanisms by which IL-2 receptor can transduce distinct signals leading to either an activated or a non-activated cell state. We speculate that this initial state differentiation, perhaps enhanced by downstream feedbacks, may eventually lead to differential cell fates.Our result shows that non-linear dynamical models can suggest resolution of a puzzling array of seemingly contradictory experimental results on IL-2 effect on proliferation and differentiation of T-cells.

CD122 signaling in CD8+ memory T cells drives costimulation-independent rejection

Interrupting T cell costimulatory signals as a strategy to control undesired immune responses, such as occur in autoimmunity or transplantation, has the potential to alleviate many of the unwanted side effects associated with current immunosuppressive therapies. Belatacept, a high-affinity version of CTLA4-Ig that blocks ligand ligation to CD28, has been approved for use in kidney transplant recipients. Despite the long-term benefits associated with its use, such as improved renal function and lower cardiovascular risk, a subset of patients treated with belatacept experience elevated rates of acute T cell-mediated rejection, tempering enthusiasm for its use. Here we demonstrate that costimulation-independent T cell alloreactivity relies on signaling through CD122, the shared IL-2 and IL-15 receptor β-chain. Combined costimulatory and CD122 blockade improved survival of transplanted tissue in mice and nonhuman primates by controlling proliferation and effector function of CD8+ T cells. The high-affinity IL-2 receptor was dispensable for memory CD8+ T cell responses, whereas signaling through CD122 as a component of the high-affinity IL-15 receptor was critical for costimulation-independent memory CD8+ T cell recall, distinguishing specific roles for IL-2 and IL-15 in T cell activation. These studies outline a novel approach for clinical optimization of costimulatory blockade strategies in transplantation by targeting CD122.

Mechanistic Insights into CpG DNA and IL-15 Synergy in Promoting B Cell Chronic Lymphocytic Leukemia Clonal Expansion

Malignant cell growth within patients with B cell chronic lymphocytic leukemia (B-CLL) is largely restricted to lymphoid tissues, particularly lymph nodes. The recent in vitro finding that TLR-9 ligand (oligodeoxynucleotide [ODN]) and IL-15 exhibit strong synergy in promoting B-CLL growth may be particularly relevant to growth in these sites. This study shows IL-15-producing cells are prevalent within B-CLL-infiltrated lymph nodes and, using purified B-CLL cells from blood, investigates the mechanism for ODN and IL-15 synergy in driving B-CLL growth. ODN boosts baseline levels of phospho-RelA(S529) in B-CLL and promotes NF-κB-driven increases in IL15RA and IL2RB mRNA, followed by elevated IL-15Rα and IL-2/IL-15Rβ (CD122) protein. IL-15→CD122 signaling during a critical interval, 20 to 36-48 h following initial ODN exposure, is required for optimal induction of the cycling process. Furthermore, experiments with neutralizing anti-IL-15 and anti-CD122 mAbs indicate that clonal expansion requires continued IL-15/CD122 signaling during cycling. The latter is consistent with evidence of heightened IL2RB mRNA in the fraction of recently proliferated B-CLL cells within patient peripheral blood. Compromised ODN+IL-15 growth with limited cell density is consistent with a role for upregulated IL-15Rα in facilitating homotypic trans IL-15 signaling, although there may be other explanations. Together, the findings show that ODN and IL-15 elicit temporally distinct signals that function in a coordinated manner to drive B-CLL clonal expansion.

Engineering a Single-Agent Cytokine/Antibody Fusion That Selectively Expands Regulatory T Cells for Autoimmune Disease Therapy

IL-2 has been used to treat diseases ranging from cancer to autoimmune disorders, but its concurrent immunostimulatory and immunosuppressive effects hinder efficacy. IL-2 orchestrates immune cell function through activation of a high-affinity heterotrimeric receptor (composed of IL-2Rα, IL-2Rβ, and common γ [γ_c]). IL-2Rα, which is highly expressed on regulatory T (T_Reg) cells, regulates IL-2 sensitivity. Previous studies have shown that complexation of IL-2 with the JES6-1 Ab preferentially biases cytokine activity toward T_Reg cells through a unique mechanism whereby IL-2 is exchanged from the Ab to IL-2Rα. However, clinical adoption of a mixed Ab/cytokine complex regimen is limited by stoichiometry and stability concerns. In this study, through structure-guided design, we engineered a single agent fusion of the IL-2 cytokine and JES6-1 Ab that, despite being covalently linked, preserves IL-2 exchange, selectively stimulating T_Reg expansion and exhibiting superior disease control to the mixed IL-2/JES6-1 complex in a mouse colitis model. These studies provide an engineering blueprint for resolving a major barrier to the implementation of functionally similar IL-2/Ab complexes for treatment of human disease.

Brain 18F-FDG PET of SIV-infected macaques after treatment interruption or initiation

BACKGROUND

Although rates of severe HIV-associated neurocognitive disorders have declined in the post-antiretroviral treatment (ART) era, subtle deficits persist, possibly exacerbated by treatment non-adherence. The actual effects of ART interruption/initiation on brain glucose metabolism as a reflection of viral replication and neuroinflammation remain unclear. Our study investigates how treatment initiation and interruption alter brain glucose metabolism in SIV-infected macaques, using 18F-FDG PET in correlation with plasma and CSF viral loads (VL) and cytokine levels.

METHODS

SIV-infected macaques (n = 7) underwent ART initiation only, ART interruption only, or both. Five uninfected animals served as controls. 18F-FDG PET imaging was performed at baseline and 1, 3, and 6 months after treatment modification. Mean and maximum standardized uptake values (SUV) for the whole-brain and subregions were calculated. Plasma and CSF VL and cytokine levels were measured. Paired t tests evaluated acute changes in whole-brain SUV from baseline to 1 month, while mixed-effect linear regression models evaluated changes over multiple timepoints and correlated SUV values with disease markers.

RESULTS

ART interruption was associated with increased SUVmean and SUVmax acutely, after 1 month (SUVmean 95% CI [0.044-0.786 g/ml], p = 0.037; SUVmax 95% CI [0.122-3.167 g/ml], p = 0.041). The correlation between SUV and time, however, was not significant when evaluated across all timepoints. Increased SUVmean and SUVmax correlated with decreased CD4+ and CD8+ T-cell counts and increased plasma VL. SUVmax was positively associated with increases in CSF VL, and there were borderline positive associations between SUVmax and IL-2, and between SUVmean and IL-15. The treatment initiation group showed no associations between imaging and disease biomarkers despite viral suppression, reduced cytokine levels, and increased CD4+ and CD8+ T-cell counts.

CONCLUSIONS

ART interruption is associated with increased brain glucose metabolism within 1 month of treatment cessation, which, in concert with increased levels of pro-inflammatory cytokines in the CSF, may reflect neuroinflammation in the setting of viral rebound. Although we cannot assert neurologic damage in association with cerebral hypermetabolism, it is a concerning outcome of ART non-adherence. Treatment initiation, meanwhile, did not result in significant changes in brain metabolism. HIV-induced neuroinflammation may require a longer period to abate than our follow-up period allowed.

Discovery of a novel IL-15 based protein with improved developability and efficacy for cancer immunotherapy

Interleukin-15 (IL-15) can promote both innate and adaptive immune reactions by stimulating CD8⁺/CD4⁺ T cells and natural killer cells (NK) while showing no effect in activating T-regulatory (Treg) cells or inducing activation-associated death among effector T cells and NK cells. Thus, IL-15 is considered as one of the most promising molecules for antitumor immune therapy. To improve the drug-like properties of natural IL-15, we create an IL-15-based molecule, named P22339, with the following characteristics: 1) building a complex of IL-15 and the Sushi domain of IL-15 receptor α chain to enhance the agonist activity of IL-15 via transpresentation; 2) through a rational structure-based design, creating a disulfide bond linking the IL-15/Sushi domain complex with an IgG1 Fc to augment its half-life. P22339 demonstrates excellent developability, pharmacokinetic and pharmacodynamic properties as well as antitumor efficacy in both in vitro assessments and in vivo studies. It significantly suppresses tumor growth and metastasis in rodent models, and activates T effector cells and NK cells in cynomolgus monkey. Overall, these data suggest that P22339 has a great potential for cancer immunotherapy.

IL-15 supports the generation of protective lung-resident memory CD4 T cells

Tissue-resident memory T cells (TRM) provide optimal defense at the sites of infection, but signals regulating their development are unclear, especially for CD4 T cells. Here we identify two distinct pathways that lead to the generation of CD4 TRM in the lungs following influenza infection. The TRM are transcriptionally distinct from conventional memory CD4 T cells and share a gene signature with CD8 TRM. The CD4 TRM are superior cytokine producers compared with conventional memory cells, can protect otherwise naive mice against a lethal influenza challenge, and display functional specialization by inducing enhanced inflammatory responses from dendritic cells compared with conventional memory cells. Finally, we demonstrate than an interleukin (IL)-2-dependent and a novel IL-2-independent but IL-15-dependent pathway support the generation of cohorts of lung TRM.

New interleukin-15 superagonist (IL-15SA) significantly enhances graft-versus-tumor activity

Interleukin-15 (IL-15) is a potent cytokine that increases CD8⁺ T and NK cell numbers and function in experimental models. However, obstacles remain in using IL-15 therapeutically, specifically its low potency and short in vivo half-life. To help overcome this, a new IL-15 superagonist complex comprised of an IL-15N72D mutation and IL-15RαSu/Fc fusion (IL-15SA, also known as ALT-803) was developed. IL-15SA exhibits a significantly longer serum half-life and increased in vivo activity against various tumors. Herein, we evaluated the effects of IL-15SA in recipients of allogeneic hematopoietic stem cell transplantation. Weekly administration of IL-15SA to transplant recipients significantly increased the number of CD8⁺ T cells (specifically CD44⁺ memory/activated phenotype) and NK cells. Intracellular IFN-γ and TNF-α secretion by CD8⁺ T cells increased in the IL-15SA-treated group. IL-15SA also upregulated NKG2D expression on CD8⁺ T cells. Moreover, IL-15SA enhanced proliferation and cytokine secretion of adoptively transferred CFSE-labeled T cells in syngeneic and allogeneic models by specifically stimulating the slowly proliferative and nonproliferative cells into actively proliferating cells.

We then evaluated IL-15SA's effects on anti-tumor activity against murine mastocytoma (P815) and murine B cell lymphoma (A20). IL-15SA enhanced graft-versus-tumor (GVT) activity in these tumors following T cell infusion. Interestingly, IL-15 SA administration provided GVT activity against A20 lymphoma cells in the murine donor leukocyte infusion (DLI) model without increasing graft versus host disease. In conclusion, IL-15SA could be a highly potent T- cell lymphoid growth factor and novel immunotherapeutic agent to complement stem cell transplantation and adoptive immunotherapy.

A novel strategy to produce high level and high purity of bioactive IL15 fusion proteins from mammalian cells

IL15, a member of the common γ chain receptor (γc) cytokine family, is gaining attention in recent years as one of the most promising anti-tumor agents. IL15 regulates T cell activation and proliferation, promotes the survival of CD8⁺ CD44^hi memory T cells and is also essential for NK cell expansion and development. Despite the attraction of developing IL15 as an anti-cancer agent, production of recombinant IL15 has proven to be difficult due to the stringent control of IL15 expression at the transcriptional, translational and the post-translational levels. Furthermore, the bioactivity of IL15 fused to an extra functional domain that is isolated from mammalian cells is generally inferior to recombinant IL15 produced by E. coli. In this study, we report that Lysine 86 in IL15 is responsible for the instability in mammalian cells when its C-terminus is fused to the albumin binding scFv (IL15-A10m3). We demonstrate that K86A or K86R mutants increased the expression of the fusion protein from HEK293 cells. When the wild type IL15 is used for the fusion, no recombinant IL15 fusion was detected in the culture media. Additionally, we determined that the residue 112 in IL15 is critical for the bioactivity of IL15-A10m3. Examination of single and double mutants provides a better understanding of how IL15 engages with its receptor complex to achieve full signaling capacity. The results of our experiments were successfully applied to scale up production to levels up to 50 mg/L and >10 mg/L of >95% pure monomeric recombinant fusion proteins after a 2-step purification from culture media. More importantly, the recombinant fusion protein produced is fully active in stimulating T cell proliferation, when compared to the recombinant wild type IL15.

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.

CD122 blockade restores immunological tolerance in autoimmune type 1 diabetes via multiple mechanisms

Signaling through IL-2/IL-15Rβ (CD122) is essential for the differentiation and function of T cells and NK cells. A mAb against CD122 has been implicated to suppress autoimmune type 1 diabetes (T1D) development in animal models. However, the mechanisms remain poorly understood. We find that in vivo administration of an anti-CD122 mAb (CD122 blockade) restores immune tolerance in nonobese diabetic (NOD) mice via multiple mechanisms. First, CD122 blockade selectively ablates pathogenic NK cells and memory phenotype CD8+ T cells from pancreatic islets. In contrast, islet CD4+Foxp3+ Tregs are only mildly affected. Second, CD122 blockade suppresses IFN-γ production in islet immune cells. Third, CD122 blockade inhibits the conversion of islet Th17 cells into diabetogenic Th1 cells. Furthermore, a combination of anti-CD122 mAb and Treg-trophic cytokines (IL-2 or IL-33) enhances the abundance and function of islet Tregs. In summary, these data provide crucial mechanistic insights into CD122 blockade-mediated immunoregulation and support therapeutic benefits of this combinational treatment in T1D.

A novel immunization strategy using cytokine/chemokines induces new effective systemic immune responses, and frequent complete regressions of human metastatic melanoma

Immune responses have been elicited by a variety of cancer vaccines, but seldom induce regressions of established cancers in humans. As a novel therapeutic immunization strategy, we tested the hypothesis that multiple cytokines/chemokines secreted early in secondary responses ex-vivo might mimic the secretory environment guiding new immune responses. The early development of immune responses is regulated by multiple cytokines/chemokines acting together, which at physiologic concentrations act locally in concert with antigen to have non-specific effects on adjacent cells, including the maturation of dendritic cells, homing and retention of T cells at the site of antigen, and the differentiation and expansion of T cell clones with appropriate receptors. We postulated that repeated injections into a metastasis of an exogenous chemokine/cytokine mixture might establish the environment of an immune response and allow circulating T cell clones to self- select for mutant neo-epitopes in the tumor and generate systemic immune responses. To test this idea we injected some metastases in patients with multiple cutaneous melanoma nodules while never injecting other control metastases in the same patient. New immune responses were identified by the development of dense lymphocytic infiltrates in never-injected metastases, and the frequent complete regression of never-injected metastases, a surprising observation. 70% of subjects developed dense infiltrates of cytotoxic CD8 cells in the center and margin of never-injected metastases; 38% of subjects had complete and often durable regressions of all metastases, without the use of check-point inhibitors, suggesting that, as a proof-of-principle, an immunization strategy can control advanced human metastatic melanoma.

Role of IL-15 in the modulation of TGF-β1-mediated inflammation in asthma

Transforming growth factor (TGF)-β1 has an essential role in bronchitis and the induction of bronchial remodelling, which are critical processes in the pathogenesis of asthma. However, the role of interleukin (IL)-15 in asthma inflammation remains unclear. The aim of the present study was to evaluate the effect of TGF-β1 mRNA expression on IL-15 mRNA expression in asthmatic patients and to assess the role of IL-15 in the clinical course of asthma. The study included 221 participants, comprising 130 patients with asthma and 91 healthy volunteers. The participants were subjected to testing using spirometry, as well as the Asthma Control Test™ and Borg Scale. The expression of TGF-β1 and IL-15 mRNA was analyzed in blood samples using reverse transcription-quantitative polymerase chain reaction. Statistical analysis indicated that IL-15 and TGF-β1 mRNA expression each differed significantly between the patient and control groups (P=0.0016 and P=0.033, respectively). A significant correlation was identified between IL-15 expression and TGF-β1 expression (R=0.41, P=0.0005). No correlation was observed between IL-15 expression and the degree of asthma severity, the results of spirometric examination or the frequency of asthma exacerbations. Further analysis revealed that IL-15 expression was elevated following the administration of inhaled glucocorticosteroids (iGCs; P=0.024), and reduced following methylxanthine treatment (P<0.001). The occurrence of dyspnoea differed between the study and control groups, and this was not found to be associated with IL-15 expression. Since IL-15 expression was correlated with TGF-β1 expression among asthmatic patients, and IL-15 expression was elevated following iGC administration, the results of the study suggest that IL-15 activity might be associated with the pathogenesis of asthma.

Discovery and characterization of a novel humanized anti-IL-15 antibody and its relevance for the treatment of refractory celiac disease and eosinophilic esophagitis

Interleukin-15 (IL-15) is a critical regulator of immune responses, especially at mucosal interfaces within the gastro-intestinal tract. Here, we describe the discovery and characterization of a humanized antibody to IL-15. Data from its epitope and mode of action, cell biology and primate pharmacology, as well as translational studies in human samples and in vivo proof-of-concept experiments in mouse models demonstrate the therapeutic potential of this new antibody targeting IL-15 for refractory celiac disease and eosinophilic esophagitis.

Cutting Edge: Differential Fine-Tuning of IL-2- and IL-15-Dependent Functions by Targeting Their Common IL-2/15Rβ/γc Receptor

Interleukin 2 and IL-15 are two closely related cytokines, displaying important functions in the immune system. They share the heterodimeric CD122/CD132 receptor to deliver their signals within target cells. Their specificity of action is conferred by their α receptor chains, IL-2Rα and IL-15Rα. By combining an increased affinity for CD122 and an impaired recruitment of CD132, we have generated an original molecule named IL-2Rβ/γ (CD122/CD132) inhibitor (BiG), targeting the CD122/CD132 receptor. BiG efficiently inhibited IL-15- and IL-2-dependent functions of primary cells, including CD8 T and NK cells, in vitro and in vivo. We also report a differential dynamic of action of these cytokines by highlighting a major role played by the IL-2Rα receptor. Interestingly, due to the presence of IL-2Rα, BiG had no impact on IL-2-dependent regulatory T cell proliferation. Thus, by acting as a fine switch in the immune system, BiG emphasizes the differential roles of these two cytokines.

Diacylglycerol Kinase ζ Limits Cytokine-dependent Expansion of CD8+ T Cells with Broad Antitumor Capacity

Interleukin-2 and -15 drive expansion/differentiation of cytotoxic CD8⁺ T cells that eliminate targets via antigen-independent killing. This property is clinically relevant for the improvement of T cell-based antitumor therapies. Diacylglycerol kinase α and ζ (DGKα/ζ) metabolize the diacylglycerol generated following antigen recognition by T lymphocytes. Enhanced expression of these two lipid kinases in tumor-infiltrating CD8⁺ T cells promotes a hyporesponsive state that contributes to tumor immune escape. Inhibition of these two enzymes might thus be of interest for potentiating conventional antigen-directed tumor elimination. In this study, we sought to characterize the contribution of DGKα and ζ to antigen-independent cytotoxic functions of CD8⁺ T cells. Analysis of DGKζ-deficient mice showed an increase in bystander memory-like CD8⁺ T cell populations not observed in DGKα-deficient mice. We demonstrate that DGKζ limits cytokine responses in an antigen-independent manner. Cytokine-specific expansion of DGKζ-deficient CD8⁺ T cells promoted enhanced differentiation of innate-like cytotoxic cells in vitro, and correlated with the more potent in vivo anti-tumor responses of DGKζ-deficient mice engrafted with the murine A20 lymphoma. Our studies reveal a isoform-specific function for DGKζ downstream of IL-2/IL-15-mediated expansion of innate-like cytotoxic T cells, Pharmacological manipulation of DGKζ activity is of therapeutic interest for cytokine-directed anti-tumor treatments.

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DGKζ, a well-characterized negative regulator of TCR signals, also limits IL-2/15 function.

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DGKζ impairs cytokine-induced differentiation of cytotoxic T cell populations with innate-like ability to kill targets.

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As a result, DGKζ-deficient mice demonstrate enhanced rejection of implanted B cell lymphoma compared to wild type mice.

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Targeting DGKζ activity might be of interest to enhance cytokine-mediated antitumor therapies.

The immune system defends the body from foreign invaders. In cancer, tumors disguise as self-body cells and evade immune attack. For this reason it is important to identify the mechanism that stop T lymphocytes from recognize and destroy tumors. In this study we investigate the role of Diacylglycerol kinase zeta (DGKζ) as an inhibitor of antitumor T cell functions. We demonstrate that lymphoma cells injected in mice genetically modified to lack DGKζ expression develop smaller tumors that resolve more rapidly than those grown in normal mice. Our studies suggest that inhibition of DGKζ could help to reinforce the antitumor capacity of immune T lymphocytes.