Presensitizing with a Toll-like receptor 3 ligand impairs CD8 T-cell effector differentiation and IL-33 responsiveness

The Janus Face of IL-33 Signaling in Tumor Development and Immune Escape

Simple Summary

Interleukin-33 (IL-33) is often released from damaged cells, acting as a danger signal. IL-33 exerts its function by interacting with its receptor suppression of tumorigenicity 2 (ST2) that is constitutively expressed on most immune cells. Therefore, IL-33/ST2 signaling can modulate immune responses to participate actively in a variety of pathological conditions, such as cancer. Like a two-faced Janus, which faces opposite directions, IL-33/ST2 signaling may play contradictory roles on its impact on cancer progression through both immune and nonimmune cellular components. Accumulating evidence demonstrates both pro- and anti-tumorigenic properties of IL-33, depending on the complex nature of different tumor immune microenvironments. We summarize and discuss the most recent studies on the contradictory effects of IL-33 on cancer progression and treatment, with a goal to better understanding the various ways for IL-33 as a therapeutic target.

Abstract

Interleukin-33 (IL-33), a member of the IL-1 cytokine family, plays a critical role in maintaining tissue homeostasis as well as pathological conditions, such as allergy, infectious disease, and cancer, by promoting type 1 and 2 immune responses. Through its specific receptor ST2, IL-33 exerts multifaceted functions through the activation of diverse intracellular signaling pathways. ST2 is expressed in different types of immune cells, including Th2 cells, Th1 cells, CD8⁺ T cells, regulatory T cells (T_reg), cytotoxic NK cells, group 2 innate lymphoid cells (ILC2s), and myeloid cells. During cancer initiation and progression, the aberrant regulation of the IL-33/ST2 axis in the tumor microenvironment (TME) extrinsically and intrinsically mediates immune editing via modulation of both innate and adaptive immune cell components. The summarized results in this review suggest that IL-33 exerts dual-functioning, pro- as well as anti-tumorigenic effects depending on the tumor type, expression levels, cellular context, and cytokine milieu. A better understanding of the distinct roles of IL-33 in epithelial, stromal, and immune cell compartments will benefit the development of a targeting strategy for this IL-33/ST2 axis for cancer immunotherapy.

The Contradictory Role of Interleukin-33 in Immune Cells and Tumor Immunity

Interleukin (IL)-33 is a member of the IL-1 superfamily and is a crucial cytokine playing the role of a dual-function molecule. IL-33 mediates its function by interacting with its receptor suppression of tumorigenicity 2 (ST2), which is constitutively expressed on T helper (Th)1 cells, Th2 cells, and other immune cells. Previously, we summarized findings on IL-33 and performed an intensive study of the correlation between IL-33 and tumor. IL-33 enables anti-tumor immune responses through Th1 cells and natural killer (NK) cells and plays a role in tumor immune escape in cancers via Th2 cells and regulatory T cells. Herein, we discuss the contradictory role of IL-33 in immune cells in different cancer, and our summaries may be helpful for better understanding of the development of research on IL-33 and tumor immunity.

Costimulation Induces CD4 T Cell Antitumor Immunity via an Innate-like Mechanism

Chronic exposure to tumor-associated antigens inactivates cognate T cells, restricting the repertoire of tumor-specific effector T cells. This problem was studied here by transferring TCR transgenic CD4 T cells into recipient mice that constitutively express a cognate self-antigen linked to MHC II on CD11c-bearing cells. Immunotherapeutic agonists to CD134 plus CD137, "dual costimulation," induces specific CD4 T cell expansion and expression of the receptor for the Th2-associated IL-1 family cytokine IL-33. Rather than producing IL-4, however, they express the tumoricidal Th1 cytokine IFNγ when stimulated with IL-33 or IL-36 (a related IL-1 family member) plus IL-12 or IL-2. IL-36, which is induced within B16-F10 melanomas by dual costimulation, reduces tumor growth when injected intratumorally as a monotherapy and boosts the efficacy of tumor-nonspecific dual costimulated CD4 T cells. Dual costimulation thus enables chronic antigen-exposed CD4 T cells, regardless of tumor specificity, to elaborate tumoricidal function in response to tumor-associated cytokines.

Interleukin (IL)-33: an orchestrator of immunity from host defence to tissue homeostasis

Interleukin (IL)-33, a member of the IL-1 superfamily, functions as an alarm signal, which is released upon cell injury or tissue damage to alert the immune system. It has emerged as a chief orchestrator in immunity and has a broad pleiotropic action that influences differentiation, maintenance and function of various immune cell types via the ST2 receptor. Although it has been strongly associated with immunopathology, critically, IL-33 is involved in host defence, tissue repair and homeostasis. In this review, we provide an overview of the signalling pathway of IL-33 and highlight its regulatory functions in immune cells. Furthermore, we attempt a broader discussion of the emerging functions of IL-33 in host defence, tissue repair, metabolism, inflammatory disease and cancer, suggesting potential avenues to manoeuvre IL-33/ST2 signalling as treatment options.

B cells with aberrant activation of Notch1 signaling promote Treg and Th2 cell-dominant T-cell responses via IL-33

The Notch-signaling pathway in a variety of mature B-cell neoplasms is often activated by gene alterations, but its role remains unclear. Here, we show that B cells harboring dysregulated activation of Notch1 signaling have an immunomodulatory effect on T cells by amplifying regulatory T (Treg) and T helper 2 (Th2) cell responses in an interleukin-33 (IL-33)-dependent manner. A conditional mouse model, in which constitutive expression of an active form of Notch1 is induced in B cells by Aicda gene promoter-driven Cre recombinase, revealed no obvious phenotypic changes in B cells; however, mice demonstrated an expansion of Treg and Th2 cell subsets and a decrease in cytokine production by Th1 and CD8⁺ T cells. The mice were susceptible to soft tissue sarcoma and defective production of CD8⁺ T cells specific for inoculated tumor cells, suggesting impaired antitumor T-cell activity. Gene-expression microarray revealed that altered T-cell responses were due to increased IL-33 production by Notch1-activated B cells. Knockout of IL33 or blockade of IL-33 by a receptor-blocking antibody abrogated the Treg and Th2 cell-dominant T-cell response triggered by B cells. Gene-expression data derived from human diffuse large B-cell lymphoma (DLBCL) samples showed that an activated Notch-signaling signature correlates positively with IL33 expression and Treg cell-rich gene-expression signatures. These findings indicate that B cells harboring dysregulated Notch signaling alter T-cell responses via IL-33, and suggest that aberrant activation of Notch signaling plays a role in fostering immune privilege in mature B-cell neoplasms.

IL-33 Released in the Liver Inhibits Tumor Growth via Promotion of CD4+ and CD8+ T Cell Responses in Hepatocellular Carcinoma

IL-33 released by epithelial cells and immune cells functions as an alarmin and can induce both type 1 and type 2 immune responses. However, the role of IL-33 release in tumor development is still not clear. In this study, we examined the function of released IL-33 in murine hepatocellular carcinoma (HCC) models by hydrodynamically injecting either IL-33-expressing tumor cells or IL-33-expressing plasmids into the liver of tumor-bearing mice. Tumor growth was greatly inhibited by IL-33 release. This antitumor effect of IL-33 was dependent on suppression of tumorigenicity 2 (ST2) because it was diminished in ST2^-/- mice. Moreover, HCC patients with high IL-33 expression have prolonged overall survival compared with the patients with low IL-33 expression. Further study showed that there were increased percentages and numbers of activated and effector CD4⁺ and CD8⁺ T cells in both spleen and liver in IL-33-expressing tumor-bearing mice. Moreover, IFN-γ production of the CD4⁺ and CD8⁺ T cells was upregulated in both spleen and liver by IL-33. The cytotoxicity of CTLs from IL-33-expressing mice was also enhanced. In vitro rIL-33 treatment could preferentially expand CD8⁺ T cells and promote CD4⁺ and CD8⁺ T cell activation and IFN-γ production. Depletion of CD4⁺ and CD8⁺ T cells diminished the antitumor activity of IL-33, suggesting that the antitumor function of released IL-33 was mediated by both CD4⁺ and CD8⁺ T cells. Taken together, we demonstrated in murine HCC models that IL-33 release could inhibit tumor development through its interaction with ST2 to promote antitumor CD4⁺ and CD8⁺ T cell responses.

IL-33 in Tumor Immunity: Nothing to Sneeze At

Although immunotherapy has been at the forefront of cancer therapy for the last several years, better clinical responses are still desired. Interleukin-33 is perhaps one of the most overlooked antitumor cytokines. Its ability to promote type 1 immune responses, which control tumor growth in preclinical animal models is overshadowed by its association with type 2 immunity and poor prognosis in some human cancers. Accumulating evidence shows that IL-33 is a powerful new tool for restoring and enhancing the body's natural antitumor immunity cycle. Furthermore, the antitumor mechanisms of IL-33 are two-fold, as it can directly boost CD8+ T cell function and restore dendritic cell dysfunction in vivo. Mechanistic studies have identified a novel pathway induced by IL-33 and its receptor ST2 in which dendritic cells avoid dysfunction and retain cross-priming abilities in tumor-bearing conditions. Here, we also comment on IL-33 data in human cancers and explore the idea that endogenous IL-33 may not deserve its reputation for promoting tumor growth. In fact, tumors may hijack the IL-33/ST2 axis to avoid immune surveillance and escape antitumor immunity.

Epithelial-derived IL-33 promotes intestinal tumorigenesis in Apc Min/+ mice

Increased expression of Interleukin (IL)-33 has been detected in intestinal samples of patients with ulcerative colitis, a condition associated with increased risk for colon cancer, but its role in the development of colorectal cancer has yet to be fully examined. Here, we investigated the role of epithelial expressed IL-33 during development of intestinal tumors. IL-33 expression was detected in epithelial cells in colorectal cancer specimens and in the Apc^Min/+ mice. To better understand the role of epithelial-derived IL-33 in the intestinal tumorigenesis, we generated transgenic mice expressing IL-33 in intestinal epithelial cells (V33 mice). V33 Apc^Min/+ mice, resulting from the cross of V33 with Apc^Min/+ mice, had increased intestinal tumor burden compared with littermate Apc^Min/+ mice. Consistently, Apc^Min/+ mice deficient for IL-33 receptor (ST2), had reduced polyp burden. Mechanistically, overexpression of IL-33 promoted expansion of ST2⁺ regulatory T cells, increased Th2 cytokine milieu, and induced alternatively activated macrophages in the gut. IL-33 promoted marked changes in the expression of antimicrobial peptides, and antibiotic treatment of V33 Apc^Min/+ mice abrogated the tumor promoting-effects of IL-33 in the colon. In conclusion, elevated IL-33 signaling increases tumor development in the Apc^Min/+ mice.

Cytokines and metabolic factors regulate tumoricidal T-cell function during cancer immunotherapy

Recent advances in cancer biology and genetics have fostered precision therapies targeting tumor-specific attributes. Immune-based therapies that elicit cytolytic T cells (CTL) specific for tumor antigens can provide therapeutic benefit to cancer patients, however, cure rates are typically low. This largely results from immunosuppressive mechanisms operating within the tumor microenvironment, many of which inflict metabolic stresses upon CTL. Conversely, immunotherapies can mitigate specific metabolic stressors. For instance, dual costimulation immunotherapy with CD134 (OX40) plus CD137 (4-1BB) agonists appears to mediate tumor control in part by engaging cytokine networks that enable infiltrating CTL to compete for limiting supplies of glucose. Future efforts combining modalities that endow CTL with complimentary metabolic advantages should improve therapeutic efficacies.

TLR3-Induced Maturation of Murine Dendritic Cells Regulates CTL Responses by Modulating PD-L1 Trafficking

Targeting TLR3 through formulations of polyI:C is widely studied as an adjuvant in cancer immunotherapy. The efficacy of such targeting has been shown to increase in combination with anti-PD-L1 treatment. Nevertheless, the mechanistic details of the effect of polyI:C on DC maturation and the impact on T-DC interactions upon PD-L1 blockade is largely unknown. Here we found that although DC treatment with polyI:C induced a potent inflammatory response including the production of type I interferon, polyI:C treatment of DCs impaired activation of peptide specific CD8+ T cells mainly due to PD-L1. Interestingly, we found that PD-L1 trafficking to the cell surface is regulated in two waves in polyI:C-treated DCs. One induced upon overnight treatment and a second more rapid one, specific to polyI:C treatment, was induced upon CD40 signaling leading to a further increase in surface PD-L1 in DCs. The polyI:C-induced cell surface PD-L1 reduced the times of contact between DCs and T cells, potentially accounting for limited T cell activation. Our results reveal a novel CD40-dependent regulation of PD-L1 trafficking induced upon TLR3 signaling that dictates its inhibitory activity. These results provide a mechanistic framework to understand the efficacy of anti-PD-L1 cancer immunotherapy combined with TLR agonists.

Interleukin-33 in tumorigenesis, tumor immune evasion, and cancer immunotherapy

Interleukin-33 (IL-33) is a member of the IL-1 gene family and mainly expressed in the nucleus of tissue lining cells, stromal cells, and activated myeloid cells. IL-33 is considered a damage-associated molecular pattern (DAMP) molecule and plays an important role in many physiological and pathological settings such as tissue repair, allergy, autoimmune disease, infectious disease, and cancer. The biological functions of IL-33 include maintaining tissue homeostasis, enhancing type 1 and 2 cellular immune responses, and mediating fibrosis during chronic inflammation. IL-33 exerts diverse functions through signaling via its receptor ST2, which is expressed in many types of cells including regulatory T cells (Treg), group 2 innate lymphoid cells (ILC2s), myeloid cells, cytotoxic NK cells, Th2 cells, Th1 cells, and CD8(+) T cells. Tumor development results in downregulation of IL-33 in epithelial cells but upregulation of IL-33 in the tumor stroma and serum. The current data suggest that IL-33 expression in tumor cells increases immunogenicity and promotes type 1 antitumor immune responses through CD8(+) T cells and NK cells, whereas IL-33 in tumor stroma and serum facilitates immune suppression via Treg and myeloid-derived suppressor cell (MDSC). Understanding the role of IL-33 in cancer immunobiology sheds lights on targeting this cytokine for cancer immunotherapy.

Costimulation Endows Immunotherapeutic CD8 T Cells with IL-36 Responsiveness during Aerobic Glycolysis

CD134- and CD137-primed CD8 T cells mount powerful effector responses upon recall, but even without recall these dual-costimulated T cells respond to signal 3 cytokines such as IL-12. We searched for alternative signal 3 receptor pathways and found the IL-1 family member IL-36R. Although IL-36 alone did not stimulate effector CD8 T cells, in combination with IL-12, or more surprisingly IL-2, it induced striking and rapid TCR-independent IFN-γ synthesis. To understand how signal 3 responses functioned in dual-costimulated T cells we showed that IL-2 induced IL-36R gene expression in a JAK/STAT-dependent manner. These data help delineate a sequential stimulation process where IL-2 conditioning must precede IL-36 for IFN-γ synthesis. Importantly, this responsive state was transient and functioned only in effector T cells capable of aerobic glycolysis. Specifically, as the effector T cells metabolized glucose and consumed O2, they also retained potential to respond through IL-36R. This suggests that T cells use innate receptor pathways such as the IL-36R/axis when programmed for aerobic glycolysis. To explore a function for IL-36R in vivo, we showed that dual costimulation therapy reduced B16 melanoma tumor growth while increasing IL-36R gene expression. In summary, cytokine therapy to eliminate tumors may target effector T cells, even outside of TCR specificity, as long as the effectors are in the correct metabolic state.

Tumor-Unrelated CD4 T Cell Help Augments CD134 plus CD137 Dual Costimulation Tumor Therapy

The ability of immune-based cancer therapies to elicit beneficial CD8(+) CTLs is limited by tolerance pathways that inactivate tumor-specific CD4 Th cells. A strategy to bypass this problem is to engage tumor-unrelated CD4 Th cells. Thus, CD4 T cells, regardless of their specificity per se, can boost CD8(+) CTL priming as long as the cognate epitopes are linked via presentation on the same dendritic cell. In this study, we assessed the therapeutic impact of engaging tumor-unrelated CD4 T cells during dual costimulation with CD134 plus CD137 that provide help via the above-mentioned classical linked pathway, as well as provide nonlinked help that facilitates CTL function in T cells not directly responding to cognate Ag. We found that engagement of tumor-unrelated CD4 Th cells dramatically boosted the ability of dual costimulation to control the growth of established B16 melanomas. Surprisingly, this effect depended upon a CD134-dependent component that was extrinsic to the tumor-unrelated CD4 T cells, suggesting that the dual costimulated helper cells are themselves helped by a CD134(+) cell(s). Nevertheless, the delivery of therapeutic help tracked with an increased frequency of tumor-infiltrating granzyme B(+) effector CD8 T cells and a reciprocal decrease in Foxp3(+)CD4(+) cell frequency. Notably, the tumor-unrelated CD4 Th cells also infiltrated the tumors, and their deletion several days following initial T cell priming negated their therapeutic impact. Taken together, dual costimulation programs tumor-unrelated CD4 T cells to deliver therapeutic help during both the priming and effector stages of the antitumor response.

ST2 contributes to T-cell hyperactivation and fatal hemophagocytic lymphohistiocytosis in mice

Cytokine storm syndromes, such as familial hemophagocytic lymphohistiocytosis (FHL), are lethal disorders caused by uncontrolled, systemic immune activation. In the murine model of FHL, in which perforin-deficient (Prf1(-/-)) mice are infected with lymphocytic choriomeningitis virus (LCMV), disease is driven by overabundant interferon (IFN)γ-producing LCMV-specific CD8(+) T cells thought to arise from excessive antigen stimulation through the T-cell receptor. However, this paradigm is insufficient to explain several fundamental aspects of FHL, namely, the inability of many pathogenic antigens to induce hyperinflammation, and the previously identified role of MyD88 in the disease. We now show a novel role for the MyD88-dependent interleukin-33 (IL-33) receptor, ST2, in FHL. Expression of IL-33 and ST2 is upregulated in LCMV-infected Prf1(-/-) mice. Blockade of ST2 markedly improves survival of LCMV-infected Prf1(-/-) mice and reduces the severity of multiple disease parameters, including serum levels of IFNγ. This decrease in IFNγ corresponds to a reduction in both the frequency of IFNγ(+) LCMV-specific CD8(+) and CD4(+) T cells and the magnitude of IFNγ expression in these cells. These findings demonstrate that disruption of ST2 signaling in the murine model of FHL reduces T cell-mediated production of IFNγ and suggest a revised paradigm in which danger signals such as IL-33 are crucial amplifiers of immune dysregulation in FHL. Furthermore, this study provides evidence to support blockade of ST2 as a novel therapeutic strategy for FHL.

Interleukin-33 acts as a transcriptional repressor and extracellular cytokine in fibroblast-like synoviocytes in patients with rheumatoid arthritis

The present study aimed to assess the functions of interleukin (IL)-33 in fibroblast-like synoviocytes (FLS) from patients with rheumatoid arthritis (RA). Enzyme-linked immunosorbent assays (ELISAs) were used to quantify interleukin (IL)-33 in plasma obtained from patients with RA and osteoarthritis (OA). To evaluate functions of intracellular IL-33, levels of inflammatory mediators and matric metalloproteinases (MMPs) were measured in RA FLS transfected with IL-33 small- interfering RNA (siRNA) or plasmids, and changes in the expression and regulation of nuclear factor kappaB (NF-κB) were determined using western blotting and reporter gene assays. In addition, to examine the extracellular effects of IL-33, IP10 and receptor activator of NF-κB ligand (RANKL) mRNA levels were measured after treatment with IL-33 and blocking antibodies to ST2, the IL-33 receptor. To evaluate whether extracellular IL-33 regulated osteoclastogenesis, human CD14(+) monocytes cocultured with IL-33-stimulated FLS were stained with tartrate-resistant acid phosphatase (TRAP). IL-33 levels were higher in plasma obtained from patients with RA than in those obtained from patients with OA. The expression levels of IL-33 were elevated in RA FLS that had been stimulated with poly I:C, IL-1β, and tumor necrosis factor (TNF)-α. Silencing of IL-33 increased the levels of pro-inflammatory molecules and MMPs, promoted inhibitor of kappaB (IκBα) degradation, and increased NF-κB activity; these effects were reversed in IL-33 plasmid-transfected FLS. Stimulation with exogenous IL-33 increased RANKL and IP-10 mRNA expression. These increases were blocked by anti-ST2 treatment. Furthermore, we confirmed that extracellular IL-33 stimulated the formation of TRAP(+) multinucleated osteoclasts through RA FLS. These results suggested that intracellular IL-33 acted as a transcriptional repressor of NF-κB, which may provide negative feedback against inflammatory responses, whereas, extracellular IL-33 functioned as an activator of osteoclastogenesis. Therefore, increased plasma IL-33 levels in patients with RA could be a possible biomarker to reflect the potential risks of bone erosion.

Trace Levels of Staphylococcal Enterotoxin Bioactivity Are Concealed in a Mucosal Niche during Pulmonary Inflammation

Pathogen and cellular by-products released during infection or trauma are critical for initiating mucosal inflammation. The localization of these factors, their bioactivity and natural countermeasures remain unclear. This concept was studied in mice undergoing pulmonary inflammation after Staphylococcal enterotoxin A (SEA) inhalation. Highly purified bronchoalveolar lavage fluid (BALF) fractions obtained by sequential chromatography were screened for bioactivity and subjected to mass spectrometry. The Inflammatory and inhibitory potentials of the identified proteins were measured using T cells assays. A potent pro-inflammatory factor was detected in BALF, and we hypothesized SEA could be recovered with its biological activity. Highly purified BALF fractions with bioactivity were subjected to mass spectrometry. SEA was the only identified protein with known inflammatory potential, and unexpectedly, it co-purified with immunosuppressive proteins. Among them was lactoferrin, which inhibited SEA and anti-CD3/-CD28 stimulation by promoting T cell death and reducing TNF synthesis. Higher doses of lactoferrin were required to inhibit effector compared to resting T cells. Inhibition relied on the continual presence of lactoferrin rather than a programming event. The data show a fraction of bioactive SEA resided in a mucosal niche within BALF even after the initiation of inflammation. These results may have clinical value in human diagnostic since traces levels of SEA can be detected using a sensitive bioassay, and may help pinpoint potential mediators of lung inflammation when molecular approaches fail.

IL-36γ Transforms the Tumor Microenvironment and Promotes Type 1 Lymphocyte-Mediated Antitumor Immune Responses

Cytokines play a pivotal role in regulating tumor immunogenicity and antitumor immunity. IL-36γ is important for the IL-23/IL-17-dominated inflammation and anti-BCG Th1 immune responses. However, the impact of IL-36γ on tumor immunity is unknown. Here we found that IL-36γ stimulated CD8(+) T cells, NK cells, and γδ T cells synergistically with TCR signaling and/or IL-12. Importantly, IL-36γ exerted profound antitumor effects in vivo and transformed the tumor microenvironment in favor of tumor eradication. Furthermore, IL-36γ strongly increased the efficacy of tumor vaccination. Moreover, IL-36γ expression inversely correlated with the progression of human melanoma and lung cancer. Our study establishes a role of IL-36γ in promoting antitumor immune responses and suggests its potential clinical translation into cancer immunotherapy.

Responses to LPS boost effector CD8 T-cell accumulation outside of signals 1 and 2

Immunization with adjuvant plus antigen induces durable T-cell immunity and is a mainstay of vaccines. Here, the consequence of separating antigen stimulation of T cells from the adjuvant response was studied in a re-transfer model. Effector CD8 T cells in recipient mice were exposed to lipopolysaccharide (LPS), the Toll-like receptor 4 (TLR4) ligand, which significantly increased persistence. While accumulation in lymphoid and non-lymphoid organs was evident, this result depended upon the timing of LPS administration and the presence of the TLR4 adaptor TRIF in the recipient mice. Interestingly, there was very little impact of the LPS response on subset differentiation, which rather appeared to be programmed by antigen and costimulation. To discern factors that limit accumulation, interleukin 10 (IL-10) was targeted since it is a product of TLR4 triggering and mitigates inflammation. Blockade of IL-10 increased accumulation even though the effector CD8 T cells were well past the priming phase, but upon recall interferon-γ secretion was not affected as would be expected when IL-10 is present during priming. Thus, the adjuvant-altered microenvironment is effective not only in the presence of antigen but also during a window of effector CD8 T-cell stasis, suggesting that pathogen-associated molecular pattern molecules released during co-infection, or by vaccines, could alter the survival fate of specific effector T cells.Cellular & Molecular Immunology advance online publication, 20 July 2015; doi:10.1038/cmi.2015.69.

Atherosclerosis severity is not affected by a deficiency in IL-33/ST2 signaling

Interleukin (IL)‐33 is a cytokine of the IL‐1 family, which signals through the ST2 receptor. Previous work demonstrated that the systemic administration of recombinant IL‐33 reduces the development of atherosclerosis in apolipoprotein E‐deficient (ApoE^−/−) mice by inducing a Th1‐to‐Th2 shift. The objective of our study was to examine the role of endogenous IL‐33 and ST2 in atherosclerosis. ApoE^−/−, IL‐33^−/−ApoE^−/−, and ST2^−/−ApoE^−/− mice were fed with a cholesterol‐rich diet for 10 weeks. Additionally, a group of ApoE^−/− mice was injected with a neutralizing anti‐ST2 or an isotype control antibody during the period of the cholesterol‐rich diet. Atherosclerotic lesion development was measured by Oil Red O staining in the thoracic‐abdominal aorta and the aortic sinus. There were no significant differences in the lipid‐staining area of IL‐33^−/−ApoE^−/−, ST2^−/−ApoE^−/−, or anti‐ST2 antibody‐treated ApoE^−/− mice, compared to ApoE^−/− controls. The absence of IL‐33 signaling had no major and consistent impact on the Th1/Th2 cytokine responses in the supernatant of in vitro‐stimulated lymph node cells. In summary, deficiency of the endogenously produced IL‐33 and its receptor ST2 does not impact the development of atherosclerosis in ApoE‐deficient mice.

IL-33 isoforms: their future as vaccine adjuvants?

The identification and characterization of cytokine isoforms is likely to provide critical important new insight into immunobiology. Cytokine isoforms can provide additional diversity to their complex biological effects that participate in control and protection against different foreign pathogens. Recently, IL-33 has been identified as a proinflammatory cytokine having several different biologically active isoform products. Originally associated with Th2 immunity, new evidence now supports the role of two IL-33 isoforms to facilitate the generation of protective Th1 and CD8 T cell immunity against specific pathogens. Therefore, a better understanding of the IL-33 isoforms will inform us on how to utilize them to facilitate their development as tools as vaccine adjuvants for immune therapy.

Severe neutrophil-dominated inflammation and enhanced myelopoiesis in IL-33-overexpressing CMV/IL33 mice

IL-33 is a cytokine of the IL-1 family, which signals through the ST2 receptor. Previous studies emphasized a role for IL-33 in shaping innate and adaptive immune responses. IL-33 was also reported to modulate myelopoiesis and myeloid cell activity. In this article, we describe IL-33-overexpressing CMV/IL33 and LysM/IL33 mice, which display an inflammatory phenotype associated with growth retardation and paw swelling. The phenotype of CMV/IL33 mice is dependent on activation of the ST2 receptor and is characterized by extensive neutrophil infiltration into different organs, including the paws. Local or systemic levels of proinflammatory mediators such as IL-1β, Cxcl-1, G-CSF, and IL-6 are increased. CMV/IL-33 mice also suffer from anemia, thrombocytosis, and a marked dysregulation of myelopoiesis, leading to an important increase in myeloid cell production or accumulation in bone marrow (BM), spleen, and peripheral blood. Consistently, recombinant IL-33 induced proliferation of myeloid lineage cells in BM-derived granulocyte cultures, whereas IL-33 knockout mice exhibited minor deficiencies in spleen and BM myeloid cell populations. Our observations reveal a neutrophil-dominated inflammatory phenotype in IL-33-overexpressing CMV/IL33 and LysM/IL33 mice, and highlight important regulatory effects of IL-33 on myelopoiesis in vitro and in vivo, where excessive IL-33 signaling can translate into the occurrence of a myeloproliferative disorder.

Tumoral expression of IL-33 inhibits tumor growth and modifies the tumor microenvironment through CD8+ T and NK cells

Cancer immunotherapy has shown great promise as a new standard cancer therapeutic modality. However, the response rates are limited for current approach that depends on enhancing spontaneous antitumor immune responses. Therefore, increasing tumor immunogenicity by expressing appropriate cytokines should further improve the current immunotherapy. IL-33 is a member of the IL-1 family of cytokines and is released by necrotic epithelial cells or activated innate immune cells and is thus considered a "danger" signal. The role of IL-33 in promoting type 2 immune responses and tissue inflammation has been well established. However, whether IL-33 drives antitumor immune responses is controversial. Our previous work established that IL-33 promoted the function of CD8(+) T cells. In this study, we showed that the expression of IL-33 in two types of cancer cells potently inhibited tumor growth and metastasis. Mechanistically, IL-33 increased numbers and IFN-γ production by CD8(+) T and NK cells in tumor tissues, thereby inducing a tumor microenvironment favoring tumor eradication. Importantly, IL-33 greatly increased tumor Ag-specific CD8(+) T cells. Furthermore, both NK and CD8(+) T cells were required for the antitumor effect of IL-33. Moreover, depletion of regulatory T cells worked synergistically with IL-33 expression for tumor elimination. Our studies established "alarmin" IL-33 as a promising new cytokine for tumor immunotherapy through promoting cancer-eradicating type 1 immune responses.

Betting on improved cancer immunotherapy by doubling down on CD134 and CD137 co-stimulation

The ability of T cells to recognize a vast array of antigens enables them to destroy tumor cells while inflicting minimal collateral damage. Nevertheless, tumor antigens often are a form of self-antigen, and thus tumor immunity can be dampened by tolerance mechanisms that evolved to prevent autoimmunity. Since tolerance can be induced by steady-state antigen-presenting cells that provide insufficient co-stimulation, the exogenous administration of co-stimulatory agonists can favor the expansion and tumoricidal functions of tumor-specific T cells. Agonists of the co-stimulatory tumor necrosis factor receptor (TNFR) family members CD134 and CD137 exert antitumor activity in mice, and as monotherapies have exhibited encouraging results in clinical trials. This review focuses on how the dual administration of CD134 and CD137 agonists synergistically boosts T-cell priming and elaborates a multi-pronged antitumor immune response, as well as how such dual co-stimulation might be translated into effective anticancer therapies.

STING-dependent type I IFN production inhibits cell-mediated immunity to Listeria monocytogenes

Infection with Listeria monocytogenes strains that enter the host cell cytosol leads to a robust cytotoxic T cell response resulting in long-lived cell-mediated immunity (CMI). Upon entry into the cytosol, L. monocytogenes secretes cyclic diadenosine monophosphate (c-di-AMP) which activates the innate immune sensor STING leading to the expression of IFN-β and co-regulated genes. In this study, we examined the role of STING in the development of protective CMI to L. monocytogenes. Mice deficient for STING or its downstream effector IRF3 restricted a secondary lethal challenge with L. monocytogenes and exhibited enhanced immunity that was MyD88-independent. Conversely, enhancing STING activation during immunization by co-administration of c-di-AMP or by infection with a L. monocytogenes mutant that secretes elevated levels of c-di-AMP resulted in decreased protective immunity that was largely dependent on the type I interferon receptor. These data suggest that L. monocytogenes activation of STING downregulates CMI by induction of type I interferon.

Targeting TLR3 with no RIG-I/MDA5 activation is effective in immunotherapy for cancer

INTRODUCTION

Many forms of RNA duplexes with agonistic activity for pattern-recognition receptors have been reported, some of which are candidates for adjuvant immunotherapy for cancer. These RNA duplexes induce cytokines, interferons (IFNs) and cellular effectors mainly via two distinct pathways, TLR3/TICAM-1 and MDA5/MAVS.

AREAS COVERED

We determined which pathway of innate immunity predominantly participates in evoking tumor immunity in response to RNA adjuvants.

EXPERT OPINION

In knockout (KO) mouse studies, robust cytokine or IFN production is dependent on systemic activation of the MAVS pathway, whereas maturation of dendritic cells (DCs) to drive cellular effectors (i.e., NK and CTL) depends on the TICAM-1 pathway in DCs. MAVS activation often causes endotoxin-like cytokinemia, while the TICAM-1 activation does not. Unlike the TLR/MyD88 pathway, this TICAM-1 pathway barely accelerates tumor progression. Although the therapeutic effect in human patients of MAVS-activating or TICAM-1-activating RNA duplexes remains undetermined, the design of a TLR3 agonist with optimized toxicity and dose is an important goal for human immunotherapy. Here we summarize current knowledge on available RNA duplex formulations, and offer a possible approach to developing a promising RNA duplex for clinical tests.

Implications for Interleukin-33 in solid organ transplantation

Interleukin(IL)-33 is a member of the IL-1 cytokine family that has been attributed T helper (Th) type 2 immunity-promoting capacity. However, new studies indicate that IL-33 is a multifunctional protein that acts as transcriptional/signaling repressor, functions as an alarmin alerting the immune system to necrosis, as well as serves as a cytokine that targets cells expressing ST2, the IL-33 receptor. Interestingly, IL-33 is also emerging as a pleiotropic cytokine. Depending on the innate or adaptive immune cells targeted by IL-33, it can not only promote type 2, but also IFN-γ dominated type 1 immunity. In addition, IL-33 expands regulatory T cells. In this review, we assimilate the current knowledge of IL-33 immunobiology and discuss how IL-33 may mediate such diverse roles in the immune response to pathogens and development of immune-mediated pathologies. The function of IL-33 in shaping alloimmune responses to transplanted organs is poorly explored, but a particularly beneficial role of IL-33 in experimental heart transplant models is summarized. Finally, given the implication of IL-33 in pathologies of the lung and intestine, we discuss how IL-33 may contribute to the comparatively poor outcomes following transplantation of these two organs.

Anti-viral CD8 T cells and the cytokines that they love

Viral infections cause an immunological disequilibrium that provokes CD8 T cell responses. These cells play critical roles in purging acute infections, limiting persistent infections, and conferring life-long protective immunity. At every stage of the response anti-viral CD8 T cells are sensitive to signals from cytokines. Initially cytokines operate as immunological warning signs that inform of the presence of an infection, and also influence the developmental choices of the responding cells. Later during the course of the response other sets of cytokines support the survival and maintenance of the differentiated anti-viral CD8 T cells. Although many cytokines promote virus-specific CD8 T cells, other cytokines can suppress their activities and thus favor viral persistence. In this review we discuss how select cytokines act to regulate anti-viral CD8 T cells throughout the response and influence the outcome of viral infections.

Disease severity in K/BxN serum transfer-induced arthritis is not affected by IL-33 deficiency

Introduction

Interleukin (IL)-33 is a cytokine of the IL-1 family, which signals through the ST2 receptor. Previous work suggested implication of the IL-33/ST2 axis in the pathogenesis of human and mouse arthritis. Here, we directly investigated the role of endogenous IL-33 in K/BxN serum transfer-induced arthritis by using IL-33 knockout (KO) mice.

Methods

Arthritis was induced by injection of complete K/BxN serum or purified IgG. Disease severity was monitored by clinical and histological scoring.

Results

K/BxN serum transfer induced pronounced arthritis with similar incidence and severity in IL-33 KO and wild-type (WT) mice. In contrast, disease development was significantly reduced in ST2 KO mice. IL-33 expression in synovial tissue was comparable in arthritic WT and ST2 KO mice, and absent in IL-33 KO mice. Transfer of purified arthritogenic IgG instead of complete K/BxN serum also resulted in similar arthritis severity in IL-33 KO and WT mice, excluding a contribution of IL-33 contained in the serum of donor mice to explain this result. We investigated additional potential confounding factors, including purity of genetic background, but the mechanisms underlying reduced arthritis in ST2 KO mice remained unclear.

Conclusions

The data obtained with IL-33 KO mice indicate that endogenous IL-33 is not required for the development of joint inflammation in K/BxN serum transfer-induced arthritis. On the contrary, arthritis severity was reduced in ST2 KO mice. This observation might relate to IL-33 independent effects of ST2, and/or reveal the existence of confounding variables affecting the severity of joint inflammation in these KO strains.

Codelivery of zoledronic acid and doublestranded RNA from core-shell nanoparticles

Background

Zoledronic acid, an inhibitor of osteoclast-mediated bone resorption, has been shown to have both direct and indirect antitumor activity. However, its use in extraskeletal malignancy is limited due to rapid uptake and accumulation within bone. Polyinosinic acid-polycytidylic acid [poly (I:C)] is a synthetic double-stranded RNA with direct antitumor cytotoxicity if it can be delivered to tumor cells intracellularly.

Methods

Cationic lipid-coated calcium phosphate nanoparticles (LCP) were developed to enable intracellular codelivery of zoledronic acid and poly (I:C). LCP codelivering zoledronic acid and poly (I:C) were prepared using an ethanol injection method. Briefly, the ethanol solution of lipids was rapidly injected into newly formed calcium phosphate crystals containing poly (I:C) and zoledronic acid, and the mixture was then sonicated briefly to form LCP. The LCP were fully characterized for mean diameter size and zeta potential, efficiency in loading zoledronic acid, cytotoxic effect in a B16BL6 melanoma cell line in vitro, and antitumor effect in B16BL6 melanoma-bearing mice.

Results

LCP with a mean diameter around 200 nm and a narrow size distribution (polydispersity index 0.17) and high zoledronic acid encapsulation efficiency (94%) were achieved. LCP loaded with zoledronic acid and poly (I:C) had significantly greater antitumor activity than the free drugs in the B16BL6 melanoma cell line (P < 0.05). Furthermore, codelivery of zoledronic acid and poly (I:C) by LCP had higher cytotoxicity than delivering poly (I:C) alone by LCP (P < 0.05), indicating a synergism between zoledronic acid and poly (I:C). Finally, the antitumor study in melanoma-bearing mice also demonstrated synergism between zoledronic acid and poly (I:C) codelivered by LCP.

Conclusion

Cationic lipid-coated calcium phosphate nanoparticles constructed for codelivery of zoledronic acid and double-stranded RNA poly (I:C) had better antitumor activity both in vitro and in vivo. Future preclinical development of LCP encapsulating zoledronic acid and poly (I:C) for the treatment of human cancer is under way.