IL-36 receptor deletion attenuates lung injury and decreases mortality in murine influenza pneumonia

Influence of serum IL-36 subfamily cytokines on clinical manifestations of asthma

Background

The IL-36 subfamily, a member of the IL-1 superfamily, is thought to promote type 2 (T2) and non-T2 inflammation and involved in autoimmune and airway disease progression. However, its role in asthma remains unclear.

Objective

We sought to determine the contribution of the IL-36 subfamily to the clinical manifestation of asthma.

Methods

The levels of serum IL-36α, IL-36β, and IL-36γ, recognized as IL-36 subfamily agonists, and IL-36 receptor antagonist (IL-36Ra) and IL-38, recognized as IL-36 subfamily antagonists, were measured by ELISA in 110 asthma patients (55 with nonsevere and 55 with severe asthma) aged ≥20 years and 31 healthy individuals. The association of IL-36 with clinical indices and inflammatory mediators was examined. The characteristics of high and low IL-36 subgroups were explored.

Results

IL-36α, IL-36γ, and IL-36Ra levels were significantly higher in asthma patients, especially patients with severe asthma, than in healthy controls. The high IL-36γ group exhibited lower Asthma Control Test scores (P = .01), more frequent asthma exacerbations (AEs), and higher hazard ratio for AEs. The high IL-36Ra group exhibited higher values of forced expiratory volume in 1 second, more frequent severe AEs, and higher hazard ratio for severe exacerbations. The IL-36 cytokine levels, except for IL 36α, were positively correlated with IL-6, IL-13, IL-17, and/or IFN-γ levels. IL-36Ra was positively correlated with age-adjusted forced expiratory volume and forced vital capacity.

Conclusion

A systemically high IL-36 level is associated with asthma severity and with both T2 and non-T2 cytokines, and it implies poor condition and enhancement of risk of AEs in asthma patients.

The complex roles of IL-36 and IL-38 in cancer: friends or foes?

The interleukin-36 (IL-36) family comprises of three pro-inflammatory receptor agonists (IL-36α, IL-36β and IL-36γ), two anti-inflammatory receptor antagonists (IL-36RA and IL-38) along with the IL-36 receptor (IL-36R). Part of the IL-1 cytokine superfamily, the IL-36 family was discovered in the early 2000s due to the homology of its member sequences to the IL-1 cytokines. As pro- and anti-inflammatory cytokines, respectively, IL-36α, IL-36β, IL-36γ and IL-38 aid in maintaining homoeostasis by reciprocally regulating the body's response to damage and disease through IL-36R-associated signalling. With the significant roles of IL-36α, IL-36β and IL-36γ in regulating the immune response realised, interest has grown in investigating their roles in cancer. While initial studies indicated solely tumour-suppressing roles, more recent work has identified tumour-promoting roles in cancer, suggesting a more complex dual functionality of the IL-36 cytokines. The activity of IL-38 in cancer is similarly complex, with the receptor antagonist displaying distinct tumour-suppressive roles, particularly in colorectal cancer (CRC), in addition to broad tumour-promoting roles in various other malignancies. This review provides a comprehensive overview of the IL-36 and IL-38 cytokines, their activation and IL-36R signalling, the physiological functions of these cytokines, and their activity in cancer.

Advances in understanding the role of interleukins in pulmonary fibrosis (Review)

Pulmonary fibrosis (PF) is a progressive, irreversible disease characterized by heterogeneous interstitial lung tissue damage. It originates from persistent or repeated lung epithelial injury and leads to the activation and differentiation of fibroblasts into myofibroblasts. Interleukins (ILs) are a group of lymphokines crucial for immunomodulation that are implicated in the pathogenesis of PF. However, different types of ILs exert disparate effects on PF. In the present review, based on the effect on PF, ILs are classified into three categories: i) Promotors of PF; ii) inhibitors of PF; and iii) those that exert dual effects on PF. Several types of ILs can promote PF by provoking inflammation, initiating proliferation and transdifferentiation of epithelial cells, exacerbating lung injury, while other ILs can inhibit PF through suppressing expression of inflammatory factors, modulating the Th1/Th2 balance and autophagy. The present review summarizes the association of ILs and PF, focusing on the roles and mechanisms of ILs underlying PF.

The role of interleukin-36 in health and disease states

The interleukin (IL)-1 superfamily upregulates immune responses and maintains homeostasis between the innate and adaptive immune systems. Within the IL-1 superfamily, IL-36 plays a pivotal role in both innate and adaptive immune responses. Of the four IL-36 isoforms, three have agonist activity (IL-36α, IL-36β, IL-36γ) and the fourth has antagonist activity (IL-36 receptor antagonist [IL-36Ra]). All IL-36 isoforms bind to the IL-36 receptor (IL-36R). Binding of IL-36α/β/γ to the IL-36R recruits the IL-1 receptor accessory protein (IL-1RAcP) and activates downstream signalling pathways mediated by nuclear transcription factor kappa B and mitogen-activated protein kinase signalling pathways. Antagonist binding of IL-36Ra to IL-36R inhibits recruitment of IL-1RAcP, blocking downstream signalling pathways. Changes in the balance within the IL-36 cytokine family can lead to uncontrolled inflammatory responses throughout the body. As such, IL-36 has been implicated in numerous inflammatory diseases, notably a type of pustular psoriasis called generalized pustular psoriasis (GPP), a chronic, rare, potentially life-threatening, multisystemic skin disease characterised by recurrent fever and extensive sterile pustules. In GPP, IL-36 is central to disease pathogenesis, and the prevention of IL-36-mediated signalling can improve clinical outcomes. In this review, we summarize the literature describing the biological functions of the IL-36 pathway. We also consider the evidence for uncontrolled activation of the IL-36 pathway in a wide range of skin (e.g., plaque psoriasis, pustular psoriasis, hidradenitis suppurativa, acne, Netherton syndrome, atopic dermatitis and pyoderma gangrenosum), lung (e.g., idiopathic pulmonary fibrosis), gut (e.g., intestinal fibrosis, inflammatory bowel disease and Hirschsprung's disease), kidney (e.g., renal tubulointerstitial lesions) and infectious diseases caused by a variety of pathogens (e.g., COVID-19; Mycobacterium tuberculosis, Pseudomonas aeruginosa, Streptococcus pneumoniae infections), as well as in cancer. We also consider how targeting the IL-36 signalling pathway could be used in treating inflammatory disease states.

Molecular aspects of Interleukin-36 cytokine activation and regulation

Interleukin-36 (IL-36) cytokines are structurally similar to other Interleukin-1 superfamily members and are essential to convey inflammatory responses at epithelial barriers including the skin, lung, and gut. Due to their potent effects on immune cells, IL-36 cytokine activation is regulated on multiple levels, from expression and activation to receptor binding. Different IL-36 isoforms convey specific responses as a consequence of particular danger- or pathogen-associated molecular patterns. IL-36 expression and activation are regulated by exogenous pathogens, including fungi, viruses and bacteria but also by endogenous factors such as antimicrobial peptides or cytokines. Processing of IL-36 into potent bioactive forms is necessary for host protection but can elevate tissue damage. Indeed, exacerbated IL-36 signalling and hyperactivation are linked to the pathogenesis of diseases such as plaque and pustular psoriasis, emphasising the importance of understanding the molecular aspects regulating IL-36 activation. Here, we summarise facets of the electrochemical properties, regulation of extracellular cleavage by various proteases and receptor signalling of the pro-inflammatory and anti-inflammatory IL-36 family members. Additionally, this intriguing cytokine subfamily displays many characteristics that are unique from prototypical members of the IL-1 family and these key distinctions are outlined here.

Floxed Il1rl2 Locus with mCherry Reporter Element Reveals Distinct Expression Patterns of the IL-36 Receptor in Barrier Tissues

IL-36 cytokines are emerging as beneficial in immunity against pathogens and cancers but can also be detrimental when dysregulated in autoimmune and autoinflammatory conditions. Interest in targeting IL-36 activity for therapeutic purposes is rapidly growing, yet many unknowns about the functions of these cytokines remain. Thus, the availability of robust research tools is essential for both fundamental basic science and pre-clinical studies to fully access outcomes of any manipulation of the system. For this purpose, a floxed Il1rl2, the gene encoding the IL-36 receptor, mouse strain was developed to facilitate the generation of conditional knockout mice. The targeted locus was engineered to contain an inverted mCherry reporter sequence that upon Cre-mediated recombination will be flipped and expressed under the control of the endogenous Il1rl2 promoter. This feature can be used to confirm knockout in individual cells but also as a reporter to determine which cells express the IL-36 receptor IL-1RL2. The locus was confirmed to function as intended and further used to demonstrate the expression of IL-1RL2 in barrier tissues. Il1rl2 expression was detected in leukocytes in all barrier tissues. Interestingly, strong expression was observed in epithelial cells at locations in direct contact with the environment such as the skin, oral mucosa, the esophagus, and the upper airways, but almost absent from epithelial cells at more inward facing sites, including lung alveoli, the small intestine, and the colon. These findings suggest specialized functions of IL-1RL2 in outward facing epithelial tissues and cells. The generated mouse model should prove valuable in defining such functions and may also facilitate basic and translational research.

Interleukin-36 receptor antagonist stimulation in vitro inhibits peripheral and lung-resident T cell response isolated from patients with ventilator-associated pneumonia

Interleukin-36 (IL-36) cytokine family members play an immunomodulatory function to immune cells through IL-36 receptor signaling pathway. However, the regulatory role of IL-36 exerted on T cells is not completely elucidated in patients with ventilator-associated pneumonia (VAP). For this purpose, this study enrolled 51 VAP patients and 27 controls. IL-36 levels were measured by ELISA. The mRNA levels of IL-36 receptor subunits were determined by real-time PCR. CD4+ and CD8+ T cells were enriched, and stimulated with recombinant IL-36 receptor antagonist (IL-36RA). The influence of IL-36RA on transcription factors and cytokine secretions by CD4+ T cells was investigated. The modulatory function of IL-36RA on CD8+ T cells was assessed by measuring target cell death and cytokine secretions. There were no significant differences in serum IL-36 levels between VAP patients and controls. Only IL-36RA, but not IL-36α, IL-36β, or IL-36γ, in bronchoalveolar lavage fluid was elevated in infection site of VAP patients. IL-36 receptor subunits in CD4+ and CD8+ T cells were comparable between VAP patients and controls. 10 ng/mL of IL-36RA stimulation dampened peripheral effector CD4+ T cell response isolated from both VAP patients and controls. Target cell death mediated by CD8+ T cells isolated from BAFL of VAP patients was suppressed by 100 ng/mL of IL-36RA stimulation in vitro. The down-regulations of perforin, granzyme B, interferon-γ, tumor necrosis factor-α, and Fas ligand following IL-36RA stimulation in vitro were responsible for reduced CD8+ T cell-mediated cytotoxicity. IL-36RA revealed an immunosuppressive property for T cell response in vitro, and may be involved in the protective mechanism in VAP patients.

Emerging functions and therapeutic targets of IL-38 in central nervous system diseases

Interleukin (IL)-38 is a newly discovered cytokine of the IL-1 family, which binds various receptors (i.e., IL-36R, IL-1 receptor accessory protein-like 1, and IL-1R1) in the central nervous system (CNS). The hallmark physiological function of IL-38 is competitive binding to IL-36R, as does the IL-36R antagonist. Emerging research has shown that IL-38 is abnormally expressed in the serum and brain tissue of patients with ischemic stroke (IS) and autism spectrum disorder (ASD), suggesting that IL-38 may play an important role in neurological diseases. Important advances include that IL-38 alleviates neuromyelitis optica disorder (NMOD) by inhibiting Th17 expression, improves IS by protecting against atherosclerosis via regulating immune cells and inflammation, and reduces IL-1β and CXCL8 release through inhibiting human microglial activity post-ASD. In contrast, IL-38 mRNA is markedly increased and is mainly expressed in phagocytes in spinal cord injury (SCI). IL-38 ablation attenuated SCI by reducing immune cell infiltration. However, the effect and underlying mechanism of IL-38 in CNS diseases remain inadequately characterized. In this review, we summarize the biological characteristics, pathophysiological role, and potential mechanisms of IL-38 in CNS diseases (e.g., NMOD, Alzheimer's disease, ASD, IS, TBI, and SCI), aiming to explore the therapeutic potential of IL-38 in the prevention and treatment of CNS diseases.

Emerging Role of the IL-36/IL-36R Axis in Multiple Inflammatory Skin Diseases

IL-36 is a most recent member of the IL-1 cytokine family, primarily expressed at barrier sites of the body such as the skin, lungs, and intestine. It plays a vital role in inflammation and is implicated in the development of various cutaneous; intestinal; and pulmonary disorders, including psoriasis, inflammatory bowel disease, and chronic obstructive pulmonary disease. IL-36 comprises 4 isoforms: the proinflammatory IL-36α, IL-36β, and IL-36γ and the anti-inflammatory IL-36R antagonist. An imbalance between proinflammatory and anti-inflammatory IL-36 isoforms can contribute to the inflammatory fate of cells and tissues. IL-36 cytokines signal through an IL-36R heterodimer mediating their function through canonical signaling cacade, including the NF-B pathway. Prominent for its role in psoriasis, IL-36 has recently been associated with disease mechanisms in atopic dermatitis, hidradenitis suppurativa, neutrophilic dermatoses, autoimmune blistering disease, and Netherton syndrome. The major cutaneous source of IL-36 cytokines is keratinocytes, pointing to its role in the communication between the epidermis, innate (neutrophils, dendritic cells) immune system, and adaptive (T helper [Th]1 cells, Th17) immune system. Thus, cutaneous IL-36 signaling is crucial for the immunopathological outcome of various skin diseases. Consequently, the IL-36/IL-36R axis has recently been recognized as a promising drug target for the treatment of inflammatory disorders beyond psoriasis. This review summarizes the current update on IL-36 cytokines in inflammatory skin diseases.

The pathogenesis of influenza in intact alveoli: virion endocytosis and its effects on the lung's air-blood barrier

Lung infection by influenza A virus (IAV) is a major cause of global mortality from lung injury, a disease defined by widespread dysfunction of the lung's air-blood barrier. Endocytosis of IAV virions by the alveolar epithelium - the cells that determine barrier function - is central to barrier loss mechanisms. Here, we address the current understanding of the mechanistic steps that lead to endocytosis in the alveolar epithelium, with an eye to how the unique structure of lung alveoli shapes endocytic mechanisms. We highlight where future studies of alveolar interactions with IAV virions may lead to new therapeutic approaches for IAV-induced lung injury.

Interleukin-36gamma Mediates the In Vitro Activation of CD8+ T Cells from Patients Living with Chronic Human Immunodeficiency Virus-1 Infection

Interleukin-36 (IL-36) signaling plays an important role in promoting CD8+ T cell-mediated antitumor immune responses. The role of IL-36 signaling in CD8+ T cells that are involved in host immune responses during human immunodeficiency virus-1 (HIV-1) infection has not been characterized. Sixty-one patients living with chronic HIV-1 infection and 23 controls were enrolled in this study. The levels of IL-36 cytokine family members were measured by enzyme-linked immunosorbent assay. Purified CD8+ T cells were stimulated with recombinant IL-36gamma (1 or 10 ng/mL). The expression of inhibitory receptors, the secretion of cytotoxic molecules and interferon-gamma, and the mRNA levels of apoptosis-related ligands were assessed to evaluate the effect of IL-36gamma on CD8+ T cell function in vitro. There were no significant differences in IL-36alpha, IL-36beta, or IL-36 receptor antagonist levels between patients living with chronic HIV-1 infection and controls. Plasma IL-36gamma levels were reduced in patients living with chronic HIV-1 infection. Perforin, granzyme B, and granulysin secretion, as well as tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and Fas ligand (FasL) mRNA expression, but not programmed death-1 (PD-1) or cytotoxic T lymphocyte-associated protein-4 (CTLA-4) expression was downregulated in CD8+ T cells from patients living with chronic HIV-1 infection. The addition of both 1 and 10 ng/mL IL-36gamma enhanced perforin, granzyme B, granulysin, and interferon-gamma secretion by CD8+ T cells without affecting PD-1/CTLA-4 or TRAIL/FasL mRNA expression in CD8+ T cells from patients living with chronic HIV-1 infection. The addition of 1 ng/mL IL-36gamma also promoted perforin and granzyme B secretion by HIV-1-specific CD8+ T cells from patients living with chronic HIV-1 infection. The reduced IL-36gamma levels in patients living with chronic HIV-1 infection might be insufficient for the activation of CD8+ T cells, leading to CD8+ T cell exhaustion.

IL-36 signaling pathway dysregulation in Crimean-Congo hemorrhagic fever virus patients: A potential therapeutic avenue

Crimean-Congo hemorrhagic fever (CCHF) is a severe viral disease. The scientific literature is growing, emphasizing the significance of the interleukin (IL)-36 family in the proinflammatory signaling pathway. However, to date, no research has explored the potential of IL-36 family members as biomarkers in CCHF. This study aims to bridge this gap by evaluating IL-36α, IL-36β, and IL-36γ levels in CCHF patients and healthy controls and investigating their association with disease severity and prognosis. Sixty confirmed CCHF patients and 29 healthy controls were enrolled in this case-control study. Serum levels of IL-36α, IL-36β, and IL-36γ were measured using enzyme-linked immunosorbent assays. Significantly higher levels of IL-36α and IL-36β were observed in CCHF patients compared to healthy controls (p < 0.05). However, no statistically significant changes were found in IL-36γ levels between the two groups. Among the CCHF patients, those who did not survive exhibited significantly elevated IL-36α and IL-36γ levels compared to survivors (p < 0.01). Positive correlations were identified between IL-36α and IL-36γ levels with activated partial thromboplastin time, and D-dimer (p < 0.01). Conversely, platelet levels showed a negative correlation with IL-36α and IL-36γ levels (p < 0.01). The increased levels of IL-36α, IL-36β, and IL-36γ in patients indicate their participation in proinflammatory reactions in CCHF patients. Understanding the role of IL-36 family members in CCHF pathogenesis could offer valuable insights into disease progression and facilitate the development of targeted therapeutic strategies.

Interleukin-36 Receptor Signaling Attenuates Epithelial Wound Healing in C57BL/6 Mouse Corneas

The IL-36 cytokines are known to play various roles in mediating the immune and inflammatory response to tissue injury in a context-dependent manner. This study investigated the role of IL-36R signaling in mediating epithelial wound healing in normal (NL) and diabetic (DM) C57BL/6 mouse corneas. The rate of epithelial wound closure was significantly accelerated in IL-36 receptor-deficient (IL-36R-/-) compared to wild-type (WT) mice. Wounding increased IL-36α and -36γ but repressed IL-36R antagonist (IL-36Ra) expression in B6 mouse corneal epithelial cells. The wound-induced proinflammatory cytokines CXCL1 and CXCL2 were dampened, while the antimicrobial peptides (AMPs) S100A8 and A9 were augmented in IL-36R-/- mouse corneas. Intriguingly, the expression of AMP LCN2 was augmented at the mRNA level. LCN2 deficiency resulted in an acceleration of epithelial wound healing. IL-36R deficiency also greatly increased the healing rate of the corneal epithelial wound in DM mice. IL-36R deficiency also suppressed IL-1β, IL-1Ra, and ICAM expression in unwounded-DM mice and wounded NL corneas. Opposing IL-1β and ICAM, the expression of IL-Ra in DM corneas of IL-36R-/- mice was augmented. The presence of recombinant IL-1Ra and IL-36Ra accelerated epithelial wound closure in T1DM corneas of B6 mice. Our study revealed an unprecedented role of IL-36R signaling in controlling corneal epithelial wound healing in normal (NL) and diabetic (DM) mice. Our data suggest that IL-36Ra, similar to IL-1Ra, might be a therapeutic reagent for improving wound healing and reducing wound-associated ulceration, particularly in the cornea and potentially in the skin of DM patients.

Influenza B Virus (IBV) Immune-Mediated Disease in C57BL/6 Mice

Influenza B viruses (IBV) primarily infect humans, causing seasonal epidemics. The absence of an animal reservoir limits pandemic concern, but IBV infections may cause severe respiratory disease, predominantly in young children and the elderly. The IBV disease burden is largely controlled by seasonal influenza vaccination; however, immunity due to vaccination is sometimes incomplete, a feature linked to antigenic mismatches. Thus, understanding the features that contribute to disease pathogenesis is important, particularly immune-mediated versus virus-mediated outcomes. Unexpectedly, C57BL/6 (B6) mice intranasally infected with a low multiplicity of infection of B/Florida/04/2006 developed substantial morbidity and mortality. To address the cause, B6 mice were treated daily with dexamethasone to dampen the immune and pro-inflammatory response to IBV infection, allowing the determination of whether the responses were immune- and/or virus-associated. As expected, dexamethasone (DEX)-treated mice had a lower pro-inflammatory response and reduced lung pathology despite the presence of high viral lung titers, but mortality was comparable to PBS-treated mice, indicating that mortality may be linked to lung virus replication. The results showed that the immune response to IBV is the major cause of morbidity, mortality, lung pathology, and viral clearance. Importantly, the results suggest that a robust lung CTL response and associated leukocyte influx contribute to disease.

IL-36 Cytokines: Their Roles in Asthma and Potential as a Therapeutic

Interleukin (IL)-36 cytokines are members of the IL-1 superfamily, which consists of three agonists (IL-36α, IL-36β and IL-36γ) and an IL-36 receptor antagonist (IL-36Ra). IL-36 cytokines are crucial for immune and inflammatory responses. Abnormal levels of IL-36 cytokine expression are involved in the pathogenesis of inflammation, autoimmunity, allergy and cancer. The present study provides a summary of recent reports on IL-36 cytokines that participate in the pathogenesis of inflammatory diseases, and the potential mechanisms underlying their roles in asthma. Abnormal levels of IL-36 cytokines are associated with the pathogenesis of different types of asthma through the regulation of the functions of different types of cells. Considering the important role of IL-36 cytokines in asthma, these may become a potential therapeutic target for asthma treatment. However, existing evidence is insufficient to fully elucidate the specific mechanism underlying the action of IL-36 cytokines during the pathological process of asthma. The possible mechanisms and functions of IL-36 cytokines in different types of asthma require further studies.

Imbalance between IL-36 receptor agonist and antagonist drives neutrophilic inflammation in COPD

Current treatments fail to modify the underlying pathophysiology and disease progression of chronic obstructive pulmonary disease (COPD), necessitating alternative therapies. Here, we show that COPD subjects have increased IL-36γ and decreased IL-36 receptor antagonist (IL-36Ra) in bronchoalveolar and nasal fluid compared to control subjects. IL-36γ is derived from small airway epithelial cells (SAEC) and further induced by a viral mimetic, whereas IL-36RA is derived from macrophages. IL-36γ stimulates release of the neutrophil chemoattractants CXCL1 and CXCL8, as well as elastolytic matrix metalloproteinases (MMPs) from small airway fibroblasts (SAF). Proteases released from COPD neutrophils cleave and activate IL-36γ thereby perpetuating IL-36 inflammation. Transfer of culture media from SAEC to SAF stimulated release of CXCL1, that was inhibited by exogenous IL-36RA. The use of a therapeutic antibody that inhibits binding to the IL-36 receptor (IL-36R) attenuated IL-36γ driven inflammation and cellular cross talk. We have demonstrated a mechanism for the amplification and propagation of neutrophilic inflammation in COPD and that blocking this cytokine family via a IL-36R neutralizing antibody could be a promising new therapeutic strategy in the treatment of COPD.

Interleukin-1 Receptor-Like 2: One Receptor, Three Agonists, and Many Implications

The interleukin (IL)-1 superfamily of cytokines comprises 11 pro- and anti-inflammatory cytokines, which play essential roles during the immune response. Several pathogenic pathways are initiated by IL-1RL2 (interleukin 1 receptor-like 2) signaling, also known as IL-36R, in the skin, lungs, and gut. IL-36 cytokines promote the secretion of proinflammatory cytokines and chemokines, upregulation of antimicrobial peptides, proliferation mediators, and adhesion molecules on endothelial cells. In addition, the IL-36-IL-1RL2 axis has an essential role against viral infections, including a potential role in COVID-19 pathology. The evidence presented in this review highlights the importance of the axis IL-36-IL-1RL2 in the development of several inflammation-related diseases and the healing process. It suggests that IL-1RL2 ligands have specific roles depending on the tissue or cell source. However, there is still much to discover about this cytokine family, their functions in other organs, and how they accomplish a dual effect in inflammation and healing.

The Protective Role of IL-36/IL-36R Signal in Con A-Induced Acute Hepatitis

The IL-36 family, including IL-36α, IL-36β, IL-36γ, and IL-36R antagonist, belong to the IL-1 superfamily. It was reported that IL-36 plays a role in immune diseases. However, it remains unclear how IL-36 regulates inflammation. To determine the role of IL-36/IL-36R signaling pathways, we established an acute hepatitis mouse model (C57BL/6) by i.v. injection of the plant lectin Con A. We found that the levels of IL-36 were increased in the liver after Con A injection. Our results demonstrated the infiltrated neutrophils, but not the hepatocytes, were the main source of IL-36 in the liver. Using the IL-36R-/- mouse model (H-2b), we surprisingly found that the absence of IL-36 signals led to aggravated liver injury, as evidenced by increased mortality, elevated serum alanine aminotransferase and aspartate aminotransferase levels, and severe liver pathological changes. Further investigations demonstrated that a lack of IL-36 signaling induced intrahepatic activation of CD4+ and CD8+ T lymphocytes and increased the production of inflammatory cytokines. In addition, IL-36R-/- mice had reduced T regulatory cell numbers and chemokines in the liver. Together, our results from the mouse model suggested a vital role of IL-36 in regulating T cell function and homeostasis during liver inflammation.

The role of IL-36 subfamily in intestinal disease

Interleukin (IL)-36 is a subfamily, of the IL-1 super-family and includes IL-36α, IL-36β, IL-36γ, IL-38 and IL-36Ra. IL-36 cytokines are involved in the pathology of multiple tissues, including skin, lung, oral cavity, intestine, kidneys and joints. Recent studies suggest that IL-36 signaling regulates autoimmune disease in addition to antibacterial and antiviral responses. Most research has focused on IL-36 in skin diseases such as psoriasis, however, studies on intestinal diseases are also underway. This review outlines what is known about the bioactivity of the IL-36 subfamily and its role in the pathogenesis of intestinal diseases such as inflammatory bowel disease, colorectal cancer, gut dysbacteriosis and infection, and proposes that IL-36 may be a target for novel therapeutic strategies to prevent or treat intestinal diseases.

Interleukin-36 Cytokines in Infectious and Non-Infectious Lung Diseases

The IL-36 family of cytokines were identified in the early 2000’s as a new subfamily of the IL-1 cytokine family, and since then, the role of IL-36 cytokines during various inflammatory processes has been characterized. While most of the research has focused on the role of these cytokines in autoimmune skin diseases such as psoriasis and dermatitis, recent studies have also shown the importance of IL-36 cytokines in the lung inflammatory response during infectious and non-infectious diseases. In this review, we discuss the biology of IL-36 cytokines in terms of how they are produced and activated, as well as their effects on myeloid and lymphoid cells during inflammation. We also discuss the role of these cytokines during lung infectious diseases caused by bacteria and influenza virus, as well as other inflammatory conditions in the lungs such as allergic asthma, lung fibrosis, chronic obstructive pulmonary disease, cystic fibrosis and cancer. Finally, we discuss the current therapeutic advances that target the IL-36 pathway and the possibility to extend these tools to treat lung inflammatory diseases.

IL-36α Enhances Host Defense against Pseudomonas aeruginosa Keratitis in C57BL/6 Mouse Corneas

The IL-36 cytokines are known to play various roles in mediating the immune response to infection in a tissue- and pathogen-dependent manner. The present study seeks to investigate the role of IL-36R signaling in C57BL/6 mouse corneas in response to Pseudomonas aeruginosa infection. IL-36α-/-, IL-36γ-/-, and IL-36R-/- mice had significantly more severe keratitis than wild-type mice. At six hours postinfection, IL-36α pretreatment augmented P. aeruginosa-induced expression of IL-1Ra, IL-36γ, LCN2, and S100A8/A9. At one day postinfection, exogenous IL-36α suppressed, whereas IL-36α deficiency promoted, the expression of IL-1β. At three days postinfection, exogenous IL-36α suppressed Th1 but promoted Th2 immune response. IL-36α stimulated the infiltration of IL-22-expressing immune cells, and IL-22 neutralization resulted in more severe keratitis. IL-36α alone stimulated dendritic cell infiltration in B6 mouse corneas. Taken together, our study suggests that IL-36R signaling plays a protective role in the pathogenesis of P. aeruginosa keratitis by promoting the innate immune defense, Th2, and/or Th22/IL-22 immune responses. Exogenous IL-36α might be a potential therapy for improving the outcome of P. aeruginosa keratitis.

IL-36 in chronic inflammation and fibrosis - bridging the gap?

IL-36 is a member of the IL-1 superfamily and consists of three agonists and one receptor antagonist (IL-36Ra). The three endogenous agonists, IL-36α, -β, and -γ, act primarily as proinflammatory cytokines, and their signaling through the IL-36 receptor (IL-36R) promotes immune cell infiltration and secretion of inflammatory and chemotactic molecules. However, IL-36 signaling also fosters secretion of profibrotic soluble mediators, suggesting a role in fibrotic disorders. IL-36 isoforms and IL-36 have been implicated in inflammatory diseases including psoriasis, arthritis, inflammatory bowel diseases, and allergic rhinitis. Moreover, IL-36 has been connected to fibrotic disorders affecting the kidney, lung, and intestines. This review summarizes the expression, cellular source, and function of IL-36 in inflammation and fibrosis in various organs, and proposes that IL-36 modulation may prove valuable in preventing or treating inflammatory and fibrotic diseases and may reveal a mechanistic link between inflammation and fibrosis.

The clinical role of host and bacterial-derived extracellular vesicles in pneumonia

Pneumonia is among the leading causes of morbidity and mortality worldwide. Due to constant evolution of respiratory bacteria and viruses, development of drug resistance and emerging pathogens, it constitutes a considerable health care threat. To enable development of novel strategies to control pneumonia, a better understanding of the complex mechanisms of interaction between host cells and infecting pathogens is vital. Here, we review the roles of host cell and bacterial-derived extracellular vesicles (EVs) in these interactions. We discuss clinical and experimental as well as pathogen-overarching and pathogen-specific evidence for common viral and bacterial elicitors of community- and hospital-acquired pneumonia. Finally, we highlight the potential of EVs for improved management of pneumonia patients and discuss the translational steps to be taken before they can be safely exploited as novel vaccines, biomarkers, or therapeutics in clinical practice.

IL-36 cytokines in inflammatory and malignant diseases: not the new kid on the block anymore

The IL-36 family of cytokines were first identified in 2000 based on their sequence homology to IL-1 cytokines. Over subsequent years, the ability of these cytokines to either agonise or antagonise an IL-1R homologue, now known as the IL-36 Receptor (IL-36R), was identified and these cytokines went through several cycles of renaming with the current nomenclature being proposed in 2010. Despite being identified over 20 years ago, it is only during the last decade that the function of these cytokines in health and disease has really begun to be appreciated, with both homeostatic functions in wound healing and response to infection, as well as pathological functions now ascribed. In the disease context, over activation of IL-36 has now been associated with many inflammatory diseases including Psoriasis and inflammatory bowel diseases, with roles in cancer also now being investigated. This review summarises the current knowledge of IL-36 biology, its role in inflammatory diseases and focuses on an emerging role for IL-36 in cancer.

The Role of IL-36 in Infectious Diseases: Potential Target for COVID-19?

IL-36 is a member of the interleukin 1 cytokine family, which is currently experiencing a renaissance due to the growing understanding of its context-dependent roles and advances in our understanding of the inflammatory response. The immunological role of IL-36 has revealed its profound and indispensable functional roles in psoriasis, as well as in several inflammatory diseases, including inflammatory bowel disease (IBD), systemic lupus erythematosus, rheumatoid arthritis (RA) and cancer. More recently, an increasing body of evidence suggests that IL-36 plays a crucial role in viral, bacterial and fungal infections. There is a growing interest as to whether IL-36 contributes to host protective immune responses against infection as well as the potential implications of IL-36 for the development of new therapeutic strategies. In this review, we summarize the recent progress in understanding cellular expression, regulatory mechanisms and biological roles of IL-36 in infectious diseases, which suggest more specific strategies to maneuver IL-36 as a diagnostic or therapeutic target, especially in COVID-19.

Role of IL-36γ/IL-36R Signaling in Corneal Innate Defense Against Candida albicans Keratitis

PURPOSE

Interleukin (IL)-36 cytokines have been shown to play either beneficial or detrimental roles in the infection of mucosal tissues in a pathogen-dependent manner, but their involvement in fungal keratitis remains elusive. We herein investigated their expression and function in mediating corneal innate immunity against Candida albicans infection.

METHODS

Gene expression in mouse corneas with or without C. albicans infection was determined by regular RT- and real-time (q)-PCR, Western blot analysis, ELISA or proteome profile assay. The severity of C. albicans keratitis was assessed using clinical scoring, bacterial counting, and myeloperoxidase (MPO) activity as an indicator of neutrophil infiltration. IL36R knockout mice and IL-33-specific siRNA were used to assess the involvement IL-33 signaling in C. albicans-infected corneas. B6 CD11c-DTR mice and clodronate liposomes were used to define the involvement of dendritic cells (DCs) and macrophages in IL-36R signaling and C. albicans keratitis, respectively.

RESULTS

IL-36γ were up-regulated in C57BL6 mouse corneas in response to C. albicans infection. IL-36 receptor-deficient mice display increased severity of keratitis, with a higher fungal load, MPO, and IL-1β levels, and lower soluble sIL-1Ra and calprotectin levels. Exogenous IL-36γ prevented fungal keratitis pathogenesis with lower fungal load and MPO activity, higher expression of sIL-1Ra and calprotectin, and lower expression of IL-1β, at mRNA or protein levels. Protein array analysis revealed that the expression of IL-33 and REG3G were related to IL-36/IL36R signaling, and siRNA downregulation of IL-33 increased the severity of C. albicans keratitis. Depletion of dendritic cells or macrophages resulted in severe C. albicans keratitis and yet exhibited minimal effects on exogenous IL-36γ-induced protection against C. albicans infection in B6 mouse corneas.

CONCLUSIONS

IL-36/IL36R signaling plays a protective role in fungal keratitis by promoting AMP expression and by suppressing fungal infection-induced expression of proinflammatory cytokines in a dendritic cell- and macrophage-independent manner.

IL-36 cytokines and gut immunity

Interleukin 36 (IL-36) constitutes a group of cytokines that belong to the IL-1 superfamily. Emerging evidence has suggested a role of IL-36 in the pathogenesis of many inflammatory disorders. Intriguingly, in the gastrointestinal tract, IL-36 has a rather complex function. IL-36 receptor ligands are overexpressed in both animal colitis models and human IBD patients and may play both pathogenic and protective roles, depending on the context. IL-36 cytokines comprise three receptor agonists: IL-36α, IL-36β and IL-36γ, and two receptor antagonists: IL-36Ra and IL-38. All IL-36 receptor agonists bind to the IL-36R complex and exert pleiotropic effects during inflammatory settings. Here, we first briefly review the processing and secretion of IL-36 cytokines. We then focus on the current understanding of the immunology effects of IL-36 in gut immunity. In addition, we also discuss the ongoing trials that aim to blockage IL-36R signalling for treating chronic intestinal inflammation and present some unexplored questions regarding IL-36 research.

Interleukin-38 ameliorates poly(I:C) induced lung inflammation: therapeutic implications in respiratory viral infections

Interleukin-38 has recently been shown to have anti-inflammatory properties in lung inflammatory diseases. However, the effects of IL-38 in viral pneumonia remains unknown. In the present study, we demonstrate that circulating IL-38 concentrations together with IL-36α increased significantly in influenza and COVID-19 patients, and the level of IL-38 and IL-36α correlated negatively and positively with disease severity and inflammation, respectively. In the co-cultured human respiratory epithelial cells with macrophages to mimic lung microenvironment in vitro, IL-38 was able to alleviate inflammatory responses by inhibiting poly(I:C)-induced overproduction of pro-inflammatory cytokines and chemokines through intracellular STAT1, STAT3, p38 MAPK, ERK1/2, MEK, and NF-κB signaling pathways. Intriguingly, transcriptomic profiling revealed that IL-38 targeted genes were associated with the host innate immune response to virus. We also found that IL-38 counteracts the biological processes induced by IL-36α in the co-culture. Furthermore, the administration of recombinant IL-38 could mitigate poly I:C-induced lung injury, with reduced early accumulation of neutrophils and macrophages in bronchoalveolar lavage fluid, activation of lymphocytes, production of pro-inflammatory cytokines and chemokines and permeability of the alveolar-epithelial barrier. Taken together, our study indicates that IL-38 plays a crucial role in protection from exaggerated pulmonary inflammation during poly(I:C)-induced pneumonia, thereby providing the basis of a novel therapeutic target for respiratory viral infections.

Analysis of Il36a induction by C/EBPβ via a half-CRE•C/EBP element in murine macrophages in dependence of its CpG methylation level

Interleukin-36α is a novel member of the IL-1 cytokine family that is highly expressed in epithelial tissues and several myeloid-derived cell types after induction. The transcription factor (TF) C/EBPβ binds specifically to an essential half-CRE•C/EBP motif in the Il36a promoter to induce Il36a expression upon LPS stimulation. C/EBPs regulate gene expression by binding to recognition sequences that can contain 5'-cytosine-phosphate-guanine-3' dinucleotides (CpG), whose methylation can influence TF binding and gene expression. Herein we show that the half-CRE•C/EBP element in the Il36a promoter is differentially methylated in the murine RAW264.7 macrophage cell line and in primary murine macrophages. We demonstrate that C/EBPβ binding to the half-CRE•C/EBP element in the Il36a promoter following LPS stimulation is insensitive to CpG methylation and that methylation of the CpG in the half-CRE•C/EBP element does not alter LPS-induced Il36a promoter activity which correlated with similar Il36a mRNA copy numbers and pro-IL-36α protein amount in both cell types. Taken together, our data indicate that C/EBPβ binding to the half-CRE•C/EBP element and subsequent gene activation occurs independently of the CpG methylation status of the half-CRE•C/EBP motif and underlines the potential of C/EBPs to recognize methylated as well as unmethylated motifs.

Role of IL-36 Cytokines in the Regulation of Angiogenesis Potential of Trophoblast Cells

IL-36 cytokines (the agonists IL-36α, IL-36β, IL-36γ, and the antagonist IL-36Ra) are expressed in the mouse uterus and associated with maternal immune response during pregnancy. Here, we characterize the expression of IL-36 members in human primary trophoblast cells (PTC) and trophoblastic cell lines (HTR-8/SVneo and JEG-3) and upon treatment with bacterial and viral components. Effects of recombinant IL-36 on the migration capacity of trophoblastic cells, their ability to interact with endothelial cells and the induction of angiogenic factors and miRNAs (angiomiRNAs) were examined. Constitutive protein expression of IL-36 (α, β, and γ) and their receptor (IL-36R) was found in all cell types. In PTC, transcripts for all IL-36 subtypes were found, whereas in trophoblastic cell lines only for IL36G and IL36RN. A synthetic analog of double-stranded RNA (poly I:C) and lipopolysaccharide (LPS) induced the expression of IL-36 members in a cell-specific and time-dependent manner. In HTR-8/SVneo cells, IL-36 cytokines increased cell migration and their capacity to interact with endothelial cells. VEGFA and PGF mRNA and protein, as well as the angiomiRNAs miR-146a-3p and miR-141-5p were upregulated as IL-36 response in PTC and HTR-8/SVneo cells. In conclusion, IL-36 cytokines are modulated by microbial components and regulate trophoblast migration and interaction with endothelial cells. Therefore, a fundamental role of these cytokines in the placentation process and in response to infections may be expected.

High Expression of IL-36γ in Influenza Patients Regulates Interferon Signaling Pathway and Causes Programmed Cell Death During Influenza Virus Infection

As a severe complication of influenza infection, acute respiratory distress syndrome (ARDS) has higher morbidity and mortality. Although IL-36γ has been proven to promote inflammation at epithelial sites and protect against specific pathogen infection, the detailed roles in severe influenza infection remain poorly understood. In this study, we have found that the expression of IL-36γ is higher in influenza-induced ARDS patients than healthy individuals. IL-36γ was induced in human lung epithelial cells and peripheral blood mononuclear cells by Influenza A virus (IAV) infection, and its induction was synergistically correlated with initiation of the cyclooxygenase-2 (COX-2)/Prostaglandin E2 (PGE2) axis. We also have found that expression of superficial IL-36R was elevated in severe influenza patients and in IAV-stimulated cells. Furthermore, although IL-36γ enhanced the induction of type I and III interferons (IFNs), which promoted IAV-mediated IFN-stimulated STAT1 and STAT2 phosphorylated inhibition in lung epithelial cells, the downstream interferon-stimulated genes (ISGs) were not affected. Finally, we have revealed that IL-36γ treatment could promote apoptosis and inhibit autophagy in the early stages of IAV infection. Overall, these findings demonstrated IL-36γ is a critical host immune factor in response to IAV infection. It has potential activity in the regulation of the interferon signaling pathway and was involved in different types of programmed cell death in human airway epithelial cells as well.

IL-36 family cytokines in protective versus destructive inflammation

The IL-1 family of cytokines and receptors are critical regulators of inflammation. Within the IL-1 family and in contrast to its IL-1 and IL-18 subfamilies, the IL-36 subfamily is still poorly characterized. Three pro-inflammatory agonists IL-36α, IL-36β, IL-36γ, one IL-36 receptor (IL-1R6) antagonist, IL-36RA, and one putative IL-1R6 antagonist, IL-38, have been grouped into the IL-36 cytokine subfamily. IL-36 agonists signal through a common receptor complex to serve as early triggers of inflammatory responses by activating and cross-regulating a number of inflammatory pathways including NF-κB, MAPK and IFN signaling. IL-36RA binds to IL-1R6 to limit inflammatory signaling, while IL-38 may be an antagonist of more than one IL-1 family receptor. Expression patterns of IL-36 family cytokines, being most prominently expressed in epithelial barrier tissues such as the skin and intestines as well as in immune cells, suggest a role in protecting these barriers from infection. Dysregulation of IL-36 family cytokine signaling at physiological barriers, most prominently the skin, induces autoimmune inflammation. However, transferring the potential of IL-36 to induce tissue damage to tumors might benefit cancer patients. Here we summarize signaling pathways regulated by IL-36 family cytokines, including IL-38, and the consequences for physiological protective and pathophysiological destructive inflammation. Moreover, we discuss the limits of current knowledge on IL-36 family function to open potential avenues for research in the future.

Interleukin-36: Structure, Signaling and Function

The IL-36 family belongs to a larger IL-1 superfamily and consists of three agonists (IL-36α/β/γ), one antagonist (IL-36Ra), one cognate receptor (IL-36R) and one accessory protein (IL-1RAcP). The receptor activation follows a two-step mechanism in that the agonist first binds to IL-36R and the resulting binary complex recruits IL-1RAcP. Assembled ternary complex brings together intracellular TIR domains of receptors which activate downstream NF-κB and MAPK signaling. Antagonist IL-36Ra inhibits the signaling by binding to IL-36R and preventing recruitment of IL-1RAcP. Members of IL-36 are normally expressed at low levels. Upon stimulation, they are inducted and act on a variety of cells including epithelial and immune cells. Protease mediated N-terminal processing is needed for cytokine activation. In the skin, the functional role of IL-36 is to contribute to host defense through inflammatory response. However, when dysregulated, IL-36 stimulates keratinocyte and immune cells to enhance the Th17/Th23 axis and induces psoriatic-like skin disorder. Genetic mutations of the antagonist IL-36Ra are associated with occurrence of generalized pustular psoriasis, a rare but life-threatening skin disease. Anti-IL-36 antibodies attenuate IMQ or IL-23 induced skin inflammation in mice, illustrating IL-36's involvement in mouse model of psoriasis. Other organs such as the lungs, the intestine, the joints and the brain also express IL-36 family members upon stimulation. The physiological and pathological roles of IL-36 are less well defined in these organs than in the skin. In this chapter, current progress on IL-36 protein and biology is reviewed with a discussion on investigative tools for this novel target.

IL-36γ Is a Key Regulator of Neutrophil Infiltration in the Vaginal Microenvironment and Limits Neuroinvasion in Genital HSV-2 Infection

HSV-2 is a neurotropic virus that causes a persistent, lifelong infection that increases risk for other sexually transmitted infections. The vaginal epithelium is the first line of defense against HSV-2 and coordinates the immune response through the secretion of immune mediators, including the proinflammatory cytokine IL-36γ. Previously, we showed that IL-36γ treatment promoted transient polymorphonuclear cell infiltration to the vaginal cavity and protected against lethal HSV-2 challenge. In this report, we reveal that IL-36γ specifically induces transient neutrophil infiltration but does not impact monocyte and macrophage recruitment. Using IL-36γ^-/- mice in a lethal HSV-2 challenge model, we show that neutrophil counts are significantly reduced at 1 and 2 d postinfection and that KC-mediated mature neutrophil recruitment is impaired in IL-36γ^-/- mice. Additionally, IL-36γ^-/- mice develop genital disease more rapidly, have significantly reduced survival time, and exhibit an increased incidence of hind limb paralysis that is linked to productive HSV-2 infection in the brain stem. IL-36γ^-/- mice also exhibit a significant delay in clearance of the virus from the vaginal epithelium and a more rapid spread of HSV-2 to the spinal cord, bladder, and colon. We further show that the decreased survival time and increased virus spread observed in IL-36γ^-/- mice are not neutrophil-dependent, suggesting that IL-36γ may function to limit HSV-2 spread in the nervous system. Ultimately, we demonstrate that IL-36γ is a key regulator of neutrophil recruitment in the vaginal microenvironment and may function to limit HSV-2 neuroinvasion.

IL-36 promotes anti-viral immunity by boosting sensitivity to IFN-α/β in IRF1 dependent and independent manners

The functions of the IL-36 cytokines remain poorly understood. We report a previously unrecognized mechanism whereby IL-36 promotes innate antiviral immunity in mouse and human models of herpes simplex virus-1 (HSV-1) infections. HSV-1 actively suppresses production of type I interferon (IFN); our data reveal that IL-36 overcomes this immune evasion strategy by increasing cellular sensitivity to IFN. IL-36β deficient mice display impaired IFN responses and poorly restrict viral replication in skin keratinocytes. In mouse and human keratinocytes IL-36 elicits an antiviral state driven by STAT1 and STAT2 via enhanced expression of IFNAR1 and IFNAR2 subunits of the type I IFN receptor. The degree of IFN regulatory factor 1 (IRF1) involvement is species dependent, with IRF1 playing a more prominent role in human cells. Similar mechanisms are activated by IL-1. Overall, IL-36 acts as an antiviral cytokine by potentiating type I IFN signaling and thereby upholds immune responses to viruses that limit the production of IFNs.

Spondyloarthritis: new insights into clinical aspects, translational immunology and therapeutics

PURPOSE OF REVIEW

The spondyloarthopathies (SpA), which encompass related diseases that were originally viewed as autoimmune, are now known to have a strong innate immune or autoinflammatory initiation phase characterized by disease localization to tissue-specific sites based on the nuances and microanatomy and immunology of those sites. This review covers recent translational advances in the field of SpA.

RECENT FINDINGS

Imaging studies in SpA continue to add support for the pivotal role of enthesitis in disease initiation and expression. Although in its infancy, there is growing evidence for microbiotal intestinal dysbiosis in ankylosing spondylitis and psoriatic arthritis. The role of cytokines beyond tumour necrosis factor (TNF) continues to grow with support for the interleukin (IL)-23/17 axis being key to disease and emergent evidence for the importance of the IL-36 pathway. The treatment of inflammatory bowel disease (IBD) with vedolizumab an α4β7-integrin blocker has been associated with arthritis flares and small molecules with Janus kinase inhibition appear to be as effective as the anti-TNFs. The disparate response of different domains in SpA points towards immunological heterogeneity even within what was considered a homogeneous disease.

SUMMARY

The clinical aspects and translational immunology and therapeutics of SpA continue to evolve and indicate the complexity of diagnosis and treatment of these conditions.

IL-36γ regulates mediators of tissue homeostasis in epithelial cells

IL-36 cytokines are critical regulators of mucosal inflammation and homeostasis. IL-36γ regulates the expression of inflammatory cytokines and antimicrobial proteins by gingival epithelial cells (e.g. TIGK cells). Here, we show that IL-36γ also regulates the expression of matrix metalloproteinase 9 (MMP9) and neutrophil gelatinase-associated lipocalin (NGAL), important mediators of antimicrobial immunity and tissue homeostasis in mucosal epithelia. MMP9 and NGAL were not similarly induced by IL-17 or IL-22, thus indicating the importance of IL-36γ in the regulation of MMP9 and NGAL. Mechanistically, MMP9 and NGAL expression was demonstrated to be induced in an IRAK1- and NF-κB-dependent manner. Furthermore, signaling by p38 MAP kinase may enable their expression to be independently regulated by IL-36γ. The stronger IL-36γ-inducible expression of MMP9 and NGAL in terminally differentiating TIGK cells suggests that control of their expression is associated with the maturation of the gingival epithelium. Although MMP9 and NGAL expression in epithelial cells can also be induced by bacteria, their expression in TIGK cells was not induced by the periodontal pathogen Porphyromonas gingivalis, most likely due to antagonism by the gingipain proteinase virulence factors. This study advances our understanding of how IL-36γ may promote oral mucosal immunity and tissue homeostasis, and how this may be dysregulated by bacterial pathogens.

Overlapping Roles for Interleukin-36 Cytokines in Protective Host Defense against Murine Legionella pneumophila Pneumonia

Legionella pneumophila causes life-threatening pneumonia culminating in acute lung injury. Innate and adaptive cytokines play an important role in host defense against L. pneumophila infection. Interleukin-36 (IL-36) cytokines are recently described members of the larger IL-1 cytokine family known to exert potent inflammatory effects. In this study, we elucidated the role for IL-36 cytokines in experimental pneumonia caused by L. pneumophila Intratracheal (i.t.) administration of L. pneumophila induced the upregulation of both IL-36α and IL-36γ mRNA and protein production in the lung. Compared to the findings for L. pneumophila-infected wild-type (WT) mice, the i.t. administration of L. pneumophila to IL-36 receptor-deficient (IL-36R^-/-) mice resulted in increased mortality, a delay in lung bacterial clearance, increased L. pneumophila dissemination to extrapulmonary organs, and impaired glucose homeostasis. Impaired lung bacterial clearance in IL-36R^-/- mice was associated with a significantly reduced accumulation of inflammatory cells and the decreased production of proinflammatory cytokines and chemokines. Ex vivo, reduced expression of costimulatory molecules and impaired M1 polarization were observed in alveolar macrophages isolated from infected IL-36R^-/- mice compared to macrophages from WT mice. While L. pneumophila-induced mortality in IL-36α- or IL-36γ-deficient mice was not different from that in WT animals, antibody-mediated neutralization of IL-36γ in IL-36α^-/- mice resulted in mortality similar to that observed in IL-36R^-/- mice, indicating redundant and overlapping roles for these cytokines in experimental murine L. pneumophila pneumonia.

Recent advances in the biology of IL-1 family cytokines and their potential roles in development of sepsis

The IL-1 family comprises two anti-inflammatory cytokines (IL-37, IL-38), two receptor antagonists (IL-1ra, IL-36ra), and seven ligand agonists (IL-1α, IL-1β, IL-33, IL-36α, IL-36β, IL-36γ). The members of this family exert pleiotropic effects on intercellular signaling, leading to pro- or anti-inflammatory responses. They initiate potent inflammatory and immune responses by binding to specific receptors in the IL-1 receptor family, and their activities are repressed by naturally occurring inhibitors. Various immune cells produce and are regulated by these crucial molecules, which appear to be involved in the pathogenesis of diverse diseases including cancer as well as inflammatory and autoimmune disorders. Recent decades have seen substantial progress in understanding how the IL-1 family contributes to the development of sepsis. In this review, we will briefly introduce the IL-1 family and discuss its critical role in inflammatory and immune responses. The potential significance of IL-1 members in sepsis will also be explored, together with the clinical implications for treating this dangerous condition.

Endothelial Extracellular Vesicles in Pulmonary Function and Disease

The pulmonary vascular endothelium is involved in the pathogenesis of acute and chronic lung diseases. Endothelial cell (EC)-derived products such as extracellular vesicles (EVs) serve as EC messengers that mediate inflammatory as well as cytoprotective effects. EC-EVs are a broad term, which encompasses exosomes and microvesicles of endothelial origin. EVs are comprised of lipids, nucleic acids, and proteins that reflect not only the cellular origin but also the stimulus that triggered their biogenesis and secretion. This chapter presents an overview of the biology of EC-EVs and summarizes key findings regarding their characteristics, components, and functions. The role of EC-EVs is specifically delineated in pulmonary diseases characterized by endothelial dysfunction, including pulmonary hypertension, acute respiratory distress syndrome and associated conditions, chronic obstructive pulmonary disease, and obstructive sleep apnea.

Regulation of the Peptidoglycan Amidase PGLYRP2 in Epithelial Cells by Interleukin-36γ

Interleukin-36 (IL-36) cytokines are important regulators of mucosal homeostasis and inflammation. We have previously established that oral epithelial cells upregulate IL-36γ expression in response to the bacterial pathogen Porphyromonas gingivalis Here, we have established that IL-36γ can stimulate the gene expression of mechanistically distinct antimicrobial proteins, including the peptidoglycan amidase PGLYRP2, in oral epithelial cells (e.g., TIGK cells). PGLYRP2 gene expression was not stimulated by either IL-17 or IL-22, thus demonstrating selectivity in the regulation of PGLYRP2 by IL-36γ. The IL-36γ-inducible expression of PGLYRP2 was shown to be mediated by IRAK1- and p38 mitogen-activated protein (MAP) kinase-dependent signaling. Furthermore, our finding that IL-36γ-inducible PGLYRP2 expression was reduced in proliferating TIGK cells but increased in terminally differentiating cells suggests that control of PGLYRP2 expression is associated with the maturation of the oral epithelium. PGLYRP2 expression in TIGK cells can also be directly stimulated by oral bacteria. However, the extracellular gingipain proteases (Kgp and RgpA/B) produced by P. gingivalis, which are critical virulence factors, can antagonize PGLYRP2 expression. Thus, the expression of IL-36γ by oral epithelial cells in response to P. gingivalis might enable the subsequent autocrine stimulation of PGLYRP2 expression. In summary, our data identify how IL-36γ may promote oral mucosal homeostasis by regulating PGLYRP2 expression.

IL-36γ induces a transient HSV-2 resistant environment that protects against genital disease and pathogenesis

Herpes simplex virus 2 (HSV-2) causes a persistent, lifelong infection that increases risk for sexually transmitted infection acquisition. Both the lack of a vaccine and the need for chronic suppressive therapies to control infection presents the need to further understand immune mechanisms in response to acute HSV-2 infection. The IL-36 cytokines are recently identified members of the IL-1 family and function as inflammatory mediators at epithelial sites. Here, we first used a well-characterized three-dimensional (3-D) human vaginal epithelial cell (VEC) model to understand the role of IL-36γ in the context of HSV-2 infection. In 3-D VEC, IL-36γ is induced by HSV-2 infection, and pretreatment with exogenous IL-36γ significantly reduced HSV-2 replication. To assess the impact of IL-36γ treatment on HSV-2 disease pathogenesis, we employed a lethal genital infection model. We showed that IL-36γ treatment in mice prior to lethal intravaginal challenge significantly limited vaginal viral replication, delayed disease onset, decreased disease severity, and significantly increased survival. We demonstrated that IL-36γ treatment transiently induced pro-inflammatory cytokines, chemokines, and antimicrobial peptides in murine lower female reproductive tract (FRT) tissue and vaginal lavages. Induction of the chemokines CCL20 and KC in IL-36γ treated mice also corresponded with increased polymorphonuclear (PMN) leukocyte infiltration observed in vaginal smears. Altogether, these studies demonstrate that IL-36γ drives the transient production of immune mediators and promotes PMN recruitment in the vaginal microenvironment that increases resistance to HSV-2 infection and disease. Our data indicate that IL-36γ may participate as a key player in host defense mechanisms against invading pathogens in the FRT.

Regulation and function of interleukin-36 cytokines

The interleukin (IL)-36 cytokines include 3 agonists, IL-36α, IL-36β, and IL-36γ that bind to a common receptor composed of IL-36R and IL-1RAcP to stimulate inflammatory responses. IL-36Ra is a natural antagonist that binds to IL-36R, but does not recruit the co-receptor IL-1RAcP and does not stimulate any intracellular responses. The IL-36 cytokines are expressed predominantly by epithelial cells and act on a number of cells including immune cells, epithelial cells, and fibroblasts. Processing of the N-terminus is required for full agonist or antagonist activity for all IL-36 members. The role of IL-36 has been extensively demonstrated in the skin where it can act on keratinocytes and immune cells to induce a robust inflammatory response that has been implicated in psoriatic disorders. Emerging data also suggest a role for this cytokine family in pulmonary and intestinal physiology and pathology.

IL-36γ Protects against Severe Influenza Infection by Promoting Lung Alveolar Macrophage Survival and Limiting Viral Replication

Although influenza virus infection remains a concerning disease for public health, the roles of individual cytokines during the immune response to influenza infection are not fully understood. We have identified IL-36γ as a key mediator of immune protection during both high- and low-pathogenesis influenza infection. Il36g mRNA is upregulated in the lung following influenza infection, and mice lacking IL-36γ have greatly increased morbidity and mortality upon infection with either H1N1 or H3N2 influenza. The increased severity of influenza infection in IL-36γ-knockout (KO) mice is associated with increased viral titers, higher levels of proinflammatory cytokines early in infection, and more diffuse pathologic conditions late in the disease course. Interestingly, the increased severity of disease in IL-36γ-KO mice correlates with a rapid loss of alveolar macrophages following infection. We find that the alveolar macrophages from naive IL-36γ-KO mice have higher expression of M2-like surface markers compared with wild-type (WT) mice and show increased apoptosis within 24 h of infection. Finally, transfer of WT alveolar macrophages to IL-36γ-KO mice restores protection against lethal influenza challenge to levels observed in WT mice. Together, these data identify a critical role for IL-36γ in immunity against influenza virus and demonstrate the importance of IL-36γ signaling for alveolar macrophage survival during infection.

IL-36 and IL-1/IL-17 Drive Immunity to Oral Candidiasis via Parallel Mechanisms

Protection against microbial infection by the induction of inflammation is a key function of the IL-1 superfamily, including both classical IL-1 and the new IL-36 cytokine families. Candida albicans is a frequent human fungal pathogen causing mucosal infections. Although the initiators and effectors important in protective host responses to C. albicans are well described, the key players in driving these responses remain poorly defined. Recent work has identified a central role played by IL-1 in inducing innate Type-17 immune responses to clear C. albicans infections. Despite this, lack of IL-1 signaling does not result in complete loss of immunity, indicating that there are other factors involved in mediating protection to this fungus. In this study, we identify IL-36 cytokines as a new player in these responses. We show that C. albicans infection of the oral mucosa induces the production of IL-36. As with IL-1α/β, induction of epithelial IL-36 depends on the hypha-associated peptide toxin Candidalysin. Epithelial IL-36 gene expression requires p38-MAPK/c-Fos, NF-κB, and PI3K signaling and is regulated by the MAPK phosphatase MKP1. Oral candidiasis in IL-36R-/- mice shows increased fungal burdens and reduced IL-23 gene expression, indicating a key role played by IL-36 and IL-23 in innate protective responses to this fungus. Strikingly, we observed no impact on gene expression of IL-17 or IL-17-dependent genes, indicating that this protection occurs via an alternative pathway to IL-1-driven immunity. Thus, IL-1 and IL-36 represent parallel epithelial cell-driven protective pathways in immunity to oral C. albicans infection.