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

Opposing Effects of IL-1Ra and IL-36Ra on Innate Immune Response to Pseudomonas aeruginosa Infection in C57BL/6 Mouse Corneas

Pseudomonas aeruginosa keratitis is characterized by severe corneal ulceration and may lead to blindness if not treated properly in a timely manner. Although the roles of the IL-1 subfamily of cytokines are well established, as a newly discovered subfamily, IL-36 cytokine regulation, immunological relevance, and relation with IL-1 cytokines in host defense remain largely unknown. In this study, we showed that P. aeruginosa infection induces the expression of IL-36α and IL-36γ, as well as IL-1β and secreted IL-1Ra (sIL-1Ra), but not IL-36Ra. Downregulation of IL-1Ra increases, whereas downregulation of IL-36Ra decreases the severity of P. aeruginosa keratitis. IL-1R and IL-36Ra downregulation have opposing effects on the expression of IL-1β, sIL-1Ra, IL-36γ, S100A8, and CXCL10 and on the infiltration of innate immune cells. Administration of recombinant IL-1Ra improved, whereas IL-36Ra worsened the outcome of P. aeruginosa keratitis. Local application of IL-36γ stimulated the expression of innate defense molecules S100A9, mouse β-defensin 3, but suppressed IL-1β expression in B6 mouse corneas. IL-36γ diminished the severity of P. aeruginosa keratitis, and its protective effects were abolished in the presence of S100A9 neutralizing Ab and partially affected by CXCL10 and CXCR3 neutralizations. Thus, our data reveal that IL-1Ra and IL-36Ra have opposing effects on the outcome of P. aeruginosa keratitis and suggest that IL-36 agonists may be used as an alternative therapeutic to IL-1β-neutralizing reagents in controlling microbial keratitis and other mucosal infections.

MEK-ERK signaling diametrically controls the stimulation of IL-23p19 and EBI3 expression in epithelial cells by IL-36γ

Interleukin (IL)-36 cytokines are important regulators of mucosal homeostasis and inflammation. We previously established that oral epithelial cells strongly upregulate IL-36γ expression in response to the bacterial pathogen Porphyromonas gingivalis. Here, we have established that IL-36γ stimulates the expression of the IL-12 cytokine family members, IL-23p19 and Epstein-Barr Virus-Induced Gene 3 (EBI3), by oral epithelial cells; their expression was also selectively stimulated by IL-36α. Notably, IL-23p19 and EBI3 expression was not stimulated by P. gingivalis, thus suggesting that their expression by the oral epithelium in response to P. gingivalis is likely to be mediated in an autocrine manner by IL-36γ. The IL-36γ-inducible expression of IL-23p19 and EBI3 was found to be diametrically regulated by the mitogen-activated protein kinase/extracellular signal regulated kinase (MEK)-extracellular signal-regulated kinase 1/2 (ERK1/2) pathway, whereby the activation of MEK-ERK signaling likely functions as a negative feedback mechanism to limit EBI3 expression. Furthermore, epidermal growth factor receptor (EGFR) signaling, which is important for mucosal homeostasis, was demonstrated to modulate, in a MEK-ERK-dependent manner, the stimulation of IL-23p19 and EBI3 expression by IL-36γ. IL-23p19 and EBI3 have recently been shown to heterodimerize to form the novel cytokine IL-39 and promote neutrophil expansion. EBI3 has been shown to also have IL-12 cytokine family independent functions (e.g. mediating IL-6 trans-signaling). Thus, this study not only advances our understanding of how IL-36 cytokines may control mucosal inflammation, but also establishes EGFR signaling as a potentially important modulator of IL-36 cytokine function.

The enigmatic role of IL-38 in inflammatory diseases

IL-38 is the most recently discovered cytokine of the IL-1 family and is considered a potential inhibitor of the IL-1 and Toll-like receptor families. IL-38 exerts anti-inflammatory properties, especially on macrophages, by inhibiting secretion of pro-inflammatory cytokines, leading to reduced T-lymphocyte TH17 maturation. IL-38 has been studied most extensively in the context of chronic inflammatory diseases, particularly arthritis, where it is considered an attractive new drug candidate. IL-38 research has entered a new phase, with the realization that IL-38 is important in the pathophysiology of TH17 dependent-diseases (psoriasis, psoriatic arthritis and ankylosing spondylitis). In this review, we provide a critical evaluation of several controversial issues concerning IL-38 function and regulation. There is effectively contrasting data regarding IL-38: it is produced in conditions such as apoptosis, necrosis or inflammation, but data is lacking regarding IL-38 processing and biological function. Furthermore, the receptor for IL-38 has yet to be identified, although three candidate receptors - IL-1R1, IL-36R and IL-1RAPL1-have been proposed. Future studies will hopefully uncover new aspects of this enigmatic cytokine.

Interleukin-36γ and IL-36 receptor signaling mediate impaired host immunity and lung injury in cytotoxic Pseudomonas aeruginosa pulmonary infection: Role of prostaglandin E2

Pseudomonas aeruginosa is a Gram-negative pathogen that can lead to severe infection associated with lung injury and high mortality. The interleukin (IL)-36 cytokines (IL-36α, IL-36β and IL-36γ) are newly described IL-1 like family cytokines that promote inflammatory response via binding to the IL-36 receptor (IL-36R). Here we investigated the functional role of IL-36 cytokines in the modulating of innate immune response against P. aeruginosa pulmonary infection. The intratracheal administration of flagellated cytotoxic P. aeruginosa (ATCC 19660) upregulated IL-36α and IL-36γ, but not IL-36β, in the lungs. IL-36α and IL-36γ were expressed in pulmonary macrophages (PMs) and alveolar epithelial cells in response to P. aeruginosa in vitro. Mortality after bacterial challenge in IL-36 receptor deficient (IL-36R-/-) mice and IL-36γ deficient (IL-36γ-/-) mice, but not IL-36α deficient mice, was significantly lower than that of wild type mice. Decreased mortality in IL-36R-/- mice and IL-36γ-/- mice was associated with reduction in bacterial burden in the alveolar space, bacterial dissemination, production of inflammatory cytokines and lung injury, without changes in lung leukocyte influx. Interestingly, IL-36γ enhanced the production of prostaglandin E2 (PGE2) during P. aeruginosa infection in vivo and in vitro. Treatment of PMs with recombinant IL-36γ resulted in impaired bacterial killing via PGE2 and its receptor; EP2. P. aeruginosa infected EP2 deficient mice or WT mice treated with a COX-2-specific inhibitor showed decreased bacterial burden and dissemination, but no change in lung injury. Finally, we observed an increase in IL-36γ, but not IL-36α, in the airspace and plasma of patients with P. aeruginosa-induced acute respiratory distress syndrome. Thus, IL-36γ and its receptor signal not only impaired bacterial clearance in a possible PGE2 dependent fashion but also mediated lung injury during P. aeruginosa infection.

Elevated production of IL-36α in chronic hepatitis B virus-infected patients correlates with viral load

Chronic hepatitis B (CHB) infection is a typical inflammatory disease characterized by a dysregulated expression of cytokines, which contributes to the pathogenesis of chronic Hepatitis B virus (HBV) infection. IL-36 cytokines (IL-36α, IL-36β, IL-36γ and IL-36Ra) are important players in infection and immunity. However, their roles in the pathogenesis of chronic HBV infection remain unknown. Here the circulating concentrations of IL-36 cytokines from 50 CHB patients and 30 healthy controls were determined by enzyme-linked immunosorbent assay (ELISA). Sera concentrations of IL-36α were found to be significantly elevated in CHB patients, while the concentrations of IL-36β, IL-36γ and IL-36Ra were not significantly different in comparison to healthy donors. Furthermore, increased IL-36α concentrations correlated positively with HBV-DNA levels in CHB patients. Our study suggests that IL-36α production was up-regulated during CHB infection, which could be directly related to HBV-DNA loads in CHB patients. The immunoregulatory role of IL-36α in the pathogenesis of chronic HBV infection should be further studied.

IL-36γ Induced by the TLR3-SLUG-VDR Axis Promotes Wound Healing via REG3A

IL-36 family members are highly expressed in hyperproliferative keratinocytes and play an important role in the pathogenesis of skin diseases such as psoriasis. However, whether and how IL-36 cytokines are induced to promote wound healing remains unknown. Here we showed that skin injury increased the expression of IL-36γ to promote wound healing. Mechanistically, the expression of IL-36γ was induced by RNAs from damaged cells via the activation of toll-like receptor 3 (TLR3) and TIR-domain-containing adapter-inducing IFN-β (TRIF) followed by the induction of a zinc finger protein SLUG to abrogate the inhibitory effect of vitamin D receptor (VDR) on the promoter of IL-36γ gene. IL-36γ acted back on keratinocytes to induce REG3A, which regulated keratinocyte proliferation and differentiation, thus promoting wound re-epithelialization. These observations show that skin injury increases IL-36γ via the activation of TLR3-SLUG-VDR axis and that IL-36γ induces REG3A to promote wound healing. These findings also provide insights into pathways contributing to wound repair.

Interleukin-36β provides protection against HSV-1 infection, but does not modulate initiation of adaptive immune responses

Interleukin-36 (IL-36) represents three cytokines, IL-36α, IL-36β and IL-36γ, which bind to the same receptor, IL-1RL2; however, their physiological function(s) remain poorly understood. Here, the role of IL-36 in immunity against HSV-1 was examined using the flank skin infection mouse model. Expression analyses revealed increased levels of IL-36α and IL-36β mRNA in infected skin, while constitutive IL-36γ levels remained largely unchanged. In human keratinocytes, IL-36α mRNA was induced by HSV-1, while IL-1β and TNFα increased all three IL-36 mRNAs. The dominant alternative splice variant of human IL-36β mRNA was isoform 2, which is the ortholog of the known mouse IL-36β mRNA. Mice deficient in IL-36β, but not IL-36α or IL-36γ, succumbed more frequently to HSV-1 infection than wild type mice. Furthermore, IL-36β^−/− mice developed larger zosteriform skin lesions along infected neurons. Levels of HSV-1 specific antibodies, CD8⁺ cells and IFNγ-producing CD4⁺ cells were statistically equal in wild type and IL-36β^−/− mice, suggesting similar initiation of adaptive immunity in the two strains. This correlated with the time at which HSV-1 genome and mRNA levels in primary skin lesions started to decline in both wild type and IL-36β^−/− mice. Our data indicate that IL-36β has previously unrecognized functions protective against HSV-1 infection.