Neutrophil Elastase-mediated proteolysis activates the anti-inflammatory cytokine IL-36 Receptor antagonist

IL-36, IL-37, and IL-38 Cytokines in Skin and Joint Inflammation: A Comprehensive Review of Their Therapeutic Potential

The interleukin (IL)-1 family of cytokines is composed of 11 members, including the most recently discovered IL-36α, β, γ, IL-37, and IL-38. Similar to IL-1, IL-36 cytokines are initiators and amplifiers of inflammation, whereas both IL-37 and IL-38 display anti-inflammatory activities. A few studies have outlined the role played by these cytokines in several inflammatory diseases. For instance, IL-36 agonists seem to be relevant for the pathogenesis of skin psoriasis whereas, despite being expressed within the synovial tissue, their silencing or overexpression do not critically influence the course of arthritis in mice. In this review, we will focus on the state of the art of the molecular features and biological roles of IL-36, IL-37, and IL-38 in representative skin- and joint-related inflammatory diseases, namely psoriasis, rheumatoid arthritis, and psoriatic arthritis. We will then offer an overview of the therapeutic potential of targeting the IL-36 axis in these diseases, either by blocking the proinflammatory agonists or enhancing the physiologic inhibitory feedback on the inflammation mediated by the antagonists IL-37 and IL-38.

Neutrophil exosomes enhance the skin autoinflammation in generalized pustular psoriasis via activating keratinocytes

Generalized pustular psoriasis (GPP) is a rare and severe inflammatory skin disease that can be life-threatening. Gene mutations are found in some cases, but its immune pathogenesis is largely unknown. Here, we observed that the neutrophil:lymphocyte ratio in patients with GPP was higher than that in healthy controls and decreased after effective treatment. Neutrophils isolated from patients with GPP induced higher expressions of inflammatory genes including IL-1β, IL-36G, IL-18, TNF-α, and C-X-C motif chemokine ligands in keratinocytes than normal neutrophils did. Moreover, neutrophils from patients with GPP secreted more exosomes than controls, which were then rapidly internalized by keratinocytes, increasing the expression of these inflammatory molecules via activating NF-κB and MAPK signaling pathways. The proteomic profiles in neutrophil exosomes further characterized functional proteins and identified olfactomedin 4 as the critical differentially expressed protein that mediates the autoimmune inflammatory responses of GPP. These results demonstrate that neutrophil exosomes have an immune-regulatory effect on keratinocytes, which modulates immune cell migration and autoinflammation in GPP.-Shao, S., Fang, H., Zhang, J., Jiang, M., Xue, K., Ma, J., Zhang, J., Lei, J., Zhang, Y., Li, B., Yuan, X., Dang, E., Wang, G. Neutrophil exosomes enhance the skin autoinflammation in generalized pustular psoriasis via activating keratinocytes.

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.

A Guide to IL-1 family cytokines in adjuvanticity

Growing awareness of the multiplicity of roles for the IL-1 family in immune regulation has prompted research exploring these cytokines in the context of vaccine-induced immunity. While tightly regulated, cytokines of the IL-1 family are normally released in response to cellular stress and in combination with other danger-/damage-associated molecular patterns (DAMPs), triggering potent local and systemic immune responses. In the context of infection or autoimmunity, engagement of IL-1 family receptors links robust innate responses to adaptive immunity. Clinical and experimental evidence has revealed that many vaccine adjuvants induce the release of one or multiple IL-1 family cytokines. The coordinated release of IL-1 family members in response to adjuvant-induced damage or cell death may be a determining factor in the transition from local inflammation to the induction of an adaptive response. Here, we analyse the effects of IL-1 family cytokines on innate and adaptive immunity with a particular emphasis on activation of antigen-presenting cells and induction of T cell-mediated immunity, and we address in detail the contribution of these cytokines to the modes of action of vaccine adjuvants including those currently approved for human use.

Identification of small-molecule elastase inhibitors as antagonists of IL-36 cytokine activation

IL‐1 family cytokines act as apical initiators of inflammation in many settings and can promote the production of a battery of inflammatory cytokines, chemokines and other inflammatory mediators in diverse cell types. IL‐36α, IL‐36β and IL‐36γ, which belong to the extended IL‐1 family, have been implicated as key initiators of skin inflammation in psoriasis. IL‐36γ is highly upregulated in lesional skin from psoriatic individuals, and heritable mutations in the natural IL‐36 receptor antagonist result in a severe form of psoriasis. IL‐36 family cytokines are initially expressed as inactive precursors that require proteolytic processing for activation. The neutrophil granule‐derived protease elastase proteolytically processes and activates IL‐36α and IL‐36γ, increasing their biological activity ~ 500‐fold, and also robustly activates IL‐1α and IL‐33 through limited proteolytic processing. Consequently, inhibitors of elastase activity may have potential as anti‐inflammatory agents through antagonizing the activation of multiple IL‐1 family cytokines. Using in silico screening approaches, we have identified small‐molecule inhibitors of elastase that can antagonize activation of IL‐36γ by the latter protease. The compounds reported herein may have utility as lead compounds for the development of inhibitors of elastase‐mediated activation of IL‐36 and other IL‐1 family cytokines in inflammatory conditions, such as psoriasis.

Suppressing IL-36-driven inflammation using peptide pseudosubstrates for neutrophil proteases

Sterile inflammation is initiated by molecules released from necrotic cells, called damage-associated molecular patterns (DAMPs). Members of the extended IL-1 cytokine family are important DAMPs, are typically only released through necrosis, and require limited proteolytic processing for activation. The IL-1 family cytokines, IL-36α, IL-36β, and IL-36γ, are expressed as inactive precursors and have been implicated as key initiators of psoriatic-type skin inflammation. We have recently found that IL-36 family cytokines are proteolytically processed and activated by the neutrophil granule-derived proteases, elastase, and cathepsin G. Inhibitors of IL-36 processing may therefore have utility as anti-inflammatory agents through suppressing activation of the latter cytokines. We have identified peptide-based pseudosubstrates for cathepsin G and elastase, based on optimal substrate cleavage motifs, which can antagonize activation of all three IL-36 family cytokines by the latter proteases. Human psoriatic skin plaques displayed elevated IL-36β processing activity that could be antagonized by peptide pseudosubstrates specific for cathepsin G. Thus, antagonists of neutrophil-derived proteases may have therapeutic potential for blocking activation of IL-36 family cytokines in inflammatory conditions such as psoriasis.

IL-36γ Is a Strong Inducer of IL-23 in Psoriatic Cells and Activates Angiogenesis

The IL-1 family member cytokine IL-36γ is recognised as key mediator in the immunopathology of psoriasis, hallmarks of which involve the activation of both resident and infiltrating inflammatory myeloid cells and aberrant angiogenesis. This research demonstrates a role for IL-36γ in both myeloid activation and angiogenesis. We show that IL-36γ induces the production of psoriasis-associated cytokines from macrophages (IL-23 and TNFα) and that this response is enhanced in macrophages from psoriasis patients. This effect is specific for IL-36γ and could not be mimicked by other IL-1 family cytokines such as IL-1α. IL-36γ was also demonstrated to induce endothelial tube formation and branching, in a VEGF-A-dependent manner. Furthermore, IL-36γ-stimulated macrophages potently activated endothelial cells and led to increased adherence of monocytes, effects that were markedly more pronounced for psoriatic macrophages. Interestingly, regardless of stimulus, psoriasis monocytes showed increased adherence to both the stimulated and unstimulated endothelium when compared with monocytes from healthy individuals. Collectively, these findings show that IL-36γ has the potential to enhance endothelium directed leucocyte infiltration into the skin and strengthen the IL-23/IL-17 pathway adding to the growing evidence of pathogenetic roles for IL-36γ in psoriatic responses. Our findings also point to a cellular response, which could potentially explain cardiovascular comorbidities in psoriasis in the form of endothelial activation and increased monocyte adherence.

Evaluation of a range of mammalian and mosquito cell lines for use in Chikungunya virus research

Chikungunya virus (CHIKV) is becoming an increasing global health issue which has spread across the globe and as far north as southern Europe. There is currently no vaccine or anti-viral treatment available. Although there has been a recent increase in CHIKV research, many of these in vitro studies have used a wide range of cell lines which are not physiologically relevant to CHIKV infection in vivo. In this study, we aimed to evaluate a panel of cell lines to identify a subset that would be both representative of the infectious cycle of CHIKV in vivo, and amenable to in vitro applications such as transfection, luciferase assays, immunofluorescence, western blotting and virus infection. Based on these parameters we selected four mammalian and two mosquito cell lines, and further characterised these as potential tools in CHIKV research.

Mutation in IL36RN impairs the processing and regulatory function of the interleukin-36-receptor antagonist and is associated with DITRA syndrome

The identification of loss-of-function mutations of the IL36RN gene encoding the interleukin-36 receptor antagonist (IL-36Ra) in generalized pustular psoriasis (GPP) emphasized the key role of this pathway in skin innate immunity and systemic inflammation. It has been previously shown in vitro that removal of the N-terminal amino acid IL36Ra (M1) is critical to its biological activity, but the in vivo contribution of this processing remains unknown. We report herein a new homozygous (c4G>T, pV2F) missense IL36RN mutation segregating in a family with three GPP-affected patients. The V2F mutation does not alter IL-36Ra protein expression but was devoid of any antagonist activity. Mass spectrometry showed that the V2F IL-36Ra mutant retains its first N-terminal methionine. These results provide the first in vivo demonstration that removal of N-terminal methionine of native IL-36Ra is a mandatory step to reach optimal antagonist activity and to prevent sustained skin and systemic inflammation in humans.

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.

IL-1 and IL-36 are dominant cytokines in generalized pustular psoriasis

BACKGROUND

Generalized pustular psoriasis (GPP) is a rare, debilitating, and often life-threatening inflammatory disease characterized by episodic infiltration of neutrophils into the skin, pustule development, and systemic inflammation, which can manifest in the presence or absence of chronic plaque psoriasis (PV). Current treatments are unsatisfactory and warrant a better understanding of GPP pathogenesis.

OBJECTIVE

We sought to understand better the disease mechanism of GPP to allow improved targeted therapies.

METHODS

We performed a gene expression study on formalin-fixed paraffin-embedded GPP (n = 28) and PV (n = 12) lesional biopsies and healthy control (n = 20) skin. Differential gene expression was analyzed using gene ontology and enrichment analysis. Gene expression was validated with quantitative RT-PCR and immunohistochemistry, and a potential disease mechanism was investigated using primary human cell culture.

RESULTS

Compared with healthy skin, GPP lesions yielded 479 and PV 854 differentially expressed genes, respectively, with 184 upregulated in both diseases. We detected significant contributions of IL-17A, TNF, IL-1, IL-36, and interferons in both diseases; although GPP lesions furnished higher IL-1 and IL-36 and lower IL-17A and IFN-γ mRNA expression than PV lesions did. We detected prominent IL-36 expression by keratinocytes proximal to neutrophilic pustules, and we show that both neutrophils and neutrophil proteases activate IL-36. Suggesting another mechanism regulating IL-36 activity, the protease inhibitors serpin A1 and A3, which inhibit elastase and cathepsin G, respectively, were upregulated in both diseases and inhibited activation of IL-36.

CONCLUSIONS

Our data indicate sustained activation of IL-1 and IL-36 in GPP, inducing neutrophil chemokine expression, infiltration, and pustule formation, suggesting that the IL-1/IL-36 inflammatory axis is a potent driver of disease pathology in GPP.

Pharmacological Stimulation of Phagocytosis Enhances Amyloid Plaque Clearance; Evidence from a Transgenic Mouse Model of ATTR Neuropathy

Hereditary ATTR V30M amyloidosis is a lethal autosomal dominant sensorimotor and autonomic neuropathy caused by deposition of aberrant transthyretin (TTR). Immunohistochemical examination of sural nerve biopsies in patients with amyloidotic neuropathy show co-aggregation of TTR with several proteins; including apolipoprotein E, serum amyloid P and components of the complement cascade. Complement activation and macrophages are increasingly recognized to play a crucial role in amyloidogenesis at the tissue bed level. In the current study we test the effect of two C5a receptor agonists and a C5a receptor antagonist (PMX53) on disease phenotype in ATTR V30M mice. Our results indicate that amyloid deposition was significantly reduced following treatment with the C5a receptor agonists, while treatment with the antagonist resulted in a significant increase of amyloid load. Administration of the C5a receptor agonists triggered increased recruitment of phagocytic cells resulting in clearance of amyloid deposits.

Correlation between IL36α and IL17 and Activity of the Disease in Selected Autoimmune Blistering Diseases

Dermatitis herpetiformis (DH), bullous pemphigoid (BP), and pemphigus vulgaris (PV) are autoimmune bullous skin conditions with eosinophilic and neutrophilic infiltrations. While cytokines are crucial for the affinity and activation of different leukocyte cells in the inflammation and blister formation, there are no studies concerning a role of IL-36. The goal of the study was to analyze whether interleukin 36 is involved in pathogenesis of DH, BP, and PV. And the second aim of the study was the estimation of correlation between Il-36 and IL-17 and titers of specific antibodies in these diseases. Expression of IL-36 and IL-17 was detected in serum in all DH, BP, and PV samples. Serum levels of IL-36 and IL-17α were statistically higher in DH, BP, and PV groups as compared to the control group. IL-36α levels were statistically higher in DH patients, as compared to patients with PV and BP. Our results showed that IL-36 may be helpful in the diagnostic and monitoring of the activity of the disease. IL 36 may play a relevant role of enrolling eosinophils and neutrophils in DH, BP, and PV and finally provoke tissue injury.

Cathepsin S is the major activator of the psoriasis-associated proinflammatory cytokine IL-36γ

The proinflammatory cytokine IL-36γ is highly expressed in epithelial cells and is a pivotal mediator of epithelial inflammation. In particular, IL-36γ is strongly associated with the inflammatory skin disease psoriasis. As with other IL-1 cytokines, IL-36γ is expressed as an inactive precursor and must be processed by specific proteases to become bioactive. Our aim therefore was to identify protease/s capable of IL-36γ activation and explore the importance of this activation in psoriasis. Using a keratinocyte-based activity assay in conjunction with small-molecule inhibitors and siRNA gene silencing, cathepsin S was identified as the major IL-36γ-activating protease expressed by epithelial cells. Interestingly, cathepsin S activity was strongly up-regulated in samples extracted from psoriasis patients relative to healthy controls. In addition, IL-36γ-Ser18, identified as the main product of cathepsin S-dependent IL-36γ cleavage, induced psoriasiform changes in human skin-equivalent models. Together, these data provide important mechanistic insights into the activation of IL-36γ and highlight that cathepsin S-mediated activation of IL-36γ may be important in the development of numerous IL-36γ-driven pathologies, in addition to psoriasis.

Sharpening Host Defenses during Infection: Proteases Cut to the Chase

The human immune system consists of an intricate network of tightly controlled pathways, where proteases are essential instigators and executioners at multiple levels. Invading microbial pathogens also encode proteases that have evolved to manipulate and dysregulate host proteins, including host proteases during the course of disease. The identification of pathogen proteases as well as their substrates and mechanisms of action have empowered significant developments in therapeutics for infectious diseases. Yet for many pathogens, there remains a great deal to be discovered. Recently, proteomic techniques have been developed that can identify proteolytically processed proteins across the proteome. These “degradomics” approaches can identify human substrates of microbial proteases during infection in vivo and expose the molecular-level changes that occur in the human proteome during infection as an operational network to develop hypotheses for further research as well as new therapeutics. This Perspective Article reviews how proteases are utilized during infection by both the human host and invading bacterial pathogens, including archetypal virulence-associated microbial proteases, such as the Clostridia spp. botulinum and tetanus neurotoxins. We highlight the potential knowledge that degradomics studies of host–pathogen interactions would uncover, as well as how degradomics has been successfully applied in similar contexts, including use with a viral protease. We review how microbial proteases have been targeted in current therapeutic approaches and how microbial proteases have shaped and even contributed to human therapeutics beyond infectious disease. Finally, we discuss how, moving forward, degradomics research can greatly contribute to our understanding of how microbial pathogens cause disease in vivo and lead to the identification of novel substrates in vivo, and the development of improved therapeutics to counter these pathogens.

Neutrophil plasticity enables the development of pathological microenvironments: implications for cystic fibrosis airway disease

INTRODUCTION

The pathological course of several chronic inflammatory diseases, including cystic fibrosis, chronic obstructive pulmonary disease, and rheumatoid arthritis, features an aberrant innate immune response dominated by neutrophils. In cystic fibrosis, neutrophil burden and activity of neutrophil elastase in the extracellular fluid have been identified as strong predictors of lung disease severity.

REVIEW

Although neutrophils are generally considered to be rigid, pre-programmed effector leukocytes, recent studies suggest extensive plasticity in how neutrophil functions unfold upon recruitment to peripheral tissues, and how they choose their ultimate fate. Indeed, upon migration to cystic fibrosis airways, neutrophils display dysregulated lifespan, metabolic activation, and altered effector and regulatory functions, consistent with profound adaptation and phenotypic reprogramming. Licensed by signals present in cystic fibrosis airway microenvironment to survive and develop these novel functions, neutrophils orchestrate, in partnership with the epithelium and with the resident microbiota, the evolution of a pathological microenvironment. This microenvironment is defined by altered proteolytic, redox, and metabolic balance and the presence of stable luminal structures in which neutrophils and microbes coexist.

CONCLUSIONS

The elucidation of molecular mechanisms driving neutrophil plasticity in vivo will open new treatment opportunities designed to modulate, rather than block, the crucial adaptive functions fulfilled by neutrophils. This review aims to outline emerging mechanisms of neutrophil plasticity and their participation in the building of pathological microenvironments in the context of cystic fibrosis and other diseases with similar features.

Are neutrophilic dermatoses autoinflammatory disorders?

Neutrophils constitute essential players in inflammatory responses and are the first line of defence against harmful stimuli. However, dysregulation of neutrophil homeostasis can result in excessive inflammation and subsequent tissue damage. Neutrophilic dermatoses are a spectrum of inflammatory disorders characterized by skin lesions resulting from a neutrophil-rich inflammatory infiltrate in the absence of infection. The exact molecular pathophysiology of neutrophilic dermatoses has long been poorly understood. Interestingly, neutrophil-rich cutaneous inflammation is also a cardinal feature of several autoinflammatory diseases with skin involvement, the latter being caused by aberrant innate immune responses. Overactivation of the innate immune system leading to increased production of interleukin-1 family members and 'sterile' neutrophil-rich cutaneous inflammation are features of both inherited autoinflammatory syndromes with skin involvement and an increasing number of neutrophilic dermatoses. Therefore, we propose that autoinflammation may be a cause of neutrophilic dermatoses.

The emergence of the IL-36 cytokine family as novel targets for inflammatory diseases

The recently discovered interleukin (IL)-36 family of cytokines form part of the broader IL-1 family and are emerging as important mediators of inflammatory disease. The IL-36 subfamily consists of three ligands-IL-36α, IL-36β, and IL-36γ-and the natural antagonist IL-36Ra. The cytokines exert their effects through a specific IL-36 receptor consisting of IL-36R and IL-1RAcP chains. IL-36 cytokines can direct both innate and adaptive immune responses by acting on parenchymal, stromal, and specific immune cell subsets. In humans, inactivating mutations in the gene encoding the IL-36R antagonist, which lead to unregulated IL-36R signaling, lead to an autoinflammatory condition termed deficiency of the IL-36R antagonist, which primarily manifests as a severe form of pustular psoriasis. While such discoveries have prompted deeper mechanistic studies highlighting the important role of IL-36 cytokines in psoriatic skin inflammation, it is now evident that IL-36 cytokines can also play important roles in inflammatory disorders in other organs, such as the gastrointestinal tract and the lungs. Given these emerging roles, strategies to specifically target the expression and activity of the IL-36 family have the potential to uncover novel therapeutic approaches aimed at treating inflammatory diseases in humans.

Estrous Cycle and Gestational Age-Dependent Expression of Members of the Interleukin-36 Subfamily in a Semi-Allogeneic Model of Infected and Non-Infected Murine Pregnancy

The IL-36 subfamily is a recently described group of cytokines with pro-inflammatory behavior, comprising three agonists (α, β, and γ), its receptor (R), and one antagonist (Ra). The expression and function of IL-36 subfamily members in the estrous cycle in healthy and infected pregnancy has not been described. We evaluated mRNA and protein expression of IL-36 family members during the estrous cycle, implantation, fetal development, and post-labor periods in a model of allogenic pregnancy in mice. We also explored the ability of Listeria monocytogenes to modulate the expression of IL-36 subfamily members during pregnancy. Expression of IL-36 subfamily members showed different expression during the estrous cycle and pregnancy but was induced at estrous, 16.5 days post coitum (dpc), 18.5 dpc, and labor. IL-36 subfamily members showed a characteristic distribution in the glandular epithelium, perimetrium, myometrium, and stratum vasculare. Infection with L. monocytogenes during pregnancy induced strong production of IL-36 subfamily members, an observation that correlated with an increasing prevalence of fetal loss. In conclusion, IL-36 agonists showed specific patterns of mRNA and protein expression that might suggest functional specialization or specific target cells. Infection with L. monocytogenes during pregnancy induced strong production of IL-36 subfamily members.

NET Confusion

Neutrophils are arguably the most important white blood cell for defense against bacterial and fungal infections. These leukocytes are produced in high numbers on a daily basis in humans and are recruited rapidly to injured/infected tissues. Phagocytosis and subsequent intraphagosomal killing and digestion of microbes have historically been the accepted means by which neutrophils carry out their role in innate host defense. Indeed, neutrophils contain and produce numerous cytotoxic molecules, including antimicrobial peptides, proteases, and reactive oxygen species, that are highly effective at killing the vast majority of ingested microbes. On the other hand, it is these characteristics - high numbers and toxicity - that endow neutrophils with the potential to injure and destroy host tissues. This potential is borne out by many inflammatory processes and diseases. Therefore, it is not surprising that host mechanisms exist to control virtually all steps in the neutrophil activation process and to prevent unintended neutrophil activation and/or lysis during the resolution of inflammatory responses or during steady-state turnover. The notion that neutrophil extracellular traps (NETs) form by cytolysis as a standard host defense mechanism seems inconsistent with these aforementioned neutrophil "containment" processes. It is with this caveat in mind that we provide perspective on the role of NETs in human host defense and disease.