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

Inflammatory Mediators Suppress FGFR2 Expression in Human Keratinocytes to Promote Inflammation

Fibroblast growth factors (FGFs) are key orchestrators of development, tissue homeostasis and repair. FGF receptor (FGFR) deficiency in mouse keratinocytes causes an inflammatory skin phenotype with similarities to atopic dermatitis, but the human relevance is unclear. Therefore, we generated human keratinocytes with a CRISPR/Cas9-induced knockout of FGFR2. Loss of this receptor promoted the expression of interferon-stimulated genes and pro-inflammatory cytokines under homeostatic conditions and in particular in response to different inflammatory mediators. Expression of FGFR2 itself was strongly downregulated in cultured human keratinocytes exposed to various pro-inflammatory stimuli. This is relevant in vivo, because bioinformatics analysis of bulk and single-cell RNA-seq data showed strongly reduced expression of FGFR2 in lesional skin of atopic dermatitis patients, which likely aggravates the inflammatory phenotype. These results reveal a key function of FGFR2 in human keratinocytes in the suppression of inflammation and suggest a role of FGFR2 downregulation in the pathogenesis of atopic dermatitis and possibly other inflammatory diseases.

Evidence on Hidradenitis Suppurativa as an Autoinflammatory Skin Disease

Hidradenitis suppurativa (HS) is a chronic and debilitating inflammatory skin disease that often exhibits heterogeneity in its clinical presentation, especially in the context of its rare syndromic forms. The pathogenesis of HS results from a complex interplay of genetic predisposition, innate and adaptive immunity dysregulation, smoking, obesity and environmental factors. In the early phase of the disease, the innate immune system is hyperactivated, contributing to tissue damage and triggering the activation and amplification of the adaptive immune response, which plays a pivotal role in the chronic stages of the disease. Recent studies focused on elucidating the importance of innate immunity impairment and autoinflammation in HS and increasing evidence has emerged on the occurrence of the disease in the context of well-known monogenic and polygenic autoinflammatory syndromes (AIDs). This review provides a comprehensive examination of the current scientific background supporting the contribution of autoinflammation to HS etiology, including genetic data, molecular studies and clinical evidence, as well as the association between HS and AIDs. However, further research is needed to shed light on the pathogenic mechanism of this challenging condition and to identify potential perspectives for future therapeutic approaches.

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.

Identification of plasma protein markers of allergic disease risk: a mendelian randomization approach to proteomic analysis

BACKGROUND

While numerous allergy-related biomarkers and targeted treatment strategies have been developed and employed, there are still signifcant limitations and challenges in the early diagnosis and targeted treatment for allegic diseases. Our study aims to identify circulating proteins causally associated with allergic disease-related traits through Mendelian randomization (MR)-based analytical framework.

METHODS

Large-scale cis-MR was employed to estimate the effects of thousands of plasma proteins on five main allergic diseases. Additional analyses including MR Steiger analyzing and Bayesian colocalisation, were performed to test the robustness of the associations; These findings were further validated utilizing meta-analytical methods in the replication analysis. Both proteome- and transcriptome-wide association studies approach was applied, and then, a protein-protein interaction was conducted to examine the interplay between the identified proteins and the targets of existing medications.

RESULTS

Eleven plasma proteins were identified with links to atopic asthma (AA), atopic dermatitis (AD), and allergic rhinitis (AR). Subsequently, these proteins were classified into four distinct target groups, with a focus on tier 1 and 2 targets due to their higher potential to become drug targets. MR analysis and extra validation revealed STAT6 and TNFRSF6B to be Tier 1 and IL1RL2 and IL6R to be Tier 2 proteins with the potential for AA treatment. Two Tier 1 proteins, CRAT and TNFRSF6B, and five Tier 2 proteins, ERBB3, IL6R, MMP12, ICAM1, and IL1RL2, were linked to AD, and three Tier 2 proteins, MANF, STAT6, and TNFSF8, to AR.

CONCLUSION

Eleven Tier 1 and 2 protein targets that are promising drug target candidates were identified for AA, AD, and AR, which influence the development of allergic diseases and expose new diagnostic and therapeutic targets.

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.