The excellent clinical efficacy of anti-interleukin 17A (IL-17A) biologics on psoriasis indicates a crucial pathogenic role of IL-17A in this autoinflammatory skin disease. IL-17A accelerates the proliferation of epidermal keratinocytes. Keratinocytes produce a myriad of antimicrobial peptides and chemokines, such as CXCL1, CXCL2, CXCL8, and CCL20. Antimicrobial peptides enhance skin inflammation. IL-17A is capable of upregulating the production of these chemokines and antimicrobial peptides in keratinocytes. CXCL1, CXCL2, and CXCL8 recruit neutrophils and CCL20 chemoattracts IL-17A-producing CCR6+ immune cells, which further contributes to forming an IL-17A-rich milieu. This feed-forward pathogenic process results in characteristic histopathological features, such as epidermal hyperproliferation, intraepidermal neutrophilic microabscess, and dermal CCR6+ cell infiltration. In this review, we focus on IL-17A and keratinocyte interaction regarding psoriasis pathogenesis.

Plaque psoriasis and pustular psoriasis are overlapping, but distinct, disorders. The therapeutic response to biologics supports the pivotal role of the tumour necrosis alpha (TNF-?)/ interleukin (IL)-23/IL-17/IL-22 axis in the pathogenesis of these disorders. Recently, functional activation of the IL-36 receptor (IL-36R) was discovered to be another driving force in the pathogenesis of psoriasis. This was first highlighted by the discovery that a loss-of-function mutation of the IL-36R antagonist (IL-36Ra) causes pustular psoriasis. Although the TNF-?/IL-23/IL-17/IL-22 axis and the functional activation of IL-36R are fundamentally involved in plaque psoriasis and pustular psoriasis, respectively, the 2 pathways are closely related and mutually reinforced, resulting in full-blown clinical manifestations. This review summarizes current topics on how IL-36 agonists (IL-36?, IL-36?, IL-36?) signal IL-36R, the pathological expression of IL-36 agonists and IL-36Ra in plaque and pustular psoriatic lesions, and the cross-talk between the TNF-?/IL-23/IL-17/IL-22 axis and the functional activation of IL-36R in the epidermal milieu.

Summary Despite sharing interleukin‐4 receptor α (IL‐4Rα) in their signaling cascades, IL‐4 and IL‐13 have different functions in atopic inflammation. IL‐13 preferentially participates in the peripheral tissues because tissue‐resident group 2 innate lymphoid cells produce IL‐13 but not IL‐4. In contrast, lymph node T follicular helper cells express IL‐4 but not IL‐13 to regulate B‐cell immunity. The dominant microenvironment of IL‐13 is evident in the lesional skin of atopic dermatitis (AD). The IL‐13‐rich local milieu causes barrier dysfunction by down‐regulating the OVOL1–filaggrin (FLG) axis and up‐regulating the periostin–IL‐24 axis. Genome‐wide association studies also point to the crucial involvement of the IL‐13, OVOL1 and FLG genes in the pathogenesis of AD. Biologics targeting IL‐13, such as the anti‐IL‐4Rα antibody dupilumab and the anti‐IL‐13 antibody tralokinumab, successfully improve AD lesions and further highlight the importance of IL‐13 in the pathogenesis of AD.

SummaryPsoriasis is characterized by widespread scaly erythematous plaques that cause significant physical and psychological burdens for the affected individuals. Accelerated inflammation driven by the tumour necrosis factor-a/ interleukin-23/interleukin-17 axis is now known to be the major mechanism in the development of psoriasis. In addition, psoriasis has an autoimmune nature that manifests as autoreactive T cells and is comorbid with other autoimmune diseases, such as autoimmune bullous diseases, vitiligo, alopecia and thyroiditis. In this article, we review the recent topics on autoimmunity and autoimmune co-morbidities in psoriasis.

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