Deficiency of IL-22 Contributes to a Chronic Inflammatory Disease: Pathogenetic Mechanisms in Acne Inversa

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Abstract
Overexpression of the T cell cytokine IL-22 is linked to the development of some chronic diseases, but little is known about IL-22 deficiency in humans. As demonstrated in this study, acne inversa (AI; also designated as Hidradenitis suppurativa) lesions show a relative deficiency of IL-22 and IL-20, but not of IL-17A, IL-26, IFN-γ, IL-24, or IL-1β. Moreover, AI lesions had reduced expression of membranous IL-22 and IL-20 receptors and increased expression of the natural IL-22 inhibitor, IL-22 binding protein. AI is a chronic inflammatory skin disease with prevalence up to 4% of the population and in which cutaneous bacterial persistence represents an important pathogenetic factor. Accordingly, we also found a relative deficiency of antimicrobial proteins (AMPs) in AI lesions and a positive correlation between lesional IL-22 and IL-20 versus AMP levels. IL-22, like its tissue cell downstream mediator IL-20, upregulated AMPs in reconstituted human epidermis and was critical for increased AMP levels under inflammatory conditions. The relative IL-22 deficiency in AI was not linked to lesional T cell numbers or Th22/Th1/Th17 subset markers and -inducing cytokines. However, IL-10 was highly expressed in AI lesions and correlated negatively with IL-22 expression. Moreover, IL-10 inhibited IL-22 but not IL-17 production in vitro. The IL-10 overexpression, in turn, was not associated with an elevated presence of regulatory T cells but with the enhanced presence of an IL-10–inducing cytokine. We conclude that IL-22 deficiency may contribute to the pathogenesis of certain chronic disorders as postulated in this paper for AI.