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

Abstract

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.T he interleukin (IL)-1 family cytokines are fundamental regulators of the innate immune system and orchestrate multiple inflammatory responses (1, 2). IL-1 cytokines are produced rapidly following infection or injury and are capable of potently inducing a range of beneficial proinflammatory processes, including additional cytokine expression, antigen-presenting cell migration, and leukocyte activation and infiltration (3-5). The aberrant expression and regulation of IL-1 cytokines is associated with a broad range of immuopathologies, ranging from autoinflammatory to autoimmune disorders (6-8). Therefore, a greater insight into the regulation and function of IL-1 cytokines is not only of academic interest but also of significant therapeutic importance.IL-36α, IL-36β, and IL-36γ are agonistic cytokines and the most recently discovered of the IL-1 family (9). Interestingly, there is growing evidence to suggest that these cytokines are important for the development of several inflammatory disorders, including psoriasis (10). In psoriatic lesions, the IL-36 cytokines have been shown to be among the most specific and highly up-regulated mRNAs relative to other inflammatory skin diseases and healthy controls (11-13). Moreover, hypomorphic mutations in the IL-36 receptor antagonist (IL-36Ra) cause the severe and potentially lethal subtype of psoriasis called pustular psoriasis in a number of cohorts (14, 15). Mouse models further support these observations, showing that IL-36 overexpression in keratinocytes results in a transient inflammatory skin condition resembling psoriasis (16). In addition, IL-36 receptor-deficient mice have been found to be resistant to Imiquimod-induced psoriasiform dermatitis (17). Interestingly, recent studies have also demonstrated a role for the IL-36 recept...

Interleukin-36 cytokines may overcome microbial immune evasion strategies that inhibit interleukin-1 family signaling

“…3B) do not resemble caspase-1 sites (73,74). In vitro, neutrophil-derived cathepsin G, elastase, and proteinase-3 can truncate full-length recombinant IL-36 and IL-36Ra proteins (74)(75)(76); however, whether this processing is activating or inactivating is currently controversial (74)(75)(76). Note that several of these proteases have been reported to also cleave IL-1b (Fig.…”

301 The psoriasis-associated IL-17A induces and cooperates with IL-36 cytokines to control keratinocyte differentiation and function

“…This indicated that sufficient IL-17A was still present in these supernatants that stimulated gene expression. As mentioned above, IL-36 cytokines require processing to gain biological activity [34][35][36] . IL-36 can be processed and activated by proteases derived from neutrophils 25,36 .…”

“…3e and Supplementary Fig. S3a) 25,[34][35][36]53 . Together, these observations suggest that IL-36 and its processing factors are likely to be important in psoriasis.…”

“…It was noted that removing N-terminal amino acids increased their signaling capacity 35 . New results describe neutrophil-derived proteases as well as cathespsin S, a lysosomal protease, as IL-36 cytokine processing enzymes 25,34,36 . In both situations the cleavage and activation of IL-36 appears to occur extracellularly, unlike other IL-1 family members that are primarily processed intracellularly, prior to secretion by non-conventional mechanisms 53 .…”

“…Importantly and similar to other IL-1 family cytokines, the different IL-36 proteins need to be processed to become biologically active. This occurs by truncating the N-termini, removing a few amino acids in response to several different proteases [34][35][36] . Thus, in addition to IL-17, IL-36 cytokines are also thought to participate in the inflammatory progression of psoriasis.…”