Oxidation-sensitive cysteines drive IL-38 amyloid formation.

Interleukin (IL)-1 family cytokines are essential for host defense at epithelial barriers. The IL-1 family member IL-33 was recently linked to stress granules (SGs). Formation of SGs and other biomolecular condensates is promoted by proteins containing low-complexity regions (LCRs). Computational analysis predicts LCRs in six of the 11 IL-1 family members. Among these, IL-38 contains a long LCR including two amyloid cores. IL-38 localizes to intracellular granules in keratinocytes under oxidative stress (OS) and forms OS-induced amyloid aggregates in cells and in vitro. Interestingly, soluble and aggregated IL-38 are released from keratinocytes in an exosome-enriched extracellular vesicle fraction. Disulfide-bond mapping, in silico modeling, and mutational analysis suggest that oxidation-sensitive cysteines act as redox switches to alter IL-38 conformation and promote its aggregation. Finally, the presence of IL-38 granules in human epidermis facing environmental OS suggests that oxidation-induced amyloidogenesis, as an intrinsic property of IL-38, supports barrier function.