Caspase-mediated interleukin-18 maturation promotes membrane-associated unconventional secretion and signal transduction in a teleost fish

Interleukin-18 (IL-18) is an important proinflammatory cytokine essential for immune modulation. Unlike most cytokines, it is synthesized as an inactive precursor, with its maturation and secretion being critical for its functionality. As an evolutionarily ancient cytokine, it can be traced back to teleosts, but not zebrafish. However, the regulatory mechanism of IL-18 in early vertebrates remains largely elusive. The present study reports the maturation and secretion of IL-18 along with its role in signal transduction in a teleost fish half-smooth tongue sole (Cynoglossus semilaevis). We found that pro-IL-18 was cleaved by caspase-1, caspase-3/7, and caspase-6 at different N-terminal sites, generating three forms of the mature IL-18. In contrast to the negatively charged pro-IL-18, the positively charged mature IL-18 is highly enriched in the cytoplasmic membrane. It is enclosed within membrane-associated microvesicles, which facilitate secretion to the extracellular milieu. Once secreted, it binds specifically to the IL-18 receptor α (IL-18Rα) present on the cell surface and recruits IL-18Rβ to form a functionally active heterotrimeric complex. Bacterial challenge induces the maturation and secretion of IL-18, which upregulates the expression of the proinflammatory cytokines. Activation of the IL-18-mediated signaling pathway enhances antimicrobial immunity and reduces infection-induced mortality. Our findings collectively reveal a unique mechanism of IL-18 maturation, unconventional secretion, and immune regulation in a teleost fish, which provides new insights into the role of IL-18-based signaling in immune regulation.