Functional characterization of IL-18 receptor subunits, IL-18Rα and IL-18Rβ, and its natural inhibitor, IL-18 binding protein (IL-18BP) in rainbow trout Oncorhynchus mykiss

As an important proinflammation and immunomodulatory cytokine, IL-18 has been reported in several species of fish, but its receptor subunits, IL-18Rα and IL-18Rβ, and its decoy receptor, IL-18BP, have not been functionally characterized in fish. In the present study, IL-18Rα, IL-18Rβ and IL-18BP were cloned from rainbow trout Oncorhynchus mykiss, and they possess common conserved domains with their mammalian orthologues. In tested organs/tissues, IL-18Rα and IL-18Rβ exhibit basal expression levels, and IL-18BP has a pattern of constitutive expression. When transfected with different combinations of chimeric receptors in HEK293T cells, recombinant IL-18 (rIL-18) can induce the activation of NF-κB only when pcDNA3.1-IL-18Rα/IL-1R1 and pcDNA3.1-IL-18Rβ/IL-1RAP were both expressed. On the other hand, recombinant receptors, including rIL-18BP, rIL-18Rα-ECD-Fc and rIL-18Rβ-ECD-Fc can down-regulate significantly the activity of NF-κB, suggesting the participation of IL-18Rα, IL-18Rβ and IL-18BP in rainbow trout IL-18 signal transduction. Co-IP assays indicated that IL-18Rβ may form a complex with MyD88, IRAK4, IRAK1, TRAF6 and TAB2 in HEK293T cells, indicating that IL-18Rβ, in IL-18 signalling pathway, is associated with these signalling molecules. In conclusion, IL-18Rα, IL-18Rβ and IL-18BP in rainbow trout are conserved in function and signalling pathway with their mammalian orthologues.