Functional characterization of IL-18 receptor subunits, IL-18R? and IL-18R?, 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-18R alpha and IL-18R beta, and its decoy receptor, IL-18BP, have not been functionally characterized in fish. In the present study, IL-18R alpha, IL-18R beta 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-18R alpha and IL-18R beta 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-kappa B only when pcDNA3.1-IL-18R alpha/IL-1R1 and pcDNA3.1-IL-18R beta/ IL-1RAP were both expressed. On the other hand, recombinant receptors, including rIL-18BP, rIL-18R alpha-ECD-Fc and rIL-18R beta-ECD-Fc can down-regulate significantly the activity of NF-kappa B, suggesting the participation of IL18R alpha, IL-18R beta and IL-18BP in rainbow trout IL-18 signal transduction. Co-IP assays indicated that IL-18R beta may form a complex with MyD88, IRAK4, IRAK1, TRAF6 and TAB2 in HEK293T cells, indicating that IL-18R beta, in IL 18 signalling pathway, is associated with these signalling molecules. In conclusion, IL-18R alpha, IL-18R beta and IL-18BP in rainbow trout are conserved in function and signalling pathway with their mammalian orthologues.