Increased Cytokine Production in Interleukin-18 Receptor α-deficient Cells Is Associated with Dysregulation of Suppressors of Cytokine Signaling

Since interleukin (IL)-18 is a proinflammatory cytokine, mice lacking IL-18 or its ligand-binding receptor (IL-18R) should exhibit decreased cytokine and chemokine production. Indeed, production of IL-1 alpha, IL-6, and MIP-1 alpha was reduced in IL-18 knock-out (ko) mouse embryonic fibroblast (MEF)-like cells. Unexpectedly, we observed a paradoxical 10-fold increase in IL-1 beta-induced IL-6 production in MEF cells from mice deficient in the IL-18R alpha-chain (IL-18R alpha) compared with wild type MEF. Similar increases were observed for IL-1 alpha, MIP-1 alpha, and prostaglandin E-2. Likewise, coincubation with a specific IL-18R alpha-blocking antibody augmented IL-1 beta-induced cytokines in wild type and IL-18 ko MEF. Stable lines of IL-18R alpha-depleted human A549 cells were generated using shRNA, resulting in an increase of IL-1 beta-induced IL-1 alpha, IL-6, and IL-8 compared to scrambled small hairpin RNA. In addition, we silenced IL-18R alpha with small interfering RNA in primary human blood cells and observed up to 4-fold increases in the secretion of lipopolysaccharide- and IL-12/IL-18-induced IL-1 beta, IL-6, interferon-gamma, and CD40L. Mechanistically, despite increases in Stat1 and IL-6, induction of SOCS1 and -3 (suppressor of cytokine signaling 1 and 3) was markedly reduced in the absence of IL-18R alpha. Consistent with these observations, activation of the p38 alpha/beta and ERK1/2 MAPKs and of protein kinase B/Akt increased in IL-18R alpha ko MEF, whereas the negative feedback kinase MSK2 was more active in IL-18 ko cells. These data reveal a role for SOCS1 and -3 in the seemingly paradoxical hyperresponsive state in cells deficient in IL-18R alpha, supporting the concept that IL-18R alpha participates in both pro-and anti-inflammatory responses and that an endogenous ligand engages IL-18R alpha to deliver an inhibitory signal.