Role of IL-18 Signaling in Dendritic Precursor Cells

Mature dendritic cells (DC) regulate T cell responses through cytokine production. Increased concentrations of IL-18 are correlated with gingival sulcular depth and pathogenesis of periodontitis. The KG1 human monocytic cell line is a DC precursor that can be differentiated into functional antigen presenting cells by IL-18. DC activity and maturation is associated with the induction of IFNg. IL-18 binds to IL-18Receptor a/b to secrete IFNg in KG1 cells. Objectives: 1) to dissect the mechanism(s) by which IL-18 induces DC maturation; 2) identify the role of TNFa and TGFb1 in regulating IL-18 signaling and IL-18R expression in KG1 cells; 3) investigate the role of p38MAPK, ERK1/2, and Tbet; 4) determine the effect of IL-18 inhibition by soluble human (sh)IL-18RabFc. Methods: FACS and QPCR was used to look at the effect of TNFa or TGFb1 on IL-18R expression in KG1 cells. Cells were stimulated with rhIL-18 w/wo shIL-18RabFc to determine p38MAPK and ERK1/2 protein phosphorylation by Western blotting. Cells treated with p38MAPK inhibitor (SB203580) or ERK1/2 inhibitor (PD89059) secreted IFNg measured by ELISA. Tbet expression was detected by Western blotting in cells stimulated w/wo TNFa and/or TGFb1 followed by IL-18. KG1 cells were primed with TNFa before stimulation with IL-18, together with increasing doses of shIL-18RaFc, shIL-18RbFc or shIL-18RabFc and IFNg secreted was determined by ELISA. Results: 1) IL-18Ra mRNA and protein expression was induced by TNFa. 2) IL-18 induced rapid phosphorylation of p38MAPK but not ERK1/2. 3) Blocking p38MAPK using specific inhibitors completely abolished IFNg while inhibitors for ERK1/2 had no significant effect. 4) IL-18 induced Tbet expression that was upregulated by TNFa and downregulated by TGFb1. 5) shIL-18RabFc blocked human IL-18 and reduced p38MAPK activation. Conclusions: Characterizing the molecular events leading to IL-18 signaling is important in identifying potential targets for blocking protein-protein interactions to manage inflammatory processes therapeutically.