Blockade of IL-18 signaling downregulates homing of effector CD4+ T cells to the gut mucosa and provides protection from experimental IBD

Abstract The immunological complexity of inflammatory bowel disease (IBD) results in persistent intestinal inflammation in millions of patients. As an epithelial cell-derived cytokine essential in the differentiation of CD4+ T helper cells, interleukin 18 (IL-18) is emerging as a therapeutic target for IBD. When naïve CD4+ T cells are stimulated with IL-18, the production of IFNγ and differentiation of Th1 cells increase. IL18R1−/− CD4+ T cells produce less IFNγ and inflammatory cytokines compared to WT. To examine the viability of targeting IL-18 in vivo, colitis was induced in WT, IL18−/−, and IL18R1−/− mice by DSS. In both knockouts, lower proportions of IFNγ- and IL17-producing CD4+ T cells were observed in the colon along with lesser overall severity relative to WT. To further investigate, we examined the interplay of antigen presenting cells and CD4+ T cells in vitro. IL18R1−/− Th1 cells had decreased α4β7 and GPR15 expression when co-cultured with WT BMDCs. Meanwhile, IL18R1−/− Tregs had decreased IFNγ and CCR9 expression when co-cultured with WT BMDCs that were stimulated with LPS and retinoic acid, suggesting that IL-18 may play a role in the differential gut homing of CD4+ T cell subsets. When CD4+ T cells are transferred to Rag2−/−, mice receiving IL18R1−/− T cells possess significantly lower proportions of α4β7+ Th1 cells while the proportion of Tregs in the colon are increased. Treatment of mice with DSS or Mdr1a−/− colitis with an oral IL-18 inhibitor (20 mg/kg) resulted in decreased Th1 cells and neutrophils, lower histological scores and improved disease severity. From converging data, we deduce that inhibition of IL-18 may alleviate exacerbated immune responses associated with IBD, yielding a favorable therapeutic.