G2A Signaling Dampens Colitic Inflammation via Production of IFN-γ

Proinflammatory consequences have been described for lysophosphatidylcholine, a lipid product of cellular injury, signaling via the G protein-coupled receptor G2A on myeloid and lymphoid inflammatory cells. This prompted the hypothesis that genetic deletion of G2A would limit intestinal inflammation in a mouse model of colitis induced by dextran sodium sulfate. Surprisingly, G2A(-/-) mice exhibited significantly worsened colitis compared with wild-type mice, as demonstrated by disease activity, colon shortening, histology, and elevated IL-6 and IL-5 in colon tissues. Investigation of inflammatory cells recruited to inflamed G2A(-/-) colons showed significantly more TNF-alpha(+) and Ly6C(hi)MHCII(-) proinflammatory monocytes and eosinophils than in wild-type colons. Both monocytes and eosinophils were pathogenic as their depletion abolished the excess inflammation in G2A(-/-) mice. G2A(-/-) mice also had less IFN-gamma in inflamed colon tissues than wild-type mice. Fewer CD4(+) lymphocytes were recruited to inflamed G2A(-/-) colons, and fewer colonic lymphocytes produced IFN-gamma upon ex vivo stimulation. Administration of IFN-gamma to G2A(-/-) mice during dextran sodium sulfate exposure abolished the excess colitic inflammation and reduced colonic IL-5 and eosinophil numbers to levels seen in wild-type mice. Furthermore, IFN-gamma reduced the numbers of TNF-alpha(+) monocyte and enhanced their maturation from Ly6C(hi)MHCII(-) to Ly6C(int)MHCII(+). Taken together, the data suggest that G2A signaling serves to dampen intestinal inflammation via the production of IFN-gamma, which, in turn, enhances monocyte maturation to a less inflammatory program and ultimately reduces eosinophil-induced injury of colonic tissues.