Transcriptional regulation and function of GPR18 in macrophage subtype specification

Resolution of inflammation is mediated in part by agonists that engage immune cells to dampen inflammatory signaling, promote phagocytic functions, and facilitate tissue repair. G protein-coupled receptor 18 (GPR18) has emerged as an important proresolving receptor that is activated by endogenous mediators including Resolvin D2. However, very little is known about how GPR18 expression is controlled in immune cells, and its cell-autonomous roles during resolution of inflammation remain of interest. Here, we elucidated transcriptional and epigenetic regulation of Gpr18 in macrophages (MFs). We identified binding of monocyte/macrophage lineage-determining transcription factors (LDTFs) including PU.1, IRF8, and RUNX1 at the Gpr18 locus in the basal state in murine bone marrow-derived macrophages (BMDM). We found that Gpr18 is highly induced in BMDM by inflammatory stimuli including interferon gamma (IFNγ) and ligands for multiple toll-like receptors. Motif analysis predicted enrichment for LDTFs and signal-dependent TFs including nuclear factor κB (NFκB) and interferon regulatory factors (IRF) at the Gpr18 locus. Stimulation of BMDM with lipopolysaccharide (LPS) or IFNγ increased the binding of NFκB p65 and IRF1 at the Gpr18 locus, which played a causal role in the induction of Gpr18 by LPS or IFNγ, respectively. Mice with myeloid deficiency of Gpr18 had increased tissue swelling and accumulation of "inflammatory" Ly6C^hi monocytes/MFs during contact hypersensitivity, which was recapitulated during acute peritonitis and led to altered transcriptomes of resolution-phase MFs. These results demonstrate that inflammatory signaling primes MFs for resolution in part by inducing Gpr18, which plays a cell-autonomous role to regulate MF phenotypic transitions.