Leukotriene B4 receptor 2 governs macrophage migration during tissue inflammation
Chronic inflammation is the underlying cause of many diseases, including type 1 diabetes, obesity, and non-alcoholic fatty liver disease. Macrophages are continuously recruited to tissues during chronic inflammation where they exacerbate or resolve the pro-inflammatory environment. Although leukotriene B4 receptor 2 (BLT2) has been characterized as a low affinity receptor to several key eicosanoids and chemoattractants, its precise roles in the setting of inflammation and macrophage function remain incompletely understood. Here we used zebrafish and mouse models to probe the role of BLT2 in macrophage function during inflammation. We detected BLT2 expression in bone marrow derived and peritoneal macrophages of mouse models. Transcriptomic analysis of Ltb4r2-/- and WT macrophages suggested a role for BLT2 in macrophage migration, and studies in vitro confirmed that whereas BLT2 does not mediate macrophage polarization, it is required for chemotactic function, possibly mediated by downstream genes Ccl5 and Lgals3. Using a zebrafish model of tailfin injury, we demonstrated that antisense morpholino-mediated knockdown of blt2a or chemical inhibition of BLT2 signaling impairs macrophage migration. We further replicated these findings in zebrafish models of islet injury and liver inflammation. Moreover, we established the applicability of our zebrafish findings to mammals by showing that macrophages of Ltb4r2-/- mice have defective migration during lipopolysaccharide stimulation in vivo. Collectively, our results demonstrate that BLT2 mediates macrophage migration during inflammation, which implicates it as a potential therapeutic target for inflammatory pathologies. Overall design: Bone marrow-derived macrophages (BMDM) were isolated as described previously and cultured for 7 days in complete medium (RPMI containing 10% FBS, 10 mM HEPES, and 100 U/ml penicillin/ streptomycin) supplemented with 10 ng/ml M-CSF.  On day 7, BMDM were stimulated with 10 ng/ml LPS and 25 ng/mL IFN-? for M1-like polarization and 10 ng/mL IL-4 for M2-like macrophage polarization for 16 h. RNA was isolated using RNAeasy Mini kit (Qiagen) and did RNA sequencing experiments.