Background: Polyunsaturated fatty acids (PUFAs) play essential roles in mediating inflammation and its resolution. PUFA metabolites generated by the cytochrome P450 (CYP) - soluble epoxide hydrolase (sEH) axis are known to regulate macrophage activation/polarization but little is known about their role in the resolution of inflammation.

Methods: Monocytes were isolated from murine bone marrow or human peripheral blood and differentiated to naïve macrophages (M0). Thereafter cells were polarized using LPS and IFNγ (M1), IL-4 (M2a), or TGFβ1 (M2c). Expression profile analysis was performed by RNA sequencing, RT-qPCR and Western blotting. Phagocytosis of zymosan and oxo-LDL was also assessed.

Results: The expression of sEH was comparable in M0, M1 and M2a macrophages but markedly elevated in M2c polarized cells. The increase in sEH expression elicited by TGF-b relied on the TGFβ receptor ALK5 and the phosphorylation of SMAD2, which was able to bind to the sEH promoter. In macrophages lacking sEH, M2c polarization was incomplete and characterized by lower levels of pro-resolving phagocytosis associated receptors (Tlr2 and Mrc1), as well as higher levels of the pro-inflammatory markers; Nlrp3, IL-1β and TNFα. Fitting with the failure to upregulate phagocytosis associated receptors, the phagocytosis of zymosan and ox-LDL were less efficient in M2c macrophages from sEH-/- mice, which also demonstrated a retarded resolution of inflammation (zymosan-induced peritonitis) in vivo. At the molecular level, the failure to upregulate Tlr2 and Mrc1 expression as well as the maintained expression of NLRP3 in sEH-deficient macrophages were accounted by the decreased expression of PPARγ.

Conclusions: Taken together, our data indicates that sEH expression is required for the effective M2c polarization of macrophages and thus the resolution of inflammation.