Docosahexaenoic acid induces M2 macrophage polarization through peroxisome proliferator-activated receptor γ activation

Aims: Impaired resolution of acute inflammation results in development of chronic inflammatory disorders such as atherosclerosis, asthma and arthritis. Clearance of apoptotic neutrophils by M2 macrophages, the process termed efferocytosis, is critical for complete resolution of inflammation as it prevents secondary necrosis caused by disgorgement of toxic contents from apoptotic cells in the inflamed site. In the present study, we investigated the effect of docosahexaenoic acid (DHA) on efferocytosis.

Main methods: To determine the effect of DHA on efferocytosis, murine macrophage-like RAW264.7 cells were co-incubated with apoptotic Jurkat T cells, and efferocytosis was assessed by flow cytometry. The expression and production of M1 and M2 markers were determined by RT-PCR, ELISA and flow cytometry. To demonstrate the involvement of peroxisome proliferator-activated receptor γ (PPARγ) in DHA-mediated effects, siRNA against PPARγ was utilized. The expression of PPARγ was examined by semiquantitative reverse-transcription polymerase chain reaction (RT-PCR), Western blot analysis and immunocytochemistry. The PPARγ activation was measured by the electrophilic gel shift assay.

Key findings: DHA enhanced the efferocytic ability of RAW264.7 cells, and induced their M2 polarization. Notably, knockdown of PPARγ abolished the stimulatory effect of DHA on M2 polarization as well as efferocytosis. Furthermore, lipopolysaccharide-induced production of pro-inflammatory cytokines was significantly inhibited by DHA, suggesting that DHA alters the macrophage phenotype in favor of M2 while it suppresses M1 polarization.

Significance: These findings indicate that DHA can promote resolution of inflammation by facilitating efferocytosis through M2 macrophage polarization. Therefore, DHA may have a therapeutic potential in the management of inflammatory diseases which are related to impaired resolution of inflammation.