Polarization of Macrophages toward M2 Phenotype Is Favored by Reduction in iPLA2β (Group VIA Phospholipase A2).

Macrophages are important in innate and adaptive immunity. Macrophage participation in inflammation or tissue repair is directed by various extracellular signals and mediated by multiple intracellular pathways. Activation of group VIA phospholipase A (iPLAβ) causes accumulation of arachidonic acid, lysophospholipids, and eicosanoids that can promote inflammation and pathologic states. We examined the role of iPLAβ in peritoneal macrophage immune function by comparing wild type (WT) and iPLAβ mouse macrophages. Compared with WT, iPLAβ macrophages exhibited reduced proinflammatory M1 markers when classically activated. In contrast, anti-inflammatory M2 markers were elevated under naïve conditions and induced to higher levels by alternative activation in iPLAβ macrophages compared with WT. Induction of eicosanoid (12-lipoxygenase (12-LO) and cyclooxygenase 2 (COX2))- and reactive oxygen species (NADPH oxidase 4 (NOX4))-generating enzymes by classical activation pathways was also blunted in iPLAβ macrophages compared with WT. The effects of inhibitors of iPLAβ, COX2, or 12-LO to reduce M1 polarization were greater than those to enhance M2 polarization. Certain lipids (lysophosphatidylcholine, lysophosphatidic acid, and prostaglandin E) recapitulated M1 phenotype in iPLAβ macrophages, but none tested promoted M2 phenotype. These findings suggest that (a) lipids generated by iPLAβ and subsequently oxidized by cyclooxygenase and 12-LO favor macrophage inflammatory M1 polarization, and (b) the absence of iPLAβ promotes macrophage M2 polarization. Reducing macrophage iPLAβ activity and thereby attenuating macrophage M1 polarization might cause a shift from an inflammatory to a recovery/repair milieu.