Phagocytosis is differentially regulated by LPS in M1- and M2-like macrophages via PGE ₂ formation and EP4 signaling

Phagocytosis is a key process in human innate immune response. Human macrophages are important phagocytes engulfing and neutralizing pathogens and cell debris. In addition, they modulate the inflammatory process by releasing cytokines and lipid mediators. However, the link between oxylipins and phagocytosis in different macrophage phenotypes remains poorly understood.

In order to better understand the link between phagocytosis and the arachidonic acid (ARA) cascade, we established a phagocytosis assay in primary human ‘inflammatory’ M1- and ‘anti-inflammatory’ M2-like macrophages from PBMCs, representing extremes of macrophage phenotypes. The branches of the ARA cascade were investigated by quantitative targeted proteomics and metabolomics.

M1-like macrophages show a higher abundance of cyclooxygenase (COX)-2 and its products particularly after LPS stimulus compared to M2-like macrophages. LPS increased phagocytosis in M2-like, but not in M1-like macrophages. We demonstrate that the COX product PGE ₂ modulates the differential effects of LPS on phagocytosis: Via the EP4 receptor PGE ₂ signaling suppresses phagocytosis in primary human macrophages. Thus, blockage of COX, e.g. by NSAID, leads to an increase of phagocytosis also in ‘inflammatory’ M1-like macrophages and may shift the macrophages towards a more pro-resolving phenotype. This supports the well-described anti-inflammatory effects of these drugs.