Differential regulation of phagocytosis by LPS via Prostaglandin E2 formation and EP4 signaling in primary human macrophages

Phagocytosis is a key mechanism of human host defense engulfing and neutralizing pathogens and cell debris. In addition, phagocytes play an essential role in the innate immune system by releasing cytokines and lipid mediators such as oxylipins. However, the link between oxylipins and phagocytosis in different macrophage phenotypes remains subject of ongoing research.

In order to better understand the impact of the arachidonic acid (ARA) cascade on phagocytosis, we established a phagocytosis assay in primary human ‘inflammatory’ M1- and ‘anti-inflammatory’ M2-like macrophages. Macrophages were differentiated from peripheral blood mononuclear cells (PBMC) using GM-CSF and IFNγ (M1) or M-CSF and IL-4 (M2) representing extremes of macrophage phenotypes. Quantitative targeted proteomics and metabolomics were used to investigate the branches of the ARA cascade in the cells.

LPS increased phagocytosis in M2-like, but not in M1-like macrophages. M1-like macrophages have a higher abundance and activity of cyclooxygenase (COX)-2, particularly after LPS stimulus, compared to M2-like. We show, that the COX product prostaglandin E2 (PGE₂) contributes to the differential effects of LPS on phagocytosis: PGE₂ suppresses phagocytosis via EP4 signaling in the cells. Hence, inhibiting COX, e.g., by non-steroidal anti-inflammatory drugs (NSAID), results in an increase of phagocytosis also in ‘inflammatory’ M1-like macrophages. This supports the well-described anti-inflammatory effects of these drugs and underscores the importance of the link between the COX branch of the ARA cascade and the regulation of phagocytosis in human macrophages.