Rationale: A crucial step in atherogenesis is the infiltration of the subendothelial space of large arteries by monocytes where they differentiate into macrophages and transform into lipid-loaded foam cells. Macrophages are heterogeneous cells that adapt their response to environmental cytokines. Th1 cytokines promote monocyte differentiation into M1 macrophages, whereas Th2 cytokines trigger an "alternative" M2 phenotype.
Objective: We previously reported the presence of CD68(+) mannose receptor (MR)(+) M2 macrophages in human atherosclerotic plaques. However, the function of these plaque CD68(+)MR(+) macrophages is still unknown.
Methods and results: Histological analysis revealed that CD68(+)MR(+) macrophages locate far from the lipid core of the plaque and contain smaller lipid droplets compared to CD68(+)MR(-) macrophages. Interleukin (IL)-4-polarized CD68(+)MR(+) macrophages display a reduced capacity to handle and efflux cellular cholesterol because of low expression levels of the nuclear receptor liver x receptor (LXR)α and its target genes, ABCA1 and apolipoprotein E, attributable to the high 15-lipoxygenase activity in CD68(+)MR(+) macrophages. By contrast, CD68(+)MR(+) macrophages highly express opsonins and receptors involved in phagocytosis, resulting in high phagocytic activity. In M2 macrophages, peroxisome proliferator-activated receptor (PPAR)γ activation enhances the phagocytic but not the cholesterol trafficking pathways.
Conclusions: These data identify a distinct macrophage subpopulation with a low susceptibility to become foam cells but high phagocytic activity resulting from different regulatory activities of the PPARγ-LXRα pathways.