Activated macrophages displaying an M1 phenotype have a reduced capacity to efflux cholesterol.

Macrophages are phagocytic cells that play an essential role in the clearance of cellular debris. Microglia, specialized phagocytic cells of the brain, likewise play a critical role in the clearance of cellular debris, particularly amyloid beta, which is involved in the pathogenesis of Alzheimer's disease. However, activated macrocytic cells displaying a pro-inflammatory or M1 phenotype are associated with neuroinflammation and have a reduced capacity to phagocytose. Studies have shown that the ability of microglia to remove amyloid beta is influenced by their cellular cholesterol clearance capacity. However, it is not known whether activated macrocytic cells have inherently reduced cholesterol efflux capacity (CEC).In this study the CEC of human THP-1 monocytes differentiated into macrophages using phorbol-12-myristate-13-acetate (PMA), achieving a resting state, were polarized to an M1 activated state using lipopolysaccharide (LPS) and interferon gamma (IFNγ) and an M2 alternatively activated state using interleukin 4 (IL4) and macrophage colony-stimulating factor (M-CSF). The capacity to remove fluorescently-labeled cholesterol was compared across all three macrophage states at 3 time points, in the presence and absence of the cholesterol acceptor high-density lipoprotein (HDL). HDL particles were isolated by ultracentrifugation followed by size exclusion chromatography and added at a concentration of 0.1 mg/mL. Macrophage phenotype was confirmed by qPCR using primers to amplify TNF-α, IL-1β, IL-6, CD206, ARG1 and IGF1.Compared to 12 and 72 hours, 36 hours of cytokine treatment resulted in the largest reduction in CEC in M1 and M2 macrophages compared to resting macrophages. The CEC of M1 and M2 macrophages was 50% and 60% lower, respectively, than the CEC of resting macrophages. This reduction was apparent in both the presence and absence of HDL acceptor.These findings suggest that in conditions of macrocytic activation, such as Alzheimer's disease, macrocytic cells involved in the clearance of cellular debris have a diminished capacity to efflux cholesterol. Whether this reduction of CEC in activated versus resting macrophages is also observed in activated versus resting microglia, and whether it is related to reduced phagocytic capacity and/or involved in the pathogenesis of Alzheimer's disease needs to be determined in future studies.