NLRP3 mediates lipid-driven macrophage proliferation in established atherosclerosis

An increased number of macrophages in the atherosclerotic plaque is associated with plaque instability and plaque progression. Lowering systemic cholesterol levels suppresses local macrophage proliferation and leads to plaque regression. However, the pathways regulating macrophage proliferation remain poorly understood. We investigated the cellular processes that underlie lipid-triggered local macrophage proliferation in the atherosclerotic plaque in transgenic mice and in human plaque tissue samples. Macrophages from mice with genetic deficiencies in scavenger receptors Cd36^-/- and Msr1^-/- showed reduced lipid uptake, lower intracellular lipid content, and lower proliferation compared to wild type macrophages. Double knockouts for the cholesterol exporters Abca1 and Abcg1 (MAC-ABC-DKO) showed increased rates of macrophage proliferation and apoptosis. In Cd36^-/-, Msr1^-/-, and MAC-ABC-DKO mixed bone marrow chimeras, no differences in chimerism were observed in blood or aorta after 4 weeks on a high-cholesterol diet. After 12 weeks of atherogenic diet, wild type macrophages predominated in the aorta since they proliferated more than neighboring Cd36^-/- or Msr1^-/- macrophages, and were less apoptotic than ABC-DKO macrophages, respectively. Knockout of NLRP3, but not ASC, Caspase 1 or IL-1 receptor, limited macrophage proliferation; indicating an NLRP3-dependent, but inflammasome-independent, effect. Inhibition of NLRP3 by MCC950 in human carotid artery plaque tissue cultures resulted in the suppression of intra-plaque macrophage proliferation and IL-1β release consistent with murine in vivo data. We identified a novel role for NLRP3 in driving macrophage proliferation in atherosclerotic plaques. NLRP3 inhibition may represent an ideal therapeutic target in atherosclerosis by combining anti-inflammasome and anti-proliferative effects in macrophages.