Inhibition of macrophage proliferation dominates plaque regression in response to cholesterol lowering

Abstract

Statins induce plaque regression characterized by reduced macrophage content in humans, but the underlying mechanisms remain speculative. Studying the translational APOE*3-Leiden.CETP mouse model with a humanized lipoprotein metabolism, we find that systemic cholesterol lowering by oral atorvastatin or dietary restriction inhibits monocyte infiltration, and reverses macrophage accumulation in atherosclerotic plaques. Contrary to current believes, none of (1) reduced monocyte influx (studied by cell fate mapping in thorax-shielded irradiation bone marrow chimeras), (2) enhanced macrophage egress (studied by fluorescent bead labeling and transfer), or (3) atorvastatin accumulation in murine or human plaque (assessed by mass spectrometry) could adequately account for the observed loss in macrophage content in plaques that undergo phenotypic regression. Instead, suppression of local proliferation of macrophages dominates phenotypic plaque regression in response to cholesterol lowering: the lower the levels of serum LDL-cholesterol and lipid contents in murine aortic and human carotid artery plaques, the lower the rates of in situ macrophage proliferation. Our study identifies macrophage proliferation as the predominant turnover determinant and an attractive target for inducing plaque regression.

Myeloid cell-specific Irf5 deficiency stabilizes atherosclerotic plaques in Apoe mice

“…In these mice, Irf5 -deficient and competent monocytes compete directly for accumulation in atherosclerotic lesions, and allow for assessing the intrinsic impact of Irf5 deficiency in monocytes and their progeny, macrophages, in the plaque. Chimerism of Irf5 WT and KO cells was determined by flow cytometric staining for CD45.1 and CD45.2 in the blood and aortic tissue cell suspensions ( Figure 4 A) as previously described [ 3 , 6 ]. In the blood of reconstituted Apoe –/– mice fed a HCD for 4 weeks, 48.05 ± 1.14% of circulating Ly6C high monocytes were CD45.2 + and thus derived from the Irf5 KO bone marrow, while Irf5 WT bone marrow gave rise to CD45.1 + Ly6C high monocytes (51.95 ± 1.14%).…”

“…Expression of Cd36 , an oxLDL uptake mediating scavenger receptor, was reduced in M0-and M1-polarized BMDM in the absence of Irf5 , and cholesterol exporter Abca1 expression was increased, providing a mechanistic explanation for decreased foam cell formation and lipid deposition in the atherosclerotic plaques of myeloid cell-specific Irf5 -KO mice. Notably, lipid uptake stimulates macrophage proliferation in the plaque, and CD36 cell surface expression was relatively reduced in Irf5 −/− aortic macrophages compared to Irf5 +/+ macrophages in our chimeras [ 3 , 6 ]. Whether, in addition, IRF5 can regulate the transcription of cell cycle genes in macrophages, directly, as reported in B cells [ 45 ], is unknown.…”

“…Aortic arches were sectioned longitudinally, and plaques were analyzed along a 2 mm section of the lesser curvature. Sections were stained with Oil-red O (Sigma Aldrich, St. Louis, MO, USA), Masson Trichrome (Sigma Aldrich, St. Louis, MO, USA), anti-CD68 (clone FA-11, BioRad AbD Serotec, Purchheim, Germany), secondary antibodies rabbit-anti rat biotin conjugated (BA-4001) according to the manufacturers’ instructions (see [ 3 ]. Images were recorded with the Axioplan 2 imaging light-/fluorescence microscope (Carl Zeiss MicroImaging GmbH, Göttingen, Germany).…”

“…Despite optimal secondary prevention, CV deaths account for almost one third of all deaths worldwide [ 1 ]. Atherosclerosis is the underlying pathomechanism in most cases, considered a chronic, lipid-driven inflammatory disease [ [2] , [3] , [4] , [5] , [6] ].…”

“…In the developing plaque, monocytes infiltrate the arterial intima and differentiate into macrophages which proliferate locally, driven by oxidized low density lipoprotein (LDL) uptake [ 3 , 4 , 6 ]. Macrophages in the plaque are thought to preserve plasticity, changing their phenotype according to the cytokine composition in the environment [ 7 ].…”