Atherosclerotic Plaque Healing

Cellular metabolism is increasingly recognized as a controller of immune cell fate and function. MicroRNA-33 (miR-33) regulates cellular lipid metabolism and represses genes involved in cholesterol efflux, HDL biogenesis, and fatty acid oxidation. Here, we determined that miR-33-mediated disruption of the balance of aerobic glycolysis and mitochondrial oxidative phosphorylation instructs macrophage inflammatory polarization and shapes innate and adaptive immune responses. Macrophage-specific Mir33 deletion increased oxidative respiration, enhanced spare respiratory capacity, and induced an M2 macrophage polarization-associated gene profile. Furthermore, miR-33-mediated M2 polarization required miR-33 targeting of the energy sensor AMP-activated protein kinase (AMPK), but not cholesterol efflux. Notably, miR-33 inhibition increased macrophage expression of the retinoic acid-producing enzyme aldehyde dehydrogenase family 1, subfamily A2 (ALDH1A2) and retinal dehydrogenase activity both in vitro and in a mouse model. Consistent with the ability of retinoic acid to foster inducible Tregs, miR-33-depleted macrophages had an enhanced capacity to induce forkhead box P3 (FOXP3) expression in naive CD4(+) T cells. Finally, treatment of hypercholesterolemic mice with miR-33 inhibitors for 8 weeks resulted in accumulation of inflammation-suppressing M2 macrophages and FOXP3(+) Tregs in plaques and reduced atherosclerosis progression. Collectively, these results reveal that miR-33 regulates macrophage inflammation and demonstrate that miR-33 antagonism is atheroprotective, in part, by reducing plaque inflammation by promoting M2 macrophage polarization and Treg induction.

Diversity and plasticity are hallmarks of mononuclear phagocytes, which are reflected in plaque formation and evolution. Different monocyte subsets, which differentially contribute to plaque infiltration and to atherosclerosis complications, have been identified. Similarly, depending on different environmental signals plaque-associated macrophages can express polarized pro- and antiatherogenic programs by influencing lipid metabolism, inflammatory responses, and plaque stability. Thus, a "macrophage balance" plays a major role in the pathogenesis of atherosclerotic plaques and affects evolution and complications of atherosclerosis.

Macrophages, a key cellular component of inflammation, become functionally polarized in a signal- and context-specific manner. Th2 cytokines such as interleukin 4 (IL-4) polarize macrophages to a state of alternative activation that limits inflammation and promotes wound healing. Alternative activation is mediated by a transcriptional program that is influenced by epigenomic modifications, including histone acetylation. Here we report that macrophages lacking histone deacetylase 3 (HDAC3) display a polarization phenotype similar to IL-4-induced alternative activation and, furthermore, are hyperresponsive to IL-4 stimulation. Throughout the macrophage genome, HDAC3 deacetylates histone tails at regulatory regions, leading to repression of many IL-4-regulated genes characteristic of alternative activation. Following exposure to Schistosoma mansoni eggs, a model of Th2 cytokine-mediated disease that is limited by alternative activation, pulmonary inflammation was ameliorated in mice lacking HDAC3 in macrophages. Thus, HDAC3 functions in alternative activation as a brake whose release could be of benefit in the treatment of multiple inflammatory diseases.