Macrophage Activation in the Development and Progression of Atherosclerosis

Napoleon Busbuso

Introduction. Cardiovascular disease originating from atherosclerosis is the leading cause of death worldwide1. In the setting of dyslipidemia, LDL accumulates within the intima of arteries, and subsequent oxidation of LDL by free radicals incites a persistent inflammatory reaction that ultimately produces an atherosclerotic plaque2. Macrophages appear in the intima to clear the oxidized LDL and, with time, convert to lipid-laden foam cells that recruit more immune cells and further develop the plaque2. Characterizing and ultimately preventing the persistent epigenetic changes that underlie this conversion represents a potential avenue for slowing the progression of atherosclerosis3. Additionally, the NLRP3 inflammasome of macrophages is hypothesized to sense oxidized LDL and induce inflammation through production of IL-1ß4,5. Methods. To assess the ability of oxidized LDL to induce persistent changes in proinflammatory gene expression in macrophages, isolated human monocytes were incubated with oxidized LDL for 24 hours with subsequent 24-hour washout and were then stimulated to differentiate into macrophages with LPS3. Methylation status of promoters of proinflammatory genes were analyzed by chromatin immunoprecipitation3. In a similar protocol, expression of proinflammatory cytokines was quantified by mRNA3. To determine the role of the NLRP3 inflammasome in plaque formation, mice lacking both NLRP3 and the LDL receptor were generated4. Atherosclerotic plaques within the aorta were then assessed after 8-weeks of feeding with western diet, which is 21.2% fat and 48.5% carbohydrate in composition4. Results. Promoters of proinflammatory genes showed increased methylation in isolated human monocytes and were largely consistent with increased expression when exposed to oxidized LDL prior to stimulation with LPS3. mRNA of proinflammatory cytokines were elevated in a similar protocol3. Mice lacking both NLRP3 and the LDL receptor showed reduced atherogenesis within the aorta following 8-week western diet feeding compared to mice lacking only the LDL receptor4. Conclusions. The data suggests that blocking the epigenetic changes in macrophages that result in persistently elevated production of inflammatory cytokines would slow the development of a plaque3. In addition, the NLRP3 inflammasome and subsequent production of IL-1ß appears to be the bridge between oxidized LDL and the persistent inflammation seen in atherosclerosis4. Further work to target delivery of therapeutic agents (histone methyltransferase inhibitors and IL1-RAs) only to desired cells is needed to bring these approaches to slowing the course of atherosclerosis closer to use in the clinic4,5.

  1. Wolf D, Ley K. Immunity and Inflammation in Atherosclerosis. Circulation Research. 2019;124(2):315-327. doi:10.1161/circresaha.118.313591
  2. Rhoads JP, Major AS. How Oxidized Low-Density Lipoprotein Activates Inflammatory Responses. Critical Reviews in Immunology. 2018;38(4):333-342. doi:10.1615/critrevimmunol.2018026483
  3. Bekkering S, Quintin J, Joosten LA, Meer JWVD, Netea MG, Riksen NP. Oxidized Low-Density Lipoprotein Induces Long-Term Proinflammatory Cytokine Production and Foam Cell Formation via Epigenetic Reprogramming of Monocytes. Arteriosclerosis, Thrombosis, and Vascular Biology. 2014;34(8):1731-1738. doi:10.1161/atvbaha.114.303887.
  4. Christ A, Günther P, Lauterbach MA, et al. Western Diet Triggers NLRP3-Dependent Innate Immune Reprogramming. Cell. 2018;172(1-2). doi:10.1016/j.cell.2017.12.013.
  5. Sheedy FJ, Grebe A, Rayner KJ, et al. CD36 coordinates NLRP3 inflammasome activation by facilitating intracellular nucleation of soluble ligands into particulate ligands in sterile inflammation. Nature Immunology. 2013;14(8):812-820. doi:10.1038/ni.2639.