Lipid‐Free Apolipoprotein A‐I Reduces Progression of Atherosclerosis by Mobilizing Microdomain Cholesterol and Attenuating the Number of CD131 Expressing Cells: Monitoring Cholesterol Homeostasis Using the Cellular Ester to Total Cholesterol Ratio

BACKGROUND: Atherosclerosis is a chronic inflammatory disorder whose development is inversely correlated with high‐density lipoprotein concentration. Current therapies involve pharmaceuticals that significantly elevate plasma high‐density lipoprotein cholesterol concentrations. Our studies were conducted to investigate the effects of low‐dose lipid‐free apolipoprotein A‐I (apoA‐I) on chronic inflammation. The aims of these studies were to determine how subcutaneously injected lipid‐free apoA‐I reduces accumulation of lipid and immune cells within the aortic root of hypercholesterolemic mice without sustained elevations in plasma high‐density lipoprotein cholesterol concentrations. METHODS AND RESULTS: Ldlr (−/−) and Ldlr (−/−) apoA‐I (−/−) mice were fed a Western diet for a total of 12 weeks. After 6 weeks, a subset of mice from each group received subcutaneous injections of 200 μg of lipid‐free human apoA‐I 3 times a week, while the other subset received 200 μg of albumin, as a control. Mice treated with lipid‐free apoA‐I showed a decrease in cholesterol deposition and immune cell retention in the aortic root compared with albumin‐treated mice, regardless of genotype. This reduction in atherosclerosis appeared to be directly related to a decrease in the number of CD131 expressing cells and the esterified cholesterol to total cholesterol content in several immune cell compartments. In addition, apoA‐I treatment altered microdomain cholesterol composition that shifted CD131, the common β subunit of the interleukin 3 receptor, from lipid raft to nonraft fractions of the plasma membrane. CONCLUSIONS: ApoA‐I treatment reduced lipid and immune cell accumulation within the aortic root by systemically reducing microdomain cholesterol content in immune cells. These data suggest that lipid‐free apoA‐I mediates beneficial effects through attenuation of immune cell lipid raft cholesterol content, which affects numerous types of signal transduction pathways that rely on microdomain integrity for assembly and activation.