In atherogenesis, macrophage foam cell formation is modulated by pathways involving both the uptake and efflux of cholesterol. We recently showed that interleukin-10 (IL-10) modulates lipid metabolism by enhancing both uptake and efflux of cholesterol in macrophages. However, the mechanistic details of these properties in vivo have been unclear. Thus, the purpose of this study was to determine whether expression of IL-10 in macrophages would alter susceptibility to atherosclerosis and whether IL-10 exerts its antiatherosclerotic properties by modulating lipid metabolism in macrophages. We utilized a macrophage-specific retroviral vector that allows long-term in vivo expression of IL-10 in macrophages through transplantation of retrovirally transduced bone marrow cells (BMCs). IL-10 expressed by macrophages derived from transduced BMCs inhibited atherosclerosis in LDLR(-/-) mice by reducing cholesteryl ester accumulation in atherosclerotic sites. Experiments with primary macrophages indicated that macrophage source of IL-10 stimulated both the uptake (by up-regulating scavenger receptors) and efflux of cholesterol (by activating the PPARgamma-LXR-ABCA1/ABCG1 pathway), thereby reducing inflammation and apoptosis in atherosclerosis. These findings indicate that BMC-transduced macrophage IL-10 production can act as a strong antiatherogenic agent, and they highlight a novel antiatherosclerotic therapy using a simple, yet effective, stem cell transduction system that facilitates long-term expression of IL-10 in macrophages.