Characteristics of plaque lipid-associated macrophages and their possible roles in the pathogenesis of atherosclerosis

Purpose of review Recent findings from single-cell transcriptomic studies prompted us to revisit the role of plaque foamy macrophages in the pathogenesis of atherosclerosis. In this review, we compared the gene expression profile of plaque foamy macrophages with those of other disease-associated macrophages and discussed their functions in the pathogenesis of atherosclerosis. Recent findings To understand the phenotypes of macrophages in atherosclerotic aorta, many research groups performed single-cell RNA sequencing analysis and found that there are distinct phenotypic differences among intimal foamy, nonfoamy and adventitial macrophages. Especially, the plaque foamy macrophages express triggering receptor expressed on myeloid cells 2 (TREM2), a key common feature of disease-associated macrophages in Alzheimer's disease, obesity, cirrhosis and nonalcoholic steatohepatitis. These TREM2(+) macrophages seem to be protective against chronic inflammation. As the gene expression profile of plaque foamy macrophages is highly comparable to that of lipid-associated macrophages from obesity, we named the plaque foamy macrophages as plaque lipid-associated macrophages (PLAMs). PLAMs have a high level of gene expression related to phago/endocytosis, lysosome, lipid metabolism and oxidative phosphorylation. Considering the protective function of lipid-associated macrophages against adipose tissue inflammation, PLAMs may suppress atherosclerotic inflammation by removing modified lipids and cell debris in the plaque.