Studies on functions of macrophage-derived foam cells in atherosclerosis

Atherosclerosis is the major cause of death in western countries and is also becoming a serious health care burden in developing countries. The atherosclerotic lesions are characterised by the accumulation of the macrophage-derived foam cells, lymphocytes, smooth muscle cells, and extracellular matrix as wells the deposition of intra- and extra-cellular lipids. Several aspects of atherosclerosis may be modulated by macrophages that are resident in the lesions. The macrophages do not only take up lipids in the lesions, transforming themselves into foam cells, they also have the capacity to secrete a large number of products, such as cytokines, lipases, proteolytic enzymes and free radicals. Thus, the functional state of the macrophages in the lesions may be critical for the development of atherosclerosis. In this study, some functions of the macrophages, that may be relevant to the initiation and progression of the lesions, were studied. The way in which the lipids and lipid oxidation products may regulate these functions, was also explored.In this study, it was demonstrated by immunohistochemical staining, that macrophages in human atherosclerotic lesions are mainly located at the shoulder region and around the necrotic core. Most of these cells contain lipids. Utilizing a new gating technique in flow cytometry, it was found that the expression of several functional epitopes, on macrophages from human atherosclerotic lesions, was lower than that on the cultured human monocytes and monocyte-derived macrophages. These epitopes include CD36, a receptor which is supposed to be involved in mediating the uptake of oxidised LDL and apoptotic cells. There was also a low expression of the cell-adhesion molecule VLA-4. On the other hand, most of the cells expressed high levels of HLA-DR, indicating the cells capacity to participate in the local immune response. Furthermore, high levels of IL-8 were found in the macrophages isolated from human atherosclerotic lesions. IL-8 is a potent chemoattractant for both lymphocytes and smooth muscle cells. The macrophages could still respond to lipopolysaccharide to produce more IL-8. It was also found in this study, that monocyte-derived macrophages, in vitro, respond to oxysterols by enhancing the production of IL-8. Oxysterols are generated during oxidative modification of LDL, which is a modification that most likely occurs in vivo in the atherosclerotic lesions. Another product, which is generated during oxidative modification of LDL, is lysophosphatidylcholine (lysoPC). It was found in this study that lysoPC could augment the production IL-1( in human macrophages in culture. IL-1( is a pro-inflammatory cytokine that, among other things, may stimulate the smooth muscle cell proliferation and induce the expression of cell adhesion molecules on the endothelium. Thus, we have found that two of the lipid oxidation products in modified LDL may affect some important functions of the macrophages. Lipid accumulation or lipid oxidation products may also modify other functions of macrophages. When human monocyte-derived macrophages were loaded with lipids in vitro, they showed a decreased capacity to take up apoptotic cells. This may be one of the explanations for the presence of abundant apoptotic cells in human atherosclerotic lesions.Taking together, the results of this study confirms the heterogeneity of the macrophage population in atherosclerotic lesions, in terms of cellular lipid content, morphology and functional aspects. The lipid oxidation products may be important modulators of macrophage function, as they can induce the production of several inflammatory cytokines in macrophage. Moreover, the intracellular lipid accumulation may inhibit the capacity of the macrophages to take up apoptotic cells. Lipids and the lipid oxidation products may thus be central for several aspects of the macrophage functions in atherosclerotic lesions, and may therefore affect the initiation and progression of the lesions.Key words: Atherosclerosis, foam cells, macrophages, cytokines, IL-8, IL-1(, cell isolation, oxysterols, lysophosphatidylcholine, flow cytometry, apoptosis