Human atherosclerotic plaques are heterogeneous tissues containing a number of different cell types, including macrophages, smooth muscle, endothelial and other undefined mesenchymal-appearing cells. Significant numbers of macrophages are found in human atherosclerotic plaques and have been postulated to be a major source of growth factor production during atherogenesis. In vitro evidence suggested that macrophages synthesize PDGF and might therefore contribute to the growth of the vessel wall in atherosclerosis. However, examination of PDGF synthesis in human atheroma by in situ hybridization revealed that while smooth muscle, mesenchymal, and endothelial cells synthesize this growth factor macrophages did not. Our inability to detect PDGF mRNA in macrophages was not due to any problems with hybridization to this cell type. In situ hybridization studies on human atherosclerotic plaques have demonstrated that plaque macrophages contain many different mRNAs other than PDGF including tissue factor, factor XIII, apoprotein E, transforming growth factor beta, and tumor necrosis factor. Recent studies have indicated that macrophages may be a major source as well of another group of inflammatory cytokines which are members of the RANTES/SIS cytokine family. In situ hybridization studies on human carotid endarterectomy specimens using probes specific for the inflammatory cytokines RANTES, LD78, HIMAP, and MCP-1 revealed numerous cells containing the mRNAs encoding for these proteins (5%, 13%, 8%, and 16% of plaque cells respectively). This is in contrast to generally low level expression found in normal human arteries (<1% of normal medial cells contain these mRNAs). Cells expressing these cytokines were often found associated with inflammatory zones in human atherosclerotic plaques. Serial section immunohistochemistry suggests that macrophages and/or T cells may synthesize these proteins. In addition to localization to macrophages MCP-1 expression was also detected in smooth muscle cells and mesenchymal-appearing cells with many of the morphological characteristics of cells previously seen to express PDGF. In vitro evidence suggests that these proteins may be chemotactic to monocytes and lymphocytes. The finding of increased expression of these mRNAs in human atheroma suggests they may play a role in monocyte trafficking into the atherosclerotic plaque.
In the present study, we demonstrated gene transcription of c-fms in smooth muscle cells isolated from an experimental rabbit model of arteriosclerosis (intimal smooth muscle cells), although there was no gene transcription of c-fms detected in medial smooth muscle cells. On immunocytochemical analysis, both types of smooth muscle cells similarly reacted with an antibody specific to muscle cells (HHF 35), but did not react with an antibody specific to rabbit macrophages (RAM 11). Intimal smooth muscle cells bound to acetylated LDL and resulting foam cell formation was observed. In response to M-CSF, an increased rate of cell proliferation was observed in intimal smooth muscle cells, but not in medial smooth muscle cells. These results indicated that intimal smooth muscle cells have monocytemacrophages characteristics such as the expression of c-fms and scavenger receptor gene.
We studied the effects of interferon gamma (IFN-γ), a T-cell lymphokine, on the proliferation and chemotaxis of vascular smooth muscle cells (SMC). Recombinant human IFN-γ dose-dependently inhibited the proliferation of SMC cultured in the presence of 20% fetal calf serum. It also inhibited PDGF-induced chemotaxis of SMC. Similar concentrations of IFN-γ induced DNA-synthesis of SMC cultured in mitogen-depleted medium for 5 days. The inhibition and the stimulation of SMC proliferation were accompanied by concomitant decrease and increase in the number of PDGF receptors. Our study indicated that IFN-γ is a bidirectional regulator of SMC proliferation.