Abstract 582: Vascular Smooth Muscle-derived Macrophage are a Major Source of MCP1 in Atherosclerosis

Smooth muscle cells (SMC) are conventionally thought to promote atherosclerotic plaque stability by investing within the fibrous cap and protecting the blood flow from the harmful inflammatory plaque core. However, lineage tracing studies have shown that SMC in plaque can undergo a Klf4-dependent transition in vivo to a macrophage-like state, characterized by loss of SMC markers and expression of multiple macrophage markers, lipid accumulation, and phagocytic activity. Of major interest, SMC induced to the macrophage state in vitro by cholesterol loading also show increased production of the atheropromoting cytokine MCP1 (Monocyte chemoattractant protein 1). These results are surprising since it has been largely assumed that monocyte derived macrophages are the major source of MCP1 and that this acts to exacerbate lesion pathogenesis by inducing recruitment of additional circulating monocytes. Using MCP1-mCherry reporter mice on an ApoE -/- background, we show that MCP1 is highly expressed by medial and fibrous cap SMC. Further, we show that SMC-specific KLF4 knockout mice, which show marked reductions in the frequency of SMC-derived macrophages, show a 50% decrease in MCP1 immunostaining in late stage atherosclerotic lesions, suggesting that these cells are a major source of MCP-1 and produce it via a Klf4-dependent mechanism. Consistent with this possibility, we demonstrated that SMC-specific tamoxifen conditional MCP1 knockout mice on an ApoE -/- background (Myh11-CreERT2 eYFP ApoE -/- MCP1-mCherry), showed greatly decreased total MCP1 protein level within the aorta. We are currently completing examination of the effects of SMC specific MCP1 KO on late stage lesion pathogenesis in our Myh11-CreERT2 eYFP ApoE -/- MCP1-mCherry mice following 18 weeks of Western diet feeding. Taken together results provide evidence that SMC derived cells are an unexpected major source of MCP1 within lesions.