Platelets and Smooth Muscle Cells Affecting the Differentiation of Monocytes

BACKGROUND: Atherosclerosis is characterised by the formation of plaques. Monocytes play a pivotal role in plaque development as they differentiate into foam cells, a component of the lipid core whilst smooth muscle cells (SMC) are the principal cell identified in the cap. Recently, the ability of monocytes to differentiate into a myriad of other cell types has been reported. In lieu of these findings the ability of monocytes to differentiate into SMCs/smooth muscle (SM)-like cells was investigated. METHOD AND RESULTS: Human monocytes were co-cultured with platelets or human coronary aortic SMCs and then analysed to assess their differentiation into SMCs/SM-like cells. The differentiated cells expressed a number of SMC markers and genes as determined by immunofluorescence staining and quantitative polymerase chain reaction (qPCR). CD array analysis identified marker expression profiles that discriminated them from monocytes, macrophages and foam cells as well as the expression of markers which overlapped with fibroblast and mesenchymal cells. Electron microscopy studies identified microfilaments and increased amounts of rough endoplasmic reticulum indicative of the SM- like cells, fibroblasts. CONCLUSIONS: In the appropriate environmental conditions, monocytes can differentiate into SM-like cells potentially contributing to cap formation and plaque stability. Thus, monocytes may play a dual role in the development of plaque formation and ultimately atherosclerosis.