Differing association of macrophage subsets with atherosclerotic plaque stability

While initial research suggests that M2 macrophages are athero-protective, more recently, proatherogenic functions, such as a greater uptake of lipid than M1 macrophages, have been demonstrated, raising the question of their actual association with plaque stability. The present study, therefore, assessed the association between macrophage subset and plaque stability. Furthermore, it examined whether the fibrocyte, that we have previously identified in the plaque, represents a subset of M2 macrophages.

Twenty human carotid atherosclerotic plaque specimens were examined for the presence of macrophages using immunohistochemistry for pan macrophages (CD68), M1 (CD64, CD86) and M2 (CD163, CD206) subsets. The slides were assessed by digital whole slide scanning/image analysis to quantify the expression of these markers in the plaque. Comparisons in marker distribution and quantity relative to plaque stability were made. Adoption of a fibrocyte phenotype was assessed by double immunofluorescence staining of the markers with procollagen I.

M1 and M2 macrophages were present throughout the plaque including the core and cap. While the levels of CD68 (pan macrophage maker) and CD86 negatively correlated with cap thickness, the levels of the M2 marker, CD163, did not and moreover, did not differ between plaques when they were separated into stable and unstable groups. Notably, collagen production was evident in most but not all M2 macrophages.

Our findings demonstrate that while macrophage levels in general negatively correlate with plaque cap thickness, levels of M2 macrophages do not. This may be in part due to their ability to produce collagen (ie adopt a fibrocyte phenotype) in the plaque.