Specialized membrane contacts between immunocompetent cells in human atherosclerotic plaques.

Several immunohistochemical studies have revealed closely apposed T cells and macrophages in the various developmental stages of atherosclerosis. The present ultrastructural study was undertaken to evaluate the morphologic features of the cell-cell contacts of immunocompetent cells and smooth muscle cells in human atherosclerotic plaques. Sixteen biopsies were taken at autopsy from small raised plaques in the coronary and carotid arteries. Four sites (coronary artery 2, carotid artery 2), each with a cell-rich lesion, were selected for ultrastructural evaluation. Close contacts between macrophages and lymphocytes could be detected in the superficial parts of the atherosclerotic plaque and the adjacent thickened intima but were absent in the deeper parts of the plaque. Close examination of these cellular contacts revealed lymphocytes partially engulfed by macrophages, intermingling of cytoplasmic extensions of both cells, and deep extensions of cells in clathrin-coated pits of apposed cells. In addition, contacts occurred involving only lymphocytes and only macrophages. Membrane contacts between smooth muscle cells and mononuclear cells were absent. The occurrence of these specialized membrane contacts provides additional support for a role of activated T cells and macrophages in atherosclerosis. The interactions among immunocompetent cells suggest their involvement in an immune regulatory process. The absence of specialized contacts between smooth muscle cells and mononuclear cells implies that they probably are influenced in a paracrine way only.