Subset of Vascular Dendritic Cells Transforming into Foam Cells in Human Atherosclerotic Lesions

It has been previously demonstrated that S-100 positive vascular dendritic cells are involved in human atherosclerosis and they usually show a low level of accumulation of lipids in their cytoplasm, even though they located among foam cells and cellular debris in atherosclerotic lesions. During ongoing immunohistochemical investigations, however, we have found that a few S-100 positive cells exhibited a foam cell appearance. Therefore, we undertook an electronmicroscopic examination to see if any foam cells exhibit the distinctive features of vascular dendritic cells such as the presence of dense granules and a tubulovesicular system uniquely found in well differentiated dendritic cells. Foam cells exhibiting the typical characteristics of vascular dendritic cells were indeed found. Their cytoplasm contained a large number of lipid vacuoles and cisterns of the tubulovesicular system as well as dense granules which, in contrast to lysosomes present in macrophages, did not transform into phagolysosomes. The formation of a central lamina inside cisterns of the tubulovesicular system was also detected. These pentalaminal structures, comprised of two parallel limiting membranes and a central lamina, are similar to the Birbeck granules present in human epidermal Langerhans cells. From our present observations we speculate that the defense mechanisms against extensive lipid accumulation may be broken in some vascular dendritic cells, causing them to transform into foam cells.