EXTRACELLULAR MAST-CELL GRANULES CARRY APOLIPOPROTEIN B-100-CONTAINING LIPOPROTEINS INTO PHAGOCYTES IN HUMAN ARTERIAL INTIMA - FUNCTIONAL COUPLING OF EXOCYTOSIS AND PHAGOCYTOSIS IN NEIGHBORING CELLS

In experimental studies in vitro, mast cells have induced uptake of ap olipoprotein B-100 (apoB-100)-containing low-density lipoproteins by m acrophages, with the subsequent formation of foam cells, the hallmarks of atherosclerosis. Recently, increased numbers of activated, ie, deg ranulated, mast cells were found to be present in human coronary fatty streaks and atheromas. We therefore sought evidence of a connection b etween mast cells and foam cell formation in vivo. In electron microsc opic studies of human aortic and coronary fatty streaks and atheromas, exocytosed cytoplasmic secretory granules of mast cells were detected in the vicinity of their parent cells. These exocytosed granules had bound apoB-100-containing lipoproteins, as indicated by their positive staining with MB 47, a monoclonal antibody against apoB-100. A smooth muscle cell was observed to be in the process of phagocytosing one su ch exocytosed granule, and in the vicinity of a degranulated mast cell a foam cell contained an ingested mast cell granule. Therefore, the m icrographs show that exocytosed granules of intimal mast cells may con tribute to intimal foam cell formation and suggest a role for mast cel ls in human atherogenesis. More generally, the findings provide eviden ce that phagocytosis of apoB-100-carrying particles is one mechanism b y which lipoproteins enter human arterial intimal cells.