Cellular distribution of CD63 antigen in platelets and in three megakaryocytic cell lines

CD63 is a 53 kDa lysosomal membrane glycoprotein that has been identified as a platelet activation molecule. We investigated the localization of CD63 antigen in platelets and in three megakaryocytic cell lines (K562, HEL and CMK11-5) using flow cytometry and immunoelectron microscopy. Flow cytometry showed that a monoclonal antibody directed against CD63 bound to 8.1% of unstimulated platelets and 59.2% of thrombin-stimulated platelets. Immunoelectron microscopy demonstrated that CD63 antigen was distributed randomly inside unstimulated platelets, while it was localized in the open canalicular system of washed platelets and on the cell membranes of thrombin-stimulated platelets. Flow cytometry detected CD63 on 16.4% of HEL cells, 31.2% of K562 cells, and 43.2% of CMK11-5 cells. Immunoelectron microscopy demonstrated that CD63 was localized in the granules and on the surface membranes of HEL cells, in the vesicles and on the membranes of K562 cells, and in the granules and vesicles as well as on the membranes of CMK11-5 cells. Thus, the distribution of CD63 differed markedly among these three megakaryocytic cell lines.