The ultrastructure of sarcoma I cells and immune macrophages during their interaction in the peritoneal cavities of immune C57BL-6 mice.

C57BL/6 mice that had recently rejected a primary Sarcoma I (Sal) ascitic tumor were given a second i.p. dose of Sal cells. The interaction of the immune macrophages of the host and the Sal target cells was investigated by electron microscopy. The macrophages rapidly adhered to the target cells and spread over their surfaces. The interaction of the immune macrophages and Sal cells was manifested by interdigitation of their surfaces and the extension of Sal microvilli into deep invaginations of the immune macrophages. The spreading of the immune macrophages over the surfaces of the Sal cells resulted in the apparent phagocytosis of viable tumor cells. Such phagocytized Sal cells were contained within phagocytic vacuoles with the vacuolar membrane of the macrophage usually closely applied to the plasma membrane of the Sal cell. Amorphous material was observed in the vacuolar space, particularly in the mildly dilated areas. These findings suggest that the phagocytosis of Sal cells by immune C57BL/6 macrophages plays an important role in the rejection of the ascitic form of the tumor during the secondary immune response.