Quantitative and ultrastructural changes of peritoneal macrophages in immunized rats after intraperitoneal injection of Novikoff hepatoma.

The response of peritoneal cells to transplantable Novikoff hepatoma was studied in immunized and nonimmunized inbred male SD rats to elucidate the role of macrophages in syngeneic tumor rejection. Specific immunization was induced by double sc challenge with viable tumor cells. Peritoneal cells were harvested by lavage at intervals of 6 hours and 1, 2, 3, 4, and 9 days after ip tumor injection. Some of the lavage fluid was used to determine the total number of cells, and the remainder was prepared for ultrastructural study. Different counts were made at the electron microscope level, and the ultrastructures of macrophages and tumor cells were studied. The results indicated that during the early intervals after tumor injection, the numbers of macrophages and lymphocytes were significantly increased in immunized animals. The mononuclear cells accumulated more slowly in the peritoneal cavities of nonimmunized rats. Morphologic heterogeneity was observed in the macrophage population of immunized rats up to day 4. Concurrent with these changes in macrophages were the decreased mitotic activity and ultrastructural alterations in tumor cells followed by their complete disappearance. In nonimmunized animals tumor cells proliferated actively and by day 9 constituted 85% of the peritoneal population.