Destruction of circulating leukemia cells by phagocytosis in rats with myelogenous leukemia

Acute myelogenous leukemia was induced in outbred Long-Evans rats by iv injections of leukemia cells from a subcutaneous tumor of Shay myelogenous leukemia. In rats with this leukemia the peripheral white blood cell (WBC) counts varied from 2.4 to 700 X 10(9)/liter. No differences were found in the bone marrow of the rats with the high WBC counts and that of rats with low WBC counts. This observation could explain the large variations in the number of circulating leukemia cells caused by differences in cell proliferation or delivery of cells into the circulation. Massive phagocytosis of leukemia cells occurred in animals with low WBC counts (less than 12 X 10(9)/liter) but not in animals with high WBC counts (greater than 150 X 10(9)/liter). This phagocytosis was directed against circulating leukemia cells. The main phagocytes were Kupffer's cells of the liver and macrophages of the spleen parenchyma. In addition, phagocytosis occurred in the spleens and bone marrow by intravascular macrophages, which were derived from extravascular sites. The endothelium of the postcapillary venules of the lymph nodes participated in the phagocytosis of circulating leukemia cells while continuing to be the locus of lymphocytic return from circulation to lymphatic parenchyma. The factors underlying the differences in macrophage activity between the rats with high and low WBC counts were unknown.