In vitro evidence for an unusual progenitor cell in acute monocytic leukemia.

The peripheral blood of 2 patients with acute monocytic leukemia of the undifferentiated type was studied for the presence of leukemic progenitor cells in colony forming assays. The peripheral blood of both patients contained only one type of progenitor cell as determined with these assay systems. Daughter cells of these progenitor cells morphologically and histochemically resembled monoblasts and immature macrophages. Similar progenitor cells were not encountered in the study of 5 patients with acute myelogenous leukemia whose peripheral blood cells formed colonies in the same system. The finding of this unusual progenitor cell supports the existence of acute monocytic leukemia as a separate clinicopathologic entity, and suggests that it represents a malignant transformation of a progenitor cell of the monocyte/macrophage series. An alternative explanation of these observations would be provided by single phenotypic expressions of a multi-potent stem cell which has undergone malignant transformation.