The nature of surface immunoglobulin in pure monocytic (M5) leukemia

The presence of endogenous monoclonal surface immunoglobulin (SIg) on circulating neoplastic cells of certain B-cells disorders such as the leukemic phase of lymphoma and B-cell acute lymphocytic leukemia is well established. The paradoxical appearance of prominent SIg on blasts from adults with acute nonlymphocytic leukemia has also been observed. In some cases, myeloblasts and myelomonoblasts have been characterized by polyclonal SIg of exogenous origin. Neoplastic cells from two patients with pure monocytic leukemia, a rare variant of acute nonlymphocytic leukemia, were studied with respect to functional properties. All blasts from both patients demonstrated polyclonal SIg. This surface fluorescence was not artifactual, since identical results were obtained when cells were stained with F(ab')₂ fragments and whole test antisera. Trypsin digestion removed the immunoglobulin, and it was not reconstituted on the cell surface after short-term culture. The SIg observed was apparently of exogenous origin, bound to the Fc or cytophilic antibody receptor. Thus, SIg on leukemic blasts does not necessarily imply B-lymphocyte cytogenesis. The use of trypsin digestion to distinguish between exogenous and endogenous SIg may be useful in the subclassification of acute nonlymphocytic leukemia.