A complement independent erythropoietic inhibitor acting on the progenitor cell in refractory anemia.

An erythropoietic inhibitor was detected in the serum of a patient with refractory anemia. Using an in vitro heme synthesis method, the patient's serum produced tenfold inhibition of erythropoietin-stimulated radioactive iron (Fe59) incorporation into heme of normal human marrow at 72 hours, as compared with AB serum. In a separate experiment the patient's serum produced threefold inhibition, whereas immunoglobulin G (IgG) prepared from the same serum sample produced 12-fold inhibition. To identify the site of action of the inhibitor, serum was tested in a cell culture system whereby human marrow cells, grown in a plasma clot, respond to exogenous erythropoietin with the appearance of nucleated erythroid colonies. Each colony arises from a committed erythroid progenitor. The patient's serum produced a two- or tenfold reduction in the number of colonies from normal human marrow. The effect was also demonstrated on autologous marrow obtained when the patient was in "partial clinical remission". Serum samples obtained at various times during the course of the patient's illness all demonstrated a suppressive effect on colony growth. All serums were heat-inactivated, and total hemolytic complement could not be detected in either culture system. It is concluded that the anemia is due to an inhibitor, probably of IgG class, that acts on the erythroid progenitor cell. The absence of heat-labile complement components in the culture systems suggests that the mechanism is not due to immune cytolysis.