The marrow cells of a patient with
pure red cell aplasia markedly increased their rate of
heme synthesis when they were freed from the host environment and were incubated in vitro. When the red cell aplasia was treated with
cyclophosphamide and
prednisone, marrow cell incorporation of (59)Fe into
heme in vitro increased several weeks before a
reticulocytosis was apparent, and was the earliest effect noted. The plasma gammaG-
globulins of this patient inhibited
heme synthesis by normal marrow cells or the patient's own marrow cells obtained after remission of the disease. Since the inhibition of
heme synthesis could be the result of damage to erythroblasts, the patient's posttreatment marrow cells or normal marrow cells were labeled with (59)Fe and were then incubated with the patient's pretreatment, treatment, and posttreatment gammaG-
globulins as well as normal gammaG-
globulins. At the end of this incubation the supernatant and cells were separated and counted.
Heme was extracted and also was counted. Treatment of the cells with the patient's pretreatment gammaG-
globulins resulted in a release of 40% of the radioactive
heme from the cells. This represented the loss of radioactive
hemoglobin and was an index of erythroblast cytotoxicity. A progressive disappearance of the cytotoxic factor in the gammaG-
globulins occurred in the 3 wk period preceding the onset of reticulocytes in the patient's blood. Posttreatment and normal gammaG-
globulins did not produce this effect and increased injury of red cells and lymphocytes was not produced by the patient's pretreatment gammaG-
globulins. These studies demonstrate a method for measuring erythroblast cytoxicity and show that red cell aplasia is associated with gammaG-
globulins that specifically damage erythroblasts. Whether interference with new erythroblast development also occurs and contributes to the inhibition of
heme synthesis has not yet been ascertained.