Twenty-two cases of idiopathic chronic pure red cell aplasia (PRCA) in adults have been studied to evaluate their erythroid progenitors in vitro using the plasma clot technique. Three types of culture growth patterns were observed and classified as follows. Type I: showing a normal number of autologous CFU-E; type II: CFU-E and BFU-E were detectable but constantly decreased; type III: CFU-E and BFU-E were undetectable. The results were reproducible when patients were studied on two or more occasions. A strong correlation was found between the in vitro growth of autologous erythroid colonies and the results of immunomodulating therapy in 18 evaluable patients. A constant response to immunomodulating treatment was observed in type I patients. A constant failure of treatment was observed in type III patients, whereas results of therapy were unpredictable in type II patients. Two patients with chronic PRCA associated with thymoma and three with chronic myeloproliferative disorders were also studied. Patients with PRCA and thymoma behaved in vitro like type I patients. Patients with chronic myeloproliferative disorders exhibited very low numbers or no CFU-E or BFU-E (type II or III). These data support the hypothesis that at least two mechanisms are responsible for PRCA--one immunologically mediated and the other resulting from a stem cell defect. Moreover, they suggest that the study of erythroid progenitors in vitro might be useful in predicting the immunosuppressive therapy effect in adult chronic PRCA.

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