Circulating suppressor cells in aplastic anemia

In vitro proliferation and differentiation of hematopoietic stem cells was studied in a patient with aplastic anemia. Prior to therapy his peripheral blood contained a very low number of myeloid progenitors but a normal number of cells forming lymphoid colonies. Moreover, peripheral blood lymphocytes of this patient were able to suppress in vitro formation of myeloid colonies but not lymphoid colonies. This suppression effect was found to be sensitive to prednisolone and antithymocytic globulin. Following treatment with prednisolone, during which an apparent hematological recovery was observed, the level of lymphoid progenitors fell, but myeloid committed cells returned to normal and hematopoietic suppression was no longer detectable. These results indicate that cells suppressing hematopoiesis may circulate in the peripheral blood of some patients with aplastic anemia and the detection and testing of suceptibility of these cells to immunosuppressive drugs may be helpful in monitoring treatment and prognosis.