Anti-thymocyte globulin effective in the treatment of aplastic anemia does not stimulate granulocyte precursor cells.

To find out whether the preparations of anti-lymphocyte or anti-thymocyte globulin (ATG), successfully used in our center for the treatment of patients with aplastic anemia, were stimulatory for hematopoietic precursor cells, we studied the effect on CFUC in 30 normal bone marrow samples. Although in 2 out of 30 cases stimulation was observed, the overall result for both the horse anti-lymphocyte globulin and the rabbit anti-thymocyte globulin was dose-dependent and complement-dependent inhibition. Neither in the absence, nor in the presence of complement was there any indication of consistent stimulation of CFUC. When bone marrow was depleted of E-rosette forming cells, the incubation of the depleted fraction with ATG did not result in stimulation. The incubation of the E-rosette positive fraction with ATG, followed by the addition of these treated cells to untreated E-rosette depleted cells was equally ineffective in giving any stimulation of CFUC. Our data suggest that the effect of ATG in the treatment of aplastic anemia is not due to direct stimulation of hematopoietic precursor cells.