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Interaction of anti-thymocyte serum with hematopoietic stem cells. I. Effects in vitro and in vivo.


 In vitro incubation of spleen cell suspensions in dilutions of anti-thymocyte globulin (ATG) leads to abrogation of the capacity of such cells to mount a graft-vs-host reaction; however, such incubation leads to considerable inactivation in the content of the hematopoietic stem cells. By prior absorption of the ATG preparation with mouse liver or spleen cells, it was found possible to eliminate the toxicity of ATG toward hematopoietic stem cells without any reduction in its immunosuppressive potency. Administration of ATG to intact animals was found to stimulate endogenous colony formation. Intravenous injection of ATG led to early and significant elevation in the serum level of colony-stimulating factor in the injected animals when compared to that after similar injection of normal horse globulin. Short or long term in vivo administration of anti-thymocyte serum did not lead to reduction in the number of spleen-colonizing stem cells in the bone marrow of the treated animals or in the capacity of such cells to reconstitute lethally irradiated syngeneic animals.