Antithymocyte globulin stimulates human hematopoietic progenitor cells

Antithymocyte globulin (ATG), a horse antihuman thymus antiserum highly effective in the majority of patients with aplastic anemia, was studied for its in vitro effects on hematopoietic progenitor cells. Marrow cells isolated by an immunoadherence technique with the HPCA-1 (human progenitor cell antigen) monoclonal antibody after removal of contaminating T cells and macrophages formed erythroid colonies in methyl cellulose media in the presence of ATG at concentrations of 25-50 micrograms/ml. ATG also stimulated continuous production of hemoglobin-containing erythroid colonies beyond 35 days of culture when it was added to the culture weekly. ATG also had an indirect effect on myeloid (granulocytic and macrophagic) colony growth in vitro. At a concentration of 10 micrograms/ml, ATG stimulated late but not early myeloid progenitor cells to form mature colonies. This effect required the participation of lymphocytes containing the Leu-11 antigen and macrophages or supernatant fluids made from these two types of cells that had been preincubated with ATG for 3 hr and then cultured for 5 additional days. The supernatant fluids produced in such a manner showed characteristics similar to granulocyte colony-stimulating factor and had an activity peak that eluted at a volume corresponding to a 20,000 Da molecular mass protein by high-resolution liquid chromatography.