Evaluation of antithymocyte globulin for human bone marrow transplantation. Antihematopoietic and immunosuppressive activity.

Antihematopoietic stem cell activity and immunosuppressive activity were evaluated before and after absorption of horse antihuman thymocyte globulin (ATG) with mature granulocytes and fetal liver cells. Unabsorbed ATG was cytotoxic to committed myeloid-macrophage progenitors (CFU-C). Absorption of ATG removed cytotoxicity to CFU-C. The immunosuppressive potentials of absorbed ATGs were identical to those of unabsorbed ATG in several tests. Primary response to trinitrophenyl (TNP)-modified autologous and allogeneic cells could be suppressed by treatment with ATG alone whereas suppression of secondary and cytotoxic responses required treatment with ATG and complement. These data indicate that unsensitized or undifferentiated T cells may be more sensitive to ATG than are sensitized or differentiated T cells.