Treatment of aplastic anemia by bone marrow transplantation in identical twins.

Four patients with aplastic anemia were treated with intravenous infusion of 0.8-3.5 x lO nucleated bone marrow cells/kg obtained from their healthy identical twin. In two cases. simple marrow infusion led to prompt recovery of bone marrow function. In the other two cases. bone marrow infusion alone was ineffective. but a second infusion after treatment with cyclophosphamide (CV) led to prompt complete recovery. Coculture studies using patients’ peripheral blood mononuclear cells and donor bone marrow or peripheral blood cells were performed for three of the patients prior to bone marrow transplantation. Cells from the two patients who ultimately required CV appeared to inhibit granulocytic or erythroid colony growth by donor marrow, whereas cells from the one patient who was studied who responded to marrow infusion alone did not. The response of patient peripheral leukocytes to twin cells in mixed leukocyte culture was compared to the response of patient cells to a cell pool from unrelated donors and expressed as a relative response. The response was abnormally high in the two cases requiring CV and normal in the other two cases. The results suggest that aplastic anemia in some cases reflects a qualitative or quantitative marrow cell defect correctable by the simple infusion of twin marrow, but in other cases reflects active suppression of hematopoiesis that can be overcome by CV and bone marrow transplantation.