Immunosuppression with busulphan: the effect on spleen, marrow and thymus cells of mice

The immunosuppressive activity of busulphan, an anti‐tumor agent, was studied using cell‐transfer experiments. Spleen cells from busulphan‐treated donors were many times less active than normal cells, while normal cells could restore competence to busulphan‐treated mice. It was concluded that the drug acts directly upon immunocompetent cells, but has no effect upon the environment in which they work. Further experiments showed that spleen from busulphan‐treated donors did not show synergy with normal marrow or thymus cells, and that neither marrow nor thymus cells from these donors show synergy with the reciprocal normal cell. It was concluded that busulphan affects both thymus‐dependent (T) and thymus‐independent (B) cells. However, marrow alone restored competence to busulphan‐treated recipients which, therefore, must contain an excess of T cells. This may be because mice normally contain an excess of T cells, or because there is a subpopulation of T cells outside the thymus and spleen, unaffected by busulphan.