Effect of specific anti-human leukaemia immunotoxins on the colony formation by human haematopoietic progenitors and by human leukaemia cells.

The specific cytotoxic activity of anti-human T cell leukaemia immunotoxins (IT) was investigated for their effects on in vitro colony formation of leukaemic cells and normal haematopoietic progenitors, i.e., CFU-GM, CFU-E and CFU-GEMM. These IT were prepared by conjugating ricin A chain with the monoclonal antibodies SN1 and SN2. These antibodies define two unique human T cell leukaemia antigens. This study reveals that while these IT are only marginally cytotoxic to normal haematopoietic progenitors, they strongly suppress colony formation of human T leukaemia cells. The latter suppression is augmented by the presence of 10 mM NH4Cl which results in total suppression. The present results indicate the therapeutic usefulness of these IT for in vitro eradication of leukemia cells in the bone marrow of patients with T cell leukaemia. Potentially, such a purged bone marrow can be used in autologous transplantation in leukaemia patients.