T cell depletion in bone marrow transplantation for leukemia: current results and future directions

T cell depletion reduces the incidence and severity of graft-versus-host disease (GVHD) following bone marrow transplantation in man. Graft-versus-host disease of more than grade 2 severity is decreased from about 45% to about 10% in recipients of HLA-identical transplants. However, T cell depletion also increases the frequency of graft failure and leukemia relapse. Graft failure increases from about 1 to 10% following HLA-identical transplants. In patients with acute leukemia in first remission or with chronic myelogenous leukemia in chronic phase, leukemia relapse increases from about 20% to about 40%. Thus, although T cell depletion decreases GVHD, it increases graft failure and leukemia relapse such that survival is not convincingly improved. Several approaches to these problems are possible including increased pre- or post-transplant immune suppression, more effective antileukemia therapy or selective T cell depletion. Preliminary results of these approaches are discussed and new directions suggested.