Graft-versus-leukemia reactions in allogeneic chimeras.

There is a strong graft-versus-leukemia (GVL) effect of allogeneic stem cell trans- plantation (SCT) due to elimination of tumor cells by alloimmune effector lym- phocytes. When leukemia relapses after allogeneic SCT, donor lymphocyte trans- fusions (DLTs) can induce sustained re- missions in some patients. This review summarizes the current status on clinical use of DLT, the basis of GVL reactions, problems associated with this therapy, and new strategies to improve DLT. Sev- eral multicenter surveys demonstrated that the GVL effect of DLT is most effec- tive in chronic myelogenous leukemia (CML), whereas it is less pronounced in acute leukemia and myeloma. Cytokine stimulation to induce differentiation of myeloid progenitor cells or to up-regulate costimulatory molecules on tumor cells may improve the efficacy of DLT. Infec- tions and graft-versus-host disease (GVHD) are major complications of DLT. Control of GVHD may be improved using suicide gene–modified T cells for DLT, allowing T-cell elimination if severe GVHD develops. Hopefully, in the future, GVL effect can be separated from GVHD through adoptive transfer of selected T cells that recognize leukemia-specific an- tigens or minor histocompatibility anti- gens, which are expressed predominantly on hematopoietic cells, thereby preclud- ing attack of normal tissues. In patients with leukemia and lymphomas with fast progression, tumor growth may outpace development of effector T cells. Here it may be preferable to select stem cell transplant donors with HLA-mismatches that allow alloreactive natural killer cells, which appear early after transplantation, to retain their cytolytic function. New ap- proaches for adoptive immune therapy of leukemia, which promise a better progno- sis for these patients, are being devel- oped