The graft versus leukemia effect: possible mechanisms and clinical significance to the biologic therapy of leukemia.

An anti-leukemic effect of allogeneic bone marrow has been repeatedly demonstrated in experimental animal models. Clinical data supporting this "graft versus leukemia" (GVL) effect are derived from several different observations which include: 1) the association of GVHD (acute and chronic) with decreased leukemic relapses; 2) identical twin transplants are associated with a higher relapse rate compared to allogeneic MHC-matched sibling transplants; 3) T cell depletion of donor bone marrow decreases GVHD and increases leukemic relapse rates; 4) Allogeneic BMT without GVHD have a lower leukemic relapse rate compared to identical twin transplants and T cell depleted transplants. The mechanisms of this GVL effect remain poorly understood, but clearly involve the immune system. It is hoped that current advances in basic understanding of the immune system and its activation will enable the "antileukemic" components of the GVL effect to be prospectively controlled and intentionally used as leukemia therapy.