PERFORIN GRANZYME-DEPENDENT AND INDEPENDENT MECHANISMS ARE BOTH IMPORTANT FOR THE DEVELOPMENT OF GRAFT-VERSUS-HOST DISEASE AFTER MURINE BONE-MARROW TRANSPLANTATION/

Graft-versus-host disease (GVHD) is the major limiting toxicity of all ogeneic bone marrow transplantation. T cells are important mediators o f GVHD, but the molecular mechanisms that they use to induce GVHD are controversial. Three effector pathways have been described for cytotox ic T lymphocytes: one requires perforin and granzymes, the second Fas (APO-I; CD95) and its ligand. Thirdly, secreted molecules (e.g., TNF-a lpha, gamma-IFN) can also mediate cytotoxicity, Together, these mechan isms appear to account for virtually all cytotoxicity induced by activ ated CTL in standard in vitro lytic assays. Using transplants across h istocompatibility barriers, we were able to analyze the contributions of these effector molecules to cell-mediated cytotoxicity in vivo in a GvHD model. We found that Fas ligand is an important independent medi ator of class II-restricted acute murine GVHD, while perforin/granzyme -dependent mechanisms have only a minor role in that compartment. In c ontrast, perforin/granzyme-dependent mechanisms are required for class I-restricted acute murine GVHD, while Fas ligand is not. The perforin /granzyme pathway may therefore represent a novel target for anti-GvHD drug design. In support of this approach, we provide additional data suggesting that specific perforin/granzyme inhibitors should not adver sely affect hematopoietic recovery after transplantation.