APG101 - a Novel Inhibitor of CD95/CD95L Signaling Prevents Graft-Versus-Host Disease without Disabling T Cell Effector Function and Anti-Tumor Activity,

Abstract 4019

Allogeneic bone marrow transplantation (BMT) is a curative treatment modality for haematological malignancies since mature allogeneic T cells in the transplant efficiently eliminate residual tumor cells (graft-versus-tumor (GVT) effect). However, these donor T cells are also responsible for the induction of graft-versus-host disease (GVHD) by attacking recipient tissue leading to significant morbidity and mortality. Interfering with T cell effector mechanisms responsible for GVHD-induction without preventing T cell functions towards third-party or opportunistic pathogens and while maintaining anti-tumor cytotoxicity might lead to new treatment strategies after allogeneic BMT. The CD95/CD95L system significantly contributes to GVHD-associated tissue damage and intervention into this signaling pathway might therefore inhibit GVHD-development without disturbing the GVT-effect. APG101 is a human soluble fusion protein consisting of the extracellular domain of CD95 and the Fc portion of IgG blocking the interaction of CD95L with its cognate receptor. We tested the effect of APG101 on T cell function, GVHD development and anti-tumor cytotoxicity in an allogeneic parent into F1 (C57BL/6 (B6) (H-2^b) into B6D2F1 (H-2^bxd)) BMT model with a mismatch in HLA-Class I and II molecules. In vitro, APG101 efficiently inhibited CD95L-mediated apoptosis of primary naïve mouse CD4⁺ and CD8⁺ T cells but did not interfere with their proliferative or cytotoxic capacity. To test APG101 as a therapeutic agent in BMT we induced acute GVHD in lethally irradiated B6D2F1 mice by transplantation of B6-derived BM together with allogeneic spleen cells. APG101 treatment was conducted intraperitoneally twice a week until the end of the experiment, starting either one day before BMT (= prophylactic treatment) or starting six or thirteen days after BMT (= therapeutic treatment). Prophylactic and early (day 6), but not late therapeutic treatment (day 13), effectively protected recipients from clinical GVHD and significantly improved survival. Although APG101 efficiently prevented GVHD-induction, homing and proliferation of allogeneic T cells into lymphoid organs and GVHD-target organs was unaltered compared to non-treated animals. Since CD95L is also known as a reverse signaling co-stimulator in T cell activation, APG101 might interfere with T cell function in vivo. However, no differences in phenotype, proliferation or cytokine expression of transplanted allogeneic T cells was observed in APG101-treated mice compared to non-treated mice. Most importantly, the GVT effect was preserved in APG101-treated mice. Anti-tumor cytotoxicity of transplanted allogeneic T cells was not abrogated by APG101 injection independent whether the murine mastocytoma cell line P815 or Bcr-Abl transduced primary B-ALL cells were injected. In summary, our findings convincingly show that early APG101 treatment is a promising approach to prevent GVHD without abrogating T cell effector functions and while preserving the GvT effect after allogeneic BMT.