IMMUNE REGULATION AND TOLERANCE II

P.E. Cippa¹, A.K. Kraus¹, S. Segerer¹, J. Chen¹, P.D. Bardwell², T. Fehr^1
1Division Of Nephrology And Institute Of Physiology, University of Zürich, Zürich/SWITZERLAND, ², Abbott Bioresearch Center, Worcester/UNITED STATES OF AMERICA

Body: Introduction: Novel, highly effective, but less toxic immunosuppressive drugs are urgently needed to improve long-term outcome after solid organ transplantation. Considering the crucial role ofapoptosis in the regulation of the adaptive immune response, pharmacological inhibition of Bcl-2 proteins - major regulators of the intrinsic apoptotic pathway - represents an innovativeimmunosuppressive concept. Here we tested for the first time the antagonist of anti-apoptotic Bcl-2 proteins ABT-737 as an inhibitor of allogeneic immune reactions in vitro and in vivo. Methods: Invitro, ABT-737 was tested in a mixed lymphocyte reaction model with BM3.3 transgenic responder cells (CBA background; expressing on CD8 T cells a transgenic T cell receptor specific for the MHC classI molecule Kb) reacting against B6 stimulators. In this alloantigen-specific model, phenotypic T cell activation (CD25 and CD69 expression), proliferation (CFSE dilution), interferon-gamma production(ELISA, intracellular staining) and cytotoxicity (51Cr release) were measured. In vivo, skin graft rejection was studied in an MHC class I mismatched mouse model (bm1 to B6). Recipient mice wereinjected daily with ABT-737 (50 mg/Kg/day i.p.) or vehicle, starting at day 5 before skin transplantation. To assess tissue selectivity of ABT-737 in vivo, we analyzed by immunohistochemistry thenumber of apoptotic cells in different organs after injection of ABT-737 or vehicle. Results: ABT-737 potently suppressed allogeneic immune reactions in vitro on every level of T cell effectorfunction (proliferation, cytokine production and cytotoxicity). A more detailed analysis of the immunosuppressive mechanism demonstrated, that ABT-737 selectively induced apoptosis in T cellsreducing the alloreactive clone size, but did not impair the physiological functions of the remaining T cells. Moreover, the pro-apoptotic effect of ABT-737 in vivo was selective to lymphocytes, asshown by an increased number of apoptotic cells in the spleen, but not in the kidney or the liver 12 hours after treatment. Eventually, ABT-737 significantly prolonged skin allograft survival in anMHC class I mismatched model (median skin graft survival 18 vs. 13 days, p=0.029). Conclusion: Bcl-2 inhibition represents a novel and promising strategy to suppress allogeneic immune reactions andto prevent rejection after solid organ transplantation. The high selectivity of ABT-737 for lymphocytes suggests a low toxicity profile of the novel immunosuppressive treatment.

Disclosure: All authors have declared no conflicts of interest.