Introduction: The search for improved outcomes in organ transplantation has led to the exploration of innovative therapeutic strategies aimed at promoting transplant tolerance, thereby reducing the reliance on long-term immunosuppression and its associated complications. The anti-CD3 fusion protein A-dmDT390-bisFv(UCHT1) or VG712, (also known as Resimmune®) has been shown to be an effective in vivo T cell depleting reagent in cutaneous T cell lymphoma patients. Porcine and non-human primate versions of this recombinant CD3 immunotoxin (CD3IT) are available for pre-clinical research.  Thymoglobulin® (anti-thymocyte globulin (ATG)) is the most used medication for induction immunosuppression for kidney transplantation but is relatively ineffective at reducing acute rejection episodes and causes prolonged and unselective T-cell depletion that leaves patients susceptible to infection. The prolonged half-life of ATG contributes to delayed immune reconstitution in patients, disrupting regulatory mechanisms and increasing the likelihood of infectious complications and cancer. There is evidence from rodent studies that ATG induction therapy may increase potential for alloantibody responses post transplantation. We have been studying CD3IT as a component of transplantation tolerance induction therapy in large animals for over 20 years. Our data demonstrate that CD3IT has distinct beneficial characteristics from ATG that support clinical development of VG712 for transplantation induction therapy.
Method: CD3IT was administered intravenously to experimental animals as part of our conditioning protocol at 25-50 μg/kg twice daily on four consecutive days prior to hematopoietic cell and/or organ transplantation. T cell depletion and recovery were monitored by flow cytometry (in peripheral blood) and immunohistochemistry (lymph node). Animals were challenged with either donor cells or organs (skin). Anti-donor alloantibodies were evaluated using complement dependent antibody mediated cytotoxicity assay.
Results: CD3IT has distinctly different properties from other lymphocyte depleting agents currently used in transplantation induction therapy such as Thymoglobulin® (anti-thymocyte globulin (ATG)) and Alemtuzumab (Campath). These properties include a potent ability to rapidly deplete T cells within tissues as well as in the peripheral blood, a very short half-life (<2 hours), which allows rapid T cell recovery with improved immune competence, and a relative sparing of T regulatory cells (Figure 1). CD3IT in our studies was demonstrated to be more selective, better preserving the total white blood cell count and B cell count, while just as efficiently depleting T cells. Our results using CD3IT induction in the swine model suggest that the unique properties of CD3IT contribute to an immune regulatory response to the infused donor cells which may prevent alloantibody development and facilitate immune tolerance induction.

Conclusion: Our data suggest that induction therapy using VG712 will result in rapid effector T cell depletion with a relative sparing of T regulatory cells in kidney transplant patients.  Immune reconstitution is also expected to occur rapidly with earlier and greater functional immune recovery compared with ATG. VG712 is being developed for clinical use in transplantation through Virogen Biotechnology Inc.