Inhibition of acute allogaft rejection - experimental transplant studies with biological and chemical immunosuppressants

In clinical transplantation immunosuppressive agents are used in combination with the objective to achieve additive effects and less toxicity. A large number of novel drugs are candidates for clinical introduction at this time and experiments in animal models have an important role to play. The aim of this study was in brief to evaluate the capacity to inhibit the acute rejection process with biological and chemical immunosuppressants including anti-CD4 monoclonal antibody (mAb), and leflunomide and its analogues malononitrilamides (MNA) 279 and 715, given alone or in combination with cyclosporine or tacrolimus. A rat cardiac transplant model was used and immunosuppressive drugs given as 10 days of induction. Graft survival was the most important variable. Flow cytometry was used for analyses of donor-specific antibody production and histopathology for evaluation of acute rejection changes at various time points posttransplant. Unresponsiveness was tested with retransplantation and with harvesting of peritoneal excudate cells (PEC) following intraperitoneal challenge with donor-specific or third party cells. Various methods for analysis of combined drug effects were used including enhancement of rejection with an immunomodulator, Linomide, and with presensitisation, and iso-effect curves of drugs given alone or in combination. Major findings of this study was as follows. A single administration of anti-CD4 mAb induced unresponsiveness in a low-responder rat strain combination, whereas the effect was minimal in high-responders. Level of response was demonstrated to depend on selection of recipient as well as donor. Unresponsiveness induced by a low dose of CsA induction was donor-specific and PEC revealed a corresponding pattern of cytotoxicity. Exogenous administration of interleukin-2 could restore cytotoxicity against donor target cells suggesting a state of anergy. Leflunomide was shown to prolong graft survival and in combination with CsA to overcome the challenge of Linomide. The MNA 279 and 715 were successfully used to reverse on-going acute rejection and inhibit allospecific antibody production. Combined therapy with CsA and MNA, given on the day of retransplantation in presensitised rats, prevented accelerated acute rejection. Using iso-effect curves for tacrolimus and MNA, alone or in combination, additive effects of the two drugs were demonstrated