Histological assessment of gastrointestinal lesions in xenotransplanted primates of Macaca fascicularis recipients hDAF porcine kidneys

Introduction: Methotrexate (MTX) and Cyclophosphamide (CYP) are routinely used as immunosuppressants in induction or maintenance protocols for a large variety of xenotransplantation models. Combining the use of transgenic porcine organs, expressing human Decay Accelerating Factor (hDAF), with immunosuppressive therapy, using CYP or MTX, the survival of primate recipients of life-supporting renal xenografts has been prolonged. However, both drugs can cause significant systemic toxicity and, in particular gastrointestinal (GI) toxicity, but only limited data have been reported on the histopathological features deriving from the use of such agents in non-human primates. Material and Methods: Forty one cynomolgus monkeys (Macaca fascicularis), bilaterally nephrectomized, received a life-supporting kidney from hDAF transgenic pigs. The immunosuppressive regimen was divided into induction therapy with CYP (n=31) or MTX (n=10), followed by maintenance immunosuppression using cyclosporine A, sodium mycophenolate and steroids. For each recipient, following euthanasia and post mortem examination, paraffin-embedded sections of a range of tissues and organs, in the presence or absence of obvious lesions, were prepared, stained with hematoxylin & eosin and analyzed microscopically by two independent pathologists. Results: Following such a comprehensive analysis, severe GI tract lesions were consistently observed in the majority of transplant recipients. In some cases, lesions were observed within the first few days post-transplant. In CYP-treated animals, lesions observed in the GI-tract were characterized by a diffuse, severe lymphoplasmocytic mucosal inflammatory infiltrate. Variable degrees of mucosal atrophy, GALT and goblet cell hyperplasia, and atrophy and blunting of villi were also observed. In contrast, in MTX-treated primates, a more severe histopathological profile was observed. Findings were consistent with severe mucosal atrophy associated with mild-to-moderate disseminated lymphoplasmocytic infiltration. The presence of such lesions showed no correlation with graft survival, being observed in both long and short-term surviving recipients. Conclusions: In hDAF transgenic pig-to-cynomolgus monkey kidney transplantation, GI-tract lesions are an early and consistent finding when CYP or MTX are used as induction therapy. Moreover, aside from renal failure, GI-tract complications are the main cause of premature euthanasia. In conclusion, this study indicates that, albeit with a different histopathological profile, both CYP and MTX have severe toxic side-effects on the GI-tract that prevents their use as long-term immunosuppressants in pig-to-primate xenotransplantation. In our laboratory both drugs demonstrated efficient immunosuppression. However, MTX showed higher GI toxicity than CYP in this model. Since the prevention and successful treatment of GI lesions could significantly prolong the survival of pig organs transplanted into non-human primates, the identification of more specific, less toxic immunosuppressive approaches is eagerly awaited.