Enhancement of rat islet tolerance with bone marrow transplantation using a non-myeloablative procedure II: failure despite the presence of lymphocyte microchimerism in the fully allogeneic Lewis/Brown-Norway model.

Transplantation of donor bone marrow cells (BMTx) has been proven to be capable of including allogeneic transplant tolerance. In our previous experiments we reported the positive effect of BMTx together with short-term tacrolimus/hydrocortisone therapy on pancreatic islet survival in recipients haploidentical with donors. In this project we used the same transplant protocol to further investigate this effect in a fully mean histocompatibility system-mismatched model. Lewis male rats and Brown-Norway female rats were used as donors and recipients, respectively. Diabetic animals were treated according to four different protocols. Recipients in group I (n = 12) underwent islet transplantation (ITx) only. Group II (n = 12) and group III (n = 11) included animals treated for 52 days with tacrolimus (0.5 mg/kg) and hydrocortisone (2 mg/kg). Diabetes was induced by intravenously applied streptozocin (50 mg/kg). Seven days later islets were injected intrahepatically through the portal vein. In addition, rats in group III underwent BMTx on day 10. In group IV (n = 6) tacrolimus therapy, ITx and BMTx were used according to the previously published protocol of Ricordi et al. After more than 120 days, cumulative survival rates were 56% and 64% for recipients in groups II and III, respectively (p > 0.05). All animals in group I became hyperglycemic by day 11 following transplantation. Despite positive detection of lymphocyte microchimerism, we did not observe improved survival of allogeneic islets in animals treated with BMTx. Surprisingly, better islet survival was not found in group IV either (survival rate at 100 days: 33%). We conclude that demonstration of lymphocyte microchimerism, as detected by a sensitive polymerase chain reaction method, did not improve allogeneic islet survival in vivo and was not able to block mixed lymphocyte reaction in vitro. Whether a larger amount of transplanted hematopoietic cells could induce tolerance in this model remains to be evaluated.