Role of intra-islet endothelial cells in islet allo-immunity

Background. Intra-islet endothelial cells (IECs) express high levels of major histocompatibility complex (MHC) and are pivotal for posttransplant islet revascularization. We postulated that donor-specific sensitization would result in hyperacute rejection of IECs and prevent islet engraftment. Furthermore, ligation of endothelial cells with subsaturating concentrations of anti-MHC class I antibody (Ab) results in "accommodation" conferring protection against Ab/complement- mediated lysis. Therefore, we investigated whether accommodation of IECs would prevent hyperacute rejection of islets in sensitized recipients. Methods. Islets were transplanted beneath the kidney capsule and allograft survival monitored using daily blood glucose (diabetes >300 mg/dL, normoglycemia < 150 mg/dL). Recipients were presensitized with donor islets, spleno-cytes, or skin. Accommodation was induced by incubating human or murine islets with varying concentrations of anti-MHC class I Ab ex vivo. Results. Isografts remained functional for > 100 days, whereas allografts were rejected by day 14. Islet allo-transplantation induced donor-specific but not third-party anti-MHC Abs. Donor-specific, but not third-party, sensitization induced hyperacute rejection of subsequent islet allografts (median survival I day) associated with complement deposition. Preincubation of islets with subsaturating concentrations of anti-MHC-I Abs (1-100 ng/mL) up-regulated Bcl-2, Bcl-xl, and HO-1 within CD31(+) IEC. These accommodated islets were resistant against hyperacute rejection when transplanted into donor- (splenocyte) sensitized recipients without any immunosuppression (median survival 6 days). Conclusions. Pretransplant sensitization against donor antigens results in hyperacute rejection of murine islets. IECs may play a crucial role in development of donor-specific immunity after islet transplantation. Significantly, accommodation of IEC may confer resistance to hyperacute rejection in sensitized recipients.