Introduction: Allogeneic islet transplantation is a viable clinical approach to treat type 1 diabetes. The need for chronic immunosuppression to control rejection is a major impediment for the broad application of allogeneic islet transplantation.  T effector cells responding to alloantigens initiate and coordinate graft rejection. These cells upregulate Fas receptor following activation and become sensitive to Fas-mediated apoptosis. In a rodent model, we previously reported the efficacy of PEG microgels engineered to display a novel form of FasL (SA-FasL) on the surface in inducing localized tolerance to allogeneic islets. We herein report the efficacy of this immunomodulatory regimen in preventing rejection of allogeneic islets in a NHP model in the absence of chronic use of immunosuppression.

Methods: PEG microgels formulated with biotin were engineered to display SA-FasL protein on the surface.  Engineered microgels were co-transplanted at 2:1 ratio with islets (PEG/:islet) into the omentum of STZ-diabetic allogeneic cynomolgus recipients.  Un-engineered microgels without SA-FasL co-transplanted with islets served as controls.  Recipients were kept on a 3-month rapamycin regimen as the only maintenance therapy. Animals were monitored for graft survival up to 6 months and subjected to IVGTT and surgical removal of the graft to assess function.

Results: Immunomodulation with PEG presenting SA-FasL resulted in graft survival in all recipients (n=4) for ~180-day experimental end-point.  In marked contrast, all control recipients (n=3) under the same rapamycin regimen rejected the graft in an acute fashion. Graft recipients promptly achieved excellent glycemic control with normal fasting blood glucose levels.  Intravenous glucose tolerance test performed at 3- and 6-month post-transplantation revealed excellent islet function, comparable to naive animals. Importantly, surgical removal of the transplant resulted in prompt hyperglycemia, demonstrating graft function, which was further confirmed by fasting and stimulated insulin and c-peptide levels. Graft survival was associated with increased number of FoxP3+ T regulatory cells in the graft site with no significant changes in the systemic frequency of various T cell subsets. Furthermore, T cells from long-term graft recipients responded to donor antigens at comparable levels to pre-transplant T cells, indicating the localized nature of induced tolerance.  This immunomodulatory protocol did not result in significant alterations in the liver and kidney metabolic function, demonstrating safety.

Conclusions: Immunomodulation with PEG microgels presenting SA-FasL is effective in overcoming rejection of allogeneic islet grafts in NHPs in the absence of chronic immunosuppression.  Thus, PEG microgels presenting SA-FasL as an off-the-shelf product has considerable clinical potential for the treatment of type 1 diabetes.