Induction of Tolerance to Human Allogeneic Islet Transplants in Humanized NOD/SCID Mice

Introduction: A recombinant soluble form of the inhibitory receptor ILT3 (ILT3Fc) was tested as an immunosuppressive treatment in a pancreatic islet transplantation model in humanized NOD/SCID mice. We sought to determine its efficacy in blocking allo-reactions against transplanted pancreatic islets and to determine its mechanism of action. Methods: Diabetes was induced in NOD/SCID mice in by streptozotocin injection. Mice were then transplanted with human pancreatic islets (1500 IEQ). Upon restoration of euglycemia, the mice were ‘humanized’ with allogeneic peripheral blood mononuclear cells. Treatment with ILT3Fc, control human IgG, or saline was initiated immediately, at day 15 or day 20 after humanization. Rejection was diagnosed when two glycemic readings >350 mg/dl were recorded. Histological studies of the tolerated and rejected grafts were performed. CD4 and CD8 T-cells from the spleen were assayed for suppressor activity, cytokine production, and costimulatory phenotypic profiles. Results: Although human T-cell engraftment was similar in all groups, ILT3Fc-treated mice tolerated the islets for the entire period of observation, while untreated mice rejected the graft within 7 weeks (P < 0.0001). Initiation of treatment at day 15 reversed rejection and rescued 70% of grafted mice, however, initiation at day 20 resulted in loss of all grafts. ILT3Fc treatment suppressed Th1 and Th2 cytokine production and costimulatory molecules. Human CD8+ T cells (but not CD4+ T cells) isolated from tolerant mice exhibited suppressor activity inhibiting Th alloreactivity. Histochemical studies showed a striking difference between the insulin production and T cell infiltration of human pancreatic islets from tolerant, ILT3Fc-treated mice and non-ILT3Fc treated controls. Conclusion: ILT3-Fc is an effective immunoregulatory agent that suppressed or reversed islet allograft rejection in this humanized mouse model by induction of allospecific CD8 T suppressor cells.