Cotransplantation With Human AAT-MSCs Enhances Human Islet Graft Survival in Humanized Mice by Modulating B- and T-Cell Responses

Immune-mediated rejection remains a major barrier to long-term success of islet/stem cell-derived islet transplantation for type 1 diabetes. We evaluated whether cotransplantation of human islets with human α-1 antitrypsin-overexpressing mesenchymal stromal/stem cells (AAT-MSCs) improves islet graft survival by modulating human immune responses in a humanized diabetic mouse model. Humanized NOD/SCID/IL2Rγ-/- (NSG) mice were generated by irradiation and transfer of human CD34+ hematopoietic stem/progenitor cells, resulting in stable T- and B-cell reconstitution. Diabetes was induced by multiple doses of low-dose streptozotocin (STZ), after which mice received intrahepatic transplants of human islets alone or together with AAT-MSCs and were monitored for 100 days. Cotransplantation with AAT-MSCs markedly improved glycemic control, prolonged graft survival, and reduced the risk of rejection. These beneficial effects were associated with lower serum IgM levels, which inversely correlated with graft rejection, and with expansion of regulatory B cells (Bregs), followed by durable suppression of T helper 1, T helper 17, and cytotoxic T cell responses. Cotransplant grafts showed improved insulin preservation and enrichment of interleukin (IL) 10+ cells within islets. In vitro, AAT-MSCs promoted Breg differentiation, shifted cytokine production toward IL-10 over IL-6, and attenuated activated B cell-mediated cytotoxicity against human islets. Collectively, these findings demonstrate that AAT-MSC cotransplantation promotes immune tolerance and human islet graft function, supporting its translational potential to reduce reliance on systemic immunosuppression in the clinic. ARTICLE HIGHLIGHTS: Immune-mediated rejection limits the long-term success of islet and stem cell-derived islet transplantation in type 1 diabetes, and strategies that protect the graft without systemic immunosuppression are needed. We asked whether cotransplanting human islets with α-1 antitrypsin-overexpressing mesenchymal stromal/stem cells (AAT-MSCs) improves graft outcomes in humanized mice, and how human immune responses are affected. AAT-MSC cotransplantation improved glycemic control, prolonged graft survival, and shifted immune responses toward a more regulatory, less inflammatory profile. These findings support further development of AAT-MSCs as an adjunct to reduce reliance on systemic immunosuppression in islet transplantation.