Transplant tolerance to pancreatic islets is initiated in the graft and sustained in the spleen

The immune system is comprised of several CD4+ T regulatory (Treg) cell types, of which two, the Foxp3+ Treg and T regulatory type 1 (Tr1) cells, have frequently been associated with transplant tolerance. However, whether and how these two Treg-cell types synergize to promote allograft tolerance remains unknown. We previously developed a mouse model of allogeneic transplantation in which a specific immunomodulatory treatment leads to transplant tolerance through both Foxp3+ Treg and Tr1 cells. Here, we show that Foxp3+ Treg cells exert their regulatory function within the allograft and initiate engraftment locally and in a non-antigen (Ag) specific manner. Whereas CD4+CD25− T cells, which contain Tr1 cells, act from the spleen and are key to the maintenance of long-term tolerance. Importantly, the role of Foxp3+ Treg and Tr1 cells is not redundant once they are simultaneously expanded/induced in the same host. Moreover, our data show that long-term tolerance induced by Foxp3+ Treg-cell transfer is sustained by splenic Tr1 cells and functionally moves from the allograft to the spleen.