Tr1 cell immunotherapy promotes transplant tolerance via de novo Tr1 cell induction in mice and is safe and effective during acute viral infection

Tr1 cell therapy is considered an emerging approach to improve transplant tolerance and enhance allogeneic graft survival. However, it remains unclear how Tr1 cells promote transplant tolerance and whether they will be safe and stable in the face of an acute viral infection. By employing a mouse model of pancreatic islet transplantation, we report that Tr1 cell therapy promoted transplant tolerance via de novo induction of Tr1 cells in the recipients. Acute viral infection with lymphocytic choriomeningitis virus (LCMV) had no impact on Tr1 cell number and function, neither on the Tr1 cells infused nor on the ones induced, and that was reflected in the robust maintenance of the graft. Moreover, Tr1 cell immunotherapy had no detrimental effect on CD8 and CD4 anti‐LCMV effector T‐cell responses and viral control. Together, these data suggest that Tr1 cells did not convert to effector cells during acute infection with LCMV, maintained transplant tolerance and did not inhibit antiviral immunity. Tr1 cell immunotherapy induces transplant tolerance to pancreatic islets via the de novo induction of Tr1 cells in the spleen. Tolerance established via Tr1 cell immunotherapy does not inhibit acute antiviral T‐cell responses. Viral Infection does not break transplant tolerance established by Tr1 cell immunotherapy.