In Vivo Expansion of Regulatory T Cells by Low-Dose Interleukin-2 Treatment Increases Allograft Survival in Corneal Transplantation

Background. Corneal allograft survival dramatically decreases in hosts with inflamed or vascularized recipient beds. We have previously shown that in rejected corneal allografts regulatory Tcells (Treg) demonstrate diminished Foxp3 expression and immunoregulatory function. Treatment with low doses of IL-2 selectively expands Treg and has been proposed for the treatment of autoimmune diseases. In this study, we investigated the effect of low-dose IL-2 administration on Treg function and corneal allograft survival. Methods. Allogeneic corneal transplantation was performed on inflamed host beds. Low-dose systemic IL-2 was administered starting 3 days before grafting until 6 weeks after transplantation. Frequencies of Treg and their immunosuppressive function and antigen specificity were assessed using flow cytometry, in vitro proliferation assays, and adoptive transfer experiments. Frequencies of effector T cells (Teff) and graft infiltrating immune cells were measured at 2 weeks posttransplantation. Long-term allograft survival was evaluated for up to 9 weeks using Kaplan-Meier survival analysis. Results. Treatment with low-dose IL-2 significantly increased frequencies of CD4(+)CD25(+)Foxp3(+) Treg and their immunosuppressive function. It also suppressed alloimmune response as shown by the decreased CD4(+)IFN gamma(+) T cell frequencies and graft infiltration of CD45(+) and CD4(+) cells. Clinical evaluation of the grafts showed significant improvement in long-term corneal allograft survival in the IL-2 treated group compared with controls. Conclusions. Our study is the first to report that treatment with low-dose IL-2 increases survival of corneal allografts. We propose that IL-2-mediated Treg expansion can be an effective tool to prevent alloimmunity and to improve long-term allograft survival in transplantation.