Attenuation of Donor‐Reactive T Cells Allows Effective Control of Allograft Rejection Using Regulatory T Cell Therapy

Regulatory T cells (Tregs) are essential for the establishment and maintenance of immune tolerance, suggesting a potential therapeutic role for Tregs in transplantation. However, Treg administration alone is insufficient in inducing long‐term allograft survival in normal hosts, likely due to the high frequency of alloreactive T cells. We hypothesized that a targeted reduction of alloreactive T effector cells would allow a therapeutic window for Treg efficacy. Here we show that preconditioning recipient mice with donor‐specific transfusion followed by cyclophosphamide treatment deleted 70–80% donor‐reactive T cells, but failed to prolong islet allograft survival. However, infusion of either 5 × 106 Tregs with direct donor reactivity or 25 × 106 polyclonal Tregs led to indefinite survival of BALB/c islets in more than 70% of preconditioned C57BL/6 recipients. Notably, protection of C3H islets in autoimmune nonobese diabetic mice required islet autoantigen‐specific Tregs together with polyclonal Tregs. Treg therapy led to significant reduction of CD8+ T cells and concomitant increase in endogenous Tregs among graft‐infiltrating cells early after transplantation. Together, these results demonstrate that reduction of the donor‐reactive T cells will be an important component of Treg‐based therapies in transplantation. This study demonstrates that donor‐reactive regulatory T cells are more potent than polyclonal regulatory T cells in preventing rejection; however, both types require depletion of donor‐reactive T effector cells for optimal efficacy. See editorial by Bradley on page 5.