Impact of Interleukin-2–expanded Regulatory T Cells in Various Allogeneic Combinations on Mouse Skin Graft Survival

The impact of in vivo regulatory T cells (Treg) expansion using short-term injections of interleukin-2 (IL-2) coupled to a specific anti–IL-2 antibody was examined in various allogeneic combinations of murine skin transplantations. In a model of a single major histocompatibility complex (MHC) class II disparity, the IL-2–expanded Tregs infiltrated the transplanted skin, inhibited Th1 alloreactivity, and prevented acute graft rejection. However, in the presence of increased load of CD4-recognized alloantigens, exogenous IL-2 only moderately prolonged graft survival as attested by CD8 T cell-depletion in full minor plus major mismatched recipients treated with IL-2. If direct CD8 alloreactivity remained intact, the IL-2/anti-IL-2–mediated Tregs expansion failed to delay allograft rejection. This observation was confirmed by the inability of expanded Tregs to delay rejection of multiple minor disparate (MHC matched) skin allografts. Altogether, these results warn that cross-reactive CD8+ T cells represent an important hurdle to Treg-based tolerance induction.