Depletion of Foxp3⁺ T cells abrogates tolerance of skin and heart allografts in murine mixed chimeras without the loss of mixed chimerism

The relative contribution of central and peripheral mechanisms to the generation and maintenance of allograft tolerance is of considerable interest. Here, we present new evidence that regulatory T cells (Foxp3⁺) maintain skin and heart allograft tolerance in mixed hematopoietic chimeric mice. Transient depletion of both donor-and recipient-derived Foxp3⁺ cells was necessary and sufficient to induce decisive rejection of long-accepted skin and heart allografts. In contrast, stable hematopoietic chimerism remained, and there was no detectable induction of donor-specific reactivity to hematopoietic cells. Foxp3⁺ cell depletion did not result in the rejection of skin grafts of only MHC-disparate donors (B6.C-H2^d/bByJ), indicating that MHC antigens were not the target in the graft. We conclude that two different mechanisms of tolerance are present in mixed chimeras. Hematopoietic chimerism, resistant to Foxp3⁺ depletion, is probably due to deletional tolerance to MHC antigens, as supported by previous studies. In contrast, regulatory tolerance mechanisms involving Foxp3⁺ cells are required to control reactivity against non-MHC antigens not present on hematopoietic lineages.