Donor-specific transplantation tolerance: The paradoxical behavior of CD4⁺CD25⁺ T cells

To investigate the antigen specificity of regulatory T cells capable of preventing transplant rejection, we have developed two different strategies to achieve tolerance to fully mismatched skin grafts in euthymic mice. A combination of nondepleting Abs targeting CD4, CD8, and CD154 (CD40 ligand) induces dominant transplantation tolerance to fully mismatched skin allografts. Such tolerance is antigen-specific, mediated by regulatory T cells, and can be extended through linked suppression to naïve lymphocytes. The same protocol, when combined with allogeneic bone marrow, enables the development of mixed hematopoietic chimerism and deletional tolerance. Although we cannot exclude that some regulatory T cells may persist in chimeric mice, these cells are insufficient to mediate linked suppression. CD4⁺CD25⁺ T cells, whether taken from naïve mice or from mice tolerized through either treatment protocol, were always able to prevent rejection of skin grafts by naïve CD4⁺ T cells, and did so with no demonstrable specificity for the tolerizing donor antigens. Such data question whether CD4⁺CD25⁺ regulatory T cells alone can account for the antigen specificity of dominant transplantation tolerance.