CD4(+)CD25(+) immune regulatory cells are required for induction of tolerance to alloantigen via costimulatory blockade

Immune regulatory CD4(+)CD25(+) cells play a vital role in the induction an d maintenance of self-tolerance and are essential for T cell homeostasis an d the prevention of autoimmunity. Induction of tolerance to allogeneic dono r grafts is a clinically desirable goal in bone marrow and solid organ tran splantation To determine whether CD4(+)CD25(+) cells regulate T cell respon ses to alloantigen and are critical for tolerance induction, murine CD4(+) T cells were tolerized to alloantigen via ex vivo CD40 ligand (CD40L)/CD40 or CD28/cytotoxic T lymphocyte-associated antigen 4/B7 blockade resulting i n secondary mixed leukocyte reaction hyporesponsiveness and tolerance to al loantigen in vivo. CD4(+)CD25(+) T cells were found to be potent regulators of alloresponses. Depletion of CD4(+)CD25(+) T cells from the CD4(+) respo nder population completely abrogated ex vivo tolerance induction to alloant igen as measured by intact responses to alloantigen restimulation in vitro and in vivo. Addback of CD4(+)CD25(+) T cells to CD4(+)CD25(-) cultures res tored tolerance induction. These data are the first to indicate that CD4(+) CD25(+) cells are essential for the induction of tolerance to alloantigen a nd have important implications for tolerance-inducing strategies targeted a t T cell costimulatory pathways.