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 and maintenance of self-tolerance and are essential for T cell homeostasis and the prevention of autoimmunity. Induction of tolerance to allogeneic donor grafts is a clinically desirable goal in bone marrow and solid organ transplantation. To determine whether CD4+CD25+ cells regulate T cell responses 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 in secondary mixed leukocyte reaction hyporesponsiveness and tolerance to alloantigen in vivo. CD4+CD25+ T cells were found to be potent regulators of alloresponses. Depletion of CD4+CD25+ T cells from the CD4+ responder population completely abrogated ex vivo tolerance induction to alloantigen as measured by intact responses to alloantigen restimulation in vitro and in vivo. Addback of CD4+CD25+ T cells to CD4+CD25− cultures restored tolerance induction. These data are the first to indicate that CD4+CD25+ cells are essential for the induction of tolerance to alloantigen and have important implications for tolerance-inducing strategies targeted at T cell costimulatory pathways.