CD4+CD25+ regulatory cells in acquired MHC tolerance.

Tolerance to self-antigens is an ongoing process that begins centrally during T-cell maturation in the thymus and continues throughout the cell's life in the periphery by a network of regulated restraints. Remaining self-reactive T-cells that escape intrathymic deletion may be silenced within the peripheral immune system by specialized regulatory CD4+ cells. By analogy, regulatory CD4+ cells that control immunity to "acquired self" should arise in circumstances where the immune system acquires tolerance to foreign MHC, such as the tolerance that develops following the exposure to foreign MHC antigens during the neonatal period. We have used this classic model of neonatal tolerance to examine the role of regulatory CD4+ cells in acquired tolerance to disparate class I and class II MHC. Adoptive transfer of unfractionated but not CD4+-depleted spleen cells from neonatal tolerant mice into SCID recipients inhibited skin graft rejection by immunocompetent CD8+ T cells. Using 5-bromo-2'-deoxyuridine incorporation, standard cytotoxic T-lymphocyte assays, short-term interferon-gamma ELISPOT, and intracellular FACS analysis to study CD8+ T-cell effector function, we demonstrated that neonatal tolerant mice contain CD4+CD25+ cells that suppress the development of anti-donor CD8+ T-cell responses in vitro. We conclude that regulatory CD4+CD25+ cells initiate and/or maintain tolerance by preventing the development of CD8+ T-cell alloreactivity.