Veto suppression: the peripheral way of T cell tolerization.

Cells with veto activity induce a state of tolerance in T cell precursors with specificity for antigen determinants expressed on the surface of the veto-active cell. This state of tolerance is not strictly defined, but results in altered responses to specific antigen, such as decreased proliferation, decreased development of cytotoxicity and secretion of interleukins, down-regulated ability to reject grafts and expression of T cell and IL-2 receptors. Both clonal anergy and clonal deletion has been shown to operate in vetoed T cells. Veto-induced tolerance can be established in vitro and in vivo for both MHC class I and II as well as minor histocompatibility antigens. The most powerful veto activity is present in mature activated cytotoxic CD8+ T cells, but other cells including noncytotoxic cells are also capable of acting as veto cells. Thus it appears that veto activity per se is not confined to a certain cellular entity, but rather reflects a constitutively expressed immunoregulatory capability inherent to a broad array of activated T cell and non-T cell categories with their own distinct functions not related to their eventual veto activity.