T cell recognition of self and altered self antigens.

T lymphocytes bearing alpha/beta TCR recognize antigens in the context of self MHC molecules, and this recognition leads to growth, differentiation, and effector functions. Recently, it has become clear that altered peptides generated by single amino acid substitution of the antigenic peptide can alter the patterns of differentiation and effector functions of the responding T lymphocytes. By defining the pattern of recognition and residues of the cognate ligand that bind to the TCR, altered peptide ligands (APLs) have been generated by selectively substituting the TCR contact residues in the antigenic peptide. These APLs have been utilized in vitro to study the biology of T cell function and alterations in the T cell signaling pathway. In vivo APLs have been utilized to study the mechanism of positive selection in the thymus and in regulation of autoimmune diseases. With this basic knowledge, APLs that can either hypo- or hyper-stimulate T cell function can be generated that can specifically alter (inhibit or enhance) immune responses in vivo in autoimmune diseases and cancers.