Specific T cell recognition of kinetic isomers in the binding of peptide to class II major histocompatibility complex

Helper T cells are triggered by molecular complexes of antigenic peptides and class II proteins of the major histocompatibility complex. The formation of stable complexes between class II major histocompatibility complex proteins and antigenic peptides is often accompanied by the formation of a short-lived complex. In this report, we describe T cell recognition of two distinct complexes, one short-lived and the other long-lived, formed during the binding of an altered myelin basic protein peptide to I-A(k). One myelin basic protein-specific T cell clone is triggered by only the short- lived complex, and another is triggered by only the stable complex. Thus, a single peptide bound to a particular class II molecule can activate different T cells depending on the conditions of the binding reaction.