A kinetic intermediate in the reaction of an antigenic peptide and I-E^k

Helper T cells are triggered by molecular complexes of antigenic peptides and cell surface glycoproteins of the MHC (gene products of the major histocompatibility complex) on antigen-presenting cells¹. There is now a lot of evidence that the complexes between isolated class II MHC molecules and selected peptides have long half-lives of approximately one day^2,3. The reported equilibrium binding constants between antigenic peptides and class II MHC molecules however, are only micromolar^4,5, suggesting that the association rate constants are very low. The only reported² association rate constant is for a chicken ovalbumin peptide (OVA323-339) binding to I-A^d, and is indeed remarkably low, about 1 litre per mole per second. Prompted by these unusual data, we have used the pigeon cytochrome-c peptide pCytc(88-104) and I-E^k reconstituted in planar lipid bilayers on glass slides to investigate further the kinetics of peptide-MHC reactions. We report the formation of two IE^k-pCytc peptide complexes. One complex has slow apparent association and dissociation kinetics, very similar to those reported previously for the chicken ovalbumin peptide and I-A^d. The second complex forms and dissociates about a hundred times more rapidly. The short-lived complex shows peptide-MHC specificity and is a kinetic intermediate in the formation of the long-lived complex; the long-lived complex is recognized by specific T-helper cells.