Isolation of a functional antigen-Ia complex.

The helper T-cell recognition of globular protein antigens requires that the antigen be processed and presented by an I-region associated (Ia)-expressing antigen-presenting cell (APC). Processing involves the uptake of antigen into an intracellular, proteolytic, acidic compartment; release of peptide fragments containing the T-cell antigenic determinant; association of these peptides with Ia; and presentation of these complexes on the cell surface for recognition by the specific T cells. The molecular mechanisms by which processed antigenic peptides associate with Ia within the APC are poorly understood. To date, functional antigen-Ia complexes have not been isolated from cells that have processed native antigens, although the resolution of the structure of a major histocompatibility complex (MHC) class I protein indicates that peptide is bound in a groove between two alpha-helical regions of the molecule and synthetic peptides have been demonstrated to bind purified MHC both in detergent solution and incorporated into planar membranes, where the MHC-peptide complexes function to activate specific T cells. Here we demonstrate that Ia purified from APCs that have processed the native globular protein antigen cytochrome c, when incorporated into lipid membranes, stimulates cytochrome c-specific T cells in the absence of exogenous antigenic peptide. The T-cell response to Ia purified from cytochrome c-pulsed APCs shows the same MHC restriction and antigen fine specificity as the response to antigen-pulsed APCs. Indeed, T-cell recognition of pigeon cytochrome c (Pc) shows a well documented high-affinity heteroclitic cross-reaction to insect cytochromes c-namely, those of Drosophila melanogaster (DMc) and tobacco hornworm moth (THMc). The same heteroclitic response is observed when purified Ia from unpulsed cells, incorporated into lipid membranes, is used to present antigenic peptides of Pc and of THMc. Significantly, Ia purified from APCs that have processed DMc is approximately 50-fold more active in stimulating specific T cells compared to Ia purified from APCs that have processed Pc. The peptide-Ia complex isolated here may provide the necessary material for analysis of the physiochemical properties of the processed form of the antigen that is produced by the APC and associates with Ia.