Analysis of the association of peptides of optimal length to class I molecules on the surface of cells.

The association of major histocompatibility complex (MHC) class I molecules on the surface of cells with synthetic antigenic peptides of eight or nine amino acid residues was examined. Peptides were synthesized that correspond to the antigenic sequences from ovalbumin and influenza nucleoprotein believed to be naturally processed and presented by cells with Kb and Db MHC class I molecules, respectively. Consistent with the results of others, these peptides were 10(3)-10(5) times more active in stimulating specific T cells as compared to peptides of longer sequences. When cells are incubated with these peptides at less than 0.01-0.1 microM, the association of the peptides with class I molecules is dependent on (i) the reassociation of free beta 2-microglobulin from the extracellular fluids, (ii) a process that requires cells to be metabolically active, or (iii) stabilization of class I heterodimers by chemical crosslinking. In contrast, when cells are incubated with these peptides at greater than 0.1-1.0 microM, the peptides associate with class I molecules in the absence of exogenous beta 2-microglobulin, energy, or chemical crosslinking. Antigen competition experiments suggest that the class I molecules that bind peptides offered at high concentration become only transiently receptive to binding peptide. The concentration of peptides required for presentation to T cells under these conditions corresponds to those that stabilize Kb molecules on the surface of RMA-S mutant cells in the absence of exogenous beta 2-microglobulin. These results support the concept that the receptivity of class I molecules on cells is determined by the dissociation of beta 2-microglobulin from MHC class I that lacks bound peptides.