Cellular mechanisms of artificial peptides binding to HLA

On the basis of the consideration that cell-free models cannot precisely mimic the complexity of the intracellular environment, we used a system to investigate the mechanisms that enable antigen-presenting cells (APC) to bind exogenous peptides through their human leukocyte antigen (HLA) molecules. We evaluated the uptake of the radiolabeled peptide 17-29-Tyr of influenza virus matrix protein by B-EBV cell lines, under various conditions. The results can be summarized as follows: a) the kinetics of peptide binding and release are very fast in living, fully competent cells; b) the peptide-HLA complexes are short-living and the DR molecules continuously undergo peptidic exchange; c) using glutaraldehyde-fixed cells, the kinetics of the two phenomena are slow, closely resembling those observed with the same peptide and purified, immobilized DR molecules. The data suggest that in APC, cellular mechanisms are operative that increase the efficiency of both loading and unloading of Class II HLA with exogenous peptides. This is likely to be related to the recycling of Class II molecules to intracellular compartments, were binding takes place. The observation that the HLA-peptide complex is a dynamic structure, suggests the possibility of replacing natural peptides with synthetic ones at this level, in order to regulate the immune response.