The role of intracellular acidification in antigen processing.

The experiments described here were designed to investigate the role of intracellular acidic organelles in the processing of foreign antigens. Simultaneous addition of antigen and the inactive precursor of an acid protease, cathepsin L, greatly diminished the stimulation of a T-cell hybridoma specific for pigeon cytochrome c. The effect occurred during antigen processing as the presentation of a peptide fragment was unaffected by precursor-cathepsin L. The effect required uptake of the enzyme via mannose-6-PO4 receptors and was specific for antigenic determinants cleavable by the enzyme. The results confirm, without any pharmacologic manipulation, that foreign antigen moves into an acidic environment during some phase of its processing. We also explored the role in antigen processing of one acidic compartment, the early endosome, by examining CHO cells expressing a temperature-sensitive defect in endosomal acidification. These cells were transfected with MHC class II genes to convert them into APC for CD4+T cells. When these cells were incubated at the nonpermissive temperature, their ability to process antigen was impaired. There was no negative effect on presentation of antigenic peptides nor on the uptake and degradation of the antigen. Wildtype CHO cells similarly transfected and tested did not show any aberrancy in antigen processing. The inhibition of processing by the mutant cells was only partial, even under stringent conditions, and the residual processing was eliminated by chloroquine. Our findings provide the first evidence for a contribution by acidified early endosomes to the pathway of antigen processing and suggest that other acidified compartments are also involved.