Antigen processing: approaches for dissecting subcellular mechanisms that form the basis for T-cell responses...

Antigen processing determines the production of peptides from antigens, including allergens, and their binding to class II major histocompatibility complex (MHC-II) molecules, which stimulate T-cell responses. Our studies have addressed the cell biology and biochemistry of the MHC-II antigen processing pathway using subcellular fractionation of macrophages on Percoll density gradients, coupled with other techniques. We have isolated a high density, late endocytic antigen processing compartment, with lysosomal properties, that contains a high level of MHC-II molecules, as assessed by several techniques. Moreover, the direct formation of peptide MHC-II complexes was demonstrated within this compartment, using a T hybridoma assay for peptide MHC-II complexes present in subcellular fractions of macrophages previously exposed to the model antigen, hen's egg-white lysozyme. These observations support an important role for this compartment in the class II major histocompatibility complex antigen processing pathway.