Characterization of MHC class II-presented peptides generated from an antigen targeted to different endocytic compartments

We evaluated the capacity of the secretory pathway or of different endocyti c compartments in B cell lines to generate MHC class Ii-presented peptides from the antigen ovalbumin (OVA). Sorting signals from the transferrin rece ptor (TFR), targeted a chimeric OVA fusion protein to early endosomes and l ed to the generation of 8 of 12 presented peptides. Sorting signals from th e lysosome-associated membrane protein 1 (LAMP-1), targeted an OVA fusion p rotein to lysosomes, and led to the generation of 9 of 12 peptides. In cont rast, OVA with only a signal sequence led to the generation of only 2 prese nted peptides. There were both qualitative and quantitative differences in the generation of peptides from the different fusion proteins, suggesting t hat multiple distinct compartments are involved in generating different epi topes. One peptide was presented better from the TFR fusion protein, while all others were presented better from the LAMP-1 construct. Twelve peptides were generated from exogenously supplied OVA, including 3 peptides that we re not generated from any of the fusion proteins. Since most endogenously s ynthesized foreign antigens are rarely presented on class II molecules, the se studies further suggest a strategy whereby antigens in DNA-based vaccine s could be targeted to endocytic compartments to enhance immunogenicity.