Dominant contribution of the proteasome and metalloproteinases to TAP-independent MHC-I peptide repertoire

Tumors frequently display defects in the MHC-I antigen processing machinery, such as deficiency of the peptide transporter TAP. Interestingly, the residual peptide repertoire contains neo-antigens which are not presented by processing-proficient cells. We termed these immunogenic peptides TEIPP ('T-cell epitopes associated with impaired peptide processing') and were interested to unravel their TAP-independent processing pathways. With an array of chemical inhibitors we assessed the participation of numerous proteases to TAP-independent peptides and found that the previously described catalytic enzymes signal peptidase and furin contributed in a cell-type and MHC-I allele-specific way. In addition, a dominant role for the proteasome and metallopeptidases was observed. These findings raised the question how these proteasome products get access to MHC-I molecules. A novel TEIPP peptide-epitope that represented this intracellular route revealed that the lysosomal peptide transporter ABCB9 ('TAP-like') was dispensable for its presentation. Interestingly, prevention of endolysosomal vesicle acidification by bafilomycin enhanced the surface display of this TEIPP peptide, suggesting that this proteasome-dependent pathway intersects endolysosomes and that these antigens are merely destroyed there. In conclusion, the proteasome has a surprisingly dominant role in shaping the TAP-independent MHC-I peptide repertoire and some of these antigens might be targeted to the endocytic vesicular pathway.