Insights into the processing of MHC class I ligands gained from the study of human tumor epitopes

Abstract

The molecular definition of tumor antigens recognized by cytolytic T lymphocytes (CTL) started in the late 1980s, at a time when the MHC class I antigen processing field was in its infancy. Born together, these two fields of science evolved together and provided each other with critical insights. Over the years, stimulated by the potential interest of tumor antigens for cancer immunotherapy, scientists have identified and characterized numerous antigens recognized by CTL on human tumors. These studies have provided a wealth of information relevant to the mode of production of antigenic peptides presented by MHC class I molecules. A number of tumor antigenic peptides were found to result from unusual mechanisms occurring at the level of transcription, translation or processing. Although many of these mechanisms occur in the cell at very low level, they are relevant to the immune system as they determine the killing of tumor cells by CTL, which are sensitive to low levels of peptide/MHC complexes. Moreover, these unusual mechanisms were found to occur not only in tumor cells but also in normal cells. Thereby, the study of tumor antigens has illuminated many aspects of MHC class I processing. We review here those insights into the MHC I antigen processing pathway that result from the characterization of human tumor antigens recognized by CTL.

Fig. 1

Pathways for processing and presentation of class I-restricted tumor antigens. CTL recognize on tumor cells antigenic peptides that are produced through a variety of unconventional processes, including aberrant transcription or mRNA splicing (1, 2), translation of alternative or cryptic open reading frames (3) or post-translational modifications (12). Proteins are targeted for degradation by the proteasome after the addition of ubiquitin chains by ubiquitin ligases (4). Surprisingly, proteasomes also produce antigenic peptides by splicing fragments that are originally non-contiguous in the parental protein (5). In the cytosol, additional proteases were found to produce or destroy antigenic peptides (6, 7). Peptides produced in the cytosol are then transported in the ER lumen by the transporter TAP (8), where they can be further trimmed by ER aminopeptidases such as ERAP1 or 2 (9). Peptides with the appropriate length and HLA binding motif are loaded onto MHC I molecules with the help of the peptide loading complex (10). The signal sequences of ER-targeted proteins also contain antigenic peptides: following cleavage of the signal sequence by the signal peptidase and the signal peptide peptidase (11), those peptides are released in the ER lumen and loaded onto MHC I independently of TAP. Membrane or secreted proteins also produce MHC class I binding peptides. Tyrosinase is a type I transmembrane protein, which supplies an antigenic peptide containing an aspartate instead of the genetically encoded asparagine. This peptide is produced after glycosylation of the tyrosinase protein in the ER, its retrotranslocation into the cytosol where it is deglycosylated and degraded by proteasome. Removal of the glycan moiety by peptide N-glycanase leads to deamidation of the asparagine into an aspartate residue, which is contained in the antigenic peptide presented by melanoma cells (12, 13). A peptide originating from secreted matrix metalloprotease-2 was found to be processed by the proteasome after endocytosis. After secretion, activation of pro-MMP-2 into mature MMP2 appears to be required for adequate processing of the antigenic peptide. Following endocytosis, the MMP-2 protein would be released in the cytosol and degraded by the proteasome (14)

Fig. 2

Model of the peptide splicing reaction inside the proteasome. Mechanism of production of the spliced the peptide RTKQLYPEW derived from gp100 (a) and the spliced peptide SLPRGTSTPK derived from SP110 (b). The balls represent the catalytic β-subunits of proteasomes with the hydroxyl group of the side chain of the N-terminal threonine