The pioneer round of translation and MHC-I peptides presented to cytotoxic T lymphocytes

Cytotoxic T lymphocytes (CTLs) detect disease-associated antigens through the presence of unique peptides displayed at the cell surface by MHC-I molecules. To allow immune surveillance, the entire pool of the cell`s proteins is constantly subjected to the MHC-I processing and presentation machinery. Perhaps the earliest protein products of most cellular genes are synthesized during the pioneer round of translation (PRT), a key step in nonsense-mediated mRNA decay (NMD) which allows scanning of new transcripts for the presence of a premature termination codon (PTC). It has been demonstrated that at least some PRT degradation products can be targeted to MHC-I presentation thus T cell activation.

To gain new insight into this putative PRT-to-MHC-I route we established an experimental system based on two pairs of reporter genes, where the two genes in each pair encode an identical fusion protein between a model antigenic peptide and EGFP, one of which harbors a PTC. We expressed these genes in different mouse and human cell lines and confirmed enhanced NMD activity for the PTC(+) gene in each pair by monitoring the effect of cycloheximide on the level of the respective mRNA. We then exploited several strategies for establishing the ratio between level of peptide presentation and total amount of protein product. We consistently observed significantly higher ratios for the PTC(+) mRNAs compared to the PTC(-) ones, pointing to correlation between the turnover of otherwise identical proteins and the fate of their template mRNA. Using confocal microscopy we showed a clear link between NMD, the presence of misfolded EGFP polypeptides and enhanced MHC-I peptide presentation. Our research provides, for the first time, strong evidence that peptides derived from degraded PRT products that are not defective, can still be efficiently directed to the MHC-I processing and presentation pathway for T cells. As the PRT does not discriminate between defective and correct transcripts and all newly synthesized mRNAs in the cell are potential substrates for scanning, our work suggests that the PRT can be an important source for MHC-I peptides which represent all cellular proteins.