Self or foreign cellular proteins provide peptides for presentation by major histocompatibility complex (MHC) class I molecules on the surface of antigen presenting cells (APC). Surprisingly, several studies have shown that T-cells can recognize APC transfected with antigen genes that were not present in the appropriate translational context. To understand the basis of this phenomenon, APC were transfected with DNA constructs encoding the OVA257-264 (SL8) peptide, but with varying translation initiation codons. We report that, in addition to ATG, 6 other codons (ATT, ACG, CTG, GCG, TGG, GAT) also allowed presentation to SL8K-specific T-cells. Significantly, this set includes 3 of 4 known non-ATG translation initiation codons strongly suggesting that cryptic translation accounts for this phenomenon. Although expression of the SL8K complex was readily detected by T-cell activation, the amount of processed peptides was below detection limit (<30 copies/cell) in cell extracts. Thus, the fortuitous presence of these cryptic translation initiation sites in transcribed genes can explain how peptideMHC complexes were obtained in sufficient amounts for T-cell activation. The translation initiation codons identified here could also be useful for identifying potential open reading frames that possess biological and/or immunological activities.

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