Cryptic open reading frames in plasmid vector backbone sequences can provide highly immunogenic cytotoxic T-lymphocyte epitopes.
Murine tumor cells obtained through transfection of expression plasmids carrying activated cellular and/or viral oncogenes constitute formidable tools for immunological tumor research. As reported previously, mouse embryo cells of C57BL/6 origin, transformed by mutated p53 or human papilloma virus type 16 (HPV16), present, at their surface, MHC-bound peptides that are derived from the p53 and the HPV16 E7 oncoproteins, respectively, which can serve as a target for a highly effective antitumor T-cell response. Here, we describe the identification, through molecular cloning, of an additional, highly immunodominant peptide that is presented by the aforementioned HPV16- and p53-transformed cells. This peptide is encoded by a cryptic open reading frame in the backbone sequences of the plasmids that had been used to generate these cells. Considerable amounts of transcripts encompassing this open reading frame were detected in the cells concerned. These transcripts were the result of the bidirectional nature of the retroviral long terminal repeat (LTR) present in the expression plasmids used for transfection, which resulted in transcription of the gene of interest, as well as in transcription of the vector sequences positioned at the other side of the LTR. Due to this mechanism, all tumor cells harboring LTR-driven expression plasmids expressed the highly immunogenic peptide, whereas cells containing plasmids driven by more unidirectional promoters exhibited lower levels of this peptide. LTR-driven expression plasmids were also shown to encode this peptide epitope when used for DNA vaccination, as mice vaccinated with such a plasmid developed a CTL response against this peptide. Our data show that awareness of plasmid backbone-derived epitopes is of crucial importance for the correct interpretation of preclinical experiments and for the design of DNA vaccines.