Vaccination strategy determines the emergence and dominance of CD8+ T-cell epitopes in a FVB/N rat HER-2/neu mouse model of breast cancer

The HER-2/neu oncogene has > 25 HLA epitopes, yet only one FVB/N mouse CD8(+) T-cell epitope has been mapped to date. This epitope has been termed the immunodominant epitope for the FVB/N mouse, but we propose that the vaccination strategy determines the dominance of epitopes. Using a series of overlapping peptides, we have mapped another CD8(+) T-cell epitope that emerges in the FVB/N mouse following vaccination with Listeria monocytogenes-based vaccines that express fragments of HER-2/neu. Following the identification of this novel H-2K(q)-restricted epitope, we sought to compare the T-cell response to this epitope with the previously identified PDSLRDLSVF epitope. This newly identified epitope and the previously identified epitope lie within fragments contained in different vaccines, the PDSLRDLSVF epitope in Lm-LLO-EC2 and the newly identified PYNYLSTEV epitope in Lm-LLO-EC1; thus, it has been possible to compare the responses of these epitopes independent of any competing response between the epitopes. CTL analysis of individual peptide-pulsed target cells and intracellular cytokine stain for IFN-gamma produced by splenocytes from Lm-LLO-EC1 compared with Lm-LLO-EC2 vaccinated FVB/N mice shows that there is no difference between the responses generated to either of these epitopes. We also show that the avidity of the CD8(+) T cells for either of these epitopes is similar based on the concentration of peptide necessary to mediate similar levels of lysis of target cells. In addition, HER-2/neu DNA vaccination followed by CTL analysis further showed that both of these peptides can emerge as epitopes.