LAUSR

Identifying appropriate antigens is essential to developing effective cancer vaccines. Protein phosphorylation plays a major role in signal transduction pathways that are dysregulated and contribute to the malignant phenotype of cancer cells. Our lab has shown that phosphopeptides derived from these proteins can be presented by major histocompatibility complex Class-I molecules and represent a new class of cancer-associated neoantigens. We previously reported that healthy donors without evident prior cancer had unusually robust CD8 T-cell responses to a cohort of HLA-B7+ leukemia-associated phosphopeptides, suggesting that they might be due to pre-existing phosphopeptide specific memory T cells. To test this, we evaluated CD8+CD45RO+ memory T cells purified from the blood of healthy donors. These cells showed robust responses to 2-25% of HLA-A2+ and 0-23% of HLA-B7+ cancer-associated phosphopeptides. This demonstrates that healthy individuals with no evident cancer history have pre-existing immune memory to some cancer-associated phosphopeptides. We identified 3 “immunodominant” HLA-A2 associated phosphopeptides, in that memory responses were evident to them in the majority of analyzed donors. Such immunodominance suggests that these phosphopeptides result from signaling processes that occur commonly in transformation- or viral infection-related processes. In contrast, immunodominant phosphopeptides were not evident in the context of HLA-B7. While 2 donors showed pre-existing immune memory to a large number of shared targets, other donors showed memory to only small numbers of nonoverlapping phosphopeptides. This suggests that the mechanisms driving expression of these phosphopeptide antigens among healthy donors are much less common. We also found that responses to some phosphopeptides could be detected directly ex vivo. In some cases, the phosphopeptide targets of these ex vivo responses did not stimulate responses after 2 weeks in culture. These results suggest that some phosphopeptides in some donors may be the targets of ongoing immune responses composed of effector or effector memory cells that do not persist long-term in culture. These results provide evidence of regular, ongoing immune surveillance in healthy individuals to cancer-associated phosphopeptides. Citation Format: Amanda M. Lulu, Kara L. Cummings, Victor H. Engelhard. Pre-existing immune memory to cancer-associated phosphopeptides in healthy donors [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2018 Nov 27-30; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(4 Suppl):Abstract nr B68.