Evidence of Ongoing Immune Surveillance to Cancer-Associated Phosphopeptides in Healthy Individuals

Abstract Dysregulated protein phosphorylation plays a major role in cancer cell malignancy. We showed that phosphopeptides are presented by MHC-I molecules, representing a new class of cancer neoantigens. We found that healthy donors with no evident prior cancer had robust CD8+ T cell responses to a cohort of HLA-B7+ leukemia-associated phosphopeptides. Here we report that CD8+CD45RO+ T cells purified from blood of different healthy donors showed responses to 2–25% of HLA-A2+ and 0–23% of HLA-B7+ cancer-associated phosphopeptides, confirming that individuals with no evident cancer history have pre-existing phosphopeptide specific immune memory. Six of 59 HLA-A2+ phosphopeptides were “immunodominant,” with memory responses evident in the majority of analyzed donors. This suggests that these phosphopeptides result from processes common in cellular transformation or viral infection. In contrast, no immunodominant HLA-B7+ phosphopeptides were identified among the 168 analyzed, suggesting that mechanisms driving their expression in healthy donors are less common. Robust day 14 responses were observed against a small cohort of HLA-B7+ phosphopeptides in 2 donors. Responses to some of these phosphopeptides could be detected ex vivo and were evident over 3 years, suggesting they reflected chronic antigen exposures. In contrast, responses to 2 phosphopeptides evident ex vivo diminished or disappeared over 14 days in culture, suggesting a recent acute immune response composed of terminal effectors unable to persist long term. These results provide evidence of ongoing immune surveillance in healthy individuals to phosphopeptides known to be expressed on cancers.