Immunity to phosphopeptides in healthy donors and cancer patients

MHC-I-presented phosphopeptides have been identified as cancer-expressed antigens and sometimes as targets of immune responses in healthy donors. It is unknown to what extent responses reflect memory or effector responses, and from what types of immunogenic exposures MHC-phosphopeptides arise in healthy donors. We characterized responses in 15 healthy donors to a total of 205 HLA-A2 or HLA-B7-restricted phosphopeptides previously identified on one or more cancer types. Most healthy donors demonstrated some pre-existing immune memory to MHC-phosphopeptides. Wide heterogeneity in the specific phosphopeptides recognized in each donor suggested that most immunity arose due to encountering uncommon immunogens, such as more rare infectious agents or transformed cells. Donor-to-donor in the number of phosphopeptides recognized suggested variations in exposure histories, tolerance, and/or antigen presentation. Despite this great heterogeneity, we identified three immunodominant phosphopeptides that were recognized by most HLA-A2+ healthy donors, suggesting a common source of exposure such as an infectious agent. Analyses in two healthy donors demonstrated predominantly resting TCM memory, as well as incidents of active responses to a subset of phosphopeptides, consistent with recent re-exposures to the immunogen. In the absence of evident illness at the time T cells were collected from these donors, active responses may suggest that the phosphopeptides arose on transformed cells.

We also examined responses to immunodominant phosphopeptides and tumor-expressed phosphopeptides in a cohort of 10 HLA-A2+ ovarian cancer patients. Responses to all phosphopeptides, regardless of whether they were expressed on the patient’s tumor and including 2 of the 3 immunodominant phosphopeptides, were negligible. Despite this, 4 of the 10 patients responded to the immunodominant, pIRS21098-1105, including VTB239, whose tumor expressed this phosphopeptide. VTB239’s response was skewed toward an active TEM/TEMRA response and was seen in both PBMCs and TILs, suggesting an active response and demonstrating the ability of phosphopeptide-specific T cells to infiltrate a tumor. Melanoma patients were also impaired in responding to a subset of phosphopeptides. However, vaccination was able to induce responses in most patients. These data support continued investigation into treatments that induce or augment T cell targeting of MHC-phosphopeptides as a part of cancer immunotherapy. Therapies that boost phosphopeptide expression on patients’ tumor may also be considered as part of a combinatorial approach.