Characteristics of Immune Memory and Effector Activity to Cancer-Expressed MHC Class I Phosphopeptides Differ in Healthy Donors and Ovarian Cancer Patients

Abstract

Elevated immunity to cancer-expressed antigens can be detected in people with no history of cancer and may contribute to cancer prevention. We have previously reported that MHC-restricted phosphopeptides are cancer-expressed antigens and targets of immune recognition. However, the extent to which this immunity reflects prior or ongoing phosphopeptide exposures was not investigated. In this study, we found that preexisting immune memory to cancer-expressed phosphopeptides was evident in most healthy donors, but the breadth among donors was highly variable. Although three phosphopeptides were recognized by most donors, suggesting exposures to common microbial/infectious agents, most of the 205 tested phosphopeptides were not recognized by peripheral blood mononuclear cells (PBMC) from any donor and the remainder were recognized by only 1 to 3 donors. In longitudinal analyses of 2 donors, effector immune response profiles suggested active reexposures to a subset of phosphopeptides. These findings suggest that the immunogens generating most phosphopeptide-specific immune memory are rare infectious agents or incipient cancer cells with distinct phosphoproteome dysregulations, and that repetitive immunogenic exposures occur in individual donors. Phosphopeptide-specific immunity in PBMCs and tumor-infiltrating lymphocytes from ovarian cancer patients was limited, regardless of whether the phosphopeptide was expressed on the tumor. However, 4 of 10 patients responded to 1 to 2 immunodominant phosphopeptides, and 1 showed an elevated effector response to a tumor-expressed phosphopeptide. As the tumors from these patients displayed many phosphopeptides, these data are consistent with lack of prior exposure or impaired ability to respond to some phosphopeptides and suggest that enhancing phosphopeptide-specific T-cell responses could be a useful approach to improve tumor immunotherapy.

The landscape of MHC-presented phosphopeptides yields actionable shared tumor antigens for cancer immunotherapy across multiple HLA alleles

“…1E), we found previously undescribed phosphopeptides pGTF3C2, pPPP1R12A, pPIM1, pMYBBP1A, and pSRRM1 were presented by multiple EBV-BLCL and EBV-LPD, but not healthy B cells. Since some phosphopeptides have been shown to be immunogenic in A2+ donors[10,17,34], we sought to determine if A11+ donors also harbor phosphopeptide-specific T cells. We selected phosphopeptides and normal peptides presented by donor AP’s EBV-BLCL, and found that only pGTF3C2 elicited peptide- specific IFNg production by sensitized T cells from this donor (Fig.…”

“…Phosphopeptides represent an emerging class of HLA ligands that have potential to be targeted as shared tumor antigens produced not by mutations, but by cancer associated post-translational modifications. Since T cell responses to phosphopeptides have been studied comprehensively in the context of HLA-A*02:01 and -B*07:02 [10,34,42], we sought to expand the phosphopeptidome by systematically reanalyzing large immunopeptidomics datasets containing phosphopeptides, yielding a new set of phosphopeptides presented by the HLA-A3 supertype as well as C*07:01. We further used healthy tissue databases [40] to determine the extent to which these phosphopeptides are found on healthy tissue.…”

“…The immunogenicity of HLA-A*0201-presented phosphopeptides in healthy donors has been previously shown to derive from memory, rather than naïve, T cells, unlike other responses to self-antigens [10,34]. Given the prevalence of phosphopeptide-specific T cells (PP-CTL), we sought to detect PP-CTL via phosphopeptide dextramers assembled using an improved method [28].…”