The role of phosphopeptide neoantigens in cancer immunotherapy

The identification of tumor-specific antigens provides the basis for the development of an efficient targeted immunotherapy. Phosphopeptides have been proposed as promising novel tumor antigens. This is because phosphoproteins are integrated in most signalling pathways and malignant dysregulation of these pathways leads to aberrant and increased phosphorylation of proteins. These phosphoproteins can be degraded and the derived peptide fragments are presented via MHC-class I molecules on the cell surface of altered cells leading to CD8 T cell recognition.

Using a mass spectrometry approach, until today we have identified hundreds of phosphopeptides (MHC-I-pP) on several cancer entities, including hepatocellular carcinoma (HCC). MHC-I-pP were found predominantly on tumorous tissues when compared to adjacent healthy tissues. Notably, many of the underlying proteins play an important role in tumor progression or survival of HCC and other cancer entities, making them especially interesting for immunotherapeutic strategies. Based on their potential involvement in tumor progression and development, 37 HLA-A2 and HLA-B7 MHC-I-pP were further selected for immunological testing. Peripheral blood lymphocytes (PBMCs) from healthy individuals, patients with chronic liver diseases or HCC patients were isolated and stimulated with the MHC-I-pP for 7 days followed by intracellular cytokine staining. CD8 T cell responses against MHC-I-pP were phosphate dependent, sequence specific and in quantity and quality comparable to those seen against immunodominant viral epitopes. MHC-I-pP specific CD8 T cell responses were found in PBMCs in patients with high risk for cancer development, but were lost after cancer formation. Additionally, first data indicate that patients responding to checkpoint therapy show MHC-I-pP specific CD8 T cell responses. Therefore, we are currently evaluating MHC-I-pP specific CD8 T cell responses in patients with HCC and malignant melanoma before and during the course of checkpoint inhibition therapy.

Taken together our results suggest that MHC-I-pP may be the target of cancer immune surveillance in liver disease and represent an attractive target for future cancer immunotherapies. Additionally, measuring immunity against MHC-I-pP might serve as a predictive biomarker for a response to checkpoint inhibition therapy.