Abstract A23: Proteogenomic discovery of novel tumor proteins as neoantigens for personalized T-cell immunotherapy in pediatric medulloblastoma

Medulloblastoma is a common childhood brain tumor for which current therapies fail to cure high-risk subgroups, including metastatic and recurrent disease. Toxicity limits further intensification of conventional chemo-radiation therapy, and most survivors endure significant lifelong sequelae from treatment. There is an urgent need to develop new tools to combat this cancer without increasing the late effects burden. Neoantigens, which are exclusively expressed on tumor cells, are one of the main targets of an effective antitumor T-cell response. The ideal target antigen is abundantly expressed by tumor cells but not by normal tissues, in order to limit off-target effects. Tumors translate a host of highly novel transcripts that are the result of aberrations in tumor DNA and the unmasking of alternative or novel exons. We used a novel proteogenomic approach to identify tumor-restricted peptides and used them to select and expand T cells capable of mounting a tumor-specific cytotoxic immune response, with no off-target effects for patients with MB. Using RNA-seq and WGS data, we created personalized custom searchable databases containing predicted novel proteins from somatic mutations, novel isoforms, and fusion proteins from 56 medulloblastoma tumors. By searching this database with raw mass spectrometry data from the paired medulloblastoma tumor, we have identified tens of peptides arising from the translation of tumor-specific transcripts; these peptides are potential neoantigens. Our data indicate that in cases of tumors with low mutation rates, such as pediatric brain tumors, novel isoforms are the main source of neoantigens. In a pilot study, we have tested these peptides for their ability to select and expand polyclonal populations of T cells from the same patient whose tumor was the source of the peptides. The immunogenicity of each individual peptide was then determined. Flow cytometry cellular characterization reveals populations of both CD4+ and CD8+ cells with an activation profile marked by IFN-γ and TNF-α production. Immunosuppressive marker profiles have also been characterized for these cells. Using cytotoxicity assays, we demonstrated that tumor-specific T cells can eliminate neoantigen-bearing tumor cells. These findings demonstrate an initial proof of principle that proteogenomics can be used to identify immunogenic tumor specific peptides and lay the groundwork for a personalized, targeted T-cell therapy for children with high-risk medulloblastoma. Citation Format: Samuel Rivero-Hinojosa, Melanie Grant, Aswini Panigrahi, Huizhen Zhang, Veronika Caisova, Catherine Bollard, Brian Rood. Proteogenomic discovery of novel tumor proteins as neoantigens for personalized T-cell immunotherapy in pediatric medulloblastoma [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2019 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(3 Suppl):Abstract nr A23.