P480: DNMT AND HDAC INHIBITION INDUCES IMMUNOGENIC NEOANTIGENS FROM HUMAN ENDOGENOUS RETROVIRAL ELEMENT-DERIVED TRANSCRIPTS

Background: Immunotherapies targeting cancer-specific neoantigens have revolutionized the treatment of cancer patients. Recent evidence suggests that epigenetic therapies synergize with immunotherapies, mediated by the de-repression of endogenous retroviral element (ERV)-encoded promoters, and the resulting transcripts. These ERV-derived transcripts can splice into protein-coding exons and encode for chimeric open reading frames (ORFs) that were hypothesized to encode for putative neoantigens. However, the existence of these neoantigens has never been demonstrated experimentally. Aims: To test the hypothesis that treatment with DNA methyl transferase inhibitor (DNMTi) or histone deacetylase inhibitor (HDACi) can lead to the induction of neoantigens encoded in ERV-derived transcripts. Methods: RNA sequencing (RNA-seq), Ribosomal profiling (Ribo-seq), whole cell proteomics, Immunopeptidomics, Artificial antigen-presenting cells (aAPCs)-based induction of neoantigens specific CD8+ T cells, T cell cytotoxicity assay, and Enzyme-linked immunosorbent spot (ELISpot) assay. Results: We used RNA-seq from cancer cell lines treated with DNMTi and/or HDACi to assemble a de novo transcriptome and identified ERV-derived, treatment-induced novel polyadenylated transcripts (TINPATs) coding for novel open reading frames. Using immunopeptidomics, we further demonstrate the human leukocyte antigen (HLA) presentation of treatment-induced neopeptides (t-neopeptides) derived from novel ORFs encoded by TINPATs. We illustrate the potential of the identified t-neopeptides to elicit a T-cell response and cancer cell killing. The presence of t- neopeptides was further verified in AML patient samples after in vivo treatment with the DNMT inhibitor Decitabine. Summary/Conclusion: ERV-derived double-stranded RNAs contribute to the anti-neoplastic activities of epigenetic inhibitors like Decitabine, through the induction of viral mimicry. This study describes a novel category of ERV-derived immunogenic neopeptides, specifically induced upon epigenetic therapy that could serve as prime targets for novel combinatorial immunotherapeutic approaches in cancer patients. Keywords: Acute myeloid leukemia, decitabine, Cancer immunotherapy, Antigen presentation