Abstract B123: Preclinical development of ErVac01, a viral-vectored vaccine encoding tumor epitopes derived from endogenous retrovirus sequences
Authors: Bonaventura Paola , Tabone Olivier , Dubois Clarisse , Mutez Virginie , Etchegaray Ema , Estornes Yann , Valladeau-Guilemond Jenny , Depil Stephane 
Published: February 2025

Abstract
Targeting shared tumor epitopes using potent delivery methods to elicit a robust CD8+ T cell response is key for developing universal off-the-shelf cancer vaccines. Human endogenous retroviruses (HERVs) are aberrantly expressed in cancer cells and provide a source of unconventional T cell epitopes, especially in tumors with low mutational burden (TMB). Among them, HML2 family members are overexpressed in different cancer subtypes, including triple-negative breast cancer (TNBC) and ovarian cancer. We first performed a proof of concept using HLA-A2-restricted HML2-derived epitopes selected from transcriptomic analysis. Targeted immunopeptidomics showed that these epitopes are presented on HLA molecules on the surface of tumor cells but not on primary normal cells derived from critical tissues. We demonstrated that these epitopes induce high-avidity CD8+ T cell clones that specifically recognize and kill tumor cells both in vitro and in vivo, while sparing normal cells. Using stringent criteria of tumor selectivity, we then developed algorithms combining transcriptomic, proteomic and immunopeptidomic analyses to select additional tumor-specific epitopes derived from HML2 Gag antigen and binding multiple HLA alleles (as confirmed in vitro) to cover &gt;80% of the worldwide population. The sequences encoding these epitopes were combined in a transgene with a proteasome signal and a CD4+ helper peptide sequence, then inserted into a gorilla adenoviral vector ("GRAd01"). HLA-A2 transgenic mice were used to evaluate the vaccination with GRAd01 or with a synthetic long peptide (SLP) containing the HLA-A2-restricted HML2 epitopes. GRAd01 induced a strong cytotoxic T cell response against the vaccine epitopes. Multimer staining showed that &gt;1% CD8+ T cell splenocytes were specific for at least one of the 5 HLA-A2 epitopes in mice vaccinated with GRAd01. This T cell response was characterized by an increase of effector memory CD8+ T cells. A specific IFN-γ production was observed using Elispot assay in response to 4 out of 5 of these peptides. Interestingly, a single GRAd01 administration generated stronger CD8+ T cell responses than a prime-boost SLP vaccination. A heterologous vaccine ("ErVac01") based on GRAd01 and a MVA containing the same epitopes is under manufacturing for clinical development. We have developed an original proteogenomics-based approach to select a panel of multi-HLA tumor-specific epitopes derived from HML-2 Gag. A heterologous prime-boost vaccination combining GRAd01 and a MVA containing the selected HML2-derived epitopes will be evaluated in the clinic in tumors with low/moderate TMB, such as breast or ovarian cancer.
Citation Format: Paola Bonaventura, Olivier Tabone, Clarisse Dubois, Virginie Mutez, Ema Etchegaray, Yann Estornes, Jenny Valladeau-Guilemond, Stephane Depil. Preclinical development of ErVac01, a viral-vectored vaccine encoding tumor epitopes derived from endogenous retrovirus sequences [abstract]. In: Proceedings of the AACR IO Conference: Discovery and Innovation in Cancer Immunology: Revolutionizing Treatment through Immunotherapy; 2025 Feb 23-26; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Immunol Res 2025;13(2 Suppl):Abstract nr B123.