Immunological targeting of the endogenous retroviral protein ERVMER34-1 in carcinomas promotes epitope spread and enhances response to PD-L1 blockade 2306293

Introduction Endogenous retroviruses (ERVs) are remnants of germline retroviral infections that occurred over evolution and make up 5—8% of the human genome. Although ERVs are typically epigenetically silenced in healthy adult tissues, they are overexpressed in carcinomas and may represent a novel pool of immunotherapeutic targets. This study characterizes the ERV envelope protein ERVMER34-1 as a therapeutic target in cancer. Methods The expression of ERVMER34-1 in healthy adult and cancer tissues was assessed by IHC. Its immunogenicity in human PBMCs was evaluated by ELISPOT assays, and the lytic ability of specific T cells was assessed by live-cell imaging assays. In addition, a rationally designed ERVMER34-1—targeted therapeutic vaccine was tested for its ability to induce tumor clearance in two murine carcinoma models, as monotherapy or in combination with immuno-oncology agents. Results The ERVMER34-1 protein is overexpressed in several human carcinomas while being absent in most healthy adult tissues. ERVMER34-1—specific T cells were detected in PBMCs of cancer patients but not healthy donors after overnight stimulation, yet reactive T cells expand readily from both groups after 7 days of in-vitro stimulation. These T cells selectively kill human carcinoma cell lines expressing ERVMER34-1. A rationally designed therapeutic vaccine targeting ERVMER34-1 mediated tumor control in two syngeneic murine tumors. When combined with checkpoint blockade, the vaccine induced tumor regression and promoted the expansion of neoepitope-reactive T cells, whose function was further enhanced by an FDA-approved IL-15 superagonist. The strong neoepitope response associates with an inflamed tumor microenvironment marked by increased CD8+ T-cell infiltration and improved tumor control. Conclusion This study supports the clinical development of a therapeutic cancer vaccine targeting the retroviral envelope protein ERVMER34-1, which would represent a new class of therapeutic cancer vaccine targeting ERVs. Funding Source This study was supported by the Intramural Research Program of the Center for Cancer Research, NCI, NIH Topic Categories Tumor Immunology: Checkpoints, Prevention, and Treatment (TIPT)