Background Human endogenous retroviruses constitute 8% of the genome and are distributed among viral families of which HERV-K is the most recently integrated. Endogenous retroviruses are well established, natural targets for immunotherapy. Previously, we observed that encoding an endogenous variant of the murine leukemia virus as a particle-forming transgene in adenoviral vectors, allowed for curative therapy against small established cancers. In addition, immunogenicity could be further improved by point mutations of an immune suppressive domain (ISD) (WO 2019/043127).In humans, HERV-K Gag and Env genes are structurally intact, and while expression is almost absent in healthy tissues, HERV-K proteins are detected in human cancers, including on cell surfaces and exosomes. Functionally, the HERV-K Env genes are implicated in oncogenic signaling pathways, Epithelial Mesenchymal Transition and immune evasion. Consequently, we developed a particle forming HERV-K vaccine incorporating ISD mutations for treatment of cancer with a combined T and B Cell response.

Materials and Methods HERV-K Gag and Env consensus sequences were encoded in human adenovirus type 5 and 19a/64 adenoviral vectors. Expression analyses were performed on human and mouse DCs. Immune responses were analyzed by intracellular cytokine staining and tetramers. Murine colorectal cancer cells were engineered to express the HERV-K Gag and Env antigens. Immunotherapy experiments in tumor-bearing mice were performed by transplantation of selected immune cell populations obtained from vaccinated donor mice.

Results Expression of the HERV-K transgene from adenoviral vectors directed high levels of transcripts to the cellular surface and led to the formation of virus like particles. Mutations in the ISD resulted in increased expression of HERV-K in human but not in murine DCs. In addition, ISD mutations increased humoral immune responses to WT Env during recombinant protein immunizations. Adenoviral vectors expressing HERV-K Gag and Env with mutations in the ISD (HERV-K-ISDmut) were highly immunogenic with rapidly induced antibody and T cell responses in mice and break of tolerance in non-human primates. Following prime-boost immunization with selected combinations of checkpoint inhibitors, T cell responses were obtained in the range of 40–50% of circulating CD8+ T cells. As Gag and Env expressing cell lines were rejected in WT mice, we engrafted CT26 cells expressing HERV-K Gag and Env in nude mice and performed adoptive transfer immunotherapy. While initially effective, CD8+ T cells rapidly lost tumor control, whereas combinations of CD8+ T cells with CD4+ T cells and B cells exhibited rapid and sustained tumor control in most animals.

Conclusions The HERV-K ISDmut antigen holds promise for directing a broad and effective immune response to a large proportion of human cancers.

Disclosure Information E. Ragonnaud: A. Employment (full or part-time); Significant; InProTher ApS. L. Neukirch: A. Employment (full or part-time); Significant; InProTher ApS. I.S. Pedersen: A. Employment (full or part-time); Significant; InProTher ApS. J. Daradoumis: A. Employment (full or part-time); Significant; InProTher ApS. J. Daradoumis: A. Employment (full or part-time); Significant; InProTher ApS. K.V. Grunddal: A. Employment (full or part-time); Significant; InProTher ApS. L. Duvnjak: None. A.V. Bermejo: A. Employment (full or part-time); Significant; InProTher ApS. S. Schroedel: A. Employment (full or part-time); Significant; Sirion Biotech. C. Thirion: A. Employment (full or part-time); Significant; Sirion Biotech. K.N. Nielsen: A. Employment (full or part-time); Significant; InProTher ApS. A.C. Andersson: A. Employment (full or part-time); Significant; InProTher ApS. P.J. Holst: A. Employment (full or part-time); Significant; InProTher ApS. E. Ownership Interest (stock, stock options, patent or other intellectual property); Significant; InProTher ApS.