Engineering human endogenous retrovirus-K for improving immunogenicity and broad cancer targeting

HERV-K (HML-2) is the most studied transcriptionally active endogenous retrovirus in human cancers, yet immune tolerance and incomplete mechanistic understanding have impeded its clinical use as an immunotherapy target. We hypothesized that rational engineering of the HERV-K envelope putative immunosuppressive domain (ISD) could improve immunogenicity.

We performed a mutational screen across the ISD region, evaluated NF-κB activation and then analyzed antigen expression in human monocyte-derived dendritic cells (moDCs) using flow cytometry, RNA-seq, and TEM. Human PBMCs were expanded with HERV-K-transduced moDCs to assess T cell priming and expansion ex vivo. Immunogenicity was tested in mice, HERV-K transgenic mice, and cynomolgus macaques using adenoviral and LNP-mRNA vectors. Antitumor efficacy was evaluated in a murine metastatic renal carcinoma HERV-K expressing tumor model.

Engineering the HERV-K envelope ISD to Q525A (ISDmut) enhanced NF-κB activation, antigen expression, and antigen presentation and cell death associated gene expression pathways in human dendritic cells while preserving virus-like particle assembly. HERV-K-ISDmut-transduced human dendritic cells expanded CD4⁺ and CD8⁺ T cells capable of recognizing HLA-matched, HERV-K expressing tumors, demonstrating that T cells from the human repertoire can be expanded ex vivo. In vivo, mRNA and adenoviral vectors encoding HERV-K-ISDmut elicited strong immune responses in mice, broke tolerance in HERV-K transgenic mice, and induced humoral responses in cynomolgus macaques. In mice, an adenovirus vectored HERV-K-ISDmut immunization further conferred increased survival in a murine renal carcinoma HERV-K expressing tumor model and benefitted from anti-PD-1 therapy.

These results show that the rational redesign of the HERV-K ISD generates a potent immunogen capable of expanding human T cells and mediating mechanistically relevant anti-tumor immunity. We propose that HERV-K-ISDmut antigens represent a promising platform for cancer immunotherapy which should be explored in future clinical trials.