West Nile virus vaccine candidates attenuated by dinucleotide enrichment are immunogenic and protective against lethal infection.

West Nile virus (WNV) poses a global public health threat. This study demonstrates that the WNV RNA tolerates CpG and UpA dinucleotide enrichment in different genomic regions resulting in attenuation of CpG- and CpG/UpA-enriched variants. Attenuation was zinc finger antiviral protein 1 (ZAP)-dependent, and ZAP knockout (ZAP-KO) cells were used to generate high-titer stocks. Ten enriched variants, with permuted control and wild-type (WT) viruses, were screened in immunocompetent mice upon intraperitoneal injection. In contrast to lethal WNV-WT and permuted viruses, the E-MAX variant, with the RNA region encoding envelope (E) protein enriched both with CpG and UpA, caused no mortality. E-MAX was immunogenic and protective against lethal challenge. Stability of enriched dinucleotides was confirmed upon serial passaging in ZAP-WT and ZAP-KO cells, with only minor (17-21%) reversion at a single site in ZAP-WT condition. E-MAX upregulated interferon (IFN) signaling genes in human cells, suggesting that the combination of CpG/UpA-mediated attenuation, and concurrent activation of IFN responses potentially driven by CpG/UpA enrichment, may contribute to E-MAX immunogenicity. Evaluation using footpad injection in mice showed E-MAX had a promising safety and immunogenicity profile, although brain infection was occasionally detected. Then, we developed the E-MAX+ FR variant by combining CpG/UpA enrichment with two amino acid substitutions in functional domains of the E protein. This strategy eliminated neuroinvasion while maintaining immunogenicity and protection. Altogether, CpG/UpA dinucleotide enrichment in the genomic E region in combination with amino acid substitutions in the E protein yields a promising platform for vaccine development against WNV and potentially other flaviviruses.