Hypervariable Domain of nsP3 of Eastern Equine Encephalitis Virus Is a Critical Determinant of Viral Virulence

Hypervariable domains (HVDs) of alphavirus nsP3 proteins recruit host proteins into viral replication complexes. The sets of HVD-binding host factors are specific for each alphavirus, and we have previously identified those specific for EEEV. The results of this study demonstrate that the deletions of the binding sites of the G3BP and FXR protein families in the nsP3 HVD of EEEV make the virus avirulent for mice. Mutations in the nuclear localization signal in EEEV capsid protein have an additional negative effect on viral replication in vivo. Despite the inability to cause a detectable disease, the double HVD and triple HVD/capsid mutants induce high levels of neutralizing antibodies. Single immunization protects mice against infection with the highly pathogenic North American strain of EEEV. High safety, the inability to revert to wild-type phenotype, and high immunogenicity make the designed mutants attractive vaccine candidates for EEEV infection. ABSTRACT Eastern equine encephalitis virus (EEEV) is the most pathogenic member of the Alphavirus genus in the Togaviridae family. This virus continues to circulate in the New World and has a potential for deliberate use as a bioweapon. Despite the public health threat, to date no attenuated EEEV variants have been applied as live EEEV vaccines. Our previous studies demonstrated the critical function of the hypervariable domain (HVD) in EEEV nsP3 for the assembly of viral replication complexes (vRCs). EEEV HVD contains short linear motifs that recruit host proteins required for vRC formation and function. In this study, we developed a set of EEEV mutants that contained combinations of deletions in nsP3 HVD and clustered mutations in capsid protein, and tested the effects of these modifications on EEEV infection in vivo. These mutations had cumulative negative effects on viral ability to induce meningoencephalitis. The deletions of two critical motifs, which interact with the members of cellular FXR and G3BP protein families, made EEEV cease to be neurovirulent. The additional clustered mutations in capsid protein, which affect its ability to induce transcriptional shutoff, diminished EEEV’s ability to develop viremia. Most notably, despite the inability to induce detectable disease, the designed EEEV mutants remained highly immunogenic and, after a single dose, protected mice against subsequent infection with wild-type (wt) EEEV. Thus, alterations of interactions of EEEV HVD and likely HVDs of other alphaviruses with host factors represent an important direction for development of highly attenuated viruses that can be applied as live vaccines. IMPORTANCE Hypervariable domains (HVDs) of alphavirus nsP3 proteins recruit host proteins into viral replication complexes. The sets of HVD-binding host factors are specific for each alphavirus, and we have previously identified those specific for EEEV. The results of this study demonstrate that the deletions of the binding sites of the G3BP and FXR protein families in the nsP3 HVD of EEEV make the virus avirulent for mice. Mutations in the nuclear localization signal in EEEV capsid protein have an additional negative effect on viral replication in vivo. Despite the inability to cause a detectable disease, the double HVD and triple HVD/capsid mutants induce high levels of neutralizing antibodies. Single immunization protects mice against infection with the highly pathogenic North American strain of EEEV. High safety, the inability to revert to wild-type phenotype, and high immunogenicity make the designed mutants attractive vaccine candidates for EEEV infection.