A virus-like particle vaccine prevents equine encephalitis virus infection in nonhuman primates

Western, Eastern, and Venezuelan equine encephalitis viruses (WEEV, EEEV, and VEEV, respectively) are important mosquito-borne agents that pose public health and bioterrorism threats. Despite considerable advances in understanding alphavirus replication, there are currently no available effective vaccines or antiviral treatments against these highly lethal pathogens. To develop a potential countermeasure for viral encephalitis, we generated a trivalent, or three-component, EEV vaccine composed of virus-like particles (VLPs). Monovalent VLPs elicited neutralizing antibody responses and protected mice and nonhuman primates (NHPs) against homologous challenges, but they were not cross-protective. In contrast, NHPs immunized with trivalent VLPs were completely protected against aerosol challenge by each of these three EEVs. Passive transfer of IgG from immunized NHPs protected mice against aerosolized EEV challenge, demonstrating that the mechanism of protection was humoral. Because they are replication incompetent, these trivalent VLPs represent a potentially safe and effective vaccine that can protect against diverse encephalitis viruses.