Eastern equine encephalitis virus-vaccinated mice are protected against Madariaga virus despite negligible neutralizing antibody titers

Abstract

Madariaga virus (MADV) is an understudied member of the eastern equine encephalitis virus (EEEV) complex that circulates widely in Latin America and may be geographically expanding. Spillover infections can cause severe disease in humans and equids. Despite these public and animal health concerns, no MADV vaccines or treatments are available. In this context, commercially available equine vaccines against the closely related North American EEEV (NA EEEV) could potentially provide heterologous protection against MADV. However, previous studies have shown weak or undetectable MADV-neutralizing antibody responses in equids and humans immunized with inactivated NA EEEV vaccines, and whether protection can occur despite these limited neutralizing antibody responses remains unknown. We evaluated a commercial equine trivalent inactivated NA EEEV vaccine against MADV challenge in NIH Swiss mice. Animals received two vaccine doses 14 days apart and were challenged 14 days later. Vaccination provided protection against lethal disease, whereas 36% mortality occurred in the unvaccinated group. Overall, vaccinated animals developed fewer and less severe clinical signs. MADV RNAemia was not detected following vaccination, and tissue dissemination was significantly reduced compared with the unvaccinated group. Central nervous system lesions were observed only in the unvaccinated group. Despite protection, most vaccinated mice had no detectable MADV-neutralizing antibody titers, and those that seroconverted developed only low titers, whereas most unvaccinated mice developed intermediate to high titers. These findings demonstrate that an inactivated NA EEEV-containing vaccine can protect against severe MADV disease despite limited MADV-neutralizing antibody responses. IMPORTANCE Madariaga virus (MADV) is an emerging mosquito-borne alphavirus that can cause severe disease in humans and equids and may be expanding its geographic range in the Americas. Yet no vaccines or treatments targeting MADV are currently available. We show that an existing commercial equine vaccine against the related North American eastern equine encephalitis virus protected mice against severe disease and death following MADV exposure. Importantly, protection occurred even though most vaccinated animals had no detectable MADV-neutralizing antibodies, while a few had low titers. These findings raise the possibility that existing equine vaccines could provide protection during MADV outbreaks and highlight the need to determine whether similar protection occurs in equids. They also indicate that immune responses other than neutralizing antibodies may contribute to protection and should be considered when evaluating immunity elicited by inactivated vaccines.