Structural basis of neutralization of Powassan virus by monoclonal antibodies

Abstract

Abstract Powassan virus (POWV) is an emerging tick-borne flavivirus endemic to the northern and northeastern United States. Severe human POWV infection is characterized by fatal encephalitis in 10% of patients and long-term neurological sequelae in 50% of survivors. Little is known about humoral immunity to POWV, and there are currently no licensed vaccines or therapeutics available for treatment. We have generated a panel of 23 murine antibodies against POWV that separate into six epitope groups based on competitive binding. Two of the groups target domain III of the viral envelope protein; mapping by alanine scanning mutagenesis places their epitopes on the A-strand and C-C′ loop of domain III, respectively. These DIII-specific mAbs display potent neutralization in vitro and robust protection in a mouse model of POWV infection. X-ray crystallographic analysis reveals that POWV-80, a C-C′ loop antibody, adopts a binding orientation similar to a class of potently neutralizing flavivirus antibodies that target the lateral ridge epitope on domain III, while POWV-61 binds primarily to the A-strand and B-C loop. Our results indicate that antibodies to POWV can target epitope regions similar to those observed on mosquito-transmitted flaviviruses by employing unique binding modes that may facilitate enhanced neutralizing and protective efficacy. Current efforts are focused on defining the determinants of antibody potency and understanding antibody binding in the context of the virion.