THE STRUCTURE OF A NEUTRALIZED VIRUS - CANINE PARVOVIRUS COMPLEXED WITH NEUTRALIZING ANTIBODY FRAGMENT

Background: Members of the Parvovirus genus cause a variety of disease s in mammals, including humans. One of the major defences against vira l infection is the presence of neutralizing antibodies that prevent vi rus particles from infecting target cells. The mechanism of neutraliza tion is not well understood. We therefore studied the structure of can ine parvovirus (CPV) complexed with the Fab fragment of a neutralizing antibody, A3B10, using image reconstruction of electron micrographs o f vitrified samples, together with the already known structure of CPV from X-ray crystallographic data. Results: The structure of the comple x of CPV with Fab A3B10 has been determined to 23 angstrom resolution. The known CPV atomic structure was subtracted from the electron densi ty of the complex, and the difference map was used to fit the atomic c oordinates of a known Fab fragment, HyHEL-5. The long axis of each Fab molecule is oriented in a near radial direction, inclined away from t he two-fold axes. The viral epitope consists of 14 amino acid residues found in loops 1, 2 and 3 on the capsid surface, which include previo usly identified escape mutations. Conclusions: The mode of Fab binding suggests that the A3B10 neutralizing antibody cannot bind bivalently to the capsid across the two-fold axes, consistent with the observatio n that whole A3B10 antibody readily precipitates CPV. Since Fab A3B10 can also neutralize the virus, mechanisms of neutralization such as in terference with cell attachment, cell entry, or uncoating, must be ope rative.