To be successful pathogens, the capsid protein of nonenveloped viruses must balance two opposing roles: it must be structurally robust in order to protect the encapsulated genome from environmental insults outside of the host cell, yet metastable to allow egress of the viral genome upon infection. A goal of these studies was to further characterize the antibody response directed against canine parvovirus (CPV), and to understand the interplay between receptor and antibody binding to the capsid. A secondary goal was to understand what conformational changes occur to the structure of the CPV capsid during infection, and to understand the biological significance of these changes. A panel of eight antibodies directed against the virus were shown to be neutralizing as intact immunoglobulin G proteins (IgGs). However the fragment antigen binding domains (Fabs) of these IgGs differed greatly in their ability to neutralize CPV. These eight Fabs compete for receptor binding on the surface of cells, and compete with soluble receptor in solution, although neutralizing Fabs competed for binding at significantly lower Fab to capsid ratios. Structural analysis of those Fabs demonstrated that most accessible areas of the capsid are able to generate an immune response and participate in antibody binding.
CPV is a highly stable virus, and harsh conditions do not cause large structural changes in the virus. By biochemical and biophysical assays presented here, the viral capsid is unchanged until pH 4 and is thermally stable to temperatures of 70?C. Conditions that the virus would encounter during infection, such as low pH and calcium removal, do not have direct structural effects on the CPV capsid. Furthermore, transferrin receptor or antibody binding does not cause detectable changes to the capsid structure. These studies show that CPV is an extremely robust pathogen, due to its highly stable capsid. The capsid has clearly evolved to persist in harsh environmental conditions outside of the cell, and infect in the presence of circulating antibodies.