New images of canine parvovirus may help predict how virus may jump to new species

Canine parvovirus (CPV) is a highly infectious pathogen that causes severe diseases in unvaccinated dogs, including inflammation of the heart and acute gastrointestinal illness. Originating in cats, the virus is a rare example of a DNA-based virus that can jump between species, and a team of researchers' discovery may help in predicting this and the virus's ability to evolve, which could have implications for current vaccines used in dogs. 

In a new study, researchers discovered that a location on CPV where dog antibodies bind -- called an antibody binding site -- overlaps with a location on CPV where the virus attaches to cells -- called a receptor binding site -- in a fashion that is similar to SARS CoV-2 and influenza viruses. As a result, when antibodies attach to this antibody binding site on the virus, they prevent the virus from then attaching to and infecting cells. 

"Canine parvovirus first appeared in the late 1970s, rapidly leading to a pandemic in dogs," said Susan Hafenstein, director of the Center for Structural Biology, Huck Chair of Structural Virology, and professor of biochemistry and molecular biology. "Scientists developed a vaccine, but we continue to study the virus because it could mutate and render the vaccines ineffective and because it has the potential to jump to other species, perhaps even to humans. In addition, understanding species jump in this virus will shed light on similar capabilities for viruses like influenza and coronavirus."

To study the interaction between CPV and host antibodies, the team created high-resolution maps of the virus using cryogenic electron microscopy (cryo-EM), a process whereby ultra-cold temperatures are used to freeze samples in glass-like ice, allowing researchers to create three-dimensional maps of structures. They also created custom software that enabled them to overcome an issue that has long been a problem in cryo-EM studies.

"Traditionally, cryo-EM averages many individual virus particles together to obtain a high-resolution 3D reconstruction," said Samantha Hartmann, graduate student in biochemistry and molecular biology, Penn State. "This is incredibly useful, but in this case, it doesn't provide the level of detail that we wanted for this study." 

Hartmann explained that each CPV particle has 60 sites on its surface where antibodies can bind in their attempt to neutralize the virus. In some cases, all 60 sites can be occupied by antibodies, whereas in others, only a few sites might contain antibodies. When averaging many virus particles together, some with full sets of antibodies and some with only a few or even no antibodies, traditional cryo-EM fails to capture variation in antibody attachment. With their new approach, which will publish on June 8 in the journal Proceedings of the National Academy of Sciences, the researchers' novel software allowed them to acquire this detailed information. 

"With our software, we were able to break each virus particle down into smaller pieces based on our region of interest -- in this case, the sites where antibodies bind to the virus - and put them back together again into an image that contained more detailed information."

The 3D image revealed that an antibody binding site on CPV significantly overlapped its own receptor binding site. 

"This means that when an antibody attaches to this antibody binding site, it blocks the virus from attaching to the dog's cells," said Hartmann. 

In addition, the team found that when antibodies attached to the antibody binding sites of CPV viruses, they induced conformational, or structural, changes in the virus, which further facilitated neutralization. 

"It all made sense once we had the structure image," said Hartmann. "Due to the overlap of the antibody binding site with the receptor binding site, and the subsequent conformational changes, the antibodies were able to neutralize CPV by blocking the virus from binding to the host receptors and entering the cells." 

By contrast, the antibodies were not able to bind to or neutralize feline parvovirus, suggesting that antibody binding sites and receptor binding sites do not overlap in cats. 

"Since species-jumping is a rare event in DNA viruses, the emergence of an antibody that binds more avidly to the canine-adapted virus and not ancestral feline-equivalent is of special interest," said Hafenstein. "We can potentially use this information to predict how CPV might behave if it jumps to another species and how it could evolve to sidestep the vaccines that are currently used in dogs."