It is widely believed that antibodies neutralize viruses by clinging to their surface and preventing them from infecting host cells. But new research shows that this barrier method isn’t the only way antibodies turn off viruses. An international team of researchers led by Penn State has found that antibodies also distort viruses, preventing them from attaching and properly entering cells.
“Everyone thinks antibodies bind to viruses and stop them from entering cells, essentially locking them down,” said Ganesh Anand, associate professor of chemistry at Penn State. “But our research shows for the first time that antibodies can also physically distort viruses so that they are unable to attach properly and infect host cells.”
In their study, published recently in the journal Cell, Anand and his colleagues studied the interactions between the human monoclonal antibody (HMAb) C10 and two pathogenic viruses: Zika and dengue. The HMAb C10 antibodies they were using had previously been isolated from patients infected with the dengue virus and had also been shown to neutralize the Zika virus.
The researchers used a combination of techniques, including cryogenic electron microscopy (cryo-EM) to visualize viruses and hydrogen / deuterium exchange mass spectrometry (HDXMS) to understand their movement.
“Cryo-EM involves flash freezing a solution containing molecules of interest and then targeting them with electrons to generate many images of individual molecules in different orientations,” Anand explained. “These images are then integrated into a snapshot of what the molecule looks like. The technique provides much more precise images of molecules than other forms of microscopy.
To document the effects of antibodies on Zika and dengue viruses, the team collected cryo-EM snapshots of the viruses under conditions of increasing antibody concentrations.
In parallel, the team applied HDXMS, a technique in which molecules of interest – in this case Zika virus and dengue, as well as HMAb C10 antibodies – are submerged in heavy water. Heavy water, Anand explained, saw its hydrogen atoms replaced by deuterium, the heavier isotopic cousin of hydrogen.
“When you submerge a virus in heavy water, the hydrogen atoms on the surface of the virus exchange with deuterium,” he said. “We can then use mass spectrometry to measure the heaviness of the virus as a function of this deuterium exchange. In doing so, we observed that the dengue virus, but not the Zika virus, got heavier with deuterium as more antibodies were added to the solution. This suggests that for the dengue virus, the antibodies distort the virus and allow more deuterium to enter. It is as if the virus is crushed and a larger area is exposed where hydrogen can be exchanged for deuterium.
In contrast, the Zika virus did not become heavier when placed in heavy water, suggesting that its surface, although fully occupied by antibodies, is not deformed by antibodies.
Anand explained that by combining cryo-EM and HDXMS, the team were able to get a complete picture of what happens when antibodies attach to Zika and dengue viruses.
“It’s like those cartoon flipbooks, where every page has a slightly different image, and when you flip through the book you see a short film,” he said. “Imagine a flipbook with drawings of a racehorse. Cryo-EM shows you what the racehorse looks like and HDXMS shows you how fast the racehorse is moving. You need both techniques to be able to describe a running racehorse in motion. This complementary set of tools allowed us to understand how one type of antibody affects two types of virus differently.
He noted that the fact that the more antibodies they added, the more the dengue virus particles deformed, suggests that stoichiometry – the relationship between the amounts of reagents and products before, during, and after a chemical reaction – is important.
“It is not enough to have antibodies present,” he said. “The amount of antibody you add determines the extent of neutralization.”
In fact, the team found that under saturation conditions, where antibodies were added at concentrations high enough to fill all of the available binding sites on dengue viruses, 60% of the virus surfaces were deformed. This distortion was sufficient to protect the cells from infection.
“If you have enough antibodies, they will deform the virus particle enough to be preemptively destabilized before it even reaches its target cells,” Anand said.
Indeed, when scientists incubated antibody-bound dengue viruses with BHK-21 cells, a cell line derived from the kidneys of baby hamsters often used in research into viral infections, they found that 50 to 70% fewer cells were infected.
Anand explained that with some viruses, including Zika, the antibodies work by blocking exits so the passenger cannot get out of the car.
“We have found a new mechanism in the dengue virus whereby the antibodies basically total the car, so it can’t even get into a cell,” he said.
How do antibodies deform dengue virus particles?
Anand explained that unlike the now familiar SARS-CoV-2, which has peak proteins protruding in all directions, the surfaces of Zika and Dengue are smoother with peaks and valleys.
Anand noted that for the dengue virus, antibodies particularly prefer to bind to “peaks” known as quintuple peaks. Once all of the proteins on the quintuple tops have been bound, the antibodies will turn to their favorite peaks – the triple tops. Finally, they only have the double vertices left.
“Antibodies don’t like double vertices because they are very mobile and difficult to bind,” Anand said. “We found that after the five and three times highs have been fully bound to the antibodies, if we add more antibodies to the solution, the virus starts to shake. There’s this competition between the antibodies trying to get in and the virus trying to shake them. As a result, these antibodies end up burrowing into the virus rather than binding to the double tops, and we believe it is this digging into the viral particle that causes the virus to tremble and warp and ultimately become non-functional.
What is the difference between Zika and dengue?
Anand explained that Zika is a much more stable and less dynamic virus than dengue, which has a lot of moving parts.
“Dengue and Zika are similar but each has a different effect. Dengue fever may have evolved as a more mobile virus in order to avoid being caught by antibodies. Its moving parts confuse and destabilize the immune system. Unfortunately for dengue, antibodies have evolved to circumvent this problem by burrowing into the virus and distorting it. “
It appears, he said, that the same type of antibody can neutralize Zika and dengue in two different ways – one where it binds to the virus and deactivates it, which is the traditional way we do. let’s think about the activity of antibodies, and the other where it burrows and distorts the virus.
What about other viruses?
Anand said the distortion strategy his team discovered can be used by antibodies when confronted with other types of viruses as well.
“Dengue is just a model virus that we used in our experiments, but we believe this preventive destabilization strategy can be widely applicable to any virus,” he said. “Antibodies may try to neutralize viruses first by the barrier method and if they fail, they resort to the distortion method.”
Are there potential applications of the results?
The results could be useful in the design of therapeutic antibodies, Anand said.
“HMAb C10 antibodies are specific for dengue and Zika viruses and are able to neutralize Zika and dengue viruses in two different ways,” he said. “But you could potentially design therapies with the same capabilities to treat other illnesses, such as COVID-19[female[feminine. By creating therapy with antibodies that can both block and distort viruses, we can eventually achieve greater neutralization. “
He added: “You don’t want to wait for a virus to reach its target tissue, so if you can introduce a therapeutic cocktail such as a nasal spray where the virus first enters the body, you can prevent it even. By doing this you may even be able to use fewer antibodies since our research shows that it takes less antibody to neutralize a virus by the distortion method. silver.
Overall, Anand stressed that the importance of the study is that it reveals a whole new strategy that certain antibodies use to deactivate certain viruses.
“Previously all we knew about antibodies was that they bind to and neutralize viruses,” he said. “Now we know that antibodies can neutralize viruses in at least two different ways, and maybe even more. This research opens the door to a whole new avenue of exploration.
Reference: “Human C10 Antibody Neutralizes by Decreasing Zika but Improving Dengue Virus Dynamics” by Xin-Xiang Lim, Bo Shu, Shuijun Zhang, Aaron WK Tan, Thiam-Seng Ng, Xin-Ni Lim, Valerie S.-Y. Chew, Jian Shi, Gavin R. Screaton, Shee-Mei Lok and Ganesh S. Anand, November 30, 2021, Cell.
DOI: 10.1016 / j.cell.2021.11.009
Other authors on the paper include Xin-Xiang Lim, graduate student; Jian Shi, director, Cryo-EM installation; and Shee-Mei Lok, professor at the National University of Singapore. Co-authors also include Bo Shu, associate researcher; Shuijun Zhang, assistant professor; Aaron WK Tan, graduate student; Thiam-Seng Ng, graduate student; Xin-Ni Lim, graduate student; and Valerie Chew, assistant professor at the Duke-National University of Singapore Medical School. Gavin R. Screaton, Head of the Division of Medical Sciences at the University of Oxford, is also an author.
This research was funded by the National Research Foundation of Singapore, the Singapore Department of Health, and Penn State.