In mid-2020, a team of scientists capturing bats in caves in Laos discovered coronaviruses that were strikingly similar to the one that had begun wreaking havoc around the world.
In the months since then, some of those researchers have been studying one of these mysterious bat viruses in a high-security lab in Paris, hoping to uncover clues about how its cousin, SARS-CoV-2, became into a global threat that has killed an estimated 15 million people.
His work has been scientifically fruitful. Last year, scientists discovered that the bat virus was capable of attaching itself to human cells, at least in petri dishes. Last month, the team reported more reassuring news: that the virus is not particularly harmful to laboratory animals. The finding suggests that SARS-CoV-2 evolved its abilities to spread rapidly and cause deadly disease only after the two lineages diverged in the viral evolutionary tree.
If the Laos virus ever jumped from a bat to a person, the new research suggests, it could cause a mild stomach bug rather than life-threatening pneumonia. However, laboratory experiments like these reignite a long-standing debate among scientists about the wisdom of messing with viruses that are so closely related to a known pathogen.
Proponents argue that this type of data is crucial to understanding and preventing pandemics. For example, the new studies have tested whether bat viruses could develop a “furin cleavage site,” a feature of SARS-CoV-2 that allows it to efficiently infect human cells.
“Our motivation was to try to give an idea about the origin of Covid,” said Marc Eloit, a virologist at the Pasteur Institute in Paris who is leading the effort.
But critics say scientists shouldn’t be running experiments that might make viruses spread better between people, given the small but real chance these altered pathogens could infect lab workers and escape to the outside world.
“To me, the benefits of this work are outweighed by the risks,” said Dr. David Relman, a microbiologist at Stanford University.
the furin site
The bat virus at the center of Dr Eloit’s experiments came to light on an expedition to limestone caves in northern Laos in the summer of 2020. A team of Lao and French researchers caught bats flying out of the caves and took samples of their saliva. blood, urine and feces.
The scientists found genetic material from five coronaviruses closely related to SARS-CoV-2. In the feces of a Marshall’s horseshoe bat, they found whole viruses of a strain they named BANAL-236 (a code indicating the virus came from a bat anal swab).
Back in their lab, the scientists discovered that BANAL-236 can infect human cells by tightly binding to the same protein that SARS-CoV-2 uses to enter. In February, the researchers published their findings in the journal Nature.
Last month, they published a second wave of results, now under review by a scientific journal, in which they investigated the behavior of the virus in laboratory mice and monkeys.
In one experiment, scientists injected the virus into genetically modified mice that are commonly used to study covid. SARS-CoV-2 replicates rapidly in their lungs, just as it does in people, causing them to lose weight and die.
BANAL-236, by contrast, struggled to gain a foothold in the animals’ lungs and produced only about 1 percent of the virus created in a SARS-CoV-2 infection.
The researchers found that the virus was even milder when they sprayed it on the noses of two monkeys. BANAL-236 replicated primarily in his intestines, rather than his lungs.
Dr. Eloit suspects that BANAL-236 is milder because it lacks a key feature important to the success of SARS-CoV-2.
After a new SARS-CoV-2 virus is created in a cell, its spike protein changes shape, with a crossbow-like effect. When the virus binds to a new cell, the prepared spike protein fires molecular beams that draw it toward its new host.
This shape-shifting region of the spike, known as the furin cleavage site, is crucial to the success of SARS-CoV-2. When scientists have engineered viruses that lack this site, the mutants struggle to replicate in the lungs of laboratory animals or spread to new hosts.
serial passage
Obtaining a furin cleavage site may have been a crucial step in the evolution of SARS-CoV-2. To explore that possibility, Dr. Eloit and his colleagues conducted laboratory experiments to give BANAL-236 a chance to develop new traits, such as a furin cleavage site.
The team based their studies on experiments in which other scientists injected avian flu viruses into chicken eggs and waited for them to replicate. They then transferred the new viruses to new eggs and again allowed them to replicate. With each transfer, the virus had a chance to evolve. After 11 transfers, the scientists found that the flu viruses had developed cleavage sites, making them more deadly to chickens.
Similarly, the Pasteur researchers removed lung tissue from mice infected with BANAL-236 and used the tissue to infect healthy animals. They then repeated the cycle, transferring virus from mouse to mouse.
In another experiment, they infected a plate of human intestinal cells with BANAL-236 and then used the new viruses produced by the cells to infect new plates.
But for both experiments, Dr. Eloit and his colleagues decided not to go as far as 11 transfers, stopping at six.
“From a purely scientific point of view, we wanted to do more than six passages,” said Dr. Eloit. “But we didn’t want to open up the risk of adapting a bat virus to humans.”
BANAL-236 did not obtain a furin cleavage site in any of the experiments. The virus acquired other mutations, but they did not do any better at infecting the lungs of mice.
Scientists have been running such evolutionary experiments, known as “serial passages,” for more than a century. In fact, vaccines for a number of viruses such as yellow fever were created by growing them in the laboratory: the viruses evolved in Petri dishes into milder forms that were safe to inject into people.
In 2011, however, a controversy erupted over the safety of serial passage experiments that could produce new human pathogens. At the time, the researchers were studying how influenza viruses that cause intestinal infections in birds can evolve into airborne forms that can infect people.
Two teams of researchers sprayed avian flu viruses into the noses of ferrets, waited for the viruses to replicate, and then transferred the new viruses to new ferrets. Soon, viruses evolved to improve their replication in ferrets.
Some critics said the research was so reckless that it should not be published, fearing that other researchers would copy the work and accidentally release a new pandemic flu strain. The US government stopped experiments like these to develop a new policy to judge their safety.
Some of the studies have been resumed in recent years. But Stanford’s Dr. Relman and others have complained that current regulations aren’t transparent enough.
Dr. Eloit said that a Pasteur Institute committee that reviews potentially risky biological research authorized his team’s proposal to study the new bat viruses. The scientists then conducted their experiments at the same level of safety as their other work with coronaviruses, known as Biosafety Level 3, or BSL-3.
Dr. Tom Inglesby, director of the Johns Hopkins Center for Health Security at the Bloomberg School of Public Health, said it was good for scientists to think about these potential risks. But he also said that he wanted to see a clear reason for deciding that six passages were safe.
“It is not possible to know in advance whether these experiments will lead to more transmissible or more virulent viruses,” he said. “There is no hard and fast rule that six is safe and more is not.”
But Thomas Peacock, a virologist at Imperial College London, said he thought Dr Eloit and his colleagues had been prudent enough. In previous studies, he noted, researchers had found that antibodies produced by people during Covid infections were very potent against BANAL-236. That most likely meant that if the virus leaked out of a lab, it couldn’t spread very far.
“This virus would probably hit a brick wall in the general population,” Dr. Peacock said. “I really don’t have much of a problem with experiments.”
origins of covid
Other researchers agreed with Dr. Eloit that the research could shed light on how and when SARS-CoV-2 spread to people.
For Dr. Eloit, his team’s inability to produce a furin cleavage site at BANAL-236 in mice or human intestinal cells suggests that the SARS-CoV-2 lineage gained the furin site in bats before spreading to cells. people. He said it would not have been easy for the virus to obtain a furin site after jumping to another species of animal, sometimes called “intermediate hosts,” such as those sold at a market in Wuhan, China. “I don’t see a strong case for an intermediate host,” Dr. Eloit said.
But scientists who favor the market scenario see the new results in a different light. If the researchers couldn’t stimulate BANAL-236 to evolve the furin site during serial passage experiments, they reasoned, then it’s unlikely that scientists in a Wuhan lab could have done so with SARS-CoV-2, like some “laboratory” proponents. have suggested the leak theory.
“This is another nail in the coffin for the lab leak theory that should be firmly sealed in the crypt by now,” said Edward Holmes, a virologist at the University of Sydney.
Dr. Peacock was reluctant to draw strong conclusions from such small-scale experiments. “I think it’s pretty hard to ask for a furin site after a few passages,” he said.
Dr. Eloit and colleagues are now exploring the possibility that SARS-CoV-2’s ancestors obtained a furin cleavage site while still in wild bats. The virus could then have spread to an intermediate host or directly to people exposed to bats, such as those who collect bat guano, hunt bats, or eat them.
To test that idea, scientists are working to obtain more bat samples from Laos and neighboring countries. Dr. Eloit can’t say if his hypothesis is more likely than the others, but at least it’s one they can investigate.
“Our job as scientists,” he said, “is to explore the working hypotheses that we can explore.”
Source: www.nytimes.com