Photograph: David Davis / PA
Two common respiratory viruses can merge to form a hybrid virus capable of evading the human immune system and infecting lung cells – the first time such viral cooperation has been observed.
The researchers believe the findings could help explain why co-infections can lead to significantly worse disease for some patients, including hard-to-treat viral pneumonia.
Each year, approximately 5 million people worldwide are hospitalized with influenza A, while respiratory syncytial virus (RSV) is the leading cause of acute lower respiratory tract infections in children under the age of five and it can cause serious illness in some children and older adults.
Although co-infections, in which a person becomes infected with both viruses at the same time, are believed to be relatively common, it was unclear how these viruses would respond if they were found within the same cell.
‘Respiratory viruses exist as part of a community of many viruses that all affect the same region of the body, as an ecological niche,’ said Dr Joanne Haney of the University of Glasgow MRC Virus Research Center, who led the study.
“We need to understand how these infections occur in the context of each other to get a more complete picture of the biology of each individual virus.”
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To investigate, Haney and his colleagues deliberately infected human lung cells with both viruses and found that, instead of competing with each other as other viruses are known to do, they fused together to form a palm-shaped hybrid virus, with the formation of RSV the trunk and influences the leaves.
‘This type of hybrid virus has never been described before,’ said Professor Pablo Murcia, who oversaw the research, published in Nature Microbiology. “We are talking about viruses from two completely different families that combine together with the genomes and external proteins of both viruses. It’s a new type of virus pathogen ”.
Once formed, the hybrid virus was also able to infect neighboring cells, even in the presence of antibodies to the flu that would normally block the infection. Although the antibodies still attached to the influenza proteins on the surface of the hybrid virus, the virus simply used nearby RSV proteins to infect lung cells instead. Murcia said: “The flu is using hybrid viral particles like a Trojan horse.”
In addition to helping viruses evade the immune system, joining forces can also allow them to access a wider range of lung cells. While influenza usually infects cells in the nose, throat, and trachea, RSV tends to prefer cells in the trachea and lungs, although there is some overlap.
Perhaps, it could increase the chances of the flu triggering a serious and sometimes fatal lung infection called viral pneumonia, said Dr Stephen Griffin, a virologist at the University of Leeds. Although he warned that more research is needed to show that hybrid viruses are implicated in human disease. “RSV tends to go lower in the lungs than the seasonal flu virus, and you are more likely to have more serious illness as the infection goes down,” he said.
“It’s another reason to avoid being infected with multiple viruses, because this [hybridisation] it is likely to happen even more if we do not take precautions to protect our health ”.
Significantly, the team showed that hybrid viruses could infect cultured cell layers as well as single respiratory cells. “This is important because the cells are authentically attached to each other and the virus particles will have to go in and out the right way,” Griffin said.
The next step is to confirm whether hybrid viruses can form in patients with co-infections and, if so, which ones. “We need to know if this only happens with the flu and RSV, or extends to other virus combinations as well,” Murcia said. “My guess is that it does. And, I would assume it extends to the animal [viruses] also. This is just the beginning of what I think will be a long journey, hopefully very interesting discoveries “.