Two common respiratory viruses can fuse to create a hybrid virus capable of evading the human immune system and infecting lung cells. This is the first time such viral cooperation has been observed. Researchers believe these findings may help explain why concurrent infections cause significantly worse illness in some patients, including viral pneumonia that can be difficult to treat. Each year, approximately 5 million people worldwide are hospitalized due to influenza A, while respiratory syncytial virus (RSV) is the leading cause of acute lower respiratory infections in children under five, causing severe illness in some children and the elderly.
Although it is thought that coinfection—where an individual is infected with both viruses simultaneously—is relatively common, it was unclear how these viruses would respond if they found themselves within the same cell. Dr. Joan Hany from the Virus Research Center at the University of Glasgow, who led the study, stated, "Respiratory viruses exist as part of a community that includes many viruses targeting the same area of the body, like a favorable environment. We need to understand how these infections interact in context to gain a more complete picture of the biology of each virus."
To investigate, Dr. Hany and her colleagues intentionally infected human lung cells with both viruses and found that, instead of competing, as is known with some other viruses, the two viruses fused to form a hybrid virus resembling a palm tree—where RSV formed the trunk and influenza formed the leaves. Professor Pablo Murcia, who supervised the study published in Nature Microbiology, explained, "This type of hybrid virus has not been described before. We are talking about viruses from two completely different families that combine with each other's genome and surface proteins. It is a new type of viral pathogen."
Once formed, the hybrid virus was also able to infect neighboring cells—even in the presence of antibodies against influenza, which typically prevent infection. Although the antibodies remained bound to the influenza proteins on the surface of the hybrid virus, the virus used adjacent RSV proteins to infect lung cells instead. Murcia noted, "Influenza is using hybrid viral particles as a Trojan horse." In addition to helping the viruses evade the immune system, this combined strength may also enable them to access a broader range of lung cells. While influenza typically infects cells in the nose, throat, and trachea, RSV tends to preferentially infect tracheal and lung cells, although there is some overlap.
Dr. Stephen Griffin, a virologist at the University of Leeds, suggested that this may increase the chances of influenza causing severe and sometimes fatal lung infections known as viral pneumonia.
