Immune system-evading hybrid virus observed for first time

Researchers found the RSV and influenza viruses fused together to form a new type of virus pathogen

As well as helping the viruses evade the immune system, joining forces may also enable them to access more lung cells. Photograph: David Davies/PALinda Geddes Science correspondent Mon 24 Oct 2022 11.55 EDT

Two common respiratory viruses can fuse to form a hybrid virus capable of evading the human immune system, and infecting lung cells – the first time such viral cooperation has ever been observed.

Researchers believe the findings could help to explain why co-infections can lead to significantly worse disease for some patients, including hard-to-treat viral pneumonia.

Each year, about 5 million people around the world are hospitalised with influenza A, while respiratory syncytial virus (RSV) is the leading cause of acute lower respiratory tract infections in children under five years old, and can cause severe illness in some children and older adults.

Although co-infections – where a person is infected with both viruses at the same time – are thought to be relatively common, it was unclear how these viruses would respond if they found themselves inside the same cell.

“Respiratory viruses exist as part of a community of many viruses that all target the same region of the body, like an ecological niche,” said Dr Joanne Haney from the MRC-University of Glasgow centre for virus research, who led the study.

“We need to understand how these infections occur within the context of one another to gain a fuller picture of the biology of each individual virus.”Autumn Covid numbers peak at lower levels in UK – but flu cases are on the upRead more

To investigate, Haney and her colleagues deliberately infected human lung cells with both viruses and found that, rather than competing with one another as some other viruses are known to do, they fused together to form a palm tree-shaped hybrid virus – with RSV forming the trunk, and influenza the leaves.

“This kind of hybrid virus has never been described before,” said Prof Pablo Murcia, who supervised the research, published in Nature Microbiology. “We are talking about viruses from two completely different families combining together with the genomes and the external proteins of both viruses. It is a new type of virus pathogen.”

Once formed, the hybrid virus was also able to infect neighbouring cells – even in the presence of antibodies against influenza that would usually block infection. Although the antibodies still stuck to influenza proteins on the hybrid virus’s surface, the virus merely used neighbouring RSV proteins to infect lung cells instead. Murcia said: “Influenza is using hybrid viral particles as a Trojan horse.”