Some viruses like to cheat – and that may be good for our health
Some influenza viruses are freeloaders
BSIP SA / ALAMY
Even viruses have to face parasites in the form of parents’ sponge – and these cheaters can be much more common and important than biologists thought. In influenza infections, these viruses can be more numerous than normal in a third of the cases, limiting the severity of infections.
Viruses force the cells they infect to make copies of themselves. They mainly exploit existing machines in cells, but some proteins coded by the virus genome are essential.
However, mutations can remove viral genes for these key proteins, causing defective viruses that can infect cells, but cannot copy – unless another complete virus also infects the same cell, providing the viral protein or missing proteins.
The cell will then cause copies of the two viruses. In fact, it can produce more copies of the incomplete or defective virus because it has a shorter genome. It is the viral equivalent of never buying a turn in the ad, and it slows infections because infected cells produce much fewer complete viruses.
The existence of these cheating viruses, also known as defective interfering virus, was established in 1970. “We know that with most animal viruses, including flu, if we cultivate them in the laboratory, we get these cheaters,” explains the Asher leek at the University of British Columbia in Canada. “But there is this question: are they also important in nature?”
He and his colleagues decided to answer this question. We know in previous studies that cheating viruses exist in nature, but not to what extent they are abundant. This is difficult to establish because it requires sequenced many viruses present in infected individuals. Due to the dangers placed by the H5N1 bird flu, this type of sequencing has now been carried out for other purposes by the US Department of Agriculture (USDA) and the raw data rendered free of charge.
The data include infections against flu in dozens of different species, indicates the leeks. “You have ostriches, homemade cats, cows, poultry, aquatic birds, birds of prey.”
Based on the USDA sequencing, the preliminary estimates of his team, which are not yet published, suggest that cheating viruses are really abundant. “Something like 1 of the individuals infected out of 3 has at least one viral cheat sequence that we estimate to be 50% or more relative abundance,” explains the leeks. “What it means is, like a third of time, if you are infected with the flu, these non -functional viruses are the majority. They simply exceed the population.”
“It is not surprising that they are there,” he says. “It is surprising that they are there in such crazy abundances, and it is surprising that they are there in so many different host species, so many different flu subtypes.”
There is evidence that higher abundance of cheating viruses reduce the severity of infections, explains that leeks, so the test of their presence could help predict the seriousness of the disease.
Other groups explore if cheating viruses could be used to treat infections. In fact, for HIV, initial human tests are underway after successful results in monkeys.
“I do not understand therapy, but the results we have answered to the questions you would like to answer, in terms of security and their usefulness,” explains the leeks.
Rafael Sanjuán at the University of Valence in Spain says that he cannot comment on the specific results as long as the full results are not available. But it is possible that they apply to the flu only, rather than to viruses in general.
“Some viruses tend to produce more of these” cheaters “than others,” said Sanjuán. “The influenza virus, in particular, would be quite prolific in this regard.”
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