By Jonas Waldenström
One of the major news in the virology community last year was the publication in PNAS describing a completely new influenza A virus. In line with the taxonomy traditionally used for influenza viruses it got the name H17N10, illustrating that it possessed novel hemagglutinin (H17) and neuraminidase variants (N10). However, it wasn’t the numbers that was the ground breaking news, it was the fact that the virus was detected in a Central American bat, and not in a bird. A tropical bat is very far from the ‘normal’ diversity of influenza A viruses seen in wetland birds and waterfowl. Although bats and ducks both have wings, in evolutionary terms they separate a very, very long time ago in the age of dinosaurs. In fact, there are more differences than similarities between bats and gulls in ecology, physiology and aspects of cellular biology. Hence, the bat flu was a remarkable observation. A real shaker. In one sweep, the whole flu field needed to come with terms that not all viruses are bird viruses.
The initial findings also hinted that the first bat influenza virus was unlikely to be alone. An influenza-iceberg, of sorts, made up of fluffy, winged mammals. This week, a first follow-up was published in PLOS Pathogens. A crew of (mainly American) scientists analyzed samples from bats sampled in the Amazonian parts of Peru in 2010, collected as part of CDC’s tropical pathogen surveys. In total, 114 individuals of 18 bat species were taken out from the freezers and different sample types were screened with a molecular method designed to broadly pick-up the RNA of any influenza A virus. They got one hit from a fecal sample in a single bat! A lucky shot at the Tivoli, given the low sample size. Prompted by this, the authors brought in the big machinery and sequenced the totality of the genetic material in the samples from this poor, long-dead bat and used bioinformatic tools to resolve the genome of the virus that had infected its intestines. When bit by bit was added it became clear that it was indeed a completely new influenza A virus, very different from avian viruses, and similar, but still distinctly different from the earlier H17N10 bat virus. And the name? H18N11 of course!
Please take a close look at the figure below. It shows the phylogenetic relationships of each of the influenza A virus’ eight RNA segments – in black are all ‘non-bat viruses’ and in red the two new bat viruses H17N10 and H18N11. For all the segments coding for ‘internal’ proteins, i.e. those involved in the polymerase machinery or the structural properties of the virus, you see that the two bat viruses are always found in a neat little red outgroup. This signals a long evolution away from other known influenza A viruses. It is a little prematurely to say exactly how long, but the branch lengths indicate that this happened a long time ago.
Now look at the hemagglutinin and the neuraminidase trees (HA and NA, respectively). The same pattern is repeated for the NA, but not the HA. In fact, the two novel hemagglutinins are nested within avian hemagglutinins. How can we interpret this? At first this doesn’t make any sense, but one has to remember that influenza viruses don’t evolve in the same way you or me, trees, shrimps or ferns do. Influenza viruses can reassort, meaning that if two viruses of different origin infect the same cell the different RNA segments can be put in new combinations in the resulting virions. Imagine two decks of cards being shuffled, one red and one black, and that each virion randomly consists of a draw of card from the combined shuffled deck, sometimes red, sometimes black, and sometimes mixed. This is a rapid way in which new variants can arise, and a reason behind the genesis of pandemic flu in humans.
Returning to the bats, it seems that bat and avian viruses have met in a not too distant evolutionary past, and that a HA variant have sailed into the bat influenza gene pool. It will be interesting to see how the picture changes when more bat viruses are sequenced. Has there been one reassortment event, followed by drift and a subsequent separation into H17 and H18? Or, has there been many? Are there, perhaps, avian H17 and H18 to be found in South American birds? What about bats in North America, Europe, Africa and Asia?
One thing we can be sure of is that there are more viruses waiting to be detected and described. One sign of this comes from the current paper. The authors used the sequenced genomes to construct recombinant HA and NA molecules (using fancy virologist tricks) and used these to build assays (ELISAs) where bat sera could be screened for antibodies against the new HA and NA variants. Where the molecular screening yielded one positive bat, the serology approach found 55 of 110 bats showing signs of having been infected with flu earlier in life. This clearly indicates that influenza viruses are widespread in Peruvian bats, and likely in other parts of the world too. Moreover, they found cases of bats with antibodies to one of the recombinant HA or NA, but not to the other, suggesting that are more combinations of HA/NA to be found.
Finally, and perhaps the most interestingly of all results was that the hemagglutinin of bat influenza viruses does not to behave in the same way as avian hemagglutinins. When a virus is to infect a cell it needs the hemagglutinin protein to serve as a key, docking with a sialic acid receptor – the lock – on the cell. If the key and the lock don’t fit infection will not occur. For instance, a major division between human flu and avian flu is the preferred conformation of a galactose residue on the sialic acid receptors. This little difference makes it hard for avian viruses to infect humans, and vice versa. But with bat viruses it seems sialic acid receptors are not used at all! Instead bat HA uses an unknown receptor for cell entry. Holy Moses!
More to follow shortly, I suppose. Major obstacle at present is the lack of a culturing method for bat influenza viruses. Neither cell lines nor eggs have worked so far. Without the means to grow the virus it is very tricky to study it. But there are many clever virologists out there, so it is likely not too far away.
But I still prefer feathers to fur, and will stick with ducks.
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