Flu Virus binding to Receptor Cells – Credit CDC
More than a decade after it re-emerged in Vietnam, H5N1 continues to circulate widely in Asian and Middle Eastern poultry providing numerous opportunities to infect humans, and yet only a few more than 600 human infections have been identified.
Similarly, the H7N9 virus which appeared 2 years ago in China appears well distributed in Asia’s domesticated poultry population, but only about 450 human infections have been reported.
While both viruses are capable of infecting and causing severe illness in humans, neither has taken off as a human-adapted pathogen. Transmission has almost always been from bird to human, with secondary human-to-human transmission a rarity.
Despite our concerns over the future of these influenza subtypes, both viruses remain primarily adapted to avian hosts. The concern of course, is that over time, that may change.
Why some influenza viruses – like seasonal H1N1, H3N2, H2N2, and others through the years – have successfully adapted to humans, while others like H5N1, H7N9, H5N6, H10N8 haven’t remains a mystery, although researchers are making progress in figuring it out.
Avian flu viruses are preferentially adapted to birds, where it is primarily a gastrointestinal infection. What scientists look for are `mammalian adaptations’; those that favor the infection and respiratory transmission among mammals – including humans.
Unlike solving a Rubik’s cube, there is probably more than just one `winning’ combination. And a change in one part of the virus that favors adaptation may either be enhanced, or blocked, by a change somewhere else along the14,000 nucleotide chain of the influenza A virion.
Avian adapted flu viruses bind preferentially to the alpha 2,3 receptor cells found in the gastrointestinal tract of birds. So the first barrier appears to be switching the RBS, or Receptor Binding Site (the area of its genetic sequence that allows it to attach to, and infect, host cells) to `fit’ the receptor cells commonly found in the human upper respiratory tract; the alpha 2,6 receptor cell (see Study: Dual Receptor Binding H5N1 Viruses In China)
But that, we are learning, isn’t enough on its own.
Birds run `hotter’ than mammals, with a normal body temperature several degrees higher (and avian viruses replicate in the gut, which is warmer than the upper airway of humans). Which means mammalian adapted viruses must be adapted to replicate at a lower temperature.
Researchers have determined the (E627K) substitution in the (PB2) protein - the swapping out of the amino acid Glutamic acid (E) at position 627 for Lysine (K) - makes the an influenza virus better able to replicate at the lower temperatures (roughly 33C) normally found in the upper human respiratory tract (see Eurosurveillance: Genetic Analysis Of Novel H7N9 Virus).
These are just two examples of species barriers that must be overcome before an avian virus can successfully adapt to human (or mammalian) physiology. There are more, some we know about, some we probably don’t.
Complicating matters – influenza viruses constantly develop multiple amino acid changes – and their combined effects on the virulence, transmission, antiviral resistance, `fitness’, and host range of the virus are far from fully understood.
All of which serves as prelude to a study recently published in Nature Communications, that finds another PB2 amino acid substitution (K526R) enhances the effects of the E627K mutation mentioned above.
First the link and abstract with the daunting title of:
Wenjun Song, Pui Wang, Bobo Wing-Yee Mok, Siu-Ying Lau, Xiaofeng Huang, Wai-Lan Wu, Min Zheng, Xi Wen, Shigui Yang, Yu Chen, Lanjuan Li, Kwok-Yung Yuen & Honglin Chen
Host-adaptive strategies, such as the E627K substitution in the PB2 protein, are critical for replication of avian influenza A viruses in mammalian hosts. Here we show that mutation PB2-K526R is present in some human H7N9 influenza isolates, in nearly 80% of H5N1 human isolates from Indonesia and, in conjunction with E627K, in almost all seasonal H3N2 viruses since 1970.
Polymerase complexes containing PB2-526R derived from H7N9, H5N1 or H3N2 viruses exhibit increased polymerase activity. PB2-526R also enhances viral transcription and replication in cells. In comparison with viruses carrying 627K, H7N9 viruses carrying both 526R and 627K replicate more efficiently in mammalian (but not avian) cells and in mouse lung tissues, and cause greater body weight loss and mortality in infected mice. PB2-K526R interacts with nuclear export protein and our results suggest that it contributes to enhance replication for certain influenza virus subtypes, particularly in combination with 627K.
Simply put, an influenza virus carrying both the E637K and K526R mutation in it’s PB2 protein replicates more efficiently in mammalian hosts.
Interestingly, the H3N2 seasonal flu virus – which traditionally produces more severe flu seasons than does seasonal H1N1 – has also carried this dynamic duo of amino acid substitutions since the early 1970s.
HKU (Hong Kong University) – which did this study – published a press release (excerpts below) with a summary of their findings:
20 Nov 2014
In a recent study reported in the Nature Communications, a research team led by Dr. Honglin Chen, Associate Professor, and Professor Kwok-yung Yuen, Henry Fok Professor in Infectious Diseases, Chair Professor of Infectious Diseases from the Department of Microbiology, Li Ka Shing Faculty of Medicine, and State Key Laboratory for Emerging Infectious Diseases, the University of Hong Kong, found that avian influenza A H5N1 and H7N9, and seasonal H3N2 viruses may gain the ability to replicate in mammal and human cells through various adaptation changes in the viral replication enzyme complex called the PB2 subunit.
They found that H7N9 avian influenza A virus is able to utilize multiple adaptive strategies to replicate in human cells, which may explain why H7N9 is distinct in causing human infections; This study identified a novel adaption marker, PB2-526R among some H7N9 viruses and almost exclusively among all H5N1 human cases from Indonesia. It has been a puzzle why there is no known PB2 adaptation marker in the H5N1 virus from Indonesia human cases and the finding from HKU nicely explained how this Indonesian subclade of avian H5N1 virus may have adapted for human infections.
This study also found PB2-526R is able to enhance replication and pathogenicity of other types of PB2 adaptations, such as previously known PB2-627K, in H7N9 and H3N2 viruses. Since the human pandemic H3N2 virus emerged in 1968, it has gained an additional PB2-526R adaptation marker since 1970s and the PB2-526R-627K virus replicates better than the solely PB2-627K virus. It is likely that the impression of more severe disease burden caused by H3N2 than that of H1N1 may be partly attributed to the better replication ability of PB2-526R-627K virus.
These findings by HKU provided new insight for the understanding of cross species transmission and replication in human cells by avian influenza viruses. The study provides a new genetic marker for the surveillance of avian influenza A virus with potential for human infection.
While this study provides us with a new genetic marker by which to track the evolution of avian flu viruses, it alone is obviously not the only barrier to seeing H7N9, H5N1, or any other novel flu virus become a pandemic.
But given its sloppy replication habits, and promiscuous `mating habits (reassortment)’, influenza viruses get billions of throws of the genetic dice each and every day.
So the odds (and history) suggest that given enough time the `right’ combination will come up, and another novel virus will strike mammalian host gold. We’ve seen that happen four times in the past century (2009, 1968, 1957, and 1918), and there is no reason to doubt it will happen again.