Flu Virus binding to Receptor Cells – Credit CDC
A fascinating, albeit technical letter was published in the CDC’s EID Journal yesterday that takes a closer look at the H5N1 virus isolate obtained from the Canadian nurse who died after returning from a trip to China last January (see WHO GAR Update On Canadian H5N1 Fatality).
Given that this patient had no known poultry contacts during her trip (and her illness was complicated by unusual neurological symptoms) the authors sought to determine if there were any changes in the virus’s receptor-binding properties that might help explain the route of infection.
As the picture at the top of the page illustrates, flu viruses must attach themselves first to the outside of a cell before they can enter, and replicate inside, a cell. Influenza viruses have an RBS - Receptor Binding Site (an area of its genetic sequence) that – like a key slipping into a padlock -`fit’ the receptor cells of the host it needs to infect.
Avian adapted flu viruses, like the H5N1 virus, bind preferentially to the alpha 2,3 receptor cells found in the gastrointestinal tract of birds, while human adapted influenza viruses bind preferentially to alpha 2,6 receptor cells which are found in the human upper respiratory system.
While there are some alpha 2,3 cells deep in the lungs of humans, for an influenza to be successful in a human host, most researchers believe it needs to a able to bind to the a 2,6 receptor cell. This host-specific binding explains why humans aren’t usually susceptible to avian , equine flu, or canine flu.
Species that carry both types of receptor cells – such as pigs – are viewed as potentially dangerous `mixing vessels’ as they can be infected by two (or more) influenza viruses simultaneously, which can sometimes swap genetic segments and create a new, hybrid (reassortant) virus.
But specific mutations within the RBS – amino acid substitutions at specific locations – can change the receptor cell affinity of an influenza virus, allowing it to expand its host range.
It is also possible possible to end up with a virus that binds to both alpha 2,3 and alpha 2,6 receptor cells, although the attraction to one is usually much stronger than to the other (see Study: Dual Receptor Binding H5N1 Viruses In China).
These mutations can happen spontaneously in an infected host, and only affect that individual. Or . . . if the mutated virus is `biological fit’ and easily transmitted – it may move onto other hosts as well.
Complicating matters – viruses generally 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.
In today’s study (excerpts below), researchers look at two specific mutations (R193K and G225R) found in the HA sequence that have previously been associated with changes in virulence or receptor binding affinity.
Volume 20, Number 9—September 2014
To the Editor: In December 2013, influenza associated with pandemic influenza A H5N1 was reported in Canada in a patient who had traveled to China; the patient died in January 2014. This case leaves unanswered questions.
In the absence of direct poultry contact by the patient, the possible route of transmission and infection, often influenced by receptor-binding properties of the virus, requires special attention. The full genome and phylogenetic analysis by Pabbaraju et al. (1) provides a summary of what can typically be deduced from the sequence.
The authors also mention 2 novel mutations, R189K and G221R, in the hemagglutinin (HA) protein (R193K and G225R in H3 numbering, used hereafter). When mapped to the H5 HA protein structure by using FluSurver in GISAID (http://www.gisaid.org, http://flusurver.bii.a-star.edu.sg), both mutations are found in the immediate receptor-binding pocket, and G225R has been known to change specificity of an H3N2 virus toward human erythrocytes (2).
These receptor-binding pocket mutations of the virus were not seen in the most closely related Asian H5N1 sequences of clade 18.104.22.168c (1), and no human contacts were known to be affected. From the epidemiologic perspective of this isolated human case, it is possible that that this variant arose in the patient after initial infection and contributed to prolonged and severe infection and to the more unusual spread to brain tissue.
If more avian strains with G225R mutations are found, the example of Q226L in H7N9 indicates that relative receptor-binding changes alone do not necessarily imply immediate mammalian transmissibility (10). It should also be noted that G225R was not among the mutations identified by recent controversial mammalian adaptation studies, (7,8) indicating that there may be more H5N1 host specificity markers than have been identified. Consequently, the functional roles of G225R in avian influenza should be further analyzed by conducting secure experiments and, pending verification, checking closely for its potential as an avian influenza host specificity marker.
I’ve excerpted some of the more technical sections due to space and sanity considerations. Many of my readers, I’m sure, will want to read this letter in its entirety.
These findings are quite preliminary, hence the use of the word `Potential’ in the title. The role of these two mutations (R193K and G225R) is far from clear, but they have rarely been seen in H5N1 isolates, and have shown up in a case with unusual presentation.
The G225R mutation in particular has a bit of a shady past.
It has been linked to increased affinity for binding to human erythrocytes with the H3N2 virus. And the same HA position is associated with the `Norway’ mutation in the 2009 H1N1 virus (see EuroSurveillance: Revisiting The D222G Mutation In A/H1N1pdm09).
Like a suspicious character detained at the scene of a crime – it may not prove guilt – but it does show opportunity.
This study also illustrates how diverse, dynamic, and unpredictable the H5N1 virus continues to be, and how much more we need to learn.
While the upstart H7N9 and H10N8 viruses have captured most of our avian flu attention over the past year, the venerable H5N1 virus has more than a 10 year head start - and a huge catalog of clades and variants continuing to circulate and evolve in Asia and the Middle East.
Which means that while there is some comfort to be taken from the fact that the H5N1 virus hasn’t successfully adapted to humans in all this time, it is also rolling the genetic dice faster, and in greater numbers, than any of its avian competitors.