The remarkable global spread of the recently emerged HPAI H5 clade 18.104.22.168 (includes H5N8 & H5N6) viruses - covering more territory in 3 years than H5N1 has done in nearly 2 decades - has researchers wondering just what might have changed to increase its range.
Today, a potential clue, as the CDC's EID Journal has published a research article that finds mutations in this new clade have altered the virus's receptor binding specificity.
For an influenza virus to infect a host, the virus must bind (attach) itself to the surface of a cell (see graphic at top of blog).
RBD's or receptor binding domains, are that part of the virus that allows it to attach to receptor cells in a host's body. Different viruses are attracted to different types of cells, which explains why some viruses that affect man, don't affect other species, and why we don't get influenza in our big toe.
Receptor cells have stalks of sugar (carbohydrate) molecules on their surface. These carbohydrate molecules - called glycans' - form a dense sugary coating to all animal cell membranes. The composition of these stalks varies between types of cells and hosts.
When a virus meets a compatible receptor cell, they bind. And infection ensues.
Today's EID journal reports that novel clade 22.214.171.124 H5 proteins now bind to fucosylated sialosides because of specific amino acid changes (K222Q and S227R). Fucosylated sialosides refer to stalks that have taken on additional fucose sugar units, and differ from the sialic acid receptor cells (a2,3 avian or a2,6 mammalian) that influenza viruses normally bind to.
That said, H5N8 clade 126.96.36.199. (and its reassortants) continue to bind preferentially to avian receptor cells - limiting their threat to humans.
We have started to see some increased affinity for mammalian receptor cells with the H5N6 virus (see J. Virology: H5N6 Receptor Cell Binding & Transmission In Ferrets), however.
While the full impact of this change in receptor binding remains unknown, the authors suggest `Altered receptor-binding properties might affect the balance between HA and NA, enable the virus to acquire different NA subtypes, and might result in altered host range and spreading.'
And as we've seen, HPAI H5 clade 188.8.131.52. has been very promiscuous, generating a number of viable reassortants in North America and Taiwan.
Although some (and I include myself) may find the more technical aspects of this report tough sledding, I've excerpted some of the less rigorous passages below.
Highly Pathogenic Influenza A(H5Nx) Viruses with Altered H5 Receptor-Binding Specificity
Hongbo Guo1, Erik de Vries1, Ryan McBride, Jojanneke Dekkers, Kim M. Bouwman, Corwin Nycholat, M. Helene Verheije, James C. Paulson, Frank J.M. van Kuppeveld, and Cornelis A.M. de HaanComments to Author
Emergence and intercontinental spread of highly pathogenic avian influenza A(H5Nx) virus clade 184.108.40.206 is unprecedented. H5N8 and H5N2 viruses have caused major economic losses in the poultry industry in Europe and North America, and lethal human infections with H5N6 virus have occurred in Asia. Knowledge of the evolution of receptor-binding specificity of these viruses, which might affect host range, is urgently needed.
We report that emergence of these viruses is accompanied by a change in receptor-binding specificity. In contrast to ancestral clade 2.3.4 H5 proteins, novel clade 220.127.116.11 H5 proteins bind to fucosylated sialosides because of substitutions K222Q and S227R, which are unique for highly pathogenic influenza virus H5 proteins.
North American clade 18.104.22.168 virus isolates have retained only the K222Q substitution but still bind fucosylated sialosides. Altered receptor-binding specificity of virus clade 22.214.171.124 H5 proteins might have contributed to emergence and spread of H5Nx viruses.
HA proteins bind to sialoside receptors on the host cell surface. Avian and human influenza A viruses prefer binding to sialic acids linked to a penultimate galactose by an α2-3 or α2-6 linkage, respectively (18). Type and number of internal monosaccharides and their linkages determine fine specificity of virus receptors (19,20).(SNIP)
NA removes sialic acids from glycans, which enables virus particles to be released from the cell surface after assembly and from decoy receptors (e.g., in mucus). The balance between activities of HA and NA proteins has a critical role in optimal viral fitness, tropism, and transmission (21).
Changes in HA receptor-binding properties might affect virus host range and within-host virus properties. These changes might have contributed to the remarkable spread of clade 126.96.36.199 viruses.
Although a recent study (22) reported enhanced avidity of H5N6 viruses for human-type receptors, recombinant clade 188.8.131.52 highly pathogenic influenza A virus H5 proteins from virus isolates in North America show a strict avian receptor-binding preference (23). We compared receptor-binding properties for clade 184.108.40.206 H5 proteins from an H5N8 virus from Europe with those for an early ancestral clade 2.3.4 H5 protein from an H5N1 virus to identify differences in these properties.
Of particular interest are the recent outbreaks of influenza caused by H5N6 viruses in poultry and ducks in Southeast Asia, which might resulted in nonavian infections, including 13, mostly lethal, cases in humans (7). Although enhanced avidity of these H5N6 viruses for human-type receptors (carrying α2-6−linked sialosides) has been reported (22), the amino acid combination Q222/R227, which all H5N6 viruses have in their HA, is unlikely to be responsible, and other amino acid substitutions, which have been shown to contribute to binding of α2-6−linked sialosides by highly pathogenic H5N1 viruses, have not been detected in H5N6 viruses (48,49).
Two clades of H5N6 viruses have been identified by phylogenetic analysis (50), one harboring an NA with a truncated stem and the other harboring a full-length stem. Truncation of the stem has been considered a poultry-specific NA adaptation. However, both H5N6 virus clades appear to have acquired their N6 segment in independent events from H6N6 viruses (50), one of which already contained the stem deletion. Also, both clades have caused infections in wild birds, poultry, and humans, but evidence for species-specific adaptions in NA is lacking.
A longstanding paradigm in influenza A virus biology is the requirement for an optimal balance between HA binding and NA cleavage. HA binding displays a clear receptor fine specificity, but substrate fine specificity of NAs has not been extensively investigated. A recent report showed that all NA genotypes (only N4 was not tested) displayed relatively poor digestion of fucosylated receptors (44). Possibly because of tight binding of such receptors by clade 220.127.116.11 viruses, N1 of highly pathogenic H5 viruses might have lost an unknown advantage over other NA genotypes in creating an optimal HA/NA balance, which lead to the remarkable success of novel H5Nx virus reassortants within this clade.
Ms. Guo is a doctoral student in the Virology Division, Faculty of Veterinary Medicine, Utrecht University, Utrecht, the Netherlands. Her primary research interest is interactions between influenza A viruses and sialoside receptors.