#19,288
There remains much interest in exactly what changes the HPAI H5Nx virus would need in order to become a more `humanized' virus - like seasonal H3N2 and H1N1 - which have a strong affinity for the alpha 2,6 receptor cells most commonly found in the human respiratory system.While it seems likely that it would require multiple concurrent mammalian adaptations to create a pandemic strain, the ability of avian flu viruses to bind to human α2-6 receptor cells is considered one of the biggest obstacles the virus must overcome in order to successfully spread in humans.
Complicating matters, as we've discussed previously (see here, here, and here), HPAI H5 viruses encompass a large and growing array of closely related - but genetically distinct subclades, subtypes, and genotypes - and a mutation that might impact one strain might not have the same effect on another.
In December of 2024, a study in Science: A Single Mutation in Bovine Influenza H5N1 Hemagglutinin Switches Specificity to Human Receptors, suggested a single HA amino acid change (Q226L) could switch H5N1 B3.13 from binding preferentially to avian-type receptor cells to human-type receptor cells.
The authors wrote:
“Our experiments revealed that the Q226L mutation could significantly increase the virus’ ability to target and attach to human-type receptors,” explains Paulson. “This mutation gives the virus a foothold on human cells that it didn’t have before, which is why this finding is a red flag for possible adaptation to people.”
The shift alone, however, may not be enough to enable human-to-human transmission. Other genetic changes—such as mutations in polymerase basic 2 (E627K) that enhance viral replication and stability in human cells—would likely be necessary for the virus to spread efficiently among people.
The following month, in Preprint: The Q226L Mutation Can Convert a Highly Pathogenic H5 2.3.4.4e Virus to Bind Human-type Receptors, another study (later published in PNAS) found that adding Q226L to an older clade 2.3.4.4e H5N6 virus could switch receptor binding from avian to human-type receptors.
While concerning, we've not seen any evidence of efficient or sustained spread of H5N1 in humans, suggesting there are additional enhancements required.
Today we've a new study that offers an update on impact of the Q226L mutation in H5N1 B3.13, which finds that its impact varies between HPAI H5 strains.
While Q226L did shift binding in laboratory tests towards human-type glycans, those glycans were not found on the human tracheal tissues tested, and the Q226L mutant failed to bind human trachea.
The addition of another HA mutation, N224K, increased human-tracheal binding, but only sparsely and mainly to goblet cells. All of which reinforces the notion that receptor adaptation to humans in HPAI H5 is neither simple nor uniform.
Q226L appears to be decisive in some H5 strains, but the current bovine 2.3.4.4b H5N1 virus likely requires additional changes before it can efficiently bind to human receptors in the upper airway.
I've only posted the link, abstract, and some excerpts from the study. Follow the link to read it in its entirety.
Authors: María Ríos Carrasco, Mafalda F. Guerreiro Cabana, Eszter Kovács, Zoé Ducarne, Cindy G. J. Cleypool , Geert-Jan Boons, Robert P. de Vries r.vries@uu.nlAuthors Info & Affiliations
https://doi.org/10.1128/jvi.01065-26
ABSTRACT
It has been suggested that the hemagglutinin of the human-infecting cattle-derived 2.3.4.4b virus A/Texas/34 H5N1 (H5TX) requires only one mutation, namely Q226L, to switch from binding avian-type to human-type receptor preference. In this study, we examined the binding of H5TX Q226L, along with other key mutations, to sections of human trachea. We conclude that, while H5TX Q226L can bind human-type receptors, more than a single mutation is required for this protein to bind to human respiratory tract tissue.
(SNIP)We also report changes in receptor-binding specificity of another 2.3.4.4b HA mutant, H5FR Q226L (from A/duck/France/161108/16 H5N8), associated with the presence of a multibasic cleavage site. This study offers insight into the determinants of evolution toward human-type receptor binding in currently circulating H5Nx viruses. It also emphasizes the importance of testing individual strains using additional methods, including tissue-based approaches, alongside synthetic glycans.
IMPORTANCE
Currently, H5N1 influenza A viruses are responsible for numerous zoonotic spillover events, from infecting birds to other mammals, including dairy cattle. Although no human-to-human transmission has been observed, several people have been infected. This host range expansion is typically linked to changes in one of the viral surface proteins, hemagglutinin, which can switch its preference from avian-type to human-type receptors. To better understand the potential of the currently circulating H5N1 virus to transmit among humans, we evaluated the effects of the Q226L mutation, in combination with other amino acid substitutions, on binding to the human trachea. We also studied the effect of the multibasic cleavage site, a specific motif present in highly pathogenic influenza strains, on receptor-binding properties. These findings provide insight into the role of receptor binding in influenza infections.
We conclude that more than a single mutation is required for the hemagglutinin of the currently circulating 2.3.4.4b viruses to bind human respiratory tissue. This aligns with previous work showing that multiple amino acid changes are required to switch from avian- to human-type receptors in H5 strains (3). One key takeaway from this study is the significance of glycan architecture. H5TX mutants, similarly to other HAs, distinguish not only between SIA linkage but are also selective according to glycan branch length and sialylation (31).
More research is needed on the presence of asymmetrical glycans in the human respiratory tract and their role in influenza A virus hemagglutinin attachment to cells. Moreover, we emphasize the importance of not underestimating the MBCS’s influence on hemagglutinin’s receptor-binding properties.
