A classic serial passage experiment showing Host adaptation
#17,928
In the summer of 2013 Taiwan's CDC Reported the 1st Human Infection With Avian H6N1, detected in a 20 year-old female who was hospitalized for pneumonia. The case might have gone undiagnosed were it not for the enhanced surveillance for H7N9, which had broken out in Mainland China a few months earlier.Three years earlier, researchers had reported the Identification of an H6N6 swine influenza virus in southern China.
Over the next few years we'd see additional evidence suggesting that avian H6 viruses - which are ubiquitous in wild birds and poultry - could someday pose a potential threat to human health.
- In 2014 we learned that H6N1 had jumped to Taiwanese dogs (see Taiwan: Debating The Importance Of H6N1 In Dogs)
- In 2015, in EID Journal: Seropositivity For H6 Influenza Viruses In China - researchers reported on a small, but significant number of people in their survey who tested positive for H6 influenza antibodies (indicating previous exposure).
- Also in 2015, in Study: Adaptation Of H6N1 From Avian To Human Receptor-Binding, we saw a report citing changes the authors suggest are slowly moving the H6N1 virus towards preferential binding to human receptor cells instead of avian receptor cells
- In 2020's Nature: Evolution & Pathogenicity of H6 Avian Influenza Viruses, Southern China 2011-2017, we looked at H6's increasing adaptation to mammalian physiology,
- And in 2022 we saw Study: Influenza A (H6N6) Viruses Isolated from Chickens Replicate in Mice and Human lungs Without Prior Adaptation which found binding to both avian-like and human-like receptors.
More recently, in November of 2023 in Emerg. Microbes & Inf.: Epidemiology, Evolution, and Biological Characteristics of H6 Avian Influenza Viruses in China, we looked at a study by Chinese researchers which characterized the evolution and biological characteristics of H6 viruses, and produced additional evidence of increased adaptation to mammalian hosts.
While avian H6 viruses don't get a lot of press, they continue to reassort, evolve, and spillover into non-avian species. With each spillover comes an opportunity to better adapt to mammalian hosts, a process which is demonstrated by the serial passage graphic at the top of this blog.
Essentially, you inoculate a host (mouse, ferret, guinea pig, etc.) with a `wild type’ strain of a virus - let it replicate for awhile - then take the virus from the first host and inoculate a second, and then repeat the process five, ten, fifteen times or more.
Over time, the virus tends to adapt to the new host (assuming there are no species barriers to prevent it), often increasing replication, virulence, and/or transmissibility.
Which is precisely what the following study demonstrates; that after only a single passage through BALB/c mice, a wild bird-origin H6N2 virus picked up two significant mutations (PB2 E627K and HA A110V) increasing both its replication and lethality in mice.
While in this case the passage-generated mutant retained its avian α-2, 3-linked sialic acid binding property, we've seen other studies showing avian H6 viruses with an affinity for both avian and human (α-2, 6-linked sialic acid) receptor cells.
This is a lengthy and detailed report, and I've only posted the Abstract and some excerpts. Follow the link to read it in its entirety. I'll have a brief postscript when you return.
Wild Bird-Origin H6N2 Influenza Virus Acquires Enhanced Pathogenicity after Single Passage in Mice
by Siqi Tang, Bing Han, Chaofan Su, Hailing Li, Shiyuchen Zhao, Haoyu Leng, Yali Feng and Ying ZhangViruses 2024, 16(3), 357; https://doi.org/10.3390/v16030357 (registering DOI)Published: 25 February 2024(SNIP)
4. Discussion
Wild birds are generally considered to be the natural hosts of AIVs. Liaoning is located on the East Asian–Australasian flyway route and possesses several migrating stopovers and wintering areas for wild birds. In this study, we isolated and purified an H6N2 subtype wild bird-origin AIV, A/wild bird/Liaoning/DD535/2021 (DD535), in April 2021 from a wetland in Liaoning. Although DD535 exhibited limited replication ability in mice, it acquired two mutations, PB2 E627K and HA A110V, during the first round of infection. These mutations endowed this H6N2 virus with enhanced polymerase activity and decreased the pH of HA activation, ultimately enhancing lethality to mice.
H6 subtype AIVs belong to the low pathogenic AIV and have become one of the most prevalent subtypes worldwide. Previous studies have demonstrated that some of the H6 AIVs isolated from poultry were able to bind α-2,6 SA receptors and transmit among mammalian hosts [7]. The H6 subtype AIVs have already shown the potential to threaten public health.
Genetic recombination and mutations are considered to be the two major mechanisms of AIVs adapting to new hosts. Three of the four human influenza pandemics of the last century were caused by genetic reassortment of influenza viruses. Recent genetic reassortment events were observed in emerging H5N1 [34], H7N9 [35], H10N8 [36], and H5N6 [37]. Most of these reassortants acquired human infectivity. Genetic mutation could occur rapidly once AIV is introduced into other species. Numerous adaptive mutations were detected during avian-origin viruses spilling into mammalian hosts. For example, the H7N9 AIVs in 2013 in China were nonpathogenic for poultry and mice but obtained the PB2 627K or PB2 701N mutation during replication in ferrets, leading to high lethality in mice and ferrets [38]. Our results indicated that the H6N2 subtype wild bird-origin AIV could acquire two adaptive mutations, PB2 E627K and HA A110V, after only one passage in mice. The mutant strain had increased tissue tropism and pathogenicity in mice. Additionally, it exhibited increased replication ability in mammalian cells and enhanced replication efficiency in guinea pigs.
As early as 1993, PB2 E627K was recognized as a determinant of virulence and host range [39]. Subsequent studies revealed multiple functions of PB2 E627K, such as enhancing AIV polymerase activity, replication, and transmissibility in mammals [40,41,42]. In our study, PB2 E627K increased the H6N2 virus polymerase activity in HEK293T cells. HA affects multiple biological characteristics of AIVs, including receptor binding property and membrane fusion, which have been proved critical in determining the host range and pathogenicity of AIVs. Amino acid positions 226 and 228 in the HA protein are associated with receptor binding properties. Both DD535 and Mut-DD535 contained 226Q/228G, suggesting a preference for binding to the avian receptor. The receptor binding assay in our study also confirmed that HA A110V did not affect the receptor binding property of the H6N2 virus. However, our findings indicated that the HA A110V mutation was responsible for the decreased pH required for membrane fusion, which might contribute to the higher replication and pathogenicity of H6N2 viruses in mice. Whether the HA A110V and PB2 E627K mutations acted synergistically or whether host factors were involved during the mutation acquisition still deserve further investigation.
5. Conclusions
In summary, we found that the wild bird-origin H6N2 subtype AIV can infect mice without prior adaptation and can rapidly acquire adaptive mutations, HA A110V and PB2 E627K, in mice. A mutant H6N2 virus with these adaptations after a single passage in mice not only exhibited higher replication ability in vitro and in vivo but also enhanced pathogenicity in mice. We further investigated the effects of HA A110V and PB2 E627K and found that HA A110V decreased the pH of the viral membrane fusion, and PB2 E627K increased the viral polymerase activity. Our research indicated that wild bird-origin H6 viruses have zoonotic potential. Continued surveillance and investigation of the H6 influenza viruses circulating in wild birds are needed.
While the conventional wisdom is that H6 viruses are unlikely to pose a serious zoonotic threat, a dozen years ago LPAI H7 viruses were thought to be a weak cousin of HPAI H5N1, and incapable of producing the same level of virulence or spread in humans.
The emergence of LPAI H7N9 in China in 2013 - with a mortality rate (among those hospitalized) of 30% - has since dispelled that notion. A severe human infection with LPAI H7N4 in China in 2018 showed this was not a fluke.
While our attentions remain firmly fixed on HPAI H5Nx, there are a lot of `lesser' novel flu threats in the wild, quietly mutating and evolving, which could - through a lucky mutation or reassortment event - go to the top of our pandemic worry list overnight.
With influenza viruses, the only constant is change. Which is why we need to be prepared to pivot the next time the unexpected happens.
