1. Introduction
Influenza A virus (IAV) is an important respiratory pathogen that
continually impacts human public health and the animal industry. The
wild waterfowl has been thought to be the natural reservoir for IAV,
however, the viruses frequently jump species barriers and infect humans
and other mammals, such as pigs, cats, horses, and
whales(Zhu et al., 2019a). Swine is of
particular significance due to its susceptibility to avian, swine, and
human influenza viruses and has been regarded as “gene mixing vessels”
to generate virus with pandemic
potential(Ma et al., 2008,
Ito et al., 1998). Systematic surveillance
of swine influenza viruses (SIVs) is essential for early warning and
preparedness for the next potential pandemic. In China, pigs are not
vaccinated against influenza virus, and distinct lineages of SIVs, such
as classical swine H1N1 (CS H1N1), Eurasian avian-like H1N1 (EA H1N1),
and triple reassortant H3N2 (TR H3N2), have been co-circulating in pig
herds(Zhu et al., 2019b). EA H1N1 SIVs
were firstly transmitted from waterfowl to pigs in 1979 in
Europe(Pensaert et al., 1981) and
gradually became the predominant lineage in
China(Yang et al., 2016). Since 2009, the
2009 pandemic (pdm/09) H1N1 in humans quickly transmitted to
pigs(Weingartl et al., 2010,
Pereda et al., 2010), and genetic
reassortants between EA H1N1 SIVs and pdm/09 H1N1 have been frequently
reported among pigs(Zhu et al., 2011,
Cao et al., 2019). Notably, sporadic human
infections of EA H1N1 and other reassortant SIVs highlight the
importance of influenza surveillance in pigs and
humans(Yang et al., 2012,
Zhu et al., 2016b,
Xie et al., 2018).
As a multifunctional protein, a single amino acid mutation in
hemagglutinin (HA) can largely alter the biological property of virus.
For example, G225E mutation in HA has been confirmed to significantly
improve the respiratory droplet transmission of EA H1N1 SIVs in guinea
pigs(Wang et al., 2017). Neutralizing
antibodies stimulated by HA protein can protect humans or animals
against influenza virus infection. HA protein consists of two
polypeptides, HA1 and HA2, and HA1 plays much important roles than HA2
in triggering host immune response(Chi et
al., 2005). Accumulation of amino acid mutations in HA1 usually cause
antigenic drift, making the vaccine unable to offer effective protection
against antigenically mismatched circulating strains. When the prevalent
viruses show a greater than 4-fold difference in hemagglutinin
inhibition (HI) assay titer from the vaccine strain, the immunity
induced by the vaccine does not prevent the circulation of such viruses
in population(Smith et al., 2001).
Therefore, identification of the genetic determinants for antigenic
variation will be undoubtedly invaluable for vaccine development and
prevention of influenza virus outbreaks.
Currently, EA H1N1 SIVs have been frequently recombining with other
influenza viruses, such as pdm/09 H1N1 and TR
H3N2(He et al., 2018). Importantly, the
overwhelming majority of the reassortants preserved the HA and NA genes
of EA H1N1 SIVs(Sun et al., 2020),
indicating that studying the genetic basis for antigenic drift of EA
H1N1 SIVs is essential to evaluate the antigenic properties of SIVs. Our
previous study demonstrated that EA H1N1 SIVs formed two distinct
antigenic groups, and A/swine/Guangdong/104/2013 (GD/104) virus
exhibited 32~64-fold lower antigenic cross-reactivity
with antibodies against A/swine/Guangxi/18/2011 (GX/18)
virus(Yang et al., 2016). In this study,
we used GX/18 and GD/104 as models to explore the underlying mechanism
of this difference in antigenicity.