A198V HA mutation causes HI titer decrease significantly
We previously reported that the 15 mutations (as seen in Figure 1A) on
the HA protein from the passaged virus occurred when the F/98 strain
passaged continuously in SPF embryonated chicken eggs or in SPF chickens
(Jin et al., 2018; Su et al., 2020). In order to evaluate the role of
these mutations, 15 recombinant viruses, each containing a single HA
mutation from the passaged viruses occurring antigenic drift in F/98
backbone, were generated. These viruses are rF/HAK131R,
rF/HAQ133H, rF/HAS145N,
rF/HAQ164L, rF/HAA168T,
rF/HAG181E, rF/HAA198V,
rF/HAM224K, rF/HAQ234L,
rF/HAY264H, rF/HAG270R,
rF/HAG274R, rF/HAK278E,
rF/HAI386V, and rF/HAK399N. The serum
against the paternal virus F/98 in chickens was used as the reference
serum to analyze the antigenicity of the recombinant viruses by HI
assay. Compared with the paternal virus F/98, the mutations S145N,
Q164L, A168T, M224K, and Q234L slightly reduced HI titers. The mutation
A198V exhibited 6.67-fold lower HI titers, which was antigenically
distinct from F/98 (Figure 1B).
To further confirm the contribution of HA A198V mutation to the
antigenic change of F/98 strain, 5 recombinant viruses possessing
multiple mutations from HA protein of the 47thgeneration of F/98 strain (containing K131R+S145N+G181E+A198V) passaged
in SPF embryonated chicken eggs (Jin et al., 2018) in F/98 backbone were
generated, respectively, including rF/HA348 (K131R+S145N+G181E),
rF/HA349 (K131R+S145N+A198V), rF/HA389 (K131R+G181E+A198V), rF/HA489
(S145N+G181E+A198V), and rF/HA47 (K131R+S145N+G181E+A198V). The results
showed that the viruses possessing A198V mutation, including
rF/HA349, rF/HA389, rF/HA489, and
rF/HA47, displayed the same 6.67-fold lower HI titers as
rF/HAA198V (Figure 1C). These results suggested that the
HA mutation A198V is the key change related to the antigenicity of H9N2
virus.