THE CONTRIBUTION OF THE INTERNAL GENES FROM THE
H9N2 AVIAN INFLUENZA VIRUS TO THE VIRULENCE OF
nH7N9
The interferon-mediated antiviral response and the species specificity
of the NS segment are important in determining tissue tropism and
virulence of influenza viruses.
46–48
Interferon-beta (IFN-b) plays a
critical role in the defense against an influenza virus that cannot be
compensated by IFN-a.
49
The NS of HPH5N1 is able to inhibit con-stitutive IFN-brelease
50
and inducible IFN-bproduction at the pre-transcriptional
51
and post-transcriptional levels
52
and to induce
cytokine dysregulation.
53
The intense innate immune suppression
enhances tropism of HPH5N1 for human tissues and allows
HPH5N1 to invade and replicate in human tissues without the need
for the avian sialic acid alpha-2,6-galactose receptor.
54–57
Introduction of the HPH5N1 NS into the low pathogenic H7N1 avian
influenza virus enabled H7N1 to replicate efficiently in different
human cell lines without prior adaptation due to potent suppression
of IFN-b production.
58
This observation shows that the NS of
HPH5N1 is able to increase the virulence and enhance the adaptation
of avian influenza viruses to human hosts. An optimal gene constel-lation is important for the virulence of NS in HPH5N1 infection.
59,60
The viral polymerase protein PA and the nucleocapsid protein are
important for the stabilization of the CPSF30–NS1 complex.
61
The
three influenza virus polymerases (PA, PB1 and PB2) and the PB1-F2
protein enhance NS-mediated interferon antagonism.
62,63