PA-X is a virulence factor in avian H9N2 influenza virus

Abstract

H9N2 influenza viruses have been circulating worldwide in multiple avian species, and regularly infect pigs and humans. Recently, a novel protein, PA-X, produced from the PA gene by ribosomal frameshifting, was demonstrated to be an antivirulence factor in pandemic 2009 H1N1, highly pathogenic avian H5N1 and 1918 H1N1 viruses. However, a similar role of PA-X in the prevalent H9N2 avian influenza viruses has not been established. In this study, we compared the virulence and cytopathogenicity of H9N2 WT virus and H9N2 PA-X-deficient virus. Loss of PA-X in H9N2 virus reduced apoptosis and had a marginal effect on progeny virus output in human pulmonary adenocarcinoma (A549) cells. Without PA-X, PA was less able to suppress co-expressed GFP in human embryonic kidney 293T cells. Furthermore, absence of PA-X in H9N2 virus attenuated viral pathogenicity in mice, which showed no mortality, reduced progeny virus production, mild-to-normal lung histopathology, and dampened proinflammatory cytokine and chemokine response. Therefore, unlike previously reported H1N1 and H5N1 viruses, we show that PA-X protein in H9N2 virus is a pro-virulence factor in facilitating viral pathogenicity and that the pro-or antivirulence role of PA-X in influenza viruses is virus strain-dependent.

Interplay of PA-X and NS1 Proteins in Replication and Pathogenesis of a Temperature-Sensitive 2009 Pandemic H1N1 Influenza A Virus

“…9 and Table 1). These data are consistent with previous findings describing that a pH1N1 WT virus showed a slightly increased virulence in mice compared to a virus expressing lower levels of the PA-X protein (21,35). Remarkably, by using our pH1N1 LAIV virus, we have observed greater differences in virus pathogenicity than those observed using pH1N1 WT, suggesting that the LAIV virus backbone is better suited to detect differences in pathogenicity than the WT backbone.…”

“…Similarly, Gao et al (43) and Hu et al (16) reported that loss of PA-X expression increased viral replication and pathogenicity of pH1N1 and H5N1 IAVs. In contrast, recent work has described that loss of PA-X in pH1N1 or H9N2 viruses leads to a reduction in viral pathogenicity (21,35). In addition, IAV strain-specific differences in the ability to block the cellular protein synthesis have also been reported (25,(31)(32)(33).…”

“…Multiple functions have been attributed to PA-X, including degradation of host mRNA that leads to inhibition of host protein synthesis and contributes to blocking the cellular antiviral responses (15)(16)(17)(18)(19). Moreover, the PA-X protein has been shown to be involved in modulating host inflammation, the immune response, and apoptosis (15,17,(20)(21)(22). Although PA and PA-X have the same N-terminal domain, PA-X has a much stronger endonucleolytic activity, indicating that the C-terminal region is responsible for the inhibition of the host protein expression (23).…”

Comparing the functions of equine and canine influenza H3N8 virus PA-X proteins: Suppression of reporter gene expression and modulation of global host gene expression

“…Several previous reports have examined the effects of PA-X on host response by comparing wild type and mutant virus infections, and shown that PA-X was important for modulating various immune responses (2, 6–8, 12, 14). To further elucidate PA-X’s effects on the host, we transfected plasmids expressing only PA-X into cell cultures and used RNA-seq to analyze the transcripts.…”

“…This ribonuclease domain has been attributed to the N terminal region, although several recent publications have shown that the C-terminal region may also be important in regulating ribonuclease activity (10, 12, 13). Interestingly, PA-X has also been shown to modulate other virus-host interactions such as preventing stress granule formation (and thereby preventing translational arrest), increasing the accumulation of poly(A)-binding proteins within the nucleus, and exhibiting anti-apoptotic activity (5, 6, 14). …”