Highly Pathogenic Avian Influenza Viruses Do Not Inhibit Interferon Synthesis in Infected Chickens but Can Override the Interferon-Induced Antiviral State

Abstract

From infection studies with cultured chicken cells and experimental mammalian hosts, it is well known that influenza viruses use the nonstructural protein 1 (NS1) to suppress the synthesis of interferon (IFN). However, our current knowledge regarding the in vivo role of virus-encoded NS1 in chickens is much more limited. Here, we report that highly pathogenic avian influenza viruses of subtypes H5N1 and H7N7 lacking fully functional NS1 genes were attenuated in 5-week-old chickens. Surprisingly, in diseased birds infected with NS1 mutants, the IFN levels were not higher than in diseased birds infected with wild-type virus, suggesting that NS1 cannot suppress IFN gene expression in at least one cell population of infected chickens that produces large amounts of the cytokine in vivo. To address the question of why influenza viruses are highly pathogenic in chickens although they strongly activate the innate immune system, we determined whether recombinant chicken alpha interferon (IFN-␣) can inhibit the growth of highly pathogenic avian influenza viruses in cultured chicken cells and whether it can ameliorate virus-induced disease in 5-week-old birds. We found that IFN treatment failed to confer substantial protection against challenge with highly pathogenic viruses, although it was effective against viruses with low pathogenic potential. Taken together, our data demonstrate that preventing the synthesis of IFN is not the primary role of the viral NS1 protein during infection of chickens. Our results further suggest that virus-induced IFN does not contribute substantially to resistance of chickens against highly pathogenic influenza viruses.The nonstructural protein 1 (NS1) of influenza A virus serves multiple functions in the viral life cycle. It regulates viral polymerase activity, and it modulates cellular signaling pathways (for a review, see reference 14). It is well established that NS1 can suppress innate immune responses in infected cells by blocking intracellular signaling pathways that lead to the synthesis of type I and type III interferon (IFN). Specifically, NS1 inhibits ubiquitinylation of RIG-I by TRIM25, thereby preventing efficient induction of IFN in infected cells (9,12,28,35). The NS1 proteins of most influenza viruses block global posttranscriptional processing of cellular mRNAs, including mRNAs encoding 27,33,34). NS1 further inhibits specific antiviral effector proteins, such as 2Ј-5Ј oligoadenylate synthetase (OAS) (29) and double-stranded RNA (dsRNA)-activated protein kinase (PKR) (23,30). In addition, NS1 was shown to modulate host cell apoptosis (39, 55), stimulate phosphoinositol 3-kinase signaling (13), and regulate viral polymerase activity (2, 30).Current knowledge regarding the in vivo functions of NS1 is mainly based on studies in mice with targeted deletions of several key components of the IFN system. Viruses expressing defective NS1 proteins are generally strongly attenuated in hosts with an intact IFN system but retain a high degree of virulence in hosts with defective IFN syst...

Identification of the N-Terminal Domain of the Influenza Virus PA Responsible for the Suppression of Host Protein Synthesis

“…However, a recent study using NS1 mutant viruses suggests that NS1 does not suppress IFN gene expression efficiently in vivo. It was suggested that, in chickens, other functions of NS1, such as its ability to inhibit apoptosis, might be more critical for maintaining the virus in an avian host (38). It is unknown how much of an effect PA has in antagonizing the innate immune response in avian hosts.…”

Antiviral Activity of Lambda Interferon in Chickens

“…Influenza A virus strains used in this study are A/WSN/1933 H1N1 (WSN), A/Chicken/United Arabic Emirates/R66/2002 H9N2 (R66), A/FPV/Rostock/34 H7N1 (FPV Rostock), and recombinant A/swan/Germany/R65/2006 H5N1 (rR65-wt). In addition, a recombinant virus carrying genome segments HA, NP, NA, M, and NS of the avian strain R65 and segments PB1, PB2, and PA of the mammalian strain A/Puerto Rico/8/1934 H1N1 (PR8), designated rR65-PR8(123), was used (16). Furthermore, the velogenic Newcastle disease virus (NDV) strain Herts-33 and the infectious bronchitis virus (IBV) strain Massachusetts 41 (M41; kindly provided by C. Winter, Hannover, Germany) were used.…”

“…Chicken IFN-␣ (chIFN-␣) administration was shown to significantly delay Rous sarcoma virus-induced tumor formation, exemplifying its in vivo antiviral properties (15). Another study demonstrated that chIFN-␣ treatment ameliorates disease progression following experimental infection of chickens with a highly pathogenic influenza A virus (HPAIV) H5N1 strain (16).…”

Length Variations in the NA Stalk of an H7N1 Influenza Virus Have Opposite Effects on Viral Excretion in Chickens and Ducks

“…and 4 d p.i (16). A likely hypothesis is that the short-stalk H7N1 virus, with higher replication efficiency than its H1N1 counterpart, can outcompete the antiviral response in infected chickens (20). The fact that the H7N1 virus has an allele B NS1 while the H1N1 virus has an allele A NS1 could also contribute to differences in the modulation of the cytokine response (17).…”