Difference in growth behavior of human, swine, equine, and avian influenza viruses at a high temperature

Influenza A Virus Reassortment Is Limited by Anatomical Compartmentalization following Coinfection via Distinct Routes

Exchange of gene segments through reassortment is a major feature of influenza A virus evolution and frequently contributes to the emergence of novel epidemic, pandemic, and zoonotic strains. It has long been evident that viral diversification through reassortment is constrained by genetic incompatibility between divergent parental viruses. In contrast, the role of virus-extrinsic factors in determining the likelihood of reassortment has remained unclear. To evaluate the impact of such factors in the absence of confounding effects of segment mismatch, we previously reported an approach in which reassortment between wild-type (wt) and genetically tagged variant (var) viruses of the same strain is measured. Here, using wt/var systems in the A/Netherlands/602/2009 (pH1N1) and A/Panama/2007/99 (H3N2) strain backgrounds, we tested whether inoculation of parental viruses into distinct sites within the respiratory tract limits their reassortment. Using a ferret (Mustella putorius furo) model, either matched parental viruses were coinoculated intranasally or one virus was instilled intranasally whereas the second was instilled intratracheally. Dual intranasal inoculation resulted in robust reassortment for wt/var viruses of both strain backgrounds. In contrast, when infections were initiated simultaneously at distinct sites, strong compartmentalization of viral replication was observed and minimal reassortment was detected. The observed lack of viral spread between upper and lower respiratory tract tissues may be attributable to localized exclusion of superinfection within the host, mediated by innate immune responses. Our findings indicate that dual infections in nature are more likely to result in reassortment if viruses are seeded into similar anatomical locations and have matched tissue tropisms.IMPORTANCE Genetic exchange between influenza A viruses (IAVs) through reassortment can facilitate the emergence of antigenically drifted seasonal strains and plays a prominent role in the development of pandemics. Typical human influenza infections are concentrated in the upper respiratory tract; however, lower respiratory tract (LRT) infection is an important feature of severe cases, which are more common in the very young, the elderly, and individuals with underlying conditions. In addition to host factors, viral characteristics and mode of transmission can also increase the likelihood of LRT infection: certain zoonotic IAVs are thought to favor the LRT, and transmission via small droplets allows direct seeding into lower respiratory tract tissues. To gauge the likelihood of reassortment in coinfected hosts, we assessed the extent to which initiation of infection at distinct respiratory tract sites impacts reassortment frequency. Our results reveal that spatially distinct inoculations result in anatomical compartmentalization of infection, which in turn strongly limits reassortment.

Exposure to cold impairs interferon-induced antiviral defense

It is commonly believed that exposure to low temperature increases susceptibility to viral infection in the human respiratory tract, but a molecular mechanism supporting this belief has yet to be discovered. In this study, we investigated the effect of low temperature on viral infection and innate defense in cell lines from the human respiratory tract and found that interferon-induced antiviral responses were impaired at low temperatures. Cells maintained at 25°C and 33°C expressed lower levels of myxovirus resistance protein 1 (MxA) and 2'5'-oligoadenylate synthetase 1 (OAS1) mRNAs when compared to cells maintained at 37°C after infection by seasonal influenza viruses. Exogenous β-interferon treatment reduced the viral replication at 37°C, but not at 25°C. Our results suggest that the impairment of interferon-induced antiviral responses by low temperature is one of several mechanisms that could explain an increase in host susceptibility to respiratory viruses after exposure to cold temperature.

Adaptive mutations in the H5N1 polymerase complex

Adaptation of the viral polymerase to host factors plays an important role in interspecies transmission of H5N1 viruses. Several adaptive mutations have been identified that, in general, determine not only host range, but also pathogenicity and transmissibility of the virus. The available evidence indicates that most of these mutations are found in the PB2 subunit of the polymerase. Particularly prominent mutations are located in the C-terminal domain of PB2 involving the amino acid exchanges E627K and D701N. Both mutations, that are also responsible for the adaptation of other avian viruses to mammalian hosts, have been described in human H5N1 isolates. In animal models, it could be demonstrated that they enhance pathogenicity in mice and induce contact transmission in guinea pigs. Mutation E627K has also been identified as a determinant of air-borne H5N1 transmission in ferrets. We are only beginning to understand the underlying mechanisms at the molecular level. Thus, mutation D701N promotes importin-α mediated nuclear transport in mammalian cells. Mutation E627K also enhances the replication rate in an importin-α dependent fashion in mammalian cells, yet without affecting nuclear entry of PB2. Numerous other adaptive mutations, some of which compensate for the lack of PB2 E627K, have been observed in PB2 as well as in the polymerase subunit PB1, the nucleoprotein NP, and the nuclear export protein NEP (NS2).

Different incubation temperatures affect viral polymerase activity and yields of low-pathogenic avian influenza viruses in embryonated chicken eggs

Various incubation conditions (35°C-38°C, 2-7 days) have been used in surveillance studies of the prevalence of avian influenza viruses in wild birds. Here, we studied viral polymerase activity and virus growth kinetics of low-pathogenic avian influenza viruses (LPAIVs) isolated from field samples [A/duck/Hong Kong/365/1978 (H4N6) and A/duck/Nanchang/2-0480/2000 (H9N2)] during incubation at different temperatures (35°C, 37°C, and 39°C) in the allantoic cavity of 10-day-old embryonated chicken eggs (ECE). The higher incubation temperatures (37°C and 39°C) resulted in a significantly higher rate of virus growth, which is most likely a result of increased viral polymerase activity (20%-60%), than was observed at 35°C, and as much as a 100% greater virus yield (as measured by hemagglutination assay) was observed two days after inoculation. Our findings revealed that the optimal activity of the viral polymerase complex, resulting in the highest yield of LPAIV field isolates, could be obtained by incubation for two days in ECE at 37°C and 39°C.

Biochemical characterization of enzyme fidelity of influenza A virus RNA polymerase complex

BACKGROUND

It is widely accepted that the highly error prone replication process of influenza A virus (IAV), together with viral genome assortment, facilitates the efficient evolutionary capacity of IAV. Therefore, it has been logically assumed that the enzyme responsible for viral RNA replication process, influenza virus type A RNA polymerase (IAV Pol), is a highly error-prone polymerase which provides the genomic mutations necessary for viral evolution and host adaptation. Importantly, however, the actual enzyme fidelity of IAV RNA polymerase has never been characterized.

PRINCIPAL FINDINGS

Here we established new biochemical assay conditions that enabled us to assess both polymerase activity with physiological NTP pools and enzyme fidelity of IAV Pol. We report that IAV Pol displays highly active RNA-dependent RNA polymerase activity at unbiased physiological NTP substrate concentrations. With this robust enzyme activity, for the first time, we were able to compare the enzyme fidelity of IAV Pol complex with that of bacterial phage T7 RNA polymerase and the reverse transcriptases (RT) of human immunodeficiency virus (HIV-1) and murine leukemia virus (MuLV), which are known to be low and high fidelity enzymes, respectively. We observed that IAV Pol displayed significantly higher fidelity than HIV-1 RT and T7 RNA polymerase and equivalent or higher fidelity than MuLV RT. In addition, the IAV Pol complex showed increased fidelity at lower temperatures. Moreover, upon replacement of Mg(++) with Mn(++), IAV Pol displayed increased polymerase activity, but with significantly reduced processivity, and misincorporation was slightly elevated in the presence of Mn(++). Finally, when the IAV nucleoprotein (NP) was included in the reactions, the IAV Pol complex exhibited enhanced polymerase activity with increased fidelity.

SIGNIFICANCE

Our study indicates that IAV Pol is a high fidelity enzyme. We envision that the high fidelity nature of IAV Pol may be important to counter-balance the multiple rounds of IAV genome amplification per infection cycle, which provides IAV Pol with ample opportunities to generate and amplify genomic founder mutations, and thus achieve optimal viral mutagenesis for its evolution.

Attenuated strains of influenza A viruses do not induce degradation of RNA polymerase II

We have previously shown that infection with laboratory-passaged strains of influenza virus causes both specific degradation of the largest subunit of the RNA polymerase II complex (RNAP II) and inhibition of host cell transcription. When infection with natural human and avian isolates belonging to different antigenic subtypes was examined, we observed that all of these viruses efficiently induce the proteolytic process. To evaluate whether this process is a general feature of nonattenuated viruses, we studied the behavior of the influenza virus strains A/PR8/8/34 (PR8) and the cold-adapted A/Ann Arbor/6/60 (AA), which are currently used as the donor strains for vaccine seeds due to their attenuated phenotype. We have observed that upon infection with these strains, degradation of the RNAP II does not occur. Moreover, by runoff experiments we observe that PR8 has a reduced ability to inhibit cellular mRNA transcription. In addition, a hypervirulent PR8 (hvPR8) variant that multiplies much faster than standard PR8 (lvPR8) in infected cells and is more virulent in mice than the parental PR8 virus, efficiently induces RNAP II degradation. Studies with reassortant viruses containing defined genome segments of both hvPR8 and lvPR8 indicate that PA and PB2 subunits individually contribute to the ability of influenza virus to degrade the RNAP II. In addition, recently it has been reported that the inclusion of PA or PB2 from hvPR8 in lvPR8 recombinant viruses, highly increases their pathogenicity. Together, the data indicate that the capacity of the influenza virus to degrade RNAP II and inhibit the host cell transcription machinery is a feature of influenza A viruses that might contribute to their virulence.

The human H5N1 influenza A virus polymerase complex is active in vitro over a broad range of temperatures, in contrast to the WSN complex, and this property can be attributed to the PB2 subunit

Influenza A virus (IAV) replicates in the upper respiratory tract of humans at 33 degrees C and in the intestinal tract of birds at close to 41 degrees C. The viral RNA polymerase complex comprises three subunits (PA, PB1 and PB2) and plays an important role in host adaptation. We therefore developed an in vitro system to examine the temperature sensitivity of IAV RNA polymerase complexes from different origins. Complexes were prepared from human lung epithelial cells (A549) using a novel adenoviral expression system. Affinity-purified complexes were generated that contained either all three subunits (PA/PB1/PB2) from the A/Viet/1203/04 H5N1 virus (H/H/H) or the A/WSN/33 H1N1 strain (W/W/W). We also prepared chimeric complexes in which the PB2 subunit was exchanged (H/H/W, W/W/H) or substituted with an avian PB2 from the A/chicken/Nanchang/3-120/01 H3N2 strain (W/W/N). All complexes were functional in transcription, cap-binding and endonucleolytic activity. Complexes containing the H5N1 or Nanchang PB2 protein retained transcriptional activity over a broad temperature range (30-42 degrees C). In contrast, complexes containing the WSN PB2 protein lost activity at elevated temperatures (39 degrees C or higher). The E627K mutation in the avian PB2 was not required for this effect. Finally, the avian PB2 subunit was shown to confer enhanced stability to the WSN 3P complex. These results show that PB2 plays an important role in regulating the temperature optimum for IAV RNA polymerase activity, possibly due to effects on the functional stability of the 3P complex.