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

Higher-temperature-adapted dengue virus serotype 2 strain exhibits enhanced virulence in AG129 mouse model

The factors that drive dengue virus (DENV) evolution, and selection of virulent variants are yet not clear. Higher environmental temperature shortens DENV extrinsic incubation period in mosquitoes, increases human transmission, and plays a critical role in outbreak dynamics. In the present study, we looked at the effect of temperature in altering the virus virulence. We found that DENV cultured at a higher temperature in C6/36 mosquito cells was significantly more virulent than the virus grown at a lower temperature. In a mouse model, the virulent strain induced enhanced viremia and aggressive disease with a short course, hemorrhage, severe vascular permeability, and death. Higher inflammatory cytokine response, thrombocytopenia, and severe histopathological changes in vital organs such as heart, liver, and kidney were hallmarks of the disease. Importantly, it required only a few passages for the virus to acquire a quasi-species population harboring virulence-imparting mutations. Whole genome comparison with a lower temperature passaged strain identified key genomic changes in the structural protein-coding regions as well as in the 3'UTR of the viral genome. Our results point out that virulence-enhancing genetic changes could occur in the dengue virus genome under enhanced growth temperature conditions in mosquito cells.

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.

Phenotypic and Genetic Characterization of Avian Influenza H5N2 Viruses with Intra- and Inter-Duck Variations in Taiwan

Background

Human infections with avian influenza viruses (AIVs) have frequently raised global concerns of emerging, interspecies-transmissible viruses with pandemic potential. Waterfowl, the predominant reservoir of influenza viruses in nature, harbor precursors of different genetic lineages that have contributed to novel pandemic influenza viruses in the past.

Methods

Two duck influenza H5N2 viruses, DV518 and DV413, isolated through virological surveillance at a live-poultry market in Taiwan, showed phylogenetic relatedness but exhibited different replication capabilities in mammalian Madin-Darby Canine Kidney (MDCK) cells. This study characterizes the replication properties of the two duck H5N2 viruses and the determinants involved.

Results

The DV518 virus replicated more efficiently than DV413 in both MDCK and chicken DF1 cells. Interestingly, the infection of MDCK cells by DV518 formed heterogeneous plaques with great differences in size [large (L) and small (S)], and the two viral strains (p518-L and p518-S) obtained from plaque purification exhibited distinguishable replication kinetics in MDCK cells. Nonetheless, both plaque-purified DV518 strains still maintained their growth advantages over the plaque-purified p413 strain. Moreover, three amino acid substitutions in PA (P224S), PB2 (E72D), and M1 (A128T) were identified in intra-duck variations (p518-L vs p518-S), whereas other changes in HA (N170D), NA (I56T), and NP (Y289H) were present in inter-duck variations (DV518 vs DV413). Both p518-L and p518-S strains had the N170D substitution in HA, which might be related to their greater binding to MDCK cells. Additionally, polymerase activity assays on 293T cells demonstrated the role of vRNP in modulating the replication capability of the duck p518-L viruses in mammalian cells.

Conclusion

These results demonstrate that intra-host phenotypic variation occurs even within an individual duck. In view of recent human infections by low pathogenic AIVs, this study suggests possible determinants involved in the stepwise selection of virus variants from the duck influenza virus population which may facilitate inter-species transmission.

Heat shock protein 70 modulates influenza A virus polymerase activity

The role of heat shock protein 70 (Hsp70) in virus replication has been discussed for many viruses. The known suppressive role of Hsp70 in influenza virus replication is based on studies conducted in cells with various Hsp70 expression levels. In this study, we determined the role of Hsp70 in influenza virus replication in HeLa and HEK293T cells, which express Hsp70 constitutively. Co-immunoprecipitation and immunofluorescence studies revealed that Hsp70 interacted with PB2 or PB1 monomers and PB2/PB1 heterodimer but not with the PB1/PA heterodimer or PB2/PB1/PA heterotrimer and translocated into the nucleus with PB2 monomers or PB2/PB1 heterodimers. Knocking down Hsp70 resulted in reduced virus transcription and replication activities. Reporter gene assay, immunofluorescence assay, and Western blot analysis of nuclear and cytoplasmic fractions from infected cells demonstrated that the increase in viral polymerase activity during the heat shock phase was accompanied with an increase in Hsp70 and viral polymerases levels in the nuclei, where influenza virus replication takes place, whereas a reduction in viral polymerase activity was accompanied with an increase in cytoplasmic relocation of Hsp70 along with viral polymerases. Moreover, significantly higher levels of viral genomic RNA (vRNA) were observed during the heat shock phase than during the recovery phase. Overall, for the first time, these findings suggest that Hsp70 may act as a chaperone for influenza virus polymerase, and the modulatory effect of Hsp70 appears to be a sequel of shuttling of Hsp70 between nuclear and cytoplasmic compartments.

A statistical strategy to identify recombinant viral ribonucleoprotein of avian, human, and swine influenza A viruses with elevated polymerase activity

Objectives

Reassortment of influenza A viruses can give rise to viral ribonucleoproteins (vRNPs) with elevated polymerase activity and the previous three pandemic influenza viruses contained reassorted vRNPs of different origins. These suggest that reassorted vRNP may be one of the factors leading to a pandemic virus. In this study, we reconstituted chimeric vRNPs with three different viral strains isolated from avian, human and swine hosts. We applied a statistical strategy to identify the effect that the origin of a single vRNP protein subunit or the interactions between these subunits on polymerase activity.

Design

Eighty one chimeric vRNPs were reconstituted in 293T cells at different temperatures. Polymerase activity was determined by luciferase reporter assay and the results were analysed by multiway anova and other statistical methods.

Results

It was found that PB2, PB1, NP, PB2‐PB1 interaction, PB2‐PA interaction and PB1‐NP interaction had significant effect on polymerase activity at 37°C and several single subunits and interactions were identified to lead to elevation of polymerase activity. Furthermore, we studied 27 out of these 81 different chimieric vRNPs in different combinations via fractional factorial design approach. Our results suggested that the approach can identify the major single subunit or interaction factors that affect the polymerase activity without the need to experimentally reproduce all possible vRNP combinations.

Conclusions

Statistical approach and fractional factorial design are useful to identify the major single subunit or interaction factors that can modulate viral polymerase activity.

Optimization of incubation temperature in embryonated chicken eggs inoculated with H9N2 vaccinal subtype of avian influenza virus

There are little information about growth properties of low pathogenic (LP) avian influenza virus (AIV) in embryonated chicken eggs (ECEs) at different incubation temperatures. Knowledge of this information increases the quantity and quality of antigen in vaccine production process. For this purpose, 10(-5) dilution of AIV (A/Chicken/Iran/99/H9N2) was inoculated (Intra-allantoic) into 400, 11-day old specific pathogen free (SPF) ECEs in the 0.1 mL per ECE rate and incubated in 32, 33, 34, 35, 36, 37.5, 38, 39 ˚C for 72 hr in 65% humidity. Early death embryos in first 24 hr were removed. Amnio-allantoic fluid was withdrawn into the measuring cylinder, and tested for hemagglutination (HA) activity and egg infective dose 50 (EID50). The utilizable ECEs and amnio-allantoic fluid volume was significantly increased in 35 ˚C, (p < 0.05). Significant difference in HA and EID50 titers, were seen only in 39 ˚C group. Therefore, 35°C is an optimum temperature for incubation of inoculated ECEs.

A comparison of viral fitness and virulence between emergent adenovirus 14p1 and prototype adenovirus 14p strains

BACKGROUND

Epidemiological studies from the last decade have suggested that the morbidity and mortality associated with a newly emergent strain of human adenovirus (HAdV-14p1) is greater than other, more prevalent, adenovirus strains. Recent molecular analysis identified very minor genetic differences in HAdV-14p1 compared to prototype HAdV-14p. No studies have evaluated how these differences may affect virulence.

OBJECTIVE

To compare HAdV-14p1 and HAdV-14p strains for competitive fitness and virulence.

STUDY DESIGN

We performed in vitro and molecular assays to evaluate growth kinetics, cellular infectivity, cytotoxicity, and plaque morphology of the two strains.

RESULTS

Growth kinetic data showed no viral replication at 30°C and minimal differences at 37°C for both strains. Cellular infectivity data showed propagation capabilities for both strains in a diverse array of cell lines, with human lung and kidney cells having the highest propagation potential. Cytotoxicity data indicated cellular distress differences induced by both strains of virus in the first 12h, but similar distress levels between 12 and 48 h. Plaque morphology assays showed some differences in average plaque diameter.

CONCLUSIONS

These data suggest that the increase in morbidity and mortality observed in recent HAdV-14p1 infections is not due to viral growth or cellular infectivity differences from the prototypic HAdV-14 strain. While there were some statistically important differences detected between strains in cytotoxicity and plaque morphology assays, it seems more likely that other factors, such as environmental stressors, co-infections, or individual host response are likely contributing to the increase in morbidity.