Reassortment of NS segments modifies highly pathogenic avian influenza virus interaction with avian hosts and host cells

Different pathogenicity of two strains of clade 2.3.4.4c H5N6 highly pathogenic avian influenza viruses bearing different PA and NS gene in domestic ducks

H5Nx clade 2.3.4.4 highly pathogenic avian influenza viruses (HPAIVs) have been disseminated to wide geographic regions since 2014. In 2016, five distinct genotypes (C-1 to C-5) of clade 2.3.4.4c H5N6 HPAIVs were detected in South Korea. In this study, we evaluated the pathogenicity, susceptibility to infection, and transmissibility of the two strains representing the C-1 and C-4 genotypes of the H5N6 viruses, which have different PA and NS gene, in domestic ducks. Although the susceptibility to infection of domestic ducks to the two strains was similar, the C-4 genotype virus induced higher mortality in ducks than C-1 genotype virus. A higher titer of viral shedding were detected in ducks challenged with the C-4 genotype virus compared with the C-1 genotype virus. These results indicated that the reassortment of HPAIVs with prevailing low pathogenic avian influenza viruses could effect on the pathogenicity in ducks.

PR8 virus harbouring H5N1 NS gene contributed for THP-1 cell tropism

Innate immune cells are key player in immune response to influenza virus infection. Influenza infected monocytes exacerbate the disease pathology. However, monocytes differ in susceptibilities to influenza virus infection. Herein, susceptibilities of U937 and THP-1 monocytic cells to PR8 virus infection, the associated cellular factor- sialic acid (SA) receptor distribution and viral factor were determined. Moreover, amino acid sequences in hemagglutinin (HA) receptor binding domain (RBD) of PR8 virus that determine SA preferences were analysed. PR8 infected U937 cells express significantly higher numbers of nucleoprotein positive cells suggesting U937 cells being more susceptible to influenza virus than THP-1 cells. Lectin staining suggested similar pattern of SA receptor distribution in both cells. Interestingly, sequence analysis of RBD suggested their preferences for alpha 2,3 SA receptors suggesting RBD sequences are not always determining for SA preferences. Furthermore, the resistance barrier on THP-1 cells was overcome by H5N1 NS gene. In conclusion, the study demonstrated that decreased susceptibility of THP-1 cells to PR8 virus could not be related to the SA receptor distribution, and H5N1 NS gene was sufficient to determine tropism for THP-1 cells. Hence, mechanistic basis of NS gene on cell tropism and contribution of other internal genes remained to be determined.

NS Segment of a 1918 Influenza A Virus-Descendent Enhances Replication of H1N1pdm09 and Virus-Induced Cellular Immune Response in Mammalian and Avian Systems

The 2009 pandemic influenza A virus (IAV) H1N1 strain (H1N1pdm09) has widely spread and is circulating in humans and swine together with other human and avian IAVs. This fact raises the concern that reassortment between H1N1pdm09 and co-circulating viruses might lead to an increase of H1N1pdm09 pathogenicity in different susceptible host species. Herein, we explored the potential of different NS segments to enhance the replication dynamics, pathogenicity and host range of H1N1pdm09 strain A/Giessen/06/09 (Gi-wt). The NS segments were derived from (i) human H1N1- and H3N2 IAVs, (ii) highly pathogenic- (H5- or H7-subtypes) or (iii) low pathogenic avian influenza viruses (H7- or H9-subtypes). A significant increase of growth kinetics in A549 (human lung epithelia) and NPTr (porcine tracheal epithelia) cells was only noticed in vitro for the reassortant Gi-NS-PR8 carrying the NS segment of the 1918-descendent A/Puerto Rico/8/34 (PR8-wt, H1N1), whereas all other reassortants showed either reduced or comparable replication efficiencies. Analysis using ex vivo tracheal organ cultures of turkeys (TOC-Tu), a species susceptible to IAV H1N1 infection, demonstrated increased replication of Gi-NS-PR8 compared to Gi-wt. Also, Gi-NS-PR8 induced a markedly higher expression of immunoregulatory and pro-inflammatory cytokines, chemokines and interferon-stimulated genes in A549 cells, THP-1-derived macrophages (dHTP) and TOC-Tu. In vivo, Gi-NS-PR8 induced an earlier onset of mortality than Gi-wt in mice, whereas, 6-week-old chickens were found to be resistant to both viruses. These data suggest that the specific characteristics of the PR8 NS segments can impact on replication, virus induced cellular immune responses and pathogenicity of the H1N1pdm09 in different avian and mammalian host species.

Interaction of Influenza A Viruses with Oviduct Explants of Different Avian Species

Infection of poultry with low pathogenic avian influenza viruses (LPAIV) is often associated with mild respiratory symptoms but may also lead to loss in egg production in laying birds. In vivo susceptibility of the reproductive tract for LPAIV infection was reported for turkeys and chickens, but virus-interaction with epithelial cells of the oviduct and possible stimulation of the local antiviral immune responses have not been characterized. In this study, we wanted to investigate the suitability of magnum organ cultures (MOC) as an in vitro model to study virus-host interactions. We compared the susceptibility of duck (Du), chicken (Ch), and turkey (Tu) MOC for three different influenza A viruses (IAV). Overall, the course of infection and the antiviral immune response varied between strains as well as host cell origin, but MOC gave reproducible results for all investigated parameters within each species. While pandemic (p) H1N1 and H9N2 efficiently replicated in MOC-Ch and MOC-Tu, MOC-Du were significantly less susceptible to infection as indicated by a reduced replication level for both viruses (p < 0.05). Overall, virus replication levels did not correlate with interferonα (IFNα) mRNA-expression levels in neither species. H9N2-infection led to a significant upregulation of interferonλ (IFNλ) mRNA expression in MOC of all species compared to the non-infected controls (p < 0.05), while a correlation with replication levels was only seen for MOC-Tu. pH1N1-infection induced only significant upregulation of IFNλ mRNA expression in MOC-Tu at 48 hours post infection (p < 0.05), but the expression pattern did not correlate with replication levels. Our results show that MOC are a suitable model to study IAV-interaction with the mucosal surface of the avian reproductive tract. The data suggest that the reproductive tract may play a role in the pathobiology of IAV in poultry.

Composition of the Hemagglutinin Polybasic Proteolytic Cleavage Motif Mediates Variable Virulence of H7N7 Avian Influenza Viruses

Acquisition of a polybasic cleavage site (pCS) in the hemagglutinin (HA) is a prerequisite for the shift of low pathogenic (LP) avian influenza virus (AIV) to the highly pathogenic (HP) form in chickens. Whereas presence of a pCS is required for high pathogenicity, less is known about the effect of composition of pCS on virulence of AIV particularly H7N7. Here, we investigated the virulence of four avian H7N7 viruses after insertion of different naturally occurring pCS from two HPAIV H7N7 (designated pCSGE and pCSUK) or from H7N1 (pCSIT). In vitro, the different pCS motifs modulated viral replication and the HA cleavability independent on the HA background. However, in vivo, the level of virulence conferred by the different pCS varied significantly. Within the respective viral backgrounds viruses with pCSIT and pCSGE were more virulent than those coding for pCSUK. The latter showed also the most restricted spread in inoculated birds. Besides the pCS, other gene segments modulated virulence of these H7N7 viruses. Together, the specific composition of the pCS significantly influences virulence of H7N7 viruses. Eurasian LPAIV H7N7 may shift to high pathogenicity after acquisition of “specific” pCS motifs and/or other gene segments from HPAIV.

In vitro characterization of the novel H3N1 reassortant influenza viruses from quail

Quail is considered as an intermediate host for generation of the novel reassortant influenza A viruses (IAVs). In this study, we evaluated the replication ability of the three novel H3N1 reassortant viruses recovered from pandemic H1N1 2009 (pH1N1) and duck H3N2 (dkH3N2) co-infected quail generated from our previous study in embryonated chicken eggs, mammalian (MDCK) and human lung derived (A549) cells. Our study demonstrated that all of the reassortant viruses replicated efficiently in avian and mammalian cells, albeit with slightly lower titers than the parental viruses. Of note, all of the reassortant viruses showed enhanced replication in human lung derived A549 cells compared to their parental viruses. Interestingly, among the reassortant viruses tested, a reassortant virus (P(NA,NS)-DK) containing NA and NS genes derived from pH1N1 and the other genes from dkH3N2 exhibited the highest replication ability in all in vitro models, indicating a high level of gene compatibility of this reassortant virus. Our results highlight the potential role of quail as intermediate hosts for the generation of the viable reassortant viruses with ability to replicate efficiently in avian, mammalian, and particularly human lung derived cells. These findings emphasize the need for the continuous IAV surveillance in quail to prevent the risk of the emergence of the novel viable reassortant viruses.

Avian metapneumovirus infection of chicken and turkey tracheal organ cultures: comparison of virus-host interactions

Avian metapneumovirus (aMPV) is a pathogen with worldwide distribution, which can cause high economic losses in infected poultry. aMPV mainly causes infection of the upper respiratory tract in both chickens and turkeys, although turkeys seem to be more susceptible. Little is known about virus-host interactions at epithelial surfaces after aMPV infection. Tracheal organ cultures (TOC) are a suitable model to investigate virus-host interaction in the respiratory epithelium. Therefore, we investigated virus replication rates and lesion development in chicken and turkey TOC after infection with a virulent aMPV subtype A strain. Aspects of the innate immune response, such as interferon-α and inducible nitric oxide synthase mRNA expression, as well as virus-induced apoptosis were determined. The aMPV-replication rate was higher in turkey (TTOC) compared to chicken TOC (CTOC) (P < 0.05), providing circumstantial evidence that indeed turkeys may be more susceptible. The interferon-α response was down-regulated from 2 to 144 hours post infection in both species compared to virus-free controls (P < 0.05); this was more significant for CTOC than TTOC. Inducible nitric oxide synthase expression was significantly up-regulated in aMPV-A-infected TTOC and CTOC compared to virus-free controls (P < 0.05). However, the results suggest that NO may play a different role in aMPV pathogenesis between turkeys and chickens as indicated by differences in apoptosis rate and lesion development between species. Overall, our study reveals differences in innate immune response regulation and therefore may explain differences in aMPV - A replication rates between infected TTOC and CTOC, which subsequently lead to more severe clinical signs and a higher rate of secondary infections in turkeys.

Mycoplasma gallisepticum modifies the pathogenesis of influenza A virus in the avian tracheal epithelium

Multiple respiratory infections have a significant impact on health and economy. Pathogenesis of co-infecting viruses and bacteria and their interaction with mucosal surfaces are poorly characterized. In this study we established a co-infection model based on pre-incubation of tracheal organ cultures (TOC) with Mycoplasma (M.) gallisepticum and a subsequent infection with avian influenza virus (AIV). Mycoplasma gallisepticum modified the pathogenesis of AIV as demonstrated in TOC of two different avian species (chickens and turkeys). Co-infection promoted bacterial growth in tracheal epithelium. Depending on the interaction time of M. gallisepticum with the host cells, AIV replication was either promoted or suppressed. M. gallisepticum inhibited the antiviral gene expression and affected AIV attachment to the host cell by desialylation of α-2,3 linked sialic acids. Ultrastructural analysis of co-infected TOC suggests that both pathogens may attach to and possibly infect the same epithelial cell. The obtained results contribute to better understanding of the interaction dynamics between M. gallisepticum and AIV. They highlight the importance of the time interval between infections as well as the biological properties of the involved pathogens as influencing factors in the outcome of respiratory infections.