Role of hemagglutinin cleavage for the pathogenicity of influenza virus

Coexpression of avian influenza virus H5 and N1 by recombinant Newcastle disease virus and the impact on immune response in chickens

To analyze the contribution of neuraminidase (NA) toward protection against avian influenza virus (AIV) infection, three different recombinant Newcastle disease viruses (NDVs) expressing hemagglutinin (HA) or NA, or both, of highly pathogenic avian influenza virus (HPAIV) were generated. The lentogenic NDV Clone 30 was used as backbone for the insertion of HA of HPAIV strain A/chicken/Vietnam/P41/05 (H5N1) and NA of HPAIV strain A/duck/Vietnam/TG24-01/05 (H5N1). The HA was inserted between the genes encoding NDV phosphoprotein (P) and matrixprotein (M), and the NA was inserted between the fusion (F) and hemagglutinin-neuraminidase protein (HN) genes, resulting in NDVH5VmPMN1FHN. Two additional recombinants were constructed carrying the HA gene between the NDV P and M genes (NDVH5VmPM) or the NA between F and HN (NDVN1FHN). All recombinants replicated well and stably expressed the HA gene, the NA gene, or both. Chickens immunized with NDVH5VmPMN1FHN or NDVH5VmPM were protected against two different HPAIV H5N1 and also against HPAIV H5N2. In contrast, immunization of chickens with NDVN1FHN induced NDV- and AIV N1-specific antibodies but did not protect the animals against a lethal dose of HPAIV H5N1. Furthermore, expression of AIV N1, in addition to AIV H5 by NDV, did not increase protection against HPAIV H5N1.

Biased mutational pattern and quasispecies hypothesis in H5N1 virus

Like other RNA viruses, influenza viruses are subject to high mutation rates. Carrying segmented RNA genomes, their genetic variability is even higher. We aimed at analyzing the mutational events occurring during the infection of chickens by the Highly Pathogenic Avian Influenza (HPAI) H5N1 virus. We therefore studied the different sequences of two surface proteins, hemagglutinin (HA) and neuraminidase (NA), as well as two internal proteins, PB2 and NS. Three organs (lung, spleen, brain) were obtained from a chicken, experimentally infected with a lethal dose of HPAI H5N1 virus. Cloning these PCR fragments enabled us to investigate the mutations undergone by the virus after several replicative cycles. The first outcome is the presence of a strong mutational bias, resembling host-driven ADAR1 adenosine deamination, which is responsible for 81% of all mutations. Whereas the frequency of RNA dependent RNA polymerase-related mutations is compatible with the survival of the virus, the ADAR1-like activity usually strongly increases the mutation frequency into a level of “error catastrophe” in theory incompatible with virus survival. Nevertheless, the virus was successfully infective. HPAI H5N1 virus displayed traits in agreement with the quasispecies theory. The role of this quasispecies structure in successful infection and the superposition with the ADAR1-like response is discussed.

Selection of an antiviral RNA aptamer against hemagglutinin of the subtype H5 avian influenza virus

Avian influenza is an acute viral respiratory disease caused by RNA viruses of the family Orthomyxoviridae. The influenza A virus subtype H5 can cause severe illness and results in almost 100% mortality rate among livestock. Hemagglutinin (HA) present in the virus envelope plays an essential role in the initiation of viral infection. In this study, we investigated the efficacy of using HA as a target for antiviral therapy through nucleic acid aptamers. After purification of the receptor binding domain (HA1) of HA protein, activity of recombinant HA1 was confirmed by using hemagglutination assay. We selected RNA aptamer candidates after 15 rounds of iterative Systematic Evolution of Ligands by EXponential enrichment (SELEX) targeting the biologically active HA protein. The selected RNA aptamer HAS15-5, which specifically binds to HA1, exhibited significant antiviral efficacy according to the results of a hemagglutination inhibition assay using egg allantoic fluids harboring the virus. Thus, the RNA aptamer HAS15-5, which acts by blocking and inhibiting the receptor-binding domain of viral HA, can be developed as a novel antiviral agent against type H5 avian influenza virus.

Viral load of the highly pathogenic avian influenza H5N1 virus in infected human tissues

The highly pathogenic avian influenza A (H5N1) virus is a virulent virus that causes an acute febrile respiratory disease with high mortality in humans. To gain a better insight of H5N1 viral distributions in infected human tissues, the levels of viral RNA were determined in the autopsy tissues from two patients who were infected with H5N1 virus by using real-time reverse transcription-polymerase chain reaction. In one patient who died on day 6 of the illness, the viral load in the lung was extremely high, whereas the levels of viral RNA in the other organs were more than 6 log lower. In the other patient who died on day 17 of the illness, the viral load was similar in the lung and other organs, and was comparable to the viral load in the extra-pulmonary tissues of the first patient. These results suggested that while the H5N1 virus can cause disseminated infection in humans, the lung is still the major site of viral replication, and viral replication in the lung in the later stages may decrease as a result of the depletion of the available target cells. In addition, the mRNA levels of the tumor necrosis factor-α (TNF-α) were found to be associated with the viral titers.

Reassortant H9N2 influenza viruses containing H5N1-like PB1 genes isolated from black-billed magpies in Southern China

H9N2 influenza A viruses have become endemic in different types of terrestrial poultry and wild birds in Asia, and are occasionally transmitted to humans and pigs. To evaluate the role of black-billed magpies (Pica pica) in the evolution of influenza A virus, we conducted two epidemic surveys on avian influenza viruses in wild black-billed magpies in Guangxi, China in 2005 and characterized three isolated black-billed magpie H9N2 viruses (BbM viruses). Phylogenetic analysis indicated that three BbM viruses were almost identical with 99.7 to 100% nucleotide homology in their whole genomes, and were reassortants containing BJ94-like (Ck/BJ/1/94) HA, NA, M, and NS genes, SH/F/98-like (Ck/SH/F/98) PB2, PA, and NP genes, and H5N1-like (Ck/YN/1252/03, clade 1) PB1 genes. Genetic analysis showed that BbM viruses were most likely the result of multiple reassortments between co-circulating H9N2-like and H5N1-like viruses, and were genetically different from other H9N2 viruses because of the existence of H5N1-like PB1 genes. Genotypical analysis revealed that BbM viruses evolved from diverse sources and belonged to a novel genotype (B46) discovered in our recent study. Molecular analysis suggested that BbM viruses were likely low pathogenic reassortants. However, results of our pathogenicity study demonstrated that BbM viruses replicated efficiently in chickens and a mammalian mouse model but were not lethal for infected chickens and mice. Antigenic analysis showed that BbM viruses were antigenic heterologous with the H9N2 vaccine strain. Our study is probably the first report to document and characterize H9N2 influenza viruses isolated from black-billed magpies in southern China. Our results suggest that black-billed magpies were susceptible to H9N2 influenza viruses, which raise concerns over possible transmissions of reassortant H9N2 viruses among poultry and wild birds.

Rapid PCR-based molecular pathotyping of H5 and H7 avian influenza viruses

While the majority of avian influenza virus (AIV) subtypes are classified as low-pathogenicity avian influenza viruses (LPAIV), the H5 and H7 subtypes have the ability to mutate to highly pathogenic avian influenza viruses (HPAIV) in poultry and therefore are the etiological agents of notifiable AIV (NAIV). It is of great importance to distinguish HPAIV from LPAIV variants during H5/H7 outbreaks and surveillance. To this end, a novel and fast strategy for the molecular pathotyping of H5/H7 AIVs is presented. The differentiation of the characteristic hemagglutinin (HA) protein cleavage sites (CSs) of HPAIVs and LPAIVs is achieved by a novel PCR method where the samples are interrogated for all existing CSs with a 484-plex primer mixture directly targeting the CS region. CSs characteristic for HP or LP H5/H7 viruses are distinguished in a seminested duplex real-time PCR format using plexor fluorogenic primers. Eighty-six laboratory isolates and 60 characterized NAIV-positive clinical specimens from poultry infected with H5/H7 both experimentally and in the field were successfully pathotyped in the validation. The method has the potential to substitute CS sequencing in the HA gene for the determination of the molecular pathotype, thereby providing a rapid means to acquire additional information concerning NAIV outbreaks, which may be critical to their management. The new assay may be extended to the LP/HP differentiation of previously unknown H5/H7 isolates. It may be considered for integration into surveillance and control programs in both domestic and wild bird populations.

A molecular and antigenic survey of H5N1 highly pathogenic avian influenza virus isolates from smallholder duck farms in Central Java, Indonesia during 2007-2008

Background

Indonesia is one of the countries most severely affected by H5N1 highly pathogenic avian influenza (HPAI) virus in terms of poultry and human health. However, there is little information on the diversity of H5N1 viruses circulating in backyard farms, where chickens and ducks often intermingle. In this study, H5N1 virus infection occurring in 96 smallholder duck farms in central Java, Indonesia from 2007-2008 was investigated and the molecular and antigenic characteristics of H5N1 viruses isolated from these farms were analysed.

Results

All 84 characterised viruses belonged to H5N1 clade 2.1 with three virus sublineages being identified: clade 2.1.1 (1), clade 2.1.3 (80), and IDN/6/05-like viruses (3) that did not belong to any of the present clades. All three clades were found in ducks, while only clade 2.1.3 was isolated from chickens. There were no significant amino acid mutations of the hemagglutinin (HA) and neuraminidase (NA) sites of the viruses, including the receptor binding, glycosylation, antigenic and catalytic sites and NA inhibitor targets. All the viruses had polybasic amino acids at the HA cleavage site. No evidence of major antigenic variants was detected. Based on the HA gene, identical virus variants could be found on different farms across the study sites and multiple genetic variants could be isolated from HPAI outbreaks simultaneously or at different time points from single farms. HPAI virus was isolated from both ducks and chickens; however, the proportion of surviving duck cases was considerably higher than in chickens.

Conclusions

The 2.1.3 clade was the most common lineage found in this study. All the viruses had sequence characteristic of HPAI, but negligible variations in other recognized amino acids at the HA and NA proteins which determine virus phenotypes. Multiple genetic variants appeared to be circulating simultaneously within poultry communities. The high proportion of live duck cases compared to chickens over the study period suggests that ducks are more likely to survive infection and they may better suit the role of long-term maintenance host for H5N1. As some viruses were isolated from dead birds, there was no clear correlation between genetic variations and pathogenicity of these viruses.

Epidemic viral pneumonia and other emerging pathogens

Viruses cause a high percentage of community-acquired pneumonias. The advent of polymerase chain reaction and other molecular techniques has been associated with the detection of a higher prevalence of common respiratory viruses than previously suspected. Better diagnostics have shown new viral pathogens regularly in epidemics, immunocompromised patients, and occasionally children. Despite better diagnostics, treatment for all but influenza is still very limited.

In situ cleavage of baculovirus occlusion-derived virus receptor binding protein P74 in the peroral infectivity complex

Proteolytic processing of viral membrane proteins is common among enveloped viruses and facilitates virus entry. The Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) occlusion-derived virus (ODV) protein P74 is part of a complex of essential peroral infectivity factors (PIFs). Here we report that P74 is efficiently cleaved into two fragments of about equal size by an occlusion body (OB) endogenous alkaline protease during ODV release when AcMNPV OBs are derived from larvae. The cleavage is specific for P74, since the other known peroral infectivity factors in the same complex (PIF1, PIF2, and PIF3) were not cleaved under the same conditions. P74 cleavage was not observed in OBs produced in three different insect cell lines, suggesting a larval host origin of the responsible protease. P74 in OBs produced in larvae of two different host species was cleaved into fragments with the same apparent molecular mass, indicating that the virus incorporates a similar alkaline protease from different hosts. Coimmunoprecipitation analysis revealed that the two P74 subunit fragments remain associated with the recently discovered PIF complex. We propose that under in vivo ODV infection conditions, P74 undergoes two sequential cleavage events, the first one being performed by an ODV-associated host alkaline protease and the second carried out by trypsin in the host midgut.

Rapid generation of pandemic influenza virus vaccine candidate strains using synthetic DNA

Please cite this paper as: Verity et al. (2011) Rapid generation of pandemic influenza virus vaccine candidate strains using synthetic DNA. Influenza and Other Respiratory Viruses DOI:10.1111/j.1750‐2659.2011.00273.x.

Background  Vaccination is considered the most effective means of reducing influenza burden. The emergence of H5N1 and pandemic spread of novel H1N1/2009 viruses reinforces the need to have strategies in place to rapidly develop seed viruses for vaccine manufacture.

Methods  Candidate pandemic vaccine strains consisting of the circulating strain haemagglutinin (HA) and neuraminidase (NA) in an A/PR/8/34 backbone were generated using alternative synthetic DNA approaches, including site‐directed mutagenesis of DNA encoding related virus strains, and rapid generation of virus using synthetic DNA cloned into plasmid vectors.

Results  Firstly, synthetic A/Bar Headed Goose/Qinghai/1A/2005 (H5N1) virus was generated from an A/Vietnam/1194/2004 template using site‐directed mutagenesis. Secondly, A/Whooper Swan/Mongolia/244/2005 (H5N1) and A/California/04/09 (H1N1) viruses were generated using synthetic DNA encoding the viral HA and NA genes. Replication and antigenicity of the synthetic viruses were comparable to that of the corresponding non‐synthetic viruses.

Conclusions  In the event of an influenza pandemic, the use of these approaches may significantly reduce the time required to generate and distribute the vaccine seed virus and vaccine manufacture. These approaches also offer the advantage of not needing to handle wild‐type virus, potentially diminishing biocontainment requirements.

Effects of magnesium ions on recombinant human furin: selective activation of hydrolytic activity upon substrates derived from virus envelope glycoprotein

Here we report a detailed analysis of magnesium (Mg²+) ion effects on furin hydrolysis of fluorescent resonance energy transfer decapeptide substrates derived from canonical R-X-K/R-R furin cleavage motifs within certain viral envelope glycoproteins and eukaryotic proproteins. Using virus-derived sequences a selective activation of furin by Mg²+) ions was observed as a result of cooperativity between furin subsites. Furin hydrolysis of the peptides Abz-SRRHKR↓FAGV-Q-EDDnp (from measles virus fusion protein F₀ and Abz-RERRRKKR↓GLFG-Q-EDDnp (from Asian avian influenza A, H5N1) was activated between 60- and 80-fold by MgCl₂. It appears that virus envelope glycoprotein mutations have been selected to increase their susceptibility to furin within cells, a location where Mg²+ is present in adequate concentrations for activation. Both the pH profile of furin and its intrinsic fluorescence were modified by Mg²+ ions, which bind to furin with a K(d) value of 1.1 mM.

Immunogenicity of low-pH treated whole viral influenza vaccine

Low pH treatment of influenza virus hemagglutinin (HA) exposes its relatively conserved stalk domain, suggesting a potential immunogen with capability to induce broader immune responses. Here, we describe characterization, immunogenicity, antigenicity, and protective immunity induced by low pH treated inactivated whole viral vaccine in comparison with the untreated vaccine. The acidic pH treated viral vaccine showed high susceptibility to proteolytic cleavage and low hemagglutination activity indicating conformational changes. Immunization of mice with low pH treated viral vaccine induced lower levels of homologous or heterologous virus-specific binding and neutralizing antibodies compared to the untreated vaccine. Also, low pH treated influenza viral antigen showed lower antigenicity compared to the untreated influenza viral antigen. Lower efficacy of cross-protection against heterosubtypic virus was observed in the low-pH treated vaccine group. The results provide evidence that there is a correlation between protective efficacy and the stability of vaccines.

HIV-protease inhibitors block the replication of both vesicular stomatitis and influenza viruses at an early post-entry replication step

The inhibitors of HIV-1 protease (PIs) have been designed to block the activity of the viral aspartyl-protease. However, it is now accepted that this family of inhibitors can also affect the activity of cell proteases. Since the replication of many virus species requires the activity of host cell proteases, investigating the effects of PIs on the life cycle of viruses other than HIV would be of interest. Here, the potent inhibition induced by saquinavir and nelfinavir on the replication of both vesicular stomatitis and influenza viruses is described. These are unrelated enveloped RNA viruses infecting target cells upon endocytosis and intracellular fusion. The PI-induced inhibition was apparently a consequence of a block at the level of the fusion between viral envelope and endosomal membranes. These findings would open the way towards the therapeutic use of PIs against enveloped RNA viruses other than HIV.

Role of proteases in the release of porcine epidemic diarrhea virus from infected cells

Porcine epidemic diarrhea virus (PEDV), a causative agent of pig diarrhea, requires a protease(s) for multicycle replication in cultured cells. However, the potential role of proteases in the infection process remains unclear. In order to explore this, we used two different approaches: we infected either Vero cells in the presence of trypsin or Vero cells that constitutively express the membrane-associated protease TMPRSS2 (Vero/TMPRSS2 cells). We found that PEDV infection was enhanced, and viruses were efficiently released into the culture fluid, from Vero cells infected in the presence of protease, while in cells without protease, the virus grew, but its release into the culture fluid was strongly hampered. Cell-to-cell fusion of PEDV-infected cells and cleavage of the spike (S) protein were observed in cells with protease. When infected Vero cells were cultured for 3 days in the absence of trypsin but were then treated transiently with trypsin, infectious viruses were immediately released from infected cells. In addition, treatment of infected Vero/TMPRSS2 cells with the protease inhibitor leupeptin strongly blocked the release of virus into the culture fluid. Under electron microscopy, PEDV-infected Vero cells, as well as PEDV-infected Vero/TMPRSS2 cells treated with leupeptin, retained huge clusters of virions on their surfaces, while such clusters were rarely seen in the presence of trypsin and the absence of leupeptin in Vero and Vero/TMPRSS2 cells, respectively. Thus, the present study indicates that proteases play an important role in the release of PEDV virions clustered on cells after replication occurs. This unique observation in coronavirus infection suggests that the actions of proteases are reminiscent of that of the influenza virus neuraminidase protein.

Effects of bacterial microflora of the lower digestive tract of free-range waterfowl on influenza virus activation

Proteolytic cleavage activation of influenza virus hemagglutinin (HA0) is required for cell entry via receptor-mediated endocytosis. Despite numerous studies describing bacterial protease-mediated influenza A viral activation in mammals, very little is known about the role of intestinal bacterial flora of birds in hemagglutinin cleavage/activation. Therefore, the cloaca of wild waterfowl was examined for (i) representative bacterial types and (ii) their ability to cleave in a "trypsin-like" manner the precursor viral hemagglutinin molecule (HA0). Using radiolabeled HA0, bacterial secretion-mediated trypsin-like conversion of HA0 to HA1 and HA2 peptide products was observed to various degrees in 42 of 44 bacterial isolates suggestive of influenza virus activation in the cloaca of wild waterfowl. However, treatment of uncleaved virus with all bacterial isolates gave rise to substantially reduced emergent virus progeny compared with what was expected. Examination of two isolates exhibiting pronounced trypsin-like conversion of HA0 to HA1 and HA2 peptide products and low infectivity revealed lipase activity to be present. Because influenza virus possesses a complex lipid envelope, the presence of lipid hydrolase activity could in part account for the observed less-than-expected level of viable progeny. A thorough characterization of respective isolate protease HA0 hydrolysis products as well as other resident activities (i.e., lipase) is ongoing such that the role of these respective contributors in virus activation/inactivation can be firmly established.

No viremia of pandemic (H1N1) 2009 was demonstrated in blood donors who had donated blood during the probable incubation period

BACKGROUND

In the spring of 2009, the novel swine-origin influenza A (pandemic [H1N1] 2009) virus emerged and spread globally. Although no established cases of transfusion-transmitted influenza have been reported, the widespread outbreak of pandemic (H1N1) 2009 caused serious concern regarding the safety of blood products. The Japanese Red Cross Blood Centers have intercepted blood products with accompanying postdonation information indicating possible pandemic (H1N1) 2009 infection. To study the risk of transmission of pandemic (H1N1) 2009 by blood transfusion, we searched for the viral genome in such products using nucleic acid amplification technology.

STUDY DESIGN AND METHODS

Between June and December 2009, blood components were collected from 579 blood donors who were diagnosed as or strongly suspected of having pandemic (H1N1) 2009 within 7 days after donation. Viral RNA was extracted from plasma and red blood cell (RBC) products, and RNA samples were subjected to real-time reverse transcription-polymerase chain reaction of the hemagglutinin and matrix genes of the pandemic (H1N1) 2009 virus.

RESULTS

A total of 565 plasma and 413 RBC products from the 579 blood donors were available. No viral RNA of the pandemic (H1N1) 2009 was detected in any of the blood samples from the 579 blood donors.

CONCLUSION

No viremia of pandemic (H1N1) 2009 was demonstrated in any of the 579 blood donors who had most likely donated blood during the incubation period. It is considered that the risk of transmitting pandemic (H1N1) 2009 by blood transfusion is extremely low.

Insertion of a multibasic cleavage site in the haemagglutinin of human influenza H3N2 virus does not increase pathogenicity in ferrets

A multibasic cleavage site (MBCS) in the haemagglutinin (HA) protein of influenza A virus is a key determinant of pathogenicity in chickens, and distinguishes highly pathogenic avian influenza (HPAI) viruses from low pathogenic avian influenza viruses (LPAI). An MBCS has only been detected in viruses of the H5 and H7 subtypes. Here we investigated the phenotype of a human H3N2 virus with an MBCS in HA. Insertion of an MBCS in the H3N2 virus resulted in cleavage of HA and efficient replication in Madin-Darby canine kidney cells in the absence of exogenous trypsin in vitro, similar to HPAI H5N1 virus. However, studies in ferrets demonstrated that insertion of the MBCS into HA did not result in increased virus shedding, cellular host range, systemic replication or pathogenicity, as compared with wild-type virus. This study indicates that acquisition of an MBCS alone is insufficient to increase pathogenicity of a prototypical seasonal human H3N2 virus.

Evolutionary characterization of human H1N1 influenza virus hemagglutinin genes isolated from 1947 to 2009 in China

OBJECTIVE

Our aim was to reveal the molecular characteristics of human H1N1 influenza virus hemagglutinin (HA) genes from 1947 to 2009 in China.

METHODS

129 HA gene sequences were downloaded from NCBI's GenBank and analyzed by DNASTAR software. Additionally, the three-dimensional structure of HA protein was predicted by the SWISS-MODEL service.

RESULTS

First, 2009 Chinese HA genes were 99% identical to those of Mexican and American ones; their key sites remained highly conserved. Second, 50 Chinese strains from 1947 to 2009 clustered by the year of isolation, and 2009 strains had only 70% identity to 1947-2008 ones. Third, over the past 60 years, 3 receptor-binding (RB) sites and 2 of the 8 glycosylation sites (amino acids 279 and 290) underwent considerable changes while the cleavage sites remained stable. Fourth, the human HA sequences differed completely from swine and avian isolates. Finally, the mutation of cleavage sites can change the three-dimensional structures, but single mutations cannot.

CONCLUSIONS

Thus, in the past 60 years, Chinese H1N1 influenza HA genes kept stable with high affinity and low pathogenicity to human except changes in 2 glycosylation and 3 RB sites which were associated with the pandemic strength, range and host specificity of viruses.

Inhibition of lung serine proteases in mice: a potentially new approach to control influenza infection

BACKGROUND

Host serine proteases are essential for the influenza virus life cycle because the viral haemagglutinin is synthesized as a precursor which requires proteolytic maturation. Therefore, we studied the activity and expression of serine proteases in lungs from mice infected with influenza and evaluated the effect of serine protease inhibitors on virus replication both in cell culture and in infected mice.

RESULTS

Two different inbred mouse strains were investigated: DBA/2J as a highly susceptible and C57Bl/6J as a more resistant strain to influenza virus infection. The serine proteases from lung homogenates of mice exhibited pH optima of 10.00. Using the substrate Bz-Val-Gly-Arg-p-nitroanilide or in zymograms, the intensities of proteolysis increased in homogenates from both mouse strains with time post infection (p.i.) with the mouse-adapted influenza virus A/Puerto Rico/8/34 (H1N1; PR8). In zymograms at day 7 p.i., proteolytic bands were stronger and numerous in lung homogenates from DBA/2J than C57Bl/6J mice. Real-time PCR results confirmed differential expression of several lung proteases before and after infecting mice with the H1N1 virus. The most strongly up-regulated proteases were Gzma, Tmprss4, Elane, Ctrl, Gzmc and Gzmb. Pretreatment of mouse and human lung cell lines with the serine protease inhibitors AEBSF or pAB or a cocktail of both prior to infection with the H1N1 or the A/Seal/Massachusetts/1/80 (H7N7; SC35M) virus resulted in a decrease in virus replication. Pretreatment of C57Bl/6J mice with either AEBSF or a cocktail of AEBSF and pAB prior to infection with the H1N1 virus significantly reduced weight loss and led to a faster recovery of treated versus untreated mice while pAB alone exerted a very poor effect. After infection with the H7N7 virus, the most significant reduction of weight loss was obtained upon pretreatment with either the protease inhibitor cocktail or pAB. Furthermore, pretreatment of C57BL/6J mice with AEBSF prior to infection resulted in a significant reduction in the levels of both the H1N1 and H7N7 nucleoproteins in mice lungs and also a significant reduction in the levels of the HA transcript in the lungs of the H1N1--but not the H7N7-infected mice.

CONCLUSION

Multiple serine protease activities might be implicated in mediating influenza infection. Blocking influenza A virus infection in cultured lung epithelia and in mice by the used serine protease inhibitors may provide an alternative approach for treatment of influenza infection.

Molecular mechanisms of interspecies transmission and pathogenicity of influenza viruses: Lessons from the 2009 pandemic

The emergence of the 2009 H1N1 virus pandemic was unexpected, since it had been predicted that the next pandemic would be caused by subtype H5N1. We also had to learn that a pandemic does not necessarily require the introduction of a new virus subtype into the human population, but that it may result from antigenic shift within the same subtype. The new variant was derived from human and animal viruses by genetic reassortment in the pig, supporting the concept that this animal is the mixing vessel for the generation of new human influenza viruses. Although it is generally believed that the 2009 outbreak was mild, there have been severe cases particularly among the young and the middle-aged. Pathogenicity and host range are determined to a large extent by the polymerase, the haemagglutinin and the NS1 protein of influenza A viruses. There is evidence that mutations of these proteins may change the pathogenicity of the new virus.

High interferon type I responses in the lung, plasma and spleen during highly pathogenic H5N1 infection of chicken

This study shows that high pathogenic H5N1 influenza virus infection of chicken induced high levels of bioactive interferon type I in the lung (4.3 × 10⁵ U/mg tissue), plasma (1.1 × 10⁵ U/mL), and spleen (9.1 × 10⁵ U/mg tissue). In contrast, a low pathogenic attenuated H5N1 vaccine strain only induced approximately 24 times less IFN in the lung, 441 times less in the spleen and 649 less in the plasma. This was in the same range as a reassortant carrying the HA from the vaccine strain and the remaining genes from the high pathogenic virus. On the other hand, a reassortant virus with the HA from the high pathogenic H5N1 with the remaining genes from the vaccine strain had intermediate levels of IFN. The level of interferon responses related to the viral load, and those in the spleen and blood to the spread of virus to lymphoid tissue, as well as disease severity. In vitro, the viruses did not induce interferon in chicken embryonic fibroblasts, but high levels in splenocytes, with not clear relationship to pathogenicity and virulence. This, and the responses also with inactivated viruses imply the presence of plasmacytoid dendritic cell-like leukocytes within the chicken immune system, possibly responsible for the high interferon responses during H5N1 infection. Our data also indicate that the viral load as well as the cleavability of the HA enabling systemic spread of the virus are two major factors controlling systemic IFN responses in chicken.

Virulence determinants of pandemic influenza viruses

Influenza A viruses cause recurrent, seasonal epidemics and occasional global pandemics with devastating levels of morbidity and mortality. The ability of influenza A viruses to adapt to various hosts and undergo reassortment events ensures constant generation of new strains with unpredictable degrees of pathogenicity, transmissibility, and pandemic potential. Currently, the combination of factors that drives the emergence of pandemic influenza is unclear, making it impossible to foresee the details of a future outbreak. Identification and characterization of influenza A virus virulence determinants may provide insight into genotypic signatures of pathogenicity as well as a more thorough understanding of the factors that give rise to pandemics.

Characterization of a Low Pathogenic Avian Influenza Virus (H6N1) Isolated from Turkeys

An avian influenza virus (AIV), A/turkey/Israel/09 subtype H6N1, was isolated from turkey poults exhibiting typical pathology associated with AIV infection. The virus was characterized by RT-PCR using AIV subtype-specific primers and by the haemagglutination inhibition test using AIV subtype-specific antisera. The virus has an intravenous pathogenicity index of 0 and possessed a nucleotide sequence at the cleavage site of the hemagglutinin gene, PQIETR*GLF, associated with avian influenza viruses of low pathogenicity. Unlike the two previous H6N2 isolates originating from domestic ducks and mallard, the A/turkey/Israel/09 (H6N1) was isolated from turkeys. The gene sequences of the A/turkey/Israel/09 (H6N1) virus show divergence from the former Israeli H6 isolates.

Virus assembly, allostery and antivirals

Assembly of virus capsids and surface proteins must be regulated to ensure that the resulting complex is an infectious virion. In this review, we examine assembly of virus capsids, focusing on hepatitis B virus and bacteriophage MS2, and formation of glycoproteins in the alphaviruses. These systems are structurally and biochemically well-characterized and are simplest-case paradigms of self-assembly. Published data suggest that capsid and glycoprotein assembly is subject to allosteric regulation, that is regulation at the level of conformational change. The hypothesis that allostery is a common theme in viruses suggests that deregulation of capsid and glycoprotein assembly by small molecule effectors will be an attractive antiviral strategy, as has been demonstrated with hepatitis B virus.

Construction and characterization of insect cell-derived influenza VLP: cell binding, fusion, and EGFP incorporation

We have constructed virus-like particles (VLPs) harboring hemagglutinin (HA), neuraminidase (NA), matrix protein 1 (M1) ,and proton channel protein (M2) using baculovirus as a vector in the SF9 insect cell. The size of the expressed VLP was estimated to be ~100 nm by light scattering experiment and transmission electron microscopy. Recognition of HA on the VLP surface by the HA2-specific monoclonal antibody IIF4 at acidic pH, as probed by surface plasmon resonance, indicated the pH-induced structural rearrangement of HA. Uptake of the particle by A549 mediated by HA-sialylose receptor interaction was visualized by the fluorescent-labeled VLP. The HA-promoted cell-virus fusion activity was illustrated by fluorescence imaging on the Jurkat cells incubated with rhodamine-loaded VLP performed at fusogenic pH. Furthermore, the green fluorescence protein (GFP) was fused to NA to produce VLP with a pH-sensitive probe, expanding the use of VLP as an antigen carrier and a tool for viral tracking.

Novel insights into proteolytic cleavage of influenza virus hemagglutinin

The influenza virus hemagglutinin (HA) mediates the first essential step in the viral life cycle, virus entry into target cells. Influenza virus HA is synthesised as a precursor protein in infected cells and requires cleavage by host cell proteases to transit into an active form. Cleavage is essential for influenza virus infectivity and the HA‐processing proteases are attractive targets for therapeutic intervention. It is well established that cleavage by ubiquitously expressed subtilisin‐like proteases is a hallmark of highly pathogenic avian influenza viruses (HPAIV). In contrast, the nature of the proteases responsible for cleavage of HA of human influenza viruses and low pathogenic avian influenza viruses (LPAIV) is not well understood. Recent studies suggest that cleavage of HA of human influenza viruses might be a cell‐associated event and might be facilitated by the type II transmembrane serine proteases (TTSPs) TMPRSS2, TMPRSS4 and human airway trypsin‐like protease (HAT). Here, we will introduce the different concepts established for proteolytic activation of influenza virus HA, with a particular focus on the role of TTSPs, and we will discuss their implications for viral tropism, pathogenicity and antiviral intervention. Copyright © 2010 John Wiley & Sons, Ltd.

Inhibition of influenza virus infection in human airway cell cultures by an antisense peptide-conjugated morpholino oligomer targeting the hemagglutinin-activating protease TMPRSS2

Influenza A viruses constitute a major and ongoing global public health concern. Current antiviral strategies target viral gene products; however, the emergence of drug-resistant viruses highlights the need for novel antiviral approaches. Cleavage of the influenza virus hemagglutinin (HA) by host cell proteases is crucial for viral infectivity and therefore presents a potential drug target. Peptide-conjugated phosphorodiamidate morpholino oligomers (PPMO) are single-stranded-DNA-like antisense agents that readily enter cells and can act as antisense agents by sterically blocking cRNA. Here, we evaluated the effect of PPMO targeted to regions of the pre-mRNA or mRNA of the HA-cleaving protease TMPRSS2 on proteolytic activation and spread of influenza viruses in human Calu-3 airway epithelial cells. We found that treatment of cells with a PPMO (T-ex5) designed to interfere with TMPRSS2 pre-mRNA splicing resulted in TMPRSS2 mRNA lacking exon 5 and consequently the expression of a truncated and enzymatically inactive form of TMPRSS2. Altered splicing of TMPRSS2 mRNA by the T-ex5 PPMO prevented HA cleavage in different human seasonal and pandemic influenza A viruses and suppressed viral titers by 2 to 3 log(10) units, strongly suggesting that TMPRSS2 is responsible for HA cleavage in Calu-3 airway cells. The data indicate that PPMO provide a useful reagent for investigating HA-activating proteases and may represent a promising strategy for the development of novel therapeutics to address influenza infections.

Use of molecular epidemiology to trace transmission pathways for infectious salmon anaemia virus (ISAV) in Norwegian salmon farming

BACKGROUND

Infectious Salmon Anaemia (ISA) is a disease affecting farmed Atlantic salmon, and most salmon producing countries have experienced ISA outbreaks. The aim of the present study was to use epidemiological and viral sequence information to trace transmission pathways for ISA virus (ISAV) in Norwegian salmon farming.

METHODS

The study covers a period from January 2007 to July 2009 with a relatively high rate of ISA outbreaks, including a large cluster of outbreaks that emerged in Northern Norway (the North-cluster). Farms with ISA outbreaks and neighbouring salmon farms (At-risk-sites) were tested for the presence of ISAV, and epidemiological information was collected. ISAV hemagglutinin-esterase (HE) and fusion (F) protein genes were sequenced and phylogenetic analyses were performed. Associations between sequence similarities and salmon population data were analysed to substantiate possible transmission pathways.

RESULTS

There was a high degree of genetic similarity between ISAV isolates within the North-cluster. ISAV was detected in 12 of 28 At-risk-sites, and a high proportion of the viruses were identified as putative low virulent genotypes harbouring the full length highly polymorphic region (HPR); HPR0 of the HE protein and the amino acid glutamine (Q) in the F protein at position 266. The sequences from HPR0/F (Q(266)) genotypes revealed larger genetic variation, lower viral loads and lower prevalence of infection than HPR-deleted genotypes. Seaway distance between salmon farms was the only robust explanatory variable to explain genetic similarity between ISAV isolates.

DISCUSSION

We suggest that a single HPR-deleted genotype of ISAV has spread between salmon farms in the North-cluster. Furthermore, we find that HPR0/F (Q(266)) genotypes are frequently present in farmed populations of Atlantic salmon. From this, we anticipate a population dynamics of ISAV portrayed by low virulent genotypes occasionally transitioning into virulent genotypes, causing solitary outbreaks or local epidemics through local transmission.

Highly conserved cross-reactive CD4+ T-cell HA-epitopes of seasonal and the 2009 pandemic influenza viruses

Please cite this paper as: Duvvuri et al. (2010) Highly conserved cross‐reactive CD4+ T‐cell HA‐epitopes of seasonal and the 2009 pandemic influenza viruses. Influenza and Other Respiratory Viruses 4(5), 249–258.

Background The relatively mild nature of the 2009 influenza pandemic (nH1N1) highlights the overriding importance of pre‐existing immune memory. The absence of cross‐reactive antibodies to nH1N1 in most individuals suggests that such attenuation may be attributed to pre‐existing cellular immune responses to epitopes shared between nH1N1 virus and previously circulating strains of inter‐pandemic influenza A viruses.

Results We sought to identify potential CD4+ T cell epitopes and predict the level of cross‐reactivity of responding T cells. By performing large‐scale major histocompatibility complex II analyses on Hemagglutinin (HA) proteins, we investigated the degree of T‐cell cross‐reactivity between seasonal influenza A (sH1N1, H3N2) from 1968 to 2009 and nH1N1 strains. Each epitope was examined against all the protein sequences that correspond to sH1N1, H3N2, and nH1N1. T‐cell cross‐reactivity was estimated to be 52%, and maximum conservancy was found between sH1N1 and nH1N1 with a significant correlation (P < 0·05).

Conclusions  Given the importance of cellular responses in kinetics of influenza infection in humans, our findings underscore the role of T‐cell assays for understanding the inter‐pandemic variability in severity and for planning treatment methods for emerging influenza viruses.

Long-term shedding of influenza A virus in stool of immunocompromised child

In immunocompromised patients, influenza infection may progress to prolonged viral shedding from the respiratory tract despite antiviral therapy. We describe chronic influenza A virus infection in an immunocompromised child who had prolonged shedding of culturable influenza virus in stool.

Phylogenetic and molecular characterization of H9N2 influenza isolates from chickens in Northern China from 2007-2009

The repeated transmission to pigs and humans, and the long-term endemicity in terrestrial poultry of H9N2 viruses in China lend urgency to the study of their ecology and pathogenicity. In the present paper, we reported an H9N2 virus sublineage isolated from chickens in northern China from 2007 to 2009 has high lethality for mice. Phylogenetic analysis of the full genome indicated that six representative H9N2 isolates shared high homology to each other, and they clustered in the same sublineage with other H9N2 viruses isolated recently in northern China. The isolates were double-reassortant viruses containing M genes similar to A/Quail/Hong Kong/G1/97 (H9N2) and the other seven gene segments from A/Chicken/Shanghai/F/98 (H9N2). These six isolates were capable of replicating in the lungs of infected chickens without producing observable clinical signs of disease or death. However, they were highly lethal to mice with mortality rates as high as 100% (14/14) without prior adaptation. The affected mice exhibited severe respiratory syndromes and diffuse lung injury. The H9N2 viruses could be detected in multiple organs of the infected mice, including hearts, livers, spleens, lungs and kidneys. Our findings demonstrated that H9N2 viruses isolated from the chickens in northern China have established a stable sublineage with enhanced pathogenicity to mice, suggesting that urgent attention will need to be paid to the transmission of H9N2 viruses from chickens to mammals.

The essentiality of alpha-2-macroglobulin in human salivary innate immunity against new H1N1 swine origin influenza A virus

A novel strain of influenza A H1N1 emerged in the spring of 2009 and has spread rapidly throughout the world. Although vaccines have recently been developed that are expected to be protective, their availability was delayed until well into the influenza season. Although anti-influenza drugs such as neuraminidase inhibitors can be effective, resistance to these drugs has already been reported. Although human saliva was known to inhibit viral infection and may thus prevent viral transmission, the components responsible for this activity on influenza virus, in particular, influenza A swine origin influenza A virus (S-OIV), have not yet been defined. By using a proteomic approach in conjunction with beads that bind alpha-2,6-sialylated glycoprotein, we determined that an alpha-2-macroglobulin (A2M) and an A2M-like protein are essential components in salivary innate immunity against hemagglutination mediated by a clinical isolate of S-OIV (San Diego/01/09 S-OIV). A model of an A2M-based "double-edged sword" on competition of alpha-2,6-sialylated glycoprotein receptors and inactivation of host proteases is proposed. We emphasize that endogenous A2M in human innate immunity functions as a natural inhibitor against S-OIV.

Dendritic cell activation by recombinant hemagglutinin proteins of H1N1 and H5N1 influenza A viruses

Since dendritic cells may play a key role in defense against influenza virus infection, we examined the effects of recombinant hemagglutinin (HA) proteins derived from mouse-adapted H1N1 (A/WSN/1933), swine-origin 2009 pandemic H1N1 (A/Texas/05/2009), and highly pathogenic avian influenza H5N1 (A/Thailand/KAN-1/2004) viruses on mouse myeloid dendritic cells (mDCs). The results reveal that tumor necrosis factor alpha (TNF-α), interleukin-12 (IL-12) p70, and major histocompatibility complex class II (MHC-II) expression was increased in mDCs after treatment with recombinant HA proteins of H1N1 and H5N1. The specificity of recombinant HA treatments for mDC activation was diminished after proteinase K digestion. HA apparently promotes mDC maturation by enhancing CD40 and CD86 expression and suppressing endocytosis. No significant differences in mDC activation were observed among recombinant proteins of H1N1 and H5N1. The stimulation of mDCs by HA proteins of H1N1 and H5N1 was completely MyD88 dependent. These findings may provide useful information for the development of more-effective influenza vaccines.

Influenza hemagglutinin and neuraminidase membrane glycoproteins

Considerable progress has been made toward understanding the structural basis of the interaction of the two major surface glycoproteins of influenza A virus with their common ligand/substrate: carbohydrate chains terminating in sialic acid. The specificity of virus attachment to target cells is mediated by hemagglutinin, which acquires characteristic changes in its receptor-binding site to switch its host from avian species to humans. Anti-influenza drugs mimic the natural sialic acid substrate of the virus neuraminidase enzyme but utilize the much tighter binding of the drugs for efficacy. Resistance to one of the two main antiviral drugs is differentially acquired by the two distinct subsets of neuraminidase as a consequence of structural differences in the enzyme active site between the two phylogenetic groups.

Formulation of microneedles coated with influenza virus-like particle vaccine

Mortality due to seasonal and pandemic influenza could be reduced by increasing the speed of influenza vaccine production and distribution. We propose that vaccination can be expedited by (1) immunizing with influenza virus-like particle (VLP) vaccines, which are simpler and faster to manufacture than conventional egg-based inactivated virus vaccines, and (2) administering vaccines using microneedle patches, which should simplify vaccine distribution due to their small package size and possible self-administration. In this study, we coated microneedle patches with influenza VLP vaccine, which was released into skin by dissolution within minutes. Optimizing the coating formulation required balancing factors affecting the coating dose and vaccine antigen stability. Vaccine stability, as measured by an in vitro hemagglutination assay, was increased by formulation with increased concentration of trehalose or other stabilizing carbohydrate compounds and decreased concentration of carboxymethylcellulose (CMC) or other viscosity-enhancing compounds. Coating dose was increased by formulation with increased VLP concentration, increased CMC concentration, and decreased trehalose concentration, as well as increased number of dip coating cycles. Finally, vaccination of mice using microneedles stabilized by trehalose generated strong antibody responses and provided full protection against high-dose lethal challenge infection. In summary, this study provides detailed analysis to guide formulation of microneedle patches coated with influenza VLP vaccine and demonstrates effective vaccination in vivo using this system.

Pandemic influenza A (H1N1) virus infection and avian influenza A (H5N1) virus infection: a comparative analysis

The 2009 H1N1 and H5N1 influenza viruses are newly (re-) emerged influenza A viruses (2009 A(H1N1) and A(H5N1), respectively) that have recently posed tremendous health threats in many regions worldwide. With the 2009 outbreak of H1N1 influenza A, the world witnessed the first influenza pandemic of the 21st century. The disease has rapidly spread across the entire globe, and has resulted in hundreds of thousands of cases with confirmed infection. Although characterized by high transmissibility, the virulence and fatality of the 2009 A(H1N1) influenza virus have thus far remained relatively low. The reverse holds true for A(H5N1) influenza; at a fatality rate that exceeds 60%, it is known to cause severe damage to the human respiratory system, but is not presently capable of efficient transmission from human to human. Apart from the clear differences between the two types of influenza, there are some significant similarities that warrant attention. In particular, the more severe and fatal 2009 A(H1N1) influenza cases have shown symptoms similar to those reported in cases of A(H5N1) influenza. Histopathological findings for these cases, to the extent available, also appear to have similarities for both diseases in terms of damage and severity. Here we review important recent publications in this area, and we discuss some of the key commonalities and contrasts between the two influenza A types in terms of their biology, origins, clinical features, pathology and pathogenesis, and receptors and transmissibility.

Strain identification of commercial influenza vaccines by mass spectrometry

Current influenza vaccine manufacturing and testing timelines require that the constituent hemagglutinin (HA) and neuraminidase (NA) strains be selected each year approximately 10 months before the vaccine becomes available. The threat of a pandemic influenza outbreak requires that more rapid testing methods be found. We have developed a specialized on-filter sample preparation method that uses both trypsin and chymotrypsin to enzymatically digest peptide-N-glycosidase F (PNGase F)-deglycosylated proteins in vaccines. In tandem with replicate liquid chromatography-mass spectrometry (LC-MS) analyses, this approach yields sufficient protein sequencing data (>85% sequence coverage on average) for strain identification of HA and NA components. This has allowed the confirmation, and in some cases the correction, of the identity of the influenza strains in recent commercial vaccines as well as the correction of some ambiguous HA sequence annotations in available databases. This method also allows the identification of low-level contaminant egg proteins produced during the manufacturing process.

TMPRSS2 and TMPRSS4 facilitate trypsin-independent spread of influenza virus in Caco-2 cells

Proteolysis of influenza virus hemagglutinin by host cell proteases is essential for viral infectivity, but the proteases responsible are not well defined. Recently, we showed that engineered expression of the type II transmembrane serine proteases (TTSPs) TMPRSS2 and TMPRSS4 allows hemagglutinin (HA) cleavage. Here we analyzed whether TMPRSS2 and TMPRSS4 are expressed in influenza virus target cells and support viral spread in the absence of exogenously added protease (trypsin). We found that transient expression of TMPRSS2 and TMPRSS4 resulted in HA cleavage and trypsin-independent viral spread. Endogenous expression of TMPRSS2 and TMPRSS4 in cell lines correlated with the ability to support the spread of influenza virus in the absence of trypsin, indicating that these proteases might activate influenza virus in naturally permissive cells. Indeed, RNA interference (RNAi)-mediated knockdown of both TMPRSS2 and TMPRSS4 in Caco-2 cells, which released fully infectious virus without trypsin treatment, markedly reduced the spread of influenza virus, demonstrating that these proteases were responsible for efficient proteolytic activation of HA in this cell line. Finally, TMPRSS2 was found to be coexpressed with the major receptor determinant of human influenza viruses, 2,6-linked sialic acids, in human alveolar epithelium, indicating that viral target cells in the human respiratory tract express TMPRSS2. Collectively, our results point toward an important role for TMPRSS2 and possibly TMPRSS4 in influenza virus replication and highlight the former protease as a potential therapeutic target.

OpenFluDB, a database for human and animal influenza virus

Although research on influenza lasted for more than 100 years, it is still one of the most prominent diseases causing half a million human deaths every year. With the recent observation of new highly pathogenic H5N1 and H7N7 strains, and the appearance of the influenza pandemic caused by the H1N1 swine-like lineage, a collaborative effort to share observations on the evolution of this virus in both animals and humans has been established. The OpenFlu database (OpenFluDB) is a part of this collaborative effort. It contains genomic and protein sequences, as well as epidemiological data from more than 27 000 isolates. The isolate annotations include virus type, host, geographical location and experimentally tested antiviral resistance. Putative enhanced pathogenicity as well as human adaptation propensity are computed from protein sequences. Each virus isolate can be associated with the laboratories that collected, sequenced and submitted it. Several analysis tools including multiple sequence alignment, phylogenetic analysis and sequence similarity maps enable rapid and efficient mining. The contents of OpenFluDB are supplied by direct user submission, as well as by a daily automatic procedure importing data from public repositories. Additionally, a simple mechanism facilitates the export of OpenFluDB records to GenBank. This resource has been successfully used to rapidly and widely distribute the sequences collected during the recent human swine flu outbreak and also as an exchange platform during the vaccine selection procedure. Database URL: http://openflu.vital-it.ch.

Comparison of selection pressures on the HA gene of pandemic (2009) and seasonal human and swine influenza A H1 subtype viruses

The 2009 human pandemic influenza (H1N1) virus possesses the HA gene of the H1 subtype. The evolutionary process of the 2009 H1N1 virus remains to be defined. We performed genetic analyses of the HA gene by comparing the 2009 H1N1 virus with seasonal human and swine viruses. We analyzed sequences of 116 2009 H1N1 viruses, and obtained 1457 seasonal H1N1, 365 swine H1, and 1332 2009 H1N1 viruses from the database. Selection pressure for the 2009 H1N1 virus was higher than that for the swine virus and equivalent to that for the seasonal virus. Positions 206 and 264 were found to be positively selected sites. We also identified sites under different selection pressures from the seasonal or swine virus that may be involved in imparting significant biological characteristics. The evolutionary characteristics of the H1 gene of the 2009 H1N1 virus differed from those of seasonal and swine viruses.

Protective immunity against H5N1 influenza virus by a single dose vaccination with virus-like particles

We generated influenza virus-like particles (VLPs) containing the wild type (WT) H5 hemagglutinin (HA) from A/Viet Nam/1203/04 virus or a mutant H5 HA with a deletion of the multibasic cleavage motif. VLPs containing mutant H5 HA were found to be as immunogenic as VLPs containing WT HA. A single intramuscular vaccination with either type of H5 VLPs provided complete protection against lethal challenge. In contrast, the recombinant H5 HA vaccine was less immunogenic and vaccination even with a 5 fold higher dose did not induce protective immunity. VLP vaccines were superior to the recombinant HA in inducing T helper type 1 immune responses, hemagglutination inhibition titers, and antibody secreting cells, which significantly contribute to inducing protective immunity after a single dose vaccination. This study provides insights into the potential mechanisms of improved immunogenicity by H5 VLP vaccines as an approach to improve the protective efficacy against potential pandemic viruses.

Modifications to the hemagglutinin cleavage site control the virulence of a neurotropic H1N1 influenza virus

A key determinant of influenza virus pathogenesis is mutation in the proteolytic cleavage site of the hemagglutinin (HA). Typically, low-pathogenicity forms of influenza virus are cleaved by trypsin-like proteases, whereas highly pathogenic forms are cleaved by different proteases (e.g., furin). Influenza virus A/WSN/33 (WSN) is a well-studied H1N1 strain that is trypsin independent in vitro and has the ability to replicate in mouse brain. Previous studies have indicated that mutations in the neuraminidase (NA) gene allow the recruitment of an alternate protease (plasminogen/plasmin) for HA activation. In this study we have identified an additional mutation in the P2 position of the WSN HA cleavage site (S328Y) that appears to control virus spread in a plasmin-dependent manner. We reconstructed recombinant WSN viruses containing tyrosine (Y), phenylalanine (F), or serine (S) in the P2 position of the cleavage site. The Y328 and F328 viruses allowed plaque formation in the absence of trypsin, whereas the S328 virus was unable to form plaques under these conditions. In mice, Y328 and F328 viruses were able to efficiently spread following intracranial inoculation; in contrast, the S328 virus showed only limited infection of mouse brain. Following intranasal inoculation, all viruses could replicate efficiently, but with Y328 and F328 viruses showing a limited growth defect. We also show that wild-type HA (Y328) was more efficiently cleaved by plasmin than S328 HA. Our studies form the foundation for a more complete understanding of the molecular determinants of influenza virus pathogenesis and the role of the plasminogen/plasmin system in activating HA.

The complete influenza hemagglutinin fusion domain adopts a tight helical hairpin arrangement at the lipid:water interface

All but five of the N-terminal 23 residues of the HA2 domain of the influenza virus glycoprotein hemagglutinin (HA) are strictly conserved across all 16 serotypes of HA genes. The structure and function of this HA2 fusion peptide (HAfp) continues to be the focus of extensive biophysical, computational, and functional analysis, but most of these analyses are of peptides that do not include the strictly conserved residues Trp(21)-Tyr(22)-Gly(23). The heteronuclear triple resonance NMR study reported here of full length HAfp of sero subtype H1, solubilized in dodecylphosphatidyl choline, reveals a remarkably tight helical hairpin structure, with its N-terminal alpha-helix (Gly(1)-Gly(12)) packed tightly against its second alpha-helix (Trp(14)-Gly(23)), with six of the seven conserved Gly residues at the interhelical interface. The seventh conserved Gly residue in position 13 adopts a positive angle, enabling the hairpin turn that links the two helices. The structure is stabilized by multiple interhelical C(alpha)H to C=O hydrogen bonds, characterized by strong interhelical H(N)-H(alpha) and H(alpha)-H(alpha) NOE contacts. Many of the previously identified mutations that make HA2 nonfusogenic are also incompatible with the tight antiparallel hairpin arrangement of the HAfp helices.(15)N relaxation analysis indicates the structure to be highly ordered on the nanosecond time scale, and NOE analysis indicates HAfp is located at the water-lipid interface, with its hydrophobic surface facing the lipid environment, and the Gly-rich side of the helix-helix interface exposed to solvent.