Improved antigen yield in pandemic H1N1 (2009) candidate vaccine viruses with chimeric hemagglutinin molecules

High-throughput sequencing-based neutralization assay reveals how repeated vaccinations impact titers to recent human H1N1 influenza strains

The high genetic diversity of influenza viruses means that traditional serological assays have too low throughput to measure serum antibody neutralization titers against all relevant strains. To overcome this challenge, we developed a sequencing-based neutralization assay that simultaneously measures titers against many viral strains using small serum volumes using a workflow similar to traditional neutralization assays. The key innovation is to incorporate unique nucleotide barcodes into the hemagglutinin (HA) genomic segment, and then pool viruses with numerous different barcoded HA variants and quantify the infectivity of all of them simultaneously using next-generation sequencing. With this approach, a single researcher performed the equivalent of 2,880 traditional neutralization assays (80 serum samples against 36 viral strains) in approximately 1 month. We applied the sequencing-based assay to quantify the impact of influenza vaccination on neutralization titers against recent human H1N1 strains for individuals who had or had not also received a vaccine in the previous year. We found that the viral strain specificities of the neutralizing antibodies elicited by vaccination vary among individuals and that vaccination induced a smaller increase in titers for individuals who had also received a vaccine the previous year-although the titers 6 months after vaccination were similar in individuals with and without the previous-year vaccination. We also identified a subset of individuals with low titers to a subclade of recent H1N1 even after vaccination. We provide an experimental protocol (dx.doi.org/10.17504/protocols.io.kqdg3xdmpg25/v1) and computational pipeline (https://github.com/jbloomlab/seqneut-pipeline) for the sequencing-based neutralization assays to facilitate the use of this method by others.

IMPORTANCE

We describe a new approach that can rapidly measure how the antibodies in human serum inhibit infection by many different influenza strains. This new approach is useful for understanding how viral evolution affects antibody immunity. We apply the approach to study the effect of repeated influenza vaccination.

An avian-origin internal backbone effectively increases the H5 subtype avian influenza vaccine candidate yield in both chicken embryonated eggs and MDCK cells

Inactivated vaccines play an important role in preventing and controlling the epidemic caused by the H5 subtype avian influenza virus. The vaccine strains are updated in response to alterations in surface protein antigens, while an avian-derived vaccine internal backbone with a high replicative capacity in chicken embryonated eggs and MDCK cells is essential for vaccine development. In this study, we constructed recombinant viruses using the clade 2.3.4.4d A/chicken/Jiangsu/GY5/2017(H5N6, CkG) strain as the surface protein donor and the clade 2.3.4.4b A/duck/Jiangsu/84512/2017(H5N6, Dk8) strain with high replicative ability as an internal donor. After optimization, the integration of the M gene from the CkG into the internal genes from Dk8 (8GM) was selected as the high-yield vaccine internal backbone, as the combination improved the hemagglutinin1/nucleoprotein (HA1/NP) ratio in recombinant viruses. The r8GMΔG with attenuated hemagglutinin and neuraminidase from the CkG exhibited high-growth capacity in both chicken embryos and MDCK cell cultures. The inactivated r8GMΔG vaccine candidate also induced a higher hemagglutination inhibition antibody titer and microneutralization titer than the vaccine strain using PR8 as the internal backbone. Further, the inactivated r8GMΔG vaccine candidate provided complete protection against wild-type strain challenge. Therefore, our study provides a high-yield, easy-to-cultivate candidate donor as an internal gene backbone for vaccine development.

High-throughput sequencing-based neutralization assay reveals how repeated vaccinations impact titers to recent human H1N1 influenza strains

The high genetic diversity of influenza viruses means that traditional serological assays have too low throughput to measure serum antibody neutralization titers against all relevant strains. To overcome this challenge, we have developed a sequencing-based neutralization assay that simultaneously measures titers against many viral strains using small serum volumes via a workflow similar to traditional neutralization assays. The key innovation is to incorporate unique nucleotide barcodes into the hemagglutinin (HA) genomic segment, and then pool viruses with numerous different barcoded HA variants and quantify infectivity of all of them simultaneously using next-generation sequencing. With this approach, a single researcher performed the equivalent of 2,880 traditional neutralization assays (80 serum samples against 36 viral strains) in approximately one month. We applied the sequencing-based assay to quantify the impact of influenza vaccination on neutralization titers against recent human H1N1 strains for individuals who had or had not also received a vaccine in the previous year. We found that the viral strain specificities of the neutralizing antibodies elicited by vaccination vary among individuals, and that vaccination induced a smaller increase in titers for individuals who had also received a vaccine the previous year-although the titers six months after vaccination were similar in individuals with and without the previous-year vaccination. We also identified a subset of individuals with low titers to a subclade of recent H1N1 even after vaccination. This study demonstrates the utility of high-throughput sequencing-based neutralization assays that enable titers to be simultaneously measured against many different viral strains. We provide a detailed experimental protocol (DOI: https://dx.doi.org/10.17504/protocols.io.kqdg3xdmpg25/v1) and a computational pipeline (https://github.com/jbloomlab/seqneut-pipeline) for the sequencing-based neutralization assays to facilitate the use of this method by others.

Identification of the properties of H5 influenza vaccine viruses with high hemagglutinin yields

Manufactured influenza vaccines have to contain a defined amount of hemagglutinin (HA) antigen. Therefore, vaccine viruses with a high HA antigen yield (HAY) are preferable for manufacturing vaccines, particularly vaccines in response to a pandemic, when vaccines need to be rapidly produced. However, the viral properties associated with a high HAY have not yet been fully clarified. To identify the HAY-associated traits, we first propagated 26 H5 candidate vaccine viruses (CVVs) in eggs, which were previously developed based on genetic reassortment methods using master viruses, to determine their total protein yield (TPY), ratio of HA to total viral protein (%-HA content) and HAY. The results revealed that the HAY was correlated with the TPY but not with the %-HA content. We further found that altering the sequences of the 3' noncoding region of HA vRNA or replacing the master virus improved the HAYs and TPYs of the low-HAY CVVs to approximately double the values of the original CVVs but did not change the %-HA content, which a previous study suggested was associated with the HAY. Analyses based on real-time PCR assays and scanning electron microscopy revealed that the virus samples with an improved HAY contained more copies of the virus genome and viral particles than the original samples. The results suggest that an improvement in virus growth (i.e., an increase in the amount of viral particles) leads to an increase in the TPY and thus in the HAY, regardless of the %-HA content. The approximately twofold increase in the HAY shown in this study may not appear to represent a large improvement, but the impact will be significant given the millions of chicken eggs used to produce vaccines. These findings will be informative for developing high-HAY vaccine viruses.

Transformation of Dunaliella salina by Agrobacterium tumefaciens for the Expression of the Hemagglutinin of Avian Influenza Virus H5

Avian influenza (AI) is one of the main threats to the poultry industry worldwide. Vaccination efforts are based on inactivated, live attenuated, and recombinant vaccines, where the virus hemagglutinin (HA) is the main component of any vaccine formulation. This study uses Dunaliella salina to express the AIV HA protein of an H5 virus. D. salina offers a system of feasible culture properties, generally recognized as safe for humans (GRAS), with N-glycosylation and nuclear transformation by Agrobacterium tumefaciens. The cloning and transformation of D. salina cells with the H5HA gene was confirmed by polymerase chain reaction (PCR). SDS-PAGE and Western blot confirmed HA5r protein expression, and the correct expression and biological activity of the HA5r protein were confirmed by a hemagglutination assay (HA). This study proves the feasibility of using a different biological system for expressing complex antigens from viruses. These findings suggest that a complex protein such as HA5r from AIV (H5N2) can be successfully expressed in D. salina.

Triple reassortment increases compatibility among viral ribonucleoprotein genes of contemporary avian and human influenza A viruses

Compatibility among the influenza A virus (IAV) ribonucleoprotein (RNP) genes affects viral replication efficiency and can limit the emergence of novel reassortants, including those with potential pandemic risks. In this study, we determined the polymerase activities of 2,451 RNP reassortants among three seasonal and eight enzootic IAVs by using a minigenome assay. Results showed that the 2009 H1N1 RNP are more compatible with the tested enzootic RNP than seasonal H3N2 RNP and that triple reassortment increased such compatibility. The RNP reassortants among 2009 H1N1, canine H3N8, and avian H4N6 IAVs had the highest polymerase activities. Residues in the RNA binding motifs and the contact regions among RNP proteins affected polymerase activities. Our data indicates that compatibility among seasonal and enzootic RNPs are selective, and enzoosis of multiple strains in the animal-human interface can facilitate emergence of an RNP with increased replication efficiency in mammals, including humans.

Cell-Based Influenza A/H1N1pdm09 Vaccine Viruses Containing Chimeric Hemagglutinin with Improved Membrane Fusion Ability

The H1N1 influenza pandemic vaccine has been developed from the A/California/07/09 (Cal) virus and the well-known high-yield A/Puerto Rico/8/34 (PR8) virus by classical reassortment and reverse genetics (RG) in eggs. Previous studies have suggested that Cal-derived chimeric hemagglutinin (HA) and neuraminidase (NA) improve virus yields. However, the cell-based vaccine of the H1N1 pandemic virus has been less investigated. RG viruses that contained Cal-derived chimeric HA and NA could be rescued in Madin-Darby canine kidney cells that expressed α2,6-sialyltransferase (MDCK-SIAT1). The viral growth kinetics and chimeric HA and NA properties were analyzed. We attempted to generate various RG viruses that contained Cal-derived chimeric HA and NA, but half of them could not be rescued in MDCK-SIAT1 cells. When both the 3'- and 5'-terminal regions of Cal HA viral RNA were replaced with the corresponding regions of PR8 HA, the RG viruses were rescued. Our results were largely consistent with those of previous studies, in which the N- and C-terminal chimeric HA slightly improved virus yield. Importantly, the chimeric HA, compared to Cal HA, showed cell fusion ability at a broader pH range, likely due to amino acid substitutions in the transmembrane region of HA. The rescued RG virus with high virus yield harbored the chimeric HA capable of cell fusion at a broader range of pH.

Segment 2 from influenza A(H1N1) 2009 pandemic viruses confers temperature-sensitive haemagglutinin yield on candidate vaccine virus growth in eggs that can be epistatically complemented by PB2 701D

Candidate vaccine viruses (CVVs) for seasonal influenza A virus are made by reassortment of the antigenic virus with an egg-adapted strain, typically A/Puerto Rico/8/34 (PR8). Many 2009 A(H1N1) pandemic (pdm09) high-growth reassortants (HGRs) selected this way contain pdm09 segment 2 in addition to the antigenic genes. To investigate this, we made CVV mimics by reverse genetics (RG) that were either 6 : 2 or 5 : 3 reassortants between PR8 and two pdm09 strains, A/California/7/2009 (Cal7) and A/England/195/2009, differing in the source of segment 2. The 5 : 3 viruses replicated better in MDCK-SIAT1 cells than the 6 : 2 viruses, but the 6 : 2 CVVs gave higher haemagglutinin (HA) antigen yields from eggs. This unexpected phenomenon reflected temperature sensitivity conferred by pdm09 segment 2, as the egg HA yields of the 5 : 3 viruses improved substantially when viruses were grown at 35 °C compared with 37.5 °C, whereas the 6 : 2 virus yields did not. However, the authentic 5 : 3 pdm09 HGRs, X-179A and X-181, were not markedly temperature sensitive despite their PB1 sequences being identical to that of Cal7, suggesting compensatory mutations elsewhere in the genome. Sequence comparisons of the PR8-derived backbone genes identified polymorphisms in PB2, NP, NS1 and NS2. Of these, PB2 N701D affected the temperature dependence of viral transcription and, furthermore, improved and drastically reduced the temperature sensitivity of the HA yield from the 5 : 3 CVV mimic. We conclude that the HA yield of pdm09 CVVs can be affected by an epistatic interaction between PR8 PB2 and pdm09 PB1, but that this can be minimized by ensuring that the backbones used for vaccine manufacture in eggs contain PB2 701D.

Mutation of Influenza A Virus PA-X Decreases Pathogenicity in Chicken Embryos and Can Increase the Yield of Reassortant Candidate Vaccine Viruses

Influenza A virus is a widespread pathogen that affects both humans and a variety of animal species, causing regular epidemics and sporadic pandemics, with major public health and economic consequences. A better understanding of virus biology is therefore important. The primary control measure is vaccination, which for humans mostly relies on antigens produced in eggs from PR8-based viruses bearing the glycoprotein genes of interest. However, not all reassortants replicate well enough to supply sufficient virus antigen for demand. The significance of our research lies in identifying that mutation of the PA-X gene in the PR8 strain of virus can improve antigen yield, potentially by decreasing the pathogenicity of the virus in embryonated eggs.

The PA-X protein of influenza A virus has roles in host cell shutoff and viral pathogenesis. While most strains are predicted to encode PA-X, strain-dependent variations in activity have been noted. We found that PA-X protein from the A/PR/8/34 (PR8) strain had significantly lower repressive activity against cellular gene expression than PA-X proteins from the avian strains A/turkey/England/50-92/91 (H5N1) (T/E) and A/chicken/Rostock/34 (H7N1). Loss of normal PA-X expression, either by mutation of the frameshift site or by truncating the X open reading frame (ORF), had little effect on the infectious virus titer of PR8 or PR8 7:1 reassortants with T/E segment 3 grown in embryonated hens’ eggs. However, in both virus backgrounds, mutation of PA-X led to decreased embryo mortality and lower overall pathology, effects that were more pronounced in the PR8 strain than in the T/E reassortant, despite the low shutoff activity of the PR8 PA-X. Purified PA-X mutant virus particles displayed an increased ratio of hemagglutinin (HA) to nucleoprotein (NP) and M1 compared to values for their wild-type (WT) counterparts, suggesting altered virion composition. When the PA-X gene was mutated in the background of poorly growing PR8 6:2 vaccine reassortant analogues containing the HA and neuraminidase (NA) segments from H1N1 2009 pandemic viruses or from an avian H7N3 strain, HA yield increased up to 2-fold. This suggests that the PR8 PA-X protein may harbor a function unrelated to host cell shutoff and that disruption of the PA-X gene has the potential to improve the HA yield of vaccine viruses.

IMPORTANCE Influenza A virus is a widespread pathogen that affects both humans and a variety of animal species, causing regular epidemics and sporadic pandemics, with major public health and economic consequences. A better understanding of virus biology is therefore important. The primary control measure is vaccination, which for humans mostly relies on antigens produced in eggs from PR8-based viruses bearing the glycoprotein genes of interest. However, not all reassortants replicate well enough to supply sufficient virus antigen for demand. The significance of our research lies in identifying that mutation of the PA-X gene in the PR8 strain of virus can improve antigen yield, potentially by decreasing the pathogenicity of the virus in embryonated eggs.

Diversity of Functionally Permissive Sequences in the Receptor-Binding Site of Influenza Hemagglutinin

Influenza A virus hemagglutinin (HA) initiates viral entry by engaging host receptor sialylated glycans via its receptor-binding site (RBS). The amino acid sequence of the RBS naturally varies across avian and human influenza virus subtypes and is also evolvable. However, functional sequence diversity in the RBS has not been fully explored. Here, we performed a large-scale mutational analysis of the RBS of A/WSN/33 (H1N1) and A/Hong Kong/1/1968 (H3N2) HAs. Many replication-competent mutants not yet observed in nature were identified, including some that could escape from an RBS-targeted broadly neutralizing antibody. This functional sequence diversity is made possible by pervasive epistasis in the RBS 220-loop and can be buffered by avidity in viral receptor binding. Overall, our study reveals that the HA RBS can accommodate a much greater range of sequence diversity than previously thought, which has significant implications for the complex evolutionary interrelationships between receptor specificity and immune escape.

Manipulation of neuraminidase packaging signals and hemagglutinin residues improves the growth of A/Anhui/1/2013 (H7N9) influenza vaccine virus yield in eggs

In 2013, a novel avian-origin H7N9 influenza A virus causing severe lower respiratory tract disease in humans emerged in China, with continued sporadic cases. An effective vaccine is needed for this virus in case it acquires transmissibility among humans; however, PR8-based A/Anhui/1/2013 (Anhui/1, H7N9), a WHO-recommended H7N9 candidate vaccine virus (CVV) for vaccine production, does not replicate well in chicken eggs, posing an obstacle to egg-based vaccine production. To address this issue, we explored the possibility that PR8's hemagglutinin (HA) and neuraminidase (NA) packaging signals mediate improvement of Anhui/1 CVV yield in eggs. We constructed chimeric HA and NA genes having the coding region of Anhui/1 HA and NA flanked by the 5' and 3' packaging signals of PR8's HA and NA, respectively. The growth of CVVs containing the chimeric HA was not affected, but that of those containing the chimeric NA gene grew in embryonated chicken eggs with a more than 2-fold higher titer than that of WT CVV. Upon 6 passages in eggs further yield increase was achieved although this was not associated with any changes in the chimeric NA gene. The HA of the passaged CVV, did, however, exhibit egg-adaptive mutations and one of them (HA-G218E) improved CVV growth in eggs without significantly changing antigenicity. The HA-G218E substitution and a chimeric NA, thus, combine to provide an Anhui/1 CVV with properties more favorable for vaccine manufacture.

The PB1 segment of an influenza A virus H1N1 2009pdm isolate enhances the replication efficiency of specific influenza vaccine strains in cell culture and embryonated eggs

Influenza vaccine strains (IVSs) contain the haemagglutinin (HA) and neuraminidase (NA) genome segments of relevant circulating strains in the genetic background of influenza A/PR/8/1934 virus (PR8). Previous work has shown that the nature of the PB1 segment may be a limiting factor for the efficient production of IVSs. Here, we showed that the PB1 segment (PB1Gi) from the 2009 pandemic influenza A virus (IAV) A/Giessen/06/2009 (Gi wt, H1N1pdm) may help to resolve (some of) these limitations. We produced a set of recombinant PR8-derived viruses that contained (i) the HA and NA segments from representative IAV strains (H3N2, H5N1, H7N9, H9N2); (ii) the PB1 segment from PR8 or Gi wt, respectively; and (iii) the remaining five genome segments from PR8. Viruses containing the PB1Gi segment, together with the heterologous HA/NA segments and five PR8 segments (5+2+1), replicated to higher titres compared with their 6+2 counterparts containing six PR8 segments and the equivalent heterologous HA/NA segments. Compared with PB1PR8-containing IVSs, viruses with the PB1Gi segment replicated to higher or similar titres in both cell culture and embryonated eggs, most profoundly IVSs of the H5N1 and H7N9 subtype, which are known to grow poorly in these systems. IVSs containing either the PB1Gi or the cognate PB1 segment of the respective specific HA/NA donor strain showed enhanced or similar virus replication levels. This study suggests that substitution of PB1PR8 with the PB1Gi segment may greatly improve the large-scale production of PR8-derived IVSs, especially of those known to replicate poorly in vitro.

Development of high-yield influenza A virus vaccine viruses

Vaccination is one of the most cost-effective ways to prevent infection. Influenza vaccines propagated in cultured cells are approved for use in humans, but their yields are often suboptimal. Here, we screened A/Puerto Rico/8/34 (PR8) virus mutant libraries to develop vaccine backbones (defined here as the six viral RNA segments not encoding haemagglutinin and neuraminidase) that support high yield in cell culture. We also tested mutations in the coding and regulatory regions of the virus, and chimeric haemagglutinin and neuraminidase genes. A combination of high-yield mutations from these screens led to a PR8 backbone that improved the titres of H1N1, H3N2, H5N1 and H7N9 vaccine viruses in African green monkey kidney and Madin–Darby canine kidney cells. This PR8 backbone also improves titres in embryonated chicken eggs, a common propagation system for influenza viruses. This PR8 vaccine backbone thus represents an advance in seasonal and pandemic influenza vaccine development.

The availability of high-yield virus strains remains an important bottleneck in the rapid production of influenza vaccines. Here, the authors report the development of influenza A vaccine backbone that improves the virus yield of various seasonal and pandemic influenza vaccine strains in cell culture.

Optimization of influenza A vaccine virus by reverse genetic using chimeric HA and NA genes with an extended PR8 backbone

The yield of influenza antigen production may significantly vary between vaccine strains; for example the A/California/07/09 (H1N1)-X179A vaccine virus, prepared during 2009 influenza pandemic, presented a low antigen yield in eggs compared to other seasonal H1N1 reassortants. In this study a bi-chimeric virus expressing HA and NA genes with A/Puerto Rico/8/34 (H1N1) (PR8) and X179A domains was rescued by reverse genetics using a mixture of Vero/CHOK1 cell lines (Medina et al. [7]). The bi-chimeric virus obtained demonstrated to yield much larger amounts of HA than X179A in eggs as measured by single-radial-immunodiffusion (SRID), the reference method to quantify HA protein in influenza vaccine. Such kind of optimized virus using PR8 backbone derived chimeric glycoproteins could be used as improved seed viruses for vaccine production.

Identification of Influenza A/PR/8/34 Donor Viruses Imparting High Hemagglutinin Yields to Candidate Vaccine Viruses in Eggs

One of the important lessons learned from the 2009 H1N1 pandemic is that a high yield influenza vaccine virus is essential for efficient and timely production of pandemic vaccines in eggs. The current seasonal and pre-pandemic vaccine viruses are generated either by classical reassortment or reverse genetics. Both approaches utilize a high growth virus, generally A/Puerto Rico/8/1934 (PR8), as the donor of all or most of the internal genes, and the wild type virus recommended for inclusion in the vaccine to contribute the hemagglutinin (HA) and neuraminidase (NA) genes encoding the surface glycoproteins. As a result of extensive adaptation through sequential egg passaging, PR8 viruses with different gene sequences and high growth properties have been selected at different laboratories in past decades. The effect of these related but distinct internal PR8 genes on the growth of vaccine viruses in eggs has not been examined previously. Here, we use reverse genetics to analyze systematically the growth and HA antigen yield of reassortant viruses with 3 different PR8 backbones. A panel of 9 different HA/NA gene pairs in combination with each of the 3 different lineages of PR8 internal genes (27 reassortant viruses) was generated to evaluate their performance. Virus and HA yield assays showed that the PR8 internal genes influence HA yields in most subtypes. Although no single PR8 internal gene set outperformed the others in all candidate vaccine viruses, a combination of specific PR8 backbone with individual HA/NA pairs demonstrated improved HA yield and consequently the speed of vaccine production. These findings may be important both for production of seasonal vaccines and for a rapid global vaccine response during a pandemic.

Error-prone pcr-based mutagenesis strategy for rapidly generating high-yield influenza vaccine candidates

Vaccination is the primary strategy for the prevention and control of influenza outbreaks. However, the manufacture of influenza vaccine requires a high-yield seed strain, and the conventional methods for generating such strains are time consuming. In this study, we developed a novel method to rapidly generate high-yield candidate vaccine strains by integrating error-prone PCR, site-directed mutagenesis strategies, and reverse genetics. We used this method to generate seed strains for the influenza A(H1N1)pdm09 virus and produced six high-yield candidate strains. We used a mouse model to assess the efficacy of two of the six candidate strains as a vaccine seed virus: both strains provided complete protection in mice against lethal challenge, thus validating our method. Results confirmed that the efficacy of these candidate vaccine seed strains was not affected by the yield-optimization procedure.

Efficient generation of recombinant influenza A viruses employing a new approach to overcome the genetic instability of HA segments

Influenza A viruses (IAVs) are the most relevant and continual source of severe infectious respiratory complications in humans and different animal species, especially poultry. Therefore, an efficient vaccination that elicits protective and neutralizing antibodies against the viral hemagglutinin (HA) and neuraminidase (NA) is an important strategy to counter annual epidemics or occasional pandemics. With the help of plasmid-based reverse genetics technology, it is possible that IAV vaccine strains (IVVS) are rapidly generated. However, the genetic instability of some cloned HA-cDNAs after transformation into competent bacteria represents a major obstacle. Herein, we report efficient cloning strategies of different genetically volatile HA segments (H5- and H9-subtypes) employing either a newly constructed vector for reverse genetics (pMKPccdB) or by the use of the Escherichia coli strain HB101. Both approaches represent improved and generalizable strategies to establish functional reverse genetics systems preventing genetic changes to the cloned (HA) segments of IAV facilitating more efficient rescue of recombinant IAV for basic research and vaccine development.

Kinetics of pulmonary immune cells, antibody responses and their correlations with the viral clearance of influenza A fatal infection in mice

Fatal influenza A virus infection is a major threat to public health throughout the world. Lung macrophages and neutrophils have critical roles for both the pathogenesis and viral clearance of fatal viral infections. These are complicated by the interaction of innate immunity and adaptive immunity against viral infection. In this study, we investigated the overall kinetics of lung macrophages, neutrophils, CD4⁺T cells, CD8⁺T cells, CD38⁺ cells, and CD138⁺ cells, the levels of antibody and cytokine responses, both in the early and late phases of fatal infection with A/PR/8/34 (H1N1) virus in mice. The changes in lung viral load were also evaluated. We found that pulmonary macrophages and neutrophils both accumulated in the early and late phases of fatal infections and they positively correlated with the lung and serum antibody titers, and negatively correlated with the viral load locally. The secretion of IL-6 might relate to high numbers of macrophages and neutrophils in the early infection. The work implies that pulmonary macrophages, neutrophils and the antibody response all have an essential role in virus elimination of fatal influenza A viral infection. These findings may have implications for the development of prophylactic and therapeutic strategies in fatal influenza A viral infection. Further evaluation of the cooperation among macrophages, neutrophils and antibody responses in eliminating the virus with fatal infection is needed.

Improving influenza virus backbones by including terminal regions of MDCK-adapted strains on hemagglutinin and neuraminidase gene segments

Reverse genetics approaches can simplify and accelerate the process of vaccine manufacturing by combining the desired genome segments encoding the surface glycoproteins from influenza strains with genome segments (backbone segments) encoding internal and non-structural proteins from high-growth strains. We have developed three optimized high-growth backbones for use in producing vaccine seed viruses for group A influenza strains. Here we show that we can further enhance the productivity of our three optimized backbones by using chimeric hemagglutinin (HA) and neuraminidase (NA) genome segments containing terminal regions (non-coding regions (NCRs) and coding regions for the signal peptide (SP), transmembrane domain (TMD), and cytoplasmic tail (CT)) from two MDCK-adapted high growth strains (PR8x and Hes) and the sequences encoding the ectodomains of the A/Brisbane/10/2010 (H1N1) HA and NA proteins. Viruses in which both the HA and NA genome segments had the high-growth terminal regions produced higher HA yields than viruses that contained one WT and one chimeric HA or NA genome segment. Studies on our best-performing backbone indicated that the increases in HA yield were also reflected in an increase in HA content in partially purified preparations. Our results show that the use of chimeric HA and NA segments with high-growth backbones is a viable strategy that could improve influenza vaccine manufacturing. Possible mechanisms for the enhancement of HA yield are discussed.

Effect of natural and chimeric haemagglutinin genes on influenza A virus replication in baby hamster kidney cells

Baby hamster kidney (BHK21) cells are used to produce vaccines against various viral veterinary diseases, including rabies and foot-and-mouth-disease. Although particular influenza virus strains replicate efficiently in BHK21 cells the general use of these cells for influenza vaccine production is prohibited by the poor replication of most strains, including model strain A/PR/8/34 [H1N1] (PR8). We now show that in contrast to PR8, the related strain A/WSN/33 [H1N1] (WSN) replicates efficiently in BHK21 cells. This difference is determined by the haemagglutinin (HA) protein since reciprocal reassortant viruses with swapped HAs behave similarly with respect to growth on BHK21 cells as the parental virus from which their HA gene is derived. The ability or inability of six other influenza virus strains to grow on BHK21 cells appears to be similarly dependent on the nature of the HA gene since reassortant PR8 viruses containing the HA of these strains grow to similar titres as the parental virus from which the HA gene was derived. However, the growth to low titres of a seventh influenza strain was not due to the nature of the HA gene since a reassortant PR8 virus containing this HA grew efficiently on BHK21 cells. Taken together, these results suggest that the HA gene often primarily determines influenza replication efficiency on BHK21 cells but that in some strains other genes are also involved. High virus titres could be obtained with reassortant PR8 strains that contained a chimeric HA consisting of the HA1 domain of PR8 and the HA2 domain of WSN. HA1 contains most antigenic sites and is therefore important for vaccine efficacy. This method of producing the HA1 domain as fusion to a heterologous HA2 domain could possibly also be used for the production of HA1 domains of other viruses to enable the use of BHK21 cells as a generic platform for veterinary influenza vaccine production.

Permissible variation in the 3' non-coding region of the haemagglutinin genome segment of the H5N1 candidate influenza vaccine virus NIBRG-14 [corrected]

The candidate H5N1 vaccine virus NIBRG-14 was created in response to a call from the World Health Organisation in 2004 to prepare candidate vaccine viruses (CVVs) to combat the threat of an H5N1 pandemic. NIBRG-14 was created by reverse genetics and is composed of the neuraminidase (NA) and modified haemagglutinin (HA) genes from A/Vietnam/1194/2004 and the internal genes of PR8, a high growing laboratory adapted influenza A(H1N1) strain. Due to time constraints, the non-coding regions (NCRs) of A/Vietnam/1194/2004 HA were not determined prior to creating NIBRG-14. Consequently, the sequence of the primers used to clone the modified A/Vietnam/1194/2004 HA was based upon previous experience of cloning H5N1 viruses. We report here that the HA 3′ NCR sequence of NIBRG-14 is different to that of the parental wild type virus A/Vietnam/1194/2004; however this does not appear to impact on its growth or antigen yield. We introduced additional small changes into the 3′NCR of NIBRG-14; these had only minor effects on viral growth and antigen content. These findings may serve to assure the influenza vaccine community that generation of CVVs using best-guess NCR sequences, based on sequence alignments, are likely to produce robust viruses.

Prospects for controlling future pandemics of influenza

Pandemic influenza remains one of the most serious threats to global public health and continued global vigilance to monitor emerging threats is crucial. Of the weapons available to control a pandemic, vaccination is potentially the most powerful, but there are currently serious limitations to timely availability of vaccine supply in an emergency. Many novel influenza vaccines are in development, some of which have the potential to deliver the massive quantities of vaccine that would be required in a pandemic in a short period of time. However, for the foreseeable future, it is likely that the principal vaccine that will be deployed in a pandemic will be an inactivated egg-derived vaccine of the kind that has been available for several decades. This review will focus on the practical hurdles that need to be surmounted to deliver large amounts of safe and effective pandemic vaccine to the general public. There needs to be a continued focus on improvement to the vaccine response system that will require close collaboration between influenza and vaccine experts, manufacturers, regulators and public health authorities around the world.