Selection of receptor-binding variants of human influenza A and B viruses in baby hamster kidney cells

Antigenic characterization of influenza and SARS-CoV-2 viruses

Antigenic characterization of emerging and re-emerging viruses is necessary for the prevention of and response to outbreaks, evaluation of infection mechanisms, understanding of virus evolution, and selection of strains for vaccine development. Primary analytic methods, including enzyme-linked immunosorbent/lectin assays, hemagglutination inhibition, neuraminidase inhibition, micro-neutralization assays, and antigenic cartography, have been widely used in the field of influenza research. These techniques have been improved upon over time for increased analytical capacity, and some have been mobilized for the rapid characterization of the SARS-CoV-2 virus as well as its variants, facilitating the development of highly effective vaccines within 1 year of the initially reported outbreak. While great strides have been made for evaluating the antigenic properties of these viruses, multiple challenges prevent efficient vaccine strain selection and accurate assessment. For influenza, these barriers include the requirement for a large virus quantity to perform the assays, more than what can typically be provided by the clinical samples alone, cell- or egg-adapted mutations that can cause antigenic mismatch between the vaccine strain and circulating viruses, and up to a 6-month duration of vaccine development after vaccine strain selection, which allows viruses to continue evolving with potential for antigenic drift and, thus, antigenic mismatch between the vaccine strain and the emerging epidemic strain. SARS-CoV-2 characterization has faced similar challenges with the additional barrier of the need for facilities with high biosafety levels due to its infectious nature. In this study, we review the primary analytic methods used for antigenic characterization of influenza and SARS-CoV-2 and discuss the barriers of these methods and current developments for addressing these challenges.

Characterization of Influenza Virus Binding to Receptors on Isolated Cell Membranes

An interplay between receptor-binding properties of influenza viruses (IVs) and spectrum of sialic acid-containing receptors on target cells in birds and mammals determine viral host range, tissue tropism, and pathogenicity. Here, we describe method that allows to characterize binding of IVs to biologically relevant cellular receptors using a conventional solid-phase enzyme-linked assay. In this method, we isolate plasma membranes from respiratory and intestinal epithelial cells of animal origin (Subheading 3.2). We adsorb the membranes in the wells of 96-well ELISA plates, incubate the membrane-coated wells with serially diluted IVs, and determine amounts of IVs attached to the membranes using viral ability to bind peroxidase-labeled sialoglycoprotein fetuin. Based on the concentration dependence of IV binding to the membrane, we estimate binding avidity and number of binding sites. We describe two variants of the assay in Subheadings 3.6 and 3.7 and provide examples.

Cell-Adapted Mutations and Antigenic Diversity of Influenza B Viruses in Missouri, 2019-2020 Season

Influenza B viruses (IBVs) are causing an increasing burden of morbidity and mortality, yet the prevalence of culture-adapted mutations in human seasonal IBVs are unclear. We collected 368 clinical samples from patients with influenza-like illness in Missouri during the 2019–2020 influenza season and recovered 146 influenza isolates including 38 IBV isolates. Of MDCK-CCL34, MDCK-Siat1, and humanized MDCK (hCK), hCK showed the highest virus recovery efficiency. All Missourian IBVs belonged to the Victoria V1A.3 lineage, all of which contained a three-amino acid deletion on the HA protein and were antigenically distant from the Victoria lineage IBV vaccine strain used during that season. By comparing genomic sequences of these IBVs in 31 paired samples, eight cell-adapted nonsynonymous mutations were identified, with the majority in the RNA polymerase. Analyses of IBV clinical sample–isolate pairs from public databases further showed that cell- and egg-adapted mutations occurred more widely in viral proteins, including the receptor and antibody binding sites on HA. Our study suggests that hCK is an effective platform for IBV isolation and that culture-adapted mutations may occur during IBV isolation. As culture-adapted mutations may affect subsequent virus studies and vaccine development, the knowledge from this study may help optimize strategies for influenza surveillance, vaccine strain selection, and vaccine development.

Different Aspects Concerning Viral Infection and the Role of MHC Molecules in Viral Prevention

Major Histocompatibility Complex (MHC) molecules play a crucial role in inducing an adaptive immune response. T-cell epitopes require compatible MHC molecules to form MHC-peptide Complexes (pMHC) that activate the T-cell Receptors (TCR) of T-lymphocyte clones. MHCs are polymorphic molecules with wide varieties of gene alleles. There are two classes of MHC molecules, class I and II. Both classes have three classical loci HLA-A, -B, and –C are present in class I and HLA-DP, -DQ, and -DR in class II. To induce a compatible T-lymphocyte clone, the T-cell epitope requires the association of the compatible MHC molecule to form pMHC. Each MHC variant possesses a different groove that is capable of binding a different range of antigenic epitopes. Without the compatible MHC molecule, a T cell clone cannot be activated by a particular viral epitope. With the aim of preventing viral transmission, the efficiency of a viral vaccine is related to the existence of specific MHC alleles in the individual. This article proposes the roles of the MHC molecule to prevent viral infection. In addition, the association of the viral receptor molecule with the viral infection will also be discussed.

Immune Pressure on Polymorphous Influenza B Populations Results in Diverse Hemagglutinin Escape Mutants and Lineage Switching

Mutations arise in the genomes of progeny viruses during infection. Mutations that occur in epitopes targeted by host antibodies allow the progeny virus to escape the host adaptive, B-cell mediated antibody immune response. Major epitopes have been identified in influenza B virus (IBV) hemagglutinin (HA) protein. However, IBV strains maintain a seasonal presence in the human population and changes in IBV genomes in response to immune pressure are not well characterized. There are two lineages of IBV that have circulated in the human population since the 1980s, B-Victoria and B-Yamagata. It is hypothesized that early exposure to one influenza subtype leads to immunodominance. Subsequent seasonal vaccination or exposure to new subtypes may modify subsequent immune responses, which, in turn, results in selection of escape mutations in the viral genome. Here we show that while some mutations do occur in known epitopes suggesting antibody escape, many mutations occur in other parts of the HA protein. Analysis of mutations outside of the known epitopes revealed that these mutations occurred at the same amino acid position in viruses from each of the two IBV lineages. Interestingly, where the amino acid sequence differed between viruses from each lineage, reciprocal amino acid changes were observed. That is, the virus from the Yamagata lineage become more like the Victoria lineage virus and vice versa. Our results suggest that some IBV HA sequences are constrained to specific amino acid codons when viruses are cultured in the presence of antibodies. Some changes to the known antigenic regions may also be restricted in a lineage-dependent manner. Questions remain regarding the mechanisms underlying these results. The presence of amino acid residues that are constrained within the HA may provide a new target for universal vaccines for IBV.

Plasticity of Amino Acid Residue 145 Near the Receptor Binding Site of H3 Swine Influenza A Viruses and Its Impact on Receptor Binding and Antibody Recognition

The hemagglutinin (HA), a glycoprotein on the surface of influenza A virus (IAV), initiates the virus life cycle by binding to terminal sialic acid (SA) residues on host cells. The HA gradually accumulates amino acid substitutions that allow IAV to escape immunity through a mechanism known as antigenic drift. We recently confirmed that a small set of amino acid residues are largely responsible for driving antigenic drift in swine-origin H3 IAV. All identified residues are located adjacent to the HA receptor binding site (RBS), suggesting that substitutions associated with antigenic drift may also influence receptor binding. Among those substitutions, residue 145 was shown to be a major determinant of antigenic evolution. To determine whether there are functional constraints to substitutions near the RBS and their impact on receptor binding and antigenic properties, we carried out site-directed mutagenesis experiments at the single-amino-acid level. We generated a panel of viruses carrying substitutions at residue 145 representing all 20 amino acids. Despite limited amino acid usage in nature, most substitutions at residue 145 were well tolerated without having a major impact on virus replication in vitro All substitution mutants retained receptor binding specificity, but the substitutions frequently led to decreased receptor binding. Glycan microarray analysis showed that substitutions at residue 145 modulate binding to a broad range of glycans. Furthermore, antigenic characterization identified specific substitutions at residue 145 that altered antibody recognition. This work provides a better understanding of the functional effects of amino acid substitutions near the RBS and the interplay between receptor binding and antigenic drift.IMPORTANCE The complex and continuous antigenic evolution of IAVs remains a major hurdle for vaccine selection and effective vaccination. On the hemagglutinin (HA) of the H3N2 IAVs, the amino acid substitution N 145 K causes significant antigenic changes. We show that amino acid 145 displays remarkable amino acid plasticity in vitro, tolerating multiple amino acid substitutions, many of which have not yet been observed in nature. Mutant viruses carrying substitutions at residue 145 showed no major impairment in virus replication in the presence of lower receptor binding avidity. However, their antigenic characterization confirmed the impact of the 145 K substitution in antibody immunodominance. We provide a better understanding of the functional effects of amino acid substitutions implicated in antigenic drift and its consequences for receptor binding and antigenicity. The mutation analyses presented in this report represent a significant data set to aid and test the ability of computational approaches to predict binding of glycans and in antigenic cartography analyses.

Influenza passaging annotations: what they tell us and why we should listen

Influenza databases now contain over 100,000 worldwide sequence records for strains influenza A(H3N2) and A(H1N1). Although these data facilitate global research efforts and vaccine development practices, they also represent a stumbling block for researchers because of their confusing and heterogeneous annotation. Unclear passaging annotations are particularly concerning given the recent work highlighting the presence and risk of false adaptation signals introduced by cell passaging of viral isolates. With this in mind, we aim to provide a concise outline of why viruses are passaged, a clear overview of passaging annotation nomenclature currently in use, and suggestions for a standardized nomenclature going forward. Our hope is that this summary will empower researchers and clinicians alike to more easily understand a virus sample's passage history when analyzing influenza sequences.

MONOCLONAL ANTIBODIES TO HEMAGGLUTININ OF INFLUENZA B VIRUSES VICTORIA EVOLUTIONARY LINEAGE

Co-circulation of two evolutionary distinct lineages of influenza virus in one epidemic season has led to development specific reagents for rapid identification and typing of new isolates. Panel of MAbs to hemagglutinin of influenza virus B/Brisbane/46/15 belonging to Victoria evolutionary lineage was developed. All MAbs reacted in ELISA with B/Victoria-like strains only. There were no interactions with heterologous influenza viruses of B/Yamagata lineage, seasonal and potentially pandemic influenza A viruses. All MAbs reacted in hemagglutination inhibition and virus neutralization. MAbs interacted in hemagglutination inhibition only with B/Victoria-like viruses, but did not interacted B/Yamagata-like strains. Neutralization and hemagglutination inhibition studies of viruses isolated before 1983 with MAbs revealed that MAbs 6E11, 9G5, 9B5 and 6A4 had the ability to interact with the virus B/ Russia/69 which may evidence that B strains of early isolation period (before lineage separation) have common epitope with recent Victoria lineage viruses. MAbs 7C8, 7G9, 7H8 and 8D11 were directed to a conserved epitope (or epitopes) specific for influenza hemagglutinin viruses of B/Victoria group. The presence of differences in the effectiveness of the interaction of MAbs 6A9, 7G9 and 8A8 in hemagglutination inhibition test allows the identification and differentiation of strains isolated in chicken embryos and MDCK cell culture. Thus, the developed MAbs can be successfully used for identification and antigenic analysis of B/Victoria-like strains.

Dynamic Convergent Evolution Drives the Passage Adaptation across 48 Years' History of H3N2 Influenza Evolution

Influenza viruses are often propagated in a diverse set of culturing media and additional substitutions known as passage adaptation can cause extra evolution in the target strain, leading to ineffective vaccines. Using 25,482 H3N2 HA1 sequences curated from Global Initiative on Sharing All Influenza Data and National Center for Biotechnology Information databases, we found that passage adaptation is a very dynamic process that changes over time and evolves in a seesaw like pattern. After crossing the species boundary from bird to human in 1968, the influenza H3N2 virus evolves to be better adapted to the human environment and passaging them in embryonated eggs (i.e., an avian environment) leads to increasingly stronger positive selection. On the contrary, passage adaptation to the mammalian cell lines changes from positive selection to negative selection. Using two statistical tests, we identified 19 codon positions around the receptor binding domain strongly contributing to passage adaptation in the embryonated egg. These sites show strong convergent evolution and overlap extensively with positively selected sites identified in humans, suggesting that passage adaptation can confound many of the earlier studies on influenza evolution. Interestingly, passage adaptation in recent years seems to target a few codon positions in antigenic surface epitopes, which makes it difficult to produce antigenically unaltered vaccines using embryonic eggs. Our study outlines another interesting scenario whereby both convergent and adaptive evolution are working in synchrony driving viral adaptation. Future studies from sequence analysis to vaccine production need to take careful consideration of passage adaptation.

Efficient generation of influenza virus with a mouse RNA polymerase I-driven all-in-one plasmid

BACKGROUND

The current influenza vaccines are effective against seasonal influenza, but cannot be manufactured in a timely manner for a sudden pandemic or to be cost-effective to immunize huge flocks of birds. We propose a novel influenza vaccine composing a bacterial carrier and a plasmid cargo. In the immunized subjects, the bacterial carrier invades and releases its cargo into host cells where the plasmid expresses viral RNAs and proteins for reconstitution of attenuated influenza virus. Here we aimed to construct a mouse PolI-driven plasmid for efficient production of influenza virus.

RESULTS

A plasmid was constructed to express all influenza viral RNAs and proteins. This all-in-one plasmid resulted in 10(5)-10(6) 50% tissue culture infective dose (TCID50)/mL of influenza A virus in baby hamster kidney (BHK-21) cells on the third day post-transfection, and also reconstituted influenza virus in Madin-Darby canine kidney (MDCK) and Chinese hamster ovary (CHO) cells. A 6-unit plasmid was constructed by deleting the HA and NA cassettes from the all-in-one plasmid. Cotransfection of BHK-21 cells with the 6-unit plasmid and the two other plasmids encoding the HA or NA genes resulted in influenza virus titers similar to those produced by the 1-plasmid method.

CONCLUSIONS

An all-in-one plasmid and a 3-plasmid murine PolI-driven reverse genetics systems were developed, and efficiently reconstituted influenza virus in BHK-21 cells. The all-in-one plasmid may serve as a tool to determine the factors inhibiting virus generation from a large size plasmid. In addition, we recommend a simple and robust "1 + 2" approach to generate influenza vaccine seed virus.

The roles of hemagglutinin Phe-95 in receptor binding and pathogenicity of influenza B virus

Diverged ~4000 years ago, influenza B virus has several important differences from influenza A virus, including lower receptor-binding affinity and highly restricted host range. Based on our prior structural studies, we hypothesized that a single-residue difference in the receptor-binding site of hemagglutinin (HA), Phe-95 in influenza B virus versus Tyr-98 in influenza A/H1-H15, is possibly a key determinant for the low receptor-binding affinity. Here we demonstrate that the mutation Phe95→Tyr in influenza B virus HA restores all three hydrogen bonds made by Tyr-98 in influenza A/H1-15 HA and has the potential to enhance receptor binding. However, the full realization of this potential is influenced by the local environment into which the mutation is introduced. The binding and replication of the recombinant viruses correlate well with the receptor-binding capabilities of HA. These results are discussed in relation to the roles of Phe-95 in receptor binding and pathogenicity of influenza B virus.

Cell culture-selected substitutions in influenza A(H3N2) neuraminidase affect drug susceptibility assessment

Assessment of drug susceptibility has become an integral part of influenza virus surveillance. In this study, we describe the drug resistance profile of influenza A(H3N2) virus, A/Mississippi/05/2011, collected from a patient treated with oseltamivir and detected via surveillance. An MDCK cell-grown isolate of this virus exhibited highly reduced inhibition by the neuraminidase (NA) inhibitors (NAIs) oseltamivir (8,005-fold), zanamivir (813-fold), peramivir (116-fold), and laninamivir (257-fold) in the NA inhibition assay. Sequence analysis of its NA gene revealed a known oseltamivir-resistance marker, the glutamic acid-to-valine substitution at position 119 (E119V), and an additional change, threonine to isoleucine at position 148 (T148I). Unlike E119V, T148I was not detected in the clinical sample but acquired during viral propagation in MDCK cells. Using recombinant proteins, T148I by itself was shown to cause only a 6-fold increase in the zanamivir 50% inhibitory concentration (IC50) and had no effect on inhibition by other drugs. The T148I substitution reduced NA activity by 50%, most likely by affecting the positioning of the 150 loop at the NA catalytic site. Using pyrosequencing, changes at T148 were detected in 35 (23%) of 150 MDCK cell-grown A(H3N2) viruses tested, which was lower than the frequency of changes at D151 (85%), an NA residue previously implicated in cell selection. We demonstrate that culturing of the A(H3N2) viruses (n = 11) at a low multiplicity of infection delayed the emergence of the NA variants with changes at position 148 and/or 151, especially when conducted in MDCK-SIAT1 cells. Our findings highlight the current challenges in monitoring susceptibility of influenza A(H3N2) viruses to the NAI class of antiviral drugs.

Heterogeneity of the MDCK cell line and its applicability for influenza virus research

Single-cell clones have been established from the MDCK cell line, characterized for their morphology and evaluated for their suitability for influenza virus research. Three discrete cell morphotypes were identified using light microscopy. Besides morphological features, the cell types can be distinguished by the level of expression of surface glycans recognized by peanut agglutinin (PNA). All clones were susceptible to infection by influenza viruses of different subtypes of influenza A virus (H1N1, H1N1pdm09, H3N2, H5N1) and influenza B virus, and all possessed on their surface terminally sialylated glycans with both types of glycosidic linkage (α2-3 and α2-6). The Type-1 cell lines were able to support a multicycle replication of influenza A and B viruses without help of an exogenous trypsin. In contrast, cell lines exhibiting Type-2 morphology were unable to support multicycle replication of influenza A viruses without trypsin supplementation. Western blot analysis of the hemagglutinin of H1N1 strains demonstrated that Type-2 cells were deficient in production of proteolytically activated hemagglutinin (no cleavage between HA1/HA2 was observed). HA1/HA2 cleavage of influenza B viruses in the Type-2 cells was also significantly impaired, but not completely abrogated, producing sufficient amount of activated HA to support efficient virus replication without trypsin. In contrast, all clones of Type-1 cells were able to produce proteolytically activated hemagglutinin of influenza A and B viruses. However, the growth kinetics and plaque size of influenza A viruses varied significantly in different clones. Influenza B virus also showed different plaque size, with the biggest plaque formation in the Type-2 cells, although the growth kinetics and peak infectivity titers were similar in all clones. Taken together, the study demonstrates that the population of original MDCK cells is represented by various types of cells that differ in their capacities to support replication of influenza A and B viruses.

Reduced susceptibility to all neuraminidase inhibitors of influenza H1N1 viruses with haemagglutinin mutations and mutations in non-conserved residues of the neuraminidase

OBJECTIVES

We characterized human H1N1 influenza isolate A/Hokkaido/15/02, which has haemagglutinin and neuraminidase mutations that reduce drug susceptibility to oseltamivir, zanamivir and peramivir.

METHODS

One wild-type and three mutant viruses were isolated by plaque purification. Viruses were tested in MUNANA-based enzyme assays, cell culture and receptor binding assays.

RESULTS

Two viruses had a neuraminidase Y155H mutation that reduced susceptibility in the enzyme inhibition assay to all inhibitors by 30-fold to >100-fold. The Y155H mutation reduced plaque size and affected the stability, Km and pH activity profile of the enzyme. In contrast to previous mutants, this neuraminidase demonstrated a slower rate of inhibitor binding in the IC50 kinetics assay. One virus had both the Y155H mutation and a haemagglutinin D225G mutation that rescued the small-plaque phenotype of the Y155H virus and affected receptor binding and drug susceptibility in cell culture and binding assays. We also isolated a third mutant virus, with both neuraminidase V114I and haemagglutinin D225N mutations, which affected susceptibility in the enzyme inhibition assay and receptor binding, respectively, but to lesser extents than the Y155H and D225G mutations.

CONCLUSIONS

Neither Y155 nor V114 is conserved across neuraminidase subtypes. Furthermore, Y155 is not conserved even among avian and swine N1 viruses. Structurally, both residues reside far from the neuraminidase active site. D225 forms part of the receptor binding site of the haemagglutinin. We believe this is the first demonstration of a specific haemagglutinin mutation correlating with reduced drug susceptibility in plaque assays in both Madin Darby Canine Kidney and SIAT cells.

Comparative effectiveness of isolation techniques for contemporary Influenza A virus strains circulating in exhibition swine

The current study sought to compare the effectiveness of 2 virus isolation methods for the recovery of contemporary Influenza A virus (FLUAV) strains circulating in swine at agricultural exhibitions. Following the emergence of the influenza A (H1N1)pdm09 virus, increased surveillance of FLUAV strains among swine was recommended for early detection of emerging strains that threaten animal and human health. The increase in genetic drift and genomic reassortment among FLUAV strains infecting swine since 1998 necessitates that detection protocols be periodically validated for contemporary FLUAV strains. During 2009, nasal swabs were collected from 221 clinically healthy pigs at 12 agricultural exhibitions in Ohio. Nasal swabs were tested in parallel for the presence of FLUAV strains using 3 methodologies: 2 passages through Madin-Darby canine kidney (MDCK) cells adapted to serum-free medium (SFM), 2 passages through embryonated chicken eggs (ECEs), and real-time reverse transcription polymerase chain reaction (real-time RT-PCR). Of the 221 samples, 40 (18.1%) were positive for FLUAV recovery in MDCK cell culture and 13 (5.9%) were positive in ECEs (P = 0.015). All samples positive in ECEs were also positive in MDCK cell culture. MDCK cell culture virus isolation results were in perfect agreement with results of the real-time RT-PCR. Hemagglutinin and neuraminidase combinations of the recovered isolates were H1N2 and H3N2, which were consistent with FLUAV strains circulating in U.S. pigs. Effectiveness and cost savings justify the use of SFM-adapted MDCK cell culture over ECEs for the recovery of contemporary FLUAV strains from exhibition swine.

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.

Characterization of a porcine intestinal epithelial cell line for influenza virus production

We have developed a porcine intestine epithelial cell line, designated SD-PJEC for the propagation of influenza viruses. The SD-PJEC cell line is a subclone of the IPEC-J2 cell line, which was originally derived from newborn piglet jejunum. Our results demonstrate that SD-PJEC is a cell line of epithelial origin that preferentially expresses receptors of oligosaccharides with Sia2-6Gal modification. This cell line is permissive to infection with human and swine influenza A viruses and some avian influenza viruses, but poorly support the growth of human-origin influenza B viruses. Propagation of swine-origin influenza viruses in these cells results in a rapid growth rate within the first 24 h post-infection and the titres ranged from 4 to 8 log(10) TCID(50) ml(-1). The SD-PJEC cell line was further tested as a potential alternative cell line to Madin-Darby canine kidney (MDCK) cells in conjunction with 293T cells for rescue of swine-origin influenza viruses using the reverse genetics system. The recombinant viruses A/swine/North Carolina/18161/02 (H1N1) and A/swine/Texas/4199-2/98 (H3N2) were rescued with virus titres of 7 and 8.25 log(10) TCID(50) ml(-1), respectively. The availability of this swine-specific cell line represents a more relevant substrate for studies and growth of swine-origin influenza viruses.

Neuraminidase H275Y and hemagglutinin D222G mutations in a fatal case of 2009 pandemic influenza A (H1N1) virus infection

Oseltamivir-resistant 2009 H1N1 influenza virus infections associated with neuraminidase (NA) H275Y have been identified sporadically. Strains possessing the hemagglutinin (HA) D222G mutation have been detected in small numbers of fatal 2009 H1N1 cases. We report the first clinical description of 2009 H1N1 virus infection with both NA-H275Y and HA-D222G mutations detected by pyrosequencing of bronchioalveolar lavage fluid obtained on symptom day 19. The 59-year-old immunosuppressed patient had multiple conditions conferring higher risk of prolonged viral replication and severe illness and died on symptom day 34. Further investigations are needed to determine the significance of infection with strains possessing NA-H275Y and HA-D222G.

Virus susceptibility of Chinese hamster ovary (CHO) cells and detection of viral contaminations by adventitious agent testing

Biopharmaceuticals are of increasing importance in the treatment of a variety of diseases. A remaining concern associated with their production is the potential introduction of adventitious agents into their manufacturing process, which may compromise the pathogen safety of a product and potentially cause stock-out situations for important medical supplies. To ensure the safety of biological therapeutics, regulatory guidance requires adventitious agent testing (AAT) of the bulk harvest. AAT is a deliberately promiscuous assay procedure which has been developed to indicate, ideally, the presence of any viral contaminant. One of the most important cell lines used in the production of biopharmaceuticals is Chinese hamster ovary (CHO) cells and while viral infections of CHO cells have occurred, a systematic screen of their virus susceptibility has never been published. We investigated the susceptibility of CHO cells to infection by 14 different viruses, including members of 12 families and representatives or the very species that were implicated in previously reported production cell infections. Based on our results, four different infection outcomes were distinguished, based on the possible combinations of the two factors (i) the induction, or not, of a cytopathic effect and (ii) the ability, or not, to replicate in CHO cells. Our results demonstrate that the current AAT is effective for the detection of viruses which are able to replicate in CHO cells. Due to the restricted virus susceptibility of CHO cells and the routine AAT of bulk harvests, our results provide re-assurance for the very high safety margins of CHO cell-derived biopharmaceuticals.

Scalable production of influenza virus in HEK-293 cells for efficient vaccine manufacturing

Cell culture processes offer an attractive alternative to conventional chicken egg-based influenza vaccine production methods. However, most protocols still rely on the use of adherent cells, which makes process scale-up a challenging issue. In this study, it is demonstrated that the HEK-293 human cell line is able to efficiently replicate influenza virus. Production in serum-free suspension of HEK-293 cultures resulted in high titers of infectious influenza viruses for different subtypes and variants including A/H1, A/H3 and B strains. After virus adaptation and optimization of infection conditions, production in 3-L bioreactor resulted in titers of up to 10(9)IVP/mL demonstrating the scale-up potential of the process.

Changes of the receptor-binding properties of influenza B virus B/Victoria/504/2000 during adaptation in chicken eggs

Selection of high-growth virus variants of strain B/Victoria/504/2000 by serial passage in eggs resulted in three amino acid substitutions, G141E, R162M, and D196Y, in the vicinity of the receptor-binding pocket of viral hemagglutinin. Virus variants containing the identified amino acid substitutions, individually or in various combinations, were constructed using reverse genetics and analyzed for their receptor-binding properties using glycan microarray platform. Three different patterns of virus binding were revealed. A low-growth virus variant, corresponding to the original egg-derived virus B/Victoria/504/2000 prior to acquisition of amino acid changes G141E, R162M, and D196Y, had a clear preference for the oligosaccharide chains terminated with alpha2-6-linked sialic acid with very weak binding of the glycans terminated with alpha2-3-linked sialic acid. Amino acid substitutions R162M and D196Y had similar effects, resulting in viruses that bound with high efficiency almost all terminally sialylated glycans represented on the array regardless of the type of glycosidic linkage. In contrast, substitution of G141E alone, or in combinations with the other two amino acid substitutions, significantly restricted virus glycan-binding capabilities. All virus variants possessing this substitution lost the ability to bind glycans with alpha2-6 glycosidic linkage as well as most of the glycans with alpha2-3 glycosidic linkage. Linear penta- and heptasaccharide chains represented at the non-reducing end by alpha2-3 sialylated Type-II motif (LacNAc) were the only structures bound with high affinity by the virus variants with G141E substitution. In all cases when the effects on virus binding of individual amino acid substitutions differed, the effect of R162M was subordinate to the effect of either G141E or D196Y.

Structural and evolutionary characteristics of HA, NA, NS and M genes of clinical influenza A/H3N2 viruses passaged in human and canine cells

BACKGROUND

Canine (MDCK) cells and chicken eggs are usually used for isolation of human influenza viruses. Viruses isolated by these procedures often differ from those present in the clinical specimens, since adaptive changes occur during virus transmission from the human host to cells of heterologous origin.

OBJECTIVES

To minimize these species-dependent changes, CACO-2 cells derived from human intestinal epithelium were used to isolate virus from influenza patients.

STUDY DESIGN

Influenza A viruses of subtype H3N2 were primarily isolated in CACO-2 and then passaged in parallel in CACO-2 and MDCK cells. Structural properties of passaged virus variants were compared and analyzed for evolutionary relationships.

RESULTS

Influenza viruses were isolated in CACO-2 with higher efficiency than in MDCK and chicken eggs. The following observations were made: (i) recent isolates showed an about 2-fold increase in the number of glycosylation sites of HA and NA when compared to isolates from 1968 to 1970; (ii) during passages of clinical strains in CACO-2 and MDCK cells HA and NA mutated cooperatively with strain-specific variations implying that functioning of the HA-NA complex varied from strain to strain in one influenza outbreak; (iii) there were no amino acid exchanges in the HA receptor binding site although the viruses acquired the ability to agglutinate avian erythrocytes after passage in MDCK cells, suggesting that virus adsorption is regulated by several factors; (iv) quasispecies characterized by deletion of 66 nucleotides (22 amino acids) in the stalk region of the NA gene was dominant in naso-pharyngeal washes of all patients whereas during passaging in CACO-2 cells this deleted genotype in isolates from different patients was either stably retained as prevalent quasispecies or rapidly replaced for that one containing full length NA gene; (v) the M2 protein of clinical viruses was sensitive to amantadine; (vi) the NS segment of human viruses, unlike the most of avian ones, contained an additional positive-sense open reading frame encoding a hypothetical 25kD polypeptide (negative strand protein, NSP).

CONCLUSIONS

The data suggest that (i) clinical influenza viruses can be isolated from respiratory tract of humans more effectively in human than in canine cells; (ii) heterologous virus population circulates during one influenza outbreak; (iii) increasing numbers of glycosylation sites on HA and NA and stalk shortening of NA take place during virus evolution in humans.

Sialic acid recognition is a key determinant of influenza A virus tropism in murine trachea epithelial cell cultures

Influenza A virus interacts with specific types of sialic acid during attachment and entry into susceptible cells. The precise amino acids in the hemagglutinin protein that control sialic acid binding specificity and affinity vary among antigenic subtypes. For H3 subtypes, amino acids 226 and 228 are critical for differentiating between alpha2,3- and alpha2,6-linked forms of sialic acid (SA). We demonstrate that position 190 of the HA from A/Udorn/307/72 (H3N2) plays an important role in the recognition of alpha2,3-SA, as changing the residue from a glutamic acid to an aspartic acid led to alteration of red blood cell hemagglutination and a complete loss of replication in differentiated, murine trachea epithelial cell cultures which express only alpha2,3-SA. This amino acid change had a minimal effect on virus replication in MDCK cells, suggesting subtle changes in receptor recognition by the H3 hemagglutinin can lead to significant alterations in cell and species tropism.

Increased sensitivity for detecting avian influenza-specific antibodies by a modified hemagglutination inhibition assay using horse erythrocytes

The hemagglutination inhibition (HI) assay is a widely used serological method to measure the levels of protective antibody responses against influenza viruses. However, the traditional HI assay which uses chicken erythrocytes is not sufficiently sensitive for detecting HI antibodies specific to avian influenza viruses. Previously, it was demonstrated that employing an assay using horse erythrocytes was able to increase the sensitivity of HI assay. The current report describes further optimization of this modified HI assay. It was shown that this method was able to increase detection of HI activities in rabbit sera immunized with H5 HA antigens, and proved that this increased sensitivity is useful in dissecting the strain specificity of HI antibody responses. In addition, the modified HI assay using horse erythrocytes increased the sensitivity of detecting HI antibodies specific for three major serotypes of avian influenza viruses, H5, H7 and H9, in people who may have asymptomatic infection with avian influenza viruses. Based on these results, the optimized use of horse erythrocytes should be standard practice for detecting HI activities against avian influenza viruses.

MDCK-SIAT1 cells show improved isolation rates for recent human influenza viruses compared to conventional MDCK cells

The ability to isolate and propagate influenza virus is an essential tool for the yearly surveillance of circulating virus strains and to ensure accurate clinical diagnosis for appropriate treatment. The suitability of MDCK-SIAT1 cells, engineered to express increased levels of alpha-2,6-linked sialic acid receptors, as an alternative to conventional MDCK cells for isolation of circulating influenza virus was assessed. A greater number of influenza A (H1N1 and H3N2) and B viruses from stored human clinical specimens collected between 2005 and 2007 were isolated following inoculation in MDCK-SIAT1 cells than in MDCK cells. In addition, a higher titer of virus was recovered following culture in MDCK-SIAT1 cells. All A(H1N1) viruses recovered from MDCK-SIAT1 cells were able to agglutinate both turkey and guinea pig red blood cells (RBC), while half of the A(H3N2) viruses recovered after passage in MDCK-SIAT1 cells lost the ability to agglutinate turkey RBC. Importantly, the HA-1 domain of the hemagglutinin gene was genetically stable after passaging in MDCK-SIAT1 cells, a feature not always seen following MDCK cell or embryonated chicken egg passage of human influenza virus. These data indicate that the MDCK-SIAT1 cell line is superior to conventional MDCK cells for isolation of human influenza virus from clinical specimens and may be used routinely for the isolation and propagation of current human influenza viruses for surveillance, diagnostic, and research purposes.

Generation of the influenza B viruses with improved growth phenotype by substitution of specific amino acids of hemagglutinin

Variability in growth characteristics of influenza B viruses remains a serious limitation in the manufacture of inactivated influenza vaccines. Currently, serial passage in eggs is the strategy used in most instances for selection of high growth virus variants. In previous studies we found that adaptation of the strain B/Victoria/504/2000 to high growth in eggs was associated with changes only in hemagglutinin (HA). The high growth phenotype was associated with acquisition of either two (R162M and D196Y) or three (G141E, R162M and D196Y) amino acid (AA) substitutions, predicted to be near the receptor-binding domain of HA. In the present study we analyzed, using reverse genetics, the contribution to virus growth of each of these AA substitutions and determined their effect on antigenic properties. We found that G141E and R162M were most favorable for virus growth; however, only R162M could improve virus growth without antigenic alteration. Substitution D196Y had least effect on virus growth but substantially altered antigenic properties. Additional virus variants with AA substitutions at positions 126, 129, 137 and 141 were generated and characterized. The AA changes advantageous for growth of B/Victoria/504/2000 were also tested in the context of the HA of the B/Beijing/184/93, a virus with stable low-growth phenotype. All of the tested AA substitutions improved the replicative capabilities of the corresponding viruses, but only N126D and K129E had no effect on antigenicity. The results of our studies demonstrate that introduction of specific AA substitutions into viral HA can improve viral replicative efficiency while preserving the original antigenic properties.

Specific biochemical features of replication of clinical influenza viruses in human intestinal cell culture

Influenza A viruses isolated from the respiratory tract of patients with influenza were cultured in human intestinal epithelium cells (CACO-2 line). The CACO-2 cells were found to be 100-fold more susceptible to the clinical viruses than MDCK cells and chicken embryos. On passaging in CACO-2 cells, clinical isolates of the subtype H3N2 retained the original "human" phenotype and agglutinated human but not chicken erythrocytes, whereas on passaging in MDCK cells the virus phenotype changed to the "avian" one. On comparison with laboratory strains (grown in chicken embryos or MDCK cells), the clinical viruses were characterized by higher stability of the anti-interferon protein NS1 but had a reduced synthesis of the matrix protein M1, and this could facilitate the virus adaptation and escape of the infected cells from immune attack in the human body. The increased tropism to the human CACO-2 cells correlated with higher adsorption of the clinical viruses on cellular receptors. However, in the CACO-2 and MDCK cells the ratio of sialyl-containing glycoreceptors of the 2-3 and 2-6 type was similar. These observations indicated that not only sialic acid residues were involved in the adsorption and penetration of the clinical viruses into human cells, but also the protein moiety of the cellular receptor itself and/or an additional cellular coreceptor. Thus, clinical influenza viruses are shown to possess a specific mechanism of sorption and entry into human epithelial cells, which is responsible for their higher tropism to human cells and is unlike such a mechanism in canine cells.

Influenza A virus infection of primary differentiated airway epithelial cell cultures derived from Syrian golden hamsters

The ability of several different influenza A virus strains to infect and replicate in primary, differentiated airway epithelial cell cultures from Syrian golden hamsters was investigated. All virus strains tested replicated equivalently in the cultures and displayed a preference for infecting nonciliated cells. This tropism correlated with the expression of both alpha2,3- and alpha2,6-linked sialic acid on the nonciliated cells. In contrast, the ciliated cells did not have detectable alpha2,6-linked sialic acid and expressed only low amounts of alpha2,3-linked sialic acid. In contrast to clinical isolates, laboratory strains of influenza A virus infected a limited number of ciliated cells at late times post-infection. The presence of alpha2,3- and alpha2,6-linked sialic acid residues on the same cell type suggests that Syrian golden hamsters and differentiated airway epithelial cell cultures derived from hamsters may provide a system for studying the reassortment of influenza A virus strains which utilize different forms of sialic acid as a primary virus receptor.

Establishment of immortal swine kidney epithelial cells

Using normal swine kidney epithelial (SKE) cells that were shown to be senescent at passages 12 to 14, we have established one lifespan-extended cell line and two lifespan-extended cell lines by exogenous introduction of the human catalytic subunit of telomerase (hTERT) and simian virus 40 large T-antigen (SV40LT), all of which maintain epithelial morphology and express cytokeratin, a marker of epithelial cells. SV40LT- and hTERT-transduced immortal cell lines appeared to be smaller and exhibited more uniform morphology relative to primary and spontaneously immortalized SKE cells. We determined the in vitro lifespan of primary SKE cells using a standard 3T6 protocol. There were two steps of the proliferation barrier at 12 and 20, in which a majority of primary SKE cells appeared enlarged, flattened, vacuolated, and ss-galactosidase-positive, all phenotypical characteristics of senescent cells. Lifespan-extended SKE cells were eventually established from most of the cellular foci, which is indicative of spontaneous cellular conversion at passage 23. Beyond passage 25, the rate of population doubling of the established cells gradually increased. At passage 30, immortal cell lines grew faster than primary counterpart cells in 10% FBS-DMEM culture conditions, and only SV40LT-transduced immortal cells grew faster than primary and other SKE immortal cells in 0.5% FBS-DMEM. These lifespan-extended SKE cell lines failed to grow in an anchorage-independent manner in soft-agar dishes. Hence, three immortalized swine kidney epithelial cells that are not transformed would be valuable biological tools for virus propagation and basic kidney epithelial cell research.

Receptor-binding properties of swine influenza viruses isolated and propagated in MDCK cells

To study the receptor specificities of H1 and H3 influenza viruses isolated recently from pigs, we employed the analogues of natural receptors, namely sialyloligosaccharides conjugated with polyacrylamide in biotinylated and label free forms. All Madin-Darby canine kidney (MDCK) cell-propagated viruses with human H3 or classical swine H1 hemagglutinins bound only to Neu5Acalpha2-6Galbeta1-bearing polymers, and not to Neu5Acalpha2-3Galbeta1-bearing polymers. This receptor-binding pattern is typical for human influenza viruses and it differs from the previously described receptor-binding specificity of egg-adapted swine influenza viruses. Swine virus isolates with avian-like H1 and H3 hemagglutinins displayed distinct receptor specificity by binding to both Neu5Acalpha2-6Gal- and Neu5Acalpha2-3Gal-containing receptors. These viruses, as well as egg-adapted swine and turkey viruses with a classical swine HA, differed from the related duck viruses by increased affinity to sulfated sialyloligosaccaride, Su-SiaLe(x). Except for avian-like H3 viruses, none of the studied swine viruses bound to Neu5Gc-containing sialoglycopolymers, suggesting that binding to these sialic acid species abundantly expressed in pigs may not be essential for virus replication in this host.

Binding of influenza viruses to sialic acids: reassortant viruses with A/NWS/33 hemagglutinin bind to alpha2,8-linked sialic acid

We have examined the specificity of binding of A/NWS/33 hemagglutinin (HA), exploring the effects of fucosylation, changing the Gal-GlcNAc linkage between the second and third sugars, and binding affinity for alpha2,8-linked sialic acid. The HA of A/NWS/33(HA)-Tokyo/67(NA) (NWS-Tok, H1N2) virus binds to 3'-linked sialyllactose with 10-fold higher affinity than 3' sialyllactosamine and 3-fold higher affinity than 6' sialyllactosamine. The P227H mutation in A/NWS/33(P227H)(HA)-A/Memphis/31/98(NA) (NWS-Mem/98, H1N2) results in sevenfold lower affinity for 3' sialyllactose, but binding to 6' sialyllactosamine is unchanged. The apparent switch from 3' to 6' specificity is solely due to a loss of Siaalpha2,3 binding. Fucosylation of the third sugar and changing the linkage between second and third sugars had little effect on binding by NWS-Tok, but marked effects on A/NWS/33(P227H)(HA)-tern/Australia/G70c/75(NA) (NWS-G70c, H1N9) and NWS-Mem/98. NWS-Tok, NWS-G70c, and NWS-Mem/98 bind to alpha2,8-bisialic acid with high affinity. NWS-Mem/98 can also bind to alpha2,8-trisialic acid, but with lower affinity. Together, these data show that alpha2,8-linked sialic acid, fucosylation of the third sugar, and linkage between the second and third sugars could play important roles in allowing efficient virus binding to its host cell. The finding that influenza viruses have the potential to bind to alpha2,8-linked sialic acid is a new influenza virus-receptor interaction pathway.

Receptor-binding properties of modern human influenza viruses primarily isolated in Vero and MDCK cells and chicken embryonated eggs

To study the receptor specificity of modern human influenza H1N1 and H3N2 viruses, the analogs of natural receptors, namely sialyloligosaccharides conjugated with high molecular weight (about 1500 kDa) polyacrylamide as biotinylated and label-free probes, have been used. Viruses isolated from clinical specimens were grown in African green monkey kidney (Vero) or Madin-Darby canine kidney (MDCK) cells and chicken embryonated eggs. All Vero-derived viruses had hemagglutinin (HA) sequences indistinguishable from original viruses present in clinical samples, but HAs of three of seven tested MDCK-derived isolates had one or two amino acid substitutions. Despite these host-dependent mutations and differences in the structure of HA molecules of individual strains, all studied Vero- and MDCK-isolated viruses bound to Neu5Ac alpha2-6Galbeta1-4GlcNAc (6'SLN) essentially stronger than to Neu5Acalpha2-6Galbeta1-4Glc (6'SL). Such receptor-binding specificity has been typical for earlier isolated H1N1 human influenza viruses, but there is a new property of H3N2 viruses that has been circulating in the human population during recent years. Propagation of human viruses in chicken embryonated eggs resulted in a selection of variants with amino acid substitutions near the HA receptor-binding site, namely Gln226Arg or Asp225Gly for H1N1 viruses and Leu194Ile and Arg220Ser for H3N2 viruses. These HA mutations disturb the observed strict 6'SLN specificity of recent human influenza viruses.

Overexpression of the alpha-2,6-sialyltransferase in MDCK cells increases influenza virus sensitivity to neuraminidase inhibitors

No reliable cell culture assay is currently available for monitoring human influenza virus sensitivity to neuraminidase inhibitors (NAI). This can be explained by the observation that because of a low concentration of sialyl-alpha2,6-galactose (Sia[alpha2,6]Gal)-containing virus receptors in conventional cell lines, replication of human virus isolates shows little dependency on viral neuraminidase. To test whether overexpression of Sia(alpha2,6)Gal moieties in cultured cells could make them suitable for testing human influenza virus sensitivity to NAI, we stably transfected MDCK cells with cDNA of human 2,6-sialyltransferase (SIAT1). Transfected cells expressed twofold-higher amounts of 6-linked sialic acids and twofold-lower amounts of 3-linked sialic acids than parent MDCK cells as judged by staining with Sambucus nigra agglutinin and Maackia amurensis agglutinin, respectively. After transfection, binding of a clinical human influenza virus isolate was increased, whereas binding of its egg-adapted variant which preferentially bound 3-linked receptors was decreased. The sensitivity of human influenza A and B viruses to the neuraminidase inhibitor oseltamivir carboxylate was substantially improved in the SIAT1-transfected cell line and was consistent with their sensitivity in neuraminidase enzyme assay and with the hemagglutinin (HA) receptor-binding phenotype. MDCK cells stably transfected with SIAT1 may therefore be a suitable system for testing influenza virus sensitivity to NAI.

Distinct host range of influenza H3N2 virus isolates in Vero and MDCK cells is determined by cell specific glycosylation pattern

Influenza A viruses were isolated in Vero, MDCK cells and chicken embryos. In contrast to MDCK-derived variants all H3N2 isolates obtained in Vero cells neither agglutinated chicken erythrocytes nor grew in chicken eggs. These host range differences of H3N2 Vero and MDCK isolates were noticed even in the absence of amino acid substitutions in the HA1 molecule. Evaluation of HA glycosylation pattern by treatment with endoglycosidases H and F revealed that Vero-variants contained more oligosaccharides of the high mannose type than did the corresponding MDCK-isolates. Removal of some mannose residues from the non-reducing termini of the carbohydrates by exomannosidase treatment resulted in the ability of Vero-isolates to agglutinate chicken erythrocytes. Glycosylation pattern and properties of H3N2 viruses grown in Vero cells were close to those of viruses grown in human kidney epithelial cells, whereas the H1N1 variants isolated from Vero, MDCK cells or eggs did not differ in agglutination properties, carbohydrate composition or ability to infect eggs.

Establishment and characterization of two new pig cell lines for use in virological diagnostic laboratories

Two pig cell lines derived from kidney and trachea tissues and referred to as newborn swine kidney (NSK) and newborn pig trachea (NPTr) were established following serial culture of primary cells. They were characterized by an epithelial-like morphology, high capacity to replicate and stability of the cell monolayer for several days after seeding. Their modal chromosome number was modified in comparison to that of primary swine cells and they both displayed a transforming potential in vitro and displayed oncogenicity in nude mice. Infection with pig endogenous retroviruses was detected. Almost all the swine viruses tested, i.e., pseudorabies virus, pig parvovirus, hog cholera virus, transmissible gastroenteritis virus of swine, encephalomyocarditis virus, swine vesicular disease virus and the enteroviruses, except pig reproductive respiratory syndrome virus, were capable of replicating in the new cell lines with titres similar to the ones detected in the reference culture systems. Furthermore, all the selected influenza virus sub-types isolated from human, swine and avian species replicated with cytopathic effect in NSK and NPTr cells, whereas, of all the equine influenza viruses tested only the Miami and Suffolk sub-types replicated.

Hemagglutinin residues of recent human A(H3N2) influenza viruses that contribute to the inability to agglutinate chicken erythrocytes

To identify the molecular determinants contributing to the inability of recent human influenza A(H3N2) viruses to agglutinate chicken erythrocytes, phenotypic revertants were selected upon passage in eggs or MDCK cells. The Leu194Ile or Val226Ile substitutions were detected in their hemagglutinin (HA) sequence concomitantly with the phenotypic reversion. Remarkably, as little as 3.5% of variants bearing a Val226Ile substitution was found to confer the ability to agglutinate chicken erythrocytes to the virus population. Hemadsorption assays following transient expression of mutated HA proteins showed that the successive Gln226 --> Leu --> Ile --> Val changes observed on natural isolates resulted in a progressive loss of the ability of the HA to bind chicken erythrocytes. The Val226Ile change maintained the preference of the HA for SAalpha2,6Gal over SAalpha2,3Gal and enhanced binding of the HA to alpha2,6Gal receptors present on chicken erythrocytes. In contrast, simultaneous Ser193Arg and Leu194Ile substitutions that were found to confer the ability to agglutinate sheep erythrocytes increased the affinity of the HA for SAalpha2,3Gal.

Characterization of a porcine lung epithelial cell line suitable for influenza virus studies

We established a porcine lung epithelial cell line designated St. Jude porcine lung cells (SJPL) and demonstrated that all tested influenza A and B viruses replicated in this cell line. The infectivity titers of most viruses in SJPL cells were comparable to or better than those in MDCK cells. The propagation of influenza viruses from clinical samples in SJPL cells did not lead to antigenic changes in the hemagglutinin molecule. The numbers of both Sia2-3Gal and Sia2-6Gal receptors on SJPL cells were greater than those on MDCK cells. Influenza virus infection of SJPL cells did not lead to apoptosis, as did infection of MDCK cells. No porcine endogenous retrovirus was detected in SJPL cells, and in contrast to MDCK cells, SJPL cells did not cause tumors in nude mice.

Update on influenza and other viral pneumonias

Major developments during the past 5 years concerning influenza prevention by vaccination and treatment with neuraminidase inhibitors are reviewed. These have been accompanied by increased media interest in related issues: pressures on hospital admissions, ethical concerns and controls on prescribing limiting professional autonomy. The new live attenuated influenza vaccines, adjuvanted vaccines and the emerging recombinant DNA vaccines are discussed. Recent information on neuraminidase inhibitor antivirals, surveillance for resistant viruses, the prospects for near patient tests (i.e. tests that can be used near the patient to improve immediate patient management or in the laboratory to give rapid feedback for physicians) and the clinical significance of other respiratory viruses are highlighted. The benefits of recent advances provide challenges for health care delivery and public acceptance as great as those involved in their scientific development.

The human cell line PER.C6 provides a new manufacturing system for the production of influenza vaccines

Influenza viruses for vaccine production are currently grown on embryonated eggs. This manufacturing system conveys many major drawbacks such as inflexibility, cumbersome down stream processing, inability of some strains to replicate on eggs to high enough yields, and selection of receptor-binding variants with reduced antigenicity. These limitations emphasize the need for a cell line-based production system that could replace eggs in the production of influenza virus vaccines in a pandemic proof fashion. Here we present the efficient propagation of influenza A and B viruses on the fully characterized and standardized human cell line PER.C6.

Change in receptor-binding specificity of recent human influenza A viruses (H3N2): a single amino acid change in hemagglutinin altered its recognition of sialyloligosaccharides

Human H3N2 influenza A viruses were known to preferentially bind to sialic acid (SA) in alpha2,6Gal linkage on red blood cells (RBC). However, H3N2 viruses isolated in MDCK cells after 1992 did not agglutinate chicken RBC (CRBC). Experiments with point-mutated hemagglutinin (HA) of A/Aichi/51/92, one of these viruses, revealed that an amino acid change from Glu to Asp at position 190 (E190D) was responsible for the loss of ability to bind to CRBC. A/Aichi/51/92 did not agglutinate CRBC treated with alpha2, 3-sialidase, suggesting that SAalpha2,3Gal on CRBC might not inhibit the binding of the virus to SAalpha2,6Gal on CRBC. However, the virus agglutinated derivatized CRBC resialylated with SAalpha2, 6Galbeta1,4GlcNAc. These findings suggested that the E190D change might have rendered the HA able to distinguish sialyloligosaccharides on the derivatized CRBC containing the SAalpha2,6Galbeta1,4GlcNAc sequence from those on the native CRBC.