Relative efficacy of embryonated eggs and cell culture for isolation of contemporary influenza viruses

Current Opinion in LAIV: A Matter of Parent Virus Choice

Influenza is still a frequent seasonal infection of the upper respiratory tract, which may have deadly consequences, especially for the elderly. This is in spite of the availability of vaccines suggested for persons above 65 years of age. Two types of conventional influenza vaccines are currently licensed for use-live attenuated and inactivated vaccines. Depending on local regulatory requirements, live attenuated vaccines are produced by the reverse genetics technique or by classical reassortment in embryonated chicken eggs. Sometimes, the efficiency of classical reassortment is complicated by certain properties of the wild-type parent virus. Cases of low efficacy of vaccines have been noted, which, among other reasons, may be associated with suboptimal properties of the wild-type parent virus that are not considered when recommendations for influenza vaccine composition are made. Unfortunately, knowledge surrounding the roles of properties of the circulating influenza virus and its impact on the efficacy of the reassortment process, vaccination efficiency, the infectivity of the vaccine candidates, etc., is now scattered in different publications. This review summarizes the main features of the influenza virus that may dramatically affect different aspects of the preparation of egg-derived live attenuated vaccine candidates and their effectiveness. The author expresses her personal view, which may not coincide with the opinion of other experts in the field of influenza vaccines.

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.

Extensive Accumulation of Influenza Virus NS1 Protein in the Nuclei Causes Effective Viral Growth in Vero Cells

We previously showed that modified A/Puerto Rico/8/34 (PR8) influenza master strain had improved viral rescue and growth properties in African green monkey kidney (Vero) cell line by introducing NS gene of Vero-adapted A/England/1/53 (vaEng53). In the present study, it was found that the NS1 protein derived from vaEng53 was extensively accumulated in the nuclei than that of PR8. This accumulation was caused by 7 amino acid differences in C-terminal region of NS1 protein. These results suggest that specific accumulation of NS1 protein may contribute to efficient viral replication in Vero cells.

Production of influenza H1N1 vaccine from MDCK cells using a novel disposable packed-bed bioreactor

A process for human influenza H1N1 virus vaccine production from Madin-Darby canine kidney (MDCK) cells using a novel packed-bed bioreactor is described in this report. The mini-bioreactor was used to study the relationship between cell density and glucose consumption rate and to optimize the infection parameters of the influenza H1N1 virus (A/New Caledonia/20/99). The MDCK cell culture and virus infection were then monitored in a disposable perfusion bioreactor (AmProtein Current Perfusion Bioreactor) with proportional-integral-derivative control of pH, dissolved O(2) (DO), agitation, and temperature. During 6 days of culture, the total cell number increased from 2.0 × 10(9) to 3.2 × 10(10) cells. The maximum virus titers of 768 hemagglutinin units/100 μL and 7.8 × 10(7) 50 % tissue culture infectious doses/mL were obtained 3 days after infection. These results demonstrate that using a disposable perfusion bioreactor for large-scale cultivation of MDCK cells, which allows for the control of DO, pH, and other conditions, is a convenient and stable platform for industrial-scale production of influenza vaccines.

Cloned cDNA of A/swine/Iowa/15/1930 internal genes as a candidate backbone for reverse genetics vaccine against influenza A viruses

Reverse genetics viruses for influenza vaccine production usually utilize the internal genes of the egg-adapted A/Puerto Rico/8/34 (PR8) strain. This egg-adapted strain provides high production yield in embryonated eggs but does not necessarily give the best yield in mammalian cell culture. In order to generate a reverse genetics viral backbone that is well-adapted to high growth in mammalian cell culture, a swine influenza isolate A/swine/Iowa/15/30 (H1N1) (rg1930) that was shown to give high yield in Madin-Darby canine kidney (MDCK) cells was used as the internal gene donor for reverse genetics plasmids. In this report, the internal genes from rg1930 were used for construction of reverse genetics viruses carrying a cleavage site-modified hemagglutinin (HA) gene and neuraminidase (NA) gene from a highly pathogenic H5N1 virus. The resulting virus (rg1930H5N1) was low pathogenic in vivo. Inactivated rg1930H5N1 vaccine completely protected chickens from morbidity and mortality after challenge with highly pathogenic H5N1. Protective immunity was obtained when chickens were immunized with an inactivated vaccine consisting of at least 2(9) HA units of the rg1930H5N1 virus. In comparison to the PR8-based reverse genetics viruses carrying the same HA and NA genes from an H5N1 virus, rg1930 based viruses yielded higher viral titers in MDCK and Vero cells. In addition, the reverse genetics derived H3N2 and H5N2 viruses with the rg1930 backbone replicated in MDCK cells better than the cognate viruses with the rgPR8 backbone. It is concluded that this newly established reverse genetics backbone system could serve as a candidate for a master donor strain for development of inactivated influenza vaccines in cell-based systems.

Comparative study of influenza virus replication in Vero and MDCK cell lines

The choice of a cell line for the production of influenza vaccines is determined by how well the virus is able to replicate and how easily the cell line can be maintained. Madin-Darby canine kidney (MDCK) cells have long been known to successfully support influenza growth. Vero cells are also another well studied candidate cell line. In this work, we have compared these two cell lines for their ability to propagate type A and type B cold-adapted and wild type influenza viruses. The growth of these viruses has been measured as plaque forming units (via plaque assay) as well as viral particle formation (via a novel quantitative RT-PCR assay) to assess the suitability of these cell lines to support the development of live attenuated influenza vaccines. The novel qRT-PCR assay outlined in this work was demonstrated to be an efficient, sensitive and reproducible method for measuring wild type (wt) and cold-adapted (ca) influenza strains. Replicates of six per sample consistently showed an average variation around +/-10%. In this study we have also found qRT-PCR to be a useful method for differentiating between wt and ca influenza strains based on their differing growth characteristics at varying temperatures. This can subsequently be used to assess reassortants prepared from ca donor strains for the purposes of live viral vaccine development. For type A and B influenza viruses studied in this work, MDCK cells supported a more rapid viral growth (measured in terms of genome copies) compared with Vero cells. For the type A viruses studied here, the genome copies: infectious unit (genome copy, gc:infectious unit, iu) ratio was found to be more favorable for Vero cells compared with MDCK cells. For the type B viruses studied in this work, the gc:iu was equivalent in both cell lines tested. Ultimately, however, the use of any new cell line would need to be approved by regulatory agencies prior to its commercial application.

Generation of high-yielding influenza A viruses in African green monkey kidney (Vero) cells by reverse genetics

Influenza A viruses are the cause of annual epidemics of human disease with occasional outbreaks of pandemic proportions. The zoonotic nature of the disease and the vast viral reservoirs in the aquatic birds of the world mean that influenza will not easily be eradicated and that vaccines will continue to be needed. Recent technological advances in reverse genetics methods and limitations of the conventional production of vaccines by using eggs have led to a push to develop cell-based strategies to produce influenza vaccine. Although cell-based systems are being developed, barriers remain that need to be overcome if the potential of these systems is to be fully realized. These barriers include, but are not limited to, potentially poor reproducibility of viral rescue with reverse genetics systems and poor growth kinetics and yields. In this study we present a modified A/Puerto Rico/8/34 (PR8) influenza virus master strain that has improved viral rescue and growth properties in the African green monkey kidney cell line, Vero. The improved properties were mediated by the substitution of the PR8 NS gene for that of a Vero-adapted reassortant virus. The Vero growth kinetics of viruses with H1N1, H3N2, H6N1, and H9N2 hemagglutinin and neuraminidase combinations rescued on the new master strain were significantly enhanced in comparison to those of viruses with the same combinations rescued on the standard PR8 master strain. These improvements pave the way for the reproducible generation of high-yielding human and animal influenza vaccines by reverse genetics methods. Such a means of production has particular relevance to epidemic and pandemic use.

New point of care test is highly specific but less sensitive for influenza virus A and B in children and adults

The importance of rapid diagnosis of influenza has increased with the availability of neuraminidase inhibitors, which need to be commenced within 48 hr of symptom onset. Furthermore, the recent development of influenza-like clinical syndromes with novel aetiologies (severe acute respiratory syndrome, SARS) has increased the need for rapid and accurate near-patient diagnosis. A new, modified point of care (POC) diagnostic test (ZstatFlu) was assessed on 469 nasopharyngeal aspirates (NPAs) and 260 nose/throat swabs (TS) taken from children and adults. The test was specific (77-98%) for all specimen types for influenza virus A and B, depending upon incubation conditions. However, it was less sensitive, detecting 65-77% of specimens confirmed as positive on culture, direct immunofluorescence or PCR testing. A positive test is useful, for both directing initiation of therapy in the clinician's office, and making a positive diagnosis of influenza in patients with influenza-like clinical syndromes.

Performance of virus isolation and Directigen Flu A to detect influenza A virus in experimental human infection

BACKGROUND

Few data exist to assess the sensitivity of different specimen types for viral detection during the course of influenza virus infection.

OBJECTIVES

This study assessed the relationships between quantitative influenza A virus replication and antigen detectability by the enzyme immunosorbent assay (EIA) Directigen Flu A in different type of samples during experimental human infection.

STUDY DESIGN

Fourteen volunteers were inoculated with influenza A virus A/Texas/36/91 (H1N1). Four specimens types were collected in sequence for quantitative isolation in cell culture and antigen testing from days 1 to 8 after inoculation.

RESULTS

Seventy-one (63%) of nasopharyngeal wash specimens were culture positive, compared to 51 (46%) of throat gargles, 51 (46%) of nasal swabs, and 27 (24%) of throat swabs. All subjects shed virus in their nasopharyngeal wash at least one day and 86% of subjects had a positive nasopharyngeal wash culture on day 2 after inoculation. The mean viral titers were highest on day 2 post inoculation for all specimen types and averaged 3.6 log10 TCID50/ml for nasal washes, 1.2 log10 TCID50/ml for throat gargles, 1.8 log10 TCID50/ml for the nasopharyngeal swabs, and 0.6 log10 TCID50/ml for the throat swabs. Mean viral titers in the nasal washes were significantly different (P<0.05) compared to other specimen types. The peak of sensitivity of EIA (compared to culture) was the second day after inoculation. Nasopharyngeal and throat swab results were combined for this analysis and considered positive by culture if positive in either or both samples. Thus, on day 2 the number of EIA positive samples relative to the number culture positive was 9/12 (75%) for nasopharyngeal wash specimens, 2/9 (22%) for throat gargles, and 7/11 (64%) for the combined throat and nasal swabs specimens.

CONCLUSIONS

Nasopharyngeal washes are the most sensitive sample type detecting influenza A virus in adults. For rapid diagnosis the Directigen Flu A is an alternative with a sensitivity compared to culture ranging between 64 and 78% if performed on nasopharyngeal specimens on day two or three after experimental infection in adults. However, if performed on other specimens or later in the course of infection the sensitivity is lower.

African green monkey kidney (Vero) cells provide an alternative host cell system for influenza A and B viruses

The preparation of live, attenuated human influenza virus vaccines and of large quantities of inactivated vaccines after the emergence or reemergence of a pandemic influenza virus will require an alternative host cell system, because embryonated chicken eggs will likely be insufficient and suboptimal. Preliminary studies indicated that an African green monkey kidney cell line (Vero) is a suitable system for the primary isolation and cultivation of influenza A viruses (E. A. Govorkova, N. V. Kaverin, L. V. Gubareva, B. Meignier, and R. G. Webster, J. Infect. Dis. 172:250-253, 1995). We now demonstrate for the first time that Vero cells are suitable for isolation and productive replication of influenza B viruses and determine the biological and genetic properties of both influenza A and B viruses in Vero cells; additionally, we characterize the receptors on Vero cells compared with those on Madin-Darby canine kidney (MDCK) cells. Sequence analysis indicated that the hemagglutinin of Vero cell-derived influenza B viruses was identical to that of MDCK-grown counterparts but differed from that of egg-grown viruses at amino acid positions 196 to 198. Fluorescence-activated cell sorting analysis showed that although Vero cells possess predominantly alpha2,3 galactose-linked sialic acid, they are fully susceptible to infection with either human influenza A or B viruses. Moreover, all virus-specific polypeptides were synthesized in the same proportions in Vero cells as in MDCK cells. Electron microscopic and immunofluorescence studies confirmed that infected Vero cells undergo the same morphological changes as do other polarized epithelia] cells. Taken together, these results indicate that Vero cell lines could serve as an alternative host system for the cultivation of influenza A and B viruses, providing adequate quantities of either virus to meet the vaccine requirements imposed by an emerging pandemic.

Use of PCR-enzyme immunoassay for identification of influenza A virus matrix RNA in clinical samples negative for cultivable virus

Influenza A virus infections are a major cause of morbidity and mortality worldwide. Standard diagnostic methods either are not efficient in identifying infected individuals in a timely manner or lack sensitivity. We developed a PCR-enzyme immunoassay (PCR-EIA) for the detection of influenza A virus RNA in respiratory secretions. A reverse transcription PCR was performed with oligonucleotide primers directed at a highly conserved area of the influenza A matrix gene. Amplified DNA was identified by hybridization in solution to a nested biotinylated RNA probe and quantitated in an EIA. PCR-EIA detected small quantities of RNA from the three prevalent subtypes of human influenza A virus. Influenza B and C, parainfluenza, measles, mumps, and respiratory syncytial viruses tested negative. The potential efficiency of PCR-EIA for use in clinical diagnosis was determined by testing 90 nasal wash specimens obtained daily over a 10-day period from nine human volunteers infected with influenza A virus. Thirty-seven of the postinfection samples had detectable influenza A virus RNA by PCR-EIA, whereas only 26 postinfection samples were positive by culture. PCR-EIA was particularly efficient for the identification of influenza A virus in samples obtained more than 4 days after infection. Seventeen of 45 such samples were positive, whereas virus was cultivated from 4 samples (P < 0.00005). All preinfection samples from volunteers subsequently infected with influenza A virus were negative by PCR-EIA, as were samples from a volunteer infected with parainfluenza virus type 3. Nucleic acid amplification techniques represent important tools for the timely and sensitive diagnosis of influenza A virus infections and, therefore, their management and control.

Comparison of Directigen FLU-A with viral isolation and direct immunofluorescence for the rapid detection and identification of influenza A virus

Directigen FLU-A, an enzyme immunoassay membrane test, was compared prospectively to isolation in cell culture and direct immunofluorescence (IF) for the detection of influenza A virus. One hundred ninety specimens were evaluated by Directigen FLU-A and cell culture; 184 of these specimens were also tested by direct IF. The sensitivity of Directigen FLU-A compared to isolation in cell culture and direct IF was 100%. The specificities of Directigen FLU-A compared to isolation and direct IF were identical, 91.6%. Fourteen specimens that were positive by Directigen FLU-A did not yield virus in culture; two of the specimens, however, were positive by direct IF, and four other specimens were not specimens of choice for the test. A positive Directigen result had positive predictive values of 62.6 and 75.0% compared to isolation and direct IF, respectively; a positive Directigen result with an intensity reading of 2+ or greater, however, had positive predictive values of 85 and 100% compared to isolation and direct IF, respectively. In all comparisons, the negative predictive value was 100%. There was no evidence that cross-reactivity occurred with non-influenza A antigens. Directigen FLU-A should serve as a convenient screening test for influenza A and as a rapid test supported by isolation in cell culture during an influenza outbreak.

Diagnosis of viral respiratory infections in the 1980s

The introduction of immunologic techniques for rapid viral diagnosis and the availability of high quality commercial reagents have resulted in significant advances in the laboratory diagnosis of viral respiratory infections. Laboratories just beginning in the area of diagnostic virology are advised to direct their efforts at viral isolation using cell types such as human embryonic kidney; immunofluorescence and other techniques of rapid viral diagnosis are recommended when the laboratory is prepared to expand.