A Method of Calculating Intracranial Thermal Neutron Isodose Curves

A method is described for obtaining by numerical integration a modified geometry factor that takes into account build-up and non-uniform source distribution patterns for use in calculating radiation doses for thermal neutrons.

Changes in nonstructural protein 3 are associated with attenuation in avian coronavirus infectious bronchitis virus

Full-length genome sequencing of pathogenic and attenuated (for chickens) avian coronavirus infectious bronchitis virus (IBV) strains of the same serotype was conducted to identify genetic differences between the pathotypes. Analysis of the consensus full-length genome for three different IBV serotypes (Ark, GA98, and Mass41) showed that passage in embryonated eggs, to attenuate the viruses for chickens, resulted in 34.75–43.66% of all the amino acid changes occurring in nsp 3 within a virus type, whereas changes in the spike glycoprotein, thought to be the most variable protein in IBV, ranged from 5.8 to 13.4% of all changes. The attenuated viruses did not cause any clinical signs of disease and had lower replication rates than the pathogenic viruses of the same serotype in chickens. However, both attenuated and pathogenic viruses of the same serotype replicated similarly in embryonated eggs, suggesting that mutations in nsp 3, which is involved in replication of the virus, might play an important role in the reduced replication observed in chickens leading to the attenuated phenotype.

Changes in biological source material

A change in source (or raw) material can be radical, e.g. changing the derivation of a therapeutic protein from human plasma to recombinant DNA manufacture, or the change may be more subtle, e.g. the change from a non-inactivated bovine serum growth supplement to an inactivated serum. The former type of change is usually driven by manufacturing strategy and has vast consequences for the regulation of the product concerned. The latter type of change is usually driven by a need to increase the assurance of viral safety and the regulatory implications for the product are significantly less severe. In this latter example, inadvertent alterations to the product may result from changes in cell metabolism brought about by the change in its growth conditions and these need to be addressed in comparability studies. Ultimately, the implication of any slight change in the fine structure of a biotech medicinal product on its efficacy and/or immunogenicity will have to be dealt with on a case-by-case basis.

Bovine serum--regulatory issues

For many years users of bovine serum in the manufacture of human and veterinary biological medicinal products have relied upon USDA 9CFR to ensure the viral safety of their serum. Recently, EU regulators have formalised their position by issuing guidelines on the use of bovine serum during manufacture. Additionally, the European Pharmacopoeia has drafted a monograph on bovine serum. There is good harmonisation among the recommendations and requirements although the EU CHMP guideline calls for greater attention to be paid to the potential presence of infectious bovine polyoma virus. The EU guidelines also call for various tests to assess the effect of BVDV antibodies in the detection of BVDV. However, in response to criticisms from serum suppliers and users, the stringency of these recommendations is being relaxed. The overall viral safety of bovine serum should be subject to a risk analysis as infectious virus will invariably be present in some batches of serum but remain undetected. Other factors such as the geographical source of the serum, the efficacy of viral inactivation/removal steps and the ability of specific viruses to grow in the production cells being used, should be taken into consideration.

Preparation of vaccines against H5N1 influenza

In response to the pandemic warning provided by the highly pathogenic H5N1 influenza virus infections in Hong Kong, there were world-wide attempts to develop vaccines. Three strategies were followed and although each was associated with some success, there were also some problems. Pre-clinical vaccine efficacy results are presented from one such strategy, that of using an apathogenic H5N3 avian strain for vaccine production.

[Differences in receptor specificity between the influenza A viruses isolated from the duck, chicken, and human]

The affinity of the duck, chicken, and human influenza viruses to the host cell sialosides was determined, and considerable distinctions between duck and chicken viruses were found. Duck viruses bind to a wide range of sialosides, including the short-stem gangliosides. Most of the chicken viruses, like human ones, lose the ability to bind these gangliosides, which strictly correlates with the appearance of carbohydrate at position 158-160. The affinity of the chicken viruses to sialoglycoconjugates of chicken intestine as well as chicken, monkey, and human respiratory epithelial cells exceeds that of the duck viruses. The human influenza viruses have high affinity to the same cells but do not bind at all to the duck epithelial cell. This testifies to the absence of 6'-sialylgalactose residues from the duck cells, in contrast to chicken and monkey cells. The alteration of the receptor specificity of chicken viruses in comparison with duck ones results in the similarity of the patterns of accessible cells for chicken and human influenza viruses. This may be the cause of the appearance of the line of H9N2 viruses from Hong Kong live bird markets with receptor specificity similar to that of H3N2 human viruses, and of the ability of H5N1 and H9N2 chicken influenza viruses to infect humans.

European Union guidance on the quality, safety and efficacy of DNA vaccines and regulatory requirements

A European Union note for guidance on gene transfer medicinal products is being developed by the Biotechnology Working Party (BWP) of the Committee for Proprietary Medicinal Products (CPMP) which will include guidance for DNA vaccines. The 'Note for Guidance on the Quality, Preclinical and Clinical Aspects of Gene Transfer Medicinal Products' outlines the information required to assure the quality of the plasmid DNA intended to be used as a vaccine. It also provides guidance on preclinical safety evaluation and on clinical efficacy and safety evaluation with regard to obtaining marketing authorisation according to Council Regulation No. (EEC) 2309/93. Before initiating a clinical trial, it is necessary to obtain an appraisal from the relevant central and/or local ethics committees, and the competent authority within the member state(s) concerned has to authorise, or be notified of, the clinical trial. Harmonisation of gene therapy and DNA vaccine regulations as well as co-operation between relevant ethics committees and authorities in Europe is currently being improved.

Assuring the quality, safety, and efficacy of DNA vaccines

Scientists in academia whose research is aimed at the development of a novel vaccine or approach to vaccination may not always be fully aware of the regulatory process by which a candidate vaccine becomes a licensed product. It is useful for such scientists to be aware of these processes as the development of a novel vaccine could be problematic owing to the starting material often being developed in a research laboratory under ill-defined conditions. This paper examines the regulatory process with respect to the development of a DNA vaccine. DNA vaccines present unusual safety considerations that must be addressed during preclinical safety studies, including adverse immunopathology, genotoxicity through integration into a vaccinees chromosomes, and the potential for the formation of anti-DNA antibodies.

Retroviral aspects of the characterisation of avian and mammalian cell substrates

There have been no reported incidents of adventitious retroviral infection of cell lines used for the production of vaccines and other biologicals. However, due to the unique molecular biology of retroviruses, cell lines may contain endogenous retroviral genomes and these can give rise to defective retroviral particles, e.g. murine hybridomas and CHO cells. Recently, a similar situation was reported for a cell substrate in use for decades for vaccine production, namely chick embryo fibroblasts, and this raised some concern regarding the safety of these vaccines.

Assuring the quality, safety, and efficacy of DNA vaccines

Scientists in academia whose research is aimed at the development of a novel vaccine or approach to vaccination may not always be fully aware of the regulatory process by which a candidate vaccine becomes a licensed product. This chapter will provide an overview of the regulatory process and will discuss in more detail the quality and pre-clinical safety issues of plasmid DNA vaccines intended for human use. It is useful for research scientists to be aware of these processes as the development of a novel vaccine could be problematic due to the starting material often being developed in a research laboratory under ill-defined conditions.

The influence of the host cell on standardisation of influenza vaccine potency

Conventional influenza vaccines are standardised using the single-radial-immunodiffusion (SRD) test where reagents are produced from egg-grown viruses. It is important to ensure homology between SRD antigen reagents and test vaccines. There was concern that cell-grown vaccines may differ antigenically from corresponding egg-grown vaccines, which may in turn affect vaccine standardisation. In an examination of five cell-grown vaccines from two companies, only one vaccine was affected by the specificity of the SRD test. Options for standardisation of cell-grown vaccines are considered and recommendations are made for further studies.

An overview of host cell selection

A virus with a new phenotype can arise from an apparently homogeneous population after a single passage by the application of a strong selection pressure. With less pressure, a variant population may arise gradually during repeated passage of the virus. Strong host selection has been observed when naturally occurring influenza virus is grown in embryonated hens' eggs whereas the natural virus grows apparently unrestricted in MDCK cells. However, repeated passage in MDCK cells can select for a virus more suited to replication in these cells and variants which resemble those selected in eggs have been observed in such passaged virus.

Effects of egg-adaptation on the receptor-binding properties of human influenza A and B viruses

Propagation of human influenza viruses in embryonated chicken eggs (CE) results in the selection of variants with amino acid substitutions near the receptor-binding site of the hemagglutinin (HA) molecule. To evaluate the mechanisms by which these substitutions enable human virus growth in CE, we studied the binding of 10 human influenza A (H1N1, H3N2) and B strains, isolated and propagated solely in MDCK cells, and of their egg-adapted counterparts to preparations of cellular membranes, gangliosides, sialylglycoproteins, and sialyloligosaccharides. All egg-adapted variants differed from nonadapted strains by increased binding to the plasma membranes of chorio-allantoic (CAM) cells of CE and by the ability to bind to CAM gangliosides. In addition, there was no decrease in affinity for inhibitors within allantoic fluid. These findings indicate that growth of human influenza viruses in CE is restricted because of their inefficient binding to receptors on CAM cells and that gangliosides can play an important role in virus binding and/or penetration. The effects of the egg-adaptation substitutions on the receptor-binding properties of the viruses include (i) enhancement of virus binding to the terminal Sia(alpha2-3)Gal determinant (substitutions in HA positions 190, 225 of H1N1 strains and in position 186 of H3N2 strains); (ii) a decrease of steric interference with more distant parts of the Sia(alpha2-3Gal)-containing receptors (a loss of glycosylation sites in positions 163 of H1 HA and 187 of type B HA); and (iii) enhanced ionic interactions with the negatively charged molecules due to charged substitutions at the tip of the HA [187, 189, 190 (H1), and 145, 156 (H3)]. Concomitantly with enhanced binding to Sia(alpha2-3)Gal-terminated receptors, all egg-adapted variants decreased their affinity for equine macroglobulin, a glycoprotein bearing terminal 6'-sialyl(N-acetyllactosamine)-moieties.

MIRD pamphlet no. 16: Techniques for quantitative radiopharmaceutical biodistribution data acquisition and analysis for use in human radiation dose estimates

This report describes recommended techniques for radiopharmaceutical biodistribution data acquisition and analysis in human subjects to estimate radiation absorbed dose using the Medical Internal Radiation Dose (MIRD) schema. The document has been prepared in a format to address two audiences: individuals with a primary interest in designing clinical trials who are not experts in dosimetry and individuals with extensive experience with dosimetry-based protocols and calculational methodology. For the first group, the general concepts involved in biodistribution data acquisition are presented, with guidance provided for the number of measurements (data points) required. For those with expertise in dosimetry, highlighted sections, examples and appendices have been included to provide calculational details, as well as references, for the techniques involved. This document is intended also to serve as a guide for the investigator in choosing the appropriate methodologies when acquiring and preparing product data for review by national regulatory agencies. The emphasis is on planar imaging techniques commonly available in most nuclear medicine departments and laboratories. The measurement of the biodistribution of radiopharmaceuticals is an important aspect in calculating absorbed dose from internally deposited radionuclides. Three phases are presented: data collection, data analysis and data processing. In the first phase, data collection, the identification of source regions, the determination of their appropriate temporal sampling and the acquisition of data are discussed. In the second phase, quantitative measurement techniques involving imaging by planar scintillation camera, SPECT and PET for the calculation of activity in source regions as a function of time are discussed. In addition, nonimaging measurement techniques, including external radiation monitoring, tissue-sample counting (blood and biopsy) and excreta counting are also considered. The third phase, data processing, involves curve-fitting techniques to integrate the source time-activity curves (determining the area under these curves). For some applications, compartmental modeling procedures may be used. Last, appendices are included that provide a table of symbols and definitions, a checklist for study protocol design, example formats for quantitative imaging protocols, temporal sampling error analysis techniques and selected calculational examples. The utilization of the presented approach should aid in the standardization of protocol design for collecting kinetic data and in the calculation of absorbed dose estimates.

MIRD pamphlet No. 17: the dosimetry of nonuniform activity distributions--radionuclide S values at the voxel level. Medical Internal Radiation Dose Committee

The availability of quantitative three-dimensional in vivo data on radionuclide distributions within the body makes it possible to calculate the corresponding nonuniform distribution of radiation absorbed dose in body organs and tissues. This pamphlet emphasizes the utility of the MIRD schema for such calculations through the use of radionuclide S values defined at the voxel level. The use of both dose point-kernels and Monte Carlo simulation methods is also discussed. PET and SPECT imaging can provide quantitative activity data in voxels of several millimeters on edge. For smaller voxel sizes, accurate data cannot be obtained using present imaging technology. For submillimeter dimensions, autoradiographic methods may be used when tissues are obtained through biopsy or autopsy. Sample S value tabulations for five radionuclides within cubical voxels of 3 mm and 6 mm on edge are given in the appendices to this pamphlet. These S values may be used to construct three-dimensional dose profiles for nonuniform distributions of radioactivity encountered in therapeutic and diagnostic nuclear medicine. Data are also tabulated for 131I in 0.1-mm voxels for use in autoradiography. Two examples illustrating the use of voxel S values are given, followed by a discussion of the use of three-dimensional dose distributions in understanding and predicting biologic response.

The MIRD perspective 1999. Medical Internal Radiation Dose Committee

The MIRD schema is a general approach for medical internal radiation dosimetry. Although the schema has traditionally been used for organ dosimetry, it is also applicable to dosimetry at the suborgan, voxel, multicellular and cellular levels. The MIRD pamphlets that follow in this issue and in coming issues, as well as the recent monograph on cellular dosimetry, demonstrate the flexibility of this approach. Furthermore, these pamphlets provide new tools for radionuclide dosimetry applications, including the dynamic bladder model, S values for small structures within the brain (i.e., suborgan dosimetry), voxel S values for constructing three-dimensional dose distributions and dose-volume histograms and techniques for acquiring quantitative distribution and pharmacokinetic data.

Direct sequencing of the HA gene of clinical equine H3N8 influenza virus and comparison with laboratory derived viruses

Equine influenza viruses propagated in the laboratory in alternate hosts such as embryonated hens' eggs or mammalian cell culture have been analysed by HA sequencing and antigenically and their sequence compared to the original virus present in clinical material. In contrast to clinically derived human influenza virus which generally grows in MDCK cells without change, the data for equine influenza virus were less clear in that variants of equine virus were derived in both eggs and cells. The study indicated that the current use of eggs for equine influenza virus surveillance and vaccine production is entirely appropriate, but that care should be exercised when equine influenza vaccines are produced in eggs or on mammalian cell cultures.

International standardization of gene amplification technology

The WHO international Working Group on the standardization of Gene Amplification Techniques for the Virological Safety Testing of Blood and Blood Products (SoGAT) was established in 1995 as a technical discussion group. It is the only international forum for the exchange of information on scientific aspects of the technology, evaluation of the technology for the testing of products and the organisation of international collaborative studies for the development, evaluation and provision of reference materials and working standards. Materials suitable as working reagents in hepatitis C virus (HCV) gene amplification are in widespread use and plans for a collaborative study for the establishment on an HCV RNA International Standard are well underway as are studies aimed at standardizing the technology for other viruses.

Assessing the significance of reverse transcriptase activity in chick cell-derived vaccines

A recent publication reported the detection of low levels of the enzyme reverse transcriptase (RTase) in live viral vaccines prepared in chick embryo cells. The enzyme was detected using an assay with greatly increased sensitivity compared to more conventional methods. The authors have confirmed the observation of RTase activity and demonstrate that the activity is not dependent on the production of viral vaccines in chick cells but is present ubiquitously in chick embryonic fluids. The authors have also been unable to transmit the RTase activity from chick cells to a wide variety of cells of human, monkey, rabbit and turkey origin, suggesting that the activity is not associated with an avian agent capable of infecting these cells. It is concluded that the data available present no cause for concern over the safety of vaccines derived in chick cells and current WHO requirements for such vaccines remain appropriate.

Avian influenza A viruses differ from human viruses by recognition of sialyloligosaccharides and gangliosides and by a higher conservation of the HA receptor-binding site

Avian influenza virus strains representing most hemagglutinin (HA) subtypes were compared with human influenza A (H1N1,H3N2) and B virus isolates, including those with no history of passaging in embryonated hen's eggs, for their ability to bind free N-acetylneuraminic acid (Neu5Ac) and sialylollgosaccharides in a competitive binding assay and to attach to gangliosides in a solid-phase adsorption assay. The avian viruses, irrespective of their HA subtype, showed a higher affinity for sialyl-3-lactose and the other Neu5Ac2-3Gal-terminated oligosaccharides and a lower affinity for sialyl-6-lactose than for free Neu5Ac, indicative of specific interactions between the HA and the 3-linked Gal and poor accommodation of 6-linked Gal in the avian receptor-binding site (RBS). Human H1 and H3 strains, by contrast, were unable to bind to 3-linked Gal, interacting instead with the asialic portion of sialyl-6-(N-acetyllactosamine). Different parts of this moiety were recognized by H3 and H1 subtype viruses (Gal and GlcNAc, respectively). Comparison of the HA amino acid sequences revealed that residues in positions. 138, 190, 194, 225, 226, and 228 are conserved in the avian RBS, while the human HAs harbor substitutions at these positions. A characteristic feature of avian viruses was their binding to Neu5Ac2-3Gal-containing gangliosides. This property of avian precursor viruses was preserved in early human H3 isolates, but was gradually lost with further circulation of the H3 HA in humans. Consequently, later human H3 isolates, as well as H1 and type B human strains, were unable to bind to short Neu5Ac2-3Gal-terminated gangliosides, an incompatibility that correlated with higher glycosylation of the HA globular head of human viruses. Our results suggest that the RBS is highly conserved among HA subtypes of avian influenza virus, while that of human viruses displays distinctive genotypic and phenotypic variability.

Specification of receptor-binding phenotypes of influenza virus isolates from different hosts using synthetic sialylglycopolymers: non-egg-adapted human H1 and H3 influenza A and influenza B viruses share a common high binding affinity for 6'-sialyl(N-acetyllactosamine)

Synthetic sialylglycoconjugates bearing 3'-sialyllactose, 6'-sialyllactose, or 6'-sialyl(N-acetyllactosamine) moieties attached to the polyacrylic acid carrier (P-3-SL, P-6-SL, and P-6-SLN, respectively) were prepared and tested for their ability to bind to influenza virus isolates from different hosts in a competitive solid phase assay. The virus panel included egg-grown avian and porcine strains, as well as human viruses isolated and propagated solely in mammalian (MDCK) cells and their egg-adapted variants. A clear correlation was observed between the pattern of virus binding of two glycopolymers, P-3-SL and P-6-SLN, and the host species from which the virus was derived. Avian isolates displayed a high binding affinity for P-3-SL and a two to three orders of magnitude lower affinity for P-6-SLN. By contrast, all non-egg-adapted human A and B viruses bound P-6-SLN strongly but did not bind P-3-SL. Unlike the "authentic" human strains, their egg-adapted counterparts acquired an ability to bind P-3-SL, indicative of a shift in the receptor-binding phenotype toward the recognition of Neu5Ac2-3Gal-terminated sugar sequences. Among the porcine viruses and human isolates with porcine hemagglutinin, few displayed an avian-like binding phenotype, while others differed from both avian and human strains by a reduced ability to discriminate between P-3-SL and P-6-SLN. Our data show that sialylglycopolymers may become a useful tool in studies on molecular mechanisms of interspecies transfer, tissue specificity, and other structure-function relationships of the influenza virus hemagglutinin.

Strategy for adventitious agent assays

There are three important questions to be addressed when considering a strategy for adventitious virus screening. (i) What viruses should be tested for? This will depend on the species of origin of the production cell although careful consideration has to be given also to the entire manufacturing process for the possible introduction of a contaminating virus from other biological materials used in production, e.g. bovine serum. (ii) What tests should be performed? Tests for viruses are highly varied. General tests act as "catch-all" types of assays. These are complemented by a variety of tests for individual viruses or types of viruses. (iii) At what stage(s) during the manufacturing process should virus testing be performed? Manufacturing stages which should be tested for virus contamination include extensive testing of the master cell bank and limited testing of the working cell bank and end of production cells.

Replicative advantage in tissue culture of egg-adapted influenza virus over tissue-culture derived virus: implications for vaccine manufacture

Influenza virus derived from clinical material on MDCK cells has been shown to possess haemagglutinin (HA) indistinguishable from that of the natural, uncultivated virus. In contrast, viurs derived in embryonated hens' eggs are variants with substitutions in their HA in the vicinity of the receptor binding site. We report here the superior growth of egg-adapted virus over cell-derived virus on MDCK cells in studies in which MDCK-derived virus was spiked with small amounts of egg-adapted virus and the mixture sequentially passaged on MDCK cells. Such egg-derived variants bind to and are internalized by MDCK cells with a much higher efficiency than cell-derived virus. These data imply that the natural virus, whilst able to replicate on MDCK cells, is by no means the best fit for the MDCK receptor and variants with appropriate substitutions around the receptor binding site can readily displace the natural virus. Vaccine manufacturers who are investigating the use of tissue culture for vaccine production should minimize passage levels of cell-derived virus and beware of the displacement of the original virus with variants similar to those derived in eggs, which are often antigenically distinct.

A collaborative study on DNA quantitation in biological products

In 1988, a collaborative study was set up to examine the sensitivity and reproducibility of assays for the detection of DNA in biologicals derived from continuous cell lines. Fifteen laboratories analyzed 12 samples containing different amounts of DNA and protein. When no or very little DNA was present in the test samples, false positives or overestimation was common. In contrast, when higher levels of DNA were present underestimation was the norm. The study also revealed a high degree of variability between laboratories.

Osteoradionecrosis of the hyoid induced by combined modality therapy for laryngeal carcinoma

Contemporary management of laryngeal carcinoma often incorporates multiple modalities of therapy. We report a case of osteoradionecrosis of the hyoid bone in a patient treated with surgery, chemotherapy, and radiation therapy for a supraglottic squamous cell carcinoma. A discussion regarding pathophysiology, radiation dosimetry and treatment options is also presented.

Safety considerations for nucleic acid vaccines

Whilst it is premature to formulate guidelines for the manufacturing requirements of a nucleic acid vaccine, it can never be too early to discuss and debate the issues surrounding the use of a novel biotherapeutic. The major safety issues posed by nucleic acid vaccination include the possibility of transformation (or tumorigenic) events in recipients of a DNA vaccine, the potential formation of anti-DNA antibodies, and unexpected and untoward effects of long-term expression of a foreign antigen. This paper examines the extent to which these points impinge on the safety of DNA vaccines.

Mixed populations in influenza virus vaccine strains

Human influenza viruses used for vaccine production have previously been adapted to grow in eggs. During egg adaptation, variants are selected and we have observed that more than one variant may derive in a single egg resulting in a mixed population. We have now investigated the extent of heterogeneity, due to host cell selection, of virus strains used for the manufacture of influenza vaccine for the 1991/1992 and 1992/1993 seasons. The A(H1N1) vaccine virus was homogeneous with respect to substitutions in the haemagglutinin deriving from egg adaptation. However, two A(H3N2) vaccine strains and the influenza B component, B/Yamagata/16/88, consisted of mixed populations, apparently due to their cultivation in eggs. The individual variants within B/Yamagata were isolated and found to be antigenically distinct. The ratios of these variants within different manufacturers' seed stocks varied to the extent that vaccine derived from them could be distinguished antigenically. Furthermore, derivation of high-growth reassortants from the A(H3N2) strains which involves passaging at limit dilution did not necessarily lead to a homogeneous virus population. The significance of these findings for the efficacy of vaccine is not known at present.

Antigenic and molecular characterization of host cell-mediated variants of equine H3N8 influenza viruses

Antigenic differences between three of six equine influenza virus (H3N8) MDCK cell- and egg-derived pairs have been demonstrated using monoclonal and polyclonal antibodies. Sequencing of the haemagglutinin (HA) genes revealed amino acid changes in four of the six virus pairs. These data contrast with those for human isolates of influenza virus in that it was predominantly tissue culture-isolated equine virus and not egg-derived virus which displayed heterogeneity. Some of the molecular changes involved are located within the vicinity of the cell receptor-binding site (positions 156, 158 and 222) whereas others are in the vicinity of the HA1-HA2 cleavage site (positions 18 and 32 of HA1 and position 12 of HA2). Our results indicate that the host cell can play a part in selecting antigenic variants of equine influenza virus and suggest that the egg, and not cell culture as is the case for human isolates, is the preferred host for vaccine and antigenic studies.

Codominant mixtures of viruses in reference strains of influenza virus due to host cell variation

Influenza viruses grown in chicken eggs may comprise mixtures of variants, creating problems in establishing international reference strains and in preparing high growth reassortants. We therefore analyzed representative reference strains of H3N2 viruses from 1987 to 1989 by direct sequencing of HA1. Three of seven reference strains had different nucleotides at the same position in nucleotide gels, indicating the presence of codominant mixtures. These nucleotide duplications occurred at residues previously shown to code for amino acids associated with egg adaptation (156, 186, and 193 of HA1). Cloning of these viruses in chicken eggs permitted separation of the mixtures, and the majority of these cloned viruses could be distinguished with monoclonal antibodies. The remaining four reference strains were homogeneous and contained one of the two amino acids usually found at these residues in HA1 (e.g., 145 Glu or Lys, 186 Ser or Ile). Analysis of epidemic H3N2 isolates, for which mammalian cell and egg isolates are available for sequence analysis from the same patient, confirmed that multiple nucleotide changes can occur at residues associated with egg adaptation. When reference or epidemic strains containing codominant mixtures were passaged in eggs one to five times, one of the codominant strains usually became dominant. Similar results were obtained with Madin Darby Canine kidney (MDCK) cells, although the dominant virus usually differed from that in eggs. Attempts to reselect an influenza virus possessing sequence changes in HA1 characteristic of mammalian cells or the original human isolate (i.e., 156 Glu, 158 Glu, 186 Ser) by multiple passages in MDCK cells were not successful, but evidence was obtained that MDCK cells can provide a selective growth advantage. Thus, variants that are dominant in eggs are not necessarily dominant in MDCK cells. To preserve the original genotype of viruses used as reference strains, we recommend the following procedure: (i) cloning in chicken eggs of the candidate virus at a very early passage, (ii) selection and analysis of multiple clones with the same ferret polyclonal and/or monoclonal antibodies used in the initial screening, and (iii) selection of the isolate whose hemagglutinin molecule most closely resembles the clinical isolate.

The role of amniotic passage in the egg-adaptation of human influenza virus is revealed by haemagglutinin sequence analyses

Obtaining an isolate of a human influenza virus in the allantoic cavity of the embryonated hen's egg is more efficient if the clinical sample is initially passaged in the amniotic cavity. To investigate the extent to which the variants present after allantoic propagation are also selected by amniotic passage, clinical virus passaged once in the amnion has been subjected to extensive genetic and antigenic analyses. The data indicate that the natural virus can replicate unrestrictedly within the amnion. However, exposure of amniotic virus to the allantois during the incubation period, which will occur through the hole between the amniotic and allantoic cavities caused by the inoculating needle, allows for the possibility of an egg-adapted variant establishing replication within the allantois and returning to the amnion. These observations illustrate why prior passage in the amnion increases the probability of a variant successfully establishing itself during a subsequent allantoic passage.

Analysis of the restriction to the growth of nonegg-adapted human influenza virus in eggs

When nonegg-adapted human influenza virus, i.e., either the natural virus present in a clinical specimen or an isolate propagated exclusively in tissue culture cells, is first passaged in the allantoic cavity of embryonated hens' eggs, variants which have amino acid substitutions around the receptor binding site are selected. We have studied the biological basis for the restriction of MDCK cell-derived virus to growth in the egg by analyzing the relative binding activities of viruses derived from the same clinical specimen, one in MDCK cells and one in eggs, and have identified a difference between the interaction of egg- and cell-derived virus with the surface of allantoic cells. The majority population of MDCK cell-derived virus binds to but fails to be internalized by allantoic cells resulting in the selection of variants which have this ability.

Influence of host cell-mediated variation on the international surveillance of influenza A (H3N2) viruses

Growth of clinical specimens of influenza viruses in eggs can result in the selection of antigenic variants distinct from corresponding viruses grown in mammalian tissue culture. To evaluate the contribution of host cell selection on the antigenic diversity of human influenza isolates, as seen in annual surveillance studies, viruses grown in embryonated eggs were compared by antigenic and genetic analyses with their mammalian tissue culture-grown counterparts. Clinical specimens were gathered from around the world from late 1987 to 1990 and the antigenicity of isolated viruses was assessed by hemagglutination-inhibition assays using immune ferret sera as is currently performed for routine surveillance and the selection of vaccine strains. In addition, viruses were assessed using a panel of anti-H3 HA monoclonal antibodies. The extent of antigenic variation exhibited by the egg-grown strains was far greater than the relative antigenic homogeneity of the tissue culture-grown viruses. Nucleotide sequence analysis of HA1 gene PCR products of 28 MDCK cell and egg derived pairs allowed identification of amino acid substitutions responsible for the antigenic differences observed and the adaptation to growth in eggs. Among these substitutions was a change at amino acid position 186 of HA1 (Ser in tissue culture viruses and lle in egg-grown viruses) which was observed at relatively high frequency. Egg- and MDCK-grown pairs with this single amino acid difference were classified into distinct antigenic groups by ferret sera raised to WHO reference viruses. Given the additional antigenic diversity observed among egg-grown strains, considerable care should be taken in the selection of reference and vaccine strains grown in eggs. Rapid sequence comparisons of MDCK- and egg-grown viruses allow identification of variants arising through egg selection and will prove to be a useful adjunct to antigenic surveillance for the selection of reference and vaccine strains.

The haemagglutinins of influenza A (H1N1) viruses in the 'O' or 'D' phases exhibit biological and antigenic differences

Influenza A (H1N1) viruses when initially isolated in mammalian cell cultures (MDCK cells) had different agglutination reactions with chicken and guinea-pig erythrocytes compared to the same viruses after passage. On first isolation the virus HA resembled the 'O' phase viruses described originally by Burnet and Bull and agglutinated mammalian but not avian erythrocytes. After passage, the virus HA resembled a classical 'D' phase virus and agglutinated both avian and mammalian erythrocytes. Monoclonal and polyclonal antisera detected antigenic differences between the HAs of the viruses in the 'O' and 'D' phases. The 'O' phase virus HA reacted preferentially with antibodies in post infection human antisera. Viruses in the 'O' phase replicated poorly in the allantoic cavity of embryonated hens' eggs whilst 'D' phase virus replicated in both MDCK cells and in embryonated hens' eggs. At least three distinguishable subpopulations of influenza A (H1N1) viruses may co-exist in clinical throat swab material, including viruses possessing HAs in the 'O' and 'D' phases and other 'D' phase viruses cultivable in embryonated hens' eggs but antigenically distinguishable from the corresponding 'D' phase virus in MDCK cells.