[Effect of amino acid substitutions in small subunit of avian H5N2 influenza virus hemagglutinin on selection of the mutants resistant to neutralizing monoclonal antibodies]

Changes associated with the resistance to physical and chemical factors in the hemagglutinin (HA) of influenza A viruses may play an important role in the selection of different influenza variants during circulation in nature. Here, we studied the escape mutants of influenza virus A/mallard/Pennsylvania/10218/84 (H5N2) that were selected by the monoclonal antibody. The escape mutant m4F11(4) carried a single amino acid substitution in large subunit (HA1) of the HA, S145P1, and two ones, m4G10(10) and m4G10(6), had additional amino acid changes in the small subunit (HA2), namely: L124F2 and L124F2 + N79D2, respectively. As it has been found the substitutions appeared in the HA2 of m4G(10) and m4G(6) viruses compensated negative effect of the S145P1 mutation and provided a significant increase in the viral replication ability at the early stage of infection in embryonated chicken eggs as well as in HA thermostability in comparison with m4F11(4) mutant. Phenotypic properties that provide advantages in the process of virus replication can play a role of the positive selection factor in viral population.

[Effect of mutations changing the antigenic specificity on the receptor-binding activity of the influenza virus hemagglutinin of H1 and H5 subtypes]

The influenza virus hemagglutinin (HA) is an envelope virus glycoprotein responsible for the attachment of the virus particles to cells via binding terminal sialic acid residues of cell surface oligosaccharides. In our previous works on influenza A virus escape mutants, that is, mutants resistant to the neutralization effect of monoclonal antibodies, we encountered amino acid changes in the vicinity of receptor-binding pocket of the HA. In this work the degree of the affinity to both alpha-2, -3, and alpha-2, -6, -sialoglycoconjugates was assessed for escape mutants of influenza H1 and H5 viruses. The data demonstrate that the decrease of the positive electrostatic charge of the HA molecule surface resulting from amino acid changes conferring resistance to monoclonal antibodies may lead to a lowering of the affinity to sialic acid-containing analogs of cell receptors. The results are discussed in the context of the evolution of HA in natural circulation of H1 and H5 influenza viruses.

Yields of virus reassortants containing the HA gene of pandemic influenza 2009 virus

KEYWORDS

influenza virus; reassortment; virus yield; gene constellation.

[High-yield reassortant virus containing hemagglutinin and neuraminidase genes of pandemic influenza A/Moscowl/01/2009 (H1N1) virus]

The crossing of influenza A/Moscow/01/2009 (H1N1) virus and reassortant strain X31 (H3N2) containing the genes of internal and non-structural proteins of A/Puerto Rico/8/34 (H1N1) strain gave rise to reassortant virus ReM8. The reassortant contained hemagglutinin (HA) and neuraminidase (NA) genes of pandemic 2009 influenza virus and 6 genes of high-yield A/Puerto Rico/8/34 (H1N1) strain. The reassortant ReM8 produced higher yields in the embryonated chicken eggs than the parent pandemic virus, as suggested by infectivity and HA activity titration as well as by ELISA and the measurement of HA protein content by scanning electrophoresis in polyacrylamide gel slabs. High immunogenicity of ReM8 reassortant was demonstrated by immune protection studies in mice. The reassortant virus ReM8 is suitable as a candidate strain for the production of inactivated and subunit influenza vaccines.

[Impact of mutations in the hemagglutinin gene of H5N1 influenza virus on antigenicity and virulence as revealed by site-specific mutagenesis]

In our earlier studies, we have shown that amino acid changes in the hemagglutinin (HA) of influenza H5N1 virus escape mutants conferring resistance to monoclonal antibodies (MAbs) may correlate with a decrease of virus virulence for mice and that the virulence can be restored to the initial level by serial passages. In the present study, the mutations identical to those observed in the HA of a low-virulent escape mutant and its readapted variant were introduced into the HA gene by site-specific mutagenesis. The viruses produced by plasmid transfection and containing the HA gene either of A/Vietnam/1203/2004 (H5N1) virus with a deletion at the cleavage site, or of a low-virulent escape mutants, or of its readapted variant, in the presence of 6 genome segments of A/Puerto Rico/8/34 (H1N1) virus and the NA gene of A/Vietnam/1203/2004 (H5N1) virus, were assayed for virulence. Determination of virulence for mice indicated that amino acid substitution in the HA gene of a low-virulent escape mutant produced a decrease of virulence whereas the additional mutation identical to that acquired by the escape mutant in the course of readaptation restored the virulence to initial level. The findings are the first strong evidence for lower H5N1 virus virulence resulting from the amino acid substitution changing the antigenic specificity of HA and for restored virulence arising from compensating mutation in the HA gene.

Antigenic epitopes in the hemagglutinin of Qinghai-type influenza H5N1 virus

The highly pathogenic avian influenza H5N1 viruses have become widespread and evolved into several clades. In our previous studies, the antigenic sites of the H5 hemagglutinin (HA) were characterized by selection and sequencing of escape mutants. In the present studies we analyzed the antigenic epitopes recognized by monoclonal antibodies against avian influenza A/Duck/Novosibirsk/56/05 (H5N1) virus isolated in western Siberia and belonging to subclade 2.2 of the H5N1 viruses. The analysis revealed several antigenically relevant positions of amino acid residues in the globular head of the HA not encountered earlier in the escape mutants of the H5 subtype. The newly recognized positions (113, 117, 118, 120, and 123, mature H5 numbering) are concentrated in an area adjacent to the region described in earlier studies as corresponding to site B in H3 HA, but extending far beyond this area. The amino acid positions recognized by the monoclonal antibodies against A/Duck/Novosibirsk/56/05 (H5N1) virus differ from the positions recognized by the monoclonal antibodies against H5N2 influenza viruses. The data suggest that the evolution of the HA of H5 avian influenza viruses is associated not only with the changes of antigenic epitopes recognized by antibodies, but also with a change in the dominance of the immunogenicity of different sites in the HA.

[Amino acid substitutions in the hemagglutinin of H5 influenza virus changing the antigenic specificity and virus virulence]

In our earlier studies, we mapped the hemagglutinin antigenic epitopes of H5 influenza virus by selecting mutants resistant to the neutralizing effect of the antibody (escape mutants). Several escape mutants were shown to have a lowered virulence for mice. The readaptation of low-virulent escape mutants to mice resulted in the restoration of virulence. In the present communication. We present data on the assay of virulence of single-gene reassortants containing HA genes of the wild-type virus, low-virulent escape mutant, or re-adapted variant, and the other genes of a mouse-adapted H9N2 Influenza virus. The results demonstrate that the amino acid change S145F (H3 numbering) in the hemagglutinin ensuring the resistance to a monoclonal antibody can be deleterious to virulence, and that the damaging effect on virulence may be compensated for by additional amino acid changes in position 186 in the hemagglutinin arising in the course of virus passaging in mice. The data indicate that the compensational mutations restoring the pathogenic potential of antigenic variants may be regarded as an additional factor in the evolution of influenza virus hemagglutinin.

Location of antigenic sites recognized by monoclonal antibodies in the influenza A virus nucleoprotein molecule

The locations of amino acid positions relevant to antigenic variation in the nucleoprotein (NP) of influenza virus are not conclusively known. We analysed the antigenic structure of influenza A virus NP by introducing site-specific mutations at amino acid positions presumed to be relevant for the differentiation of strain differences by anti-NP monoclonal antibodies. Mutant proteins were expressed in a prokaryotic system and analysed by performing ELISA with monoclonal antibodies. Four amino acid residues were found to determine four different antibody-binding sites. When mapped in a 3D X-ray model of NP, the four antigenically relevant amino acid positions were found to be located in separate physical sites of the NP molecule.

[Differential incorporation of genomic segments into the influenza A virus reassortants in mixed infection]

The gene composition of the viral population obtained via mixed infection of embryonated chick eggs with influenza viruses at a high multiplicity of infection was analyzed. In mixed infection caused by influenza A/WSN/33 (H1N1) and A/Duck/Czechoslovakia/56 (H4N6) viruses, the population showed a preponderance of the reassortants whose content of genomic segments originating from either of the parent virus deviated strongly from the random pattern: the hemagglutinin (HA) gene of A/WSN/33 (H1N1) virus and the NP gene of A/Duck/Czechoslovakia/56 (H4N6) virus were prevalent in the gene composition of the reassortants. The mixed infection produced by influenza A/Udorn/72 (H3N2) virus and the reassortant R8 containing the HA gene of A/Duck/Ho Chi Minh/014/78 (H5N3) virus, the population of reassortants contained mainly the HA gene of A/Udorn/72 (H3N2) virus and the NP gene of the reassortant R8. The findings are discussed due to the problem of specific recognition of gene segments when incorporated into the viral particles.

Post-reassortment amino acid change in the hemagglutinin of a human-avian influenza H5N1 reassortant virus alters its antigenic specificity

It was shown earlier that the reassortant influenza virus having hemagglutinin (HA) gene of A/Duck/Primorie/2621/2001 (H5N2) virus and 7 genes of A/Puerto Rico/8/34 (H1N1) virus produced low yields in embryonated chicken eggs. We found that a variant reassortant selected by serial passages in eggs produced higher yields than the initial reassortant. The variant reassortant had an amino acid substitution in the hemagglutinin N244D (H3 numbering). In this report we demonstrated that the post-reassortment amino acid substitution N244D altered the antigenic specificity of HA as revealed by the loss of reactivity with an anti-H5 monoclonal antibody in hemagglutination-inhibition (HI) test. The results are discussed in association with the evolution of H5 hemagglutinin.

[Optimization of the gene composition of influenza H5 virus hemagglutinin-containing reassortants and their efficacy in immune cross-protection experiments]

The reassortant described in the authors' previous paper contained 6 genes originating from the high-yield virus A/Puerto Rico/8/34 (H1N1) and the genes of hemagglutinin (HA) and neuraminidase (NA) of the low-pathogenic avian influenza A/Duck/Primorie/2621/2001 (H5N2) (6:2 reassortant). The reassortant was used for the backcrossing with the parent avian virus in order to optimize the gene composition. Genotyping of the highest-yield second-generation reassortment indicated that it had obtained the PB1, HA, and NA genes from the virus A/Duck/Primorie/ 2621/2001 and the other genes received the genes from the virus A/Puerto Rico/8/34 (5:3 reassortant). The yield produced in the embryonated chicken eggs by the 5:3 reassortant was higher than that produced by the 6:2 reassortant although it did not achieve the reproduction of the parent virus A/Puerto Rico/8/34. Murine immunization with the inactivated reassortant containing the HA and NA genes of the virus A/Duck/Primorie/2621/2001 (H5N2) provided an efficient protection against the virus containing HA and NA of a recent H5N1 strain.

Epitope mapping of the hemagglutinin molecule of a highly pathogenic H5N1 influenza virus by using monoclonal antibodies

We mapped the hemagglutinin (HA) antigenic epitopes of a highly pathogenic H5N1 influenza virus on the three-dimensional HA structure by characterizing escape mutants of a recombinant virus containing A/Vietnam/1203/04 (H5N1) deltaHA and neuraminidase genes in the genetic background of A/Puerto Rico/8/34 (H1N1) virus. The mutants were selected with a panel of eight anti-HA monoclonal antibodies (MAbs), seven to A/Vietnam/1203/04 (H5N1) virus and one to A/Chicken/Pennsylvania/8125/83 (H5N2) virus, and the mutants' HA genes were sequenced. The amino acid changes suggested three MAb groups: four MAbs reacted with the complex epitope comprising parts of the antigenic site B of H3 HA and site Sa of H1 HA, two MAbs reacted with the epitope corresponding to the antigenic site A in H3 HA, and two MAbs displayed unusual behavior: each recognized amino acid changes at two widely separate antigenic sites. Five changes were detected in amino acid residues not previously reported as changed in H5 escape mutants, and four others had substitutions not previously described. The HA antigenic structure differs substantially between A/Vietnam/1203/04 (H5N1) virus and the low-pathogenic A/Mallard/Pennsylvania/10218/84 (H5N2) virus we previously characterized (N. V. Kaverin et al., J. Gen. Virol. 83:2497-2505, 2002). The hemagglutination inhibition reactions of the MAbs with recent highly pathogenic H5N1 viruses were consistent with the antigenic-site amino acid changes but not with clades and subclades based on H5 phylogenetic analysis. These results provide information on the recognition sites of the MAbs widely used to study H5N1 viruses and demonstrate the involvement of the HA antigenic sites in the evolution of highly pathogenic H5N1 viruses, findings that can be critical for characterizing pathogenesis and vaccine design.

Effect of gene constellation and postreassortment amino acid change on the phenotypic features of H5 influenza virus reassortants

Reassortants between a low-pathogenic avian influenza virus strain A/Duck/Primorie/2621/2001 (H5N2) and a high-yield human influenza virus strain A/Puerto Rico/8/34 (H1N1) were generated, genotyped and analyzed with respect to their yield in embryonated chicken eggs, pathogenicity for mice, and immunogenicity. A reassortant having HA and NA genes from A/Duck/Primorie/2621/2001 virus and 6 genes from A/Puerto Rico/8/34 virus (6:2 reassortant) replicated efficiently in embryonated chicken eggs, the yields being intermediate between the yields of the avian parent virus and those of the A/Puerto Rico/8/34 parent strain. The reassortant having the HA gene from A/Duck/Primorie/2621/2001 virus and 7 genes from A/Puerto Rico/8/34 virus (7:1 reassortant) produced low yields. A variant of the 7:1 reassortant selected by serial passages in eggs had an amino acid substitution in the hemagglutinin (N244D, H3 numbering). The variant produced yields similar to those of the 6:2 reassortant. A 5:3 reassortant generated by a back-cross of the 6:2 reassortant with the avian parent and having PB1, HA and NA genes of A/Duck/Primorie/2621/2001 virus produced higher yields than the 7:1 or 6:2 reassortants, although still lower than the yields of A/Puerto Rico/8/34 virus. The 7:1, 6:2 and 5:3 reassortants were pathogenic for mice, with the level of virulence close to A/Puerto Rico/8/34 virus, in contrast to the extremely low pathogenicity of the A/Duck/Primorie/2621/2001 parent strain. Immunization of mice with an inactivated 6:2 H5N2 reassortant provided efficient immune protection against a reassortant virus containing the HA and NA genes of a recent H5N1 isolate. The results are discussed in connection with the problem of the improvement of vaccine strains against the threatening H5N1 pandemic.

[Reassortment and gene interactions in the crossing of low-pathogenic avian influenza H5 virus with human influenza virus]

The reassortants obtained via the crossing of highly productive influenza virus A/Puerto Rico/8/34 (H1N1) strain and the low pathogenic avian influenza virus A/Duck/Primorie/2621/2001 (H5N2) strain were genotyped and characterized. The H5N2 reassortant having 6 genes from A/Puerto Rico/8/34 virus has the high level of reproduction in chick embryos, while slightly more moderate than in the parent A/Puerto Rico/8/34 strain. The reproduction of the H5N1 reassortant that had 7 genes from A/Puerto Rico/8134 virus was very low. The serial passage selection allowed the investigators to obtain the H5N1 strain that was reproductively close to the H5N2 reassortant. This variant had one amino acid substitution in hemagglutinin (N244D, H3 numbering) and a lower affinity for fetuin. By the level of virulence to mice, the H5N1 and H5N2 reassortants were close to A/Puerto Rico/8/34 virus and greatly differed in this respect from low virulent A/Duck/Primorie/2621/2001 (H5N2). The results are discussed in connection with the problem of vaccination when there is a threat for H5N1 virus subtype-caused pandemic.

Restoration of virulence of escape mutants of H5 and H9 influenza viruses by their readaptation to mice

Antigenic mapping of the haemagglutinin (HA) molecule of H5 and H9 influenza viruses by selecting escape mutants with monoclonal anti-HA antibodies and subjecting the selected viruses to immunological analysis and sequencing has previously been performed. The viruses used as wild-type strains were mouse-adapted variants of the original H5 and H9 isolates. Phenotypic characterization of the escape mutants revealed that the amino acid change in HA that conferred resistance to a monoclonal antibody was sometimes associated with additional effects, including decreased virulence for mice. In the present study, the low-virulence H5 and H9 escape mutants were readapted to mice. Analysis of the readapted variants revealed that the reacquisition of virulence was not necessarily achieved by reacquisition of the wild-type HA gene sequence, but was also associated either with the removal of a glycosylation site (the one acquired previously by the escape mutant) without the exact restoration of the initial wild-type amino acid sequence, or, for an H5 escape mutant that had no newly acquired glycosylation sites, with an additional amino acid change in a remote part of the HA molecule. The data suggest that such 'compensating' mutations, removing the damaging effects of antibody-selected amino acid changes, may be important in the course of influenza virus evolution.

[Evolution of avian influenza viruses H5N1 (1997-2004) in southern and south-eastern Asia]

Highly pathogenic avian influenza viruses of the H5N1 subtype are widespread and have become endemic in poultry in southern and southeastern Asia. An unprecedented epizootic was caused by these viruses in 8 countries in the winter of 2003 to 2004. This fact along with more frequent human cases of the infection with unusually high mortality rates in Vietnam and Thailand raises concern that these H5N1 events may lead to a new influenza A virus pandemic. This review summarizes the results of studies dealing with the ecology and evolution of avian influenza H5N1 viruses in southern and southeastern Asia since 1997. The pathogenesis of the infection in human beings and laboratory animals and possible determinants of the high pathogenicity of H5N1 viruses in mammals are considered. A scheme for designing modified H5N1 vaccines using the latest advances in reverse genetics of influenza viruses is given.

Postreassortment changes in a model system: HA-NA adjustment in an H3N2 avian-human reassortant influenza virus

In our previous studies we described the postreassortment changes in the hemagglutinin (HA) of H2N1, H3N1, H4N1 and H13N1 influenza A virus reassortants with HAs derived from avian viruses and low-functional neuraminidase (NA) of a human parent virus A/USSR/90/77 (H1N1). The changes involved amino acid substitutions that increased the negative local charge in the vicinity of the receptor-binding pocket and decreased the affinity of HA to sialic acid receptors. In the present report we describe the studies performed with H3N2 reassortant viruses having HA of A/Duck/Ukraine/1/63 (H3N8) virus and NA of A/Aichi/2/68 (H3N2) virus. Amino acid changes in the HA gene registered in virus variants selected in the course of serial passages resulted in a decrease in the affinity to sialic acid-containing substrates and cell receptors. However, the decrease was less expressed than in the reassortants containing the low-functional NA of N1 subtype described in our earlier studies, and the amino acid changes were not necessarily associated with an increase of negative charge. In one passage variant an amino acid substitution in NA was detected. The relevance of these results for the evolution of the H3N2 virus of the 1968 pandemic is discussed.

Antigenically relevant amino acid positions as revealed by reactions of monoclonal antibodies with the nucleoproteins of closely related influenza A virus strains

The nucleoproteins (NP) of a group of influenza A virus strains were analyzed with the use of a panel of anti-NP monoclonal antibodies (Mabs) in the radioimmunosorbtion reaction with subsequent polyacrylamide gel electrophoresis of the immune complexes, and in the immunoblotting. The group included 2 pairs of closely related viruses having minimal amino acid differences in NP sequence. The analysis of the results of the immune reactions in comparison with the known amino acid sequences of NP allowed us to suggest that the Mab 150/4 recognizes the epitope containing amino acid residues in positions 196 and/or 290, whereas the amino acid residue in position 353 participates in the formation of the antigenic epitope reacting with the Mab 7/3.

Influenza H5 virus escape mutants: immune protection and antibody production in mice

Avian H5N1 influenza A viruses are considered to be of high pandemic potential as they are able to cross the avian-human species barrier and cause disease in humans. In the present study we assessed the impact of amino acid substitutions in the hemagglutinin (HA) of antigenic escape mutants of influenza A/Mallard/Pennsylvania/10218/84 (H5N2) (Mld/PA/84-MA) virus on the level of neutralizing antibodies and the ability to protect mice against challenge with the wild type H5 influenza virus. beta-Propiolactone-inactivated vaccines prepared from eight different H5 escape mutants could be separated into two groups based on levels of protection. One group of escape mutants [m46(7), m46(7)-24B9, m46(7)-55, and m46(7)-55-24B9] was characterized by providing high levels of protection (90.0-95.4% survival) to mice against subsequent challenge with 5 LD(50) of wild type Mld/PA/84-MA virus. The other group of escape mutants [m176/26, m55(2), m55(2)-24B9, and m24B9-176/26] provided moderate level of protection (57.1-66.6% survival) in mice. Analysis of the amino acid substitutions in the HA revealed that two amino acid changes in antigenic site B of the HA molecule (D(126)-->N and K(152)-->N) were associated for decreases in the levels of antibody and the immune protection afforded by vaccination with these H5 virus escape mutants. The phenotypic effects of mutations in HA gene of H5 virus may be of importance to appraise the extent and direction of H5 influenza viruses antigenic evolution.

Structural differences among hemagglutinins of influenza A virus subtypes are reflected in their antigenic architecture: analysis of H9 escape mutants

We used a panel of monoclonal antibodies to H9 hemagglutinin to select 18 escape mutants of mouse-adapted influenza A/Swine/Hong Kong/9/98 (H9N2) virus. Cross-reactions of the mutants with the antibodies and the sequencing of hemagglutinin genes revealed two minimally overlapping epitopes. We mapped the amino acid changes to two areas of the recently reported three-dimensional structure of A/Swine/Hong Kong/9/98 hemagglutinin. The grouping of the antigenically relevant amino acid positions in H9 hemagglutinin differs from the pattern observed in H3 and H5 hemagglutinins. Several positions in site B of H3 hemagglutinin are distributed in two sites of H9 hemagglutinin. Unlike any subtype analyzed so far, H9 hemagglutinin does not contain an antigenic site corresponding to site A in H3 hemagglutinin. Positions 145 and 193 (H3 numbering), which in H3 hemagglutinin belong to sites A and B, respectively, are within one site in H9 hemagglutinin. This finding is consistent with the peculiarity of the three-dimensional structure of the H9 molecule, that is, the absence from H9 hemagglutinin of the lateral loop that forms site A in H3 and the equivalent site in H5 hemagglutinins. The escape mutants analyzed displayed phenotypic variations, including decreased virulence for mice and changes in affinity for sialyl substrates. Our results demonstrate a correlation between intersubtype differences in three-dimensional structure and variations among subtypes in the distribution of antigenic areas. Our findings also suggest that covariation and pleiotropic effects of antibody-selected mutations may be important in the evolution of H9 influenza virus, a possible causative agent of a future pandemic.

[Antigenic structure of influenza A virus subtype H5 hemagglutinin: mechanism of acquiring stability to monoclonal antibodies in escape-mutants]

The analysis of escape mutants of the avian influenza virus of H5 subtype (strain A/Mallard/Pennsylvania/10218/84) revealed the location and structure of two antigenic sites in the hemagglutinin (HA) molecule. Several escape mutants exhibited unusual features in the reactions with monoclonal antibodies (Mabs), being completely resistant in the infectivity neutralization test to the Mabs used for their selection, and retaining the ability to bind the Mabs as revealed by enzyme-linked immunosorbent assay. An enhancement of the binding by an amino acid change in a different antigenic site was demonstrated, as well as a complete abolishment of the binding by a mutation selected by passage in the presence of an excess of the non-neutralizing Mab of high binding ability. The observed effects did not result from the changes in the affinity of the mutant HA toward sialic receptors. The data suggest that one amino acid change in HA may prevent the virus neutralization by different mechanisms for different Mabs: either the binding of the Mab to HA is prevented, or the bound Mab is unable to block the receptor-binding pocket of HA. Different mechanisms of the acquisition of resistance to Mabs in the course of the selection of escape mutants are discussed.

[Interspecies transmission of influenza A viruses and influenza pandemics]

Molecular and genetic data are summarized on the origin of influenza A virus pandemic variants. Conceptual modifications of the reassortment theory of the origin of pandemic strains are discussed in connection with the appearance of new H5 and H9 avian influenza viruses, which caused the respiratory infection in man and which are presently in the focus of attention as possible agents of future pandemic.

[Variation of primary structure and conformation of NP protein from influenza A virus, recognized by monoclonal antibodies]

The antigenic structure of influenza A NP protein was studied by using a panel of 12 monoclonal antibodies (mAbs). A group of strains of influenza-A virus with the known NP amino acid sequence was analyzed by radioimmunoprecipitation with a subsequent analysis of the immune complexes by electrophoresis in polyacrylamide gel. The examined group of strains comprised pairs of closely related variants like A/USSR/90/77 and A/Brazil/1/78 as well as A/Puerto Rico/8/34 (Mount Sinai variant) and A Puerto Rico/8/34 Cambridge 1 variant). The results made it possible to identify position 353 as a part of antigenic site. The combined results of radioimmunoprecipitation and immunoblotting suggest that the mentioned site is recognized by mAb IVE8 as a linear epitope and by mAb as a confirmation epitope. Amino acids in positions 196 and/or 290 are involved in the formation of another antigenic site. The N-terminal part of NP, removed in the course of protein processing, is not involved in the differentiation of avian and human strains by mAb 315. None of the mAbs used in our study recognized the N-terminal part of NP as a linear epitope.

[Functional interactions of the influenza virus glycoproteins]

Hemagglutinin (HA) and neuraminidase (NA) are functionally related coat glycoproteins of the influenza virus (Flu). HA interacts with terminal sialyl residues of oligosaccharides and ensures the binding of the virus particle to the cell surface. NA is a receptor-destroying enzyme that removes sialyl residues from oligosaccharides contained in cell and virus components and thereby prevents aggregation of virus particles. Analysis of reasortants combining low-functional NA of human Flu with HA of avian Flu showed that sialyl residues are not completely removed in some cases. With high HA affinity for sialyl substrates, such virus particles aggregate, aggregates accumulate on the cell surface, and virus yield decreases. Serial passaging of low-yield aggregating reassortants may lead to selection of high-yield variants, which do not aggregate. A loss of aggregation is due to a decrease in HA affinity for high-molecular-weight sialyl substrates. On evidence of sequencing of the HA gene in original reassortants and their nonaggregating variants, HA affinity is reduced and aggregation lost owing to a mechanism common for different HA antigenic subtypes (H2, H3, H4, and H13). This is an increase in the negative charge as a result of an amino acid substitution in the vicinity of the receptor-binding pocket of HA. Taken together, these findings suggest a way of postreassortment adaptation which improves the functional match of HA and NA. The experimental system employed provides a model of natural processes associated with generation of Flu variants having a pandemic potential.

Structure of antigenic sites on the haemagglutinin molecule of H5 avian influenza virus and phenotypic variation of escape mutants

To elucidate the structure of the antigenic sites of avian H5 influenza virus haemagglutinin (HA) we analysed escape mutants of a mouse-adapted variant of the H5N2 strain A/Mallard/Pennsylvania/10218/84. A panel of five anti-H5 monoclonal antibodies (mAbs) was used to select 16 escape mutants. The mutants were tested by ELISA and haemagglutination inhibition with this panel of anti-H5 mAbs and the HA genes of the mutants were sequenced. The sequencing demonstrated that the amino acid changes were grouped in two antigenic sites. One corresponded to site A in the H3 HA. The other contained areas that are separated in the amino acid sequence but are topographically close in the three-dimensional structure and partially overlap in the reactions with mAbs. This site corresponds in part to site B in the H3 structure; it also includes a region not involved in site B that partially overlaps site Sa in the H1 HA and an antigenic area in H2 HA. Mutants with the amino acid change K152N, as well as those with the change D126N, showed reduced lethality in mice. The substitution D126N, creating a new glycosylation site, was accompanied by an increase in the sensitivity of the mutants to normal mouse serum inhibitors. Several amino acid changes in the H5 escape mutants occurred at the positions of reported changes in H2 drift variants. This coincidence suggests that the antigenic sites described and analysed here may be important for drift variation if H5 influenza virus ever appears as a pathogen circulating in humans.

[Reduction of the functional match of influenza virus hemagglutinin and neuraminidase after reassortation of genes]

Influenza virus A (FluA) reassortants with low-functional neuraminidase (NA) of subtype N1 and hemagglutinin (HA) of subtypes H2, H3, H4, and H13 display virion aggregation and accumulate to a lower titer because sialyl residues are not completely removed from virion components. Nonaggregating variants of FluA (H13N1) were shown to result from a mutation that reduces the HA affinity for sialyl substrates. Amino acid substitution K156E, which increases a negative charge at the edge of the receptor-binding pocket of HA large subunit (HA1), was revealed in two independent variants. This substitution was the only difference between HA1 of the original reassortant and one of its variants and, therefore, accounted for restoration of the functional match between HA and NA.

Cross-protection and reassortment studies with avian H2 influenza viruses

In order to assess the degree of immune cross-protection among avian H2 influenza virus strains, mice were immunised with beta-propiolactone-inactivated virus preparations and infected intranasally with mouse-adapted variant of A/Black Duck/New Jersey/1580/78 (H2N3) strain. The experiments with 11 avian H2 strains revealed that both Eurasian and American H2 avian influenza viruses exhibit either high or moderate degree of cross-protection. The grouping of the strains in accordance with their cross-protection efficiency does not coincide with H2 phylogenetic branches. Several reassortant clones were obtained with the use of A/Pintail Duck/Primorie/695/76 (H2N3) strain and high-yield X-67 reassortant as parent viruses, among them a high-yield H2N3 reassortant. Taking into account the data on cross-protection among avian H2 strains, the high-yield H2N3 reassortant may be regarded as a prototype strain to be used for the preparation of killed vaccines in the case of a new appearance of avian H2 haemagglutinin in circulation in humans.

Intergenic HA-NA interactions in influenza A virus: postreassortment substitutions of charged amino acid in the hemagglutinin of different subtypes

In our previous studies influenza A virus reassortants having neuraminidase (NA) gene of A/USSR/90/77 (H1N1) strain and hemagglutinin (HA) genes of H3, H4 and H13 subtypes were shown to produce a low virus yield and to exhibit a strong tendency to virion aggregation. More detailed studies with the use of a H3N1 reassortant and its high-yield non-aggregating variants revealed that NA of A/USSR/90/77 strain is inefficient in the removal of the terminal sialic acid residues from the virion components, and that the inefficiency of NA may be compensated by mutations in HA gene leading to a decrease of the receptor-binding affinity (Kaverin, N.V. , Gambaryan, A.S., Bovin, N.V., Rudneva, I.A., Shilov, A.A., Khodova, O.M., Varich, N.L., Sinitsin, B.V., Makarova, N.L., Kaverin, N.V., 1998. Postreassortment changes in influenza virus hemagglutinin restoring HA-NA functional match, Virology 244, 315-321). The present report describes studies performed with the use of H2N1 and H4N1 reassortants having HA genes of A/Pintail/Primorie/695/76 (H2N3) and A/Duck/Czechoslovakia/56 (H4N6) strains respectively and NA gene of A/USSR/90/77 strain. The low-yield reassortants and their high-yield non-aggregating variants were studied in both direct and competitive binding assays with sialic acid-containing substrates. The non-aggregating variants were shown to have a decreased affinity as compared to the initial reassortants toward high-molecular-weight sialic acid-containing substrates. The sequencing of HA genes revealed that all non-aggregating variants of H2N1 and H4N1 reassortants had amino acid substitutions increasing the negative charge of the HA molecule in the vicinity of the receptor-binding pocket. The results suggest that the influenza virus reassortants containing low-functional NA undergo similar postreassortment changes irrespective of the HA subtype: their receptor-binding activity decreased due to negatively charged amino acid substitutions in the vicinity of the receptor-binding pocket.

Transmission of Eurasian avian H2 influenza virus to shorebirds in North America

Influenza A virus of the H2 subtype caused a serious pandemic in 1957 and may cause similar outbreaks in the future. To assess the evolution and the antigenic relationships of avian influenza H2 viruses, we sequenced the haemagglutinin (HA) genes of H2 isolates from shorebirds, ducks and poultry in North America and derived a phylogenetic tree to establish their interrelationships. This analysis confirmed the divergence of H2 HA into two geographical lineages, American and Eurasian. One group of viruses isolated from shorebirds in North America had HA belonging to the Eurasian lineage, indicating an interregional transmission of the H2 gene. Characterization of HA with a monoclonal antibody panel revealed that the antigenicity of the Delaware strains differed from the other avian strains analysed. The data emphasizes the importance of avian influenza surveillance.

[Dependence of antigenic properties of human and avian influenza A virus NP proteins on strain variations in the amino acid sequence]

Human and avian influenza A strains with a known amino acid sequence of NP protein were studied in radioimmunoprecipitation test with a panel of anti-NP monoclonal antibodies. Two of 7 MAbs (315 and IVE8) reacted with variable epitopes. One of the epitopes was present only in human strains, while the other in both human and avian strains, but absent in gull strains and in one human strain, A/Puerto Rico/8/34 (H1N1). The variations recognized by antibodies 315 and IVE8 correlated with amino acid substitutes in positions 16 and 353, respectively.

Strain-specific differences in the effect of influenza A virus neuraminidase on vector-expressed hemagglutinin

In order to evaluate the efficiency of the removal of sialic acid residues from the influenza virus hemagglutinin by the viral neuraminidase in the course of the virus replication cycle, CV-1 cells expressing the hemagglutinin of H7 subtype from an SV40-based vector were superinfected with influenza virus strain A/Duck/Ukraine/63 (H3N8) or A/USSR/90/77 (H1N1). Vector-expressed hemagglutinin was immunoprecipitated from cell lysates and analyzed by polyacrylamide gel electrophoresis. The data indicate that the removal of sialic acid residues from the vector-expressed H7 hemagglutinin by N1 neuraminidase of A/USSR/90/77 virus in the course of the virus replication cycle is incomplete. The results are discussed in connection with previously published data showing that the low activity of NA in wild-type influenza virus results in incomplete removal of sialic acid residues from virion components.

Inhibition of influenza A virus reproduction by a ribozyme targeted against PB1 mRNA

A ribozyme gene directed at a specific cleavage of mRNA coding for PB1 protein, a component of RNA-dependent RNA-polymerase of influenza A virus, was constructed. The avian adenovirus CELO virus-associated RNA (VA RNA CELO) promoter and human cytomegalovirus (CMV) promoter were used for the permanent expression of the ribozyme in cell lines. The cells were infected with influenza A virus strains A/Singapore/1/57 and A/WSN/33, and the suppression of the virus reproduction and virus-specific protein synthesis was measured. The maximal level of the inhibition of virus reproduction as compared to the reproduction in non-transformed cells was 93.5%. Defective recombinant adenoviruses were constructed carrying the genes of functional and non-functional ribozymes under the control of human cytomegalovirus (CMV) promoter. The reproduction of A/WSN/33 virus in CV-1 cells preinfected with recombinant adenoviruses was shown to be suppressed.

Postreassortment changes in influenza A virus hemagglutinin restoring HA-NA functional match

An important function of influenza virus neuraminidase (NA) is the removal of sialic acid residues from virion components in order to prevent the aggregation of virus particles. In previous communications we have reported that reassortant viruses containing the NA gene of A/USSR/90/77 (H1N1) virus and HA genes of H3, H4, H10, or H13 subtypes had a tendency to virion aggregation at 4 degrees C and that the virion clusters irreversibly dissociated after the treatment with bacterial neuraminidase. It was concluded that in such reassortants the removal of sialic acid residues is inefficient. Nonaggregating variants of the reassortants were selected in the course of serial passages in embryonated chicken eggs. In the present paper a reassortant virus, R2, having the HA gene of A/Duck/Ukraine/1/63 (H3N8) virus and the other genes of A/USSR/90/77 (H1N1) virus, as well as its non-aggregating passage variants and both parent viruses, have been studied in order to reveal the presence of unremoved sialic acid residues in the virions. An assay of sialic acid content by high-performance liquid chromatography with fluorescent detection has revealed the presence of sialic acid in the purified virus preparations of A/USSR/90/77 (H1N1) virus and the R2 reassortant and its nonaggregating variants, whereas only trace amounts of sialic acid have been detected in the A/Duck/Ukraine/1/63 (H3N8) parent virus. The data obtained with the use of the labeled "indicator" virus suggest that the unremoved sialic acid residues are present at the virion surface. The nonaggregating variants have been shown to possess a lower affinity toward high-molecular-weight sialic acid-containing substrates compared to the initial reassortant R2. Sequencing of HA genes has revealed amino acid changes in the nonaggregating variants compared to the initial reassortant. One substitution, N248D in HA1, is the same in two independently selected nonaggregating variants. The presented data suggest that the complete removal of sialic acid residues by viral NA from the virion components is not obligatory for the absence of virus particle aggregation: the latter may be achieved (in the reassortants and, presumably, in the wild-type virus) through a balance between the degree of HA affinity toward the sialic acid-containing receptors and the extent of the removal of sialic acid residues by NA.

[Borna disease virus--a possible etiological agent of human psychoneurological disorders?]

The characteristics of Borna disease virus are described, specifically, the structure and replicative features. This virus differs from other representatives of Mononegavirales order and therefore cannot be referred to any of its families. There are grounds to propose that Borna disease virus is the forefather of a new family: Bornaviridae. The review presents data on neurological and behavioral disorders caused by this virus in animals. A relatively high level of antiviral antibodies was detected in humans with mental disorders; moreover, virus antigens and virus RNA were found in the brain postmortem. Contribution of Borna disease virus to mental diseases of man is discussed.

[A mechanism for limiting reproduction influenza A virus reassortants with incomplete functional compatibility of hemagglutinin and neuraminidase gene products]

The mechanism of decrease in the level of virus accumulation in reassortants with hemagglutinin (HA) and neuraminidase (NA) genes from different parents is studied. The reassortant viruses and their passage variants do not differ by the rate of virus protein production or their stability in infected cells. Electron microscopy and titration of infectious virus in culture fluid and cell-associated virus showed that the variants selected by serial passages accumulated mainly in the culture fluid, whereas the initial reassortant virions were predominantly cell-associated. These data suggest that incomplete removal of sialic acid residues by viral neuraminidase N1 in some reassortants results in re-attachment of virions to the infected cells and thus impairs the virus dissemination, which may be regarded as a reassortant-limiting factor probably significant for virus evolution.

[Study of incorporation of highly-cleaved H7 hemagglutinin, expressed by a recombinant vector in virions of superinfecting influenza viruses]

CV-1 cells were preinfected with H7-SV40 recombinant vector and superinfected with influenza viruses A/USSR/90/77 (H1N1), A/Duck/Czechoslovakia/56 (H4N6), or B/Hongkong/73. A fraction of the yields of influenza A viruses was infectious without trypsin treatment due to the incorporation of cleaved H7 hemagglutinin into virions. The specificity of the effect was confirmed by neutralization of the infectivity with anti-H7 serum. At a low multiplicity of infection the incorporation of H7 hemagglutinin into virions allowed the virus to replicate without trypsin in a multi-cycle manner. The fraction of the infectious virus and the kinetics of the multi-cycle infection in H7-expressing cells were similar in the cells superinfected with A/USSR/90/77 and with A/Duck/Czechoslovakia/56 viruses, thus demonstrating a low specificity of interaction between H7 hemagglutinin and heterologous M2 proteins. The superinfection of H7-expressing cells with influenza B/Hongkong/73 virus did not produce infectious phenotypically mixed virus, which may be regarded as an indication of inability of influenza B virus NB protein to replace functionally the influenza A virus M2 protein in its interaction with hemagglutinin.

Phenotypic expression of HA-NA combinations in human-avian influenza A virus reassortants

Human-avian and human-mammalian influenza A virus reassortant clones with the neuraminidase (NA) gene of the A/USSR/90/77 (H1N1) strain and hemagglutinin (HA) genes of H3, H4 and H13 subtypes had been shown in an earlier publication to produce low HA yields in the embryonated chicken eggs. The low HA titers had been shown to be due, at least in part, to the formation of virion clusters at 4 degrees C; the clustering was removed by the treatment with bacterial neuraminidase [Rudneva et al., Arch. Virol (1993) 133: 437-450]. By serial passages of the reassortants in chick embryos non-aggregating variants were selected: the variants produced HA titers of the same order as A/USSR/90/77 parent virus. The assessment of the virus yields by the analysis of the partially purified virus preparations from fixed volumes of the allantoic fluid revealed that actual virion yields of the initial reassortants were lower than the yields of their passaged variants or of the parent viruses. The passaged variant of a reassortant possessing the HA gene of A/Duck/Ukraine/1/63 (H3N2) virus differed from the original (non-passaged) reassortant and from the parent A/Duck/Ukraine/1/63 virus in the reaction with a panel of monoclonal antibodies against H3 hemagglutinin. The data suggest that some HA-NA combinations may lead to an incomplete functional match between HA and NA and to the formation of low-yield reassortants, thus representing a possible limiting factor in the emergence of new HA-NA combinations in natural conditions.

Phenotypic mixing with recombinant haemagglutinin of high cleavability mediates multi-cycle replication of human influenza virus in cell culture

When CV-1 cells expressing haemagglutinin (HA) of fowl plague virus A/FPV/34/Rostock(H7) (FPV) from an SV40-based recombinant vector were superinfected with the human influenza virus A/FM/1/47(H1N1)(FM1), phenotypically mixed progeny virus was observed. It contained cleaved FPV HA and uncleaved FM1 HA, was infectious without trypsin treatment and its infectivity was neutralizable by anti-FPV serum. When superinfection of H7 HA-expressing CV-1 cells was performed at a low multiplicity of infection, multi-cycle replication occurred. Control cells preinfected with an SV40-based recombinant not expressing FPV HA did not allow multi-cycle replication. Multi-cycle replication of FM1 virus was also observed when cells were preinfected with a vector expressing a highly cleavable mutant of influenza virus A/Port Chalmers/1/73(H3) HA carrying an insert of four arginine residues at the cleavage site. This was not the case when cells expressing uncleaved wild-type H3 HA were used. The results show that by phenotypic mixing with recombinant HA of high cleavability, a human influenza virus can be obtained in infectious form from cells lacking a suitable protease to activate this virus.

Replication of influenza A viruses in a green monkey kidney continuous cell line (Vero)

A Vero cell line was investigated as a suitable host system for primary isolation and cultivation of influenza A viruses. The efficiency of primary isolation for currently circulating (H3N2) strains was similar in Vero and MDCK cells. Of 72 egg-adapted strains investigated, 90.3% were detectable hy hemagglutinin (HA) titration in Vero cells after the first passage and 51.4% after the second. The amino acid sequences of the HA1 region of influenza A viruses isolated and passaged in Vero cells were identical to those of their MDCK-grown counterparts. At low MOI, high yields of influenza virus were achieved in Vero cells by multiple additions of trypsin to the medium. After 20 passages of A/England/1/53 (H1N1) in Vero cells, the titer of infectious virus was 8.37 log10 TCID50/mL, and virus protein yields were as high as in MDCK cells.

Impairment of multicycle influenza virus growth in Vero (WHO) cells by loss of trypsin activity

We demonstrated that influenza virus replication in Vero (WHO) cells, a subline of African green monkey kidney cells, is impaired by rapid inactivation of trypsin in the culture fluids. Trypsin inactivation was caused by a factor secreted by Vero cells into the media. Repeated addition of trypsin to the culture medium of influenza virus-infected Vero cells restores the multicycle growth pattern of influenza A virus strains, allowing high yields to be obtained at a low multiplicity of infection. These findings may permit efficient use of Vero (WHO) cells in the production of influenza vaccines.

The effect of cell density on influenza virus replication in CV-1 cells

The number of hemadsorbing cells in CV-1 cell monolayers infected with influenza A virus was higher in semi-confluent cultures than in dense contact-inhibited monolayers. The level of virus-specific protein synthesis as well as the accumulation of virus progeny were also inversely correlated with cell density. At a high multiplicity of infection the majority of these cells in dense monolayers did not express HA on the cell surface, did not synthesize virus-specific proteins and survived at least 96 hours after infection. However, these cells developed a partial shut-off of cell protein synthesis and could not be efficiently superinfected. Analysis of the data of virus protein synthesis, progeny virus accumulation, and the assessment of hemadsorption suggest that CV-1 cells in dense monolayers can be infected at a sufficiently high multiplicity of infection, but only a fraction of the infected cells is capable of amplification of virus protein synthesis and progeny virus accumulation. No cell-density effects of comparable extent could be observed in MDCK cells.

Influenza A virus reassortants with surface glycoprotein genes of the avian parent viruses: effects of HA and NA gene combinations on virus aggregation

A series of 33 human-avian and human-mammalian influenza virus reassortant clones possessing either HA or both HA and NA genes of the avian or mammalian virus was obtained by crosses of A/USSR/90/77 (H1N1) human virus with 5 avian and 1 mammalian influenza virus strains. All of the reassortants possessing NA genes of the H1N1 human parent virus and HA gene of an avian or mammalian parent virus had high values of infectivity/HA activity ratio. Since this feature could result from a limited virion aggregation, several reassortants were analyzed by velocity sucrose gradient centrifugation. In all cases tested, the reassortants of H3N1, H4N1, H10N1 and H13N1 composition were shown to be aggregated, whereas the preparations of the parent H1N1 virus and the reassortants possessing both HA and NA genes from the avian parents were represented mostly by single virions. The aggregates were formed at 4 degrees C and dissociated at 37 degrees C. The dissociation was blocked by an inhibitor of neuraminidase activity (2-deoxy-2,3-dehydro-N-acetyl-neuraminic acid). The dissociation was reversible since the virions reaggregated at 4 degrees C; however, treatment with bacterial neuraminidase led to an irreversible dissociation of the aggregates. The tendency of the reassortants to aggregate correlates with an increased infectivity/HA ratio. No regular decrease in the neuraminidase activity in the virions of reassortants as compared to the parent H1N1 virus was revealed. The most likely explanation of the observed phenomenon seems to be an inefficient removal of sialic acid residues from the avian virus hemagglutinin by the human virus N1 neuraminidase.

[Properties of intracellular virus-specific ribonucleoproteins containing negative and positive hepatitis A virus RNA]

The characteristics of the intracellular virus-specific nucleocapsids containing either a negative or a positive RNA strand were studied. The immunosorption of nucleocapsids by the monoclonal antibodies against the three epitopes of NP protein failed to reveal any antigenic difference between the negative strand or positive strand-containing nucleocapsids. On the other hand, the sensitivity of virus-specific RNA in the nucleocapsids to digestion by the pancreatic ribonuclease proved to be lower for the positive strand-containing nucleocapsids.

Effects of UV-irradiation upon Venezuelan equine encephalomyelitis virus

Venezuelan equine encephalomyelitis virus labelled with [14C]aminoacids or [3H]uridine was purified and UV-irradiated. The irradiation led to the formation of uracil photodimers and the covalent linking of the nucleocapsid protein C to virion RNA. The inactivation of infectivity correlated with the formation of uracil dimers, whereas the RNA-protein links were formed at much higher doses of UV irradiation. The analysis of covalent RNA-protein complexes suggests that a fairly large fraction (at least one third) of the whole content of C protein is able to participate in the formation of UV-induced links, suggesting extensive contacts of RNA with protein with the nucleocapsid.

The genetic aspects of influenza virus filamentous particle formation

We analysed the genetic content of reassortants between parent viruses differing in their ability to form filaments. The results suggest that primarily HA, M, and NP genes are involved in the control of the filament forming ability. A lower buoyant density of the filamentous forms as compared to spherical particles allowed us to obtain a sufficiently pure population of filaments. A difference in the UV-inactivation kinetics between filaments and spherical virions suggests that the infectious filamentous forms are probably represented by multigenomic particles or partial heterozygotes.

[The role of polymerase protein genes of influenza A virus in the transition from the early to late stage of replication]

Regulation of influenza virus RNA replication was studied with the use of A/FPV/Rostock/34 strain ts-mutants. Mutants C44, C15, C45 possessing the ts-defects in the PB2, PB1 and PA genes respectively were used for the infection of chick embryo cultured cells and H-uridine-labelled nucleocapsid-associated RNA was analysed in polyacrylamide gel electrophoresis to assess the kinetics of vRNA synthesis. A typical early-late transition of the pattern of vRNA synthesis was observed in the cells infected with C44, whereas the other two mutants exhibited a slightly changed (C15) or strongly distorted (C45) pattern. In shift up experiments after the transfer to non-permissive temperature all the mutants exhibited partial reversion to an early pattern of vRNA synthesis. The results are discussed in connection with the mechanism of the early-late transition of influenza virus-specific RNA synthesis.