Postexposure vaccination massively increases the prevalence of gamma-herpesvirus-specific CD8+ T cells but confers minimal survival advantage on CD4-deficient mice

Mice that lack CD4(+) T cells remain clinically normal for more than 60 days after respiratory challenge with the murine gamma-herpesvirus 68 (gammaHV-68), then develop symptoms of a progressive wasting disease. The gammaHV-68-specific CD8(+) T cells that persist in these I-A(b-/-) mice are unable to prevent continued, but relatively low level, virus replication. Postexposure challenge with recombinant vaccinia viruses expressing gammaHV-68 lytic cycle epitopes massively increased the magnitude of the gammaHV-68-specific CD8(+) population detectable by staining with tetrameric complexes of MHC class I glycoprotein + peptide, or by interferon-gamma production subsequent to in vitro restimulation with peptide. The boosting effect was comparable for gammaHV-68-infected I-A(b-/-) and I-A(b+/+) mice within 7 days of challenge, and took more than 110 days to return to prevaccination levels in the I-A(b+/+) controls. Although the life-span of the I-A(b-/-) mice was significantly increased, there was no effect on long-term survival. A further boost with a recombinant influenza A virus failed to improve the situation. Onset of weight loss was associated with a decline in gammaHV-68-specific CD8(+) T cell numbers, though it is not clear whether this was a cause or an effect of the underlying pathology. Even very high levels of virus-specific CD8(+) T cells thus provide only transient protection against the uniformly lethal consequences of gammaHV-68 infection under conditions of CD4(+) T cell deficiency.

A gamma-herpesvirus sneaks through a CD8(+) T cell response primed to a lytic-phase epitope

To determine whether established CD8(+) T cell memory to an epitope prominent during the replicative phase of a gamma-herpesvirus infection protects against subsequent challenge, mice were primed with a recombinant vaccinia virus expressing the p56 peptide and then boosted by intranasal exposure to an influenza A virus incorporating p56 in the neuraminidase protein. Clonally expanded populations of functional, p56-specific CD8(+) T cells were present at high frequency in both the lung and the lymphoid tissue 1 month later, immediately before respiratory challenge with gammaHV-68. This prime-and-boost regime led to a massive reduction of productive gammaHV-68 infection in the respiratory tract and, initially, to much lower levels of latency in both the regional lymph nodes and the spleen. The CD8(+) T cell response to another epitope (p79) was diminished, there was less evidence of B cell activation, and the onset of the CD4(+) T cell-dependent splenomegaly was delayed. Within 3-4 weeks of the gammaHV-68 challenge, however, the extent of latent infection in the lymph nodes and spleen was equivalent, and both groups developed the prominent infectious mononucleosis-like syndrome that is characteristic of this infection. The reverse protocol (influenza then vaccinia) seemed to be slightly less effective. Even though immune CD8(+) T cells may be present at the time and site of virus challenge, establishing a high level of CD8(+) T cell memory to lytic-phase epitopes alone does not protect against the longer-term consequences of this gammaHV infection.

Amino acid residues contributing to the substrate specificity of the influenza A virus neuraminidase

Influenza A viruses possess two glycoprotein spikes on the virion surface: hemagglutinin (HA), which binds to oligosaccharides containing terminal sialic acid, and neuraminidase (NA), which removes terminal sialic acid from oligosaccharides. Hence, the interplay between these receptor-binding and receptor-destroying functions assumes major importance in viral replication. In contrast to the well-characterized role of HA in host range restriction of influenza viruses, there is only limited information on the role of NA substrate specificity in viral replication among different animal species. We therefore investigated the substrate specificities of NA for linkages between N-acetyl sialic acid and galactose (NeuAcalpha2-3Gal and NeuAcalpha2-6Gal) and for different molecular species of sialic acids (N-acetyl and N-glycolyl sialic acids) in influenza A viruses isolated from human, avian, and pig hosts. Substrate specificity assays showed that all viruses had similar specificities for NeuAcalpha2-3Gal, while the activities for NeuAcalpha2-6Gal ranged from marginal, as represented by avian and early N2 human viruses, to high (although only one-third the activity for NeuAcalpha2-3Gal), as represented by swine and more recent N2 human viruses. Using site-specific mutagenesis, we identified in the earliest human virus with a detectable increase in NeuAcalpha2-6Gal specificity a change at position 275 (from isoleucine to valine) that enhanced the specificity for this substrate. Valine at position 275 was maintained in all later human viruses as well as swine viruses. A similar examination of N-glycolylneuraminic acid (NeuGc) specificity showed that avian viruses and most human viruses had low to moderate activity for this substrate, with the exception of most human viruses isolated between 1967 and 1969, whose NeuGc specificity was as high as that of swine viruses. The amino acid at position 431 was found to determine the level of NeuGc specificity of NA: lysine conferred high NeuGc specificity, while proline, glutamine, and glutamic acid were associated with lower NeuGc specificity. Both residues 275 and 431 lie close to the enzymatic active site but are not directly involved in the reaction mechanism. This finding suggests that the adaptation of NA to different substrates occurs by a mechanism of amino acid substitutions that subtly alter the conformation of NA in and around the active site to facilitate the binding of different species of sialic acid.

The role of influenza A virus hemagglutinin residues 226 and 228 in receptor specificity and host range restriction

Influenza A viruses can be isolated from a variety of animals, but their range of hosts is restricted. For example, human influenza viruses do not replicate in duck intestine, the major replication site of avian viruses in ducks. Although amino acids at positions 226 and 228 of hemagglutinin (HA) of the H3 subtype are known to be important for this host range restriction, the contributions of specific amino acids at these positions to restriction were not known. Here, we address this issue by generating HAs with site-specific mutations of a human virus that contain different amino acid residues at these positions. We also let ducks select replication-competent viruses from a replication-incompetent virus containing a human virus HA by inoculating animals with 10(10.5) 50% egg infectious dose of the latter virus and identified a mutation in the HA. Our results showed that the Ser-to-Gly mutation at position 228, in addition to the Leu-to-Gln mutation at position 226 of the HA of the H3 subtype, is critical for human virus HA to support virus replication in duck intestine.

Molecular basis for the generation in pigs of influenza A viruses with pandemic potential

Genetic and biologic observations suggest that pigs may serve as "mixing vessels" for the generation of human-avian influenza A virus reassortants, similar to those responsible for the 1957 and 1968 pandemics. Here we demonstrate a structural basis for this hypothesis. Cell surface receptors for both human and avian influenza viruses were identified in the pig trachea, providing a milieu conducive to viral replication and genetic reassortment. Surprisingly, with continued replication, some avian-like swine viruses acquired the ability to recognize human virus receptors, raising the possibility of their direct transmission to human populations. These findings help to explain the emergence of pandemic influenza viruses and support the need for continued surveillance of swine for viruses carrying avian virus genes.

Influence of host species on the evolution of the nonstructural (NS) gene of influenza A viruses

The matrix (M) and nonstructural (NS) genes of influenza A viruses each encode two overlapping proteins. In the M gene, evolution of one protein affects that of the other. To determine whether or not this evolutionary influence operating between the two M proteins also occurs in the NS gene, we sequenced the NS genes of 36 influenza A viruses isolated from a broad spectrum of animal species (wild and domestic birds, horses, pigs, humans, and sea mammals) and analyzed them phylogenetically, together with other previously published sequences. These analyses enabled us to conclude the following host species-related points that are not found in the other influenza A virus genes and their gene products. (1) The evolution of the two overlapping proteins encoded by the NS gene are lineage-dependent, unlike the M gene where evolutionary constraints on the Ml protein affect the evolution of the M2 protein (Ito et al.. J. Virol. 65 (1991) 5491 5498). (2) The gull-specific lineage contained nonH13 gull viruses and the non-gull avian lineage contained H13 gull viruses, indicating that the gull-specific lineage does not link to the H13 HA subtype in the NS gene unlike findings with other genes. (3) The branching topology of the recent equine lineage (H7N7 viruses isolated after 1973 and H3N8) indicates recent introduction of the NS, M, and PB2 genes into horses from avian sources by genetic reassortment.

The M2 ectodomain is important for its incorporation into influenza A virions

M2 is an integral protein of influenza A virus that functions as an ion channel. The ratio of M2 to HA in influenza A virions differs from that found on the cell surface, suggesting selective incorporation of M2 and HA into influenza virions. To examine the sequences that are important for M2 incorporation into virions, we used an incorporation assay that involves expressing M2 from a plasmid, transfecting the plasmid into recipient cells, and then infecting those cells with influenza virus. To test the importance of the different regions of the protein (extracellular, transmembrane, and cytoplasmic) in determining M2 incorporation, we created chimeric mutants of M2 and Sendai virus F proteins, exchanging corresponding extracellular, transmembrane, and cytoplasmic domains. Of the six possible chimeric mutants, only three were expressed on the cell surface. Of these three chimeric proteins, only one mutant (with the extracellular domain from M2 and the rest from F) was incorporated into influenza virions. These results suggest that the extracellular domain of M2 is important for its incorporation into virions.

Nuclear import and export of influenza virus nucleoprotein

Influenza virus nucleoprotein (NP) shuttles between the nucleus and the cytoplasm. A nuclear localization signal (NLS) has been identified in NP at amino acids 327 to 345 (J. Davey et al., Cell 40:667-675, 1985). However, some NP mutants that lack this region still localize to the nucleus, suggesting an additional NLS in NP. We therefore investigated the nucleocytoplasmic transport of NP from influenza virus A/WSN/33 (H1N1). NP deletion constructs lacking the 38 N-terminal amino acids, as well as those lacking the 38 N-terminal amino acids and the previously identified NLS, localized to both the cytoplasm and the nucleus. Nuclear localization of a protein containing amino acids 1 to 38 of NP fused to LacZ proved that these 38 amino acids function as an NLS. Within this region, we identified two basic amino acids, Lys7 and Arg8, that are crucial for NP nuclear import. After being imported into the nucleus, the wild-type NP and the NP-LacZ fusion construct containing amino acids 1 to 38 of NP were both transported back to the cytoplasm, where they accumulated. These data indicate that NP has intrinsic structural features that allow nuclear import, nuclear export, and cytoplasmic accumulation in the absence of any other viral proteins. Further, the information required for nuclear import and export is located in the 38 N-terminal amino acids of NP, although other NP nuclear export signals may exist. Treatment of cells with a protein kinase C inhibitor increased the amounts of nuclear NP, whereas treatment of cells with a phosphorylation stimulator increased the amounts of cytoplasmic NP. These findings suggest a role of phosphorylation in nucleocytoplasmic transport of NP.

The cysteine residues of the M2 protein are not required for influenza A virus replication

The M2 protein of influenza A virus functions as an ion channel. It contains three cysteine residues: cysteines 17 and 19, which form disulfide bonds in the ectodomain, and cysteine 50 which is acylated. To understand the role of these cysteine residues in virus replication, we used reverse genetics to create influenza viruses in which the individual cysteines were mutated and a virus in which all three cysteines were changed to serine. The M2 cysteine mutants that lacked either of the cysteine residues in the ectodomain and the mutant that lacked all three residues had appreciably lower amounts of M2 oligomers than did the wild-type virus when examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. None of the mutants, however, were defective in replication, either in vitro or in ferrets and mice. These findings demonstrate that noncovalent interactions are sufficient for the M2 protein to form functional oligomers for virus replication and that its cysteine residues are dispensable for influenza virus replication in vitro and in vivo.

HEL-Flu: an influenza virus containing the hen egg lysozyme epitope recognized by CD4+ T cells from mice transgenic for an alphabeta TCR

Reverse genetics was used to modify the influenza virus genome by inserting the p46-63 sequence of hen egg lysozyme (HEL) into the neuraminidase stalk of the virus. The resulting virus, HEL-Flu, contained the epitopes recognized by CD4+ T cells from 3A9-TCR transgenic mice (C3HTg). Here, we show that HEL-Flu was infectious in the respiratory tract of both C3H and C3HTg mice, the latter animals showing an early, transient morbidity. Splenic dendritic cells and certain cloned populations of splenic macrophages and brain microglia constitutively presented infectious and inactivated HEL-Flu to the T cells in an Ag-specific and MHC class II-restricted manner. These results demonstrate the utility of HEL-Flu in assessing the APC activity for naive T cells; they also extend the previous studies showing that discrete populations of macrophages and microglia constitutively process and present Ag to naive T cells.

HEL-Flu: an influenza virus containing the hen egg lysozyme epitope recognized by CD4+ T cells from mice transgenic for an alphabeta TCR

Reverse genetics was used to modify the influenza virus genome by inserting the p46-63 sequence of hen egg lysozyme (HEL) into the neuraminidase stalk of the virus. The resulting virus, HEL-Flu, contained the epitopes recognized by CD4+ T cells from 3A9-TCR transgenic mice (C3HTg). Here, we show that HEL-Flu was infectious in the respiratory tract of both C3H and C3HTg mice, the latter animals showing an early, transient morbidity. Splenic dendritic cells and certain cloned populations of splenic macrophages and brain microglia constitutively presented infectious and inactivated HEL-Flu to the T cells in an Ag-specific and MHC class II-restricted manner. These results demonstrate the utility of HEL-Flu in assessing the APC activity for naive T cells; they also extend the previous studies showing that discrete populations of macrophages and microglia constitutively process and present Ag to naive T cells.

Continued evolution of H1N1 and H3N2 influenza viruses in pigs in Italy

Swine influenza viruses possessing avian genes were first detected in Europe in 1979 (Scholtissek et al., 1983, Virology, 129, 521-523) and continue to circulate in pigs in that region of the world. To characterize the molecular epidemiology of swine influenza viruses currently circulating in Europe, we used dot-blot hybridization and sequence analysis to determine the origin of the genes encoding the nonsurface proteins ("internal" genes) of 10 H1N1 and 11 H3N2 swine influenza viruses isolated in Italy between 1992 and 1995. All of the 126 genes examined were of avian origin; thus the currently circulating H3N2 strains which possess A/Port Chalmers/1/73-like surface glycoproteins appear to be descendants of the reassortant human-avian viruses that emerged between 1983 and 1985 in Italy. Sequence analysis of matrix (M), nonstructural, and nucleoprotein genes, as well as phylogenetic analysis of M gene showed that the H1N1 and H3N2 viruses from the pigs were closely related to recent isolates of the avian-like swine H1N1 influenza strain currently circulating in northern Europe and were distinguishable from the genes of viruses isolated from European swine in 1979. To evaluate the frequency of transmission of swine H1N1 and H3N2 viruses to man, we tested 123 human sera for hemagglutination-inhibiting antibodies against avian and mammalian H1N1 and H3N2 virus strains. Our findings indicate that swine influenza viruses possessing A/Port Chalmers/1/73-like hemagglutinin may have transmitted to approximately 20% of young persons under 20 years of age who had contact with pigs. Thus, H3N2 swine viruses, possibly possessing avian-derived internal genes, may be entering humans more often than was previously thought. We strongly recommend that pigs be regularly monitored as a potential early warning system for detection of future pandemic strains.

Immunogenicity of influenza vaccine (1993-94 winter season) in HIV-seropositive and -seronegative ex-intravenous drug users

The humoral response (haemagglutination inhibiting antibodies) to trivalent split influenza vaccine for the 1993-94 winter season (A/Beijing/32/92 (H3N2), A/Singapore/6/86 (H1N1) and B/Panama/45/90) was evaluated in a group of young HIV-seropositive ex-intravenous heroin users and compared with responses measured in HIV-seronegative individuals with a similar history. HIV-negative volunteers showed an overall positive response suggesting that previous heroin use did not influence their humoral response to influenza vaccine. Comparable results were obtained in HIV-positive subjects with CD4+ lymphocyte counts > 500 microliters-1, whereas impaired reactivity was found in HIV-positive volunteers with CD4+ counts < 500 microliters-1. Booster vaccination did not increase antibody levels in any of the groups studied, although the data did not exclude a positive influence of a second vaccine dose on persistence of antibody at 120 days after the first dose. No significant changes were observed in p24 antigenemia levels in HIV-positive individuals after vaccination.

The cytoplasmic tail of influenza A virus neuraminidase (NA) affects NA incorporation into virions, virion morphology, and virulence in mice but is not essential for virus replication

In this study, we investigated the role of the conserved neuraminidase (NA) cytoplasmic tail residues in influenza virus replication. Mutants of influenza A virus (A/WSN/33 [H1N1]) with deletions of the NA cytoplasmic tail region were generated by reverse genetics. The resulting viruses, designated NOTAIL, contain only the initiating methionine of the conserved six amino-terminal residues. The mutant viruses grew much less readily and produced smaller plaques than did the wild-type virus. Despite similar levels of NA cell surface expression by the NOTAIL mutants and wild-type virus, incorporation of mutant NA molecules into virions was decreased by 86%. This reduction resulted in less NA activity per virion, leading to the formation of large aggregates of progeny mutant virions on the surface of infected cells. A NOTAIL virus containing an additional mutation (Ser-12 to Pro) in the transmembrane domain incorporated three times more NA molecules into virions than did the NOTAIL parent but approximately half of the amount incorporated by the wild-type virus. However, aggregation of the progeny virions still occurred at the cell surface. All NOTAIL viruses were attenuated in mice. We conclude that the cytoplasmic tail of NA is not absolutely essential for virus replication but exerts important effects on the incorporation of NA into virions and thus on the aggregation and virulence of progeny virus. In addition, the relative abundance of long filamentous particles formed by the NOTAIL mutants, compared with the largely spherical wild-type particles, indicates a role for the NA cytoplasmic tail in virion morphogenesis.

Reverse genetics system for generation of an influenza A virus mutant containing a deletion of the carboxyl-terminal residue of M2 protein

We established a reverse genetics system for the M gene of influenza A virus, using amantadine resistance as a selection criterion. Transfection of an artificial M ribonucleoprotein complex of A/Puerto Rico/8/34 (H1N1), a naturally occurring amantadine-resistant virus, and superinfection with amantadine-sensitive A/equine/Miami/1/63 (H3N8), followed by cultivation in the presence of the drug, led to the generation of a transfectant virus with the A/Puerto Rico/8/34 (H1N1) M gene. With this system, we attempted to generate a virus containing a deletion in an M-gene product (M2 protein). Viruses lacking the carboxyl-terminal Glu of M2, but not those lacking 5 or 10 carboxyl-terminal residues, were rescued in the presence of amantadine. These findings indicate that carboxyl-terminal residues of the M2 protein play an important role in influenza virus replication. The M-gene-based reverse genetics system will allow the study of different M-gene mutations to achieve a balance between attenuation and virus replication, thus facilitating the production of live vaccine strains.

Protection against lethal lymphocytic choriomeningitis virus (LCMV) infection by immunization of mice with an influenza virus containing an LCMV epitope recognized by cytotoxic T lymphocytes

The reverse genetics system has made it possible to modify the influenza virus genome. By this method, we were able to assess influenza virus as a vaccine vector for protecting BALB/c mice against otherwise lethal lymphocytic choriomeningitis virus (LCMV) infection. A single dose of influenza virus [A/WSN/33 (H1N1)] bearing a cytotoxic T-lymphocyte-specific epitope of the LCMV nucleoprotein (residues 116 to 127) in the neuraminidase stalk protected mice against LCMV challenge for at least 4 months. The immunity was mediated by cytotoxic T lymphocytes and was haplotype specific, indicating that the observed protective response was solely a consequence of prior priming with the H-2d LCMV nucleoprotein epitope expressed in the recombinant influenza virus. We also found that as many as 58 amino acids could be inserted into the neuraminidase stalk without loss of viral function. These findings demonstrate the potential of influenza virus as a vaccine vector, with the neuraminidase stalk as a repository for foreign epitopes.

Antigenic and sequence analysis of H3 influenza virus haemagglutinins from pigs in Italy

To investigate the possible mechanism of maintenance of old human influenza A (H3N2) viruses in pigs, the haemagglutinins (HAs) of seven isolates from swine were studied by analysis of nucleotide and deduced primary amino acid sequences, as well as reactivity of the HA molecule to chicken antisera and monoclonal antibodies. The swine HAs were closely similar to the HA of the A/Victoria/3/75 human variant as regards antigenic and molecular characteristics. These findings are consistent with the hypothesis that the swine HA genes were transmitted from an early human H3 virus to pigs, where they survived with limited mutations over a period of 5 years. The sequence data were also compared with swine H3 sequences to investigate genetic relationships between the H3 genes from swine viruses isolated in different geographical areas. An evolutionary tree, constructed from the nucleotide sequences of viruses isolated from pigs in China and in Italy, illustrated that, depending on the country of their isolation, the HA genes of swine influenza A (H3N2) viruses have different origins, e.g. human and avian, and evolved independently in different lineages. The study provides direct support for the hypothesis that pigs might serve as a 'mixing vessel' for the generation of pandemic strains of human influenza viruses.

Mutations in the cytoplasmic tail of influenza A virus neuraminidase affect incorporation into virions

The significance of the conserved cytoplasmic tail sequence of influenza A virus neuraminidase (NA) was analyzed by the recently developed reverse genetics technique (W. Luytjes, M. Krystal, M. Enami, J. D. Parvin, and P. Palese, Cell 59:1107-1113, 1989). A chimeric influenza virus A/WSN/33 NA containing the influenza B virus cytoplasmic tail rescued influenza A virus infectivity. The transfectant virus had less NA incorporated into virions than A/WSN/33, indicating that the cytoplasmic tail of influenza virus NA plays a role in incorporation of NA into virions. However, these results also suggest that the influenza A virus and influenza B virus cytoplasmic tail sequences share common features that lead to the production of infectious virus. Transfectant virus was obtained with all cytoplasmic tail mutants generated by site-directed mutagenesis of the influenza A virus tail, except for the mutant resulting from substitution of the conserved proline residue, presumably because of its contribution to the secondary structure of the tail. No virus was rescued when the cytoplasmic tail was deleted, indicating that the cytoplasmic tail is essential for production of the virus. The virulence of the transfectant viruses in mice was directly proportional to the amount of NA incorporated. The importance of the NA cytoplasmic tail in virus assembly and virulence has implications for use in developing antiviral strategies.

Concurrent antigenic analysis of recent epidemic influenza A and B viruses and quantitation of antibodies in population serosurveys in Italy

Laboratory investigations of virus isolation and serum antibodies in a Mediterranean country (Italy) demonstrated that influenza A and B viruses, and often both, circulated every winter in Italy. The winter 1987/88 was characterized by a low level of influenza activity, as shown by the limited number (47) of influenza virus isolates, the majority of which (61%) belonged to the influenza B type. In contrast, the 1988/89 influenza season was exclusively associated with the circulation of influenza type A viruses. The A(H1H1) subtype was largely predominant (97%), as compared to the low incidence of the A(H3N2) subtype (3%). During the 1989/90 winter a co-circulation of A and B influenza viruses was observed, A(H3N2) strains being responsible for 96% of the virologically confirmed cases. Antigenic analysis of the virus isolates showed some antigenic variation in influenza A viruses of both H1N1 and H3N2 subtypes, whilst antigenic stability was found among the influenza B virus isolates. Overall, the above virological findings correlate with the data concerning the pattern of influenza virus circulation in Northern Europe and the UK during the three years surveyed. The results of serum antibody surveys conducted in each post-epidemic period are also reported.

Genetic reassortment between avian and human influenza A viruses in Italian pigs

Pandemic strains of influenza A virus arise by genetic reassortment between avian and human viruses. To examine the possibility that pigs serve as "mixing vessels" for such reassortment events (Scholtissek et al., Virology 147, 287-294, 1985), we phylogenetically analyzed the internal protein genes of classic H1N1, avian-like H1N1, and human-like H3N2 viruses circulating among Italian pigs. The results show that human-like H3N2 strains isolated from 1985 to 1989 contained the internal protein genes of avian-like H1N1 viruses, whereas those isolated in 1977 and 1983 did not. Thus, at some time between 1983 and 1985, genetic reassortment took place between avian- and human-like viruses in Italian pigs. This study provides the first evidence supporting genetic reassortment between human and avian viruses in a natural swine environment.

Biologic importance of neuraminidase stalk length in influenza A virus

To investigate the biologic importance of the neuraminidase (NA) stalk of influenza A virus, we generated mutant viruses of A/WSN/33 (H1N1) with stalks of various lengths (0 to 52 amino acids), by using the recently developed reverse genetics system. These mutant viruses, including one that lacked the entire stalk, replicated in tissue culture to the level of the parent virus, whose NA stalk contains 24 amino acid residues. In eggs, however, the length of the stalk was correlated with the efficiency of virus replication: the longer the stalk, the better the replication. This finding indicates that the length of the NA stalk affects the host range of influenza A viruses. The NA stalkless mutant was highly attenuated in mice; none of the animals died even after intranasal inoculation of 10(6) PFU of the virus (the dose of the parent virus required to kill 50% of mice was 10(2.5) PFU). Moreover, the stalkless mutant replicated only in the respiratory organs, whereas the parent virus caused systemic infection in mice. Thus, attenuation of the virus with the deletion of the entire NA stalk raises the possibility of its use as live vaccines.

Influenza--a model of an emerging virus disease

Influenza A viruses continue to emerge from the aquatic avian reservoir and cause pandemics. Phylogenetic analysis of the nucleotide sequence of all eight influenza A virus RNA segments indicate that all of the influenza viruses in mammalian hosts originate from the avian gene pool. In contrast to the rapid progressive changes in both the nucleotide and amino acid sequences of mammalian virus gene lineages, avian virus genes show far less variation and, in most cases, appear to be in evolutionary stasis. There are periodic exchanges of influenza virus genes or whole viruses between species giving rise to pandemics of diseases in humans, lower animals and birds. The periodic emergence of influenza viruses in mammalian species has been illustrated by the appearance of a new influenza virus in horses in northern China in 1989. Phylogenetic analysis of classical H1N1, avian-like H1N1 and human H3N2 viruses circulating in Italian pigs reveals that genetic reassortment is taking place between avian- and human-like viruses in the European pig population. These studies provide evidence supporting the possibility that pigs serve as a mixing vessel for reassortment between influenza viruses in mammalian and avian hosts and raise the question of whether the next pandemic of influenza will emerge in Europe!

Attenuation of influenza A virus by insertion of a foreign epitope into the neuraminidase

As the initial step in generating a live attenuated influenza A vaccine, we attempted to substitute an unrelated amino acid sequence (FLAG) for a portion of the neuraminidase (NA) molecule in influenza virus A/WSN/33 (H1N1), using a recently developed technique (reverse genetics [W. Luytjes, M. Krystal, M. Enami, J. D. Parvin, and P. Palese, Cell 59:1107-1113, 1989]). This technique allowed us to rescue the NA molecules containing the FLAG sequence (Asp-Tyr-Lys-Asp-Asp-Asp-Asp-Lys) at the bottom portion of the boxlike head of the molecule immediately above the stalk region (amino acid residues 63 to 70 [WSN NA numbering]). An anti-FLAG monoclonal antibody immunoprecipitated the NA molecules with the FLAG sequence, demonstrating that the foreign epitope was exposed on the virion surface. The dose of FLAG-containing transfectant virus required to kill 50% of mice was 100-fold higher than the required dose of parent virus. The FLAG sequence was stably maintained in the NA molecule during passage of the virus in tissue culture and in mice. These findings demonstrate that live influenza A vaccine strains with stable attenuating mutations in the coding region of the viral genes can be generated by reverse genetics.

Immunization of elderly volunteers with the 1988-89 inactivated whole influenza vaccine: assessment of antibody responses by haemagglutination inhibition and single radial haemolysis tests

The immunogenicity of inactivated whole trivalent influenza vaccines (A/Taiwan/1/86 (H1N1), A/Sichuan/2/87 (H3N2), and B ijing/1/87) recommended for the 1988-89 winter season was evaluated in 236 elderly (mean age 71 years) high risk volunteers. An overall significant increase in the number of subjects with protective haemagglutination inhibiting (HI) antibodies (titer > 1:40) against vaccine components was observed after vaccination. Nevertheless, a percentage of individuals (ranging from 56% to 62%) remained without protective antibodies and the number of people showing a positive response was limited (from 32% to 41%). By the comparative analysis of the results obtained examining the presence of protective levels of antibody in the sera from 91 volunteers using HI versus the single radial haemolysis (SRH) test, we obtained evidence for a higher sensitivity of SRH technique especially against B antigen.

Detection of two antigenic subpopulations of A(H1N1) influenza viruses from pigs: antigenic drift or interspecies transmission?

Serological analysis of a group of 63 influenza H1N1 viruses isolated from pigs in Italy in the period 1976-1988 revealed the presence of two distinct antigenic subpopulations: some viruses possessed a haemagglutinin indistinguishable from that of viruses typically associated with pigs, i.e., A/New Jersey/8/76 (H1N1), whereas others showed a close antigenic relatedness with the haemagglutinin of avian-like H1 viruses. These findings represent further evidence that influenza A viruses from avian species may be transmitted to mammals. The surface and internal proteins of some of these viruses were also analyzed biochemically to evaluate the molecular relatedness among viruses circulating in non-human hosts.