Influenza receptors, polymerase and host range

Influenza infection is initiated by virus attachment to sialic acid-containing cell-surface receptors. The spectrum of sialylglycoconjugates varies substantially between viral host species as well as target tissues and cell types of the same species, leading to variations in the receptor-binding specificity of viruses circulating in these hosts. Therefore, receptor specificity plays an important role in the viral cell and tissue tropism, interspecies transmission and adaptation to a new host, and a poor fit of avian viruses to receptors in humans limits the emergence of new pandemic strains. Adaptation of an avian virus to a mammalian host also involves enhanced activity of the viral polymerase in mammalian cells which, in part, is the result of improved binding of the polymerase to the nuclear import machinery of the cell. These findings suggest that host range and virulence are the result of optimised molecular interactions between viral proteins and cellular factors. Future transmission studies with animals may reveal to what extent haemagglutinin receptor-binding mutations and polymerase-activity-enhancing mutations together enable influenza A viruses to cross species barriers.

Differential polymerase activity in avian and mammalian cells determines host range of influenza virus

As recently shown, mutations in the polymerase genes causing increased polymerase activity in mammalian cells are responsible for the adaptation of the highly pathogenic avian influenza virus SC35 (H7N7) to mice (G. Gabriel et al., Proc. Natl. Acad. Sci. USA 102:18590-18595, 2005). We have now compared mRNA, cRNA, and viral RNA levels of SC35 and its mouse-adapted variant SC35M in avian and mammalian cells. The increase in levels of transcription and replication of SC35M in mammalian cells was linked to a decrease in avian cells. Thus, the efficiency of the viral polymerase is a determinant of both host specificity and pathogenicity.

The viral polymerase mediates adaptation of an avian influenza virus to a mammalian host

Mammalian influenza viruses are descendants of avian strains that crossed the species barrier and underwent further adaptation. Since 1997 in southeast Asia, H5N1 highly pathogenic avian influenza viruses have been causing severe, even fatal disease in humans. Although no lineages of this subtype have been established until now, such repeated events may initiate a new pandemic. As a model of species transmission, we used the highly pathogenic avian influenza virus SC35 (H7N7), which is low-pathogenic for mice, and its lethal mouse-adapted descendant SC35M. Specific mutations in SC35M polymerase considerably increase its activity in mammalian cells, correlating with high virulence in mice. Some of these mutations are prevalent in chicken and mammalian isolates, especially in the highly pathogenic H5N1 viruses from southeast Asia. These activity-enhancing mutations of the viral polymerase complex demonstrate convergent evolution in nature and, therefore, may be a prerequisite for adaptation to a new host paving the way for new pandemic viruses.

A new approach to an influenza live vaccine: modification of the cleavage site of hemagglutinin

A promising approach to reduce the impact of influenza is the use of an attenuated, live virus as a vaccine. Using reverse genetics, we generated a mutant of strain A/WSN/33 with a modified cleavage site within its hemagglutinin, which depends on proteolytic activation by elastase. Unlike the wild-type, which requires trypsin, this mutant is strictly dependent on elastase. Both viruses grow equally well in cell culture. In contrast to the lethal wild-type virus, the mutant is entirely attenuated in mice. At a dose of 10(5) plaque-forming units, it induced complete protection against lethal challenge. This approach allows the conversion of any epidemic strain into a genetically homologous attenuated virus.

Universal primer set for the full-length amplification of all influenza A viruses

To systematically identify and analyze the 15 HA and 9 NA subtypes of influenza A virus, we need reliable, simple methods that not only characterize partial sequences but analyze the entire influenza A genome. We designed primers based on the fact that the 15 and 21 terminal segment specific nucleotides of the genomic viral RNA are conserved between all influenza A viruses and unique for each segment. The primers designed for each segment contain influenza virus specific nucleotides at their 3'-end and non-influenza virus nucleotides at the 5'-end. With this set of primers, we were able to amplify all eight segments of N1, N2, N4, N5, and N8 subtypes. For N3, N6, N7, and N9 subtypes, the segment specific sequences of the neuraminidase genes are different. Therefore, we optimized the primer design to allow the amplification of those neuraminidase genes as well. The resultant primer set is suitable for all influenza A viruses to generate full-length cDNAs, to subtype viruses, to sequence their DNA, and to construct expression plasmids for reverse genetics systems.

Characterization of the influenza A virus gene pool in avian species in southern China: was H6N1 a derivative or a precursor of H5N1?

In 1997, an H5N1 influenza virus outbreak occurred in chickens in Hong Kong, and the virus was transmitted directly to humans. Because there is limited information about the avian influenza virus reservoir in that region, we genetically characterized virus strains isolated in Hong Kong during the 1997 outbreak. We sequenced the gene segments of a heterogeneous group of viruses of seven different serotypes (H3N8, H4N8, H6N1, H6N9, H11N1, H11N9, and H11N8) isolated from various bird species. The phylogenetic relationships divided these viruses into several subgroups. An H6N1 virus isolated from teal (A/teal/Hong Kong/W312/97 [H6N1]) showed very high (>98%) nucleotide homology to the human influenza virus A/Hong Kong/156/97 (H5N1) in the six internal genes. The N1 neuraminidase sequence showed 97% nucleotide homology to that of the human H5N1 virus, and the N1 protein of both viruses had the same 19-amino-acid deletion in the stalk region. The deduced hemagglutinin amino acid sequence of the H6N1 virus was most similar to that of A/shearwater/Australia/1/72 (H6N5). The H6N1 virus is the first known isolate with seven H5N1-like segments and may have been the donor of the neuraminidase and the internal genes of the H5N1 viruses. The high homology between the internal genes of H9N2, H6N1, and the H5N1 isolates indicates that these subtypes are able to exchange their internal genes and are therefore a potential source of new pathogenic influenza virus strains. Our analysis suggests that surveillance for influenza A viruses should be conducted for wild aquatic birds as well as for poultry, pigs, and humans and that H6 isolates should be further characterized.

Independence of evolutionary and mutational rates after transmission of avian influenza viruses to swine

In 1979, an H1N1 avian influenza virus crossed the species barrier, establishing a new lineage in European swine. Because there is no direct or serologic evidence of previous H1N1 strains in these pigs, these isolates provide a model for studying early evolution of influenza viruses. The evolutionary rates of both the coding and noncoding changes of the H1N1 swine strains are higher than those of human and classic swine influenza A viruses. In addition, early H1N1 swine isolates show a marked plaque heterogeneity that consistently appears after a few passages. The presence of a mutator mutation was postulated (C. Scholtissek, S. Ludwig, and W. M. Fitch, Arch. Virol. 131:237-250, 1993) to account for these observations and the successful establishment of an avian H1N1 strain in swine. To address this question, we calculated the mutation rates of A/Mallard/New York/6750/78 (H2N2) and A/Swine/Germany/2/81 (H1N1) by using the frequency of amantadine-resistant mutants. To account for the inherent variability of estimated mutation rates, we used a probabilistic model for the statistical analysis. The resulting estimated mutation rates of the two strains were not significantly different. Therefore, an increased mutation rate due to the presence of a mutator mutation is unlikely to have led to the successful introduction of avian H1N1 viruses in European swine.

Protection against a lethal avian influenza A virus in a mammalian system

The question of how best to protect the human population against a potential influenza pandemic has been raised by the recent outbreak caused by an avian H5N1 virus in Hong Kong. The likely strategy would be to vaccinate with a less virulent, laboratory-adapted H5N1 strain isolated previously from birds. Little attention has been given, however, to dissecting the consequences of sequential exposure to serologically related influenza A viruses using contemporary immunology techniques. Such experiments with the H5N1 viruses are limited by the potential risk to humans. An extremely virulent H3N8 avian influenza A virus has been used to infect both immunoglobulin-expressing (Ig+/+) and Ig-/- mice primed previously with a laboratory-adapted H3N2 virus. The cross-reactive antibody response was very protective, while the recall of CD8(+) T-cell memory in the Ig-/- mice provided some small measure of resistance to a low-dose H3N8 challenge. The H3N8 virus also replicated in the respiratory tracts of the H3N2-primed Ig+/+ mice, generating secondary CD8(+) and CD4(+) T-cell responses that may contribute to recovery. The results indicate that the various components of immune memory operate together to provide optimal protection, and they support the idea that related viruses of nonhuman origin can be used as vaccines.

[Demonstration of intrathecal antibody formation against Borrelia burgdorferi in Lyme neuroborreliosis]

Neurologic complications are common in Lyme borreliosis. In adults in particular, detection of intrathecally formed antibodies to B. burgdorferi is a decisive diagnostic criterion. As in neurosyphilis diagnosis, the specific antibody titer against B. burgdorferi in the CSF is compared with the serum titer by calculating the specific antibody indices, taking into account the disturbed blood/CSF barrier. Deriving from these specific indices, we adapt by dilution the serum IgG level to that in the CSF of the same day, thus allowing for passive diffusion of antibody through the blood/CSF barrier. Determination of the specific antibody titer by indirect immunofluorescence demonstrates intrathecal synthesis of specific antibodies against B. burgdorferi if the CSF titer is higher than in the diluted serum tested in parallel. The individual methods are presented in a group of patients.

[Effect of estradiol on lymphatic cells in the thymus, lymph nodes and spleen]

Male and female mice were given oestradiol and its effect on the tissues in the lymphatic system were observed including the thymus, lymph nodes and spleen. The weight of these organs was determined together with the number of cells present in them, with the percent proportion of live cells, their proliferative ability and metabolic activity. The obtained results indicate that oestradiol exerts a suppressive effect on the cells of these organs both in males and females and that this effect is strongest in the thymus.

[Peripheral blood lymphocytes in female mice during successive phases of the sexual cycle]

The authors determined the lymphocyte count, nucleo-cytoplasmic ratio, nuclear diameter, and the number of nucleoli and their morphology in peripheral blood of female mice. The results indicate that lymphocyte count remains unchanged during the sexual cycle but changes occur in the nucleocytoplasmic ratio, nuclear diameter, the number and morphology of lymphocyte nucleoli and are synchronized with the cycle. The observations suggest that changes in the maturity and activity of lymphocytes depend on the phase of the sexual cycle.