Differences in the multiplication at elevated temperature of influenza virus recombinants pathogenic and nonpathogenic for chicken

A Single Amino Acid in the M1 Protein Responsible for the Different Pathogenic Potentials of H5N1 Highly Pathogenic Avian Influenza Virus Strains

Two highly pathogenic avian influenza virus strains, A/duck/Hokkaido/WZ83/2010 (H5N1) (WZ83) and A/duck/Hokkaido/WZ101/2010 (H5N1) (WZ101), which were isolated from wild ducks in Japan, were found to be genetically similar, with only two amino acid differences in their M1 and PB1 proteins at positions 43 and 317, respectively. We found that both WZ83 and WZ101 caused lethal infection in chickens but WZ101 killed them more rapidly than WZ83. Interestingly, ducks experimentally infected with WZ83 showed no or only mild clinical symptoms, whereas WZ101 was highly lethal. We then generated reassortants between these viruses and found that exchange of the M gene segment completely switched the pathogenic phenotype in both chickens and ducks, indicating that the difference in the pathogenicity for these avian species between WZ83 and WZ101 was determined by only a single amino acid in the M1 protein. It was also found that WZ101 showed higher pathogenicity than WZ83 in mice and that WZ83, whose M gene was replaced with that of WZ101, showed higher pathogenicity than wild-type WZ83, although this reassortant virus was not fully pathogenic compared to wild-type WZ101. These results suggest that the amino acid at position 43 of the M1 protein is one of the factors contributing to the pathogenicity of H5N1 highly pathogenic avian influenza viruses in both avian and mammalian hosts.

Host immune-related gene responses against highly pathogenic avian influenza virus infection in vitro differ among chicken cell lines established from different organs

Highly pathogenic avian influenza virus (HPAIV) induces acute disease in chickens causing high mortality and morbidity and is a major threat to poultry industries in Southeast Asian countries. The mechanisms of disease manifestation and host innate immune responses against HAPIV in chickens are not well understood. In this study, we examined virus replication and host gene expressions in four chicken cell lines in vitro to elucidate the impact of host innate immune responses against viral replication. It was demonstrated that viral replication efficiencies were different depending on the cell line. The viral replication appeared to be affected by the basal expression of IFN related genes. The expression of immune-related genes against the viral infection also varied in a cell line dependent manner. In non-immune derived cell lines, but not in immune derived cell lines, the expression of the CCL5 and CCL20 genes were induced by HPAIV infection. Reverse genetics HPAIV, with internal genes from avirulent avian influenza, reduced virus replication and affected immune-related gene expression in a cell line dependent manner. These results suggest the possibility that differential immune responses in different cell types in local tissues could modulate the consequences of HPAIV infection in chickens.

The critical cut-off temperature of avian influenza viruses

We have measured the pathogenicity for 6-week-old chicks of infection by H7 avian influenza viruses. One virus, strain S3 from A/FPV/Rostock/34(H7N1) showed a temperature sensitive phenotype at 41.5 degrees C and reduced pathogenicity. By analysis of reassortants made between virus S3 and A/FPV/Dobson/27(H7N7), a fully pathogenic virus, two conclusions arise. (1) The critical cut-off temperature for avian influenza virus in 6-week-old chicks is 41.5 degrees. (2) RNA segment 1 of virus S3 is responsible for the lack of pathogenicity in reassortant viruses. Nucleotide sequencing of RNA segment 1 from S3 and its parent, A/FPV/Rostock/34 has revealed a single mutation at nucleotide 1561. This results in a substitution of isoleucine for leucine at amino acid position 512 in the cap-binding protein, PB2.

Comparison of biological and physical properties of human and animal A(H1N1) influenza viruses

The study of biological properties of influenza virus strains belonging to the same subtype A(H1N1) and closely antigenically related, but isolated from different animal species (man, pig and duck), demonstrated that avian strains were more resistant than those isolated from mammals to high temperature and low pH, as shown by titration of residual infectivity in cell cultures (MDCK) and by sialidase assay. The difference in behaviour could be correlated to biological adaptation of the virus to its host. Avian body temperature is 40 degrees C and influenza virus, in ducks, is enterotropic and therefore capable of passing through the low pH values in the upper digestive tract of the animal. These results do not contradict the hypothesis of a possible filiation between avian and mammalian orthomyxoviruses.

The role of RNA segment 1 in an in vitro host restriction occurring in an avian influenza virus mutant

A temperature sensitive mutant, ts C47, derived from A/FPV/Rostock/34 and with a ts mutation in RNA segment 8, fails to form plaques in MDCK cells. From data obtained with reassortant viruses using the human influenza isolate A/FM/1/47 it was apparent that more than one mutation contributed to the temperature-sensitive (ts) and host range (hr) phenotypes of ts C47, and the phenotype of reassortants containing RNA segment 1 from A/FM/1/47 indicated that this segment was involved. A single nucleotide substitution at nucleotide 1961, resulting in valine instead of methionine in the predicted amino acid sequence of polypeptide PB2, was found in RNA segment 1 of ts C47, but this mutation did not segregate with the attenuated phenotype on gene reassortment. The following conclusions are drawn: (a) that ts C47 has at least two mutations in addition to that already known to exist in RNA segment 8, one of which (that in RNA segment 1) does not contribute to the observed ts hr phenotypes and (b) that the hr phenotype can be suppressed by substitution of RNA segment 1 by that of another strain.

The molecular biology of influenza virus pathogenicity

It is an accepted concept that the pathogenicity of a virus is of polygenic nature. Because of their segmented genome, influenza viruses provide a suitable system to prove this concept. The studies employing virus mutants and reassortants have indicated that the pathogenicity depends on the functional integrity of each gene and on a gene constellation optimal for the infection of a given host. As a consequence, virtually every gene product of influenza virus has been reported to contribute to pathogenicity, but evidence is steadily growing that a key role has to be assigned to hemagglutinin. As the initiator of infection, hemagglutinin has a double function: (1) promotion of adsorption of the virus to the cell surface, and (2) penetration of the viral genome through a fusion process among viral and cellular membranes. Adsorption is based on the binding to neuraminic acid-containing receptors, and different virus strains display a distinct preference for specific oligosaccharides. Fusion capacity depends on proteolytic cleavage by host proteases, and variations in amino acid sequence at the cleavage site determine whether hemagglutinin is activated in a given cell. Differences in cleavability and presumably also in receptor specificity are important determinants for host tropism, spread of infection, and pathogenicity. The concept that proteolytic activation is a determinant for pathogenicity was originally derived from studies on avian influenza viruses, but there is now evidence that it may also be relevant for the disease in humans because bacterial proteases have been found to promote the development of influenza pneumonia in mammals.

Aggravation of pathogenicity of an avian influenza virus by adaptation to quails

Influenza virus A/turkey/Ontario/7732/66 (H 5 N 9), which is highly pathogenic to chickens, is nonpathogenic to quails. After intratracheal or intramuscular inoculation of quails, virus replication was limited to the respiratory tract, genital organs, and pancreas. However, aggravation of the pathogenicity was achieved through adaptation only by several passages of lung homogenates in quails. The adapted virus caused a fatal generalized infection in quails as well as in chickens. The pathogenic change of the virus could not be explained by a change in the proteolytic cleavability of the hemagglutinin, because no difference was found in the cleavability between the original and the adapted viruses. The adapted virus formed larger plaques and grew a little faster than the original one in both chicken embryo and quail embryo cells. The faster multiplication of the adapted virus at the site of infection might be the reason for its change in pathogenicity. The original virus could circulate among quails by a direct contact transmission without causing disease. The shed virus, however, caused a fatal infection in chickens when they were kept in contact with the infected quails. The epidemiological significance of this observation is discussed.

Studies on the temperature sensitivity of influenza A virus reassortants nonpathogenic for chicken

Influenza A virus reassortants which are nonpathogenic for chickens are like mammalian influenza A viruses in that they are temperature sensitive for growth at 41 degrees C. We have investigated the mechanism of this temperature sensitivity using reassortants between the two highly pathogenic strains A/FPV/Rostock/34 (FPV, H7N1) and A/turkey/England/63 (TE, H7N3). These reassortants show a strict correlation between the pathogenicity for chickens and the constellation of the genes coding for the ribonucleoprotein complex, RNP. Evidence is presented which shows that all viral components are synthesized in sufficient amounts and that the block in the viral replication cycle at the nonpermissive temperature is a late one affecting virus maturation. It is suggested that the RNP, although still enzymatically functional, may lose its ability to interact normally with viral surface components, thus interfering with the process of virus maturation. Some of the nonpathogenic reassortants which possessed the neuraminidase of TE showed an interesting temperature-dependent phenomenon: the haemagglutinin synthesized at the elevated temperature could only agglutinate erythrocytes at 20 degrees C, when the neuraminidase was inhibited or the infected cells vigorously disrupted by ultrasonication. This phenomenon is possibly not directly related to the temperature-sensitive block.

Mouse neurotropic recombinants of influenza A viruses

Recombinants with known gene constellations between fowl plague virus (FPV) and various prototype influenza virus strains have been examined for neurovirulence in suckling mice. Strongly neurotropic recombinants were obtained from crosses between FPV and the strains virus N, Hong Kong, and PR8, but not between FPV and equi 2 or swine viruses. All highly neurotropic recombinants had RNA segment 4 (HA) derived from FPV and RNA segment 2 (Ptra gene) from the other prototype strain. The derivation of two other RNA segments of the polymerase complex, namely RNA segments 3 (Pol 2) and 5 (NP) and also segment 8 (NS) can modulate these properties. For example, if in recombinants between FPV and virus N in addition to RNA segment 2 also RNA segments 3 and/or 8 are derived from virus N, neurovirulence is further enhanced, while replacement of RNA segment 5 of FPV by the corresponding segment of virus N decreases or abolishes neurovirulence. The derivation of the other genes does not seem to be relevant for neurovirulence in the crosses mentioned above. Of the prototype strains tested, the turkey England (t. Engl.) strain is the only one which was highly neurotropic for suckling mice. Recombinants between FPV and t. Engl. which have kept the HA gene of t. Engl. were still neurotropic, while those with the HA gene of FPV were completely avirulent. The results obtained demonstrated that 1. the creation of influenza virus recombinants neurotropic for mice is not a rare event; 2. one of the parents should multiply well in mouse lungs; 3. the presence of a cleavable hemagglutinin is necessary, but not sufficient. In the pair FPV/turkey England the hemagglutinin of turkey England seems to determine neurovirulence.

Pathogenicity reactivation of nonpathogenic influenza virus recombinants under von Magnus conditions

The pathogenicity for the chicken of a number of nonpathogenic recombinants between fowl plague virus and various avian and mammalian influenza A viruses can be reactivated by passaging serially at high multiplicities (von Magnus conditions) at 41 degrees, which is the nonpermissive temperature of nonpathogenic recombinants. While the mechanism underlying this reactivation is unclear, it could be excluded that it was due to segregation of heterozygotes to the wild type homozygotes.

Low genetic mixing between avian influenza viruses of different geographic regions

The degree of genetic relatedness of vRNA segments 1, 2, and 3 of avian influenza A viruses was investigated by molecular hybridization. The results indicate that avian influenza A viruses isolated within a given geographic region are genetically more closely related than strains from different regions, irrespective of the year of isolation and the species from which the virus was isolated. Studies on RNA segment 4 of viruses within the subtype H7 isolated in different regions gave similar results. Thus the genetic composition of avian influenza A viruses appears to be maintained to a rather high degree within a given geographic region and the intrusion of genes from "foreign regions" appears to be taking place with low frequency. The results are discussed with respect to the worldwide distribution of influenza virus genes by migrating birds.