Biochemical studies on influenza viruses. I. Comparative analysis of equine 2 virus and virus N genes and gene products

Correlation of influenza A virus nucleoprotein genes with host species

The RNAs coding for the nucleoproteins of a panel of influenza isolates from human and nonhuman hosts were compared by RNA-RNA hybridization to determine the extent of genetic diversity of this protein and to determine if related nucleoproteins (NP) are consistently found in viruses from certain hosts. Five nucleoprotein groups were defined. Group 1 contains nearly all of the avian influenza viruses, group 2 includes only certain viruses isolated from gulls, group 3 includes all recent equine influenza strains, group 4 contains only equine/Prague/1/56, and group 5 contains all human and swine influenza isolates. The maintenance of specific nucleoproteins in viruses from certain species suggests that these proteins have evolved functionally significant differences that favor their replication in a specific host.

Sequential mutations in the NS genes of influenza virus field strains

The complete nucleotide sequences of the NS genes from three human influenza viruses, A/FM/1/47 (H1N1), A/FW/1/50 (H1N1), and A/USSR/90/77 (H1N1), were determined. Only five single-base differences were found within the sequences of the A/FW/1/50 and A/USSR/90/77 NS genes, thus confirming earlier data suggesting that the 1977 H1N1 viruses are closely related to virus strains that were circulating around 1950. Comparison of all three sequences with those from A/PR/8/34 and A/Udorn/72 viruses illustrates that these genes (with the exception of that of the A/USSR/90/77 strain) evolve through cumulative base changes along a single common lineage. A nucleotide sequence variation of approximately 2.2 to 3.4% per 10 years was determined for the NS gene segments. Extensive size variation was also observed among the NS1 proteins of the various human viruses. The A/FM/1/47 NS1 protein, which consists of 202 amino acids, is 15% shorter than the A/Udorn/72 NS1 protein, which consists of 237 amino acids.

Reassortant virus derived from avian and human influenza A viruses is attenuated and immunogenic in monkeys

An influenza A reassortant virus that contained the hemagglutinin and neuraminidase genes of a virulent human virus, A/Udorn/72 (H3N2), and the six other influenza A virus genome segments from an avirulent avian virus, A/Mallard/New York/6750/78 (H2N2), was evaluated for its level of replication is squirrel monkeys and hamsters. In monkeys, the reassortant virus was as attenuated and as restricted in its level of replication in the upper and lower respiratory tract as its avian influenza virus parent. Nonetheless, infection with the reassortant induced significant resistant to challenge with virulent human influenza virus. In hamsters, the reassortant virus replicated to a level intermediate between that of its parents. These findings suggest that the nonsurface antigen genes of the avian parental virus are the primary determinants of restriction of replication of the reassortant virus in monkeys. Attenuation of the reassortant virus for primates is achieved by inefficient functioning of the avian influenza genes in primate cells, while antigenic specificity of the human influenza virus is provided by the neuraminidase and hemagglutinin genes derived from the human virus. This approach could lead to the development of a live influenza A virus vaccine that is attenuated for man if the avian influenza genes are similarly restricted in human cells.

Virulence of avian influenza A viruses for squirrel monkeys

Ten serologically distinct avian influenza A viruses were administered to squirrel monkeys and hamsters to compare their replication and virulence with those of human influenza A virus, A/Udorn/307/72 (H3N2). In squirrel monkeys, the 10 avian influenza A viruses exhibited a spectrum of replication and virulence. The levels of virus replication and clinical response were closely correlated. Two viruses, A/Mallard/NY/6874/78 (H3N2) and A/Pintail/Alb/121/79 (H7N8), resembled the human virus in their level and duration of replication and in their virulence. At the other end of the spectrum, five avian viruses were restricted by 100- to 10,000-fold in replication in the upper and lower respiratory tract and were clearly attenuated compared with the human influenza virus. In hamsters, the 10 viruses exhibited a spectrum of replication in the nasal turbinates, ranging from viruses that replicated as efficiently as the human virus to those that were 8,000- fold restricted. Since several avian viruses were closely related serologically to human influenza viruses, studies were done to confirm the avian nature of these isolates. Each of the avian viruses plaqued efficiently at 42 degrees C, a restrictive temperature for replication of human influenza A viruses. Avian strains that had replicated either very efficiently or very poorly in squirrel monkeys still grew to high titer in the intestinal tracts of ducks, a tropism characteristic of avian, but not mammalian, influenza viruses. These observations indicate that some avian influenza A viruses grow well and cause disease in a primate host, whereas other avian viruses are very restricted in this host. These findings also provide a basis for determining the gene or genes involved in the restriction of replication that is observed with the attenuated avian viruses. Application of such information may allow the preparation of reassortant viruses derived from a virulent human influenza virus and an attenuated avian virus for possible use in a live attenuated vaccine for prevention of influenza in humans.

Genetic relatedness of the neuramindase of influenza A strains Nav2, Nav3, and Nav6

By molecular hybridization and by neuraminidase inhibition tests it is shown that all influenza A strains tested carrying an Nav3 or Nav2 neuraminidase (NA) are genetically highly related in their NA genes and cross-react serologically with specific antineuraminidase sera. The Nav6 strains exhibit a very low RNase protection after hybridization and do not cross-react serologically with Nav2 or Nav3 strains. Thus, the Nav2 and Nav3 strains comprise one group which is distinct from that of Nav6 strains.

On the origin of the gene coding for an influenze A virus nucleocapsid protein

The gene coding for the nucleocapsid protein NP of the influenza A virus recombinant strain 413 1,1 was characterized biochemically by molecular hybridization and fingerprint analysis. The data presented suggest that this NP gene has evolved by intracistronic recombination between NP genes of virus N and the fowl plague virus temperature-sensitive mutants ts 19.

Characterization of virus-like particles produced by an influenza A virus

The influenza strain 413 1,1 segregated as a stable recombinant during passage of the isolate 19/N which was obtained after double infection of chick embryo fibroblasts by virus N and the fowl plague virus (FPV) mutant ts 19. Its gene constellation was determined by molecular hybridization. Upon infection of chick embryo cells by this recombinant strain, two particle populations of high (H) and low (L) buoyant densities were produced. By biological and biochemical parameters, the H-population (delta = 1.22 g/cm3) cannot be distinguished from standard infectious influenza virus. In contrast, the noninfectious L-particles (delta = 1.14 g/cm3) lack all virus-specific glycoproteins (HA, NA) as well as the matrix protein M and are visualized by electron microscopy as spikeless particles. Significant changes in the quantitative composition of the phospholipid bilayer are evident as compared to the H-particles. In addition to the previously characterized eight genes both populations contain a variety of smaller RNA fragments which hybridize with complementary RNA and presumably represent degradation products of full-length genes.

Host range mutants of an influenza A virus

Temperature-sensitive (ts) mutants of fowl plague virus with a ts-lesion in segment 1 (ts 3, polymerase 1 gene) or segment 2 (ts 90, transport gene) do not form plaques on MDCK cells at the permissive temperature, while the wild type and ts-mutants of other groups are able to do so. This property is correlated with the ts-lesion, since revertants for the ts-lesion of ts 3 and ts 90 again form plaques on MDCK cells. The block on MDCK cells--at least for ts3--may be located in a late function, since viral RNA polymerase and hemagglutinin are formed in almost normal yields. MDCK cells infected with ts 3 or ts 90 exhibit a retarded cytopathic effect at 33 degrees C, but no cytopathic effect at 39 degrees C, at which temperature the infected cells can be passaged and super-infected with the wild type strain. Cells surviving the infection with ts 90 at 33 degrees C sometimes grow out again to a normal monolayer. It is suggested that the spread of virus is inhibited under these conditions.