Genetic studies of influenza viruses. 3. Production of plaque-type recombinants with A0 and A1 strains

Genetic recombination for antigenic markers of antigenically different strains of influenza B virus

Incorporation of trypsin in agar overlay or fluid maintenance media resulted in enhancement of plaquing efficiency and replication of influenza B viruses in primary chicken embryo fibroblasts. Using this improved technique, recombination was attempted with two serologically distinct strains of influenza B virus, B/Lee/40 and B/Massachusetts/1/71. After mixed infection, two virus clones were selected and characterized in detail. Hemagglutination inhibition and neuraminidase inhibition tests showed that these viruses are reciprocal antigenic recombinants with hemagglutinin derived from one parent and neuraminidase from the other. Serological examinations of the antisera to these recombinants confirmed the results. The frequency of recombination was high in the present system and 64% of the virus clones isolated without selection from the mixed yield were recombinants. This high recombination frequency is consistent with the genomic reassortment that is characteristic of recombination of influenza A viruses.

In pursuit of influenza: Fort Monmouth to Valhalla (and back)

In reviewing 50 years of personal research on influenza, I have journeyed, literally and figuratively, from an army camp epidemic in Fort Monmouth NJ in 1947 to a (literal and figurative) Valhalla, where I now conduct my research. Having entered the field as a physician, I have always sought practical applications of my work, yet in every instance, such applications have led me to seek further answers in basic research as new questions arose. I entered the area of influenza virus genetics by the back door through an interest in the effects of corticosteroid hormones on viral replication, used the genetic approach in analyzing the morphological variation of the virus and, in so doing, exploited the finding of a linkage of high-yield growth to spherical morphology. Today, all influenza vaccine viruses are high-yield genetic reassortants. Subsequent study of reassortant viruses facilitated the identification and isolation of the two major antigens of the virus in antigenic hybrids and showed their differing functions in the induction of immunity. In turn, a new approach to influenza vaccination has been discovered and is presently under clinical investigation.

Relationship between surface antigens of two variants of influenza A (H3N2) virus, as revealed by hemagglutination inhibition, kinetic neutralization, and neuraminidase inhibition

Rabbit antisera were raised against plaque-purified influenza virus strains of A/Victoria/75 and A/Texas/77 isolated from Seattle influenza patients. The antigenic specificity of hemagglutinins was compared by hemagglutination inhibition (HI) and kinetic neutralization tests. Anti-A/Victoria/75 had equally high HI titers and neutralization rate constants (kappa values) for A/Victoria/75 and A/Texas/77. In contrast, anti-A/Texas/77 had a high HI titer and kappa value to A/Texas/77 and a low HI titer and kappa value to A/Victoria/75. Similar results were obtained with antisera to recombinants with hemagglutinin specific for A/Victoria/3/75 or A/Texas/1/77 and with irrelevant neuraminidase. Seven wild-type isolates, three each of A/Texas and A/Victoria, and one strain characterized as a bridging strain were tested by HI and kinetic neutralization. Characterization as A/Texas or A/Victoria was confirmed by the results. No significant difference in neuraminidase specific for A/Victoria/75 or A/Texas/77 was hown when recombinants with an irrelevant hemagglutinin were compared by the neuraminidase inhibition test. These results suggest that A/Victoria/75 strains are "senior" to A/Texas/77 strains. The epidemiological implications of this observation are discussed.

Nonlinkage of neurovirulence exclusively to viral hemagglutinin or neuraminidase in genetic recombinants of A-NWS (HON1) influenza virus

Genetic recombination of the neurovirulent A/NWS/cc-p (H0N1) and the non-neurovirulent A/Jap.305/57 (H2N2) influenza viruses in which hemagglutinin and neuraminidase were segregated (H0N2, H2N1) were studied for neurovirulence in mice immunosuppressed with cyclophosphamide (CPA) which permitted full expression of virulence. Both H0N2 and H2N1 recombinants replicated in the brain (in contrast to the H2N2 parent) and both produced lethal effects in CPA-treated animals. Therefore we conclude that A/NWS (H0N1) neurovirulence is not exclusively linked with either the hemagglutinin or the neuraminidase of the virus. The H0N2 and H2N1 recombinants have revealed the existence of two separate attributes of neurovirulence: (i) the capacity of virus to initiate intracerebral infection and (ii) the capacity of infection, once initiated, to produce lethal disease. These studies provide further evidence for the polygenic nature of A/NWS neurovirulence.

Isolation and preliminary characterization of temperature-sensitive mutants of influenza virus

Isolation of temperature-sensitive (ts) mutants was attempted from the WSN strain of influenza A virus which was grown and assayed in MDBK cells. After growth of wild-type virus in the presence of 5-fluorouracil, 15 ts mutants were selected for which the ratio of plaquing efficiency at 39.5 C to that at 33 C was 10(-3) or less. In pairwise crosses of ts mutants, recombination and complementation were either very efficient or undetectable. It is suggested, therefore, that the viral genome consists of physically discrete units and recombination occurs as an exchange of these units. All 15 mutants have been assigned with certainty into five recombination groups. Three mutants are suspected to be double mutants. Any two complementing mutants always recombined with each other, and noncomplementing mutants did not recombine. In physiological tests, mutants showed diverse patterns of functional defects at the nonpermissive temperature. However, it was not always possible to correlate these physiological defects with the results of genetic characterization.

Independent variation in nature of hemagglutinin and neuraminidase antigens of influenza virus: distinctiveness of hemagglutinin antigen of Hong Kong-68 virus

Antigenic variations of the two virus-coded surface proteins of influenza virus-hemagglutinin and neuraminidase-were examined in seven strains of influenza A(2) virus (including the Hong Kong/68 strain) isolated from 1957 to 1968. Changes in the two antigens were found to occur independently in nature, resulting in new viruses which differ from older strains more with respect to one antigen than the other. The Hong Kong/68 strain is markedly different from previous A(2) strains in its hemagglutinin antigen but possesses neuraminidase indistinguishable antigenically from that of recent strains. Immunization experiments in mice provided evidence that only the neuraminidase component of an earlier A(2) strain provided protection against Hong Kong virus challenge. Segregation of hemagglutinin and neuraminidase antigens through recombination of each of the seven strains of influenza A(2) virus with A(0)/NWS virus made it possible to investigate antigenic variation of the two dissociated surface proteins independently in a common plaque assay system. Comparison of these hybrid viruses with the parent A(2) strains provided evidence that all the cross-reactivity of the Hong Kong strain with previous A(2) viruses is explicable on the basis of its similar neuraminidase component. It is proposed that the taxonomy of influenza A viruses must take into account differences in neuraminidase as well as hemagglutinin antigens.

Antiviral activity of antiserum specific for an influenza virus neuraminidase

Antiserum specific for influenza A(2) neuraminidase was produced by immunization of rabbits with the purified enzyme which had been isolated by electrophoresis from the proteins of a detergent-disrupted A(0)A(2) influenza virus recombinant [X-7 (F1)]. This recombinant contained hemagglutinin of the A(0) subtype and A(2) neuraminidase. Antiserum to the isolated A(2) neuraminidase did not react in any of four serological tests with A(0) or A(2) subtype viruses that lacked the A(2) enzyme. In contrast, the antiserum inhibited the neuraminidase activity only of wild-type and recombinant viruses containing the A(2) enzyme, regardless of the nature of their hemagglutinin proteins. The antiserum caused hemagglutination-inhibition of some, but not all, viruses bearing the A(2) enzyme, and it reduced the plaque size or plaque number of all viruses tested that contained A(2) neuraminidase. In the chick embryo and in cell culture, low dilutions of antiserum reduced the yield of virus. True neutralization of virus in the chick embryo did not occur. We conclude that an antiserum specific for A(2) neuraminidase influenced the yield and release of virus from influenza virus-infected cells.