Adaptation and growth characteristics of influenza virus at 25 degrees c

Temperature-sensitive defect of influenza A/Ann Arbor/6/60 cold-adapted variant leads to a blockage of matrix polypeptide incorporation into the plasma membrane of the infected cells

A temperature-sensitive (ts) defect in growth of the A/Ann Arbor/6/60 (A/AA/60) cold-adapted (ca) and ts variant strain has been studied. At the restrictive temperature of 38.5 degrees C, the variant synthesized all the viral polypeptides in normal amounts within the infected cells, but the virions released into the culture fluid contained greatly reduced amounts of the matrix (M1) polypeptide and showed significantly low infectivity per unit hemagglutinin activity. Cell fractionation experiments revealed that incorporation of the M1 polypeptide into plasma membranes of the variant-infected cells was selectively reduced at 38.5 degrees C, whilst it occurred normally at 34 degrees C. The ts reassortants between the A/AA/60 variant and the A/AA/1/80 wild type (wt) strain (non-ts), which had the M gene derived from the wt parent, also showed similar patterns. These results suggest that the ts defect of the variant and its ts reassortants involves the process of incorporation of the M1 polypeptide into the plasma membranes of the infected cells and that this defect is not attributable to the M gene of the variant.

Cold-adaptation of human rotavirus

A human rotavirus strain was cold-adapted for possible future use as a live vaccine. The original strain was isolated in 1980 in primary cynomolgus monkey kidney cells and has a serotype I and subgroup II antigenicity. The virus was serially passaged in African green monkey kidney cells; it was cultivated at 37 degrees C at the first stage of passages, and the cultivation temperature was then shifted down stepwise by 3 degrees C per each 10 passages. Finally the virus was passaged 10 times at 25 degrees C (total passage number of 55). The virus formed small-size plaques with irregular shaped borders at 31 degrees C. Growth at 25 degrees C of the cold-adapted virus was higher than that of the original virus. There was no difference between the migration patterns of 11 dsRNA segments in polyacrylamide gel electrophoresis of the original and the cold-adapted viruses.

Comparative studies of wild-type and cold-mutant (temperature-sensitive) influenza virus: detection of mutations in all genes of the A/Ann Arbor/6/60 (H2N2) mutant vaccine donor strain

Direct biochemical evidence has been obtained for the existence of mutations in all eight RNA segments of the A/Ann Arbor/6/60 cold-adapted (ca) mutant influenza virus strain as compared with its wild-type (wt) progenitor. Polyacrylamide gel electrophoresis (PAGE) of viral RNA revealed a change in the electrophoretic migration of RNA 2 (PB1). T1 oligonucleotide mapping revealed changes in two polymerase genes (the PB2 and PA genes), the hemagglutinin (HA) gene and the nucleoprotein (NP) gene. Analysis of S1 nuclease-treated RNA hybrids on polyacrylamide gels detected changes in the HA and neuraminidase (NA) genes. Partial DNA sequence analysis demonstrated a base sequence change in the matrix (M) protein gene that predicts an amino acid change in the M2 protein and a silent mutation in the non-structural (NS) protein gene. In addition, analysis of viral polypeptides by PAGE has so far revealed changes in the viral protein, PA. These findings directly demonstrate the existence of multiple mutations in the ca vaccine strain, a property that may provide reliably and stably attenuated vaccines that derive their six internal genes from the ca A/Ann Arbor/6/60 donor strain.

Biological characteristics of a cold-adapted influenza A virus mutation residing on a polymerase gene

The biological function of a cold-adapted (ca) mutation residing on the PB2 gene of an influenza A/Ann Arbor/6/60 (A/AA/6/60) ca variant virus in the viral replication cycle at 25 degrees C was studied. The viral polypeptide synthesis of A/AA/6/60 ca variant at 25 degrees C was evident approximately 6 hours earlier than the wild type (wt) virus and yielded twice as many products. The quantitative analysis of viral complementary RNA (cRNA), synthesized in the presence of cycloheximide, revealed that A/AA/6/60 ca variant and a single gene reassortant that contains only the PB2 gene of the ca variant with remaining genes of the wt virus produced equal amount of cRNA at 25 degrees and 33 degrees C, which was an amount approximately four fold greater than the wt virus' cRNA synthesized at 25 degrees C. These results strongly suggest that the ca mutation residing on the PB2 gene of A/AA/6/60 ca variant affects the messenger RNA synthesis at 25 degrees C in the primary transcription.

Development and characterization of cold-adapted viruses for use as live virus vaccines

Representative viruses from twelve RNA and two DNA virus genera have been successfully adapted to growth at sub-optimal temperature (cold-adapted). In almost every case, there was a correlation between acquisition of the cold-adaptation phenotype and loss of virulence in the normal host whether animal or man. Overall, the best method of cold adaptation to develop a live virus vaccine line appeared to be a stepwise lowering of the growth temperature allowing time for multiple lesions to occur and/or be selected. In addition, the starting virus should be a recent isolate not as yet adapted to a tissue culture host and the cold-adaptation process should then occur in a host heterologous to the virus' normal host. These viruses have been reviewed in the light of their cold-adaptation method and successful production of an attenuated line as virus vaccine candidate. Finally, detailed information is presented for the cold-adaptation process in influenza virus.

Serological evaluation of an influenza A virus cold-adapted reassortant live vaccine, CR-37 (H1N1), in Japanese adult volunteers

A cold-adapted influenza A virus, CR-37 (H1N1), derived from genetic reassortment between A/Ann Arbor/6/60 (H2N2) cold-adapted variant virus and A/California/10/78 (H1N1) wild-type virus, was tested in Japanese adult volunteer. The CR-37 live virus preparation induced only low-grade clinical reactions in volunteers for the first 3-4 days after inoculation. Two vaccinees who did not show any antibody changes became febrile (over 38.0 degrees C). Skin tests using the vaccine preparation and uninfected allantoic fluid were performed, and indicated that one of these two vaccines was positive for the CR-37 vaccine preparation. A high proportion of the vaccinees whose sera had a haemagglutination-inhibition (HI) antibody titre against the vaccine strain of less than or equal to 64 before inoculation, seroconverted in both HI and neuraminidase-inhibition (NAI) antibody titrations, and only a few seroconverted in the titration of antibody against type-specific internal antigens. The serological examinations against heterotypic H1N1 variants indicated that the cold-adapted live influenza virus vaccine could induce a broad spectrum of HI antibody reactivity and immunity of long duration.

Analysis of an influenza A virus mutant with a deletion in the NS segment

The influenza virus host range mutant CR43-3, derived by recombination from the A/Alaska/6/77 and the cold-adapted and temperature-sensitive A/Ann Arbor/6/60 viruses, has previously been shown to possess a defect in the NS gene. To characterize this defect, nucleotide sequence data were obtained from cloned cDNAs. The CR43-3 NS gene was found to be 854 nucleotides long and to derive from the NS gene of the A/Alaska/6/77 parent virus by an internal deletion of 36 nucleotides. Direct sequencing of RNA 8 of CR43-3 virus confirmed that the deletion in the NS1-coding region was not an artifact that was generated during the cloning procedure. Protein analysis indicated that the NS1 protein of CR43-3 virus was synthesized in equal amounts in the restrictive (MDCK) cells as well as in the permissive (PCK) host cells. Also, indirect immunofluorescence studies of virus-infected cells showed that the NS1 protein of CR43-3 virus, like that of the parent viruses, accumulates in the nuclei of both cell systems. Although no differences in synthesis or localization of the NS1 protein could be detected, a consistent reduction in M1 protein was noted in CR43-3 virus-infected, nonpermissive cells as compared with that of the permissive host. Since analysis of the CR43-3 virus required us to obtain the NS nucleotide sequence of the 1977 isolate A/Alaska/6/77, we were able to compare this sequence with those of corresponding genes of earlier strains. The result of this analysis supports the idea of a common lineage of human influenza A viruses isolated over a 43-year period.

Dose response of A/Alaska/6/77 (H3N2) cold-adapted reassortant vaccine virus in adult volunteers: role of local antibody in resistance to infection with vaccine virus

An attenuated influenza A candidate vaccine virus, derived from the A/Ann Arbor/6/60 (H2N2) cold-adapted (ca) donor virus and the A/Alaska/6/77 (H3N2) wild-type virus, was evaluated in adult seronegative volunteers (serum hemagglutination-inhibiting antibody titer, less than or equal to 1:8) for level of attenuation, infectivity, antigenicity, and genetic stability. Four groups with similar preinoculation mean titers of serum and nasal wash antibodies were inoculated intranasally with 10(4.5), 10(5.5), 10(6.5), or 10(7.5) 50% tissue culture infectious doses (TCID50) of the ca reassortant virus, and eight other seronegative adult volunteers received the wild-type virus. Only 2 of 66 vaccinees developed fever or mild and brief systemic or upper respiratory tract illness or both. Both volunteers with vaccine-related reactions received the highest dose (10(7.5) TCID50) of ca virus, which indicates that the vaccine retains some mild reactogenicity at a high dosage. In contrast, four of eight volunteers infected with the wild-type virus became ill. Each of the 54 isolates tested retained the temperature-sensitive phenotype of the vaccine virus. Thus, the ca reassortant was genetically stable and attenuated at 10(4.5) to 10(7.5) TCID50 for seronegative adults. The 50% human infective dose of ca virus was approximately 10(5.3) TCID50. Ten and one hundred 50% human infectious doses infected 73 and 83% of vaccinees, respectively, and approximately 75% developed an immunological response at these doses. The failure of the vaccine virus to infect some volunteers was correlated with the presence of pre-inoculation nasal wash immunoglobulin A hemagglutinin antibody.

Cold adaptation of parainfluenza virus type 3: induction of three phenotypic markers

In order to attenuate parainfluenza type 3 virus, a wild type strain that was isolated from a child with respiratory disease was adapted to replicate in African green monkey kidney cells at 20 degrees C. Replication at 20 degrees C was not a property of the wild type virus. The virus was serially passaged 45 times in the cold, and clones were selected following passage levels 7, 12, 18, and 45. The population of cold-adapted virus was found to be progressively enriched with temperature sensitive (ts) mutants. After 7 passages in the cold, 1 of 9, and after 12 passages in the cold, 3 of 12 clones were temperature sensitive. Following 18 passages in the cold, 80% of the clones were temperature sensitive and after 45 passages in the cold, all clones were temperature sensitive. In addition to being temperature sensitive each ts clone manifested the tiny plaque morphology. Each temperature-sensitive clone was also cold adapted. Some clones were cold adapted but were not temperature sensitive. The mutants were found to be genetically stable when serially passaged at 32 degrees, 35 degrees, or 39 degrees C. The mutants may possess the necessary degree of attenuation for use as live attenuated intranasal vaccines.

Characterization and clinical evaluation of live influenza A vaccine prepared from a recombinant of the A/USSR/92/77 (H1N1) and the cold-adapted A/Ann Arbor/6/60 (H2N2) strains

Live influenza vaccine was prepared after genetic recombination of the A/USSR/92/77 (H1N1) strain with the cold-adapted A/Ann Arbor/6/60 (H2N2) strain. The recombinant contains the genes coding for the HA and NA proteins from the A/USSR/92/77 (H1N1) strain and the genes coding for the P1, P2, P3, NP, M and NS proteins from the A/Ann Arbor/6/60 (H2N2) strain. To assess the properties of this vaccine, it was administered under double-blind conditions to 14 healthy volunteers, while another 14 healthy volunteers received placebo. The vaccine virus appeared to be sufficiently attenuated. No febrile reactions were observed. The vaccinees showed an increase in mean serum haemagglutination-inhibiting antibody level from 19 to 73 after two vaccinations. From nasal swabs and antibody responses, it was concluded that the vaccine virus showed no transmission to the placebo group under conditions of close contact. Also, the vaccine virus was found to be genetically stable. It is concluded that this live influenza virus vaccine meets the requirements for safe use in humans. However, several problems still exist which may impede a general use of life influenza vaccines.

Evaluation of A/Alaska/6/77 (H3N2) cold-adapted recombinant viruses derived from A/Ann Arbor/6/60 cold-adapted donor virus in adult seronegative volunteers

The influenza A/Ann Arbor/6/60 (H2N2) cold-adapted (ca) virus was evaluated as a donor of attenuating genes to new variants of influenza A virus. This ca donor virus was mated with the A/Alaska/6/77 (H3N2) wild-type virus, and three A/Alaska/6/77 (H3N2) ca recombinant viruses were produced. The parental origin of the genes in the three ca recombinants had been determined previously (2), and their virulence for adult seronegative volunteers was assessed in the present study to identify the genes present in the ca donor virus that confer attenuation. Each of the recombinants received the hemagglutinin and neuraminidase genes from the A/Alaska/6/77 (H3N2) wild-type parent. One ca recombinant (CR-29) received all six transferable genes from the ca parent and was found to be satisfactorily attenuated in the volunteers. The two other ca recombinants received five of the six transferable genes with a wild-type gene at the M or NS locus. The pattern of infection in humans with these latter two ca recombinants was similar to the CR-29 ca recombinant. These findings demonstrate that inheritance of a gene in ca recombinants at the M or NS locus segregates independently of attenuation and suggest that the M and NS genes present in the ca donor virus are not the major determinants of attenuation conferred by this virus.

Characterization of the temperature sensitive phenotype of the A/Ann Arbor/6/60 cold-adapted virus and its recombinants

The A/Ann Arbor/6/60 cold-adapted (ca) donor virus had a 38 degrees C shutoff temperature when tested in MDCK tissue culture. ca recombinant viruses bearing all six "internal genes" of the A/Ann Arbor/6/60 ca donor virus and the surface antigens of wild-type virus can manifest a 1,000-fold difference in plaquing efficiency at 38 degrees C. These observations suggest that the A/Ann Arbor/6/60 ca donor genes that specify the temperature sensitive phenotype of the ca recombinants can undergo genetic modification during the production and passage of the recombinants. The NS gene in the ca recombinant virus could be inherited from either parent without influencing the level of temperature sensitivity of the ca recombinant.

Evaluation of influenza A/Hong Kong/123/77 (H1N1) ts-1A2 and cold-adapted recombinant viruses in seronegative adult volunteers

Two attenuated influenza A donor viruses, the A/Udorn/72 ts-1A2 and the A/Ann Arbor/6/60 cold-adapted (ca) viruses, are being evaluated for their ability to reproducibly attenuate each new variant of influenza A virus to a specific and desired level by the transfer of one or more attenuating genes. Each of these donor viruses has been able to attenuate influenza A viruses belonging to the H3N2 subtype by the transfer of one or more attenuating genes. To determine whether these two donor viruses could attenuate a wild-type virus that belonged to a different influenza A subtype, ts-1A2 and ca recombinants of a wild-type virus representative of the A/USSR/77 (H1N1) Russian influenza strain were prepared and evaluated in adult doubly seronegative volunteers at several doses. The recombinants derived from both donor viruses were attenuated for the doubly seronegative adults. Less than 5% of infected vaccinees developed a febrile or systemic reaction, whereas five of six recipients of wild-type virus developed such a response. The 50% human infectious dose (HID(50)) for each recombinant was approximately 10(5.0) 50% tissue culture infective doses. The virus shed by the ts-1A2 and ca vaccinees retained the ts or ca phenotype, or both. This occurred despite replication of the recombinant viruses for up to 9 days. No evidence for transmission of the ca or ts-1A2 recombinant virus to controls was observed. A serum hemagglutination inhibition response was detected in less than 50% of the infected vaccinees. However, with the more sensitive enzyme-linked immunosorbent assay, a serological response was detected in 100% of the ca vaccinees given 300 HID(50) and approximately 70% of ca or ts vaccinees who received 10 to 32 HID(50) of virus. These results indicate that the recombinants derived from both donor viruses were satisfactorily attenuated and were stable genetically after replication in doubly seronegative adults although they induced a lower serum hemagglutination inhibition response than that found previously for H3N2 ts and ca recombinants.

Growth of influenza A viruses in hamsters

The growth characteristics of four temperature-sensitive or cold-adapted recombinant influenza viruses and eight recombinant influenza viruses derived in other ways, together with the wild-type, parent viruses of these strains, were tested in hamster lungs and turbinates and in embryonated eggs at different temperatures for their replicative ability. The results showed that although the temperature-sensitive and cold-adapted recombinant viruses replicated to considerably lower titres than their wild-type parent virus strains in hamster lung and at 37 degrees C in embryonated eggs, no similar pattern of growth was observed for the group of A/PR 8 and A/Okuda recombinant influenza viruses studied in these systems. The hamster model is not therefore generally applicable as a marker for attenuated influenza virus vaccine strains.

Humoral and cellular immune responses of seronegative children vaccinated with a cold-adapted influenza A/HK/123/77 (H1N1) recombinant virus

Humoral and cell-mediated immune responses of young, seronegative children were assessed after intranasal vaccination with a cold-adapted influenza. A/HK/77 (H1N1) CR 35 recombinant virus. Vaccines shedding influenza virus experienced a rise in hemagglutinin-inhibition antibody 15 to 30 days after vaccination. Vaccinees showed low but significant lymphocyte transformation to A/USSR (H1N1) by day 8 after vaccination, which decreased to prevaccination levels at 30 to 34 days. The lymphocyte transformation response occurred before serum antibody rises were detected by hemagglutinin-inhibition assay. No change in lymphocyte responsiveness was observed after vaccination as measured by phytohemagglutinin stimulation. Lymphocytes responded to in vitro incubation with inactivated influenza (H1N1) virus by producing interferon. The interferon produced was of type I and was observed in vaccinees and nonvaccinees both before and after vaccination.

Studies in man with cold-recombinant influenza virus (H1N1) live vaccines

Two cold-recombinant influenza A (H1N1) viruses were tested in several groups of human volunteers. Only minor clinical symptoms were seen and no febrile reactions occurred. With serologically primed individuals virus shedding was low, but a high proportion showed rises in serum antibody levels after vaccination and mean titres were high. With serologically unprimed volunteers shedding was high, about 75% yielding viruses but only at low titres and for a short duration. No revertant viruses were found and there was no evidence of transmission to potentially susceptible individuals housed in close contact to the vaccinees. Serum antibody responses with unprimed volunteers were, however, low. Only about one half showed increases in serum antibody titres after vaccination and mean titres were low. Nevertheless, challenge with live attenuated virus indicated a high degree of protection based on virological evidence of infection.

Cold adapted variants of influenza A. II. Comparison of the genetic and biological properties of ts mutants and recombinants of the cold adapted A/AA/6/60 strain

The genetic and biological properties of 13 recombinant influenza A clones derived at 25 degrees C from the A/AA/6/60-cold variant (by crosses with 4 different wild type strains) were compared with a set of 5-FU induced ts-mutants. The 5-FU mutants had previously been placed into 7 complementation-recombination groups; the A/AA/6/60-cold parent (PI-7) and the 12 cold recombinant clones which were ts were shown to share a lesion with only one of these groups. The parental strain and 5 recombinant clones were evaluated for replication in the lungs and nasal turbinates of hamsters. Each virus appeared to be attenuated; genetic stability correlated with the level of viral replication in the hamster lung, i.e., viruses which grew best showed a tendency to revert to the ts+ phenotype. Characterization of the ts+ revertants for the presence of the cold adaptation property revealed that these viruses exhibited a spectrum of cold adaptation properties. Two viruses, PI-7 (the parental cold variant) and the CR6 recombinant (A/Queensland/6/72) did not revert in either the lungs or nasal turbinates of hamsters.

Chemically-induced temperature sensitive mutants of dengue virus type 2. I. Isolation and partial characterization

Temperature sensitive (ts) mutants of dengue virus type 2 (DEN-2, TH-36 isolate) were induced by replication in primary hamster kidney cells treated with 5-azacytidine. Seven ts mutants were obtained from 138 clones isolated by an immunofluorescent cloning technique. Of these 7 ts mutants, 5 were sufficiently stable to permit partial characterization. Complementation was detected at very low but statistically significant levels between some ts mutants at 40 degrees C. Viral double-stranded RNA production was evaluated in LLC-MK2 cells at 30 degrees and 40 degrees C by micro-quantitative complement fixation. The results of complementation tests and RNA production tests indicated that the 4 of 5 stable ts mutants constitute 3 separate complementation groups (2 RNA+ and 1 RNA-groups), while a fifth ts mutant was RNA- but non-complementable. The data presented here indicate that a genetic system can be developed without employing traditional plaque or cytopathology methods. Further, the 5 DEN-2 ts mutants are believed to represent the only set of complementation-positive flavivirus mutants so far isolated.

Virulence of temperature-sensitive mutants of foot-and-mouth disease virus

A number of temperature-sensitive mutants isolated from two strains of foot-and-mouth disease virus were examined for their virulence in suckling mice. The majority of the mutants were found to be less virulent than the parent virus strains, ranging from slight to total attenuation, but two mutants retained parental levels of virulence. There was no correlation between mutant cut-off temperatures and virulence, or the revertant content of mutant preparations and virulence. It was not always possible to regain parental levels of virulence by isolating phenotypic revertants or wild-type recombinants from genetic crosses, but recombinants were significantly more virulent than either of the two input viruses in a genetic cross.

Comparative studies of wild-type and "cold-mutant" (temperature-sensitive) influenza viruses: polypeptide synthesis by an Asian (H2N2) strain and its cold-adapted variant

The structure and replication of a cold-adapted, temperature-sensitive (TS) mutant of an Asian (H2N2) influenza virus was compared with that of its wild-type (WT) parent. Viruses were grown in a chicken kidney cell system, and at the nonpermissive temperature of 40 C, production of infectious TS virus was about 100,000-fold less than at 35 C, in contrast to WT virus. Major structural polypeptides of each virus grown at 35 C were similar, except that the hemagglutinin glycopolypeptide (HA) of the TS virions was slightly more heterogenous than that of WT virions. Synthesis of viral polypeptides was examined by sodium dodecyl sulfate acrylamide gel electrophoresis of pulse-labeled infected cells. This revealed a defect in the synthesis of TS viral hemagglutinin that was most pronounced at the nonpermissive temperature. Other TS viral polypeptides appeared to be synthesized normally at 40 C. A defect in the TS virus hemagglutinin was also indicated by serological studies that demonstrated that TS virus hemagglutinin had lost antigenic sites present on the WT virus. Thus, it is concluded that the virus mutant examined contains lesions in the hemagglutinin gene, although the possibility of additional unrecognized lesions is not excluded.

Attenuation of human influenza A viruses

The attenuation of two human influenza A viruses has been carried out, using the selection of inhibitor-resistant strains and multiple passages at low temperatures. A virus related to A2/Tokyo/3/67 was obtained in an inhibitor-resistant form. When this was compared with the inhibitor-sensitive strain in a volunteer trial it was relatively non-pathogenic. The second virus, A2/Hongkong/1/68, was subjected to much longer treatment, but nevertheless remained slightly sensitive to serum inhibitor. When given to volunteers it was less pathogenic than before but attenuation was incomplete. A2/Hongkong/1/68 was also modified by passage at low temperatures. Many of these passages are apparently necessary for full attenuation.All attenuated viruses were infective and antigenic.