[Live influenza vaccine: laboratory markers of attenuation].

Screening for candidate reassortants is an important step in the development of live influenza vaccine (LIV). The temperature-sensitive (ts) and cold-adapted (ca) phenotypes of vaccine strains are generally determined, by employing chicken embryos, and used as ts and ca attenuation markers. However, it is difficult to use the egg-determined ts phenotypes of vaccine candidate reassortants as an attenuation marker due to a wide circulation of natural ts epidemic influenza viruses. This study used two new alternative ts and ca attenuation markers in MDCK cells. The MDCK cell line was shown to be able to differentiate cold-adapted influenza viruses from any epidemic strains whereas they were undistinguishable when using eggs. The reduced ability of influenza type A vaccine viruses to grow in the MDCK cell culture at temperatures above 37 degrees C can be successfully used as a "cell-culture" ts marker. The similar marker for influenza B viruses may serve their reduced activity in the MDCK cells at 38 degrees C. The high reproductive activity of cold-adapted viruses in the MDCK cells at 26 degrees C was shown to be a suitable ca attenuation marker. The presented attenuation markers may be included into the standard scheme of primary screening of ts reassortant candidates for commercial live influenza vaccine as additional selection factors and may be used as basic markers in the design of culture vaccine.

[Efficacy of production of reassortant of epidemic strains and cold-adapted influenza viruses in chicken embryo and MDCK cells].

Reassortant strains for modern live influenza vaccines are prepared using growing chicken embryos. It is very important to switch manufacture of influenza vaccines from chicken embryos to cell cultures, especially due to the threat of future pandemic, when there will be need of big quantities of vaccine for immunization of all age groups. Efficacy of production of reassortant strains with 6:2 vaccine formulation of genome (6 internal genes from the donor of attenuation and 2 genes coding external antigens--hemagglutinin and neuraminidase--from epidemic strain) in MDCK cell culture, using standard techniques employed for production of the vaccine in chicken embryos, was studied. It was shown that yield frequency of aforementioned reassortants of influenza A viruses did not exceed 5.7% whereas in chicken embryos vaccine 6:2 reassortants were isolated with frequency of 4%. For influenza B viruses, yield of 6:2 reassortants in growing chicken embryos exceeded 67% whereas in MDCK cell culture we were unable to produce clones with required genome composition. Thus, existing method while effective for production of vaccine reassortants in chicken embryos is low effective for isolation of 6:2 reassortants in MDCK cell culture. Fundamentally new techniques are needed for production of reassortant strains for live influenza vaccine in cell culture.

[Phenotyte of epidemic influenza B virus strains isolated in different years].

The reproducing ability at elevated temperatures (non-ts phenotype) was examined for 38 influenza B virus strains isolated in different years in different countries. Out of the 7 strains isolated in 1940 to 1973, only one showed temperature-sensitivity of reproduction (a ts phenotype). In 1984 to 1988, the proportion of temperature-sensitive strains increased up to 55% (6 of 11). Since the late 1990s, the majority (90%) of the study influenza B viruses demonstrated a pronounced ts phenotype. Influenza B virus strains were also examined for their resistance to serum inhibitors. Prior to the divergence of influenza B viruses into two lines: B/Jamagata and B/Victoria, the epidemic viruses exhibited a high resistance to nonspecific inhibitors of normal equine serum. This property was also preserved in all study B/Victoria strains; however, 83% of the B/Jamagata viruses were inhibitor-sensitive. The present study has demonstrated the heterogenicity of epidemic influenza B viruses in temperature- and inhibitor-sensitivity.

[Genetic stability of cold-adapted influenza viruses].

The stability of cold adaptation, temperature-sensitivity, and marker mutations that are typical of attenuated influenza A and B viruses--master donor strains and their based reassortant vaccine strains was studied. After 5 sequential passages in chick embryos (CE) at resolving temperatures of 32 and 37 degrees C, the master donor strains and vaccine viruses retained their adaptability and temperature sensitive phenotype. Passage at the temperatures maximally permissible for viral reproduction (39 and 38 degrees C for influenza A and B viruses, respectively, aborted infection just during the second passage. After a series of passages at all study temperatures), there was neither loss or nor substitution of the marker mutations typical of the cold-adapted and temperature-sensitive phenotype of attenuated viruses. The study supports the high genetic stability of attenuated cold-adapted influenza A and B viruses during CE passage not only at the optimum, but also at elevated incubation temperatures.