Development of high yield reassortants for influenza type B viruses and analysis of their gene compositions

A critical step in producing the annual inactivated influenza vaccine is the development of high yield (hy) seed viruses by reassortment for improved growth in ovo. Although hy reassortants for type A influenza viruses have been developed for many years, hy B influenza reassortant virus development for vaccine production has proven difficult. In this study, we have developed fourteen hy influenza type B reassortants as vaccine candidate strains with B/Lee/40 as the donor virus. Upon characterization by the Influenza Division at the Centers for Disease Control and Prevention (CDC) and the verification of HA by sequencing, all B reassortants were found to be antigenically indistinguishable from the wild type (wt) parents and suitable for vaccine production. However, only one hy reassortant seed virus from this group was used by a manufacturer for vaccine production. In general, hy reassortants showed an increase in hemagglutination (HA) titers over their wt parents by approximately 8 fold (range 1-32 fold). Gene compositions of the hy B reassortants were analyzed by restriction fragment length polymorphism (RFLP) and the wt origin of the HA and neuraminidase (NA) were confirmed. However, in contrast to hy A reassortants which require the M gene (hy donor A/PR/8/34) for high yield, all fourteen hy B reassortants obtained the NP gene from the hy donor strain (B/Lee/40). The parental source for the remaining genes varied among the hy B reassortants. The results indicate that the B/Lee/40 NP and PB1 gene segments are important contributors to high yield growth in influenza B reassortant viruses for both Yamagata and Victoria lineages. The B/Lee/40 PB2 gene along with wt NS gene also contributed to the improved growth for hy reassortants of Yamagata lineage.