Rapid confirmation by RFLP of transfer to vaccine candidate reassortment viruses of the principal 'high yield' gene of influenza A viruses

Evolving gene targets and technology in influenza detection

Influenza viruses cause recurring epidemic outbreaks every year associated with high morbidity and mortality. Despite extensive research and surveillance efforts to control influenza outbreaks, the primary mitigation treatment for influenza is the development of yearly vaccine mixes targeted for the most prevalent virus strains. Consequently, the focus of many detection technologies has evolved toward accurate identification of subtype and understanding the evolution and molecular determinants of novel and pathogenic forms of influenza. The recent availability of potential antiviral treatments are only effective if rapid and accurate diagnostic tests for influenza epidemic management are available; thus, early detection of influenza infection is still important for prevention, containment, patient management, and infection control. This review discusses the current and emerging technologies for detection and strain identification of influenza virus and their specific gene targets, as well as their implications in patient management.

Influenza virus surveillance, vaccine strain selection, and manufacture

As outlined in other chapters, the influenza virus, existing laboratory diagnostic abilities, and disease epidemiology have several peculiarities that impact on the timing and processes for the annual production of influenza vaccines. The chapter provides an overview on the key biological and other factors that influence vaccine production. They are the reason for an "annual circle race" beginning with global influenza surveillance during the influenza season in a given year to the eventual supply of vaccines 12 months later in time before the next seasonal outbreak and so on. As influenza vaccines are needed for the Northern and Southern Hemisphere outbreaks in fall and spring, respectively, global surveillance and vaccine production has become a year round business. Its highlights are the WHO recommendations on vaccine strains in February and September and the eventual delivery of vaccine doses in time before the coming influenza season. In between continues vaccine strain and epidemiological surveillance, preparation of new high growth reassortments, vaccine seed strain preparation and development of standardizing reagents, vaccine bulk production, fill-finishing and vaccine release, and in some regions, clinical trials for regulatory approval.

Gene constellation of influenza A virus reassortants with high growth phenotype prepared as seed candidates for vaccine production

Background

Influenza A virus vaccines undergo yearly reformulations due to the antigenic variability of the virus caused by antigenic drift and shift. It is critical to the vaccine manufacturing process to obtain influenza A seed virus that is antigenically identical to circulating wild type (wt) virus and grows to high titers in embryonated chicken eggs. Inactivated influenza A seasonal vaccines are generated by classical reassortment. The classical method takes advantage of the ability of the influenza virus to reassort based on the segmented nature of its genome. In ovo co-inoculation of a high growth or yield (hy) donor virus and a low yield wt virus with antibody selection against the donor surface antigens results in progeny viruses that grow to high titers in ovo with wt origin hemagglutinin (HA) and neuraminidase (NA) glycoproteins. In this report we determined the parental origin of the remaining six genes encoding the internal proteins that contribute to the hy phenotype in ovo.

Methodology

The genetic analysis was conducted using reverse transcription-polymerase chain reaction (RT-PCR) and restriction fragment length polymorphism (RFLP). The characterization was conducted to determine the parental origin of the gene segments (hy donor virus or wt virus), gene segment ratios and constellations. Fold increase in growth of reassortant viruses compared to respective parent wt viruses was determined by hemagglutination assay titers.

Significance

In this study fifty-seven influenza A vaccine candidate reassortants were analyzed for the presence or absence of correlations between specific gene segment ratios, gene constellations and hy reassortant phenotype. We found two gene ratios, 6∶2 and 5∶3, to be the most prevalent among the hy reassortants analyzed, although other gene ratios also conferred hy in certain reassortants.

A multiplex RT-PCR method for screening of reassortant live influenza vaccine virus strains

A system based on reverse transcription polymerase chain reaction (RT-PCR) of the RNA genome was established to identify genetic composition of influenza viruses generated by reassortment between an attenuated donor virus and virulent wild type virus. The primers were designed, by multiple sequence alignment of variable regions, specific for cold-adapted donor virus HTCA-A 101, as compared to other influenza A viruses. The specificity of each primer set was confirmed and the primers were combined to perform RT-PCR in multiplex manner. The multiplex PCR was adopted to distinguish the 6:2 reassortant viruses containing six internal genome segments of attenuated donor virus and two surface antigens of virulent strain from the wild type viruses. The method allowed us to optimize the reassorting process on a routine basis and to confirm the selection of reassortant clones efficiently. The method is suitable for analyzing the contribution of specific gene segments for growth and attenuating characteristics and for generation of live attenuated vaccine by annual reassortment.

Simple and rapid strategy for genetic characterization of influenza B virus reassortants

Genetic reassortment of influenza viruses is widely used for creating viruses with specific phenotypes. Reassortment of two influenza viruses, each with eight RNA segments potentially yields as many as 256 gene segment combinations. Therefore, confirmation that progeny viruses possess genomes corresponding to the specified phenotypes can be laborious and time-consuming. To establish a convenient method for genotyping influenza virus reassortants, we adapted single-strand conformation polymorphism analysis (SSCP) using standard laboratory equipment. By varying the concentration of polyacrylamide between 4-6% and the concentration of glycerol between 5-8% in the gel, together with adding PCR primers to the DNA sample during the denaturing step, optimal conditions can be found for SSCP with little effort. The described method has high accuracy and reliability, and provides a tool for rapid, cost-effective genetic screening and assessment of the purity and genetic stability of the reassortant viruses. This method should be useful in basic research applications and in preparing reassortant viruses for vaccine use.