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Review
. 2016 Dec 22;18(1):20.
doi: 10.3390/ijms18010020.

Reverse Genetics Approaches for the Development of Influenza Vaccines

Aitor Nogales  1 , Luis Martínez-Sobrido  2
Affiliations
Review

Reverse Genetics Approaches for the Development of Influenza Vaccines

Aitor Nogales et al. Int J Mol Sci. .

Abstract

Influenza viruses cause annual seasonal epidemics and occasional pandemics of human respiratory disease. Influenza virus infections represent a serious public health and economic problem, which are most effectively prevented through vaccination. However, influenza viruses undergo continual antigenic variation, which requires either the annual reformulation of seasonal influenza vaccines or the rapid generation of vaccines against potential pandemic virus strains. The segmented nature of influenza virus allows for the reassortment between two or more viruses within a co-infected cell, and this characteristic has also been harnessed in the laboratory to generate reassortant viruses for their use as either inactivated or live-attenuated influenza vaccines. With the implementation of plasmid-based reverse genetics techniques, it is now possible to engineer recombinant influenza viruses entirely from full-length complementary DNA copies of the viral genome by transfection of susceptible cells. These reverse genetics systems have provided investigators with novel and powerful approaches to answer important questions about the biology of influenza viruses, including the function of viral proteins, their interaction with cellular host factors and the mechanisms of influenza virus transmission and pathogenesis. In addition, reverse genetics techniques have allowed the generation of recombinant influenza viruses, providing a powerful technology to develop both inactivated and live-attenuated influenza vaccines. In this review, we will summarize the current knowledge of state-of-the-art, plasmid-based, influenza reverse genetics approaches and their implementation to provide rapid, convenient, safe and more effective influenza inactivated or live-attenuated vaccines.

Keywords: antivirals; influenza inactivated virus; influenza vaccines; influenza virus; live-attenuated influenza virus; recombinant influenza virus; reverse genetics; universal vaccines; vaccination.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Virion structure of IAV (A) and IBV (B): IAV and IBV are surrounded by a lipid bilayer containing the two viral glycoproteins hemagglutinin (HA), responsible for binding to sialic acid-containing receptors in the surface of susceptible cells, and neuraminidase (NA), responsible for viral release from infected cells. Furthermore, in the virion membrane is the ion channel M2 (IAV) or BM2 and NB (IBV) proteins. Under the viral lipid bilayer is a protein layer composed of the M1 protein, which plays a role in virion assembly and budding, and the nuclear export protein (NEP) involved in the nuclear export of the viral ribonucleoprotein (vRNP) complexes. The eight viral segments and protein products are indicated inside the virions. Black lines at the end of each of the eight IAV and IBV vRNAs indicate the 3′ and 5′ non-coding regions (NCR). PB1 and PB2, polymerase basic 1 and 2; PA, polymerase acid; NP, nucleoprotein; NS, nonstructural gene; M: matrix; BM2: influenza B matrix protein 2.
Figure 2
Figure 2
Schematic representation to produce inactivated (A) or live-attenuated (B) influenza vaccines by genetic reassortment in embryonated eggs: The traditional method for generating reassortant virus is based on the coinfection of two influenza viruses in eggs. Both the WHO candidate virus and the high-growth virus for influenza inactivated vaccine (IIV) (A) or the master donor virus (MDV) for live-attenuated influenza vaccine (LAIV) (B) are inoculated in eggs followed by the selection of appropriate seed viruses by amplification in the presence of antibodies against the HA and NA of the high-growth virus (A) or the MDV (B). The resulting viruses containing the HA and NA segments from the WHO-recommended strain and the six internal vRNAs of the high-growth virus (A) or the MDV (B) are used for vaccine production. PR8, Puerto Rico/8.
Figure 3
Figure 3
Influenza vRNA cloning into bi-directional rescue plasmids. (A) Schematic representation of an ambisense plasmid, influenza cDNA inserts and generation of influenza rescue plasmids: The ambisense plasmid is a bi-directional vector containing the human polymerase I promoter (hPol-I, white arrow) and the mouse Pol-I terminator (TI, white box) sequences to direct the synthesis of the influenza vRNAs. Transcription from the Pol-I cassette results in vRNAs identical to those present in influenza virus, allowing their recognition by the influenza polymerase complex. In opposite orientation to the Pol-I cassette, a polymerase II-dependent cytomegalovirus promoter (Pol-II, black arrow) and a polyadenylation sequence (aBGH, black box) direct the synthesis of influenza proteins from the same viral cDNAs; (B) Influenza plasmid-based reverse genetics: In cells transfected with the influenza ambisense plasmids, the Pol-I cassette generates the eight negative sense vRNAs (bottom) while the Pol-II directs the synthesis of the eight viral mRNAs (top) that are translated into the influenza viral proteins. After translation, influenza NP and polymerase complex PA, PB1 and PB2 associate with the vRNAs to form the viral ribonucleoprotein (vRNP) complexes and initiate transcription from the viral promoter located within the non-coding regions at the 3′ termini of the vRNAs. Transcription results in the synthesis of more mRNAs and proteins. The influenza polymerase complex also replicates the vRNAs into complementary (c)RNAs that serve as templates for the amplification of vRNAs. Newly-synthesized vRNAs, together with the structural viral proteins result in the formation of new influenza viruses. Blue and red boxes indicates the HA and NA of seasonal influenza viruses to be included in either the IIV or the LAIV vaccine, respectively. Ampr: ampicillin resistance gene; Ori: plasmid origin of replication.
Figure 4
Figure 4
Reverse genetics approaches to generate influenza vaccines: For the development of reverse genetics, influenza vRNAs are cloned into the eight bi-directional plasmids. Transfection of the eight ambisense plasmids into permissible FDA-approved for vaccine production cell lines leads to the rescue of the recombinant influenza viruses containing the six internal genome segments from the high-growth virus (A) or from the MDV (B) and two genome segments (the HA and NA encoding segments) from WHO candidate strain for their use as IIV (A) or LAIV (B), respectively. Because the viruses are derived entirely from DNA, no selection system is needed to isolate the desired reassortant. The rescued viruses can be amplified and used as seed viruses for vaccine production.
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