Establishment of Vero cell RNA polymerase I-driven reverse genetics for Influenza A virus and its application for pandemic (H1N1) 2009 influenza virus vaccine production

Research Note: Establishment of vector system harboring duck RNA polymerase I promoter for avian influenza virus

Reverse genetics (RG) systems are extensively utilized to investigate the characteristics of influenza viruses and develop vaccines, predominantly relying on human RNA polymerase I (pol I). However, the efficiency of RG systems for avian-origin influenza viruses may be compromised due to potential species-specific interactions of RNA pol I. In this study, we reported the polymerase activities of the duck RNA pol I promoter in avian cells and the generation of recombinant avian-derived influenza viruses using a novel vector system containing the duck RNA pol I promoter region to enhance the rescue efficiency of the RG system in avian cells. Initially, we explored a putative duck promoter region and identified the optimal size to improve the existing system. Subsequently, we established an RG system incorporating the duck RNA pol I promoter and compared its rescue efficiency with the human pol I system by generating recombinant influenza viruses in several cell lines. Notably, the 250-bp duck RNA pol I promoter demonstrated effective functionality in avian cells, exhibiting higher polymerase activity in a minigenome assay. The newly constructed RG system was significantly improved, enabling the rescue of influenza viruses in 293T, DF-1, and CCL141 cells. Furthermore, HPAI viruses were successfully rescued in DF-1 cells, a result that had not been achieved in previous experiments. In conclusion, our novel RG system harboring duck RNA pol I offers an additional tool for researching influenza viruses and may facilitate the development of vaccines for poultry.

Updating the National Antigen Bank in Korea: Protective Efficacy of Synthetic Vaccine Candidates against H5Nx Highly Pathogenic Avian Influenza Viruses Belonging to Clades 2.3.2.1 and 2.3.4.4

Since 2018, Korea has been building an avian influenza (AI) national antigen bank for emergency preparedness; this antigen bank is updated every 2 years. To update the vaccine strains in the antigen bank, we used reverse genetics technology to develop two vaccine candidates against avian influenza strains belonging to clades 2.3.2.1d and 2.3.4.4h, and then evaluated their immunogenicity and protective efficacy in SPF chickens challenged with H5 viruses. The two vaccine candidates, named rgCA2/2.3.2.1d and rgES3/2.3.4.4h, were highly immunogenic, with hemagglutination inhibition (HI) titers of 8.2−9.3 log2 against the vaccine strain, and 7.1−7.3 log2 against the lethal challenge viruses (in which the HA genes shared 97% and 95.4% homology with that of rgCA2/2.3.2.1d and rgES3/2.3.4.4h, respectively). A full dose of each vaccine candidate provided 100% protection against the challenge viruses, with a reduction in clinical symptoms and virus shedding. A 1/10 dose provided similar levels of protection, whereas a 1/100 dose resulted in mortality and virus shedding by 7 dpi. Moreover, immunity induced by the two vaccines was long lasting, with HI titers of >7 log2 against the vaccine strain remaining after 6 months. Thus, the two vaccine candidates show protective efficacy and can be used to update the AI national antigen bank.

Investigating Influenza Virus Polymerase Activity in Feline Cells Based on the Influenza Virus Minigenome Replication System Driven by the Feline RNA Polymerase I Promoter

Emerging influenza virus poses a health threat to humans and animals. Domestic cats have recently been identified as a potential source of zoonotic influenza virus. The influenza virus minigenome replication system based on the ribonucleic acid (RNA) polymerase I (PolI) promoter is the most widely used tool for investigating polymerase activity. It could help determine host factors or viral proteins influencing influenza virus polymerase activity in vitro. However, influenza virus polymerase activity has never been studied in feline cells thus far. In the present study, the feline RNA PolI promoter was identified in the intergenic spacer regions between adjacent upstream 28S and downstream 18S rRNA genes in the cat (Felis catus) genome using bioinformatics strategies. The transcription initiation site of the feline RNA PolI promoter was predicted. The feline RNA PolI promoter was cloned from CRFK cells, and a promoter size of 250 bp contained a sequence with sufficient PolI promoter activity by a dual-luciferase reporter assay. The influenza virus minigenome replication system based on the feline RNA PolI promoter was then established. Using this system, the feline RNA PolI promoter was determined to have significantly higher transcriptional activity than the human and chicken RNA PolI promoters in feline cells, and equine (H3N8) influenza virus presented higher polymerase activity than human (H1N1) and canine (H3N2) influenza viruses. In addition, feline myxovirus resistance protein 1 (Mx1) and baloxavir were observed to inhibit influenza virus polymerase activity in vitro in a dose-dependent manner. Our study will help further investigations on the molecular mechanism of host adaptation and cross-species transmission of influenza virus in cats.

Cross-protective efficacy of inactivated whole influenza vaccines against Korean Y280 and Y439 lineage H9N2 viruses in mice

Since June 2020, the Y280 lineage H9N2 virus, which is distinct from the previously endemic Y439 lineage, has been circulating in poultry in Korea. In this study, we developed two whole inactivated vaccines, rgHS314 and vac564, against the Y280 and Y439 lineages, respectively, and evaluated their immunogenicity and protective efficacy against homologous or heterologous viral challenge in mice. Serum neutralizing antibody titers in the rgHS314-vaccinated group were higher (68 ± 8.4 10log₂) than in the vac564-vaccinated group (18 ± 8.4 10log₂). In homologous challenge, rgHS314 conferred 100% protection, with no severe clinical signs, no body weight loss, and no viral replication in any tissues tested except the nasal turbinate. Viral replication in the lungs at 1, 3, 5, and 7 days post-infection (dpi) was significantly lower than in the sham group (p < 0.01). By contrast, all mice in the sham group were dead by 8 dpi with severe clinical signs and weight loss. Likewise, vac564 conferred 100% protection with no weight loss and with significantly lower viral replication in the lung than in the sham group at 3 dpi (p < 0.01). However, both vaccines showed partial protection in heterologous challenge. Our results suggest that both the rgHS314 and vac564 vaccines could be candidate vaccines for further evaluation in humans.

Influenza Reverse Genetics-Historical Perspective

The generation of wild-type, mutant, and reassortant influenza viruses from viral cDNAs (reverse genetics) is now a basic molecular virology technique in many influenza virus laboratories. Here, I describe the original RNA polymerase I reverse genetics system and the modifications that have been developed in past years. Together, these technologies have made possible many advances in basic and applied influenza virology that would not have been otherwise attainable, including the revival and study of extinct influenza viruses, the rapid characterization of emerging influenza viruses, the generation of conventional influenza vaccines, and the development of novel influenza vaccines.

Development of a Simian RNA Polymerase I Promoter-Driven Reverse Genetics for the Rescue of Recombinant Rift Valley Fever Virus from Vero Cells

Rift Valley fever (RVF), which has been designated as a priority disease by the World Health Organization (WHO), is one of the most pathogenic zoonotic diseases endemic to Africa and the Arabian Peninsula. Human vaccine preparation requires the use of appropriate cell substrates to support efficient production of seed vaccine with minimum concerns of tumorigenicity, oncogenicity, or adventitious agents. Vero cells, which were derived from the African green monkey kidney, represent one of the few mammalian cell lines that are used for vaccine manufacturing. This study demonstrated the rescue of RVFV MP-12 infectious clones in Vero cells using plasmids encoding the Macaca mulatta RNA polymerase I promoter. Although Vero cells demonstrated an approximately 20% transfection efficiency, only 0.5% of transfected cells showed the replication of viral genomic RNA, supported by the co-expression of RVFV N and L helper proteins. RVFV Infectious clones were detectable in the culture supernatants approximately 4 to 9 days posttransfection reaching maximum titers during the following 5 days. The re-amplification of rescued recombinant MP-12 (rMP-12) in Vero cells led to an increase in the genetic subpopulations, affecting the viral phenotype via amino acid substitutions in the NSs gene, whereas the rMP-12 re-amplified in human diploid MRC-5 cells did not increase viral sub-populations with NSs gene mutations. The strategy in which RVFV infectious clones are rescued in Vero cells and then subsequently amplified in MRC-5 cells will support the vaccine seed lot systems of live-attenuated recombinant RVFV vaccines for human use.IMPORTANCE RVF is a mosquito-transmitted, viral, zoonotic disease endemic to Africa and the Arabian Peninsula, and its spread outside of the endemic area will potentially cause devastating economic damages and serious public health problems. Different from classical live-attenuated vaccines, live-attenuated recombinant vaccines allow rational improvement of vaccine production efficiency, protective efficacy, and vaccine safety via the genetic engineering. This study demonstrated the generation of infectious Rift Valley fever (RVF) virus from cloned cDNA using Vero cells, which are one of a few mammalian cell lines used for vaccine manufacturing. Subsequent re-amplification of virus clones in Vero cells unexpectedly increased viral subpopulations encoding unfavorable mutations, whereas viral re-amplification in human diploid MRC-5 cells could minimize the emergence of such mutants. Rescue of recombinant RVFV from Vero cells and re-amplification in MRC-5 cells will support the vaccine seed lot systems of live-attenuated recombinant RVFV vaccines for human use.

Development and application of reverse genetic technology for the influenza virus

Influenza virus is a common virus in people's daily lives, and it has certain infectivity in humans and animals. Influenza viruses have the characteristics of a high mutation rate and wide distribution. Reverse genetic technology is primarily used to modify viruses at the DNA level through targeted modification of the virus cDNA. Genetically modified influenza viruses have a unique advantage when researching the transmission and pathogenicity of influenza. With the continuous development of oncolytic viruses in recent years, studies have found that influenza viruses also have certain oncolytic activity. Influenza viruses can specifically recognize tumor cells; activate cytotoxic T cells, NK cells, dendritic cells, etc.; and stimulate the body to produce an immune response, thereby killing tumor cells. This article will review the development and application of influenza virus reverse genetic technology.

Protection of layers and breeders against homologous or heterologous HPAIv by vaccines from Korean national antigen bank

Korean government has selected and stocked five type antigens of two clades as Korean national antigen bank having high possibility of introduction to Korea. We aimed to evaluate the efficacy of the clade 2.3.2.1c and 2.3.4.4c H5Nx vaccines from the Korean avian influenza (AI) national antigen bank for emergency preparedness for their potency and protective efficacy against lethal homologous and heterologous viruses in layer and breeder chickens practically. The PD₅₀ (dose of vaccine that protects 50% of chickens from viral challenge) of all vaccinated groups was >50, which was satisfied with minimum antigen requirement of OIE, and the PD₅₀ levels of the two vaccines differed depending on strain and chicken breed. In homologous challenge, all vaccinated groups exhibited 100% survival with no clinical symptoms and high levels of pre-challenge protective immunity (7.2–8.5 log₂), although they did not completely prevent virus shedding. On the other hand, against heterologous virus challenge, vaccinated animals exhibited 62.5–80% survival with lower antibody titers (2.3–3.4 log₂) and a longer period of virus shedding (14 days post infection [dpi]). Our results suggest that the clade 2.3.2.1c and 2.3.4.4c H5Nx vaccines are good candidates for emergency vaccination of commercial chickens and support the idea that close genetic matching between vaccine and challenge virus provides the best protection.

Protective efficacy of vaccines of the Korea national antigen bank against the homologous H5Nx clade 2.3.2.1 and clade 2.3.4.4 highly pathogenic avian influenza viruses

The occurrence of severe outbreaks of highly pathogenic avian influenza in Korea led to establishment of a national antigen bank for emergency preparedness. Here, we developed five vaccines for this bank (clade 2.3.2.1C, clade 2.3.4.4A, B, C, and D) by reverse genetics, inactivated them with formalin, and evaluated the protective efficacy and potency of serial dilutions against lethal homologous challenge in specific-pathogen-free chickens. After vaccination with one dose, each vaccine resulted in 100% survival, with no clinical symptoms, or lack of detectable virus shedding, and high levels of pre-challenge protective immunity (8.4-10.2 log₂). After vaccination with one-tenth of the full dose, protection was similar to that with the full dose. After vaccination with one-hundredth of the initial dose, survival was 20-80%, and all vaccines showed virus shedding. Four vaccines (excluding clade 2.3.2.1C) had satisfactory potency. In antibody-persistence tests, all vaccines maintained long-lasting protective immunity. Our results suggest that inactivated reverse-genetics vaccines genetically matched to outbreak viruses provide adequate protection after a single vaccination.

Alternative reverse genetics system for influenza viruses based on a synthesized swine 45S rRNA promoter

We generated an alternative reverse genetics (RG) system based on a synthesized swine 45S rRNA promoter to rescue the H3N2 subtype swine influenza virus. All eight flanking segment cassettes of A/swine/Henan/7/2010 (H3N2) were amplified with ambisense expression elements from RG plasmids. All segments were then recombined with the pHC2014 vector, which contained the synthesized swine 45S rRNA promoter (spol1) and its terminal sequence (t1) in a pcDNA3 backbone. As a result, we obtained a set of RG plasmids carrying the corresponding eight-segment cassettes. We efficiently generated the H3N2 virus after transfection into 293T/PK15, PK15, and 293T cells. The efficiency of spol1-driven influenza virus rescue in PK15 cells was similar to that in 293T cells by titration using the human pol1 RG system. Our approach suggests that an alternative spol1-based RG system can produce influenza viruses.

Evaluation of the Immune Responses to and Cross-Protective Efficacy of Eurasian H7 Avian Influenza Viruses

Due to increasing concerns about human infection by various H7 influenza viruses, including recent H7N9 viruses, we evaluated the genetic relationships and cross-protective efficacies of three different Eurasian H7 avian influenza viruses. Phylogenic and molecular analyses revealed that recent Eurasian H7 viruses can be separated into two different lineages, with relatively high amino acid identities within groups (94.8 to 98.8%) and low amino acid identities between groups (90.3 to 92.6%). In vivo immunization with representatives of each group revealed that while group-specific cross-reactivity was induced, cross-reactive hemagglutination inhibition (HI) titers were approximately 4-fold lower against heterologous group viruses than against homologous group viruses. Moreover, the group I (RgW109/06) vaccine protected 100% of immunized mice from various group I viruses, while only 20 to 40% of immunized mice survived lethal challenge with heterologous group II viruses and exhibited high viral titers in the lung. Moreover, while the group II (RgW478/14) vaccine also protected mice from lethal challenge with group II viruses, it failed to elicit cross-protection against group I viruses. However, it is noteworthy that vaccination with RgAnhui1/13, a virus of a sublineage of group I, cross-protected immunized mice against lethal challenge with both group I and II viruses and significantly attenuated lung viral titers. Interestingly, immune sera from RgAnhui1/13-vaccinated mice showed a broad neutralizing spectrum rather than the group-specific pattern observed with the other viruses. These results suggest that the recent human-infective H7N9 strain may be a candidate broad cross-protective vaccine for Eurasian H7 viruses.IMPORTANCE Genetic and phylogenic analyses have demonstrated that the Eurasian H7 viruses can be separated into at least two different lineages, both of which contain human-infective fatal H7 viruses, including the recent novel H7N9 viruses isolated in China since 2013. Due to the increasing concerns regarding the global public health risk posed by H7 viruses, we evaluated the genetic relationships between Eurasian H7 avian influenza viruses and the cross-protective efficacies of three different H7 viruses: W109/06 (group I), W478/14 (group II), and Anhui1/13 (a sublineage of group I). While each vaccine induced group-specific antibody responses and cross-protective efficacy, only Anhui1/13 was able to cross-protect immunized hosts against lethal challenge across groups. In fact, the Anhui1/13 virus induced not only cross-protection but also broad serum neutralizing antibody responses against both groups of viruses. This suggests that Anhui1/13-like H7N9 viruses may be viable vaccine candidates for broad protection against Eurasian H7 viruses.

Vaccine Efficacy of Inactivated, Chimeric Hemagglutinin H9/H5N2 Avian Influenza Virus and Its Suitability for the Marker Vaccine Strategy

In order to produce a dually effective vaccine against H9 and H5 avian influenza viruses that aligns with the DIVA (differentiating infected from vaccinated animals) strategy, we generated a chimeric H9/H5N2 recombinant vaccine that expressed the whole HA1 region of A/CK/Korea/04163/04 (H9N2) and the HA2 region of recent highly pathogenic avian influenza (HPAI) A/MD/Korea/W452/14 (H5N8) viruses. The chimeric H9/H5N2 virus showed in vitro and in vivo growth properties and virulence that were similar to those of the low-pathogenic avian influenza (LPAI) H9 virus. An inactivated vaccine based on this chimeric virus induced serum neutralizing (SN) antibodies against both H9 and H5 viruses but induced cross-reactive hemagglutination inhibition (HI) antibody only against H9 viruses. Thus, this suggests its compatibility for use in the DIVA strategy against H5 strains. Furthermore, the chimeric H9/H5N2 recombinant vaccine protected immunized chickens against lethal challenge by HPAI H5N8 viruses and significantly attenuated virus shedding after infection by both H9N2 and HPAI H5N8 viruses. In mice, serological analyses confirmed that HA1- and HA2 stalk-specific antibody responses were induced by vaccination and that the DIVA principle could be employed through the use of an HI assay against H5 viruses. Furthermore, each HA1- and HA2 stalk-specific antibody response was sufficient to inhibit viral replication and protect the chimeric virus-immunized mice from lethal challenge with both mouse-adapted H9N2 and wild-type HPAI H5N1 viruses, although differences in vaccine efficacy against a homologous H9 virus (HA1 head domain immune-mediated protection) and a heterosubtypic H5 virus (HA2 stalk domain immune-mediated protection) were observed. Taken together, these results demonstrate that the novel chimeric H9/H5N2 recombinant virus is a low-pathogenic virus, and this chimeric vaccine is suitable for a DIVA vaccine with broad-spectrum neutralizing antibody against H5 avian influenza viruses.IMPORTANCE Current influenza virus killed vaccines predominantly induce antihemagglutinin (anti-HA) antibodies that are commonly strain specific in that the antibodies have potent neutralizing activity against homologous strains but do not cross-react with HAs of other influenza virus subtypes. In contrast, the HA2 stalk domain is relatively well conserved among subtypes, and recently, broadly neutralizing antibodies against this domain have been isolated. Therefore, in light of the need for a vaccine strain that applies the DIVA strategy utilizing an HI assay and induces broad cross-protection against H5N1 and H9N2 viruses, we generated a novel chimeric H9/H5N1 virus that expresses the entire HA1 portion from the H9N2 virus and the HA2 region of the heterosubtypic H5N8 virus. The chimeric H9/H5N2 recombinant vaccine protected immunized hosts against lethal challenge with H9N2 and HPAI H5N1 viruses with significantly attenuated virus shedding in immunized hosts. Therefore, this chimeric vaccine is suitable as a DIVA vaccine against H5 avian influenza viruses.

Cross-protective efficacies of highly-pathogenic avian influenza H5N1 vaccines against a recent H5N8 virus

To investigate cross-protective vaccine efficacy of highly-pathogenic avian influenza H5N1 viruses against a recent HPAI H5N8 virus, we immunized C57BL/6 mice and ferrets with three alum-adjuvanted inactivated whole H5N1 vaccines developed through reverse-genetics (Rg): [Vietnam/1194/04xPR8 (clade 1), Korea/W149/06xPR8 (clade 2.2), and Korea/ES223N/03xPR8 (clade 2.5)]. Although relatively low cross-reactivities (10-40 HI titer) were observed against heterologous H5N8 virus, immunized animals were 100% protected from challenge with the 20 mLD50 of H5N8 virus, with the exception of mice vaccinated with 3.5μg of Rg Vietnam/1194/04xPR8. Of note, the Rg Korea/ES223N/03xPR8 vaccine provided not only effective protection, but also markedly inhibited viral replication in the lungs and nasal swabs of vaccine recipients within five days of HPAI H5N8 virus challenge. Further, we demonstrated that antibody-dependent cell-mediated cytotoxicity (ADCC) of an antibody-coated target cell by cytotoxic effector cells also plays a role in the heterologous protection of H5N1 vaccines against H5N8 challenge.

Efficient generation of influenza virus with a mouse RNA polymerase I-driven all-in-one plasmid

BACKGROUND

The current influenza vaccines are effective against seasonal influenza, but cannot be manufactured in a timely manner for a sudden pandemic or to be cost-effective to immunize huge flocks of birds. We propose a novel influenza vaccine composing a bacterial carrier and a plasmid cargo. In the immunized subjects, the bacterial carrier invades and releases its cargo into host cells where the plasmid expresses viral RNAs and proteins for reconstitution of attenuated influenza virus. Here we aimed to construct a mouse PolI-driven plasmid for efficient production of influenza virus.

RESULTS

A plasmid was constructed to express all influenza viral RNAs and proteins. This all-in-one plasmid resulted in 10(5)-10(6) 50% tissue culture infective dose (TCID50)/mL of influenza A virus in baby hamster kidney (BHK-21) cells on the third day post-transfection, and also reconstituted influenza virus in Madin-Darby canine kidney (MDCK) and Chinese hamster ovary (CHO) cells. A 6-unit plasmid was constructed by deleting the HA and NA cassettes from the all-in-one plasmid. Cotransfection of BHK-21 cells with the 6-unit plasmid and the two other plasmids encoding the HA or NA genes resulted in influenza virus titers similar to those produced by the 1-plasmid method.

CONCLUSIONS

An all-in-one plasmid and a 3-plasmid murine PolI-driven reverse genetics systems were developed, and efficiently reconstituted influenza virus in BHK-21 cells. The all-in-one plasmid may serve as a tool to determine the factors inhibiting virus generation from a large size plasmid. In addition, we recommend a simple and robust "1 + 2" approach to generate influenza vaccine seed virus.

Design of alternative live attenuated influenza virus vaccines

Each year due to the ever-evolving nature of influenza, new influenza vaccines must be produced to provide protection against the influenza viruses in circulation. Currently, there are two mainstream strategies to generate seasonal influenza vaccines: inactivated and live-attenuated. Inactivated vaccines are non-replicating forms of whole influenza virus, while live-attenuated vaccines are viruses modified to be replication impaired. Although it is widely believed that by inducing both mucosal and humoral immune responses the live-attenuated vaccine provides better protection than that of the inactivated vaccine, there are large populations of individuals who cannot safely receive the LAIV vaccine. Thus, safer LAIV vaccines are needed to provide adequate protection to these populations. Improvement is also needed in the area of vaccine production. Current strategies relying on traditional tissue culture-based and egg-based methods are slow and delay production time. This chapter describes experimental vaccine generation and production strategies that address the deficiencies in current methods for potential human and agricultural use.

Reverse genetics: Unlocking the secrets of negative sense RNA viral pathogens

Negative-sense RNA viruses comprise several zoonotic pathogens that mutate rapidly and frequently emerge in people including Influenza, Ebola, Rabies, Hendra and Nipah viruses. Acute respiratory distress syndrome, encephalitis and vasculitis are common disease outcomes in people as a result of pathogenic viral infection, and are also associated with high case fatality rates. Viral spread from exposure sites to systemic tissues and organs is mediated by virulence factors, including viral attachment glycoproteins and accessory proteins, and their contribution to infection and disease have been delineated by reverse genetics; a molecular approach that enables researchers to experimentally produce recombinant and reassortant viruses from cloned cDNA. Through reverse genetics we have developed a deeper understanding of virulence factors key to disease causation thereby enabling development of targeted antiviral therapies and well-defined live attenuated vaccines. Despite the value of reverse genetics for virulence factor discovery, classical reverse genetic approaches may not provide sufficient resolution for characterization of heterogeneous viral populations, because current techniques recover clonal virus, representing a consensus sequence. In this review the contribution of reverse genetics to virulence factor characterization is outlined, while the limitation of the technique is discussed with reference to new technologies that may be utilized to improve reverse genetic approaches.

Application of minigenome technology in virology research of the Paramyxoviridae family

Minigenomes (MGs) are complementary DNAs of the synthetic analogs of genomic RNA. MGs are widely used to study the life cycle of the Paramyxoviridae family of viruses. MG-based studies have provided valuable insights into the mechanisms of viral replication and transcription in this family, including the roles of viral proteins, the location and boundaries of the cis-acting elements, the functional domains of trans-acting proteins, techniques for the measurement of neutralizing antibody, virus-host interactions, and the structure and function of viral RNA. This article provides a brief overview of the principle and application of MG technology in studies involving members of the Paramyxoviridae family. The advantages, potential limitations, and future scope of MG technology are also discussed.

Influenza reverse genetics: dissecting immunity and pathogenesis

Reverse genetics systems allow artificial generation of non-segmented and segmented negative-sense RNA viruses, like influenza viruses, entirely from cloned cDNA. Since the introduction of reverse genetics systems over a decade ago, the ability to generate ‘designer’ influenza viruses in the laboratory has advanced both basic and applied research, providing a powerful tool to investigate and characterise host–pathogen interactions and advance the development of novel therapeutic strategies. The list of applications for reverse genetics has expanded vastly in recent years. In this review, we discuss the development and implications of this technique, including the recent controversy surrounding the generation of a transmissible H5N1 influenza virus. We will focus on research involving the identification of viral protein function, development of live-attenuated influenza virus vaccines, host–pathogen interactions, immunity and the generation of recombinant influenza virus vaccine vectors for the prevention and treatment of infectious diseases and cancer.

Reverse genetics of influenza virus

Reverse genetics is the creation of a virus from a full-length cDNA copy of the viral genome, referred to as an "infectious clone," and is one of the most powerful genetic tools in modern virology. Since its development in 1999, plasmid-based reverse genetics has been effectively applied to numerous aspects of influenza studies which include revolutionizing the production of seasonal and pandemic influenza vaccine seed strains. Although continual improvement in reverse genetics system is being made in different laboratories for the efficient rescue of the influenza virus, the basic concept of synthesizing viral RNA using RNA polymerase I remains the same. Coupled with in vitro mutagenesis, reverse genetics can be applied widely to accelerate progress in understanding the influenza virus life cycle, the generation of customized vaccine seed strains, development of live-attenuated vaccines, and the use of influenza virus as vaccine and gene delivery vectors.