Intranasal inoculation of a recombinant influenza virus containing exogenous nucleotides in the NS segment induces mucosal immune response against the exogenous gene product in mice

Engineering influenza viral vectors

The influenza virus is a respiratory pathogen with a negative-sense, segmented RNA genome. Construction of recombinant influenza viruses in the laboratory was reported starting in the 1980s. Within a short period of time, pioneer researchers had devised methods that made it possible to construct influenza viral vectors from cDNA plasmid systems. Herein, we discuss the evolution of influenza virus reverse genetics, from helper virus-dependent systems, to helper virus-independent 17-plasmid systems, and all the way to 3- and 1- plasmid systems. Successes in the modification of different gene segments for various applications, including vaccine and gene therapies are highlighted.

Recombinant influenza virus vectors

Newly optimized reverse genetics techniques have allowed influenza researchers to generate recombinant influenza viruses expressing mutant viral proteins, as well as foreign proteins. Approaches include the insertion of noninfluenza epitopes and polypeptides into viral glycoproteins, foreign open reading frames as additional segments, and the fusion of independent proteins into viral genes encoding glycoproteins or the nonstructural protein 1. These genetically engineered viruses have been demonstrated to be good viral vectors for mounting B- and T-cell responses and are attractive candidates for vaccine development. As the molecular biology of influenza viral infection is more fully understood, influenza vectors can be concurrently manipulated to produce designed chimeric viruses, unveiling the possibility of a prosperous future with cheap, effective and safe vaccines against different human diseases.

Vaccine potential of influenza vectors expressing Mycobacterium tuberculosis ESAT-6 protein

We generated several attenuated recombinant influenza A vectors expressing the Mycobacterium tuberculosis early secretory antigenic target (ESAT-6) protein. The ESAT-6 protein was recently identified as one of the most promising protective antigens for cell-mediated immunity. The obtained vectors appeared to be capable of inducing ESAT-6 specific Th1 immune response in mice after intranasal immunization. We found that double immunization with two influenza vectors of different subtypes provided a significant level of protection in mice, when applied as prophylactic vaccine, as well as substantial therapeutic effect in mice with pre-established tuberculosis infection. Moreover, we found a strong synergistic effect when vaccination with Flu/ESAT-6 vectors was combined with isoniazid treatment, resulting in a dramatic reduction of bacterial load in the lungs of infected mice.

Influenza virus NS vectors expressing the mycobacterium tuberculosis ESAT-6 protein induce CD4+ Th1 immune response and protect animals against tuberculosis challenge

Infection with Mycobacterium tuberculosis remains a major cause of morbidity and mortality all over the world. Since the effectiveness of the only available tuberculosis vaccine, Mycobacterium bovis bacillus Calmette-Guérin (BCG), is suboptimal, there is a strong demand to develop new tuberculosis vaccines. As tuberculosis is an airborne disease, the intranasal route of vaccination might be preferable. Live influenza virus vaccines might be considered as potential vectors for mucosal immunization against various viral or bacterial pathogens, including M. tuberculosis. We generated several subtypes of attenuated recombinant influenza A viruses expressing the 6-kDa early secretory antigenic target protein (ESAT-6) of M. tuberculosis from the NS1 reading frame. We were able to demonstrate the potency of influenza virus NS vectors to induce an M. tuberculosis-specific Th1 immune response in mice. Moreover, intranasal immunization of mice and guinea pigs with such vectors induced protection against mycobacterial challenge, similar to that induced by BCG vaccination.

Attenuation and immunogenicity in mice of temperature-sensitive influenza viruses expressing truncated NS1 proteins

It was previously shown that two mutant influenza A viruses expressing C-terminally truncated forms of the NS1 protein (NS1-81 and NS1-110) were temperature sensitive in vitro. These viruses contain HA, NA and M genes derived from influenza A/WSN/33 H1N1 virus (mouse-adapted), and the remaining five genes from human influenza A/Victoria/3/75 virus. Mice intranasally infected with the NS1 mutant viruses showed undetectable levels of virus in lungs at day 3, whereas those infected with the NS1 wild-type control virus still had detectable levels of virus at this time. Nevertheless, the temperature-sensitive mutant viruses induced specific cellular and humoral immune responses similar to those induced by the wild-type virus. Mice immunized with the NS1 mutant viruses were protected against a lethal challenge with influenza A/WSN/33 virus. These results indicate that truncations in the NS1 protein resulting in temperature-sensitive phenotypes in vitro correlate with attenuation in vivo without compromising viral immunogenicity, an ideal characteristic for live attenuated viral vaccines.

Generation of an influenza A virus vector expressing biologically active human interleukin-2 from the NS gene segment

Engineering of the influenza A virus NS1 protein became an attractive approach to the development of influenza vaccine vectors since it can tolerate large inserts of foreign proteins. However, influenza virus vectors expressing long foreign sequences from the NS1 open reading frame (ORF) are usually replication deficient in animals due to the abrogation of their NS1 protein function. In this study, we describe a bicistronic expression strategy based on the insertion of an overlapping UAAUG stop-start codon cassette into the NS gene, allowing the reinitiation of translation of a foreign sequence. Although the expression level of green fluorescent protein (GFP) from the newly created reading frame was significantly lower than that obtained previously from an influenza virus vector expressing GFP from the NS1 ORF, the bicistronic vector appeared to be replication competent in mice and showed outstanding genetic stability. All viral isolates derived from mouse lungs at 10 days postinfection were still capable of expressing GFP in infected cells. Utilizing this bicistronic approach, we constructed another recombinant influenza virus, allowing the secretion of biologically active human interleukin-2 (IL-2). Although this virus also replicated to high titers in mouse lungs, it did not display any mortality rate in infected animals, in contrast to control viruses. Moreover, the IL-2-expressing virus showed an enhanced CD8+ response to viral antigens in mice after a single intranasal immunization. These results indicate that influenza viruses could be engineered for the expression of biologically active molecules such as cytokines for immune modulation purposes.

Rescue of influenza virus expressing GFP from the NS1 reading frame

In this study, several influenza NS1 mutants were examined for their growth ability in interferon (IFN)-deficient Vero cells treated with human interferon alpha (IFN-alpha). Mutants with an intact RNA binding domain showed similar growth properties as the wild-type virus, whereas viruses carrying an impaired RNA binding domain were dramatically attenuated. Relying on the ability of the first half of the NS1 protein to antagonize the IFN action, we established a rescue system for the NS gene based on the transfection of one plasmid expressing recombinant NS vRNA and subsequent coinfection with an IFN sensitive helper virus followed by adding of human IFN-alpha as a selection drug. Using this method, a recombinant influenza A virus expressing green fluorescence protein (GFP) from the NS1 reading frame was rescued. To ensure the posttranslational cleavage of GFP from the N-terminal 125 amino acids (aa) of NS1 protein, a peptide sequence comprising a caspase recognition site (CRS) was inserted upstream the GFP protein. Although a rather long sequence of 275 aa was inserted into the NS1 reading frame, the rescued recombinant vector appeared to be genetically stable while passaging in Vero cells and was able to replicate in PKR knockout mice.

Expression of a foreign gene by stable recombinant influenza viruses harboring a dicistronic genomic segment with an internal promoter

Based on the observation that an internally located 3' promoter sequence can be functional (R. Flick and G. Hobom, Virology, 1999, 262(1), 93-103), we generated transfectant influenza A viruses harboring a dicistronic segment containing the CAT gene (660 nt) or a fragment of the Mengo virus VP0 capsid gene (306 nt) under the control of a duplicated 3' promoter sequence. Despite slightly reduced NA expression, the transfectant viruses replicated efficiently and proved to be stable upon both serial passage in vitro in MDCK cells and in vivo replication in the pulmonary tissue of infected mice. Internal initiation of replication and transcription from the second, internal, 3' promoter directed the synthesis of subgenomic vRNA and mRNA and therefore permitted expression of the foreign gene product, e.g., the CAT enzyme. The design of this vector may prove particularly appropriate for the utilization of influenza virus for the expression of heterologous proteins in their native form.