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

Recombinant influenza A viruses as vaccine vectors

INTRODUCTION

Various viruses, including poxviruses, adenoviruses and vesicular stomatitis virus, have been considered as vaccine vectors for the delivery of antigens of interest in the development of vaccines against newly emerging pathogens.

AREAS COVERED

Here, we review results that have been obtained with influenza A viruses (IAV) as vaccine vectors. With the advent of reverse genetics technology, IAV-based recombinant vaccine candidates have been constructed that induce protective immunity to a variety of different pathogens of interest, including West Nile virus, Plasmodium falciparum and respiratory syncytial virus. The various cloning strategies to produce effective and attenuated, safe to use IAV-based viral vectors are discussed.

EXPERT COMMENTARY

It was concluded that IAV-based vector system has several advantages and holds promise for further development.

Recombinant canine adenovirus type-2 expressing TgROP16 provides partial protection against acute Toxoplasma gondii infection in mice

We previously demonstrated that the survival time of BALB/c mice challenged with Toxoplasma gondii RH strain was prolonged by immunising the mice with a eukaryotic vector expressing the protein ROP16 of T. gondii. Building upon previous findings, we are exploring improved vaccination strategies to enhance protection. In this work, a novel recombinant canine adenovirus type 2 expressing ROP16 (CAV-2-ROP16) of T. gondii was constructed and identified to express ROP16 in Madin-Darby canine kidney cells (MDCK) cells by western blot (WB) and indirect immunofluorescence (IFA) assays. Intramuscular immunisation of BALB/c mice with CAV-2-ROP16 was performed to evaluate the humoral and cellular immune responses. This vaccination triggered significant humoral and cellular responses, including ROP16-stimulated lymphoproliferation (P<0.05). Compared to control groups, the CAV-2-ROP16 immunised mice had high production of IFN-γ, IL-2 and IL-12 (P<0.05), with a predominance of IgG2a production, but not IL-10 (P>0.05), revealing that a predominant Th1-type response had developed. The cell-mediated cytotoxic activity with high levels of IFN-γ and TNF-α was significantly increased in both CD4⁺ and CD8⁺ T-cell compartments in the mice immunised with CAV-2-ROP16 (P<0.05), compared to three control groups. In addition, when immunised mice were challenged with the RH strain of T. gondii, they showed a significantly increased survival rate (25%) 80days post infection compared with control mice that all died within seven days (P<0.05). The 25% protection rate elicited by the recombinant virus CAV-2-ROP16 has not been achieved in the field of anti-T. gondii vaccination until now. Our work presents the successful use of recombinant virus CAV-2-ROP16 in vaccination protocols to protect against intraperitoneal challenge with the virulent RH strain of T. gondii. This system was shown to be extremely efficient in eliciting humoral and cellular immune responses that led to a significant improvement in survival time in mice.

Replication-Competent Influenza A Viruses Expressing Reporter Genes

Influenza A viruses (IAV) cause annual seasonal human respiratory disease epidemics. In addition, IAV have been implicated in occasional pandemics with inordinate health and economic consequences. Studying IAV, in vitro or in vivo, requires the use of laborious secondary methodologies to identify virus-infected cells. To circumvent this requirement, replication-competent IAV expressing an easily traceable reporter protein can be used. Here we discuss the development and applications of recombinant replication-competent IAV harboring diverse fluorescent or bioluminescent reporter genes in different locations of the viral genome. These viruses have been employed for in vitro and in vivo studies, such as the screening of neutralizing antibodies or antiviral compounds, the identification of host factors involved in viral replication, cell tropism, the development of vaccines, or the assessment of viral infection dynamics. In summary, reporter-expressing, replicating-competent IAV represent a powerful tool for the study of IAV both in vitro and in vivo.

A genetically adjuvanted influenza B virus vector increases immunogenicity and protective efficacy in mice

The existence of multiple antigenically distinct types and subtypes of influenza viruses allows the construction of a multivalent vector system for the mucosal delivery of foreign sequences. Influenza A viruses have been exploited successfully for the expression of extraneous antigens as well as immunostimulatory molecules. In this study, we describe the development of an influenza B virus vector whose functional part of the interferon antagonist NS1 was replaced by human interleukin 2 (IL2) as a genetic adjuvant. We demonstrate that IL2 expressed by this viral vector displays immune adjuvant activity in immunized mice. Animals vaccinated with the IL2 viral vector showed an increased hemagglutination inhibition antibody response and higher protective efficacy after challenge with a wild-type influenza B virus when compared to mice vaccinated with a control virus. Our results demonstrate that it is feasible to construct influenza B vaccine strains expressing immune-potentiating foreign sequences from the NS genomic segment. Based on these data, it is now hypothetically possible to create a trivalent (or quadrivalent) live attenuated influenza vaccine in which each component expresses a selected genetic adjuvant with tailored expression levels.

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.

Protective immunity and safety of a genetically modified influenza virus vaccine

Recombinant influenza viruses are promising viral platforms to be used as antigen delivery vectors. To this aim, one of the most promising approaches consists of generating recombinant viruses harboring partially truncated neuraminidase (NA) segments. To date, all studies have pointed to safety and usefulness of this viral platform. However, some aspects of the inflammatory and immune responses triggered by those recombinant viruses and their safety to immunocompromised hosts remained to be elucidated. In the present study, we generated a recombinant influenza virus harboring a truncated NA segment (vNA-Δ) and evaluated the innate and inflammatory responses and the safety of this recombinant virus in wild type or knock-out (KO) mice with impaired innate (Myd88 -/-) or acquired (RAG -/-) immune responses. Infection using truncated neuraminidase influenza virus was harmless regarding lung and systemic inflammatory response in wild type mice and was highly attenuated in KO mice. We also demonstrated that vNA-Δ infection does not induce unbalanced cytokine production that strongly contributes to lung damage in infected mice. In addition, the recombinant influenza virus was able to trigger both local and systemic virus-specific humoral and CD8+ T cellular immune responses which protected immunized mice against the challenge with a lethal dose of homologous A/PR8/34 influenza virus. Taken together, our findings suggest and reinforce the safety of using NA deleted influenza viruses as antigen delivery vectors against human or veterinary pathogens.

Influenza A virus encoding secreted Gaussia luciferase as useful tool to analyze viral replication and its inhibition by antiviral compounds and cellular proteins

Reporter genes inserted into viral genomes enable the easy and rapid quantification of virus replication, which is instrumental to efficient in vitro screening of antiviral compounds or in vivo analysis of viral spread and pathogenesis. Based on a published design, we have generated several replication competent influenza A viruses carrying either fluorescent proteins or Gaussia luciferase. Reporter activity could be readily quantified in infected cultures, but the virus encoding Gaussia luciferase was more stable than viruses bearing fluorescent proteins and was therefore analyzed in detail. Quantification of Gaussia luciferase activity in the supernatants of infected culture allowed the convenient and highly sensitive detection of viral spread, and enzymatic activity correlated with the number of infectious particles released from infected cells. Furthermore, the Gaussia luciferase encoding virus allowed the sensitive quantification of the antiviral activity of the neuraminidase inhibitor (NAI) zanamivir and the host cell interferon-inducible transmembrane (IFITM) proteins 1–3, which are known to inhibit influenza virus entry. Finally, the virus was used to demonstrate that influenza A virus infection is sensitive to a modulator of endosomal cholesterol, in keeping with the concept that IFITMs inhibit viral entry by altering cholesterol levels in the endosomal membrane. In sum, we report the characterization of a novel influenza A reporter virus, which allows fast and sensitive detection of viral spread and its inhibition, and we show that influenza A virus entry is sensitive to alterations of endosomal cholesterol levels.

Vaccination using recombinants influenza and adenoviruses encoding amastigote surface protein-2 are highly effective on protection against Trypanosoma cruzi infection

In the present study we evaluated the protection raised by immunization with recombinant influenza viruses carrying sequences coding for polypeptides corresponding to medial and carboxi-terminal moieties of Trypanosoma cruzi ´s amastigote surface protein 2 (ASP2). Those viruses were used in sequential immunization with recombinant adenovirus (heterologous prime-boost immunization protocol) encoding the complete sequence of ASP2 (Ad-ASP2) in two mouse strains (C57BL/6 and C3H/He). The CD8 effector response elicited by this protocol was comparable to that observed in mice immunized twice with Ad-ASP2 and more robust than that observed in mice that were immunized once with Ad-ASP2. Whereas a single immunization with Ad-ASP2 sufficed to completely protect C57BL/6 mice, a higher survival rate was observed in C3H/He mice that were primed with recombinant influenza virus and boosted with Ad-ASP2 after being challenged with T. cruzi. Analyzing the phenotype of CD8+ T cells obtained from spleen of vaccinated C3H/He mice we observed that heterologous prime-boost immunization protocol elicited more CD8+ T cells specific for the immunodominant epitope as well as a higher number of CD8+ T cells producing TNF-α and IFN-γ and a higher mobilization of surface marker CD107a. Taken together, our results suggest that immunodominant subpopulations of CD8+ T elicited after immunization could be directly related to degree of protection achieved by different immunization protocols using different viral vectors. Overall, these results demonstrated the usefulness of recombinant influenza viruses in immunization protocols against Chagas Disease.

Comparison of vRNA and cRNA based reporters for detection of influenza replication

In this study, RNA polymerase I expressed replicons containing EGFP and luciferase reporter genes controlled by influenza vRNA or cRNA promoters were compared side-by-side in the ability to detect influenza RNA-dependent RNA polymerase activity as an indicator of influenza replication. Results showed the vRNA based Luc reporter was more sensitive to early detection of influenza virus at 6h post infection (p<0.05), and at 10-fold lower titer (MOI=0.001). Lower sensitivity of cRNA based Luc reporter constructs was due to its background expression, 2-fold lower expression, and around 4h delay in expression of luciferase. Despite these differences, both cRNA- and vRNA-based reporters demonstrated strong correlation between MOI and luciferase signal, and can be used for effective and early detection of influenza infection in vitro. Further, we demonstrated that these reporters can be used successfully to study the kinetics of antiviral drugs including siRNA. Our results also suggest that progeny vRNAs might participate not only in secondary transcription but also in secondary replication. The developed cRNA and vRNA reporters may help with further elucidation of the replication model of influenza A virus.

Immunogenicity of a recombinant influenza virus bearing both the CD4+ and CD8+ T cell epitopes of ovalbumin

Recombinant influenza viruses that bear the single immunodominant CD8+ T cell epitope OVA_257−264 or the CD4+ T cell epitope OVA_323−339 of the model antigen ovalbumin (OVA) have been useful tools in immunology. Here, we generated a recombinant influenza virus, WSN-OVA_I/II, that bears both OVA-specific CD8+ and CD4+ epitopes on its hemagglutinin molecule. Live and heat-inactivated WSN-OVA_I/II viruses were efficiently presented by dendritic cells in vitro to OT-I TCR transgenic CD8+ T cells and OT-II TCR transgenic CD4+ T cells. In vivo, WSN-OVA_I/II virus was attenuated in virulence, highly immunogenic, and protected mice from B16-OVA tumor challenge in a prophylactic model of vaccination. Thus, WSN-OVA_I/II virus represents an additional tool, along with OVA TCR transgenic mice, for further studies on T cell responses and may be of value in vaccine design.

Establishment of a chimeric, replication-deficient influenza A virus vector by modulation of splicing efficiency

Segment 8 of the influenza A virus codes for two proteins (NS1 and NS2/NEP) via splicing. Here, we developed a viral vector expressing a cytokine or chemokine instead of the interferon antagonist NS1. To achieve both the desired genetic stability and high transgene expression levels, NS2/NEP mRNA splicing efficacy had to be fine-tuned by modification of splicing elements. Expression levels of secreted foreign proteins could be further enhanced by fusing the N-terminal 13 amino acids of NS1 with an IgK-derived secretion signal peptide. Thus, the first start codon was used for translation initiation of both NS2/NEP and the foreign protein.

Prime and boost immunization with influenza and adenovirus encoding the Toxoplasma gondii surface antigen 2 (SAG2) induces strong protective immunity

In this work, we explored an original vaccination protocol using recombinant influenza and adenovirus. We constructed recombinant influenza viruses harboring dicistronic NA segments containing the surface antigen 2 (SAG2) from Toxoplasma gondii under control of the duplicated 3' promoter. Recombinant influenza viruses were able to drive the expression of the foreign SAG2 sequence in cell culture and to replicate efficiently both in cell culture and in lungs of infected mice. In addition, mice primed with recombinant influenza virus and boosted with a recombinant adenovirus encoding SAG2 elicited both humoral and cellular immune responses specific for SAG2. Moreover, when immunized animals were challenged with the cystogenic P-Br strain of T. gondii, they displayed up to 85% of reduction in parasite burden. These results demonstrate the potential use of recombinant influenza vectors harboring the dicistronic segments in the development of vaccines against infectious diseases.

A live bivalent influenza vaccine based on a H9N2 virus strain

The purpose of this study was to construct an H9N2 virus-based bivalent live vaccine expressing the protective antigen of a different subtype of influenza virus. Reverse genetics was used to generate an influenza virus containing nine gene segments derived from the A/Chicken/Jiangsu/11/2002 (H9N2) strain, including independent M1 and M2 matrix gene segments. A recombinant virus expressing the H1N1 HA1 hemagglutinin protein was produced on this framework by substituting the extracellular domain of the H9N2 M2 gene with the H1N1 HA1 fragment from A/PR/8/34 (PR8, H1N1). The resulting hybrid virus H9N2-PR8/HA1 was genetically stable and of low pathogenicity. Intra-nasal immunization of BALB/c mice with H9N2-PR8/HA1 virus induced both anti-H9N2 virus and anti-PR8 HA antibodies and conferred protection to mice against lethal challenge (40x LD(50)) with either H1N1 or H9N2 viruses. This study provides a new influenza H9N2 virus model for the expression and/or delivery of foreign antigens.

Incorporation of influenza A virus genome segments does not absolutely require wild-type sequences

The efficient incorporation of influenza virus genome segments into virions is mediated by cis-acting regions at both ends of the viral RNAs. It was shown previously that nt 16-26 at the 3' end of the non-structural (NS) viral RNA of influenza A virus are important for efficient virion incorporation and that nt 27-56 also contribute to this process. To understand further the signalling requirements for genome packaging, this study performed linker-scanning mutagenesis in the latter region and found that nt 27-35 made an appreciable contribution to the efficient incorporation of the NS segment. An NS vRNA library was then generated composed of an RNA population with randomized nucleotides at positions 16-35 such that the virus could select the sequences it required for virion incorporation. The sequences selected differed from the wild-type sequence and no conserved nucleotides were selected. The ability of non-wild-type sequences to function in this manner indicates that the incorporation of influenza A virus genome segments does not absolutely require specific sequences.

Generation of recombinant lymphocytic choriomeningitis viruses with trisegmented genomes stably expressing two additional genes of interest

Several arenaviruses cause hemorrhagic fever disease in humans for which no licensed vaccines are available and current therapeutic intervention is limited to the off-label use of the wide-spectrum antiviral ribavirin. However, the prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) has proven to be a Rosetta stone for the investigation of virus-host interactions. Arenaviruses have a bisegmented negative-strand RNA genome. The S segment encodes for the virus nucleoprotein and glycoprotein, whereas the L segment encodes for the virus polymerase (L) and Z protein. The ability to generate recombinant LCMV (rLCMV) expressing additional foreign genes of interest would open novel avenues for the study of virus-host interactions and the development of novel vaccine strategies and high-throughput screens to identify antiarenaviral molecules. To this end, we have developed a trisegmented (1L + 2S) rLCMV-based approach (r3LCMV). Each of the two S segments in r3LCMV was altered to replace one of the viral genes by a gene of interest. All r3LCMVs examined expressing different reported genes were stable both genetically and phenotypically and exhibited wild-type growth properties in cultured cells. Reporter gene expression in r3LCMV-infected cells provided an accurate surrogate of levels of virus multiplication. Notably, some r3LCMVs displayed highly attenuated virulence in mice but induced protective immunity against a subsequent lethal challenge with wild-type LCMV, supporting the potential development of r3LCMV-based vaccines.

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.

Recombinant influenza A viruses harboring optimized dicistronic NA segment with an extended native 5' terminal sequence: induction of heterospecific B and T cell responses in mice

We generated novel recombinant influenza A viruses (vNA38) harboring dicistronic NA segments with an extended native 5' terminal sequence of 70 nucleotides comprised of the last 42 nucleotides of the NA ORF and the 5' noncoding region (5' NCR). vNA38 viruses replicated stably and more efficiently than vNA35 viruses with a dicistronic NA segment comprised of the native 5' NCR only, that we described previously (Vieira Machado, A., Naffakh, N., van der Werf, S., Escriou, N., 2003. Expression of a foreign gene by stable recombinant influenza viruses harboring a dicistronic genomic segment with an internal promoter. Virology 313, 235-249). In addition, vNA38 viruses drove the expression of higher levels of encoded heterologous proteins than corresponding vNA35 viruses, both in cell culture and in the pulmonary tissue of infected mice. These data demonstrate that a sequence overlapping 5' coding and noncoding regions of the NA segment determines efficient replication and/or propagation of the vRNA. Intranasal immunization of mice with live vNA38 viruses induced B and T cell responses specific for the heterologous protein expressed, establishing the usefulness of such recombinant influenza viruses with a dicistronic segment for the development of live bivalent vaccines.

Complete cure of persistent virus infections by antiviral siRNAs

Small interfering RNAs (siRNAs) have been developed as antiviral agents for mammalian cells. The capacity of specific siRNAs to prevent virus infections has been demonstrated, and there is evidence that these new antiviral agents could have a partial therapeutic effect a few days after infection. We investigated the possibility of curing a persistent infection, several months after becoming established, using an in vitro model of persistent poliovirus (PV) infection in HEp-2 cells. Despite high virus titers and the presence of PV mutants, repeated treatment with a mixture of two siRNAs targeting both noncoding and coding regions, one of them in a highly conserved region, resulted in the complete cure of the majority of persistently infected cultures. No escape mutants emerged in treated cultures. The antiviral effect of specific siRNAs, consistent with a mechanism of RNA interference, correlated with a decrease in the amount of viral RNA, until its complete disappearance, resulting in cultures cured of virions and viral RNA.

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.