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.

The proton translocation domain of cellular vacuolar ATPase provides a target for the treatment of influenza A virus infections.

BACKGROUND AND PURPOSE: Cellular vacuolar ATPases (v-ATPase) play an important role in endosomal acidification, a critical step in influenza A virus (IAV) host cell infection. We investigated the antiviral activity of the v-ATPase inhibitor saliphenylhalamide (SaliPhe) and compared it with several older v-ATPase inhibitors concanamycin A, bafilomycin A1, (BafA) and archazolid B targeting the subunit c of the V(0) sector. EXPERIMENTAL APPROACH: An in vitro assay was devised to quantify the anti-influenza effect of v-ATPase inhibitors by measuring green fluorescent protein fluorescence of a reporter IAV. These data were combined with cytotoxicity testing to calculate selectivity indices. Data were validated by testing v-ATPase inhibitors against wild-type IAV in vitro and in vivo in mice. KEY RESULTS: In vitro SaliPhe blocked the proliferation of pandemic and multidrug resistant viruses at concentrations up to 51-fold below its cytotoxic concentrations. At essentially non-toxic concentrations, SaliPhe protected 62.5% of mice against a lethal challenge of a mouse-adapted influenza strain, while BafA at cytotoxic concentrations showed essentially no protection against infection with IAV (SaliPhe vs. BafA P < 0.001). CONCLUSIONS AND IMPLICATIONS: Our results show that a distinct binding site of the proton translocation domain of cellular v-ATPase can be selectively targeted by a new generation v-ATPase inhibitor with reduced toxicity to treat influenza virus infections, including multi-resistant strains. Treatment strategies against influenza that target host cellular proteins are expected to be more resistant to virus mutations than drugs blocking viral proteins.

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.

Development of a live-attenuated influenza B DeltaNS1 intranasal vaccine candidate.

We discovered a unique, single amino acid mutation in the influenza B M1 protein promoting viral growth of NS1 truncation mutants in Vero cells. Due to this mutation, we were able to generate an influenza B virus lacking the complete NS1 open reading frame (DeltaNS1-B virus) by reverse genetics, which was growing to titers of 8log(10)TCID(50)/ml in a Vero cell culture-based micro-carrier fermenter. The DeltaNS1-B vaccine candidate was attenuated in IFN-competent hosts such as human alveolar epithelial cells (A549) similar to influenza A DeltaNS1 viruses. In ferrets, the DeltaNS1-B virus was replication-deficient and did not provoke any clinical symptoms. Importantly, a single intranasal immunization of ferrets at a dose as low as 6 log(10)TCID(50)/animal induced a significant HAI response and provided protection against challenge with wild-type influenza B virus. So far, the lack of a DeltaNS1-B virus component growing to high titers in cell culture has been limiting the possibility to formulate a trivalent vaccine based on deletion of the NS1 gene. Our study closes this gap and paves the way for the clinical evaluation of a seasonal, trivalent, live replication-deficient DeltaNS1 intranasal influenza vaccine.

Influenza B mutant viruses with truncated NS1 proteins grow efficiently in Vero cells and are immunogenic in mice.

Contemporary influenza B virus strains were generated encoding C-terminally truncated NS1 proteins. Viable viruses containing the N-terminal 14, 38, 57 or 80 aa of the NS1 protein were rescued in Vero cells. The influenza B virus NS1-truncated mutants were impaired in their ability to counteract interferon (IFN) production, induce antiviral pro-inflammatory cytokines early after infection and show attenuated or restricted growth in IFN-competent hosts. In Vero cells, all of the mutant viruses replicated to high titres comparable to the wild-type influenza B virus. Mice that received a single, intranasal immunization of the NS1-truncated mutants elicited an antibody response and protection against wild-type virus challenge. Therefore, these NS1-truncated mutants should prove useful as potential candidates for live-attenuated influenza virus vaccines.

Caspase-15 is autoprocessed at two sites that contain an aspartate residue in the P1' position.

Our recent characterization of porcine caspase-15 suggested that the amino terminus of the small catalytic subunit is formed by proteolytic processing between the consecutive aspartate residues D277 and D278. Since a charged residue (D278) is highly unusual in the P1' position of a caspase cleavage site, we further characterized the mechanism of caspase-15 autoproteolysis. Amino acid sequence alignments showed that D277 and D278 as well as another pair of aspartates, D270 and D271, were evolutionarily conserved among species of the mammalian clade Laurasiatheria. Site-directed mutations of these four residues and analysis of recombinant proteins showed that D270 was crucial for autoproteolysis whereas the three other aspartates were dispensable for separation of the catalytic subunits. Mutation of D270 prevented catalytic activation and abolished subsequent processing at D277. Together with previous reports, our results show that caspase-15, unlike all other caspases, efficiently cleaves sites with an aspartate in the P1' position.

Live attenuated influenza virus expressing human interleukin-2 reveals increased immunogenic potential in young and aged hosts.

Despite the reported efficacy of commercially available influenza virus vaccines, a considerable proportion of the human population does not respond well to vaccination. In an attempt to improve the immunogenicity of live influenza vaccines, an attenuated, cold-adapted (ca) influenza A virus expressing human interleukin-2 (IL-2) from the NS gene was generated. Intranasal immunization of young adult and aged mice with the IL-2-expressing virus resulted in markedly enhanced mucosal and cellular immune responses compared to those of mice immunized with the nonrecombinant ca parent strain. Interestingly, the mucosal immunoglobulin A (IgA) and CD8(+) T-cell responses in the respiratory compartment could be restored in aged mice primed with the IL-2-expressing virus to magnitudes similar to those in young adult mice. The immunomodulating effect of locally expressed IL-2 also gave rise to a systemic CD8(+) T-cell and distant urogenital IgA response in young adult mice, but this effect was less distinct in aged mice. Importantly, only mice immunized with the recombinant IL-2 virus were completely protected from a pathogenic wild-type virus challenge and revealed a stronger onset of virus-specific CD8(+) T-cell recall response. Our findings emphasize the potential of reverse genetics to improve the efficacy of live influenza vaccines, thus rendering them more suitable for high-risk age groups.

Identification of a novel exon encoding the amino-terminus of the predominant caspase-5 variants.

Caspase-5 is a caspase-1-like protease with pro-apoptotic and pro-inflammatory activities. Here we have identified a novel exon at the 5'-end of the human caspase-5 gene. This novel exon was present in six alternatively spliced caspase-5 mRNA variants expressed in human peripheral blood mononuclear cells (PBMC) and encoded the previously unknown amino-terminus of caspase-5. The genomic region upstream of this exon contained sequence elements homologous to those of the caspase-11 promoter in the mouse, and transcription of caspase-5 was upregulated by lipopolysaccharide (LPS) in a caspase-11-like manner in human PBMC in vivo. Taken together, our findings call for a revision of the structure of caspase-5 at the genomic level as well as at the mRNA and protein levels.

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.

DNA prime/protein boost immunization against HIV clade C: safety and immunogenicity in mice.

The induction of both cellular and humoral immunity is an important goal for vaccine development against HIV. As a step towards the development of an efficacious vaccine against HIV clade C, the world's most prevalent strain, a combination DNA prime/protein boost immunization strategy was tested. A DNA expression vector was prepared encoding a codon-optimized env gene derived from a pediatric HIV clade C isolate, 1084i. Mice were immunized with HIV1084i env-encoding DNA, then boosted with homologous 1084i gp160. HIV1084i Env-specific T-cell responses were induced with DNA vaccination alone, but the strongest cellular immune responses were seen after boosting with gp160. Immunization with gp160 alone induced high-titer antibodies but required two inoculations. In contrast, high-titer antibodies were seen after a single 1084i gp160 boost in DNA-primed animals. All animals given gp160 inoculations, whether DNA primed or not, developed neutralizing antibodies reactive with HIV1084i and a macaque-passaged simian/human immunodeficiency construct, SHIV-1084ip. The results demonstrate the utility of this DNA prime/protein boost approach to generate cellular immunity, as well as neutralizing antibodies, against HIV clade C env antigens.

Identification and characterization of a novel mammalian caspase with proapoptotic activity.

Caspases are essential proteases in programmed cell death and inflammation. Studies in murine and human cells have led to the characterization of 14 members of this enzyme family. Here we report the identification of caspase-15, a novel caspase that is expressed in various mammalian species including pig, dog, and cattle. The caspase-15 protein contains a catalytic domain with all amino acid residues critical for caspase activity and a prodomain that is predicted to fold into a pyrin domain structure, which is a unique feature among mammalian caspases. Recombinant porcine caspase-15 underwent autocatalytic processing into its subunits and cleaved both tetrapeptide caspase substrates and the apoptosis regulator protein Bid in vitro. Overexpression of caspase-15 in mammalian cells induced proenzyme maturation, cleavage of Bid, activation of caspase-3, and eventually cell death. Both the proteolytic and the pro-apoptotic activity of caspase-15 were abolished by mutation of the active site cysteine. Since a homolog of caspase-15 is absent in the human and the mouse genome, our results reveal an unexpected variability in the molecular apoptotic machinery of mammals.

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.

Influenza A mutant viruses with altered NS1 protein function provoke caspase-1 activation in primary human macrophages, resulting in fast apoptosis and release of high levels of interleukins 1beta and 18.

Several NS1 mutant viruses of human influenza A/PR/8/34 (H1N1) virus were tested for their ability to induce pro-inflammatory cytokines in primary human macrophages. The findings revealed a pronounced difference in the virus-induced cytokine pattern, depending on the functionality of the NS1 protein-encoded domains. The PR8/NS1-125 mutant virus, which encodes the first 125 aa of the NS1 protein, thus lacking the C-terminal domains, induced significantly higher amounts of beta interferon, interleukin (IL) 6, tumour necrosis factor alpha and CCL3 (MIP-1alpha) when compared with the A/PR/8/34 wild-type virus. However, this mutant virus was as efficient as wild-type virus in the inhibition of IL1beta and IL18 release from infected macrophages. Another group of viral mutants either lacking or possessing non-functional RNA-binding and dimerization domains induced 10-50 times more biologically active IL1beta and five times more biologically active IL18 than the wild-type or PR8/NS1-125 viruses. The hallmark of infection with this group of mutant viruses was the induction of rapid apoptosis in infected macrophages, which correlated with the enhanced activity of caspase-1. These results indicated that the NS1 protein, through the function of its N-terminal domains, might control caspase-1 activation, thus repressing the maturation of pro-IL1beta-, pro-IL18- and caspase-1-dependent apoptosis in infected primary human macrophages.

Immunogenicity and protection efficacy of replication-deficient influenza A viruses with altered NS1 genes.

We explored the immunogenic properties of influenza A viruses with altered NS1 genes (NS1 mutant viruses). NS1 mutant viruses expressing NS1 proteins with an impaired RNA-binding function or insertion of a longer foreign sequence did not replicate in murine lungs but still were capable of inducing a Th1-type immune response resulting in significant titers of virus-specific serum and mucosal immunoglobulin G2 (IgG2) and IgA, but with lower titers of IgG1. In contrast, replicating viruses elicited high titers of serum and mucosal IgG1 but less serum IgA. Replication-deficient NS1 mutant viruses induced a rapid local release of proinflammatory cytokines such as interleukin-1beta (IL-1beta) and IL-6. Moreover, these viruses also elicited markedly higher levels of IFN-alpha/beta in serum than the wild-type virus. Comparable numbers of virus-specific primary CD8(+) T cells were determined in all of the groups of immunized mice. The most rapid onset of the recall CD8(+)-T-cell response upon the wild-type virus challenge was detected in mice primed with NS1 mutant viruses eliciting high levels of cytokines. It is noteworthy that there was one NS1 mutant virus encoding NS1 protein with a deletion of 40 amino acids predominantly in the RNA-binding domain that induced the highest levels of IFN-alpha/beta, IL-6 and IL-1beta after infection. Mice that were immunized with this virus were completely protected from the challenge infection. These findings indicate that a targeted modification of the RNA-binding domain of the NS1 protein is a valuable technique to generate replication-deficient, but immunogenic influenza virus vaccines.

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.

Generation of recombinant influenza virus using baculovirus delivery vector.

A recombinant baculovirus vector containing mammalian cell-active promoters and transcription terminators was used to deliver a mutated influenza NS gene into Vero cells. In addition to the influenza NS gene, the baculovirus contained a reporter gene expression cassette (Green fluorescent protein, GFP), allowing to monitor the Vero cell transduction efficiency. More than 90% of Vero cells were expressing GFP 24-48 h post transduction. After infecting baculovirus transduced cells with influenza helper virus, progeny of attenuated influenza virus carrying the recombinant NS gene could be selected. Baculovirus delivery was highly reproducible and efficient in Vero cells. This new method for influenza gene delivery could contribute to influenza virus research and vaccine development.