Reduced levels of neuraminidase of influenza A viruses correlate with attenuated phenotypes in mice

Interferon mediated prophylactic protection against respiratory viruses conferred by a prototype live attenuated influenza virus vaccine lacking non-structural protein 1

The influenza A non-structural protein 1 (NS1) is known for its ability to hinder the synthesis of type I interferon (IFN) during viral infection. Influenza viruses lacking NS1 (ΔNS1) are under clinical development as live attenuated human influenza virus vaccines and induce potent influenza virus-specific humoral and cellular adaptive immune responses. Attenuation of ΔNS1 influenza viruses is due to their high IFN inducing properties, that limit their replication in vivo. This study demonstrates that pre-treatment with a ΔNS1 virus results in an antiviral state which prevents subsequent replication of homologous and heterologous viruses, preventing disease from virus respiratory pathogens, including SARS-CoV-2. Our studies suggest that ΔNS1 influenza viruses could be used for the prophylaxis of influenza, SARS-CoV-2 and other human respiratory viral infections, and that an influenza virus vaccine based on ΔNS1 live attenuated viruses would confer broad protection against influenza virus infection from the moment of administration, first by non-specific innate immune induction, followed by specific adaptive immunity.

Prophylactic protection against respiratory viruses conferred by a prototype live attenuated influenza virus vaccine

The influenza A non-structural protein 1 (NS1) is known for its ability to hinder the synthesis of type I interferon (IFN) during viral infection. Influenza viruses lacking NS1 (ΔNS1) are under clinical development as live attenuated human influenza virus vaccines and induce potent influenza virus-specific humoral and cellular adaptive immune responses. Attenuation of ΔNS1 influenza viruses is due to their high IFN inducing properties, that limit their replication in vivo. This study demonstrates that pre-treatment with a ΔNS1 virus results in an immediate antiviral state which prevents subsequent replication of homologous and heterologous viruses, preventing disease from virus respiratory pathogens, including SARS-CoV-2. Our studies suggest that ΔNS1 influenza viruses could be used for the prophylaxis of influenza, SARS-CoV-2 and other human respiratory viral infections, and that an influenza virus vaccine based on ΔNS1 live attenuated viruses would confer broad protection against influenza virus infection from the moment of administration, first by non-specific innate immune induction, followed by specific adaptive immunity.

Inventory of molecular markers affecting biological characteristics of avian influenza A viruses

Avian influenza viruses (AIVs) circulate globally, spilling over into domestic poultry and causing zoonotic infections in humans. Fortunately, AIVs are not yet capable of causing sustained human-to-human infection; however, AIVs are still a high risk as future pandemic strains, especially if they acquire further mutations that facilitate human infection and/or increase pathogenesis. Molecular characterization of sequencing data for known genetic markers associated with AIV adaptation, transmission, and antiviral resistance allows for fast, efficient assessment of AIV risk. Here we summarize and update the current knowledge on experimentally verified molecular markers involved in AIV pathogenicity, receptor binding, replicative capacity, and transmission in both poultry and mammals with a broad focus to include data available on other AIV subtypes outside of A/H5N1 and A/H7N9.

Effect of the fourth nucleotide at the 3' end of neuraminidase and matrix viral genomic RNA on the pathogenicity of influenza virus A/PR/8/34

Twelve nucleotides located at the 3′ end of viral genomic RNA (vRNA) are conserved among influenza A viruses (IAV) and have a promoter function. Hoffmann's 8-plasmid reverse genetics vector system introduced mutations at position 4, C nucleotide (C4) to U nucleotide (U4), of the 3′ ends of neuraminidase (NA) and matrix (M) vRNAs of wild-type A/PR/8/34 (PR8). This resulted in a constellation of C4 and U4 vRNAs coding for low (polymerases) and relatively high (all others) copy number proteins, respectively. U4 has been reported to increase promoter activity in comparison to C4, but the constellation effect on the replication efficiency and pathogenicity of reverse genetics PR8 (rgPR8) has not been fully elucidated. In the present study, we generated 3 recombinant viruses with C4 in the NA and/or M vRNAs and rgPR8 by using reverse genetics and compared their pathobiological traits. The mutant viruses showed lower replication efficiency than rgPR8 due to the low transcription levels of NA and/or M genes. Furthermore, C4 in the NA and/or M vRNAs induced lower PR8 virus pathogenicity in BALB/c mice. The results suggest that the constellation of C4 and U4 among vRNAs may be one of the multigenic determinants of IAV pathogenicity.

Recombinant influenza A viruses with enhanced levels of PB1 and PA viral protein expression

Influenza A viruses containing the promoter mutations G3A/C8U in a given segment express increased levels of the corresponding viral protein during infection due to increased levels of mRNA or cRNA species. The replication of these recombinant viruses is attenuated, and they have an enhanced shedding of noninfectious particles and are incapable of antagonizing interferon (IFN) effectively. Our findings highlight the possibility of increasing influenza virus protein expression and the need for a delicate balance between influenza viral replication, protein expression, and assembly.

Protective effect of low-concentration chlorine dioxide gas against influenza A virus infection

Influenza virus infection is one of the major causes of human morbidity and mortality. Between humans, this virus spreads mostly via aerosols excreted from the respiratory system. Current means of prevention of influenza virus infection are not entirely satisfactory because of their limited efficacy. Safe and effective preventive measures against pandemic influenza are greatly needed. We demonstrate that infection of mice induced by aerosols of influenza A virus was prevented by chlorine dioxide (ClO2) gas at an extremely low concentration (below the long-term permissible exposure level to humans, namely 0.1 p.p.m.). Mice in semi-closed cages were exposed to aerosols of influenza A virus (1 LD50) and ClO2 gas (0.03 p.p.m.) simultaneously for 15 min. Three days after exposure, pulmonary virus titre (TCID50) was 102.6±1.5 in five mice treated with ClO2, whilst it was 106.7±0.2 in five mice that had not been treated (P=0.003). Cumulative mortality after 16 days was 0/10 mice treated with ClO2 and 7/10 mice that had not been treated (P=0.002). In in vitro experiments, ClO2 denatured viral envelope proteins (haemagglutinin and neuraminidase) that are indispensable for infectivity of the virus, and abolished infectivity. Taken together, we conclude that ClO2 gas is effective at preventing aerosol-induced influenza virus infection in mice by denaturing viral envelope proteins at a concentration well below the permissible exposure level to humans. ClO2 gas could therefore be useful as a preventive means against influenza in places of human activity without necessitating evacuation.

Influenza A virus derived from persistently virus-infected cells shows attenuated cytotoxicity in cultured cells but virulent pathogenicity in mice

The IVpi-43 strain of influenza A virus, a progeny virus derived from persistently virus-infected Madin-Darby canine kidney (MDCK) cells, showed a more attenuated nature in cytopathology in cultured cells than the parental wild-type influenza virus (IVwt) that was used for establishment of the virus carrier culture. Upon infection of MDCK cells, growth of the IVpi-43 virus was restrained with an impaired synthesis of virus structural proteins in the cells. Apoptosis induced by IVpi-43 virus was confined at a low level. The IVpi-43 virus was able to easily cause persistent infection in fresh MDCK cells. In contrast to the in vitro phenotype, the IVpi-43 virus proved highly virulent in mice, with massive and broadly disseminated virus multiplication in the lungs. It was suggested that impaired activity of the neuraminidase molecule of the IVpi-43 virus was responsible for the delayed and faint appearance of apoptosis in the IVpi-43 virus-infected respiratory cells, which made it possible for the virus to replicate for a longer period and to spread to a broader area of the lungs and that abundant numbers of the virus-infected lung cells were killed within a short period by the subsequently established virus-specific immune responses, leading to unrecoverable serious pneumonia.

Amino acid change 335 E to K affects the sialic-acid-binding and neuraminidase activities of Urabe AM9 mumps virus hemagglutinin-neuraminidase glycoprotein

A mutation coding for the amino acid change E335 to K is frequently found in the hemagglutinin-neuraminidase (HN) gene of Urabe AM9 mumps viruses isolated during post-vaccination meningitis cases. To identify if this mutation modifies the biological activities of the HN glycoprotein, two variants of Urabe AM9 vaccine differing at amino acid 335 (HN-E335 and HN-K335) were isolated and their receptor-binding specificity was determined by means of competence assays. Pre-incubation of the viruses with sialic acids inhibited both syncytia formation in Vero cells and replication in SH-SY5Y cells. Thus, HN-K335 showed higher affinity towards sialylalpha2,6lactose, whereas HN-G335 preferred sialylalpha2,3lactose. These results are relevant because a high expression of sialylalpha2,6lactose in nerve cells was confirmed by means of Sambucus nigra lectin-cytochemistry. In addition, kinetics assays showed that HN-K335 and HN-E335 also differ in their hydrolysis rate (Vmax values of 37.5 vs. 3.5 nmol min-1mg-1, respectively). Therefore, HN-K335 variant presented a neuraminidase activity level 11-fold higher than that of HN-E335 variant. In conclusion, the mutation affects the receptor-binding and neuraminidase activities of Urabe AM9 mumps virus variants.

Characterization of a mitochondrial-targeting signal in the PB2 protein of influenza viruses

Influenza virus RNA polymerase is a heterotrimeric complex consisting of PB1, PB2, and PA subunits. These polymerase subunits accumulate in the nucleus of infected cells. We report here that PB2, from both human and avian influenza viruses, could also localize to mitochondria in transfected cells. Importantly, cells infected with influenza A virus also displayed mitochondrial PB2. We show that an N-terminal motif composed of 120 amino acids is sufficient for localization of PB2 to mitochondria. In particular, leucine residues at positions 7 and 10 were essential for mitochondrial targeting. Recombinant influenza A/WSN/33 viruses expressing PB2 proteins with L7A and/or L10A mutations showed reduced viral titers, but unaffected levels of transcription, replication, and protein expression. The introduction of L7A and/or L10A mutations into recombinant viruses correlated with reduced mitochondrial membrane potential in infected cells, suggesting that mitochondrial localization of PB2 contributes to the preservation of mitochondrial function during influenza virus infection.

The morphology and composition of influenza A virus particles are not affected by low levels of M1 and M2 proteins in infected cells

We generated a recombinant influenza A virus (Mmut) that produced low levels of matrix (M1) and M2 proteins in infected cells. Mmut virus propagated to significantly lower titers than did wild-type virus in cells infected at low multiplicity. By contrast, virion morphology and incorporation of viral proteins and vRNAs into virus particles were similar to those of wild-type virus. We propose that a threshold amount of M1 protein is needed for the assembly of viral components into an infectious particle and that budding is delayed in Mmut virus-infected cells until sufficient levels of M1 protein accumulate at the plasma membrane.

Protection afforded by intranasal immunization with the neuraminidase-lacking mutant of influenza A virus in a ferret model

Protective efficacy of the intranasal immunization with the neuraminidase (NA)-deficient mutant of the influenza A virus was investigated in ferrets. Despite the highly attenuated replication in vivo, the mutant completely protected the animals against the wild type virus challenge. When challenge was done with antigenic drift variants, significant reductions in the viral titers, inflammatory cell counts, and protein concentrations were observed in the nasal washes of the immunized animals. The genetically engineered NA-deficient mutant also protected animals against the challenge and induced humoral immune response against the foreign protein that replaced the NA. We conclude that the NA as antigen is dispensable in the live attenuated influenza virus vaccine and that the NA-lacking mutant can be used as a virus vector.

Mucosal immunization with attenuated Shigella flexneri harboring an influenza hemagglutinin DNA vaccine protects mice against a lethal influenza challenge

Mucosal surfaces are important for the induction of immunity against influenza virus. In a murine intranasal immunization model, we demonstrated that the attenuated Shigella flexneri Deltaasd strain 15D, carrying a DNA construct encoding the influenza virus hemagglutinin (HA), induces protective immunity against a lethal respiratory challenge with influenza A/WSN/33. Influenza virus-specific IFN-gamma T cells were detected among splenocytes, and anti-HA IgG was detected in serum post-immunization, albeit at low levels. Following influenza virus challenge, an accelerated anti-HA IgA antibody response was detected in bronchoalveolar lavage (BAL) washings from mice vaccinated with attenuated shigella containing the HA construct. These results suggest that S. flexneri Deltaasd strain 15D is a promising vector for mucosal DNA vaccine immunization against influenza virus and other mucosal pathogens.

Bunyamwera bunyavirus RNA synthesis requires cooperation of 3'- and 5'-terminal sequences

Bunyamwera virus (BUNV) is the prototype of both the Orthobunyavirus genus and the Bunyaviridae family of segmented negative-sense RNA viruses. The tripartite BUNV genome consists of small (S), medium (M), and large (L) segments that are each transcribed to yield a single mRNA and are replicated to generate an antigenome that acts as a template for synthesis of further genomic strands. As for all negative-sense RNA viruses, the 3'- and 5'-terminal nontranslated regions (NTRs) of the BUNV S, M, and L segments exhibit nucleotide complementarity and, except for one conserved U-G pairing, this complementarity extends for 15, 18, and 19 nucleotides, respectively. We investigated whether the complementarity of 3' and 5' NTRs reflected a functional requirement for terminal cooperation to promote BUNV RNA synthesis or, alternatively, was a consequence of genomic and antigenomic NTRs having similar functions requiring sequence conservation. We show that cooperation between 3'- and 5'-NTR sequences is required for BUNV RNA synthesis, and our results suggest that this cooperation is due to nucleotide complementarity allowing 3' and 5' NTRs to associate through base-pairing interactions. To examine the importance of complementarity in promoting BUNV RNA synthesis, we utilized a competitive replication assay able to examine the replication ability of all possible combinations of interacting nucleotides within a defined region of BUNV 3' and 5' NTRs. We show here that maximal RNA replication was signaled when sequences exhibiting perfect complementarity within 3' and 5' NTRs were selected.

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.

Alternative base pairs attenuate influenza A virus when introduced into the duplex region of the conserved viral RNA promoter of either the NS or the PA gene

The development of plasmid-based rescue systems for influenza virus has allowed previous studies of the neuraminidase (NA) virion RNA (vRNA) promoter to be extended, in order to test the hypothesis that alternative base pairs in the conserved influenza virus vRNA promoter cause attenuation when introduced into other gene segments. Influenza A/WSN/33 viruses with alternative base pairs in the duplex region of the vRNA promoter of either the polymerase acidic (PA) or the NS (non-structural 1, NS1, and nuclear export, NEP, -encoding) gene have been rescued. Virus growth in MDBK cells demonstrated that one of the mutations, the D2 mutation (U-A replacing G-C at nucleotide positions 12'-11), caused significant virus attenuation when introduced into either the PA or the NS gene. The D2 mutation resulted in the reduction of PA- or NS-specific vRNA and mRNA levels in PA- or NS-recombinant viruses, respectively. Since the D2 mutation attenuates influenza virus when introduced into either the PA or the NS gene segments, or the NA gene segment, as demonstrated previously, this suggests that this mutation will lead to virus attenuation when introduced into any of the eight gene segments. Such a mutation may be useful in the production of live-attenuated viruses.

Prophylaxis and treatment of influenza virus infection

Influenza virus infections remain an important cause of morbidity and mortality. Furthermore, a recurrence of pandemic influenza remains a real possibility. There are now effective ways to both prevent and treat influenza. Prevention of infection is most effectively accomplished by vaccination. Vaccination with the inactivated, intramuscular influenza vaccine has been clearly demonstrated to reduce serious morbidity and mortality associated with influenza infection, especially in groups of patients at high risk (e.g. the elderly). However, the inactivated, intramuscular vaccine does not strongly induce cell-mediated or mucosal immune responses, and protection induced by the vaccine is highly strain specific. Live, attenuated influenza vaccines administered intranasally have been studied in clinical trials and shown to elicit stronger mucosal and cell-mediated immune responses. Live, attenuated vaccines appear to be more effective for inducing protective immunity in children or the elderly than inactivated, intramuscular vaccines. Additionally, novel vaccine methodologies employing conserved components of influenza virus or viral DNA are being developed. Preclinical studies suggest that these approaches may lead to methods of vaccination that could induce immunity against diverse strains or subtypes of influenza. Because of the limitations of vaccination, antiviral therapy continues to play an important role in the control of influenza. Two major classes of antivirals have demonstrated ability to prevent or treat influenza in clinical trials: the adamantanes and the neuraminidase inhibitors. The adamantanes (amantadine and rimantadine) have been in use for many years. They inhibit viral uncoating by blocking the proton channel activity of the influenza A viral M2 protein. Limitations of the adamantanes include lack of activity against influenza B, toxicity (especially in the elderly), and the rapid development of resistance. The neuraminidase inhibitors were designed to interfere with the conserved sialic acid binding site of the viral neuraminidase and act against both influenza A and B with a high degree of specificity when administered by the oral (oseltamivir) or inhaled (zanamivir) route. The neuraminidase inhibitors have relatively low toxicity, and viral resistance to these inhibitors appears to be uncommon. Additional novel antivirals that target other phases of the life cycle of influenza are in preclinical development. For example, recombinant collectins inhibit replication of influenza by binding to the viral haemagglutinin as well as altering phagocyte responses to the virus. Recombinant techniques have been used for generation of antiviral proteins (e.g. modified collectins) or oligonucleotides. Greater understanding of the biology of influenza viruses has already resulted in significant advances in the management of this important pathogen. Further advances in vaccination and antiviral therapy of influenza should remain a high priority.