Influenza A and B viruses expressing altered NS1 proteins: A vaccine approach

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.

Host responses from innate to adaptive immunity after vaccination: molecular and cellular events

The availability of effective vaccines has had the most profound positive effect on improving the quality of public health by preventing infectious diseases. Despite many successful vaccines, there are still old and new emerging pathogens against which there is no vaccine available. A better understanding of how vaccines work for providing protection will help to improve current vaccines as well as to develop effective vaccines against pathogens for which we do not have a proper means to control. Recent studies have focused on innate immunity as the first line of host defense and its role in inducing adaptive immunity; such studies have been an intense area of research, which will reveal the immunological mechanisms how vaccines work for protection. Toll-like receptors (TLRs), a family of receptors for pathogen-associated molecular patterns on cells of the innate immune system, play a critical role in detecting and responding to microbial infections. Importantly, the innate immune system modulates the quantity and quality of longterm T and B cell memory and protective immune responses to pathogens. Limited studies suggest that vaccines which mimic natural infection and/or the structure of pathogens seem to be effective in inducing long-term protective immunity. A better understanding of the similarities and differences of the molecular and cellular events in host responses to vaccination and pathogen infection would enable the rationale for design of novel preventive measures against many challenging pathogens.

Rotavirus NSP1 inhibits NFkappaB activation by inducing proteasome-dependent degradation of beta-TrCP: a novel mechanism of IFN antagonism

Mechanisms by which viruses counter innate host defense responses generally involve inhibition of one or more components of the interferon (IFN) system. Multiple steps in the induction and amplification of IFN signaling are targeted for inhibition by viral proteins, and many of the IFN antagonists have direct or indirect effects on activation of latent cytoplasmic transcription factors. Rotavirus nonstructural protein NSP1 blocks transcription of type I IFNalpha/beta by inducing proteasome-dependent degradation of IFN-regulatory factors 3 (IRF3), IRF5, and IRF7. In this study, we show that rotavirus NSP1 also inhibits activation of NFkappaB and does so by a novel mechanism. Proteasome-mediated degradation of inhibitor of kappaB (IkappaBalpha) is required for NFkappaB activation. Phosphorylated IkappaBalpha is a substrate for polyubiquitination by a multisubunit E3 ubiquitin ligase complex, Skp1/Cul1/F-box, in which the F-box substrate recognition protein is beta-transducin repeat containing protein (beta-TrCP). The data presented show that phosphorylated IkappaBalpha is stable in rotavirus-infected cells because infection induces proteasome-dependent degradation of beta-TrCP. NSP1 expressed in isolation in transiently transfected cells is sufficient to induce this effect. Targeted degradation of an F-box protein of an E3 ligase complex with a prominent role in modulation of innate immune signaling and cell proliferation pathways is a unique mechanism of IFN antagonism and defines a second strategy of immune evasion used by rotaviruses.

Swine influenza viruses a North American perspective

Influenza is a zoonotic viral disease that represents a health and economic threat to both humans and animals worldwide. Swine influenza (SI) was first recognized clinically in pigs in the Midwestern U.S., in 1918, coinciding with the human influenza pandemic known as the Spanish flu. Since that time SI has remained of importance to the swine industry throughout the world. In this review, the epidemiology of swine influenza virus (SIV) infection in North American pigs is described in detail. The first 80 years of SI remained relatively static, whereas the last decade has become dynamic with the establishment of many emerging subtypes. With the increasing number of novel subtypes and genetic variants, the control of SI has become increasingly difficult and innovative strategies to combat this economically important zoonotic disease are critical. Therefore, protective immune responses against influenza virus infections as well as new paradigms of vaccine development in pigs are discussed in the review. It is expected that the dynamic evolutionary changes of SIVs in North American pigs will continue, making currently available prophylactic approaches of limited use to control the spread and economic losses associated with this important swine pathogen.

Influenza a virus induces an immediate cytotoxic activity in all major subsets of peripheral blood mononuclear cells

Background

A replication defective influenza A vaccine virus (delNS1 virus) was developed. Its attenuation is due to potent stimulation of the innate immune system by the virus. Since the innate immune system can also target cancer cells, we reasoned that delNS1 virus induced immune-stimulation should also lead to the induction of innate cytotoxic effects towards cancer cells.

Methodology/Principal Findings

Peripheral blood mononuclear cells (PBMCs), isolated CD56+, CD3+, CD14+ and CD19+ subsets and different combinations of the above subsets were stimulated by delNS1, wild type (wt) virus or heat inactivated virus and co-cultured with tumor cell lines in the presence or absence of antibodies against the interferon system. Stimulation of PBMCs by the delNS1 virus effectively induced cytotoxicity against different cancer cell lines. Surprisingly, virus induced cytotoxicity was exerted by all major subtypes of PBMCs including CD56+, CD3+, CD14+ and CD19+ cells. Virus induced cytotoxicity in CD3+, CD14+ and CD19+ cells was dependent on virus replication, whereas virus induced cytotoxicity in CD56+ cells was only dependent on the binding of the virus. Virus induced cytotoxicity of isolated cell cultures of CD14+, CD19+ or CD56+ cells could be partially blocked by antibodies against type I and type II (IFN) interferon. In contrast, virus induced cytotoxicity in the complete PBMC preparation could not be inhibited by blocking type I or type II IFN, indicating a redundant system of activation in whole blood.

Conclusions/Significance

Our data suggest that apart from their well known specialized functions all main subsets of peripheral blood cells also initially exert a cytotoxic effect upon virus stimulation. This closely links the innate immune system to the adaptive immune response and renders delNS1 virus a potential therapeutic tool for viro-immunotherapy of cancer.

Attenuated influenza virus vaccines with modified NS1 proteins

The development of reverse genetics techniques allowing the rescue of influenza virus from plasmid DNA has opened up the possibility of inserting mutations into the genome of this virus for the generation of novel live attenuated influenza virus vaccines. Modifications introduced into the viral NS1 gene via reverse genetics have resulted in attenuated influenza viruses with promising vaccine potential. One of the main functions of the NS1 protein of influenza virus is the inhibition of the innate host type I interferon-mediated antiviral response. Upon viral infection, influenza viruses with modified NS1 genes induce a robust local type I interferon response that limits their replication, resulting in disease attenuation in different animal models. Nevertheless, these viruses can be grown to high titers in cell- and egg-based substrates with deficiencies in the type I IFN system. Intranasal inoculation of mice, pigs, horses, and macaques with NS1-modified influenza virus strains induced robust humoral and cellular immune responses, and generated immune protection against challenge with wild-type virus. This protective response was not limited to homologous strains of influenza viruses, as reduced replication of heterologous strains was also demonstrated in animals vaccinated with NS1-modified viruses, indicating the induction of a broad cross-neutralizing response by these vaccine candidates. The immunogenicity of NS1-modified viruses correlated with enhanced activation of antigen-presenting cells. While further studies on their safety and efficacy are still needed, the results obtained so far indicate that NS1-modified viruses could represent a new generation of improved influenza virus vaccines, and they suggest that modifying viral interferon antagonists in other virus families is a promising strategy for the generation of live attenuated virus vaccines.

Live attenuated vaccines for pandemic influenza

In this chapter, we will review the development of and clinical experience with the currently licensed seasonal live attenuated influenza vaccines (LAIV) and preclinical studies of H5, H7, and H9 live attenuated pandemic influenza vaccine candidates. Vectored vaccine approaches will not be reviewed in this chapter. Experience with seasonal influenza vaccination has demonstrated the safety and efficacy of LAIV in both children and adults; moreover, cross-protection among antigenically distinct viruses within the same subtype may be induced by LAIV. While clinical studies and further characterization of the immunologic response to avian influenza viruses are still needed, the experience with seasonal LAIV underscores the potential of live attenuated vaccines to play an important role in the event of a pandemic.

Reovirus mu2 protein inhibits interferon signaling through a novel mechanism involving nuclear accumulation of interferon regulatory factor 9

The secreted cytokine alpha/beta interferon (IFN-alpha/beta) binds its receptor to activate the Jak-STAT signal transduction pathway, leading to formation of the heterotrimeric IFN-stimulated gene factor 3 (ISGF3) transcription complex for induction of IFN-stimulated genes (ISGs) and establishment of an antiviral state. Many viruses have evolved countermeasures to inhibit the IFN pathway, thereby subverting the innate antiviral response. Here, we demonstrate that the mildly myocarditic reovirus type 1 Lang (T1L), but not the nonmyocarditic reovirus type 3 Dearing, represses IFN induction of a subset of ISGs and that this repressor function segregates with the T1L M1 gene. Concordantly, the T1L M1 gene product, mu2, dramatically inhibits IFN-beta-induced reporter gene expression. Surprisingly, T1L infection does not degrade components of the ISGF3 complex or interfere with STAT1 or STAT2 nuclear translocation as has been observed for other viruses. Instead, infection with T1L or reassortant or recombinant viruses containing the T1L M1 gene results in accumulation of interferon regulatory factor 9 (IRF9) in the nucleus. This effect has not been previously described for any virus and suggests that mu2 modulates IRF9 interactions with STATs for both ISGF3 function and nuclear export. The M1 gene is a determinant of virus strain-specific differences in the IFN response, which are linked to virus strain-specific differences in induction of murine myocarditis. We find that virus-induced myocarditis is associated with repression of IFN function, providing new insights into the pathophysiology of this disease. Together, these data provide the first report of an increase in IRF9 nuclear accumulation associated with viral subversion of the IFN response and couple virus strain-specific differences in IFN antagonism to the pathogenesis of viral myocarditis.

Innate sensors of influenza virus: clues to developing better intranasal vaccines

Mucosal immunity acquired by natural infection with influenza viruses at the respiratory tract is more effective and cross-protective against subsequent variant virus infection than systemic immunity induced by parenteral immunization with inactivated vaccines. To develop an effective influenza vaccine, it is beneficial to mimic the process of natural infection that bridges innate and adaptive immune systems. The innate immune system that recognizes influenza virus infection consists of several classes of pattern-recognition receptors, including the Toll-like receptors, the retinoic acid-inducible gene-I-like receptors and the NOD-like receptors. Here, we review our current understanding of the mechanism of innate recognition of influenza and how the signals emanating from the innate sensors control adaptive immunity. Further, we discuss the potential roles of these receptors in developing intranasal influenza vaccines.

Live attenuated influenza viruses containing NS1 truncations as vaccine candidates against H5N1 highly pathogenic avian influenza

Due to the high mortality associated with recent, widely circulating strains of H5N1 influenza virus in poultry, the recurring introduction of H5N1 viruses from birds to humans, and the difficulties in H5N1 eradication by elimination of affected flocks, an effective vaccine against HPAI (highly pathogenic avian influenza) is highly desirable. Using reverse genetics, a set of experimental live attenuated vaccine strains based on recombinant H5N1 influenza virus A/Viet Nam/1203/04 was generated. Each virus was attenuated through expression of a hemagglutinin protein in which the polybasic cleavage site had been removed. Viruses were generated which possessed a full-length NS1 or a C-terminally truncated NS1 protein of 73, 99, or 126 amino acids. Viruses with each NS genotype were combined with a PB2 polymerase gene which carried either a lysine or a glutamic acid at position 627. We predicted that glutamic acid at position 627 of PB2 would attenuate the virus in mammalian hosts, thus increasing the safety of the vaccine. All recombinant viruses grew to high titers in 10-day-old embryonated chicken eggs but were attenuated in mammalian cell culture. Induction of high levels of beta interferon by all viruses possessing truncations in the NS1 protein was demonstrated by interferon bioassay. The viruses were each found to be highly attenuated in a mouse model. Vaccination with a single dose of any virus conferred complete protection from death upon challenge with a mouse lethal virus expressing H5N1 hemagglutinin and neuraminidase proteins. In a chicken model, vaccination with a single dose of a selected virus encoding the NS1 1-99 protein completely protected chickens from lethal challenge with homologous HPAI virus A/Viet Nam/1203/04 (H5N1) and provided a high level of protection from a heterologous virus, A/egret/Egypt/01/06 (H5N1). Thus, recombinant influenza A/Viet Nam/1203/04 viruses attenuated through the introduction of mutations in the hemagglutinin, NS1, and PB2 coding regions display characteristics desirable for live attenuated vaccines and hold potential as vaccine candidates in poultry as well as in mammalian hosts.

Novel influenza virus NS1 antagonists block replication and restore innate immune function

The innate immune system guards against virus infection through a variety of mechanisms including mobilization of the host interferon system, which attacks viral products mainly at a posttranscriptional level. The influenza virus NS1 protein is a multifunctional facilitator of virus replication, one of whose actions is to antagonize the interferon response. Since NS1 is required for efficient virus replication, it was reasoned that chemical inhibitors of this protein could be used to further understand virus-host interactions and also serve as potential new antiviral agents. A yeast-based assay was developed to identify compounds that phenotypically suppress NS1 function. Several such compounds exhibited significant activity specifically against influenza A virus in cell culture but had no effect on the replication of another RNA virus, respiratory syncytial virus. Interestingly, cells lacking an interferon response were drug resistant, suggesting that the compounds block interactions between NS1 and the interferon system. Accordingly, the compounds reversed the inhibition of beta interferon mRNA induction during infection, which is known to be caused by NS1. In addition, the compounds blocked the ability of NS1 protein to inhibit double-stranded RNA-dependent activation of a transfected beta interferon promoter construct. The effects of the compounds were specific to NS1, because they had no effect on the ability of the severe acute respiratory syndrome coronavirus papainlike protease protein to block beta interferon promoter activation. These data demonstrate that the function of NS1 can be modulated by chemical inhibitors and that such inhibitors will be useful as probes of biological function and as starting points for clinical drug development.

The multifunctional NS1 protein of influenza A viruses

The non-structural (NS1) protein of influenza A viruses is a non-essential virulence factor that has multiple accessory functions during viral infection. In recent years, the major role ascribed to NS1 has been its inhibition of host immune responses, especially the limitation of both interferon (IFN) production and the antiviral effects of IFN-induced proteins, such as dsRNA-dependent protein kinase R (PKR) and 2'5'-oligoadenylate synthetase (OAS)/RNase L. However, it is clear that NS1 also acts directly to modulate other important aspects of the virus replication cycle, including viral RNA replication, viral protein synthesis, and general host-cell physiology. Here, we review the current literature on this remarkably multifunctional viral protein. In the first part of this article, we summarize the basic biochemistry of NS1, in particular its synthesis, structure, and intracellular localization. We then discuss the various roles NS1 has in regulating viral replication mechanisms, host innate/adaptive immune responses, and cellular signalling pathways. We focus on the NS1-RNA and NS1-protein interactions that are fundamental to these processes, and highlight apparent strain-specific ways in which different NS1 proteins may act. In this regard, the contributions of certain NS1 functions to the pathogenicity of human and animal influenza A viruses are also discussed. Finally, we outline practical applications that future studies on NS1 may lead to, including the rational design and manufacture of influenza vaccines, the development of novel antiviral drugs, and the use of oncolytic influenza A viruses as potential anti-cancer agents.

Attenuated influenza A viruses with modified cleavage sites in hemagglutinin as live vaccines

Influenza A viruses are a public-health concern as they cause annual epidemics and may initiate a pandemic. Common inactivated influenza A vaccines induce a serum antibody response, which may not be protective against virus variation in the field. In contrast to conventional vaccines, the intranasally administered live influenza vaccine may have the potential to induce long-lived and heterosubtypic immunity. In this perspective, attenuated hemagglutinin cleavage-site mutants are discussed in view of usage as influenza live vaccines. This approach allows the convertion of any influenza A strain into an attenuated vaccine virus. The mutated hemagglutinin can serve as a component of a multiple live-attenuated influenza vaccine and would prevent reassortment into circulating viruses.

Influenza B virus NS1-truncated mutants: live-attenuated vaccine approach

Type B influenza viruses can cause substantial morbidity and mortality in the population, and vaccination remains by far the best means of protection against infections with these viruses. Here, we report the construction of mutant influenza B viruses for potential use as improved live-virus vaccine candidates. Employing reverse genetics, we altered the NS1 gene, which encodes a type I interferon (IFN) antagonist. The resulting NS1 mutant viruses induced IFN and, as a consequence, were found to be attenuated in vitro and in vivo. The absence of pathogenicity of the NS1 mutants in both BALB/c and C57BL/6 PKR(-/-) mice was confirmed. We also provide evidence that influenza B virus NS1 mutants induce a self-adjuvanted immune response and confer effective protection against challenge with both homologous and heterologous B virus strains in mice.

Structural basis for suppression of a host antiviral response by influenza A virus

Influenza A viruses are responsible for seasonal epidemics and high mortality pandemics. A major function of the viral NS1A protein, a virulence factor, is the inhibition of the production of IFN-beta mRNA and other antiviral mRNAs. The NS1A protein of the human influenza A/Udorn/72 (Ud) virus inhibits the production of these antiviral mRNAs by binding the cellular 30-kDa subunit of the cleavage and polyadenylation specificity factor (CPSF30), which is required for the 3' end processing of all cellular pre-mRNAs. Here we report the 1.95-A resolution X-ray crystal structure of the complex formed between the second and third zinc finger domain (F2F3) of CPSF30 and the C-terminal domain of the Ud NS1A protein. The complex is a tetramer, in which each of two F2F3 molecules wraps around two NS1A effector domains that interact with each other head-to-head. This structure identifies a CPSF30 binding pocket on NS1A comprised of amino acid residues that are highly conserved among human influenza A viruses. Single amino acid changes within this binding pocket eliminate CPSF30 binding, and a recombinant Ud virus expressing an NS1A protein with such a substitution is attenuated and does not inhibit IFN-beta pre-mRNA processing. This binding pocket is a potential target for antiviral drug development. The crystal structure also reveals that two amino acids outside of this pocket, F103 and M106, which are highly conserved (>99%) among influenza A viruses isolated from humans, participate in key hydrophobic interactions with F2F3 that stabilize the complex.

Immune response in the absence of neurovirulence in mice infected with m protein mutant vesicular stomatitis virus

Matrix (M) protein mutants of vesicular stomatitis virus (VSV), such as rM51R-M virus, are less virulent than wild-type (wt) VSV strains due to their inability to suppress innate immunity. Studies presented here show that when inoculated intranasally into mice, rM51R-M virus was cleared from nasal mucosa by day 2 postinfection and was attenuated for spread to the central nervous system, in contrast to wt VSV, thus accounting for its reduced virulence. However, it stimulated an antibody response similar to that in mice infected with the wt virus, indicating that it has the ability to induce adaptive immunity in vivo without causing disease. These results support the use of M protein mutants of VSV as vaccine vectors.

Characterization of influenza virus variants with different sizes of the non-structural (NS) genes and their potential as a live influenza vaccine in poultry

From a stock of A/turkey/Oregon/71-delNS1 (H7N3) virus, which has a 10 nucleotide deletion in the coding region of the NS1 gene, we found that several variants with different sizes of NS genes could be produced by passaging the virus in 10- and 14-day-old embryonating chicken eggs (ECE), but not in 7-day-old ECE or Vero cells. We were able to rescue the reassortant virus that has different sizes of the NS genes and confirmed that those NS genes are genetically stable. By conducting in vivo studies in 2-week-old chickens, we found two plaque purified variants (D-del pc3 and pc4) which can be used as a potential live-attenuated vaccine. The variants were highly attenuated in chickens and did not transmit the virus from infected chickens to uninoculated cage mates. At the same time, the variants induced relatively high antibody titers which conferred good protection against a high dose heterologous virus challenge. Our study indicates that naturally selected NS1 deletion variants might be useful in the development of live-attenuated influenza vaccines in poultry. Furthermore, deletion in the NS1 protein can be potentially useful as a negative marker for a differentiating infected from vaccinated animals (DIVA) approach.

Cellular and molecular basis of antibody-dependent enhancement in human dengue pathogenesis

Dengue fever is gaining increased attention as a major global health problem. It occurs annually in 50–100 million people in more than 100 countries, and places half a million people at risk of life-threatening diseases: dengue hemorrhagic fever and dengue shock syndrome (DHF/DSS). The pathogenic mechanisms causing DHF/DSS are not clearly understood. This article reviews cellular and molecular mechanisms that might be responsible for the initiation of the pathogenic processes, including hypotheses for DHF/DSS, dengue-permissive target cells, putative dengue receptors, neutralizing and enhancing antibodies to dengue virus, mechanisms of vascular plasma leakage, innate immune response in dengue infection and antibody-dependent enhancement of dengue infection. While reviewing the literature, the article also gives the author’s opinion on perceived areas of importance for future research in human dengue pathogenesis.

Virus growth and antibody responses following respiratory tract infection of ferrets and mice with WT and P/V mutants of the paramyxovirus Simian Virus 5

P/V gene substitutions convert the non-cytopathic paramyxovirus Simian Virus 5 (SV5), which is a poor inducer of host cell responses in human tissue culture cells, into a mutant (P/V-CPI−) that induces high levels of apoptosis, interferon (IFN)-beta, and proinflammatory cytokines. However, the effect of SV5-P/V gene mutations on virus growth and adaptive immune responses in animals has not been determined. Here, we used two distinct animal model systems to test the hypothesis that SV5-P/V mutants which are more potent activators of innate responses in tissue culture will also elicit higher antiviral antibody responses. In mouse cells, in vitro studies identified a panel of SV5-P/V mutants that ranged in their ability to limit IFN responses. Intranasal infection of mice with these WT and P/V mutant viruses elicited equivalent anti-SV5 IgG responses at all doses tested, and viral titers recovered from the respiratory tract were indistinguishable. In primary cultures of ferret lung fibroblasts, WT rSV5 and P/V-CPI− viruses had phenotypes similar to those established in human cell lines, including differential induction of IFN secretion, IFN signaling and apoptosis. Intranasal infection of ferrets with a low dose of WT rSV5 elicited ~ 500 fold higher anti-SV5 serum IgG responses compared to the P/V-CPI− mutant, and this correlated with overall higher viral titers for the WT virus in tracheal tissues. There was a dose-dependent increase in antibody response to infection of ferrets with P/V-CPI−, but not with WT rSV5. Together our data indicate that WT rSV5 and P/V mutants can elicit distinct innate and adaptive immunity phenotypes in the ferret animal model system, but not in the mouse system. We present a model for the effect of P/V gene substitutions on SV5 growth and immune responses in vivo.

A seven-segmented influenza A virus expressing the influenza C virus glycoprotein HEF

Influenza viruses are classified into three types: A, B, and C. The genomes of A- and B-type influenza viruses consist of eight RNA segments, whereas influenza C viruses only have seven RNAs. Both A and B influenza viruses contain two major surface glycoproteins: the hemagglutinin (HA) and the neuraminidase (NA). Influenza C viruses have only one major surface glycoprotein, HEF (hemagglutinin-esterase fusion). By using reverse genetics, we generated two seven-segmented chimeric influenza viruses. Each possesses six RNA segments from influenza virus A/Puerto Rico/8/34 (PB2, PB1, PA, NP, M, and NS); the seventh RNA segment encodes either the influenza virus C/Johannesburg/1/66 HEF full-length protein or a chimeric protein HEF-Ecto, which consists of the HEF ectodomain and the HA transmembrane and cytoplasmic regions. To facilitate packaging of the heterologous segment, both the HEF and HEF-Ecto coding regions are flanked by HA packaging sequences. When introduced as an eighth segment with the NA packaging sequences, both viruses are able to stably express a green fluorescent protein (GFP) gene, indicating a potential use for these viruses as vaccine vectors to carry foreign antigens. Finally, we show that incorporation of a GFP RNA segment enhances the growth of seven-segmented viruses, indicating that efficient influenza A viral RNA packaging requires the presence of eight RNA segments. These results support a selective mechanism of viral RNA recruitment to the budding site.

Novel approach to the development of effective H5N1 influenza A virus vaccines: use of M2 cytoplasmic tail mutants

Outbreaks of highly pathogenic H5N1 influenza viruses in avian species began in Asia and have since spread to other continents. Concern regarding the pandemic potential of these viruses in humans is clearly warranted, and there is an urgent need to develop effective vaccines against them. Previously, we and others demonstrated that deletions of the M2 cytoplasmic tail caused a growth defect in A/WSN/33 (H1N1) influenza A virus in vitro (K. Iwatsuki-Horimoto, T. Horimoto, T. Noda, M. Kiso, J. Maeda, S. Watanabe, Y. Muramoto, K. Fujii, and Y. Kawaoka, J. Virol. 80:5233-5240, 2006; M. F. McCown and A. Pekosz, J. Virol. 79:3595-3605, 2005; M. F. McCown and A. Pekosz, J. Virol. 80:8178-8189, 2006). We therefore tested the feasibility of using M2 tail mutants as live attenuated vaccines against H5N1 virus. First we generated a series of highly pathogenic H5N1 (A/Vietnam/1203/04 [VN1203]) M2 cytoplasmic tail deletion mutants and examined their growth properties in vitro and in vivo. We found that one mutant, which contains an 11-amino-acid deletion from the C terminus (M2del11 virus), grew as well as the wild-type virus but replicated in mice less efficiently. We then generated a recombinant VN1203M2del11 virus whose hemagglutinin (HA) gene was modified by replacing sequences at the cleavage site with those of an avirulent type of HA (M2del11-HAavir virus). This M2del11-HAavir virus protected mice against challenge with lethal doses of homologous (VN1203; clade 1) and antigenically distinct heterologous (A/Indonesia/7/2005; clade 2) H5N1 viruses. Our results suggest that M2 cytoplasmic tail mutants have potential as live attenuated vaccines against H5N1 influenza viruses.

Interferons and viruses: an interplay between induction, signalling, antiviral responses and virus countermeasures

The interferon (IFN) system is an extremely powerful antiviral response that is capable of controlling most, if not all, virus infections in the absence of adaptive immunity. However, viruses can still replicate and cause disease in vivo, because they have some strategy for at least partially circumventing the IFN response. We reviewed this topic in 2000 [Goodbourn, S., Didcock, L. & Randall, R. E. (2000). J Gen Virol 81, 2341-2364] but, since then, a great deal has been discovered about the molecular mechanisms of the IFN response and how different viruses circumvent it. This information is of fundamental interest, but may also have practical application in the design and manufacture of attenuated virus vaccines and the development of novel antiviral drugs. In the first part of this review, we describe how viruses activate the IFN system, how IFNs induce transcription of their target genes and the mechanism of action of IFN-induced proteins with antiviral action. In the second part, we describe how viruses circumvent the IFN response. Here, we reflect upon possible consequences for both the virus and host of the different strategies that viruses have evolved and discuss whether certain viruses have exploited the IFN response to modulate their life cycle (e.g. to establish and maintain persistent/latent infections), whether perturbation of the IFN response by persistent infections can lead to chronic disease, and the importance of the IFN system as a species barrier to virus infections. Lastly, we briefly describe applied aspects that arise from an increase in our knowledge in this area, including vaccine design and manufacture, the development of novel antiviral drugs and the use of IFN-sensitive oncolytic viruses in the treatment of cancer.

Extending the cytoplasmic tail of the influenza a virus M2 protein leads to reduced virus replication in vivo but not in vitro

A carboxy-terminal epitope tag introduced into the coding region of the A/WSN/33 M2 protein resulted in a recombinant virus (rWSN M2myc) which replicated to titers similar to those of the parental virus (rWSN) in MDCK cells. The rWSN M2myc virus was attenuated in its ability to induce mortality and weight loss after the intranasal inoculation of BALB/c mice, indicating that the M2 cytoplasmic tail plays a role in virus virulence. Mice infected with rWSN M2myc were completely protected from subsequent challenge with rWSN, suggesting that epitope tagging of the M2 protein may be a useful way of attenuating influenza A virus strains.

A naturally occurring deletion in its NS gene contributes to the attenuation of an H5N1 swine influenza virus in chickens

In 2001 and 2003, we isolated two H5N1 viruses, A/swine/Fujian/1/01 (SW/FJ/01) and A/swine/Fujian/1/03 (SW/FJ/03), from pigs in Fujian Province, southern China. Genetically, these two viruses are similar, although the NS gene of the SW/FJ/03 virus has a 15-nucleotide deletion at coding positions 612 to 626. The SW/FJ/01 virus is highly lethal for chickens, whereas the SW/FJ/03 virus is nonpathogenic for chickens when administrated intravenously or intranasally. To understand the molecular basis for the difference in virulence, we used reverse genetics to create a series of single-gene recombinants of both viruses. We found that a recombinant virus containing the mutated NS gene from the SW/FJ/03 virus in the SW/FJ/01 virus background was completely attenuated in chickens. We also found that viruses expressing the mutant NS1 protein of SW/FJ/03 did not antagonize the induction of interferon (IFN) protein. Conversely, only the recombinant virus containing the wild-type SW/FJ/01 NS gene in the SW/FJ/03 background was lethal in chickens and antagonized IFN protein levels. Further, we proved that the NS1 genes of the two viruses differ in their stabilities in the host cells and in their abilities to interact with the chicken cleavage and polyadenylation specificity factor. These results indicate that the deletion of amino acids 191 to 195 of the NS1 protein is critical for the attenuation of the SW/FJ/03 virus in chickens and that this deletion affects the ability of the virus to antagonize IFN induction in host cells.

Coronavirus non-structural protein 1 is a major pathogenicity factor: implications for the rational design of coronavirus vaccines

Attenuated viral vaccines can be generated by targeting essential pathogenicity factors. We report here the rational design of an attenuated recombinant coronavirus vaccine based on a deletion in the coding sequence of the non-structural protein 1 (nsp1). In cell culture, nsp1 of mouse hepatitis virus (MHV), like its SARS-coronavirus homolog, strongly reduced cellular gene expression. The effect of nsp1 on MHV replication in vitro and in vivo was analyzed using a recombinant MHV encoding a deletion in the nsp1-coding sequence. The recombinant MHV nsp1 mutant grew normally in tissue culture, but was severely attenuated in vivo. Replication and spread of the nsp1 mutant virus was restored almost to wild-type levels in type I interferon (IFN) receptor-deficient mice, indicating that nsp1 interferes efficiently with the type I IFN system. Importantly, replication of nsp1 mutant virus in professional antigen-presenting cells such as conventional dendritic cells and macrophages, and induction of type I IFN in plasmacytoid dendritic cells, was not impaired. Furthermore, even low doses of nsp1 mutant MHV elicited potent cytotoxic T cell responses and protected mice against homologous and heterologous virus challenge. Taken together, the presented attenuation strategy provides a paradigm for the development of highly efficient coronavirus vaccines.

Prevention of viral diseases by vaccination aims for controlled induction of protective immune responses against viral pathogens. Live viral vaccines consist of attenuated, replication-competent viruses that are believed to be superior in the induction of broad immune responses, including cell-mediated immunity. The recent proceedings in the area of virus reverse genetics allows for the rational design of recombinant vaccines by targeting, i.e., inactivating, viral pathogenicity factors. For coronaviruses, a major pathogenicity factor has now been identified. The effect of coronavirus non-structural protein 1 on pathogenicity has been analyzed in a murine model of coronavirus infection. By deleting a part of this protein, a recombinant virus has been generated that is greatly attenuated in vivo, while retaining immunogenicity. In particular, the mutant virus retained the ability to replicate in professional antigen-presenting cells and fulfilled an important requirement of a promising vaccine candidate: the induction of a protective long-lasting, antigen-specific cellular immune response. This study has implications for the rational design of live attenuated coronavirus vaccines aimed at preventing coronavirus-induced diseases of veterinary and medical importance, including the potentially lethal severe acute respiratory syndrome.

Functional genomic and serological analysis of the protective immune response resulting from vaccination of macaques with an NS1-truncated influenza virus

We are still inadequately prepared for an influenza pandemic due to the lack of a vaccine effective for subtypes to which the majority of the human population has no prior immunity and which could be produced rapidly in sufficient quantities. There is therefore an urgent need to investigate novel vaccination approaches. Using a combination of genomic and traditional tools, this study compares the protective efficacy in macaques of an intrarespiratory live influenza virus vaccine produced by truncating NS1 in the human influenza A/Texas/36/91 (H1N1) virus with that of a conventional vaccine based on formalin-killed whole virus. After homologous challenge, animals in the live-vaccine group had greatly reduced viral replication and pathology in lungs and reduced upper respiratory inflammation. They also had lesser induction of innate immune pathways in lungs and of interferon-sensitive genes in bronchial epithelium. This postchallenge response contrasted with that shortly after vaccination, when more expression of interferon-sensitive genes was observed in bronchial cells from the live-vaccine group. This suggested induction of a strong innate immune response shortly after vaccination with the NS1-truncated virus, followed by greater maturity of the postchallenge immune response, as demonstrated with robust influenza virus-specific CD4+ T-cell proliferation, immunoglobulin G production, and transcriptional induction of T- and B-cell pathways in lung tissue. In conclusion, a single respiratory tract inoculation with an NS1-truncated influenza virus was effective in protecting nonhuman primates from homologous challenge. This protection was achieved in the absence of significant or long-lasting adverse effects and through induction of a robust adaptive immune response.

Activation of phosphatidylinositol 3-kinase signaling by the nonstructural NS1 protein is not conserved among type A and B influenza viruses

Recently it has been shown by several laboratories that the influenza A virus nonstructural protein 1 (A/NS1) binds and activates phosphatidylinositol 3-kinase (PI3K). This function of the protein is likely to prevent premature apoptosis induction during viral propagation. Here we show that the B/NS1 protein completely lacks the capacity to induce PI3K signaling. Thus, PI3K activation is another unique function of A/NS1 that is different from the action of its influenza B virus counterpart.

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.

Role of the influenza virus nonstructural 1 protein in evasion of immunity

The influenza virus nonstructural (NS)1 protein is a potent immune modulator that has multiple inhibitory functions in the infected cells. The NS1 protein blocks the production of interferon in infected cells by multiple actions, including the inhibition of transcription factors, such as nuclear factor-κB and interferon regulatory factor 3, and the cytoplasmic RNA sensor, retinoic acid-inducible gene-I. Additionally, our recent studies have demonstrated that the NS1 protein of influenza virus is able to inhibit both innate and adaptive immunity by targeting a very specific set of genes and proteins in dendritic cells (DCs). These genes are crucial for the activation of DCs and facilitate their interaction with T cells for the initiation of antiviral immune responses in the infected host. Thus, the NS1 protein is a dual-immune modulator that affects DC function profoundly.

Viral suppression of the interferon system

Type I interferons (IFN-alpha/beta) were originally discovered by their strong and direct antiviral activity [A. Isaacs, J. Lindenmann, Virus interference. I. The interferon, Proc. R. Soc. Lond. B Biol. Sci. 147 (1957) 258-267]. (see review by J. Lindenmann on p. 719, in this issue). Nevertheless, only very recently it was entirely realized that viruses would not succeed without efficient tools to undermine this potent host defense system. Current investigations are revealing an astonishing variety of viral IFN antagonistic strategies targeting virtually all parts of the IFN system, often in a highly specific manner. Viruses were found to interfere with induction of IFN synthesis, IFN-induced signaling events, the antiviral effector proteins, or simply shut off the host cell macromolecule synthesis machinery to avoid booting of the antiviral host defense. Here, we will describe a few well-characterized examples to illustrate the sophisticated and often multi-layered anti-IFN mechanisms employed by viruses.

The threat of avian influenza A (H5N1). Part IV: development of vaccines

Among emerging and re-emerging infectious diseases, influenza constitutes one of the major threats to mankind. In this review series epidemiologic, virologic and pathologic concerns raised by infections of humans with avian influenza virus A/H5N1 are discussed. This fourth part focuses on vaccine development. Several phase I clinical studies with vaccines against H5 viruses have demonstrated limited efficacy compared to seasonal influenza vaccines. To induce protective immunity two immunisations with increased amounts of H5N1 vaccine were required. Novel vaccination strategies that are egg- and adjuvant-independent, broadly cross-reactive and long-lasting are highly desirable.

Development of a mucosal vaccine for influenza viruses: preparation for a potential influenza pandemic

Highly pathogenic avian H5N1 influenza A virus has caused influenza outbreaks in poultry and migratory birds in Southeast Asia, Africa and Europe, and there is concern that it could cause a new pandemic. This fear of an emerging pandemic of a new influenza strain underscores the urgency of preparing effective vaccines to meet the pandemic. One way to mitigate current concerns is to develop an influenza vaccine that is fully functional against drift influenza viruses. In our current situation, in which we cannot predict which strain will cause a pandemic, cross-protective immunity using potential and novel mucosal vaccines plays a particularly important role in preventing the spread of highly pathogenic influenza virus.

Pandemic vaccines: promises and pitfalls

Prototype vaccines against influenza A/H5N1 may be poorly immunogenic, and two or more doses may be required to induce levels of neutralising antibody that are deemed to be protective. The actual levels of antibody required to protect against a highly pathogenic virus that potentially can spread beyond the large airways is unknown. The global capacity for vaccine manufacture in eggs or tissue culture is considerable, but the number of doses that can theoretically be produced in a pandemic context will only be sufficient for a small fraction of the world's population, even less if a high antigen content is required. The safety of new pandemic vaccines should be addressed in an internationally coordinated way. Steps are underway through the Therapeutic Goods Administration to evaluate mock-up vaccines now, so that the time to registration of a new product can be minimised. It will be 3-6 months into the pandemic before an effective vaccine becomes available, so other control measures will be important in the early stages of a pandemic. The primary goal of a pandemic influenza vaccine must be to prevent death, and not necessarily to prevent infection.

Amelioration of influenza virus pathogenesis in chickens attributed to the enhanced interferon-inducing capacity of a virus with a truncated NS1 gene

Avian influenza virus (AIV) A/turkey/Oregon/71-SEPRL (TK/OR/71-SEPRL) (H7N3) encodes a full-length NS1 protein and is a weak inducer of interferon (IFN). A variant, TK/OR/71-delNS1 (H7N3), produces a truncated NS1 protein and is a strong inducer of IFN. These otherwise genetically related variants differ 20-fold in their capacities to induce IFN in primary chicken embryo cells but are similar in their sensitivities to the action of IFN. Furthermore, the weak IFN-inducing strain actively suppresses IFN induction in cells that are otherwise programmed to produce it. These phenotypic differences are attributed to the enhanced IFN-inducing capacity that characterizes type A influenza virus strains that produce defective NS1 protein. The pathogenesis of these two variants was evaluated in 1-day-old and 4-week-old chickens. The cell tropisms of both viruses were similar. However, the lesions in chickens produced by the weak IFN inducer were more severe and differed somewhat in character from those observed for the strong IFN inducer. Differences in lesions included the nature of inflammation, the rate of resolution of the infection, and the extent of viral replication and/or virus dissemination. The amelioration of pathogenesis is attributed to the higher levels of IFN produced by the variant encoding the truncated NS1 protein and the antiviral state subsequently induced by that IFN. The high titer of virus observed in kidney tissue ( approximately 10(9) 50% embryo lethal doses/g) from 1-day-old chickens infected intravenously by the weak IFN-inducing strain is attributed to the capacity of chicken kidney cells to activate the hemagglutinin fusion peptide along with their unresponsiveness to inducers of IFN as measured in vitro. Thus, the IFN-inducing capacity of AIV appears to be a significant factor in regulating the pathogenesis, virulence, and viral transmission of AIV in chickens. This suggests that the IFN-inducing and IFN induction suppression phenotypes of AIV should be considered when characterizing strains of influenza virus.

Capsid protein of eastern equine encephalitis virus inhibits host cell gene expression

Eastern equine encephalitis virus (EEEV) causes sporadic but often severe cases of human and equine neurological disease in North America. To determine how EEEV may evade innate immune responses, we screened individual EEEV proteins for the ability to rescue the growth of a Newcastle disease virus expressing green fluorescent protein (NDV-GFP) from the antiviral effects of interferon (IFN). Only expression of the EEEV capsid facilitated NDV-GFP replication. Inhibition of the antiviral effects of IFN by the capsid appears to occur through a general inhibition of cellular gene expression. For example, the capsid inhibited the expression of several reporter genes under the control of RNA polymerase II promoters. In contrast, capsid did not inhibit expression from a T7 RNA polymerase promoter construct, suggesting that the inhibition of gene expression is specific and is not a simple manifestation of toxicity. The inhibition correlated both with capsid-induced phosphorylation of eukaryotic initiation factor 2 alpha and with capsid-mediated inhibition of cellular mRNA accumulation. Mapping analysis identified the N terminus as the region important for the inhibition of host gene expression, suggesting that this inhibition is independent of capsid protease activity. Finally, when cell lines containing EEEV replicons encoding capsid were selected, replicons consistently acquired mutations that deleted all or part of the capsid, for example, amino acids 18 to 135. Given that the amino terminus of the capsid is required to inhibit host cell gene expression, these data suggest that capsid expression from the replicons is ultimately toxic to host cells, presumably because of its ability to inhibit gene expression.

Prostate tumor cells infected with a recombinant influenza virus expressing a truncated NS1 protein activate cytolytic CD8+ cells to recognize noninfected tumor cells

Many viral oncolytic approaches against cancer are based on the ability of specific viruses to replicate in tumors expressing components of the constitutively activated Ras/mitogen-activated protein kinase (MAPK) pathways and/or inhibited or dysregulated alpha/beta interferon (IFN-alpha/beta) response pathways. A major issue when considering these approaches is their applicability to tumors that lack activated Ras. To identify the effector mechanisms activated by oncolytic viruses, we investigated inhibition of proliferation of the prostate cancer line LNCap by the recombinant TR-NS1 influenza A virus, a genetically attenuated influenza A/PR8/34 virus expressing a truncated nonstructural protein (NS1) of 126 amino acids. LNCap cells lack constitutively activated MAPK, extracellular signal-regulated kinase (ERK), and p38 and are resistant to death by IFN-alpha. Truncation of the NS1 protein of influenza viruses is known to result in viral attenuation due to a reduced ability of the NS1 to inhibit the IFN-alpha/beta response. Infection with TR-NS1 virus rapidly activated ERK-1 more than ERK-2 in LNCap cells. Importantly, TR-NS1 virus infection transiently inhibited cell proliferation and induced apoptosis in LNCap cells. Addition of peripheral blood mononuclear cells (PBMC) and interleukin 12 (IL-12) to TR-NS1 virus-infected LNCap cells (TR-NS1-LNCap) resulted in faster elimination of TR-NS1-LNCap cells compared with LNCap cells. Moreover, TR-NS1-LNCap cells induced IFN-gamma in PBMC. The levels of IFN-gamma were amplified by IL-12. TR-NS1-LNCap cells also induced tumor-lytic cytotoxic T lymphocytes (CTL). These CTL lysed noninfected LNCap cells in a CD8-dependent manner. Activation of cellular immunity to tumor cells by viruses is an intriguing effector pathway, which should be especially significant for elimination of human tumors that lack activated Ras.

Influenza A virus NS1 protein activates the PI3K/Akt pathway to mediate antiapoptotic signaling responses

Recently we have shown that influenza A virus infection leads to activation of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway and that this cellular reaction is dependent on the expression of the viral nonstructural protein 1 (NS1). These data also suggested that PI3K activation confers a virus-supporting activity at intermediate stages of the infection cycle. So far it is not known which process is regulated by the kinase that supports virus replication. It is well established that upon infection with influenza A virus, the expression of the viral NS1 keeps the induction of beta interferon and the apoptotic response within a tolerable limit. On a molecular basis, this activity of NS1 has been suggested to preclude the activation of cellular double-stranded RNA receptors as well as impaired modulation of mRNA processing. Here we present a novel mode of action of the NS1 protein to suppress apoptosis induction. NS1 binds to and activates PI3K, which results in the activation of the PI3K effector Akt. This leads to a subsequent inhibition of caspase 9 and glycogen synthase-kinase 3beta and limitation of the virus-induced cell death program. Thus, NS1 not only blocks but also activates signaling pathways to ensure efficient virus replication.

Inhibition of retinoic acid-inducible gene I-mediated induction of beta interferon by the NS1 protein of influenza A virus

The retinoic acid-inducible gene I product (RIG-I) has been identified as a cellular sensor of RNA virus infection resulting in beta interferon (IFN-beta) induction. However, many viruses are known to encode viral products that inhibit IFN-beta production. In the case of influenza A virus, the viral nonstructural protein 1 (NS1) prevents the induction of the IFN-beta promoter by inhibiting the activation of transcription factors, including IRF-3, involved in IFN-beta transcriptional activation. The inhibitory properties of NS1 appear to be due at least in part to its binding to double-stranded RNA (dsRNA), resulting in the sequestration of this viral mediator of RIG-I activation. However, the precise effects of NS1 on the RIG-I-mediated induction of IFN-beta have not been characterized. We now report that the NS1 of influenza A virus interacts with RIG-I and inhibits the RIG-I-mediated induction of IFN-beta. This inhibition was apparent even when a mutant RIG-I that is constitutively activated (in the absence of dsRNA) was used to trigger IFN-beta production. Coexpression of RIG-I, its downstream signaling partner, IPS-1, and NS1 resulted in increased levels of RIG-I and NS1 within an IPS-1-rich, solubilization-resistant fraction after cell lysis. These results suggest that RIG-I, IPS-1, and NS1 become part of the same complex. Consistent with this idea, NS1 was also found to inhibit IFN-beta promoter activation by IPS-1 overexpression. Our results indicate that, in addition to sequestering dsRNA, the NS1 of influenza A virus binds to RIG-I and inhibits downstream activation of IRF-3, preventing the transcriptional induction of IFN-beta.

Influenza A virus PB1-F2 protein contributes to viral pathogenesis in mice

The influenza virus PB1-F2 protein is a novel protein previously shown to be involved in induction of cell death. Here we characterize the expression and the function of the protein within the context of influenza viral infection in tissue culture and a mouse model. We show that the C-terminal region of the protein can be expressed from a downstream initiation codon and is capable of interaction with the full-length protein. Using this knowledge, we generated influenza viruses knocked out for the expression of PB1-F2 protein and its downstream truncation products. Knocking out the PB1-F2 protein had no effect on viral replication in tissue culture but diminished virus pathogenicity and mortality in mice. The viruses replicated to similar levels in mouse lungs by day 3 postinfection, suggesting that the knockout did not impair viral replication. However, while the PB1-F2 knockout viruses were cleared after day 5, the wild-type viruses were detectable in mouse lungs until day 7, implying that expression of PB1-F2 resulted in delayed clearance of the viruses by the host immune system. Based on our findings and on the fact that the PB1 genomic segment was always newly introduced into some pandemic influenza viruses of the last century, we speculate that the PB1-F2 protein plays an important role in pathogenesis of influenza virus infection and may be an important contributor to pathogenicity of pandemic influenza viruses.

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.

The NS1 gene contributes to the virulence of H5N1 avian influenza viruses

In the present study, we explored the genetic basis underlying the virulence and host range of two H5N1 influenza viruses in chickens. A/goose/Guangdong/1/96 (GS/GD/1/96) is a highly pathogenic virus for chickens, whereas A/goose/Guangdong/2/96 (GS/GD/2/96) is unable to replicate in chickens. These two H5N1 viruses differ in sequence by only five amino acids mapping to the PA, NP, M1, and NS1 genes. We used reverse genetics to create four single-gene recombinants that contained one of the sequence-differing genes from nonpathogenic GS/GD/2/96 and the remaining seven gene segments from highly pathogenic GS/GD/1/96. We determined that the NS1 gene of GS/GD/2/96 inhibited the replication of GS/GD/1/96 in chickens, while the substitution of the PA, NP, or M gene did not change the highly pathogenic properties of GS/GD/1/96. Conversely, of the recombinant viruses generated in the GS/GD/2/96 background, only the virus containing the NS1 gene of GS/GD/1/96 was able to replicate and cause disease and death in chickens. The single-amino-acid difference in the sequence of these two NS1 genes resides at position 149. We demonstrate that a recombinant virus expressing the GS/GD/1/96 NS1 protein with Ala149 is able to antagonize the induction of interferon protein levels in chicken embryo fibroblasts (CEFs), but a recombinant virus carrying a Val149 substitution is not capable of the same effect. These results indicate that the NS1 gene is critical for the pathogenicity of avian influenza virus in chickens and that the amino acid residue Ala149 correlates with the ability of these viruses to antagonize interferon induction in CEFs.

Vaccination of pigs against swine influenza viruses by using an NS1-truncated modified live-virus vaccine

Swine influenza viruses (SIV) naturally infect pigs and can be transmitted to humans. In the pig, genetic reassortment to create novel influenza subtypes by mixing avian, human, and swine influenza viruses is possible. An SIV vaccine inducing cross-protective immunity between different subtypes and strains circulating in pigs is highly desirable. Previously, we have shown that an H3N2 SIV (A/swine/Texas/4199-2/98 [TX98]) containing a deleted NS1 gene expressing a truncated NS1 protein of 126 amino acids, NS1black triangle126, was attenuated in swine. In this study, 4-week-old pigs were vaccinated with the TX98 NS1black triangle126 modified live virus (MLV). Ten days after boosting, pigs were challenged with wild-type homologous H3N2 or heterosubtypic H1N1 SIV and sacrificed 5 days later. The MLV was highly attenuated and completely protected against challenge with the homologous virus. Vaccinated pigs challenged with the heterosubtypic H1N1 virus demonstrated macroscopic lung lesions similar to those of the unvaccinated H1N1 control pigs. Remarkably, vaccinated pigs challenged with the H1N1 SIV had significantly lower microscopic lung lesions and less virus shedding from the respiratory tract than did unvaccinated, H1N1-challenged pigs. All vaccinated pigs developed significant levels of hemagglutination inhibition and enzyme-linked immunosorbent assay titers in serum and mucosal immunoglobulin A antibodies against H3N2 SIV antigens. Vaccinated pigs were seronegative for NS1, indicating the potential use of the TX98 NS1black triangle126 MLV as a vaccine to differentiate infected from vaccinated animals.

Influenza virus evades innate and adaptive immunity via the NS1 protein

Both antibodies and T cells contribute to immunity against influenza virus infection. However, the generation of strong Th1 immunity is crucial for viral clearance. Interestingly, we found that human dendritic cells (DCs) infected with influenza A virus have lower allospecific Th1-cell stimulatory abilities than DCs activated by other stimuli, such as lipopolysaccharide and Newcastle disease virus infection. This weak stimulatory activity correlates with a suboptimal maturation of the DCs following infection with influenza A virus. We next investigated whether the influenza A virus NS1 protein could be responsible for the low levels of DC maturation after influenza virus infection. The NS1 protein is an important virulence factor associated with the suppression of innate immunity via the inhibition of type I interferon (IFN) production in infected cells. Using recombinant influenza and Newcastle disease viruses, with or without the NS1 gene from influenza virus, we found that the induction of a genetic program underlying DC maturation, migration, and T-cell stimulatory activity is specifically suppressed by the expression of the NS1 protein. Among the genes affected by NS1 are those coding for macrophage inflammatory protein 1beta, interleukin-12 p35 (IL-12 p35), IL-23 p19, RANTES, IL-8, IFN-alpha/beta, and CCR7. These results indicate that the influenza A virus NS1 protein is a bifunctional viral immunosuppressor which inhibits innate immunity by preventing type I IFN release and inhibits adaptive immunity by attenuating human DC maturation and the capacity of DCs to induce T-cell responses. Our observations also support the potential use of NS1 mutant influenza viruses as live attenuated influenza virus vaccines.

Type 1 interferons and the virus-host relationship: a lesson in détente

The interface between an infectious agent and its host represents the ultimate battleground for survival: The microbe must secure a niche for replication, whereas the host must limit the pathogen's advance. Among the host's arsenal of antimicrobial factors, the type 1 interferons (IFNs) induce potent defense mechanisms against viruses and are key in the host-virus standoff. Viruses have evolved multiple tricks to avoid the immediate antiviral effects of IFNs and, in turn, hosts have adapted use of this innate cytokine system to galvanize multiple additional layers of immune defense. The plasticity that exists in these interactions provides us with a lesson in détente.

Making better influenza virus vaccines?

Better influenza vaccines are possible and necessary.

Killed and live influenza virus vaccines are effective in preventing and curbing the spread of disease, but new technologies such as reverse genetics could be used to improve them and to shorten the lengthy process of preparing vaccine seed viruses. By taking advantage of these new technologies, we could develop live vaccines that would be safe, cross-protective against variant strains, and require less virus per dose than conventional vaccines. Furthermore, pandemic vaccines against highly virulent strains such as the H5N1 virus can only be generated by reverse genetics techniques. Other technologic breakthroughs should result in effective adjuvants for use with killed and live vaccines, increasing the number of available doses. Finally, universal influenza virus vaccines seem to be within reach. These new strategies will be successful if they are supported by regulatory agencies and if a robust market for influenza virus vaccines against interpandemic and pandemic threats is made and sustained.

A simian virus 5 (SV5) P/V mutant is less cytopathic than wild-type SV5 in human dendritic cells and is a more effective activator of dendritic cell maturation and function

Human epithelial cells infected with the parainfluenza virus simian virus 5 (SV5) show minimal activation of host cell interferon (IFN), cytokine, and cell death pathways. In contrast, a recombinant SV5 P/V gene mutant (rSV5-P/V-CPI-) overexpresses viral gene products and is a potent inducer of IFN, proinflammatory cytokines, and apoptosis in these cells. In this study, we have compared the outcomes of wild-type (WT) SV5 and rSV5-P/V-CPI- infections of primary human dendritic cells (DC), important antigen-presenting cells for initiating adaptive immune responses. We have tested the hypothesis that a P/V mutant which activates host antiviral responses will be a more potent inducer of DC maturation and function than WT rSV5, which suppresses host cell responses. Infection of peripheral blood mononuclear cell-derived immature DC with WT rSV5 resulted in high levels of viral protein and progeny virus but very little increase in cell surface costimulatory molecules or secretion of IFN and proinflammatory cytokines. In contrast, immature DC infected with the rSV5-P/V-CPI- mutant produced only low levels of viral protein and progeny virus, but these infected cells were induced to secrete IFN-alpha and other cytokines and showed elevated levels of maturation markers. Unexpectedly, DC infected with WT rSV5 showed extensive cytopathic effects and increased levels of active caspase-3, while infection of DC with the P/V mutant was largely noncytopathic. In mixed-culture assays, WT rSV5-infected DC were impaired in the ability to stimulate proliferation of autologous CD4+ T cells, whereas DC infected with the P/V mutant were very effective at activating T-cell proliferation. The addition of a pancaspase inhibitor to DC infected with WT rSV5 reduced cytopathic effects and resulted in higher surface expression levels of maturation markers. Our finding that the SV5 P/V mutant has both a reduced cytopathic effect in human DC compared to WT SV5 and an enhanced ability to induce DC function has implications for the rational design of novel recombinant paramyxovirus vectors based on engineered mutations in the viral P/V gene.

The role of reverse genetics in the development of vaccines against respiratory viruses

Despite their significance, the only available vaccines against respiratory viruses are those for the prevention of influenza. Attempts have been made to produce vaccines against other respiratory viruses using traditional techniques, but have met with little success. Reverse genetics, although still a r-elatively new tool for the manipulation of negative-strand RNA viruses, has great potential for the preparation of vaccines against many of the common respiratory viruses. In the preparation of live vaccines, reverse genetics s-ystems allow the direct modification of the specific regions in the genomes of negative-stranded RNA viruses concerned with attenuation; the ultimate goal is the introduction of site-specific mutations through a cDNA intermediate in order to develop strains with the requisite attenuation, antigenic and growth properties needed in a vaccine. These techniques can also be used to disarm potentially highly pathogenic viruses, such as emerging H5N1 avian influenza viruses, in order to facilitate large-scale preparation of viruses for use in inactivated vaccines under conditions of manufacturing safety. Before these vaccines become available, residual issues concerned with intellectual property rights to the technology and its application will need to be resolved.

The interferon response circuit: induction and suppression by pathogenic viruses

Type I interferons (IFN-α/β) are potent antiviral cytokines and modulators of the adaptive immune system. They are induced by viral infection or by double-stranded RNA (dsRNA), a by-product of viral replication, and lead to the production of a broad range of antiviral proteins and immunoactive cytokines. Viruses, in turn, have evolved multiple strategies to counter the IFN system which would otherwise stop virus growth early in infection. Here we discuss the current view on the balancing act between virus-induced IFN responses and the viral counterplayers.