Expression of the nonstructural protein NS1 of equine influenza A virus: detection of anti-NS1 antibody in post infection equine sera

Live attenuated influenza A virus vaccines with modified NS1 proteins for veterinary use

Influenza A viruses (IAV) spread rapidly and can infect a broad range of avian or mammalian species, having a tremendous impact in human and animal health and the global economy. IAV have evolved to develop efficient mechanisms to counteract innate immune responses, the first host mechanism that restricts IAV infection and replication. One key player in this fight against host-induced innate immune responses is the IAV non-structural 1 (NS1) protein that modulates antiviral responses and virus pathogenicity during infection. In the last decades, the implementation of reverse genetics approaches has allowed to modify the viral genome to design recombinant IAV, providing researchers a powerful platform to develop effective vaccine strategies. Among them, different levels of truncation or deletion of the NS1 protein of multiple IAV strains has resulted in attenuated viruses able to induce robust innate and adaptive immune responses, and high levels of protection against wild-type (WT) forms of IAV in multiple animal species and humans. Moreover, this strategy allows the development of novel assays to distinguish between vaccinated and/or infected animals, also known as Differentiating Infected from Vaccinated Animals (DIVA) strategy. In this review, we briefly discuss the potential of NS1 deficient or truncated IAV as safe, immunogenic and protective live-attenuated influenza vaccines (LAIV) to prevent disease caused by this important animal and human pathogen.

Development of an Inactivated H7N9 Subtype Avian Influenza Serological DIVA Vaccine Using the Chimeric HA Epitope Approach

H7N9 avian influenza virus (AIV) is an emerging zoonotic pathogen, and it is necessary to develop a differentiating infected from vaccinated animals (DIVA) vaccine for the purpose of eradication. H7N9 subtype AIV hemagglutinin subunit 2 glycoprotein (HA2) peptide chips and antisera of different AIV subtypes were used to screen H7N9 AIV-specific epitopes. A selected specific epitope in the HA2 protein of H7N9 AIV strain A/Chicken/Huadong/JD/17 (JD/17) was replaced with an epitope from an H3N2 subtype AIV strain by reverse genetics. The protection and serological DIVA characteristics of the recombinant H7N9 AIV strain were evaluated. The results showed that a specific epitope on the HA2 protein of H7N9 AIV, named the H7-12 peptide, was successfully screened. The recombinant H7N9 AIV with a modified epitope in the HA2 protein was rescued and named A/Chicken/Huadong/JD-cHA/17 (JD-cHA/17). The HA titer of JD-cHA/17 was 10 log₂, and the 50% egg infective dose (EID₅₀) titer was 9.67 log₁₀ EID₅₀/ml. Inactivated JD-cHA/17 induced a hemagglutination inhibition (HI) antibody titer similar that of the parent strain and provided 100% protection against high-pathogenicity or low-pathogenicity H7N9 AIV challenge. A peptide chip coated with H7-12 peptide was successfully applied to detect the seroconversion of chickens infected or vaccinated with JD/17, while there was no reactivity with antisera of chickens vaccinated with JD-cHA/17. Therefore, the marked vaccine candidate JD-cHA/17 can be used as a DIVA vaccine against H7N9 avian influenza when combined with an H7-12 peptide chip, making it a useful tool for stamping out the H7N9 AIV.

IMPORTANCE DIVA vaccine is a useful tool for eradicating avian influenza, especially for highly pathogenic avian influenza. Several different DIVA strategies have been proposed for avian influenza inactivated whole-virus vaccine, involving the neuraminidase (NA), nonstructural protein 1 (NS1), matrix protein 2 ectodomain (M2e), or HA2 gene. However, virus reassortment, residual protein in a vaccine component, or reduced vaccine protection may limit the application of these DIVA strategies. Here, we constructed a novel chimeric H7N9 AIV, JD-cHA/17, that expressed the entire HA protein with substitution of an H3 AIV epitope in HA2. The chimeric H7N9 recombinant vaccine provides full clinical protection against high-pathogenicity or low-pathogenicity H7N9 AIV challenge. Combined with a short-peptide-based microarray chip containing the H7N9 AIV epitope in HA2, our finding is expected to be useful as a marker vaccine designed for avian influenza.

Immuno-reactivity of recombinant non-structural protein 3 N-terminus (rNS3Nt) in indirect-ELISA for detection of bluetongue viral antibodies in serum samples

Bluetongue, an arthropod borne non-contagious disease of ruminants especially sheep, is caused by bluetongue virus (BTV). Detection of BTV antibodies in susceptible hosts is considered to be of significance in disease diagnosis and differentiation. In the present study, a partial NS3 gene encoding for non-structural protein-3 N-terminus (1MT117 aa) of BTV-23, produced as purified recombinant NS3Nt fusion protein (~32 kDa) using prokaryotic expression system (Escherichia coli), was evaluated as a candidate antigen in an indirect-ELISA (rNS3Nt-ELISA) to measure the serologic response to NS3 protein in small ruminants. The rNS3Nt fusion protein obtained in sufficient quantity and quality has good reactivity in detecting NS3 specific antibodies in field serum samples by indirect-ELISA. As NS3 protein is highly conserved, rNS3Nt-ELISA has potential for NS3 specific detection of antibodies in BTV affected animals irrespective of different viral serotypes. In comparison to structural protein (VP7) based c-ELISA kit and i-ELISA kit, the diagnostic sensitivity (85.1%, 86.2%) and specificity (92.5%, 93.2%) of rNS3Nt-ELISA were found to be relatively lower, respectively. Nevertheless, the study indicated the potential utility of rNS3Nt-ELISA as an alternate assay in routine sero-diagnosis of BTV infection and possible sero-surveillance of ruminants under DIVA strategy.

Avian Influenza Virus and DIVA Strategies

Vaccination is becoming a more acceptable option in the effort to eradicate avian influenza viruses (AIV) from commercial poultry, especially in countries where AIV is endemic. The main concern surrounding this option has been the inability of the conventional serological tests to differentiate antibodies produced due to vaccination from antibodies produced in response to virus infection. In attempts to address this issue, at least six strategies have been formulated, aiming to differentiate infected from vaccinated animals (DIVA), namely (i) sentinel birds, (ii) subunit vaccine, (iii) heterologous neuraminidase (NA), (iv) nonstructural 1 (NS1) protein, (v) matrix 2 ectodomain (M2e) protein, and (vi) haemagglutinin subunit 2 (HA2) glycoprotein. This short review briefly discusses the strengths and limitations of these DIVA strategies, together with the feasibility and practicality of the options as a part of the surveillance program directed toward the eventual eradication of AIV from poultry in countries where highly pathogenic avian influenza is endemic.

Duration of the protective immune response after prime and booster vaccination of yearlings with a live modified cold-adapted viral vaccine against equine influenza

We previously created a live vaccine against equine influenza based the new reassortant cold-adapted (Ca) strain A/HK/Otar/6:2/2010. The live vaccine contains surface proteins (HA, NA) from the wild-type virus A/equine/Otar/764/2007 (Н3N8; American Lineage Florida Clade 2), and internal proteins (PB2, PB1, PA, NP, M, NS) from the attenuated Ca donor virus A/Hong Kong/1/68/162/35CA (H3N2). To determine the safety and duration of the protective immune responses, 90 yearlings were intranasally vaccinated in single mode, double mode at an interval of 42 days (10(7.0) EID50/animal for both vaccinations), or with PBS (control group). Ten animals from each group were challenged with the homologous wild-type virus A/equine/Otar/764/07 (Н3N8) at 1, 2, 3, 4, 5, 6, 9 and 12 months after vaccination. Similarly, 10 animals from each group were challenged with the heterologous wild-type virus A/equine/Sydney/2888-8/07 (Н3N8; American Lineage Florida Clade 1) 12 months after vaccination. The vaccine was completely safe, and single intranasal vaccination of yearlings was capable of inducing statistically significant (from P=0.03 to P<0.0001) clinical and virological protection against the homologous virus; however, only double mode vaccination generated significant (from P=0.02 to P<0.0001) protection against the heterologous virus at 12 months (observation period). Interestingly, this vaccine enables the differentiation of infected and vaccinated animals. On this basis of this study, we recommend double intranasal administration of this vaccine at an interval of 42 days in veterinary practice.

The multifaceted effect of PB1-F2 specific antibodies on influenza A virus infection

PB1-F2 is a small influenza A virus (IAV) protein encoded by an alternative reading frame of the PB1 gene. During IAV infection, antibodies to PB1-F2 proteins are induced. To determine their function and contribution to virus infection, three distinct approaches were employed: passive transfer of anti-PB1-F2 MAbs and polyclonal antibodies, active immunization with PB1-F2 peptides and DNA vaccination with plasmids expressing various parts of PB1-F2. Mostly N-terminal specific antibodies were detected in polyclonal sera raised to complete PB1-F2. Passive and active immunization revealed that antibodies recognizing the N-terminal part of the PB1-F2 molecule have no remarkable effect on the course of IAV infection. Interestingly antibodies against the C-terminal region of PB1-F2, obtained by immunization with KLH-PB1-F2 C-terminal peptide or DNA immunization with pC-ter.PB1-F2 plasmid, partially protected mice against virus infection. To our knowledge, this is the first report demonstrating the biological relevance of humoral immunity against PB1-F2 protein in vivo.

DIVA vaccination strategies for avian influenza virus

Vaccination for both low pathogenicity avian influenza and highly pathogenic avian influenza is commonly used by countries that have become endemic for avian influenza virus, but stamping-out policies are still common for countries with recently introduced disease. Stamping-out policies of euthanatizing infected and at-risk flocks has been an effective control tool, but it comes at a high social and economic cost. Efforts to identify alternative ways to respond to outbreaks without widespread stamping out has become a goal for organizations like the World Organisation for Animal Health. A major issue with vaccination for avian influenza is trade considerations because countries that vaccinate are often considered to be endemic for the disease and they typically lose their export markets. Primarily as a tool to promote trade, the concept of DIVA (differentiate infected from vaccinated animals) has been considered for avian influenza, but the goal for trade is to differentiate vaccinated and not-infected from vaccinated and infected animals because trading partners are unwilling to accept infected birds. Several different strategies have been investigated for a DIVA strategy, but each has advantages and disadvantages. A review of current knowledge on the research and implementation of the DIVA strategy will be discussed with possible ways to implement this strategy in the field. The increased desire for a workable DIVA strategy may lead to one of these ideas moving from the experimental to the practical.

Influenza A virus NS1 induces G0/G1 cell cycle arrest by inhibiting the expression and activity of RhoA protein

Influenza A virus is an important pathogenic virus known to induce host cell cycle arrest in G(0)/G(1) phase and create beneficial conditions for viral replication. However, how the virus achieves arrest remains unclear. We investigated the mechanisms underlying this process and found that the nonstructural protein 1 (NS1) is required. Based on this finding, we generated a viable influenza A virus (H1N1) lacking the entire NS1 gene to study the function of this protein in cell cycle regulation. In addition to some cell cycle regulators that were changed, the concentration and activity of RhoA protein, which is thought to be pivotal for G(1)/S phase transition, were also decreased with overexpressing NS1. And in the meantime, the phosphorylation level of cell cycle regulator pRb, downstream of RhoA kinase, was decreased in an NS1-dependent manner. These findings indicate that the NS1 protein induces G(0)/G(1) cell cycle arrest mainly through interfering with the RhoA/pRb signaling cascade, thus providing favorable conditions for viral protein accumulation and replication. We further investigated the NS1 protein of avian influenza virus (H5N1) and found that it can also decrease the expression and activity of RhoA, suggesting that the H5N1 virus may affect the cell cycle through the same mechanism. The NS1/RhoA/pRb cascade, which can induce the G(0)/G(1) cell cycle arrest identified here, provides a unified explanation for the seemingly different NS1 functions involved in viral replication events. Our findings shed light on the mechanism of influenza virus replication and open new avenues for understanding the interaction between pathogens and hosts.

A DNA vaccine expressing PB1 protein of influenza A virus protects mice against virus infection

Although influenza DNA vaccine research has focused mainly on viral hemagglutinin and has led to promising results, other virion proteins have also shown some protective potential. In this work, we explored the potential of a DNA vaccine based on the PB1 protein to protect BALB/c mice against lethal influenza A virus infection. The DNA vaccine consisted of pTriEx4 plasmid expressing PB1. As a positive control, a pTriEx4 plasmid expressing influenza A virus HA was used. Two weeks after three subcutaneous doses of DNA vaccine, the mice were challenged intranasally with 1 LD50 of A/Puerto Rico/8/34 (H1N1) virus, and PB1- and HA-specific antibodies, survival rate, body weight change, viral mRNA load, infectious virus titer in the lungs, cytokines IL-2, IL-4 and IL-10, and granzyme-B were measured. The results showed that (i) the PB1-expressing DNA vaccine provided a fair protective immunity in the mouse model and (ii) viral structural proteins such as PB1 represent promising antigens for DNA vaccination against influenza A.

Development of DIVA (differentiation of infected from vaccinated animals) vaccines utilizing heterologous NA and NS1 protein strategies for the control of triple reassortant H3N2 influenza in turkeys

Since 2003, triple reassortant (TR) swine H3N2 influenza viruses containing gene segments from human, avian, and swine origins have been detected in the U.S. turkey populations. The initial outbreak that occurred involved birds that were vaccinated with the currently available H3 swine- and avian-origin influenza vaccines. Antigenically, all turkey swine-lineage TR H3N2 isolates are closely related to each other but show little or no antigenic cross-reactivity with the avian origin or swine origin influenza vaccine strains that are currently being used in turkey operations. These results call for re-evaluation of currently available influenza vaccines being used in turkey flocks and development of more effective DIVA (differentiation of infected from vaccinated animals) vaccines. In this study, we selected one TR H3N2 strain, A/turkey/OH/313053/04 (H3N2) that showed broad cross reactivity with other recent TR turkey H3N2 isolates, and created NA- and NS-based DIVA vaccines using traditional reassortment as well as reverse genetics methods. Protective efficacy of those vaccines was determined in 2-week-old and 80-week-old breeder turkeys. The reassortant DIVA vaccines significantly reduced the presence of challenge virus in the oviduct of breeder turkeys as well as trachea and cloaca shedding of both young and old breeder turkeys, suggesting that proper vaccination could effectively prevent egg production drop and potential viral contamination of eggs in infected turkeys. Our results demonstrate that the heterologous NA and NS1 DIVA vaccines together with their corresponding serological tests could be useful for the control of TR H3N2 influenza in turkeys.

Field application of the H9M2e enzyme-linked immunosorbent assay for differentiation of H9N2 avian influenza virus-infected chickens from vaccinated chickens

Vaccination for control of H9N2 low-pathogenicity avian influenza (LPAI) in chickens began in 2007 in South Korea where the H9N2 virus is prevalent. Recently, an enzyme-linked immunosorbent assay (ELISA) using the extracellular domain of the M2 protein (M2e ELISA) was developed as another strategy to differentiate between vaccinated and infected chickens. Here, an ELISA using the extracellular domain of the M2 protein of H9N2 LPAI virus (H9M2e ELISA) was applied to differentiate infected from vaccinated chickens using the H9N2 LPAI virus M2 peptide. The specificity and sensitivity of the optimized H9M2e ELISA were 96.1% and 83.8% (the absorbance of the sample to the absorbance for the positive control [S/P ratio] ≥ 0.6), respectively, with the cutoff value (S/P ratio = 0.6), and the criterion of avian influenza (AI) infection in a chicken house was established as >20% reactivity of anti-M2e antibody per house with this cutoff value. After infection in naïve chickens and once-vaccinated chickens with a hemagglutination inhibition (HI) assay titer of 9.25 ± 0.75 log(2) units, the sera from infected chickens were confirmed as AI infected when the chickens were 1 week old in both groups, and AI infection lasted for 24 weeks and 9 weeks in naïve and once-vaccinated chickens, respectively, although in twice-vaccinated chickens with a higher HI titer of 11.17 ± 0.37 log(2) units, anti-M2e antibody in infected sera did not reach a level indicating AI infection. In field application, anti-M2e antibody produced in infected chickens after vaccination or in reinfected chickens could be identified as AI infection, although HI test could not distinguish infected from vaccinated sera. These results indicate the utility of H9M2e ELISA as a surveillance tool in control of H9N2 LPAI infections.

Validation of diagnostic tests for detection of avian influenza in vaccinated chickens using Bayesian analysis

Vaccination is an attractive tool for the prevention of outbreaks of highly pathogenic avian influenza in domestic birds. It is known, however, that under certain circumstances vaccination may fail to prevent infection, and that the detection of infection in vaccinated birds can be problematic. Here, we investigate the characteristics of three serological tests (immunofluorescent antibody test (iIFAT), neuraminidase inhibition (NI) assay, and NS1 ELISA) that are able to differentiate infected from vaccinated animals. To this end, data of H7N7 infection experiments are analyzed using Bayesian methods of inference. These Bayesian methods enable validation of the tests in the absence of a gold standard, and allow one to take into account that infected birds do not always develop antibodies after infection. The results show that the N7 iIFAT and the NI assay have sensitivities for detecting antibodies of 0.95 (95% CI: 0.89-0.98) and 0.93 (95% CI: 0.78-0.99), but substantially lower sensitivities for detecting infection: 0.64 (95% CI: 0.52-0.75) and 0.63 (95% CI: 0.49-0.75). The NS1 ELISA has a low sensitivity for both detecting antibodies 0.55 (95% CI: 0.34-0.74) and infection 0.42 (95% CI: 0.28-0.56). The estimated specificities of the N7 iIFAT and the NI assay are 0.92 (95% CI: 0.87-0.95) and 0.91 (95% CI: 0.85-0.95), and 0.82 (95% CI: 0.74-0.87) for the NS1 ELISA. Additionally, our analyses suggest a strong association between the duration of virus excretion of infected birds and the probability to develop antibodies.

A Closer Look at the NS1 of Influenza Virus

The Non-Structural 1 (NS1) protein is a multifactorial protein of type A influenza viruses that plays an important role in the virulence of the virus. A large amount of what we know about this protein has been obtained from studies using human influenza isolates and, consequently, the human NS1 protein. The current global interest in avian influenza, however, has highlighted a number of sequence and functional differences between the human and avian NS1. This review discusses these differences in addition to describing potential uses of NS1 in the management and control of avian influenza outbreaks.

Antibodies to PB1-F2 protein are induced in response to influenza A virus infection

PB1-F2 is a small influenza A virus (IAV) protein encoded by an alternative (+1) reading frame of the PB1 gene. While dispensable for IAV replication in cultured cells, PB1-F2 has been implicated in IAV pathogenicity. To better understand PB1-F2 expression in vivo and its immunogenicity, we analyzed anti-PB1-F2 antibodies (Abs) in sera of mice infected intranasally (i.n.) with A/PR/8/34 (H1N1) virus and human acute and convalescent sera collected from the influenza H3N2 winter 2003-2004 epidemic. We explored a number of methods for detecting anti-PB1-F2 Abs, finding that PB1-F2-specific Abs could clearly be detected via immunoprecipitation or immunofluorescence assays using both immune mouse and human convalescent sera. Importantly, paired human sera exhibited similar increases in HI titers and PB1-F2-specific Abs. This study indicates that PB1-F2 is expressed in sufficient quantities in mice and humans infected with IAV to elicit an Ab response, supporting the biological relevance of this intriguing accessory protein.

Broad influenza-specific CD8+ T-cell responses in humanized mice vaccinated with influenza virus vaccines

The development of novel human vaccines would be greatly facilitated by the development of in vivo models that permit preclinical analysis of human immune responses. Here, we show that nonobese diabetic severe combined immunodeficiency (NOD/SCID) beta(2) microglobulin(-/-) mice, engrafted with human CD34+ hematopoietic progenitors and further reconstituted with T cells, can mount specific immune responses against influenza virus vaccines. Live attenuated trivalent influenza virus vaccine induces expansion of CD8+ T cells specific to influenza matrix protein (FluM1) and nonstructural protein 1 in blood, spleen, and lungs. On ex vivo exposure to influenza antigens, antigen-specific CD8+ T cells produce IFN-gamma and express cell-surface CD107a. FluM1-specific CD8+ T cells can be also expanded in mice vaccinated with inactivated trivalent influenza virus vaccine. Expansion of antigen-specific CD8+ T cells is dependent on reconstitution of the human myeloid compartment. Thus, this humanized mouse model permits preclinical testing of vaccines designed to induce cellular immunity, including those against influenza virus. Furthermore, this work sets the stage for systematic analysis of the in vivo functions of human DCs. This, in turn, will allow a new approach to the rational design and preclinical testing of vaccines that cannot be tested in human volunteers.

Avian influenza virus: prospects for prevention and control by vaccination

Although vaccination does not always prevent infection of avian influenza (AI) virus, the clear benefit of vaccination is in its ability to prevent disease and to reduce the amount of virus in circulation. Thus, judicious use of vaccination can be an important component of an AI control program. However, the long-term use of vaccination without eradication may result in the selection of the antigenically divergent strains, which compromises the value of vaccination. In this review, the effectiveness of currently available and future AI vaccines is discussed with suggestions for the ideal use of vaccination even with antigenic drift of the virus.

Progressive truncation of the Non-Structural 1 gene of H7N1 avian influenza viruses following extensive circulation in poultry

In order to support eradication efforts of avian influenza (AI) infections in poultry, the implementation of "DIVA" vaccination strategies, enabling the Differentiation of Infected from Vaccinated Animals have been recommended by international organisations. A system, based on the detection of antibodies to the Non-Structural 1 (NS1) protein of AI has been proposed but the success of such a system lies in the conservation of the NS1 protein among different AI isolates. With this in mind, the ns1 gene of 40 influenza A viruses isolated from a spectrum of avian species was sequenced and compared phylogenetically. The isolates included both low pathogenicity (LPAI) (n=22) and highly pathogenic (HPAI) (n=18) viruses of the H7 subtype and were representative of the avian influenza viruses that circulated in Northern Italy from 1999 to 2003. Size variation in the predicted amino acid sequence of each NS1 was revealed with two different levels of carboxy-terminal truncation being observed. Of the 40 isolates analysed, 16 had a full-length NS1 protein of 230 aa, 6 had a truncated protein of 220 aa and 18 had an intermediate truncation resulting in a protein of 224 aa. All of the H7N1 HPAI isolates possessed the intermediate carboxy-terminal truncation. In addition, all of the H7N1 LPAI viruses circulating at the beginning of the epidemic had a full length NS1 while those circulating towards the end of the period had a truncated protein. To determine whether modifications to NS1 could be a result of laboratory manipulation, two strains (A/ty/Italy/977/99 and A/ck/Italy/1082/99) with a full length NS1 were inoculated into 10-day-old embryonated chicken and 12-day-old embryonated turkey eggs via the allantoic route for 20 blind passages and sequenced at passages 3, 10, and 20. No truncation was observed following these serial passages. To determine whether the truncation involved an immunogenic region of the NS1 protein a peptide spanning residues 219 aa to 230 aa was synthesized and tested in an indirect ELISA against sera obtained from turkeys experimentally infected with a virus strain known to have a full length NS1 protein. The peptide proved to be immunogenic highlighting the fact that the variations of the NS1 protein presented in this work must to be taken into consideration when developing a diagnostic test based on the identification of antibodies to the NS1 protein.

Detection of Antibodies to the Nonstructural Protein (NS1) of Avian Influenza Viruses Allows Distinction Between Vaccinated and Infected Chickens

To differentiate avian influenza virus (AIV)-infected chickens vs. chickens immunized with inactivated avian influenza virus, an enzyme-linked immunosorbent assay (ELISA) was developed using a recombinant nonstructural protein (NS1) as the diagnostic antigen, which was cloned from an AIV H9N2 subtype strain isolated during the avian influenza outbreak of 2003-04 and expressed in Escherichia coli. Antibodies to the AIV NS1 protein was only detected in the sera of chickens experimentally infected with AIV but not in the sera of chickens immunized with inactivated vaccine. This ELISA is useful for serological diagnosis to distinguish chickens infected with influenza viruses from those immunized with inactivated vaccine.

Overview of avian influenza DIVA test strategies

The use of vaccination in poultry to control avian influenza has been increasing in recent years. Vaccination has been primarily with killed whole virus-adjuvanted vaccines. Proper vaccination can reduce or prevent clinical signs, reduce virus shedding in infected birds, and increase the resistance to infection. Historically, one limitation of the killed vaccines is that vaccinated birds cannot be differentiated serologically from naturally infected birds using the commonly available diagnostic tests. Therefore, surveillance for avian influenza becomes much more difficult and often results in trade restrictions because of the inability to differentiate infected from vaccinated animals (DIVA). Several different DIVA strategies have been proposed for avian influenza to overcome this limitation. The most common is the use of unvaccinated sentinels. A second approach is the use of subunit vaccines targeted to the hemagglutinin protein that allows serologic surveillance to the internal proteins. A third strategy is to vaccinate with a homologous hemagglutinin to the circulating field strain, but a heterologous neuraminidase subtype. Serologic surveillance can then be performed for the homologous NA subtype as evidence of natural infection. The fourth strategy is to measure the serologic response to the nonstructural protein 1 (NS1). The NS1 protein is produced in large quantities in infected cells, but it is not packaged in the virion. Since killed vaccines for influenza are primarily made with whole virions, a differential antibody response can be seen between naturally infected and vaccinated animals. However, poultry vaccines are not highly purified, and they contain small amounts of the NS1 protein. Although vaccinated chickens will produce low levels of antibody to the NS1 protein, virus infected chickens will produce higher levels of NS1 antibody, and the two groups can be differentiated. All four DIVA strategies have advantages and disadvantages, and further testing is needed to identify the best strategy to make vaccination a more viable option for avian influenza.

Characterization of the humoral immune response of experimentally infected and vaccinated pigs to swine influenza viral proteins

The value of serologic tests for diagnosis of swine influenza virus (SIV) infection has been diminished by the emergence of new subtypes and by antigenic drift within subtype. The intensive use of vaccination also has complicated interpretation of serology results. Serologic assays are needed that can detect infection regardless of subtype or antigenic variation and that can differentiate antibody induced by infection from that induced by vaccination. In this study, the antibody responses to specific viral proteins in pigs infected by or vaccinated for SIV were characterized by Western immunoblot. Both IgM and IgG against hemagglutinin, nucleoprotein, NS1 and NS2 were detected in experimentally infected pigs by 7 days post inoculation (DPI). IgG against these proteins was still detectable at the end of the study (28 DPI). In contrast, IgG against neuraminidase and M1 was not detected until 14 DPI and no IgM against these proteins was detected. In vaccinated pigs, no antibody against NS1 was detected while antibody responses to other proteins were identical to those in exposed pigs. In conclusion, nucleoprotein may be a suitable antigen for use in a subtype-unrestricted serologic assay. NS1 protein may be suitable for a serologic assay that differentiates between infected and vaccinated pigs.

Diagnostic approach for differentiating infected from vaccinated poultry on the basis of antibodies to NS1, the nonstructural protein of influenza A virus

Vaccination programs for the control of avian influenza (AI) in poultry have limitations due to the problem of differentiating between vaccinated and virus-infected birds. We have used NS1, the conserved nonstructural protein of influenza A virus, as a differential diagnostic marker for influenza virus infection. Experimentally infected poultry were evaluated for the ability to induce antibodies reactive to NS1 recombinant protein produced in Escherichia coli or to chemically synthesized NS1 peptides. Immune sera were obtained from chickens and turkeys inoculated with live AI virus, inactivated purified vaccines, or inactivated commercial vaccines. Seroconversion to positivity for antibodies to the NS1 protein was achieved in birds experimentally infected with multiple subtypes of influenza A virus, as determined by enzyme-linked immunosorbent assay (ELISA) and Western blot analysis. In contrast, animals inoculated with inactivated gradient-purified vaccines had no seroconversion to positivity for antibodies to the NS1 protein, and animals vaccinated with commercial vaccines had low, but detectable, levels of NS1 antibodies. The use of a second ELISA with diluted sera identified a diagnostic test that results in seropositivity for antibodies to the NS1 protein only in infected birds. For the field application phase of this study, serum samples were collected from vaccinated and infected poultry, diluted, and screened for anti-NS1 antibodies. Field sera from poultry that received commercial AI vaccines were found to possess antibodies against AI virus, as measured by the standard agar gel precipitin (AGP) test, but they were negative by the NS1 ELISA. Conversely, diluted field sera from AI-infected poultry were positive for both AGP and NS1 antibodies. These results demonstrate the potential benefit of a simple, specific ELISA for anti-NS1 antibodies that may have diagnostic value for the poultry industries.

Antibody responses of naive cattle to two inactivated bovine viral diarrhoea virus vaccines, measured by indirect and blocking ELISAS and virus neutralisation

Two groups of naive heifers were given primary courses of two inactivated bovine viral diarrhoea (BVD) virus vaccines licensed for use in the UK. Their humoral responses in serum and milk were assayed by means of an indirect ELISA detecting antibodies to structural viral glycoproteins, a blocking ELISA specific for antibodies to the non-structural protein NS2-3 and the virus neutralisation test (VNT). For each assay, the numbers of serum or milk samples testing positive at each sample point and the mean values were determined. In both vaccine groups, serum antibody responses were detected by the indirect ELISA and the VNT, with both the numbers of seropositive animals and mean values peaking five weeks after the second vaccination. In the 23 heifers vaccinated with Bovilis BVD, the mean NS2-3-specific ELISA values remained low throughout the trial, with no serum or milk samples testing positive. In the 24 heifers vaccinated with Bovidec, the mean NS2-3 responses peaked below the level of positivity five weeks after the second vaccination, before declining again; NS2-3-specific antibodies were detected in one serum sample and one milk sample from two heifers in this group. A pooled milk sample from each vaccine group tested negative by both ELISAS 12 weeks after the second vaccination.

Vaccination: a management tool in veterinary medicine

Conventional vaccines have been used for some 200 years, primarily to control infectious diseases. It is envisaged that such vaccines will continue to be used and new ones developed using conventional technology. However, in addition to conventional vaccines, novel approaches using biotechnology are already in use and many more are in various stages of development. These novel vaccines are not only being used to control infectious diseases, but also to improve productivity of livestock by modulating hormones, for gender selection, as well as in controlling ectoparasites. The recent developments in vaccination technology in all of these areas are described.

Equine influenza virus infections: an update

Equine influenza is one of the most economically important contagious respiratory diseases of horses. In this paper the current state of knowledge of equine influenza virus and the most important aspects of these virus infections, e.g. epidemiology, clinical aspects, pathogenesis and pathology, immunity, diagnosis, treatment, management and vaccination, are reviewed with an emphasis on epidemiology, diagnosis and vaccinology. Many questions have remained and with the advent of improved technology new questions have arisen. Consequently, research priorities should be set in an attempt to answer them. Therefore, this review ends with some personal recommendations for important priorities for future research.

Hyperattenuated recombinant influenza A virus nonstructural-protein-encoding vectors induce human immunodeficiency virus type 1 Nef-specific systemic and mucosal immune responses in mice

We have generated recombinant influenza A viruses belonging to the H1N1 and H3N2 virus subtypes containing an insertion of the 137 C-terminal amino acid residues of the human immunodeficiency virus type 1 (HIV-1) Nef protein into the influenza A virus nonstructural-protein (NS1) reading frame. These viral vectors were found to be genetically stable and capable of growing efficiently in embryonated chicken eggs and tissue culture cells but did not replicate in the murine respiratory tract. Despite the hyperattenuated phenotype of influenza/NS-Nef viruses, a Nef and influenza virus (nucleoprotein)-specific CD8(+)-T-cell response was detected in spleens and the lymph nodes draining the respiratory tract after a single intranasal immunization of mice. Compared to the primary response, a marked enhancement of the CD8(+)-T-cell response was detected in the systemic and mucosal compartments, including mouse urogenital tracts, if mice were primed with the H1N1 subtype vector and subsequently boosted with the H3N2 subtype vector. In addition, Nef-specific serum IgG was detected in mice which were immunized twice with the recombinant H1N1 and then boosted with the recombinant H3N2 subtype virus. These findings may contribute to the development of alternative immunization strategies utilizing hyperattenuated live recombinant influenza virus vectors to prevent or control infectious diseases, e.g., HIV-1 infection.

Detection of antibodies to the nonstructural protein (NS1) of influenza A virus allows distinction between vaccinated and infected horses

Antibodies to the nonstructural protein (NS1) of A/equine/Miami/1/63 (H3N8) influenza virus were detected exclusively in the sera of mice experimentally infected with A/Aichi/2/68 (H3N2) and horses infected with A/equine/Kentucky/1/81 (H3N8) or A/equine/La Plata/1/93 (H3N8), but not in those of the animals immunized with the inactivated viruses, by enzyme-linked immunosorbent assay (ELISA) using a recombinant NS1 as antigen. The results indicate that the present method is useful for serological diagnosis to distinguish horses infected with equine H3 influenza viruses from those immunized with the inactivated vaccine.

Present and future of veterinary viral vaccinology: a review

This review deals briefly with some key developments in veterinary vaccinology, lists the types of vaccines that are used for vaccinations commonly performed in food animals as well as in companion animals, and indicates that the practising veterinarian can select the best vaccine by comparing the results of efficacy studies. Diva (Differentiating Infected from Vaccinated Animals; also termed marker) vaccines and companion diagnostic tests have been developed that can be used for progammes aimed to control or eradicate virus infections. Vaccine-induced herd immunity, which can be measured relatively easily when diva vaccines are used, is a crucial issue in such programmes. Current vaccine research follows many routes towards novel vaccines, which can be divided into non-replicating ('killed') and replicating ('live') vaccines. Promising trends are the development of DNA vaccination, vector vaccines, and attenuation of DNA and RNA viruses by DNA technology. The lack of (in vitro) correlates of vaccine protection markedly hampers progress in vaccine research. Various characteristics of an 'ideal' vaccine are listed, such as multivalency and the induction of lifelong immunity after one non-invasive administration in animals with maternal immunity. Future research should be aimed at developing vaccines that approach the ideal as closely as possible and which are directed against diseases not yet controlled by vaccination and against newly emerging diseases.

Antibody to the nonstructural protein NS1 of Japanese encephalitis virus: potential application of mAb-based indirect ELISA to differentiate infection from vaccination

An indirect enzyme-linked immunosorbent assay (ELISA) was developed to detect and differentiate the antibody responses to Japanese encephalitis (JE) virus nonstructural protein NS1 between infected and vaccinated individuals. The results showed that all convalescent sera from JE patients contained NS1-specific IgG antibodies, while 65 and 40% of these sera showed detectable NS1-specific IgM and IgA antibodies, respectively. Specificity analysis showed that NS1-specific IgM and IgA antibodies from JE patients do not cross-react to dengue virus NS1 glycoprotein, while IgG antibodies from 10% of JE patients showed significant cross-reaction to dengue virus NS1 glycoprotein. To differentiate infection from vaccination, the immune sera from 24 children vaccinated with inactivated JE vaccine were analyzed. The data showed that none of these immune sera had detectable NS1-specific IgG antibodies. The results demonstrated the potential application of JE NS1-specific indirect ELISA to differentiate infection from vaccination.

Diva vaccines that reduce virus transmission

This brief review deals with the effect of diva (Differentiating Infected from VAccinated individuals) vaccines (also termed marker vaccines) on transmission of herpesviruses and pestiviruses in swine and cattle. Pseudorabies and bovine herpesvirus 1 diva vaccines have been demonstrated to reduce transmission of wild-type virus in populations of pigs and cattle in the laboratory as well as in the field. A subunit diva vaccine based on the immunodominant E2 protein of classical swine fever virus that is expressed in the baculovirus system may reduce transmission of wild-type virus among pigs and also transmission from mother to foetuses. A similar diva vaccine against bovine virus diarrhoea infections protected sheep against transplacental transmission of antigenically homologous wild-type virus. Diva vaccines along with their companion diagnostic tests can play a role in control of infections, ultimately leading to eradication of viruses.

Vaccination of cattle against bovine viral diarrhoea

This brief review describes types and quality (efficacy and safety) of bovine viral diarrhoea virus (BVDV) vaccines that are in the market or under development. Both conventional live and killed vaccines are available. The primary aim of vaccination is to prevent congenital infection, but the few vaccines tested are not highly efficacious in this respect, as shown in vaccination-challenge experiments. Vaccination to prevent severe postnatal infections may be indicated when virulent BVDV strains are prevalent. Live BVDV vaccines have given rise to safety problems. A complication for the development of BVDV vaccines is the wide antigenic diversity among wild-type BVDV. There is ample room for improvement of both the efficacy and safety of BVDV vaccines, and it may be expected that better vaccines, among which marker vaccines, will be launched in the future.