The attenuated NYCBH vaccinia virus deleted for the immune evasion gene, E3L, completely protects mice against heterologous challenge with ectromelia virus

The New York City Board of Health (NYCBH) vaccinia virus (VACV) vaccine strain was deleted for the immune evasion gene, E3L, and tested for its pathogenicity and ability to protect mice from heterologous challenge with ectromelia virus (ECTV). NYCBHΔE3L was found to be highly attenuated for pathogenicity in a newborn mouse model and showed a similar attenuated phenotype as the NYVAC strain of vaccinia virus. Scarification with one or two doses of the attenuated NYCBHΔE3L was able to protect mice equally as well as NYCBH from death, weight loss, and viral spread to visceral organs. A single dose of NYCBHΔE3L resulted in low poxvirus-specific antibodies, and a second dose increased levels of poxvirus-specific antibodies to a level similar to that seen in animals vaccinated with a single dose of NYCBH. However, similar neutralizing antibody titers were observed following one or two doses of NYCBHΔE3L or NYCBH. Thus, NYCBHΔE3L shows potential as a candidate for a safer human smallpox vaccine since it protects mice from challenge with a heterologous poxvirus.     

Dose response effects of avian influenza (H7N7) vaccination of chickens: Serology,clinical protection and reduction of virus excretion

Knowledge of the relation between the antigen content of inactivated avian influenza (AI) vaccines, the serological response after vaccination and protection of vaccinated animals is important for the choice of optimal vaccines and vaccination regimes as well as for the assessment of criteria for the licensing of new AI-vaccines. We studied this relation in a dose response study using inactivated H7N7 avian influenza vaccines with varying antigen content. The serological response depended on the antigen content of the vaccines. Anti-AI antibodies were detected most frequently with ELISA, followed by the virus neutralisation test and the haemagglutination inhibition (HI) assay. Chickens with measurable HI-antibody titers, using homologous H7N7 antigen, were all protected against clinical disease after challenge with highly pathogenic A/chicken/Netherlands/621557/03 H7N7 virus. However, in these chickens high levels of virus could still be present on days 2–4 after challenge. The reduction of virus titers after challenge, depended on the antigen content of the vaccines as well as on the serum antibody titers. While 10 haemagglutinating units (HAU), equivalent to 0.8 μg haemagglutinin (HA) protein, per vaccine dose was sufficient for prevention of clinical disease, 128 HAU (9 μg HA) per dose was required for reduction of virus titers in all chickens to 10³ egg-infectious dose 50% (EID₅₀) or less. In order to reduce virus titers below 10³ EID₅₀ per swab a HI-antibody titer of 64 was required. After use of the vaccine with the highest antigen content, challenge still induced a booster of antibody titers which is indicative of replication of challenge virus.     

Vaccinia viruses with mutations in the E3L gene as potential replication-competent, attenuated vaccines: scarification vaccination

In this study, we evaluated the efficacy of vaccinia virus (VACV) containing mutations in the E3L virulence gene to protect mice against a lethal poxvirus challenge after vaccination by scarification. VACV strains with mutations in the E3L gene had significantly decreased pathogenicity, even in immune deficient mice, yet retained the ability to produce a potent Th1-dominated immune response in mice after vaccination by scarification, while protecting against challenge with wild type, pathogenic VACV. Initial experiments were done using the mouse-adapted, neurovirulent Western Reserve (WR) strain of vaccinia virus. Testing of the full E3L deletion mutation in the Copenhagen and NYCBH strains of VACV, which are more appropriate for use in humans, produced similar results. These results suggest that highly attenuated strains of VACV containing mutations in E3L have the potential for use as scarification administered vaccines.     

Squalene-adjuvanted H7N9 virus vaccine induces robust humoral immune response against H7N9 and H7N7 viruses

Recent cases of avian influenza H7N9 have caused great concerns that virus may become transmittable between humans. It is imperative to develop an effective vaccine to fight against the pandemic potential of this H7N9 influenza virus to protect human from the disease. This study aims to investigate an optimized formulation for the development of H7N9 vaccines. Various doses of H7N9 inactivated whole or split-virus antigens (0.5, 1.5, or 3 μg based on hemagglutinin content) combined with squalene-based adjuvant (AddaVAX), aluminum hydroxide Al(OH)₃ or without adjuvant were evaluated for the efficacy of H7N9 vaccine regiments in mice. With either H7N9 whole or split-virus based vaccines, AddaVAX-adjuvanted formulations were the most immunogenic in eliciting significant humoral immune response against H7N9 virus and exhibited strong cross-reactive response in hemagglutination inhibition (HAI) and viral-neutralization assays against H7N7 virus as well. In contrast, formulations with Al(OH)₃ or without adjuvant were less immunogenic and elicited lower titers of HAI and microneutralization assays against both viruses. Dose-sparing experiments suggested that the formulation with as low as 0.004 μg of split or whole virus vaccine antigens together with 50% AddaVAX provided sufficient sero-protective HAI titers and achieved essential virus-neutralizing antibody titers against H7-subtype influenza viruses in mice. Protection experiments demonstrated that the formulation of 0.004 μg to 0.5 μg of split-virion vaccines with AddaVAX conferred full protection against viral challenge up to 100 LD50 of wild-type H7N9 virus, with 0% survival in placebo group. Taken together, our study demonstrates that squalene-based adjuvant can significantly enhance the protective efficacy of H7N9 virus vaccine and provides a useful strategy to confront the potential pandemic outbreaks of H7N9 virus.     

A protein-based smallpox vaccine protects non-human primates from a lethal monkeypox virus challenge

Concerns about infections caused by orthopoxviruses, such as variola and monkeypox viruses, drive ongoing efforts to develop novel smallpox vaccines that are both effective and safe to use in diverse populations. A subunit smallpox vaccine comprising vaccinia virus membrane proteins A33, B5, L1, A27 and aluminum hydroxide (alum) ± CpG was administered to non-human primates, which were subsequently challenged with a lethal intravenous dose of monkeypox virus. Alum adjuvanted vaccines provided only partial protection but the addition of CpG provided full protection that was associated with a more homogeneous antibody response and stronger IgG1 responses. These results indicate that it is feasible to develop a highly effective subunit vaccine against orthopoxvirus infections as a safer alternative to live vaccinia virus vaccination.     

A novel vaccine against Streptococcus equi ssp. zooepidemicus infections: the recombinant swinepox virus expressing M-like protein

To develop a safer, more immunogenic and efficacious vaccine against Streptococcus equi ssp. zooepidemicus (SEZ) infections, the gene of M-like protein (SzP) was placed under the strong vaccinia virus promoter P28 and then inserted into swinepox virus (SPV) genome. The recombinant swinepox virus (rSPV-szp) was isolated in a non-selective medium by the co-expression of Escherichia coli LacZ gene and verified by PCR, western blotting and immunofluorescence assays. To evaluate the immunogenicity of this rSPV-szp, ICR mice were immunized with the rSPV-szp, inactivated SEZ vaccine (positive control), wild type SPV (negative control), or PBS (challenge control). All mice were intraperitoneally challenged with 5 LD₅₀ of homogenous ATCC 35246 strain 14 days post-vaccination. The results showed that at least 70% mice in rSPV-szp-vaccinated group were protected against homogenous ATCC 35246 challenge, the survival rate was significantly higher compared with mice in the negative control group and the challenge control group (P < 0.001). The antibody titers of the rSPV-szp-vaccinated group were significantly higher (P < 0.05) than the other three groups. Passive immune protection assays showed that the hyperimmune sera against M-like protein could provide mice with complete protection against challenge of ATCC 35246. Semi-quantitative RT-PCR analysis showed a marked increased in levels of IL-4 and IFN-γ mRNA in immunized mice. The results suggested that the recombinant rSPV-szp provided mice with significant protection from the SEZ infections. It is a promising candidate for the vaccine development against SEZ infections.     

Priming with a DNA vaccine delivered by attenuated Salmonella typhimurium and boosting with a killed vaccine confers protection of chickens against infection with the H9 subtype of avian influenza virus

Control of the circulation of H9 low-pathogenic avian influenza virus (LPAIV) is a major concern for both animal and public health. To improve vaccine efficacy against H9 LPAIV, we have utilized a novel prime–boost vaccination strategy. Specific-pathogen free (SPF) chickens were first orally immunized with a hemagglutinin (HA) DNA vaccine delivered by attenuated Salmonella typhimurium, followed by boosting with a killed avian influenza (AI) vaccine. Chickens in this combined vaccination group were completely protected against both oropharyngeal and cloacal virus shedding after intranasal challenge with H9N2 AIV, while viruses were detected from these sites in other vaccination groups. Prior to challenge, chickens in the prime–boost group also had higher (P < 0.05) serum hemagglutination inhibition (HI) titers and intestinal mucosal IgA ELISA titers against AIV, and higher lymphoproliferation stimulation indices than those from other groups. Thus, we have demonstrated the efficacy of a novel prime–boost vaccination strategy against H9N2 avian influenza virus, which could be also applied for the development of vaccines against other mucosally infectious pathogens.     

A combination HIV vaccine based on Tat and Env proteins was immunogenic and protected macaques from mucosal SHIV challenge in a pilot study

HIV native Tat and V2 loop-deleted Env (EnvΔV2) proteins already proved safe and immunogenic in phase I clinical testing as single vaccine components. Further, a phase II vaccine trial with Tat showed intensification of the therapeutic effects of HAART in successfully treated HIV-infected individuals. Here a pilot study assessed the immunogenicity and protective efficacy of an HIV/AIDS vaccine based on the combination of Tat and EnvΔV2 proteins in cynomolgus macaques against homologous intrarectal challenge with 35 MID₅₀ (monkey infectious dose 50) of an R5 simian-human immunodeficiency virus (SHIV_SF162P4cy).Upon challenge, three of four macaques immunized with Tat and EnvΔV2, and two of three monkeys immunized with EnvΔV2 alone were protected from infection. In contrast, all three control animals, which had been either administered with the adjuvants only or left untreated, and an additional monkey immunized with Tat alone became systemically infected. Protection of the macaques vaccinated with EnvΔV2 or Tat/EnvΔV2 correlated with higher peak titers of pre-challenge neutralizing antibodies obtained during the immunization period (between 70 and 3 weeks before challenge) and with anti-Env V3 loop binding antibodies assessed 3 weeks before challenge.Compared to EnvΔV2 alone, the Tat and EnvΔV2 combined vaccine elicited faster antibody responses (IgM) with a trend, early in the vaccination schedule, after the second immunization including EnvΔV2, towards broader anti-Env IgG epitope specificity and a higher ratio of neutralizing to Env-binding antibody titers. As the number of immunizations increased, vaccination with EnvΔV2 approached the immune response assessed after two inocula with the Tat/EnvΔV2 combined vaccine, even though some differences remained between groups, as indicated by anti-Env IgG epitope mapping. In fact, three weeks before challenge, plasma IgG of animals in the EnvΔV2 group showed a trend towards stronger specificity for the V1 loop and V5 loop-C5 regions of Env, whereas the Tat/EnvΔV2 group displayed an overall higher reactivity for epitopes within the Env V3 loop throughout the immunization period.Although differences in terms of protection rate were not found between the EnvΔV2 or Tat/EnvΔV2 vaccination groups in this pilot study, vaccination with Tat/EnvΔV2 appeared to accelerate the induction of potentially protective antibody responses to Env. In particular, antibodies to the Env V3 loop, whose levels at pre-challenge correlated with protection, were already higher early in the vaccination schedule in monkeys immunized with Tat/EnvΔV2 as compared to EnvΔV2 alone.Further studies including larger vaccination groups and fewer immunizations with these two vaccine candidates are needed to confirm these findings and to assess whether the Tat/EnvΔV2 vaccine may afford superior protection against infection.     

Enhanced immune sera and vaccine: safe approach to treat scorpion envenoming

Irradiation of Androctonus australis hector venom using a dose of 2 kGy has successfully abolished toxicity without reducing its antigenic or immunogenic properties. Toxicity of irradiated antigen was abolished until 20 times of LD₅₀ of native venom. Analysis of physiopathological effects induced by native and irradiated venoms was assessed by the analysis of tissue damage, immunohistochemistry and metabolical analysis in the organs (heart, lungs and liver). Immunological response of Aah venom using native or irradiated venom showed high titers of IgG1 in the plasma of immunized animals with native venom suggesting that Th2 cells were predominantly involved in the immune response. In the other hand, irradiated venom induced high titers of IgG2, indicating a predominantly Th1 type response. A protective effect of immunized mice with irradiated venom was evaluated. Immunized mice were protected from the toxic effects of native venom doses at one, three and six months after immunization. Mice were protected against a challenge of 4 LD₅₀ doses of native venom, one month after immunization. This protective effect was improved and effective at 3 and 6 months, all immunized mice were protected respectively against 6 and 10 LD₅₀ of native venom. At the one-month time point, the protective effect of mice was associated with high levels of antibodies in the plasma of immunized mice. However, despite the persistence of higher protection levels, the antibody titers decreased in a time-dependent manner. These results suggest that additional factors other than circulating antibodies provided the long-term protective activity produced by immunization with irradiated venom.     

DNA-mediated vaccination conferring protection against infectious bursal disease in broiler chickens in the presence of maternal antibody

The objective of the present study was to determine if a DNA vaccine carrying large segment gene of infectious bursal disease virus (IBDV) could confer protection against infectious bursal disease (IBD) in broiler chickens in the presence of maternal antibody. Broiler chickens with maternal antibody titers to IBDV were intramuscularly injected with a DNA plasmid coding for VP2, VP3, and VP4 genes of IBDV strain variant E (VE) (P/VP243/E) at 1-day, 1-week, and/or 2 weeks old. The dose of P/VP243/E used ranging from 400 μg to 10 mg. Broiler chickens at 3 weeks old were orally challenged with IBDV strain (VE) and observed for 10 days. Only broiler chickens vaccinated with 7.5 or 10 mg of P/VP243/E 3 times had 90 or 100% protection against challenge by IBDV strain VE and protected broiler chickens had significantly higher (P < 0.05) bursa weight/body weight (B/B) ratios, significantly lower (P < 0.05) bursal lesion scores, and the absence of IBDV antigens in bursae determined by immunofluorescent antibody assay (IFA). Antibody titers to IBDV as determined by enzyme-linked immunosorbent assay (ELISA) or virus neutralization (VN) assay in chickens of each group in each trial were gradually decreased prior to challenge. There was no significant difference (P > 0.05) in ELISA or VN titers to IBDV among all groups of broiler chickens or among the groups of broiler chickens vaccinated with various dose of P/VP243/E before challenge. Broiler chickens in the groups receiving 7.5 or 10 mg of P/VP243/E had significantly lower (P < 0.05) ELISA or VN titers to IBDV than those in the challenge control (CC) groups or the other groups vaccinated with various dose of P/VP243/E after challenge. Broiler chickens in the groups vaccinated with 10 mg of P/VP243/E 3 times had significantly higher (P < 0.05) stimulation indices for IBDV-stimulated lymphocyte proliferation response than those in the vector control (VC) or CC group at 14, 21, 24, or 31 days after first DNA vaccination. The results indicated that DNA vaccination with DNA encoding large segment gene of IBDV confers protection against challenge by IBDV in broiler chickens with maternal antibody to IBDV.     

Immune correlates of protection against yellow fever determined by passive immunization and challenge in the hamster model

Live, attenuated yellow fever (YF) 17D vaccine is highly efficacious but causes rare, serious adverse events resulting from active replication in the host and direct viral injury to vital organs. We recently reported development of a potentially safer β-propiolactone-inactivated whole virion YF vaccine (XRX-001), which was highly immunogenic in mice, hamsters, monkeys, and humans [10] and [11]. To characterize the protective efficacy of neutralizing antibodies stimulated by the inactivated vaccine, graded doses of serum from hamsters immunized with inactivated XRX-001 or live 17D vaccine were transferred to hamsters by the intraperitoneal (IP) route 24 h prior to virulent, viscerotropic YF virus challenge. Neutralizing antibody (PRNT₅₀) titers were determined in the sera of treated animals 4 h before challenge and 4 and 21 days after challenge. Neutralizing antibodies were shown to mediate protection. Animals having 50% plaque reduction neutralization test (PRNT₅₀) titers of ≥40 4 h before challenge were completely protected from disease as evidenced by viremia, liver enzyme elevation, and protection against illness (weight change) and death. Passive titers of 10-20 were partially protective. Immunization with the XRX-001 vaccine stimulated YF neutralizing antibodies that were equally effective (based on dose response) as antibodies stimulated by live 17D vaccine. The results will be useful in defining the level of seroprotection in clinical studies of new yellow fever vaccines.     

Cross-protection against influenza virus infection by intranasal administration of M1-based vaccine with chitosan as an adjuvant

The antigenic variation of influenza virus represents a major health problem, thus continuous efforts have been made to develop broad-spectrum vaccines against influenza virus. Matrix protein 1 (M1) protein is highly conserved in all influenza A strains. In this study, M1 protein was efficiently expressed in Escherichia coli (E. coli), then purified and used for immunization of BALB/c mice by intranasal drip using chitosan as adjuvant. The M1 protein was administered intranasally to mice in combination with chitosan adjuvant twice at an interval of 3 weeks. Three weeks after the second immunization, the mice were challenged with a lethal dose (5 × LD₅₀) of A/Chicken/Jiangsu/7/2002 (H9N2) virus, PR8 (H1N1) virus and A/Chicken/Henan/12/2004 (H5N1) virus. The protective immunity of the vaccine was evaluated by determining the survival rates, residual lung virus titers, bodyweight, and the serum antibody titers of the mice. The results showed that nasal administration of 100 μg M1 in combination with chitosan could not only completely protect the mice effectively against the challenge of the homologous virus but also protect 70% and 30% of the mice against the heterologous H1N1 and H5N1 viruses, respectively. The study indicated that the M1 protein was a candidate antigen for a broad-spectrum influenza virus vaccine and the adjuvant chitosan significantly improved the efficacy of the M1 vaccine.     

Development and characterization of non-glycosylated E and NS1 mutant viruses as a potential candidate vaccine for West Nile virus

West Nile virus is an arthropod-borne flavivirus that has caused substantial morbidity and mortality to animals as well as humans since its introduction in to the New York area in 1999. Given that there are no antiviral drugs available for treatment of the disease, vaccines provide an efficacious alternative to control this disease. Herein we describe an attenuated WNV strain developed by the ablation of the glycosylation sites in the envelope (E) and non-structural 1 (NS1) proteins. This E_154S/NS1_{130A/175A/207A} strain showed modest reduction in multiplication kinetics in cell culture and small plaque phenotype compared to the parental NY99 strain yet displayed greater than a 200,000-fold attenuation for mouse neuroinvasiveness compared to the parental strain. Mice infected with 1000 PFU of E_154S/NS1_{130A/175A/207A} showed undectable viremia at either two or three days post infection; nonetheless, high titer neutralizing antibodies were detected in mice inoculated with low doses of this virus and protected against lethal challenge with a 50% protective dose of 50 PFU.     

Long term immunity in African cattle vaccinated with a recombinant capripox-rinderpest virus vaccine

Cattle were vaccinated with a recombinant capripox-rinderpest vaccine designed to protect cattle from infection with either rinderpest virus (RPV) or lumpy skin disease virus (LSDV). Vaccination did not induce any adverse clinical responses or show evidence of transmission of the vaccine virus to in-contact control animals. Approximately 50% of the cattle were solidly protected from challenge with a lethal dose of virulent RPV 2 years after vaccination while at 3 years approx. 30% were fully protected. In the case of LSDV, all of 4 vaccinated cattle challenged with virulent LSDV at 2 years were completely protected from clinical disease while 2 of 5 vaccinated cattle were completely protected at 3 years. The recombinant vaccine showed no loss of potency when stored lyophylized at 4 degrees C for up to 1 year. These results indicate that capripoxvirus is a suitable vector for the development of safe, effective and stable recombinant vaccines for cattle.     

Chimeric alphavirus vaccine candidates protect mice from intranasal challenge with western equine encephalitis virus

We developed two types of chimeric Sindbis virus (SINV)/western equine encephalitis virus (WEEV) alphaviruses to investigate their potential use as live virus vaccines against WEE. The first-generation vaccine candidate, SIN/CO92, was derived from structural protein genes of WEEV strain CO92-1356, and two second-generation candidates were derived from WEEV strain McMillan. For both first- and second-generation vaccine candidates, the nonstructural protein genes were derived from SINV strain AR339. Second-generation vaccine candidates SIN/SIN/McM and SIN/EEE/McM included the envelope glycoprotein genes from WEEV strain McMillan; however, the amino-terminal half of the capsid, which encodes the RNA-binding domain, was derived from either SINV or eastern equine encephalitis virus (EEEV) strain FL93-939. All chimeric viruses replicated efficiently in mammalian and mosquito cell cultures and were highly attenuated in 6-week-old mice. Vaccinated mice developed little or no detectable disease and showed little or no evidence of challenge virus replication; however, all developed high titers of neutralizing antibodies. Upon intranasal challenge with high doses of virulent WEEV strains, mice vaccinated with ≥10⁵ PFU of SIN/CO92 or ≥10⁴ PFU of SIN/SIN/McM or SIN/EEE/McM were completely protected from disease. These findings support the potential use of these live-attenuated vaccine candidates as safe and effective vaccines against WEE.     

A protective chimeric antibody to tick-borne encephalitis virus

The efficiency of several mouse monoclonal antibodies (mAbs) specific to the tick-borne encephalitis virus (TBEV) glycoprotein E in post-exposure prophylaxis was assessed, and mAb14D5 was shown to be the most active of all those studied. It was proven that the hybridoma cell line 14D5 produced one immunoglobulin H chain and two L chains. They were used to construct chimeric antibodies ch14D5a and ch14D5b, the affinity constants of which were 2.6 × 10¹⁰ M⁻¹ and 1.0 × 10⁷ M⁻¹, respectively, according to the SPR-based ProteOn biosensor assay. The neutralization index (IC₅₀) of ch14D5a was 0.04 μg/ml in the focus reduction neutralization test. In in vivo experiments, ch14D5a at a dose of 10 μg/mouse resulted in a 100% survival of the mice infected with 240 LD₅₀ of TBEV. This chimeric antibody is promising for further development of prevention and therapeutic drugs against TBEV.     

Glycoprotein E2 of classical swine fever virus expressed by baculovirus induces the protective immune responses in rabbits

Classical swine fever (CSF) caused by CSF virus (CSFV) is a highly contagious and devastating disease that affects the pig industry worldwide. The glycoprotein E2 of CSFV is the principal immunogenic protein that induces neutralizing antibodies and protective immunity. Several CSFV genotypes, including 1.1, 2.1, 2.2, and 2.3, have been identified in Mainland China. The glycoprotein E2 of genotypes 1.1 and 2.1 was expressed by using a baculovirus system and tested for its protective immunity in rabbits to develop novel CSF vaccines that elicit a broad immune response. Twenty CSFV seronegative rabbits were randomly divided into five groups. Each rabbit was intramuscularly immunized with E2 of genotypes 1.1 (CSFV-1.1E2), 2.1 (CSFV-2.1E2), or their combination (CSFV-1.1 + 2.1E2). A commercial CSF vaccine (C-strain) and phosphate-buffered saline (PBS) were used as positive or negative controls, respectively. All animals were challenged with CSFV C-strain at 4 weeks and then boosted with the same dose. All rabbits inoculated with CSFV-1.1E2, CSFV-2.1E2, and CSFV-1.1 + 2.1E2 elicited high levels of ELISA antibody, neutralizing antibody, and lymphocyte proliferative responses to CSFV. The rabbits inoculated with CSFV-1.1E2 and CSFV-1.1 + 2.1E2 received complete protection against CSFV C-strain. Two of the four rabbits vaccinated with CSFV-2.1E2 were completely protected. These results demonstrate that CSFV-1.1E2 and CSFV-1.1 + 2.1E2 not only elicit humoral and cell-mediated immune responses but also confer complete protection against CSFV C-strain in rabbits. Therefore, CSFV-1.1E2 and CSFV-1.1 + 2.1E2 are promising candidate subunit vaccines against CSF.     

Comparative evaluation of vaccine efficacy of recombinant Marek's disease virus vaccine lacking Meq oncogene in commercial chickens

Marek's disease virus (MDV) oncogene meq has been identified as the gene involved in tumorigenesis in chickens. We have recently developed a Meq-null virus, rMd5ΔMeq, in which the oncogene meq was deleted. Vaccine efficacy experiments conducted in Avian Disease and Oncology Laboratory (ADOL) 15I₅ × 7₁ chickens vaccinated with rMd5ΔMeq virus or an ADOL preparation of CVI988/Rispens indicated that rMd5ΔMeq provided superior protection than CVI988/Rispens when challenged with the very virulent plus MDV 648A strain. In the present study we set to investigate the vaccine efficacy of rMd5ΔMeq in the field compared to several commercial preparations of CVI988/Rispens. Three large-scale field experiments, in which seeder chickens were inoculated with a very virulent plus strain of 686, vv+ MDV, were conducted in a model developed by Hy-Line International. In addition, comparisons were made with bivalent vaccine (HVT + SB-1), HVT alone and several serotype 3 HVT-vectored vaccines individually or in combination with CVI988/Rispens. Experimental results showed that addition of HVT to either of the two commercial CVI988/Rispens preparations tested (A or B) did not enhance protection conferred by CVI988/Rispens alone and that rMd5ΔMeq was a better or equal vaccine compared to any of the CVI988/Rispens vaccines tested under the conditions of the field trials presented herein. Our results also emphasized the complexity of factors affecting vaccine efficacy and the importance of challenge dose in protection.     

Protective immunity against foot-and-mouth disease virus induced by a recombinant vaccinia virus

We report the construction of a recombinant vaccinia virus expressing the precursor for the four structural proteins of FMD virus (FMDV) (P1) strain C3Arg85 using a procedure for isolation of recombinant vaccinia viruses based solely on plaque formation. Adult mice vaccinated with this recombinant vaccinia virus elicited high titers of neutralizing antibodies against both the homologous FMDV and vaccinia virus, measured by neutralization assays. Liquid phase blocking sandwich enzyme-linked immunosorbent assays (ELISAs) using whole virus as antigen showed high total antibody titers against homologous FMDV, similar to those induced by the conventional inactivated vaccine. When ELISAs were carried out with heterologous strains A79 or O1Caseros as antigens, sera from animals vaccinated with the recombinant virus cross-reacted. Mice boosted once with the recombinant vaccinia virus were protected against challenge with infectious homologous virus. These results indicate that recombinant vaccinia viruses are efficient immunogens against FMDV when used as a live vaccine in a mouse model.