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Introduction
Brucellosis is a severe and acute febrile disease caused by infection with a Brucella species, a gram-negative bacterium of the genus Brucella [521]. It remains one of the important worldwide zoonotic diseases [3]. In 1955, bovine brucellosis was reported among dairy cattle imported to the Republic of Korea [1725]. Thereafter, a total of 85,521 Brucella reactor animals have been identified in 14,215 outbreaks between 2001 and 2011. The numbers of brucellosis cases at both the individual animal and farm levels increased after 2003, peaked in 2006, and slightly decreased thereafter because of implementation of a ‘test-and-slaughter’ policy [1623].
It is important to control brucellosis in animal population because humans can be directly or indirectly infected by infected animals; thus immunization against Brucella in animals has a critical role in human health. Brucella abortus S19 and Brucella melitensis Rev. 1 vaccines have been widely used in developed countries. However, those vaccines have induced abortions in pregnant animals [1822]. Moreover, they elicit anti-Brucella antibodies that interfere with serodiagnosis [13].
Antigen delivery systems become necessary when antigens are inefficiently transported to appropriate sites or presented to the immune system [20]. Brucella protective immunogens can be delivered to critical immunological sites by using a Salmonella-based vector. Therefore, an intracellular pathogen, such as an attenuated Salmonella strain, can be licensed as a vector to deliver Brucella antigens. As Salmonella can produce infection in a manner similar to that of Brucella, it is a pragmatic choice for an anti-Brucella vaccine-delivery platform. The attenuated Salmonella Typhimurium delivery-based Brucella vaccine used in this study was previously developed and has proven to be a suitably vectored Brucella vaccine when applied through different routes of immunization in mice [11]. That vaccine's protective efficiency was improved by combining four heterologous Brucella antigens: Brucella lumazine synthase (BLS), proline racemase subunit A (PrpA), outer-membrane protein 19 (Omp19), and Cu/Zn superoxide dismutase (SOD) proteins [9]. However, a livestock vaccine trial to show the actual response of the natural host to the Brucella vaccine delivered by an attenuated Salmonella vector has not been reported. Although large-animal experimental trials are uncommon because of limited resources, they are important for characterization of the equivalent immune responses in livestock hosts (natural host) in order to consider the commercial use of a developed vaccine. Therefore, this study reports on the use of a novel Salmonella system delivering four Brucella antigens (SOD, BLS, PrpA, and Omp19) to determine whether combinations of individual vaccines can efficiently regulate brucellosis in goats.
Materials and Methods
Ethics and biosafety statement
All animal experimental procedures were approved (CBNU 2016-98) by the Chonbuk National University Animal Ethics Committee in accordance with the guidelines of the Korean Council on Animal Care and Korean Animal Protection Law (2007, Article 13: Experiments with animals). All goats used in the study were housed and maintained humanely. The biosafety level-3 organism B. abortus strain 544 was handled with the required safety precautions and under the supervision of the Ministry of Health & Welfare, South Korea.
Bacteria strains, plasmids, and construction of vaccine
B. abortus strain 544 (ATCC23448) was used as the virulent challenge strain [712]. The bacterial strains, plasmids, and primers used in this study are listed in Table 1. Construction and validation of Salmonella Typhimurium strains expressing Brucella immunogenic frames were undertaken as previously described [8911]. The challenge strain (B. abortus strain 544) was prepared for the challenge experiment by brief culture in Brucella broth (Becton, Dickinson and Company, USA) at 37℃ for 24 h and then resuspended to approximately 5 × 108 CFU/mL. Goats were conjunctively challenged with 100 µL of the challenge strain in saline.
Immunization and challenge of B. abortus biotype 1
A total of 10-month-old goats (n = 18) that were seronegative for brucellosis based on a Rose-Bengal plate agglutination test were used in the study. The animals were divided into 4 groups, fasted for 24 h, and immunized according to the scheme in Table 2. Upon vaccination, animals were monitored for immunization-induced morbidity or mortality and underwent immunological profiling. The negative control (NC) group (n = 3) was subcutaneously (SC) inoculated with 0.1 mL of saline. Group A (n = 5), as the vector control group, was inoculated SC with approximately 5 × 109 colony-forming units (CFU/mL) of Salmonella Typhimurium delivery strain containing pJHL65 only in 1 mL. Group B (n = 5) and Group C (n = 5) were immunized SC using the Salmonella Typhimurium system with inoculations of approximately 5 × 109 CFU/mL and 5 × 1010 CFU/mL, respectively, of mixtures containing four different delivery strains in 1 mL. At 6 weeks post-immunization, Groups A, B, and C were conjunctively challenged with goat-passaged virulent B. abortus strain 544 at a dosage of 5 × 108 CFU/mL in 100 µL (50 µL of inoculum per eye). Animals were kept in a restricted large-animal isolation facility with no contact between groups and were observed daily.
Humoral immune responses of the vaccinated goats
To determine the level of anti-Brucella antibodies generated, the humoral response of the control and immunized goats were investigated by performing goat immunoglobulin G (IgG) enzyme-linked immunosorbent assay (ELISA). Serum samples were collected at 3-week intervals after immunization. A standard ELISA was carried out in serum to evaluate the immune response against the BLS, PrpA, Omp19, and SOD antigens in goats according to the modified method previously reported [6]. Briefly, 96-well microtiter plates (Nunc, Denmark) were coated overnight with pre-titrated recombinant BLS (5 µg/mL), Omp19 (5 µg/mL), PrpA (5 µg/mL), or SOD (5 µg/mL) proteins in phosphate-buffered saline (PBS), blocked for 30 min using PBS containing 1% bovine serum albumin (diluent; 200 µL/well), and washed with PBS containing 0.05% Tween-20. Serum samples were diluted at 1:100 in diluent. The plate was treated with horseradish peroxidase (HRP)-conjugated rabbit anti-goat IgG-Fc detection antibody (1:150,000; Bethyl Lab, USA). For IgG samples, colorimetric changes resulting from the action of HRP on o-phenylenediamine (Sigma-Aldrich, USA) were measured through an automated ELISA spectrophotometer (Multiskan GO; Thermo Fisher Scientific, Finland) at 492 nm, 10 min after development. The values for binding of IgG to the respective antigens in each group were expressed as the mean optical density (OD) ± SE.
Interferon (IFN)-γ quantification by goat IFN-γ ELISA kit
Measurement of IFN-γ from peripheral blood mononuclear cells (PBMCs) was done at 6 weeks post-immunization. PBMCs were prepared by modifying a previously described method [4]. PBMCs were prepared using Ficoll sodium diatrizoate gradient (Sigma-Aldrich), washed twice with PBS and resuspended in RPMI 1640 medium to a level of 2.5 × 106 viable cells per milliliter as determined by trypan blue dye exclusion. The PBMCs (5 × 105 cells/well) were stimulated in vitro with BLS, PrpA, SOD, or Omp19 antigen (4 µg/well) for 24 h. The supernatants were collected and used for cytokine measurement as previously described [19]. Antigen-specific induction of IFN-γ was measured by using an IFN-γ ELISA kit with biotin-conjugated anti-IFN-γ primary antibody. HRP-avidin (1:1,000; Cusabio Biotech, China) was used to bind the primary antibody. The OD of each well was measured by using a microplate reader set to 450 nm.
Bacterial culture
At 8 weeks post-infection, the experimental animals were humanely sacrificed, and tissues from each animal were freshly collected. Sampled tissues included spleen, liver, lung, kidney, heart, testis, epididymis, mandibular lymph node (LN), parotid LN, retropharyngeal LN, superficial cervical LN, bronchial LN, portal LN, and mesenteric LN. Each sample (5 g) was collected aseptically in 5 mL of PBS and immediately homogenized with a sterile wooden applicator. Inoculum (200 µL) was seeded and spread on Brucella agar media containing an antibiotic supplement and incubated. Cultures were examined daily for presence of colonies and counted after incubation for 7 days at 37℃ in a 5% CO2 atmosphere.
DNA extraction and relative real-time polymerase chain reaction (PCR) measurement of B. abortus antigens in tissue samples
DNA was extracted from tissue by using the GeneAll genomic DNA (gDNA) extraction kit (Seoul, Korea) according to the manufacturer's recommendation. The total gDNA concentration and purity (A260/A280) were measured by using an e-spect Malcom spectrophotometer (ES-2 model; Malcom, Japan) at a 250 nm wavelength and using the Nanodrop method. An A260/A280 absorbance ratio of 1.8 to 2.0 was regarded as indicating pure gDNA. A 20 µL reaction mixture containing 2 µL gDNA was used as a template in the assay, which used 500 nmol B. abortus-specific primers. Real-time PCR conditions were set as follows: denaturation and polymerase activation step at 50℃ for 2 min and 95℃ for 10 min, followed by 40 cycles of 95℃ for 15 sec and 60℃ for 1 min, concluding with derivation of dissociation curves on a CFX96 real-time PCR detection system (Bio-Rad Laboratories, USA) using SYBR green I as the double-strand DNA-specific binding dye. After reaction completion, specificity was verified by performing melting curve analysis. Quantification was performed by comparing cycle threshold (Ct) values of each sample with that of glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Sequences of the real-time PCR primers are summarized in Table 3. The oligonucleotide primers used in the study were purchased from Bioneer (Korea).
Microscopic observation of tissue sections
Tissues in 10% neutral buffered formalin were routinely processed by using a Shandon Citadel 1000 tissue processor and were embedded in paraffin. Tissue sections (5 µm) were obtained by using a microtome (HM-340E; Thermo Fisher Scientific, USA) and placed on glass slides. Hematoxylin and eosin staining was performed according to standard techniques [2]. A microphotograph was taken by using an Olympus microscope (BX53F; Olympus, Japan) and digital imaging software (Olympus). Goat liver sections were examined for hepatic microgranulomas by counting the number of foci in 10 microscopic fields in each tissue sample at 100× magnification.
Results
Brucella antigen-specific humoral immune responses increased in vaccinated goats
The systemic humoral immune responses elicited by the combined vaccine strains were investigated. As shown in Fig. 1, all immunized goats (Groups B and C) developed serum IgG antibodies production against the individual antigens, including Brucella BLS, Brucella SOD, Brucella Omp19, and Brucella PrpA. Serum IgG titers against the individual antigens in Group C were significantly higher than those in the non-immunized NC group and vector control Group A at 3 and 6 weeks after the vaccine injection. Individual antigen-specific serum IgG titers in Group B were also significantly higher than those of the non-immunized NC group and vector control Group A at 6 weeks after vaccine injection, but only the SOD and Omp19 antigen-specific IgG levels were significantly increased at 3 weeks after vaccine injection. However, relative serum IgG titers against all antigens in Group C were not statistically different from those of Group B at 3 and 6 weeks after the vaccine injection. There were no statistical differences between the non-immunized NC group and the vector control Group A. These results indicate that treatment by our combination vaccine at each of the two dosages tested can fully induce humoral immune responses targeting individual Brucella antigens at 6 weeks post-vaccination.
Treatment with the combination vaccine increased the secretion of IFN-γ from PBMCs
Antigen-specific induction of IFN-γ production from PBMCs stimulated with individual antigens (BLS, Omp19, PrpA, or SOD) was measured by using an ELISA at 6 weeks post-vaccination. As shown in Fig. 2, significantly higher IFN-γ production occurred in PBMCs of goats in Groups B and C than in the non-immunized NC group and vector control Group A (p < 0.05). Although levels of IFN-γ were higher in Group C than in Group B after stimulation with the individual antigens, no significant differences were noted between those two groups. These results indicate that treatment with our combination vaccine at two different dosages can fully induce cell-mediated immune responses against individual Brucella antigens at 6 weeks after immunization.
Salmonella delivery vaccine immunization protected against the challenge of B. abortus
The protective efficacy of immunization with the Salmonella Typhimurium-based Brucella antigen vaccine was investigated in a goat model. At 8 weeks after infection, the magnitude of the challenge bacteria load in the spleen, heart, kidney, liver, lung, testis, epididymis, parotid LN, portal LN, retropharyngeal LN, superficial cervical LN, mandibular LN, mesenteric LN, and bronchial LN reflected the efficacy of the immunization. Compared to the Group A vector control goats, the immunized goats showed significantly higher protection and a lower bacterial load in most of the above-mentioned tissues of goats challenged with B. abortus virulent strain 544 (Fig. 3). Generally, the degree of protection in the Group C goats was higher than that in the Group B goats, but the bacteria load in Group B was still lower in most of the collected tissue types than that of Group A. As expected the non-infected NC group showed no bacterial loads.
Relative real-time PCR technique was used to compare the presence of B. abortus antigens in all collected samples. As shown in Fig. 4, expression of B. abortus antigens was lower in the spleen, liver, kidney, and lung of Groups B and C than those of Group A, although no significant difference was noted between Groups B, C, and the NC group. Such a pattern of decreased antigen expression in the vaccine-treated groups was not observed in the other collected tissues.
Treatment with combination vaccine ameliorated histopathological lesions induced by B. abortus strain 544 infection
At 8 weeks after infection with B. abortus strain 544, aggregations of inflammatory cells that consisted predominantly of lymphocytes (microgranulomas) in the liver and distinctly visible trabeculae in the spleen were microscopically observed in the examined goats (panel A in Fig. 5). As shown in panel B in Fig. 5, the incidence of microgranuloma foci lesions was significantly low in the livers of goats in Groups B and C. Levels of distinct visibility of trabeculae in the spleen were lower in Groups B and C than that in Group A, indicating lower septicemia associated with the vaccination. These results suggest that the combination vaccine treatment ameliorated B. abortus infection-related lesions.
Discussion
Immunization of animals and humans has been successfully practiced as a means to control infectious diseases for centuries [7]. In order to avoid the residual virulence of Brucella live vaccines, an alternative approach of delivering Brucella subunit components via live vaccine vectors has been used. Safety is the major concern when using attenuated live vaccine vectors delivering Brucella antigens compared to the use of live attenuated Brucella vaccine. Attenuated Salmonella Typhimurium live vector strains delivering various heterogeneous antigens to the immune system can be used as a combination vaccine vehicle against a variety of targeted pathogens at a relatively low cost [10].
The success of immunization with a mixture of antigens using attenuated Salmonella Typhimurium live vaccine was anticipated based on the results of previous studies. However, the immune response in livestock was based much on the precept that live attenuated Salmonella vaccine expressing recombinant rBL protein can induce significant protective effects in a mouse model [27]. In this study, the attenuated Salmonella Typhimurium Δlon, ΔcpxR, and Δasd bacterial delivery vector was used with the pBP65 plasmid encoding the asd gene to deliver the Brucella antigens at two different dosages. The goats inoculated with 5 × 1010 CFU/mL (Group C) showed significantly increased serum IgG titers against all antigens when compared to the vector control group. However, the lower inoculant concentration (5 × 109 CFU/mL) should not be overlooked, as it also produced higher serum IgG titers than that in Group A. Humoral response results of the two inoculation dosages showed the effectiveness of delivering antigens by the attenuated Salmonella Typhimurium vaccine. Furthermore, IFN-γ concentration was evaluated in supernatants of PBMCs following re-stimulation with heat-inactivated B. abortus antigens of goats immunized with vaccine candidates. The levels of IFN-γ were significantly elevated in both Group C (5 × 1010 CFU/mL) and Group B (5 × 109 CFU/mL) over that in Group A. The results suggest that a Th1-type immune response is strongly induced by both inoculation dosages. It seems logical to favor the higher inoculation for sufficient protection, but the lower inoculation dose also produces a significant immune response. In addition, economics must be considered because the lower dosage (5 × 109 CFU/mL) may offer a cost-effective alternative for the production of effective vaccines against brucellosis.
To examine cross-reactivity, Brucella spp. have been identified by performing amplification of a specific region of its genome, as has been done for other closely related bacteria [1419]. The IS711 region of the Brucella genome has been used to identify B. abortus, B. melitensis, and Brucella suis biovar 1. Similar primers and probes exploiting the IS711 repetitive element have been used for real-time detection in some studies [1415]. This study explored using real-time PCR and SYBR green I, a double-stranded DNA intercalating dye [14]. Real-time PCR was used as a confirmatory diagnostic alternative to problematic culturing of Brucella spp. The high detection sensitivity of real-time PCR can be explained by the fact that it detects DNA from bacteria that are damaged or nonviable and which therefore cannot be isolated by applying conventional culture techniques. In addition, relative real-time PCR can indicate which tissues have higher/lower levels of bacterial expression. Our results showed that the relative expression of B. abortus antigens was more decreased in spleen, kidney, and lung of immunized goats than in the non-immunized group. However, in contrast to the bacteria load recovery results, such a decrease in antigen expression in the vaccine groups was not observed in other collected tissues, especially not in the various LNs that were examined. Based on these results, we speculate that many nonviable Brucella in the LNs of immunized goats were measured by real-time PCR, since macrophages phagocytose antigens and then migrate to draining LNs. Therefore, we believe that the challenge strain-tissue recovery model is a gold standard for evaluating the efficacy of our combination vaccine, and that results obtained by using real-time PCR should be used for reference purposes only.
In this study, the experimental goats did not show any gross lesions, including splenomegaly, when necropsy was performed, thus indicating that severe brucellosis did not develop as a result of the experimental infection. Even though few gross lesions were observed, distinctly visible trabeculae was microscopically evident in the spleen of non-immunized goats after Brucella infection. This suggests that the spleen can respond to septicemia through active hyperemia [26], but the hyperactive spleen may not always be enlarged [8]. In addition, the experimentally infected goats revealed aggregations of inflammatory cells (microgranulomas), and fewer neutrophils were observed in the liver. The recruitment of lymphocytes in the liver was evident based on the formation of microgranulomas, supporting the observations in a previous study that showed an inflammatory response in the liver of mice [24]. These two microscopic changes after Brucella infection were decreased in our vaccine-treated groups, indicating that such histopathological parameters can be used for evaluating the severity of brucellosis and the efficacy of a vaccine in goat.
In conclusion, we have shown that SC vaccination with an attenuated Salmonella Typhimurium live vector strain delivering a mixture of four Brucella immunogens (BLS, PrpA, Omp19, and SOD) can induce a protective immune response against infection by B. abortus. It was apparent that vaccination at the higher dose (Group C) was more efficacious than that at the lower dose (Group B); regardless, vaccination at the lower dose also had a significant protective effect. It is likely that a Salmonella-based delivery system could be adapted to develop multivalent recombinant Salmonella vaccines against brucellosis. Further work is needed to compare the efficacy of our combination vaccine with that of vaccines already in use, such as RB51.
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