Evaluation of Brucellosis Vaccines: A Comprehensive Review

Brucellosis is a bacterial zoonosis caused by Brucella spp. which can lead to heavy economic losses and severe human diseases. Thus, controlling brucellosis is very important. Due to humans easily gaining brucellosis from animals, animal brucellosis control programs can help the eradication of human brucellosis. There are two popular vaccines against animal brucellosis. Live attenuated Brucella abortus strain 19 (S19 vaccine) is the first effective and most extensively used vaccine for the prevention of brucellosis in cattle. Live attenuated Brucella melitensis strain Rev.1 (Rev.1 vaccine) is the most effective vaccine against caprine and ovine brucellosis. Although these two vaccines provide good immunity for animals against brucellosis, the expense of persistent serological responses is one of the main problems of both vaccines. The advantages and limitations of Brucella vaccines, especially new vaccine candidates, have been less studied. In addition, there is an urgent need for new strategies to control and eradicate this disease. Therefore, this narrative review aims to present an updated overview of the available different types of brucellosis vaccines.

Immunization with a combination of recombinant Brucella abortus proteins induces T helper immune response and confers protection against wild-type challenge in BALB/c mice

Protective efficiency of a combination of four recombinant Brucella abortus (B. abortus) proteins, namely, ribosomal protein L7/L12, outer membrane protein (OMP) 22, OMP25 and OMP31, was evaluated as a combined subunit vaccine (CSV) against B. abortus infection in RAW 264.7 cell line and murine model. Four proteins were cloned, expressed and purified, and their immunocompetence was analysed. BALB/c mice were immunized subcutaneously with single subunit vaccines (SSVs) or CSV. Cellular and humoral immune responses were determined by ELISA. Results of immunoreactivity showed that these four recombinant proteins reacted with Brucella‐positive serum individually but not with Brucella‐negative serum. A massive production of IFN‐γ and IL‐2 but low degree of IL‐10 was observed in mice immunized with SSVs or CSV. In addition, the titres of IgG2a were heightened compared with IgG1 in SSV‐ or CSV‐immunized mice, which indicated that SSVs and CSV induced a typical T‐helper‐1‐dominated immune response in vivo. Further investigation of the CSV showed a superior protective effect in mice against brucellosis. The protection level induced by CSV was significantly higher than that induced by SSVs, which was not significantly different compared with a group immunized with RB51. Collectively, these antigens of Brucella could be potential candidates to develop subunit vaccines, and the CSV used in this study could be a potential candidate therapy for the prevention of brucellosis.

Protection against virulent Brucella spp. by gamma-irradiated B. ovis in BALB/c mice model

Purpose

Brucella spp. is a zoonosis that causes undulant fever in humans and abortion in livestock worldwide. Lately, it was conveyed that vaccines developed by irradiation have induced a strong cellular and humoral immune response which have made these types of vaccines highly effective.

Materials and Methods

In this study, we aimed to use the gamma-irradiated B. ovis as a vaccine and to study the humoral immune response and cytokines production in order to evaluate it for protecting mice against B. abortus 544, B. melitensis 16M, and B. ovis.

Results

The humoral immune response in immunized mice with gamma-irradiated B. ovis showed a lasting for 8 weeks after immunization. Moreover, immunoglobulin G (IgG), IgG1, IgG2a, and IgG2b isotypes antibodies against B. ovis were observed after 4 and 8 weeks of the last immunization. It was noticed that the production of tumor necrosis factor-α, interferon-γ, and interleukin (IL)-10 continued after 4 and 8 weeks by splenocytes from immunized BALB/c mice, while no production of IL-4 or IL-5 was observed.

Conclusion

Our results indicate that the protection of BALB/c mice against B. melitensis 16M, B. abortus 544, and B. ovis was induced and the developed vaccine at our laboratory could stimulate similar protection to those induced by the traditional vaccine.

Immunogenicity and protective response induced by recombinant Brucella abortus proteins Adk, SecB and combination of these two recombinant proteins against a virulent strain B. abortus 544 infection in BALB/c mice

In this study, two recombinant proteins encoded by Brucella abortus genes Adk and SecB were evaluated as single subunit vaccine (SSV) as well as combined subunit vaccine (CSV) against B. abortus infection in BALB/c mice. These genes were cloned into pcold-TF expression system and recombinant proteins were expressed in Escherichia coli DH5α. The immunoreactivity of purified rAdk and rSecB was analyzed by immunoblotting showing that purified rAdk and rSecB as well as pcold-TF vector strongly reacted with Brucella-positive serum. Mice were immunized intraperitoneally with SSVs, CSV, pcold-TF, RB51 and PBS. The analysis of cytokine revealed that SSVs and CSV can strongly induce production of proinflammatory cytokines TNF and IL-6, suggesting that these subunit vaccines elicited innate immune response, particularly, activated antimicrobial mechanism of macrophages to limit the initial infection. On the other hand, immunization with SSVs and CSV elicited strong IFN-γ production and decreased IL-10 production compared to PBS group. The secretion profiles of IFN-γ and IL-10 together with an enhancement of blood CD4⁺ population and significantly induced specific IgG1 and IgG2a antibodies indicated that SSVs and CSV induced not only humoral immunity but also T helper 1 T cell immunity. Finally, spleen proliferation and bacterial burden in the spleen of mice vaccinated with these subunit vaccines were significantly lower than those of PBS group, which conferred significant protection against B. abortus infection. Altogether, the potential of these antigens of B. abortus could be prospective candidates to develop subunit vaccines against brucellosis.

The advances in brucellosis vaccines

Brucellosis is a worldwide zoonosis affecting animal and human health. Till now, there is no effective vaccine licensed for brucellosis in humans. Although M5, H38 and 45/20 vaccines were used to prevent animal brucellosis in the early stages, the currently used animal vaccines are S19, Rev.1, S2, RB51 and SR82. However, these vaccines still have several drawbacks such as residual virulence and interfering conventional serological tests. With the development of DNA recombination technologies and the completion of the sequence of Brucella genome, much research focuses on the search for potential safer and more effective vaccines. Preliminary studies have demonstrated that new vaccines, including genetically engineered attenuated vaccines, subunit vaccines and other potential vaccines, have higher levels of protection, but there are still some problems. In this paper, we briefly review the main vaccines that have been used in controlling the brucellosis for decades and the progress in the development of new brucellosis vaccines.

Interleukin 1 alpha (IL-1α) restricts Brucella abortus 544 survival through promoting lysosomal-mediated killing and NO production in macrophages

The interleukin-1 (IL-1) family of cytokines, particularly IL-1α and IL-1β, are potent regulators of innate immunity that play key roles in host defense against infection, hence we evaluated the role of these cytokines in the control of brucellosis within RAW 264.7 cells. Marked expression and secretion of IL-1α and IL-1β were observed during Brucella infection in macrophages. Blocking of IL-1α and IL-1β reduced induction of IL-10, IL-1β and TNF, and IL-6 and TNF, respectively. However, interference of IL-1α and not IL-1β signaling notably augmented susceptibility of macrophages to Brucella infection which indicates that IL-1α is required for a downstream signaling cascade of innate immunity for efficient clearance of Brucella. This protection requires binding to interleukin-1 receptor (IL-1R) mediated by myeloid differentiation factor 88 (MyD88) signaling and associated with increased lysosomal-mediated killing and nitric oxide (NO) production. Expression of pro-inflammatory cytokines was observed to be mediated via NF-κB-p50, HIF-1α and CEBPA, but negatively controlled by CEBPB while transcription of some important phagolysosomal genes was regulated via CEBPA and c-Jun which indicates the important role of these transcription factors in the control of Brucella infection in macrophages via IL-1α signaling pathway.

Induction of Th2-related immune responses and production of systemic IgA in mice intranasally immunized with Brucella abortus malate dehydrogenase loaded chitosan nanoparticles

The aim of this study was to investigate the induction of mucosal immune responses by an important Brucella abortus antigen, malate dehydrogenase (Mdh), loaded in mucoadhesive chitosan nanoparticles (CNs) and immunized intranasally in a BALB/c mouse model. The production of cytokines was investigated in human leukemic monocyte cells (THP-1 cells) after stimulation with the nanoparticles. Mdh-loaded CNs (CNs-Mdh) induced higher interleukin (IL)-6 production than unloaded antigens and TF loaded CNs (CNs-TF). Using ELISpot to quantify cytokines and antibody-secreting cells in the intranasally immunized mice, IL-4 and IgG-secreting cells were found to be significantly increased at 4 weeks and 6 weeks post-immunization in the CNs-Mdh immunized group, respectively. Increases in Mdh-specific IgG, IgG1, and IgG2a antibodies were confirmed at 6 weeks after immunization, indicating a predominant IgG1 response. Analysis of the mucosal immune response in the intranasally immunized mice revealed, Mdh-specific IgA and total IgA in the nasal washes, genital secretions, fecal extracts and sera that were remarkably increased in the CNs-Mdh-immunized group compared to the CNs-TF-immunized group except total IgA of nasal wash. Therefore, the results indicated that the intranasal immunization of CNs-loaded B. abortus Mdh antigen effectively induced antigen-specific mucosal immune responses through the elicitation of Th2-related immune responses.

Comparative Analysis of Immune Responses to Outer Membrane Antigens OMP10, OMP19, and OMP28 of Brucella abortus

Brucella infection is accompanied by cytokine production, which serves as an important factor to evaluate the innate and adaptive immune responses. Several researchers have been investigating the mechanisms involved in Brucella infection in the host. Here, we conducted an analytical study to define pathogenic pathways and immune mechanisms involved in Brucella infection by investigating the antigenic efficacy of recombinant outer membrane protein 10 (rOMP10), outer membrane protein 19 (rOMP19), and outer membrane protein 28 (rOMP28) in vitro and in vivo upon stimulation/immunization. Cytokine production was analyzed by nitric oxide (NO) assay and enzyme-linked immunosorbent assay (ELISA) after stimulation of RAW 264.7 cells and naive splenocytes with the recombinant proteins. Our results show that levels of NO, tumor necrosis factor (TNF)-α, and interleukin (IL)-6 increased in RAW 264.7 cells in a time-dependent manner following recombinant protein stimulation. In contrast, levels of interferon (IFN)-γ and IL-2 increased in naive splenocytes after stimulation with proteins. ELISA and ELISpot assays were performed after immunization of mice with recombinant proteins. rOMP28 greatly increased IFN-γ, IL-2, and TNF-α levels than IL-4 and IL-6 levels in vitro. Of the recombinant proteins, rOMP19 elicited a mixed Th1/Th2 immune response by increasing the number of IgG-secreting cells in vivo.

Immunization of BALB/c mice with a combination of four recombinant Brucella abortus proteins, AspC, Dps, InpB and Ndk, confers a marked protection against a virulent strain of Brucella abortus

In this study, we assessed the protective efficacy of single subunit vaccines, encoded by the B. abortus 544 genes aspC, dps, yaeC and inpB, against B. abortus infection in mice. First, immunization with these antigens, with the exception of the YaeC protein, was found to elicit both humoral and cellular immune responses with IgG2a being dominant over IgG1. In addition, a massive production of IFN-γ but lower degree of IL-10 was observed, suggesting that all three antigens were able to induce predominantly cell-mediated immunity in response to B. abortus infection. Further investigation of a combined subunit vaccine (CSV) consisting of purified AspC, Dps, InpB and Ndk proteins showed a superior protective effect in mice against brucellosis. The intraperitoneal injection of this combination was shown to induce a remarkable production of IFN-γ and IL-2, which occurred in conjunction with an increase of blood CD4⁺ and CD8⁺ T cell proportions. In addition, the higher titer of IgG2a compared to IgG1 elicited by this CSV was obtained, suggesting that this CSV induced a typical T-helper-1-dominated immune response in vivo. Furthermore, the protection level induced by this combination was significantly higher than that induced by single antigens and was not significantly different compared to a group immunized with a live attenuated vaccine (RB51). Altogether, our findings suggest that the combination of different immunogenic antigens could be a useful approach for the development of a new, effective and safe brucellosis vaccine that can replace current vaccine strains.

Interleukin 10 suppresses lysosome-mediated killing of Brucella abortus in cultured macrophages

Brucella abortus is a Gram-negative zoonotic pathogen for which there is no 100% effective vaccine. Phagosomes in B. abortus-infected cells fail to mature, allowing the pathogen to survive and proliferate. Interleukin 10 (IL10) promotes B. abortus persistence in macrophages by mechanisms that are not fully understood. In this study, we investigated the regulatory role of IL10 in the immune response to B. abortus infection. B. abortus-infected macrophages were treated with either IL10 siRNA or recombinant IL10 (rIL10), and the expression of phagolysosome- or inflammation-related genes was evaluated by qRT-PCR and Western blotting. Phagolysosome fusion was monitored by fluorescence microscopy. We found that the synthesis of several membrane-trafficking regulators and lysosomal enzymes was suppressed by IL10 during infection, resulting in a significant increase in the recruitment of hydrolytic enzymes by Brucella-containing phagosomes (BCPs) when IL10 signaling was blocked. Moreover, blocking IL10 signaling also enhanced proinflammatory cytokine production. Finally, concomitant treatment with STAT3 siRNA significantly reduced the suppression of proinflammatory brucellacidal activity but not phagolysosome fusion by rIL10. Thus, our data provide the first evidence that clearly indicates the suppressive role of IL10 on phagolysosome fusion and inflammation in response to B. abortus infection through two distinct mechanisms, STAT3-independent and -dependent pathways, respectively, in murine macrophages.

Lipocalin 2 (Lcn2) interferes with iron uptake by Brucella abortus and dampens immunoregulation during infection of RAW 264.7 macrophages

Lipocalin 2 (Lcn2) is an important innate immunity component against bacterial pathogens. In this study, we report that Lcn2 is induced by Brucella (B.) abortus infection and significantly contributes to the restriction of intracellular survival of Brucella in macrophages. We found that Lcn2 prevented iron uptake by B. abortus through two distinct mechanisms. First, Lcn2 is secreted to capture bacterial siderophore(s) and abrogate iron import by Brucella. Second, Lcn2 decreases the intracellular iron levels during Brucella infection, which probably deprives the invading Brucella of the iron source needed for growth. Suppression of Lcn2 signalling resulted in a marked induction of anti-inflammatory cytokine, interleukin 10, which was shown to play a major role in Lcn2-induced antibrucella immunity. Similarly, interleukin 6 was also found to be increased when Lcn2 signalling is abrogated; however, this induction was thought to be an alternative pathway that rescues the cell from infection when the effective Lnc2 pathway is repressed. Furthermore, Lcn2 deficiency also caused a marked decrease in brucellacidal effectors, such as reactive oxygen species and nitric oxide but not the phagolysosome fusion. Taken together, our results indicate that Lcn2 is required for the efficient restriction of intracellular B. abortus growth that is through limiting iron acquisition and shifting cells to pro-inflammatory brucellacidal activity in murine macrophages.

Simultaneous RNA-seq based transcriptional profiling of intracellular Brucella abortus and B. abortus-infected murine macrophages

Brucella is a zoonotic pathogen that survives within macrophages; however the replicative mechanisms involved are not fully understood. We describe the isolation of sufficient Brucella abortus RNA from primary host cell environment using modified reported methods for RNA-seq analysis, and simultaneously characterize the transcriptional profiles of intracellular B. abortus and bone marrow-derived macrophages (BMM) from BALB/c mice at 24 h (replicative phase) post-infection. Our results revealed that 25.12% (801/3190) and 16.16% (515/3190) of the total B. abortus genes were up-regulated and down-regulated at >2-fold, respectively as compared to the free-living B. abortus. Among >5-fold differentially expressed genes, the up-regulated genes are mostly involved in DNA, RNA manipulations as well as protein biosynthesis and secretion while the down-regulated genes are mainly involved in energy production and metabolism. On the other hand, the host responses during B. abortus infection revealed that 14.01% (6071/43,346) of BMM genes were reproducibly transcribed at >5-fold during infection. Transcription of cytokines, chemokines and transcriptional factors, such as tumor necrosis factor (Tnf), interleukin-1α (Il1α), interleukin-1β (Il1β), interleukin-6 (Il6), interleukin-12 (Il12), chemokine C-X-C motif (CXCL) family, nuclear factor kappa B (Nf-κb), signal transducer and activator of transcription 1 (Stat1), that may contribute to host defense were markedly induced while transcription of various genes involved in cell proliferation and metabolism were suppressed upon B. abortus infection. In conclusion, these data suggest that Brucella modulates gene expression in hostile intracellular environment while simultaneously alters the host pathways that may lead to the pathogen's intracellular survival and infection.

Activation of NF-kB-Mediated TNF-Induced Antimicrobial Immunity Is Required for the Efficient Brucella abortus Clearance in RAW 264.7 Cells

In this study, we explore the regulatory roles of pro-inflammatory cytokine tumor necrosis factor alpha (TNF) in the innate immunity of macrophages against B. abortus infection. We show that infection of macrophage with B. abortus induces marked expression and secretion of TNF which subsequently binds to TNF receptor 1 (TNFR-1) and activates a downstream signaling cascade of the innate immunity. Blocking of TNF signaling resulted in a notable increase of B. abortus survival which was associated with an increase of anti-inflammatory cytokine interleukin 10 (IL-10), a beneficial effector of Brucella survival, as well as remarkable decrease of reactive oxygen species (ROS) and nitric oxide (NO), antibrucella molecules. However, surprisingly, the interference of TNF did not show any influence on phagolysosome and cell death events. Furthermore, the transcriptional factor NF-kB was found to be a main mediator of TNF signaling when blocking of NF-kB pathway drastically suppressed the TNF-induced brucellacidal effect. Taken together, these findings clearly indicate that the immune cascade activated by TNF/TNFR-1 is required for the sufficient resistance to B. abortus survival in macrophages.

Proteomics Analysis of Three Different Strains of Mycobacterium tuberculosis under In vitro Hypoxia and Evaluation of Hypoxia Associated Antigen's Specific Memory T Cells in Healthy Household Contacts

In vitro mimicking conditions are thought to reflect the environment experienced by Mycobacterium tuberculosis inside the host granuloma. The majority of in vitro dormancy experimental models use laboratory-adapted strains H37Rv or Erdman instead of prevalent clinical strains involved during disease outbreaks. Thus, we included the most prevalent clinical strains (S7 and S10) of M. tuberculosis from south India in addition to H37Rv for our in vitro oxygen depletion (hypoxia) experimental model. Cytosolic proteins were prepared from hypoxic cultures, resolved by two-dimensional electrophoresis and protein spots were characterized by mass spectrometry. In total, 49 spots were characterized as over-expressed or newly emergent between the three strains. Two antigens (ESAT-6, Lpd) out of the 49 characterized spots were readily available in recombinant form in our lab. Hence, these two genes were overexpressed, purified and used for in vitro stimulation of whole blood collected from healthy household contacts (HHC) and active pulmonary tuberculosis patients (PTB). Multicolor flow cytometry analysis showed high levels of antigen specific CD4(+) central memory T cells in the circulation of HHC compared to PTB (p < 0.005 for ESAT-6 and p < 0.0005 for Lpd). This shows proteins that are predicted to be up regulated during in vitro hypoxia in most prevalent clinical strains would indicate possible potential immunogens. In vitro hypoxia experiments with most prevalent clinical strains would also elucidate the probable true representative antigens involved in adaptive mechanisms.