Outer Membrane Protein 25 of Brucella Activates Mitogen-Activated Protein Kinase Signal Pathway in Human Trophoblast Cells

Evasion of host defense by Brucella

Brucella , an adept intracellular pathogen, causes brucellosis, a zoonotic disease leading to significant global impacts on animal welfare and the economy. Regrettably, there is currently no approved and effective vaccine for human use. The ability of Brucella to evade host defenses is essential for establishing chronic infection and ensuring stable intracellular growth. Brucella employs various mechanisms to evade and undermine the innate and adaptive immune responses of the host through modulating the activation of pattern recognition receptors (PRRs), inflammatory responses, or the activation of immune cells like dendritic cells (DCs) to inhibit antigen presentation. Moreover, it regulates multiple cellular processes such as apoptosis, pyroptosis, and autophagy to establish persistent infection within host cells. This review summarizes the recently discovered mechanisms employed by Brucella to subvert host immune responses and research progress on vaccines, with the aim of advancing our understanding of brucellosis and facilitating the development of more effective vaccines and therapeutic approaches against Brucella .

Immune Responses Potentially Involved in the Gestational Complications of Brucella Infection

Infection by Brucella species in pregnant animals and humans is associated with an increased risk of abortion, preterm birth, and transmission of the infection to the offspring. The pathogen has a marked tropism for the placenta and the pregnant uterus and has the ability to invade and replicate within cells of the maternal-fetal unit, including trophoblasts and decidual cells. Placentitis is a common finding in infected pregnant animals. Several proinflammatory factors have been found to be increased in both the placenta of Brucella-infected animals and in trophoblasts or decidual cells infected in vitro. As normal pregnancies require an anti-inflammatory placental environment during most of the gestational period, Brucella-induced placentitis is thought to be associated with the obstetric complications of brucellosis. A few studies suggest that the blockade of proinflammatory factors may prevent abortion in these cases.

Outer membrane protein 25 of Brucella suppresses TLR-mediated expression of proinflammatory cytokines through degradation of TLRs and adaptor proteins

Toll-like receptors (TLRs) are essential components of innate immunity that serves as the first line of defense against the invaded microorganisms. However, successful infectious pathogens subvert TLR signaling to suppress the activation of innate and adaptive responses. Brucella species are infectious intracellular bacterial pathogens causing the worldwide zoonotic disease, brucellosis, that impacts economic growth of many countries. Brucella species are considered as stealthy bacterial pathogens as they efficiently evade or suppress host innate and adaptive immune responses for their chronic persistence. However, the bacterial effectors and their host targets for modulating the immune responses remain obscure. Brucella encodes various outer membrane proteins (Omps) that facilitate their invasion, intracellular replication, and immunomodulation. Outer membrane protein 25 (Omp25) of Brucella plays an important role in the immune modulation through suppression of proinflammatory cytokines. However, the mechanism and the signaling pathways that are targeted by Omp25 to attenuate the production of proinflammatory cytokines remain obscure. Here, we report that Omp25 and its variants, viz. Omp25b, Omp25c, and Omp25d, suppress production of proinflammatory cytokines that are mediated by various TLRs. Furthermore, we demonstrate that Omp25 and its variants promote enhanced ubiquitination and degradation of TLRs and their adaptor proteins to attenuate the expression of proinflammatory cytokines. Targeting multiple TLRs and adaptor proteins enables Omp25 to effectively suppress the expression of proinflammatory cytokines that are induced by diverse pathogen-associated molecular patterns. This can contribute to the defective adaptive immune response and the chronic persistence of Brucella in the host.

A designed peptide-based vaccine to combat Brucella melitensis, B. suis and B. abortus: Harnessing an epitope mapping and immunoinformatics approach

Vaccines against Brucella abortus, B. melitensis and B. suis have been based on weakened or killed bacteria, however there is no recombinant vaccine for disease prevention or therapy. This study attempted to predict IFN-γ epitopes, T cell cytotoxicity, and T lymphocytes in order to produce a multiepitope vaccine based on BtpA, Omp16, Omp28, virB10, Omp25, and Omp31 antigens against B. melitensis, B. abortus, and B. suis. AAY, GPGPG, and EAAAK peptides were used as epitope linkers, while the PADRE sequence was used as a Toll-like receptor 2 (TLR2) and TLR4 agonist. The final construct included 389 amino acids, and was a soluble protein with a molecular weight of 41.3 kDa, and nonallergenic and antigenic properties. Based on molecular docking studies, molecular dynamics simulations such as Gyration, RMSF, and RMSD, as well as tertiary structure validation methods, the modeled protein had a stable structure capable of interacting with TLR2/4. As a result, this novel vaccine may stimulate immune responses in B and T cells, and could prevent infection by B. suis, B. abortus, and B. melitensis.

Brucella melitensis outer membrane protein 25 interacts with ferritin heavy polypeptide 1 in human trophoblast cells

Outer membrane protein 25 (OMP25) is involved in Brucella virulence and serves an important role in Brucella pathogenesis during infection. However, the protein interacting with OMP25 in host cells remains to be elucidated. The present study constructed a cDNA library from Brucella melitensis 16M‑infected human placenta trophoblastic cells (HPT‑8) and identified and confirmed the interaction between Brucella OMP25 and ferritin heavy polypeptide 1 (FTH1) of HPT‑8 using yeast two‑hybrid and co‑immunoprecipitation technologies. The expression of Toll‑like receptor (TLR)4, myeloid differentiation primary response protein MyD88 (MyD88) mRNA and inflammatory factors was detected by RNA interference. The results showed that OMP25 interacted with FTH1. Subsequently, TLR4 and MyD88 mRNA expression levels and inflammatory factors, such as nitric oxide, lactate dehydrogenase and TNF‑α, significantly increased after inserting OMP25 into the HPT‑8 cells. Notably, a low dose of OMP25 resulted in immunological protection, whereas high dose of OMP25 resulted in a cytotoxic effect on the HPT‑8 cells. It is suggested that OMP25 and FTH1 serve important roles in intracellular parasitism of Brucella and inhibition of expression.

Design of a multi-epitope vaccine candidate against Brucella melitensis

Brucella is a typical facultative intracellular bacterium that can cause zoonotic infections. For Brucella, it is difficult to eliminate with current medical treatment. Therefore, a multi-epitope vaccine (MEV) should be designed to prevent Brucella infection. For this purpose, we applied the reverse vaccinology approach from Omp10, Omp25, Omp31 and BtpB. Finally, we obtained 13 cytotoxic T lymphocyte (CTL) epitopes, 17 helper T lymphocyte (HTL) epitopes, 9 linear B cell epitopes, and 2 conformational B cell epitopes for further study. To keep the protein folded normally, we linked AAY, GPGPG, and KK to CTL epitopes, HTL epitopes, and B cell epitopes, respectively. The N-terminal of the vaccine peptide is supplemented with appropriate adjuvants to enhance immunogenicity. To evaluate its immunogenicity, stability, safety, and feasibility, a final MEV containing 806 amino acids was constructed by linking linkers and adjuvants. In addition, molecular docking and molecular dynamics simulations were performed to verify the affinity and stability of the MEV-TLR4. Then, codon adaptation and in silico cloning studies were carried out to identify the possible codons for expressing the MEV. In animal experiments, the results demonstrated that the MEV had high immunogenicity. Collectively, this study provided a theoretical basis for the development of a Brucella vaccine.

Kinetics of Placental Infection by Different Smooth Brucella Strains in Mice

Abortion and reproductive failures induced by Brucella are the main symptoms of animal brucellosis. Laboratory animal models are essential tools of research to study the Brucella pathogenesis before experimentation in natural hosts. To extend the existing knowledge, we studied B. melitensis 16M (virulent) and Rev1 (attenuated) as well as B. suis bv2 infections in pregnant mice. Here, we report new information about kinetics of infection (in spleens, blood, placentas, vaginal shedding, and foetuses), serum cytokine profiles, and histopathological features in placentas and the litter throughout mice pregnancy. Both B. melitensis strains showed a marked placental tropism and reduced viability of pups (mainly in 16M infections), which was preceded by an intense Th1-immune response during placental development. In contrast, B. suis bv2 displayed lower placental tropism, mild proinflammatory immune response, and scarce bacterial transmission to the litter, thus allowing foetal viability. Overall, our studies revealed three different smooth Brucella patterns of placental and foetal pathogenesis in mice, providing a useful animal model for experimental brucellosis.

Immune-metabolic receptor GPR84 surrogate and endogenous agonists, 6-OAU and lauric acid, alter Brucella abortus 544 infection in both in vitro and in vivo systems

Brucella abortus, one of the most important members of the genus Brucella responsible for human disease, is an intracellular pathogen capable of avoiding or interfering components of the host immune responses that are critical for its virulence. GPR84, on the other hand, is a seven-transmembrane GPCR involved in the inflammatory response and its induced expression was associated with B. abortus infection of RAW264.7 cells. Here we examined the effects of the reported GPR84 surrogate and endogenous agonists, namely 6-n-octylaminouracil (6-OAU) and lauric acid (LU), respectively in the progression of B. abortus infection in a cell and mouse models. The in vitro studies revealed the LU had bactericidal effect against Brucella starting at 24 h post-incubation. Adhesion of Brucella to RAW264.7 cells was attenuated in both 6-OAU and LU treatments. Brucella uptake was observed to be inhibited in a dose and time-dependent manner in 6-OAU but only at the highest non-cytotoxic concentration in LU-treated cells. However, survival of Brucella within the cells was reduced only in LU-treated cells. We also investigated the possible inhibitory effects of the agonist in other Gram-negative bacterium, Salmonella Typhimurium and we found that both adhesion and uptake were inhibited in 6-OAU treatment and only the intracellular survival for LU treatment. Furthermore, 6-OAU treatment reduced ERK phosphorylation and MCP-1 secretion during Brucella infection as well as reduced MALT1 protein expression and ROS production in cells without infection. LU treatment attenuated ERK and JNK phosphorylation, MCP-1 secretion and NO accumulation but increased ROS production during infection, and similar pattern with MALT1 protein expression. The in vivo studies showed that both treatments via oral route augmented resistance to Brucella infection but more pronounced with 6-AOU as observed with reduced bacterial proliferation in spleens and livers. At 7 d post-treatment and 14 d post-infection, 6-OAU-treated mice displayed reduced IFN-γ serum level. At 7 d post-infection, high serum level of MCP-1 was observed in both treatments with the addition of TNF-α in LU group. IL-6 was increased in both treatments at 14 d post-infection with higher TNF-α, MCP-1 and IL-10 in LU group. Taken together, 6-OAU and LU are potential candidates representing pharmaceutical strategy against brucellosis and possibly other intracellular pathogens or inflammatory diseases.

Uncovering the Hidden Credentials of Brucella Virulence

Bacteria in the genus Brucella are important human and veterinary pathogens. The abortion and infertility they cause in food animals produce economic hardships in areas where the disease has not been controlled, and human brucellosis is one of the world's most common zoonoses. Brucella strains have also been isolated from wildlife, but we know much less about the pathobiology and epidemiology of these infections than we do about brucellosis in domestic animals. The brucellae maintain predominantly an intracellular lifestyle in their mammalian hosts, and their ability to subvert the host immune response and survive and replicate in macrophages and placental trophoblasts underlies their success as pathogens. We are just beginning to understand how these bacteria evolved from a progenitor alphaproteobacterium with an environmental niche and diverged to become highly host-adapted and host-specific pathogens. Two important virulence determinants played critical roles in this evolution: (i) a type IV secretion system that secretes effector molecules into the host cell cytoplasm that direct the intracellular trafficking of the brucellae and modulate host immune responses and (ii) a lipopolysaccharide moiety which poorly stimulates host inflammatory responses. This review highlights what we presently know about how these and other virulence determinants contribute to Brucella pathogenesis. Gaining a better understanding of how the brucellae produce disease will provide us with information that can be used to design better strategies for preventing brucellosis in animals and for preventing and treating this disease in humans.

Integrative Bioinformatics Indentification of the Autophagic Pathway-Associated miRNA-mRNA Networks in RAW264.7 Macrophage Cells Infected with ∆Omp25 Brucella melitensis

Brucellosis is a zoonotic infectious disease caused by Brucella infection. Outer membrane protein 25 (Omp25) is closely related to the virulence and immunogenicity of Brucella. However, the molecular mechanism of Omp25 affecting Brucella-mediated macrophage autophagy remains unclear. Our previous study reported that four miRNAs (the upregulation of mmu-miR-146a-5p and mmu-miR-155-5p and downregulation of mmu-miR-149-3p and mmu-miR-5126) were confirmed and revealed the differentially expressed genes (DEGs) profile in RAW264.7 macrophage cells infected with Brucella melitensis Omp25 deletion mutant (∆Omp25 B. melitensis). Here, we predicted the target genes of the four miRNAs by TargetScan, miRanda, and PicTar. GO and KEGG were used for functional enrichment analysis of DEGs profile to reveal the autophagic pathway-associated genes. The overlapped genes, which drawn the autophagic pathway-associated miRNA-mRNA networks by cytoscape software, were identified by intersecting with the predicted target genes and autophagic pathway-associated DEGs. qRT-PCR was performed to validate the mRNAs of networks. The results showed that the autophagic pathway-associated networks of mmu-miR-149-3p-Ptpn5, mmu-miR-149-3p-Ppp2r3c, and mmu-miR-146a-5p-Dusp16 were identified in RAW264.7 macrophage cells infected with ∆Omp25 B. melitensis. Our findings are of great significance in elucidating the function of Omp25, revealing the infection mechanism of Brucella and prophylaxising and treating brucellosis.

HMGB1 release from trophoblasts contributes to inflammation during Brucella melitensis infection

Brucella melitensis infection causes acute necrotizing inflammation in pregnant animals; however, the pathophysiological mechanisms leading to placentitis are unknown. Here, we demonstrate that high-mobility group box 1 (HMGB1) acts as a mediator of placenta inflammation in B. melitensis-infected pregnant mice model. HMGB1 levels were increased in trophoblasts or placental explant during B. melitensis infection. Inhibition of HMGB1 activity with neutralising antibody significantly reduced the secretion of inflammatory cytokines in B. melitensis-infected trophoblasts or placenta, whereas administration of recombinant HMGB1 (rHMGB1) increased the inflammatory response. Mechanistically, this decreased inflammatory response results from inhibition of HMGB1 activity, which cause the suppression of both mitogen-activated protein kinases and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) activation. Moreover, neutralising antibody to HMGB1 prevented B. melitensis infection-induced activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase in trophoblasts. In contrast, in vitro stimulation of trophoblasts with rHMGB1 caused activation of NADPH oxidase and increased the production of ROS, which contributes to high bacterial burden within trophoblasts or placenta. In vivo, treatment with anti-HMGB1 antibody increases the number of Brucella survival within placenta in B. melitensis-infected pregnant mice but successfully reduced the severity of placentitis and abortion.

Immunogenic and protective antigens of Brucella as vaccine candidates

Brucella is an intracellular pathogen that causes abortion in domestic animals and undulant fever in humans. Due to the lack of a human vaccine against brucellosis, animal vaccines play an important role in the management of animal and human brucellosis for decades. Strain 19, RB51 and Rev1 are the approved Brucella spp. vaccine strains that are most commonly used to protect livestock against infection and abortion. However, due to some disadvantages of these vaccines, numerous studies have been conducted for the development of effective vaccines that could also be used in other susceptible animals. In this review, we compare different aspects of immunogenic antigens that have been a candidate for the brucellosis vaccine.

Learning from the master: targets and functions of the CtrA response regulator in Brucella abortus and other alpha-proteobacteria

The α-proteobacteria are a fascinating group of free-living, symbiotic and pathogenic organisms, including the Brucella genus, which is responsible for a worldwide zoonosis. One common feature of α-proteobacteria is the presence of a conserved response regulator called CtrA, first described in the model bacterium Caulobacter crescentus, where it controls gene expression at different stages of the cell cycle. Here, we focus on Brucella abortus and other intracellular α-proteobacteria in order to better assess the potential role of CtrA in the infectious context. Comparative genomic analyses of the CtrA control pathway revealed the conservation of specific modules, as well as the acquisition of new factors during evolution. The comparison of CtrA regulons also suggests that specific clades of α-proteobacteria acquired distinct functions under its control, depending on the essentiality of the transcription factor. Other CtrA-controlled functions, for instance motility and DNA repair, are proposed to be more ancestral. Altogether, these analyses provide an interesting example of the plasticity of a regulation network, subject to the constraints of inherent imperatives such as cell division and the adaptations to diversified environmental niches.