Outer membrane protein A (OmpA) from Shigella flexneri 2a: a promising subunit vaccine candidate

Shigellosis is the leading cause of childhood mortality and morbidity. Despite many years of extensive research a practical vaccine is not yet available against the disease. Recent studies illustrate that bacterial outer membrane proteins are budding target as vaccine antigen. Outer membrane proteins A (OmpA) are among the most immunodominant antigens in the outer membrane of gram negative bacteria and possess many characteristics desired of a vaccine candidate. We observe that OmpA of Shigella flexneri 2a is crossreactive and common antigen among Shigella spp. and the epitope is widely exposed on the cell surface as well as capable of evoking protective immunity in mice. The protective immunity involves participation of both the humoral and cellular immune responses, since OmpA boosts rapid induction of IgG and IgA in both the systemic and mucosal compartments and also activates Th1 cells. The immunopotentiating activity of OmpA is mediated by its ability to bind and stimulate macrophages and up-regulate the surface expression of MHCII, CD80 and CD40, leading to activation of CD4(+) T cells to secrete cytokines and express chemokine receptor and IL-12Rβ2, thereby orchestrating the bridge between innate and adaptive immune responses. This ability is dependent on Toll-like receptor 2 (TLR2), as demonstrated by lack of response by TLR2 knockdown macrophages to OmpA. Hence this property of OmpA to link innate and adaptive immunity via TLR2 offers a novel vista to develop vaccine against shigellosis.

The periplasmic domain of Escherichia coli outer membrane protein A can undergo a localized temperature dependent structural transition

Gram-negative bacteria such as Escherichia coli are surrounded by two membranes with a thin peptidoglycan (PG)-layer located in between them in the periplasmic space. The outer membrane protein A (OmpA) is a 325-residue protein and it is the major protein component of the outer membrane of E. coli. Previous structure determinations have focused on the N-terminal fragment (residues 1-171) of OmpA, which forms an eight stranded transmembrane β-barrel in the outer membrane. Consequently it was suggested that OmpA is composed of two independently folded domains in which the N-terminal β-barrel traverses the outer membrane and the C-terminal domain (residues 180-325) adopts a folded structure in the periplasmic space. However, some reports have proposed that full-length OmpA can instead refold in a temperature dependent manner into a single domain forming a larger transmembrane pore. Here, we have determined the NMR solution structure of the C-terminal periplasmic domain of E. coli OmpA (OmpA(180-325)). Our structure reveals that the C-terminal domain folds independently into a stable globular structure that is homologous to the previously reported PG-associated domain of Neisseria meningitides RmpM. Our results lend credence to the two domain structure model and a PG-binding function for OmpA, and we could indeed localize the PG-binding site on the protein through NMR chemical shift perturbation experiments. On the other hand, we found no evidence for binding of OmpA(180-325) with the TonB protein. In addition, we have also expressed and purified full-length OmpA (OmpA(1-325)) to study the structure of the full-length protein in micelles and nanodiscs by NMR spectroscopy. In both membrane mimetic environments, the recombinant OmpA maintains its two domain structure that is connected through a flexible linker. A series of temperature-dependent HSQC experiments and relaxation dispersion NMR experiments detected structural destabilization in the bulge region of the periplasmic domain of OmpA above physiological temperatures, which may induce dimerization and play a role in triggering the previously reported larger pore formation.

Development of a cost-effective vaccine candidate with outer membrane vesicles of a tolA-disrupted Shigella boydii strain

Our previous studies on outer membrane vesicles based vaccine development against shigellosis, revealed the inability of Shigella to release significant amount of vesicles naturally, during growth. Disruption of tolA, one of the genes of the Tol-Pal system of Gram negative bacterial membrane, has increased the vesicle release rate of a Shigella boydii type 4 strain to approximately 60% higher. We also noticed the vesicles, released from tolA-disrupted strain captured more OmpA protein and lipopolysaccharide, compared to the vesicles released from its wild type prototype. Six to seven weeks old BALB/c mice, immunized with 25 μg of three oral doses of the vesicles, released by tolA mutant, conferred 100% protection against lethal homologous challenge through nasal route, compared to only 60% protection after the same dose of wild type immunogen. Mice, immunized with the vesicles from tolA-mutant, manifested significant secretion of mucosal IgG and IgA. A sharp and significant response of pro-inflammatory cytokines (TNF-α, IL-6, IFN-γ) were also observed in the lung lavage of these groups of mice, within 6h post challenge; but at 24h, these inflammatory cytokines showed the sign of subsidence and the system was taken over by the release of anti-inflammatory cytokines (IL-4 and IL-10). Studies with naïve peritoneal macrophages, proved further, the potency of these vesicles to stimulate nitric oxide and TNF-α, IL-12p70, IL-6 and IL-10 productions in-vitro. The ability of these vesicles to trigger polarization of CD4(+) T cells toward Th1 adaptive immune response, had also been observed along with the presence of anti-inflammatory cytokines in the system. Our study demonstrated, the vesicles from tolA-disrupted Shigella were able to suppress Shigella-mediated inflammation in the host and could balance between inflammation and anti-inflammation, promoting better survival and health of the infected mice. Outer membrane vesicles from tolA-mutant, could be a potential cost-effective vaccine candidate against shigellosis.

Outer membrane protein A: An immunogenic protein induces highly protective efficacy against Vibrio ichthyoenteri

Vibrio ichthyoenteri was an important causative agent of bacterial enteritis in flounder (Paralichthys olivaceus). Outer membrane protein A (OmpA) of Gram-negative pathogen was a major cell surface antigen. In the present study, OmpA of V. ichthyoenteri was recombinantly expressed in Escherichia coli, and the immunogenicity of OmpA was identified by western blotting using flounder anti-rOmpA and anti-V. ichthyoenteri antibodies. The vaccine potential of rOmpA was tested in a flounder model, and a high relative percentage of survival rate was obtained with 73.1% after challenge with V. ichthyoenteri. Meanwhile, the immune response of flounder induced by rOmpA was also investigated, and the results showed that the sIg + lymphocytes in blood, spleen, and pronephros significantly proliferated, and the peak levels occurred at week 4 after immunization. Moreover, rOmpA could induce higher levels of specific serum antibodies than the control group after immunization, and the peak level occurred at week 5 after immunization. Meanwhile, qRT-PCR analysis showed that the expressions of CD4-1, CD8α, IL-1β, IFN-γ, MHCIα and MHCIIα genes were significantly up-regulated after immunization with rOmpA. Taking together, these results demonstrated that rOmpA could evoke highly protective effects against V. ichthyoenteri challenge and induce strong immune response of flounder, which indicated that OmpA was a promising vaccine candidate.

Biochemical and functional characterization of the periplasmic domain of the outer membrane protein A from enterohemorrhagic Escherichia coli

Outer membrane protein A (OmpA) plays multiple roles in the physiology and pathogenesis of the zoonotic pathogen enterohemorrhagic Escherichia coli (EHEC). The N-terminus of OmpA forms a transmembrane domain (OmpA™), and the roles of this domain in bacterial pathogenesis have been well studied. However, how its C-terminal domain (OmpAper), which is located at the periplasmic space in the bacterial membrane, contributes to virulence remains unclear. Herein, we report that OmpAper forms a dimer and binds to peptidoglycan in vitro. Furthermore, OmpAper is responsible for bacterial resistance to acidic conditions, high osmotic pressure and high SDS environments. In addition, OmpAper contributes to the adhesion of bacteria to HeLa cells in vitro and ex vivo. These results provide an additional understanding of the role of OmpA in EHEC physiology and pathogenesis.

OmpA of Klebsiella pneumoniae ATCC 13883 induces pyroptosis in HEp-2 cells, leading to cell-cycle arrest and apoptosis

Klebsiella pneumoniae is an opportunistic pathogenic bacterium that commonly causes pneumonia in elderly people. OmpA, a toxin that is highly expressed in the outer membrane of the bacterium, is one of the primary factors implicated in the pulmonary pathogenesis of K. pneumoniae. To evaluate the associated pyroptosis mechanism of infection, the ompA gene was cloned, and the protein was expressed, extracted, and used to treat human larynx epithelial cells. We observed that OmpA induces reactive oxygen species production and cell-cycle arrest in the G2/M phase in host cells, leading to subsequent apoptosis. Moreover, OmpA was found to induce IL-1β and IL-18 production in host cells, resulting in caspase-1 activation, which simultaneously stimulated pyroptosis, thus leading to the death of the host cells. We next sought to examine differential gene expression via RNA sequencing to better elucidate the mechanisms associated with these cellular changes, and found that genes associated with these pathways were more highly expressed in OmpA-treated cells than in K. pneumoniae-infected cells. Thus, cell-cycle arrest, apoptosis, and pyroptosis may serve as the primary defenses employed by host cells against OmpA. These results provide novel insights into the host defense against K. pneumoniae infection.

Outer membrane protein a in Acinetobacter baumannii induces autophagy through mTOR signalling pathways in the lung of SD rats

Outer membrane protein A (OmpA) of Acinetobacter baumannii (A. baumannii) is associated with autophagy, which plays an important role in its pathogenicity. However, its exact pathophysiological role in the process of lung tissue cell autophagy remains unclear. In this study, animal and cell infection models were established by wild A. baumannii strain and An OmpA knockout mutant (OmpA-/- A. baumannii) strain. The expression levels of markers autophagy, histological change, cell viability and protein expression levels of inflammatory cytokines were examined. OmpA-/-A. baumannii was successfully constructed. The capacities of bacterial adhesion and invasion to host cells increased more obviously in the AB group and the AB + Rapa group than in the OmpA-/- AB group and AB + CQ group. The AB group and AB + Rapa group could produce double membrane vacuoles, endoplasmic reticulum dilation, mitochondrial ridge rupture, and mitochondrial vacuoles. OmpA could lead to increased LC3, AMPK, and PAMPK protein release, and decreased levels of P62, mTOR and pmTOR proteins in vivo and in vitro. OmpA caused lung pathology and the release of inflammatory cytokines. A. baumannii OmpA promotes autophagy in lung cells through the mTOR signalling pathway, which increases the bacterial colonization ability in the double-layer membrane autophagosome formed by the autophagy reaction to escape the clearance of bacteria by the host, promote the release of inflammatory mediators and aggravate the damage to the host.

The OmpA of commensal Escherichia coli of CRC patients affects apoptosis of the HCT116 colon cancer cell line

BACKGROUND

Colorectal cancer ranks third globally among all types of cancers. Dysbiosis of the gut microbiota of people with CRC is one of the effective agents in the tumorigenesis and metastasis in this type of cancer. The population of Escherichia coli strains, a component of gut microbiota, is increased in the gut of people with CRC compared with healthy people. So, E.coli strains isolated from these patients may have a role in tumorigenesis. Because the most isolated strains belong to the B2 phylogenuetic group, there seems to be a linkage between the bacterium components and malignancy.

MATERIAL AND METHODS

In this study, the proteomic comparison between isolated Ecoli from CRC patients and healthy people was assayed. The isolated spot was studied by Two-dimensional gel electrophoresis (2DE) and Liquid chromatography-mass spectrometry (LC-MS). The results showed that the expression of Outer membrane protein A (OmpA) protein increased in the commensal E.coli B2 phylogenetic group isolated from CRC patients. Additionally, we analyzed the effect of the OmpA protein on the expression of the four genes related to apoptosis in the HCT116 colon cancer cell line.

RESULTS

This study identified that OmpA protein was overexpressed in the commensal E.coli B2 phylogenetic group isolated from CRC patients compared to the E.coli from the control group. This protein significantly decreased the expression of Bax and Bak, pro-apoptotic genes, as well as the expression of P53 in the HCT116 Cell Line, P < 0.0001. LC-MS and protein bioinformatics results confirmed that this protein is outer membrane protein A, which can bind to nucleic acid and some of the organelle proteins on the eukaryotic cell surface.

CONCLUSIONS

According to our invitro and insilico investigations, OmpA of gut E.coli strains that belong to the B2 phylogenetic group can affect the eukaryotic cell cycle.

Evaluation of the protective immunity of Riemerella anatipestifer OmpA

Riemerella anatipestifer is responsible for an economically important disease of commercially raised ducks. No or only few cross-protection was observed between different serotypes of R. anatipestifer strains, and so far no protective antigen in this bacterium has been identified. OmpA is a predominant immunogenic protein of R. anatipestifer, and within the 1467 bp ompA ORF (ompA1467), there is another 1164 bp ORF (ompA1164) with the same C-terminal. In this study, our results showed that the full sequence of ompA1467 from some R. anatipestifer strains with different serotypes shared the same amino acid sequence. Animal experiments showed that the soluble recombinant protein rOmpA1164, but not rOmpA1467, could provide partial protective immunity against challenge. Moreover, there was no significant difference in protective immunity between ducklings immunized with Th4△ompA bacterin and those immunized with Th4 bacterin. In addition, OmpA1467 was the main existing form of OmpA in R. anatipestifer cells by gel electrophoresis and western blot analyses. The results suggested that OmpA1467 was not a protective antigen of R. anatipestifer, and antibodies against proteins other than OmpA play a critical role in the process of anti-R. anatipestifer infection.

Efficacy of indirect ELISA for serodiagnosis of melioidosis using immunodominant antigens from non-pathogenic Burkholderia thailandensis

Melioidosis caused by gram negative bacteria, B. pseudomallei, is a fatal disease in the tropical and sub-tropical regions. However, sporadic cases have been reported in elsewhere. Early detection is imperative to reduce the mortality rate. Serological tests have being substantially developed using recombinant proteins as specific targeted antigens to melioidosis antibodies. In the present study, we focus on a truncated flagellin fragment (FLAG300) and outer membrane protein A (OmpABT) of B. thailandensis E264 as potential antigens for developing indirect ELISA to improve the serodiagnosis of melioidosis. Recombinant proteins were overexpressed and purified by immobilized metal affinity chromatography with denaturing conditions. The sensitivity and specificity of individual test were calculated within culture-confirmed melioidosis sera (n = 42) and non-melioidosis serum samples (n = 241) using the cut-off point at average of absorbance plus 2 standard deviations. The results demonstrated that a FLAG 300 based indirect ELISA showed 90.48 % sensitivity and 87.14 % specificity and an OmpABT based this assay revealed sensitivity of 80.95 % and specificity of 89.21 %. Their use in a double-antigen ELISA resulted in improve specificity (92.95 %) and still high degree of sensitivity (85.71 %). These data suggest a facile method for serodiagnosis of melioidosis by the use of antigens from a non-pathogenic strain.

Oral administration of recombinant outer membrane protein A-based nanovaccine affords protection against Aeromonas hydrophila in zebrafish

Aeromonas hydrophila, an opportunistic warm water pathogen, has always been a threat to aquaculture, leading to substantial economic losses. Vaccination of the cultured fish would effectively prevent Aeromoniasis, and recent advancements in nanotechnology show promise for efficacious vaccines. Oral delivery would be the most practical and convenient method of vaccine delivery in a grow-out pond. This study studied the immunogenicity and protective efficacy of a nanoparticle-loaded outer membrane protein A from A. hydrophila in the zebrafish model. The protein was over-expressed, purified, and encapsulated using poly lactic-co-glycolic acid (PLGA) nanoparticles via the double emulsion method. The PLGA nanoparticles loaded with recombinant OmpA (rOmpA) exhibited a size of 295 ± 15.1 nm, an encapsulation efficiency of 72.52%, and a polydispersity index of 0.292 ± 0.07. Scanning electron microscopy confirmed the spherical and isolated nature of the PLGA-rOmpA nanoparticles. The protective efficacy in A. hydrophila-infected zebrafish after oral administration of the nanovaccine resulted in relative percentage survival of 77.7. Gene expression studies showed significant upregulation of immune genes in the vaccinated fish. The results demonstrate the usefulness of oral administration of nanovaccine-loaded rOmpA as a potential vaccine since it induced a robust immune response and conferred adequate protection against A. hydrophila in zebrafish, Danio rerio.

A genetic engineering strategy to enhance outer membrane vesicle-mediated extracellular electron transfer of Geobacter sulfurreducens

Bioelectrochemical systems (BESs) are unique devices that harness the metabolic activity of electroactive microorganisms (EAMs) to convert chemical energy stored in organic substrates into electrical energy. Enhancing electron transfer efficiency between EAMs and electrodes is the key to practical implementation of BESs. Considering the role of outer membrane vesicles (OMVs) in mediating electron transfer of EAMs, a genetic engineering strategy to achieve OMVs overproduction was explored to enhance electron transfer efficiency and the underlying mechanisms were investigated. This study constructed a mutant strain of Geobacter sulfurreducens that lacked the ompA gene encoding an outer membrane protein. Experimental results showed that the mutant strain produced more OMVs and possessed higher electron transfer efficiency in Fe(III) reduction, dye degradation and current generation in BESs than the wild-type strain. More cargoes such as c-type cytochromes, functional proteins, eDNA, polysaccharides and signaling molecules that might be favorable for electron transfer and biofilm formation were found in OMVs produced by ompA-deficient anodic biofilm, which possibly contributed to the improved electron transfer efficiency of ompA-deficient biofilm. The results indicate that overproduction of OMVs in EAMs might be a potential strategy to enhance BESs performance.

Polyvinylpyrrolidone capped silver nanoparticles enhance the autophagic clearance of Acinetobacter baumannii from human pulmonary cells

Acinetobacter baumannii, an opportunistic pathogen has shown an upsurge in its multi-drug resistant isolates. OmpA of A. baumannii induces incomplete autophagy and apoptosis in host cells. Various therapeutic alternatives are under investigation against A. baumannii. Here, the major emphasis has been laid on comparing the efficacy of AgNP with different capping agents. OmpA targeted lead, Ivermectin capped AgNP (IVM-AgNP) has been compared with the antibacterial polyvinylpyrrolidone capped AgNP (PVP-AgNP) for their role in the modulations of host autophagy. Upregulation of p62 and LC3B confirmed by real-time PCR analysis indicated an increased autophagic flux upon the treatment with AgNPs. The elongation and closure of autophagic vacuoles was also supported by upregulated Atg genes (Atg4, Atg3, Atg5) in A. baumannii infected cells after treatment with AgNP. Autophagic flux increased on treatment with PVP-AgNP as suggested by the rise in mcherryLC3B fluorescence in A549 cells treated with PVP-AgNP as compared to the GFP-LC3B of IVM-AgNP. This suggests that PVP-AgNP treatment more effectively promotes the elongation and maturation stages of autophagy by increasing autophagic flux. These results indicate that capped AgNPs have the efficiency to revert the incomplete autophagy induced by A. baumannii back to normal autophagic levels.

Preparation of polyclonal antibody against a universal bacterial antigen OmpA deduced by bioinformatic analysis and preliminary evaluation of concentration effects on foodborne pathogens

Rapid and ultrasensitive microbial detection in actual samples have challenges because of target pathogen diversity and low abundance. In this study, we attempted to capture and concentrate multiple pathogens by combining magnetic beads with polyclonal antibodies against a universal antigen of ompA, LAMOA-1, before further detection. A protein sequence consisting of 241 amino acids with spatial conformation similar to E. coli ompA was identified and expressed as a recombinant protein in prokaryotes according to the results of sequence alignment among 432 sequences of ompA belonging to intestinal bacteria from gram-negative bacteria. Purified from immunized rabbits, the anti-LAMOA-1 antibody was shown to effectively recognize 12 foodborne bacterial species. Antibody-conjugated beads were used to concentrate the bacteria when the bacterial concentration in artificially contaminated samples is between 10 and 100 CFU/mL, which shortens detection duration by 8-24 h. The enrichment strategy is potentially beneficial for detection of foodborne pathogens.

Effect of clinically relevant antibiotics on bacterial extracellular vesicle release from Escherichia coli

Sepsis, a leading cause of death in hospitals, can be defined as a dysregulated host inflammatory response to infection, which can lead to tissue damage, organ failure and cardiovascular complications. Although there is no cure for sepsis, the condition is typically managed with broad-spectrum antibiotics to eliminate any potential bacterial source of infection. However, a potential side effect of antibiotic treatment is the enhanced release of bacterial extracellular vesicles (BEVs), membrane-bound nanoparticles containing proteins and other biological molecules from their parent bacterium. Some of the Gram-negative extracellular vesicle (EV) cargo, including peptidoglycan associated lipoprotein and outer membrane protein A, have been shown to induce both acute and chronic inflammation in host tissue. It was hypothesized that the antibiotic concentration and mechanism of action may affect the amount of released BEVs, which could potentially exacerbate the host inflammatory response. This study evaluated nine clinically relevant antibiotics for their effect on EV release from Escherichia coli. Several beta-lactam antibiotics caused significantly more EV release, while quinolone and aminoglycoside antibiotics caused less vesiculation. Further study is warranted to corroborate the correlation between an antibiotic's mechanism of action and its effect on EV release, but these results underline the importance of antibiotic choice when treating patients with sepsis.