BackgroundSimilar to Gram-negative organisms, Borrelia spirochetes are dual-membrane organisms with both an inner and outer membrane. Although the outer membrane contains integral membrane proteins, few of the borrelial outer membrane proteins (OMPs) have been identified and characterized to date. Therefore, we utilized a consensus computational network analysis to identify novel borrelial OMPs.ResultsUsing a series of computer-based algorithms, we selected all protein-encoding sequences predicted to be OM-localized and/or to form β-barrels in the borrelial OM. Using this system, we identified 41 potential OMPs from B. burgdorferi and characterized three (BB0838, BB0405, and BB0406) to confirm that our computer-based methodology did, in fact, identify borrelial OMPs. Triton X-114 phase partitioning revealed that BB0838 is found in the detergent phase, which would be expected of a membrane protein. Proteolysis assays indicate that BB0838 is partially sensitive to both proteinase K and trypsin, further indicating that BB0838 is surface-exposed. Consistent with a prior study, we also confirmed that BB0405 is surface-exposed and associates with the borrelial OM. Furthermore, we have shown that BB0406, the product of a co-transcribed downstream gene, also encodes a novel, previously uncharacterized borrelial OMP. Interestingly, while BB0406 has several physicochemical properties consistent with it being an OMP, it was found to be resistant to surface proteolysis. Consistent with BB0405 and BB0406 being OMPs, both were found to be capable of incorporating into liposomes and exhibit pore-forming activity, suggesting that both proteins are porins. Lastly, we expanded our computational analysis to identify OMPs from other borrelial organisms, including both Lyme disease and relapsing fever spirochetes.ConclusionsUsing a consensus computer algorithm, we generated a list of candidate OMPs for both Lyme disease and relapsing fever spirochetes and determined that three of the predicted B. burgdorferi proteins identified were indeed novel borrelial OMPs. The combined studies have identified putative spirochetal OMPs that can now be examined for their roles in virulence, physiology, and disease pathogenesis. Importantly, the studies described in this report provide a framework by which OMPs from any human pathogen with a diderm ultrastructure could be cataloged to identify novel virulence factors and vaccine candidates.Electronic supplementary materialThe online version of this article (doi:10.1186/s12866-016-0762-z) contains supplementary material, which is available to authorized users.
Immunoproteomic analyses were used to characterize the outer membrane proteome of Mannheimia haemolytica, formerly Pasteurella haemolytica, serotype 1, and determine potential vaccine candidate proteins. 2-DE of M. haemolytica outer membranes was followed by immunoblot analyses using naïve and convalescent bovine sera. Proteins were identified using MALDI-TOF and LC-MS/MS. Spectral data was used to mine M. haemolytica protein database and 132 immunoreactive proteins were identified. Bioinformatic analysis using PSORTb, SubLoc, LipoP, BOMP, MCMBB, and TMB-Hunt/BBTM to predict subcellular localization of immunoreactive proteins and beta-barrels narrowed the list down to 55 candidates. Functional characterization of 55 proteins predicted 16 (29%) are involved in cell structure, 13 (23.6%) in transport/virulence, ten (18.2%) as unknown, six (10.9%) in general metabolism, four (7.27%) in cell process, two (3.64%) in translation, and one (1.8%) each in DNA replication, regulation, transcription, and virulence. Prediction of beta-barrel formation was between 11 and 31 immunoreactive proteins depending on the bioinformatic tool employed. Some of these proteins have potentials to be developed into stand-alone vaccines or components of vaccines. Of those proteins, several have already been characterized. Finally, although characteristics of many of M. haemolytica immunoreactive proteins identified in this study were obtained from published data and predictions using bioinformatics tools, five proteins previously listed in the published M. haemolytica sequence as unidentified were found to have correlates with functional proteins in other bacterial species.
b Mannheimia haemolytica, a major causative agent in bovine respiratory disease, inflicts extensive losses each year on cattle producers. Commercially available vaccines are only partially efficacious. Immunity to M. haemolytica requires antibodies to secreted toxins and outer membrane proteins (OMPs) of the bacterium. Gram-negative bacteria produce membrane blebs or vesicles, the membrane components of which are primarily derived from OMPs. Accordingly, vesicles have been used as immunogens with various degrees of success. This study characterized components of M. haemolytica vesicles and determined their immunogenicity in mice and cattle. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis of vesicles from this bacterium identified 226 proteins, of which 58 (25.6%) were OMPs and periplasmic and one (0.44%) was extracellular. Vesicles were used to vaccinate dairy calves and BALB/c mice. Analyses of sera from calves and mice by enzyme-linked immunosorbent assay (ELISA) showed that circulating antibodies against M. haemolytica whole cells and leukotoxin were significantly higher on days 21 and 28 (P < 0.05) than on day 0. For control calves and mice, there were no significant differences in serum anti-whole-cell and leukotoxin antibody levels from days 0 and 21 or 28, respectively. Lesion scores of lungs from vaccinated calves (15.95%) were significantly (P < 0.05) lower than those from nonvaccinated calves (42.65%). Sera from mice on day 28 and calves on day 21 showed 100% serum bactericidal activity. Sera from vesicle-vaccinated mice neutralized leukotoxin.
We recently identified the Borrelia burgdorferi outer membrane protein (OMP) BB0406 and found that the gene encoding this OMP was cotranscribed with the gene encoding the OMP BB0405. Interestingly, BB0405 and BB0406 share 59% similarity and are grouped into the same B. burgdorferi paralogous gene family. Given their overall similarity, it is plausible that both OMPs have similar or overlapping functions in this pathogenic spirochete. BB0405 was recently shown to be required for mammalian infection despite the observations that BB0405 is poorly immunogenic and not recognized during mouse or human infection. BB0405 orthologs have also been shown to bind the complement regulator protein factor H. Therefore, to better elucidate the role of BB0405 and its paralog BB0406 during infection and in serum resistance, we examined both proteins in animal infection, factor H binding, and serum sensitivity assays. Our combined results suggest that BB0405-and BB0406-specific antibodies are borreliacidal and that both OMPs are immunogenic during nonhuman primate infection. Additionally, while BB0405 was found to be required for establishing mouse infection, BB0406 was not found to be essential for infectivity. In contrast to data from previous reports, however, neither OMP was found to bind human factor H or to be required for enhancing serum resistance of B. burgdorferi in vitro.KEYWORDS Borrelia burgdorferi, Lyme disease, outer membrane proteins, spirochete L yme disease is a multisystem infection caused by pathogenic spirochetes belonging to the Borrelia burgdorferi sensu lato complex (1, 2). It is the most prevalent arthropod-borne disease in the United States and has become a major public health concern (3, 4). The enzootic life cycle of the spirochete is complex and involves tick vectors of the Ixodes genus that transmit the spirochete to a wide range of mammalian host reservoirs in nature. Humans are accidental hosts and play no significant role in the spirochete life cycle (5). The primary manifestations of Lyme disease are a localized rash, termed erythema migrans, and flu-like symptoms (6). In the absence of antibiotic treatment, spirochetes can disseminate to multiple organs, including the heart, joints, skin, and nervous system (7). To disseminate to these distant sites, the spirochete must evade serum-mediated killing and the subsequent adaptive immune response that ensues in the mammalian host (5,6,8).Given that B. burgdorferi is an extracellular pathogen, the outer membrane (OM) of this organism is the interface between the spirochete and the host during infection. The two major classes of proteins present in the borrelial OM are (i) outer surface lipoproteins (Osps) that are anchored to the outer leaflet of the OM bilayer by their N-terminal lipid moieties and (ii) integral outer membrane proteins (OMPs) that contain membrane-spanning domains. Membrane-anchored Osps have been well studied be-
We previously identified Mannheimia haemolytica outer membrane proteins (OMPs) that may be important immunogens by using immunoproteomic analyses. Genes for serotype 1-specific antigen (SSA-1), OmpA, OmpP2, and OmpD15 were cloned and expressed, and recombinant proteins were purified. Objective 1 of this study was to demonstrate immunogenicity of the four recombinant OMPs in mice and cattle. Objective 2 was to determine if the addition of individual recombinant OMPs or combinations of them would modify immune responsiveness of mice to the recombinant chimeric protein SAC89, containing the main epitope from M. haemolytica outer membrane lipoprotein PlpE and the neutralizing epitope of M. haemolytica leukotoxin. Mice vaccinated with recombinant OmpA (rOmpA), rSSA-1, rOmpD15, and rOmpP2 developed significant antibody responses to M. haemolytica outer membranes and to the homologous recombinant OMP. Cattle vaccinated with rOmpA and rSSA-1 developed significant antibodies to M. haemolytica outer membranes by day 28, whereas cattle vaccinated with rOmpD15 and rOmpP2 developed only minimal responses. Sera from cattle vaccinated with each of the recombinant proteins stimulated complement-mediated killing of the bacterium. Concurrent vaccination with SAC89 plus any of the four rOMPs singly resulted in increased endpoint anti-SAC89 titers, and for the SAC89/rSSA-1 vaccinees, the response was increased significantly. In contrast, the SAC89/P2/SSA-1 and SAC89/OmpA/P2/D15/SSA-1 combination vaccines resulted in significant decreases in anti-SAC89 antibodies compared to SAC89 vaccination alone. In conclusion, under the conditions of these experiments, vaccination of mice and cattle with rOmpA and rSSA-1 stimulated high antibody responses and may have protective vaccine potential.The major cause of severe bacterial pneumonia in cattle is Mannheimia haemolytica serotype 1 (S1), and current vaccines against M. haemolytica are only moderately efficacious (9, 36). Shewen and Wilkie (43) demonstrated that immunity against M. haemolytica requires antibodies against leukotoxin (LKT), which causes necrosis, apoptosis, or activation of ruminant leukocytes, as well as antibodies against bacterial cell surface antigens. The important surface immunogens needed to stimulate protective immunity against M. haemolytica appear to be outer membrane proteins (OMPs) (13,33,34,37,38).Our laboratory demonstrated that high antibody responses to a major 45-kDa outer membrane lipoprotein, PlpE, correlated with resistance against experimental challenge, and PlpE proteins were nearly identical among serotype 1 and serotype 6 isolates (2). Cattle vaccinated with commercial M. haemolytica vaccines to which 100 g of recombinant M. haemolytica S1 PlpE (rPlpE) was added had significantly greater resistance against experimental challenge with either S1 or S6 than did cattle vaccinated with the commercial vaccine alone (11, 12). The major epitope region of M. haemolytica S1 PlpE, designated region 2 (R2), consists of 8 imperfect hexapeptide repeats of QAQNAP loc...
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