Immunogenic Burkholderia pseudomallei outer membrane proteins as potential candidate vaccine targets

Burkholderia pseudomallei Complex Subunit and Glycoconjugate Vaccines and Their Potential to Elicit Cross-Protection to Burkholderia cepacia Complex

Burkholderia are a group of Gram-negative bacteria that can cause a variety of diseases in at-risk populations. B. pseudomallei and B. mallei, the etiological agents of melioidosis and glanders, respectively, are the two clinically relevant members of the B. pseudomallei complex (Bpc). The development of vaccines against Bpc species has been accelerated in recent years, resulting in numerous promising subunits and glycoconjugate vaccines incorporating a variety of antigens. However, a second group of pathogenic Burkholderia species exists known as the Burkholderia cepacia complex (Bcc), a group of opportunistic bacteria which tend to affect individuals with weakened immunity or cystic fibrosis. To date, there have been few attempts to develop vaccines to Bcc species. Therefore, the primary goal of this review is to provide a broad overview of the various subunit antigens that have been tested in Bpc species, their protective efficacy, study limitations, and known or suspected mechanisms of protection. Then, we assess the reviewed Bpc antigens for their amino acid sequence conservation to homologous proteins found in Bcc species. We propose that protective Bpc antigens with a high degree of Bpc-to-Bcc sequence conservation could serve as components of a pan-Burkholderia vaccine capable of protecting against both disease-causing groups.

Designing mRNA- and Peptide-Based Vaccine Construct against Emerging Multidrug-Resistant Citrobacter freundii: A Computational-Based Subtractive Proteomics Approach

Background and Objectives: Citrobacter freundii (C. freundii) is an emerging and opportunistic Gram-negative bacteria of the human gastrointestinal tract associated with nosocomial and severe respiratory tract infections. It has also been associated with pneumonia, bloodstream, and urinary tract infections. Intrinsic and adaptive virulence characteristics of C. freundii have become a significant source of diarrheal infections and food poisoning among immune-compromised patients and newborns. Impulsive usage of antibiotics and these adaptive virulence characteristics has modulated the C. freundii into multidrug-resistant (MDR) bacteria. Conventional approaches are futile against MDR C. freundii. Materials and Methods: The current study exploits the modern computational-based vaccine design approach to treat infections related to MDR C. freundii. A whole proteome of C. freundii (strain: CWH001) was retrieved to screen pathogenic and nonhomologous proteins. Six proteins were shortlisted for the selection of putative epitopes for vaccine construct. Highly antigenic, nonallergen, and nontoxic eleven B-cell, HTL, and TCL epitopes were selected for mRNA- and peptide-based multi-epitope vaccine construct. Secondary and tertiary structures of the multi-epitope vaccine (MEVC) were designed, refined, and validated. Results: Evaluation of population coverage of MHC-I and MHC-II alleles were 72% and 90%, respectively. Docking MEVC with TLR-3 receptor with the binding affinity of 21.46 (kcal/mol) occurred through the mmGBSA process. Further validations include codon optimization with an enhanced CAI value of 0.95 and GC content of about 51%. Immune stimulation and molecular dynamic simulation ensure the antibody production upon antigen interaction with the host and stability of the MEVC construct, respectively. Conclusions: These interpretations propose a new strategy to combat MDR C. freundii. Further, in vivo and in vitro trials of this vaccine will be valuable in combating MDR pathogens.

Layered and integrated medical countermeasures against Burkholderia pseudomallei infections in C57BL/6 mice

Burkholderia pseudomallei, the gram-negative bacterium that causes melioidosis, is notoriously difficult to treat with antibiotics. A significant effort has focused on identifying protective vaccine strategies to prevent melioidosis. However, when used as individual medical countermeasures both antibiotic treatments (therapeutics or post-exposure prophylaxes) and experimental vaccine strategies remain partially protective. Here we demonstrate that when used in combination, current vaccine strategies (recombinant protein subunits AhpC and/or Hcp1 plus capsular polysaccharide conjugated to CRM197 or the live attenuated vaccine strain B. pseudomallei 668 ΔilvI) and co-trimoxazole regimens can result in near uniform protection in a mouse model of melioidosis due to apparent synergy associated with distinct medical countermeasures. Our results demonstrated significant improvement when examining several suboptimal antibiotic regimens (e.g., 7-day antibiotic course started early after infection or 21-day antibiotic course with delayed initiation). Importantly, this combinatorial strategy worked similarly when either protein subunit or live attenuated vaccines were evaluated. Layered and integrated medical countermeasures will provide novel treatment options for melioidosis as well as diseases caused by other pathogens that are refractory to individual strategies, particularly in the case of engineered, emerging, or re-emerging bacterial biothreat agents.

Vaccinomics to Design a Multi-Epitopes Vaccine for Acinetobacter baumannii

Antibiotic resistance (AR) is the result of microbes’ natural evolution to withstand the action of antibiotics used against them. AR is rising to a high level across the globe, and novel resistant strains are emerging and spreading very fast. Acinetobacter baumannii is a multidrug resistant Gram-negative bacteria, responsible for causing severe nosocomial infections that are treated with several broad spectrum antibiotics: carbapenems, β-lactam, aminoglycosides, tetracycline, gentamicin, impanel, piperacillin, and amikacin. The A. baumannii genome is superplastic to acquire new resistant mechanisms and, as there is no vaccine in the development process for this pathogen, the situation is more worrisome. This study was conducted to identify protective antigens from the core genome of the pathogen. Genomic data of fully sequenced strains of A. baumannii were retrieved from the national center for biotechnological information (NCBI) database and subjected to various genomics, immunoinformatics, proteomics, and biophysical analyses to identify potential vaccine antigens against A. baumannii. By doing so, four outer membrane proteins were prioritized: TonB-dependent siderphore receptor, OmpA family protein, type IV pilus biogenesis stability protein, and OprD family outer membrane porin. Immuoinformatics predicted B-cell and T-cell epitopes from all four proteins. The antigenic epitopes were linked to design a multi-epitopes vaccine construct using GPGPG linkers and adjuvant cholera toxin B subunit to boost the immune responses. A 3D model of the vaccine construct was built, loop refined, and considered for extensive error examination. Disulfide engineering was performed for the stability of the vaccine construct. Blind docking of the vaccine was conducted with host MHC-I, MHC-II, and toll-like receptors 4 (TLR-4) molecules. Molecular dynamic simulation was carried out to understand the vaccine-receptors dynamics and binding stability, as well as to evaluate the presentation of epitopes to the host immune system. Binding energies estimation was achieved to understand intermolecular interaction energies and validate docking and simulation studies. The results suggested that the designed vaccine construct has high potential to induce protective host immune responses and can be a good vaccine candidate for experimental in vivo and in vitro studies.

Evaluation of the Whole Proteome of Achromobacter xylosoxidans to Identify Vaccine Targets for mRNA and Peptides-Based Vaccine Designing Against the Emerging Respiratory and Lung Cancer-Causing Bacteria

Achromobacter xylosoxidans is a rod-shaped Gram-negative bacterium linked with causing several infections which mostly includes hematological malignancies. It has been recently reported to be associated with the development and progression of lung cancer and is an emerging respiratory disease-causing bacterium. The treatment of individuals infected with A. xylosoxidans bacteremia is difficult due to the fact that this pathogen has both intrinsic and acquired resistance mechanisms, typically resulting in a phenotype of multidrug resistance (MDR). Efforts are needed to design effective therapeutic strategies to curtail the emergence of this bacterium. Computational vaccine designing has proven its effectiveness, specificity, safety, and stability compared to conventional approaches of vaccine development. Therefore, the whole proteome of A. xylosoxidans was screened for the characterization of potential vaccine targets through subtractive proteomics pipeline for therapeutics design. Annotation of the whole proteome confirmed the three immunogenic vaccine targets, such as (E3HHR6), (E3HH04), and (E3HWA2), which were used to map the putative immune epitopes. The shortlisted epitopes, specific against Cytotoxic T Lymphocytes, Helper T-cell Lymphocytes, and linear B-Cell, were used to design the mRNA and multi-epitopes vaccine (MEVC). Initial validations confirmed the antigenic and non-allergenic properties of these constructs, followed by docking with the immune receptor, TLR-5, which resulted in robust interactions. The interaction pattern that followed in the docking complex included formation of 5 hydrogen bonds, 2 salt bridges, and 165 non-bonded contacts. This stronger binding affinity was also assessed through using the mmGBSA approach, showing a total of free binding energy of -34.64 kcal/mol. Further validations based on in silico cloning revealed a CAI score of 0.98 and an optimal percentage of GC contents (54.4%) indicated a putatively higher expression of the vaccine construct in Escherichia coli. Moreover, immune simulation revealed strong antibodies production upon the injection of the designed MEVC that resulted in the highest peaks of IgM+ IgG production (>3,500) between 10 and 15 days. In conclusion the current study provide basis for vaccine designing against the emerging A. xylosoxidans, which demands further experimental studies for in vitro and in vivo validations.

Recent Progress in Shigella and Burkholderia pseudomallei Vaccines

Significant advancement has been made in the development of vaccines against bacterial pathogens. However, several roadblocks have been found during the evaluation of vaccines against intracellular bacterial pathogens. Therefore, new lessons could be learned from different vaccines developed against unrelated intracellular pathogens. Bacillary dysentery and melioidosis are important causes of morbidity and mortality in developing nations, which are caused by the intracellular bacteria Shigella and Burkholderia pseudomallei, respectively. Although the mechanisms of bacterial infection, dissemination, and route of infection do not provide clues about the commonalities of the pathogenic infectious processes of these bacteria, a wide variety of vaccine platforms recently evaluated suggest that in addition to the stimulation of antibodies, identifying protective antigens and inducing T cell responses are some additional required elements to induce effective protection. In this review, we perform a comparative evaluation of recent candidate vaccines used to combat these two infectious agents, emphasizing the common strategies that can help investigators advance effective and protective vaccines to clinical trials.

Predictive and Experimental Immunogenicity of Burkholderia Collagen-like Protein 8-Derived Antigens

Burkholderia pseudomallei is an infectious bacterium of clinical and biodefense concern, and is the causative agent of melioidosis. The mortality rate can reach up to 50% and affects 165,000 people per year; however, there is currently no vaccine available. In this study, we examine the antigen-specific immune response to a vaccine formulated with antigens derived from an outer membrane protein in B. pseudomallei, Bucl8. Here, we employed a number of bioinformatic tools to predict Bucl8-derived epitopes that are non-allergenic and non-toxic, but would elicit an immune response. From these data, we formulated a vaccine based on two extracellular components of Bucl8, the β-barrel loops and extended collagen and non-collagen domains. Outbred CD-1 mice were immunized with vaccine formulations—composed of recombinant proteins or conjugated synthetic peptides with adjuvant—to assess the antigen-specific immune responses in mouse sera and lymphoid organs. We found that mice vaccinated with either Bucl8-derived components generated a robust TH2-skewed antibody response when antigen was combined with the adjuvant AddaVax, while the TH1 response was limited. Mice immunized with synthetic loop peptides had a stronger, more consistent antibody response than recombinant protein antigens, based on higher IgG titers and recognition of bacteria. We then compared peptide-based vaccines in an established C57BL/6 inbred mouse model and observed a similar TH2-skewed response. The resulting formulations will be applied in future studies examining the protection of Bucl8-derived vaccines.

The Peptidoglycan-associated lipoprotein Pal contributes to the virulence of Burkholderia mallei and provides protection against lethal aerosol challenge

Burkholderia mallei

is a highly pathogenic bacterium that causes the fatal zoonosis glanders. The organism specifies multiple membrane proteins, which represent prime targets for the development of countermeasures given their location at the host-pathogen interface. We investigated one of these proteins, Pal, and discovered that it is involved in the ability of B. mallei to resist complement-mediated killing and replicate inside host cells in vitro, is expressed in vivo and induces antibodies during the course of infection, and contributes to virulence in a mouse model of aerosol infection. A mutant in the pal gene of the B. mallei wild-type strain ATCC 23344 was found to be especially attenuated, as BALB/c mice challenged with the equivalent of 5,350 LD₅₀ completely cleared infection. Based on these findings, we tested the hypothesis that a vaccine containing the Pal protein elicits protective immunity against aerosol challenge. To achieve this, the pal gene was cloned in the vaccine vector Parainfluenza Virus 5 (PIV5) and mice immunized with the virus were infected with a lethal dose of B. mallei. These experiments revealed that a single dose of PIV5 expressing Pal provided 80% survival over a period of 40 days post-challenge. In contrast, only 10% of mice vaccinated with a PIV5 control virus construct survived infection. Taken together, our data establish that the Peptidoglycan-associated lipoprotein Pal is a critical virulence determinant of B. mallei and effective target for developing a glanders vaccine.

Use of Recombinant Escherichia coli Strains in Immunofluorescence Assays for Melioidosis Diagnosis

Burkholderia pseudomallei is a Gram-negative bacterium and the causative agent of melioidosis in humans and animals in the tropics. The clinical manifestations of melioidosis are diverse, ranging from localized infections to whole-body sepsis. The effective serological method is crucial for the point-of-care diagnosis of melioidosis. The aim of this study was to develop indirect immunofluorescence assay (IFA)-based methods for detecting immunoglobulin G (IgG) antibodies in melioidosis patients. These methods use whole-cell antigens made from recombinant E. coli strains that express major B. pseudomallei antigens, including TssM, OmpH, AhpC, BimA, and Hcp1. A total of 271 serum samples from culture-confirmed melioidosis patients (n = 81), patients with other known infections (n = 70), and healthy donors (n = 120) were tested. Our study showed that the recombinant TssM strain had the highest performance, with 92.6% sensitivity, 100% specificity, 100% positive predictive value, 96.9% negative predictive value, 97.8% efficiency, 97.0% accuracy, and no cross-reactivity. The method agreement analysis based on k efficiency calculations showed that all five IFA methods perfectly agreed with the standard culturing method, while the traditional indirect hemagglutination (IHA) method moderately agreed with the culture. In summary, our investigations showed that the TssM-IFA method could be used for melioidosis diagnosis.

Thinking Outside the Bug: Targeting Outer Membrane Proteins for Burkholderia Vaccines

Increasing antimicrobial resistance due to misuse and overuse of antimicrobials, as well as a lack of new and innovative antibiotics in development has become an alarming global threat. Preventative therapeutics, like vaccines, are combative measures that aim to stop infections at the source, thereby decreasing the overall use of antibiotics. Infections due to Gram-negative pathogens pose a significant treatment challenge because of substantial multidrug resistance that is acquired and spread throughout the bacterial population. Burkholderia spp. are Gram-negative intrinsically resistant bacteria that are responsible for environmental and nosocomial infections. The Burkholderia cepacia complex are respiratory pathogens that primarily infect immunocompromised and cystic fibrosis patients, and are acquired through contaminated products and equipment, or via patient-to-patient transmission. The Burkholderia pseudomallei complex causes percutaneous wound, cardiovascular, and respiratory infections. Transmission occurs through direct exposure to contaminated water, water-vapors, or soil, leading to the human disease melioidosis, or the equine disease glanders. Currently there is no licensed vaccine against any Burkholderia pathogen. This review will discuss Burkholderia vaccine candidates derived from outer membrane proteins, OmpA, OmpW, Omp85, and Bucl8, encompassing their structures, conservation, and vaccine formulation.

Burkholderia pseudomallei OMVs derived from infection mimicking conditions elicit similar protection to a live-attenuated vaccine

Burkholderia pseudomallei is a Gram-negative, facultative intracellular bacillus that causes the disease melioidosis. B. pseudomallei expresses a number of proteins that contribute to its intracellular survival in the mammalian host. We previously demonstrated that immunization with OMVs derived from B. pseudomallei grown in nutrient-rich media protects mice against lethal disease. Here, we evaluated if OMVs derived from B. pseudomallei grown under macrophage-mimicking growth conditions could be enriched with intracellular-stage proteins in order to improve the vaccine. We show that OMVs produced in this manner (M9 OMVs) contain proteins associated with intracellular survival yet are non-toxic to living cells. Immunization of mice provides significant protection against pulmonary infection similar to that achieved with a live attenuated vaccine and is associated with increased IgG, CD4⁺, and CD8⁺ T cells. OMVs possess inherent adjuvanticity and drive DC activation and maturation. These results indicate that M9 OMVs constitute a new promising vaccine against melioidosis.

Current Advances in Burkholderia Vaccines Development

The genus Burkholderia includes a wide range of Gram-negative bacterial species some of which are pathogenic to humans and other vertebrates. The most pathogenic species are Burkholderia mallei, Burkholderia pseudomallei, and the members of the Burkholderia cepacia complex (Bcc). B. mallei and B. pseudomallei, the cause of glanders and melioidosis, respectively, are considered potential bioweapons. The Bcc comprises a subset of Burkholderia species associated with respiratory infections in people with chronic granulomatous disease and cystic fibrosis. Antimicrobial treatment of Burkholderia infections is difficult due to the intrinsic multidrug antibiotic resistance of these bacteria; prophylactic vaccines provide an attractive alternative to counteract these infections. Although commercial vaccines against Burkholderia infections are still unavailable, substantial progress has been made over recent years in the development of vaccines against B. pseudomallei and B. mallei. This review critically discusses the current advances in vaccine development against B. mallei, B. pseudomallei, and the Bcc.

A DUF4148 family protein produced inside RAW264.7 cells is a critical Burkholderia pseudomallei virulence factor

Burkholderia pseudomallei: is the etiological agent of the disease melioidosis and is a Tier 1 select agent. It survives and replicates inside phagocytic cells by escaping from the endocytic vacuole, replicating in the cytosol, spreading to other cells via actin polymerization and promoting the fusion of infected and uninfected host cells to form multinucleated giant cells. In this study, we utilized a proteomics approach to identify bacterial proteins produced inside RAW264.7 murine macrophages and host proteins produced in response to B. pseudomallei infection. Cells infected with B. pseudomallei strain K96243 were lysed and the lysate proteins digested and analyzed using nanoflow reversed-phase liquid chromatography and tandem mass spectrometry. Approximately 160 bacterial proteins were identified in the infected macrophages, including BimA, TssA, TssB, Hcp1 and TssM. Several previously uncharacterized B. pseudomallei proteins were also identified, including BPSS1996 and BPSL2748. Mutations were constructed in the genes encoding these novel proteins and their relative virulence was assessed in BALB/c mice. The 50% lethal dose for the BPSS1996 mutant was approximately 55-fold higher than that of the wild type, suggesting that BPSS1996 is required for full virulence. Sera from B. pseudomallei-infected animals reacted with BPSS1996 and it was found to localize to the bacterial surface using indirect immunofluorescence. Finally, we identified 274 host proteins that were exclusively present or absent in infected RAW264.7 cells, including chemokines and cytokines involved in controlling the initial stages of infection.

Prediction of Burkholderia pseudomallei DsbA substrates identifies potential virulence factors and vaccine targets

Identification of bacterial virulence factors is critical for understanding disease pathogenesis, drug discovery and vaccine development. In this study we used two approaches to predict virulence factors of Burkholderia pseudomallei, the Gram-negative bacterium that causes melioidosis. B. pseudomallei is naturally antibiotic resistant and there are no clinically available melioidosis vaccines. To identify B. pseudomallei protein targets for drug discovery and vaccine development, we chose to search for substrates of the B. pseudomallei periplasmic disulfide bond forming protein A (DsbA). DsbA introduces disulfide bonds into extra-cytoplasmic proteins and is essential for virulence in many Gram-negative organism, including B. pseudomallei. The first approach to identify B. pseudomallei DsbA virulence factor substrates was a large-scale genomic analysis of 511 unique B. pseudomallei disease-associated strains. This yielded 4,496 core gene products, of which we hypothesise 263 are DsbA substrates. Manual curation and database screening of the 263 mature proteins yielded 81 associated with disease pathogenesis or virulence. These were screened for structural homologues to predict potential B-cell epitopes. In the second approach, we searched the B. pseudomallei genome for homologues of the more than 90 known DsbA substrates in other bacteria. Using this approach, we identified 15 putative B. pseudomallei DsbA virulence factor substrates, with two of these previously identified in the genomic approach, bringing the total number of putative DsbA virulence factor substrates to 94. The two putative B. pseudomallei virulence factors identified by both methods are homologues of PenI family β-lactamase and a molecular chaperone. These two proteins could serve as high priority targets for future B. pseudomallei virulence factor characterization.

Immunoinformatics and analysis of antigen distribution of Ureaplasma diversum strains isolated from different Brazilian states

BACKGROUND

Ureaplasma diversum has numerous virulence factors that contribute to pathogenesis in cattle, including Lipid-associated membrane proteins (LAMPs). Therefore, the objectives of this study were to evaluate in silico important characteristics for immunobiological applications and for heterologous expression of 36 LAMPs of U. diversum (UdLAMPs) and, also, to verify by conventional PCR the distribution of these antigens in strains of Brazilian states (Bahia, Minas Gerais, São Paulo, and Mato Grosso do Sul). The Manatee database was used to obtain the gene and peptide sequences of the antigens. Similarity and identity studies were performed using BLASTp and direct antigenicity was evaluated by the VaxiJen v2.0 server. Epitope prediction for B lymphocytes was performed on the BepiPred v2.0 and CBTOPE v1.0 servers. NetBoLApan v1.0 was used to predict CD8⁺ T lymphocyte epitopes. Subcellular location and presence of transmembrane regions were verified by the software PSORTb v3.0.2 and TMHMM v2.2 respectively. SignalP v5.0, SecretomeP v2.0, and DOLOP servers were used to predict the extracellular excretion signal. Physico-chemical properties were evaluated by the web-software ProtParam, Solpro, and Protein-sol.

RESULTS

In silico analysis revealed that many UdLAMPs have desirable properties for immunobiological applications and heterologous expression. The proteins gudiv_61, gudiv_103, gudiv_517, and gudiv_681 were most promising. Strains from the 4 states were PCR positive for antigens predicted with immunogenic and/or with good characteristics for expression in a heterologous system.

CONCLUSION

These works contribute to a better understanding of the immunobiological properties of the UdLAMPs and provide a profile of the distribution of these antigens in different Brazilian states.

Identification of novel therapeutic target and epitopes through proteome mining from essential hypothetical proteins in Salmonella strains: An In silico approach towards antivirulence therapy and vaccine development

Salmonella strains are responsible for a huge mortality rate through foodborne ailment in the world that necessitated the discovery of novel drugs and vaccines. Essential hypothetical proteins (EHPs), whose structures and functions were previously unknown, could serve as potential therapeutic and vaccine targets. Antivirulence therapy shall emerge as a superior therapeutic approach that uses virulence factors as drug targets. This study annotated the biological functions of 96 out of total 106 essential hypothetical proteins in five strains of Salmonella and classified into nine important protein categories. 34 virulence factors were predicted among the EHPs, out of which, 11 were identified to be pathogen specific potential drug targets for antivirulence therapy. These targets were non-homologous to both human and gut microbiota proteome to avoid cross-reactivity with them. Seven identified targets had druggable property, while the rest four targets were novel targets. Four identified targets (DEG10320148, DEG10110027, DEG10110040 and DEG10110142) had antigenic properties and were further classified as: two membrane-bound Lipid-binding transmembrane proteins, a Zinc-binding membrane protein and an extracellular glycosylase. These targets could be potentially used for the development of subunit vaccines. The study further identified 11 highly conserved and exposed epitope sequences from these 4 vaccine targets. The three-dimensional structures of the vaccine targets were also elucidated along with highlighting the conformation of the epitopes. This study identified potential therapeutic targets for antivirulence therapy against Salmonella. It would therefore instigate in novel drug designing as well as provide important leads to new Salmonella vaccine development.

Development of an immunoassay using recombinant outer membrane protein A and flagellin for diagnosis of goats with melioidosis

Among domestic animals, melioidosis is one of the most common diseases reported in goat, sheep, and swine. To evaluate the specific antibodies in goats with melioidosis, we developed a serology test using recombinant outer membrane protein A (OmpA) and flagellin (FliC) of Burkholderia pseudomallei as antigens. DNA corresponding to each antigen was cloned into a pET32a vector and expressed in Escherichia coli. Essentially, the recombinant OmpA and FliC were expressed in a soluble form that could be isolated with 95% homogeneity. Both recombinants could be recognized by rabbit antibodies prepared against heat-inactivated B. pseudomallei (1:1,000) on a Western blot. Subsequently, we demonstrated that both recombinants could capture the antibodies present in goat with naturally occurring melioidosis (optimized titer 1:40) while not cross-reacting with the serum samples of goats naturally infected by Corynebacterium pseudotuberculosis or Staphylococcus aureus. Finally, an enzyme-linked immunosorbent assay (ELISA) using 20 goat serum samples without melioidosis and 10 goat serum samples with melioidosis demonstrated that the infected group has significantly higher antibody titer levels than the normal group (P<0.001) when using either OmpA or FliC as an antigen. However, the sensitivity (100%) of the assay using OmpA was superior to that (90%) from using FliC. Serological tests that are commonly used often rely on antigens from crude cell extracts, which pose risks for laboratory-acquired infections and inconsistency in their preparation; however, use of recombinant OmpA is safe; it can potentially be used as a reagent in testing for goat melioidosis.

Immunogenicity and protective efficacy of Burkholderia pseudomallei BLF1-N and BLF1-C terminal domains against BLF1 toxin

Burkholderia lethal factor 1 (BLF1), a glutamine deamidase, is a key virulence factor that plays significant role in B. pseudomallei pathogenesis. To elucidate the BLF1 immunological responses, two truncated BLF1 structural units, BLF1-C (90-211 amino acids) with structural similarity to T. maritima Chemoreceptor glutamine deamidase (CheD) protein, and BLF1-N (1-89 amino acids) disparate to CheD were identified from the 23 kDa BLF1 protein. Both the components were devoid of toxicity in mice and elicited an antibody titer of 1:16,000 that reacted with the respective truncated proteins and BLF1. A549 cell lines supplemented with anti BLF1-N and BLF1-C antibodies exhibited 73.47% and 83.24% survival when treated with BLF1 toxin. Passive i.p. transfer with antibodies elicited by BLF1-C that contained LSGC active site resulted in 80% protection while anti BLF1-N (devoid of LSGC) antibodies provided 51.4% protection, establishing the role of BLF1-N terminal also in deamidase action. The truncated proteins also elicited cell mediated immune responses through proliferation of CD4+ T cells, IFN-γ and IL-4 cytokines but with bias towards Th2 subsets. BLF1-C and BLF1-N immunization resulted in 80% and 60% active protection when challenged with BLF1 toxin while the sham immunized mice exhibited severe histopathological changes like necrosis in liver, lung, spleen and kidney similar to that observed in melioidosis and were killed within 7 days post challenge. The higher level of active and passive protection by BLF1-C protein could be attributed to the comparatively higher level of immune responses and inclusion of LSGC residues.

Novel multi-component vaccine approaches for Burkholderia pseudomallei

Burkholderia pseudomallei is the causative agent of melioidosis. Historically believed to be a relatively rare human disease in tropical countries, a recent study estimated that, worldwide, there are approximately 165 000 human melioidosis cases per year, more than half of whom die. The bacterium is inherently resistant to many antibiotics and treatment of the disease is often protracted and ineffective. There is no licensed vaccine against melioidosis, but a vaccine is predicted to be of value if used in high-risk populations. There has been progress over the last decade in the pursuit of an effective vaccine against melioidosis. Animal models of disease including mouse and non-human primates have been developed, and these models show that antibody responses play a key role in protection against melioidosis. Surprisingly, although B. pseudomallei is an intracellular pathogen there is limited evidence that CD8⁺  T cells play a role in protection. It is evident that a multi-component vaccine, incorporating one or more protective antigens, will probably be essential for protection because of the pathogen's sophisticated virulence mechanisms as well as strain heterogeneity. Multi-component vaccines in development include glycoconjugates, multivalent subunit preparations, outer membrane vesicles and other nano/microparticle platforms and live-attenuated or inactivated bacteria. A consistent finding with vaccine candidates tested in mice is the ability to induce sterilizing immunity at low challenge doses and extended time to death at higher challenge doses. Further research to identify ways of eliciting more potent immune responses might provide a path for licensing an effective vaccine.

BPSL1626: Reverse and Structural Vaccinology Reveal a Novel Candidate for Vaccine Design against Burkholderia pseudomallei

Due to significant advances in computational biology, protein prediction, together with antigen and epitope design, have rapidly moved from conventional methods, based on experimental approaches, to in silico-based bioinformatics methods. In this context, we report a reverse vaccinology study that identified a panel of 104 candidate antigens from the Gram-negative bacterial pathogen Burkholderia pseudomallei, which is responsible for the disease melioidosis. B. pseudomallei can cause fatal sepsis in endemic populations in the tropical regions of the world and treatment with antibiotics is mostly ineffective. With the aim of identifying potential vaccine candidates, we report the experimental validation of predicted antigen and type I fimbrial subunit, BPSL1626, which we show is able to recognize and bind human antibodies from the sera of Burkholderia infected patients and to stimulate T-lymphocytes in vitro. The prerequisite for a melioidosis vaccine, in fact, is that both antibody- and cell-mediated immune responses must be triggered. In order to reveal potential antigenic regions of the protein that may aid immunogen re-design, we also report the crystal structure of BPSL1626 at 1.9 Å resolution on which structure-based epitope predictions were based. Overall, our data suggest that BPSL1626 and three epitope regions here-identified can represent viable candidates as potential antigenic molecules.

Effect of temperature on Burkholderia pseudomallei growth, proteomic changes, motility and resistance to stress environments

Burkholderia pseudomallei is a flagellated, gram-negative environmental bacterium that causes melioidosis, a severe infectious disease of humans and animals in tropical areas. We hypothesised that B. pseudomallei may undergo phenotypic adaptation in response to an increase in growth temperature. We analysed the growth curves of B. pseudomallei strain 153 cultured in Luria–Bertani broth at five different temperatures (25 °C–42 °C) and compared the proteomes of bacteria cultured at 37 °C and 42 °C. B. pseudomallei exhibited the highest growth rate at 37 °C with modest reductions at 30 °C, 40 °C and 42 °C but a more marked delay at 25 °C. Proteome analysis revealed 34 differentially expressed protein spots between bacterial cultures at 42 °C versus 37 °C. These were identified as chaperones (7 spots), metabolic enzymes (12 spots), antioxidants (10 spots), motility proteins (2 spots), structural proteins (2 spots) and hypothetical proteins (1 spot). Of the 22 down-regulated proteins at 42 °C, redundancy in motility and antioxidant proteins was observed. qRT-PCR confirmed decreased expression of fliC and katE. Experiments on three B. pseudomallei strains demonstrated that these had the highest motility, greatest resistance to H₂O₂ and greatest tolerance to salt stress at 37 °C. Our data suggest that temperature affects B. pseudomallei motility and resistance to stress.

Six genes of ompA family shuffling for development of polyvalent vaccines against Vibrio alginolyticus and Edwardsiella tarda

Polyvalent vaccines against more than one species of pathogens are especially important due to the complex ecosystem in aquaculture. We have previously shown that the development of polyvalent vaccines by shuffling six ompA genes from different bacteria with V. parahaemolyticus VP0764 primers. Here, we used the same 6 genes, V. alginolyticus VA0764 and VA1186, V. parahaemolyticus VP0764 and VP1186, E. tarda ompA and E. coli ompA, but with E. tarda ompA primers to develop new polyvalent vaccines. By this approach, we identified 7 potential polyvalent vaccines that were effective against both V. alginolyticus and E. tarda infections. Furthermore, the innate immunity triggered by the vaccines were also explored in three groups, no protection (group I), protection against V. alginolyticus (group II), and protection against both V. alginolyticus and E. tarda (group III). The transcription of IL-1β, IL-6, IL-8, C3b and NF-kB were significantly increased in group II and group III but not group I, where the expression level of group III was higher than group II. In addition, differential activities of succinate dehydrogenase were detected among the three groups. These results indicate the expansion of polyvalent vaccine reservoir with the same shuffling genes but different primers, and promote the understanding of the mechanisms of polyvalent vaccines based on vaccine-induced innate immunity.

Construction, immune protection and innate immune response of shuffled polyvalent ompAs vaccines

Our previous studies demonstrated that molecular breeding via DNA shuffling directs the evolution of polyvalent vaccines with desired traits, which leads to generation of polyvalent ompA vaccines using Vibrio alginolyticus VA0764 primers. Here, we replaced VA0764 primers with Edwardsiella tarda ompA primers to generate new polyvalent ompA vaccines by DNA shuffling of the same five ompA genes from four species of bacteria E. tarda, V. parahaemolyticus, V. alginolyticus and Escherichia coli. We identified four polyvalent vaccine candidates from a eukaryotic expressing library EompAs-FE containing 82 ompAs using active immune protection against V. alginolyticus and E. tarda. Furthermore, we explored mechanisms of polyvalent vaccine candidates by investigation of the innate immune response to these ompAs, and found that expression of IL-1β, IL-8, IL-15, COX-2, IFN-γ, TLR-1, TLR-3 and C3b genes was elevated as a characteristic feature of these polyvalent vaccine candidates. These results indicate that use of different primers to construct a DNA library selects new evolution of polyvalent vaccines with desired traits, and polyvalent ompA vaccines elicit high innate immune response.

Production and evaluation of recombinant Burkholderia pseudomallei GroEL and OmpA proteins for serodiagnosis of melioidosis

Melioidosis is a bacterial infectious disease caused by Burkholderia pseudomallei (B. pseudomallei). The variable clinical manifestations of this disease make its diagnosis difficult. The gold standard strategy for diagnosis is bacterial culture and identification, which is time-consuming and often too late for early medical intervention. Therefore, a rapid and accurate diagnosis of melioidosis is needed for effective treatment. This study aimed to produce and evaluate two purified B. pseudomallei recombinant proteins (rGroEL and rOmpA) as potential antigens for melioidosis diagnosis by ELISA. A total of 218 serum samples from Thailand was analysed. The study includes melioidosis patients' serum samples confirmed by bacterial culture (n=38); sera from patients with various bacterial infections but negative for B. pseudomallei (disease control, n=55); and sera from healthy individuals from non-endemic (n=77) and endemic (n=48) regions. The rGroEL ELISA achieved a good sensitivity of 92%, which was higher than that of rOmpA ELISA (76%). The specificities of rGroEL and rOmpA ELISAs were 88% and 90%, respectively. Both antigens demonstrated good diagnostic accuracy, at 89% (rGroEL) and 88% (rOmpA). There was less reactivity of sera from healthy individuals with rGroEL than rOmpA antigens. These data highlight the usefulness of rGroEL purified protein as a potential antigen for the serodiagnosis of melioidosis by ELISA and may be useful in the development of additional methods, such as dot blot ELISA or immunochromatographic tests (ICT), for a rapid, simple, cost-effective and efficient diagnosis for use in poorly resourced regions where melioidosis is endemic.

Designing Probes for Immunodiagnostics: Structural Insights into an Epitope Targeting Burkholderia Infections

Structure-based epitope prediction drives the design of diagnostic peptidic probes to reveal specific antibodies elicited in response to infections. We previously identified a highly immunoreactive epitope from the peptidoglycan-associated lipoprotein (Pal) antigen from Burkholderia pseudomallei, which could also diagnose Burkholderia cepacia infections. Here, considering the high phylogenetic conservation within Burkholderia species, we ask whether cross-reactivity can be reciprocally displayed by the synthetic epitope from B. cenocepacia. We perform comparative analyses of the conformational preferences and diagnostic performances of the corresponding epitopes from the two Burkholderia species when presented in the context of the full-length proteins or as isolated peptides. The effects of conformation on the diagnostic potential and cross-reactivity of Pal peptide epitopes are rationalized on the basis of the 1.8 Å crystal structure of B. cenocepacia Pal and through computational analyses. Our results are discussed in the context of designing new diagnostic molecules for the early detection of infectious diseases.

Prediction of vaccine candidates against Pseudomonas aeruginosa: An integrated genomics and proteomics approach

Pseudomonas aeruginosa is among top critical nosocomial infectious agents due to its persistent infections and tendency for acquiring drug resistance mechanisms. To date, there is no vaccine available for this pathogen. We attempted to exploit the genomic and proteomic information of P. aeruginosa though reverse-vaccinology approaches to unveil the prospective vaccine candidates. P. aeruginosa strain PAO1 genome was subjected to sequential prioritization approach following genomic, proteomics and structural analyses. Among, the predicted vaccine candidates: surface components of antibiotic efflux pumps (Q9HY88, PA2837), chaperone-usher pathway components (CupC2, CupB3), penicillin binding protein of bacterial cell wall (PBP1a/mrcA), extracellular component of Type 3 secretory system (PscC) and three uncharacterized secretory proteins (PA0629, PA2822, PA0978) were identified as potential candidates qualifying all the set criteria. These proteins were then analyzed for potential immunogenic surface exposed epitopes. These predicted epitopes may provide a basis for development of a reliable subunit vaccine against P. aeruginosa.

Within-Host Evolution of Burkholderia pseudomallei during Chronic Infection of Seven Australasian Cystic Fibrosis Patients

Cystic fibrosis (CF) is a genetic disorder characterized by progressive lung function decline. CF patients are at an increased risk of respiratory infections, including those by the environmental bacterium Burkholderia pseudomallei, the causative agent of melioidosis. Here, we compared the genomes of B. pseudomallei isolates collected between ~4 and 55 months apart from seven chronically infected CF patients. Overall, the B. pseudomallei strains showed evolutionary patterns similar to those of other chronic infections, including emergence of antibiotic resistance, genome reduction, and deleterious mutations in genes involved in virulence, metabolism, environmental survival, and cell wall components. We documented the first reported B. pseudomallei hypermutators, which were likely caused by defective MutS. Further, our study identified both known and novel molecular mechanisms conferring resistance to three of the five clinically important antibiotics for melioidosis treatment. Our report highlights the exquisite adaptability of microorganisms to long-term persistence in their environment and the ongoing challenges of antibiotic treatment in eradicating pathogens in the CF lung. Convergent evolution with other CF pathogens hints at a degree of predictability in bacterial evolution in the CF lung and potential targeted eradication of chronic CF infections in the future.IMPORTANCEBurkholderia pseudomallei, the causative agent of melioidosis, is an environmental opportunistic bacterium that typically infects immunocompromised people and those with certain risk factors such as cystic fibrosis (CF). Patients with CF tend to develop chronic melioidosis infections, for reasons that are not well understood. This report is the first to describe B. pseudomallei evolution within the CF lung during chronic infection. We show that the pathways by which B. pseudomallei adapts to the CF lung are similar to those seen in better-studied CF pathogens such as Pseudomonas aeruginosa, Staphylococcus aureus, and Burkholderia cepacia complex species. Adaptations include the accumulation of antibiotic resistance, loss of nonessential genes, metabolic alterations, and virulence factor attenuation. Known and novel mechanisms of resistance to three of the five antibiotics used in melioidosis treatment were identified. Similar pathways of evolution in CF pathogens, including B. pseudomallei, provide exciting avenues for more-targeted treatment of chronic, recalcitrant infections.

Boosting of post-exposure human T-cell and B-cell recall responses in vivo by Burkholderia pseudomallei-related proteins

Burkholderia pseudomallei is the causative agent of melioidosis, an infectious disease with high incidence and mortality in South East Asia and northern Australia. To date there is no protective vaccine and antibiotic treatment is prolonged and not always effective. Most people living in endemic areas have been exposed to the bacteria and have developed some immunity, which may have helped to prevent disease. Here, we used a humanized mouse model (hu-PBL-SCID), reconstituted with human peripheral blood mononuclear cells from seropositive donors, to illustrate the potential of three known antigens (FliC, OmpA and N-PilO2) for boosting both T-cell and B-cell immune responses. All three antigens boosted the production of specific antibodies in vivo, and increased the number of antibody and interferon-γ-secreting cells, and induced antibody affinity maturation. Moreover, antigen-specific antibodies isolated from either seropositive individuals or boosted mice, were found to enhance phagocytosis and oxidative burst activities from human polymorphonuclear cells. Our study demonstrates that FliC, OmpA and N-PilO2 can stimulate human memory T and B cells and highlight the potential of the hu-PBL-SCID system for screening and evaluation of novel protein antigens for inclusion in future vaccine trials against melioidosis.

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.

DNA shuffling approach for recombinant polyvalent OmpAs against V. alginolyticus and E. tarda infections

Molecular breeding via DNA shuffling directs the evolution of vaccines with desired traits. In the present study, polyvalent OmpA vaccines were generated by DNA shuffling of five ompA genes from four species of bacteria Vibrio parahaemolyticus, V. alginolyticus, Edwardsiella tarda and Escherichia coli. First, a new hybrid OmpA was constructed using VA0764 primers and used for construction of a prokaryotic expressing library PompAs-FV containing 84 ompAs, which were validated by PCR and SDS/PAGE. Then, the 84 ompAs were used to construct a eukaryotic expressing library EompAs-FV for preparing DNA vaccines. Third, extracellular bacterium V. alginolyticus challenge post active immunization using these DNA vaccines was carried out to identify genes with high immunoprotection. Among the 84 ompAs, 17 showed higher or equal immune protection against infection caused by V. alginolyticus than control VA0764. Finally, immune protection against infection caused by intracellular bacterium Edwardsiella tarda was assessed further using the top seven out of the 17 ompAs. This led to identification of three efficient polyvalent vaccines against infections caused by the extracellular bacterium V. alginolyticus and intracellular bacterium E. tarda. In addition, we sequenced genes for understanding mechanisms of the polyvalent vaccines, but association of immune protection with mutation of gene and amino acids is not determined. These results indicate that DNA shuffling is an efficient way to develop polyvalent vaccines against microbial infections.

Rapid and Sensitive Multiplex Detection of Burkholderia pseudomallei-Specific Antibodies in Melioidosis Patients Based on a Protein Microarray Approach

Background

The environmental bacterium Burkholderia pseudomallei causes the infectious disease melioidosis with a high case-fatality rate in tropical and subtropical regions. Direct pathogen detection can be difficult, and therefore an indirect serological test which might aid early diagnosis is desirable. However, current tests for antibodies against B. pseudomallei, including the reference indirect haemagglutination assay (IHA), lack sensitivity, specificity and standardization. Consequently, serological tests currently do not play a role in the diagnosis of melioidosis in endemic areas. Recently, a number of promising diagnostic antigens have been identified, but a standardized, easy-to-perform clinical laboratory test for sensitive multiplex detection of antibodies against B. pseudomallei is still lacking.

Methods and Principal Findings

In this study, we developed and validated a protein microarray which can be used in a standard 96-well format. Our array contains 20 recombinant and purified B. pseudomallei proteins, previously identified as serodiagnostic candidates in melioidosis. In total, we analyzed 196 sera and plasmas from melioidosis patients from northeast Thailand and 210 negative controls from melioidosis-endemic and non-endemic regions. Our protein array clearly discriminated between sera from melioidosis patients and controls with a specificity of 97%. Importantly, the array showed a higher sensitivity than did the IHA in melioidosis patients upon admission (cut-off IHA titer ≥1:160: IHA 57.3%, protein array: 86.7%; p = 0.0001). Testing of sera from single patients at 0, 12 and 52 weeks post-admission revealed that protein antigens induce either a short- or long-term antibody response.

Conclusions

Our protein array provides a standardized, rapid, easy-to-perform test for the detection of B. pseudomallei-specific antibody patterns. Thus, this system has the potential to improve the serodiagnosis of melioidosis in clinical settings. Moreover, our high-throughput assay might be useful for the detection of anti-B. pseudomallei antibodies in epidemiological studies. Further studies are needed to elucidate the clinical and diagnostic significance of the different antibody kinetics observed during melioidosis.

Melioidosis is a potentially fatal infectious disease caused by the Gram-negative environmental bacterium Burkholderia pseudomallei. Since the clinical presentations of melioidosis are extremely variable and no specific signs or symptoms exist, early laboratory-based diagnosis is highly desirable to start appropriate antibiotics. Routine methods for culture detection of B. pseudomallei are highly specific but take several days for a result, and depending on the clinical sample and other factors, sensitivity can be low. The standard serology test for melioidosis is an indirect hemagglutination assay (IHA) based on crude B. pseudomallei antigen preparations. Due to the variable prevalence of background seropositivity in endemic areas and the low diagnostic sensitivity of the IHA upon admission, the test is currently not recommended for the diagnosis of melioidosis, but still widely used. Thus, we generated a protein array containing 20 B. pseudomallei antigens previously shown to have serodiagnostic potential. Our array allows highly specific and sensitive antibody recognition in blood sera and plasmas from patients with melioidosis. The standardized microarray device is simple to use and fast, and is thus applicable in a routine diagnostic laboratory. In this study, the multiplex testing of antibodies in melioidosis sera from different time points after admission allowed the detection of short- and long-term antibodies to various antigens. Further studies will examine the potential role of those antibodies to discriminate different stages of the disease. Furthermore, the protein microarray might be useful in studies aimed at elucidating the exposure of humans and animals to B. pseudomallei in different parts of the world.

Identification of an OmpW homologue in Burkholderia pseudomallei, a protective vaccine antigen against melioidosis

Burkholderia pseudomallei is the causative agent of melioidosis, which is associated with a range of clinical manifestations, including sepsis and fatal pneumonia and is endemic in Southeast Asia and Northern Australia. Treatment can be challenging and control of infection involves prolonged antibiotic therapy, yet there are no approved vaccines available to prevent infection. Our aim was to develop and assess the potential of a prophylactic vaccine candidate targeted against melioidosis. The identified candidate is the 22kDa outer membrane protein, OmpW. We previously demonstrated that this protein was immunoprotective in mouse models of Burkholderia cepacia complex (Bcc) infections. We cloned Bp_ompW in Escherichia coli, expressed and purified the protein. Endotoxin free protein administered with SAS adjuvant protected Balb/C mice (75% survival) relative to controls (25% survival) (p<0.05). A potent serological response was observed with IgG2a to IgG1 ratio of 6.0. Furthermore C57BL/6 mice were protected for up to 80 days against a lethal dose of B. pseudomallei and surpassed the efficacy of the live attenuated 2D2 positive control. BpompW is homologous across thirteen sequenced B. pseudomallei strains, indicating that it should be broadly protective against B. pseudomallei. In conclusion, we have demonstrated that BpOmpW is able to induce protective immunity against melioidosis and is likely to be an effective vaccine antigen, possibly in combination with other subunit antigens.

Proteomic analysis and identification of cell surface-associated proteins of Clostridium chauvoei

Blackleg is a highly fatal disease of cattle and sheep, caused by Clostridium chauvoei, a Gram positive, anaerobic, spore forming bacteria. Cell surface-associated proteins play a major role in inducing the protective immunity. However, the identity of a majority of cell surface-associated proteins of C. chauvoei is not known. In the present investigation, we have used SDS-PAGE, 2D-gel electrophoresis and Western blotting followed by mass spectrometry to identify cell surface-associated proteins of C. chauvoei. Among the identified proteins, which have shown to offer protective antigencity in other bacteria, Enolase, Chaperonin, Ribosomal protein L10, Glycosyl Hydrolase and Flavoprotein were characterized by sequencing and their overexpression in Escherichia coli. In conclusion, cell surface-associated proteins were identified using proteomic approach and the genes for the immunoreactive proteins were expressed, which may prove to be potential diagnostic or vaccine candidates.

Construction and immune protection evaluation of recombinant polyvalent OmpAs derived from genetically divergent ompA by DNA shuffling

A wide variety of bacterial infections is a major challenge in aquaculture. Development of polyvalent vaccines that can fight against as many pathogens as possible is especially necessary. The present study uses DNA shuffling to create a new hybrid OmpA with improved cross-protection against Vibrio alginolyticus and Edwardsiella tarda through the recombination of six OmpA genes from Vibrio parahaemolyticus, V. alginolyticus, E. tarda and Escherichia coli. Out of the 43 recombinant chimeras genes constructed using VA0764 primers, EompAs-19 was demonstrated as an ideal polyvalent vaccine against infections caused V. alginolyticus and E. tarda. Compared with VA0764, OmpAs-19 had three mutations, which may be a molecular basis of EompAs-19 as an efficient polyvalent vaccine against both V. alginolyticus and E. tarda infections. These results develop a polyvalent vaccine that prevents the infections caused by extracellular and intracellular bacteria. Thus, the present study highlights the way to develop polyvalent vaccines against microbial infections by DNA shuffling.

Development of repeatable arrays of proteins using immobilized DNA microplate (RAPID-M) technology

Background

Protein microarrays have enormous potential as in vitro diagnostic tools stemming from the ability to miniaturize whilst generating maximum evaluation of diagnostically relevant information from minute amounts of sample. In this report, we present a method known as repeatable arrays of proteins using immobilized DNA microplates (RAPID-M) for high-throughput in situ protein microarray fabrication. The RAPID-M technology comprises of cell-free expression using immobilized DNA templates and in situ protein purification onto standard microarray slides.

Results

To demonstrate proof-of-concept, the repeatable protein arrays developed using our RAPID-M technology utilized green fluorescent protein (GFP) and a bacterial outer membrane protein (OmpA) as the proteins of interest for microarray fabrication. Cell-free expression of OmpA and GFP proteins using beads-immobilized DNA yielded protein bands with the expected molecular sizes of 27 and 30 kDa, respectively. We demonstrate that the beads-immobilized DNA remained stable for at least four cycles of cell-free expression. The OmpA and GFP proteins were still functional after in situ purification on the Ni–NTA microarray slide.

Conclusion

The RAPID-M platform for protein microarray fabrication of two different representative proteins was successfully developed.

Towards New Drug Targets? Function Prediction of Putative Proteins of Neisseria meningitidis MC58 and Their Virulence Characterization

Neisseria meningitidis is a Gram-negative aerobic diplococcus, responsible for a variety of meningococcal diseases. The genome of N. meningitidis MC58 is comprised of 2114 genes that are translated into 1953 proteins. The 698 genes (∼35%) encode hypothetical proteins (HPs), because no experimental evidence of their biological functions are available. Analyses of these proteins are important to understand their functions in the metabolic networks and may lead to the discovery of novel drug targets against the infections caused by N. meningitidis. This study aimed at the identification and categorization of each HP present in the genome of N. meningitidis MC58 using computational tools. Functions of 363 proteins were predicted with high accuracy among the annotated set of HPs investigated. The reliably predicted 363 HPs were further grouped into 41 different classes of proteins, based on their possible roles in cellular processes such as metabolism, transport, and replication. Our studies revealed that 22 HPs may be involved in the pathogenesis caused by this microorganism. The top two HPs with highest virulence scores were subjected to molecular dynamics (MD) simulations to better understand their conformational behavior in a water environment. We also compared the MD simulation results with other virulent proteins present in N. meningitidis. This study broadens our understanding of the mechanistic pathways of pathogenesis, drug resistance, tolerance, and adaptability for host immune responses to N. meningitidis.

Characterization of Burkholderia pseudomallei Strains Using a Murine Intraperitoneal Infection Model and In Vitro Macrophage Assays

Burkholderia pseudomallei, the etiologic agent of melioidosis, is a gram-negative facultative intracellular bacterium. This bacterium is endemic in Southeast Asia and Northern Australia and can infect humans and animals by several routes. It has also been estimated to present a considerable risk as a potential biothreat agent. There are currently no effective vaccines for B. pseudomallei, and antibiotic treatment can be hampered by nonspecific symptomology, the high incidence of naturally occurring antibiotic resistant strains, and disease chronicity. Accordingly, there is a concerted effort to better characterize B. pseudomallei and its associated disease. Before novel vaccines and therapeutics can be tested in vivo, a well characterized animal model is essential. Previous work has indicated that mice may be a useful animal model. In order to develop standardized animal models of melioidosis, different strains of bacteria must be isolated, propagated, and characterized. Using a murine intraperitoneal (IP) infection model, we tested the virulence of 11 B. pseudomallei strains. The IP route offers a reproducible way to rank virulence that can be readily reproduced by other laboratories. This infection route is also useful in distinguishing significant differences in strain virulence that may be masked by the exquisite susceptibility associated with other routes of infection (e.g., inhalational). Additionally, there were several pathologic lesions observed in mice following IP infection. These included varisized abscesses in the spleen, liver, and haired skin. This model indicated that commonly used laboratory strains of B. pseudomallei (i.e., K96243 and 1026b) were significantly less virulent as compared to more recently acquired clinical isolates. Additionally, we characterized in vitro strain-associated differences in virulence for macrophages and described a potential inverse relationship between virulence in the IP mouse model of some strains and in the macrophage phagocytosis assay. Strains which were more virulent for mice (e.g., HBPU10304a) were often less virulent in the macrophage assays, as determined by several parameters such as intracellular bacterial replication and host cell cytotoxicity.

Curcumin rescues Caenorhabditis elegans from a Burkholderia pseudomallei infection

The tropical pathogen Burkholderia pseudomallei requires long-term parenteral antimicrobial treatment to eradicate the pathogen from an infected patient. However, the development of antibiotic resistance is emerging as a threat to this form of treatment. To meet the need for alternative therapeutics, we proposed a screen of natural products for compounds that do not kill the pathogen, but in turn, abrogate bacterial virulence. We suggest that the use of molecules or compounds that are non-bactericidal (bacteriostatic) will reduce or abolish the development of resistance by the pathogen. In this study, we adopted the established Caenorhabditis elegans-B. pseudomallei infection model to screen a collection of natural products for any that are able to extend the survival of B. pseudomallei infected worms. Of the 42 natural products screened, only curcumin significantly improved worm survival following infection whilst not affecting bacterial growth. This suggested that curcumin promoted B. pseudomallei-infected worm survival independent of pathogen killing. To validate that the protective effect of curcumin was directed toward the pathogen, bacteria were treated with curcumin prior to infection. Worms fed with curcumin-treated bacteria survived with a significantly extended mean-time-to-death (p < 0.0001) compared to the untreated control. In in vitro assays, curcumin reduced the activity of known virulence factors (lipase and protease) and biofilm formation. To determine if other bacterial genes were also regulated in the presence of curcumin, a genome-wide transcriptome analysis was performed on curcumin-treated pathogen. A number of genes involved in iron acquisition and transport as well as genes encoding hypothetical proteins were induced in the presence of curcumin. Thus, we propose that curcumin may attenuate B. pseudomallei by modulating the expression of a number of bacterial proteins including lipase and protease as well as biofilm formation whilst concomitantly regulating iron transport and other proteins of unknown function.

From crystal structure to in silico epitope discovery in the Burkholderia pseudomallei flagellar hook-associated protein FlgK

Melioidosis, caused by the Gram-negative bacterium Burkholderia pseudomallei, is a potentially fatal infection that is endemic in Southeast Asia and Northern Australia that is poorly controlled by antibiotics. Research efforts to identify antigenic components for a melioidosis vaccine have led to the identification of several proteins, including subunits forming the flagella that mediate bacterial motility, host colonization, and virulence. This study focuses on the B. pseudomallei flagellar hook-associated protein (FlgK(Bp)), and provides the first insights into the 3D structure of FlgK proteins as targets for structure-based antigen engineering. The FlgK(Bp) crystal structure (presented here at 1.8-Å resolution) reveals a multidomain fold, comprising two small β-domains protruding from a large elongated α-helical bundle core. The evident structural similarity to flagellin, the flagellar filament subunit protein, suggests that, depending on the bacterial species, flagellar hook-associated proteins are likely to show a conserved, elongated α-helical bundle scaffold coupled to a variable number of smaller domains. Furthermore, we present immune serum recognition data confirming, in agreement with previous findings, that recovered melioidosis patients produce elevated levels of antibodies against FlgK(Bp), in comparison with seronegative and seropositive healthy subjects. Moreover, we show that FlgK(Bp) has cytotoxic effects on cultured murine macrophages, suggesting an important role in bacterial pathogenesis. Finally, computational epitope prediction methods applied to the FlgK(Bp) crystal structure, coupled with in vitro mapping, allowed us to predict three antigenic regions that locate to discrete protein domains. Taken together, our results point to FlgK(Bp) as a candidate for the design and production of epitope-containing subunits/domains as potential vaccine components.

Cloning, expression and purification of outer membrane protein (OmpA) of Burkholderia pseudomallei and evaluation of its potential for serodiagnosis of melioidosis

Melioidosis is an emerging infectious disease in India and caused by gram-negative, soil saprophyte bacteria Burkholderia pseudomallei. This disease is endemic in Southeast Asia and northern Australia, and sporadic cases of melioidosis are also reported from southern states of India. The present study reports the cloning, expression, and purification of recombinant protein outer membrane protein A (OmpA) of B. pseudomallei and its evaluation in indirect enzyme-linked immunosorbent assay (ELISA) format with 87 serum samples collected from Manipal, Karnataka, India. Twenty-three samples from culture confirmed cases (n=23) of melioidosis, 25 serum samples from patients of other febrile illness and pyrexia of unknown origin (n=25), and 39 serum samples from healthy blood donors (n=39) from Kasturba Medical College, Manipal, were tested in this assay format. The assay showed sensitivity of 82.6% and specificity of 93.75%. The recombinant OmpA based indirect ELISA will be a useful tool for serodiagnosis of melioidosis in large scale rapid screening of clinical samples.

Comparison of the protective effects of killed Burkholderia pseudomallei and CpG oligodeoxynucleotide against live challenge

Melioidosis is a fatal disease caused by Burkholderia pseudomallei. Currently there is no vaccine available. Synthetic oligodeoxynucleotides with unmethylated CpG dinucleotide motifs (CpG ODN) can stimulate vertebrate immune cells and clear certain pathogens that are susceptible to a strong Th1 response. In our previous study, pretreatment with CpG ODN alone or CpG-ODN with cationic liposomes for 2-10 or 30 days before B. pseudomallei infection in mice conferred 80-100% protection. In the present study we investigated the protective effect of CpG-ODN together with heat-killed (HK) or paraformaldehyde-killed B. pseudomallei (PP). HK or PP were used to immunize BALB/c mice twice at 15-day intervals before intra-peritoneal challenge with 5LD50 of B. pseudomallei and observed for 30 days. We found that PP could significantly protect mice (60%) with an increased survival time (24.8±11.63 days) while in the HK and PBS groups, all infected mice died within 6 days. Although either CpG ODN or PP conferred significant protection, giving them in combination did not enhance it further. Serum IFN-γ levels on day-5 (before challenge) of the PP and PP+CpG ODN groups were significantly higher than those of the PBS control group. The results further support the importance of IFN-γ in host protection against B. pseudomallei and suggest further study on paraformaldehyde-killed bacteria as a component of a future B. pseudomallei vaccine.

Immunoproteomics: the key to discovery of new vaccine antigens against bacterial respiratory infections

The increase in antibiotic resistance and the shortage of new antimicrobials to prevent difficult bacterial infections underlines the importance of prophylactic therapies to prevent infection by bacterial pathogens. Vaccination has reduced the incidence of many serious diseases, including respiratory bacterial infections. However, there are many pathogens for which no vaccine is available and some vaccines are not effective among all age groups or among immunocompromised individuals. Immunoproteomics is a powerful technique which has been used to identify potential vaccine candidates to protect against pathogenic bacteria. The combination of proteomics with the detection of immunoreactive antigens using serum highlights immunogenic proteins that are expressed during infection. This is particularly useful when patient serum is used as the antigens that promote a humoral response during human infection are identified. This review outlines examples of vaccine candidates that have been identified using immunoproteomics and have successfully protected animals against challenge when tested in immunisation studies. Many immunoreactive proteins are common to several unrelated pathogens, however some of these are not always protective in animal immunisation and challenge studies. Furthermore, examples of well-established immunogens, including Bordetella pertussis antigen FHA were not detected in immunoproteomics studies, indicating that this technology may underrepresent the immunoreactive proteins in a pathogen. Although only one step in the pathway towards an efficacious approved vaccine, immunoproteomics is an important technology in the identification of novel vaccine antigens.

Exploiting the Burkholderia pseudomallei acute phase antigen BPSL2765 for structure-based epitope discovery/design in structural vaccinology

We solved the crystal structure of Burkholderia pseudomallei acute phase antigen BPSL2765 in the context of a structural vaccinology study, in the area of melioidosis vaccine development. Based on the structure, we applied a recently developed method for epitope design that combines computational epitope predictions with in vitro mapping experiments and successfully identified a consensus sequence within the antigen that, when engineered as a synthetic peptide, was selectively immunorecognized to the same extent as the recombinant protein in sera from melioidosis-affected subjects. Antibodies raised against the consensus peptide were successfully tested in opsonization bacterial killing experiments and antibody-dependent agglutination tests of B. pseudomallei. Our strategy represents a step in the development of immunodiagnostics, in the production of specific antibodies and in the optimization of antigens for vaccine development, starting from structural and physicochemical principles.

Identification of gingipain-specific I-A(b) -restricted CD4+ T cells following mucosal colonization with Porphyromonas gingivalis in C57BL/6 mice

Chronic periodontitis is associated with Porphyromonas gingivalis infection. Although virulence factors of P. gingivalis are hypothesized to contribute to the pathogenesis of periodontitis, it is unclear whether the local CD4(+) T-cell-mediated response they elicit prevents or contributes to periodontal bone destruction. We hypothesize that major histocompatibility complex class II I-A(b) -binding peptides existing in Kgp and RgpA are presented to CD4(+) T cells during P. gingivalis oral colonization. The protein sequences of gingipains RgpA and Kgp, and OMP40 and OMP41 of P. gingivalis were scanned using an I-A(b) -binding matrix. From this analysis we identified 53 candidate peptides that had the potential to engage the peptide-binding groove of the I-A(b) molecule of C57BL/6 mice. An ELISpot-based screen revealed those peptide-primed effector/memory CD4(+) T cells that could be re-stimulated in vitro with P. gingivalis or the peptide itself to produce interleukin-17A or interferon-γ. Two immunodominant peptides, Kgp467-477 (pKgp) and RgpA1054-1064 /Kgp1074-1084 (pR/Kgp) were identified and engineered to be displayed on I-A(b) molecular tetramers. Peptide pR/Kgp is conserved across all sequenced P. gingivalis strains. C57BL/6 mice were orally inoculated with P. gingivalis strain 53977 and cervical lymph node cells were stained with phycoerythrin-conjugated pKgp::I-A(b) and pR/Kgp::I-A(b) tetramers. We found that only pR/Kgp::I-A(b) bound with the desired specificity to gingipain-specific CD4(+) T cells. The pR/Kgp::I-A(b) tetramer complex will allow the identification of effector/memory CD4(+) T cells specific for two virulence factors of P. gingivalis strains associated with periodontal disease.

Induction of mouse melioidosis with meningitis by CD11b+ phagocytic cells harboring intracellular B. pseudomallei as a Trojan horse

BACKGROUND

Approximately 3-5% of patients with melioidosis manifest CNS symptoms; however, the clinical data regarding neurological melioidosis are limited.

METHODS AND FINDINGS

We established a mouse model of melioidosis with meningitis characterized by neutrophil infiltration into the meninges histologically and B. pseudomallei in the cerebrospinal fluid (CSF) by bacteriological culturing methods. As the disease progresses, the bacteria successively colonize the spleen, liver, bone marrow (BM) and brain and invade splenic and BM cells by days 2 and 6 post-infection, respectively. The predominant cell types intracellularly infected with B. pseudomallei were splenic and BM CD11b(+) populations. The CD11b(+)Ly6C(high) inflamed monocytes, CD11b(+)Ly6C(low) resident monocytes, CD11b(+)Ly6G(+) neutrophils, CD11b(+)F4/80(+) macrophages and CD11b(+)CD19(+) B cells were expanded in the spleen and BM during the progression of melioidosis. After adoptive transfer of CD11b populations harboring B. pseudomallei, the infected CD11b(+) cells induced bacterial colonization in the brain, whereas CD11b(-) cells only partially induced colonization; extracellular (free) B. pseudomallei were unable to colonize the brain. CD62L (selectin) was absent on splenic CD11b(+) cells on day 4 but was expressed on day 10 post-infection. Adoptive transfer of CD11b(+) cells expressing CD62L (harvested on day 10 post-infection) resulted in meningitis in the recipients, but transfer of CD11b(+) CD62L-negative cells did not.

CONCLUSIONS/SIGNIFICANCE

We suggest that B. pseudomallei-infected CD11b(+) selectin-expressing cells act as a Trojan horse and are able to transmigrate across endothelial cells, resulting in melioidosis with meningitis.

Multiple-antigen ELISA for melioidosis--a novel approach to the improved serodiagnosis of melioidosis

Background

Burkholderia pseudomallei, the causative agent of melioidosis, is endemic to Southeast Asia and northern Australia. Clinical manifestations of disease are diverse, ranging from chronic infection to acute septicaemia. The current gold standard of diagnosis involves bacterial culture and identification which is time consuming and often too late for early medical intervention. Hence, rapid diagnosis of melioidosis is crucial for the successful management of melioidosis.

Methods

The study evaluated 4 purified B. pseudomallei recombinant proteins (TssD-5, Omp3, smBpF4 and Omp85) as potential diagnostic agents for melioidosis. A total of 68 sera samples from Malaysian melioidosis patients were screened for the presence of specific antibodies towards these proteins using enzyme-linked immunosorbent assay (ELISA). Sera from patients with various bacterial and viral infections but negative for B. pseudomallei, as well as sera from healthy individuals, were also included as non-melioidosis controls. The Mann Whitney test was performed to compare the statistical differences between melioidosis patients and the non-melioidosis controls.

Results

TssD-5 demonstrated the highest sensitivity of 71% followed by Omp3 (59%), smBpF4 (41%) and Omp85 (19%). All 4 antigens showed equally high specificity (89-96%). A cocktail of all 4 antigens resulted in slightly reduced sensitivity of 65% but improved specificity (99%). Multiple-antigen ELISA provided improved sensitivity of 88.2% whilst retaining good specificity (96%). There was minimal reactivity with sera from healthy individuals proposing the utility of these antigens to demarcate diseased from non-symptomatic individuals in an endemic country.

Conclusions

TssD-5 demonstrated high detection sensitivity and specificity and the results were obtained within a few hours compared to time consuming culture and IFAT methods commonly used in a clinical setting. The use of multiple-antigens resulted in improved sensitivity (88.2%) whilst maintaining superior specificity. These data highlight the use of TssD-5 and other recombinant antigens tested in this study as potential serodiagnostic agents for melioidosis.