Immunization with the recombinant Burkholderia pseudomallei outer membrane protein Omp85 induces protective immunity in mice

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.

Hcp1-loaded staphylococcal membrane vesicle vaccine protects against acute melioidosis

Burkholderia pseudomallei is the causal agent of melioidosis, a deadly tropical infectious disease that lacks a vaccine. On the basis of the attenuated Staphylococcus aureus RN4220-Δagr (RN), we engineered the RN4220-Δagr/pdhB-hcp1 strain (RN-Hcp1) to generate B. pseudomallei hemolysin-coregulated protein 1 (Hcp1)-loaded membrane vesicles (^hcp1MVs). The immunization of BALB/c mice with ^hcp1MVs mixed with adjuvant by a three-dose regimen increased the serum specific IgG production. The serum levels of inflammatory factors, including TNF-α and IL-6, in ^hcp1MV-vaccinated mice were comparable with those in PBS-challenged mice. The partial adjuvant effect of staphylococcal MVs was observed with the elevation of specific antibody titer in ^hcp1MV-vaccinated mice relative to those that received the recombinant Hcp1 protein (rHcp1) or MVs derived from RN strain (^ΔagrMVs). The ^hcp1MVs/adjuvant vaccine protected 70% of mice from lethal B. pseudomallei challenge. Immunization with ^hcp1MVs only protected 60% of mice, whereas vaccination with rHcp1 or ^ΔagrMVs conferred no protection. Moreover, mice that received ^hcp1MVs/adjuvant and ^hcp1MVs immunization had low serum TNF-α and IL-6 levels and no inflammatory infiltration in comparison with other groups. In addition, all surviving mice in ^hcp1MVs/adjuvant and ^hcp1MVs groups exhibited no culturable bacteria in their lungs, livers, and spleens five days postinfection. Overall, our data highlighted a new strategy for developing B. pseudomallei vaccine and showed that Hcp1-incorporated staphylococcal MV is a promising candidate for the prevention of acute melioidosis.

In silico Designing of an Epitope-Based Vaccine Against Common E. coli Pathotypes

Escherichia coli (E. coli) is a Gram-negative bacterium that belongs to the family Enterobacteriaceae. While E. coli can stay as an innocuous resident in the digestive tract, it can cause a group of symptoms ranging from diarrhea to live threatening complications. Due to the increased rate of antibiotic resistance worldwide, the development of an effective vaccine against E. coli pathotypes is a major health priority. In this study, a reverse vaccinology approach along with immunoinformatics has been applied for the detection of potential antigens to develop an effective vaccine. Based on our screening of 5,155 proteins, we identified lipopolysaccharide assembly protein (LptD) and outer membrane protein assembly factor (BamA) as vaccine candidates for the current study. The conservancy of these proteins in the main E. coli pathotypes was assessed through BLASTp to make sure that the designed vaccine will be protective against major E. coli pathotypes. The multitope vaccine was constructed using cytotoxic T lymphocyte (CTL), helper T lymphocyte (HTL), and B cell lymphocyte (BCL) epitopes with suitable linkers and adjuvant. Following that, it was analyzed computationally where it was found to be antigenic, soluble, stable, and non-allergen. Additionally, the adopted docking study, as well as all-atom molecular dynamics simulation, illustrated the promising predicted affinity and free binding energy of this constructed vaccine against the human Toll-like receptor-4 (hTLR-4) dimeric state. In this regard, wet lab studies are required to prove the efficacy of the potential vaccine construct that demonstrated promising results through computational validation.

Transmembrane β-barrel proteins of bacteria: From structure to function

The outer membrane of Gram-negative bacteria is a specialized organelle conferring protection to the cell against various environmental stresses and resistance to many harmful compounds. The outer membrane has a number of unique features, including an asymmetric lipid bilayer, the presence of lipopolysaccharides and an individual proteome. The vast majority of the integral transmembrane proteins in the outer membrane belongs to the family of β-barrel proteins. These evolutionarily related proteins share a cylindrical, anti-parallel β-sheet core fold spanning the outer membrane. The loops and accessory domains attached to the β-barrel allow for a remarkable versatility in function for these proteins, ranging from diffusion pores and transporters to enzymes and adhesins. We summarize the current knowledge on β-barrel structure and folding and give an overview of their functions, evolution, and potential as drug targets.

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.

Interactions Between Pathogenic Burkholderia and the Complement System: A Review of Potential Immune Evasion Mechanisms

The genus Burkholderia contains over 80 different Gram-negative species including both plant and human pathogens, the latter of which can be classified into one of two groups: the Burkholderia pseudomallei complex (Bpc) or the Burkholderia cepacia complex (Bcc). Bpc pathogens Burkholderia pseudomallei and Burkholderia mallei are highly virulent, and both have considerable potential for use as Tier 1 bioterrorism agents; thus there is great interest in the development of novel vaccines and therapeutics for the prevention and treatment of these infections. While Bcc pathogens Burkholderia cenocepacia, Burkholderia multivorans, and Burkholderia cepacia are not considered bioterror threats, the incredible impact these infections have on the cystic fibrosis community inspires a similar demand for vaccines and therapeutics for the prevention and treatment of these infections as well. Understanding how these pathogens interact with and evade the host immune system will help uncover novel therapeutic targets within these organisms. Given the important role of the complement system in the clearance of bacterial pathogens, this arm of the immune response must be efficiently evaded for successful infection to occur. In this review, we will introduce the Burkholderia species to be discussed, followed by a summary of the complement system and known mechanisms by which pathogens interact with this critical system to evade clearance within the host. We will conclude with a review of literature relating to the interactions between the herein discussed Burkholderia species and the host complement system, with the goal of highlighting areas in this field that warrant further investigation.

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.

Interactions Between Pathogenic Burkholderia and the Complement System: A Review of Potential Immune Evasion Mechanisms

The genus Burkholderia contains over 80 different Gram-negative species including both plant and human pathogens, the latter of which can be classified into one of two groups: the Burkholderia pseudomallei complex (Bpc) or the Burkholderia cepacia complex (Bcc). Bpc pathogens Burkholderia pseudomallei and Burkholderia mallei are highly virulent, and both have considerable potential for use as Tier 1 bioterrorism agents; thus there is great interest in the development of novel vaccines and therapeutics for the prevention and treatment of these infections. While Bcc pathogens Burkholderia cenocepacia, Burkholderia multivorans, and Burkholderia cepacia are not considered bioterror threats, the incredible impact these infections have on the cystic fibrosis community inspires a similar demand for vaccines and therapeutics for the prevention and treatment of these infections as well. Understanding how these pathogens interact with and evade the host immune system will help uncover novel therapeutic targets within these organisms. Given the important role of the complement system in the clearance of bacterial pathogens, this arm of the immune response must be efficiently evaded for successful infection to occur. In this review, we will introduce the Burkholderia species to be discussed, followed by a summary of the complement system and known mechanisms by which pathogens interact with this critical system to evade clearance within the host. We will conclude with a review of literature relating to the interactions between the herein discussed Burkholderia species and the host complement system, with the goal of highlighting areas in this field that warrant further investigation.

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.

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.

Immunogenicity of a bivalent protein as a vaccine against Edwardsiella anguillarum and Vibrio vulnificus in Japanese eel (Anguilla japonica)

The OMPs A (OmpA)—of Edwardsiella anguillarum and OmpU of V. vulnificus have been proven to be good antigens. In this study, after construction of a vector, a new recombinant Omp (rOMP) containing both OmpA and OmpU was expressed and purified. Then, the Japanese eels (Anguilla japonica) were intraperitoneally (i.p.) injected with the phosphate‐buffered saline (PBS group), formalin‐killed‐cell (FKC group) or the recombinant Omp (rOMP group). The stimulation index of the whole blood cells in eels from FKC group was significantly higher than the eels from PBS and rOMP groups at 28 dpi; serum titers of anti‐E. anguillarum and anti‐V. vulnificus antibody of eels from FKC and rOMP group increased significantly at 21 and 28 dpi; in the rOMP group, eels serum titer stayed at a high level on 42 dpi. The activities of lysozyme in skin mucus, liver, kidney, and serum in three groups exhibited considerable changes. The relative percent survival (RPS) rate of eels from rOMP group were 100% and 83% when challenged with V. vulnificus or E. anguillarum. These results indicated that inoculation of rOMP would protect Japanese eels against the infection by E. anguillarum and V. vulnificus.

A targeted boost-and-sort immunization strategy using Escherichia coli BamA identifies rare growth inhibitory antibodies

Outer membrane proteins (OMPs) in Gram-negative bacteria are essential for a number of cellular functions including nutrient transport and drug efflux. Escherichia coli BamA is an essential component of the OMP β-barrel assembly machinery and a potential novel antibacterial target that has been proposed to undergo large (~15 Å) conformational changes. Here, we explored methods to isolate anti-BamA monoclonal antibodies (mAbs) that might alter the function of this OMP and ultimately lead to bacterial growth inhibition. We first optimized traditional immunization approaches but failed to identify mAbs that altered cell growth after screening >3000 hybridomas. We then developed a “targeted boost-and-sort” strategy that combines bacterial cell immunizations, purified BamA protein boosts, and single hybridoma cell sorting using amphipol-reconstituted BamA antigen. This unique workflow improves the discovery efficiency of FACS + mAbs by >600-fold and enabled the identification of rare anti-BamA mAbs with bacterial growth inhibitory activity in the presence of a truncated lipopolysaccharide layer. These mAbs represent novel tools for dissecting the BamA-mediated mechanism of β-barrel folding and our workflow establishes a new template for the efficient discovery of novel mAbs against other highly dynamic membrane proteins.

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.

A Burkholderia pseudomallei Outer Membrane Vesicle Vaccine Provides Cross Protection against Inhalational Glanders in Mice and Non-Human Primates

Burkholderia mallei is a Gram-negative, non-motile, facultative intracellular bacillus and the causative agent of glanders, a highly contagious zoonotic disease. B. mallei is naturally resistant to multiple antibiotics and there is concern for its potential use as a bioweapon, making the development of a vaccine against B. mallei of critical importance. We have previously demonstrated that immunization with multivalent outer membrane vesicles (OMV) derived from B. pseudomallei provide significant protection against pneumonic melioidosis. Given that many virulence determinants are highly conserved between the two species, we sought to determine if the B. pseudomallei OMV vaccine could cross-protect against B. mallei. We immunized C57Bl/6 mice and rhesus macaques with B. pseudomallei OMVs and subsequently challenged animals with aerosolized B. mallei. Immunization with B. pseudomallei OMVs significantly protected mice against B. mallei and the protection observed was comparable to that achieved with a live attenuated vaccine. OMV immunization induced the production of B.mallei-specific serum IgG and a mixed Th1/Th17 CD4 and CD8 T cell response in mice. Additionally, immunization of rhesus macaques with B. pseudomallei OMVs provided protection against glanders and induced B.mallei-specific serum IgG in non-human primates. These results demonstrate the ability of the multivalent OMV vaccine platform to elicit cross-protection against closely-related intracellular pathogens and to induce robust humoral and cellular immune responses against shared protective antigens.

Vaccines for the Prevention of Melioidosis and Glanders

Purpose of review

Burkholderia pseudomallei's and Burkholderia mallei's high rate of infectivity, limited treatment options, and potential use as biological warfare agents underscore the need for development of effective vaccines against these bacteria. Research efforts focused on vaccines against these bacteria are in pre-clinical stages, with no approved formulations currently on the market.

Recent findings

Several live attenuated and subunit vaccine formulations have been evaluated in animal studies, with no reports of significant long term survival after lethal challenge.

Summary

This review encompasses the most current vaccine strategies to prevent B. pseudomallei and B. mallei infections while providing insight for successful vaccines moving forward.

Proteomics-driven Antigen Discovery for Development of Vaccines Against Gonorrhea

Expanding efforts to develop preventive gonorrhea vaccines is critical because of the dire possibility of untreatable gonococcal infections. Reverse vaccinology, which includes genome and proteome mining, has proven very successful in the discovery of vaccine candidates against many pathogenic bacteria. However, progress with this approach for a gonorrhea vaccine remains in its infancy. Accordingly, we applied a comprehensive proteomic platform-isobaric tagging for absolute quantification coupled with two-dimensional liquid chromatography and mass spectrometry-to identify potential gonococcal vaccine antigens. Our previous analyses focused on cell envelopes and naturally released membrane vesicles derived from four different Neisseria gonorrhoeae strains. Here, we extended these studies to identify cell envelope proteins of N. gonorrhoeae that are ubiquitously expressed and specifically induced by physiologically relevant environmental stimuli: oxygen availability, iron deprivation, and the presence of human serum. Together, these studies enabled the identification of numerous potential gonorrhea vaccine targets. Initial characterization of five novel vaccine candidate antigens that were ubiquitously expressed under these different growth conditions demonstrated that homologs of BamA (NGO1801), LptD (NGO1715), and TamA (NGO1956), and two uncharacterized proteins, NGO2054 and NGO2139, were surface exposed, secreted via naturally released membrane vesicles, and elicited bactericidal antibodies that cross-reacted with a panel of temporally and geographically diverse isolates. In addition, analysis of polymorphisms at the nucleotide and amino acid levels showed that these vaccine candidates are highly conserved among N. gonorrhoeae strains. Finally, depletion of BamA caused a loss of N. gonorrhoeae viability, suggesting it may be an essential target. Together, our data strongly support the use of proteomics-driven discovery of potential vaccine targets as a sound approach for identifying promising gonococcal antigens.

Melioidosis and glanders modulation of the innate immune system: barriers to current and future vaccine approaches

Burkholderia pseudomallei and Burkholderia mallei are pathogenic bacteria causing fatal infections in animals and humans. Both organisms are classified as Tier 1 Select Agents owing to their highly fatal nature, potential/prior use as bioweapons, severity of disease via respiratory exposure, intrinsic resistance to antibiotics, and lack of a current vaccine. Disease manifestations range from acute septicemia to chronic infection, wherein the facultative intracellular lifestyle of these organisms promotes persistence within a broad range of hosts. This ability to thrive intracellularly is thought to be related to exploitation of host immune response signaling pathways. There are currently considerable gaps in our understanding of the molecular strategies employed by these pathogens to modulate these pathways and evade intracellular killing. A better understanding of the specific molecular basis for dysregulation of host immune responses by these organisms will provide a stronger platform to identify novel vaccine targets and develop effective countermeasures.

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.

A Homology Model Reveals Novel Structural Features and an Immunodominant Surface Loop/Opsonic Target in the Treponema pallidum BamA Ortholog TP_0326

We recently demonstrated that TP_0326 is a bona fide rare outer membrane protein (OMP) in Treponema pallidum and that it possesses characteristic BamA bipartite topology. Herein, we used immunofluorescence analysis (IFA) to show that only the β-barrel domain of TP_0326 contains surface-exposed epitopes in intact T. pallidum. Using the solved structure of Neisseria gonorrhoeae BamA, we generated a homology model of full-length TP_0326. Although the model predicts a typical BamA fold, the β-barrel harbors features not described in other BamAs. Structural modeling predicted that a dome comprised of three large extracellular loops, loop 4 (L4), L6, and L7, covers the barrel's extracellular opening. L4, the dome's major surface-accessible loop, contains mainly charged residues, while L7 is largely neutral and contains a polyserine tract in a two-tiered conformation. L6 projects into the β-barrel but lacks the VRGF/Y motif that anchors L6 within other BamAs. IFA and opsonophagocytosis assay revealed that L4 is surface exposed and an opsonic target. Consistent with B cell epitope predictions, immunoblotting and enzyme-linked immunosorbent assay (ELISA) confirmed that L4 is an immunodominant loop in T. pallidum-infected rabbits and humans with secondary syphilis. Antibody capture experiments using Escherichia coli expressing OM-localized TP_0326 as a T. pallidum surrogate further established the surface accessibility of L4. Lastly, we found that a naturally occurring substitution (Leu(593) → Gln(593)) in the L4 sequences of T. pallidum strains affects antibody binding in sera from syphilitic patients. Ours is the first study to employ a "structure-to-pathogenesis" approach to map the surface topology of a T. pallidum OMP within the context of syphilitic infection.

IMPORTANCE

Previously, we reported that TP_0326 is a bona fide rare outer membrane protein (OMP) in Treponema pallidum and that it possesses the bipartite topology characteristic of a BamA ortholog. Using a homology model as a guide, we found that TP_0326 displays unique features which presumably relate to its function(s) in the biogenesis of T. pallidum's unorthodox OM. The model also enabled us to identify an immunodominant epitope in a large extracellular loop that is both an opsonic target and subject to immune pressure in a human population. Ours is the first study to follow a structure-to-pathogenesis approach to map the surface topology of a T. pallidum rare OMP within the context of syphilitic infection.

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.

Production of a recombinant vaccine candidate against Burkholderia pseudomallei exploiting the bacterial N-glycosylation machinery

Vaccines developing immune responses toward surface carbohydrates conjugated to proteins are effective in preventing infection and death by bacterial pathogens. Traditional production of these vaccines utilizes complex synthetic chemistry to acquire and conjugate the glycan to a protein. However, glycoproteins produced by bacterial protein glycosylation systems are significantly easier to produce, and could possible be used as vaccine candidates. In this work, we functionally expressed the Burkholderia pseudomallei O polysaccharide (OPS II), the Campylobacter jejuni oligosaccharyltransferase (OTase), and a suitable glycoprotein (AcrA) in a designer E. coli strain with a higher efficiency for production of glycoconjugates. We were able to produce and purify the OPS II-AcrA glycoconjugate, and MS analysis confirmed correct glycan was produced and attached. We observed the attachment of the O-acetylated deoxyhexose directly to the acceptor protein, which expands the range of substrates utilized by the OTase PglB. Injection of the glycoprotein into mice generated an IgG immune response against B. pseudomallei, and this response was partially protective against an intranasal challenge. Our experiments show that bacterial engineered glycoconjugates can be utilized as vaccine candidates against B. pseudomallei. Additionally, our new E. coli strain SDB1 is more efficient in glycoprotein production, and could have additional applications in the future.

Correlates of immune protection following cutaneous immunization with an attenuated Burkholderia pseudomallei vaccine

Infections with the Gram-negative bacterium Burkholderia pseudomallei (melioidosis) are associated with high mortality, and there is currently no approved vaccine to prevent the development of melioidosis in humans. Infected patients also do not develop protective immunity to reinfection, and some individuals will develop chronic, subclinical infections with B. pseudomallei. At present, our understanding of what constitutes effective protective immunity against B. pseudomallei infection remains incomplete. Therefore, we conducted a study to elucidate immune correlates of vaccine-induced protective immunity against acute B. pseudomallei infection. BALB/c and C57BL/6 mice were immunized subcutaneously with a highly attenuated, Select Agent-excluded purM deletion mutant of B. pseudomallei (strain Bp82) and then subjected to intranasal challenge with virulent B. pseudomallei strain 1026b. Immunization with Bp82 generated significant protection from challenge with B. pseudomallei, and protection was associated with a significant reduction in bacterial burden in lungs, liver, and spleen of immunized mice. Humoral immunity was critically important for vaccine-induced protection, as mice lacking B cells were not protected by immunization and serum from Bp82-vaccinated mice could transfer partial protection to nonvaccinated animals. In contrast, vaccine-induced protective immunity was found to be independent of both CD4 and CD8 T cells. Tracking studies demonstrated uptake of the Bp82 vaccine strain predominately by neutrophils in vaccine-draining lymph nodes and by smaller numbers of dendritic cells (DC) and monocytes. We concluded that protection following cutaneous immunization with a live attenuated Burkholderia vaccine strain was dependent primarily on generation of effective humoral immune responses.

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.

Meningococcal omp85 in detergent-extracted outer membrane vesicle vaccines induces high levels of non-functional antibodies in mice

The vaccine potential of meningococcal Omp85 was studied by comparing the immune responses of genetically modified deoxycholate-extracted outer membrane vesicles, expressing five-fold higher levels of Omp85, with wild-type vesicles. Groups (n = 6-12) of inbred and outbred mouse strains (Balb/c, C57BL/6, OFI and NMRI) were immunized with the two vaccines, and the induced antibody levels and bactericidal and opsonic activities measured. Except for Balb/c mice, which were low responders, the genetically modified vaccine raised high Omp85 antibody levels in all mouse strains. In comparison, the wild-type vaccine gave lower antibody levels, but NMRI mice responded to this vaccine with the same high levels as the modified vaccine in the other strains. Although the vaccines induced strain-dependent Omp85 antibody responses, the mouse strains showed high and similar serum bactericidal titres. Titres were negligible with heterologous or PorA-negative meningococcal target strains, demonstrating the presence of the dominant bactericidal PorA antibodies. The two vaccines induced the same opsonic titres. Thus, the genetically modified vaccine with high Omp85 antibody levels and the wild-type vaccine induced the same levels of functional activities related to protection against meningococcal disease, suggesting that meningococcal Omp85 is a less attractive vaccine antigen.

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.

Development of Burkholderia mallei and pseudomallei vaccines

Burkholderia mallei and Burkholderia pseudomallei are Gram-negative bacteria that cause glanders and melioidosis, respectively. Inhalational infection with either organism can result in severe and rapidly fatal pneumonia. Inoculation by the oral and cutaneous routes can also produce infection. Chronic infection may develop after recovery from acute infection with both agents, and control of infection with antibiotics requires prolonged treatment. Symptoms for both meliodosis and glanders are non-specific, making diagnosis difficult. B. pseudomallei can be located in the environment, but in the host, B. mallei and B. psedomallei are intracellular organisms, and infection results in similar immune responses to both agents. Effective early innate immune responses are critical to controlling the early phase of the infection. Innate immune signaling molecules such as TLR, NOD, MyD88, and pro-inflammatory cytokines such as IFN-γ and TNF-α play key roles in regulating control of infection. Neutrophils and monocytes are critical cells in the early infection for both microorganisms. Both monocytes and macrophages are necessary for limiting dissemination of B. pseudomallei. In contrast, the role of adaptive immune responses in controlling Burkholderia infection is less well understood. However, T cell responses are critical for vaccine protection from Burkholderia infection. At present, effective vaccines for prevention of glanders or meliodosis have not been developed, although recently development of Burkholderia vaccines has received renewed attention. This review will summarize current and past approaches to develop B. mallei and B. pseudomalllei vaccines, with emphasis on immune mechanisms of protection and the challenges facing the field. At present, immunization with live attenuated bacteria provides the most effective and durable immunity, and it is important therefore to understand the immune correlates of protection induced by live attenuated vaccines. Subunit vaccines have typically provided less robust immunity, but are safer to administer to a wider variety of people, including immune compromised individuals because they do not reactivate or cause disease. The challenges facing B. mallei and B. pseudomalllei vaccine development include identification of broadly protective antigens, design of efficient vaccine delivery and adjuvant systems, and a better understanding of the correlates of protection from both acute and chronic infection.

Haemophilus influenzae acquires vitronectin via the ubiquitous Protein F to subvert host innate immunity

Acquisition of the complement inhibitor vitronectin (Vn) is important for the respiratory tract pathogen nontypeable Haemophilus influenzae (NTHi) to escape complement-mediated killing. NTHi actively recruits Vn, and we previously showed that this interaction involves Protein E (PE). Here we describe a second Vn-binding protein, a 30 kDa Yersinia YfeA homologue designated as Protein F (PF). An isogenic NTHi 3655Δhpf mutant devoid of PF displayed a reduced binding of Vn, and was consequently more sensitive to killing by human serum compared with the wild type. Surface expression of PF on Escherichia coli conferred binding of Vn that resulted in a serum resistant phenotype. Molecular analyses revealed that the N-terminal of PF (Lys23-Glu48) bound to the C-terminal of Vn (Phe352-Ser374) without disrupting the inhibitory role of Vn on the membrane attack complex. The PF-Vn complex actively delayed C9 deposition on PF-expressing bacteria. Comparative studies of binding affinity and multiple mutants demonstrated that both PE and PF contribute individually to NTHi serum survival. PF was highly conserved and ubiquitously expressed in a series of randomly selected NTHi clinical isolates (n = 18). In conclusion, the multifaceted binding of Vn is beneficial for NTHi survival in serum and may contribute to successful colonization and consequently infection.

Burkholderia vaccines: are we moving forward?

The genus Burkholderia consists of diverse species which includes both "friends" and "foes." Some of the "friendly" Burkholderia spp. are extensively used in the biotechnological and agricultural industry for bioremediation and biocontrol. However, several members of the genus including B. pseudomallei, B. mallei, and B. cepacia, are known to cause fatal disease in both humans and animals. B. pseudomallei and B. mallei are the causative agents of melioidosis and glanders, respectively, while B. cepacia infection is lethal to cystic fibrosis (CF) patients. Due to the high rate of infectivity and intrinsic resistance to many commonly used antibiotics, together with high mortality rate, B. mallei and B. pseudomallei are considered to be potential biological warfare agents. Treatments of the infections caused by these bacteria are often unsuccessful with frequent relapse of the infection. Thus, we are at a crucial stage of the need for Burkholderia vaccines. Although the search for a prophylactic therapy candidate continues, to date development of vaccines has not advanced beyond research to human clinical trials. In this article, we review the current research on development of safe vaccines with high efficacy against B. pseudomallei, B. mallei, and B. cepacia. It can be concluded that further research will enable elucidation of the potential benefits and risks of Burkholderia vaccines.

Distinct human antibody response to the biological warfare agent Burkholderia mallei

The genetic similarity between Burkholderia mallei (glanders) and Burkholderia pseudomallei (melioidosis) had led to the general assumption that pathogenesis of each bacterium would be similar. In 2000, the first human case of glanders in North America since 1945 was reported in a microbiology laboratory worker. Leveraging the availability of pre-exposure sera for this individual and employing the same well-characterized protein array platform that has been previously used to study a large cohort of melioidosis patients in southeast Asia, we describe the antibody response in a human with glanders. Analysis of 156 peptides present on the array revealed antibodies against 17 peptides with a > 2-fold increase in this infection. Unexpectedly, when the glanders data were compared with a previous data set from B. pseudomallei infections, there were only two highly increased antibodies shared between these two infections. These findings have implications in the diagnosis and treatment of B. mallei and B. pseudomallei infections.

Melioidosis vaccines: a systematic review and appraisal of the potential to exploit biodefense vaccines for public health purposes

Background

Burkholderia pseudomallei is a Category B select agent and the cause of melioidosis. Research funding for vaccine development has largely considered protection within the biothreat context, but the resulting vaccines could be applicable to populations who are at risk of naturally acquired melioidosis. Here, we discuss target populations for vaccination, consider the cost-benefit of different vaccination strategies and review potential vaccine candidates.

Methods and Findings

Melioidosis is highly endemic in Thailand and northern Australia, where a biodefense vaccine might be adopted for public health purposes. A cost-effectiveness analysis model was developed, which showed that a vaccine could be a cost-effective intervention in Thailand, particularly if used in high-risk populations such as diabetics. Cost-effectiveness was observed in a model in which only partial immunity was assumed. The review systematically summarized all melioidosis vaccine candidates and studies in animal models that had evaluated their protectiveness. Possible candidates included live attenuated, whole cell killed, sub-unit, plasmid DNA and dendritic cell vaccines. Live attenuated vaccines were not considered favorably because of possible reversion to virulence and hypothetical risk of latent infection, while the other candidates need further development and evaluation. Melioidosis is acquired by skin inoculation, inhalation and ingestion, but routes of animal inoculation in most published studies to date do not reflect all of this. We found a lack of studies using diabetic models, which will be central to any evaluation of a melioidosis vaccine for natural infection since diabetes is the most important risk factor.

Conclusion

Vaccines could represent one strand of a public health initiative to reduce the global incidence of melioidosis.

The designation of Burkholderia pseudomallei as a category B select agent has resulted in considerable research funding to develop a protective vaccine. This bacterium also causes a naturally occurring disease (melioidosis), an important cause of death in many countries including Thailand and Australia. In this study, we explored whether a vaccine could be used to provide protection from melioidosis. An economic evaluation based on its use in Thailand indicated that a vaccine could be a cost-effective intervention if used in high-risk populations such as diabetics and those with chronic kidney or lung disease. A literature search of vaccine studies in animal models identified the current candidates, but noted that models failed to take account of the common routes of infection in natural melioidosis and major risk factors for infection, primarily diabetes. This review highlights important areas for future research if biodefence-driven vaccines are to play a role in reducing the global incidence of melioidosis.

Development of vaccines against burkholderia pseudomallei

Burkholderia pseudomallei is a Gram-negative bacterium which is the causative agent of melioidosis, a disease which carries a high mortality and morbidity rate in endemic areas of South East Asia and Northern Australia. At present there is no available human vaccine that protects against B. pseudomallei, and with the current limitations of antibiotic treatment, the development of new preventative and therapeutic interventions is crucial. This review considers the multiple elements of melioidosis vaccine research including: (i) the immune responses required for protective immunity, (ii) animal models available for preclinical testing of potential candidates, (iii) the different experimental vaccine strategies which are being pursued, and (iv) the obstacles and opportunities for eventual registration of a licensed vaccine in humans.

A naturally derived outer-membrane vesicle vaccine protects against lethal pulmonary Burkholderia pseudomallei infection

Burkholderia pseudomallei, and other members of the Burkholderia, are among the most antibiotic-resistant bacterial species encountered in human infection. Mortality rates associated with severe B. pseudomallei infection approach 50% despite therapeutic treatment. A protective vaccine against B. pseudomallei would dramatically reduce morbidity and mortality in endemic areas and provide a safeguard for the U.S. and other countries against biological attack with this organism. In this study, we investigated the immunogenicity and protective efficacy of B. pseudomallei-derived outer membrane vesicles (OMVs). Vesicles are produced by Gram-negative and Gram-positive bacteria and contain many of the bacterial products recognized by the host immune system during infection. We demonstrate that subcutaneous (SC) immunization with OMVs provides significant protection against an otherwise lethal B. pseudomallei aerosol challenge in BALB/c mice. Mice immunized with B. pseudomallei OMVs displayed OMV-specific serum antibody and T-cell memory responses. Furthermore, OMV-mediated immunity appears species-specific as cross-reactive antibody and T cells were not generated in mice immunized with Escherichia coli-derived OMVs. These results provide the first compelling evidence that OMVs represent a non-living vaccine formulation that is able to produce protective humoral and cellular immunity against an aerosolized intracellular bacterium. This vaccine platform constitutes a safe and inexpensive immunization strategy against B. pseudomallei that can be exploited for other intracellular respiratory pathogens, including other Burkholderia and bacteria capable of establishing persistent infection.

Chloroplast Omp85 proteins change orientation during evolution

The majority of outer membrane proteins (OMPs) from gram-negative bacteria and many of mitochondria and chloroplasts are β-barrels. Insertion and assembly of these proteins are catalyzed by the Omp85 protein family in a seemingly conserved process. All members of this family exhibit a characteristic N-terminal polypeptide-transport-associated (POTRA) and a C-terminal 16-stranded β-barrel domain. In plants, two phylogenetically distinct and essential Omp85's exist in the chloroplast outer membrane, namely Toc75-III and Toc75-V. Whereas Toc75-V, similar to the mitochondrial Sam50, is thought to possess the original bacterial function, its homolog, Toc75-III, evolved to the pore-forming unit of the TOC translocon for preprotein import. In all current models of OMP biogenesis and preprotein translocation, a topology of Omp85 with the POTRA domain in the periplasm or intermembrane space is assumed. Using self-assembly GFP-based in vivo experiments and in situ topology studies by electron cryotomography, we show that the POTRA domains of both Toc75-III and Toc75-V are exposed to the cytoplasm. This unexpected finding explains many experimental observations and requires a reevaluation of current models of OMP biogenesis and TOC complex function.