Monoclonal antibodies passively protect BALB/c mice against Burkholderia mallei aerosol challenge

Characterization of immunoglobulin and cytokine responses in Burkholderia mallei infected equids

Burkholderia mallei causes a highly fatal infectious disease in equines known as glanders. It is one of the OIE listed notifiable diseases, which entails strict control policy measures once B. mallei infection is confirmed in the susceptible hosts. Humans, especially equine handlers, veterinary professionals and laboratory workers are at greater risk to acquire the B. mallei infection directly through prolonged contact with glanderous equines, and indirectly through unprotected handling of B. mallei contaminated materials. Further, natural resistance of B. mallei to multiple antibiotics, aerosol transmission, lack of effective vaccine and treatment make this organism a potential agent of biological warfare. Results of experimental B. mallei infection in mouse and non-human primates and immunization with live attenuated B. mallei strains demonstrated that activation of early innate and adaptive immune responses play a critical role in controlling B. mallei infection. However, the immune response elicited by the primary hosts (equids) B. mallei infection is poorly understood. Therefore, we aimed to investigate immune responses in glanders affected horses (n = 23) and mules (n = 1). In this study, chronically infected equids showed strong humoral responses (IgM, IgG and IgA) specific to B. mallei type 6 secretory proteins such as Hcp1, TssA and TssB. The infected equids also elicited robust cellular responses characterized by significantly elevated levels of IFN-γ, TNF-α, IL-12, IL-17 and IL-6 in PBMCs. In addition, stimulation of equine PBMCs by Hcp1 resulted in the further elevation of these cytokines. Thus, the present study indicated that antibody response and T helper cell (Th) type 1-associated cytokines were the salient features of chronic B. mallei infection in horses. The immune responses also suggest further evaluation of these proteins as potential vaccine candidates.

Comparison of three non-human primate aerosol models for glanders, caused by Burkholderia mallei

Burkholderia mallei is a gram-negative obligate animal pathogen that causes glanders, a highly contagious and potentially fatal disease of solipeds including horses, mules, and donkeys. Humans are also susceptible, and exposure can result in a wide range of clinical forms, i.e., subclinical infection, chronic forms with remission and exacerbation, or acute and potentially lethal septicemia and/or pneumonia. Due to intrinsic antibiotic resistance and the ability of the organisms to survive intracellularly, current treatment regimens are protracted and complicated; and no vaccine is available. As a consequence of these issues, and since B. mallei is infectious by the aerosol route, B. mallei is regarded as a major potential biothreat agent. To develop optimal medical countermeasures and diagnostic tests, well characterized animal models of human glanders are needed. The goal of this study was to perform a head-to-head comparison of models employing three commonly used nonhuman primate (NHP) species, the African green monkey (AGM), Rhesus macaque, and the Cynomolgus macaque. The natural history of infection and in vitro clinical, histopathological, immunochemical, and bacteriological parameters were examined. The AGMs were the most susceptible NHP to B. mallei; five of six expired within 14 days. Although none of the Rhesus or Cynomolgus macaques succumbed, the Rhesus monkeys exhibited abnormal signs and clinical findings associated with B. mallei infection; and the latter may be useful for modeling chronic B. mallei infection. Based on the disease progression observations, gross and histochemical pathology, and humoral and cellular immune response findings, the AGM appears to be the optimal model of acute, lethal glanders infection. AGM models of infection by B. pseudomallei, the etiologic agent of melioidosis, have been characterized recently. Thus, the selection of the AGM species provides the research community with a single NHP model for investigations on acute, severe, inhalational melioidosis and glanders.

Laser Scanning Confocal Microscopy Was Used to Validate the Presence of Burkholderia Pseudomallei or B. Mallei in Formalin-Fixed Paraffin Embedded Tissues

Burkholderia pseudomallei and B. mallei are Gram-negative, facultative intracellular bacteria that cause melioidosis and glanders, respectively. Currently, there are no vaccines for these two diseases. Animal models have been developed to evaluate vaccines and therapeutics. Tissues from infected animals, however, must be fixed in formalin and embedded in paraffin (FFPE) before analysis. A brownish staining material in infected tissues that represents the exopolysaccharide of the pathogen was seen by bright field microscopy but not the actual microorganism. Because of these results, FFPE tissue was examined by laser scanning confocal microscopy (LSCM) in an attempt to see the microorganism. Archival FFPE tissues were examined from ten mice, and five nonhuman primates after exposure to B. pseudomallei or B.mallei by LSCM. Additionally, a historical spleen biopsy from a human suspected of exposure to B. mallei was examined. B. pseudomallei was seen in many of the infected tissues from mice. Four out of five nonhuman primates were positive for the pathogen. In the human sample, B. mallei was seen in pyogranulomas in the spleen biopsy. Thus, the presence of the pathogen was validated by LSCM in murine, nonhuman primate, and human FFPE tissues.

Antibodies Are Major Drivers of Protection against Lethal Aerosol Infection with Highly Pathogenic Burkholderia spp

Burkholderia pseudomallei and Burkholderia mallei are the causative agents of melioidosis and glanders, respectively. There is no vaccine to protect against these highly pathogenic bacteria, and there is concern regarding their emergence as global public health (B. pseudomallei) and biosecurity (B. mallei) threats. In this issue of mSphere, an article by Khakhum and colleagues (N. Khakhum, P. Bharaj, J. N. Myers, D. Tapia, et al., mSphere 4:e00570-18, 2019, https://doi.org/10.1128/mSphere.00570-18) describes a novel vaccination platform with excellent potential for cross-protection against both Burkholderia species. The report also highlights the importance of antibodies in immunity against these facultative intracellular organisms.

Cancer Immunotherapy: The Dawn of Antibody Cocktails

Since the approval of the first monoclonal antibody (mAb), rituximab, for hematological malignancies, almost 30 additional mAbs have been approved in oncology. Despite remarkable advances, relatively weak responses and resistance to antibody monotherapy remain major open issue. Overcoming resistance might require combinations of drugs blocking both the major target and the emerging secondary target. We review clinically approved combinations of antibodies and either cytotoxic regimens (chemotherapy and irradiation) or kinase inhibitors. Thereafter, we focus on the most promising and currently very active arena that combines mAbs inhibiting immune checkpoints or growth factor receptors. Clinically approved and experimental oligoclonal mixtures of mAbs targeting different antigens (hetero-combinations) or different epitopes of the same antigen (homo-combinations) are described. Effective oligoclonal mixtures of antibodies that mimic the polyclonal immune response will likely become a mainstay of cancer therapy.

Melioidosis patient serum-reactive synthetic tetrasaccharides bearing the predominant epitopes ofBurkholderia pseudomalleiandBurkholderia malleiO-antigens

Tetrasaccharides mimickingBurkholderia pseudomalleiandBurkholderia malleilipopolysaccharide O-antigens were synthesized and found to be highly reactive with Thai melioidosis patient serum, highlighting their potential as vaccine candidates.

Potential targets for next generation antimicrobial glycoconjugate vaccines

Cell surface carbohydrates have been proven optimal targets for vaccine development. Conjugation of polysaccharides to a carrier protein triggers a T-cell-dependent immune response to the glycan moiety. Licensed glycoconjugate vaccines are produced by chemical conjugation of capsular polysaccharides to prevent meningitis caused by meningococcus, pneumococcus and Haemophilus influenzae type b. However, other classes of carbohydrates (O-antigens, exopolysaccharides, wall/teichoic acids) represent attractive targets for developing vaccines. Recent analysis from WHO/CHO underpins alarming concern toward antibiotic-resistant bacteria, such as the so called ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter spp.) and additional pathogens such as Clostridium difficile and Group A Streptococcus. Fungal infections are also becoming increasingly invasive for immunocompromised patients or hospitalized individuals. Other emergencies could derive from bacteria which spread during environmental calamities (Vibrio cholerae) or with potential as bioterrorism weapons (Burkholderia pseudomallei and mallei, Francisella tularensis). Vaccination could aid reducing the use of broad-spectrum antibiotics and provide protection by herd immunity also to individuals who are not vaccinated.

This review analyzes structural and functional differences of the polysaccharides exposed on the surface of emerging pathogenic bacteria, combined with medical need and technological feasibility of corresponding glycoconjugate vaccines.

Deciphering minimal antigenic epitopes associated with Burkholderia pseudomallei and Burkholderia mallei lipopolysaccharide O-antigens

Burkholderia pseudomallei (Bp) and Burkholderia mallei (Bm), the etiologic agents of melioidosis and glanders, respectively, cause severe disease in both humans and animals. Studies have highlighted the importance of Bp and Bm lipopolysaccharides (LPS) as vaccine candidates. Here we describe the synthesis of seven oligosaccharides as the minimal structures featuring all of the reported acetylation/methylation patterns associated with Bp and Bm LPS O-antigens (OAgs). Our approach is based on the conversion of an L-rhamnose into a 6-deoxy-L-talose residue at a late stage of the synthetic sequence. Using biochemical and biophysical methods, we demonstrate the binding of several Bp and Bm LPS-specific monoclonal antibodies with terminal OAg residues. Mice immunized with terminal disaccharide-CRM197 constructs produced high-titer antibody responses that crossreacted with Bm-like OAgs. Collectively, these studies serve as foundation for the development of novel therapeutics, diagnostics, and vaccine candidates to combat diseases caused by Bp and Bm.Melioidosis and glanders are multifaceted infections caused by gram-negative bacteria. Here, the authors synthesize a series of oligosaccharides that mimic the lipopolysaccharides present on the pathogens' surface and use them to develop novel glycoconjugates for vaccine development.

Antibodies against In Vivo-Expressed Antigens Are Sufficient To Protect against Lethal Aerosol Infection with Burkholderia mallei and Burkholderia pseudomallei

Burkholderia mallei, a facultative intracellular bacterium and tier 1 biothreat, causes the fatal zoonotic disease glanders. The organism possesses multiple genes encoding autotransporter proteins, which represent important virulence factors and targets for developing countermeasures in pathogenic Gram-negative bacteria. In the present study, we investigated one of these autotransporters, BatA, and demonstrate that it displays lipolytic activity, aids in intracellular survival, is expressed in vivo, elicits production of antibodies during infection, and contributes to pathogenicity in a mouse aerosol challenge model. A mutation in the batA gene of wild-type strain ATCC 23344 was found to be particularly attenuating, as BALB/c mice infected with the equivalent of 80 median lethal doses cleared the organism. This finding prompted us to test the hypothesis that vaccination with the batA mutant strain elicits protective immunity against subsequent infection with wild-type bacteria. We discovered that not only does vaccination provide high levels of protection against lethal aerosol challenge with B. mallei ATCC 23344, it also protects against infection with multiple isolates of the closely related organism and causative agent of melioidosis, Burkholderia pseudomallei Passive-transfer experiments also revealed that the protective immunity afforded by vaccination with the batA mutant strain is predominantly mediated by IgG antibodies binding to antigens expressed exclusively in vivo Collectively, our data demonstrate that BatA is a target for developing medical countermeasures and that vaccination with a mutant lacking expression of the protein provides a platform to gain insights regarding mechanisms of protective immunity against B. mallei and B. pseudomallei, including antigen discovery.

Characterization of in vitro phenotypes of Burkholderia pseudomallei and Burkholderia mallei strains potentially associated with persistent infection in mice

Burkholderia pseudomallei (Bp) and Burkholderia mallei (Bm), the agents of melioidosis and glanders, respectively, are Tier 1 biothreats. They infect humans and animals, causing disease ranging from acute and fatal to protracted and chronic. Chronic infections are especially challenging to treat, and the identification of in vitro phenotypic markers which signal progression from acute to persistent infection would be extremely valuable. First, a phenotyping strategy was developed employing colony morphotyping, chemical sensitivity testing, macrophage infection, and lipopolysaccharide fingerprint analyses to distinguish Burkholderia strains. Then mouse spleen isolates collected 3–180 days after infection were characterized phenotypically. Isolates from long-term infections often exhibited increased colony morphology differences and altered patterns of antimicrobial sensitivity and macrophage infection. Some of the Bp and Bm persistent infection isolates clearly displayed enhanced virulence in mice. Future studies will evaluate the potential role and significance of these phenotypic markers in signaling the establishment of a chronic infection.

Recombinant Salmonella Expressing Burkholderia mallei LPS O Antigen Provides Protection in a Murine Model of Melioidosis and Glanders

Burkholderia pseudomallei and Burkholderia mallei are the etiologic agents of melioidosis and glanders, respectively. These bacteria are highly infectious via the respiratory route and can cause severe and often fatal diseases in humans and animals. Both species are considered potential agents of biological warfare; they are classified as category B priority pathogens. Currently there are no human or veterinary vaccines available against these pathogens. Consequently efforts are directed towards the development of an efficacious and safe vaccine. Lipopolysaccharide (LPS) is an immunodominant antigen and potent stimulator of host immune responses. B. mallei express LPS that is structurally similar to that expressed by B. pseudomallei, suggesting the possibility of constructing a single protective vaccine against melioidosis and glanders. Previous studies of others have shown that antibodies against B. mallei or B. pseudomallei LPS partially protect mice against subsequent lethal virulent Burkholderia challenge. In this study, we evaluated the protective efficacy of recombinant Salmonella enterica serovar Typhimurium SL3261 expressing B. mallei O antigen against lethal intranasal infection with Burkholderia thailandensis, a surrogate for biothreat Burkholderia spp. in a murine model that mimics melioidosis and glanders. All vaccine-immunized mice developed a specific antibody response to B. mallei and B. pseudomallei O antigen and to B. thailandensis and were significantly protected against challenge with a lethal dose of B. thailandensis. These results suggest that live-attenuated SL3261 expressing B. mallei O antigen is a promising platform for developing a safe and effective vaccine.

A gold nanoparticle-linked glycoconjugate vaccine against Burkholderia mallei

Burkholderia mallei are Gram-negative bacteria, responsible for the disease glanders. B. mallei has recently been classified as a Tier 1 agent owing to the fact that this bacterial species can be weaponised for aerosol release, has a high mortality rate and demonstrates multi-drug resistance. Furthermore, there is no licensed vaccine available against this pathogen. Lipopolysaccharide (LPS) has previously been identified as playing an important role in generating host protection against Burkholderia infection. In this study, we present gold nanoparticles (AuNPs) functionalised with a glycoconjugate vaccine against glanders. AuNPs were covalently coupled with one of three different protein carriers (TetHc, Hcp1 and FliC) followed by conjugation to LPS purified from a non-virulent clonal relative, B. thailandensis. Glycoconjugated LPS generated significantly higher antibody titres compared with LPS alone. Further, they improved protection against a lethal inhalation challenge of B. mallei in the murine model of infection.

FROM THE CLINICAL EDITOR

Burkholderia mallei is associated with multi-drug resistance, high mortality and potentials for weaponization through aerosol inhalation. The authors of this study present gold nanoparticles (AuNPs) functionalized with a glycoconjugate vaccine against this Gram negative bacterium demonstrating promising results in a murine model even with the aerosolized form of B. Mallei.

Animal models for Francisella tularensis and Burkholderia species: scientific and regulatory gaps toward approval of antibiotics under the FDA Animal Rule

The development and regulatory approval of medical countermeasures (MCMs) for the treatment and prevention of bacterial threat agent infections will require the evaluation of products in animal models. To obtain regulatory approval, these models must accurately recapitulate aspects of human disease, including, but not necessarily limited to, route of exposure, time to disease onset, pathology, immune response, and mortality. This article focuses on the state of animal model development for 3 agents for which models are largely immature: Francisella tularensis, Burkholderia mallei, and Burkholderia pseudomallei. An overview of available models and a description of scientific and regulatory gaps are provided.

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.

Development of novel O-polysaccharide based glycoconjugates for immunization against glanders

Burkholderia mallei the etiologic agent of glanders, causes severe disease in humans and animals and is a potential agent of biological warfare and terrorism. Diagnosis and treatment of glanders can be challenging, and in the absence of chemotherapeutic intervention, acute human disease is invariably fatal. At present, there are no human or veterinary vaccines available for immunization against disease. One of the goals of our research, therefore, is to identify and characterize protective antigens expressed by B. mallei and use them to develop efficacious glanders vaccine candidates. Previous studies have demonstrated that the O-polysaccharide (OPS) expressed by B. mallei is both a virulence factor and a protective antigen. Recently, we demonstrated that Burkholderia thailandensis, a closely related but non-pathogenic species, can be genetically manipulated to express OPS antigens that are recognized by B. mallei OPS-specific monoclonal antibodies (mAbs). As a result, these antigens have become important components of the various OPS-based subunit vaccines that we are currently developing in our laboratory. In this study, we describe a method for isolating B. mallei-like OPS antigens from B. thailandensis oacA mutants. Utilizing these purified OPS antigens, we also describe a simple procedure for coupling the polysaccharides to protein carriers such as cationized bovine serum albumin, diphtheria toxin mutant CRM197 and cholera toxin B subunit. Additionally, we demonstrate that high titer IgG responses against purified B. mallei LPS can be generated by immunizing mice with the resulting constructs. Collectively, these approaches provide a rational starting point for the development of novel OPS-based glycoconjugates for immunization against glanders.

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.

Natural history of inhalation melioidosis in rhesus macaques (Macaca mulatta) and African green monkeys (Chlorocebus aethiops)

Burkholderia pseudomallei, the causative agent of melioidosis, is recognized as a serious health threat due to its involvement in septic and pulmonary infections in areas of endemicity and is recognized by the Centers for Disease Control and Prevention as a category B biothreat agent. An animal model is desirable to evaluate the pathogenesis of melioidosis and medical countermeasures. A model system that represents human melioidosis infections is essential in this process. A group of 10 rhesus macaques (RMs) and 10 African green monkeys (AGMs) was exposed to aerosolized B. pseudomallei 1026b. The first clinical signs were fever developing 24 to 40 h postexposure followed by leukocytosis resulting from a high percentage of neutrophils. Dyspnea manifested 2 to 4 days postexposure. In the AGMs, an increase in interleukin 1β (IL-1β), IL-6, IL-8, gamma interferon (IFN-γ), and tumor necrosis factor alpha (TNF-α) was observed. In the RMs, IL-1β, IL-6, and TNF-α increased. All the RMs and AGMs had various degrees of bronchopneumonia, with inflammation consisting of numerous neutrophils and a moderate number of macrophages. Both the RMs and the AGMs appear to develop a melioidosis infection that closely resembles that seen in acute human melioidosis. However, for an evaluation of medical countermeasures, AGMs appear to be a more appropriate model.

Antibodies for biodefense

Potential bioweapons are biological agents (bacteria, viruses, and toxins) at risk of intentional dissemination. Biodefense, defined as development of therapeutics and vaccines against these agents, has seen an increase, particularly in the US following the 2001 anthrax attack. This review focuses on recombinant antibodies and polyclonal antibodies for biodefense that have been accepted for clinical use. These antibodies aim to protect against primary potential bioweapons, or category A agents as defined by the Centers for Disease Control and Prevention (Bacillus anthracis, Yersinia pestis, Francisella tularensis, botulinum neurotoxins, smallpox virus, and certain others causing viral hemorrhagic fevers) and certain category B agents. Potential for prophylactic use is presented, as well as frequent use of oligoclonal antibodies or synergistic effect with other molecules. Capacities and limitations of antibodies for use in biodefense are discussed, and are generally applicable to the field of infectious diseases.

Production and characterization of chimeric monoclonal antibodies against Burkholderia pseudomallei and B. mallei using the DHFR expression system

Burkholderia pseudomallei (BP) and B. mallei (BM) are closely related gram-negative, facultative anaerobic bacteria which cause life-threatening melioidosis in human and glanders in horse, respectively. Our laboratory has previously generated and characterized more than 100 mouse monoclonal antibodies (MAbs) against BP and BM, according to in vitro and in vivo assay. In this study, 3 MAbs (BP7 10B11, BP7 2C6, and BP1 7F7) were selected to develop into chimeric mouse-human monoclonal antibodies (cMAbs) against BP and/or BM. For the stable production of cMAbs, we constructed 4 major different vector systems with a dihydrofolate reductase (DHFR) amplification marker, and optimized transfection/selection conditions in mammalian host cells with the single-gene and/or double-gene expression system. These 3 cMAbs were stably produced by the DHFR double mutant Chinese hamster ovarian (CHO)-DG44 cells. By ELISA and Western blot analysis using whole bacterial antigens treated by heat (65°C/90 min), sodium periodate, and proteinase K, the cMAb BP7 10B11 (cMAb CK1) reacted with glycoproteins (34, 38, 48 kDa in BP; 28, 38, 48 kDa in BM). The cMAb BP7 2C6 (cMAb CK2) recognized surface-capsule antigens with molecular sizes of 38 to 52 kDa, and 200 kDa in BM. The cMAb CK2 was weakly reactive to 14∼28, 200 kDa antigens in BP. The cMAb BP1 7F7 (cMAb CK3) reacted with lipopolysaccharides (38∼52 kDa in BP; 38∼60 kDa in B. thailandensis). Western blot results with the outer surface antigens of the 3 Burkholderia species were consistent with results with the whole Burkholderia cell antigens, suggesting that these immunodominant antigens reacting with the 3 cMAbs were primarily present on the outer surface of the Burkholderia species. These 3 cMAbs would be useful for analyzing the role of the major outer surface antigens in Burkholderia infection.

In Vitro and In Vivo studies of monoclonal antibodies with prominent bactericidal activity against Burkholderia pseudomallei and Burkholderia mallei

Our laboratory has developed more than a hundred mouse monoclonal antibodies (MAbs) against Burkholderia pseudomallei and Burkholderia mallei. These antibodies have been categorized into different groups based on their specificities and the biochemical natures of their target antigens. The current study first examined the bactericidal activities of a number of these MAbs by an in vitro opsonic assay. Then, the in vivo protective efficacy of selected MAbs was evaluated using BALB/c mice challenged intranasally with a lethal dose of the bacteria. The opsonic assay using dimethyl sulfoxide-treated human HL-60 cells as phagocytes revealed that 19 out of 47 tested MAbs (40%) have prominent bactericidal activities against B. pseudomallei and/or B. mallei. Interestingly, all MAbs with strong opsonic activities are those with specificity against either the capsular polysaccharides (PS) or the lipopolysaccharides (LPS) of the bacteria. On the other hand, none of the MAbs reacting to bacterial proteins or glycoproteins showed prominent bactericidal activity. Further study revealed that the antigenic epitopes on either the capsular PS or LPS molecules were readily available for binding in intact bacteria, while the epitopes on proteins/glycoproteins were less accessible to the MAbs. Our in vivo study showed that four MAbs reactive to either the capsular PS or LPS were highly effective in protecting mice against lethal bacterial challenge. The result is compatible with that of our in vitro study. The MAbs with the highest protective efficacy are those reactive to either the capsular PS or LPS of the Burkholderia bacteria.

Molecular insights into Burkholderia pseudomallei and Burkholderia mallei pathogenesis

Burkholderia pseudomallei and Burkholderia mallei are closely related gram-negative bacteria that can cause serious diseases in humans and animals. This review summarizes the current and rapidly expanding knowledge on the specific virulence factors employed by these pathogens and their roles in the pathogenesis of melioidosis and glanders. In particular, the contributions of recently identified virulence factors are described in the context of the intracellular lifestyle of these pathogens. Throughout this review, unique and shared virulence features of B. pseudomallei and B. mallei are discussed.

Burkholderia thailandensis oacA mutants facilitate the expression of Burkholderia mallei-like O polysaccharides

Previous studies have shown that the O polysaccharides (OPS) expressed by Burkholderia mallei are similar to those produced by Burkholderia thailandensis except that they lack the 4-O-acetyl modifications on their 6-deoxy-α-l-talopyranosyl residues. In the present study, we describe the identification and characterization of an open reading frame, designated oacA, expressed by B. thailandensis that accounts for this phenomenon. Utilizing the B. thailandensis and B. mallei lipopolysaccharide (LPS)-specific monoclonal antibodies Pp-PS-W and 3D11, Western immunoblot analyses demonstrated that the LPS antigens expressed by the oacA mutant, B. thailandensis ZT0715, were antigenically similar to those produced by B. mallei ATCC 23344. In addition, immunoblot analyses demonstrated that when B. mallei ATCC 23344 was complemented in trans with oacA, it synthesized B. thailandensis-like LPS antigens. To elucidate the structure of the OPS moieties expressed by ZT0715, purified samples were analyzed via nuclear magnetic resonance spectroscopy. As predicted, these studies demonstrated that the loss of OacA activity influenced the O acetylation phenotype of the OPS moieties. Unexpectedly, however, the results indicated that the O methylation status of the OPS antigens was also affected by the loss of OacA activity. Nonetheless, it was revealed that the LPS moieties expressed by the oacA mutant reacted strongly with the B. mallei LPS-specific protective monoclonal antibody 9C1-2. Based on these findings, it appears that OacA is required for the 4-O acetylation and 2-O methylation of B. thailandensis OPS antigens and that ZT0715 may provide a safe and cost-effective source of B. mallei-like OPS to facilitate the synthesis of glanders subunit vaccine candidates.

Protective efficacy of heat-inactivated B. thailandensis, B. mallei or B. pseudomallei against experimental melioidosis and glanders

Burkholderia pseudomallei and Burkholderia mallei are gram-negative bacilli that are the causative agents of melioidosis and glanders, respectively. Both humans and animals are susceptible to both diseases. There is currently no vaccine available for the prevention of disease. We report the protective efficacy of heat-inactivated Burkholderia thailandensis, B. mallei or B. pseudomallei cells as vaccines against murine melioidosis and glanders. Immunisation with heat-inactivated B. pseudomallei cells provided the highest levels of protection against either melioidosis or glanders. These studies indicate the longer term potential for heat-inactivated bacteria to be developed as vaccines against melioidosis and glanders.

Strategies toward vaccines against Burkholderia mallei and Burkholderia pseudomallei

Burkholderia mallei and Burkholderia pseudomallei are Gram-negative, rod-shaped bacteria, and are the causative agents of the diseases glanders and melioidosis, respectively. These bacteria have been recognized as important pathogens for over 100 years, yet a relative dearth of available information exists regarding their virulence determinants and immunopathology. Infection with either of these bacteria presents with nonspecific symptoms and can be either acute or chronic, impeding rapid diagnosis. The lack of a vaccine for either bacterium also makes them potential candidates for bioweaponization. Together with their high rate of infectivity via aerosols and resistance to many common antibiotics, both bacteria have been classified as category B priority pathogens by the US NIH and US CDC, which has spurred a dramatic increase in interest in these microorganisms. Attempts have been made to develop vaccines for these infections, which would not only benefit military personnel, a group most likely to be targeted in an intentional release, but also individuals who may come in contact with glanders-infected animals or live in areas where melioidosis is endemic. This review highlights some recent attempts of vaccine development for these infections and the strategies used to improve the efficacy of vaccine approaches.

Host immunity in the protective response to vaccination with heat-killed Burkholderia mallei

Background

We performed initial cell, cytokine and complement depletion studies to investigate the possible role of these effectors in response to vaccination with heat-killed Burkholderia mallei in a susceptible BALB/c mouse model of infection.

Results

While protection with heat-killed bacilli did not result in sterilizing immunity, limited protection was afforded against an otherwise lethal infection and provided insight into potential host protective mechanisms. Our results demonstrated that mice depleted of either B cells, TNF-α or IFN-γ exhibited decreased survival rates, indicating a role for these effectors in obtaining partial protection from a lethal challenge by the intraperitoneal route. Additionally, complement depletion had no effect on immunoglobulin production when compared to non-complement depleted controls infected intranasally.

Conclusion

The data provide a basis for future studies of protection via vaccination using either subunit or whole-organism vaccine preparations from lethal infection in the experimental BALB/c mouse model. The results of this study demonstrate participation of B220⁺ cells and pro-inflammatory cytokines IFN-γ and TNF-α in protection following HK vaccination.

Glanders: off to the races with Burkholderia mallei

Burkholderia mallei, the etiologic agent of the disease known as glanders, is primarily a disease affecting horses and is transmitted to humans by direct contact with infected animals. The use of B. mallei as a biological weapon has been reported and currently, there is no vaccine available for either humans or animals. Despite the history and highly infective nature of B. mallei, as well as its potential use as a bio-weapon, B. mallei research to understand the pathogenesis and the host responses to infection remains limited. Therefore, this minireview will focus on current efforts to elucidate B. mallei virulence, the associated host immune responses elicited during infection and discuss the feasibility of vaccine development.

The type IV pilin of Burkholderia mallei is highly immunogenic but fails to protect against lethal aerosol challenge in a murine model

Burkholderia mallei is the cause of glanders and a proven biological weapon. We identified and purified the type IV pilin protein of this organism to study its potential as a subunit vaccine. We found that purified pilin was highly immunogenic. Furthermore, mice infected via sublethal aerosol challenge developed significant increases in titers of antibody against the pilin, suggesting that it is expressed in vivo. Nevertheless, we found no evidence that high-titer antipilin antisera provided passive protection against a sublethal or lethal aerosol challenge and no evidence of protection afforded by active immunization with purified pilin. These results contrast with the utility of type IV pilin subunit vaccines against other infectious diseases and highlight the need for further efforts to identify protective responses against this pathogen.

Burkholderia Hep_Hag autotransporter (BuHA) proteins elicit a strong antibody response during experimental glanders but not human melioidosis

BACKGROUND

The bacterial biothreat agents Burkholderia mallei and Burkholderia pseudomallei are the cause of glanders and melioidosis, respectively. Genomic and epidemiological studies have shown that B. mallei is a recently emerged, host restricted clone of B. pseudomallei.

RESULTS

Using bacteriophage-mediated immunoscreening we identified genes expressed in vivo during experimental equine glanders infection. A family of immunodominant antigens were identified that share protein domain architectures with hemagglutinins and invasins. These have been designated Burkholderia Hep_Hag autotransporter (BuHA) proteins. A total of 110/207 positive clones (53%) of a B. mallei expression library screened with sera from two infected horses belonged to this family. This contrasted with 6/189 positive clones (3%) of a B. pseudomallei expression library screened with serum from 21 patients with culture-proven melioidosis.

CONCLUSION

Members of the BuHA proteins are found in other Gram-negative bacteria and have been shown to have important roles related to virulence. Compared with other bacterial species, the genomes of both B. mallei and B. pseudomallei contain a relative abundance of this family of proteins. The domain structures of these proteins suggest that they function as multimeric surface proteins that modulate interactions of the cell with the host and environment. Their effect on the cellular immune response to B. mallei and their potential as diagnostics for glanders requires further study.