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

Decoration of Burkholderia Hcp1 protein to virus-like particles as a vaccine delivery platform

Virus-like particles (VLPs) are protein-based nanoparticles frequently used as carriers in conjugate vaccine platforms. VLPs have been used to display foreign antigens for vaccination and to deliver immunotherapy against diseases. Hemolysin-coregulated proteins 1 (Hcp1) is a protein component of the Burkholderia type 6 secretion system, which participates in intracellular invasion and dissemination. This protein has been reported as a protective antigen and is used in multiple vaccine candidates with various platforms against melioidosis, a severe infectious disease caused by the intracellular pathogen Burkholderia pseudomallei. In this study, we used P22 VLPs as a surface platform for decoration with Hcp1 using chemical conjugation. C57BL/6 mice were intranasally immunized with three doses of either PBS, VLPs, or conjugated Hcp1-VLPs. Immunization with Hcp1-VLPs formulation induced Hcp1-specific IgG, IgG1, IgG2c, and IgA antibody responses. Furthermore, the serum from Hcp1-VLPs immunized mice enhanced the bacterial uptake and opsonophagocytosis by macrophages in the presence of complement. This study demonstrated an alternative strategy to develop a VLPs-based vaccine platform against Burkholderia species.

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.

Shotgun Immunoproteomic Approach for the Discovery of Linear B-Cell Epitopes in Biothreat Agents Francisella tularensis and Burkholderia pseudomallei

Peptide-based subunit vaccines are coming to the forefront of current vaccine approaches, with safety and cost-effective production among their top advantages. Peptide vaccine formulations consist of multiple synthetic linear epitopes that together trigger desired immune responses that can result in robust immune memory. The advantages of linear compared to conformational epitopes are their simple structure, ease of synthesis, and ability to stimulate immune responses by means that do not require complex 3D conformation. Prediction of linear epitopes through use of computational tools is fast and cost-effective, but typically of low accuracy, necessitating extensive experimentation to verify results. On the other hand, identification of linear epitopes through experimental screening has been an inefficient process that requires thorough characterization of previously identified full-length protein antigens, or laborious techniques involving genetic manipulation of organisms. In this study, we apply a newly developed generalizable screening method that enables efficient identification of B-cell epitopes in the proteomes of pathogenic bacteria. As a test case, we used this method to identify epitopes in the proteome of Francisella tularensis (Ft), a Select Agent with a well-characterized immunoproteome. Our screen identified many peptides that map to known antigens, including verified and predicted outer membrane proteins and extracellular proteins, validating the utility of this approach. We then used the method to identify seroreactive peptides in the less characterized immunoproteome of Select Agent Burkholderia pseudomallei (Bp). This screen revealed known Bp antigens as well as proteins that have not been previously identified as antigens. Although B-cell epitope prediction tools Bepipred 2.0 and iBCE-EL classified many of our seroreactive peptides as epitopes, they did not score them significantly higher than the non-reactive tryptic peptides in our study, nor did they assign higher scores to seroreactive peptides from known Ft or Bp antigens, highlighting the need for experimental data instead of relying on computational epitope predictions alone. The present workflow is easily adaptable to detecting peptide targets relevant to the immune systems of other mammalian species, including humans (depending upon the availability of convalescent sera from patients), and could aid in accelerating the discovery of B-cell epitopes and development of vaccines to counter emerging biological threats.

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

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

Human Melioidosis

The causative agent of melioidosis, Burkholderia pseudomallei, a tier 1 select agent, is endemic in Southeast Asia and northern Australia, with increased incidence associated with high levels of rainfall. Increasing reports of this condition have occurred worldwide, with estimates of up to 165,000 cases and 89,000 deaths per year. The ecological niche of the organism has yet to be clearly defined, although the organism is associated with soil and water. The culture of appropriate clinical material remains the mainstay of laboratory diagnosis. Identification is best done by phenotypic methods, although mass spectrometric methods have been described. Serology has a limited diagnostic role. Direct molecular and antigen detection methods have limited availability and sensitivity. Clinical presentations of melioidosis range from acute bacteremic pneumonia to disseminated visceral abscesses and localized infections. Transmission is by direct inoculation, inhalation, or ingestion. Risk factors for melioidosis include male sex, diabetes mellitus, alcohol abuse, and immunosuppression. The organism is well adapted to intracellular survival, with numerous virulence mechanisms. Immunity likely requires innate and adaptive responses. The principles of management of this condition are drainage and debridement of infected material and appropriate antimicrobial therapy. Global mortality rates vary between 9% and 70%. Research into vaccine development is ongoing.