Developing Vault Nanoparticles as a Delivery Platform for Burkholderia pseudomallei and Burkholderia mallei Antigens

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ABSTRACT Burkholderia pseudomallei (Bp) is endemic to tropical regions and causes the often-fatal disease melioidosis in humans and animals, posing a significant threat to native residents, tourists, and deployed U.S. military personnel. In highly-endemic areas of southeast Asia, melioidosis is projected to surpass tuberculosis as the second most common cause of infectious disease mortality after HIV/AIDS. There are currently no effective approaches for providing immunity, and treatment confounded by intrinsic antibiotic resistance. The ability to survive intracellularly and utilize membrane fusion as a means for spreading from cell to cell shelters the bacterium from the humoral immune system. Prior efforts to provide protective immunity against Bp rely on live-attenuated strains or bacterial subunits that are only partially protective. Therefore, an antigen-delivery strategy to elicit both antibody and cellular responses is needed for B. pseudomallei, and the factors that promote survival inside mammalian cells will likely constitute effective antigens. This is supported by current evidence in animal models and recovered melioidosis patients, where cytotoxic T lymphocytes (CTLs) play a central role by recognizing and killing Bp-infected cells, with antibody playing an ancillary role. Our goal is to utilize the endogenous human vault nanoparticle as a novel, safe platform for delivering Burkholderia antigens that can elicit strong cell-mediated and humoral responses against Bp. Vaults efficiently deliver antigens and provide remarkable immune responses without additional adjuvants. Vaults are large, naturally-occurring, cytoplasmic riboprotein particles found in all nucleated mammalian cells. Recent observations suggest that vaults function in immune surveillance, delivering antigens from dying cells to facilitate adaptive immunity. Recombinant vaults efficiently deliver antigens to both the MHC class I and II pathways, and have adjuvant-like effects in APCs, eliciting protective CD4 and CD8 T cell responses, while also generating antibody responses. In contrast, native or "empty" vaults do not activate inflammasome or TLR-dependent inflammatory reactions and are non-immunogenic. We have exploited these properties of vaults to elicit robust immune responses in several systems, including a mouse model of mucosal infection by intracellular Chlamydia, which like Bp, is an intracellular pathogen. We will combine our considerable knowledge of pathogenic mechanisms and practiced production approaches for making recombinant vaults containing Burkholderia antigens (BurkAgs). BurkAgs have been identified based on properties that are hypothesized to correlate with robust immunogenicity and the potential to elicit broad responses against diverse strains of Bp. The ability of recombinant BurkVaults to induce antigen-specific antibody, CD4 and CD8 T cell responses following i.n. or s.c. immunization will be determined in established, quantitative assays. Recombinant vaults that display optimal structural, compositional, and immunogenic profiles will be advanced for studies against lethal challenge with Bp in BALB/c mice.