Burkholderia pseudomallei ΔtonB Δhcp1 Live Attenuated Vaccine Strain Elicits Full Protective Immunity against Aerosolized Melioidosis Infection

Burkholderia pseudomallei is a Gram-negative facultative intracellular bacterium and the causative agent of melioidosis, a severe infectious disease found throughout the tropics. This organism is closely related to Burkholderia mallei, the etiological agent of glanders disease which primarily affects equines. These two pathogenic bacteria are classified as Tier 1 select agents due to their amenability to aerosolization, limited treatment options, and lack of an effective vaccine. We have previously successfully demonstrated the immunogenicity and protective efficacy of a live attenuated vaccine strain, B. mallei ΔtonB Δhcp1 (CLH001). Thus, we applied this successful approach to the development of a similar vaccine against melioidosis by constructing the B. pseudomallei ΔtonB Δhcp1 (PBK001) strain. C57BL/6 mice were vaccinated intranasally with the live attenuated PBK001 strain and then challenged with wild-type B. pseudomallei K96243 by the aerosol route. Immunization with strain PBK001 resulted in full protection (100% survival) against acute aerosolized melioidosis with very low bacterial burden as observed in the lungs, livers, and spleens of immunized mice. PBK001 vaccination induced strong production of B. pseudomallei-specific serum IgG antibodies and both Th1 and Th17 CD4(+) T cell responses. Further, humoral immunity appeared to be essential for vaccine-induced protection, whereas CD4(+) and CD8(+) T cells played a less direct immune role. Overall, PBK001 was shown to be an effective attenuated vaccine strain that activates a robust immune response and offers full protection against aerosol infection with B. pseudomallei. IMPORTANCE In recent years, an increasing number of melioidosis cases have been reported in several regions where melioidosis is endemic and in areas where melioidosis had not commonly been diagnosed. Currently, the estimated burden of disease is around 165,000 new cases annually, including 89,000 cases that have fatal outcomes. This life-threatening infectious disease is caused by B. pseudomallei, which is classified as a Tier 1 select agent. Due to the high case fatality rate, intrinsic resistance to multiple antibiotic treatments, susceptibility to infection via the aerosol route, and potential use as a bioweapon, we have developed an effective live attenuated PBK001 vaccine capable of protecting against aerosolized melioidosis.