Construction, characterization, and evaluation of CLH001 as a vaccine candidate against respiratory glanders
Hatcher, Christopher Lawrence
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Burkholderia mallei is the causative agent of glanders, an incapacitating disease with high mortality rates in respiratory cases. Its endemicity and ineffectual treatment options emphasize its public health threat and highlight the need for a vaccine. In this study, we constructed and characterized B. mallei ∆tonB ∆hcp1 (CLH001), a strain deficient in iron uptake and type six secretion functions, and investigated its ability to protect against acute respiratory glanders infection. When compared to wild-type (wt), CLH001 exhibited decreased growth kinetics in both culture media (LBG) and RAW 264.7 murine macrophages. Additionally unlike wt, CLH001 was deficient in Hcp1 production and was unable to induce multinucleated giant cells (MNGC) formation in both phagocytic and non-phagocytic cell lines. Intranasal (i.n.) administration of CLH001 (1.5x104 CFU) to BALB/c and NSG mice resulted in 100% survival with no detectable colonization or abnormal histopathology in the lungs, liver or spleen of vaccinated mice. BALB/c mice immunized i.n. with 1.5x105 CFU of CLH001 in a prime/boost regimen showed full protection post-challenge with 1.5x104 CFU of B. mallei lux wt strain. Organs from surviving mice were clear of bacterial colonization and histopathological abnormalities. Immunized mice showed high B. mallei-specific IgG serum titers and a Th1-biased response (IgG2a:IgG1 ratio = 4.0), a good predictor of protection. Additionally, pre-challenge sera displayed significant bactericidal activity over naïve serum (p=0.0062). Vaccinated BALB/c mice were also significantly protected (87.5% survival; p= < 0.0001) against higher dose (3.5x105 CFU) of B. mallei 23344 challenge. Our studies show that CLH001 is attenuated and safe, and effective at providing protection against lethal B. mallei challenge. CLH001 is not only a viable vaccine platform for advancement into pre-clinical studies, but also represents the first Tier 1 Select Agent-excluded B. mallei strain.