Virulence associated functions and vaccine potential of the burkholderia mallei type three secretion effector BopA

Abstract

The detailed mechanism(s) of Burkholderia mallei pathogenesis is virtually unknown, with the production of a polysaccharide capsule and presence of secretion systems as the only known mechanisms associated with virulence. Various gram-negative pathogens utilize a secretion system known as type III, which is used to deliver bacterial (effector) proteins into host cells to modulate immunological and cellular responses. The intracellular survival of B. mallei within murine macrophage J774.2 cells requires the type III secretion system (TTSS), although the effector protein(s) involved are unknown. Additionally, previous reports have documented that B. mallei TTSS is required for phagosomal membrane lysis and bacterial escape into the macrophage cytoplasm. While progress has been made in demonstrating the importance of the TTSS contribution to B. mallei pathogenicity, additional work is needed to identify the potential secreted (effector) molecule(s) involved. BopA, a predicted effector protein of B. mallei, shares 24% amino acid homology to the TTS effector IcsB of Shigella flexneri, which is responsible for intercellular spread and host cell invasion. Characterization of the involvement of this putative TTS protein with bacterial survival strategies will provide crucial information for the development and assessment of a candidate vaccine. The work presented herein identified an N-terminal portion of BopA sufficient to drive secretion throughout a TTS apparatus and characterized the potential virulence-associated functions of B. mallei TTS protein BopA to subvert the host cell and promote intracellular replication and survival utilizing an in vitro respiratory cell model. Furthermore, in vivo studies assessed the attenuation of virulence offered by the B. mallei bopA mutant. Additionally, evaluation of the immune effectors involved in the protective host response to B. mallei infection identified key cellular and humoral components. Finally, a recombinant BopA sub-unit vaccine was utilized to assess the efficacy as a potential vaccine candidate.