Burkholderia pseudomallei causes the tropical
infection melioidosis.
Pneumonia is a common manifestation of
melioidosis and is associated with high mortality. Understanding the key elements of host defense is essential to developing new
therapeutics for
melioidosis. As a flagellated bacterium encoding
type III secretion systems, B. pseudomallei may trigger numerous host pathogen recognition receptors. TLR5 is a
flagellin sensor located on the plasma membrane. NLRC4, along with NAIP
proteins, assembles a canonical caspase-1-dependent
inflammasome in the cytoplasm that responds to
flagellin (in mice) and
type III secretion system components (in mice and humans). In a murine model of respiratory
melioidosis, Tlr5 and Nlrc4 each contributed to survival. Mice deficient in both Tlr5 and Nlrc4 were not more susceptible than single knockout animals. Deficiency of Casp1/Casp11 resulted in impaired bacterial control in the lung and spleen; in the lung much of this effect was attributable to Nlrc4, despite relative preservation of pulmonary IL-1β production in Nlrc4(-/-) mice. Histologically, deficiency of Casp1/Casp11 imparted more severe
pulmonary inflammation than deficiency of Nlrc4. The human NLRC4 region polymorphism rs6757121 was associated with survival in
melioidosis patients with pulmonary involvement. Co-inheritance of rs6757121 and a functional TLR5 polymorphism had an additive effect on survival. Our results show that NLRC4 and TLR5, key components of two
flagellin sensing pathways, each contribute to host defense in respiratory
melioidosis.