A type VI secretion system in <i>Burkholderia</i> species <i>cenocepacia</i> and <i>orbicola</i> triggers distinct macrophage death pathways independent of the pyrin inflammasome

The <i>Burkholderia cepacia</i> complex contains opportunistic pathogens that cause chronic infections and inflammation in the lungs of people with cystic fibrosis. Two closely related species within this complex are <i>Burkholderia cenocepacia</i> and the recently classified <i>Burkholderia orbicola. B. cenocepacia</i> and <i>B. orbicola</i> encode a type VI secretion system and the effector TecA, which is detected by the pyrin/caspase-1 inflammasome, and triggers macrophage inflammatory death. We previously showed that the pyrin inflammasome was dispensable for lung inflammation in mice infected with <i>B. orbicola</i> AU1054<i>,</i> indicating this species activates an alternative pathway of macrophage inflammatory death. Notably, <i>B. cenocepacia</i> strains J2315 and K56-2 can damage macrophage phagosomes, and K56-2 triggers activation of the caspase-11 inflammasome, which detects cytosolic lipopolysaccharide. Here, we investigated inflammatory cell death in pyrin- (<i>Mefv</i>^-/-) or caspase-1/caspase-11- (<i>Casp1/11^-/-</i>) deficient mouse macrophages infected with <i>B. cenocepacia</i> J2315 or K56-2 or <i>B. orbicola</i> AU1054 or PC184. Macrophage inflammatory death was measured by cleavage of gasdermin D protein, the release of cytokines IL-1α and IL-1β, and plasma membrane rupture. We found that J2315 and K56-2 are detected by the caspase-11 inflammasome in <i>Mefv</i>^-/- macrophages, resulting in IL-1β release. By contrast, inflammasome activation was not detected in <i>Mefv</i>^-/- macrophages infected with AU1054 or PC184. Instead, AU1054 triggered an alternative macrophage inflammatory death pathway that required TecA and resulted in plasma membrane rupture and IL-1α release. Structural modeling of TecA orthologs in <i>B. cenocepacia</i> and <i>B. orbicola</i> suggested that amino acid changes in the latter may underlie its ability to trigger a non-inflammasome macrophage death pathway.