A Type VI Secretion System inBurkholderiaSpeciescenocepaciaandorbicolaTriggers Distinct Macrophage Death Pathways Independent of the Pyrin Inflammasome

Abstract

AbstractTheBurkholderia cepaciacomplex contains opportunistic pathogens that cause chronic infections and inflammation in lungs of people with cystic fibrosis. Two closely related species within this complex areBurkholderia cenocepaciaand the recently classifiedBurkholderia orbicola.B. cenocepaciaandB. orbicolaencode a type VI secretion system and the effector TecA, which is detected by the pyrin/caspase-1 inflammasome, and triggers macrophage inflammatory death. In our earlier study the pyrin inflammasome was dispensable for lung inflammation in mice infected withB. orbicolaAU1054, indicating this species activates an alternative pathway of macrophage inflammatory death. Notably,B. cenocepaciaJ2315 and K56-2 can damage macrophage phagosomes and K56-2 triggers activation of the caspase-11 inflammasome, which detects cytosolic LPS. Here we investigated inflammatory cell death in pyrin-deficient (Mefv−/−) mouse macrophages infected withB. cenocepaciaJ2315 or K56-2 orB. orbicolaAU1054 or PC184. Macrophage inflammatory death was measured by cleavage of gasdermin D protein, release of cytokines IL-1α and IL-1β and plasma membrane rupture. Findings suggest that J2315 and K56-2 are detected by the caspase-11 inflammasome inMefv−/−macrophages, resulting in IL-1β release. In contrast, inflammasome activation is not detected inMefv−/−macrophages infected with AU1054 or PC184. Instead, AU1054 triggers an alternative macrophage inflammatory death pathway that requires TecA and results in plasma membrane rupture and IL-1α release. Amino acid variation between TecA isoforms inB. cenocepaciaandB. orbicolamay explain how the latter species triggers a non-inflammasome macrophage death pathway.