title: Burkholderia cenocepacia type VI secretion system mediates escape of type II secreted proteins into the cytoplasm of infected macrophages.
abstract: Burkholderia cenocepacia is an opportunistic pathogen that survives intracellularly in macrophages and causes serious respiratory infections in patients with cystic fibrosis. We have previously shown that bacterial survival occurs in bacteria-containing membrane vacuoles (BcCVs) resembling arrested autophagosomes. Intracellular bacteria stimulate IL-1β secretion in a caspase-1-dependent manner and induce dramatic changes to the actin cytoskeleton and the assembly of the NADPH oxidase complex onto the BcCV membrane. A Type 6 secretion system (T6SS) is required for these phenotypes but surprisingly it is not required for the maturation arrest of the BcCV. Here, we show that macrophages infected with B. cenocepacia employ the NLRP3 inflammasome to induce IL-1β secretion and pyroptosis. Moreover, IL-1β secretion by B. cenocepacia-infected macrophages is suppressed in deletion mutants unable to produce functional Type VI, Type IV, and Type 2 secretion systems (SS). We provide evidence that the T6SS mediates the disruption of the BcCV membrane, which allows the escape of proteins secreted by the T2SS into the macrophage cytoplasm. This was demonstrated by the activity of fusion derivatives of the T2SS-secreted metalloproteases ZmpA and ZmpB with adenylcyclase. Supporting this notion, ZmpA and ZmpB are required for efficient IL-1β secretion in a T6SS dependent manner. ZmpA and ZmpB are also required for the maturation arrest of the BcCVs and bacterial intra-macrophage survival in a T6SS-independent fashion. Our results uncover a novel mechanism for inflammasome activation that involves cooperation between two bacterial secretory pathways, and an unanticipated role for T2SS-secreted proteins in intracellular bacterial survival.
authors: Centre for Human Immunology, Department of Microbiology and Immunology, The University of Western Ontario, London, Ontario, Canada.
authors: Roberto
authors: R
authors: Rosales-Reyes
authors: Daniel F
authors: DF
authors: Aubert
authors: Jennifer S
authors: JS
authors: Tolman
authors: Amal O
authors: AO
authors: Amer
authors: Miguel A
authors: MA
authors: Valvano
mesh_terms: D000818:Animals; D001426:Bacterial Proteins / Q000378:metabolism*; D058947:Bacterial Secretion Systems; D057508:Burkholderia cenocepacia / Q000378:metabolism / Q000502:physiology*; D002352:Carrier Proteins / Q000378:metabolism; D016923:Cell Death; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism / Q000382:microbiology; D003593:Cytoplasm / Q000382:microbiology*; D058847:Inflammasomes / Q000378:metabolism; D053583:Interleukin-1beta / Q000378:metabolism; D008264:Macrophages / Q000166:cytology* / Q000382:microbiology*; D051379:Mice; D008810:Mice, Inbred C57BL; D050296:Microbial Viability; D000071199:NLR Family, Pyrin Domain-Containing 3 Protein; D021381:Protein Transport
chemical_list: D001426:Bacterial Proteins; D058947:Bacterial Secretion Systems; D002352:Carrier Proteins; D058847:Inflammasomes; D053583:Interleukin-1beta; D000071199:NLR Family, Pyrin Domain-Containing 3 Protein; C492716:Nlrp3 protein, mouse