The long-term aim of research in the Bliska Lab is to understand how bacterial effectors that are secreted into infected leukocytes promote pathogenesis or elicit host protection. Our current research is focused on bacterial pathogens that use type III secretion (T3S) or type VI secretion (T6S) systems to deliver effectors into phagocytic leukocytes (macrophages, monocytes, neutrophils). Effectors are proteins, usually enzymes, that manipulate phagocyte processes to promote bacterial pathogenesis. The pathogenic activities of some effectors can induce an immune response in phagocytes by a mechanism referred to as effector-triggered immunity (ETI). A common mechanism of ETI involves assembly of a multiprotein complex called an inflammasome. Inflammasomes coordinate the processing and release of pro-inflammatory cytokines through pores formed in the plasma membrane of the phagocyte. Inflammasome activation can also lead to death of phagocytes by a process called pyroptosis, and dying neutrophils can extrude chromatin as extracellular traps (NETs). The following projects explore mechanisms of inflammasome regulation and ETI at the molecular, cellular and immunological levels in the Gram-negative bacterial pathogens Yersinia, Pseudomonas and Burkholderia.
Yersinia
Yersinia species (e.g. Y. pseudotuberculosis) are virulent bacteria that cause invasive infections (yersiniosis or plague) in healthy humans. Yersinia are primarily extracellular pathogens that use T3S to deliver effectors across the plasma membrane of phagocytes. We study Yersinia effectors that interact with the RhoA-pyrin signaling pathway in phagocytes. Two Yersinia effectors (YopE, YopT) promote pathogenesis by inactivating RhoA, a small GTPase and regulator of cellular processes such as actin dynamics. Inactivation of RhoA by these effectors triggers an ETI in phagocytes in which the guard protein pyrin assembles an inflammasome. Because cytokine and NET release promotes host protection in this context, Yersinia evolved a third effector (YopM) that inhibits the pyrin inflammasome, to block ETI. Current goals in this project are to determine how pyrin is positively and negatively regulated by Yersinia effectors in phagocytes and the immunological basis of ETI.
Pseudomonas
Pseudomonas aeruginosa is an opportunistic pathogen responsible for chronic lung infections in people with cystic fibrosis (CF). CF is a genetic disease associated with multiple immune deficiencies underlying the susceptibility to P. aeruginosa and other pathogens. P. aeruginosa is primarily an extracellular pathogen that uses T3S to deliver effectors across the plasma membrane of phagocytes. We study the effector ExoS which has broad substrate specificity and inhibits phagocytosis and superoxide production in neutrophils. ExoS is epidemiologically linked to successful establishment of P. aeruginosa infections in people with CF. Current goals in this project are to determine if ExoS regulates inflammasome responses and ETI in neutrophils infected with P. aeruginosa. We are also investigating the use of synthetic immunology to engineer phagocytes to function as immunotherapeutics targeting P. aeruginosa.
Burkholderia
Burkholderia species (e.g. B. cenocepacia, B. orbicola) are opportunistic pathogens that cause chronic inflammatory lung infections in people with CF. Burkholderia are primarily intracellular pathogens that reside in macrophages after they are taken up by phagocytosis. Defects in superoxide production by CF macrophages may underlie the susceptibility of these cells to colonization by Burkholderia. Burkholderia use T6S to deliver the effector TecA across the phagosomal membrane. TecA inactivates RhoA, leading to assembly of the pyrin inflammasome in macrophages. We have determined that Burkholderia triggers two additional inflammasome mechanisms in macrophages. Our results suggest that macrophage inflammasome responses to Burkholderia in the lung is not protective and instead results in immunopathogenesis. We call this effector-triggered immuno-pathogenesis (ETP). Current goals in this project are to understand how TecA is delivered across the phagosome membrane, to define the ETP mechanisms caused by Burkholderia in macrophages and the role of these processes in lung disease.