Date Published: May 23, 2018
Publisher: Public Library of Science
Author(s): Jinyong Wang, Manoranjan Sahoo, Louis Lantier, Jonathan Warawa, Hector Cordero, Kelly Deobald, Fabio Re, Denise M. Monack.
Infection with Burkholderia pseudomallei or B. thailandensis triggers activation of the NLRP3 and NLRC4 inflammasomes leading to release of IL-1β and IL-18 and death of infected macrophages by pyroptosis, respectively. The non-canonical inflammasome composed of caspase-11 is also activated by these bacteria and provides protection through induction of pyroptosis. The recent generation of bona fide caspase-1-deficient mice allowed us to reexamine in a mouse model of pneumonic melioidosis the role of caspase-1 independently of caspase-11 (that was also absent in previously generated Casp1-/- mice). Mice lacking either caspase-1 or caspase-11 were significantly more susceptible than wild type mice to intranasal infection with B. thailandensis. Absence of caspase-1 completely abolished production of IL-1β and IL-18 as well as pyroptosis of infected macrophages. In contrast, in mice lacking caspase-11 IL-1β and IL-18 were produced at normal level and macrophages pyroptosis was only marginally affected. Adoptive transfer of bone marrow indicated that caspase-11 exerted its protective action both in myeloid cells and in radio-resistant cell types. B. thailandensis was shown to readily infect mouse lung epithelial cells triggering pyroptosis in a caspase-11-dependent way in vitro and in vivo. Importantly, we show that lung epithelial cells do not express inflammasomes components or caspase-1 suggesting that this cell type relies exclusively on caspase-11 for undergoing cell death in response to bacterial infection. Finally, we show that IL-18’s protective action in melioidosis was completely dependent on its ability to induce IFNγ production. In turn, protection conferred by IFNγ against melioidosis was dependent on generation of ROS through the NADPH oxidase but independent of induction of caspase-11. Altogether, our results identify two non-redundant protective roles for caspase-1 and caspase-11 in melioidosis: Caspase-1 primarily controls pyroptosis of infected macrophages and production of IL-18. In contrast, caspase-11 mediates pyroptosis of infected lung epithelial cells.
Burkholderia pseudomallei is a Gram-negative flagellated bacterium that causes melioidosis, a diseases endemic to South-East Asia and other tropical regions and the most common cause of pneumonia-derived sepsis in Thailand [1, 2]. Due to global warming and increased international travel, cases of melioidosis are increasingly being reported outside the endemic areas. B. pseudomallei infection can be contracted through ingestion, inhalation, or subcutaneous inoculation and leads to broad-spectrum disease forms including pneumonia, septicemia, and organ abscesses. Although not pathogenic to humans, Burkholderia thailandensis possesses several of the B. pseudomallei virulence factors, causes morbidity and mortality in mice, and is often used as a model for melioidosis [3–5]. Following infection of macrophages and other non-phagocytic cell types, Burkholderia is able to escape the phagosome and invade and replicate in the host cell cytoplasm. Macrophages and IFNγ have been shown to play a critical role in protection from melioidosis [6–8]and several B. pseudomallei virulence factors have been identified. Analysis of mouse strains with different susceptibility to B. pseudomallei infection indicates that the early phases of the infection are crucial for survival, emphasizing the necessity for better understanding of innate immune responses during melioidosis.
The innate immune response to lung infection with Burkholderia species has been examined in a few papers but much remains to be learned. Here we have analyzed the role of the canonical and non-canonical inflammasomes and of the IL-18-IFNγ axis in a mouse model of melioidosis. We and others have previously shown that processing and secretion of the mature form of IL-1β and IL-18 in response to Burkholderia infection was dependent on caspase-1 [12, 14, 15]. The caveat of those studies is that they were performed using mice that also lacked caspase-11. The recent generation of bona fide caspase-1-deficient mice  allowed us to examine for the first time the role of this caspase independently of concomitant absence of caspase-11. Our data conclusively demonstrate that processing and secretion of IL-1β and IL-18 in response to B. thailandensis infection in vivo or in vitro is completely dependent on caspase-1 but unaffected by absence of caspase-11. The fact that IL-1β secretion is not reduced in absence of caspase-11 also indicates that activation of the NLRP3 inflammasome, which we previously showed exclusively controls IL-1β and IL-18 secretion in response to Burkholderia species infection [12, 13], does not occur as a consequence of caspase-11-mediated pyroptosis and potassium efflux, as in other circumstances . Our results also show that pyroptosis of macrophages infected with B. thailandensis or B. pseudomallei and restriction of intracellular bacteria replication is primarily mediated by caspase-1 with minor involvement of caspase-11. Previous works have attributed a more prominent role to caspase-11 in the pyroptosis of B. thailandensis-infected BMM . It should be noted that those studies relied on Asc-/-Nlrc4-/- cells or Casp1-/-/Casp11-/- cells reconstituted with transgenic human caspase-4 as proxy of bone fide caspase-1 deficient cells, two models that may not faithfully represent caspase-1 absence. Our results also indicate that experimental variables, such as the length of IFNγ priming, may lead to discordant conclusions regarding the involvement of caspase-11 in the pyroptosis of myeloid cells. In fact, it has been proposed that caspase-11 may function as a back-up mechanism to trigger pyroptosis in situations where caspase-1 may be inactivated .