Preventing bacterial DNA release and absent in melanoma 2 inflammasome activation by a Legionella effector functioning in membrane trafficking

Abstract

Legionella pneumophila, the causative agent of Legionnaires' pneumonia, resides in a distinct vacuole structure called Legionella-containing vacuole (LCV). The LCV resists fusion with the lysosome and permits efficient bacterial replication in host macrophages, which requires a Dot/Icm type IVB secretion system. Dot/Icm-translocated effector SdhA is critical for L. pneumophila intracellular growth and functions to prevent host cell death. Here, we show that the absence of SdhA resulted in elevated caspase-1 activation and IL-1β secretion as well as macrophage pyroptosis during Legionella infection. These inflammasome activation phenotypes were independent of the established flagellin-NAIP5-NLRC4 axis, but relied on the DNA-sensing AIM2 inflammasome. We further demonstrate that Legionella DNA was released into macrophage cytosol, and this effect was significantly exaggerated by the absence of SdhA. SdhA bears a functional Golgi-targeting GRIP domain that is required for preventing AIM2 inflammasome activation. Ectopically expressed SdhA formed a unique ring-shape membrane structure, further indicating a role in membrane trafficking and maintaining LCV membrane integrity. Our data together suggest a possible link, mediated by the function of SdhA, between LCV trafficking/maturation and suppression of host innate immune detection.

The Type II Secretion System of Legionella pneumophila Dampens the MyD88 and Toll-Like Receptor 2 Signaling Pathway in Infected Human Macrophages

“…During L. pneumophila infection of murine macrophages, activation of the inflammasome occurs through NAIP5/NLRC4, triggered by flagellin, as well as through an apoptosis-associated speck-like protein containing a CARD (ASC)-dependent pathway, resulting in the production of cytokines via an IL-1 autocrine loop (22,23,(53)(54)(55)(56). Humans lack the NAIP5 allele present in murine cells (57); however, the canonical ASC-dependent inflammasome is still activated upon L. pneumophila infection of human macrophages (58).…”

Evasion of inflammasome activation by microbial pathogens

“…The Dot/Icm-translocated effector molecule SdhA is required for L. pneumophila intracellular growth. Mutant bacteria deficient in SdhA induced elevated IL-1β secretion and macrophage pyroptosis that was dependent on AIM2 inflammasome activation, but independent of the flagellin-sensing NLRC4 inflammasome pathway (52). Interestingly, SdhA was required to maintain LCV membrane integrity, and thus its absence drove Legionella DNA release into the cytosol and increased AIM2 but not NLRC4 inflammasome activation (52).…”

Creating a customized intracellular niche: subversion of host cell signaling byLegionellatype IV secretion system effectors

“…These bacterial molecules, in particular Legionella DNA, are detected by cytosolic intracellular pattern recognition receptors, such as AIM2. This triggers a type I interferon immune response and rapid inflammasome-mediated caspase-1 activation and proinflammatory cell death, called pyroptosis, depriving Legionella of its intracellular niche before it can efficiently replicate (Monroe et al 2009;Ge et al 2012). Deletion of SdhA results in a growth defect of L. pneumophila in the Galleria mellonella infection model, in the lungs of mice, in murine bone-marrowderived macrophages and, to a lesser extent, human U937 macrophage-like cells, and in amoebae (Laguna et al 2006;Harding et al 2013b).…”

“…The molecular details of the interplay between these 2 effectors have not been defined yet. However, breakdown of the LCV releases Legionella and Legionella-derived molecules into the cytoplasm (Creasey and Isberg 2012;Ge et al 2012). These bacterial molecules, in particular Legionella DNA, are detected by cytosolic intracellular pattern recognition receptors, such as AIM2.…”