Molecular mechanisms of inflammasome activation during microbial infections

Abstract

The innate immune system plays a crucial role in the rapid recognition and elimination of invading microbes. Detection of microbes relies on germ-line encoded pattern recognition receptors (PRRs) that recognize essential bacterial molecules, so-called pathogen-associated molecular patterns (PAMPs). A subset of PRRs, belonging to the NOD-like receptor (NLR) and the PYHIN protein families, detects viral and bacterial pathogens in the cytosol of host cells and induces the assembly of a multi-protein signaling platform called the inflammasome. The inflammasome serves as an activation platform for the mammalian cysteine protease caspase-1, a central mediator of innate immunity. Active caspase-1 promotes the maturation and release of interleukin-1β (IL-1β) and IL-18 as well as protein involved in cytoprotection and tissue repair. In addition, caspase-1 initiates a novel form of cell death called pyroptosis. Here, we discuss latest advances and our insights on inflammasome stimulation by two model intracellular pathogens, Francisella tularensis and Salmonella typhimurium. Recent studies on these pathogens have significantly shaped our understanding of the molecular mechanisms of inflammasome activation and how microbes can evade or manipulate inflammasome activity. In addition, we review the role of the inflammasome adapter ASC in caspase-1 autoproteolysis and new insights into the structure of the inflammasome complex.

Fig. 1. Inflammasome receptors recognize a variety of microbial pathogens and danger signals

NLRP3 responds to numerous stimuli. Common terminal signals appear to involve lysosomal rupture and the release of Cathepsins, potassium release and the production of reactive oxygen species (ROS). AIM2 functions as a cytosolic DNA sensor, detecting DNA introduced by transfection, infection with the cytosolic bacterial pathogens F. tularensis or L. monocytogenes or DNA viruses. Human NLRP1 responds to muramyl dipeptide, while Anthrax lethal toxin triggers murine Nlrp1b. NLRC4 detects flagellin or the T3SS rod subunit in the cytosol. LRR, leucine-rich repeats; NBD, nucleotide binding and oligomerization domain; PYD, Pyrin-like domain; CARD, caspase activation and recruitment domain.

Fig. 2. Model of innate immune recognition of F. tularensis by macrophages

(1) F. tularensis is phagocytosed by macrophages and quickly escapes from the FCV (Francisella containing vacuole) into the cytosol. In the cytosol, F. tularensis releases an unknown ligand, which is recognized by an unknown cytoplasmic receptor, leading to STING- and IRF3-dependent production of type I interferons. (2) Autocrine and paracrine signaling through the type-I-interferon receptor (IFNAR) leads to STAT1/2 dependent expression of interferon inducible genes, among them AIM2. (3) A subset of cytocolic F. tularensis lyse, releasing bacterial DNA, which is recognized by AIM2. DNA-AIM2 complexes serve as a nucleation point for rapid ASC oligomerization, which leads to the formation of an ASC focus. (4) Pro-caspase-1 is recruited and activated in the ASC focus and promotes cell death and cytokine maturation.

Fig. 3. Model of S. typhimurium-induced inflammasome activation in cultured macrophages

Distinct growth conditions of S. typhimurium results in different innate immune responses in cultured macrophages. (A). Growth of S. typhimurium to logarithmic phase results in the expression of the SPI-1 T3SS (red). Infection of bone-marrow derived macrophages under this condition leads to a rapid recognition of flagellin and PrgJ by NLRC-4 resulting in inflammasome activation within 1-2 hours post-infection. (B). Growth of S. typhimurium to stationary phase leads to the downregulation of SPI-1 expression and the up-regulation of the SPI-2 T3SS (blue). Infection under these condition results in a slow activation of the NLRC4 and NLRP3 inflammasomes starting approximately 10 hours and a peak at 16-20 hours post-infection. NLRC4 responds to SPI-2 T3SS-dependent translocation of flagellin into the cytosol, while NLRP3 responds to a T3SS-independent signal. LRR, leucine-rich repeats; NBD, nucleotide binding and oligomerization domain; PYD, Pyrin-like domain; CARD, caspasea activation and recruitment domain.

Fig. 4. Model for the formation of functionally distinct inflammasomes by CARD-containing receptors

(1) Recruitment of ASC to activated receptors is followed by rapid oligomerization of ASC and the formation of the macro-molecular ASC focus through homotypic CARD-CARD and PYD-PYD interactions. This complex recruits and activates pro-caspase-1 through proximity-induced dimerization. In the ASC focus, conformational changes in the pro-caspase-1 dimer allow for auto-proteolytic processing into the p20 and p10 subunits. Only fully processed Caspase-1 efficiently promotes cytokine maturation. (2) Recruitment of pro-caspase-1 to activated receptors leads to the formation of a “Death-complex”. Pro-caspase-1 is not autoproteolytically processed in this type of a complex. Nevertheless, pro-caspase-1 is activated in this complex and promotes pyroptosis and inefficient cytokine processing. LRR, leucine-rich repeats; NBD, nucleotide binding and oligomerization domain; PYD, Pyrin-like domain; CARD, caspase activation and recruitment domain.