Recognition of flagellin by NLRC4 is likely indirect and mediated through host cellular factors, which trigger inflammasome activation since there is no evidence to date for a direct interaction between NLRC4 and flagellin. NLRC4 DAPT can sense additional molecules besides flagellin as certain aflagellated bacteria including S. flexneri14 and Mycobacterium tuberculosis21 activate caspase-1 via NLRC4. The NLR protein Naip5 is also critical for the sensing
of a conserved C-terminal portion of flagellin from L. pneumophila and for NLRC4-dependent caspase-1 activation 22. Remarkably, Naip5 is not required for caspase-1 activation triggers by S. typhimurium or P. aeruginosa infection 22. The mechanism by which Naip5 regulates the NLRC4 inflammasome activated by L. pneumophila remains
unclear 23. Because caspase-1 is critical for restricting the replication of L. pneumophila in the host cytosol, these studies suggest that both Naip5 and NLRC4 control the susceptibility to L. pneumophila through the sensing of flagellin and caspase-1 activation. Alternatively, Naip5 may have additional NLRC4-independent roles EPZ-6438 cost that are important in restricting the growth of L. pneumophila in macrophages. Recent studies suggest that caspase-7 which is activated by the NLRC4 inflammasome is an important factor in restricting L. pneumophila replication, although the mechanism involved remains elusive PD184352 (CI-1040) 24. While the NLRC4 inflammasome
is activated primarily by cytosolic flagellin, a plethora of microbial and non-microbial stimuli have been reported to activate caspase-1 via NLRP3. These include multiple TLR agonists and the Nod2 agonist, MDP 25, 26. In addition, large particles including urate crystals, silica, asbestos, β-amyloid and aluminum hydroxide activate the NLRP3 inflammasome in phagocytes pre-stimulated with microbial ligands such as LPS 6. Unlike TLR ligands, these particulate and crystalline molecules can activate the inflammasome in the absence of extracellular ATP 6. Although the critical cellular events remain poorly understood, disruption of the lysosomal membrane and/or production of ROS 27 have been suggested to be important for particulate matter-induced NLRP3 activation 28. The ability of multiple pathogen-associated molecular patterns to activate the NLRP3 inflammasome is puzzling because most of the molecules including TLR ligands are structurally unrelated. Recent findings suggest that most or all TLR agonists as well as MDP do not activate the NLRP3 inflammasome directly. Instead, they prime the inflammasome via NF-κB to promote caspase-1 activation 29, 30, which is consistent with previous results 31. Consistently, TNF-α and IL-1 are as effective as TLR agonists in promoting caspase-1 activation in response to ATP or silica 29.