Diverse mechanisms for inflammasome sensing of cytosolic bacteria and bacterial virulence.

The inflammasomes are emerging cytosolic defenses against bacterial infections. The inflammasomes converge on inflammatory caspases activation that triggers pyroptosis, and interleukin-1β/18 maturation in the case of caspase-1 activation. The inflammasomes not only detect major bacterial molecules but also sense bacterial virulence activity. Among the canonical caspase-1-activating inflammasomes, the NAIP subfamily of NLR proteins serves as the receptors for bacterial flagellin and type III secretion apparatus; Pyrin indirectly senses Rho modification/inactivation by various bacterial agents; NLRP1 in mice/rats detects the protease activity of anthrax lethal toxin by serving as its substrate. Caspase-11 and caspase-4/5 directly recognize bacterial LPS and then become activated. Inflammasome sensing of cytosolic bacteria employs much more diversified biochemical mechanisms, compared with Toll-like receptors-mediated recognition on the membrane.