Inflammasomes are cytosolic multiprotein complexes that assemble in response to a variety of infectious and noxious insults. Inflammasomes play a critical role in the initiation of innate immune responses, primarily by serving as platforms for the activation of inflammatory caspase proteases. One such caspase, CASPASE-1 (CASP1), initiates innate immune responses by cleaving pro-IL-1β and pro-IL-18, leading to their activation and release. CASP1 and another inflammatory caspase termed CASP11 can also initiate a rapid and inflammatory form of cell death termed pyroptosis. Several distinct inflammasomes have been described, each of which contains a unique sensor protein of the NLR (nucleotide-binding domain, leucine-rich repeat-containing) superfamily or the PYHIN (PYRIN and HIN-200 domain-containing) superfamily. Here we describe the surprisingly diverse mechanisms by which NLR/PYHIN proteins sense bacteria and initiate innate immune responses. We conclude that inflammasomes represent a highly adaptable scaffold ideally suited for detecting and initiating rapid innate responses to diverse and rapidly evolving bacteria.

The obligate intracellular parasite Toxoplasma gondii is able to infect nearly all nucleated cell types of warm-blooded animals. This is achieved through the injection of hundreds of parasite effectors into the host cell cytosol, allowing the parasite to establish a vacuolar niche for growth, replication, and persistence. Here we show that Toxoplasma infection actives an inflammasome response in mice and rats, an innate immune sensing system designed to survey the host cytosol for foreign components leading to inflammation and cell death. Oral infection with Toxoplasma triggers an inflammasome response that is protective to the host, limiting parasite load and dissemination. Toxoplasma infection is sufficient to generate an inflammasome response in germfree animals. Interleukin 1␤ (IL-1␤) secretion by macrophage requires the effector caspases 1 and 11, the adapter ASC, and NLRP1, the sensor previously described to initiate the inflammasome response to Bacillus anthracis lethal factor. The allele of NLRP1b derived from 129 mice is sufficient to enhance the B6 bone marrow-derived macrophage (BMDM) inflammasome response to Toxoplasma independent of the lethal factor proteolysis site. Moreover, N-terminal processing of NLRP1b, the only mechanism of activation known to date, is not observed in response to Toxoplasma infection. Cumulatively, these data indicate that NLRP1 is an innate immune sensor for Toxoplasma infection, activated via a novel mechanism that corresponds to a hostprotective innate immune response to the parasite.

Inflammasomes are multimeric protein complexes that respond to infection by recruitment and activation of the Caspase-1 (CASP1) protease. Activated CASP1 initiates immune defense by processing inflammatory cytokines and by causing a rapid and lytic cell death called pyroptosis. Inflammasome formation is orchestrated by members of the nucleotide-binding domain and leucine-rich repeat (NLR) or AIM2-like receptor (ALR) protein families. Certain NLRs and ALRs have been shown to function as direct receptors for specific microbial ligands, such as flagellin or DNA, but the molecular mechanism responsible for activation of most NLRs is still poorly understood. Here we determine the mechanism of activation of the NLRP1B inflammasome in mice. NLRP1B, and its ortholog in rats, is activated by the lethal factor (LF) protease that is a key virulence factor secreted by Bacillus anthracis, the causative agent of anthrax. LF was recently shown to cleave mouse and rat NLRP1 directly. However, it is unclear if cleavage is sufficient for NLRP1 activation. Indeed, other LF-induced cellular events have been suggested to play a role in NLRP1B activation. Surprisingly, we show that direct cleavage of NLRP1B is sufficient to induce inflammasome activation in the absence of LF. Our results therefore rule out the need for other LF-dependent cellular effects in activation of NLRP1B. We therefore propose that NLRP1 functions primarily as a sensor of protease activity and thus could conceivably detect a broader spectrum of pathogens than just B. anthracis. By adding proteolytic cleavage to the previously established ligand-receptor mechanism of NLR activation, our results illustrate the remarkable flexibility with which the NLR architecture can be deployed for the purpose of pathogen-detection and host defense.

Inflammasomes are cytosolic protein complexes that serve as platforms for the recruitment and activation of the pro-inflammatory CASPASE-1 protease. CASPASE-1 activation leads to processing and maturation of the cytokines interleukin-1β and interleukin-18 and a lytic form of cell death termed pyroptosis. Inflammasome assembly is initiated by cytosolic sensors in response to microbial infections. Many of these sensors, including NLRP1 (NLR family, pyrin domain containing 1), are described to form an inflammasome, but until recently, the mechanism of inflammasome activation and its physiological functions in host defense have remained unclear. In the last few years, important advances in our understanding of NLRP1 biology have been achieved. In this review, we discuss the activation of NLRP1 by various stimuli, including Bacillus anthracis lethal toxin, Toxoplasma gondii, muramyl dipeptide, and host intracellular ATP depletion. The role NLRP1 plays in pathogen recognition and resistance during infection is also discussed, as is the regulation of NLRP1 by host and viral proteins. We conclude by discussing the unexpected differences in the mechanism of NLRP1 inflammasome activation, as compared to the activation of other inflammasomes, such as the NAIP (NLR family, apoptosis inhibitory protein)/NLRC4 inflammasomes.

The NAIP/NLRC4 inflammasomes activate caspase-1 in response to bacterial type III secretion systems (T3SS). Inadvertent injection of the T3SS rod protein and flagellin into the cytosol are detected through murine NAIP2 and NAIP5/6, respectively. Here, we identify the agonist for the orphan murine NAIP1 receptor as the T3SS needle protein. NAIP1 is poorly expressed in resting mouse bone marrow-derived macrophages (BMMs), however, priming with poly(I:C) induces it, and confers needle protein sensitivity. Further, overexpression of NAIP1 in immortalized BMMs by retroviral transduction enabled needle detection. In contrast, peritoneal cavity macrophages basally express NAIP1 and respond to needle protein robustly independent of priming. Human macrophages are known to only express one NAIP gene, which detects the needle protein, but not rod or flagellin. Thus, murine NAIP1 is functionally analogous to human NAIP.