Inflammasomes are cytosolic multiprotein complexes that sense microbial infection and trigger cytokine production and cell death. However, the molecular components of inflammasomes, and what they sense, remain poorly defined. Here we demonstrate that 35 amino acids from the Cterminus of flagellin triggered inflammasome activation in the absence of bacterial contaminants or secretion systems. To further elucidate the host flagellin-sensing pathway, we generated mice deficient in Naip5. Naip5-deficient mice failed to activate the inflammasome in response to the 35 amino acids of flagellin or in response to Legionella pneumophila infection. Taken together, these data clarify the molecular basis for the cytosolic response to flagellin.Inflammasomes are cytosolic multiprotein complexes that are critical regulators of inflammation, and are required for proteolytic activation of the cysteine protease caspase-1 (refs. 1-3). Caspase-1 (A000492; http://www.signaling-gateway.org/molecule/query?afcsid=A000492) is itself required for the proteolytic processing and release of inflammatory cytokines such as interleukin 1β (IL-1β) and IL-18, as well as for induction of a necrotic-like cell death called pyroptosis1-3. The molecular components and structures of inflammasomes remain poorly defined. It is believed that multiple distinct inflammasomes may exist, each containing a key scaffold protein of the NLR (nucleotide-binding domain, leucine-rich repeat) superfamily that confers specificity for particular microbial products. For example, NLR proteins of the NLRP1 family (also called NALP1) appear to activate the inflammasome in response to anthrax lethal toxin4 and bacterial muramyl dipeptide5. In contrast, the NLR protein NLRP3 (also called NALP3 or cryopyrin) has been proposed to sense a wide range of stimuli including bacterial RNA6, viral DNA7, uric acid crystals8, muramyl dipeptide9,10, nigericin11, amyloid-beta12, and other irritants13-16. There is at present no molecular explanation for how a single NLR protein can NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript be activated by all these microbial products and the precise molecular nature of what is sensed by any inflammasome remains undefined.The inflammasome containing the NLR protein IPAF (also called NLRC4) is one of the best characterized inflammasomes, and has been proposed by several groups to respond to the presence of flagellin in the cytosol17-19. Flagellin-deficient mutants of Salmonella typhimurium and Legionella pneumophila are defective in IPAF-dependent inflammasome activation, and flagellin, purified from or expressed in bacteria, triggers IPAF-dependent caspase-1 activation when delivered to the cytosol of macrophages by use of a pore-forming toxin (listeriolysin O (LLO)) or transfection reagents17-21. It was proposed that during natural infections, flagellin triggers inflammasome activation upon secretion into the host cytosol via bacterial type III/IV secretion systems17-21. However, doubts have been expressed as to w...

Distinct patterns of immune dysfunction and interaction with sensory pathways occur in different patient groups and through different intracellular pathways. Our results indicate IBS patient subgroups would benefit from selective targeting of the immune system.

The amino‐terminal hypervariable region (HVR) of streptococcal M protein is required for the ability of this virulence factor to confer phagocytosis resistance. The function of the HVR has remained unknown, but the finding that many HVRs with extremely divergent sequences bind the human complement regulator C4b‐binding protein (C4BP) has suggested that this ligand may play a role in phagocytosis resistance. We used the M22 system to study the function of bound C4BP and provide several lines of evidence that C4BP indeed contributes to phagocytosis resistance. First, the ability of anti‐HVR antibodies to cause opsonization correlated with their ability to inhibit binding of C4BP. Secondly, a short deletion in the HVR eliminated C4BP binding and also reduced the ability of M22 to confer phagocytosis resistance. Thirdly, the addition of an excess of pure C4BP to a phagocytosis system almost completely blocked the effect of opsonizing anti‐HVR antibodies. Together, our data indicate that binding of C4BP to the HVR of M22 plays an important role in phagocytosis resistance, but other properties of M22 also contribute. This study provides the first molecular insight into the mechanisms by which the HVR of an M protein confers phagocytosis resistance.

Summary. Background: Thrombomodulin (TM) is predominantly a vascular endothelial cell plasma membrane glycoprotein that, via distinct structural domains, interacts with multiple ligands, thereby modulating coagulation, fibrinolysis, complement activation, inflammation and cell proliferation. We previously reported that by mediating signals that interfere with mitogen-activated protein kinase and nuclear factor jB pathways, the amino-terminal C-type lectin-like domain of TM has direct anti-inflammatory properties. Methods: In the current study, we use murine models of acute inflammatory arthritis and biochemical approaches to assess the mechanism by which the lectin-like domain of TM modifies disease progression. Results: Mice lacking the lectin-like domain of TM (TM LeD/LeD mice) develop inflammatory arthritis that is more rapid in onset and more severe than that developed in their wildtype counterparts. In two models of arthritis, treatment of mice with recombinant soluble lectin-like domain of TM significantly suppresses clinical evidence of disease and diminishes monocyte/macrophage infiltration into the synovium, with weaker expression of the pro-inflammatory high mobility group box chromosomal protein 1. While thrombin-TM mediated activation of thrombin activatable fibrinolysis inhibitor inactivates complement factors C3a and C5a, we show that TM has a second independent mechanism to regulate complement: the lectin-like domain of TM directly interferes with complement activation via the classical and lectin pathways. Conclusions: These data extend previous insights into the mechanisms by which TM modulates innate immunity, and highlight its potential as a therapeutic target for inflammatory diseases.