The inflammasome is a multi-protein complex that mediates activation of caspase-1 which promotes the secretion of the proinflammatory cytokines IL-1β and IL-18 as well as pyroptosis, a form of cell death induced by bacterial pathogens. Members of the Nod-like receptor family including NLRP1, NLRP3 and NLRC4 and the adaptor ASC are critical components of the inflammasome by linking microbial and endogenous danger signals to caspase-1 activation. Several diseases are associated with the dysregulated activation of caspase-1 and IL-1β secretion. Thus, understanding of inflammasome pathways may provide insights into disease pathogenesis that might serve as potential targets for therapeutic intervention.

The Nlrp3 inflammasome is critical for the activation of caspase-1 in response to danger signals and particulate matter. However, its role in sterile inflammation remains unclear because prestimulation of phagocytic cells with microbial molecules is required for caspase-1 activation. We show here that exposure of macrophages and dendritic cells to TNF-α promotes ATP- or silica-mediated caspase-1 activation and IL-1β secretion in the absence of microbial stimulation. The effect of TNF-α was abolished in macrophages deficient in TNF receptor I and II, Nlrp3, or ASC, whereas that induced by TLR ligands required MyD88/Trif. In addition to TNF-α, IL-1α and IL-1β promoted caspase-1 activation via Nlrp3 in response to ATP. Remarkably, macrophages tolerized to TNF-α, but not to LPS, retained full sensitivity to ATP stimulation via Nlrp3. These results provide a mechanism by which danger signals and particulate matter mediate inflammation via the Nlrp3 inflammasome in the absence of microbial infection.

Acute brain lesions induce profound alterations of the peripheral immune response comprising the opposing phenomena of early immune activation and subsequent immunosuppression. The mechanisms underlying this brain-immune signaling are largely unknown. We used animal models for experimental brain ischemia as a paradigm of acute brain lesions and additionally investigated a large cohort of stroke patients. We analyzed release of HMGB1 isoforms by mass spectrometry and investigated its inflammatory potency and signaling pathways by immunological in vivo and in vitro techniques. Features of the complex behavioral sickness behavior syndrome were characterized by homecage behavior analysis. HMGB1 downstream signaling, particularly with RAGE, was studied in various transgenic animal models and by pharmacological blockade. Our results indicate that the cytokine-inducing, fully reduced isoform of HMGB1 was released from the ischemic brain in the hyperacute phase of stroke in mice and patients. Cytokines secreted in the periphery in response to brain injury induced sickness behavior, which could be abrogated by inhibition of the HMGB1-RAGE pathway or direct cytokine neutralization. Subsequently, HMGB1-release induced bone marrow egress and splenic proliferation of bone marrow-derived suppressor cells, inhibiting the adaptive immune responses in vivo and vitro. Furthermore, HMGB1-RAGE signaling resulted in functional exhaustion of mature monocytes and lymphopenia, the hallmarks of immune suppression after extensive ischemia. This study introduces the HMGB1-RAGE-mediated pathway as a key mechanism explaining the complex postischemic brain-immune interactions.

Endogenous danger signals released from necrotic cells are thought to be sensed by phagocytes leading to secretion of IL-1␣ and neutrophilic recruitment. However, the mechanisms for IL-1␣ production and IL-1␣-mediated sterile inflammation remain poorly understood. We report here that necrotic cell extracts elicited little secretion of CXCL1 and IL-6 from macrophages but robust production in mesothelial cells. The induction of CXCL1 as well as activation of NF-B and MAPKs by cytosolic extracts required the presence of IL-1␣ in the necrotic cell. Conversely, expression of IL-1R and MyD88 but not IL-1␣, RICK, TLR2, TLR4, TRIF, or inflammasome components in mesothelial cells was critical for the production of CXCL1. Furthermore, IL-1␣ was critical to induce the recruitment of neutrophils in the peritoneal cavity via CXCR2. These studies show that IL-1␣ is a key danger signal released from necrotic cells to trigger CXCL1 secretion and recruitment of neutrophils via IL-1R/MyD88 on neighboring mesothelial cells.

Background: Exosomes, secreted from cells, have immunomodulatory capacities. Results: NFB-and STAT3-mediated cytokine release is triggered by various types of ex vivo exosomes in a TLR-dependent fashion. Conclusion: Exosomes have inherent signaling capacities important for global inflammatory responses.Significance: Detailed knowledge about intercellular communication in cancer and inflammatory diseases is crucial for development of new therapeutic approaches.