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Intervertebral disc degeneration (IDD) is a leading contributor to low back pain. The intervertebral disc (IVD) is composed of three tissue types: the central gelatinous nucleus pulposus (NP) tissue, the surrounding annulus fibrosus (AF) tissue, and the inferior and superior cartilage endplates. The IVD microenvironment is hypoxic, acidic, hyperosmotic, and low in nutrients because it is mostly avascular. The cellular processes that underlie IDD initiation and progression are still poorly understood. Specifically, a lack of understanding regarding NP cell metabolism and physiology hinders the development of effective therapeutics to treat IDD patients. Autophagy is a vital intracellular degradation process that removes damaged organelles, misfolded proteins, and intracellular pathogens and recycles the degraded components for cellular energy and function. NP cells have adapted to survive within their harsh tissue microenvironment using processes that are largely unknown, and we postulate autophagy is one of these undiscovered mechanisms. In this review, we describe unique features of the IVD tissue, review how physiological stressors impact autophagy in NP cells in vitro, survey the current understanding of autophagy regulation in the IVD, and assess the relationship between autophagy and IDD. Published studies confirm autophagy markers are present in IVD tissue, and IVD cells can regulate autophagy in response to cellular stressors in vitro. However, data are still lacking to determine the exact mechanisms regulating autophagy in IVD cells. More in-depth research is needed to establish whether autophagy is necessary to maintain IVD cell health and validate autophagy as a relevant therapeutic target for treating IDD.

Cell necroptosis, a form of regulated inflammatory cell death, is one of the mechanisms that controls cell release of inflammatory mediators from innate immune cells, such as polymorphonuclear neutrophils (PMNs), and critically regulates the progress of inflammation. Cell necroptosis features receptor-interacting protein (RIPK) 1 activation and necroptosome formation. This leads to loss of plasma membrane integrity, the release of cell contents into the extracellular space, and subsequent increased inflammation. Here, we report an intra-PMN mechanism of negative regulation of necroptosis mediated through TBK1/IKKe. Using an in vivo mouse model of intratracheal injection (i.t.) of LPS and in vitro LPS stimulation of mouse PMN, we found that LPS-TLR4 signaling in PMNs activates and phosphorylates TBK1 and IKKe, which in turn suppress LPS-induced formation of the RIPK1-RIPK3-MLKL (necrosome) complex. TBK1 dysfunction by knockdown or inhibitor significantly increases the phosphorylation of RIPK1 (∼67%), RIPK3 (∼68%), and MLKL (∼50%) and promotes RIPK1-RIPK3 and RIPK3-MLKL interactions and increases PMN necroptosis (∼83%) in response to LPS, with subsequent augmented lung inflammation. These findings suggest that the LPS-TLR4-TBK1 axis serves as a negative regulator for PMN necroptosis and might be a therapeutic target for modulating PMN death and inflammation.

Hemorrhagic shock with tissue trauma (HS/T) leads to the activation of a system-wide immune-inflammatory response that involves all organs and body compartments. Recent advances in single-cell analysis permit the simultaneous assessment of transcriptomic patterns in a large number of cells making it feasible to survey the landscape of immune cell responses across numerous anatomic sites. Here, we used single-cell RNA sequencing of leukocytes from the blood, liver, and spleen to identify the major shifts in gene expression by cell type and compartment in a mouse HS/T model. At 6 h, dramatic changes in gene expression were observed across multiple-cell types and in all compartments in wild-type mice. Monocytes from circulation and liver exhibited a significant upregulation of genes associated with chemotaxis and migration and a simultaneous suppression of genes associated with interferon signaling and antigen presentation. In contrast, liver conventional DC exhibited a unique pattern compared with other myeloid cells that included a pronounced increase in major histocompatibility complex class II (MHCII) gene expression. The dominant pattern across all compartments for B and T cells was a suppression of genes associated with cell activation and signaling after HS/T. Using complement factor 3 (C3) knockout mice we unveiled a role for C3 in the suppression of monocyte Major Histocompatibility Complex class II expression and activation of gene expression associated with migration, phagocytosis and cytokine upregulation, and an unexpected role in promoting interferon-signaling in a subset of B and T cells across all three compartments after HS/T. This transcriptomic landscape study of immune cells provides new insights into the host immune response to trauma, as well as a rich resource for further investigation of trauma-induced immune responses and complement in driving interferon signaling.

Traumatic injury is a major cause of morbidity and mortality worldwide, despite significant advances in treatments. Most deaths occur either very early, through massive head trauma/CNS injury or exsanguination (despite advances in transfusion medicine), or later after injury often through multiple organ failure and secondary infection. Extracellular vesicles (EVs) are known to increase in the circulation after trauma and have been used to limited extent as diagnostic and prognostic markers. More intriguingly, EVs are now being investigated as both causes of pathologies post trauma, such as trauma-induced coagulopathy, and as potential treatments. In this review, we highlight what is currently known about the role and effects of EVs in various aspects of trauma, as well as exploring current literature from investigators who have begun to use EVs therapeutically to alter the physiology and pathology of traumatic insults. The potential effectiveness of using EVs therapeutically in trauma is supported by a large number of experimental studies, but there is still some way to go before we understand the complex effects of EVs in what is already a complex disease process.

In the original publication the bands in Fig. 1J and Fig. 2B were not visible. The correct versions of Fig. 1J and Fig. 2B are provided in this correction.

High-mobility group box-1 (HMGB1), a ubiquitous nuclear protein, acts as a late mediator of lethality when released extracellularly during sepsis. The major source of circulating HMGB1 in sepsis is hepatocytes. However, the mechanism of HMGB1 release of hepatocytes during sepsis is not very clear. We have previously shown that bacterial endotoxin [lipopolysaccharide (LPS)] sensing pathways, including Toll-like receptor (TLR)4 and caspase-11, regulate hepatocyte HMGB1 release in response to LPS. Here, we report the novel function of caspase-11 and gasdermin D (GsdmD) in LPS-induced active HMGB1 released from hepatocytes. HMGB1 release during endotoxemia was caspase-11/GsdmD dependent via an active way in vivo and in vitro. Caspase-11/GsdmD was responsible for HMGB1 translocation from nucleus to the cytoplasm via calcium changing-induced phosphorylation of calcium-calmodulin kinase kinase (camkk)β during endotoxemia. Cleaved GsdmD accumulated on the endoplasmic reticulum, suggesting this may lead to calcium leak and intracellular calcium increase. Furthermore, we investigated that exosome was an important pathway for HMGB1 release from hepatocytes; this process was dependent on TLR4, independent of caspase-11 and GsdmD in vivo and in vitro. These findings provide a novel mechanism that TLR4 signaling results in an increase in caspase-11 expression, as well as increased exosome release, while caspase-11/GsdmD activation/cleavage leads to accumulation of HMGB1 in the cytoplasm through a process associated with the release of calcium from the endoplasmic reticulum and camkkβ activation.

Background Acute lung injury (ALI) is a major component of multiple organ dysfunction syndrome (MODS) following pulmonary and systemic infection. Alveolar macrophages (AMϕ) are at the center of ALI pathogenesis. Emerging evidence has shown that cell-cell interactions in the lungs play an important regulatory role in the development of acute lung inflammation. However, the underneath mechanisms remain poorly addressed. In this study, we explore a novel function of lung epithelial cells (LEPCs) in regulating the release of exosomes from AMϕ following LPS stimulation. Methods For the in vivo experiments, C57BL/6 wildtype (WT) mice were treated with lipopolysaccharide (LPS) (2 mg/kg B.W.) in 0.2 ml of saline via intratracheal aerosol administration. Bronchoalveolar lavage fluid was collected at 0–24 h after LPS treatment, and exosomes derived from AMϕ were measured. For the in vitro studies, LEPCs and bone marrow-derived Mϕ (BMDM) were isolated from WT or TLR4−/− mice and were then cocultured in the Transwell™ system. After coculture for 0–24 h, the BMDM and supernatant were harvested for the measurement of exosomes and cytokines. Results We demonstrate that LPS induces macrophages (Mϕ) to release exosomes, which are then internalized by neighboring Mϕ to promote TNF-α expression. The secreted interleukin (IL)-25 from LEPCs downregulates Rab27a and Rab27b expression in Mϕ, resulting in suppressed exosome release and thereby attenuating exosome-induced TNF-α expression and secretion. Conclusion These findings reveal a previously unidentified crosstalk pathway between LEPCs and Mϕ that negatively regulates the inflammatory responses of Mϕ to LPS. Modulating IL-25 signaling and targeting exosome release may present a new therapeutic strategy for the treatment of ALI.

Inflammation plays a significant role in protecting hosts against pathogens. Inflammation induced by noninfectious endogenous agents can be detrimental and, if excessive, can result in organ and tissue damage. The inflammasome is a major innate immune pathway that can be activated via both exogenous pathogen-associated molecular patterns (PAMPs) and endogenous damage-associated molecular patterns (DAMPs). Inflammasome activation involves formation and oligomerization of a protein complex including a nucleotide oligomerization domain (NOD)-like receptor (NLR), an adaptor protein and pro-caspase-1. This then allows cleavage and activation of caspase-1, followed by downstream cleavage and release of proinflammatory cytokines interleukin (IL)-1β and IL-18 from innate immune cells. Hyperinflammation caused by unrestrained inflammasome activation is linked with multiple inflammatory diseases, including inflammatory bowel disease, Alzheimer’s disease and multiple sclerosis. So there is an understandable rush to understand mechanisms that regulate such potent inflammatory pathways. Autophagy has now been identified as a main regulator of inflammasomes. Autophagy is a vital intracellular process involved in cellular homeostasis, recycling and removal of damaged organelles (eg, mitochondria) and intracellular pathogens. Autophagy is regulated by proteins that are important in endosomal/phagosomal pathways, as well as by specific autophagy proteins coded for by autophagy-related genes. Cytosolic components are surrounded and contained by a double-membraned vesicle, which then fuses with lysosomes to enable degradation of the contents. Autophagic removal of intracellular DAMPs, inflammasome components or cytokines can reduce inflammasome activation. Similarly, inflammasomes can regulate the autophagic process, allowing for a two-way mutual regulation of inflammation that may hold the key for treatment of multiple diseases.

Sterile liver inflammation, such as liver ischemia-reperfusion, hemorrhagic shock after trauma, and drug-induced liver injury, is initiated and regulated by endogenous mediators including DNA and reactive oxygen species. Here, we identify a mechanism for redox-mediated regulation of absent in melanoma 2 (AIM2) inflammasome activation in hepatocytes after redox stress in mice, which occurs through interaction with cytosolic high mobility group box 1 (HMGB1). We show that in liver during hemorrhagic shock in mice and in hepatocytes after hypoxia with reoxygenation, cytosolic HMGB1 associates with AIM2 and is required for activation of caspase-1 in response to cytosolic DNA. Activation of caspase-1 through AIM2 leads to subsequent hepatoprotective responses such as autophagy. HMGB1 binds to AIM2 at a non-DNA-binding site on the hematopoietic interferon-inducible nuclear antigen domain of AIM2 to facilitate inflammasome and caspase-1 activation in hepatocytes. Furthermore, binding of HMGB1 to AIM2 is stronger with fully reduced all-thiol HMGB1 than with partially oxidized disulfide-HMGB1, and binding strength corresponds to caspase-1 activation. These data suggest that HMGB1 redox status regulates AIM2 inflammasome activation. Conclusion These findings suggest a novel and important mechanism for regulation of AIM2 inflammasome activation in hepatocytes during redox stress and may suggest broader implications for how this and other inflammasomes are activated and how their activation is regulated during cell stress, as well as the mechanisms of inflammasome regulation in nonimmune cell types. (Hepatology 2017;65:253-268).

Gasdermin D (GsdmD) was recently identified as the executioner of pyroptotic inflammatory cell death, and is a substrate for caspases-1 and 11. GsdmD is detrimental in lethal endotoxemia but protective in bacterial sepsis. However, little is known about its role during noninfectious/sterile injuries. In this study, we examined the contribution of GsdmD using WT and GsdmD−/− mice in two models of noninfectious liver injury: hemorrhagic shock with resuscitation (HS/R) and acetaminophen (APAP) overdose. GsdmD−/− mice had significantly increased liver damage at 6 h after HS/R or APAP vs WT, shown by significantly elevated ALT level and extended areas of cell death in liver. Caspase-8, a mediator of multiple cell death pathways, was highly elevated in GsdmD−/− mice after injury. Significantly increased cleavage of caspase-8 and subsequent high levels of apoptosis were found in livers of GsdmD−/− mice after HS/R, a relatively mild ROS-induced liver injury. However, during more severe APAP-mediated ROS-induced liver injury, caspase-8 cleavage in GsdmD−/− liver was inhibited compared with WT, resulting in accumulation of pro-caspase-8 and increased levels of necroptosis. Our findings indicate a novel hepatoprotective role for GsdmD in noninfectious inflammation models via regulation of caspase-8 expression and downstream cell death pathways. The effects of GsdmD protection are likely injury specific and may also depend on injury severity and levels of ROS produced. These data suggest modulation of GsdmD/caspase-8 may be a novel therapeutic option in ROS-mediated liver injury.

Discussion of the potential contradictory effects of vitamin D receptor signaling in a mouse model of autoimmune hepatitis.

High mobility group box 1 (HMGB1) is a multifunctional nuclear protein, probably known best as a prototypical alarmin or damage-associated molecular pattern (DAMP) molecule when released from cells. However, HMGB1 has multiple functions that depend on its location in the nucleus, in the cytosol, or extracellularly after either active release from cells, or passive release upon lytic cell death. Movement of HMGB1 between cellular compartments is a dynamic process induced by a variety of cell stresses and disease processes, including sepsis, trauma, and hemorrhagic shock. Location of HMGB1 is intricately linked with its function and is regulated by a series of posttranslational modifications. HMGB1 function is also regulated by the redox status of critical cysteine residues within the protein, and is cell-type dependent. This review highlights some of the mechanisms that contribute to location and functions of HMGB1, and focuses on some recent insights on important intracellular effects of HMGB1 during sepsis and trauma.

Several heat shock proteins (HSPs) prime immune responses, which are, in part, a result of activation of APCs. APCs respond to these immunogenic HSPs by upregulating costimulatory molecules and secreting cytokines, including IL-1β. These HSP-mediated responses are central mediators in pathological conditions ranging from cancer, sterile inflammation associated with trauma, and rheumatoid arthritis. We tested in this study the requirement of inflammasomes in the release of IL-1β by one immunogenic HSP, gp96. Our results show that murine APCs activate NLRP3 inflammasomes in response to gp96 by K+ efflux. This is shown to initiate inflammatory conditions in vivo in the absence of additional known inflammasome activators or infection. These results document a novel mechanism by which proteins of endogenous origin, the HSPs, can modulate an inflammatory response following their release from aberrant cells.

Severe hepatic inflammation is a common cause of acute liver injury following systemic infection with Ehrlichia, obligate Gram-negative intracellular bacteria that lack lipopolysaccharide (LPS). We have previously shown that type I IFN (IFN-I) and inflammasome activation are key host-pathogenic mediators that promote excessive inflammation and liver damage following fatal Ehrlichia infection. However, the underlying signals and mechanisms that regulate protective immunity and immunopathology during Ehrlichia infection are not well understood. To address this issue, we compared susceptibility to lethal Ixodes ovatus Ehrlichia (IOE) infection between wild type (WT) and MyD88-deficient (MyD88-/-) mice. We show here that MyD88-/- mice exhibited decreased inflammasome activation, attenuated liver injury, and were more resistant to lethal infection than WT mice, despite suppressed protective immunity and increased bacterial burden in the liver. MyD88-dependent inflammasome activation was also dependent on activation of the metabolic checkpoint kinase mammalian target of rapamycin complex 1 (mTORC1), inhibition of autophagic flux, and defective mitophagy in macrophages. Blocking mTORC1 signaling in infected WT mice and primary macrophages enhanced bacterial replication and attenuated inflammasome activation, suggesting autophagy promotes bacterial replication while inhibiting inflammasome activation. Finally, our data suggest TLR9 and IFN-I are upstream signaling mechanisms triggering MyD88-mediated mTORC1 and inflammasome activation in macrophages following Ehrlichia infection. This study reveals that Ehrlichia-induced liver injury and toxic shock are mediated by MyD88-dependent inflammasome activation and autophagy inhibition., Author summary Human monocytic ehrlichiosis (HME) is the most prevalent emerging infectious disease in the United States. Ehrlichia chaffeensis, etiologic agent of HME, is a Gram negative obligate intracellular bacterium transmitted by infected tick bites and can infect different cell type. Although Ehrlichia lack lipopolysaccharide (LPS), they induce potentially life threatening HME that mimic sepsis or toxic shock associated with multi-organ failure. The clinical diagnosis of HME is difficult, and definitive diagnosis is most often retrospective. Late antibiotic treatment is frequently ineffective in preventing disease progression to fatal multi-organ failure. Liver failure is one of the most serious complications and the most frequent cause of death in patients with HME, however we only have a limited understanding of how this liver failure is caused during fatal Ehrlichia infection. The objective of this study is to determine how LPS-negative Ehrlichia activates inflammatory responses in macrophages during Ehrlichia infection to promote liver damage. We show here that MyD88-signaling causes detrimental derangement of the immune system and subsequent liver damage by regulating two key innate immune events in macrophages: autophagy and inflammasome activation. Targeting host-pathogenic pathways in ehrlichiosis can be incorporated into future design of novel therapeutic approaches for HME.

Various cell populations expressing NK1.1 contribute to innate host defense and systemic inflammatory responses, but their role in hemorrhagic shock and trauma remains uncertain. NK1.1+ cells were depleted by i.p. administration of anti-NK1.1 (or isotype control) on two consecutive days, followed by hemorrhagic shock with resuscitation and peripheral tissue trauma (HS/T). The plasma levels of IL-6, MCP-1, alanine transaminase (ALT), and aspartate aminotransferase (AST) were measured at 6 and 24 h. Histology in liver and gut were examined at 6 and 24 h. The number of NK cells, NKT cells, neutrophils, and macrophages in liver, as well as intracellular staining for TNF-α, IFN-γ, and MCP-1 in liver cell populations were determined by flow cytometry. Control mice subjected to HS/T exhibited end organ damage manifested by marked increases in circulating ALT, AST, and MCP-1 levels, as well as histologic evidence of hepatic necrosis and gut injury. Although NK1.1+ cell–depleted mice exhibited a similar degree of organ damage as nondepleted animals at 6 h, NK1.1+ cell depletion resulted in marked suppression of both liver and gut injury by 24 h after HS/T. These findings indicate that NK1.1+ cells contribute to the persistence of inflammation leading to end organ damage in the liver and gut.

Hemorrhagic shock (HS) renders patients susceptible to development of systemic inflammatory response syndrome (SIRS) and multiple organ dysfunction syndrome (MODS) through mechanisms that are, as yet, unclear. Cell necroptosis, a form of regulated inflammatory cell death, is one of the mechanisms that controls cell release of inflammatory mediators from innate immune cells, such as polymorphonuclear neutrophils (PMNs), and critically regulates the progress of inflammation. In this study, we investigated the mechanisms of alveolar macrophage (AMϕ) effects on PMN necroptosis following HS. With the use of in vivo and ex vivo HS models, we reveal a novel function of shock-activated AMϕ in promoting PMN necroptosis. We demonstrate that exosomes released from HS-activated AMϕ induce mainly NADPH oxidase-derived reactive oxygen species (ROS) production inside PMNs and subsequent promotion of necroptosis. These findings explore a previously unidentified pathway of AMϕ–PMN cross-talk, which causes enhanced PMN necroptosis and subsequent exaggerated post-HS lung inflammation. The targeting of this PMN death pathway may serve as a new therapeutic strategy for treatment of post-HS SIRS.

In elderly patients, bacterial infection often causes severe complications and sepsis. Compared to younger patients, older patients are more susceptible to sepsis caused by respiratory infection. Macrophage (Mϕ) phagocytosis of bacteria plays a critical role in the clearance of pathogens and the initiation of immune responses. It has been suggested that Mϕ exhibit age-related functional alterations, including reduced chemotaxis, phagocytosis, antibacterial defense, and the ability to generate reactive oxygen species. However, the mechanisms behind these changes remain unclear. The present study sought to determine changes in bacterial phagocytosis in aging alveolar Mϕ (AMϕ) and the underlying mechanisms. We show that bacteria initiate cytoskeleton remodeling in AMϕ through interaction with macrophage receptor with collagenous structure (MARCO), a bacterial scavenger receptor. This remodeling, in turn, promotes enhanced cell surface expression of MARCO and bacterial phagocytosis. We further demonstrate that Rac1-GTP mediates MARCO signaling and activates actin-related protein-2/3 complex, an F-actin nucleator, thereby inducing F-actin polymerization, filopodia formation, and increased cell surface expression of MARCO, all of which are essential for the execution of bacteria phagocytosis. However, AMϕ isolated from aging mice exhibit suppressed Rac1 mRNA and protein expression, which resulted in decreases in Rac1-GTP levels and actin-related protein-2/3 activation, as well as subsequent attenuation of F-actin polymerization, filopodia formation, and cell surface expression of MARCO. As a result, bacterial phagocytosis in aging AMϕ is decreased. This study highlights a previously unidentified mechanism by which aging impairs Mϕ phagocytosis of bacteria. Targeting these pathways may improve outcomes of bacterial infection in elderly patients.

Macrophages can be activated and regulated by high-mobility group box 1 (HMGB1), a highly conserved nuclear protein. Inflammatory functions of HMGB1 are mediated by binding to cell surface receptors, including the receptor for advanced glycation end products (RAGE), Toll-like receptor (TLR)2, TLR4, and TLR9. Pyroptosis is a caspase-1-dependent programmed cell death, which features rapid plasma membrane rupture, DNA fragmentation, and release of proinflammatory intracellular contents. Pyroptosis can be triggered by various stimuli, however, the mechanism underlying pyroptosis remains unclear. In this study, we identify a novel pathway of HMGB1-induced macrophage pyroptosis. We demonstrate that HMGB1, acting through RAGE and dynamin-dependent signaling, initiates HMGB1endocytosis, which in turn induces cell pyroptosis. The endocytosis of HMGB1 triggers a cascade of molecular events, including cathepsin B release from ruptured lysosomes followed by pyroptosome formation and caspase-1 activation. We further confirm that HMGB1-induced macrophage pyroptosis also occurs in vivo during endotoxemia, suggesting a pathophysiological significance for this form of pyroptosis in the development of inflammation. These findings shed light on the regulatory role of ligand-receptor internalization in directing cell fate, which may have an important role in the progress of inflammation following infection and injury.

Monocytes are critical effector cells of the innate immune system that protect the host by migrating to inflammatory sites, differentiating to macrophages and dendritic cells, eliciting immune responses, and killing pathogenic microbes. MCP-1, also known as CCL2, plays an important role in monocyte activation and migration. The chemotactic function of MCP-1 is mediated by binding to the CCR2 receptor, a member of the G protein–coupled receptor (GPCR) family. Desensitization of GPCR chemokine receptors is an important regulator of the intensity and duration of chemokine stimulation. GPCR kinases (GRKs) induce GPCR phosphorylation, and this leads to GPCR desensitization. Regulation of subcellular localization of GRKs is considered an important early regulatory mechanism of GRK function and subsequent GPCR desensitization. Chemokines and LPS are both present during Gram-negative bacterial infection, and LPS often synergistically exaggerates leukocyte migration in response to chemokines. In this study, we investigated the role and mechanism of LPS–TLR4 signaling on the regulation of monocyte chemotaxis. We demonstrate that LPS augments MCP-1–induced monocyte migration. We also show that LPS, through p38 MAPK signaling, induces phosphorylation of GRK2 at serine 670, which, in turn, suppresses GRK2 translocation to the membrane, thereby preventing GRK2-initiated internalization and desensitization of CCR2 in response to MCP-1. This results in enhanced monocyte migration. These findings reveal a novel function for TLR4 signaling in promoting innate immune cell migration.

MDSC-derived nitric oxide supports the development of Th17 cells in ovarian cancer dependent on the induction of endogenous NOS2 and the cGMP–cGK pathway in Th17 cells., Nitric oxide (NO) is a ubiquitous mediator of inflammation and immunity, involved in the pathogenesis and control of infectious diseases, autoimmunity, and cancer. We observed that the expression of nitric oxide synthase-2 (NOS2/iNOS) positively correlates with Th17 responses in patients with ovarian cancer (OvCa). Although high concentrations of exogenous NO indiscriminately suppress the proliferation and differentiation of Th1, Th2, and Th17 cells, the physiological NO concentrations produced by patients’ myeloid-derived suppressor cells (MDSCs) support the development of RORγt(Rorc)+IL-23R+IL-17+ Th17 cells. Moreover, the development of Th17 cells from naive-, memory-, or tumor-infiltrating CD4+ T cells, driven by IL-1β/IL-6/IL-23/NO-producing MDSCs or by recombinant cytokines (IL-1β/IL-6/IL-23), is associated with the induction of endogenous NOS2 and NO production, and critically depends on NOS2 activity and the canonical cyclic guanosine monophosphate (cGMP)–cGMP-dependent protein kinase (cGK) pathway of NO signaling within CD4+ T cells. Inhibition of NOS2 or cGMP–cGK signaling abolishes the de novo induction of Th17 cells and selectively suppresses IL-17 production by established Th17 cells isolated from OvCa patients. Our data indicate that, apart from its previously recognized role as an effector mediator of Th17-associated inflammation, NO is also critically required for the induction and stability of human Th17 responses, providing new targets to manipulate Th17 responses in cancer, autoimmunity, and inflammatory diseases.

Hemorrhagic shock (HS) promotes the development of systemic inflammatory response syndrome and organ injury by activating and priming the innate immune system for an exaggerated inflammatory response through, as of yet, unclear mechanisms. IL-1β also plays an important role in the development of post-HS systemic inflammatory response syndrome and active IL-1β production is tightly controlled by the inflammasome. Pyrin, a protein of 781 aa with pyrin domain at the N-terminal, negatively regulates inflammasome activation through interaction with nucleotide-binding oligomerization domain–like receptor protein (NLRP). Expression of pyrin can be induced by LPS and cytokines, and IL-10 is a known potent inducer of pyrin expression in macrophages. In the current study, we tested the hypothesis that HS downregulates IL-10 and therefore decreases pyrin expression to promote inflammasome activation and subsequent IL-1β processing and secretion in the lungs. Our results show that LPS, while activating Nlrp3 inflammasome in the lungs, also induced pyrin expression, which in turn suppressed inflammasome activation. More importantly, LPS-mediated upregulation of IL-10 enhanced pyrin expression, which serves, particularly in later phases, as a potent negative-feedback mechanism regulating inflammasome activation. However, HS-mediated suppression of IL-10 expression in alveolar macrophages attenuated the upregulation of pyrin in alveolar macrophages and lung endothelial cells and thereby significantly enhanced inflammasome activation and IL-1β secretion in the lungs. This study demonstrates a novel mechanism by which HS suppresses negative-feedback regulation of Nlrp3 inflammasome to enhance IL-1β secretion in response to subsequent LPS challenge and so primes for inflammation.

High mobility group box 1 (HMGB1) is a DNA-binding protein that possesses cytokinelike, proinflammatory properties when released extracellularly in the C23-C45 disulfide form. HMGB1 also plays a key role as a mediator of acute and chronic inflammation in models of sterile injury. Although HMGB1 interacts with multiple pattern recognition receptors (PRRs), many of its effects in injury models occur through an interaction with toll-like receptor 4 (TLR4). HMGB1 interacts directly with the TLR4/myeloid differentiation protein 2 (MD2) complex, although the nature of this interaction remains unclear. We demonstrate that optimal HMGB1-dependent TLR4 activation in vitro requires the coreceptor CD14. TLR4 and MD2 are recruited into CD14-containing lipid rafts of RAW264.7 macrophages after stimulation with HMGB1, and TLR4 interacts closely with the lipid raft protein GM1. Furthermore, we show that HMGB1 stimulates tumor necrosis factor (TNF)-α release in WT but not in TLR4(-/-), CD14(-/-), TIR domain-containing adapter-inducing interferon-β (TRIF)(-/-) or myeloid differentiation primary response protein 88 (MyD88)(-/-) macrophages. HMGB1 induces the release of monocyte chemotactic protein 1 (MCP-1), interferon gamma-induced protein 10 (IP-10) and macrophage inflammatory protein 1α (MIP-1α) in a TLR4- and CD14-dependent manner. Thus, efficient recognition of HMGB1 by the TLR4/MD2 complex requires CD14.

We and others have previously shown that the inducible nitric-oxide synthase (iNOS) and nitric oxide (NO) are hepatoprotective in a number of circumstances, including endotoxemia. In vitro, hepatocytes are protected from tumor necrosis factor (TNF) α-induced apoptosis via cGMP-dependent and cGMP-independent mechanisms. We have shown that the cGMP-dependent protective mechanisms involve the inhibition of death-inducing signaling complex formation. We show here that LPS-induced iNOS expression leads to rapid TNF receptor shedding from the surface of hepatocytes via NO/cGMP/protein kinase G-dependent activation and surface translocation of TNFα-converting enzyme (TACE/ADAM17). The activation of TACE is associated with the up-regulation of iRhom2 as well as the interaction and phosphorylation of TACE and iRhom2, which are also NO/cGMP/protein kinase G-dependent. These findings suggest that one mechanism of iNOS/NO-mediated protection of hepatocytes involves the rapid shedding of TNF receptor 1 to limit TNFα signaling.

Caspase-11, a cytosolic endotoxin (lipopolysaccharide: LPS) receptor, mediates pyroptosis, a lytic form of cell death. Caspase-11-dependent pyroptosis mediates lethality in endotoxemia, but it is unclear how LPS is delivered into the cytosol for the activation of caspase-11. Here we discovered that hepatocyte-released high mobility group box 1 (HMGB1) was required for caspase-11-dependent pyroptosis and lethality in endotoxemia and bacterial sepsis. Mechanistically, hepatocyte-released HMGB1 bound LPS and targeted its internalization into the lysosomes of macrophages and endothelial cells via the receptor for advanced glycation end-products (RAGE). Subsequently, HMGB1 permeabilized the phospholipid bilayer in the acidic environment of lysosomes. This resulted in LPS leakage into the cytosol and caspase-11 activation. Depletion of hepatocyte HMGB1, inhibition of hepatocyte HMGB1 release, neutralizing extracellular HMGB1, or RAGE deficiency prevented caspase-11-dependent pyroptosis and death in endotoxemia and bacterial sepsis. These findings indicate that HMGB1 interacts with LPS to mediate caspase-11-dependent pyroptosis in lethal sepsis.

The liver is the main organ that clears lipopolysaccharide (LPS) and hepatocytes are a major cell-type involved in LPS uptake. LPS tolerance, or desensitization, is important in negative regulation of responses to LPS, but little is known about its mechanisms in hepatocytes. Primary isolated C57BL/6 hepatocytes, and liver in vivo, internalized fluorescent LPS, and this was dependent on Toll-like receptor 4 (TLR4) at the cell surface but not on TLR4-TIR signaling through MyD88. LPS clearance from plasma was also TLR4-dependent. Pretreatment of C57BL/6 hepatocytes with LPS prevented uptake of LPS 24 hours later and this LPS-mediated suppression was dependent on TLR4 signaling through MyD88. Many regulators of TLR4 signaling have been identified and implicated in LPS desensitization, including suppressor of cytokine signaling 1 (SOCS1). SOCS1 mRNA and protein expression increased after LPS stimulation in hepatocytes and in whole liver. LPS uptake in hepatocytes and liver was significantly reduced following infection with adenoviral vectors overexpressing SOCS1. Similarly, inhibition of SOCS1 using small interfering (si)RNA-mediated knockdown prevented LPS desensitization in hepatocytes. SOCS1 is known to interact with Toll/IL-1 receptor associated protein (TIRAP) and cause TIRAP ubiquitination and degradation, which regulates TLR signaling. We have also shown previously that TIRAP regulates LPS uptake in hepatocytes. SOCS1 coimmunoprecipitated with TIRAP in wild type hepatocyte cell lysates up to 8 hours after LPS stimulation, but not at later times. In the same samples, ubiquitinated TIRAP was detected after 4 hours and up to 8 hours after LPS stimulation, but not at later times. Conclusion: These data indicate hepatocytes are desensitized by LPS in a TLR4 signaling-dependent manner. LPS-induced SOCS1 upregulation increases degradation of TIRAP and prevents subsequent LPS uptake. The exploitation of these mechanisms of LPS desensitization in the liver may be important in future sepsis therapies. (HEPATOLOGY 2009.)

The liver is the main organ that clears circulating lipopolysaccharide (LPS), and hepatocytes are a major cell type involved in LPS uptake. Little is known about the mechanisms for LPS internalization in hepatocytes and what signaling pathways are involved. We show here that LPS uptake is initiated after formation of a multi-receptor complex within lipid rafts. We find that essential components for LPS uptake are CD14, TLR4, MD2, and the β2-integrin CD11b/CD18. Activation of p38 MAPK is also essential for the initiation of LPS uptake, and interestingly, we show that this activation is not through TLR4 signaling by MyD88 but through activation of TIRAP via CD11b/CD18. However, TLR4/MD2 remain essential components at the cell surface as part of the LPS receptor complex. We therefore suggest novel roles for TLR4/MD2, CD11b/CD18, TIRAP, and p38 MAPK in LPS uptake by hepatocytes.

Infection is a common clinical complication following tissue damage resulting from surgery and severe trauma. Studies have suggested that cell pre-activation by antecedent trauma/tissue damage profoundly impacts the response of innate immune cells to a secondary infectious stimulus. Cell necroptosis, a form of regulated inflammatory cell death, is one of the mechanisms that control cell release of inflammatory mediators from important innate immune executive cells such as macrophages (Mφ), which critically regulate the progress of inflammation. In this study, we investigated the mechanism and role of trauma/tissue damage in the regulation of LPS-induced Mφ necroptosis using a mouse model simulating long-bone fracture. We demonstrate that LPS acting through Toll-like receptor (TLR) 4 promotes Mφ necroptosis. However, necroptosis is ameliorated by high-mobility group box 1 (HMGB1) release from damaged tissue. We show that HMGB1 acting through cell surface receptor for advanced glycation end products (RAGE) upregulates caveolin-1 expression, which in turn induces caveolae-mediated TLR4 internalization and desensitization to decrease Mφ necroptosis. We further show that RAGE-MyD88 activation of Cdc42 and subsequent activation of transcription factor Sp1 serves as a mechanism underlying caveolin-1 transcriptional upregulation. These results reveal a previous unidentified protective role of damage-associated molecular pattern (DAMP) molecules in restricting inflammation in response to exogenous pathogen-associated molecular pattern molecules.

Although tissue-derived high mobility group box 1 (HMGB1) is involved in many aspects of inflammation and tissue injury after trauma, its role in trauma-induced immune suppression remains elusive. Using an established mouse model of peripheral tissue trauma, which includes soft tissue and fracture components, we report here that treatment with anti-HMGB1 monoclonal antibody ameliorated the trauma-induced attenuated T-cell responses and accumulation of CD11b+Gr-1+myeloid-derived suppressor cells in the spleens seen two days after injury. Our data suggest that HMGB1 released after tissue trauma contributes to signaling pathways that lead to attenuation of T-lymphocyte responses and enhancement of myeloid-derived suppressor cell expansion.

The response to lipopolysaccharide (LPS) in the liver is complex, requiring cell-to-cell interactions between hepatocytes and liver nonparenchymal cells (NPC), in particular, Kupffer cells. Previous studies show that cytokines produced by Kupffer cells stimulated with LPS can, in turn, activate hepatocytes. In the present study, we sought to examine whether the reverse, hepatocyte (HC)-NPC interactions, is important in cytokine production in mixed cell cocultures. LPS-stimulated production of tumor necrosis factor (TNF)-alpha and interleukin (IL)-6 from NPC was augmented in mixed HC-NPC cocultures, as compared with NPC monocultures. This HC-NPC interaction was not observed when hepatocytes were cocultured with NPC from TLR4-mutant (C3H/HeJ) mice or CD14-deficient mice. The effect was partially lost when hepatocytes from lipopolysaccharide-binding protein (LBP)-deficient mice were cocultured with wild-type mice. These data indicate that functional TLR4 and CD14 are required for NPC production of cytokines and that at least one of the critical components from hepatocytes is LBP. The augmented cytokine production by mixed HC-NPC cocultures was abrogated when the cells were separated by a filter system, indicating that close cell interactions are also required for this interaction. Thus, interaction between hepatocytes and NPC are critical for cytokine secretion by NPC.

Trauma is a major cause of systemic inflammatory response syndrome and multiple organ dysfunction syndrome. Macrophages (Mφ) direct trauma-induced inflammation, and Mφ death critically influences the progression of the inflammatory response. In the current study, we explored an important role of trauma in inducing mitochondrial DNA (mtDNA) damage in Mφ and the subsequent regulation of Mφ death. Using an animal pseudo-fracture trauma model, we demonstrated that tissue damage induced NADPH oxidase activation and increased the release of reactive oxygen species via cold-inducible RNA-binding protein (CIRP)–TLR4–MyD88 signaling. This in turn, activates endonuclease G, which serves as an executor for the fragmentation of mtDNA in Mφ. We further showed that fragmented mtDNA triggered both p62-related autophagy and necroptosis in Mφ. However, autophagy activation also suppressed Mφ necroptosis and pro-inflammatory responses. This study demonstrates a previously unidentified intracellular regulation of Mφ homeostasis in response to trauma.

Impairment of autophagy has been associated with liver injury. TLR4-stimulation by LPS upregulates autophagy in hepatocytes, although the signaling pathways involved remain elusive. The objective of this study was to determine the signaling pathway leading to LPS-stimulated autophagy in hepatocytes. Cell lysates from livers of wild type (WT; C57BL/6) mice given LPS (5 mg/kg-IP) and hepatocytes from WT, TLR4ko, and MyD88ko mice treated with LPS (100 ng/mL) up to 24 h were collected. LC3II, p62/SQSTM1, Nrf2, and beclin1 levels were determined by immunoblot, immunofluorescence, and qPCR. Autophagy-like activation was measured by GFP-LC3-puncta formation and LC3II-expression. Beclin1, Nrf2, p62, MyD88, and TIRAP were knocked-down using siRNA. LC3II-expression increased in both liver and hepatocytes after LPS and was dependent on TLR4. Beclin1 expression did not increase after LPS in hepatocytes and beclin1-knockdown did not affect LC3II levels. In hepatocytes given LPS, expression of p62 increased and p62 colocalized with LC3. p62-knockdown prevented LC3II puncta formation. LPS-induced LC3II/p62-puncta also required MyD88/TIRAP signaling and localization of both Nrf2 and NFκB transcription factors to the nucleus to upregulate p62-expression. Therefore, TLR4-activation by LPS in hepatocytes induces a p62-mediated, not beclin1-mediated, autophagy-like clearance pathway that is hepatoprotective by clearing aggregate-prone or misfolded proteins from the cytosol and preserving energy homeostasis under stress.

Caspase 1 activation can be induced by oxidative stress, which leads to the release of the proinflammatory cytokines IL1β and IL18 in myeloid cells and a potentially damaging inflammatory response. However, little is known about the role of caspase 1 in non-immune cells, such as hepatocytes, that express and activate the inflammasome but do not produce a significant amount of IL1β/IL18. Here we demonstrate that caspase 1 activation protects against cell death after redox stress induced by hypoxia/reoxygenation in hepatocytes. Mechanistically, we show that caspase 1 reduces mitochondrial respiration and reactive oxygen species by increasing mitochondrial autophagy and subsequent clearance of mitochondria in hepatocytes after hypoxia/reoxygenation. Caspase 1 increases autophagic flux through up-regulating autophagy initiator beclin 1 during redox stress and is an important cell survival factor in hepatocytes. We find that during hemorrhagic shock with resuscitation, an in vivo mouse model associated with severe hepatic redox stress, caspase 1 activation is also protective against liver injury and excessive oxidative stress through the up-regulation of beclin 1. Our findings suggest an alternative role for caspase 1 activation in promoting adaptive responses to oxidative stress and, more specifically, in limiting reactive oxygen species production and damage in cells and tissues where IL1β/IL18 are not highly expressed.

The release of hematopoietic progenitor cells (HPCs) from bone marrow (BM) is under tight homeostatic control. Under stress conditions, HPCs migrate from BM and egress into circulation to participate in immune response, wound repair, or tissue regeneration. Hemorrhagic shock with resuscitation (HS/R), resulting from severe trauma and major surgery, promotes HPC mobilization from BM, which, in turn, affects post-HS immune responses. In this study, we investigated the mechanism of HS/R regulation of HPC mobilization from BM. Using a mouse HS/R model, we demonstrate that the endogenous alarmin molecule high-mobility group box 1 mediates HS/R-induced granulocyte colony-stimulating factor secretion from macrophages (Mϕ in a RAGE [receptor for advanced glycation end products] signaling-dependent manner. Secreted granulocyte colony-stimulating factor, in turn, induces HPC egress from BM. We also show that activation of β-adrenergic receptors on Mϕ by catecholamine mediates the HS/R-induced release of high-mobility group box 1. These data indicate that HS/R, a global ischemia-reperfusion stimulus, regulates HPC mobilization through a series of interacting pathways that include neuroendocrine and innate immune systems, in which Mϕ play a central role.

Hemorrhagic shock (HS) due to major trauma and surgery predisposes the host to the development of systemic inflammatory response syndrome (SIRS), including acute lung injury (ALI), through activating and exaggerating the innate immune response. IL-1β is a crucial proinflammatory cytokine that contributes to the development of SIRS and ALI. Lung endothelial cells (EC) are one important source of IL-1β, and the production of active IL-1β is controlled by the inflammasome. In this study, we addressed the mechanism underlying HS activation of the inflammasome in lung EC. We show that high mobility group box 1 acting through TLR4, and a synergistic collaboration with TLR2 and receptor for advanced glycation end products signaling, mediates HS-induced activation of EC NAD(P)H oxidase. In turn, reactive oxygen species derived from NAD(P)H oxidase promote the association of thioredoxin-interacting protein with the nucleotide-binding oligomerization domain-like receptor protein NLRP3 and subsequently induce inflammasome activation and IL-1β secretion from the EC. We also show that neutrophil-derived reactive oxygen species play a role in enhancing EC NAD(P)H oxidase activation and therefore an amplified inflammasome activation in response to HS. The present study explores a novel mechanism underlying HS activation of EC inflammasome and thus presents a potential therapeutic target for SIRS and ALI induced after HS.

Stimulation of TLR4 by LPS results in factor B synthesis by macrophages, while factor B production in response to poly I:C is independent of TRL3 and TRIF. TLRs and complement are critical to the host response in sepsis, trauma, and ischemia/reperfusion. We hypothesize that TLR stimulation leads to synthesis and release of complement components by macrophages, an important source of extrahepatic complement. RAW264.7 macrophages or peritoneal macrophages from WT and TLR4-, TLR3-, TRIF-, or MyD88-deficient mice were cultured under standard conditions. In some experiments, cells were pretreated with inhibitors of MAPKs or a NF-κB inhibitor. Cells were stimulated with TLR ligands at known stimulatory concentrations. Intratracheal and i.p. injections were also performed in mice. RT-PCR, Western blotting, and immunocytochemistry were used for analysis. Using a RT-PCR-based panel, we demonstrate that of 18 complement components tested, factor B of the alternative pathway is the most robustly up-regulated complement component in macrophages in response to LPS. This up-regulation results in release of factor B into the media. Up-regulation of factor B by LPS is dependent on TLR4, TRIF, JNK, and NF-κB. A screen of other TLR ligands demonstrated that stimulation with poly I:C (dsRNA analog) also results in up-regulation of factor B, which is dependent on JNK and NF-κB but independent of TLR3 and TRIF. Up-regulation of factor B is also observed after intratracheal and i.p. injection of LPS or poly I:C in vivo. PRR stimulation profoundly influences production and release of factor B by macrophages. Understanding the mechanisms of PRR-mediated complement production may lead to strategies aimed at preventing tissue damage in diverse settings, including sepsis, trauma, and ischemia/reperfusion.

Background & Aims NOD-like receptors are recently described cytosolic pattern recognition receptors. NOD1 and NOD2 are members of this family that recognize bacterial cell wall components, diaminopimelic acid and muramyl dipeptide, respectively. Both NOD1 and NOD2 have been associated with many inflammatory diseases, although their role in liver inflammation and infection has not been well studied. Methods We investigated the role of NOD receptors in mouse liver by assessing expression and activation of NOD1 and NOD2 in liver and primary isolated hepatocytes from C57BL/6 mice. Results Both NOD1 and NOD2 mRNA and protein were highly expressed in hepatocytes and liver. RIP2, the main signaling partner for NODs, was also expressed. Stimulation of hepatocytes with NOD1 ligand (C12-iEDAP) induced NFκB activation, activation of MAP kinases and expression of chemokines CCL5 (RANTES) and CXCL1 (KC). C12-iEDAP also synergized with interferon (IFN)γ to increase iNOS expression and production of nitric oxide. Despite activating NFκB, NOD1 ligand did not upregulate hepatocyte production of the acute phase proteins lipopolysaccharide binding protein, serum amyloid A, or soluble CD14 in cell culture supernatants, or upregulate mRNA expression of lipopolysaccharide binding protein, serum amyloid A, C-reactive protein, or serum amyloid P. NOD2 ligand (MDP) did not activate hepatocytes when given alone, but did synergize with Toll-like receptor ligands, lipopolysaccharide (LPS), and polyI:C to activate NFκB and MAPK. Conclusions All together these data suggest an important role for hepatocyte NOD1 in attracting leukocytes to the liver during infection and for hepatic NLRs to augment innate immune responses to pathogens.

Endogenous damage-associated molecular pattern (DAMP) molecules are released from cells during traumatic injury, allowing them to interact with pattern recognition receptors such as the toll-like receptors (TLRs) on other cells and subsequently, to stimulate inflammatory signaling. TLR4, in particular, plays a key role in systemic and remote organ responses to hemorrhagic shock (HS) and peripheral tissue injury in the form of bilateral femur fracture. TLR4 chimeric mice were generated to investigate the cell lineage in which functional TLR4 is needed to initiate the injury response to trauma. Chimeric mice were generated by adoptive bone marrow (BM) transfer, whereby donor marrow was given to an irradiated host using reciprocal combinations of TLR4 wild-type (WT; C3H/HeOuJ) and TLR4 mutant (Mu; C3H/HeJ) mice. After a period of engraftment, chimeric mice were then subjected to HS or bilateral femur fracture. Control groups, including TLR4-WT mice receiving WT BM and TLR4-Mu mice receiving Mu BM, responded to injury in a similar pattern to unaltered HeOuJ and HeJ mice, and protection was afforded to those mice lacking functional TLR4. In contrast, TLR4-WT mice receiving Mu BM and TLR4-Mu mice receiving WT BM demonstrated intermediate inflammatory and cellular damage profiles. These data demonstrate that functional TLR4 is required in BM-derived cells and parenchymal cells for an optimal inflammatory response to trauma.

Sepsis remains a significant cause of morbidity and mortality in the intensive care unit (68), affecting more than 500,000 patients per year in the United States (39). The incidence is increasing despite the major advances in the development of antimicrobial agents and other supportive treatments (56). Septic patients often succumb to a systemic inflammatory response, of which multiple organ failure is a main complication. There is no definitive treatment for multiple organ failure other than supportive treatment, such as assisted ventilation or renal dialysis. Research efforts now heavily focus on the mechanisms of multiple organ failure, with the hope that treatment options can be identified through a better understanding of these mechanisms. One current theory implicates an inappropriate cytokine response as the main protagonist in the development of multiple organ failure following an inflammatory stimulus (35, 48). Cytokines are low-molecular-weight polypeptides or glycoproteins that regulate numerous cellular functions and allow both autocrine and paracrine signaling. Cytokines regulate many of the pathways involved in the host inflammatory response to sepsis. They influence cell differentiation, proliferation, and activation and modulate proinflammatory and anti-inflammatory responses to allow the host to react appropriately to pathogens. The binding of a cytokine to a corresponding receptor initiates intracellular signal cascades, ultimately leading to changes in gene expression. These signals must rapidly transduce an extracellular signal to the nucleus. The Janus-kinase/ signal transducers and activators of transcription (Jak/STAT) pathway is employed in the signaling of many cytokines. Cytokine receptors do not, in general, have intrinsic tyrosine kinase activity and therefore require association with receptor-associated kinases in order to propagate a phosphorylation cascade. Jaks fulfill this function by their ability to associate with many of the cytokine receptors that are involved in regulatory pathways during a sepsis response. Cytokines bind to their receptors and cause receptor dimerization. This enables Jaks to phosphorylate each other as well as the cytokine receptors. Cytokine receptor phosphorylation allows binding of STATs via a specific binding domain. Once bound to the receptor, STATs are also phosphorylated by Jaks. STATs then dissociate from the receptors and dimerize. Dimerization enables translocation of STAT into the nucleus and binding to DNA in order to alter transcription (Fig. 1). Four Jaks and seven STATs have been identified to date in mammalian cells. Multiple Jaks and STATs, as well as the ability of STATs to homoor hetero-dimerize, allow for cytokine-specific cellular responses (47). This minireview describes a number of important cytokines involved in regulating the inflammatory response to sepsis, the importance of the Jak/STAT pathways, and how these pathways are regulated.