The Endotoxin Delivery Protein HMGB1 Mediates Caspase-11-Dependent Lethality in Sepsis.

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.

GRK2 regulates group 2 innate lymphoid cell mobilization in sepsis.

BACKGROUND: Sepsis induces group 2 innate lymphoid cell (ILC2) expansion in the lung. However, the origin of these lung-recruited ILC2 and the mechanism of ILC2 expansion are unclear. This study aims to determine the origin of lung-recruited ILC2 and its underlying mechanism in sepsis. METHODS: Sepsis was induced by cecal ligation and puncture (CLP) model in wild-type, IL-33-deficient and ST2-deficient mice. The frequency, cell number and C-X-C chemokine receptor 4 (CXCR4) expression of ILC2 in bone marrow (BM), blood and lung were measured by flow cytometry. In the in vitro studies, purified ILC2 progenitor (ILC2p) were challenged with IL-33 or G protein-coupled receptor kinase 2 (GRK2) inhibitor, the CXCR4 expression and GRK2 activity were detected by confocal microscopy or flow cytometry. RESULTS: We show that IL-33 acts through its receptor, ST2, on BM ILC2p to induce GRK2 expression and subsequent downregulation of cell surface expression of CXCR4, which results in decreasing retention of ILC2p in the BM and promoting expansion of ILC2 in the lung. Importantly, we demonstrate that reduced IL-33 level in aging mice contributes to impaired ILC2 mobilization from BM and accumulation in the lung following sepsis. CONCLUSION: This study identifies a novel pathway in regulating ILC2p mobilization and expansion during sepsis and indicates BM as the main source of ILC2 in the lung following sepsis.

Maresin 1 protects the liver against ischemia/reperfusion injury via the ALXR/Akt signaling pathway.

BACKGROUND: Hepatic ischemia/reperfusion (I/R) injury can be a major complication following liver surgery contributing to post-operative liver dysfunction. Maresin 1 (MaR1), a pro-resolving lipid mediator, has been shown to suppress I/R injury. However, the mechanisms that account for the protective effects of MaR1 in I/R injury remain unknown. METHODS: WT (C57BL/6J) mice were subjected to partial hepatic warm ischemia for 60mins followed by reperfusion. Mice were treated with MaR1 (5-20 ng/mouse), Boc2 (Lipoxin A4 receptor antagonist), LY294002 (Akt inhibitor) or corresponding controls just prior to liver I/R or at the beginning of reperfusion. Blood and liver samples were collected at 6 h post-reperfusion. Serum aminotransferase, histopathologic changes, inflammatory cytokines, and oxidative stress were analyzed to evaluate liver injury. Signaling pathways were also investigated in vitro using primary mouse hepatocyte (HC) cultures to identify underlying mechanisms for MaR1 in liver I/R injury. RESULTS: MaR1 treatment significantly reduced ALT and AST levels, diminished necrotic areas, suppressed inflammatory responses, attenuated oxidative stress and decreased hepatocyte apoptosis in liver after I/R. Akt signaling was significantly increased in the MaR1-treated liver I/R group compared with controls. The protective effect of MaR1 was abrogated by pretreatment with Boc2, which together with MaR1-induced Akt activation. MaR1-mediated liver protection was reversed by inhibition of Akt. CONCLUSIONS: MaR1 protects the liver against hepatic I/R injury via an ALXR/Akt signaling pathway. MaR1 may represent a novel therapeutic agent to mitigate the detrimental effects of I/R-induced liver injury.

Return of Individual Research Results: A Guide for Biomedical Researchers Utilizing Human Biospecimens.

The recent movement toward returning individual research results to study subjects/participants generates ethical and legal challenges for laboratories performing research on human biospecimens. The concept of an individual's interest in knowing the results of testing on their tissue is pitted against individual and systemic risks and an established legal framework regulating the performance of laboratory testing for medical care purposes. This article discusses the rationale for returning individual research results to subjects, the potential risks associated with returning these results, and the legal framework in the United States that governs testing of identifiable human biospecimens. On the basis of these considerations, this article provides recommendations for investigators to consider when planning and executing human biospecimen research, with the objective of appropriately balancing the interests of research subjects, the need for ensuring integrity of the research process, and compliance with US laws and regulations.

Immune-Responsive Gene 1/Itaconate Activates Nuclear Factor Erythroid 2-Related Factor 2 in Hepatocytes to Protect Against Liver Ischemia-Reperfusion Injury.

BACKGROUND AND AIMS: Itaconate, a metabolite of the tricarboxylic acid cycle, plays anti-inflammatory roles in macrophages during endotoxemia. The mechanisms underlying its anti-inflammatory roles have been shown to be mediated by the modulation of oxidative stress, an important mechanism of hepatic ischemia-reperfusion (I/R) injury. However, the role of itaconate in liver I/R injury is unknown. APPROACH AND RESULTS: We found that deletion of immune-responsive gene 1 (IRG1), encoding for the enzyme producing itaconate, exacerbated liver injury and systemic inflammation. Furthermore, bone marrow adoptive transfer experiments indicated that deletion of IRG1 in both hematopoietic and nonhematopoietic compartments contributes to the protection mediated by IRG1 after I/R. Interestingly, the expression of IRG1 was up-regulated in hepatocytes after I/R and hypoxia/reoxygenation-induced oxidative stress. Modulation of the IRG1 expression levels in hepatocytes regulated hepatocyte cell death. Importantly, addition of 4-octyl itaconate significantly improved liver injury and hepatocyte cell death after I/R. Furthermore, our data indicated that nuclear factor erythroid 2-related factor 2 (Nrf2) is required for the protective effect of IRG1 on mouse and human hepatocytes against oxidative stress-induced injury. Our studies document the important role of IRG1 in the acute setting of sterile injury induced by I/R. Specifically, we provide evidence that the IRG1/itaconate pathway activates Nrf2-mediated antioxidative response in hepatocytes to protect liver from I/R injury. CONCLUSIONS: Our data expand on the importance of IRG1/itaconate in nonimmune cells and identify itaconate as a potential therapeutic strategy for this unfavorable postsurgical complication.

Platelet Extracellular Vesicles Drive Inflammasome-IL-1ß-Dependent Lung Injury in Sickle Cell Disease.

Rationale: Intraerythrocytic polymerization of Hb S promotes hemolysis and vasoocclusive events in the microvasculature of patients with sickle cell disease (SCD). Although platelet-neutrophil aggregate-dependent vasoocclusion is known to occur in the lung and contribute to acute chest syndrome, the etiological mechanisms that trigger acute chest syndrome are largely unknown.Objectives: To identify the innate immune mechanism that promotes platelet-neutrophil aggregate-dependent lung vasoocclusion and injury in SCD.Methods:In vivo imaging of the lung in transgenic humanized SCD mice and in vitro imaging of SCD patient blood flowing through a microfluidic system was performed. SCD mice were systemically challenged with nanogram quantities of LPS to trigger lung vasoocclusion.Measurements and Main Results: Platelet-inflammasome activation led to generation of IL-1ß and caspase-1-carrying platelet extracellular vesicles (EVs) that bind to neutrophils and promote platelet-neutrophil aggregation in lung arterioles of SCD mice in vivo and SCD human blood in microfluidics in vitro. The inflammasome activation, platelet EV generation, and platelet-neutrophil aggregation were enhanced by the presence of LPS at a nanogram dose in SCD but not control human blood. Inhibition of the inflammasome effector caspase-1 or IL-1ß pathway attenuated platelet EV generation, prevented platelet-neutrophil aggregation, and restored microvascular blood flow in lung arterioles of SCD mice in vivo and SCD human blood in microfluidics in vitro.Conclusions: These results are the first to identify that platelet-inflammasome-dependent shedding of IL-1ß and caspase-1-carrying platelet EVs promote lung vasoocclusion in SCD. The current findings also highlight the therapeutic potential of targeting the platelet-inflammasome-dependent innate immune pathway to prevent acute chest syndrome.

Hepatocyte high-mobility group box 1 protects against steatosis and cellular stress during high fat diet feeding.

BACKGROUND: Circulating high-mobility group box 1 (HMGB1) plays important roles in the pathogenesis of nonalcoholic fatty liver disease (NAFLD). Intracellular HMGB1 is critical for the biology of hepatocytes. However, the intracellular role of HMGB1 in hepatocellular steatosis is unknown. Therefore, we aimed to investigate the role of hepatocyte-specific HMGB1 (HC-HMGB1) in development of hepatic steatosis. METHODS: Wild type (WT) C57BL/6 and HC-HMGB1-/- mice were fed high-fat diet (HFD) or low-fat diet (LFD) for up to 16 weeks. RESULTS: As expected, HMGB1 translocated from nuclear into cytoplasm and released into circulation after HFD treatment. HC-HMGB1 deficiency significantly reduced circulating HMGB1, suggesting that hepatocyte is a major source of circulating HMGB1 during NAFLD. Unexpectedly, HC-HMGB1 deficiency promoted rapid weight gain with enhanced hepatic fat deposition compared with WT at as early as 4 weeks after HFD treatment. Furthermore, there was no difference between WT and HC-HMGB1-/- mice in glucose tolerance, energy expenditure, liver damage or systemic inflammation. Interestingly, hepatic gene expression related to free fatty acid (FFA) ß-oxidation was significantly down-regulated in HC-HMGB1-/- mice compared with WT, and endoplasmic reticulum (ER) stress markers were significantly higher in livers of HC-HMGB1-/- mice. In vitro experiments using primary mouse hepatocytes showed absence of HMGB1 increased FFA-induced intracellular lipid accumulation, accompanied by increased ER-stress, significant downregulation of FFA ß-oxidation, and reduced oxidative phosphorylation. CONCLUSIONS: Our findings suggest that hepatocyte HMGB1 protects against dysregulated lipid metabolism via maintenance of ß-oxidation and prevention of ER stress. This represents a novel mechanism for HMGB1-regulation of hepatocellular steatosis, and suggests that stabilizing HMGB1 in hepatocytes may be effective strategies for prevention and treatment of NAFLD.

Cutting Edge: The Heat Shock Protein gp96 Activates Inflammasome-Signaling Platforms in APCs.

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-1ß. 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-1ß 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.

Aging-Impaired Filamentous Actin Polymerization Signaling Reduces Alveolar Macrophage Phagocytosis of Bacteria.

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.

cGAS-mediated autophagy protects the liver from ischemia-reperfusion injury independently of STING.

Liver ischemia-reperfusion (I/R) injury occurs through induction of oxidative stress and release of damage-associated molecular patterns (DAMPs), including cytosolic DNA released from dysfunctional mitochondria or from the nucleus. Cyclic guanosine monophosphate-adenosine monophosphate (cGAMP) synthase (cGAS) is a cytosolic DNA sensor known to trigger stimulator of interferon genes (STING) and downstream type 1 interferon (IFN-I) pathways, which are pivotal innate immune system responses to pathogen. However, little is known about the role of cGAS/STING in liver I/R injury. We subjected C57BL/6 (WT), cGAS knockout (cGAS-/-), and STING-deficient (STINGgt/gt) mice to warm liver I/R injury and that found cGAS-/- mice had significantly increased liver injury compared with WT or STINGgt/gt mice, suggesting a protective effect of cGAS independent of STING. Liver I/R upregulated cGAS in vivo and also in vitro in hepatocytes subjected to anoxia/reoxygenation (A/R). We confirmed a previously published finding that hepatocytes do not express STING under normoxic conditions or after A/R. Hepatocytes and liver from cGAS-/- mice had increased cell death and reduced induction of autophagy under hypoxic conditions as well as increased apoptosis. Protection could be restored in cGAS-/- hepatocytes by overexpression of cGAS or by pretreatment of mice with autophagy inducer rapamycin. Our findings indicate a novel protective role for cGAS in the regulation of autophagy during liver I/R injury that occurs independently of STING. NEW & NOTEWORTHY Our studies are the first to document the important role of cGAS in the acute setting of sterile injury induced by I/R. Specifically, we provide evidence that cGAS protects liver from I/R injury in a STING-independent manner.

Redox-dependent regulation of hepatocyte absent in melanoma 2 inflammasome activation in sterile liver injury in mice.

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).

Inflammasome and autophagy regulation - a two-way street.

Inflammation plays a significant role in protecting hosts against pathogens. Inflammation induced by non-infectious, 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 NOD-like receptor (NLR), an adaptor protein (ASC) and procaspase-1. This then allows cleavage and activation of caspase-1, followed by downstream cleavage and release of proinflammatory cytokines, IL-1ß 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 (e.g. 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.

Toll-like Receptor 4 Signaling on Dendritic Cells Suppresses Polymorphonuclear Leukocyte CXCR2 Expression and Trafficking via Interleukin 10 During Intra-abdominal Sepsis.

BACKGROUND: Toll-like receptor 4 (TLR4) is a critical receptor involved in the sensing of gram-negative bacterial infection. However, the roles of TLR4 in sepsis are cell-type specific. Dendritic cells (DCs) are known to play a central role in microbial detection, alerting the immune system to the presence of infection and coordinating adaptive immune response. The goal of this study was to investigate the impact of DC-specific TLR4 signaling on host defense against intra-abdominal polymicrobial sepsis. METHODS: C57BL/6, global Tlr4 knockout, cell-specific knockout control, and CD11c-specific Tlr4(-/-) mice underwent cecal ligation and puncture (CLP). RESULTS: Specific deletion of TLR4 on DCs in mice improved survival and enhanced bacterial clearance. Deletion of TLR4 on DCs was associated with lower levels of circulating interleukin 10 (IL-10), higher polymorphonuclear leukocyte (PMN) accumulation in the peritoneal cavity, and higher expression of chemokine (C-X-C motif) receptor 2 (CXCR2) on PMNs after CLP. In vitro studies of DC and neutrophil cocultures confirmed that TLR4-dependent secretion of IL-10 from DCs regulated neutrophil CXCR2 expression. CONCLUSIONS: Our data shed light on a previously unrecognized role for TLR4 signaling on DCs in driving IL-10 secretion during sepsis and, through this pathway, regulates PMN recruitment via suppression of CXCR2 expression.

Type I interferon contributes to noncanonical inflammasome activation, mediates immunopathology, and impairs protective immunity during fatal infection with lipopolysaccharide-negative ehrlichiae.

Ehrlichia species are intracellular bacteria that cause fatal ehrlichiosis, mimicking toxic shock syndrome in humans and mice. Virulent ehrlichiae induce inflammasome activation leading to caspase-1 cleavage and IL-18 secretion, which contribute to development of fatal ehrlichiosis. We show that fatal infection triggers expression of inflammasome components, activates caspase-1 and caspase-11, and induces host-cell death and secretion of IL-1ß, IL-1α, and type I interferon (IFN-I). Wild-type and Casp1(-/-) mice were highly susceptible to fatal ehrlichiosis, had overwhelming infection, and developed extensive tissue injury. Nlrp3(-/-) mice effectively cleared ehrlichiae, but displayed acute mortality and developed liver injury similar to wild-type mice. By contrast, Ifnar1(-/-) mice were highly resistant to fatal disease and had lower bacterial burden, attenuated pathology, and prolonged survival. Ifnar1(-/-) mice also had improved protective immune responses mediated by IFN-γ and CD4(+) Th1 and natural killer T cells, with lower IL-10 secretion by T cells. Importantly, heightened resistance of Ifnar1(-/-) mice correlated with improved autophagosome processing, and attenuated noncanonical inflammasome activation indicated by decreased activation of caspase-11 and decreased IL-1ß, compared with other groups. Our findings demonstrate that IFN-I signaling promotes host susceptibility to fatal ehrlichiosis, because it mediates ehrlichia-induced immunopathology and supports bacterial replication, perhaps via activation of noncanonical inflammasomes, reduced autophagy, and suppression of protective CD4(+) T cells and natural killer T-cell responses against ehrlichiae.

Neutrophils counteract autophagy-mediated anti-inflammatory mechanisms in alveolar macrophage: role in posthemorrhagic shock acute lung inflammation.

Acute lung injury (ALI) is a major component of multiple organ dysfunction syndrome after hemorrhagic shock (HS) resulting from major surgery and trauma. The increased susceptibility in HS patients to the development of ALI suggests not yet fully elucidated mechanisms that enhance proinflammatory responses and/or suppress anti-inflammatory responses in the lung. Alveolar macrophages (AMϕ) are at the center of the pathogenesis of ALI after HS. We have previously reported that HS-activated polymorphonuclear neutrophils (PMNs) interact with macrophages to influence inflammation progress. In this study, we explore a novel function of PMNs regulating AMϕ anti-inflammatory mechanisms involving autophagy. Using a mouse "two-hit" model of HS/resuscitation followed by intratracheal injection of muramyl dipeptide, we demonstrate that HS initiates high mobility group box 1/TLR4 signaling, which upregulates NOD2 expression in AMϕ and sensitizes them to subsequent NOD2 ligand muramyl dipeptide to augment lung inflammation. In addition, upregulated NOD2 signaling induces autophagy in AMϕ, which negatively regulates lung inflammation through feedback suppression of NOD2-RIP2 signaling and inflammasome activation. Importantly, we further demonstrate that HS-activated PMNs that migrate in alveoli counteract the anti-inflammatory effect of autophagy in AMϕ, possibly through NAD(P)H oxidase-mediated signaling to enhance I-κB kinase γ phosphorylation, NF-κB activation, and nucleotide-binding oligomerization domain protein 3 inflammasome activation, and therefore augment post-HS lung inflammation. These findings explore a previously unidentified complexity in the mechanisms of ALI, which involves cell-cell interaction and receptor cross talk.

Lipopolysaccharide clearance, bacterial clearance, and systemic inflammatory responses are regulated by cell type-specific functions of TLR4 during sepsis.

The morbidity associated with bacterial sepsis is the result of host immune responses to pathogens, which are dependent on pathogen recognition by pattern recognition receptors, such as TLR4. TLR4 is expressed on a range of cell types, yet the mechanisms by which cell-specific functions of TLR4 lead to an integrated sepsis response are poorly understood. To address this, we generated mice in which TLR4 was specifically deleted from myeloid cells (LysMTLR4KO) or hepatocytes (HCTLR4KO) and then determined survival, bacterial counts, host inflammatory responses, and organ injury in a model of cecal ligation and puncture (CLP), with or without antibiotics. LysM-TLR4 was required for phagocytosis and efficient bacterial clearance in the absence of antibiotics. Survival, the magnitude of the systemic and local inflammatory responses, and liver damage were associated with bacterial levels. HCTLR4 was required for efficient LPS clearance from the circulation, and deletion of HCTLR4 was associated with enhanced macrophage phagocytosis, lower bacterial levels, and improved survival in CLP without antibiotics. Antibiotic administration during CLP revealed an important role for hepatocyte LPS clearance in limiting sepsis-induced inflammation and organ injury. Our work defines cell type-selective roles for TLR4 in coordinating complex immune responses to bacterial sepsis and suggests that future strategies for modulating microbial molecule recognition should account for varying roles of pattern recognition receptors in multiple cell populations.

TLR4 Signaling augments monocyte chemotaxis by regulating G protein-coupled receptor kinase 2 translocation.

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.

Hemorrhagic shock augments Nlrp3 inflammasome activation in the lung through impaired pyrin induction.

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-1ß also plays an important role in the development of post-HS systemic inflammatory response syndrome and active IL-1ß 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-1ß 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-1ß secretion in the lungs. This study demonstrates a novel mechanism by which HS suppresses negative-feedback regulation of Nlrp3 inflammasome to enhance IL-1ß secretion in response to subsequent LPS challenge and so primes for inflammation.

Anti-HMGB1 monoclonal antibody ameliorates immunosuppression after peripheral tissue trauma: attenuated T-lymphocyte response and increased splenic CD11b (+) Gr-1 (+) myeloid-derived suppressor cells require HMGB1.

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.