Heterotrimeric Gi/Go proteins modulate endothelial TLR signaling independent of the MyD88-dependent pathway

RGS2 is an innate immune checkpoint for TLR4 and Gαq-mediated IFNγ generation and lung injury

IFNγ, a type II interferon secreted by immune cells, augments tissue responses to injury following pathogenic infections leading to lethal acute lung injury (ALI). Alveolar macrophages (AM) abundantly express Toll-like receptor-4 and represent the primary cell type of the innate immune system in the lungs. A fundamental question remains whether AM generation of IFNg leads to uncontrolled innate response and perpetuated lung injury. LPS induced a sustained increase in IFNg levels and unresolvable inflammatory lung injury in the mice lacking RGS2 but not in RGS2 null chimeric mice receiving WT bone marrow or receiving the RGS2 gene in AM. Thus, indicating RGS2 serves as a gatekeeper of IFNg levels in AM and thereby lung's innate immune response. RGS2 functioned by forming a complex with TLR4 shielding Gaq from inducing IFNg generation and AM inflammatory signaling. Thus, inhibition of Gaq blocked IFNg generation and subverted AM transcriptome from being inflammatory to reparative type in RGS2 null mice, resolving lung injury.

Highlights

RGS2 levels are inversely correlated with IFNγ in ARDS patient's AM.RGS2 in alveolar macrophages regulate the inflammatory lung injury.During pathogenic insult RGS2 functioned by forming a complex with TLR4 shielding Gαq from inducing IFNγ generation and AM inflammatory signaling.

eToc Blurb

Authors demonstrate an essential role of RGS2 in macrophages in airspace to promoting anti-inflammatory function of alveolar macrophages in lung injury. The authors provided new insight into the dynamic control of innate immune response by Gαq and RGS2 axis to prevent ALI.

Helicobacter pylori Neutrophil-Activating Protein Directly Interacts with and Activates Toll-like Receptor 2 to Induce the Secretion of Interleukin-8 from Neutrophils and ATRA-Induced Differentiated HL-60 Cells

Helicobacter pylori neutrophil-activating protein (HP-NAP)-induced production of reactive oxygen species (ROS) by neutrophils and monocytes is regulated by pertussis toxin (PTX)-sensitive G proteins, whereas HP-NAP-induced cytokine secretion by monocytes is mediated by Toll-like receptor 2 (TLR2). However, it is unclear whether TLR2 participates in HP-NAP-induced cytokine secretion by neutrophils. Here, all-trans retinoic acid (ATRA)-induced differentiated HL-60 cells were first employed as a neutrophil model to investigate the molecular mechanisms underlying neutrophil responses to HP-NAP. HP-NAP-induced ROS production in ATRA-induced differentiated HL-60 cells is mediated by the PTX-sensitive heterotrimeric G protein-dependent activation of extracellular signal-regulated kinase 1/2 and p38-mitogen-activated protein kinase, which is consistent with the findings reported for human neutrophils. Next, whether TLR2 participated in HP-NAP-induced secretion of interleukin-8 (IL-8) was investigated in neutrophils and ATRA-induced differentiated HL-60 cells. In both cells, TLR2 participated in HP-NAP-induced IL-8 secretion but not HP-NAP-induced ROS production. Interestingly, PTX-sensitive G proteins also contributed to the HP-NAP-induced secretion of IL-8 from neutrophils and the differentiated HL-60 cells. Our ELISA-based binding assay further revealed the competitive binding of Pam₃CSK₄, a TLR2 agonist, and HP-NAP to TLR2, which suggests the presence of specific and direct interactions between HP-NAP and TLR2. Thus, HP-NAP directly interacts with and activates TLR2 to induce IL-8 secretion in neutrophils and ATRA-induced differentiated HL-60 cells.

TLR4 signaling and macrophage inflammatory responses are dampened by GIV/Girdin

Sensing of pathogens by Toll-like receptor 4 (TLR4) induces an inflammatory response; controlled responses confer immunity but uncontrolled responses cause harm. Here we define how a multimodular scaffold, GIV (a.k.a. Girdin), titrates such inflammatory response in macrophages. Upon challenge with either live microbes or microbe-derived lipopolysaccharides (a ligand for TLR4), macrophages with GIV mount a more tolerant (hypo-reactive) transcriptional response and suppress proinflammatory cytokines and signaling pathways (i.e., NFkB and CREB) downstream of TLR4 compared to their GIV-depleted counterparts. Myeloid-specific gene-depletion studies confirmed that the presence of GIV ameliorates dextran sodium sulfate-induced colitis and sepsis-induced death. The antiinflammatory actions of GIV are mediated via its C-terminally located TIR-like BB-loop (TILL) motif which binds the cytoplasmic TIR modules of TLR4 in a manner that precludes receptor dimerization; such dimerization is a prerequisite for proinflammatory signaling. Binding of GIV's TILL motif to TIR modules inhibits proinflammatory signaling via other TLRs, suggesting a convergent paradigm for fine-tuning macrophage inflammatory responses.

When Innate Immunity Meets Angiogenesis-The Role of Toll-Like Receptors in Endothelial Cells and Pulmonary Hypertension

Toll-like receptors serve a central role in innate immunity, but they can also modulate cell function in various non-immune cell types including endothelial cells. Endothelial cells are necessary for the organized function of the vascular system, and part of their fundamental role is also the regulation of immune function and inflammation. In this review, we summarize the current knowledge of how Toll-like receptors contribute to the immune and non-immune functions of the endothelial cells.

Endothelial Sash1 Is Required for Lung Maturation through Nitric Oxide Signaling

The sterile alpha motif (SAM) and SRC homology 3 (SH3) domain containing protein 1 (Sash1) acts as a scaffold in TLR4 signaling. We generated Sash1^-/- mice, which die in the perinatal period due to respiratory distress. Constitutive or endothelial-restricted Sash1 loss leads to a delay in maturation of alveolar epithelial cells causing reduced surfactant-associated protein synthesis. We show that Sash1 interacts with β-arrestin 1 downstream of the TLR4 pathway to activate Akt and endothelial nitric oxide synthase (eNOS) in microvascular endothelial cells. Generation of nitric oxide downstream of Sash1 in endothelial cells affects alveolar epithelial cells in a cGMP-dependent manner, inducing maturation of alveolar type 1 and 2 cells. Thus, we identify a critical cell nonautonomous function for Sash1 in embryonic development in which endothelial Sash1 regulates alveolar epithelial cell maturation and promotes pulmonary surfactant production through nitric oxide signaling. Lung immaturity is a major cause of respiratory distress and mortality in preterm infants, and these findings identify the endothelium as a potential target for therapy.

Pulmonary Endothelial Cell Apoptosis in Emphysema and Acute Lung Injury

Apoptosis plays an essential role in homeostasis and pathogenesis of a variety of human diseases. Endothelial cells are exposed to various environmental and internal stress and endothelial apoptosis is a pathophysiological consequence of these stimuli. Pulmonary endothelial cell apoptosis initiates or contributes to progression of a number of lung diseases. This chapter will focus on the current understanding of the role of pulmonary endothelial cell apoptosis in the development of emphysema and acute lung injury (ALI) and the factors controlling pulmonary endothelial life and death.

Ginsenoside metabolite compound-K regulates macrophage function through inhibition of β-arrestin2

Ginsenoside metabolite compound-K (C-K), which is an active metabolite of ginsenoside in vivo, can produce anti-inflammatory affects by activating glucocorticoid receptors (GRs) to inhibit the expression of β-arrestin2. Studies have shown that C-K can inhibit the function of immune cells including macrophage polarization and phagocytosis. However, the mechanism by which C-K regulates macrophage polarization is currently unclear. Toll-like receptors (TLRs) are the pattern recognition receptors on the membrane of immune cells, with TLR4 being especially important in polarization of macrophages. The Gαi-mediated activation of nuclear factor-κB (NF-κB) by TLR4 promotes inflammation and phagocytosis in macrophages by increasing the proportion of type I phenotypic macrophages (M1). Whether C-K inhibits the signal transduction of TLR4-Gαi-NF-κB and how that effects macrophage polarization regulation in murine models of RA is not reported. The coupling of G proteins with receptors is regulated by β-arrestin2, but it has been unclear whether C-K modulates the TLR4 interaction with G proteins by inhibiting the expression of β-arrestin2. To explore these questions, the collagen-induced arthritis (CIA) mouse model was employed, and mice were treated with C-K (112 mg/kg/day). The results depict that C-K treatment inhibits macrophage phagocytosis and reduces the proportion of M1. C-K decreases the overexpressed β-arrestin2, Gαi, TLR4 and NF-κB in macrophages of CIA mice, while increasing the expression of Gαs. Furthermore, C-K promotes TLR4-Gαs coupling and inhibits TLR4-Gαi coupling through β-arrestin2 regulation in macrophages, leading to a decrease in the proportion of M1 to M2 macrophages and improved outcomes in CIA mice.

RGS10 Regulates the Expression of Cyclooxygenase-2 and Tumor Necrosis Factor Alpha through a G Protein-Independent Mechanism

The small regulator of G protein signaling protein RGS10 is a key regulator of neuroinflammation and ovarian cancer cell survival; however, the mechanism for RGS10 function in these cells is unknown and has not been linked to specific G protein pathways. RGS10 is highly enriched in microglia, and loss of RGS10 expression in microglia amplifies production of the inflammatory cytokine tumor necrosis factor α (TNFα) and enhances microglia-induced neurotoxicity. RGS10 also regulates cell survival and chemoresistance of ovarian cancer cells. Cyclooxygenase-2 (COX-2)-mediated production of prostaglandins such as prostaglandin E₂ (PGE₂) is a key factor in both neuroinflammation and cancer chemoresistance, suggesting it may be involved in RGS10 function in both cell types, but a connection between RGS10 and COX-2 has not been reported. To address these questions, we completed a mechanistic study to characterize RGS10 regulation of TNFα and COX-2 and to determine if these effects are mediated through a G protein-dependent mechanism. Our data show for the first time that loss of RGS10 expression significantly elevates stimulated COX-2 expression and PGE₂ production in microglia. Furthermore, the elevated inflammatory signaling resulting from RGS10 loss was not affected by Gα_i inhibition, and a RGS10 mutant that is unable to bind activated G proteins was as effective as wild type in inhibiting TNFα expression. Similarly, suppression of RGS10 in ovarian cancer cells enhanced TNFα and COX-2 expression, and this effect did not require G_i activity. Together, our data strongly indicate that RGS10 inhibits COX-2 expression by a G protein-independent mechanism to regulate inflammatory signaling in microglia and ovarian cancer cells.

Poly(I:C) induces controlled release of IL-36γ from keratinocytes in the absence of cell death

The epithelium is part of an integrated immune system where cytokines, toll-like receptors and their ligands, and extracellular vesicles play a crucial role in initiating an innate immune response. IL-36γ is a pro-inflammatory member of the IL-1 family that is mainly expressed by epithelial cells, but regulation of its expression and release are only beginning to be understood. Previous studies reported that IL-36γ is abundant in recurrent respiratory papillomatosis, a rare but devastating disease caused by human papillomaviruses (HPV) types 6 and 11, in which papillomas recurrently grow in and block the airway. Despite the overexpression of IL-36γ, papilloma tissues show no evidence of inflammation, possibly due to suppression of its release by HPVs. We have used primary human foreskin keratinocytes as a model to study IL-36γ regulation in normal epithelial cells. Low doses of poly(I:C) mediate expression and release of IL-36γ without inducing the cell death reported by those using high doses. PKR, an enzyme required for inflammasome activation, does not contribute to controlled release of IL36γ. The keratinocytes secrete IL-36γ in two forms, soluble and in extracellular vesicles. We conclude that there are two separately regulated pathways for the controlled secretion of IL-36γ from keratinocytes, which could contribute to the modulation of both local and systemic immune responses to viruses and other pathogens.

Toll-like Receptors in the Vascular System: Sensing the Dangers Within

Toll-like receptors (TLRs) are components of the innate immune system that respond to exogenous infectious ligands (pathogen-associated molecular patterns, PAMPs) and endogenous molecules that are released during host tissue injury/death (damage-associated molecular patterns, DAMPs). Interaction of TLRs with their ligands leads to activation of downstream signaling pathways that induce an immune response by producing inflammatory cytokines, type I interferons (IFN), and other inflammatory mediators. TLR activation affects vascular function and remodeling, and these molecular events prime antigen-specific adaptive immune responses. Despite the presence of TLRs in vascular cells, the exact mechanisms whereby TLR signaling affects the function of vascular tissues are largely unknown. Cardiovascular diseases are considered chronic inflammatory conditions, and accumulating data show that TLRs and the innate immune system play a determinant role in the initiation and development of cardiovascular diseases. This evidence unfolds a possibility that targeting TLRs and the innate immune system may be a novel therapeutic goal for these conditions. TLR inhibitors and agonists are already in clinical trials for inflammatory conditions such as asthma, cancer, and autoimmune diseases, but their study in the context of cardiovascular diseases is in its infancy. In this article, we review the current knowledge of TLR signaling in the cardiovascular system with an emphasis on atherosclerosis, hypertension, and cerebrovascular injury. Furthermore, we address the therapeutic potential of TLR as pharmacological targets in cardiovascular disease and consider intriguing research questions for future study.

Modulation of endothelial function by Toll like receptors

Endothelial cells (EC) are able to actively control vascular permeability, coagulation, blood pressure and angiogenesis. Most recently, a role for endothelial cells in the immune response has been described. Therefore, the endothelium has a dual role controlling homeostasis but also being the first line for host defence and tissue damage repair thanks to its ability to mount an inflammatory response. Endothelial cells have been shown to express pattern-recognition receptors (PRR) including Toll-like receptors (TLR) that are activated in response to stimuli within the bloodstream including pathogens and damage signals. TLRs are strategic mediators of the immune response in endothelial cells but they also regulate the angiogenic process critical for tissue repair. Nevertheless, endothelial activation and angiogenesis can contribute to some pathologies. Thus, inappropriate endothelial activation, also known as endothelial dysfunction, through TLRs contributes to tissue damage during autoimmune and inflammatory diseases such as atherosclerosis, hypertension, ischemia and diabetes associated cardiovascular diseases. Also TLR induced angiogenesis is required for the growth of some tumors, atherosclerosis and rheumatoid arthritis, among others. In this review we discuss the importance of various TLRs in modulating the activation of endothelial cells and their importance in immunity to infection and vascular disease as well as their potential as therapeutic targets.

Vascular endothelial cell Toll-like receptor pathways in sepsis

The endothelium forms a vast network that dynamically regulates vascular barrier function, coagulation pathways and vasomotor tone. Microvascular endothelial cells are uniquely situated to play key roles during infection and injury, owing to their widespread distribution throughout the body and their constant interaction with circulating blood. While not viewed as classical immune cells, endothelial cells express innate immune receptors, including the Toll-like receptors (TLRs), which activate intracellular inflammatory pathways mediated through NF-κB and the MAP kinases. TLR agonists, including LPS and bacterial lipopeptides, directly upregulate microvascular endothelial cell expression of inflammatory mediators. Intriguingly, TLR activation also modulates microvascular endothelial cell permeability and the expression of coagulation pathway intermediaries. Microvascular thrombi have been hypothesized to trap microorganisms thereby limiting the spread of infection. However, dysregulated activation of endothelial inflammatory pathways is also believed to lead to coagulopathy and increased vascular permeability, which together promote sepsis-induced organ failure. This article reviews vascular endothelial cell innate immune pathways mediated through the TLRs as they pertain to sepsis, highlighting links between TLRs and coagulation and permeability pathways, and their role in healthy and pathologic responses to infection and sepsis.

Gαi1 and Gαi3 regulate macrophage polarization by forming a complex containing CD14 and Gab1

Heterotrimeric G proteins have been implicated in Toll-like receptor 4 (TLR4) signaling in macrophages and endothelial cells. However, whether guanine nucleotide-binding protein G(i) subunit alpha-1 and alpha-3 (Gαi1/3) are required for LPS responses remains unclear, and if so, the underlying mechanisms need to be studied. In this study, we demonstrated that, in response to LPS, Gαi1/3 form complexes containing the pattern recognition receptor (PRR) CD14 and growth factor receptor binding 2 (Grb2)-associated binding protein (Gab1), which are required for activation of PI3K-Akt signaling. Gαi1/3 deficiency decreased LPS-induced TLR4 endocytosis, which was associated with decreased phosphorylation of IFN regulatory factor 3 (IRF3). Gαi1/3 knockdown in bone marrow-derived macrophage cells (Gαi1/3 KD BMDMs) exhibited an M2-like phenotype with significantly suppressed production of TNF-α, IL-6, IL-12, and NO in response to LPS. The altered polarization coincided with decreased Akt activation. Further, Gαi1/3 deficiency caused LPS tolerance in mice. In vitro studies revealed that, in LPS-tolerant macrophages, Gαi1/3 were down-regulated partially by the proteasome pathway. Collectively, the present findings demonstrated that Gαi1/3 can interact with CD14/Gab1, which modulates macrophage polarization in vitro and in vivo.

H5N1 Virus Hemagglutinin Inhibition of cAMP-Dependent CFTR via TLR4-Mediated Janus Tyrosine Kinase 3 Activation Exacerbates Lung Inflammation

The host tolerance mechanisms to avian influenza virus (H5N1) infection that limit tissue injury remain unknown. Emerging evidence indicates that cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-dependent Cl⁻ channel, modulates airway inflammation. Janus tyrosine kinase (JAK) 3, a JAK family member that plays a central role in inflammatory responses, prominently contributes to the dysregulated innate immune response upon H5N1 attachment; therefore, this study aims to elucidate whether JAK3 activation induced by H5N1 hemagglutinin (HA) inhibits cAMP-dependent CFTR channels. We performed short-circuit current, immunohistochemistry and molecular analyses of the airway epithelium in Jak3^+/+ and Jak3^+/− mice. We demonstrate that H5N1 HA attachment inhibits cAMP-dependent CFTR Cl⁻ channels via JAK3-mediated adenylyl cyclase (AC) suppression, which reduces cAMP production. This inhibition leads to increased nuclear factor-kappa B (NF-κB) signaling and inflammatory responses. H5N1 HA is detected by TLR4 expressed on respiratory epithelial cells, facilitating JAK3 activation. This activation induces the interaction between TLR4 and Gαi protein, which blocks ACs. Our findings provide novel insight into the pathogenesis of acute lung injury via the inhibition of cAMP-dependent CFTR channels, indicating that the administration of cAMP-elevating agents and targeting JAK3 may activate host tolerance to infection for the management of influenza virus–induced fatal pneumonia.

Subendothelial resistin enhances monocyte transmigration in a co-culture of human endothelial and smooth muscle cells by mechanisms involving fractalkine, MCP-1 and activation of TLR4 and Gi/o proteins signaling

The cytokine resistin and the chemokine fractalkine (FKN) were found at increased levels in human atherosclerotic plaque, in the subendothelium, but their role in this location still needs to be characterized. Recently, high local resistin in the arterial vessel wall was shown to contribute to an enhanced accumulation of macrophages by mechanisms that need to be clarified. Our recent data showed that resistin activated smooth muscle cells (SMC) by up-regulating FKN and MCP-1 expression and monocyte chemotaxis by activating toll-like receptor 4 (TLR4) and Gi/o proteins. Since in the vessel wall both endothelial cells (EC) and SMC respond to cytokines and promote atherosclerosis, we questioned whether subendothelial resistin (sR) has a role in vascular cells cross-talk leading to enhanced monocyte transmigration and we investigated the mechanisms involved. To this purpose we used an in vitro system of co-cultured SMC and EC activated by sR and we analyzed monocyte transmigration. Our results indicated that: (1) sR enhanced monocyte transmigration in EC/SMC system compared to EC cultured alone; (2) sR activated TLR4 and Gi/o signaling in EC/SMC system and induced the secretion of more FKN and MCP-1 compared to EC cultured alone and used both chemokines to specifically recruit monocytes by CX3CR1 and CCR2 receptors. Moreover, FKN produced by resistin in EC/SMC system, by acting on CX3CR1 on EC/SMC specifically contributes to MCP-1 secretion in the system and to the enhanced monocyte transmigration. Our study indicates new possible targets for therapy to reduce resistin-dependent enhanced macrophage infiltration in the atherosclerotic arterial wall.

Toll-like receptors and damage-associated molecular patterns: novel links between inflammation and hypertension

Low-grade systemic inflammation is a common manifestation of hypertension; however, the exact mechanisms that initiate this pathophysiological response, thereby contributing to further increases in blood pressure, are not well understood. Aberrant vascular inflammation and reactivity via activation of the innate immune system may be the first step in the pathogenesis of hypertension. One of the functions of the innate immune system is to recognize and respond to danger. Danger signals can arise from not only pathogenic stimuli but also endogenous molecules released following cell injury and/or death [damage-associated molecular patterns (DAMPs)]. In the short-term, activation of the innate immune system is beneficial in the vasculature by providing cytoprotective mechanisms and facilitating tissue repair following injury or infection. However, sustained or excessive immune system activation, such as in autoimmune diseases, may be deleterious and can lead to maladaptive, irreversible changes to vascular structure and function. An initial source of DAMPs that enter the circulation to activate the innate immune system could arise from modest elevations in peripheral vascular resistance. These stimuli could subsequently lead to ischemic- or pressure-induced events aggravating further cell injury and/or death, providing more DAMPs for innate immune system activation. This review will address and critically evaluate the current literature on the role of the innate immune system in hypertension pathogenesis. The role of Toll-like receptor activation on somatic cells of the vasculature in response to the release of DAMPs and the consequences of this activation on inflammation, vasoreactivity, and vascular remodeling will be specifically discussed.

Toll-like receptor 2-blocking antibodies promote angiogenesis and induce ERK1/2 and AKT signaling via CXCR4 in endothelial cells

OBJECTIVE

Toll-like receptor 2 (TLR2) inhibition by function blocking antibodies (ABs) is associated with enhanced preservation of endothelial cell function during vascular disease. In the present study, we investigated the capacity of TLR2-blocking ABs to modulate the angiogenic response of endothelial cells in vitro and in vivo.

APPROACH AND RESULTS

Incubation of endothelial cells with mono- or polyclonal anti-TLR2 ABs resulted in increased tube formation, sprouting, and migration of endothelial cells compared with controls. In a mouse model of hindlimb ischemia, using TLR2-deficient or anti-TLR2 AB-treated wild-type mice resulted in increased new capillary formation and enhanced reperfusion. The effects of anti-TLR2 ABs were similar to those exerted by stromal cell-derived factor-1, and we show that anti-TLR2 ABs yet not TLR2 ligands lead to comparable activation of extracellular signal-regulated kinase1/2 and AKT but not p38 mitogen-activated protein kinase as activation of the CXCR4 canonical signal transduction pathways by stromal cell-derived factor-1. Immunoprecipitation of TLR2 revealed that anti-TLR2 ABs initiate an association of TLR2 with CXCR4 and mitogen-activated protein kinase activation. The proangiogenic properties of anti-TLR2 ABs were abolished by both G-protein inhibition and CXCR4 knockdown in endothelial cells.

CONCLUSIONS

Our results provide evidence for a proangiogenic effect of TLR2-blocking ABs on endothelial cells in vitro and in vivo. They identify a novel molecular mechanism linking TLR2 to angiogenic processes that is independent from the activation of inflammatory cascades and further support the concept of a beneficial effect of TLR2 inhibition for endothelial cell function in vascular disease.

Toll-like receptor-induced inflammatory cytokines are suppressed by gain of function or overexpression of Gα(i2) protein

Previous studies have implicated a role of Gα(i) proteins as co-regulators of Toll-like receptor (TLR) activation. These studies largely derived from examining the effect of Gα(i) protein inhibitors or genetic deletion of Gα(i) proteins. However, the effect of increased Gα(i) protein function or Gα(i) protein expression on TLR activation has not been investigated. We hypothesized that gain of function or increased expression of Gα(i) proteins suppresses TLR2- and TLR4-induced inflammatory cytokines. Novel transgenic mice with genomic "knock-in" of a regulator of G protein signaling (RGS)-insensitive Gnai2 allele (Gα(i2)(G184S/G184S) ; GS/GS) were employed. These mice express essentially normal levels of Gα(i2) protein; however, the Gα(i2) is insensitive to its negative regulator RGS thus rendering more sustained Gα(i2) protein activation following ligand/receptor binding. In subsequent studies, we generated Raw 264.7 cells that stably overexpress Gα(i2) protein (Raw Gα(i2)). Peritoneal macrophages, splenocytes, and mouse embryonic fibroblasts (MEF) were isolated from WT and GS/GS mice and were stimulated with LPS, Pam3CSK4, or Poly (I:C). We also subjected WT and GS/GS mice to endotoxic shock (LPS, 25 mg/kg i.p.) and plasma tumor necrosis factor alpha (TNF-α) and interleukin (IL)-6 production were determined. We found that in vitro LPS and Pam3CSK4-induced TNF-α, and IL-6 production are decreased in macrophages from GS/GS mice compared with WT mice (p < 0.05). In vitro, LPS and Pam3CSK4 induced IL-6 production in splenocytes, and in vivo, LPS-induced IL-6 were suppressed in GS/GS mice. Poly (I:C)-induced TNF-α, and IL-6 in vitro demonstrated no difference between GS/GS mice and WT mice. LPS-induced IL-6 production was inhibited in MEFs from GS/GS mice similarly to macrophage and splenocytes. In parallel studies, Raw Gα(i2) cells also exhibit decreased TNF-α and IL-6 production in response to LPS and Pam3CSK4. These studies support our hypothesis that Gα(i2) proteins are novel negative regulators of TLR activation.

The antitumor effect of the toll-like receptor 3 ligand polyinosinic-cytidylic acid as an adjuvant

Although polyinosinic-polycytidylic acid (poly(I:C)) has been applied in tumor immunity as a Toll-like receptor 3 (TLR3) ligand, the interaction between poly(I:C) and TLR3 is still unclear, as are the mechanisms underlying the antitumor effect of poly(I:C). Our aim was to investigate the interaction between poly(I:C) and TLR3, as well as the mechanisms underlying the antitumor effect of poly(I:C). NK92 cells were maintained in medium (untreated group), or medium containing E7(44-62) (E7 group) or E7(44-62)+poly(I:C) (poly(I:C)/E7 group), and we measured the expression of TLR3 mRNA, p-p65, and IκB-α protein. The cells were first incubated in medium alone or medium containing TLR3 monoclonal antibody, and then in medium containing poly(I:C)/E7. Finally, we measured the level of interferon-beta (INF-β) in the supernatant and determined the tumor cell-killing effect of the NK92 cells. At 1 h, the expression of TLR3 mRNA in the poly(I:C)/E7 group was markedly higher than that in the untreated and E7 groups (P < 0.05). When compared with the poly(I:C)/E7 group, the expression of IκB-α was dramatically increased in the E7 and untreated groups, and the expression of p-p65 was dramatically decreased in the E7 and untreated groups (all P < 0.05). At 24 h, INF-β content and tumor cell-killing activity in the poly(I:C)/E7 group were markedly higher than those in the untreated group (P < 0.001, <0.05, respectively). Treatment with TLR3 monoclonal antibody significantly inhibited poly(I:C)/E7-induced INF-β secretion and tumor cell-killing activity in NK92 cells (P < 0.001, <0.05, respectively). The interaction between poly(I:C) and TLR3 plays an important role in the antitumor immunity of NK92 cells. In addition, the interaction between poly(I:C) and TLR3 increases INF-β expression, which may be attributed to the activation of NFκB.

Regulation of Toll-like receptor 4-mediated immune responses through Pasteurella multocida toxin-induced G protein signalling

Background

Lipopolysaccharide (LPS)-triggered Toll-like receptor (TLR) 4-signalling belongs to the key innate defence mechanisms upon infection with Gram-negative bacteria and triggers the subsequent activation of adaptive immunity. There is an active crosstalk between TLR4-mediated and other signalling cascades to secure an effective immune response, but also to prevent excessive inflammation. Many pathogens induce signalling cascades via secreted factors that interfere with TLR signalling to modify and presumably escape the host response. In this context heterotrimeric G proteins and their coupled receptors have been recognized as major cellular targets. Toxigenic strains of Gram-negative Pasteurella multocida produce a toxin (PMT) that constitutively activates the heterotrimeric G proteins Gα_q, Gα₁₃ and Gα_i independently of G protein-coupled receptors through deamidation. PMT is known to induce signalling events involved in cell proliferation, cell survival and cytoskeleton rearrangement.

Results

Here we show that the activation of heterotrimeric G proteins through PMT suppresses LPS-stimulated IL-12p40 production and eventually impairs the T cell-activating ability of LPS-treated monocytes. This inhibition of TLR4-induced IL-12p40 expression is mediated by Gα_i-triggered signalling as well as by Gβγ-dependent activation of PI3kinase and JNK.

Taken together we propose the following model: LPS stimulates TLR4-mediated activation of the NFĸB-pathway and thereby the production of TNF-α, IL-6 and IL-12p40. PMT inhibits the production of IL-12p40 by Gα_i-mediated inhibition of adenylate cyclase and cAMP accumulation and by Gβγ-mediated activation of PI3kinase and JNK activation.

Conclusions

On the basis of the experiments with PMT this study gives an example of a pathogen-induced interaction between G protein-mediated and TLR4-triggered signalling and illustrates how a bacterial toxin is able to interfere with the host’s immune response.