TLR4 and Toll-IL-1 Receptor Domain-Containing Adapter-Inducing IFN-β, but Not MyD88, Regulate Escherichia coli-Induced Dendritic Cell Maturation and Apoptosis In Vivo

Activation, Amplification, and Ablation as Dynamic Mechanisms of Dendritic Cell Maturation

T cell responses to cognate antigens crucially depend on the specific functionality of dendritic cells (DCs) activated in a process referred to as maturation. Maturation was initially described as alterations of the functional status of DCs in direct response to multiple extrinsic innate signals derived from foreign organisms. More recent studies, conducted mainly in mice, revealed an intricate network of intrinsic signals dependent on cytokines and various immunomodulatory pathways facilitating communication between individual DCs and other cells for the orchestration of specific maturation outcomes. These signals selectively amplify the initial activation of DCs mediated by innate factors and dynamically shape DC functionalities by ablating DCs with specific functions. Here, we discuss the effects of the initial activation of DCs that crucially includes the production of cytokine intermediaries to collectively achieve amplification of the maturation process and further precise sculpting of the functional landscapes among DCs. By emphasizing the interconnectedness of the intracellular and intercellular mechanisms, we reveal activation, amplification, and ablation as the mechanistically integrated components of the DC maturation process.

Life and death of tolerogenic dendritic cells

In contrast to conventional dendritic cells (cDCs) that are constantly exposed to microbial signals at anatomical barriers, cDCs in systemic lymphoid organs are sheltered from proinflammatory stimulation in the steady state but respond to inflammatory signals by gaining specific immune functions in a process referred to as maturation. Recent findings show that, during maturation, a population of systemic tolerogenic cDCs undergoes an acute tumor necrosis factor α (TNFα)-mediated cell death, resulting in the loss of tolerance-inducing capacity. This tolerogenic cDC population is restored upon return to the homeostatic baseline. We propose that such a dynamic reshaping of cDC populations becomes the foundation of a novel framework for maintaining tolerance at the steady state while being conducive to unhampered initiation of immune responses under proinflammatory conditions.

TNF-α sculpts a maturation process in vivo by pruning tolerogenic dendritic cells

It remains unclear how the pro-immunogenic maturation of conventional dendritic cells (cDCs) abrogates their tolerogenic functions. Here, we report that the loss of tolerogenic functions depends on the rapid death of BTLAhi cDC1s, which, in the steady state, are present in systemic peripheral lymphoid organs and promote tolerance that limits subsequent immune responses. A canonical inducer of maturation, lipopolysaccharide (LPS), initiates a burst of tumor necrosis factor alpha (TNF-α) production and the resultant acute death of BTLAhi cDC1s mediated by tumor necrosis factor receptor 1. The ablation of these individual tolerogenic cDCs is amplified by TNF-α produced by neighboring cells. This loss of tolerogenic cDCs is transient, accentuating the restoration of homeostatic conditions through biological turnover of cDCs in vivo. Therefore, our results reveal that the abrogation of tolerogenic functions during an acute immunogenic maturation depends on an ablation of the tolerogenic cDC population, resulting in a dynamic remodeling of the cDC functional landscape.

Systemic bacterial infections affect dendritic cell development and function

Dendritic cells (DCs) are critical in host defense against infection. DC depletion is an early event in the course of sepsis that may impair the host defense mechanisms. Here, we addressed whether DC depletion and dysfunction are pathogen-independent, mediated via pattern recognition receptors, and are due to impaired DC development upon systemic infection with the Gram-negative bacterium Escherichia coli and the Gram-positive pathogen Staphylococcus aureus. Infection with E. coli and S. aureus led to reduced numbers of splenic DC subsets and of DC progenitors in the bone marrow (BM) with this effect persisting significantly longer in mice infected with S. aureus than with E. coli. The reduction of DC subsets and their progenitors was mainly TLR-independent as was the infection-induced monopoiesis. Moreover, de novo DC development was impaired in mice infected with S. aureus, and BM cells from E. coli or S. aureus infected mice favored macrophage differentiation in vitro. As a consequence of reduced DC numbers and their reduced expression of MHC II less CD4⁺ and CD8⁺ T cells, especially Th1 and IFN-γ producing CD8⁺ T cells, could be detected in S. aureus compared to E. coli infected mice. These differences are reflected in the rapid killing of E. coli as opposed to an increase in bacterial load in S. aureus. In summary, our study supports the idea that systemic bacterial infections generally affect the number and development of DCs and thereby the T cell responses, but the magnitude is pathogen-dependent.

Innate cell microenvironments in lymph nodes shape the generation of T cell responses during type I inflammation

Microanatomical organization of innate immune cells within lymph nodes (LNs) is critical for the generation of adaptive responses. In particular, steady-state LN-resident dendritic cells (Res cDCs) are strategically localized to intercept lymph-draining antigens. Whether myeloid cell organization changes during inflammation and how that might affect the generation of immune responses are unknown. Here, we report that during type I, but not type II, inflammation after adjuvant immunization or viral infection, antigen-presenting Res cDCs undergo CCR7-dependent intranodal repositioning from the LN periphery into the T cell zone (TZ) to elicit T cell priming. Concurrently, inflammatory monocytes infiltrate the LNs via local blood vessels, enter the TZ, and cooperate with Res cDCs by providing polarizing cytokines to optimize T cell effector differentiation. Monocyte infiltration is nonuniform across LNs, generating distinct microenvironments with varied local innate cell composition. These spatial microdomains are associated with divergent early T cell effector programming, indicating that innate microenvironments within LNs play a critical role in regulating the quality and heterogeneity of T cell responses. Together, our findings reveal that dynamic modulation of innate cell microenvironments during type I inflammation leads to optimized generation of adaptive immune responses to vaccines and infections.

The Role of Toll-Like Receptors in Osteoclastogenesis

Bone homeostasis is maintained by a balance in the levels of osteoclast and osteoblast activity. Osteoclasts are bone-resorbing cells and have been shown to act as key players in various osteolytic diseases. Osteoclasts differentiate from monocyte/macrophage lineage cells in the presence of receptor activator of nuclear factor-κB ligand and macrophage colony-stimulating factor. Osteoblasts support osteoclastogenesis by producing several osteoclast differentiation factors. Toll-like receptors (TLRs) are members of the pattern recognition receptor family that are involved in recognizing pathogen-associated molecular patterns and damage-associated molecular patterns in response to pathogen infection. TLRs regulate osteoclastogenesis and bone resorption through either the myeloid differentiation primary response 88 or the Toll/interleukin-1 receptor domain-containing adapter-inducing interferon-β signaling pathways. Since osteoclasts play a central role in the progression of osteolytic diseases, extensive research focusing on TLR downstream signaling in these cells should be conducted to advance the development of effective TLR modulators. In this review, we summarize the currently available information on the role of TLRs in osteoclast differentiation and osteolytic diseases.

Signalling, sorting and scaffolding adaptors for Toll-like receptors

Toll-like receptors (TLRs) are danger-sensing receptors that typically propagate self-limiting inflammatory responses, but can unleash uncontrolled inflammation in non-homeostatic or disease settings. Activation of TLRs by pathogen- and/or host-derived stimuli triggers a range of signalling and transcriptional pathways to programme inflammatory and anti-microbial responses, including the production of a suite of inflammatory cytokines and other mediators. Multiple sorting and signalling adaptors are recruited to receptor complexes on the plasma membrane or endosomes where they act as scaffolds for downstream signalling kinases and effectors at these sites. So far, seven proximal TLR adaptors have been identified: MyD88, MAL, TRIF (also known as TICAM1), TRAM (TICAM2), SARM (SARM1), BCAP (PIK3AP1) and SCIMP. Most adaptors tether directly to TLRs through homotypic Toll/interleukin-1 receptor domain (TIR)-TIR interactions, whereas SCIMP binds to TLRs through an atypical TIR-non-TIR interaction. In this Review, we highlight the key roles for these adaptors in TLR signalling, scaffolding and receptor sorting and discuss how the adaptors thereby direct the differential outcomes of TLR-mediated responses. We further summarise TLR adaptor regulation and function, and make note of human diseases that might be associated with mutations in these adaptors.

Dendritic cell subsets in T cell programming: location dictates function

Dendritic cells (DCs) can be viewed as translators between innate and adaptive immunity. They integrate signals derived from tissue infection or damage and present processed antigen from these sites to naive T cells in secondary lymphoid organs while also providing multiple soluble and surface-bound signals that help to guide T cell differentiation. DC-mediated tailoring of the appropriate T cell programme ensures a proper cascade of immune responses that adequately targets the insult. Recent advances in our understanding of the different types of DC subsets along with the cellular organization and orchestration of DC and lymphocyte positioning in secondary lymphoid organs over time has led to a clearer understanding of how the nature of the T cell response is shaped. This Review discusses how geographical organization and ordered sequences of cellular interactions in lymph nodes and the spleen regulate immunity.

Compartmentalization of dendritic cell and T-cell interactions in the lymph node: Anatomy of T-cell fate decisions

Upon receiving cognate and co-stimulatory priming signals from antigen (Ag)-presenting dendritic cells (DCs) in secondary lymphoid tissues, naïve CD4+ T cells differentiate into distinct effector and memory populations. These alternate cell fate decisions, which ultimately control the T-cell functional attributes, are dictated by programming signals provided by Ag-bearing DCs and by other cells that are present in the microenvironment in which T-cell priming occurs. We know that DCs can be subdivided into multiple populations and that the various DC subsets exhibit differential capacities to initiate development of the different CD4+ T-helper populations. What is less well understood is why different subanatomic regions of secondary lymphoid tissues are colonized by distinct populations of Ag-presenting DCs and how the location of these DCs influences the type of T-cell response that will be generated. Here we review how chemokine receptors and their ligands, which position allergen and nematode-activated DCs within different microdomains of secondary lymphoid tissues, contribute to the establishment of IL-4 committed follicular helper T and type 2 helper cell responses.

CSF1R regulates the dendritic cell pool size in adult mice via embryo-derived tissue-resident macrophages

Regulatory mechanisms controlling the pool size of spleen dendritic cells (DC) remain incompletely understood. DCs are continuously replenished from hematopoietic stem cells, and FLT3-mediated signals cell-intrinsically regulate homeostatic expansion of spleen DCs. Here we show that combining FLT3 and CSF1R-deficiencies results in specific and complete abrogation of spleen DCs in vivo. Spatiotemporally controlled CSF1R depletion reveals a cell-extrinsic and non-hematopoietic mechanism for DC pool size regulation. Lack of CSF1R-mediated signals impedes the differentiation of spleen macrophages of embryonic origin, and the resulted macrophage depletion during development or in adult mice results in loss of DCs. Moreover, embryo-derived macrophages are important for the physiologic regeneration of DC after activation-induced depletion in situ. In summary, we show that the differentiation of DC and their regeneration relies on ontogenetically distinct spleen macrophages, thereby providing a novel regulatory principle that may also be important for the differentiation of other hematopoietic cell types.

Understanding the Cellular Origin of the Mononuclear Phagocyte System Sheds Light on the Myeloid Postulate of Immune Paralysis in Sepsis

Sepsis, in essence, is a serious clinical condition that can subsequently result in death as a consequence of a systemic inflammatory response syndrome including febrile leukopenia, hypotension, and multiple organ failures. To date, such life-threatening organ dysfunction remains one of the leading causes of death in intensive care units, with an increasing incidence rate worldwide and particularly within the rapidly growing senior population. While most of the clinical trials are aimed at dampening the overwhelming immune response to infection that spreads through the bloodstream, based on several human immunological investigations, it is now widely accepted that susceptibility to nosocomial infections and long-term sepsis mortality involves an immunosuppressive phase that is characterized by a decrease in some subsets of dendritic cells (DCs). Only recently substantial advances have been made in terms of the origin of the mononuclear phagocyte system that is now likely to allow for a better understanding of how the paralysis of DCs leads to sepsis-related death. Indeed, the unifying view of each subset of DCs has already improved our understanding of the pivotal pathways that contribute to the shift in commitment of their progenitors that originate from the bone marrow. It is quite plausible that this anomaly in sepsis may occur at the single level of DC-committed precursors, and elucidating the immunological basis for such a derangement during the ontogeny of each subset of DCs is now of particular importance for restoring an adequate cell fate decision to their vulnerable progenitors. Last but not least, it provides a direct perspective on the development of sophisticated myelopoiesis-based strategies that are currently being considered for the treatment of immunosenescence within different tissue microenvironments, such as the kidney and the spleen.

Distinct oxysterol requirements for positioning naïve and activated dendritic cells in the spleen

Correct positioning of dendritic cells (DCs) is critical for efficient pathogen encounter and antigen presentation. Epstein-Barr virus-induced gene 2 (EBI2) has been identified as a chemoattractant receptor required for naïve CD4⁺DCIR2⁺ DC positioning in response to 7α,25-hydroxycholesterol (7α,25-HC). We now provide evidence that a second EBI2 ligand, 7α,27-HC, is involved in splenic DCIR2⁺ DC positioning and homeostasis. Cyp27a1, the enzyme uniquely required for 7α,27-HC synthesis, is expressed by stromal cells in the region of naïve DC localization. After activation, DCIR2⁺ DCs move into the T cell zone. We find that EBI2 is rapidly up-regulated in DCIR2⁺ DCs under certain activation conditions, and positioning at the B-T zone interface depends on EBI2. Under conditions of type I interferon induction, EBI2 ligand levels are elevated, causing activated DCIR2⁺ DCs to disperse throughout the T zone. Last, we provide evidence that oxysterol metabolism by Batf3-dependent DCs is important for EBI2-dependent positioning of activated DCIR2⁺ DCs. This work indicates that 7α,27-HC functions as a guidance cue in vivo and reveals a multitiered role for EBI2 in DC positioning. Deficiency in this organizing system results in defective CD4⁺ T cell responses.

Effect of glucocorticoid on cytokines TLR9 and TLR7 in peripheral blood for patients with uveitis

The effect of glucocorticoid on cytokines Toll-like receptor (TLR)9 and TLR7 in peripheral blood of patients with uveitis was explored. Forty-six patients with uveitis admitted to our hospital from April 2014 to April 2015 were selected as the research observational group. Thirty-five able-bodied individuals in the same period were selected as the control group. To treat uveitis, the observational group was injected with glucocorticoid (1–2 mg/kg/day) daily, while the control group did not receive any treatment. The quantity of expression of peripheral blood cytokines TLR9 and TLR7 were detected by the methods of fluorescence quantitative PCR, enzyme-linked immunosorbent assay and western blotting. The content of peripheral blood TLR9 and TLR7 (0.21±0.01, 0.19±0.01) decreased significantly (P<0.05) in observational group after glucocorticoid treatment. Compared with data of control group (0.21±0.01, 0.19±0.01), TLR9 and TLR7 content in peripheral blood after glucocorticoid treatment on the patients with uveitis from observation group (0.19±0.01, 0.17±0.01) did not show any significant difference, for correlation between TLR9 and TLR7 in observation group before and after treatment. It was observed that the cytokine content of TLR9 was associated with TLR7 positively (r=0.653, P=0.012). In conclusion, glucocorticoid can improve uveitis by reducing the content of cytokines TLR9 and TLR7 in peripheral blood.

Both intrinsic and extrinsic apoptotic pathways are involved in Toll-like receptor 4 (TLR4)-induced cell death in monocytic THP-1 cells

Our previous study showed that TLR3 induces apoptosis via both death receptors and mitochondial in human endothelial cells. We report here that the activation of TLR4 induced dose- and time-dependent cell death in moncytic THP-1 cells. LPS treatment of THP-1 cells induced the activation of both caspase 8 and 9, suggesting the involvement of intrinsic and extrinsic apoptosis pathways. TNFα was induced by TLR4 activation at both mRNA and protein levels, but its neutralization did not down-regulated TLR4-induced cell death. TLR4 activation also induced the up-regulation of TRAIL and its receptors DR4 and DR5, and the neutralization of TRAIL ameliorated TLR4 induced apoptosis, suggesting the involvement of TRAIL and its receptors DR4 and DR5 in LPS-induced cell death. Meanwhile, LPS treatment down-regulated the expression of FLICE inhibitory protein (FLIP), a suppressor of death receptor-induced cell death. In addition, TLR4 activation down-regulated the anti-apoptotic protein bcl-2, and up-regulated the pro-apoptotic proteins Noxa and Puma, suggesting that mitochondrial apoptotic pathway was also involved in LPS-induced cell death. Furthermore, we found that TAP63α might confer to the activation of intrinsic and extrinsic apoptotic pathways. The treatment of THP-1 cells with LPS induced the translocation of TAP63α from cytoplasm to nucleus. Taken together, our study suggested that both death receptors and mitochondial were involved in TLR4-induced cell death, and TAP63α may be a target for the prevention of LPS-induced cell death.

Differential Intrasplenic Migration of Dendritic Cell Subsets Tailors Adaptive Immunity

Evidence suggests that distinct splenic dendritic cell (DC) subsets activate either CD4+ or CD8+ T cells in vivo. This bias has been partially ascribed to differential antigen presentation; however, all DC subsets can activate both T cell lineages in vitro. Therefore, we tested whether the organization of DC and T cell subsets in the spleen dictated this preference. We discovered that CD4+ and CD8+ T cells segregated within splenic T cell zones prior to immunization. After intravenous immunization, the two major conventional DC populations, distinguished by 33D1 and XCR1 staining, migrated into separate regions of the T cell zone: 33D1+ DCs migrated into the CD4+ T cell area, whereas XCR1+ DCs migrated into the CD8+ T cell area. Thus, the post-immunization location of each DC subset correlated with the T cell lineage it preferentially primes. Preventing this co-localization selectively impaired either CD4+ or CD8+ T cell immunity to blood-borne antigens.

Splenic Dendritic Cells Survey Red Blood Cells for Missing Self-CD47 to Trigger Adaptive Immune Responses

Sheep red blood cells (SRBCs) have long been used as a model antigen for eliciting systemic immune responses, yet the basis for their adjuvant activity has been unknown. Here, we show that SRBCs failed to engage the inhibitory mouse SIRPα receptor on splenic CD4(+) dendritic cells (DCs), and this failure led to DC activation. Removal of the SIRPα ligand, CD47, from self-RBCs was sufficient to convert them into an adjuvant for adaptive immune responses. DC capture of Cd47(-/-) RBCs and DC activation occurred within minutes in a Src-family-kinase- and CD18-integrin-dependent manner. These findings provide an explanation for the adjuvant mechanism of SRBCs and reveal that splenic DCs survey blood cells for missing self-CD47, a process that might contribute to detecting and mounting immune responses against pathogen-infected RBCs.

Discovery of Intermediary Genes between Pathways Using Sparse Regression

The use of pathways and gene interaction networks for the analysis of differential expression experiments has allowed us to highlight the differences in gene expression profiles between samples in a systems biology perspective. The usefulness and accuracy of pathway analysis critically depend on our understanding of how genes interact with one another. That knowledge is continuously improving due to advances in next generation sequencing technologies and in computational methods. While most approaches treat each of them as independent entities, pathways actually coordinate to perform essential functions in a cell. In this work, we propose a methodology based on a sparse regression approach to find genes that act as intermediary to and interact with two pathways. We model each gene in a pathway using a set of predictor genes, and a connection is formed between the pathway gene and a predictor gene if the sparse regression coefficient corresponding to the predictor gene is non-zero. A predictor gene is a shared neighbor gene of two pathways if it is connected to at least one gene in each pathway. We compare the sparse regression approach to Weighted Correlation Network Analysis and a correlation distance based approach using time-course RNA-Seq data for dendritic cell from wild type, MyD88-knockout, and TRIF-knockout mice, and a set of RNA-Seq data from 60 Caucasian individuals. For the sparse regression approach, we found overrepresented functions for shared neighbor genes between TLR-signaling pathway and antigen processing and presentation, apoptosis, and Jak-Stat pathways that are supported by prior research, and compares favorably to Weighted Correlation Network Analysis in cases where the gene association signals are weak.

Insights how monocytes and dendritic cells contribute and regulate immune defense against microbial pathogens

The immune system protects from infections primarily by detecting and eliminating invading pathogens. Beside neutrophils, monocytes and dendritic cells (DCs) have been recently identified as important sentinels and effectors in combating microbial pathogens. In the steady state mononuclear phagocytes like monocytes and DCs patrol the blood and the tissues. Mammalian monocytes contribute to antimicrobial defense by supplying tissues with macrophage and DC precursors. DCs recognize pathogens and are essential in presenting antigens to initiate antigen-specific adaptive immune responses, thereby bridging the innate and adaptive immune systems. Both, monocytes and DCs play distinct roles in the shaping of immune response. In this review we will focus on the contributions of monocytes and lymphoid organ DCs to immune defense against microbial pathogens in the mouse and their dynamic regulation from steady state to infection.

Role of extracellular RNA and TLR3-Trif signaling in myocardial ischemia-reperfusion injury

Background

Toll‐like receptor 3 (TLR3) was originally identified as the receptor for viral RNA and represents a major host antiviral defense mechanism. TLR3 may also recognize extracellular RNA (exRNA) released from injured tissues under certain stress conditions. However, a role for exRNA and TLR3 in the pathogenesis of myocardial ischemic injury has not been tested. This study examined the role of exRNA and TLR3 signaling in myocardial infarction (MI), apoptosis, inflammation, and cardiac dysfunction during ischemia‐reperfusion (I/R) injury.

Methods and Results

Wild‐type (WT), TLR3^−/−, Trif^−/−, and interferon (IFN) α/β receptor‐1 deficient (IFNAR1^−/−) mice were subjected to 45 minutes of coronary artery occlusion and 24 hours of reperfusion. Compared with WT, TLR3^−/− or Trif^−/− mice had smaller MI and better preserved cardiac function. Surprisingly, unlike TLR(2/4)‐MyD88 signaling, lack of TLR3‐Trif signaling had no impact on myocardial cytokines or neutrophil recruitment after I/R, but myocardial apoptosis was significantly attenuated in Trif^−/− mice. Deletion of the downstream IFNAR1 had no effect on infarct size. Importantly, hypoxia and I/R led to release of RNA including microRNA from injured cardiomyocytes and ischemic heart, respectively. Necrotic cardiomyocytes induced a robust and dose‐dependent cytokine response in cultured cardiomyocytes, which was markedly reduced by RNase but not DNase, and partially blocked in TLR3‐deficient cardiomyocytes. In vivo, RNase administration reduced serum RNA level, attenuated myocardial cytokine production, leukocytes infiltration and apoptosis, and conferred cardiac protection against I/R injury.

Conclusion

TLR3‐Trif signaling represents an injurious pathway during I/R. Extracellular RNA released during I/R may contribute to myocardial inflammation and infarction.

DAMP molecule S100A9 acts as a molecular pattern to enhance inflammation during influenza A virus infection: role of DDX21-TRIF-TLR4-MyD88 pathway

Pathogen-associated molecular patterns (PAMPs) trigger host immune response by activating pattern recognition receptors like toll-like receptors (TLRs). However, the mechanism whereby several pathogens, including viruses, activate TLRs via a non-PAMP mechanism is unclear. Endogenous “inflammatory mediators” called damage-associated molecular patterns (DAMPs) have been implicated in regulating immune response and inflammation. However, the role of DAMPs in inflammation/immunity during virus infection has not been studied. We have identified a DAMP molecule, S100A9 (also known as Calgranulin B or MRP-14), as an endogenous non-PAMP activator of TLR signaling during influenza A virus (IAV) infection. S100A9 was released from undamaged IAV-infected cells and extracellular S100A9 acted as a critical host-derived molecular pattern to regulate inflammatory response outcome and disease during infection by exaggerating pro-inflammatory response, cell-death and virus pathogenesis. Genetic studies showed that the DDX21-TRIF signaling pathway is required for S100A9 gene expression/production during infection. Furthermore, the inflammatory activity of extracellular S100A9 was mediated by activation of the TLR4-MyD88 pathway. Our studies have thus, underscored the role of a DAMP molecule (i.e. extracellular S100A9) in regulating virus-associated inflammation and uncovered a previously unknown function of the DDX21-TRIF-S100A9-TLR4-MyD88 signaling network in regulating inflammation during infection.

The lung disease severity following influenza A virus (IAV) infection is dependent on the extent of inflammation in the respiratory tract. Severe inflammation in the lung manifests in development of pneumonia. Therefore, it is very critical to identify cellular factors and dissect the molecular/cellular mechanism controlling inflammation in the respiratory tract during IAV infection. Knowledge derived from these studies will be instrumental in development of therapeutics to combat the lung disease associated with IAV infection. Towards that end, in the current study we have identified a cellular factor S100A9 which is responsible for enhanced inflammation during IAV infection. In addition, we have characterized a signal transduction pathway involving various cellular receptors and signaling adaptors that are involved in mediating S100A9-dependent inflammatory response. Thus, our studies have illuminated a cellular/molecular mechanism that can be intervened by therapeutics to reduce and control IAV-associated lung inflammatory disease like pneumonia.

Cell death triggered by Yersinia enterocolitica identifies processing of the proinflammatory signal adapter MyD88 as a general event in the execution of apoptosis

Many pathogenic microorganisms have evolved tactics to modulate host cell death or survival pathways for establishing infection. The enteropathogenic bacterium Yersinia enterocolitica deactivates TLR-induced signaling pathways, which triggers apoptosis in macrophages. In this article, we show that Yersinia-induced apoptosis of human macrophages involves caspase-dependent cleavage of the TLR adapter protein MyD88. MyD88 was also cleaved when apoptosis was mediated by overexpression of the Toll-IL-1R domain-containing adapter inducing IFN-β in epithelial cells. The caspase-processing site was mapped to aspartate-135 in the central region of MyD88. MyD88 is consequently split by caspases in two fragments, one harboring the death domain and the other the Toll-IL-1R domain. Caspase-3 was identified as the protease that conferred the cleavage of MyD88 in in vitro caspase assays. In line with a broad role of caspase-3 in the execution of apoptosis, the processing of MyD88 was not restricted to Yersinia infection and to proapoptotic Toll-IL-1R domain-containing adapter inducing IFN-β signaling, but was also triggered by staurosporine treatment. The cleavage of MyD88 therefore seems to be a common event in the advanced stages of apoptosis, when caspase-3 is active. We propose that the processing of MyD88 disrupts its scaffolding function and uncouples the activation of TLR and IL-1Rs from the initiation of proinflammatory signaling events. The disruption of MyD88 may consequently render dying cells less sensitive to proinflammatory stimuli in the execution phase of apoptosis. The cleavage of MyD88 could therefore be a means of conferring immunogenic tolerance to apoptotic cells to ensure silent, noninflammatory cell demise.

Sodium methyldithiocarbamate exerts broad inhibition of cellular signaling and expression of effector molecules of inflammation

Sodium methyldithiocarbamate (SMD) is one of the most abundantly used conventional pesticides in the United States. At dosages relevant to occupational exposure, it causes major effects on the immune system in mice, including a decreased resistance to sepsis. This lab has identified some of the mechanisms of action of this compound and some of the immunological parameters affected, but the global effects have not previously been assessed. The purpose of the present study was to conduct transcriptomic analysis of the effects of SMD on lipopolysaccharide-induced expression of mediators important in innate immunity and inflammation. The results revealed broad effects on expression of transcription factors in both branches of Toll-like receptor 4 (TLR4) signaling (MyD88 and TRIF). However, TLR3 and interferon signaling pathways were decreased to a greater extent, and assessment of the effects of SMD on polyinosinic polycytidylic acid-induced cytokine and chemokine production revealed that these responses mediated by TLR3 were indeed sensitive to the effects of SMD, with inhibition occurring at lower dosages than required to inhibit responses to other immunological stimuli tested in our previous studies. In the downstream signaling pathways of these TLRs, functional analysis also revealed that NF-κB activation was inhibited by SMD, as indicated by gene expression analysis and a reporter construct in mice. A previously unreported effect on luteinizing hormone and follicle-stimulating hormone pathways was also observed.

Effect of TRIF on permeability and apoptosis in bovine microvascular endothelial cells exposed to lipopolysaccharide

Bovine respiratory disease complex (BRDC) can be caused by several Gram negative bacteria. Lung endothelial cells may be damaged by the release of lipopolysaccharide (LPS) from these organisms. Toll-like receptor (TLR-4) signaling pathways include the myeloid differentiation primary response gene 88 (MyD88) and the Toll/interleukin (IL)-1 receptor (TIR) domain-containing adapter-inducing interferon-β (TRIF) pathways. The aim of this study was to determine which of these pathways is responsible for permeability changes, apoptosis and cytokine production in bovine lung microvascular cells exposed to LPS. Bovine lung endothelial cells were treated with a peptide to inhibit MyD88 signaling or small interfering RNA (siRNA) to inhibit TRIF signaling. Effects were measured using trans-well endothelial electrical resistance to determine cell monolayer permeability, annexin staining to estimate apoptosis and real-time PCR to measure levels of expression of IL-1β and tumor necrosis factor (TNF)-α mRNA. Inhibition of TRIF signaling reduced permeability changes and apoptosis in endothelial cells exposed to LPS. In contrast, MyD88 inhibition reduced expression of IL-1β and TNF-α mRNA in LPS treated cells, but had no effect on permeability. It was concluded that TRIF signaling in LPS-stimulated lung endothelial cells results in permeability changes and apoptosis.

EBI2-mediated bridging channel positioning supports splenic dendritic cell homeostasis and particulate antigen capture

Splenic dendritic cells (DCs) present blood-borne antigens to lymphocytes to promote T cell and antibody responses. The cues involved in positioning DCs in areas of antigen exposure in the spleen are undefined. Here we show that CD4⁺ DCs highly express EBI2 and migrate to its oxysterol ligand, 7α,25-OHC. In mice lacking EBI2 or the enzymes needed for generating normal distributions of 7α,25-OHC, CD4⁺ DCs are reduced in frequency and the remaining cells fail to situate in marginal zone bridging channels. The CD4⁺ DC deficiency can be rescued by LTβR agonism. EBI2-mediated positioning in bridging channels promotes DC encounter with blood-borne particulate antigen. Upon exposure to antigen, CD4⁺ DCs move rapidly to the T-B zone interface and promote induction of helper T cell and antibody responses. These findings establish an essential role for EBI2 in CD4⁺ DC positioning and homeostasis and in facilitating capture and presentation of blood-borne particulate antigens.

DOI:http://dx.doi.org/10.7554/eLife.00757.001

One of the main roles of the spleen is to make the antibodies that protect the body against viruses, bacteria and other microorganisms. Antibodies are made by B cells, which are a type of white blood cell, after they have been exposed to antigens. For most antibody responses, it is also necessary for the B cells to get help from other white blood cells called T cells that have been exposed to antigens. Specialized cells called dendritic cells have a central role in bringing the antigens—which are usually fragments of the infectious agents that have invaded the body—to the T cells.

One subset of dendritic cells, called CD4⁺ dendritic cells, are found in large numbers in a part of the spleen called the bridging channel, but the process by which these cells become localized in this channel has not been fully understood. Now, Yi and Cyster show that a receptor called EBI2, which is found on the surface of CD4⁺ dendritic cells, binds to a type of organic molecule called an oxysterol that is produced in the bridging channel.

In mice that had been genetically engineered to lack EBI2 or the enzymes needed to make this particular oxysterol—which is known as 7α,25-dihydroxycholesterol, or 7α,25-OHC for short—the CD4⁺ dendritic cells were no longer clustered in the bridging channel and their number was markedly decreased. This showed that the interaction between EBI2 and the oxysterol was essential for ensuring that the CD4⁺ dendritic cells were in the right place. The correct positioning of the CD4⁺ dendritic cells was, in turn, necessary for maintaining cell numbers. Moreover, these mice had a weakened immune response because of the very low number of antigens that were being presented to the T cells.

A number of autoimmune diseases, such as lupus, are caused by the body developing an immune response to its own cells and tissues. One implication of the work of Yi and Cyster is that if small molecule inhibitors of EBI2 could be designed, they might be able to suppress the onset of such autoimmune responses.

DOI:http://dx.doi.org/10.7554/eLife.00757.002

Role of interleukin-6 in hemopoietic and non-hemopoietic synergy mediating TLR4-triggered late murine ileus and endotoxic shock

BACKGROUND

Early murine endotoxin-induced ileus at 6 h is exclusively mediated by non-hemopoietic TLR4/MyD88 signaling despite molecular activation of hemopoietic cells which included a significant IL-6 mRNA induction. Our objective was to define the role of hemopoietic cells in LPS/TLR4-triggered ileus and inflammation over time, and identify mechanisms of ileus.

METHODS

CSF-1(-/-) , TLR4 non-chimera and TLR4 chimera mice were single-shot intraperitoneal injected with ultrapure lipopolysaccharide (UP-LPS) and studied up to 4 days. Subgroups of TLR4(WT) mice were additionally intravenously injected with exogenous recombinant IL-6 (rmIL-6) or murine soluble IL-6 receptor blocking antibody (anti-sIL-6R mAB).

KEY RESULTS

Hemopoietic TLR4 signaling independently mediated UP-LPS-induced ileus at 24 h, but chemotactic muscularis neutrophil extravasation was not causatively involved and mice lacking CSF-1-dependent macrophages died prematurely. Synergy of hemopoietic and non-hemopoietic cells determined ileus severity and mortality which correlated with synergistic cell lineage specific transcription of inflammatory mediators like IL-6 within the intestinal muscularis. Circulating IL-6 levels were LPS dose dependent, but exogenous rmIL-6 did not spark off a self-perpetuating inflammatory response triggering ileus. Sustained therapeutic inhibition of functional IL-6 signaling efficiently ameliorated late ileus while preemptive antibody-mediated IL-6R blockade was marginally effective in mitigating ileus. However, IL-6R blockade did not prevent endotoxin-associated mortality nor did it alter circulating IL-6 levels.

CONCLUSIONS & INFERENCES

A time-delayed bone marrow-driven mechanism of murine endotoxin-induced ileus exists, and hemopoietic cells synergize with non-hemopoietic cells thereby prolonging ileus and fueling intestinal inflammation. Importantly, IL-6 signaling via IL-6R/gp130 drives late ileus, yet it did not regulate mortality in endotoxic shock.

In situ microscopy analysis reveals local innate immune response developed around Brucella infected cells in resistant and susceptible mice

Brucella are facultative intracellular bacteria that chronically infect humans and animals causing brucellosis. Brucella are able to invade and replicate in a broad range of cell lines in vitro, however the cells supporting bacterial growth in vivo are largely unknown. In order to identify these, we used a Brucella melitensis strain stably expressing mCherry fluorescent protein to determine the phenotype of infected cells in spleen and liver, two major sites of B. melitensis growth in mice. In both tissues, the majority of primary infected cells expressed the F4/80 myeloid marker. The peak of infection correlated with granuloma development. These structures were mainly composed of CD11b⁺ F4/80⁺ MHC-II⁺ cells expressing iNOS/NOS2 enzyme. A fraction of these cells also expressed CD11c marker and appeared similar to inflammatory dendritic cells (DCs). Analysis of genetically deficient mice revealed that differentiation of iNOS⁺ inflammatory DC, granuloma formation and control of bacterial growth were deeply affected by the absence of MyD88, IL-12p35 and IFN-γ molecules. During chronic phase of infection in susceptible mice, we identified a particular subset of DC expressing both CD11c and CD205, serving as a reservoir for the bacteria. Taken together, our results describe the cellular nature of immune effectors involved during Brucella infection and reveal a previously unappreciated role for DC subsets, both as effectors and reservoir cells, in the pathogenesis of brucellosis.

Inhibition of mammalian target of rapamycin augments lipopolysaccharide-induced lung injury and apoptosis

Acute lung injury during bacterial infection is associated with neutrophilic inflammation, epithelial cell apoptosis, and disruption of the alveolar-capillary barrier. TLR4 is required for lung injury in animals exposed to bacterial LPS and initiates proinflammatory responses in part via the transcription factor NF-κB. Ligation of TLR4 also initiates a proapoptotic response by activating IFN-β and STAT1-dependent genes. We recently demonstrated that mammalian target of rapamycin (mTOR), a key controller of cell growth and survival, can physically interact with STAT1 and suppress the induction of STAT1-dependent apoptosis genes. We therefore hypothesized that the mTOR inhibitor rapamycin would increase LPS-induced apoptosis and lung injury in vivo. Rapamycin increased lung injury and cellular apoptosis in C57BL/6J mice exposed to intratracheal LPS for 24 h. Rapamycin also augmented STAT1 activation, and the induction of STAT1-dependent genes that mediate cellular apoptosis (i.e., Fas, caspase-3). LPS-induced lung injury was attenuated in STAT1 knockout mice. In addition, LPS and IFN-β-induced apoptosis was absent in cultured cells lacking STAT1, and, unlike in wild-type cells, a permissive effect of rapamycin was not observed. In contrast to its effect on STAT1, rapamycin inhibited NF-κB activation in vivo and reduced selected markers of inflammation (i.e., neutrophils in the bronchoalveolar lavage fluid, TNF-α). Therefore, although it inhibits NF-κB and neutrophilic inflammation, rapamycin augments LPS-induced lung injury and apoptosis in a mechanism that involves STAT1 and the induction of STAT1-dependent apoptosis genes.

Depletion of dendritic cells enhances innate anti-bacterial host defense through modulation of phagocyte homeostasis

Dendritic cells (DCs) as professional antigen-presenting cells play an important role in the initiation and modulation of the adaptive immune response. However, their role in the innate immune response against bacterial infections is not completely defined. Here we have analyzed the role of DCs and their impact on the innate anti-bacterial host defense in an experimental infection model of Yersinia enterocolitica (Ye). We used CD11c-diphtheria toxin (DT) mice to deplete DCs prior to severe infection with Ye. DC depletion significantly increased animal survival after Ye infection. The bacterial load in the spleen of DC-depleted mice was significantly lower than that of control mice throughout the infection. DC depletion was accompanied by an increase in the serum levels of CXCL1, G-CSF, IL-1α, and CCL2 and an increase in the numbers of splenic phagocytes. Functionally, splenocytes from DC-depleted mice exhibited an increased bacterial killing capacity compared to splenocytes from control mice. Cellular studies further showed that this was due to an increased production of reactive oxygen species (ROS) by neutrophils. Adoptive transfer of neutrophils from DC-depleted mice into control mice prior to Ye infection reduced the bacterial load to the level of Ye-infected DC-depleted mice, suggesting that the increased number of phagocytes with additional ROS production account for the decreased bacterial load. Furthermore, after incubation with serum from DC-depleted mice splenocytes from control mice increased their bacterial killing capacity, most likely due to enhanced ROS production by neutrophils, indicating that serum factors from DC-depleted mice account for this effect. In summary, we could show that DC depletion triggers phagocyte accumulation in the spleen and enhances their anti-bacterial killing capacity upon bacterial infection.

Type I interferon drives dendritic cell apoptosis via multiple BH3-only proteins following activation by PolyIC in vivo

Background

DC are activated by pathogen-associated molecular patterns (PAMPs), and this is pivotal for the induction of adaptive immune responses. Thereafter, the clearance of activated DC is crucial to prevent immune pathology. While PAMPs are of major interest for vaccine science due to their adjuvant potential, it is unclear whether and how PAMPs may affect DC viability. We aimed to elucidate the possible apoptotic mechanisms that control activated DC lifespan in response to PAMPs, particularly in vivo.

Methodology/Principal Findings

We report that polyinosinic:polycytidylic acid (PolyIC, synthetic analogue of dsRNA) induces dramatic apoptosis of mouse splenic conventional DC (cDC) in vivo, predominantly affecting the CD8α subset, as shown by flow cytometry-based analysis of splenic DC subsets. Importantly, while Bim deficiency conferred only minor protection, cDC depletion was prevented in mice lacking Bim plus one of three other BH3-only proteins, either Puma, Noxa or Bid. Furthermore, we show that Type I Interferon (IFN) is necessary and sufficient for DC death both in vitro and in vivo, and that TLR3 and MAVS co-operate in IFNß production in vivo to induce DC death in response to PolyIC.

Conclusions/Significance

These results demonstrate for the first time in vivo that apoptosis restricts DC lifespan following activation by PolyIC, particularly affecting the CD8α cDC subset. Such DC apoptosis is mediated by the overlapping action of pro-apoptotic BH3-only proteins, including but not solely involving Bim, and is driven by Type I IFN. While Type I IFNs are important anti-viral factors, CD8α cDC are major cross-presenting cells and critical inducers of CTL. We discuss such paradoxical finding on DC death with PolyIC/Type I IFN. These results could contribute to understand immunosuppression associated with chronic infection, and to the optimization of DC-based therapies and the clinical use of PAMPs and Type I IFNs.

A composite MyD88/CD40 switch synergistically activates mouse and human dendritic cells for enhanced antitumor efficacy

The in vivo therapeutic efficacy of DC-based cancer vaccines is limited by suboptimal DC maturation protocols. Although delivery of TLR adjuvants systemically boosts DC-based cancer vaccine efficacy, it could also increase toxicity. Here, we have engineered a drug-inducible, composite activation receptor for DCs (referred to herein as DC-CAR) comprising the TLR adaptor MyD88, the CD40 cytoplasmic region, and 2 ligand-binding FKBP12 domains. Administration of a lipid-permeant dimerizing ligand (AP1903) induced oligomerization and activation of this fusion protein, which we termed iMyD88/CD40. AP1903 administration to vaccinated mice enabled prolonged and targeted activation of iMyD88/CD40-modified DCs. Compared with conventionally matured DCs, AP1903-activated iMyD88/CD40-DCs had increased activation of proinflammatory MAPKs. AP1903-activated iMyD88/CD40-transduced human or mouse DCs also produced higher levels of Th1 cytokines, showed improved migration in vivo, and enhanced both antigen-specific CD8+ T cell responses and innate NK cell responses. Furthermore, treatment with AP1903 in vaccinated mice led to robust antitumor immunity against preestablished E.G7-OVA lymphomas and aggressive B16.F10 tumors. Thus, the iMyD88/CD40 unified "switch" effectively and safely replaced exogenous adjuvant cocktails, allowing remote and sustained DC activation in vivo. DC "licensing" through iMyD88/CD40 may represent a mechanism by which to exploit the natural synergy between the TLR and CD40 signaling pathways in DCs using a single small molecule drug and could augment the efficacy of antitumor DC-based vaccines.

Metabolic stress boosts humoral responses in vivo independently of inflammasome and inflammatory reaction

Adjuvant formulations boost humoral responses by acting through several, yet incompletely elucidated pathways. In this study, we show that oligomycin or 5-aminoimidazole-4-carboxamide-1-β-D-ribonucleoside (AICAR) enhances Ab production when coinjected with T cell-dependent Ags. Oligomycin and AICAR lead to intracellular ATP reduction, suggesting that metabolic stress could be sensed by immune cells and leads to increased humoral responses. AICAR promotes IL-4 and IL-21 by naive Th cells but does not affect dendritic cell activation/maturation in vitro or in vivo. Accordingly, the adjuvant effect of AICAR or oligomycin does not require MyD88 or caspase-1 expression in vivo. Because AICAR is well tolerated in humans, this compound could represent a novel and safe adjuvant promoting humoral responses in vivo with a minimal reactogenicity.

Immune evasion by Yersinia enterocolitica: differential targeting of dendritic cell subpopulations in vivo

CD4(+) T cells are essential for the control of Yersinia enterocolitica (Ye) infection in mice. Ye can inhibit dendritic cell (DC) antigen uptake and degradation, maturation and subsequently T-cell activation in vitro. Here we investigated the effects of Ye infection on splenic DCs and T-cell proliferation in an experimental mouse infection model. We found that OVA-specific CD4(+) T cells had a reduced potential to proliferate when stimulated with OVA after infection with Ye compared to control mice. Additionally, proliferation of OVA-specific CD4(+) T cells was markedly reduced when cultured with splenic CD8α(+) DCs from Ye infected mice in the presence of OVA. In contrast, T-cell proliferation was not impaired in cultures with CD4(+) or CD4(-)CD8α(-) DCs isolated from Ye infected mice. However, OVA uptake and degradation as well as cytokine production were impaired in CD8α(+) DCs, but not in CD4(+) and CD4(-)CD8α(-) DCs after Ye infection. Pathogenicity factors (Yops) from Ye were most frequently injected into CD8α(+) DCs, resulting in less MHC class II and CD86 expression than on non-injected CD8α(+) DCs. Three days post infection with Ye the number of splenic CD8α(+) and CD4(+) DCs was reduced by 50% and 90%, respectively. The decreased number of DC subsets, which was dependent on TLR4 and TRIF signaling, was the result of a faster proliferation and suppressed de novo DC generation. Together, we show that Ye infection negatively regulates the stimulatory capacity of some but not all splenic DC subpopulations in vivo. This leads to differential antigen uptake and degradation, cytokine production, cell loss, and cell death rates in various DC subpopulations. The data suggest that these effects might be caused directly by injection of Yops into DCs and indirectly by affecting the homeostasis of CD4(+) and CD8α(+) DCs. These events may contribute to reduced T-cell proliferation and immune evasion of Ye.

Inhibition of dynamin-dependent endocytosis interferes with type III IFN expression in bacteria-infected human monocyte-derived DCs

Type I IFNs (IFN-α/βs) and type III IFNs (IFN-λ1-3) play an important role in host defense against viral infections. The induction of type I IFNs has recently been found to take place also in bacterial infections, and therefore, this study focuses on analyzing the regulation of type III IFNs in response to bacterial stimulation. We found by quantitative RT-PCR that the expression of IFN-λ1 and IFN-λ2/3 mRNAs, as well as that of IFN-β, was similarly up-regulated in response to stimulation with live Salmonella typhimurium or TLR4 agonist LPS in human moDCs. The induction of IFN-λ mRNAs did not require ongoing protein synthesis, and only IFN-λ1 was detected at the protein level. The induction of IFN-λ mRNAs was sensitive to SB202190, Ly294002, and PDTC, which inhibit p38 MAPK, PI3K, and NF-κB activation, respectively. Furthermore, we observed that blocking dynamin-dependent endocytosis pathways with dynasore led to decreased cell surface expression of CD86 and HLA class II molecules and reduced production of IFN-λ1, CXCL10, and IL-6 when the cells were infected with S. typhimurium. Cytokine production was also impaired in dynasore-treated, Streptococcus thermophilus-stimulated cells. Further, inhibition of dynamin prevented S. typhimurium-induced phosphorylation of IRF3 and the internalization of the bacteria. In summary, induction of type III IFNs in bacteria-infected human moDCs requires multiple signaling pathways and involves bacterial phagocytosis.

Acinetobacter baumannii outer membrane protein A induces dendritic cell death through mitochondrial targeting

Acinetobacter baumannii outer membrane protein A (AbOmpA) is a potential virulence factor that induces epithelial cell death, but its pathologic effects on the immune system have yet to be determined. The present study investigated the pathologic events occurring in dendritic cells (DCs) exposed to a cytotoxic concentration of AbOmpA. AbOmpA induced early-onset apoptosis and delayed-onset necrosis in DCs. AbOmpA targeted the mitochondria and induced the production of reactive oxygen species (ROS). ROS were directly responsible for both apoptosis and necrosis of AbOmpA-treated DCs. These results demonstrate that the AbOmpA secreted from A. baumannii induces DC death, which may impair T cell biology to induce adaptive immune responses against A. baumannii.

Autophagy in innate and adaptive immunity

Autophagy (self-eating) is an evolutionary conserved simple process by which cells target their own cellular organelles and long-lived proteins for degradation. Recently, this simple ancient process has proved to be involved in many biological aspects, including host defense, cell survival and death, innate and adaptive immunity, and cancer. The implications of aberrant regulation of autophagy in human diseases are just beginning to unravel. This is a brief review of recent progress in the association of autophagy with innate and adaptive immunity relevant to lung biology and disease.

TLR4-mediated podosome loss discriminates gram-negative from gram-positive bacteria in their capacity to induce dendritic cell migration and maturation

Chronic infections are caused by microorganisms that display effective immune evasion mechanisms. Dendritic cell (DC)-dependent T cell-mediated adaptive immunity is one of the mechanisms that have evolved to prevent the occurrence of chronic bacterial infections. In turn, bacterial pathogens have developed strategies to evade immune recognition. In this study, we show that gram-negative and gram-positive bacteria differ in their ability to activate DCs and that gram-negative bacteria are far more effective inducers of DC maturation. Moreover, we observed that only gram-negative bacteria can induce loss of adhesive podosome structures in DCs, a response necessary for the induction of effective DC migration. We demonstrate that the ability of gram-negative bacteria to trigger podosome turnover and induce DC migration reflects their capacity to selectively activate TLR4. Examining mice defective in TLR4 signaling, we show that this DC maturation and migration are mainly Toll/IL-1 receptor domain-containing adaptor-inducing IFNbeta-dependent. Furthermore, we show that these processes depend on the production of PGs by these DCs, suggesting a direct link between TLR4-mediated signaling and arachidonic metabolism. These findings demonstrate that gram-positive and gram-negative bacteria profoundly differ in their capacity to activate DCs. We propose that this inability of gram-positive bacteria to induce DC maturation and migration is part of the armamentarium necessary for avoiding the induction of an effective cellular immune response and may explain the frequent involvement of these pathogens in chronic infections.

Toll-like receptors 2 and 4 contribute to sepsis-induced depletion of spleen dendritic cells

Depletion of dendritic cells (DC) in secondary lymphoid organs is a hallmark of sepsis-induced immune dysfunction. In this setting, we investigated if Toll-like receptor (TLR)-dependent signaling might modulate the maturation process and the survival of DC. Using a model of sublethal polymicrobial sepsis induced by cecal ligation and puncture, we investigated the quantitative and functional features of spleen DC in wild-type, TLR2(-/-), TLR4(-/-), and TLR2(-/-) TLR4(-/-) mice. By 24 h, a decrease in the relative percentage of CD11c(high) spleen DC occurred in wild-type mice but was prevented in TLR2(-/-), TLR4(-/-), and TLR2(-/-) TLR4(-/-) mice. In wild-type mice, sepsis dramatically affected both CD11c(+) CD8alpha(+) and CD11c(+) CD8alpha(-) subsets. In all three types of knockout mice studied, the CD11c(+) CD8alpha(+) subset followed a depletion pattern similar to that for wild-type mice. In contrast, the loss of CD11c(+) CD8alpha(-) cells was attenuated in TLR2(-/-) and TLR4(-/-) mice and completely prevented in TLR2(-/-) TLR4(-/-) mice. Accordingly, apoptosis of spleen DC was increased in septic wild-type mice and inhibited in knockout mice. In addition we characterized the functional features of spleen DC obtained from septic mice. As shown by increased expression of major histocompatibility complex class II and CD86, polymicrobial sepsis induced maturation of DC, with subsequent increased capacity to prime T lymphocytes, similarly in wild-type and knockout mice. In response to CpG DNA stimulation, production of interleukin-12 was equally impaired in DC obtained from wild-type and knockout septic mice. In conclusion, although dispensable for the DC maturation process, TLR2 and TLR4 are involved in the mechanisms leading to depletion of spleen DC following polymicrobial sepsis.

Type I IFN regulate DC turnover in vivo

DC are the most potent antigen-presenting cells that recognise signs of infection and serve as the main activators of naïve T cells. We have previously shown that type I IFN (IFN-I) are produced by DC and can act in an autocrine manner to activate DC. In the present study, we have investigated the role of IFN-I in regulating the turnover and lifespan of DC. We found that DC, especially the CD8alpha(+) subset, from type I IFN receptor knock out (IFNAR KO) mice, display a reduced turnover rate when compared with DC from WT mice, as revealed by BrdU labelling kinetics. In vitro, IFNAR KO BM precursor cells cultured in the presence of GM-CSF generated CD11c(+) DC less efficiently than WT BM, and the IFNAR KO DC that arose displayed reduced migratory ability. Interestingly, splenic DC from IFNAR KO mice exhibited a higher survival rate in short-term culture compared with control DC. Exposure to IFN-I in vivo markedly increased the turnover rate of splenic DC, particularly CD8alpha(+) DC, which was preceded by a transient induction of apoptosis. In accordance with this, IFN-I stimulated the apoptosis of splenic DC in vitro. Overall, our data indicate that IFN-I are important regulators of DC turnover in vivo and suggest that these cytokines may exert this function through the modulation of multiple processes involving DC apoptosis, proliferation and migration.

Plasmacytoid and conventional dendritic cells are early producers of IL-12 in Neospora caninum-infected mice

Neospora caninum is a coccidian parasite causative of clinical infections in a wide range of animal hosts. The maturation and activation of splenic conventional dendritic cells (cDCs) and plasmacytoid dendritic cells (pDCs) were studied here in BALB/c mice challenged intraperitoneal with N. caninum tachyzoites. The number of cDCs was found to decrease in the spleen of the infected mice 12 h and 2 days after the parasitic challenge, whereas at day 5 after infection it was significantly above that of mock-infected controls. In contrast, the number of splenic pDCs did not change significantly on infection. In the infected mice, both cell subtypes displayed an activated phenotype with upregulation of costimulatory and MHC class II molecules. This stimulatory effect was more marked at the earliest assessed time point after infection, 12 h, when a clear increase in the frequency of cDCs (CD8alpha(+) and CD8alpha(-)) and pDCs producing interleukin-12 (IL-12) was also observed. N. caninum tachyzoites could be observed by confocal microscopy associated with sorted DCs. Overall, these results present the first evidence that both cDCs and pDCs mediate in vivo the innate immune response to N. caninum infection through the production of IL-12, a key cytokine for host resistance to neosporosis.

Cellular cardiomyoplasty: what have we learned?

Restoring blood flow, improving perfusion, reducing clinical symptoms, and augmenting ventricular function are the goals after acute myocardial infarction. Other than cardiac transplantation, no standard clinical procedure is available to restore damaged myocardium. Since we first reported cellular cardiomyoplasty in 1989, successful outcomes have been confirmed by experimental and clinical studies, but definitive long-term efficacy requires large-scale placebo-controlled double-blind randomized trials. On meta-analysis, stem cell-treated groups had significantly improved left ventricular ejection fraction, reduced infarct scar size, and decreased left ventricular end-systolic volume. Fewer myocardial infarctions, deaths, readmissions for heart failure, and repeat revascularizations were additional benefits. Encouraging clinical findings have been reported using satellite or bone marrow stem cells, but understanding of the benefit mechanisms demands additional studies. Adult mammalian ventricular myocardium lacks adequate regeneration capability, and cellular cardiomyoplasty offers a new way to overcome this; the poor retention and engraftment rate and high apoptotic rate of the implanted stem cells limit outcomes. The ideal type and number of cells, optimal timing of cell therapy, and ideal cell delivery method depend on determining the beneficial mechanisms. Cellular cardiomyoplasty has progressed rapidly in the last decade. A critical review may help us to better plan the future direction.

Toll-like receptors and cancer

Toll-like receptors (TLRs) are a family of pattern recognition receptors that are best-known for their role in host defence from infection. Emerging evidence also suggests that TLRs have an important role in maintaining tissue homeostasis by regulating the inflammatory and tissue repair responses to injury. The development of cancer has been associated with microbial infection, injury, inflammation and tissue repair. Here we discuss how the function of TLRs may relate to these processes in the context of carcinogenesis.

Regulation of STAT pathways and IRF1 during human dendritic cell maturation by TNF-alpha and PGE2

Maturation of dendritic cells (DCs) by TLR ligands induces expression of IFN-beta and autocrine activation of IFN-inducible Stat1-dependent genes important for DC function. In this study, we analyzed the regulation of STAT signaling during maturation of human DCs by TNF-alpha and PGE2, which induced maturation of human DCs comparably with LPS but did not induce detectable IFN-beta production or Stat1 tyrosine phosphorylation. Consistent with these results, TNF-alpha and PGE2 did not induce Stat1 DNA binding to a standard Stat1-binding oligonucleotide. Instead, TNF-alpha and PGE2 increased Stat1 serine phosphorylation and Stat4 tyrosine phosphorylation and activated expression of the NF-kappaB and Stat1 target gene IFN regulatory factor 1 (IRF1), which contributes to IFN responses. TNF-alpha and PGE2 induced a complex that bound an oligonucleotide derived from the IRF1 promoter that contains a STAT-binding sequence embedded in a larger palindromic sequence, and this complex was recognized by Stat1 antibodies. These results suggest that TNF-alpha and PGE2 activate STAT-mediated components of human DC maturation by alternative pathways to the IFN-beta-mediated autocrine loop used by TLRs.

Uropathogenic Escherichia coli block MyD88-dependent and activate MyD88-independent signaling pathways in rat testicular cells

Uropathogenic Escherichia coli (UPEC) is the most common etiological cause of urogenital tract infections and represents a considerable cause of immunological male infertility. We examined TLR 1-11 expression profiles in testicular cells and the functional response to infection with UPEC. All testicular cell types expressed mRNAs for at least two TLRs and, in particular, synthesis of TLR4 was induced in testicular macrophages (TM), Sertoli cells (SC), peritubular cells (PTC), and peritoneal macrophages (PM) after UPEC exposure. Even though MyD88-dependent pathways were activated as exemplified by phosphorylation of mitogen-activated protein kinases in TM, SC, PTC, and PM and by the degradation of IkappaBalpha and the nuclear translocation of NF-kappaB in PTC and PM, treatment with UPEC did not result in secretion of the proinflammatory cytokines IL-1alpha, IL-6, and TNF-alpha in any of the investigated cells. Moreover, stimulated production of these cytokines by nonpathogenic commensal E. coli or LPS in PM was completely abolished after coincubation with UPEC. Instead, in SC, PTC, TM, and PM, UPEC exposure resulted in activation of MyD88-independent signaling as documented by nuclear transfer of IFN-related factor-3 and elevated expression of type I IFNs alpha and beta, IFN-gamma-inducible protein 10, MCP-1, and RANTES. We conclude that in this in vitro model UPEC can actively suppress MyD88-dependent signaling at different levels to prevent proinflammatory cytokine secretion by testicular cells. Thus, testicular innate immune defense is shifted to an antiviral-like MyD88-independent response.