Dendritic cells respond to influenza virus through TLR7- and PKR-independent pathways

Signals from intestinal microbiota mediate the crosstalk between the lung-gut axis in an influenza infection mouse model

Introduction

Introduction: The influenza virus primarily targets the respiratory tract, yet both the respiratory and intestinal systems suffer damage during infection. The connection between lung and intestinal damage remains unclear.

Methods

Our experiment employs 16S rRNA technology and Liquid Chromatography-Mass Spectrometry (LC-MS) to detect the impact of influenza virus infection on the fecal content and metabolites in mice. Additionally, it investigates the effect of influenza virus infection on intestinal damage and its underlying mechanisms through HE staining, Western blot, Q-PCR, and flow cytometry.

Results

Our study found that influenza virus infection caused significant damage to both the lungs and intestines, with the virus detected exclusively in the lungs. Antibiotic treatment worsened the severity of lung and intestinal damage. Moreover, mRNA levels of Toll-like receptor 7 (TLR7) and Interferon-b (IFN-b) significantly increased in the lungs post-infection. Analysis of intestinal microbiota revealed notable shifts in composition after influenza infection, including increased Enterobacteriaceae and decreased Lactobacillaceae. Conversely, antibiotic treatment reduced microbial diversity, notably affecting Firmicutes, Proteobacteria, and Bacteroidetes. Metabolomics showed altered amino acid metabolism pathways due to influenza infection and antibiotics. Abnormal expression of indoleamine 2,3-dioxygenase 1 (IDO1) in the colon disrupted the balance between helper T17 cells (Th17) and regulatory T cells (Treg cells) in the intestine. Mice infected with the influenza virus and supplemented with tryptophan and Lactobacillus showed reduced lung and intestinal damage, decreased Enterobacteriaceae levels in the intestine, and decreased IDO1 activity.

Discussion

Overall, influenza infection caused damage to lung and intestinal tissues, disrupted intestinal microbiota and metabolites, and affected Th17/Treg balance. Antibiotic treatment exacerbated these effects. Supplementation with tryptophan and Lactobacillus improved lung and intestinal health, highlighting a new understanding of the lung-intestine connection in influenza-induced intestinal disease.

Expression of Toll-Like Receptors in the Lung Tissue of Mouse Fetuses Generated by in vitro Embryo Culture and Embryo Transfer

Mouse fetuses generated by in vitro embryo culture and embryo transfer exhibit impaired lung development, altered composition of pulmonary epithelial cells associated with downregulation of several genes involved in lung development and toll-like receptor (TLR) signaling pathway. The aims of the present study were to determine the expression of all TLRs and to examine if the expression of TLRs, along with genes involved in TLR signaling pathway, is altered in the lung tissue of mouse fetuses generated through embryo culture and embryo transfer. Two experimental (EGs) and one control (CG) group were included in the study. Embryos cultured at 5% CO2-95% air for 95 h or less than 24 h were transferred to pseudo-pregnant females to obtain fetuses comprising EGin vitro (n = 18) and EGin vivo (n = 18), respectively. Fetuses obtained from naturally ovulating females on day 18 of pregnancy served as the CG (n = 18). Western blot and immunohistochemistry were used to determine the expression of TLR proteins. The expression of transcripts encoding TLRs, and the genes involved in TLR signaling pathway (Lbp, Pik3r1, Pik3cb, Nfkbia, and Fos), was determined using qRT-PCR. While all TLRs were expressed by cells lining the bronchial/bronchiolar epithelium of lung tissues in all groups, some of the TLRs were expressed in a specific pattern. When compared to CG, the expression of transcripts encoding TLR-2, -3, -4, -5, -7, -8, -9, -12, -13, Lbp, Pik3r1, Pik3cb, Nfkbia, and Fos was significantly downregulated in both EGs. It appears that stress imposed on embryos at preimplantation stages of development is associated with downregulation of TLRs, along with some of the genes involved in TLR signaling pathway, in the lung tissue during the perinatal period. It remains to be determined if downregulation of TLRs, along with the genes involved in TLR signaling pathway, has any functional consequences in the adult lung tissue.

Complexing CpG adjuvants with cationic liposomes enhances vaccine-induced formation of liver TRM cells

Tissue resident memory T cells (T_RM cells) can provide effective tissue surveillance and can respond rapidly to infection. Vaccination strategies aimed at generating T_RM cells have shown promise against a range of pathogens. We have previously shown that the choice of adjuvant critically influences CD8⁺ T_RM cell formation in the liver. However, the range of adjuvants tested was limited. Here, we assessed the ability of a broad range of adjuvants stimulating membrane (TLR4), endosomal (TLR3, TLR7 and TLR9) and cytosolic (cGAS, RIG-I) pathogen recognition receptors for their capacity to induce CD8⁺ T_RM formation in a subunit vaccination model. We show that CpG oligodeoxynucleotides (ODN) remain the most efficient inducers of liver T_RM cells among all adjuvants tested. Moreover, their combination with the cationic liposome DOTAP further enhances the potency, particularly of the class B ODN CpG 1668 and the human TLR9 ligand CpG 2006 (CpG 7909). This study informs the design of efficient liver T_RM-based vaccines for their potential translation.

Fueling influenza and the immune response: Implications for metabolic reprogramming during influenza infection and immunometabolism

Recent studies support the notion that glycolysis and oxidative phosphorylation are rheostats in immune cells whose bioenergetics have functional outputs in terms of their biology. Specific intrinsic and extrinsic molecular factors function as molecular potentiometers to adjust and control glycolytic to respiratory power output. In many cases, these potentiometers are used by influenza viruses and immune cells to support pathogenesis and the host immune response, respectively. Influenza virus infects the respiratory tract, providing a specific environmental niche, while immune cells encounter variable nutrient concentrations as they migrate in response to infection. Immune cell subsets have distinct metabolic programs that adjust to meet energetic and biosynthetic requirements to support effector functions, differentiation, and longevity in their ever-changing microenvironments. This review details how influenza coopts the host cell for metabolic reprogramming and describes the overlap of these regulatory controls in immune cells whose function and fate are dictated by metabolism. These details are contextualized with emerging evidence of the consequences of influenza-induced changes in metabolic homeostasis on disease progression.

Current Insights into the Host Immune Response to Respiratory Viral Infections

Respiratory viral infections often lead to severe illnesses varying from mild or asymptomatic upper respiratory tract infections to severe bronchiolitis and pneumonia or/and chronic obstructive pulmonary disease. Common viral infections, including but not limited to influenza virus, respiratory syncytial virus, rhinovirus and coronavirus, are often the leading cause of morbidity and mortality. Since the lungs are continuously exposed to foreign particles, including respiratory pathogens, it is also well equipped for recognition and antiviral defense utilizing the complex network of innate and adaptive immune cells. Immediately upon infection, a range of proinflammatory cytokines, chemokines and an interferon response is generated, thereby making the immune response a two edged sword, on one hand it is required to eliminate viral pathogens while on other hand it's prolonged response can lead to chronic infection and significant pulmonary damage. Since vaccines to all respiratory viruses are not available, a better understanding of the virus-host interactions, leading to the development of immune response, is critically needed to design effective therapies to limit the severity of inflammatory damage, enhance viral clearance and to compliment the current strategies targeting the virus. In this chapter, we discuss the host responses to common respiratory viral infections, the key players of adaptive and innate immunity and the fine balance that exists between the viral clearance and immune-mediated damage.

Structural analysis reveals TLR7 dynamics underlying antagonism

Toll-like receptor 7 (TLR7) recognizes both microbial and endogenous RNAs and nucleosides. Aberrant activation of TLR7 has been implicated in several autoimmune diseases including systemic lupus erythematosus (SLE). Here, by modifying potent TLR7 agonists, we develop a series of TLR7-specific antagonists as promising therapeutic agents for SLE. These compounds protect mice against lethal autoimmunity. Combining crystallography and cryo-electron microscopy, we identify the open conformation of the receptor and reveal the structural equilibrium between open and closed conformations that underlies TLR7 antagonism, as well as the detailed mechanism by which TLR7-specific antagonists bind to their binding pocket in TLR7. Our work provides small-molecule TLR7-specific antagonists and suggests the TLR7-targeting strategy for treating autoimmune diseases.

A series of Toll-like receptor 7 (TLR7)-specific antagonists and extensive structural analysis reveal the open conformation of the receptor and the structural basis of TLR7 antagonism. One of the compounds shows efficacy in treating mouse model of systemic lupus erythematosus.

Promoter methylation status of ASC/TMS1/PYCARD is associated with decreased overall survival and TNM status in patients with early stage non-small cell lung cancer (NSCLC)

BACKGROUND

Lung cancer is the leading cause of cancer-related death worldwide, with 5-year overall survival less than 15%. Therefore, it is essential to find biomarkers for early detection and prognosis. Aberrant DNA methylation is a common feature of human cancers and its utility is already recognized in cancer management. The aim of this study was to explore the diagnostic and prognostic value of the promoter methylation status of the ASC/TMS1/PYCARD and MyD88 genes, key adaptor molecules in the activation of the innate immune response and apoptosis pathways.

METHODS

A total of 50 non-small cell lung cancer (NSCLC) patients were enrolled in the study. Methylation of bisulphite converted DNA was quantified by pyrosequencing in fresh frozen malignant tissues and adjacent non-malignant tissues. Associations between methylation and lung function, tumor grade and overall survival were evaluated using receiver-operating characteristics (ROC) analysis and statistical tests of hypothesis.

RESULTS

Methylation level of tested genes is generally low but significantly decreased in tumor tissues (ASC/TMS1/PYCARD, P<0.0001; MyD88, P<0.0002), which correlates with increased protein expression. Three CpG sites were identified as promising diagnostic marker candidates; CpG11 (-63 position) in ASC/TMS1/PYCARD and CpG1 (-253 position) and 2 (-265 position) in MyD88. The association study showed that the methylation status of the ASC/TMS1 CpG4 site (-34 position) in malignant and non-malignant tissues is associated with the overall survival (P=0.019) and the methylation status of CpG8 site (-92 position) is associated with TNM-stage (P=0.011).

CONCLUSIONS

The methylation status of the ASC/TMS1/PYCARD and MyD88 promoters are promising prognostic biomarker candidates. However, presented results should be considered as a preliminary and should be confirmed on the larger number of the samples.

Sources of Type I Interferons in Infectious Immunity: Plasmacytoid Dendritic Cells Not Always in the Driver's Seat

Type I Interferons (IFNs) are hallmark cytokines produced in immune responses to all classes of pathogens. Type I IFNs can influence dendritic cell (DC) activation, maturation, migration, and survival, but also directly enhance natural killer (NK) and T/B cell activity, thus orchestrating various innate and adaptive immune effector functions. Therefore, type I IFNs have long been considered essential in the host defense against virus infections. More recently, it has become clear that depending on the type of virus and the course of infection, production of type I IFN can also lead to immunopathology or immunosuppression. Similarly, in bacterial infections type I IFN production is often associated with detrimental effects for the host. Although most cells in the body are thought to be able to produce type I IFN, plasmacytoid DCs (pDCs) have been termed the natural "IFN producing cells" due to their unique molecular adaptations to nucleic acid sensing and ability to produce high amounts of type I IFN. Findings from mouse reporter strains and depletion experiments in in vivo infection models have brought new insights and established that the role of pDCs in type I IFN production in vivo is less important than assumed. Production of type I IFN, especially the early synthesized IFNβ, is rather realized by a variety of cell types and cannot be mainly attributed to pDCs. Indeed, the cell populations responsible for type I IFN production vary with the type of pathogen, its tissue tropism, and the route of infection. In this review, we summarize recent findings from in vivo models on the cellular source of type I IFN in different infectious settings, ranging from virus, bacteria, and fungi to eukaryotic parasites. The implications from these findings for the development of new vaccination and therapeutic designs targeting the respectively defined cell types are discussed.

CD8+ T Cells Orchestrate pDC-XCR1+ Dendritic Cell Spatial and Functional Cooperativity to Optimize Priming

Adaptive cellular immunity is initiated by antigen-specific interactions between T lymphocytes and dendritic cells (DCs). Plasmacytoid DCs (pDCs) support antiviral immunity by linking innate and adaptive immune responses. Here we examined pDC spatiotemporal dynamics during viral infection to uncover when, where, and how they exert their functions. We found that pDCs accumulated at sites of CD8⁺ T cell antigen-driven activation in a CCR5-dependent fashion. Furthermore, activated CD8⁺ T cells orchestrated the local recruitment of lymph node-resident XCR1 chemokine receptor-expressing DCs via secretion of the XCL1 chemokine. Functionally, this CD8⁺ T cell-mediated reorganization of the local DC network allowed for the interaction and cooperation of pDCs and XCR1⁺ DCs, thereby optimizing XCR1⁺ DC maturation and cross-presentation. These data support a model in which CD8⁺ T cells upon activation create their own optimal priming microenvironment by recruiting additional DC subsets to the site of initial antigen recognition.

The Modified JiuWei QiangHuo Decoction Alleviated Severe Lung Injury Induced by H1N1 Influenza Virus by Regulating the NF- κ B Pathway in Mice

A new approach to treat infections of highly pathogenic influenza virus is to inhibit excessive innate immune response. JiuWei QiangHuo decoction has been used for centuries for the treatment of pulmonary disorders in China. In this study, we evaluated the anti-inflammatory activities of the modified JiuWei QiangHuo (MJWQH) decoction in the treatment of influenza A (H1N1) virus-induced severe pneumonia in mice. The results showed that MJWQH significantly increased the survival rate of H1N1-infected mice and suppressed the production of TNF-α, IL-1, IL-6, MCP-1, RANTES, and IFN-α on day 4 after infection. Moreover, oral administration of MJWQH efficiently inhibited virus replication and alleviated the severity of lung injuries. The results also showed that MJWQH may have potential therapeutic effect on severe lung injury induced by H1N1 virus by regulating the NF-κB pathway. Our study suggested that MJWQH might be an alternative therapy for the treatment of viral pneumonia.

Innate immune sensing and response to influenza

Influenza viruses pose a substantial threat to human and animal health worldwide. Recent studies in mouse models have revealed an indispensable role for the innate immune system in defense against influenza virus. Recognition of the virus by innate immune receptors in a multitude of cell types activates intricate signaling networks, functioning to restrict viral replication. Downstream effector mechanisms include activation of innate immune cells and, induction and regulation of adaptive immunity. However, uncontrolled innate responses are associated with exaggerated disease, especially in pandemic influenza virus infection. Despite advances in the understanding of innate response to influenza in the mouse model, there is a large knowledge gap in humans, particularly in immunocompromised groups such as infants and the elderly. We propose here, the need for further studies in humans to decipher the role of innate immunity to influenza virus, particularly at the site of infection. These studies will complement the existing work in mice and facilitate the quest to design improved vaccines and therapeutic strategies against influenza.

Enhanced immune response of BMDCs pulsed with H9N2 AIV and CpG

Dendritic cells (DCs), professional antigen presenting cells, have demonstrated effective in controlling the initial of innate immune, while CpG could improve the performance of immune system. To explore the mechanism of CpG enhancing the immune response, we compared different stimulated mouse DCs with systemic approach microarrays. Analysis revealed 1840 differentially expressed genes in H9N2 stimulated group, more than 1728 altered genes in inactive H9N2 group. Investigation also proved that CpG/inactive H9N2 co-stimulation changed 2140 genes, more than that in H9N2 group, strongly demonstrated that CpG improved the performance of inactive H9N2 vaccination. Pathways analysis founded that DCs response rapid to shift in their maturation state, which involved Toll-like receptor (TLR) pathway significantly. Microarrays results were also verified by qRT-PCR with 14 elected representative genes. Further analysis proved that co-stimulatory molecules (CD40, CD80, CD86 and MHC-II), regulatory protein (IRF-7 and TRAF-6) and pro-inflammatory cytokines (IL-1, IL-6 and IL-12) were all changed and involved in DCs maturation. At last we demonstrated TLR signalling pathway in chicken bone marrow-derived dendritic cells (chBM-DCs) stimulated with CpG. The distinct transcriptional profiles of DCs pulsed with various stimuli expanded our understanding of how DCs respond and recognize influenza.

ISG15 functions as an interferon-mediated antiviral effector early in the murine norovirus life cycle

Human noroviruses (HuNoV) are the leading cause of nonbacterial gastroenteritis worldwide. Similar to HuNoV, murine noroviruses (MNV) are enteric pathogens spread via the fecal-oral route and have been isolated from numerous mouse facilities worldwide. Type I and type II interferons (IFN) restrict MNV-1 replication; however, the antiviral effectors impacting MNV-1 downstream of IFN signaling are largely unknown. Studies using dendritic cells, macrophages, and mice deficient in free and conjugated forms of interferon-stimulated gene 15 (ISG15) revealed that ISG15 conjugation contributes to protection against MNV-1 both in vitro and in vivo. ISG15 inhibited a step early in the viral life cycle upstream of viral genome transcription. Directly transfecting MNV-1 RNA into IFN-stimulated mouse embryonic fibroblasts (MEFs) and bone marrow-derived dendritic cells (BMDC) lacking ISG15 conjugates bypassed the antiviral activity of ISG15, further suggesting that ISG15 conjugates restrict the MNV-1 life cycle at the viral entry/uncoating step. These results identify ISG15 as the first type I IFN effector regulating MNV-1 infection both in vitro and in vivo and for the first time implicate the ISG15 pathway in the regulation of early stages of MNV-1 replication.

IMPORTANCE

Type I IFNs are important in controlling murine norovirus 1 (MNV-1) infections; however, the proteins induced by IFNs that restrict viral growth are largely unknown. This report reveals that interferon-stimulated gene 15 (ISG15) mitigates MNV-1 replication both in vitro and in vivo. In addition, it shows that ISG15 inhibits MNV-1 replication by targeting an early step in the viral life cycle, MNV-1 entry and/or uncoating. These results identify ISG15 as the first type I IFN effector regulating MNV-1 infection both in vitro and in vivo and for the first time implicate the ISG15 pathway in the regulation of viral entry/uncoating.

Leucomycin A3, a 16-membered macrolide antibiotic, inhibits influenza A virus infection and disease progression

Severe respiratory disease arising from influenza virus infection has a high fatality rate. Neutrophil myeloperoxidase (MPO) has been implicated in the pathogenesis of severe influenza-induced pneumonia because extracellularly released MPO mediates the production of hypochlorous acid, a potent tissue injury factor. To search for candidate anti-influenza compounds, we screened leucomycin A3 (LM-A3), spiramycin (SPM), an erythromycin derivative (EM900, in which anti-bacterial activity has been eliminated), and clarithromycin (CAM), by analyzing their ability to inhibit MPO release in neutrophils from mice and humans. When each candidate was injected into mice infected with a lethal dose of A/H1N1 influenza virus (PR-8), LM-A3 produced the highest survival rate (80.9%). We found that LM-A3 induced beneficial effects on lung pathology and viral proliferation involved in the regulatory activity of MPO release, pro-inflammatory cytokines and interferon-α production in the lung. SPM and EM900 also induced positive survival effects in the infected mice, whereas CAM did not. We further found that these compounds inhibit virus proliferation in human pneumonia epithelial A549 cells in vitro. LM-A3 showed effective action against influenza A virus infection with high anti-viral activity in human host cells, indicating the possibility that LM-A3 is a prospective lead compound for the development of a drug for human influenza. The positive survival effect induced by EM900 suggests that pharmacological architectures between anti-bacterial and anti-influenza virus activities can be dissociated in macrolide derivatives. These observations provide valuable evidence for the potential development of novel macrolide derivatives that have strong anti-viral but no anti-bacterial activity.

Efficient influenza A virus replication in the respiratory tract requires signals from TLR7 and RIG-I

Induction of a proinflammatory response is the hallmark of host innate defense against invading pathogens. Host recognition of influenza A virus (IAV) infection relies on pattern-recognition receptors, including Toll-like receptor 7 (TLR7) and retinoic acid inducible gene-1 (RIG-I) for the activation of innate-immune responses. Here, we show that following a physiological low dose of IAV infection, viral sensing by either TLR7 or RIG-I induces a proinflammatory program that promotes viral replication. Transfer of bronchoalveolar lavage from infected wild-type mice into the airway of mice deficient in TLR7 and RIG-I pathways was sufficient to restore viral replication efficiency. Comparison of IAV-infected cells revealed that inflammatory mediators elicited by TLR7 and RIG-I signaling recruit viral target cells to the airway, thereby enhancing viral load within the respiratory tract. Our data suggest that IAV uses physiological levels of inflammatory responses for its replicative advantage and highlight the complex interplay between viruses and the host innate-immune responses.

Microbiota regulates the TLR7 signaling pathway against respiratory tract influenza A virus infection

Although intestinal flora are crucial in maintaining immune homeostasis of the intestine, the role of intestinal flora in immune responses at other mucosal surfaces remains less clear. Here, we show that intestinal flora composition critically regulates the toll-like receptor 7 (TLR7) signaling pathway following respiratory influenza virus infection. TLR7 ligands rescued the immune impairment in antibiotic-treated mice. Intact microbiota provided signals leading to the expression of mRNA for TLR7, MyD88, IRAK4, TRAF6, and NF-κB at steady state. Significant changes in the composition of culturable commensal bacteria reduced the expression levels of components of the TLR7 signaling pathway. Our results reveal the importance of intestinal flora in regulating immunity in the respiratory mucosa through the upregulation of the TLR7 signaling pathway for the proper activation of inflammasomes.

Pathogen-triggered activation of plasmacytoid dendritic cells induces IL-10-producing B cells in response to Staphylococcus aureus

Induction of polyclonal B cell activation is a phenomenon observed in many types of infection, but its immunological relevance is unclear. In this study we show that staphylococcal protein A induces T cell-independent human B cell proliferation by enabling uptake of TLR-stimulating nucleic acids via the V(H)3(+) BCR. We further demonstrate that Staphylococcus aureus strains with high surface protein A expression concomitantly trigger activation of human plasmacytoid dendritic cells (pDC). Sensitivity to chloroquine, cathepsin B inhibition, and a G-rich inhibitory oligodeoxynucleotide supports the involvement of TLR9 in this context. We then identify pDC as essential cellular mediators of B cell proliferation and Ig production in response to surface protein A-bearing S. aureus. The in vivo relevancy of these findings is confirmed in a human PBMC Nod/scid(Prkdc)/γc(-/-) mouse model. Finally, we demonstrate that co-operation of pDC and B cells enhances B cell-derived IL-10 production, a cytokine associated with immunosuppression and induction of IgG4, an isotype frequently dominating the IgG response to S. aureus. IL-10 release is partially dependent on TLR2-active lipoproteins, a hallmark of the Staphylococcus species. Collectively, our data suggest that S. aureus exploits pDC and TLR to establish B cell-mediated immune tolerance.

Mechanism of action of inhibition of allergic immune responses by a novel antedrug TLR7 agonist

Triggering innate immune responses through TLRs is expected to be a novel therapeutic strategy for the treatment of allergic diseases. TLR agonists are able to modulate Th2 immune responses through undefined mechanisms. We investigated the mechanism of action of the suppression of Th2 immune responses with a novel antedrug TLR7 agonist. The antedrug is rapidly metabolized by plasma esterases to an acid with reduced activity to limit systemic responses. Topical administration of this compound inhibited features of the allergic airway inflammatory response in rat and murine allergic airways model. Type I IFN played a role in the suppression of Th2 cytokines produced from murine splenocytes. Inhibition of Th2 immune responses with the antedrug TLR7 agonist was shown to be via a type I IFN-dependent mechanism following short-term exposure to the compound, although there might be type I IFN-independent mechanisms following long-term exposure. We have demonstrated that local type I IFN signaling and plasmacytoid dendritic cells, but not Th1 immune responses, are required for in vivo efficacy against murine airway Th2-driven eosinophilia. Furthermore, migration of dendritic cell subsets into the lung was related to efficacy and is dependent on type I IFN signaling. Thus, the mechanism of action at the cytokine and cellular level involved in the suppression of Th2 allergic responses has been characterized, providing a potential new approach to the treatment of allergic disease.

Study on the anti-H1N1 virus effects of quercetin and oseltamivir and their mechanism related to TLR7 pathway

The antivirus effect of quercetin and oseltamivir on the Toll-like receptor 7 (TLR7) signaling pathway was observed when dendritic cells and macrophages were infected with H1N1. Leukomonocytes were obtained from umbilical cord blood and harvested after stimulation by recombinant human Granulocyte-Macrophage Colony-Stimulating Factor (rhGM-CSF) and recombinant human Interleukin 4 (rhIL-4). Virus-infected cell model was established by human bronchial epithelial cells (16HBE) infected with H1N1. After immunological cells and virus-infected cells were co-cultured, quercetin and oseltamivir were also added into the medium as a treatment intervention. Then the immunological cells were collected for Real Time PCR (RT-PCR) and Western blot to determine the expression levels of genes related to TLR7 pathway. Viral infection led to cell death and increased the gene expression levels of TLR7 signal pathway. Quercetin and oseltamivir increased cell viability and reduced the expression levels of TLR7 signal pathway.

Sex differences in the response to viral infections: TLR8 and TLR9 ligand stimulation induce higher IL10 production in males

Background

Susceptibility to viral infections as well as their severity are higher in men than in women. Heightened antiviral responses typical of women are effective for rapid virus clearance, but if excessively high or prolonged, can result in chronic/inflammatory pathologies. We investigated whether this variability could be in part attributable to differences in the response to the Toll-Like Receptors (TLR) more involved in the virus recognition.

Methods

Cytokine production by peripheral blood mononuclear cells (PBMCs) from male and female healthy donors after stimulation with Toll-like receptors (TLR) 3, 7, 8, 9 ligands or with viruses (influenza and Herpes-simplex-1) was evaluated.

Results

Compared to females, PBMCs from males produced not only lower amounts of IFN-α in response to TLR7 ligands but also higher amounts of the immunosuppressive cytokine IL10 after stimulation with TLR8 and TLR9 ligands or viruses. IL10 production after TLR9 ligands or HSV-1 stimulation was significantly related with plasma levels of sex hormones in both groups, whereas no correlation was found in cytokines produced following TLR7 and TLR8 stimulation.

Conclusions

Given the role of an early production of IL10 by cells of innate immunity in modulating innate and adaptive immune response to viruses, we suggest that sex-related difference in its production following viral nucleic acid stimulation of TLRs may be involved in the sex-related variability in response to viral infections.

Lipid-derived nanoparticles for immunostimulatory RNA adjuvant delivery

The specific activation of Toll-like receptors (TLRs) has potential utility for a variety of therapeutic indications including antiviral immunotherapy and as vaccine adjuvants. TLR7 and TLR 8 may be activated by their native ligands, single-stranded RNA, or by small molecules of the imidazoquinoline family. However the use of TLR7/8 agonists for in vivo therapy is limited by instability, in the case of RNA, or systemic biodistribution and toxicity in the case of small molecule agonists. We hypothesized that unique lipid-like materials, termed "lipidoids," could be designed to efficiently deliver immunostimulatory RNA (isRNA) to TLR-expressing cells to drive innate and adaptive immune responses. A library of lipidoids was synthesized and screened for the ability to induce type I IFN activation in human peripheral blood mononuclear cells when combined with isRNA oligonucleotides. Effective lipidoid-isRNA nanoparticles, when tested in mice, stimulated strong IFN-α responses following subcutaneous injection, had robust antiviral activity that suppressed influenza virus replication, and enhanced antiovalbumin humoral and cell-mediated responses when used as a vaccine adjuvant. Further, we demonstrate that whereas all immunological activity was MyD88-dependent, certain materials were found to engage both TLR7-dependent and TLR7-independent activity in the mouse suggestive of cell-specific delivery. These lipidoid formulations, which are materials designed specifically for delivery of isRNA to Toll-like receptors, were superior to the commonly used N-[1-(2,3-dioleoyloxy)propyl]-N,N,N-trimethylammonium methylsulfate-RNA delivery system and may provide new tools for the manipulation of TLR responses in vitro and in vivo.

The human side of influenza

A clear understanding of immunity in individuals infected with influenza virus is critical for the design of effective vaccination and treatment strategies. Whereas myriad studies have teased apart innate and adaptive immune responses to influenza infection in murine models, much less is known about human immunity as a result of the ethical and technical constraints of human research. Still, these murine studies have provided important insights into the critical correlates of protection and pathogenicity in human infection and helped direct the human studies that have been conducted. Here, we examine and review the current literature on immunity in humans infected with influenza virus, noting evidence offered by select murine studies and suggesting directions in which future research is most warranted.

SHARPIN is essential for cytokine production, NF-κB signaling, and induction of Th1 differentiation by dendritic cells

Spontaneous mutations of the Sharpin (SHANK-associated RH domain-interacting protein, other aliases: Rbckl1, Sipl1) gene in mice result in systemic inflammation that is characterized by chronic proliferative dermatitis and dysregulated secretion of T helper1 (Th1) and Th2 cytokines. The cellular and molecular mechanisms underlying this inflammatory phenotype remain elusive. Dendritic cells may contribute to the initiation and progression of the phenotype of SHARPIN-deficient mice because of their pivotal role in innate and adaptive immunity. Here we show by flow cytometry that SHARPIN- deficiency did not alter the distribution of different DC subtypes in the spleen. In response to TOLL-like receptor (TLR) agonists LPS and poly I:C, cultured bone marrow-derived dendritic cells (BMDC) from WT and mutant mice exhibited similar increases in expression of co-stimulatory molecules CD40, CD80, and CD86. However, stimulated SHARPIN-deficient BMDC had reduced transcription and secretion of pro-inflammatory mediators IL6, IL12P70, GMCSF, and nitric oxide. Mutant BMDC had defective activation of NF-κB signaling, whereas the MAPK1/3 (ERK1/2) and MAPK11/12/13/14 (p38 MAP kinase isoforms) and TBK1 signaling pathways were intact. A mixed lymphocyte reaction showed that mutant BMDC only induced a weak Th1 immune response but stimulated increased Th2 cytokine production from allogeneic naïve CD4(+) T cells. In conclusion, loss of Sharpin in mice significantly affects the immune function of DC and this may partially account for the systemic inflammation and Th2-biased immune response.

Antiviral applications of Toll-like receptor agonists

In the past, antiviral research has focused mainly on viral targets. As the search for effective and differentiated antiviral therapies continues, cellular targets are becoming more common, bringing with them a variety of challenges and concerns. Toll-like receptors (TLRs) provide a unique mechanism to induce an antiviral state in the host. In this review we introduce TLRs as targets for the pharmaceutical industry, including how they signal and thereby induce an antiviral state through the production of type I interferons. We examine how TLRs are being therapeutically targeted and discuss several clinically precedented agents for which efficacy and safety data are available. We describe some of the chemistries that have been applied to both small molecule and large molecule leads to tune agonist potency, and offer a differentiated safety profile through targeting certain compartments such as the gut or the lung, thereby limiting systemic drug exposure and affecting systemic cytokine levels. The application of low-dose agonists of TLRs as vaccine adjuvants or immunoprotective agents is also presented. Some of the challenges presented by this approach are then discussed, including viral evasion strategies and mechanism-linked inflammatory cytokine induction.

Host gene targets for novel influenza therapies elucidated by high-throughput RNA interference screens

Influenza virus encodes only 11 viral proteins but replicates in a broad range of avian and mammalian species by exploiting host cell functions. Genome-wide RNA interference (RNAi) has proven to be a powerful tool for identifying the host molecules that participate in each step of virus replication. Meta-analysis of findings from genome-wide RNAi screens has shown influenza virus to be dependent on functional nodes in host cell pathways, requiring a wide variety of molecules and cellular proteins for replication. Because rapid evolution of the influenza A viruses persistently complicates the effectiveness of vaccines and therapeutics, a further understanding of the complex host cell pathways coopted by influenza virus for replication may provide new targets and strategies for antiviral therapy. RNAi genome screening technologies together with bioinformatics can provide the ability to rapidly identify specific host factors involved in resistance and susceptibility to influenza virus, allowing for novel disease intervention strategies.

Toll like receptors in diseases of the lung

The lung is in continuous contact with a diverse array of infectious agents, foreign antigens, and host-derived danger signals. To sample this expansive internal and external milieu, both resident myeloid and stromal/structure cells of the lung express a full complement of toll like receptors (TLRs) which recognize pathogen-associated molecular patterns (PAMPs) and endogenous danger-associated molecular patterns (DAMPs). TLRs play a vital role in immune host defense against bacterial, mycobacterial, fungal, and viral pathogens of the lung. Additionally, TLRs contribute to disease pathogenesis in non-infectious pulmonary disorders, including airway disease, acute lung injury, and interstitial lung disease. In this review, TLR biology in the context of experimental infectious and non-infectious lung disease is discussed, and correlates to human lung disease, including therapeutic implications of these findings, are defined.

Defining the anatomical localisation of subsets of the murine mononuclear phagocyte system using integrin alpha X (Itgax, CD11c) and colony stimulating factor 1 receptor (Csf1r, CD115) expression fails to discriminate dendritic cells from macrophages

The murine mononuclear phagocyte (MNP) system comprises a diverse population of cells, including monocytes, dendritic cells (DC) and macrophages. Derived from the myeloid haematopoietic lineage, this group of cells express a variety of well characterized surface markers. Expression of the integrin alpha X (Itgax, CD11c) is commonly used to identify classical DC, and similarly expression of colony stimulating factor 1 receptor (Csf1r, CD115) to identify macrophages. We have characterized the expression of these markers using a variety of transgenic mouse models. We confirmed previous observations of Itgax expression in anatomically defined subsets of MNPs in secondary lymphoid organs, including all MNPs identified within the germinal centres. The majority of MNPs in the intestinal lamina propria and lung express Itgax. All mucosal Itgax expressing cells also express Csf1r suggesting Csf1-dependent haematopoietic derivation. This double-positive population included germinal centre MNPs. These data reveal that Itgax expression alone does not specifically define classical DC. These results suggest more cautious interpretation of Itgax-dependent experimentation and direct equation with uniquely DC-mediated activities, particularly in the functioning of non-lymphoid MNPs within the intestinal lamina propria.

TLR7/9 versus TLR3/MDA5 signaling during virus infections and diabetes

IFN-I are pleiotropic cytokines that impact innate and adaptive immune responses. In this article, we discuss TLR7/9 versus TLR3/MDA5 signaling in antiviral responses and diabetes. pDCs are thought to have a critical role in antiviral defense because of their ability to rapidly secrete large amounts of IFN-I through TLR7/9 signaling. A recent study demonstrates that although pDCs are a source of IFN-I in vivo, their overall contribution to viral containment is limited and time-dependent, such that additional cellular sources of IFN-I are required to fully control viral infections. dsRNA sensors, such as TLR3 and MDA5, provide another important trigger for antiviral IFN-I responses, which can be exploited to enhance immune responses to vaccines. In the absence of infection, IFN-I production by pDCs or from signaling through dsRNA sensors has been implicated in the pathogenesis of autoimmune diseases such as diabetes. However, recent data demonstrate that IFN-I production via TLR3 and MDA5 is critical to counter diabetes caused by a virus with preferential tropism for pancreatic β-cells. This highlights the complexity of the host antiviral response and how multiple cellular and molecular components balance protective versus pathological responses.

Toll-Like Receptors Dysregulation after Influenza Virus Infection: Insights into Pathogenesis of Subsequent Bacterial Pneumonia

The innate immune system utilizes an intricate network to aid in fighting foreign invaders. Recent insight and understanding of toll-like receptors (TLRs) has been critical in providing key information about early responses to infection, and more recently, understanding dysregulation of TLRs has shed light on pathogenic states. This paper addresses the importance of innate immunity and TLR regulation of immune responses to the presence of influenza infection and its role in the subsequent bacterial infections.

Plasmacytoid dendritic cells promote host defense against acute pneumovirus infection via the TLR7-MyD88-dependent signaling pathway

Human respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract infection in infants. In human infants, plasmacytoid dendritic cells (pDC) are recruited to the nasal compartment during infection and initiate host defense through the secretion of type I IFN, IL-12, and IL-6. However, RSV-infected pDC are refractory to TLR7-mediated activation. In this study, we used the rodent-specific pathogen, pneumonia virus of mice (PVM), to determine the contribution of pDC and TLR7 signaling to the development of the innate inflammatory and early adaptive immune response. In wild-type, but not TLR7- or MyD88-deficient mice, PVM inoculation led to a marked infiltration of pDC and increased expression of type I, II, and III IFNs. The delayed induction of IFNs in the absence of TLR7 or MyD88 was associated with a diminished innate inflammatory response and augmented virus recovery from lung tissue. In the absence of TLR7, PVM-specific CD8(+) T cell cytokine production was abrogated. The adoptive transfer of TLR7-sufficient, but not TLR7-deficient pDC to TLR7 gene-deleted mice recapitulated the antiviral responses observed in wild-type mice and promoted virus clearance. In summary, TLR7-mediated signaling by pDC is required for appropriate innate responses to acute pneumovirus infection. It is conceivable that as-yet-unidentified defects in the TLR7 signaling pathway may be associated with elevated levels of RSV-associated morbidity and mortality among otherwise healthy human infants.

Bovine plasmacytoid dendritic cells are the major source of type I interferon in response to foot-and-mouth disease virus in vitro and in vivo

Type I interferons (alpha/beta interferons [IFN-α/β]) are the main innate cytokines that are able to induce a cellular antiviral state, thereby limiting viral replication and disease pathology. Plasmacytoid dendritic cells (pDCs) play a crucial role in the control of viral infections, especially in response to viruses that have evolved mechanisms to block the type I IFN signal transduction pathway. Using density gradient separation and cell sorting, we have highly enriched a population of bovine cells capable of producing high levels of biologically active type I IFN. These cells represented less than 0.1% of the total lymphocyte population in blood, pseudoafferent lymph, and lymph nodes. Phenotypic analysis identified these cells as bovine pDCs (CD3(-) CD14(-) CD21(-) CD11c(-) NK(-) TCRδ(-) CD4(+) MHC II(+) CD45RB(+) CD172a(+) CD32(+)). High levels of type I IFN were generated by these cells in vitro in response to Toll-like receptor 9 (TLR-9) agonist CpG and foot-and-mouth disease virus (FMDV) immune complexes. In contrast, immune complexes formed with UV-inactivated FMDV or FMDV empty capsids failed to elicit a type I IFN response. Depletion of CD4 cells in vivo resulted in levels of type I IFN in serum early during FMDV infection that were significantly lower than those for control animals. In conclusion, pDCs interacting with immune-complexed virus are the major source of type I interferon production during acute FMDV infection in cattle.

Hepatitis C Virus Evasion from RIG-I-Dependent Hepatic Innate Immunity

Exposure to hepatitis C virus (HCV) usually results in persistent infection that often develops into chronic liver disease. Interferon-alpha (IFN) treatment comprises the foundation of current approved therapy for chronic HCV infection but is limited in overall efficacy. IFN is a major effector of innate antiviral immunity and is naturally produced in response to viral infection when viral pathogen-associated molecular patterns (PAMPs) are recognized as nonself and are bound by cellular pathogen recognition receptors (PRRs), including Toll-like receptors (TLRs) and the RIG-I-like receptors (RLRs). Within hepatocytes, RIG-I is a major PRR of HCV infection wherein PAMP interactions serve to trigger intracellular signaling cascades in the infected hepatocyte to drive IFN production and the expression of interferon-stimulated genes (ISGs). ISGs function to limit virus replication, modulate the immune system, and to suppress virus spread. However, studies of HCV-host interactions have revealed several mechanisms of innate immune regulation and evasion that feature virus control of PRR signaling and regulation of hepatic innate immune programs that may provide a molecular basis for viral persistence.

Respiratory virus-induced TLR7 activation controls IL-17-associated increased mucus via IL-23 regulation

The response to respiratory syncytial virus (RSV), negative strand ssRNA virus, depends upon the ability to recognize specific pathogen-associated targets. In the current study, the role of TLR7 that recognizes ssRNA was examined. Using TLR7(-/-) mice, we found that the response to RSV infection in the lung was more pathogenic as assessed by significant increases in inflammation and mucus production. Although there appeared to be no effect of TLR7 deficiency on type I IFN, the pathology was associated with an alteration in T cell responses with increases in mucogenic cytokines IL-4, IL-13, and IL-17. Examination of dendritic cells from TLR7(-/-) animals indicated a preferential activation of IL-23 (a Th17-promoting cytokine) and a decrease in IL-12 production. Neutralization of IL-17 in the TLR7(-/-) mice resulted in a significant decrease in the mucogenic response in the lungs of the RSV-infected mice. Thus, without TLR7-mediated responses, an altered immune environment ensued with a significant effect on airway epithelial cell remodeling and goblet cell hyper/metaplasia, leading to increased mucus production.

Dendritic cells in viral bronchiolitis

Dendritic cells (DCs) are major antigen-presenting cells that constitute a link between innate and adaptive immune responses, and are critical in the processes of control and elimination of viral infections. On the other hand, there is a large body of data strongly implicating respiratory viruses in morbidity during infancy through the induction of lower respiratory tract infections, such as bronchiolitis, and later on in childhood and adult life, mainly due to their association with asthma exacerbations. Little is known, however, about the precise role of DCs in human respiratory tract infections. This review focuses on current data, both from in vivo and in vitro studies, that highlight the interplay between DCs and the viral causes of bronchiolitis.

Evasion of innate and adaptive immune responses by influenza A virus

Host organisms have developed sophisticated antiviral responses in order to defeat emerging influenza A viruses (IAVs). At the same time IAVs have evolved immune evasion strategies. The immune system of mammals provides several lines of defence to neutralize invading pathogens or limit their replication. Here, we summarize the mammalian innate and adaptive immune mechanisms involved in host defence against viral infection and review strategies by which IAVs avoid, circumvent or subvert these mechanisms. We highlight well-characterized, as well as recently described features of this intriguing virus-host molecular battle.

Influenza viruses control the vertebrate type I interferon system: factors, mechanisms, and consequences

Virus research of the past decades showed that the vertebrate type I interferon (IFN) system is in principle capable of slowing down the replication and spread of most if not all viruses at early stages of infection. However, influenza viruses and other viral pathogens have evolved gene products to subvert this innate defense to enable efficient reproduction and thereby cause disease. Inhibition of type I IFN also impairs adaptive immune reactions as those cytokines function in the development of pathogen-specific cellular immunity. This article highlights the recent progress in our understanding of key interactions of influenza viruses with the type I IFN defense, which are central to viral virulence. A main focus is on corruptions of IFN-dependent antiviral functions mediated by the cellular receptors RIG-I and PKR and their blockade by the viral NS1 protein.

Dendritic Cells in Innate and Adaptive Immune Responses against Influenza Virus

Dendritic cells (DC) are major players in both innate and adaptive immune responses against influenza virus. These immune responses, as well as the important interface between the innate and adaptive systems, are orchestrated by specialized subsets of DC, including conventional steady-state DC, migratory DC and plasmacytoid DC. The characteristics and efficacy of the responses are dependent on the relative activity of these DC subsets, rendering DC crucial for the development of both naïve and memory immune responses. However, due to their critical role, DC also contribute to the immunopathological processes observed during acute influenza, such as that caused by the pathogenic H5N1 viruses. Therein, the role of different DC subsets in the induction of interferon type I, pro-inflammatory cytokine and chemokine responses is important for the outcome of interaction between the virus and host immune defences. The present review will present current knowledge on this area, relating to the importance of DC activity for the induction of efficacious humoral and cell-mediated immune responses. This will include the main viral elements associated with the triggering or inhibition of DC activation. Finally, the current knowledge on understanding how differences in various vaccines influence the manner of immune defence induction will be presented.

Co-ordinating innate and adaptive immunity to viral infection: mobility is the key

The host counters a viral infection through a complex response made up of components belonging to both the innate and the adaptive immune system. In this report, we review the mechanisms underlying this response, how it is induced and how it is co-ordinated. As cell-cell communication represents the very essence of immune system physiology, a key to a rapid, efficient and optimally regulated immune response is the ability of the involved cells to rapidly shift between a stationary and a mobile state, combined with stringent regulation of cell migration during the mobile state. Through the co-ordinated recruitment of different cell types intended to work in concert, cellular co-operation is optimized particularly under conditions that may involve rare cells. Consequently, a major focus is placed on presenting an overview of the co-operative events and the associated cell migration, which is essential in mounting an efficient host response and co-ordinating innate and adaptive immunity during a primary viral infection.

Sabotage of antiviral signaling and effectors by influenza viruses

Vertebrate cells activate multiple signaling modules upon virus infection to eliminate the invading pathogen and to prevent the establishment of a persistent infection. A major immediate response pathway is controlled by the RNA helicases RIG-I and MDA5, which, after recognition of viral nucleic acids, signal induction of the interferon (IFN)-alpha/beta cytokine family that upregulates numerous antiviral effector proteins. Virulent viruses, in contrast, have learned during co-evolution with their hosts to manipulate or avoid this response in order to prevail in a repulsive environment. Focusing on the influenza viruses and their IFN-antagonistic NS1 proteins, we summarize recent progress in this rapidly evolving field at the intersection of virology and immunobiology involving studies of how viral pathogens induce and sabotage cellular defenses.

Regulation of T-helper-cell lineage development by osteopontin: the inside story

Studies of osteopontin (OPN)-dependent regulation of immune responses have focused on the cytokine activities of the secreted form of this protein. Recent evidence has revealed that an intracellular form of OPN expressed by dendritic cells regulates the expression of pro-inflammatory cytokines and the differentiation of T helper (T(H))-cell lineages. In this Opinion article, we discuss the properties of both OPN isoforms and their respective contributions to the immune response. We propose that cell-type-specific expression of secreted and intracellular OPN regulates the development of distinct effector T(H) cells, including that of T(H)1 and T(H)17 cells.

Role of T cells for cytokine production and outcome in a model of acute septic peritonitis

Although it is generally accepted that early defense mechanisms are controlled by cells of the innate immune system, T cells were found to be crucial for host resistance against acute septic peritonitis. However, the mechanisms by which T cells mediate protection are not fully understood. Here, we demonstrate mice deficient for recombinase-activating gene (RAG) 1, which lack mature B and T cells, showed enhanced susceptibility and impaired bacterial clearance in a model of acute septic peritonitis. Whereas B-cell-deficient muMT mice showed no significant difference in the survival rate after peritonitis induction, T-cell-deficient Balb/c nude mice exhibited reduced survival. Importantly, analysis of cytokine production in both RAG-1-deficient and T-cell-deficient nude mice indicated strongly attenuated production of IL-12, interferon (IFN) gamma, and IL-10 during sepsis. Reduced cytokine levels were detected both in serum and in organ extracts of septic mice. Direct analysis of T cells isolated from septic mice demonstrated that T cells respond to an acute septic challenge by increased production of IFN-gamma and IL-10. Moreover, bacterial numbers in spleens of septic RAG-1-deficient mice were significantly increased as compared with controls, suggesting that T cells are engaged in the early antibacterial immune defense during sepsis, possibly via the production of IFN-gamma. In summary, these results imply that T cells contribute to protective immune responses against acute systemic infections via their ability to produce crucial immune mediators.

Staphylococcus aureus-induced plasmacytoid dendritic cell activation is based on an IgG-mediated memory response

Type I IFNs represent a major antimicrobial defense mechanism due to their property of enhancing immune responses by priming both innate and adaptive immune cells. Plasmacytoid dendritic cells (pDC) are the major source of type I IFN in the human body and represent innate immune cells involved in first-line defense against invading pathogens. Although pDC activation has been extensively studied upon stimulation with synthetic TLR ligands, viruses, and intracellular bacteria, there is only scarce information on extracellular bacteria. In this study we show that the triggering of human pDC-derived IFN-alpha secretion by Staphylococcus aureus is independent of TLR2 and specific for coagulase-positive staphylococci. Specificity of the pDC response to S. aureus is independent of the bacterial virulence factors protein A and alpha-toxin but is mediated by Ag-specific IgG and CD32. S. aureus-induced pDC activation can be blocked by inhibitory DNA oligonucleotides and chloroquine, suggesting that engagement of TLR7/9 by bacterial nucleic acids after CD32-mediated uptake of these compounds may play a central role in this process. Altogether, we propose that in marked contrast to nonselective TLR2-dependent activation of most innate immune cells, pDC activation by S. aureus represents an Ag-specific memory response since it requires the presence of class-switched immunoglobulins.

Superior immunogenicity of inactivated whole virus H5N1 influenza vaccine is primarily controlled by Toll-like receptor signalling

In the case of an influenza pandemic, the current global influenza vaccine production capacity will be unable to meet the demand for billions of vaccine doses. The ongoing threat of an H5N1 pandemic therefore urges the development of highly immunogenic, dose-sparing vaccine formulations. In unprimed individuals, inactivated whole virus (WIV) vaccines are more immunogenic and induce protective antibody responses at a lower antigen dose than other formulations like split virus (SV) or subunit (SU) vaccines. The reason for this discrepancy in immunogenicity is a long-standing enigma. Here, we show that stimulation of Toll-like receptors (TLRs) of the innate immune system, in particular stimulation of TLR7, by H5N1 WIV vaccine is the prime determinant of the greater magnitude and Th1 polarization of the WIV-induced immune response, as compared to SV- or SU-induced responses. This TLR dependency largely explains the relative loss of immunogenicity in SV and SU vaccines. The natural pathogen-associated molecular pattern (PAMP) recognized by TLR7 is viral genomic ssRNA. Processing of whole virus particles into SV or SU vaccines destroys the integrity of the viral particle and leaves the viral RNA prone to degradation or involves its active removal. Our results show for a classic vaccine that the acquired immune response evoked by vaccination can be enhanced and steered by the innate immune system, which is triggered by interaction of an intrinsic vaccine component with a pattern recognition receptor (PRR). The insights presented here may be used to further improve the immune-stimulatory and dose-sparing properties of classic influenza vaccine formulations such as WIV, and will facilitate the development of new, even more powerful vaccines to face the next influenza pandemic.

The rise and spread of the highly pathogenic avian H5N1 influenza virus has seriously increased the risk of a new influenza pandemic. However, the number of vaccine doses that can be produced with today's production capacity will fall short of the demand in times of a pandemic. Use of inactivated whole virus (WIV) vaccines, which are more immunogenic than split virus or subunit vaccines in an unprimed population, could contribute to a dose-sparing strategy. Yet, the mechanisms underlying the superior immunogenicity of WIV vaccine formulations are unknown. Here, we demonstrate that the viral RNA present in inactivated virus particles is crucial for the improved immunogenic properties of WIV in mice. By triggering Toll-like receptor 7 (TLR7), the viral RNA activates innate immune mechanisms that augment and determine subsequent adaptive responses. Efficient TLR7 signalling is lost in split virus and subunit vaccines with the processing steps that lead to disruption of the integrity of the virus particle and exclusion of the RNA. Our results prove for the first time to our knowledge that the immune-potentiating mechanism of a classic vaccine is based on activation of the innate immune system by one of its structural components. These findings may reflect a general principle for viral vaccines and provide a rational basis for further improvement of influenza vaccines, which are urgently needed in the face of the current H5N1 pandemic threat.

Influenza-induced expression of functional tumor necrosis factor-related apoptosis-inducing ligand on human peripheral blood mononuclear cells

The immunologic response to influenza virus infection, like many other viruses, is characterized by robust production of proinflammatory cytokines, including type I and II interferon (IFN), which induce a number of antiviral effects and are essential for priming the innate and adaptive cellular components of the immune response. Here, we demonstrate that influenza virus infection induces the expression of functional tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) on human peripheral blood mononuclear cell (PBMC) populations. Consistent with previous studies examining TRAIL upregulation, increased TRAIL expression correlated with increased type I and II IFN levels in PBMC cultures. Interestingly, dilution of these cytokines resulted in decreased expression of TRAIL. TRAIL upregulation was not dependent on active viral infection, and TRAIL was observed on NS-1-negative cells. Furthermore, influenza virus infection of lung adenocarcinoma cells (A549) resulted in increased sensitization to TRAIL-induced apoptosis compared with uninfected A549. Infected PBMC expressing TRAIL preferentially killed infected A549, but did not affect uninfected cells, and the addition of soluble TRAIL-R2:Fc blocked the lysis of infected cells, demonstrating TRAIL-dependent killing of infected cells. Collectively, these data demonstrate that TRAIL expression is induced on primary human innate and adaptive immune cells in response to cytokines produced during influenza infection and that TRAIL sensitivity is increased in influenza virus-infected cells. These data also suggest that TRAIL is a primary mechanism used by influenza-stimulated human PBMC to kill influenza-infected target cells and reinforce the importance of cytokines produced in response to TLR agonists in enhancing cellular immune effector functions.

Accessing the therapeutic potential of immunostimulatory nucleic acids

Short synthetic CpG oligodesoxynucleotides (CpG ODN) that activate plasmacytoid dendritic cells (PDC) and B-cells via the endosomal Toll-like receptor (TLR) 9 are the prototypical immunostimulatory nucleic acids (NAs), and their therapeutic potential is currently evaluated in numerous clinical trials. In recent years, NA recognition has emerged as a general means of virus detection by the innate immune system that involves at least seven receptors located in either the endosome or the cytosol. Endosomal TLRs are expressed selectively, and predominantly by immune cell types. By contrast, the expression of cytosolic NA receptors is ubiquitous, significantly widening the range of cell types that can be stimulated therapeutically by NAs to include even tumor cells. Here we discuss unique properties of each of these receptors, and argue that an understanding of the molecular basis of receptor-ligand interactions, and the development of chemically defined, selective ligands is required in order to fully realize the promise that NA immunetherapeutics hold.

MDA-5 recognition of a murine norovirus

Noroviruses are important human pathogens responsible for most cases of viral epidemic gastroenteritis worldwide. Murine norovirus-1 (MNV-1) is one of several murine noroviruses isolated from research mouse facilities and has been used as a model of human norovirus infection. MNV-1 infection has been shown to require components of innate and adaptive immunity for clearance; however, the initial host protein that recognizes MNV-1 infection is unknown. Because noroviruses are RNA viruses, we investigated whether MDA5 and TLR3, cellular sensors that recognize dsRNA, are important for the host response to MNV-1. We demonstrate that MDA5−/− dendritic cells(DC) have a defect in cytokine response to MNV-1. In addition, MNV-1 replicates to higher levels in MDA5−/− DCs as well as in MDA5−/− mice in vivo. Interestingly, TLR3−/− DCs do not have a defect in vitro, but TLR3−/− mice have a slight increase in viral titers. This is the first demonstration of an innate immune sensor for norovirus and shows that MDA5 is required for the control of MNV-1 infection. Knowledge of the host response to MNV-1 may provide keys for prevention and treatment of the human disease.

Gastroenteritis is a common disease in both developed and developing countries. The two main causes of this affliction are bacteria and viruses. The primary viruses implicated in gastroenteritis have been shown to be noroviruses, which include Norwalk virus, notorious for numerous recent outbreaks on cruise ships. We are interested in how the innate immune system detects viral infection and prepares the host to respond to the threat. To understand how the host responds to norovirus infection, we studied two classes of proteins, both of which are thought to detect signs of viral infection. We discovered that one of these proteins, melanoma differentiation associated protein-5 (MDA-5), is responsible for detecting a mouse norovirus that is genetically related to the human pathogen. These findings allow us to better understand the pathogenesis of norovirus infection and may provide clues for controlling the human disease.

Toll-like receptor-mediated activation of neutrophils by influenza A virus

Influenza virus infection of the respiratory tract is characterized by a neutrophil infiltrate accompanied by inflammatory cytokine and chemokine production. We and others have reported that Toll-like receptor (TLR) proteins are present on human neutrophils and that granulocyte-macrophage colony-stimulating factor (GM-CSF) treatment enhances IL-8 (CXCL8) secretion in response to stimulation with TLR ligands. We demonstrate that influenza virus can induce IL-8 and other inflammatory cytokines from GM-CSF-primed human neutrophils. Using heat inactivation of influenza virus, we show that viral entry but not replication is required for cytokine induction. Furthermore, endosomal acidification and viral uncoating are necessary. Finally, using single-cell analysis of intracellular cytokine accumulation in neutrophils from knockout mice, we prove that TLR7 is essential for influenza viral recognition and inflammatory cytokine production by murine neutrophils. These studies demonstrate neutrophil activation by influenza virus and highlight the importance of TLR7 and TLR8 in that response.