Differential role for TLR3 in respiratory syncytial virus-induced chemokine expression

Genetic replacement of surfactant protein-C reduces respiratory syncytial virus induced lung injury

Background

Individuals with deficiencies of pulmonary surfactant protein C (SP-C) develop interstitial lung disease (ILD) that is exacerbated by viral infections including respiratory syncytial virus (RSV). SP-C gene targeted mice (Sftpc -/-) lack SP-C, develop an ILD-like disease and are susceptible to infection with RSV.

Methods

In order to determine requirements for correction of RSV induced injury we have generated compound transgenic mice where SP-C expression can be induced on the Sftpc -/- background (SP-C/Sftpc -/-) by the administration of doxycycline (dox). The pattern of induced SP-C expression was determined by immunohistochemistry and processing by Western blot analysis. Tissue and cellular inflammation was measured following RSV infection and the RSV-induced cytokine response of isolated Sftpc +/+ and -/- type II cells determined.

Results

After 5 days of dox administration transgene SP-C mRNA expression was detected by RT-PCR in the lungs of two independent lines of bitransgenic SP-C/Sftpc -/- mice (lines 55.3 and 54.2). ProSP-C was expressed in the lung, and mature SP-C was detected by Western blot analysis of the lavage fluid from both lines of SP-C/Sftpc -/- mice. Induced SP-C expression was localized to alveolar type II cells by immunostaining with an antibody to proSP-C. Line 55.3 SP-C/Sftpc -/- mice were maintained on or off dox for 7 days and infected with 2.6x10⁷ RSV pfu. On day 3 post RSV infection total inflammatory cell counts were reduced in the lavage of dox treated 55.3 SP-C/Sftpc -/- mice (p = 0.004). The percentage of neutrophils was reduced (p = 0.05). The viral titers of lung homogenates from dox treated 55.3 SP-C/Sftpc -/- mice were decreased relative to 55.3 SP-C/Sftpc -/- mice without dox (p = 0.01). The cytokine response of Sftpc -/- type II cells to RSV was increased over that of Sftpc +/+ cells.

Conclusions

Transgenic restoration of SP-C reduced inflammation and improved viral clearance in the lungs of SP-C deficient mice. The loss of SP-C in alveolar type II cells compromises their response to infection. These findings show that the restoration of SP-C in Sftpc -/- mice in response to RSV infection is a useful model to determine parameters for therapeutic intervention.

Learning from the messengers: innate sensing of viruses and cytokine regulation of immunity - clues for treatments and vaccines

Virus infections are a major global public health concern, and only via substantial knowledge of virus pathogenesis and antiviral immune responses can we develop and improve medical treatments, and preventive and therapeutic vaccines. Innate immunity and the shaping of efficient early immune responses are essential for control of viral infections. In order to trigger an efficient antiviral defense, the host senses the invading microbe via pattern recognition receptors (PRRs), recognizing distinct conserved pathogen-associated molecular patterns (PAMPs). The innate sensing of the invading virus results in intracellular signal transduction and subsequent production of interferons (IFNs) and proinflammatory cytokines. Cytokines, including IFNs and chemokines, are vital molecules of antiviral defense regulating cell activation, differentiation of cells, and, not least, exerting direct antiviral effects. Cytokines shape and modulate the immune response and IFNs are principle antiviral mediators initiating antiviral response through induction of antiviral proteins. In the present review, I describe and discuss the current knowledge on early virus–host interactions, focusing on early recognition of virus infection and the resulting expression of type I and type III IFNs, proinflammatory cytokines, and intracellular antiviral mediators. In addition, the review elucidates how targeted stimulation of innate sensors, such as toll-like receptors (TLRs) and intracellular RNA and DNA sensors, may be used therapeutically. Moreover, I present and discuss data showing how current antimicrobial therapies, including antibiotics and antiviral medication, may interfere with, or improve, immune response.

Respiratory syncytial virus infection: mechanisms of redox control and novel therapeutic opportunities

Respiratory syncytial virus (RSV) is one of the most important causes of upper and lower respiratory tract infections in infants and young children, for which no effective treatment is currently available. Although the mechanisms of RSV-induced airway disease remain incompletely defined, the lung inflammatory response is thought to play a central pathogenetic role. In the past few years, we and others have provided increasing evidence of a role of reactive oxygen species (ROS) as important regulators of RSV-induced cellular signaling leading to the expression of key proinflammatory mediators, such as cytokines and chemokines. In addition, RSV-induced oxidative stress, which results from an imbalance between ROS production and airway antioxidant defenses, due to a widespread inhibition of antioxidant enzyme expression, is likely to play a fundamental role in the pathogenesis of RSV-associated lung inflammatory disease, as demonstrated by a significant increase in markers of oxidative injury, which correlate with the severity of clinical illness, in children with RSV infection. Modulation of ROS production and oxidative stress therefore represents a potential novel pharmacological approach to ameliorate RSV-induced lung inflammation and its long-term consequences.

Innate immune responses to respiratory syncytial virus infection

The innate immune response has a critical role in the initial stages of respiratory syncytial virus (RSV) infection and provides important instructional control that determines the direction of the acquired immune response and the severity of subsequent disease. Contributions to innate immunity include responses initiated in epithelial cells, dendritic cells, and macrophages. The initiation and the intensity of the response depends upon the recognition of pathogen-associated molecular patterns (PAMPs) that activate various pattern recognition receptors (PRRs) such as toll-like receptors (TLR), RIG-I-like receptors (RLR), and NOD-like receptors (NLR), that induce innate cytokines and chemokines that promote inflammation and direct the recruitment of immune cells as well as promote anti-viral responses. In this review, we summarize the results of numerous studies that have characterized the innate immune responses that contribute to the RSV-induced responses and may be important considerations for the development of efficacious vaccine strategies.

Advances in understanding respiratory syncytial virus infection in airway epithelial cells and consequential effects on the immune response

This article reviews aspects of respiratory syncytial virus (RSV) infection in airway epithelial cells (AECs), including cytopathogenesis, entry, replication and the induction of immune response to the virus, including a new role for thymic stromal lymphopoietin in RSV immunopathology.

The role of UNC93B1 protein in surface localization of TLR3 receptor and in cell priming to nucleic acid agonists

Translocation of nucleic acid-sensing (NAS) Toll-like receptors (TLRs) to endosomes is essential for response to microbial nucleic acids as well as for prevention of the autoimmune response. The accessory protein UNC93B1 is indispensable for activation of NAS TLRs because it regulates their response through trafficking to endosomes. We observed that poly(I:C) up-regulates transcription of UNC93B1 and promotes trafficking of TLR3 to the plasma membrane in human epithelial cell line. Up-regulation of UNC93B1 is triggered through TLR3 activation by poly(I:C). Further studies revealed that expression of UNC93B1 promotes trafficking of differentially glycosylated TLR3, but not other NAS TLRs, to the plasma membrane. UNC93B1 promoter region contains binding sites for poly(I:C)- and type I interferon-inducible regulatory elements. UNC93B1 also increases the protein lifetime of TLR3 and TLR9 and augments signaling of all NAS TLRs. Furthermore, we discovered that poly(I:C) pretreatment primes B-cells to the activation by ssDNA via up-regulation of UNC93B1. Our findings identified TLR3 as the important regulator of UNC93B1 that in turn governs the responsiveness of all NAS TLRs.

Respiratory syncytial virus infection augments NOD2 signaling in an IFN-β-dependent manner in human primary cells

Respiratory syncytial virus (RSV) is a major cause of lower respiratory tract infections in infants, with remarkable variability in disease severity. An exaggerated proinflammatory response and influx of leukocytes is part of the pathogenesis of severe RSV disease. Here, we show an increase in proinflammatory cytokine production by human immune cells after stimulation with RSV and muramyl dipeptide (MDP), which is recognized by nucleotide-binding oligomerization domain containing 2 (NOD2). PBMCs from Crohn's disease patients homozygous for the 3020insC mutation in the NOD2 gene did not show a synergistic response to stimulation with RSV and MDP, suggesting that NOD2 is essential for the observed synergy. Further experiments aimed at identifying the viral ligand indicated that viral RNA plays an essential role in the recognition of RSV. Stimulation with RSV or Poly(I:C) induced IFN-β expression, which resulted in an increased expression of the viral receptors TLR3 and RIG-I, as well as an increased NOD2 expression. Our data indicate that IFN-β induction by viral RNA is an essential first step in the increased proinflammatory response to MDP. We hypothesize that the enhanced proinflammatory response to MDP following RSV infection may be an important factor in determining the outcome of the severity of disease.

Preexposure to CpG protects against the delayed effects of neonatal respiratory syncytial virus infection

Severe respiratory viral infection in early life is associated with recurrent wheeze and asthma in later childhood. Neonatal immune responses tend to be skewed toward T helper 2 (Th2) responses, which may contribute to the development of a pathogenic recall response to respiratory infection. Since neonatal Th2 skewing can be modified by stimulation with Toll-like receptor (TLR) ligands, we investigated the effect of exposure to CpG oligodeoxynucleotides (TLR9 ligands) prior to neonatal respiratory syncytial virus (RSV) infection in mice. CpG preexposure was protective against enhanced disease during secondary adult RSV challenge, with a reduction in viral load and an increase in Th1 responses. A similar Th1 switch and reduction in disease were observed if CpG was administered in the interval between neonatal infection and challenge. In neonates, CpG pretreatment led to a transient increase in expression of major histocompatibility complex class II (MHCII) and CD80 on CD11c-positive cells and gamma interferon (IFN-γ) production by NK cells after RSV infection, suggesting that the protective effects may be mediated by antigen-presenting cells (APC) and NK cells. We conclude that the adverse effects of early-life respiratory viral infection on later lung health might be mitigated by conditions that promote TLR activation in the infant lung.

Poly (I:C), an agonist of toll-like receptor-3, inhibits replication of the Chikungunya virus in BEAS-2B cells

Background

Double-stranded RNA (dsRNA) and its mimic, polyinosinic acid: polycytidylic acid [Poly (I:C)], are recognized by toll-like receptor 3 (TLR3) and induce interferon (IFN)-β in many cell types. Poly (I:C) is the most potent IFN inducer. In in vivo mouse studies, intraperitoneal injection of Poly (I:C) elicited IFN-α/β production and natural killer (NK) cells activation. The TLR3 pathway is suggested to contribute to innate immune responses against many viruses, including influenza virus, respiratory syncytial virus, herpes simplex virus 2, and murine cytomegalovirus. In Chikungunya virus (CHIKV) infection, the viruses are cleared within 7–10 days postinfection before adaptive immune responses emerge. The innate immune response is important for CHIKV clearance.

Results

The effects of Poly (I:C) on the replication of CHIKV in human bronchial epithelial cells, BEAS-2B, were studied. Poly (I:C) suppressed cytopathic effects (CPE) induced by CHIKV infection in BEAS-2B cells in the presence of Poly (I:C) and inhibited the replication of CHIKV in the cells. The virus titers of Poly (I:C)-treated cells were much lower compared with those of untreated cells. CHIKV infection and Poly (I:C) treatment of BEAS-2B cells induced the production of IFN-β and increased the expression of anti-viral genes, including IFN-α, IFN-β, MxA, and OAS. Both Poly (I:C) and CHIKV infection upregulate the expression of TLR3 in BEAS-2B cells.

Conclusions

CHIKV is sensitive to innate immune response induced by Poly (I:C). The inhibition of CHIKV replication by Poly (I:C) may be through the induction of TLR3, which triggers the production of IFNs and other anti-viral genes. The innate immune response is important to clear CHIKV in infected cells.

Pattern recognition receptors for respiratory syncytial virus infection and design of vaccines

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract illness in infants and young children. Host immune response has been implicated in both the protection and immunopathological mechanisms. Pattern recognition receptors (PRRs) expressed on innate immune cells during RSV infection recognize the RSV-associated molecular patterns and activate innate immune cells as well as mediate airway inflammation, protective immune response, and pulmonary immunopathology. The resident and recruited innate immune cells play important roles in the protection and pathogenesis of an RSV disease by expressing these PRRs. Agonist-binding PRRs are the basis of many adjuvants that are essential for most vaccines. In the present review, we highlight recent advances in the innate immune recognition of and responses to RSV through PRRs, including toll-like receptors (TLRs), retinoic acid-inducible gene (RIG)-I-like receptors (RLRs), and nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs). We also describe the role of PRRs in the design of RSV vaccines.

TLR3 and RIG-I gene variants: associations with functional effects on receptor expression and responses to measles virus and vaccine in vaccinated infants

Measles virus causes severe morbidity and mortality, despite the availability of measles vaccines. Successful defence against viral pathogens requires early recognition of virus-specific patterns by innate receptors like Toll-like receptor (TLR)3 and the RNA helicase, retinoic acid inducible gene-I (RIG-I). Genetic differences in these receptors may influence the primary immune responses to measles and the efficacy of measles vaccine. In 1-year-old Australian infants after their first measles vaccine dose, we investigated functional effects of TLR3 and RIG-I polymorphisms on intracellular protein expression using flow cytometry, cytokine responses to receptor ligands and measles lysate, and post-vaccination measles IgG levels. We found that TLR3 Leu412Phe was significantly associated with IFN-α/β response after stimulation with TLR3 ligand, poly(I:C) (P=0.024). Downregulation of TLR3 protein expression in NK cells after poly(I:C) was also associated with this variant (P=0.011). In contrast, measles-specific expression, cytokine responses and antibody responses were not associated with TLR3 polymorphisms. No associations were found with RIG-I variants. These results suggest that a TLR3 polymorphism has functional effects on receptor expression and cytokine response. However, this did not translate to an effect on specific responses to measles virus or vaccine. We found no evidence that RIG-I polymorphisms were involved in measles immune responses.

Differential expression of cytokines in response to respiratory syncytial virus infection of calves with high or low circulating 25-hydroxyvitamin D3

Deficiency of serum levels of 25-hydroxyvitamin D₃ has been related to increased risk of lower respiratory tract infections in children. Respiratory syncytial virus (RSV) is a leading cause of low respiratory tract infections in infants and young children. The neonatal calf model of RSV infection shares many features in common with RSV infection in infants and children. In the present study, we hypothesized that calves with low circulating levels of 25-hydroxyvitamin D₃ (25(OH)D₃) would be more susceptible to RSV infection than calves with high circulating levels of 25(OH)D₃. Calves were fed milk replacer diets with different levels of vitamin D for a 10 wk period to establish two treatment groups, one with high (177 ng/ml) and one with low (32.5 ng/ml) circulating 25(OH)D₃. Animals were experimentally infected via aerosol challenge with RSV. Data on circulating 25(OH)D₃ levels showed that high and low concentrations of 25(OH)D₃ were maintained during infection. At necropsy, lung lesions due to RSV were similar in the two vitamin D treatment groups. We show for the first time that RSV infection activates the vitamin D intracrine pathway in the inflamed lung. Importantly, however, we observed that cytokines frequently inhibited by this pathway in vitro are, in fact, either significantly upregulated (IL-12p40) or unaffected (IFN-γ) in the lungs of RSV-infected calves with high circulating levels of 25(OH)D₃. Our data indicate that while vitamin D does have an immunomodulatory role during RSV infection, there was no significant impact on pathogenesis during the early phases of RSV infection. Further examination of the potential effects of vitamin D status on RSV disease resolution will require longer-term studies with immunologically sufficient and deficient vitamin D levels.

Sensing of RNA viruses: a review of innate immune receptors involved in recognizing RNA virus invasion

Our knowledge regarding the contribution of the innate immune system in recognizing and subsequently initiating a host response to an invasion of RNA virus has been rapidly growing over the last decade. Descriptions of the receptors involved and the molecular mechanisms they employ to sense viral pathogen-associated molecular patterns have emerged in great detail. This review presents an overview of our current knowledge regarding the receptors used to detect RNA virus invasion, the molecular structures these receptors sense, and the involved downstream signaling pathways.

The Anti-Apoptotic Effect of Respiratory Syncytial Virus on Human Peripheral Blood Neutrophils is Mediated by a Monocyte Derived Soluble Factor

Respiratory Syncytial Virus (RSV) causes annual epidemics of respiratory disease particularly affecting infants. The associated airway inflammation is characterized by an intense neutrophilia. This neutrophilic inflammation appears to be responsible for much of the pathology and symptoms. Previous work from our group had shown that there are factors within the airways of infants with RSV bronchiolitis that inhibit neutrophil apoptosis. This study was undertaken to determine if RSV can directly affect neutrophil survival.

Neutrophils were isolated from citrated venous blood (collected from healthy adult volunteers) by discontinuous plasma: Percoll gradient centrifugation and, in some experiments, further purified by negative immunomagnetic bead selection. The effect of RSV on neutrophil survival was measured by Annexin V-PE /To-Pro-3 staining and by morphological changes, using Dif-Quick staining of cytospins.

Inhibition of neutrophil apoptosis was observed in neutrophils isolated by standard plasma:Percoll gradient when exposed to RSV but not in ultra pure neutrophil preparations. Adding monocytes back to ultra purified preparations restored the effect. The inhibition of apoptosis was observed with both active and UV inactivated virus. The effect is dependent on a soluble factor and appears to be dependent on CD14 receptors on the monocytes.

Identifying a role for Toll-like receptor 3 in the innate immune response to Chlamydia muridarum infection in murine oviduct epithelial cells

Because epithelial cells are the major cell type productively infected with Chlamydia during genital tract infections, the overall goal of our research was to understand the contribution of infected epithelial cells to the host defense. We previously showed that Toll-like receptor 3 (TLR3) is the critical pattern recognition receptor in oviduct epithelial (OE) cells that is stimulated during Chlamydia infection, resulting in the synthesis of beta interferon (IFN-β). Here, we present data that implicates TLR3 in the expression of a multitude of other innate-inflammatory immune modulators including interleukin-6 (IL-6), CXCL10, CXCL16, and CCL5. We demonstrate that Chlamydia-induced expression of these cytokines is severely disrupted in TLR3-deficient OE cells, whereas Chlamydia replication in the TLR3-deficient cells is more efficient than in wild-type OE cells. Pretreatment of the TLR3-deficient OE cells with 50 U of IFN-β/ml prior to infection diminished Chlamydia replication and restored the ability of Chlamydia infection to induce IL-6, CXCL10, and CCL5 expression in TLR3-deficient OE cells; however, CXCL16 induction was not restored by IFN-β preincubation. Our findings were corroborated in pathway-focused PCR arrays, which demonstrated a multitude of different inflammatory genes that were defectively regulated during Chlamydia infection of the TLR3-deficient OE cells, and we found that some of these genes were induced only when IFN-β was added prior to infection. Our OE cell data implicate TLR3 as an essential inducer of IFN-β and other inflammatory mediators by epithelial cells during Chlamydia infection and highlight the contribution of TLR3 to the inflammatory cytokine response.

Resveratrol-mediated gamma interferon reduction prevents airway inflammation and airway hyperresponsiveness in respiratory syncytial virus-infected immunocompromised mice

Respiratory syncytial virus (RSV) is the most important cause of severe, lower respiratory tract infections in infants, and RSV infections have been associated with chronic wheezing and asthma during childhood. However, the mechanism of RSV-induced airway inflammation and airway hyperresponsiveness (AHR) is poorly understood. Furthermore, there are presently neither effective vaccines nor drugs available for the prevention or treatment of RSV infections. In this study, we investigated the effect of the plant extract resveratrol as a means of preventing airway inflammation and attenuating RSV-induced AHR. Our data showed that resveratrol reduced RSV lung titers and the number of infiltrating lymphocytes present in bronchoalveolar lavage fluid (BALF) and reduced inflammation. Furthermore, resveratrol attenuated airway responses to methacholine following RSV infection and significantly decreased gamma interferon (IFN-γ) levels in BALF of RSV-infected mice. Data presented in this report demonstrated that resveratrol controlled Toll-like receptor 3 (TLR3) expression, inhibited the TRIF signaling pathway, and induced M2 receptor expression following RSV infection. These data support a role for the use of resveratrol as a means of reducing IFN-γ levels associated with RSV-mediated airway inflammation and AHR, which may be mediated via TLR3 signaling.

Label-free quantitative proteomics reveals regulation of interferon-induced protein with tetratricopeptide repeats 3 (IFIT3) and 5'-3'-exoribonuclease 2 (XRN2) during respiratory syncytial virus infection

ABSTRACT: A large quantitative study was carried out to compare the proteome of respiratory syncytial virus (RSV) infected versus uninfected cells in order to determine novel pathways regulated during viral infection. RSV infected and mock-infected HEp2 cells were lysed and proteins separated by preparative isoelectric focussing using offgel fractionation. Following tryptic digestion, purified peptides were characterized using label-free quantitative expression profiling by nano-ultra performance liquid chromatography coupled to electrospray ionisation mass spectrometry with collision energy ramping for all-ion fragmentation (UPLC-MSE). A total of 1352 unique cellular proteins were identified and their abundance compared between infected and non-infected cells. Ingenuity pathway analysis revealed regulation of several central cellular metabolic and signalling pathways during infection. Selected proteins that were found regulated in RSV infected cells were screened by quantitative real-time PCR for their regulation on the transcriptional level. Synthesis of interferon-induced protein with tetratricopeptide repeats 3 (IFIT3) and 5'-3'-exoribonuclease 2 (XRN2) mRNAs were found to be highly induced upon RSV infection in a time dependent manner. Accordingly, IFIT3 protein levels accumulated during the time course of infection. In contrast, little variation was observed in XRN2 protein levels, but different forms were present in infected versus non-infected cells. This suggests a role of these proteins in viral infection, and analysis of their function will shed further light on mechanisms of RNA virus replication and the host cell defence machinery.

Th17 cytokines are critical for respiratory syncytial virus-associated airway hyperreponsiveness through regulation by complement C3a and tachykinins

Respiratory syncytial virus (RSV) infection is associated with serious lung disease in infants and immunocompromised individuals and is linked to development of asthma. In mice, acute RSV infection causes airway hyperresponsiveness (AHR), inflammation, and mucus hypersecretion. Infected cells induce complement activation, producing the anaphylatoxin C3a. In this paper, we show RSV-infected wild-type mice produce Th17 cytokines, a response not previously associated with viral infections. Mice deficient in the C3aR fail to develop AHR following acute RSV infection, and production of Th17 cytokines was significantly attenuated. Tachykinin production also has been implicated in RSV pathophysiology, and tachykinin receptor-null mice were similarly protected from developing AHR. These animals were also deficient in production of Th17 cytokines. Tachykinin release was absent in mice deficient in C3aR, whereas C3a levels were unchanged in tachykinin receptor-null animals. Thus, our data reveal a crucial sequence following acute RSV infection where initial C3a production causes tachykinin release, followed by activation of the IL-17A pathway. Deficiency of either receptor affords protection from AHR, identifying two potential therapeutic targets.

Autophagy-mediated dendritic cell activation is essential for innate cytokine production and APC function with respiratory syncytial virus responses

The regulation of innate immune responses during viral infection is a crucial step to promote antiviral reactions. Recent studies have drawn attention to a strong relationship of pathogen-associated molecular pattern recognition with autophagy for activation of APC function. Our initial observations indicated that autophagosomes formed in response to respiratory syncytial virus (RSV) infection of dendritic cells (DC). To further investigate whether RSV-induced DC activation and innate cytokine production were associated with autophagy, we used several methods to block autophagosome formation. Using 3-MA, small interfering RNA inhibition of LC3, or Beclin(+/-) mouse-derived DC, studies established a relationship between RSV-induced autophagy and enhanced type I IFN, TNF, IL-6, and IL-12p40 expression. Moreover, autophagosome formation induced by starvation also promoted innate cytokine expression in DC. The induction of starvation-induced autophagy in combination with RSV infection synergistically enhanced DC cytokine expression that was blocked by an autophagy inhibitor. The latter synergistic responses were differentially altered in DC from MyD88(-/-) and TRIF(-/-) mice, supporting the concept of autophagy-mediated TLR signaling. In addition, blockade of autophagy in RSV-infected DC inhibited the maturation of DC as assessed by MHC class II and costimulatory molecule expression. Subsequently, we demonstrated that inhibition of autophagy in DC used to stimulate primary OVA-induced and secondary RSV-infected responses significantly attenuated cytokine production by CD4(+) T cells. Thus, these studies have outlined that autophagy in DC after RSV infection is a crucial mechanism for driving innate cytokine production, leading to altered acquired immune responses.

Ribosomal Protein L19 and L22 Modulate TLR3 Signaling

Background

Toll-like receptor 3 (TLR3) recognizes double-stranded RNA (dsRNA) and induces inflammation. In this study we attempted to ascertain if there are endogenous host molecules controlling the production of cytokines and chemokines. Two candidates, ribosomal protein L19 and L22, were analyzed to determine if they influence cytokine production followed by TLR3 activation. In this study we report that L19 acts upon production of IP-10 or IL-8 differently in glioblastoma cells.

Methods

L19 or L22 was transfected into HEK293-TLR3, A549 or A172 cells. After treatment with several inhibitors of NF-kB, PI3K, p38 or ERK, production of IL-8 or IP-10 was measured by ELISA. siRNA was introduced to suppress expression of L19. After Vesicular stomatitis virus infection, viral multiplication was measured by western blot.

Results

L19 increased ERK activation to produce IL-8. In A172 cells, in which TLR3 is expressed at endosomes, L19 inhibited interferon regulatory factor 3 (IRF3) activation and IP-10 production to facilitate viral multiplication, whereas L19 inhibited viral multiplication in A549 cells bearing TLR3 on their cell membrane.

Conclusion

Our results suggest that L19 regulates TLR3 signaling, which is cell type specific and may be involved in pathogenesis of autoimmune diseases and chronic inflammatory diseases.

CCL21/CCR7 promotes G2/M phase progression via the ERK pathway in human non-small cell lung cancer cells

C-C chemokine receptor 7 (CCR7) contributes to the survival of certain cancer cell lines, but its role in the proliferation of human non-small cell lung cancer (NSCLC) cells remains vague. Proliferation assays performed on A549 and H460 NSCLC cells using Cell Counting Kit-8 indicated that activation of CCR7 by its specific ligand, exogenous chemokine ligand 21 (CCL21), was associated with a significant linear increase in cell proliferation with duration of exposure to CCL21. The CCL21/CCR7 interaction significantly increased the fraction of cells in the G₂/M phase of the cell cycle as measured by flow cytometry. In contrast, CCL21/CCR7 had no significant influence on the G₀/G₁ and S phases. Western blot and real-time PCR indicated that CCL21/CCR7 significantly upregulated expression of cyclin A, cyclin B1, and cyclin-dependent kinase 1 (CDK1), which are related to the G₂/M phase transition. The expression of cyclin D1 and cyclin E, which are related to the G₀/G₁ and G₁/S transitions, was not altered. The CCL21/CCR7 interaction significantly enhanced phosphorylation of extracellular signal-regulated kinase (P-ERK) but not Akt, as measured by Western blot. LY294002, a selective inhibitor of PI3K that prevents activation of the downstream Akt, did not weaken the effect of CCL21/CCR7 on P-ERK. Coimmunoprecipitation further confirmed that there was an interaction between P-ERK and cyclin A, cyclin B1, or CDK1, particularly in the presence of CCL21. CCR7 small interfering RNA or PD98059, a selective inhibitor of MEK that disrupts the activation of downstream ERK, significantly abolished the effects of exogenous CCL21. These results suggest that CCL21/CCR7 contributes to the time-dependent proliferation of human NSCLC cells by upregulating cyclin A, cyclin B1, and CDK1 potentially via the ERK pathway.

Identifying innate immune pathways of the chicken may lead to new antiviral therapies

Zoonotic viruses, such as highly pathogenic avian influenza (HPAI), present a significant threat to both the poultry industry and public health. The present method of controlling avian influenza (AI) relies on good farming practice with limited use of vaccination in some countries. However, new ways to control disease outbreaks might be possible with additional knowledge of the natural host response to virus. Moreover, manipulation of the innate immune system in mammals improves the outcomes following viral infection. A similar approach might be applied to the chicken, nevertheless, a greater knowledge of the chicken innate immune system is required. This review outlines important mammalian antiviral mechanisms that have been modulated to strengthen viral immunity and highlights the potential application of these strategies in the chicken, especially in regards, to AI.

TLRs, NLRs and RLRs: innate sensors and their impact on allergic diseases--a current view

Charles Janeway first wrote 1989 about how important recognition of "certain characteristics or patterns common on infectious agents but absent from the host" would be for our immune response [1]. Surprisingly, it almost took 10 years before his ideas would lead to the revolutionary findings that fundamentally changed the view of the innate immune system over the past decade. Recognition of invading microorganisms belongs to the primary tasks of the innate immune system and is achieved through different families of innate immune sensors. Among these, Toll-like receptors (TLRs), nucleotide-binding domain and leucine-rich repeat containing receptors (NLRs) and Rig-I-like receptors (RLRs) have drawn major interests over the last decade. These receptor families are targeted by overlapping classes of pathogens and share functional domains and signal transduction pathways (see Fig. 1 and Table 1 for an overview of their structural organization, ligands, adaptors and activated pathways). This current view describes our present knowledge about these three main innate immune receptor families and their importance for adaptive immune responses such as asthma and allergy.

Toll-like receptor, RIG-I-like receptors and the NLRP3 inflammasome: key modulators of innate immune responses to double-stranded RNA viruses

Double-stranded RNA (dsRNA), the genetic material for many RNA viruses, induces robust host immune responses via pattern recognition receptors, which include Toll-like receptor 3 (TLR3), retinoic acid-inducible gene-I-like receptors (RLRs) and the multi-protein NLRP3 inflammasome complex. The engagement of dsRNA receptors or inflammasome activation by viral dsRNA initiates complex intracellular signaling cascades that play essential roles in inflammation and innate immune responses, as well as the resultant development of adaptive immunity. This review focuses on signaling pathways mediated by TLR3, RLRs and the NLRP3 inflammasome, as well as the potential use of agonists and antagonists that target these pathways to treat disease.

Toll-like receptor modulators: a patent review (2006-2010)

INTRODUCTION

The immune response is mediated via two parallel immune components, innate and adaptive, whose effector functions are highly integrated and coordinated for the protection of the human body against invading pathogens and transformed cells. The discovery of pathogen recognition receptors (PRRs), most notably toll-like receptors (TLRs), in innate immunity has evoked increased interest in the therapeutic handling of the innate immune system. TLRs are germ line-encoded receptors that play a potent role in the recognition of a diverse variety of ligands ranging from hydrophilic nucleic acids to lipopolysaccharide (LPS) or peptidoglycan (PGN) structures in pathogens.

AREAS COVERED

This review discusses recent updates (2006-2010) in completed, ongoing and planned clinical trials of TLR immunomodulator-based therapies for the treatment of infectious diseases, inflammatory disorders and cancer.

EXPERT OPINION

Since the discovery of human TLRs, modulating immune responses using TLR agonists or antagonists for therapeutic purposes has provoked intense activity in the pharmaceutical industry. The ability of TLRs to initiate and propagate inflammation makes them attractive therapeutic targets. We are now at the stage of evaluating such molecules in human diseases. Additionally, there is also extensive literature available on TLRs in diseased states. These data provide a basis for the identification of novel immunomodulators (agonists and antagonists) for the therapeutic targeting of TLRs.

Role of Nrf2 in host defense against influenza virus in cigarette smoke-exposed mice

Influenza virus is a common respiratory tract viral infection. Although influenza can be fatal in patients with chronic pulmonary diseases such as chronic obstructive pulmonary disease, its pathogenesis is not fully understood. The Nrf2-mediated antioxidant system is essential to protect the lungs from oxidative injury and inflammation. In the present study, we investigated the role of Nrf2 in protection against influenza virus-induced pulmonary inflammation after cigarette smoke exposure with both in vitro and in vivo approaches. For in vitro analyses, peritoneal macrophages isolated from wild-type and Nrf2-deficient mice were treated with poly(I:C) and/or cigarette smoke extract. For in vivo analysis, these mice were infected with influenza A virus with or without exposure to cigarette smoke. In Nrf2-deficient macrophages, NF-κB activation and the induction of its target inflammatory genes were enhanced after costimulation with cigarette smoke extract and poly(I:C) compared with wild-type macrophages. The induction of antioxidant genes was observed for the lungs of wild-type mice but not those of Nrf2-deficient mice after cigarette smoke exposure. Cigarette smoke-exposed Nrf2-deficient mice showed higher rates of mortality than did wild-type mice after influenza virus infection, with enhanced peribronchial inflammation, lung permeability damage, and mucus hypersecretion. Lung oxidant levels and NF-κB-mediated inflammatory gene expression in the lungs were also enhanced in Nrf2-deficient mice. Our data indicate that the antioxidant pathway controlled by Nrf2 is pivotal for protection against the development of influenza virus-induced pulmonary inflammation and injury under oxidative conditions.

Antiviral responses induced by the TLR3 pathway

Antiviral responses are successively induced in virus-infected animals, and include primary innate immune responses such as type I interferon (IFN) and cytokine production, secondary natural killer (NK) cell responses, and final cytotoxic T lymphocyte (CTL) responses and antibody production. The endosomal Toll-like receptors (TLRs) and cytoplasmic RIG-I-like receptors (RLRs), which recognize viral nucleic acids, are responsible for virus-induced type I IFN production. RLRs are expressed in most tissues and cells and are primarily implicated in innate immune responses against various viruses through type I IFN production, whereas nucleic acid-sensing TLRs, TLRs 3, 7, 8 and 9, are expressed on the endosomal membrane of dendritic cells (DCs) and play distinct roles in antiviral immunity. TLR3 recognizes viral double-stranded RNA taken up into the endosome and serves to protect the host against viral infection by the induction of a range of responses including type I IFN production and DC-mediated activation of NK cells and CTLs, although the deteriorative role of TLR3 has also been reported in some virus infections. Here, we review the current knowledge on the role of TLR3 during viral infection, and the current understanding of the TLR3-signalling cascade that operates via the adaptor protein TICAM-1 (also called TRIF).

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.

Local cytokine response upon respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is the leading cause of childhood hospitalization and respiratory distress and has been recognized for several decades as a major health and economic burden worldwide. This virus has developed several virulence mechanisms to impair the establishment of a protective immune response to re-infection. Accordingly, inefficient immunological memory is usually generated after exposure to this pathogen. Furthermore, it has been shown that RSV can actively promote the induction of an inadequate cellular immune response at the site of infection that causes exacerbated inflammation in the respiratory tract. Such an inflammatory response is both inefficient for clearing the virus and can be responsible for detrimental symptoms, such as asthma and wheezing. Recent data suggest that RSV possesses molecular mechanisms to induce the secretion of pro-inflammatory cytokines that modulate the immune response and impair viral clearance by reducing IFN-γ production. Here, we discuss recent research leading to the identification of RSV virulence factors that are responsible of promoting a pro-inflammatory environment at the airways and their implications on pathogenicity.

Subversion of innate defenses by the interplay between DENV and pre-existing enhancing antibodies: TLRs signaling collapse

Background

The phenomenon of antibody dependent enhancement as a major determinant that exacerbates disease severity in DENV infections is well accepted. While the detailed mechanism of antibody enhanced disease severity is unclear, evidence suggests that it is associated with both increased DENV infectivity and suppression of the type I IFN and pro-inflammatory cytokine responses. Therefore, it is imperative for us to understand the intracellular mechanisms altered during ADE infection to decipher the mechanism of severe pathogenesis.

Methodology/Principal Findings

In this present work, qRT-PCR, immunoblotting and gene array analysis were conducted to determine whether DENV-antibody complex infection exerts a suppressive effect on the expression and/or function of the pathogen recognition patterns, focusing on the TLR-signaling pathway. We show here that FcγRI and FcγRIIa synergistically facilitated entry of DENV-antibody complexes into monocytic THP-1 cells. Ligation between DENV-antibody complexes and FcR not only down regulated TLRs gene expression but also up regulated SARM, TANK, and negative regulators of the NF-κB pathway, resulting in suppression of innate responses but increased viral production. These results were confirmed by blocking with anti-FcγRI or anti-FcγRIIa antibodies which reduced viral production, up-regulated IFN-β synthesis, and increased gene expression in the TLR-dependent signaling pathway. The negative impact of DENV-ADE infection on the TLR-dependent pathway was strongly supported by gene array screening which revealed that both MyD88-dependent and –independent signaling molecules were down regulated during DENV-ADE infection. Importantly, the same phenomenon was seen in PBMC of secondary DHF/DSS patients but not in PBMC of DF patients.

Conclusions/Significance

Our present work demonstrates the mechanism by which DENV uses pre-existing immune mediators to defeat the principal activating pathway of innate defense resulting in suppression of an array of innate immune responses. Interestingly, this phenomenon specifically occurred during the severe form of DENV infection but not in the mild form of disease.

Dengue is the most common vector-borne viral disease in humans, with 50–100 million infections per year. The severity of dengue ranges from an acute febrile illness, DF, to a life-threatening vascular leakage syndrome with or without shock, DHF/DSS. Determinants of these syndromes are mainly host factors including non protective but cross reactive antibodies which are known as preexisting enhancing antibodies. These antibodies enhance disease severity through increasing the virus infected cell mass and facilitating intracellular virus replication. Here we demonstrate that DENV exploits preexisting subneutralizing antibodies to defeat the pathogen recognition system and to down regulate the TLR signaling pathway resulting in suppression of an array of innate immune responses. Furthermore, we also show that this phenomenon specifically occurs in the severe form of dengue but not in the mild form of disease.

Characterization of innate immune signalling receptors in virus-induced acute asthma

BACKGROUND

The role of toll-like receptors (TLRs) and innate immune activation in clinical asthma exacerbations and their relationship to virus infection are unclear.

OBJECTIVE

This study aimed to characterize TLR expression and innate immune activity during virus infection in acute asthma.

METHODS

Subjects with acute asthma, stable asthma and healthy controls were recruited and underwent spirometry and sputum induction with isotonic saline. Selected sputum was dispersed with dithiothreitol and total and differential leucocyte counts were performed. Selected sputum was also used for quantitative real-time PCR for TLR2, TLR3, TLR4, IL-10 and IP-10mRNA expression. Sputum supernatant was used for the measurement of innate immune markers, including IL-8, matrix metalloproteinase-9 and neutrophil elastase activity. Viruses were detected using real-time and gel-based PCR.

RESULTS

Sputum TLR2 mRNA expression was up-regulated in both acute and stable asthma compared with healthy controls and decreased 4-6 weeks after acute exacerbation. Sputum TLR2 mRNA expression was elevated in viral, compared with non-viral, acute asthma. Sputum TLR3 mRNA expression was similar in controls, stable and acute asthma. However, in acute asthma, subjects with virus-induced acute asthma had significantly higher sputum TLR3 mRNA expression. Induced sputum gene expression for IP-10 and IL-10 were increased in viral, compared with non-viral, acute asthma. In virus-induced acute asthma, levels of IP-10 and IL-10 mRNA expression were correlated with the mRNA expression of TLR2 and TLR3.

CONCLUSIONS AND CLINICAL RELEVANCE

Virus-induced acute asthma leads to specific induction of TLR2, TLR3, IP-10 and IL-10, suggesting that signalling via TLRs may play an important role in mediating airway inflammation, via both innate and adaptive pathways, in virus-induced exacerbations. These mediators may provide potential treatment targets for virus-induced asthma. They may also be useful in diagnosing the nature of acute asthma exacerbations and monitoring treatment responses, which would be useful in the clinical management of asthma exacerbations.

The role of toll-like receptors in acute and chronic lung inflammation

By virtue of its direct contact with the environment, the lung is constantly challenged by infectious and non-infectious stimuli that necessitate a robust yet highly controlled host response coordinated by the innate and adaptive arms of the immune system. Mammalian Toll-like receptors (TLRs) function as crucial sentinels of microbial and non-infectious antigens throughout the respiratory tract and mediate host innate immunity. Selective induction of inflammatory responses to harmful environmental exposures and tolerance to innocuous antigens are required to maintain tissue homeostasis and integrity. Conversely, dysregulated innate immune responses manifest as sustained and self-perpetuating tissue damage rather than controlled tissue repair. In this article we review aspects of Toll-like receptor function that are relevant to the development of acute lung injury and chronic obstructive lung diseases as well as resistance to frequently associated microbial infections.

Activation of interferon response through toll-like receptor 3 impacts viral pathogenesis and pulmonary toll-like receptor expression during respiratory syncytial virus and influenza infections in the cotton rat Sigmodon hispidus model

Interferon (IFN) therapy in humans often causes flu-like symptoms by an unknown mechanism. Poly ICLC is a synthetic dsRNA and a Toll-like receptor 3 (TLR3) agonist with a strong IFN-inducing ability. In this work, we analyzed the effect of poly ICLC on pulmonary responses to influenza and respiratory syncytial virus (RSV) infections in the cotton rat (Sigmodon hispidus) model. Viral replication, pulmonary inflammation, and expression of IFN, TLR, and chemokines were monitored and compared. Antiviral effect of poly ICLC against influenza virus and RSV was best achieved at high poly ICLC concentrations that, in the absence of virus infection, induced a strong IFN response. The antiviral doses of poly ICLC, however, also increased lung inflammation, an unexpected finding because of the reported poly ICLC safety in BALB/c mice. Similarly, in contrast to murine model, pathology of RSV infection was increased in cotton rats treated with poly ICLC. Augmented lung inflammation was accompanied by an earlier induction of IFN and TLR responses and a stronger chemokine expression. Overall, these findings indicate significant association between antiviral IFN action and pulmonary inflammation and highlight important animal model-specific variations in the potential of IFN to cause pathology.

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.

Differential expression of cytokine transcripts in neonatal and adult ovine alveolar macrophages in response to respiratory syncytial virus or toll-like receptor ligation

Alveolar macrophages (AMvarphis) secrete regulatory molecules that are believed to be critical in maintaining normal lung homeostasis. However, in response to activating signals, AMvarphis have been shown to become highly phagocytic cells capable of secreting significant levels of pro-inflammatory cytokines. There is evidence to suggest that susceptibility of Mvarphi subpopulations to viral infection, and their subsequent cytokine/chemokine response, is dependent on age of the host. In the present study, we compared bovine respiratory syncytial virus (BRSV) replication and induction of cytokine responses in neonatal ovine AMvarphis to those cells isolated from adult animals. While neonatal AMvarphis could be infected with BRSV, viral replication was limited as previously shown for AMvarphis from mature animals. Interestingly, following BRSV infection, peak mRNA levels of IL-1beta and IL-8 in neonatal AMvarphi were several fold higher than levels induced in adult AMvarphis. In addition, peak mRNA expression for the cytokines examined occurred at earlier time points in neonatal AMvarphis compared to adult AMvarphis. However, the data indicated that viral replication was not required for the induction of specific cytokines in either neonatal or adult AMvarphis. TLR3 and TLR4 agonists induced significantly higher levels of cytokine transcripts than BRSV in both neonatal and adult AMvarphis. It was recently proposed that immaturity of the neonatal immune system extends from production of pro-inflammatory cytokines to regulation of such responses. Differential regulation of cytokines in neonatal AMvarphis compared to adult AMvarphis in response to RSV could be a contributory factor to more severe clinical episodes seen in neonates.

Respiratory syncytial virus: immunopathology and control

Respiratory syncytial virus (RSV) is the primary cause of serious upper and lower respiratory tract infections in infants and children worldwide. RSV infection in infancy may lead to the onset of asthma or other health problems later in life. An effective vaccine is not yet available against RSV infection. Humans respond to RSV infection by mounting an immune response, but the antiviral immunity is incomplete, thus repeat RSV infections continue throughout life. The precise mechanism of RSV-induced infection and immunopathology remains unclear. The limited knowledge of RSV immunity is a major problem in designing a protective vaccine. In this review, the biology of RSV infection, its immunopathology, the role of innate and adaptive immunity, as well as the understanding of how to control RSV infection based on prophylactic and therapeutic approaches are discussed.

The role of viruses in acute exacerbations of asthma

Viral respiratory infections are the most common cause of an acute asthma exacerbation in both children and adults and represent a significant global health burden. An increasing body of evidence supports the hypothesis that these infections cause a greater degree of morbidity in asthmatic subjects than in the healthy population, emphasizing a discrepancy in the antiviral response of asthmatics. In this review we discuss why such a discrepancy might exist, examining the role of the bronchial epithelium as well as the main inflammatory cells, mediators, and molecular pathways that are involved in the immune response. In addition, the potential impact of virus-induced asthma exacerbations on airway remodelling is reviewed and we explore which therapeutic options might be of benefit in preventing the deterioration of asthma control seen following viral infection.

New insights for development of a safe and protective RSV vaccine

Respiratory Syncytial Virus (RSV) is the leading cause of pneumonia and bronchiolitis in infants and children <1 year old, resulting in significant morbidity and mortality worldwide. There is currently no RSV vaccine. In the 1960s, a formalin-inactivated RSV (FI-RSV) vaccine trial led to exacerbated disease upon natural infection of vaccinees, including two deaths. The causes involved in the disastrous results of these vaccine trials are still unclear but they remain the engine for searching new avenues to develop a safe vaccine that can provide long-term protection against this important pathogen. This article reviews some of the early history of RSV vaccine development,as well as more recent information on the interaction between RSV and the host innate and adaptive immune responses. A safe and efficacious vaccine for RSV will require "re-education" of the host immune response against RSV to prevent vaccine-enhanced or severe RSV disease.

Human CD141+ (BDCA-3)+ dendritic cells (DCs) represent a unique myeloid DC subset that cross-presents necrotic cell antigens

The characterization of human dendritic cell (DC) subsets is essential for the design of new vaccines. We report the first detailed functional analysis of the human CD141⁺ DC subset. CD141⁺ DCs are found in human lymph nodes, bone marrow, tonsil, and blood, and the latter proved to be the best source of highly purified cells for functional analysis. They are characterized by high expression of toll-like receptor 3, production of IL-12p70 and IFN-β, and superior capacity to induce T helper 1 cell responses, when compared with the more commonly studied CD1c⁺ DC subset. Polyinosine-polycytidylic acid (poly I:C)–activated CD141⁺ DCs have a superior capacity to cross-present soluble protein antigen (Ag) to CD8⁺ cytotoxic T lymphocytes than poly I:C–activated CD1c⁺ DCs. Importantly, CD141⁺ DCs, but not CD1c⁺ DCs, were endowed with the capacity to cross-present viral Ag after their uptake of necrotic virus-infected cells. These findings establish the CD141⁺ DC subset as an important functionally distinct human DC subtype with characteristics similar to those of the mouse CD8α⁺ DC subset. The data demonstrate a role for CD141⁺ DCs in the induction of cytotoxic T lymphocyte responses and suggest that they may be the most relevant targets for vaccination against cancers, viruses, and other pathogens.

Cellular response to influenza virus infection: a potential role for autophagy in CXCL10 and interferon-alpha induction

Historically, influenza pandemics have arisen from avian influenza viruses. Avian influenza viruses H5N1 and H9N2 are potential pandemic candidates. Infection of humans with the highly pathogenic avian influenza H5N1 virus is associated with a mortality in excess of 60%, which has been attributed to dysregulation of the cytokine system. Human macrophages and epithelial cells infected with some genotypes of H5N1 and H9N2 viruses express markedly elevated cytokine and chemokine levels when compared with seasonal influenza A subtype H1N1 virus. The mechanisms underlying this cytokine and chemokine hyperinduction are not fully elucidated. In the present study, we demonstrate that autophagy, a tightly regulated homeostatic process for self-digestion of unwanted cellular subcomponents, plays a role in cytokine induction. Autophagy is induced to a greater extent by H9N2/G1, in association with cytokine hyperinduction, compared with H1N1 and the novel pandemic swine-origin influenza A/H1N1 viruses. Using 3-methyladenine to inhibit autophagy and small interfering RNA to silence the autophagy gene, Atg5, we further show that autophagic responses play a role in influenza virus-induced CXCL10 and interferon-α expression in primary human blood macrophages. Our results provide new insights into the pathogenic mechanisms of avian influenza viruses.

A role for Toll-like receptor 3 variants in host susceptibility to enteroviral myocarditis and dilated cardiomyopathy

The innate antiviral response is mediated, at least in part, by Toll-like receptors (TLRs). TLR3 signaling is activated in response to viral infection, and the absence of TLR3 in mice significantly increases mortality after infection with enteroviruses that cause myocarditis and/or dilated cardiomyopathy. We screened TLR3 in patients diagnosed with enteroviral myocarditis/cardiomyopathy and identified a rare variant in one patient as well as a significantly increased occurrence of a common polymorphism compared with controls. Expression of either variant resulted in significantly reduced TLR3-mediated signaling after stimulation with synthetic double-stranded RNA. Furthermore, Coxsackievirus B3 infection of cell lines expressing mutated TLR3 abrogated activation of the type I interferon pathway, leading to increased viral replication. TLR3-mediated type I interferon signaling required cellular autophagy and was suppressed by 3-methyladenine and bafilomycin A1, by inhibitors of lysosomal proteolysis, and by reduced expression of Beclin 1, Atg5, or microtubule-associated protein 1 light chain 3beta (MAP1LC3beta). However, TLR3-mediated signaling was restored upon exogenous expression of Beclin 1 or a variant MAP1LC3beta fusion protein refractory to RNA interference. These data suggest that individuals harboring these variants may have a blunted innate immune response to enteroviral infection, leading to reduced viral clearance and an increased risk of cardiac pathology.

TLR3-dependent upregulation of RIG-I leads to enhanced cytokine production from cells infected with the parainfluenza virus SV5

Here we address the role of RIG-I and TLR3 in differential cytokine responses against Simian Virus 5 (SV5) and two distinct cytokine inducing SV5 mutants. IFN-beta and IL-6 secretion was induced by infection with P/V-CPI-, an SV5 mutant with P/V substitutions, and were reduced by either siRNA-mediated knockdown of RIG-I expression or by expression of a dsRNA-binding protein. TLR3 overexpression did not alter cytokine secretion induced by P/V-CPI- or by Le-(U5C, A14G), an SV5 promoter mutant. TLR3 signaling by addition of exogenously added dsRNA was not blocked by WT SV5 or either SV5 mutant. Unexpectedly, TLR3 activation in infected cells led to enhanced IL-8 secretion, which correlated with increased RIG-I expression. Dominant negative RIG-I and TRIF supported a model whereby TLR3 activation upregulates RIG-I expression and in turn hypersensitizes cells to RIG-I-mediated cytokine secretion. Implications for crosstalk between different innate immunity pathways in mounting antiviral responses to paramyxoviruses are discussed.

Respiratory viral infections in infants: causes, clinical symptoms, virology, and immunology

In global terms, respiratory viral infection is a major cause of morbidity and mortality. Infancy, in particular, is a time of increased disease susceptibility and severity. Early-life viral infection causes acute illness and can be associated with the development of wheezing and asthma in later life. The most commonly detected viruses are respiratory syncytial virus (RSV), rhinovirus (RV), and influenza virus. In this review we explore the complete picture from epidemiology and virology to clinical impact and immunology. Three striking aspects emerge. The first is the degree of similarity: although the infecting viruses are all different, the clinical outcome, viral evasion strategies, immune response, and long-term sequelae share many common features. The second is the interplay between the infant immune system and viral infection: the immaturity of the infant immune system alters the outcome of viral infection, but at the same time, viral infection shapes the development of the infant immune system and its future responses. Finally, both the virus and the immune response contribute to damage to the lungs and subsequent disease, and therefore, any prevention or treatment needs to address both of these factors.

Vitamin D decreases respiratory syncytial virus induction of NF-kappaB-linked chemokines and cytokines in airway epithelium while maintaining the antiviral state

Epidemiological studies suggest that low vitamin D levels may increase the risk or severity of respiratory viral infections. In this study, we examined the effect of vitamin D on respiratory syncytial virus (RSV)-infected human airway epithelial cells. Airway epithelium converts 25-hydroxyvitamin D3 (storage form) to 1,25-dihydroxyvitamin D3 (active form). Active vitamin D, generated locally in tissues, is important for the nonskeletal actions of vitamin D, including its effects on immune responses. We found that vitamin D induces IkappaBalpha, an NF-kappaB inhibitor, in airway epithelium and decreases RSV induction of NF-kappaB-driven genes such as IFN-beta and CXCL10. We also found that exposing airway epithelial cells to vitamin D reduced induction of IFN-stimulated proteins with important antiviral activity (e.g., myxovirus resistance A and IFN-stimulated protein of 15 kDa). In contrast to RSV-induced gene expression, vitamin D had no effect on IFN signaling, and isolated IFN induced gene expression. Inhibiting NF-kappaB with an adenovirus vector that expressed a nondegradable form of IkappaBalpha mimicked the effects of vitamin D. When the vitamin D receptor was silenced with small interfering RNA, the vitamin D effects were abolished. Most importantly we found that, despite inducing IkappaBalpha and dampening chemokines and IFN-beta, there was no increase in viral mRNA or protein or in viral replication. We conclude that vitamin D decreases the inflammatory response to viral infections in airway epithelium without jeopardizing viral clearance. This suggests that adequate vitamin D levels would contribute to reduced inflammation and less severe disease in RSV-infected individuals.

Pneumoviruses infect eosinophils and elicit MyD88-dependent release of chemoattractant cytokines and interleukin-6

Eosinophils are recruited to the lung in response to infection with pneumovirus pathogens and have been associated with both the pathophysiologic sequelae of infection and, more recently, with accelerated virus clearance. Here, we demonstrate that the pneumovirus pathogens, respiratory syncytial virus (RSV) and pneumonia virus of mice (PVM), can infect human and mouse eosinophils, respectively, and that virus infection of eosinophils elicits the release of disease-related proinflammatory mediators from eosinophils. RSV replication in human eosinophils results in the release of infectious virions and in the release of the proinflammatory mediator, interleukin-6 (IL-6). PVM replication in cultured bone marrow eosinophils (bmEos) likewise results in release of infectious virions and the proinflammatory mediators IL-6, IP-10, CCL2, and CCL3. In contrast to the findings reported in lung tissue of RSV-challenged mice, PVM replication is accelerated in MyD88 gene-deleted bmEos, whereas release of cytokines is diminished. Interestingly, exogenous IL-6 suppresses virus replication in MyD88 gene-deleted bmEos, suggesting a role for a MyD88-dependent cytokine-mediated feedback circuit in modulating this response. Taken together, our findings suggest that eosinophils are targets of virus infection and may have varied and complex contributions to the pathogenesis and resolution of pneumovirus disease.

Viral sensors: diversity in pathogen recognition

Summary:  Innate sensors of viral infection detect viral products and initiate the signal cascades that lead to the antiviral response. Several proteins have been identified to play a role in this process, mostly members of the Toll‐like receptor and retinoic acid‐inducible gene I‐like receptor families. These receptors have been demonstrated to function in part by recognizing a diverse yet unique repertoire of nucleic acid substrates. Upon recognition of their ligands, these sensors activate distinct signaling pathways that lead to the secretion of type I interferon and inflammatory cytokines. It remains to be seen, however, if these sensors are redundant or whether each serves a unique function. In this work, we review the current knowledge of viral sensors, speculate on how they may function in vivo, and explore the potential reasons for their diversity.

Recognizing the importance of respiratory syncytial virus in chronic obstructive pulmonary disease

Acute exacerbations of chronic obstructive pulmonary disease (COPD) are responsible for a large proportion of the health care dollar expenditure, morbidity, and mortality related to COPD. Respiratory infections are the most common cause of acute exacerbations, but recent evidence indicates that the importance of respiratory syncytial virus (RSV) infection in COPD is under-appreciated. Improved detection of RSV using techniques based on the polymerase chain reaction accounts for much of the increased recognition of the importance of this virus in COPD patients. Furthermore, COPD patients may be more susceptible to RSV infection, possibly due to RSV-or immune response-induced pulmonary effects that are altered by age, environmental exposures, genetics, COPD itself, or a combination of these. However, although RSV infection occurs throughout life, viral and host factors that place COPD patients at increased risk are unclear. The complexities of RSV effects in COPD present opportunities for research with the goal of developing approaches to selectively modify damaging viral effects (e.g., altered airway function), while retaining beneficial immunity (e.g., clearance of virus) in COPD patients. This review explores the role RSV plays in acute exacerbations of COPD, the potential for RSV disease in chronic stable COPD, and newer concepts in RSV diagnosis, epidemiology, and host defense.

Gene expression of nucleic acid-sensing pattern recognition receptors in children hospitalized for respiratory syncytial virus-associated acute bronchiolitis

Given the critical role of pattern recognition receptors (PRRs) in acid nucleic recognition in the initiation of innate immunity and the orchestration of adaptive immunity, the aim of this study was to determine whether any heterogeneity of PRR expression in the airway tracts of infants with respiratory syncytial virus (RSV) infection might explain the broad clinical spectrum of RSV-associated bronchiolitis in infants. For this purpose, the levels of melanoma differentiation-associated protein-5 (MDA-5), retinoic acid inducible gene-1 (RIG-1), and Toll-like receptor 3 (TLR-3), TLR-7, TLR-8, and TLR-9 mRNAs were evaluated, using TaqMan quantitative reverse transcription-PCR, in cells from nasopharyngeal washes collected from 157 infants suffering from acute bronchiolitis whether or not they were associated with respiratory viruses. High interindividual variability was observed in both virus-positive and -negative infants; however, the relative gene expression levels of MDA-5, RIG-1, TLR-7, and TLR-8 were significantly higher in the virus-infected group, whereas the expression levels of TLR-3 and TLR-9 were not significantly different. The differences in the gene expression of MDA-5, RIG-1, TLR-7, and TLR-8 were more evident in infants with RSV infection than in those with bocavirus or rhinovirus infection. In RSV-infected infants, PRR-mRNA levels also were analyzed in relation to interferon protein levels, viral load, clinical severity, days of hospitalization, age, and body weight. A significant positive correlation was observed only between RSV viral load and RIG-1 mRNA levels. These findings provide the first direct evidence that, in infants with respiratory virus-associated bronchiolitis, especially RSV, there are substantial changes in PRR gene expression; this likely is an important determinant of the clinical outcome of bronchiolitis.