Recognition of viruses by innate immunity

A review of the fetal brain cytokine imbalance hypothesis of schizophrenia

Maternal infection during pregnancy increases the risk of schizophrenia and other brain disorders of neurodevelopmental origin in the offspring. A multitude of infectious agents seem to be involved in this association. Therefore, it has been proposed that factors common to the immune response to a wide variety of bacterial and viral pathogens may be the critical link between prenatal infection and postnatal brain and behavioral pathology. More specifically, it has been suggested that the maternal induction of pro-inflammatory cytokines may mediate the neurodevelopmental effects of maternal infections. Here, we review recent findings from in vitro and in vivo investigations supporting this hypothesis and further emphasize the influence of enhanced anti-inflammatory cytokine signaling on early brain development. Disruption of the fetal brain balance between pro- and anti-inflammatory cytokine signaling may thus represent a key mechanism involved in the precipitation of schizophrenia-related pathology following prenatal maternal infection and innate immune imbalances.

Viral RNA and DNA trigger common antiviral responses in mesangial cells

Extrarenal viral infections commonly trigger glomerulonephritis, usually in association with immune complex disease. The Ig component of immune complexes can activate glomerular cell Fc receptors, but whether complexed viral nucleic acids contribute to glomerular inflammation remains unknown. Because of the types of Toll-like receptors (Tlrs) expressed by glomerular mesangial cells, we hypothesized that viral single-stranded RNA and DNA would activate mesangial cells via Tlr-independent pathways and trigger overlapping antiviral immune responses. Consistent with this hypothesis, 5'-triphosphate RNA (3P-RNA) and non-CpG DNA activated murine primary glomerular mesangial cells to secrete Cxcl10 and Il-6 even in cells derived from mice deficient in the Tlr adaptor proteins Myd88 and Trif. Transcriptome analysis revealed that 3P-RNA and non-CpG-DNA triggered almost identical gene expression programs, especially the proinflammatory cytokine Il-6, several chemokines, and genes related to type I IFN. We observed similar findings in glomerular preparations after injecting 3P-RNA and non-CpG-DNA in vivo. These effects depended on the formation of complexes with cationic lipids, which enhanced nucleic acid uptake into the cytosol of mesangial cells. Small interfering RNA studies revealed that 3P-RNA recognition involves Rig-1, whereas non-CpG-DNA did not require Rig-1 or Dai to activate glomerular mesangial cells. We conclude that 3P-RNA and double-stranded DNA trigger a common, TLR-independent, antiviral response in glomerular mesangial cells, which may promote glomerulonephritis in the setting of viral infection.

Focal distribution of hepatitis C virus RNA in infected livers

Background

Hepatitis C virus (HCV) is a plus-strand RNA virus that replicates by amplification of genomic RNA from minus strands leading to accumulation of almost one thousand copies per cell under in vitro cell culture conditions. In contrast, HCV RNA copy numbers in livers of infected patients appear to be much lower, estimated at a few copies per cell.

Methodology/Principal Findings

To gain insights into mechanisms that control HCV replication in vivo, we analyzed HCV RNA levels as well as expression of interferon beta (IFNβ) and several interferon stimulated genes (ISGs) from whole liver sections and micro-dissected subpopulations of hepatocytes in biopsy samples from 21 HCV-infected patients. The results showed that intrahepatic HCV RNA levels range form less than one copy per hepatocyte to a maximum of about eight. A correlation existed between viral RNA levels and IFNβ expression, but not between viral RNA and ISG levels. Also, IFNβ expression did not correlate with ISGs levels. Replication of HCV RNA occurred in focal areas in the liver in the presence of a general induction of ISGs.

Conclusion/Significance

The low average levels of HCV RNA in biopsy samples can be explained by focal distribution of infected hepatocytes. HCV replication directly induces IFNβ, which then activates ISGs. The apparent lack of a correlation between levels of IFNβ and ISG expression indicates that control of the innate immune response during HCV infections depends on multiple factors.

Combined genome-wide expression profiling and targeted RNA interference in primary mouse macrophages reveals perturbation of transcriptional networks associated with interferon signalling

Background

Interferons (IFNs) are potent antiviral cytokines capable of reprogramming the macrophage phenotype through the induction of interferon-stimulated genes (ISGs). Here we have used targeted RNA interference to suppress the expression of a number of key genes associated with IFN signalling in murine macrophages prior to stimulation with interferon-gamma. Genome-wide changes in transcript abundance caused by siRNA activity were measured using exon-level microarrays in the presence or absence of IFNγ.

Results

Transfection of murine bone-marrow derived macrophages (BMDMs) with a non-targeting (control) siRNA and 11 sequence-specific siRNAs was performed using a cationic lipid transfection reagent (Lipofectamine2000) prior to stimulation with IFNγ. Total RNA was harvested from cells and gene expression measured on Affymetrix GeneChip Mouse Exon 1.0 ST Arrays. Network-based analysis of these data revealed six siRNAs to cause a marked shift in the macrophage transcriptome in the presence or absence IFNγ. These six siRNAs targeted the Ifnb1, Irf3, Irf5, Stat1, Stat2 and Nfkb2 transcripts. The perturbation of the transcriptome by the six siRNAs was highly similar in each case and affected the expression of over 600 downstream transcripts. Regulated transcripts were clustered based on co-expression into five major groups corresponding to transcriptional networks associated with the type I and II IFN response, cell cycle regulation, and NF-KB signalling. In addition we have observed a significant non-specific immune stimulation of cells transfected with siRNA using Lipofectamine2000, suggesting use of this reagent in BMDMs, even at low concentrations, is enough to induce a type I IFN response.

Conclusion

Our results provide evidence that the type I IFN response in murine BMDMs is dependent on Ifnb1, Irf3, Irf5, Stat1, Stat2 and Nfkb2, and that siRNAs targeted to these genes results in perturbation of key transcriptional networks associated with type I and type II IFN signalling and a suppression of macrophage M1 polarization.

The extrinsic RNA-sensing pathway for adjuvant immunotherapy of cancer

Infection with RNA viruses presents a typical pattern of virus products, double-stranded RNA (dsRNA), and induces the maturation of antigen-presenting dendritic cell (mDC). There are several dsRNA sensors that are differentially distributed on the cell membrane and in the cytoplasm and are variably expressed depending on the cell type. Among these sensors, TLR3 links to the adaptor TICAM-1 (TRIF), which is characterized by its unique multipronged signaling cascades for cytokine/chemokine production, apoptosis and autophagy in both immune and tumor cells. In the context of mDC maturation, various cellular events are further induced in response to dsRNA; these include cross-priming followed by CD8+ CTL induction, NK activation and proliferation of CD4+ T cells including Th1, Th2, Treg and Th17 cells. In this review, we focus on the potential role of dsRNA in modulating the inflammatory milieu around mDCs and tumor-associated antigens to drive specific cellular effectors against the tumor.

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.

Pattern recognition receptors and control of adaptive immunity

The mammalian immune system effectively fights infection through the cooperation of two connected systems, innate and adaptive immunity. Germ-line encoded pattern recognition receptors (PRRs) of the innate immune system sense the presence of infection and activate innate immunity. Some PRRs also induce signals that lead to the activation of adaptive immunity. Adaptive immunity is controlled by PRR-induced signals at multiple checkpoints dictating the initiation of a response, the type of response, the magnitude and duration of the response, and the production of long-term memory. PRRs thus instruct the adaptive immune system on when and how to best respond to a particular infection. In this review, we discuss the roles of various PRRs in control of adaptive immunity.

Oncolytic viruses from the perspective of the immune system

Cancer treatment with oncolytic viruses is at a crucial intersection from where two very different routes can be taken. The key role of the immune system needs to be addressed proactively to succeed. An immunocentric point of view posits that the intense immunosuppression induced by tumors can be outbalanced by the natural immunogenicity of viruses. To their advantage, viruses can be safely armed to be even more immunostimulatory. The microbe-associated inflammatory response is optimal for antigen presentation and helps to reveal the hidden tumor antigens. The induced immune effector cells patrol the organs to destroy disseminated tumor cells out of the reach of the oncolytic virus. However, as tumor immunosuppression is localized, this concept needs to be revisited because every tumor focus will have to be reached by the oncolytic virus. By contrast, virocentrics see the immune system as an obstacle to virotherapy. A virus is so immunogenic that it dominates all the elicited immunity to the detriment of a response towards tumor antigens. For them immunosuppression is the way to go, and the intense immunosuppression in and around the tumor is now an advantage, offering a privileged site for virus replication. A better oncolytic virus evades the immune system, but such a virus should be very tumor-selective to be safe. Although the trend favors immunocentrics, clinical results have been more often documented in immunocompromised patients. Trials of comparative interventions on the immune system will validate immunocentrism or virocentrism. What seems clear is that at this intersection one should take one route or the other to overcome the current limitations of virotherapy.

Functions of the cytoplasmic RNA sensors RIG-I and MDA-5: key regulators of innate immunity

The innate immune system responds within minutes of infection to produce type I interferons and pro-inflammatory cytokines. Interferons induce the synthesis of cell proteins with antiviral activity, and also shape the adaptive immune response by priming T cells. Despite the discovery of interferons over 50 years ago, only recently have we begun to understand how cells sense the presence of a virus infection. Two families of pattern recognition receptors have been shown to distinguish unique molecules present in pathogens, such as bacterial and fungal cell wall components, viral RNA and DNA, and lipoproteins. The first family includes the membrane-bound toll-like receptors (TLRs). Studies of the signaling pathways that lead from pattern recognition to cytokine induction have revealed extensive and overlapping cascades that involve protein-protein interactions and phosphorylation, and culminate in activation of transcription proteins that control the transcription of genes encoding interferons and other cytokines. A second family of pattern recognition receptors has recently been identified, which comprises the cytoplasmic sensors of viral nucleic acids, including MDA-5, RIG-I, and LGP2. In this review we summarize the discovery of these cytoplasmic sensors, how they recognize nucleic acids, the signaling pathways leading to cytokine synthesis, and viral countermeasures that have evolved to antagonize the functions of these proteins. We also consider the function of these cytoplasmic sensors in apoptosis, development and differentiation, and diabetes.

Lack of evidence for Litopenaeus vannamei Toll receptor (lToll) involvement in activation of sequence-independent antiviral immunity in shrimp

Injection of non-specific dsRNA initiates a broad-spectrum innate antiviral immune response in the Pacific white shrimp, Litopenaeus vannamei, however, the receptor involved in recognition of this by-product of viral infections remains unknown. In vertebrates, dsRNA sensing is mediated by a class of Toll-like receptors (TLRs) and results in activation of the interferon system. Because a TLR (lToll) was recently characterized in L. vannamei, we investigated its potential role in dsRNA recognition. We showed that injection of non-specific RNA duplexes did not modify lToll gene expression. A reverse genetic approach was therefore implemented to study its role in vivo. Silencing of lToll did not impair the ability of non-specific dsRNA to trigger protection from white spot syndrome virus and did not increase the shrimp susceptibility to viral infection, when compared to controls. In contrast, gene-specific dsRNA injected to specifically silence lToll expression activated an antiviral response. These data strongly suggest that shrimp lToll plays no role in dsRNA-induced antiviral immunity.

Ebola Zaire virus blocks type I interferon production by exploiting the host SUMO modification machinery

Ebola Zaire virus is highly pathogenic for humans, with case fatality rates approaching 90% in large outbreaks in Africa. The virus replicates in macrophages and dendritic cells (DCs), suppressing production of type I interferons (IFNs) while inducing the release of large quantities of proinflammatory cytokines. Although the viral VP35 protein has been shown to inhibit IFN responses, the mechanism by which it blocks IFN production has not been fully elucidated. We expressed VP35 from a mouse-adapted variant of Ebola Zaire virus in murine DCs by retroviral gene transfer, and tested for IFN transcription upon Newcastle Disease virus (NDV) infection and toll-like receptor signaling. We found that VP35 inhibited IFN transcription in DCs following these stimuli by disabling the activity of IRF7, a transcription factor required for IFN transcription. By yeast two-hybrid screens and coimmunoprecipitation assays, we found that VP35 interacted with IRF7, Ubc9 and PIAS1. The latter two are the host SUMO E2 enzyme and E3 ligase, respectively. VP35, while not itself a SUMO ligase, increased PIAS1-mediated SUMOylation of IRF7, and repressed Ifn transcription. In contrast, VP35 did not interfere with the activation of NF-κB, which is required for induction of many proinflammatory cytokines. Our findings indicate that Ebola Zaire virus exploits the cellular SUMOylation machinery for its advantage and help to explain how the virus overcomes host innate defenses, causing rapidly overwhelming infection to produce a syndrome resembling fulminant septic shock.

Ebola Zaire virus causes severe hemorrhagic fever in humans that is fatal in almost 90% of cases. The rapid spread of the virus to macrophages and dendritic cells results in the release of high levels of inflammatory cytokines, causing shock and bleeding. The ability of Ebola virus to overwhelm host defenses is believed to result from its suppression of the type I interferon (IFN) response. The Ebola viral protein VP35 is known to block IFN responses, but the precise mechanisms have not been identified. We expressed VP35 in mouse dendritic cells and found that the cells failed to develop a normal IFN response when infected with Newcastle Disease virus. By a yeast two-hybrid system and other biochemical experiments, we showed that the blockade resulted from the conjugation of a Small Ubiquitin-like Modifier (SUMO) protein to IRF-7, the principal cellular factor required for IFN gene expression. However, the cells were still able to activate NF-κB, a transcription factor responsible for the release of proinflammatory cytokines. Our findings provide a first example where a virus hijacks the host SUMO system to undermine innate immunity, and help to explain how Ebola virus spreads rapidly in lymphoid tissues to cause a lethal inflammatory syndrome.

Measure and countermeasure: type I IFN (IFN-alpha/beta) antiviral response against West Nile virus

As a first line of defense after viral infection, host cells develop an intrinsic immune response to control virus dissemination and protect against serious infection. Recent experiments have shown a dominant role of the IFN-alpha/beta response in protection against lethal West Nile virus (WNV) by limiting the cellular and tissue tropism of infection. This review will focus on advances in identifying the host sensors that detect WNV and the adaptor molecules and signaling pathways that regulate the induction of IFN-alpha/beta defenses that limit WNV replication, spread and pathogenesis.

TLR agonists prevent the establishment of allogeneic hematopoietic chimerism in mice treated with costimulation blockade

Activation of TLR4 by administration of LPS shortens the survival of skin allografts in mice treated with costimulation blockade through a CD8 T cell-dependent, MyD88-dependent, and type I IFN receptor-dependent pathway. The effect of TLR activation on the establishment of allogeneic hematopoietic chimerism in mice treated with costimulation blockade is not known. Using a costimulation blockade protocol based on a donor-specific transfusion (DST) and a short course of anti-CD154 mAb, we show that LPS administration at the time of DST matures host alloantigen-presenting dendritic cells, prevents the establishment of mixed allogeneic hematopoietic chimerism, and shortens survival of donor-specific skin allografts. LPS mediates its effects via a mechanism that involves both CD4(+) and CD8(+) T cells and results from signaling through either the MyD88 or the type I IFN receptor pathways. We also document that timing of LPS administration is critical, as injection of LPS 24 h before treatment with DST and anti-CD154 mAb does not prevent hematopoietic engraftment but administration the day after bone marrow transplantation does. We conclude that TLR4 activation prevents the induction of mixed allogeneic hematopoietic chimerism through type I IFN receptor and MyD88-dependent signaling, which leads to the up-regulation of costimulatory molecules on host APCs and the generation of alloreactive T cells. These data suggest that distinct but overlapping cellular and molecular mechanisms control the ability of TLR agonists to block tolerance induction to hematopoietic and skin allografts in mice treated with costimulation blockade.

REUL is a novel E3 ubiquitin ligase and stimulator of retinoic-acid-inducible gene-I

RIG-I and MDA5 are cytoplasmic sensors that recognize different species of viral RNAs, leads to activation of the transcription factors IRF3 and NF-kappaB, which collaborate to induce type I interferons. In this study, we identified REUL, a RING-finger protein, as a specific RIG-I-interacting protein. REUL was associated with RIG-I, but not MDA5, through its PRY and SPRY domains. Overexpression of REUL potently potentiated RIG-I-, but not MDA5-mediated downstream signalling and antiviral activity. In contrast, the RING domain deletion mutant of REUL suppressed Sendai virus (SV)-induced, but not cytoplasmic polyI:C-induced activation of IFN-beta promoter. Knockdown of endogenous REUL by RNAi inhibited SV-triggered IFN-beta expression, and also increased VSV replication. Full-length RIG-I, but not the CARD domain deletion mutant of RIG-I, underwent ubiquitination induced by REUL. The Lys 154, 164, and 172 residues of the RIG-I CARD domain were critical for efficient REUL-mediated ubiquitination, as well as the ability of RIG-I to induce activation of IFN-beta promoter. These findings suggest that REUL is an E3 ubiquitin ligase of RIG-I and specifically stimulates RIG-I-mediated innate antiviral activity.

Innate immune sensing of modified vaccinia virus Ankara (MVA) is mediated by TLR2-TLR6, MDA-5 and the NALP3 inflammasome

Modified vaccinia virus Ankara (MVA) is an attenuated double-stranded DNA poxvirus currently developed as a vaccine vector against HIV/AIDS. Profiling of the innate immune responses induced by MVA is essential for the design of vaccine vectors and for anticipating potential adverse interactions between naturally acquired and vaccine-induced immune responses. Here we report on innate immune sensing of MVA and cytokine responses in human THP-1 cells, primary human macrophages and mouse bone marrow-derived macrophages (BMDMs). The innate immune responses elicited by MVA in human macrophages were characterized by a robust chemokine production and a fairly weak pro-inflammatory cytokine response. Analyses of the cytokine production profile of macrophages isolated from knockout mice deficient in Toll-like receptors (TLRs) or in the adapter molecules MyD88 and TRIF revealed a critical role for TLR2, TLR6 and MyD88 in the production of IFNbeta-independent chemokines. MVA induced a marked up-regulation of the expression of RIG-I like receptors (RLR) and the IPS-1 adapter (also known as Cardif, MAVS or VISA). Reduced expression of RIG-I, MDA-5 and IPS-1 by shRNAs indicated that sensing of MVA by RLR and production of IFNbeta and IFNbeta-dependent chemokines was controlled by the MDA-5 and IPS-1 pathway in the macrophage. Crosstalk between TLR2-MyD88 and the NALP3 inflammasome was essential for expression and processing of IL-1beta. Transcription of the Il1b gene was markedly impaired in TLR2(-/-) and MyD88(-/-) BMDM, whereas mature and secreted IL-1beta was massively reduced in NALP3(-/-) BMDMs or in human THP-1 macrophages with reduced expression of NALP3, ASC or caspase-1 by shRNAs. Innate immune sensing of MVA and production of chemokines, IFNbeta and IL-1beta by macrophages is mediated by the TLR2-TLR6-MyD88, MDA-5-IPS-1 and NALP3 inflammasome pathways. Delineation of the host response induced by MVA is critical for improving our understanding of poxvirus antiviral escape mechanisms and for designing new MVA vaccine vectors with improved immunogenicity.

Activation of melanoma differentiation-associated gene 5 causes rapid involution of the thymus

In the course of infection, the detection of pathogen-associated molecular patterns by specialized pattern recognition receptors in the host leads to activation of the innate immune system. Whereas the subsequent induction of adaptive immune responses in secondary lymphoid organs is well described, little is known about the effects of pathogen-associated molecular pattern-induced activation on primary lymphoid organs. Here we show that activation of innate immunity through the virus-sensing melanoma differentiation-associated gene 5 (MDA-5) receptor causes a rapid involution of the thymus. We observed a strong decrease in thymic cellularity associated with characteristic alterations in thymic subpopulations and microanatomy. In contrast, immune stimulation with potent TLR agonists did not lead to thymic involution or induce changes in thymic subpopulations, demonstrating that thymic pathology is not a general consequence of innate immune activation. We determined that suppression of thymocyte proliferation and enhanced apoptosis are the essential cellular mechanisms involved in the decrease in thymic size upon MDA-5 activation. Further, thymic involution critically depended on type I IFN. Strikingly however, no direct action of type I IFN on thymocytes was required, given that the decrease in thymic size was still observed in mice with a selective deletion of the type I IFN receptor on T cells. All changes observed were self-limiting, given that cessation of MDA-5 activation led to a rapid recovery of thymic size. We show for the first time that the in vivo activation of the virus-sensing MDA-5 receptor leads to a rapid and reversible involution of the thymus.

Actin and RIG-I/MAVS signaling components translocate to mitochondria upon influenza A virus infection of human primary macrophages

Influenza A virus is one of the most important causes of respiratory infection. During viral infection, multiple cell signaling cascades are activated, resulting in the production of antiviral cytokines and initiation of programmed cell death of virus-infected cells. In the present study, we have used subcellular proteomics to reveal the host response to influenza A infection at the protein level in human macrophages. Macrophages were infected with influenza A virus, after which the cytosolic and mitochondrial cell fractions were prepared and analyzed by using two-dimensional electrophoresis for protein separation and mass spectrometry for protein identification. In cytosolic proteomes, the level of several heat shock proteins and fragments of cytoskeletal proteins was clearly up-regulated during influenza A virus infection. In mitochondrial proteomes, simultaneously with the expression of viral proteins, the level of intact actin and tubulin was highly up-regulated. This was followed by translocation of the components of antiviral RNA recognition machinery, including RIG-I (retinoic acid-inducible protein I), TRADD (TNFR1-associated death domain protein), TRIM25 (tripartite motif protein 25), and IKKepsilon (inducible IkappaB kinase), onto the mitochondria. Cytochalasin D, a potent inhibitor of actin polymerization, clearly inhibited influenza A virus-induced expression of IFN-beta, IL-29, and TNF-alpha, suggesting that intact actin cytoskeleton structure is crucial for proper activation of antiviral response. At late phases of infection mitochondrial fragmentation of actin was seen, indicating that actin fragments, fractins, are involved in disruption of mitochondrial membranes during apoptosis of virus-infected cells. In conclusion, our results suggest that actin network interacts with mitochondria to regulate both antiviral and cell death signals during influenza A virus infection.

Type I interferon in HIV treatment: from antiviral drug to therapeutic target

Type I interferons (IFNs) are soluble molecules that exert potent antiviral activity and are currently used for the treatment of a panel of viral infections. In the case of HIV, the use of type I IFN has had limited success, and has almost been abandoned. During the last decade, a series of studies has highlighted how HIV infection may cause overactivation of type I IFN production, which contributes to the exhaustion of the immune system and to disease progression. This review describes the transition from the proposed use of type I IFN as antiviral drugs in HIV infection, to the idea that blocking their activity or production may provide an immunologic benefit of much greater importance than their antiviral activity.

Brome mosaic virus capsid protein regulates accumulation of viral replication proteins by binding to the replicase assembly RNA element

Viruses provide valuable insights into the regulation of molecular processes. Brome mosaic virus (BMV) is one of the simplest entities with four viral proteins and three genomic RNAs. Here we report that the BMV capsid protein (CP), which functions in RNA encapsidation and virus trafficking, also represses viral RNA replication in a concentration-dependent manner by inhibiting the accumulation of the RNA replication proteins. Expression of the replication protein 2a in trans can partially rescue BMV RNA accumulation. A mutation in the CP can decrease the repression of translation. Translation repression by the CP requires a hairpin RNA motif named the B Box that contains seven loop nucleotides (nt) within the 5' untranslated regions (UTR) of BMV RNA1 and RNA2. Purified CP can bind directly to the B Box RNA with a K (d) of 450 nM. The secondary structure of the B Box RNA was determined to contain a highly flexible 7-nt loop using NMR spectroscopy, native gel analysis, and thermal denaturation studies. The B Box is also recognized by the BMV 1a protein to assemble the BMV replicase, suggesting that the BMV CP can act to regulate several viral infection processes.

An antiserum against Atlantic salmon IFNa1 detects IFN and neutralizes antiviral activity produced by poly I:C stimulated cells

Type I interferons (IFNs) play a crucial role in innate immune responses against virus infections in vertebrates. Two IFNs (IFNa1 and IFNa2) have previously been cloned from Atlantic salmon. In the present work a polyclonal antiserum, which was generated against salmon IFNa1 was used to study its production in cells by immunoblot detection and neutralization of antiviral activity. The antiserum was first confirmed to detect and neutralize the antiviral activity of recombinant salmon IFNa1 produced in HEK293 cells. The antiserum also detected IFNa1 and neutralized 95-98% of the antiviral activity in supernatants of poly I:C stimulated salmon TO cells. This suggests that IFNa1/IFNa2 are the major IFNs produced by poly I:C stimulated TO cells. The antiserum neutralized most of the IFN activity in poly I:C stimulated head kidney leucocytes from three of five individuals, but in stimulated leucocytes from the other two individuals only 75% of the antiviral activity was neutralized. This shows that although IFNa1/IFNa2 are major IFNs secreted by poly I:C stimulated leucocytes, these cells can also produce additional molecules with IFN-like activity.

Identification of an Atlantic salmon IFN multigene cluster encoding three IFN subtypes with very different expression properties

A cluster of 11 interferon (IFN) genes were identified in the Atlantic salmon genome linked to the growth hormone 1 gene. The genes encode three different IFN subtypes; IFNa (two genes), IFNb (four genes) and IFNc (five genes), which show 22-32% amino acid sequence identity. Expression of the fish IFNs were studied in head kidney, leukocytes or TO cells after stimulation with the dsRNA poly I:C or the imidazoquinoline S-27609. In mammals, poly I:C induces IFN-beta through the RIG-I/MDA5 or the TLR3 pathway, both of which are dependent on NF-kB. In contrast, S-27609 induces mammalian IFN-alpha in plasmacytoid dendritic cells through the TLR7 pathway independent of NF-kappaB. The presence of an NF-kappaB site in their promoters and their strong up-regulation by poly I:C, suggest that salmon IFNa1/IFNa2 are induced through similar pathways as IFN-beta. In contrast, the apparent lack of NF-kappaB motif in the promoter and the strong upregulation by S-27609 in head kidney and leukocytes, suggest that IFNb genes are induced through a pathway similar to mammalian IFN-alpha. IFNc genes showed expression patterns different from both IFNa and IFNb. Taken together, salmon IFNa and IFNb are not orthologs of mammalian IFN-beta and IFN-alpha, respectively, but appear to utilize similar induction pathways.

Modulation of dendritic cell maturation and function with mono- and bifunctional small interfering RNAs targeting indoleamine 2,3-dioxygenase

Antigen-presenting cells expressing indoleamine 2,3-dioxygenase (IDO) play a critical role in maintaining peripheral tolerance. Strategies to inhibit IDO gene expression and enhance antigen-presenting cell function might improve anti-tumour immunity. Here we have designed highly effective anti-IDO small interfering (si) RNAs that function at low concentrations. When delivered to human primary immune cells such as monocytes and dendritic cells (DCs), they totally inhibited IDO gene expression without impairing DC maturation and function. Depending on the design and chemical modifications, we show that it is possible to design either monofunctional siRNAs devoid of immunostimulation or bifunctional siRNAs with gene silencing and immunostimulatory activities. The latter are able to knockdown IDO expression and induce cytokine production through either endosomal Toll-like receptor 7/8 or cytoplasmic retinoid acid-inducible gene 1 helicase. Inhibition of IDO expression with both classes of siRNAs inhibited DC immunosuppressive function on T-cell proliferation. Immature monocyte-derived DCs that had been transfected with siRNA-bearing 5'-triphosphate activated T cells, indicating that, even in the absence of external stimuli such as tumour necrosis factor-alpha, those DCs were sufficiently mature to initiate T-cell activation. Collectively, our data highlight the potential therapeutic applications of this new generation of siRNAs in immunotherapy.

Cytomegalovirus microRNAs

MicroRNAs (miRNAs) are small, non-coding RNA molecules that regulate gene expression at a post-transcriptional level in virtually all eukaryotic organisms and some viruses, particularly herpesviruses. miRNAs are non-immunogenic, stealthy tools for viruses to regulate their as well as host gene expression. The human cytomegalovirus (HCMV) is the major cause of morbidity in immunocompromised patients and allogenic bone-marrow or organ-transplant recipients and the leading cause of congenital birth defects. HCMV miRNAs may provide valuable targets for new urgently needed antiviral drugs. This review focuses on recent findings for viral miRNAs expressed by cytomegaloviruses (CMV) including data from human, chimpanzee, and murine CMV. These are discussed in the context of findings for other viruses to highlight potentially conserved roles exerted by viral miRNAs.

Autophagy, antiviral immunity, and viral countermeasures

The autophagy pathway likely evolved not only to maintain cellular and tissue homeostasis but also to protect cells against microbial attack. This conserved mechanism by which cytoplasmic cargo is delivered to the endolysosomal system is now recognized as a central player in coordinating the host response to diverse intracellular pathogens, including viruses. As an endolysosomal delivery system, autophagy functions in the transfer of viruses from the cytoplasm to the lysosome where they are degraded, in the transfer of viral nucleic acids to endosomal sensors for the activation of innate immunity, and in the transfer of endogenous viral antigens to MHC class II compartments for the activation of adaptive immunity. Viruses have, in turn, evolved different strategies to antagonize, and potentially, to exploit the host autophagic machinery. Moreover, through mechanisms not yet well understood, autophagy may dampen host innate immune and inflammatory responses to viral infection. This review highlights the roles of autophagy in antiviral immunity, viral strategies to evade autophagy, and potential negative feedback functions of autophagy in the host antiviral response.

Hantaan virus triggers TLR3-dependent innate immune responses

Immediately after viral infection, innate responses including expression of IFN-alpha/beta and IFN-stimulated genes (ISGs) are elicited ubiquitously by recruitment of specific pathogen recognition receptors. The velocity to induce IFN-alpha/beta and ISGs in response to an infection is often decisive for virulence. Interestingly, in primary endothelial cells ISGs are induced later by hantaviruses pathogenic to humans than those considered to be nonpathogenic or of low virulence. Here we demonstrate that pathogenic Hantaan (HTNV) and putatively nonpathogenic Prospect Hill hantavirus (PHV) differentially activate innate responses in the established cell lines A549 and HuH7. STAT1alpha phosphorylation was detectable 3 h after PHV inoculation but not within the first 2 days after HTNV inoculation. The velocity to induce the ISGs MxA and ISG15 correlated inversely with amounts of virus produced. Moreover, expression of the inflammatory chemokine CCL5 was also induced differentially. Both hantaviruses induced innate responses via TRAF3 (TNF receptor-associated factor 3), and TLR3 was required for HTNV-induced expression of MxA, but not for the MxA induction triggered by PHV. Infection of RIG-I-deficient HuH7.5 cells revealed that RIG-I (retinoic acid receptor I) was not necessary for induction of innate responses by PHV. Taken together, these data suggest that HTNV and PHV elicit different signaling cascades that converge via TRAF3. Early induction of antiviral responses might contribute to efficient elimination of PHV. Subsequent to clearance of the infection, innate responses most likely cease; vice versa, retarded induction of antiviral responses could lead to increased HTNV replication and dissemination, which might cause a prolonged inflammatory response and might contribute to the in vivo virulence.

Nucleotide sequences and modifications that determine RIG-I/RNA binding and signaling activities

Cytoplasmic viral RNAs with 5' triphosphates (5'ppp) are detected by the RNA helicase RIG-I, initiating downstream signaling and alpha/beta interferon (IFN-alpha/beta) expression that establish an antiviral state. We demonstrate here that the hepatitis C virus (HCV) 3' untranslated region (UTR) RNA has greater activity as an immune stimulator than several flavivirus UTR RNAs. We confirmed that the HCV 3'-UTR poly(U/UC) region is the determinant for robust activation of RIG-I-mediated innate immune signaling and that its antisense sequence, poly(AG/A), is an equivalent RIG-I activator. The poly(U/UC) region of the fulminant HCV JFH-1 strain was a relatively weak activator, while the antisense JFH-1 strain poly(AG/A) RNA was very potent. Poly(U/UC) activity does not require primary nucleotide sequence adjacency to the 5'ppp, suggesting that RIG-I recognizes two independent RNA domains. Whereas poly(U) 50-nt or poly(A) 50-nt sequences were minimally active, inserting a single C or G nucleotide, respectively, into these RNAs increased IFN-beta expression. Poly(U/UC) RNAs transcribed in vitro using modified uridine 2' fluoro or pseudouridine ribonucleotides lacked signaling activity while functioning as competitive inhibitors of RIG-I binding and IFN-beta expression. Nucleotide base and ribose modifications that convert activator RNAs into competitive inhibitors of RIG-I signaling may be useful as modulators of RIG-I-mediated innate immune responses and as tools to dissect the RNA binding and conformational events associated with signaling.

DNA sensors in innate immune system

Microbial sensing mediated by pattern recognition receptors (PRRs) is the first key step to trigger innate immune responses, represented by the induction of type I interferons (IFNs), proinflammatory cytokines and chemokines. This innate signaling elicits an efficient activation of more specific responses in adaptive immunity. Such coordinated responses in the two systems are essential for the optimal elimination of invading microbes. Despite a major advance in our understanding of RNA sensors, TLR9 remained the only known sensor of DNA. On the other hand, there has been accumulating evidence supporting the existence of TLR9-independent DNA recognition mechanism. In this regard, DAI (also termed as DLM-1/ZBP1), the first sensor of cytosolic DNA, has recently been identified with its activation of IFN-regulatory factors(IRFs) and NF-kappaB transcriptional factors. Several recent papers suggest the involvement of an additional cytosolic DNA sensor(s). There is also a recent report that cytosolic microbial and host DNA can trigger pro-inflammatory responses via the TLR- and IRF-indepnedent pathway mediated by the inflammasome, which is consisted of NLR family members together with the adaptor protein ASC and caspase-1. In addition, evidence has been provided that host- and virus-derived proteins, which contain DNA-binding motifs (Zalpha and/or Zbeta) similar to those of DAI(DLM-1/ZBP1), negatively regulates the immune response that is activated by cytosolic DNA. Thus, these recent findings reveal the complex DNA-sensing mechanism for triggering the activation of innate immunity, and the breakdown of this sensing mechanism may lead to autoimmune abnormalities.

Contribution of IRF-3 mediated IFNbeta production to DNA vaccine dependent cellular immune responses

The mechanism(s) by which DNA vaccines activate Ag-specific cellular immune responses is incompletely understood. Current findings indicate that IRF-3 plays an important role in this process. The IRF-3 dependent signaling pathway is triggered by the presence of intracytoplasmic DNA, and culminates in the production of type I IFNs. DNA vaccination of IRF-3 KO mice elicits a strong Ag-specific humoral response, yet CD4 and CD8 T cell responses (including the production of Th1, Th2 and Th 17 cytokines) are severely impaired. Although expression of the immunogenic protein encoded by the DNA vaccine was similar in IRF-3 KO vs wild type mice, antigen presentation was severely impaired in the KO animals. This defect was remedied by the co-delivery of an IFNbeta encoding plasmid. These findings suggest that the IRF-3/IFNbeta pathways are key to the induction of cellular immunity following DNA vaccination.

A recent perspective on alcohol, immunity, and host defense

BACKGROUND

Multiple line of clinical and experimental evidence demonstrates that both acute, moderate, and chronic, excessive alcohol use result in various abnormalities in the functions of the immune system.

METHODS

Medline and PubMed databases were used to identify published reports with particular interest in the period of 2000-2008 in the subject of alcohol use, infection, inflammation, innate, and adaptive immunity.

RESULTS

This review article summarizes recent findings relevant to acute or chronic alcohol use-induced immunomodulation and its consequences on host defense against microbial pathogens and tissue injury. Studies with in vivo and in vitro alcohol administration are both discussed. The effects of alcohol on lung infections, trauma and burn injury, liver, pancreas, and cardiovascular diseases are evaluated with respect to the role of immune cells. Specific changes in innate immune response and abnormalities in adaptive immunity caused by alcohol intake are detailed.

CONCLUSION

Altered inflammatory cell and adaptive immune responses after alcohol consumption result in increased incidence and poor outcome of infections and other organ-specific immune-mediated effects.

Viral infection: a potent barrier to transplantation tolerance

Transplantation of allogeneic organs has proven to be an effective therapeutic for a large variety of disease states, but the chronic immunosuppression that is required for organ allograft survival increases the risk for infection and neoplasia and has direct organ toxicity. The establishment of transplantation tolerance, which obviates the need for chronic immunosuppression, is the ultimate goal in the field of transplantation. Many experimental approaches have been developed in animal models that permit long-term allograft survival in the absence of chronic immunosuppression. These approaches function by inducing peripheral or central tolerance to the allograft. Emerging as some of the most promising approaches for the induction of tolerance are protocols based on costimulation blockade. However, as these protocols move into the clinic, there is recognition that little is known as to their safety and efficacy when confronted with environmental perturbants such as virus infection. In animal models, it has been reported that virus infection can prevent the induction of tolerance by costimulation blockade and, in at least one experimental protocol, can lead to significant morbidity and mortality. In this review, we discuss how viruses modulate the induction and maintenance of transplantation tolerance.

Modulation of innate immune signalling pathways by viral proteins

In recent years an explosion of information on the various strategies viruses employ to penetrate and hijack the host cell has led to an increased understanding of both viruses themselves and the host immune response. Despite their simplicity viruses have evolved a number of strategies to not only evade the host immune response but also modulate immune signalling to favour their replication and survival within the cell. The innate immune response provides the host with an early reaction against viruses. This response relies heavily upon the recognition of pathogen-associated molecular patterns (PAMPs) by a number of host pattern recognition receptors (PRRs), leading to activation of innate signalling pathways and altered gene expression. In this chapter we outline the signalling pathways that respond to viral infection and the various methods that viruses utilize to evade detection and modulate the innate immune response to favour their survival.

RIG-I and dsRNA-induced IFNbeta activation

Except for viruses that initiate RNA synthesis with a protein primer (e.g., picornaviruses), most RNA viruses initiate RNA synthesis with an NTP, and at least some of their viral _pppRNAs remain unblocked during the infection. Consistent with this, most viruses require RIG-I to mount an innate immune response, whereas picornaviruses require mda-5. We have examined a SeV infection whose ability to induce interferon depends on the generation of capped dsRNA (without free 5′ tri-phosphate ends), and found that this infection as well requires RIG-I and not mda-5. We also provide evidence that RIG-I interacts with poly-I/C in vivo, and that heteropolymeric dsRNA and poly-I/C interact directly with RIG-I in vitro, but in different ways; i.e., poly-I/C has the unique ability to stimulate the helicase ATPase of RIG-I variants which lack the C-terminal regulatory domain.

Cloning and characterization of SARI (suppressor of AP-1, regulated by IFN)

We describe a novel basic leucine zipper containing type I IFN-inducible early response gene SARI (Suppressor of AP-1, Regulated by IFN). Steady-state SARI mRNA expression was detected in multiple lineage-specific normal cells, but not in their transformed/tumorigenic counterparts. In normal and cancer cells, SARI expression was induced 2 h after fibroblast IFN (IFN-beta) treatment with 1 U/ml of IFN-beta. Antisense inhibition of SARI protected HeLa cells from IFN-beta-mediated growth inhibition. As a corollary, overexpression of SARI inhibited growth and induced apoptosis in cancer cells, but not in normal cells. SARI interacted with c-Jun via its leucine zipper, resulting in inhibition of DNA binding of activator protein (AP-1) complex and consequently AP-1-dependent gene expression. Transformed cells relying on AP-1 activity for proliferative advantage demonstrated increased susceptibility to SARI-mediated growth inhibition. These findings uncover a novel mode of IFN-induced anti-tumor growth suppression and suggest potential gene therapy applications for SARI.

New horizons in adjuvants for vaccine development

Over the last decade, there has been a flurry of research on adjuvants for vaccines, and several novel adjuvants are now in licensed products or in late stage clinical development. The success of adjuvants in enhancing the immune response to recombinant antigens has led many researchers to re-focus their vaccine development programs. Successful vaccine development requires knowing which adjuvants to use and knowing how to formulate adjuvants and antigens to achieve stable, safe and immunogenic vaccines. For the majority of vaccine researchers this information is not readily available, nor is access to well-characterized adjuvants. In this review, we outline the current state of adjuvant research and development and how formulation parameters can influence the effectiveness of adjuvants.

FLN29 deficiency reveals its negative regulatory role in the Toll-like receptor (TLR) and retinoic acid-inducible gene I (RIG-I)-like helicase signaling pathway

FLN29 was identified as an interferon (IFN)-inducible gene, and it has been shown to suppress Toll-like receptor 4-mediated NF-kappaB activation by binding to TRAF6. To elucidate the physiological roles of FLN29, we generated FLN29-deficient mice. FLN29 deficiency resulted in hyper-response to LPS both in vivo and in vitro, demonstrating the negative regulatory role of FLN29 in TLR4 signaling. Furthermore, we found that FLN29(-/-) mice exhibited increased susceptibility to poly(I:C)-induced septic shock compared with WT mice. FLN29(-/-) fibroblasts were highly resistant to vesicular stomatitis virus infection, and these cells produced more IFN-beta than WT cells did in response to not only intracellular poly(I:C) but also overexpression of IPS-1. Forced expression of FLN29 inhibited the IPS-1-dependent activation of both NF-kappaB and IRF3. We also found that FLN29 could interact with TRIF, IPS-1, TRAF3, and TRAF6. Together, these results suggest that FLN29, in addition to playing a negative regulatory role in the TLR4 signaling pathway, negatively regulates the RIG-I-like helicase signaling pathway at the level of IPS-1/TRAF6 and IPS-1/TRAF3 complexes.

Estrogen inhibits dendritic cell maturation to RNA viruses

Dendritic cells (DCs) play a central role in initiating and polarizing the immune response. Therefore, DC maturation represents a key control point in the shift from innate to adaptive immunity. It is suspected that during pregnancy, hormones are critical factors that modulate changes reported to occur in maternal immunity. Here we examined the effect of 17-beta-estradiol (E2) on the maturational response triggered by virus in human DCs and its influence on their ability to activate naive T cells. We developed an in vitro system to measure the response of DCs to virus infection with Newcastle disease virus (NDV) after a 24-hour E2 treatment. Using this system, we demonstrated that E2 pretreatment down-regulated the antiviral response to RNA viruses in DCs by profoundly suppressing type I interferon (IFN) synthesis and other important inflammatory products. In addition, the DCs capacity to stimulate naive CD4 T cells was also reduced. These results suggest an important role for E2 in suppressing the antiviral response and provide a mechanism for the reduced immunity to virus infection observed during pregnancy.

Systems biology approach predicts immunogenicity of the yellow fever vaccine in humans

A major challenge in vaccinology is to prospectively determine vaccine efficacy. Here we have used a systems biology approach to identify early gene 'signatures' that predicted immune responses in humans vaccinated with yellow fever vaccine YF-17D. Vaccination induced genes that regulate virus innate sensing and type I interferon production. Computational analyses identified a gene signature, including complement protein C1qB and eukaryotic translation initiation factor 2 alpha kinase 4-an orchestrator of the integrated stress response-that correlated with and predicted YF-17D CD8(+) T cell responses with up to 90% accuracy in an independent, blinded trial. A distinct signature, including B cell growth factor TNFRS17, predicted the neutralizing antibody response with up to 100% accuracy. These data highlight the utility of systems biology approaches in predicting vaccine efficacy.

Specific inhibition of the PKR-mediated antiviral response by the murine cytomegalovirus proteins m142 and m143

Double-stranded RNA (dsRNA) produced during viral infection activates several cellular antiviral responses. Among the best characterized is the shutoff of protein synthesis mediated by the dsRNA-dependent protein kinase (PKR) and the oligoadenylate synthetase (OAS)/RNase L system. As viral replication depends on protein synthesis, many viruses have evolved mechanisms for counteracting the PKR and OAS/RNase L pathways. The murine cytomegalovirus (MCMV) proteins m142 and m143 have been characterized as dsRNA binding proteins that inhibit PKR activation, phosphorylation of the translation initiation factor eIF2alpha, and a subsequent protein synthesis shutoff. In the present study we analyzed the contribution of the PKR- and the OAS-dependent pathways to the control of MCMV replication in the absence or presence of m142 and m143. We show that the induction of eIF2alpha phosphorylation during infection with an m142- and m143-deficient MCMV is specifically mediated by PKR, not by the related eIF2alpha kinases PERK or GCN2. PKR antagonists of vaccinia virus (E3L) or herpes simplex virus (gamma34.5) rescued the replication defect of an MCMV strain with deletions of both m142 and m143. Moreover, m142 and m143 bound to each other and interacted with PKR. By contrast, an activation of the OAS/RNase L pathway by MCMV was not detected in the presence or absence of m142 and m143, suggesting that these viral proteins have little or no influence on this pathway. Consistently, an m142- and m143-deficient MCMV strain replicated to high titers in fibroblasts lacking PKR but did not replicate in cells lacking RNase L. Hence, the PKR-mediated antiviral response is responsible for the essentiality of m142 and m143.

Persistent fetal infection with bovine viral diarrhea virus differentially affects maternal blood cell signal transduction pathways

The consequences of viral infection during pregnancy include impact on fetal and maternal immune responses and on fetal development. Transplacental infection in cattle with noncytopathic bovine viral diarrhea virus (ncpBVDV) during early gestation results in persistently infected (PI) fetuses with life-long viremia and susceptibility to infections. Infection of the fetus during the third trimester or after birth leads to a transient infection cleared by a competent immune system. We hypothesized that ncpBVDV infection and presence of an infected fetus would alter immune response and lead to downregulation of proinflammatory processes in pregnant dams. Naïve pregnant heifers were challenged with ncpBVDV2 on day 75 (PI fetus) and day 175 [transiently infected (TI) fetus] or kept uninfected (healthy control fetus). Maternal blood samples were collected up to day 190 of gestation. Genome-wide microarray analysis of gene expression in maternal peripheral white blood cells, performed on days 160 and 190 of gestation, revealed multiple signal transduction pathways affected by ncpBVDV infection. Acute infection and presence of a TI fetus caused upregulation of the type I interferon (IFN) pathway genes, including dsRNA sensors and IFN-stimulated genes. The presence of a PI fetus caused prolonged downregulation of chemokine receptor 4 (CXCR4) and T cell receptor (TCR) signaling in maternal blood cells. We conclude that: 1) infection with ncpBVDV induces a vigorous type I IFN response, and 2) presence of a PI fetus causes downregulation of important signaling pathways in the blood of the dam, which could have deleterious consequences on fetal development and the immune response.

Activation of Toll-like receptor 3 augments myofibroblast differentiation

Airway remodeling is observed in the airways of patients with asthma, and differentiation of fibroblasts to myofibroblasts plays a critical role in the progress of airway remodeling. Viral infection induces not only the disease development and exacerbations but also airway remodeling. The aim of this study was to evaluate whether the activation of Toll-like receptor 3 (TLR3) can affect the differentiation of fibroblasts to myofibroblasts and the extracellular matrix (ECM) protein production. Human fetal lung fibroblasts (HFL-1) and adult lung fibroblasts were treated with a synthetic double-stranded RNA, polyinosine-polycytidylic acid (poly[I:C]) and the expression of alpha-smooth muscle actin (alpha-SMA), a marker of myofibroblast differentiation, was evaluated. The release of transforming growth factor-beta(1) (TGF-beta(1)) and ECM protein production were assessed. The effect of anti-TGF-beta antibody on the alpha-SMA and ECM production was also assessed. Poly(I:C) significantly augmented the alpha-SMA expression (P < 0.01) and release of TGF-beta(1) (P < 0.01) compared with control. Bafilomycin, an inhibitor of TLR3 signaling, diminished poly(I:C)-augmented TGF-beta(1) release. Anti-TGF-beta(1) antibody inhibited the poly(I:C)-augmented alpha-SMA expression. Poly(I:C) enhanced translocation of nuclear factor-kB (NF-kappaB) and interferon regulatory factor-3 (IRF-3) into the nucleus. Poly(I:C)-augmented TGF-beta(1) release was almost completely blocked by NF-kappaB inhibitors, but not by silencing IRF-3. The production of fibronectin and collagen I expression were significantly increased by poly(I:C) (P < 0.01) and they were inhibited by anti-TGF-beta antibody. These results suggest that activation of TLR3 can affect the differentiation to myofibroblasts and enhance ECM production via the NF-kappaB-TGF-beta(1)-dependent pathway.

Dendritic cells and macrophages in the genitourinary tract

Dendritic cells (DCs) and macrophages are antigen-presenting cells (APCs) that are important in innate immune defense as well as in the generation and regulation of adaptive immunity against a wide array of pathogens. The genitourinary (GU) tract, which serves an important reproductive function, is constantly exposed to numerous agents of sexually transmitted infections (STIs). To combat these STIs, several subsets of DCs and macrophages are strategically localized within the GU tract. In the female genital mucosa, recruitment and function of these APCs are uniquely governed by sex hormones. This review summarizes the latest advances in our understanding of DCs and macrophages in the GU tract with respect to their subsets, lineage, and function. In addition, we discuss the divergent roles of these cells in immune defense against STIs as well as in maternal tolerance to the fetus.

Evolution of MDA-5/RIG-I-dependent innate immunity: independent evolution by domain grafting

Type I Interferons (IFNs) are requisite components in antiviral innate immunity. Classically, a Toll-like receptor-dependent pathway induces type I interferons. However, recent recognition of melanoma differentiation associated gene-5 (MDA-5) and retinoic acid inducible gene-I (RIG-I) as primary sensors of RNA viruses for type I interferon induction highlights a potentially unique pathway for innate immunity. Our present investigation tracing the phylogenetic origin of MDA-5 and RIG-I domain arrangement (CARD1-CARD2-helicase-DEAD/DEAH) indicates that these proteins originated specifically in mammals, firmly linking this family of proteins with interferons in a highly derived evolutionary development of innate immunity. MDA-5, but not RIG-I, orthologs are found in fish, indicating that MDA-5 might have evolved before RIG-I. Our analyses also reveal that the MDA-5 and RIG-I domain arrangement evolved independently by domain grafting and not by a simple gene-duplication event of the entire four-domain arrangement, which may have been initiated by differential sensitivity of these proteins to viral infection.

Expression of interferon and interferon-induced genes in salmonids in response to virus infection, interferon-inducing compounds and vaccination

Interferons (IFNs) involved in innate immunity against viruses have recently been cloned from Atlantic salmon and rainbow trout. Moreover, several IFN-stimulated genes (ISGs) have been cloned from salmonids although only Mx has been shown to possess antiviral properties. Much less in known about how viruses induce IFNs in salmonids, but synthetic ligands for some of the main mammalian viral sensors also induce IFNs and ISGs in salmonids. Analysis of the promoters of the salmon IFN-alpha1 and IFN-alpha2 genes shows that activation is dependent on both NFkappaB and IRFs similar to human IFN-beta. Furthermore, several IFN-stimulated genes (ISGs) have been cloned from salmonids although only Mx has been shown to possess antiviral properties. The synthetic compounds poly I:C, imidazoquinolines and CpG oligonucleotides induce IFNs and ISGs in salmonids, probably through the same pathways as in mammals. Salmonid viruses show potent ability to stimulate expression of IFN and ISGs in vivo. Differences between viruses in the ability to stimulate host gene expression are often more evident in cell culture, but more work is needed to pinpoint how salmonid viruses antagonize the IFN system of their host. Finally, existing data suggest that IFNs play a role in the early non-specific protection observed after vaccination of salmonids with rhabdoviral DNA vaccines and conventional polyvalent vaccines.

STING is an endoplasmic reticulum adaptor that facilitates innate immune signalling

We report here the identification, following expression cloning, of a molecule, STING (STimulator of INterferon Genes) that regulates innate immune signaling processes. STING, comprising 5 putative transmembrane (TM) regions, predominantly resides in the endoplasmic reticulum (ER) and is able to activate both NF-κB and IRF3 transcription pathways to induce type I IFN and exert a potent anti-viral state following expression. In contrast, loss of STING rendered murine embryonic fibroblasts (STING ^−/−MEFs) extremely susceptible to negative-stranded virus infection, including vesicular stomatitis virus, VSV. Further, STING ablation abrogated the ability of intracellular B-form DNA, as well as members of the herpes virus family, to induce IFNβ, but did not significantly affect the Toll-like receptor (TLR pathway). Yeast-two hybrid and co-immunprecipitation studies indicated that STING interacts with RIG-I and with Ssr2/TRAPβ, a member of the translocon-associated protein (TRAP) complex required for protein translocation across the ER membrane following translation[1, 2]. RNAi ablation of TRAPβ and translocon adaptor Sec61β was subsequently found to inhibit STING’s ability to stimulate IFNβ. Thus, aside from identifying a novel regulator of innate immune signaling, this data implicates for the first time a potential role for the translocon in innate signaling pathways activated by select viruses as well as intracellular DNA.

Negative regulation of cytoplasmic RNA-mediated antiviral signaling

The recent, rapid progress in our understanding of cytoplasmic RNA-mediated antiviral innate immune signaling was initiated by the discovery of retinoic acid-inducible gene I (RIG-I) as a sensor of viral RNA. It is now widely recognized that RIG-I and related RNA helicases, melanoma differentiation-associated gene-5 (MDA5) and laboratory of genetics and physiology-2 (LGP2), can initiate and/or regulate RNA and virus-mediated type I IFN production and antiviral responses. As with other cytokine systems, production of type I IFN is a transient process, and can be hazardous to the host if unregulated, resulting in chronic cellular toxicity or inflammatory and autoimmune diseases. In addition, the RIG-I-like receptor (RLR) system is a fundamental target for virus-encoded immune suppression, with many indirect and direct examples of interference described. In this article, we review the current understanding of endogenous negative regulation in RLR signaling and explore direct inhibition of RLR signaling by viruses as a host immune evasion strategy.

West Nile virus infection of Drosophila melanogaster induces a protective RNAi response

To determine if West Nile virus (WNV) infection of insect cells induces a protective RNAi response, Drosophila melanogaster S2 and Aedes albopictus C6/36 cells were infected with WNV, and the production of WNV-homologous small RNAs was assayed as an indicator of RNAi induction. A distinct population of approximately 25 nt WNV-homologous small RNAs was detected in infected S2 cells but not C6/36 cells. RNAi knockdown of Argonaute 2 in S2 cells resulted in slightly increased susceptibility to WNV infection, suggesting that some WNV-homologous small RNAs produced in infected S2 cells are functional small interfering RNAs. WNV was shown to infect adult D. melanogaster, and adult flies containing mutations in each of four different RNAi genes (Argonaute 2, spindle-E, piwi, and Dicer-2) were significantly more susceptible to WNV infection than wildtype flies. These results combined with the analysis of WNV infection of S2 and C6/36 cells support the conclusion that WNV infection of D. melanogaster, but perhaps not Ae. albopictus, induces a protective RNAi response.

Multiple Nod-like receptors activate caspase 1 during Listeria monocytogenes infection

Listeria monocytogenes escapes from the phagosome of macrophages and replicates within the cytosolic compartment. The macrophage responds to L. monocytogenes through detection pathways located on the cell surface (TLRs) and within the cytosol (Nod-like receptors) to promote inflammatory processes aimed at clearing the pathogen. Cytosolic L. monocytogenes activates caspase 1, resulting in post-translational processing of the cytokines IL-1beta and IL-18 as well as caspase 1-dependent cell death (pyroptosis). We demonstrate that the presence of L. monocytogenes within the cytosolic compartment induces caspase 1 activation through multiple Nod-like receptors, including Ipaf and Nalp3. Flagellin expression by cytosolic L. monocytogenes was detected through Ipaf in a dose-dependent manner. Concordantly, detection of flagellin promoted bacterial clearance in a murine infection model. Finally, we provide evidence that suggests cytosolic L. monocytogenes activates caspase 1 through a third pathway, which signals through the adaptor protein ASC. Thus, L. monocytogenes activates caspase 1 in macrophages via multiple pathways, all of which detect the presence of bacteria within the cytosol.

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.

Expression and significance of interferon regulatory factor 3 of peripheral blood dendritic cells in HBV-infected patients

AIM: To elucidate the roles of interferon regulatory factor-3 (IRF3) of peripheral blood dendritic cells (DCs ) in immune response induced by hepatitis B virus (HBV) .

METHODS: Monocytes CD14⁺ were separated in 28 chronic HBV-infected patients (CH) group and 27 healthy volunteers group (control group) using magnetic activated cell sorting. Then, monocytes were placed on a culture medium of RPMI 1640, which were later induced into premature MoDC using hGM-CSF and hIL-4. We stimulated DCs with PolyI:C. The mRNA expressions of IRF3, TLR3 and IFN-β were detected using real time PCR, TLR3 expression was detected using flow cytometry and levels of IFN-β in supernatant were evaluated by ELISA.

RESULTS: In the control group, there was significant elevation of IRF3 and IFN-β level at 12 h following stimulation of PolyI:C compared with that at 0 h. Expression level of IRF3 rose and then declined gradually at 24h and 48 h (86.27% ± 14.74% vs 70.78% ± 11.16%, P < 0.05). In CH group, there was no significant up-regulation of TLR 3 at 0, 12, 24 h while a significant up-regulation was observed at 48 h (85.46% ± 6.87% vs 69.17% ± 20.43%, P < 0.05). The concentration of IFN-β had no significant changes at 0 h, 12 h, 24 h and 48 h in CH groups, while there was a significant higher up-regulation of IFN-β mRNA expression level at 0, 12, 24 h in the control group than in CH group (P < 0.05).

CONCLUSION: Chronic HBV-infected patients fail to activate IRF 3 following virus contraction and thereby are unable to secrete enough IFN-β to eradicate HBV virus, which may partly contribute to persistent infection of HBV.