Regulation of virus-triggered type I interferon signaling by cellular and viral proteins

Host pattern recognition receptors (PRRs) recognize invading viral pathogens and initiate a series of signaling cascades that lead to the expression of type I interferons (IFNs) and inflammatory cytokines. During the past decade, significant progresses have been made to characterize PRRs such as Toll-like receptors (TLRs) and RIG-I-like receptors (RLRs) and elucidate the molecular mechanisms of TLR- and RLR-mediated signaling. To avoid excessive and harmful immune effects caused by over-activation of these signaling pathways, host cells adopt a number of strategies to regulate them. In addition, invading viruses also employ a variety of mechanisms to inhibit the production of type I IFNs, thereby evading the supervision and clearance by the host. In this review, we briefly summarize the TLR- and RLR-mediated type I IFN signaling and then focus on the mechanisms by which host cellular and viral components regulate the expression of type I IFNs.     

Viral infection augments Nod1/2 signaling to potentiate lethality associated with secondary bacterial infections

Secondary bacterial infection is a common sequela to?viral infection and is associated with increased lethality and morbidity. However, the underlying mechanisms remain poorly understood. We show that the TLR3/MDA5 agonist poly I:C or viral infection dramatically augments signaling via the NLRs Nod1 and Nod2 and enhances the production of proinflammatory cytokines. Enhanced Nod1 and Nod2 signaling by poly I:C required the TLR3/MDA5 adaptors TRIF and IPS-1 and was mediated by type I IFNs. Mechanistically, poly I:C or IFN-β induced the expression of Nod1, Nod2, and the Nod-signaling adaptor Rip2. Systemic administration of poly I:C or IFN-β or infection with murine norovirus-1 promoted inflammation and lethality in mice superinfected with E.?coli, which was independent of bacterial burden but attenuated in the absence of Nod1/Nod2 or Rip2. Thus, crosstalk between type I IFNs and Nod1/Nod2 signaling promotes bacterial recognition, but induces harmful effects in the virally infected host.     

MyD88 and STING signaling pathways are required for IRF3-mediated IFN-β induction in response to Brucella abortus infection

Type I interferons (IFNs) are cytokines that orchestrate diverse immune responses to viral and bacterial infections. Although typically considered to be most important molecules in response to viruses, type I IFNs are also induced by most, if not all, bacterial pathogens. In this study, we addressed the role of type I IFN signaling during Brucella abortus infection, a facultative intracellular bacterial pathogen that causes abortion in domestic animals and undulant fever in humans. Herein, we have shown that B. abortus induced IFN-β in macrophages and splenocytes. Further, IFN-β induction by Brucella was mediated by IRF3 signaling pathway and activates IFN-stimulated genes via STAT1 phosphorylation. In addition, IFN-β expression induced by Brucella is independent of TLRs and TRIF signaling but MyD88-dependent, a pathway not yet described for Gram-negative bacteria. Furthermore, we have identified Brucella DNA as the major bacterial component to induce IFN-β and our study revealed that this molecule operates through a mechanism dependent on RNA polymerase III to be sensed probably by an unknown receptor via the adaptor molecule STING. Finally, we have demonstrated that IFN-αβR KO mice are more resistant to infection suggesting that type I IFN signaling is detrimental to host control of Brucella. This resistance phenotype is accompanied by increased IFN-γ and NO production by IFN-αβR KO spleen cells and reduced apoptosis.     

Double-stranded RNA induces similar pulmonary dysfunction to respiratory syncytial virus in BALB/c mice

Both respiratory syncytial virus (RSV) and influenza A virus induce nucleotide/P2Y purinergic receptor-mediated impairment of alveolar fluid clearance (AFC), which contributes to formation of lung edema. Although genetically dissimilar, both viruses generate double-stranded RNA replication intermediates, which act as Toll-like receptor (TLR)-3 ligands. We hypothesized that double-stranded RNA/TLR-3 signaling underlies nucleotide-mediated inhibition of amiloride-sensitive AFC in both infections. We found that addition of the synthetic double-stranded RNA analog poly-inosinic-cytidylic acid [poly(I:C)] (500 ng/ml) to the AFC instillate resulted in nucleotide/P2Y purinergic receptor-mediated inhibition of amiloride-sensitive AFC in BALB/c mice but had no effect on cystic fibrosis transmembrane regulator (CFTR)-mediated Cl(-) transport. Poly(I:C) also induced acute keratinocyte cytokine-mediated AFC insensitivity to stimulation by the β-adrenergic agonist terbutaline. Inhibitory effects of poly(I:C) on AFC were absent in TLR-3(-/-) mice and were not replicated by addition to the AFC instillate of ligands for other TLRs except TLR-2. Intranasal poly(I:C) administration (250 μg/mouse) similarly induced nucleotide-dependent AFC inhibition 2-3 days later, together with increased lung water content and neutrophilic inflammation. Intranasal treatment of BALB/c mice with poly(I:C) did not induce airway hyperresponsiveness at day 2 but did result in insensitivity to airway bronchodilation by β-adrenergic agonists. These findings suggest that viral double-stranded RNA replication intermediates induce nucleotide-mediated impairment of amiloride-sensitive AFC in both infections, together with β-adrenergic agonist insensitivity. Both of these effects also occur in RSV infection. However, double-stranded RNA replication intermediates do not appear to be sufficient to induce either adenosine-mediated, CFTR-dependent Cl(-) secretion in the lung or severe, lethal hypoxemia, both of which are features of influenza infection.     

TLR3 is essential for the induction of protective immunity against Punta Toro Virus infection by the double-stranded RNA (dsRNA), poly(I:C12U), but not Poly(I:C): differential recognition of synthetic dsRNA molecules

In the wake of RNA virus infections, dsRNA intermediates are often generated. These viral pathogen-associated molecular patterns can be sensed by a growing number of host cell cytosolic proteins and TLR3, which contribute to the induction of antiviral defenses. Recent evidence indicates that melanoma differentiation-associated gene-5 is the prominent host component mediating IFN production after exposure to the dsRNA analog, poly(I:C). We have previously reported that Punta Toro virus (PTV) infection in mice is exquisitely sensitive to treatment with poly(I:C(12)U), a dsRNA analog that has a superior safety profile while maintaining the beneficial activity of the parental poly(I:C) in the induction of innate immune responses. The precise host factor(s) mediating protective immunity following its administration remain to be elucidated. To assess the role of TLR3 in this process, mice lacking the receptor were used to investigate the induction of protective immunity, type I IFNs, and IL-6 following treatment. Unlike wild-type mice, those lacking TLR3 were not protected against PTV infection following poly(I:C(12)U) therapy and failed to produce IFN-alpha, IFN-beta, and IL-6. In contrast, poly(I:C) treatment significantly protected TLR3(-/-) mice from lethal challenge despite some deficiencies in cytokine induction. There was no indication that the lack of protection was due to the fact that TLR3-deficient mice had a reduced capacity to fight infection because they were not found to be more susceptible to PTV. We conclude that TLR3 is essential to the induction of antiviral activity elicited by poly(I:C(12)U), which does not appear to be recognized by the cytosolic sensor of poly(I:C), melanoma differentiation-associated gene-5.     

Immunomodulatory functions of type I interferons

Interferon-α (IFNα) and IFNβ, collectively known as type I IFNs, are the major effector cytokines of the host immune response against viral infections. However, the production of type I IFNs is also induced in response to bacterial ligands of innate immune receptors and/or bacterial infections, indicating a broader physiological role for these cytokines in host defence and homeostasis than was originally assumed. The main focus of this Review is the underappreciated immunomodulatory functions of type I IFNs in health and disease. We discuss their function in the regulation of innate and adaptive immune responses, the response to bacterial ligands, inflammasome activation, intestinal homeostasis and inflammatory and autoimmune diseases.     

Microbial pathogenesis and type III interferons

The innate immune system possesses a multitude of pathways to sense and respond to microbial pathogens. One such family are the interferons (IFNs), a family of cytokines that are involved in several cellular functions. Type I IFNs are appreciated to be important in several viral and bacterial diseases, while the recently identified type III IFNs (IFNL1, IFNL2, IFNL3, IFNL4) have been studied primarily in the context of viral infection. Viral and bacterial infections however are not mutually exclusive, and often the presence of a viral pathogen increases the pathogenesis of bacterial infection. The role of type III IFN in bacterial and viral-bacterial co-infections has just begun to be explored. In this mini review we discuss type III IFN signaling and its role in microbial pathogenesis with an emphasis on the work that has been conducted with bacterial pathogens.     

Induction of Interferon-Stimulated Genes by Chlamydia pneumoniae in Fibroblasts Is Mediated by Intracellular Nucleotide-Sensing Receptors

Background

Recognition of microorganisms by the innate immune system is mediated by pattern recognition receptors, including Toll-like receptors and cytoplasmic RIG-I-like receptors. Chlamydia, which include several human pathogenic species, are obligate intracellular gram-negative bacteria that replicate in cytoplasmic vacuoles. The infection triggers a host response contributing to both bacterial clearance and tissue damage. For instance, type I interferons (IFN)s have been demonstrated to exacerbate the course of Chlamydial lung infections in mice.

Methods/Principal Findings

Here we show that Chlamydia pneumoniae induces expression of IFN-stimulated genes (ISG)s dependent on recognition by nucleotide-sensing Toll-like receptors and RIG-I-like receptors, localized in endosomes and the cytoplasm, respectively. The ISG response was induced with a delayed kinetics, compared to virus infections, and was dependent on bacterial replication and the bacterial type III secretion system (T3SS).

Conclusions/Significance

Activation of the IFN response during C. pneumoniae infection is mediated by intracellular nucleotide-sensing PRRs, which operate through a mechanism dependent on the bacterial T3SS. Strategies to inhibit the chlamydial T3SS may be used to limit the detrimental effects of the type I IFN system in the host response to Chlamydia infection.     

Role of type I interferons in the activation of autoreactive B cells

Type I interferons (IFNs) are a family of cytokines involved in the defense against viral infections that play a key role in the activation of both the innate and adaptive immune system. IFNs both directly and indirectly enhance the capacity of B lymphocytes to respond to viral challenge and produce cytotoxic and neutralizing antibodies. However, prolonged type I IFN exposure is not always beneficial to the host. If not regulated properly IFN can drive autoantibody production as well as other parameters of systemic autoimmune disease. Type I IFNs impact B-cell function through a variety of mechanisms, including effects on receptor engagement, Toll-like receptor expression, cell migration, antigen presentation, cytokine responsiveness, cytokine production, survival, differentiation and class-switch recombination. Type I IFNs are also cytotoxic for a variety of cell types and thereby contribute to the accumulation of cell debris that serves as a potential source for autoantigens. Type I IFN engagement of a variety of accessory cells further promotes B-cell survival and activation, as exemplified by the capacity of type I IFNs to increase the level of B-cell survival factors, such as B lymphocyte stimulator, produced by dendritic cells. Therefore, it is not surprising that the loss of expression of the type I IFN receptor can have dramatic effects on the production of autoantibodies and on the clinical features of systemic autoimmune diseases such as systemic lupus erythematosus.     

Dissection of a type I interferon pathway in controlling bacterial intracellular infection in mice

Defence mechanisms against intracellular bacterial pathogens are incompletely understood. Our study characterizes a type I IFN-dependent cell-autonomous defence pathway directed against Legionella pneumophila, an intracellular model organism and frequent cause of pneumonia. We show that macrophages infected with L. pneumophila produced IFNβ in a STING- and IRF3- dependent manner. Paracrine type I IFNs stimulated upregulation of IFN-stimulated genes and a cell-autonomous defence pathway acting on replicating and non-replicating Legionella within their specialized vacuole. Our infection experiments in mice lacking receptors for type I and/or II IFNs show that type I IFNs contribute to expression of IFN-stimulated genes and to bacterial clearance as well as resistance in L. pneumophila pneumonia in addition to type II IFN. Overall, our study shows that paracrine type I IFNs mediate defence against L. pneumophila, and demonstrates a protective role of type I IFNs in in vivo infections with intracellular bacteria.     

Ⅲ型干扰素——新型抗病毒细胞因子

干扰素（IFN）是抗病毒感染的第一道防线,Ⅰ型和Ⅱ型干扰素不仅可抑制病毒,而且还能参与天然免疫反应和获得性免疫反应。最近干扰素家族增添一位新成员：Ⅲ型干扰素,即IFN-λ,因其具有类似干扰素的抗病毒活性且能诱导干扰素相关基因的表达而命名。IFN-λ受体与Ⅰ型干扰素的受体不同,但具有与Ⅰ型干扰素类似的诱导表达方式和信号转导通路,并能激活一系列相似的干扰素刺激基因。就IFN-λ家族及其受体、基因表达和信号转导机制、抗病毒作用等进行综述。     

Polyinosinic-polycytidylic acid treatment of Friend retrovirus-infected mice improves functional properties of virus-specific T cells and prevents virus-induced disease

The induction of type I IFN is the most immediate host response to viral infections. Type I IFN has a direct antiviral activity mediated by antiviral enzymes, but it also modulates the function of cells of the adaptive immune system. Many viruses can suppress type I IFN production, and in retroviral infections, the initial type I IFN is weak. Thus, one strategy of immunotherapy in viral infection is the exogenous induction of type I IFN during acute viral infection by TLR ligands. Along these lines, the TLR3/MDA5 ligand polyinosinic-polycytidylic acid [poly(I:C)] has already been used to treat viral infections. However, the immunological mechanisms underlying this successful therapy have not been defined until now. In this study, the Friend retrovirus (FV) mouse model was used to investigate the mode of action of poly(I:C) in antiretroviral immunotherapy. Postexposure, poly(I:C) treatment of FV-infected mice resulted in a significant reduction in viral loads and protection from virus-induced leukemia. This effect was IFN dependent because type I IFN receptor-deficient mice could not be protected by poly(I:C). The poly(I:C)-induced IFN response resulted in the expression of antiviral enzymes, which suppressed FV replication. Also, the virus-specific T cell response was augmented. Interestingly, it did not enhance the number of virus-specific CD4(+) and CD8(+) T cells, but rather the functional properties of these cells, such as cytokine production and cytotoxic activity. The results demonstrate a direct antiviral and immunomodulatory effect of poly(I:C) and, therefore, suggests its potential for clinical treatment of retroviral infections.     

The transforming growth factor-beta-Smad3/4 signaling pathway acts as a positive regulator for TLR2 induction by bacteria via a dual mechanism involving functional cooperation with NF-kappaB and MAPK phosphatase 1-dependent negative cross-talk with p38 MAPK

The transforming growth factor beta (TGF-beta) pathway represents an important signaling pathway involved in the regulation of diverse biological processes, including cell proliferation, differentiation, and apoptosis. Despite the known role of TGF-betaR-mediated signaling in suppressing immune response, its role in regulating human Toll-like receptors (TLRs), key host defense receptors that recognize invading bacterial pathogens, however, remains unknown. Here, we show for the first time that TGF-betaR-Smad3/4 signaling pathway acts as a positive regulator for TLR2 induction by bacterium nontypeable Hemophilus influenzae (NTHi) in vitro and in vivo. The positive regulation of TLR2 induction by TGF-betaR is mediated via a dual mechanism involving distinct signaling pathways. One mechanism involves functional cooperation between the TGF-betaR-Smad3/4 pathway and NF-kappaB pathway. Another involves MAP kinase phosphatase 1 (MKP-1)-dependent inhibition of p38 MAPK, a known negative regulator for TLR2 induction. Moreover, we showed that TbetaR-mediated signaling is probably activated by NTHi-derived TGF-beta mimicry molecule via an autocrine-independent mechanism. Thus, our study provides new insights into the role of TGF-beta signaling in positively regulating host defense response by tightly controlling the expression level of TLR2 during bacterial infections and may lead to new therapeutic strategies for modulating host defense and inflammatory response.     

The dsRNA-mimetic poly (I:C) and IL-18 synergize for IFNγ and TNFα expression

Interleukin (IL)-18 bioactivity and dsRNA sensing by receptors of innate immunity are key components of anti-viral host defense. Despite extensive data on signal transduction activated by both pathways knowledge on cross-communication is incomplete. By using human PBMC and predendritic KG1 cells, as prototypic IL-18-responsive cellular models, we sought to assess cytokine production under the influence of IL-18 and the dsRNA-mimetic poly (I:C). Here, we report on potent synergy between both mediators concerning pro-inflammatory IFNγ and TNFα production. KG1 data revealed that synergistic induction likely relied on TLR3 and was associated with prolonged/increased activation of NF-κB, as detected by IκB analysis and luciferase reporter assays, respectively. Moreover, extended activation of JNK was mediated by IL-18/poly (I:C). Although vital for innate immunity, overwhelming induction of inflammatory cytokines during viral infections poses the threat of serious collateral tissue damage. The stunning synergism inherent to IL-18/dsRNA-induced TNFα/IFNγ detected herein may contribute to this pathological phenomenon.     

Induction of type I interferons by bacteria

Type I interferons (IFNs) are secreted cytokines that orchestrate diverse immune responses to infection. Although typically considered to be most important in the response to viruses, type I IFNs are also induced by most, if not all, bacterial pathogens. Although diverse mechanisms have been described, bacterial induction of type I IFNs occurs upon stimulation of two main pathways: (i) Toll‐like receptor (TLR) recognition of bacterial molecules such as lipopolysaccharide (LPS); (ii) TLR‐independent recognition of molecules delivered to the host cell cytosol. Cytosolic responses can be activated by two general mechanisms. First, viable bacteria can secrete stimulatory ligands into the cytosol via specialized bacterial secretion systems. Second, ligands can be released from bacteria that lyse or are degraded. The bacterial ligands that induce the cytosolic pathways remain uncertain in many cases, but appear to include various nucleic acids. In this review, we discuss recent advances in our understanding of how bacteria induce type I interferons and the roles type I IFNs play in host immunity.     

Activation of Toll-like receptor 3 impairs the dengue virus serotype 2 replication through induction of IFN-β in cultured hepatoma cells

Toll-like receptors (TLRs) play an important role in innate immunity against invading pathogens. Although TLR signaling has been indicated to protect cells from infection of several viruses, the role of TLRs in Dengue virus (DENV) replication is still unclear. In the present study, we examined the replication of DENV serotype 2 (DENV2) by challenging hepatoma cells HepG2 with different TLR ligands. Activation of TLR3 showed an antiviral effect, while pretreatment of other TLR ligands (including TLR1/2, TLR2/6, TLR4, TLR5 or TLR7/8) did not show a significant effect. TLR3 ligand poly(I:C) treatment prior to viral infection or simultaneously, but not post-treatment, significantly down-regulated virus replication. Pretreatment with poly(I:C) reduced viral mRNA expression and viral staining positive cells, accompanying an induction of the type I interferon (IFN-β) and type III IFN (IL-28A/B). Intriguingly, neutralization of IFN-β alone successfully restored the poly(I:C)-inhibited replication of DENV2. The poly(I:C)-mediated effects, including IFN induction and DENV2 suppression, were significantly reversed by IKK inhibitor, further suggesting that IFN-β is the dominant factor involved in the poly(I:C) mediated antiviral effect. Our study presented the first evidence to show that activation of TLR3 is effective in blocking DENV2 replication via IFN-β, providing an experimental clue that poly(I:C) may be a promising immunomodulatory agent against DENV infection and might be applicable for clinical prevention.     

Intrapulmonary TNF gene therapy reverses sepsis-induced suppression of lung antibacterial host defense

Sepsis syndrome is frequently complicated by the development of nosocomial infections, particularly Gram-negative pneumonia. Although TNF-alpha (TNF) has been shown to mediate many of the pathophysiologic events in sepsis, this cytokine is a critical component of innate immune response within the lung. Therefore, we hypothesized that the transient transgenic expression of TNF within the lung during the postseptic period could augment host immunity against nosocomial pathogens. To test this, mice underwent 26-gauge cecal ligation and puncture (CLP) as a model of abdominal sepsis, followed 24 h later by intratracheal (i.t.) administration of PSEUDOMONAS: aeruginosa. In animals undergoing sham surgery followed by bacterial challenge, PSEUDOMONAS: were nearly completely cleared from the lungs by 24 h. In contrast, mice undergoing CLP were unable to clear P. aeruginosa and rapidly developed bacteremia. Alveolar macrophages (AM) recovered from mice 24 h after CLP produced significantly less TNF ex vivo, as compared with AM from sham animals. Furthermore, the adenoviral mediated transgenic expression of TNF within the lung increased survival in CLP animals challenged with PSEUDOMONAS: from 25% in animals receiving control vector to 91% in animals administered recombinant murine TNF adenoviral vector. Improved survival in recombinant murine TNF adenoviral vector-treated mice was associated with enhanced lung bacterial clearance and proinflammatory cytokine expression, as well as enhanced AM phagocytic activity and cytokine expression when cultured ex vivo. These observations suggest that intrapulmonary immunostimulation with TNF can reverse sepsis-induced impairment in antibacterial host defense.     

Critical role for alpha/beta and gamma interferons in persistence of lymphocytic choriomeningitis virus by clonal exhaustion of cytotoxic T cells

Under conditions of high antigenic load during infection with invasive lymphocytic choriomeningitis virus (LCMV) strains, virus can persist by selective clonal exhaustion of antigen-specific CD8(+) T cells. In this work we studied the down-regulation of the virus-specific CD8(+)-T-cell response during a persistent infection of adult mice, with particular emphasis on the contribution of the interferon response in promoting host defense. Studies were conducted by infecting mice deficient in receptors for type I (alpha/beta interferon [IFN-alpha/beta]), type II (IFN-gamma), and both type I and II IFNs with LCMV isolates that vary in their capacity to induce T-cell exhaustion. The main conclusions of this study are as follows. (i) IFNs play a critical role in LCMV infection by reducing viral loads in the initial stages of infection and thus modifying both the extent of CD8(+)-T-cell exhaustion and the course of infection. The importance of IFNs in this context varies with the biological properties of the LCMV strain. (ii) An inverse correlation exists between antigen persistence and responsiveness of virus-specific CD8(+) T cells. This results in distinct programs of activation or tolerance (functional unresponsiveness and/or physical elimination of antigen-specific cells) during acute and chronic virus infections, respectively. (iii) A successful immune response associated with definitive viral clearance requires an appropriate balance between cellular and humoral components of the immune system. We discuss the role of IFNs in influencing virus-specific T cells that determine the outcome of persistent infections.     

Anti-immunology: evasion of the host immune system by bacterial and viral pathogens

Multicellular organisms possess very sophisticated defense mechanisms that are designed to effectively counter the continual microbial insult of the environment within the vertebrate host. However, successful microbial pathogens have in turn evolved complex and efficient methods to overcome innate and adaptive immune mechanisms, which can result in disease or chronic infections. Although the various virulence strategies used by viral and bacterial pathogens are numerous, there are several general mechanisms that are used to subvert and exploit immune systems that are shared between these diverse microbial pathogens. The success of each pathogen is directly dependant on its ability to mount an effective anti-immune response within the infected host, which can ultimately result in acute disease, chronic infection, or pathogen clearance. In this review, we highlight and compare some of the many molecular mechanisms that bacterial and viral pathogens use to evade host immune defenses.