Interferon induction and function at the mucosal surface

Disease-promoting effects of type I interferons in viral, bacterial, and coinfections

While type I interferons (IFNs) are universally acknowledged for their antiviral and immunostimulatory functions, there is increasing appreciation of the detrimental effects of inappropriate, excessive, or mistimed type I IFN responses in viral and bacterial infections. The underlying mechanisms by which type I IFNs promote susceptibility or severity include direct tissue damage by apoptosis induction or suppression of proliferation in tissue cells, immunopathology due to excessive inflammation, and cell death induced by TRAIL- and Fas-expressing immune cells, as well as immunosuppression through IL-10, IL-27, PD-L1, IL-1Ra, and other regulatory molecules that antagonize the induction or action of IL-1, IL-12, IL-17, IFN-γ, KC, and other effectors of the immune response. Bacterial superinfections following influenza infection are a prominent example of a situation where type I IFNs can misdirect the immune response. This review discusses current understanding of the parameters of signal strength, duration, timing, location, and cellular recipients that determine whether type I IFNs have beneficial or detrimental effects in infection.

The role of type I interferons in intestinal infection, homeostasis, and inflammation

Type I interferons are a widely expressed family of effector cytokines that promote innate antiviral and antibacterial immunity. Paradoxically, they can also suppress immune responses by driving production of anti-inflammatory cytokines, and dysregulation of these cytokines can contribute to host-mediated immunopathology and disease progression. Recent studies describe their anti-inflammatory role in intestinal inflammation and the locus containing IFNAR, a heterodimeric receptor for the type I interferons has been identified as a susceptibility region for human inflammatory bowel disease. This review focuses on the role of type I IFNs in the intestine in health and disease and their emerging role as immune modulators. Clear understanding of type I IFN-mediated immune responses may provide avenues for fine-tuning existing IFN treatment for infection and intestinal inflammation.

Role of Interferon-λ in Allergic Asthma

Type III interferons (IFNs), or IFN-λ, are known to have potent antiviral and antiproliferative activities. It inhibits viral replication and upregulates cytotoxic responses to virally infected cells. Besides these characteristics, IFN-λ also has additional activities in the immune system. In fact, it induces the proliferation of Foxp3-expressing regulatory T cells mediated in part by dendritic cells and inhibit the production of IL-5 and IL-13 in vitro. Regulatory T cells and the Th2 cytokines like IL-5 and IL-13 play important roles in the pathogenesis of allergic asthma. In humans, there seems to be an inverse link between IFN-λ and the severity of allergic asthma and allergic asthma exacerbations. Asthmatic patients, without a detectable viral infection show an inverse correlation between IL-28 and IL-29 mRNA levels and severity of allergic responses in the airways. These additional features of IFN-λ that affect the adaptive immune system make it a potential immunotherapeutic agent for the treatment of allergic asthma.

Host detection and the stealthy phenotype in influenza virus infection

The innate host response to influenza virus infection plays a critical role in determining the subsequent course of infection and the clinical outcome of disease. The host has a diverse array of detection and effector mechanisms that are able to recognize and initiate effective antiviral responses. In opposition, the virus utilizes a number of distinct mechanisms to evade host detection and effector activity in order to remain "stealthy" throughout its replication cycle. In this review, we describe these host and viral mechanisms, including the major pattern recognition receptor families (the TLRs, NLRs, and RLRs) in the host and the specific viral proteins such as NS1 that are key players in this interaction. Additionally, we explore nonreductive mechanisms of viral immune evasion and propose areas important for future inquiry.

Immunology of bovine respiratory syncytial virus in calves

Bovine respiratory syncytial virus (BRSV) is an important cause of respiratory disease in young calves. The virus is genetically and antigenically closely related to human (H)RSV, which is a major cause of respiratory disease in young infants. As a natural pathogen of calves, BRSV infection recapitulates the pathogenesis of respiratory disease in man more faithfully than semi-permissive, animal models of HRSV infection. With the increasing availability of immunological reagents, the calf can be used to dissect the pathogenesis of and mechanisms of immunity to RSV infection, to analyse the ways in which the virus proteins interact with components of the innate response, and to evaluate RSV vaccine strategies. Passively transferred, neutralising bovine monoclonal antibodies, which recognise the same epitopes in the HRSV and BRSV fusion (F) protein, can protect calves against BRSV infection, and depletion of different T cells subsets in calves has highlighted the importance of CD8(+) T cells in viral clearance. Calves can be used to model maternal-antibody mediated suppression of RSV vaccine efficacy, and to increase understanding of the mechanisms responsible for RSV vaccine-enhanced respiratory disease.

The role of macrophage IL-10/innate IFN interplay during virus-induced asthma

Activation through different signaling pathways results in two functionally different types of macrophages, the pro-inflammatory (M1) and the anti-inflammatory (M2). The polarization of macrophages toward the pro-inflammatory M1 phenotype is considered to be critical for efficient antiviral immune responses in the lung. Among the various cell types that are present in the asthmatic airways, macrophages have emerged as significant participants in disease pathogenesis, because of their activation during both the inflammatory and resolution phases, with an impact on disease progression. Polarized M1 and M2 macrophages are able to reversibly undergo functional redifferentiation into anti-inflammatory or pro-inflammatory macrophages, respectively, and therefore, macrophages mediate both processes. Recent studies have indicated a predominance of M2 macrophages in asthmatic airways. During a virus infection, it is likely that M2 macrophages would secrete higher amounts of the suppressor cytokine IL-10, and less innate IFNs. However, the interactions between IL-10 and innate IFNs during virus-induced exacerbations of asthma have not been well studied. The possible role of IL-10 as a therapy in allergic asthma has already been suggested, but the divergent roles of this suppressor molecule in the antiviral immune response raise concerns. This review attempts to shed light on macrophage IL-10-IFNs interactions and discusses the role of IL-10 in virus-induced asthma exacerbations. Whereas IL-10 is important in terminating pro-inflammatory and antiviral immune responses, the presence of this immune regulatory cytokine at the beginning of virus infection could impair the response to viruses and play a role in virus-induced asthma exacerbations.

In vitro anti-influenza A activity of interferon (IFN)-λ1 combined with IFN-β or oseltamivir carboxylate

Influenza viruses, which can cross species barriers and adapt to new hosts, pose a constant potential threat to human health. The influenza pandemic of 2009 highlighted the rapidity with which an influenza virus can spread worldwide. Currently available antivirals have a number of limitations against influenza, and novel antiviral strategies, including novel drugs and drug combinations, are urgently needed. Here, we evaluated the in vitro effects of interferon (IFN)-β, IFN-λ1, oseltamivir carboxylate (a neuraminidase (NA) inhibitor), and combinations of these agents against two seasonal (i.e., H1N1 and H3N2) influenza A viruses. We observed that A/California/04/09 (H1N1) and A/Panama/2007/99 (H3N2) isolates were equally sensitive to the antiviral activity of IFN-β and oseltamivir carboxylate in A549 and Calu-3 cells. In contrast, IFN-λ1 exhibited substantially lower protective potential against the H1N1 strain (64-1030-fold ↓, P<0.05), and was ineffective against H3N2 virus in both cell lines. Three dimensional analysis of drug-drug interactions revealed that IFN-λ1 interacted with IFN-β and oseltamivir carboxylate in an additive or synergistic manner, respectively, to inhibit influenza A virus replication in human airway epithelial cells. Overall, the present study demonstrated that anti-influenza agents with different mechanisms of action (e.g., a NA inhibitor combined with IFN-λ1) exerted a significantly greater (P<0.05) synergistic effect compared to co-treatment with drugs that target the same signaling pathway (i.e., IFN-β plus IFN-λ1) in vitro. Our findings provide support for the combined use of interferon plus oseltamivir as a potential means for treating influenza infections.

Polymerase discordance in novel swine influenza H3N2v constellations is tolerated in swine but not human respiratory epithelial cells

Swine-origin H3N2v, a variant of H3N2 influenza virus, is a concern for novel reassortment with circulating pandemic H1N1 influenza virus (H1N1pdm09) in swine because this can lead to the emergence of a novel pandemic virus. In this study, the reassortment prevalence of H3N2v with H1N1pdm09 was determined in swine cells. Reassortants evaluated showed that the H1N1pdm09 polymerase (PA) segment occurred within swine H3N2 with ∼80% frequency. The swine H3N2-human H1N1pdm09 PA reassortant (swH3N2-huPA) showed enhanced replication in swine cells, and was the dominant gene constellation. Ferrets infected with swH3N2-huPA had increased lung pathogenicity compared to parent viruses; however, swH3N2-huPA replication in normal human bronchoepithelial cells was attenuated - a feature linked to expression of IFN-β and IFN-λ genes in human but not swine cells. These findings indicate that emergence of novel H3N2v influenza constellations require more than changes in the viral polymerase complex to overcome barriers to cross-species transmission. Additionally, these findings reveal that while the ferret model is highly informative for influenza studies, slight differences in pathogenicity may not necessarily be indicative of human outcomes after infection.

Passive broad-spectrum influenza immunoprophylaxis

Influenza is a perennial problem affecting millions of people annually with the everpresent threat of devastating pandemics. Active prophylaxis by vaccination against influenza virus is currently the main countermeasure supplemented with antivirals. However, disadvantages of this strategy include the impact of antigenic drift, necessitating constant updating of vaccine strain composition, and emerging antiviral drug resistance. The development of other options for influenza prophylaxis, particularly with broad acting agents able to provide protection in the period between the onset of a pandemic and the development of a strain specific vaccine, is of great interest. Exploitation of broad-spectrum mediators could provide barricade protection in the early critical phase of influenza virus outbreaks. Passive immunity has the potential to provide immediate antiviral effects, inhibiting virus replication, reducing virus shedding, and thereby protecting vulnerable populations in the event of an impending influenza pandemic. Here, we review passive broad-spectrum influenza prophylaxis options with a focus on harnessing natural host defenses, including interferons and antibodies.

Sendai virus pathogenesis in mice is prevented by Ifit2 and exacerbated by interferon

The type I/III interferon (IFN) system has major roles in regulating viral pathogenesis, usually ameliorating pathogenesis by impairing virus replication through the antiviral actions of one or more IFN-induced proteins. Ifit2 is one such protein which can be induced by IFN or virus infection, and it is responsible for protecting mice from neuropathogenesis caused by vesicular stomatitis virus. Here, we show that Ifit2 also protects mice from pathogenesis caused by the respirovirus Sendai virus (SeV). Mice lacking Ifit2 (Ifit2(-/-)) suffered severe weight loss and succumbed to intranasal infection with SeV strain 52 at a dose that killed only a few wild-type mice. Viral RNA was detectable only in lungs, and SeV titers were higher in Ifit2(-/-) mice than in wild-type mice. Similar infiltration of immune cells was found in the lungs of both mouse lines, corresponding to similar levels of many induced cytokines and chemokines. In contrast, IFN-β and IFN-λ3 expression were considerably higher in the lungs of Ifit2(-/-) mice. Surprisingly, type I IFN receptor knockout (IFNAR(-/-)) mice were less susceptible to SeV than Ifit2(-/-) mice, although their pulmonary virus titers were similarly high. To test the intriguing possibility that type I IFN action enhances pathogenesis in the context of elevated SeV replication in lungs, we generated Ifit2/IFNAR(-/-) double knockout mice. These mice were less susceptible to SeV than Ifit2(-/-) mice, although viral titers in their lungs were even higher. Our results indicate that high SeV replication in the lungs of infected Ifit2(-/-) mice cooperates with elevated IFN-β induction to cause disease.

IMPORTANCE

The IFN system is an innate defense against virus infections. It is triggered quickly in infected cells, which then secrete IFN. Via their cell surface receptors on surrounding cells, they induce transcription of numerous IFN-stimulated genes (ISG), which in turn protect these cells by inhibiting virus life cycles. Hence, IFNs are commonly considered beneficial during virus infections. Here, we report two key findings. First, lack of a single ISG in mice, Ifit2, resulted in high mortality after SeV infection of the respiratory tract, following higher virus loads and higher IFN production in Ifit2(-/-) lungs. Second, mortality of Ifit2(-/-) mice was reduced when mice also lacked the type I IFN receptor, while SeV loads in lungs still were high. This indicates that type I IFN exacerbates pathogenesis in the SeV model, and that limitation of both viral replication and IFN production is needed for effective prevention of disease.

Molecular cloning, expression and characterization of Pekin duck interferon-λ

Interferons (IFNs) are the first line of defense against viral infections in vertebrates. Type III interferon (IFN-λ) is recognized for its key role in innate immunity of tissues of epithelial origin. Here we describe the identification of the Pekin duck IFN-λ ortholog (duIFN-λ). The predicted duIFN-λ protein has an amino acid identity of 63%, 38%, 37% and 33% with chicken IFN-λ and human IFN-λ3, IFN-λ2 and IFN-λ1, respectively. The duck genome contains a single IFN-λ gene that is comprised of five exons and four introns. Recombinant duIFN-λ up-regulated OASL and Mx-1 mRNA in primary duck hepatocytes. Our observations suggest evolutionary conservation of genomic organization and structural features implicated in receptor binding and antiviral activity. The identification and expression of duIFN-λ will facilitate further study of the role of type III IFN in antiviral defense and inflammatory responses of the Pekin duck, a non-mammalian vertebrate and pathogen host with relevance for human and animal health.

Sustained inflammation and differential expression of interferons type I and III in PVM-infected interferon-gamma (IFNγ) gene-deleted mice

Interferon gamma (IFNγ) has complex immunomodulatory and antiviral properties. While IFNγ is detected in the airways in response to infection with the pneumovirus pathogen, pneumonia virus of mice (PVM; Family Paramyxoviridae), its role in promoting disease has not been fully explored. Here, we evaluate PVM infection in IFNγ(-/-) mice. Although the IFNγ gene-deletion has no impact on weight loss, survival or virus kinetics, expression of IFNβ, IFNλ2/3 and IFN-stimulated 2-5' oligoadenylate synthetases was significantly diminished compared to wild-type counterparts. Furthermore, PVM infection in IFNγ(-/-) mice promoted prominent inflammation, including eosinophil and neutrophil infiltration into the airways and lung parenchyma, observed several days after peak virus titer. Potential mechanisms include over-production of chemoattractant and eosinophil-active cytokines (CXCL1, CCL11, CCL3 and IL5) in PVM-infected IFNγ(-/-) mice; likewise, IFNγ actively antagonized IL5-dependent eosinophil survival ex vivo. Our results may have clinical implications for pneumovirus infection in individuals with IFNγ signaling defects.

Type I and III interferon production in response to RNA viruses

The biology of RNA viruses is closely linked to the type I and type III interferon (IFN) response of the host. These viruses display a range of molecular patterns that may be detected by host cells resulting in the induction of IFNs. Consequently, there are many examples of mechanisms employed by RNA viruses to block or delay IFN induction and reduce the expression of IFN-stimulated genes (ISGs), a necessary step in the virus lifecycle because of the capacity of IFNs to block virus replication. Efficient transmission of viruses depends, in part, on maintaining a balance between virus replication and host survival; specialized host cells, such as plasmacytoid dendritic cells, can sense viral molecular patterns and produce IFNs to help maintain this balance. There are now many examples of RNA viruses inducing type I and type III IFNs, and although these IFNs act through different receptors, in many systems studied, they induce a similar spectrum of genes. However, there may be a difference in the temporal expression pattern, with more prolonged expression of ISGs in response to type III IFN compared with type I IFN. There are also examples of synergy between type I and type III IFNs to induce antiviral responses. Clearly, it is important to understand the different roles of these IFNs in the antiviral response in vivo. One of the most striking differences between these 2 IFN systems is the distribution of the receptors: type I IFN receptors are expressed on most cells, yet type III receptor expression is restricted primarily to epithelial cells but has also been demonstrated on other cells, including dendritic cells. There is increasing evidence that type III IFNs are a key control mechanism against RNA viruses that infect respiratory and enteric epithelia.

The bacterial pathogen Listeria monocytogenes and the interferon family: type I, type II and type III interferons

Interferons (IFNs) are secreted proteins of the cytokine family that regulate innate and adaptive immune responses to infection. Although the importance of IFNs in the antiviral response has long been appreciated, their role in bacterial infections is more complex and is currently a major focus of investigation. This review summarizes our current knowledge of the role of these cytokines in host defense against the bacterial pathogen Listeria monocytogenes and highlights recent discoveries on the molecular mechanisms evolved by this intracellular bacterium to subvert IFN responses.

Alpha/beta interferon receptor signaling amplifies early proinflammatory cytokine production in the lung during respiratory syncytial virus infection

Type I interferons (IFNs) are produced early upon virus infection and signal through the alpha/beta interferon (IFN-α/β) receptor (IFNAR) to induce genes that encode proteins important for limiting viral replication and directing immune responses. To investigate the extent to which type I IFNs play a role in the local regulation of inflammation in the airways, we examined their importance in early lung responses to infection with respiratory syncytial virus (RSV). IFNAR1-deficient (IFNAR1^−/−) mice displayed increased lung viral load and weight loss during RSV infection. As expected, expression of IFN-inducible genes was markedly reduced in the lungs of IFNAR1^−/− mice. Surprisingly, we found that the levels of proinflammatory cytokines and chemokines in the lungs of RSV-infected mice were also greatly reduced in the absence of IFNAR signaling. Furthermore, low levels of proinflammatory cytokines were also detected in the lungs of IFNAR1^−/− mice challenged with noninfectious innate immune stimuli such as selected Toll-like receptor (TLR) agonists. Finally, recombinant IFN-α was sufficient to potentiate the production of inflammatory mediators in the lungs of wild-type mice challenged with innate immune stimuli. Thus, in addition to its well-known role in antiviral resistance, type I IFN receptor signaling acts as a central driver of early proinflammatory responses in the lung. Inhibiting the effects of type I IFNs may therefore be useful in dampening inflammation in lung diseases characterized by enhanced inflammatory cytokine production.

IMPORTANCE The initial response to viral infection is characterized by the production of interferons (IFNs). One group of IFNs, the type I IFNs, are produced early upon virus infection and signal through the IFN-α/β receptor (IFNAR) to induce proteins important for limiting viral replication and directing immune responses. Here we examined the importance of type I IFNs in early responses to respiratory syncytial virus (RSV). Our data suggest that type I IFN production and IFNAR receptor signaling not only induce an antiviral state but also serve to amplify proinflammatory responses in the respiratory tract. We also confirm this conclusion in another model of acute inflammation induced by noninfectious stimuli. Our findings are of relevance to human disease, as RSV is a major cause of infant bronchiolitis and polymorphisms in the IFN system are known to impact disease severity.

Systems biology unravels interferon responses to respiratory virus infections

Interferon production is an important defence against viral replication and its activation is an attractive therapeutic target. However, it has long been known that viruses perpetually evolve a multitude of strategies to evade these host immune responses. In recent years there has been an explosion of information on virus-induced alterations of the host immune response that have resulted from data-rich omics technologies. Unravelling how these systems interact and determining the overall outcome of the host response to viral infection will play an important role in future treatment and vaccine development. In this review we focus primarily on the interferon pathway and its regulation as well as mechanisms by which respiratory RNA viruses interfere with its signalling capacity.

Interferons and their stimulated genes in the tumor microenvironment

Constitutive expression of interferons (IFNs) and activation of their signaling pathways have pivotal roles in host responses to malignant cells in the tumor microenvironment. IFNs are induced by the innate immune system and in tumors through stimulation of Toll-like receptors (TLRs) and through other signaling pathways in response to specific cytokines. Although in the oncologic context IFNs have been thought of more as exogenous pharmaceuticals, the autocrine and paracrine actions of endogenous IFNs probably have even more critical effects on neoplastic disease outcomes. Through high-affinity cell surface receptors, IFNs modulate transcriptional signaling, leading to regulation of more than 2,000 genes with varying patterns of temporal expression. Induction of the gene products by both unphosphorylated and phosphorylated STAT1 after ligand binding results in alterations in tumor cell survival, inhibition of angiogenesis, and augmentation of actions of T, natural killer (NK), and dendritic cells. The interferon-stimulated gene (ISG) signature can be a favorable biomarker of immune response but, in a seemingly paradoxical finding, a specific subset of the full ISG signature indicates an unfavorable response to DNA-damaging interventions such as radiation. IFNs in the tumor microenvironment thus can alter the emergence, progression, and regression of malignancies.