Wide range of susceptibility to rhabdoviruses in homozygous clones of rainbow trout

Inbred lines differentially susceptible to diseases are a powerful tool to get insights into the mechanisms of genetic resistance to pathogens. In fish, chromosome manipulation techniques allow a quick production of such homozygous lines. Using gynogenesis, we produced nine homozygous clones of rainbow trout from a domestic population (INRA Sy strain). We examined the variability between clones for resistance to two rhabdoviruses, the viral haemorrhagic septicaemia virus (VHSV) and the infectious haematopoietic necrosis virus (IHNV). Intraperitoneal injections and waterborne infections were performed in parallel for both viruses. No survival was recorded after intraperitoneal injection of VHSV or IHNV, indicating that fish from all clones were fully susceptible to both viruses by this route of infection. In contrast, the different clones showed a wide range of survival frequency after waterborne infection. The resistance levels to VHSV ranged from 0 to 99% and resistance was not abrogated when resistant and sensitive animals were mixed and subjected to waterborne infection. VHSV was recovered from 10% of resistant fish after waterborne infection, confirming that virus replication was possible in this context but effective only in a low proportion of the population. The different clones also exhibited a wide range of survival (0-68%) after a waterborne infection with IHNV. Although VHSV-resistant clones were not fully resistant to IHNV, the susceptibility to IHNV and VHSV tended to be correlated, suggesting that non-specific mechanisms common to both viruses were involved.     

The implication of SUMO in intrinsic and innate immunity

Since its discovery, SUMOylation has emerged as a key post-translational modification involved in the regulation of host-virus interactions. SUMOylation has been associated with the replication of a large number of viruses, either through the direct modification of viral proteins or through the modulation of cellular proteins implicated in antiviral defense. SUMO can affect protein function via covalent or non-covalent binding. There is growing evidence that SUMO regulates several host proteins involved in intrinsic and innate immunity, thereby contributing to the process governing interferon production during viral infection; as well as the interferon-activated Jak/STAT pathway. Unlike the interferon-mediated innate immune response, intrinsic antiviral resistance is mediated by constitutively expressed antiviral proteins (defined as restriction factors), which confer direct viral resistance through a variety of mechanisms. The aim of this review is to evaluate the role of SUMO in intrinsic and innate immunity; highlighting the involvement of the TRIM family proteins, with a specific focus on the mechanism through which SUMO affects i- interferon production upon viral infection, ii-interferon Jak/STAT signaling and biological responses, iii-the relationship between restriction factors and RNA viruses.     

Interferon-lambda contributes to innate immunity of mice against influenza A virus but not against hepatotropic viruses

Virus-infected cells secrete a broad range of interferon (IFN) subtypes which in turn trigger the synthesis of antiviral factors that confer host resistance. IFN-alpha, IFN-beta and other type I IFNs signal through a common universally expressed cell surface receptor, whereas IFN-lambda uses a distinct receptor complex for signaling that is not present on all cell types. Since type I IFN receptor-deficient mice (IFNAR1(0/0)) exhibit greatly increased susceptibility to various viral diseases, it remained unclear to which degree IFN-lambda might contribute to innate immunity. To address this issue we performed influenza A virus infections of mice which carry functional alleles of the influenza virus resistance gene Mx1 and which, therefore, develop a more complete innate immune response to influenza viruses than standard laboratory mice. We demonstrate that intranasal administration of IFN-lambda readily induced the antiviral factor Mx1 in mouse lungs and efficiently protected IFNAR1(0/0) mice from lethal influenza virus infection. By contrast, intraperitoneal application of IFN-lambda failed to induce Mx1 in the liver of IFNAR1(0/0) mice and did not protect against hepatotropic virus infections. Mice lacking functional IFN-lambda receptors were only slightly more susceptible to influenza virus than wild-type mice. However, mice lacking functional receptors for both IFN-alpha/beta and IFN-lambda were hypersensitive and even failed to restrict usually non-pathogenic influenza virus mutants lacking the IFN-antagonistic factor NS1. Interestingly, the double-knockout mice were not more susceptible against hepatotropic viruses than IFNAR1(0/0) mice. From these results we conclude that IFN-lambda contributes to inborn resistance against viral pathogens infecting the lung but not the liver.     

Disruption of Type I Interferon Induction by HIV Infection of T Cells

Our main objective of this study was to determine how Human Immunodeficiency Virus (HIV) avoids induction of the antiviral Type I Interferon (IFN) system. To limit viral infection, the innate immune system produces important antiviral cytokines such as the IFN. IFN set up a critical roadblock to virus infection by limiting further replication of a virus. Usually, IFN production is induced by the recognition of viral nucleic acids by innate immune receptors and subsequent downstream signaling. However, the importance of IFN in the defense against viruses has lead most pathogenic viruses to evolve strategies to inhibit host IFN induction or responses allowing for increased pathogenicity and persistence of the virus. While the adaptive immune responses to HIV infection have been extensively studied, less is known about the balance between induction and inhibition of innate immune defenses, including the antiviral IFN response, by HIV infection. Here we show that HIV infection of T cells does not induce significant IFN production even IFN I Interferon production. To explain this paradox, we screened HIV proteins and found that two HIV encoded proteins, Vpu and Nef, strongly antagonize IFN induction, with expression of these proteins leading to loss of expression of the innate immune viral RNA sensing adaptor protein, IPS-1 (IFN-β promoter stimulator-1). We hypothesize that with lower levels of IPS-1 present, infected cells are defective in mounting antiviral responses allowing HIV to replicate without the normal antiviral actions of the host IFN response. Using cell lines as well as primary human derived cells, we show that HIV targeting of IPS-1 is key to limiting IFN induction. These findings describe how HIV infection modulates IFN induction providing insight into the mechanisms by which HIV establishes infection and persistence in a host.     

Effect of Cecropin B and a Synthetic Analogue on Propagation of Fish Viruses In Vitro

Abstract Cecropins and other natural antimicrobial peptides are widely distributed in animals from insects to mammals. These proteins have been shown to be major constituents of the innate immune systems of animals for nonspecific defense of the host against various bacteria and parasites. Therefore, exploitation of this natural innate defense system may lead to the development of effective methods for protecting fish from invasion by microbial pathogens. Recently, we have demonstrated that the introduction of cecropin transgenes into Japanese medaka (Oryzias latipes) conferred resistance to infection by fish bacterial pathogens. Aside from a few reports documenting the antiviral effect of antimicrobial peptides including cecropins against mammalian viruses, there is no evidence for the effect of these peptides against fish viruses. In this article we present results of in vitro characterization of native cecropin B and a synthetic analogue, CF17, against several important fish viral pathogens—namely, infectious hematopoietic necrosis virus (IHNV), viral hemorrhagic septicemia virus (VHSV), snakehead rhabdovirus (SHRV), and infectious pancreatic necrosis virus (IPNV). Upon coincubation of these peptides and viruses, the viral titers yielded in fish cells were reduced from several fold to 104-fold. Direct disruption of the viral envelope and disintegration of the viral capsids may be involved in the inhibition of viral replication by the peptides. Results of our studies demonstrate the potential of manipulating antimicrobial peptide genes by transgenesis to combat viral infection in fish.     

Epstein-Barr virus LF2: an antagonist to type I interferon

Upon viral infection, the major defense mounted by the host immune system is activation of the interferon (IFN)-mediated antiviral pathway, which is mediated by IFN regulatory factors (IRFs). In order to complete their life cycle, viruses must modulate host IFN-mediated immune responses. Despite its association with significant human health problems, activities of Epstein-Barr virus (EBV), a human tumor-inducing herpesvirus, to evade host IFN-mediated innate immunity have not been well characterized. To search for EBV genes that block IFN signal transduction, we carried out a screening of EBV open reading frames for their abilities to block IFN-α/β-mediated luciferase expression upon Sendai virus infection. This screening demonstrates that EBV LF2 tegument protein specifically interacts with the central inhibitory association domain of IRF7, and this interaction leads to inhibition of the dimerization of IRF7, which suppresses IFN-α production and IFN-mediated immunity. This demonstrates a novel immune evasion mechanism of EBV LF2 in blocking cellular IRF7-mediated innate immunity.     

Paramyxovirus Activation and Inhibition of Innate Immune Responses

Paramyxoviruses represent a remarkably diverse family of enveloped nonsegmented negative-strand RNA viruses, some of which are the most ubiquitous disease-causing viruses of humans and animals. This review focuses on paramyxovirus activation of innate immune pathways, the mechanisms by which these RNA viruses counteract these pathways, and the innate response to paramyxovirus infection of dendritic cells (DC). Paramyxoviruses are potent activators of extracellular complement pathways, a first line of defense that viruses must face during natural infections. We discuss mechanisms by which these viruses activate and combat complement to delay neutralization. Once cells are infected, virus replication drives type I interferon (IFN) synthesis that has the potential to induce a large number of antiviral genes. Here we describe four approaches by which paramyxoviruses limit IFN induction: by limiting synthesis of IFN-inducing aberrant viral RNAs, through targeted inhibition of RNA sensors, by providing viral decoy substrates for cellular kinase complexes, and through direct blocking of the IFN promoter. In addition, paramyxoviruses have evolved diverse mechanisms to disrupt IFN signaling pathways. We describe three general mechanisms, including targeted proteolysis of signaling factors, sequestering cellular factors, and upregulation of cellular inhibitors. DC are exceptional cells with the capacity to generate adaptive immunity through the coupling of innate immune signals and T cell activation. We discuss the importance of innate responses in DC following paramyxovirus infection and their consequences for the ability to mount and maintain antiviral T cells.     

Interferon response following infection with genetically similar isolates of viral haemorrhagic septicaemia virus (VHSV) exhibiting contrasting virulence in rainbow trout

Isolates of viral haemorrhagic septicaemia virus (VHSV) were identified which are genetically similar yet, based on their isolation history were considered likely to differ in virulence in juvenile rainbow trout. An experimental infection study was performed in order to verify this hypothesis and provide an experimental infectivity model with which to investigate the basis for susceptibility of rainbow trout to this commercially important virus. Significant differences in mortality were obtained following both intraperitoneal (IP) injection and immersion challenges with an early marine (DK-M.Rhabdo) and early rainbow trout VHSV isolate (DK-F1) respectively. Expression of Type I IFN, Mx1 (an IFN-inducible protein), and viral genes (encoding nucleo-, phospho-, matrix, glyco- and non-viron proteins) was studied in sequential tissue samples using real-time quantitative PCR (QPCR). Resulting data revealed a significant increase in IFN and Mx1 expression detected in fish challenged by IP injection with both isolates. Expression levels of these genes were directly related to the degree of viral replication as measured by the expression of VHSV RNAs. In immersion-challenged fish a significant increase in Mx1 was observed only when using the virulent isolate DK-F1; however no elevated host response was detectable in fish challenged with the marine isolate DK-M.Rhabdo. Quintessentially the inability to detect any virus in trout challenged with the marine isolate via immersion suggests the virus was incapable of establishing infection. The mechanisms for this appear to be more related to initial cellular entry and replication rather than due to the overcoming of initial infection via an elevated host innate immune response.     

Inhibition of interferon regulatory factor 7 (IRF7)-mediated interferon signal transduction by the Kaposi's sarcoma-associated herpesvirus viral IRF homolog vIRF3

Upon viral infection, the major defense mounted by the host immune system is activation of the interferon (IFN)-mediated antiviral pathway that is mediated by IFN regulatory factors (IRFs). In order to complete their life cycle, viruses must modulate the host IFN-mediated immune response. Kaposi's sarcoma-associated herpesvirus (KSHV), a human tumor-inducing herpesvirus, has developed a unique mechanism for antagonizing cellular IFN-mediated antiviral activity by incorporating viral homologs of the cellular IRFs, called vIRFs. Here, we report a novel immune evasion mechanism of KSHV vIRF3 to block cellular IRF7-mediated innate immunity in response to viral infection. KSHV vIRF3 specifically interacts with either the DNA binding domain or the central IRF association domain of IRF7, and this interaction leads to the inhibition of IRF7 DNA binding activity and, therefore, suppression of alpha interferon (IFN-alpha) production and IFN-mediated immunity. Remarkably, the central 40 amino acids of vIRF3, containing the double alpha helix motifs, are sufficient not only for binding to IRF7, but also for inhibiting IRF7 DNA binding activity. Consequently, the expression of the double alpha helix motif-containing peptide effectively suppresses IRF7-mediated IFN-alpha production. This demonstrates a remarkably efficient means of viral avoidance of host antiviral activity.     

Identification of lncRNA‐155 encoded by MIR155HG as a novel regulator of innate immunity against influenza A virus infection

Long noncoding RNAs (lncRNAs) are single‐stranded RNA molecules longer than 200 nt that regulate many cellular processes. MicroRNA 155 host gene (MIR155HG) encodes the microRNA (miR)‐155 that regulates various signalling pathways of innate and adaptive immune responses against viral infections. MIR155HG also encodes a lncRNA that we call lncRNA‐155. Here, we observed that expression of lncRNA‐155 was markedly upregulated during influenza A virus (IAV) infection both in vitro (several cell lines) and in vivo (mouse model). Interestingly, robust expression of lncRNA‐155 was also induced by infections with several other viruses. Disruption of lncRNA‐155 expression in A549 cells diminished the antiviral innate immunity against IAV. Furthermore, knockout of lncRNA‐155 in mice significantly increased IAV replication and virulence in the animals. In contrast, overexpression of lncRNA‐155 in human cells suppressed IAV replication, suggesting that lncRNA‐155 is involved in host antiviral innate immunity induced by IAV infection. Moreover, we found that lncRNA‐155 had a profound effect on expression of protein tyrosine phosphatase 1B (PTP1B) during the infection with IAV. Inhibition of PTP1B by lncRNA‐155 resulted in higher production of interferon‐beta (IFN‐β) and several critical interferon‐stimulated genes (ISGs). Together, these observations reveal that MIR155HG derived lncRNA‐155 can be induced by IAV, which modulates host innate immunity during the virus infection via regulation of PTP1B‐mediated interferon response.     

Host factors involved in resistance to retroviral infection

Viral replication requires the help of host cell factors, whose species specificity may affect viral tropism. On the other hand, there exist host factors that restrict viral replication. The anti-viral system mediated by some of these restriction factors, which is termed intrinsic immunity and is distinguished from conventional innate and adaptive immunity, has been described as playing an important role in making species-specific barriers against viral infection. Here, we describe the current progress in understanding of such restriction factors against retroviral replication, focusing on TRIM5alpha and APOBEC, whose anti-retroviral effects have recently been recognized. Additionally, we mention cyclophilin A, which is essential for HIV-1 replication in human cells and may affect viral tropism. Understanding of these host factors would contribute to identification of the determinants for viral tropism.     

Understanding Interferon Subtype Therapy for Viral Infections: Harnessing the Power of the Innate Immune System.

Type I and III interferons (IFNs) of the innate immune system belong to a polygenic family, however the individual subtype mediators of the antiviral response in viral infections have been hindered by a lack of reagents. Evaluation studies using different IFN subtypes have distinguished distinct protein properties with different efficacies towards different viruses, opening promising avenues for immunotherapy. This review largely focuses on the application of IFN-α/β and IFN-λ therapies for viral infections, influenza, herpes, HIV and hepatitis. Such IFN subtype therapies may help to cure patients with virus infections where no vaccine exists. The ability of cell types to secrete a number of IFN subtypes from a multi-gene family may be an intuitive counterattack on viruses that evade IFN subtype responses. Hence, clinical use of virus-targeted IFN subtypes may restore antiviral immunity in viral infections. Accumulating evidence suggests that individual IFN subtypes have differential efficacies in selectively activating immune cell subsets to enhance antiviral immune responses leading to production of sustained B and T cell memory. Cytokine therapy can augment innate immunity leading to clearance of acute virus infections but such treatments may have limited effects on chronic virus infections that establish lifelong latency. Therefore, exploiting individual IFN subtypes to select those with the ability to sculpt protective responses as well as reinstating those targeted by viral evasion mechanisms may inform development of improved antiviral therapy.