Dual modulation of type I interferon response by bluetongue virus.

UNLABELLED: Bluetongue virus (BTV) is a double-stranded RNA (dsRNA) virus that causes an economically important disease in ruminants. BTV infection is a strong inducer of type I interferon (IFN-I) in multiple cell types. It has been shown recently that BTV and, more specifically, the nonstructural protein NS3 of BTV are able to modulate the IFN-I synthesis pathway. However, nothing is known about the ability of BTV to counteract IFN-I signaling. Here, we investigated the effect of BTV on the IFN-I response pathway and, more particularly, the Janus tyrosine kinase (JAK)/signal transducer and activator of transcription protein (STAT) signaling pathway. We found that BTV infection triggered the expression of IFN-stimulated genes (ISGs) in A549 cells. However, when BTV-infected cells were stimulated with external IFN-I, we showed that activation of the IFN-stimulated response element (ISRE) promoter and expression of ISGs were inhibited. We found that this inhibition involved two different mechanisms that were dependent on the time of infection. After overnight infection, BTV blocked specifically the phosphorylation and nuclear translocation of STAT1. This inhibition correlated with the redistribution of STAT1 in regions adjacent to the nucleus. At a later time point of infection, BTV was found to interfere with the activation of other key components of the JAK/STAT pathway and to induce the downregulation of JAK1 and TYK2 protein expression. Overall, our study indicates for the first time that BTV is able to interfere with the JAK/STAT pathway to modulate the IFN-I response. IMPORTANCE: Bluetongue virus (BTV) causes a severe disease in ruminants and has an important impact on the livestock economy in areas of endemicity such as Africa. The emergence of strains, such as serotype 8 in Europe in 2006, can lead to important economic losses due to commercial restrictions and prophylactic measures. It has been known for many years that BTV is a strong inducer of type I interferon (IFN-I) in vitro and in vivo in multiple cell types. However, the ability of BTV to interact with the IFN-I system remains unclear. Here, we report that BTV is able to modulate the IFN-I response by interfering with the Janus tyrosine kinase (JAK)/signal transducer and activator of transcription protein (STAT) signaling pathway. These findings contribute to knowledge of how BTV infection interferes with the host's innate immune response and becomes pathogenic. This will also be important for the design of efficacious vaccine candidates.

Induction and control of the type I interferon pathway by Bluetongue virus.

The innate immune response is the first line of defence against viruses, involving the production of type I IFN (IFN-α/ß) and other pro-inflammatory cytokines that control the infection. It also shapes the adaptive immune response generated by both T and B cells. Production of type I IFN occurs both in vivo and in vitro in response to Bluetongue virus (BTV), an arthropod-borne virus. However, the mechanisms responsible for the production of IFN-ß in response to BTV remained unknown until recently and are still not completely understood. In this review, we describe the recent advances in the identification of cellular sensors and signalling pathways involved in this process. The RNA helicases retinoic acid-inducible gene-I (RIG-I) and melanoma differentiation-associated gene 5 (MDA5) were shown to be involved in the expression of IFN-ß as well as in the control of BTV infection in non-haematopoietic cells. In contrast, induction of IFN-α/ß synthesis in sheep primary plasmacytoid dendritic cells (pDCs) required the MyD88 adaptor independently of the Toll-like receptor 7 (TLR7), as well as the kinases dsRNA-activated protein kinase (PKR) and stress-activated protein kinase (SAPK)/Jun N-terminal protein kinase (JNK). As type I IFN is essential for the establishment of an antiviral cellular response, most of viruses have elaborated counteracting mechanisms to hinder its action. This review also addresses the ability of BTV to interfere with IFN-ß synthesis and the recent findings describing the non-structural viral protein NS3 as a powerful antagonist of the host cellular response.

NS3 of bluetongue virus interferes with the induction of type I interferon.

Upon infection with Bluetongue virus (BTV), an arthropod-borne virus, type I interferon (IFN-I) is produced in vivo and in vitro. IFN-I is essential for the establishment of an antiviral cellular response, and most if not all viruses have elaborated strategies to counteract its action. In this study, we assessed the ability of BTV to interfere with IFN-I synthesis and identified the nonstructural viral protein NS3 as an antagonist of the IFN-I system.

Sensing and control of bluetongue virus infection in epithelial cells via RIG-I and MDA5 helicases.

Bluetongue virus (BTV), an arthropod-borne member of the Reoviridae family, is a double-stranded RNA virus that causes an economically important livestock disease that has spread across Europe in recent decades. Production of type I interferon (alpha/beta interferon [IFN-α/ß]) has been reported in vivo and in vitro upon BTV infection. However, the cellular sensors and signaling pathways involved in this process remain unknown. Here we studied the mechanisms responsible for the production of IFN-ß in response to BTV serotype 8. Upon BTV infection of A549 cells, expression of IFN-ß and other proinflammatory cytokines was strongly induced at both the protein and mRNA levels. This response appeared to be dependent on virus replication, since exposure to UV-inactivated virus failed to induce IFN-ß. We also demonstrated that BTV infection activated the transcription factors IFN regulatory factor 3 and nuclear factor κB. We investigated the role of several pattern recognition receptors in this response and showed that expression of IFN-ß was greatly reduced after small-interfering-RNA-mediated knockdown of the RNA helicase encoded by retinoic acid-inducible gene I (RIG-I) or melanoma differentiation-associated gene 5 (MDA5). In contrast, silencing of MyD88, Toll-like receptor 3, or the recently described DexD/H-box helicase DDX1 sensor had no or a weak effect on IFN-ß induction, suggesting that the RIG-I-like receptor pathway is specifically engaged for BTV sensing. Moreover, we also showed that overexpression of either RIG-I or MDA5 impaired BTV expression in infected A549 cells. Overall, this indicates that RIG-I and MDA5 can both contribute to the recognition and control of BTV infection.

The double-stranded RNA bluetongue virus induces type I interferon in plasmacytoid dendritic cells via a MYD88-dependent TLR7/8-independent signaling pathway.

Dendritic cells (DCs), especially plasmacytoid DCs (pDCs), produce large amounts of alpha/beta interferon (IFN-α/ß) upon infection with DNA or RNA viruses, which has impacts on the physiopathology of the viral infections and on the quality of the adaptive immunity. However, little is known about the IFN-α/ß production by DCs during infections by double-stranded RNA (dsRNA) viruses. We present here novel information about the production of IFN-α/ß induced by bluetongue virus (BTV), a vector-borne dsRNA Orbivirus of ruminants, in sheep primary DCs. We found that BTV induced IFN-α/ß in skin lymph and in blood in vivo. Although BTV replicated in a substantial fraction of the conventional DCs (cDCs) and pDCs in vitro, only pDCs responded to BTV by producing a significant amount of IFN-α/ß. BTV replication in pDCs was not mandatory for IFN-α/ß production since it was still induced by UV-inactivated BTV (UV-BTV). Other inflammatory cytokines, including tumor necrosis factor alpha (TNF-α), interleukin-6 (IL-6), and IL-12p40, were also induced by UV-BTV in primary pDCs. The induction of IFN-α/ß required endo-/lysosomal acidification and maturation. However, despite being an RNA virus, UV-BTV did not signal through Toll-like receptor 7 (TLR7) for IFN-α/ß induction. In contrast, pathways involving the MyD88 adaptor and kinases dsRNA-activated protein kinase (PKR) and stress-activated protein kinase (SAPK)/Jun N-terminal protein kinase (JNK) were implicated. This work highlights the importance of pDCs for the production of innate immunity cytokines induced by a dsRNA virus, and it shows that a dsRNA virus can induce IFN-α/ß in pDCs via a novel TLR-independent and Myd88-dependent pathway. These findings have implications for the design of efficient vaccines against dsRNA viruses.