Dengue virus (DENV) is a pathogen with a high impact on human health. It replicates in a wide range of cells involved in the immune response. To efficiently infect humans, DENV must evade or inhibit fundamental elements of the innate immune system, namely the type I interferon response. DENV circumvents the host immune response by expressing proteins that antagonize the cellular innate immunity. We have recently documented the inhibition of type I IFN production by the proteolytic activity of DENV NS2B3 protease complex in human monocyte derived dendritic cells (MDDCs). In the present report we identify the human adaptor molecule STING as a target of the NS2B3 protease complex. We characterize the mechanism of inhibition of type I IFN production in primary human MDDCs by this viral factor. Using different human and mouse primary cells lacking STING, we show enhanced DENV replication. Conversely, mutated versions of STING that cannot be cleaved by the DENV NS2B3 protease induced higher levels of type I IFN after infection with DENV. Additionally, we show that DENV NS2B3 is not able to degrade the mouse version of STING, a phenomenon that severely restricts the replication of DENV in mouse cells, suggesting that STING plays a key role in the inhibition of DENV infection and spread in mice. Dengue virus (DENV) is a pathogen with a high impact in human health that replicates in a wide range of cells of the immune system. To efficiently infect humans, DENV must evade or inhibit fundamental elements of the innate immune system, namely the type I interferon response (IFN). Thus, DENV can inhibit type I IFN signaling (described by several groups), and type I IFN production (described by our group). We documented the inhibition of type I IFN production in human monocyte derived dendritic cells (MDDCs) with an otherwise strong cytokine and chemokine profile in those cells and that the NS2B3 protease complex of DENV functions as an antagonist of type I IFN production, and its proteolytic activity is necessary for this event. Here we identify the human adaptor molecule STING as a target of the NS2B3 protease complex and characterize the mechanism of inhibition of the type I IFN production in primary human MDDCs mediated by this viral factor. We also describe that DENV NS2B3 cannot degrade the mouse version of STING, a phenomenon that strictly restricts the replication of DENV in mouse cells, suggesting that STING plays a key role in the inhibition of DENV infection and spread in mice.

An estimated 50 million dengue virus (DENV) infections occur annually and more than forty percent of the human population is currently at risk of developing dengue fever (DF) or dengue hemorrhagic fever (DHF). Despite the prevalence and potential severity of DF and DHF, there are no approved vaccines or antiviral therapeutics available. An improved understanding of DENV immune evasion is pivotal for the rational development of anti-DENV therapeutics. Antagonism of type I interferon (IFN-I) signaling is a crucial mechanism of DENV immune evasion. DENV NS5 protein inhibits IFN-I signaling by mediating proteasome-dependent STAT2 degradation. Only proteolytically-processed NS5 can efficiently mediate STAT2 degradation, though both unprocessed and processed NS5 bind STAT2. Here we identify UBR4, a 600-kDa member of the N-recognin family, as an interacting partner of DENV NS5 that preferentially binds to processed NS5. Our results also demonstrate that DENV NS5 bridges STAT2 and UBR4. Furthermore, we show that UBR4 promotes DENV-mediated STAT2 degradation, and most importantly, that UBR4 is necessary for efficient viral replication in IFN-I competent cells. Our data underscore the importance of NS5-mediated STAT2 degradation in DENV replication and identify UBR4 as a host protein that is specifically exploited by DENV to inhibit IFN-I signaling via STAT2 degradation. Dengue virus (DENV) is the leading cause of mosquito-borne viral illness and death in humans. At present, there are no vaccines and no specific antiviral therapeutics to prevent or treat DENV infections. We previously described that the NS5 protein of DENV inhibits type I interferon signaling in virus-infected cells by mediating STAT2 degradation. This property allows DENV to overcome the antiviral effects of type I interferon, contributing to viral replication in the host. We have now obtained new insight into the mechanism by which DENV NS5 induces STAT2 degradation. NS5 bridges STAT2 with the cellular protein UBR4, a member of a family of predicted E3 ligases, resulting in UBR4-mediated STAT2 degradation. Elimination of UBR4 or mutations in NS5 that prevent its binding to UBR4 prevent NS5 from inducing STAT2 degradation. Importantly, UBR4 is required for optimal DENV replication in the presence of a competent type I interferon system. Our data demonstrate the requirement of a host factor, UBR4, for DENV to overcome the antiviral interferon response. This information might be important for the design of specific DENV inhibitors that prevent dengue virus from evading innate immunity.

Both antibodies and T cells contribute to immunity against influenza virus infection. However, the generation of strong Th1 immunity is crucial for viral clearance. Interestingly, we found that human dendritic cells (DCs) infected with influenza A virus have lower allospecific Th1-cell stimulatory abilities than DCs activated by other stimuli, such as lipopolysaccharide and Newcastle disease virus infection. This weak stimulatory activity correlates with a suboptimal maturation of the DCs following infection with influenza A virus. We next investigated whether the influenza A virus NS1 protein could be responsible for the low levels of DC maturation after influenza virus infection. The NS1 protein is an important virulence factor associated with the suppression of innate immunity via the inhibition of type I interferon (IFN) production in infected cells. Using recombinant influenza and Newcastle disease viruses, with or without the NS1 gene from influenza virus, we found that the induction of a genetic program underlying DC maturation, migration, and T-cell stimulatory activity is specifically suppressed by the expression of the NS1 protein. Among the genes affected by NS1 are those coding for macrophage inflammatory protein 1β, interleukin-12 p35 (IL-12 p35), IL-23 p19, RANTES, IL-8, IFN-α/β, and CCR7. These results indicate that the influenza A virus NS1 protein is a bifunctional viral immunosuppressor which inhibits innate immunity by preventing type I IFN release and inhibits adaptive immunity by attenuating human DC maturation and the capacity of DCs to induce T-cell responses. Our observations also support the potential use of NS1 mutant influenza viruses as live attenuated influenza virus vaccines.