Effects of Filovirus Interferon Antagonists on Responses of Human Monocyte-Derived Dendritic Cells to RNA Virus Infection

Abstract

Dendritic cells (DCs) are major targets of filovirus infection in vivo. Previous studies have shown that the filoviruses Ebola virus (EBOV) and Marburg virus (MARV) suppress DC maturation in vitro.Both viruses also encode innate immune evasion functions. The EBOV VP35 (eVP35) and the MARV VP35 (mVP35) proteins each can block RIG-I-like receptor signaling and alpha/ beta interferon (IFN-␣/␤) production. The EBOV VP24 (eVP24) and MARV VP40 (mVP40) proteins each inhibit the production of IFN-stimulated genes (ISGs) by blocking Jak-STAT signaling; however, this occurs by different mechanisms, with eVP24 blocking nuclear import of tyrosine-phosphorylated STAT1 and mVP40 blocking Jak1 function. MARV VP24 (mVP24) has been demonstrated to modulate host cell antioxidant responses. Previous studies demonstrated that eVP35 is sufficient to strongly impair primary human monocyte-derived DC (MDDC) responses upon stimulation induced through the RIG-I-like receptor pathways. We demonstrate that mVP35, like eVP35, suppresses not only IFN-␣/␤ production but also proinflammatory responses after stimulation of MDDCs with RIG-I activators. In contrast, eVP24 and mVP40, despite suppressing ISG production upon RIG-I activation, failed to block upregulation of maturation markers or T cell activation. mVP24, although able to stimulate expression of antioxidant response genes, had no measurable impact of DC function. These data are consistent with a model where filoviral VP35 proteins are the major suppressors of DC maturation during filovirus infection, whereas the filoviral VP24 proteins and mVP40 are insufficient to prevent DC maturation. Zaire ebolavirus (EBOV) and Marburg marburgvirus (MARV) belong to the Filovirus family of emerging, zoonotic negativesense RNA viruses. Both EBOV and MARV cause severe, frequently fatal disease in humans (1). The West African Ebola epidemic, which was first recognized in early 2014, is the largest filovirus outbreak on record with over 28,000 cases worldwide and a reported case fatality rate of approximately 40% (2). The severity of EBOV and MARV infections reflects, at least in part, robust systemic virus replication that is not effectively restrained by host immune responses (3).Both EBOV and MARV encode multiple proteins that impair innate antiviral immune responses and likely contribute to excessive virus replication (3, 4). Both viruses encode VP35, a doublestranded RNA (dsRNA)-binding protein that carries out multiple functions critical for virus propagation. One such function is the inhibition of signaling by RIG-I and MDA5 through (RLR) signaling, which otherwise triggers alpha/beta interferon (IFN-␣/␤) gene expression (5-13). For both EBOV VP35 (eVP35) and MARV VP35 (mVP35), point mutations that disrupt VP35 dsR-NA-binding activity also greatly impair RLR inhibition activity (5,9,14,15). In vitro studies suggest that dsRNA-binding allows eVP35 to sequester RIG-I stimulatory dsRNAs (8, 9). eVP35 also interacts with the cellular protein PACT, which binds to and facilitates activatio...

Conservation of Structure and Immune Antagonist Functions of Filoviral VP35 Homologs Present in Microbat Genomes

“…The most intensively studied, eVP35, inhibits IFN responses through several mechanisms (Basler et al, 2000; Leung et al, 2010; Luthra et al, 2013; Prins et al, 2009, 2010b; Yen and Basler, 2016). One crucial mechanism is thought to be the interaction of EBOV and mVP35 with dsRNA via their IID, sequestering immunostimulatory dsRNA from recognition by RLRs (Dilley et al, 2017).…”

The role of mononuclear phagocytes in Ebola virus infection

“…A more recent study looked more specifically at transcript levels in PBMCs in an experimental macaque infection with EBOV and found over 100 genes up‐regulated by EBOV infection, most of which were interferon stimulated genes (ISGs) and many of which were not reported previously to be up‐regulated during EBOV infection . Extensive production of ISGs during EBOV infection is somewhat surprising because two EBOV proteins, VP35 and VP24, serve as robust IFN antagonists . However, as the authors suggest, uninfected neighboring cells may be responsible for ISG production.…”

Molecular pathogenesis of viral hemorrhagic fever

“…In DCs for example, VP35 appears to suppress not only IFN production but also inflammatory cytokine production [61,62]. In contrast, numerous studies have infected peripheral blood mononuclear cells (PBMC), monocytes or macrophages and detected robust cytokine responses, suggesting that these cells may be important sources of damaging soluble mediators in vivo [34,22,42,33,77,78].…”