Filovirus VP24 Proteins Differentially Regulate RIG-I and MDA5-Dependent Type I and III Interferon Promoter Activation

Antiviral defense against filovirus infections: targets and evasion mechanisms

Filoviruses include a number of serious human pathogens, infections with which result in the development of hemorrhagic fevers with high case fatality rates. As for other RNA viruses, viral replication generates both protein and RNA species that can serve as danger signals, leading to the activation of antiviral defense pathways. However, in order to be able to efficiently infect humans these viruses have developed mechanisms that allow them to evade diverse host antiviral defense mechanisms. Consequently, in addition to their functions within the viral lifecycle many filovirus proteins have been shown to have accessory functions involved in the regulation of diverse host pathways. These include those of the type-I interferon response, other pathways involved in dsRNA-sensing, as well as the selective inhibition of interferon stimulated gene activities. Further, filoviruses have developed mechanisms to subvert recognition of infected cells and the generation of neutralizing antibodies. This review focuses on bringing together the evidence to date supporting the existence of diverse mechanisms aimed at regulating these pathways as well as providing details of the mechanisms involved.

Novel filoviruses: indication of a global threat or cause to reassess our risk perception?

Filoviruses such as Ebola virus are widely known as causative agents of severe human disease, although apathogenic filoviruses also exist. There is now increasing evidence that filoviruses circulate in almost all parts of the world, where they are being discovered in an expanding range of sometimes unexpected host species. Here we summarize the current knowledge regarding these novel filoviruses, and open questions that need answering to assess and prepare for the risk they pose.

The Role of Ebola Virus VP24 Nuclear Trafficking Signals in Infectious Particle Production

Ebola virus (EBOV) protein VP24 carries out at least two critical functions. It promotes condensation of viral nucleocapsids, which is crucial for infectious virus production, and it suppresses interferon (IFN) signaling, which requires interaction with the NPI-1 subfamily of importin-α (IMPA) nuclear transport proteins. Interestingly, over-expressed IMPA leads to VP24 nuclear accumulation and a carboxy-terminus nuclear export signal (NES) has been reported, suggesting that VP24 may undergo nuclear trafficking. For the first time, we demonstrate that NPI-1 IMPA overexpression leads to the nuclear accumulation of VP24 during EBOV infection. To assess the functional impact of nuclear trafficking, we generated tetracistronic minigenomes encoding VP24 nuclear import and/or export signal mutants. The minigenomes, which also encode Renilla luciferase and viral proteins VP40 and GP, were used to generate transcription and replication competent virus-like particles (trVLPs) that can be used to assess EBOV RNA synthesis, gene expression, entry and viral particle production. With this system, we confirmed that NES or IMPA binding site mutations altered VP24 nuclear localization, demonstrating functional trafficking signals. While these mutations minimally affected transcription and replication, the trVLPs exhibited impaired infectivity and formation of shortened nucleocapsids for the IMPA binding mutant. For the NES mutants, infectivity was reduced approximately 1000-fold. The NES mutant could still suppress IFN signaling but failed to promote nucleocapsid formation. To determine whether VP24 nuclear export is required for infectivity, the residues surrounding the wildtype NES were mutated to alanine or the VP24 NES was replaced with the Protein Kinase A Inhibitor NES. While nuclear export remained intact for these mutants, infectivity was severely impaired. These data demonstrate that VP24 undergoes nuclear trafficking and illuminates a separate and critical role for the NES and surrounding sequences in infectivity and nucleocapsid assembly.

Reverse Genetics Systems for Filoviruses

Filoviruses are causative agents of severe hemorrhagic fevers with high case fatality rates in humans. For studies of virus biology and the subsequent development of countermeasures, reverse genetic systems, and especially those facilitating the generation of recombinant filoviruses, are indispensable. Here, we describe the generation of recombinant filoviruses from cDNA.

In vivo characterization of the novel ebolavirus Bombali virus suggests a low pathogenic potential for humans

Ebolaviruses cause outbreaks of haemorrhagic fever in Central and West Africa. Some members of this genus such as Ebola virus (EBOV) are highly pathogenic, with case fatality rates of up to 90%, whereas others such as Reston virus (RESTV) are apathogenic for humans. Bombali virus (BOMV) is a novel ebolavirus for which complete genome sequences were recently found in free-tailed bats, although no infectious virus could be isolated. Its pathogenic potential for humans is unknown. To address this question, we first determined whether proteins encoded by the available BOMV sequence found in Chaerephon pumilus were functional in in vitro assays. The correction of an apparent sequencing error in the glycoprotein based on these data then allowed us to generate infectious BOMV using reverse genetics and characterize its infection of human cells. Furthermore, we used HLA-A2-transgenic, NOD-scid-IL-2γ receptor-knockout (NSG-A2) mice reconstituted with human haematopoiesis as a model to evaluate the pathogenicity of BOMV in vivo in a human-like immune environment. These data demonstrate that not only does BOMV show a slower growth rate than EBOV in vitro, but it also shows low pathogenicity in humanized mice, comparable to previous studies using RESTV. Taken together, these findings suggest a low pathogenic potential of BOMV for humans.

Natural killer cells and their exosomes in viral infections and related therapeutic approaches: where are we?

Innate immunity is the first line of the host immune system to fight against infections. Natural killer cells are the innate immunity lymphocytes responsible for fighting against virus-infected and cancerous cells. They have various mechanisms to suppress viral infections. On the other hand, viruses have evolved to utilize different ways to evade NK cell-mediated responses. Viruses can balance the response by regulating the cytokine release pattern and changing the proportion of activating and inhibitory receptors on the surface of NK cells. Exosomes are a subtype of extracellular vesicles that are involved in intercellular communication. Most cell populations can release these nano-sized vesicles, and it was shown that these vesicles produce identical outcomes to the originating cell from which they are released. In recent years, the role of NK cell-derived exosomes in various diseases including viral infections has been highlighted, drawing attention to utilizing the therapeutic potential of these nanoparticles. In this article, the role of NK cells in various viral infections and the mechanisms used by viruses to evade these important immune system cells are initially examined. Subsequently, the role of NK cell exosomes in controlling various viral infections is discussed. Finally, the current position of these cells in the treatment of viral infections and the therapeutic potential of their exosomes are reviewed. Video Abstract.

VP24 matrix proteins of eight filoviruses downregulate innate immune response by inhibiting the interferon-induced pathway

Filoviruses encode viral protein 24 (VP24) which effectively inhibit the innate immune responses in infected cells. Here we systematically analysed the effects of nine mammalian filovirus VP24 proteins on interferon (IFN)-induced immune response. We transiently expressed Ebola, Bombali, Bundibugyo, Reston, Sudan and Taï Forest ebolavirus (EBOV, BOMV, BDBV, RESTV, SUDV, TAFV, respectively), Lloviu virus (LLOV), Mengla dianlovirus (MLAV) and Marburgvirus (MARV) VP24 proteins and analysed their ability to inhibit IFN-α-induced activation of myxovirus resistance protein 1 (MxA) and interferon-induced transmembrane protein 3 (IFITM3) promoters. In addition, we analysed the expression of endogenous MxA protein in filovirus VP24-expressing cells. Eight filovirus VP24 proteins, including the VP24s of the recently discovered MLAV, BOMV and LLOV, inhibited IFN-induced MxA and IFITM3 promoter activation. MARV VP24 was the only protein with no inhibitory effect on the activation of either promoter. Endogenous MxA protein expression was impaired in cells transiently expressing VP24s with the exception of MARV VP24. We mutated nuclear localization signal (NLS) of two highly pathogenic filoviruses (EBOV and SUDV) and two putatively non-pathogenic filoviruses (BOMV and RESTV), and showed that the inhibitory effect on IFN-induced expression of MxA was dependent on functional cluster 3 of VP24 nuclear localization signal. Our findings suggest that filovirus VP24 proteins are both genetically and functionally conserved, and that VP24 proteins of most filovirus species are capable of inhibiting IFN-induced antiviral gene expression thereby efficiently downregulating the host innate immune responses.

Ebolavirus Species-Specific Interferon Antagonism Mediated by VP24

Members of the Ebolavirus genus demonstrate a marked differences in pathogenicity in humans with Ebola (EBOV) being the most pathogenic, Bundibugyo (BDBV) less pathogenic, and Reston (RESTV) is not known to cause a disease in humans. The VP24 protein encoded by members of the Ebolavirus genus blocks type I interferon (IFN-I) signaling through interaction with host karyopherin alpha nuclear transporters, potentially contributing to virulence. Previously, we demonstrated that BDBV VP24 (bVP24) binds with lower affinities to karyopherin alpha proteins relative to EBOV VP24 (eVP24), and this correlated with a reduced inhibition in IFN-I signaling. We hypothesized that modification of eVP24-karyopherin alpha interface to make it similar to bVP24 would attenuate the ability to antagonize IFN-I response. We generated a panel of recombinant EBOVs containing single or combinations of point mutations in the eVP24-karyopherin alpha interface. Most of the viruses appeared to be attenuated in both IFN-I-competent 769-P and IFN-I-deficient Vero-E6 cells in the presence of IFNs. However, the R140A mutant grew at reduced levels even in the absence of IFNs in both cell lines, as well as in U3A STAT1 knockout cells. Both the R140A mutation and its combination with the N135A mutation greatly reduced the amounts of viral genomic RNA and mRNA suggesting that these mutations attenuate the virus in an IFN-I-independent attenuation. Additionally, we found that unlike eVP24, bVP24 does not inhibit interferon lambda 1 (IFN-λ1), interferon beta (IFN-β), and ISG15, which potentially explains the lower pathogenicity of BDBV relative to EBOV. Thus, the VP24 residues binding karyopherin alpha attenuates the virus by IFN-I-dependent and independent mechanisms.

Hemorrhagic fever viruses: Pathogenesis, therapeutics, and emerging and re-emerging potential

Hemorrhagic fever viruses (HFVs) pose a threat to global public health owing to the emergence and re-emergence of highly fatal diseases. Viral hemorrhagic fevers (VHFs) caused by these viruses are mostly characterized by an acute febrile syndrome with coagulation abnormalities and generalized hemorrhage that may lead to life-threatening organ dysfunction. Currently, the events underlying the viral pathogenicity associated with multiple organ dysfunction syndrome still underexplored. In this minireview, we address the current knowledge of the mechanisms underlying VHFs pathogenesis and discuss the available development of preventive and therapeutic options to treat these infections. Furthermore, we discuss the potential of HFVs to cause worldwide emergencies along with factors that favor their spread beyond their original niches.