Tetherin Inhibits Cell-Free Virus Dissemination and Retards Murine Leukemia Virus Pathogenesis

Small but Highly Versatile: The Viral Accessory Protein Vpu

Human and simian immunodeficiency viruses (HIVs and SIVs, respectively) encode several small proteins (Vif, Vpr, Nef, Vpu, and Vpx) that are called accessory because they are not generally required for viral replication in cell culture. However, they play complex and important roles for viral immune evasion and spread in vivo. Here, we discuss the diverse functions and the relevance of the viral protein U (Vpu) that is expressed from a bicistronic RNA during the late stage of the viral replication cycle and found only in HIV-1 and closely related SIVs. It is well established that Vpu counteracts the restriction factor tetherin, mediates degradation of the primary viral CD4 receptors, and inhibits activation of the transcription factor nuclear factor kappa B. Recent studies identified additional activities and provided new insights into the sophisticated mechanisms by which Vpu enhances and prolongs the release of fully infectious viral particles. In addition, it has been shown that Vpu prevents superinfection not only by degrading CD4 but also by modulating DNA repair mechanisms to promote degradation of nuclear viral complementary DNA in cells that are already productively infected.

The role of REC8 in the innate immune response to viral infection

REC8 meiotic recombination protein (REC8) is a member of structural maintenance of chromosome (SMC) protein partners, which play an important role in meiosis, anti-tumor, and sperm formation. As the adapter proteins of RLR signaling and cGAS-DNA signaling, the activity and stability of MAVS (also known as VISA, Cardif and IPS-1) and STING (also known as MITA) are critical for innate immunity. Here, we report that REC8 interacts with MAVS and STING, and inhibits their ubiquitination and subsequent degradation, thereby promoting innate antiviral signaling. REC8 is upregulated through the JAK-STAT signaling pathway under viral infection. Knockdown of REC8 impair the innate immune responses against VSV (Vesicular Stomatitis Virus), NDV (Newcastle disease virus) and HSV (herpes simplex virus). Mechanistically, under the infection of viruses, the SUMOylated REC8 is transferred from the nucleus to the cytoplasm and then interacts with MAVS and STING to inhibit their K48-linked ubiquitination triggered by RNF5. Moreover, REC8 promotes the recruitment of TBK1 to MAVS and STING. Thus, REC8 functions as a positive modulator of innate immunity. Our work highlights a previously undocumented role of meiosis-associated protein REC8 in regulating innate immunity. IMPORTANCE The innate immune response is crucial for the host to resist the invasion of viruses and other pathogens. STING and MAVS play a critical role in the innate immune response to DNA and RNA viral infection, respectively. In this study, REC8 promotes the innate immune response by targeting STING and MAVS. Notably, REC8 interacts with MAVS and STING in the cytoplasm and inhibits K48-linked ubiquitination of MAVS and STING triggered by RNF5, stabilizing MAVS and STING protein to promote innate immunity and gradually inhibiting viral infection. Our study provides a new insight for the study of antiviral innate immune.

Human IFITM3 restricts chikungunya virus and Mayaro virus infection and is susceptible to virus-mediated counteraction

Interferon-induced transmembrane (IFITM) proteins restrict membrane fusion and virion internalization of several enveloped viruses. The role of IFITM proteins during alphaviral infection of human cells and viral counteraction strategies are insufficiently understood. Here, we characterized the impact of human IFITMs on the entry and spread of chikungunya virus and Mayaro virus and provide first evidence for a CHIKV-mediated antagonism of IFITMs. IFITM1, 2, and 3 restricted infection at the level of alphavirus glycoprotein-mediated entry, both in the context of direct infection and cell-to-cell transmission. Relocalization of normally endosomal IFITM3 to the plasma membrane resulted in loss of antiviral activity. rs12252-C, a naturally occurring variant of IFITM3 that may associate with severe influenza in humans, restricted CHIKV, MAYV, and influenza A virus infection as efficiently as wild-type IFITM3 Antivirally active IFITM variants displayed reduced cell surface levels in CHIKV-infected cells involving a posttranscriptional process mediated by one or several nonstructural protein(s) of CHIKV. Finally, IFITM3-imposed reduction of specific infectivity of nascent particles provides a rationale for the necessity of a virus-encoded counteraction strategy against this restriction factor.

Metabolic engineering of HEK293 cells to improve transient transfection and cell budding of HIV-1 virus-like particles

HIV-1 Gag virus-like particles (VLPs) are promising candidates for the development of future vaccines. Recent viral outbreaks have manifested the need of robust vaccine production platforms able to adapt to new challenges while achieving mass production capacity. For the rapid production of VLPs, the method of transient gene expression (TGE) have proved highly efficient. Based on a previous characterization of the HEK293 cell line upon transient transfection using multiplexed quantitative proteomics, molecular production bottlenecks and metabolic pathways likely to be optimized were identified. In this study, these molecular components and metabolic pathways have been explored and modulated via transient metabolic engineering using approaches like design of experiments to fully exploit and optimize VLP production, transfection and budding efficiency. Upon overexpression of endosomal sorting complex required for transport accessory proteins like NEDD4L and CIT, VLP production increased 3.3 and 2.9-fold, respectively. Overexpression of glycosphingolipid precursor enzyme UGCG improved transfection efficiency by 17% and knocking-down the Gag-binding protein CNP improved 2.5-fold VLP specific productivity. Combining CNP inhibition and UGCG overexpression further improved budding efficiency by 37.3%. Modulating VLP production and accessory pathways like intracellular budding, demonstrated the potential of metabolic engineering to optimize and intensify the development of robust production platforms for future vaccines.

Vesicular Stomatitis Virus Transcription Is Inhibited by TRIM69 in the Interferon-Induced Antiviral State

Interferons (IFNs) induce the expression of interferon-stimulated genes (ISGs), many of which are responsible for the cellular antiviral state in which the replication of numerous viruses is blocked. How the majority of individual ISGs inhibit the replication of particular viruses is unknown. We conducted a loss-of-function screen to identify genes required for the activity of alpha interferon (IFN-α) against vesicular stomatitis virus, Indiana serotype (VSVIND), a prototype negative-strand RNA virus. Our screen revealed that TRIM69, a member of the tripartite motif (TRIM) family of proteins, is a VSVIND inhibitor. TRIM69 potently inhibited VSVIND replication through a previously undescribed transcriptional inhibition mechanism. Specifically, TRIM69 physically associates with the VSVIND phosphoprotein (P), requiring a specific peptide target sequence encoded therein. P is a cofactor for the viral polymerase and is required for viral RNA synthesis, as well as the assembly of replication compartments. By targeting P, TRIM69 inhibits pioneer transcription of the incoming virion-associated minus-strand RNA, thereby preventing the synthesis of viral mRNAs, and consequently impedes all downstream events in the VSVIND replication cycle. Unlike some TRIM proteins, TRIM69 does not inhibit viral replication by inducing degradation of target viral proteins. Rather, higher-order TRIM69 multimerization is required for its antiviral activity, suggesting that TRIM69 functions by sequestration or anatomical disruption of the viral machinery required for VSVIND RNA synthesis.IMPORTANCE Interferons are important antiviral cytokines that work by inducing hundreds of host genes whose products inhibit the replication of many viruses. While the antiviral activity of interferon has long been known, the identities and mechanisms of action of most interferon-induced antiviral proteins remain to be discovered. We identified gene products that are important for the antiviral activity of interferon against vesicular stomatitis virus (VSV), a model virus that whose genome consists of a single RNA molecule with negative-sense polarity. We found that a particular antiviral protein, TRIM69, functions by a previously undescribed molecular mechanism. Specifically, TRIM69 interacts with and inhibits the function of a particular phosphoprotein (P) component of the viral transcription machinery, preventing the synthesis of viral messenger RNAs.

A virus-packageable CRISPR screen identifies host factors mediating interferon inhibition of HIV

Interferon (IFN) inhibits HIV replication by inducing antiviral effectors. To comprehensively identify IFN-induced HIV restriction factors, we assembled a CRISPR sgRNA library of Interferon Stimulated Genes (ISGs) into a modified lentiviral vector that allows for packaging of sgRNA-encoding genomes in trans into budding HIV-1 particles. We observed that knockout of Zinc Antiviral Protein (ZAP) improved the performance of the screen due to ZAP-mediated inhibition of the vector. A small panel of IFN-induced HIV restriction factors, including MxB, IFITM1, Tetherin/BST2 and TRIM5alpha together explain the inhibitory effects of IFN on the CXCR4-tropic HIV-1 strain, HIV-1LAI, in THP-1 cells. A second screen with a CCR5-tropic primary strain, HIV-1Q23.BG505, described an overlapping, but non-identical, panel of restriction factors. Further, this screen also identifies HIV dependency factors. The ability of IFN-induced restriction factors to inhibit HIV strains to replicate in human cells suggests that these human restriction factors are incompletely antagonized.

Editorial note

This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

BST-2 controls T cell proliferation and exhaustion by shaping the early distribution of a persistent viral infection

The interferon inducible protein, BST-2 (or, tetherin), plays an important role in the innate antiviral defense system by inhibiting the release of many enveloped viruses. Consequently, viruses have evolved strategies to counteract the anti-viral activity of this protein. While the mechanisms by which BST-2 prevents viral dissemination have been defined, less is known about how this protein shapes the early viral distribution and immunological defense against pathogens during the establishment of persistence. Using the lymphocytic choriomeningitis virus (LCMV) model of infection, we sought insights into how the in vitro antiviral activity of this protein compared to the immunological defense mounted in vivo. We observed that BST-2 modestly reduced production of virion particles from cultured cells, which was associated with the ability of BST-2 to interfere with the virus budding process mediated by the LCMV Z protein. Moreover, LCMV does not encode a BST-2 antagonist, and viral propagation was not significantly restricted in cells that constitutively expressed BST-2. In contrast to this very modest effect in cultured cells, BST-2 played a crucial role in controlling LCMV in vivo. In BST-2 deficient mice, a persistent strain of LCMV was no longer confined to the splenic marginal zone at early times post-infection, which resulted in an altered distribution of LCMV-specific T cells, reduced T cell proliferation / function, delayed viral control in the serum, and persistence in the brain. These data demonstrate that BST-2 is important in shaping the anatomical distribution and adaptive immune response against a persistent viral infection in vivo.

A GXXXA Motif in the Transmembrane Domain of the Ebola Virus Glycoprotein Is Required for Tetherin Antagonism

The interferon-induced antiviral host cell protein tetherin can inhibit the release of several enveloped viruses from infected cells. The Ebola virus (EBOV) glycoprotein (GP) antagonizes tetherin, but the domains and amino acids in GP that are required for tetherin antagonism have not been fully defined. A GXXXA motif within the transmembrane domain (TMD) of EBOV-GP was previously shown to be important for GP-mediated cellular detachment. Here, we investigated whether this motif also contributes to tetherin antagonism. Mutation of the GXXXA motif did not impact GP expression or particle incorporation and only modestly reduced EBOV-GP-driven entry. In contrast, the GXXXA motif was required for tetherin antagonism in transfected cells. Moreover, alteration of the GXXXA motif increased tetherin sensitivity of a replication-competent vesicular stomatitis virus (VSV) chimera encoding EBOV-GP. Although these results await confirmation with authentic EBOV, they indicate that a GXXXA motif in the TMD of EBOV-GP is important for tetherin antagonism. Moreover, they provide the first evidence that GP can antagonize tetherin in the context of an infectious EBOV surrogate.IMPORTANCE The glycoprotein (GP) of Ebola virus (EBOV) inhibits the antiviral host cell protein tetherin and may promote viral spread in tetherin-positive cells. However, tetherin antagonism by GP has so far been demonstrated only with virus-like particles, and it is unknown whether GP can block tetherin in infected cells. Moreover, a mutation in GP that selectively abrogates tetherin antagonism is unknown. Here, we show that a GXXXA motif in the transmembrane domain of EBOV-GP, which was previously reported to be required for GP-mediated cell rounding, is also important for tetherin counteraction. Moreover, analysis of this mutation in the context of vesicular stomatitis virus chimeras encoding EBOV-GP revealed that GP-mediated tetherin counteraction is operative in infected cells. To our knowledge, these findings demonstrate for the first time that GP can antagonize tetherin in infected cells and provide a tool to study the impact of GP-dependent tetherin counteraction on EBOV spread.

Inhibiting the Ins and Outs of HIV Replication: Cell-Intrinsic Antiretroviral Restrictions at the Plasma Membrane

Like all viruses, human immunodeficiency viruses (HIVs) and their primate lentivirus relatives must enter cells in order to replicate and, once produced, new virions need to exit to spread to new targets. These processes require the virus to cross the plasma membrane of the cell twice: once via fusion mediated by the envelope glycoprotein to deliver the viral core into the cytosol; and secondly by ESCRT-mediated scission of budding virions during release. This physical barrier thus presents a perfect location for host antiviral restrictions that target enveloped viruses in general. In this review we will examine the current understanding of innate host antiviral defences that inhibit these essential replicative steps of primate lentiviruses associated with the plasma membrane, the mechanism by which these viruses have adapted to evade such defences, and the role that this virus/host battleground plays in the transmission and pathogenesis of HIV/AIDS.

The glycoprotein of vesicular stomatitis virus promotes release of virus-like particles from tetherin-positive cells

Vesicular stomatitis virus (VSV) release from infected cells is inhibited by the interferon (IFN)-inducible antiviral host cell factor tetherin (BST-2, CD317). However, several viruses encode tetherin antagonists and it is at present unknown whether residual VSV spread in tetherin-positive cells is also promoted by a virus-encoded tetherin antagonist. Here, we show that the viral glycoprotein (VSV-G) antagonizes tetherin in transfected cells, although with reduced efficiency as compared to the HIV-1 Vpu protein. Tetherin antagonism did not involve alteration of tetherin expression and was partially dependent on a GXXXG motif in the transmembrane domain of VSV-G. However, mutation of the GXXXG motif did not modulate tetherin sensitivity of infectious VSV. These results identify VSV-G as a tetherin antagonist in transfected cells but fail to provide evidence for a contribution of tetherin antagonism to viral spread.