Vaccinia virus produces two types of virions known as single-membraned intracellular mature virus (MV) and double-membraned extracellular enveloped virus (EV). EV production peaks earlier when initial MV are further wrapped and secreted to spread infection within the host. However, late during infection MV accumulate intracellularly and become important for host-to-host transmission. The process that regulates this switch remains elusive and is thought to be influenced by host factors. Here we examined the hypothesis that EV and MV production are regulated by the virus through expression of F13 and the MV-specific protein A26. By switching the promoters and altering the expression kinetics of F13 and A26, we demonstrate that A26 expression downregulates EV production and plaque size, thus limiting viral spread. This process correlates with A26 association with the MV surface protein A27 and exclusion of F13, thus reducing EV titres. Thus, MV maturation is controlled by the abundance of the viral A26 protein, independently of other factors, and is rate-limiting for EV production. The A26 gene is conserved within vertebrate poxviruses, but strikingly lost in poxviruses known to be transmitted exclusively by biting arthropods. A26-mediated virus maturation thus has the appearance to be an ancient evolutionary adaptation to enhance transmission of poxviruses that has subsequently been lost from vector-adapted species, for which it may serve as a genetic signature. The existence of virus-regulated mechanisms to produce virions adapted to fulfil different functions represents a novel level of complexity in mammalian viruses with major impact on evolution, adaptation and transmission.
Type 1 interferons (IFN-1) are pleiotropic cytokines with well-established anticancer and antiviral properties, particularly in mucosal tissues. Hence, natural IFN-1-inducing treatments are highly sought after in the clinic. Here, we report for the first time that cryptolepine, a pharmacoactive alkaloid in the medicinal plant Cryptolepis sanguinolenta, is a potent IFN-1 pathway inducer. Cryptolepine increased the transcript levels of JAK1, TYK2, STAT1, STAT2, IRF9, and OAS3, as well as increased the accumulation of STAT1 and OAS3 proteins, similar to recombinant human IFN-α. Cryptolepine effects were observed in multiple cell types including a model of human macrophages. This response was maintained in MAVS and STING-deficient cell lines, suggesting that cryptolepine effects are not mediated by nucleic acids released upon nuclear or organelle damage. In agreement, cryptolepine did not affect cell viability in concentrations that triggered potent IFN-1 activation. In addition, we observed no differences in the presence of a pharmacological inhibitor of TBK1, a pleiotropic kinase that is a converging point for Toll-like receptors (TLRs) and nucleic acid sensors. Together, our results demonstrate that cryptolepine is a strong inducer of IFN-1 response and suggest that cryptolepine-based medications such as C. sanguinolenta extract could be potentially tested in resource-limited regions of the world for the management of chronic viral infections as well as cancers.
Background The NF-κB family of transcription factors and associated signalling pathways are abundant and ubiquitous in human immune responses. Activation of NF-κB transcription factors by viral pathogen-associated molecular patterns, such as viral RNA and DNA, is fundamental to anti-viral innate immune defences and pro-inflammatory cytokine production that steers adaptive immune responses. Diverse non-viral stimuli, such as lipopolysaccharide and cytokines, also activate NF-κB and the same anti-pathogen gene networks. Viruses adapted to human cells often encode multiple proteins targeting the NF-κB pathway to mitigate the anti-viral effects of NF-κB-dependent host immunity. Results In this study we have demonstrated using a variety of assays, in a number of different cell types including primary cells, that plasmid-encoded or virus-delivered simian immunodeficiency virus (SIV) accessory protein Vpx is a broad antagonist of NF-κB signalling active against diverse innate NF-κB agonists. Using targeted Vpx mutagenesis, we showed that this novel Vpx phenotype is independent of known Vpx cofactor DCAF1 and other cellular binding partners, including SAMHD1, STING and the HUSH complex. We found that Vpx co-immunoprecipitated with canonical NF-κB transcription factor p65, but not NF-κB family members p50 or p100, preventing nuclear translocation of p65. We found that broad antagonism of NF-κB activation by Vpx was conserved across distantly related lentiviruses as well as for Vpr from SIV Mona monkey (SIVmon), which has Vpx-like SAMHD1-degradation activity. Conclusions We have discovered a novel mechanism by which lentiviruses antagonise NF-κB activation by targeting p65. These findings extend our knowledge of how lentiviruses manipulate universal regulators of immunity to avoid the anti-viral sequelae of pro-inflammatory gene expression stimulated by both viral and extra-viral agonists. Importantly our findings are also relevant to the gene therapy field where virus-like particle associated Vpx is routinely used to enhance vector transduction through antagonism of SAMHD1, and perhaps also through manipulation of NF-κB.
Modified vaccinia Ankara (MVA) is an attenuated strain of vaccinia virus (VACV), a dsDNA virus that replicates its genome in the cytoplasm and as a result is canonically sensed by the cyclic GMP-AMP synthase (cGAS) and its downstream stimulator of interferon genes (STING). MVA has a highly restricted host range due to major deletions in its genome including inactivation of immunomodulatory genes, only being able to grow in avian cells and the hamster cell line BHK21. Here we studied the interplay between MVA and the cGAS/STING DNA in this permissive cell line and determined whether manipulation of this axis could impact MVA replication and cell responses. We demonstrate that BHK21 cells retain a functional cGAS/STING axis that responds to canonical DNA sensing agonists, upregulating interferon stimulated genes (ISGs). BHK21 cells also respond to MVA, but with a distinct ISG profile. This profile remains unaltered after CRISPR/Cas9 knock-out editing of STING and ablation of cytosolic DNA responses, indicating that MVA responses are independent of the cGAS/STING axis. Furthermore, infection by MVA diminishes the ability of BHK21 cells to respond to exogenous DNA suggesting that MVA still encodes uncharacterised inhibitors of DNA sensing. This suggests that using attenuated strains in permissive cell lines may assist in identification of novel host-virus interactions that may be of relevance to disease or the therapeutic applications of poxviruses.
Cytosolic recognition of DNA has emerged as a critical cellular mechanism of host immune activation upon pathogen invasion. The central cytosolic DNA sensor cGAS activates STING, which is phosphorylated, dimerises and translocates from the ER to a perinuclear region to mediate IRF-3 activation. Poxviruses are dsDNA viruses replicating in the cytosol and hence likely to trigger cytosolic DNA sensing. Here we investigated the activation of innate immune signalling by 4 different strains of the prototypic poxvirus vaccinia virus (VACV) in a cell line proficient in DNA sensing. Infection with the attenuated VACV strain MVA activated IRF-3 via cGAS and STING, and accordingly STING dimerised and was phosphorylated during MVA infection. Conversely, VACV strains Copenhagen and Western Reserve inhibited STING dimerisation and phosphorylation during infection and in response to transfected DNA and cGAMP, thus efficiently suppressing DNA sensing and IRF-3 activation. A VACV deletion mutant lacking protein C16, thought to be the only viral DNA sensing inhibitor acting upstream of STING, retained the ability to block STING activation. Similar inhibition of DNA-induced STING activation was also observed for cowpox and ectromelia viruses. Our data demonstrate that virulent poxviruses possess mechanisms for targeting DNA sensing at the level of the cGAS-STING axis and that these mechanisms do not operate in replication-defective strains such as MVA. These findings shed light on the role of cellular DNA sensing in poxvirus-host interactions and will open new avenues to determine its impact on VACV immunogenicity and virulence.
Ubiquitylation is a covalent post-translational modification that regulates protein stability and is involved in many biological functions. Proteins may be modified with mono-ubiquitin or ubiquitin chains. Viruses have evolved multiple mechanisms to perturb the cell ubiquitin system and manipulate it to their own benefit. Here, we report ubiquitylation of vaccinia virus (VACV) protein N1. N1 is an inhibitor of the nuclear factor NF-κB and apoptosis that contributes to virulence, has a Bcl-2-like fold, and is highly conserved amongst orthopoxviruses. The interaction between N1 and ubiquitin occurs at endogenous protein levels during VACV infection and following ectopic expression of N1. Biochemical analysis demonstrated that N1 is covalently ubiquitylated, and heterodimers of ubiquitylated and non-ubiquitylated N1 monomers were identified, suggesting that ubiquitylation does not inhibit N1 dimerization. Studies with other VACV Bcl-2 proteins, such as C6 or B14, revealed that although these proteins also interact with ubiquitin, these interactions are non-covalent. Finally, mutagenesis of N1 showed that ubiquitylation occurs in a conventional lysine-dependent manner at multiple acceptor sites because only an N1 allele devoid of lysine residues remained unmodified. Taken together, we described a previously uncharacterized modification of the VACV protein N1 that provided a new layer of complexity to the biology of this virulence factor, and provided another example of the intricate interplay between poxviruses and the host ubiquitin system.