F11-mediated inhibition of RhoA signalling enhances the spread of vaccinia virus in vitro and in vivo in an intranasal mouse model of infection

Vaccinia virus induces EMT-like transformation and RhoA-mediated mesenchymal migration

The emerging outbreak of monkeypox is closely associated with the viral infection and spreading, threatening global public health. Virus-induced cell migration facilitates viral transmission. However, the mechanism underlying this type of cell migration remains unclear. Here we investigate the motility of cells infected by vaccinia virus (VACV), a close relative of monkeypox, through combining multi-omics analyses and high-resolution live-cell imaging. We find that, upon VACV infection, the epithelial cells undergo epithelial-mesenchymal transition-like transformation, during which they lose intercellular junctions and acquire the migratory capacity to promote viral spreading. After transformation, VACV-hijacked RhoA signaling significantly alters cellular morphology and rearranges the actin cytoskeleton involving the depolymerization of robust actin stress fibers, leading-edge protrusion formation, and the rear-edge recontraction, which coordinates VACV-induced cell migration. Our study reveals how poxviruses alter the epithelial phenotype and regulate RhoA signaling to induce fast migration, providing a unique perspective to understand the pathogenesis of poxviruses.

ISG15 Is Required for the Dissemination of Vaccinia Virus Extracellular Virions

Viruses have developed many different strategies to counteract immune responses, and Vaccinia virus (VACV) is one of a kind in this aspect. To ensure an efficient infection, VACV undergoes a complex morphogenetic process resulting in the production of two types of infective virions: intracellular mature virus (MV) and extracellular enveloped virus (EV), whose spread depends on different dissemination mechanisms. MVs disseminate after cell lysis, whereas EVs are released or propelled in actin tails from living cells. Here, we show that ISG15 participates in the control of VACV dissemination. Infection of Isg15-/- mouse embryonic fibroblasts with VACV International Health Department-J (IHD-J) strain resulted in decreased EV production, concomitant with reduced induction of actin tails and the abolition of comet-shaped plaque formation, compared to Isg15+/+ cells. Transmission electron microscopy revealed the accumulation of intracellular virus particles and a decrease in extracellular virus particles in the absence of interferon-stimulated gene 15 (ISG15), a finding consistent with altered virus egress. Immunoblot and quantitative proteomic analysis of sucrose gradient-purified virions from both genotypes reported differences in protein levels and composition of viral proteins present on virions, suggesting an ISG15-mediated control of viral proteome. Lastly, the generation of a recombinant IHD-J expressing V5-tagged ISG15 (IHD-J-ISG15) allowed us to identify several viral proteins as potential ISG15 targets, highlighting the proteins A34 and A36, which are essential for EV formation. Altogether, our results indicate that ISG15 is an important host factor in the regulation of VACV dissemination. IMPORTANCE Viral infections are a constant battle between the virus and the host. While the host's only goal is victory, the main purpose of the virus is to spread and conquer new territories at the expense of the host's resources. Along millions of years of incessant encounters, poxviruses have developed a unique strategy consisting in the production two specialized "troops": intracellular mature virions (MVs) and extracellular virions (EVs). MVs mediate transmission between hosts, and EVs ensure advance on the battlefield mediating the long-range dissemination. The mechanism by which the virus "decides" to shed from the primary site of infection and its significant impact in viral transmission is not yet fully established. Here, we demonstrate that this process is finely regulated by ISG15/ISGylation, an interferon-induced ubiquitin-like protein with broad antiviral activity. Studying the mechanism that viruses use during infection could result in new ways of understanding our perpetual war against disease and how we might win the next great battle.

A succession of two viral lattices drives vaccinia virus assembly

During its cytoplasmic replication, vaccinia virus assembles non-infectious spherical immature virions (IV) coated by a viral D13 lattice. Subsequently, IV mature into infectious brick-shaped intracellular mature virions (IMV) that lack D13. Here, we performed cryo-electron tomography (cryo-ET) of frozen-hydrated vaccinia-infected cells to structurally characterise the maturation process in situ. During IMV formation, a new viral core forms inside IV with a wall consisting of trimeric pillars arranged in a new pseudohexagonal lattice. This lattice appears as a palisade in cross-section. As maturation occurs, which involves a 50% reduction in particle volume, the viral membrane becomes corrugated as it adapts to the newly formed viral core in a process that does not appear to require membrane removal. Our study suggests that the length of this core is determined by the D13 lattice and that the consecutive D13 and palisade lattices control virion shape and dimensions during vaccinia assembly and maturation.

Analysis of individual HIV-1 budding event using fast AFM reveals a multiplexed role for VPS4

The assembly and budding of newly formed human immunodeficiency virus-1 (HIV-1) particles occur at the plasma membrane of infected cells. Although the molecular basis for viral budding has been studied extensively, investigation of its spatiotemporal characteristics has been limited by the small dimensions (∼100 nm) of HIV particles and the fast kinetics of the process (a few minutes from bud formation to virion release). Here we applied ultra-fast atomic force microscopy to achieve real-time visualization of individual HIV-1 budding events from wild-type (WT) cell lines as well as from mutated lines lacking vacuolar protein sorting-4 (VPS4) or visceral adipose tissue-1 protein (VTA1). Using single-particle analysis, we show that HIV-1 bud formation follows two kinetic pathways (fast and slow) with each composed of three distinct phases (growth, stationary, decay). Notably, approximately 38% of events did not result in viral release and were characterized by the formation of short (rather than tall) particles that slowly decayed back into the cell membrane. These non-productive events became more abundant in VPS4 knockout cell lines. Strikingly, the absence of VPS4B, rather than VPS4A, increased the production of short viral particles, suggesting a role for VPS4B in earlier stages of HIV-1 budding than traditionally thought.

Applications of Atomic Force Microscopy in HIV-1 Research

Obtaining an understanding of the mechanism underlying the interrelations between the structure and function of HIV-1 is of pivotal importance. In previous decades, this mechanism was addressed extensively in a variety of studies using conventional approaches. More recently, atomic force microscopy, which is a relatively new technique with unique capabilities, has been utilized to study HIV-1 biology. Atomic force microscopy can generate high-resolution images at the nanometer-scale and analyze the mechanical properties of individual HIV-1 virions, virus components (e.g., capsids), and infected live cells under near-physiological environments. This review describes the working principles and various imaging and analysis modes of atomic force microscopy, and elaborates on its distinctive contributions to HIV-1 research in areas such as mechanobiology and the physics of infection.

A generic protocol for the affinity-purification of native macromolecular complexes from poxvirus-infected cells

The functional and structural characterization of macromolecular complexes requires protocols for their native isolation. Here, we describe a protocol for this task based on the recombinant poxvirus Vaccinia expressing tagged proteins of interest in infected cells. Tagged proteins and their interactors can then be isolated via affinity chromatography. The procedure is illustrated for the Vaccinia virus encoded multi-subunit RNA polymerase. Our protocol also allows the expression and isolation of heterologous proteins and hence is suitable for a broader application.

For complete details on the use and execution of this profile, please refer to Grimm et al. (2019).

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Generation of endogenously tagged Vaccinia virus (VACV) strains

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Generation of VACV strains expressing heterologous proteins

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Protocol for the affinity purification of native macromolecular complexes

Transient Inhibition of PI3Kδ Enhances the Therapeutic Effect of Intravenous Delivery of Oncolytic Vaccinia Virus

Tumor-targeting oncolytic viruses such as vaccinia virus (VV) are attractive cancer therapeutic agents that act through multiple mechanisms to provoke both tumor lysis and anti-tumor immune responses. However, delivery of these agents remains restricted to intra-tumoral administration, which prevents effective targeting of inaccessible and disseminated tumor cells. In the present study we have identified transient pharmacological inhibition of the leukocyte-enriched phosphoinositide 3-kinase δ (PI3Kδ) as a novel mechanism to potentiate intravenous delivery of oncolytic VV to tumors. Pre-treatment of immunocompetent mice with the PI3Kδ-selective inhibitor IC87114 or the clinically approved idelalisib (CAL-101), prior to intravenous delivery of a tumor-tropic VV, dramatically improved viral delivery to tumors. This occurred via an inhibition of viral attachment to, but not internalization by, systemic macrophages through perturbation of signaling pathways involving RhoA/ROCK, AKT, and Rac. Pre-treatment using PI3Kδ-selective inhibitors prior to intravenous delivery of VV resulted in enhanced anti-tumor efficacy and significantly prolonged survival compared to delivery without PI3Kδ inhibition. These results indicate that effective intravenous delivery of oncolytic VV may be clinically achievable and could be useful in improving anti-tumor efficacy of oncolytic virotherapy.

Cell-to-cell spread of vaccinia virus is promoted by TGF-β-independent Smad4 signalling

The induction of Smad signalling by the extracellular ligand TGF-β promotes tissue plasticity and cell migration in developmental and pathological contexts. Here, we show that vaccinia virus (VACV) stimulates the activity of Smad transcription factors and expression of TGF-β/Smad-responsive genes at the transcript and protein levels. Accordingly, infected cells share characteristics to those undergoing TGF-β/Smad-mediated epithelial-to-mesenchymal transition (EMT). Depletion of the Smad4 protein, a common mediator of TGF-β signalling, results in an attenuation of viral cell-to-cell spread and reduced motility of infected cells. VACV induction of TGF-β/Smad-responsive gene expression does not require the TGF-β ligand or type I and type II TGF-β receptors, suggesting a novel, non-canonical Smad signalling pathway. Additionally, the spread of ectromelia virus, a related orthopoxvirus that does not activate a TGF-β/Smad response, is enhanced by the addition of exogenous TGF-β. Together, our results indicate that VACV orchestrates a TGF-β-like response via a unique activation mechanism to enhance cell migration and promote virus spread.

Extended sequencing of vaccine and wild-type capripoxvirus isolates provides insights into genes modulating virulence and host range

The genus Capripoxvirus in the subfamily Chordopoxvirinae, family Poxviridae, comprises sheeppox virus (SPPV), goatpox virus (GTPV) and lumpy skin disease virus (LSDV), which cause the eponymous diseases across parts of Africa, the Middle East and Asia. These diseases cause significant economic losses and can have a devastating impact on the livelihoods and food security of small farm holders. So far, only live classically attenuated SPPV, GTPV and LSDV vaccines are commercially available and the history, safety and efficacy of many have not been well established. Here, we report 13 new capripoxvirus genome sequences, including the hairpin telomeres, from both pathogenic field isolates and vaccine strains. We have also updated the genome annotations to incorporate recent advances in our understanding of poxvirus biology. These new genomes and genes grouped phenetically with other previously sequenced capripoxvirus strains, and these new alignments collectively identified several recurring alterations in genes thought to modulate virulence and host range. In particular, some of the many large capripoxvirus ankyrin and kelch-like proteins are commonly mutated in vaccine strains, while the variola virus B22R-like gene homolog has also been disrupted in many vaccine isolates. Among these vaccine isolates, frameshift mutations are especially common and clearly present a risk of reversion to wild type in vaccines bearing these mutations. A consistent pattern of gene inactivation from LSDV to GTPV and then SPPV is also observed, much like the pattern of gene loss in orthopoxviruses, but, rather surprisingly, the overall genome size of ~150 kbp remains relatively constant. These data provide new insights into the evolution of capripoxviruses and the determinants of pathogenicity and host range. They will find application in the development of new vaccines with better safety, efficacy and trade profiles.

Kinetics of Mimivirus Infection Stages Quantified Using Image Flow Cytometry

Due to the heterogeneity of viruses and their hosts, a comprehensive view of viral infection is best achieved by analyzing large populations of infected cells. However, information regarding variation in infected cell populations is lost in bulk measurements. Motivated by an interest in the temporal progression of events in virally infected cells, we used image flow cytometry (IFC) to monitor changes in Acanthamoeba polyphaga cells infected with Mimivirus. This first use of IFC to study viral infection required the development of methods to preserve morphological features of adherent amoeba cells prior to detachment and analysis in suspension. It also required the identification of IFC parameters that best report on key events in the Mimivirus infection cycle. The optimized IFC protocol enabled the simultaneous monitoring of diverse processes including generation of viral factories, transport, and fusion of replication centers within the cell, accumulation of viral progeny, and changes in cell morphology for tens of thousands of cells. After obtaining the time windows for these processes, we used IFC to evaluate the effects of perturbations such as oxidative stress and cytoskeletal disruptors on viral infection. Accurate dose‐response curves could be generated, and we found that mild oxidative stress delayed multiple stages of virus production, but eventually infection processes occurred with approximately the same amplitudes. We also found that functional actin cytoskeleton is required for fusion of viral replication centers and later for the production of viral progeny. Through this report, we demonstrate that IFC offers a quantitative, high‐throughput, and highly robust approach to study viral infection cycles and virus–host interactions. © The Authors. Cytometry Part A published by Wiley Periodicals, Inc. on behalf of International Society for Advancement of Cytometry.

lncRNA UCA1-Mediated Cdc42 Signaling Promotes Oncolytic Vaccinia Virus Cell-to-Cell Spread in Ovarian Cancer

Oncolytic vaccinia virus (OVV) has demonstrated appropriate safety profiles for clinical development. Although designed to kill cancer cells efficiently, OVV sensitivity varies in individual cancers, and predictive biomarkers of therapeutic responses have not been identified. Here we found that OVV was much more efficient in KFTX paclitaxel-resistant ovarian cancer cells compared to that in KFlow paclitaxel-sensitive cells. Microarray analysis identified long non-coding RNA urothelial carcinoma-associated 1 (UCA1) upregulation, which contributed to both enhanced paclitaxel resistance and OVV spread. In addition, UCA1 expression correlated with efficient OVV spread in other ovarian cell lines and primary cancer cell cultures. When host pathways underlying OVV spread were analyzed, differences were detected in the activation of the Rho GTPase Cdc42, suggesting that filopodia formation enhances OVV cell-to-cell spread and tumor migration. Moreover, we established a clinically relevant mouse model of peritoneal metastasis using KFTX or KFlow cells. Paclitaxel exerted anti-tumor effects on KFlow, but not KFTX, tumors. In mice bearing KFTX cells after paclitaxel failure, OVV treatment induced the regression of residual tumors and improved survival. Our findings demonstrated that UCA1 promotes OVV cell-to-cell spread in ovarian cancer, resulting in enhanced therapeutic outcome.

Vaccinia virus hijacks EGFR signalling to enhance virus spread through rapid and directed infected cell motility

Cell motility is essential for viral dissemination1. Vaccinia virus (VACV), a close relative of smallpox virus, is thought to exploit cell motility as a means to enhance the spread of infection1. A single viral protein, F11L, contributes to this by blocking RhoA signalling to facilitate cell retraction2. However, F11L alone is not sufficient for VACV-induced cell motility, indicating that additional viral factors must be involved. Here, we show that the VACV epidermal growth factor homologue, VGF, promotes infected cell motility and the spread of viral infection. We found that VGF secreted from early infected cells is cleaved by ADAM10, after which it acts largely in a paracrine manner to direct cell motility at the leading edge of infection. Real-time tracking of cells infected in the presence of EGFR, MAPK, FAK and ADAM10 inhibitors or with VGF-deleted and F11-deleted viruses revealed defects in radial velocity and directional migration efficiency, leading to impaired cell-to-cell spread of infection. Furthermore, intravital imaging showed that virus spread and lesion formation are attenuated in the absence of VGF. Our results demonstrate how poxviruses hijack epidermal growth factor receptor-induced cell motility to promote rapid and efficient spread of infection in vitro and in vivo.

Genomic Characterization of Orf Virus Strain D1701-V (Parapoxvirus) and Development of Novel Sites for Multiple Transgene Expression

The Orf virus (ORFV; Parapoxvirus) strain D1701 with an attenuated phenotype and excellent immunogenic capacity is successfully used for the generation of recombinant vaccines against different viral infections. Adaption for growth in Vero cells was accompanied by additional major genomic changes resulting in ORFV strain variant D1701-V. In this study, restriction enzyme mapping, blot hybridization and DNA sequencing of the deleted region s (A, AT and D) in comparison to the predecessor strain D1701-B revealed the loss of 7 open reading frames (ORF008, ORF101, ORF102, ORF114, ORF115, ORF116, ORF117). The suitability of deletion site D for expression of foreign genes is demonstrated using novel synthetic early promoter eP1 and eP2. Comparison of promoter strength showed that the original vegf-e promoter Pv as well as promoter eP2 display an up to 11-fold stronger expression than promoter eP1, irrespective of the insertion site. Successful integration and expression of the fluorescent marker genes is demonstrated by gene- and insertion-site specific PCR assays, fluorescence microscopy and flow cytometry. For the first time ORFV recombinants are generated simultaneously expressing transgenes in two different insertion loci. That allows production of polyvalent vaccines containing several antigens against one or different pathogens in a single vectored ORFV vaccine.

The M25 gene products are critical for the cytopathic effect of mouse cytomegalovirus

Cell rounding is a hallmark of the cytopathic effect induced by cytomegaloviruses. By screening a panel of deletion mutants of mouse cytomegalovirus (MCMV) a mutant was identified that did not elicit cell rounding and lacked the ability to form typical plaques. Altered cell morphology was assigned to the viral M25 gene. We detected an early 2.8 kb M25 mRNA directing the synthesis of a 105 kDa M25 protein, and confirmed that a late 3.1 kb mRNA encodes a 130 kDa M25 tegument protein. Virions lacking the M25 tegument protein were of smaller size because the tegument layer between capsid and viral envelope was reduced. The ΔM25 mutant did not provoke the rearrangement of the actin cytoskeleton observed after wild-type MCMV infection, and isolated expression of the M25 proteins led to cell size reduction, confirming that they contribute to the morphological changes. Yields of progeny virus and cell-to-cell spread of the ΔM25 mutant in vitro were diminished and replication in vivo was impaired. The identification of an MCMV gene involved in cell rounding provides the basis for investigating the role of this cytopathic effect in CMV pathogenesis.

RhoD Inhibits RhoC-ROCK-Dependent Cell Contraction via PAK6

RhoA-mediated regulation of myosin-II activity in the actin cortex controls the ability of cells to contract and bleb during a variety of cellular processes, including cell migration and division. Cell contraction and blebbing also frequently occur as part of the cytopathic effect seen during many different viral infections. We now demonstrate that the vaccinia virus protein F11, which localizes to the plasma membrane, is required for ROCK-mediated cell contraction from 2 hr post infection. Curiously, F11-induced cell contraction is dependent on RhoC and not RhoA signaling to ROCK. Moreover, RhoC-driven cell contraction depends on the upstream inhibition of RhoD signaling by F11. This inhibition prevents RhoD from regulating its downstream effector Pak6, alleviating the suppression of RhoC by the kinase. Our observations with vaccinia have now demonstrated that RhoD recruits Pak6 to the plasma membrane to antagonize RhoC signaling during cell contraction and blebbing.

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Vaccinia F11 protein is required for virus-induced cell contraction and blebbing

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F11-induced cell contraction depends on RhoC, but not RhoA, signaling to ROCK

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RhoD recruits Pak6 to the plasma membrane to antagonize RhoC signaling

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F11 inhibits RhoD signaling to its downstream effector Pak6

Cytoplasmic ATR Activation Promotes Vaccinia Virus Genome Replication

In contrast to most DNA viruses, poxviruses replicate their genomes in the cytoplasm without host involvement. We find that vaccinia virus induces cytoplasmic activation of ATR early during infection, before genome uncoating, which is unexpected because ATR plays a fundamental nuclear role in maintaining host genome integrity. ATR, RPA, INTS7, and Chk1 are recruited to cytoplasmic DNA viral factories, suggesting canonical ATR pathway activation. Consistent with this, pharmacological and RNAi-mediated inhibition of canonical ATR signaling suppresses genome replication. RPA and the sliding clamp PCNA interact with the viral polymerase E9 and are required for DNA replication. Moreover, the ATR activator TOPBP1 promotes genome replication and associates with the viral replisome component H5. Our study suggests that, in contrast to long-held beliefs, vaccinia recruits conserved components of the eukaryote DNA replication and repair machinery to amplify its genome in the host cytoplasm.

Divergent roles of β- and γ-actin isoforms during spread of vaccinia virus

Actin is a major component of the cytoskeleton and is present as two isoforms in non‐muscle cells: β‐ and γ‐cytoplasmic actin. These isoforms are strikingly conserved, differing by only four N‐terminal amino acids. During spread from infected cells, vaccinia virus (VACV) particles induce localized actin nucleation that propel virus to surrounding cells and facilitate cell‐to‐cell spread of infection. Here we show that virus‐tipped actin comets are composed of β‐ and γ‐actin. We employed isoform‐specific siRNA knockdown to examine the role of the two isoforms in VACV‐induced actin comets. Despite the high level of similarity between the actin isoforms, and their colocalization, VACV‐induced actin nucleation was dependent exclusively on β‐actin. Knockdown of β‐actin led to a reduction in the release of virus from infected cells, a phenotype dependent on virus‐induced Arp2/3 complex activity. We suggest that local concentrations of actin isoforms may regulate the activity of cellular actin nucleator complexes.

Viruses That Exploit Actin-Based Motility for Their Replication and Spread

The actin cytoskeleton is a crucial part of the eukaryotic cell. Viruses depend on host cells for their replication, and, as a result, many have developed ways of manipulating the actin network to promote their spread. This chapter reviews the various ways in which viruses utilize the actin cytoskeleton at discrete steps in their life cycle, from entry into the host cell, replication, and assembly of new progeny to virus release. Various actin inhibitors that function in different ways to affect proper actin dynamics can be used to parse the role of actin at these steps.

Viruses that ride on the coat-tails of actin nucleation

Actin nucleation drives a diversity of critical cellular processes and the motility of a select group of viral pathogens. Vaccinia virus and baculovirus, Autographa californica multiple nucleopolyhedrovirus, recruit and activate the cellular actin nucleator, the Arp2/3 complex, at the surface of virus particles thereby instigating highly localized actin nucleation. The extension of these filaments provides a mechanical force that bestows the ability to navigate the intracellular environment and promote their infectious cycles. This review outlines the viral and cellular proteins that initiate and regulate the signalling networks leading to viral modification of the actin cytoskeleton and summarizes recent insights into the role of actin-based virus transport.

Plaque2.0-A High-Throughput Analysis Framework to Score Virus-Cell Transmission and Clonal Cell Expansion

Classical plaque assay measures the propagation of infectious agents across a monolayer of cells. It is dependent on cell lysis, and limited by user-specific settings and low throughput. Here, we developed Plaque2.0, a broadly applicable, fluorescence microscopy-based high-throughput method to mine patho-biological clonal cell features. Plaque2.0 is an open source framework to extract information from chemically fixed cells by immuno-histochemistry or RNA in situ hybridization, or from live cells expressing GFP transgene. Multi-parametric measurements include infection density, intensity, area, shape or location information at single plaque or population levels. Plaque2.0 distinguishes lytic and non-lytic spread of a variety of DNA and RNA viruses, including vaccinia virus, adenovirus and rhinovirus, and can be used to visualize simultaneous plaque formation from co-infecting viruses. Plaque2.0 also analyzes clonal growth of cancer cells, which is relevant for cell migration and metastatic invasion studies. Plaque2.0 is suitable to quantitatively analyze virus infections, vector properties, or cancer cell phenotypes.

Coinfection with multiple strains of bovine papular stomatitis virus

Bovine papular stomatitis virus (BPSV) infects cattle and, occupationally, humans. Prevalent subclinical infections, frequent reinfections, and virus persistence in healthy animals compound a poorly understood, but likely complex, scenario of BPSV perpetuation and transmission in nature. Here, we report the isolation of multiple BPSV strains coinfecting a single animal. Whole-genome analysis of isolated BPSV strains revealed genomic variability likely affecting virus virulence and infectivity. Further, incongruent phylogenetic relationships between viruses suggested genomic recombination. These results have significant implications for parapoxvirus infection biology and virus evolution in nature.

The role of signalling and the cytoskeleton during Vaccinia Virus egress

Viruses are obligate intracellular parasites that are critically dependent on their hosts to replicate and generate new progeny. To achieve this goal, viruses have evolved numerous elegant strategies to subvert and utilise the different cellular machineries and processes of their unwilling hosts. Moreover, they often accomplish this feat with a surprisingly limited number of proteins. Among the different systems of the cell, the cytoskeleton is often one of the first to be hijacked as it provides a convenient transport system for viruses to reach their site of replication with relative ease. At the latter stages of their replication cycle, the cytoskeleton also provides an efficient means for newly assembled viral progeny to reach the plasma membrane and leave the infected cell. In this review we discuss how Vaccinia virus takes advantage of the microtubule and actin cytoskeletons of its host to promote the spread of infection into neighboring cells. In particular, we highlight how analysis of actin-based motility of Vaccinia has provided unprecedented insights into how a phosphotyrosine-based signalling network is assembled and functions to stimulate Arp2/3 complex-dependent actin polymerization. We also suggest that the formin FHOD1 promotes actin-based motility of the virus by capping the fast growing ends of actin filaments rather than directly promoting filament assembly. We have come a long way since 1976, when electron micrographs of vaccinia-infected cells implicated the actin cytoskeleton in promoting viral spread. Nevertheless, there are still many unanswered questions concerning the role of signalling and the host cytoskeleton in promoting viral spread and pathogenesis.

KSHV-TK is a tyrosine kinase that disrupts focal adhesions and induces Rho-mediated cell contraction

Paradoxically, the thymidine kinase (TK) encoded by Kaposi sarcoma-associated herpesvirus (KSHV) is an extremely inefficient nucleoside kinase, when compared to TKs from related herpesviruses. We now show that KSHV-TK, in contrast to HSV1-TK, associates with the actin cytoskeleton and induces extensive cell contraction followed by membrane blebbing. These dramatic changes in cell morphology depend on the auto-phosphorylation of tyrosines 65, 85 and 120 in the N-terminus of KSHV-TK. Phosphorylation of tyrosines 65/85 and 120 results in an interaction with Crk family proteins and the p85 regulatory subunit of PI3-Kinase, respectively. The interaction of Crk with KSHV-TK leads to tyrosine phoshorylation of this cellular adaptor. Auto-phosphorylation of KSHV-TK also induces a loss of FAK and paxillin from focal adhesions, resulting in activation of RhoA-ROCK signalling to myosin II and cell contraction. In the absence of FAK or paxillin, KSHV-TK has no effect on focal adhesion integrity or cell morphology. Our observations demonstrate that by acting as a tyrosine kinase, KSHV-TK modulates signalling and cell morphology.

Adding new dimensions: towards an integrative understanding of HIV-1 spread

In vitro studies in primary or immortalized cells continue to be used to elucidate the essential principles that govern the interactions between HIV-1 and isolated target cells. However, until recently, substantial technical barriers prevented this information from being efficiently translated to the more complex scenario of HIV-1 spread in the host in vivo, which has limited our understanding of the impact of host physiological parameters on the spread of HIV-1. In this Review, we discuss the recent development of imaging approaches to visualize HIV-1 spread and the adaptation of these approaches to organotypic ex vivo models and animal models. We focus on new concepts, including the mechanisms and in vivo relevance of cell-cell transmission for HIV-1 spread and the function of the HIV-1 pathogenesis factor Nef, which have emerged from the application of these integrative approaches in complex cell systems.

A novel mode of poxvirus superinfection exclusion that prevents fusion of the lipid bilayers of viral and cellular membranes

Superinfection exclusion is a widespread phenomenon that prevents secondary infections by closely related viruses. The vaccinia virus A56 and K2 proteins in the cell membrane can prevent superinfection by interacting with the entry-fusion complex of subsequent viruses. Here, we described another form of exclusion that is established earlier in infection and does not require the A56 or K2 protein. Cells infected with one or more infectious virions excluded hundreds of superinfecting vaccinia virus particles. A related orthopoxvirus, but neither a flavivirus nor a rhabdovirus, was also excluded, indicating selectivity. Although superinfecting vaccinia virus bound to cells, infection was inhibited at the membrane fusion step, thereby preventing core entry into the cytoplasm and early gene expression. In contrast, A56/K2 protein-mediated exclusion occurred subsequent to membrane fusion. Induction of resistance to superinfection depended on viral RNA and protein synthesis by the primary virus but did not require DNA replication. Although superinfection resistance correlated with virus-induced changes in the cytoskeleton, studies with mutant vaccinia viruses indicated that the cytoskeletal changes were not necessary for resistance to superinfection. Interferon-inducible transmembrane proteins, which can inhibit membrane fusion in other viral systems, did not prevent vaccinia virus membrane fusion, suggesting that these interferon-inducible proteins are not involved in superinfection exclusion. While the mechanism remains to be determined, the early establishment of superinfection exclusion may provide a "winner-take-all" reward to the first poxvirus particles that successfully initiate infection and prevent the entry and genome reproduction of defective or less fit particles.

IMPORTANCE

The replication of a virus usually follows a defined sequence of events: attachment, entry into the cytoplasm or nucleus, gene expression, genome replication, assembly of infectious particles, and spread to other cells. Although multiple virus particles may enter a cell at the same time, mechanisms exist to prevent infection by subsequent viruses. The latter phenomenon, known as superinfection exclusion, can occur by a variety of mechanisms that are not well understood. We showed that superinfection by vaccinia virus was prevented at the membrane fusion step, which closely followed virion attachment. Thus, neither gene expression nor genome replication of the superinfecting virus occurred. Expression of early proteins by the primary virus was necessary and sufficient to induce the superinfection-resistant state. Superinfection exclusion may be beneficial to vaccinia virus by selecting particles that can infect cells rapidly, excluding defective particles and synchronizing the replication cycle.

Vaccinia virus F5 is required for normal plaque morphology in multiple cell lines but not replication in culture or virulence in mice

Vaccinia virus (VACV) gene F5L was recently identified as a determinant of plaque morphology that is truncated in Modified Vaccinia virus Ankara (MVA). Here we show that F5L also affects plaque morphology of the virulent VACV strain Western Reserve (WR) in some, but not all cell lines, and not via previously described mechanisms. Further, despite a reduction in plaque size for VACV WR lacking F5L there was no evidence of reduced virus replication or spread in vitro or in vivo. In vivo we examined two mouse models, each with more than one dose and measured signs of disease and virus burden. These data provide an initial characterization of VACV F5L in a virulent strain of VACV. Further they show the necessity of testing plaque phenotypes in more than one cell type and provide an example of a VACV gene required for normal plaque morphology but not replication and spread.

Initial characterization of vaccinia virus B4 suggests a role in virus spread

Currently, little is known about the ankyrin/F-box protein B4. Here, we report that B4R-null viruses exhibited reduced plaque size in tissue culture, and decreased ability to spread, as assessed by multiple-step growth analysis. Electron microscopy indicated that B4R-null viruses still formed mature and extracellular virions; however, there was a slight decrease of virions released into the media following deletion of B4R. Deletion of B4R did not affect the ability of the virus to rearrange actin; however, VACV811, a large vaccinia virus deletion mutant missing 55 open reading frames, had decreased ability to produce actin tails. Using ectromelia virus, a natural mouse pathogen, we demonstrated that virus devoid of EVM154, the B4R homolog, showed decreased spread to organs and was attenuated during infection. This initial characterization suggests that B4 may play a role in virus spread, and that other unidentified mediators of actin tail formation may exist in vaccinia virus.

Rho'ing in and out of cells: viral interactions with Rho GTPase signaling

Rho GTPases are key regulators of actin and microtubule dynamics and organization. Increasing evidence shows that many viruses have evolved diverse interactions with Rho GTPase signaling and manipulate them for their own benefit. In this review, we discuss how Rho GTPase signaling interferes with many steps in the viral replication cycle, especially entry, replication, and spread. Seen the diversity between viruses, it is not surprising that there is considerable variability in viral interactions with Rho GTPase signaling. However, several largely common effects on Rho GTPases and actin architecture and microtubule dynamics have been reported. For some of these processes, the molecular signaling and biological consequences are well documented while for others we just begin to understand them. A better knowledge and identification of common threads in the different viral interactions with Rho GTPase signaling and their ultimate consequences for virus and host may pave the way toward the development of new antiviral drugs that may target different viruses.

Methodology for the efficient generation of fluorescently tagged vaccinia virus proteins

Tagging of viral proteins with fluorescent proteins has proven an indispensable approach to furthering our understanding of virus-host interactions. Vaccinia virus (VACV), the live vaccine used in the eradication of smallpox, is particularly amenable to fluorescent live-cell microscopy owing to its large virion size and the ease with which it can be engineered at the genome level. We report here an optimized protocol for generating recombinant viruses. The minimal requirements for targeted homologous recombination during vaccinia replication were determined, which allows the simplification of construct generation. This enabled the alliance of transient dominant selection (TDS) with a fluorescent reporter and metabolic selection to provide a rapid and modular approach to fluorescently label viral proteins. By streamlining the generation of fluorescent recombinant viruses, we are able to facilitate downstream applications such as advanced imaging analysis of many aspects of the virus-host interplay that occurs during virus replication.

TRAF2 facilitates vaccinia virus replication by promoting rapid virus entry

Tumor necrosis factor receptor (TNFR)-associated factor 2 (TRAF2) is a pivotal intracellular mediator of signaling pathways downstream of TNFR1 and -2 with known pro- and antiviral effects. We investigated its role in the replication of the prototype poxvirus vaccinia virus (VACV). Loss of TRAF2 expression, either through small interfering RNA treatment of HeLa cells or through genetic knockout in murine embryonic fibroblasts (MEFs), led to significant reductions in VACV growth following low-multiplicity infection. In single-cycle infections, there was delayed production of both early and late VACV proteins as well as accelerated virus-induced alterations to cell morphology, indicating that TRAF2 influences early stages of virus replication. Consistent with an early role, uncoating assays showed normal virus attachment but delayed virus entry in the absence of TRAF2. Although alterations to c-Jun N-terminal kinase (JNK) signaling were apparent in VACV-infected TRAF2^−/− MEFs, treatment of wild-type cells with a JNK inhibitor did not affect virus entry. Instead, treatment with an inhibitor of endosomal acidification greatly reduced virus entry into TRAF2^−/− MEFs, suggesting that VACV is reliant on the endosomal route of entry in the absence of TRAF2. Thus, TRAF2 is a proviral factor for VACV that plays a role in promoting efficient viral entry, most likely via the plasma membrane.

IMPORTANCE Tumor necrosis factor receptor-associated factors (TRAFs) are key facilitators of intracellular signaling with roles in innate and adaptive immunity and stress responses. We have discovered that TRAF2 is a proviral factor in vaccinia virus replication in both HeLa cells and mouse embryonic fibroblasts and that its influence is exercised through promotion of efficient virus entry.

Myxoma virus oncolytic efficiency can be enhanced through chemical or genetic disruption of the actin cytoskeleton

Myxoma virus (MYXV) is one of many animal viruses that exhibit oncolytic properties in transformed human cells. Compared to orthopoxviruses like vaccinia (VACV), MYXV spreads inefficiently, which could compromise its use in treating tumors and their associated metastases. The VACV F11 protein promotes virus exit and rapid spread by inhibiting Rho signalling, which results in a disruption of cortical actin. We have previously shown that although MYXV lacks an F11 homolog, the F11L gene can be introduced into MYXV promoting the spread of this Leporipoxvirus in natural host cells. Here we show that the F11-encoding (F11L⁺) MYXV strain replicates to higher levels in a number of human cancer cells. We also show that F11L⁺ MYXV induces better tumor control and prolonged survival of mice bearing MDA-MB-231 cancer cells. Furthermore, we show that this virus also spreads more efficiently from the site of growth in one injected tumor, to a second untreated tumor.

While we focused mostly on the use of a modified MYXV we were able to show that the effects of F11 on MYXV growth in cancer cells could be mimicked through the use of pharmacological inhibition or siRNA-mediated silencing of key regulators of cortical actin (RhoA, RhoC, mDia1, or LIMK2). These data suggest that it may be possible to increase the oncolytic efficacy of wild-type MYXV using chemical inhibitors of RhoA/C or their downstream targets. Furthermore, since all viruses must overcome barriers to exit posed by structures like cortical actin, these findings suggest that the oncolytic activity of other viruses may be enhanced through similar strategies.

Truncation of gene F5L partially masks rescue of vaccinia virus strain MVA growth on mammalian cells by restricting plaque size

Modified vaccinia virus Ankara (MVA) is a candidate vaccine vector that is severely attenuated due to mutations acquired during several hundred rounds of serial passage in vitro. A previous study used marker rescue to produce a set of MVA recombinants with improved replication on mammalian cells. Here, we extended the characterization of these rescued MVA strains and identified vaccinia virus (VACV) gene F5L as a determinant of plaque morphology but not replication in vitro. F5 joins a growing group of VACV proteins that influence plaque formation more strongly than virus replication and which are disrupted in MVA. These defective genes in MVA confound the interpretation of marker rescue experiments designed to map mutations responsible for the attenuation of this important VACV strain.

ISG15 regulates peritoneal macrophages functionality against viral infection

Upon viral infection, the production of type I interferon (IFN) and the subsequent upregulation of IFN stimulated genes (ISGs) generate an antiviral state with an important role in the activation of innate and adaptive host immune responses. The ubiquitin-like protein (UBL) ISG15 is a critical IFN-induced antiviral molecule that protects against several viral infections, but the mechanism by which ISG15 exerts its antiviral function is not completely understood. Here, we report that ISG15 plays an important role in the regulation of macrophage responses. ISG15−/− macrophages display reduced activation, phagocytic capacity and programmed cell death activation in response to vaccinia virus (VACV) infection. Moreover, peritoneal macrophages from mice lacking ISG15 are neither able to phagocyte infected cells nor to block viral infection in co-culture experiments with VACV-infected murine embryonic fibroblast (MEFs). This phenotype is independent of cytokine production and secretion, but clearly correlates with impaired activation of the protein kinase AKT in ISG15 knock-out (KO) macrophages. Altogether, these results indicate an essential role of ISG15 in the cellular immune antiviral response and point out that a better understanding of the antiviral responses triggered by ISG15 may lead to the development of therapies against important human pathogens.

Modification of proteins by ubiquitin (UB) and ubiquitin-like proteins (UBLs) are key regulatory processes of the innate and adaptive immune response. Interferon (IFN) stimulated gene product 15 (ISG15) is an ubiquitin-like protein modifier, which is reversibly conjugated to different viral and cellular proteins mediating considerable antiviral responses. In turn, many viruses, including poxviruses, have evolved strategies to block the antiviral and inflammatory effects of the innate immune responses to keep cells alive until virus replication is completed. Here, we describe a novel function of ISG15 in the control of macrophages activation, phagocytosis and apoptosis in response to viral infection. These processes are essential for the self-defense mechanism to protect animals from infectious disease and could be crucial to understand the ISG15 antiviral activity described in animal models.

Vaccinia virus F11 promotes viral spread by acting as a PDZ-containing scaffolding protein to bind myosin-9A and inhibit RhoA signaling

The vaccinia F11 protein promotes viral spread by modulating the cortical actin cytoskeleton by inhibiting RhoA signaling via an unknown mechanism. PDZ domains are widely conserved protein interaction modules whose occurrence in viral proteins is unprecedented. We found that F11 contains a central PDZ-like domain that is required to downregulate RhoA signaling and enhance viral spread. The PDZ-like domain interacts with the PDZ binding motif of the Rho GTPase-activating protein (GAP) Myosin-9A. In the absence of Myosin-9A, RhoA signaling is not inhibited, resulting in fewer actin tails and reduced virus release concomitant with less viral spread. The loss of Myosin-9A GAP activity or its ability to bind F11 also reduces actin tail formation. Furthermore, the ability of Myosin-9A to promote viral spread depends on F11 binding RhoA. Thus, F11 acts as a functional PDZ-containing scaffolding protein to inhibit RhoA signaling by binding Myosin-9A.

Liposome delivery of microRNA-145 to mesenchymal stem cells leads to immunological off-target effects mediated by RIG-I

Synthetic microRNAs regulate gene expression when transfected into cells, and may be used in strategies for molecular therapy both in vitro and in vivo. Liposomal transfection reagents are frequently used as delivery vehicles in both settings. Here, we report on the immunological off-target effects observed following liposome transfection of synthetic microRNA-145 into human mesenchymal stem cells and human articular chondrocytes (hAC). The immune response was independent on endosome delivery and toll-like receptors (TLRs) but was mediated by retinoic acid inducible-gene 1 (RIG-I). Upregulation of immune genes required liposomal delivery, as no immune response was observed after electroporation of smiR-145 directly in to the cytosol, suggesting a new role of RIG-I. Immune response was seen both with blunt ended and 2-nucleotide 3' overhang versions of synthetic miR-145, and occurred in the absence of a 5'ppp cap. Mutations in a centrally placed poly (UUUU) sequence reduced, but did not abolish the immune response. Interestingly, exposure to liposomes alone led to upregulation of several immune genes, including RIG-I mRNA. However, this process was not mediated by RIG-I. This insight is important for researchers to avoid unexpected results from gene transfer experiments in vitro and unwanted immune responses following the use of lipid-based transfection reagents in vivo.

Cell-to-cell transmission can overcome multiple donor and target cell barriers imposed on cell-free HIV

Virus transmission can occur either by a cell-free mode through the extracellular space or by cell-to-cell transmission involving direct cell-to-cell contact. The factors that determine whether a virus spreads by either pathway are poorly understood. Here, we assessed the relative contribution of cell-free and cell-to-cell transmission to the spreading of the human immunodeficiency virus (HIV). We demonstrate that HIV can spread by a cell-free pathway if all the steps of the viral replication cycle are efficiently supported in highly permissive cells. However, when the cell-free path was systematically hindered at various steps, HIV transmission became contact-dependent. Cell-to-cell transmission overcame barriers introduced in the donor cell at the level of gene expression and surface retention by the restriction factor tetherin. Moreover, neutralizing antibodies that efficiently inhibit cell-free HIV were less effective against cell-to-cell transmitted virus. HIV cell-to-cell transmission also efficiently infected target T cells that were relatively poorly susceptible to cell-free HIV. Importantly, we demonstrate that the donor and target cell types influence critically the extent by which cell-to-cell transmission can overcome each barrier. Mechanistically, cell-to-cell transmission promoted HIV spread to more cells and infected target cells with a higher proviral content than observed for cell-free virus. Our data demonstrate that the frequently observed contact-dependent spread of HIV is the result of specific features in donor and target cell types, thus offering an explanation for conflicting reports on the extent of cell-to-cell transmission of HIV.

Modulation of the myxoma virus plaque phenotype by vaccinia virus protein F11

Vaccinia virus (VACV) produces large plaques consisting of a rapidly expanding ring of infected cells surrounding a lytic core, whereas myxoma virus (MYXV) produces small plaques that resemble a focus of transformed cells. This is odd, because bioinformatics suggests that MYXV carries homologs of nearly all of the genes regulating Orthopoxvirus attachment, entry, and exit. So why does MYXV produce foci? One notable difference is that MYXV-infected cells produce few of the actin microfilaments that promote VACV exit and spread. This suggested that although MYXV carries homologs of the required genes (A33R, A34R, A36R, and B5R), they are dysfunctional. To test this, we produced MYXV recombinants expressing these genes, but we could not enhance actin projectile formation even in cells expressing all four VACV proteins. Another notable difference between these viruses is that MYXV lacks a homolog of the F11L gene. F11 inhibits the RhoA-mDia signaling that maintains the integrity of the cortical actin layer. We constructed an MYXV strain encoding F11L and observed that, unlike wild-type MYXV, the recombinant virus disrupted actin stress fibers and produced plaques up to 4-fold larger than those of controls, and these plaques expanded ∼6-fold faster. These viruses also grew to higher titers in multistep growth conditions, produced higher levels of actin projectiles, and promoted infected cell movement, although neither process was to the extent of that observed in VACV-infected cells. Thus, one reason for why MYXV produces small plaques is that it cannot spread via actin filaments, although the reason for this deficiency remains obscure. A second reason is that leporipoxviruses lack vaccinia's capacity to disrupt cortical actin.

Biological characterization and next-generation genome sequencing of the unclassified Cotia virus SPAn232 (Poxviridae)

Cotia virus (COTV) SPAn232 was isolated in 1961 from sentinel mice at Cotia field station, São Paulo, Brazil. Attempts to classify COTV within a recognized genus of the Poxviridae have generated contradictory findings. Studies by different researchers suggested some similarity to myxoma virus and swinepox virus, whereas another investigation characterized COTV SPAn232 as a vaccinia virus strain. Because of the lack of consensus, we have conducted an independent biological and molecular characterization of COTV. Virus growth curves reached maximum yields at approximately 24 to 48 h and were accompanied by virus DNA replication and a characteristic early/late pattern of viral protein synthesis. Interestingly, COTV did not induce detectable cytopathic effects in BSC-40 cells until 4 days postinfection and generated viral plaques only after 8 days. We determined the complete genomic sequence of COTV by using a combination of the next-generation DNA sequencing technologies 454 and Illumina. A unique contiguous sequence of 185,139 bp containing 185 genes, including the 90 genes conserved in all chordopoxviruses, was obtained. COTV has an interesting panel of open reading frames (ORFs) related to the evasion of host defense, including two novel genes encoding C-C chemokine-like proteins, each present in duplicate copies. Phylogenetic analysis revealed the highest amino acid identity scores with Cervidpoxvirus, Capripoxvirus, Suipoxvirus, Leporipoxvirus, and Yatapoxvirus. However, COTV grouped as an independent branch within this clade, which clearly excluded its classification as an Orthopoxvirus. Therefore, our data suggest that COTV could represent a new poxvirus genus.

An overview of the vaccinia virus infectome: a survey of the proteins of the poxvirus-infected cell

We have quantitatively profiled the proteins of vaccinia virus-infected HEK293T cells early and late during vaccinia virus infection. Proteins corresponding to 4,326 accessions were identified, the products of 3,798 genes. One hundred thirty-six of the proteins were vaccinia virus-encoded (∼64% of the known vaccinia virus proteome). The remaining accessions were from the host cell. A total of 3,403 of the 4,326 accessions could be confidently quantitated at the precursor peptide level. Although vaccinia virus gene products spanned the entire abundance dynamic range of the cellular proteome, nearly all of the proteome dynamics observed as a result of infection were manifest in the virus gene products with very little plasticity in the host cell proteome. The vaccinia virus gene products could be grouped into four kinetic classes (i.e., four combinations of pre- and postreplicative expression). These protein kinetic classes reflected, almost entirely, the corresponding gene classes within the recently characterized vaccinia virus transcriptome map. The few cellular gene products that showed notable changes in abundance upon vaccinia virus infection were concentrated largely in just a few functional groups. After all of the quantitated cellular gene products were assigned to Gene Ontology (GO)-specific groups, quantitation values for a number of these GO-specific groups were significantly skewed toward over- or underabundance with respect to the global distribution of quantitation values. Quantitative analysis of host cell functions reflected several known facets of virus infection, along with some novel observations.

The vaccinia virus-encoded Bcl-2 homologues do not act as direct Bax inhibitors

Many viruses, including members of several poxvirus genera, encode inhibitors that block apoptosis by simultaneously binding the proapoptotic Bcl-2 proteins Bak and Bax. The Orthopoxvirus vaccinia virus encodes the Bcl-2-like F1 protein, which sequesters Bak but not Bax. However, N1, a potent virulence factor, is reported to be antiapoptotic and to interact with Bax. Here we investigated whether vaccinia virus inhibits Bak/Bax-dependent apoptosis via the cooperative action of F1 and N1. We found that Western Reserve (WR) and ΔN1L viruses inhibited drug- and infection-induced apoptosis equally. Meanwhile, infections with ΔF1L or ΔN1L/F1L virus resulted in similar levels of Bax activation and apoptosis. Outside the context of infection, N1 did not block drug- or Bax-induced cell death or interact with Bax. In addition to F1 and N1, vaccinia virus encodes further structural homologs of Bcl-2 proteins that are conserved in orthopoxviruses, including A46, A52, B14, C1, C6, C16/B22, K7, and N2. However, we found that these do not associate with Bax or inhibit drug-induced cell death. Based on our findings that N1 is not an antiapoptotic protein, we propose that the F1 orthologs represent the only orthopoxvirus Bcl-2 homolog to directly inhibit the Bak/Bax checkpoint.

Small rho GTPases and cholesterol biosynthetic pathway intermediates in African swine fever virus infection

The integrity of the cholesterol biosynthesis pathway is required for efficient African swine fever virus (ASFV) infection. Incorporation of prenyl groups into Rho GTPases plays a key role in several stages of ASFV infection, since both geranylgeranyl and farnesyl pyrophosphates are required at different infection steps. We found that Rho GTPase inhibition impaired virus morphogenesis and resulted in an abnormal viral factory size with the accumulation of envelope precursors and immature virions. Furthermore, abundant defective virions reached the plasma membrane, and filopodia formation in exocytosis was abrogated. Rac1 was activated at early ASFV infection stages, coincident with microtubule acetylation, a process that stabilizes microtubules for virus transport. Rac1 inhibition did not affect the viral entry step itself but impaired subsequent virus production. We found that specific Rac1 inhibition impaired viral induced microtubule acetylation and viral intracellular transport. These findings highlight that viral infection is the result of a carefully orchestrated modulation of Rho family GTPase activity within the host cell; this modulation results critical for virus morphogenesis and in turn, triggers cytoskeleton remodeling, such as microtubule stabilization for viral transport during early infection.

Subversion of the actin cytoskeleton during viral infection

Viral infection converts the normal functions of a cell to optimize viral replication and virion production. One striking observation of this conversion is the reconfiguration and reorganization of cellular actin, affecting every stage of the viral life cycle, from entry through assembly to egress. The extent and degree of cytoskeletal reorganization varies among different viral infections, suggesting the evolution of myriad viral strategies. In this Review, we describe how the interaction of viral proteins with the cell modulates the structure and function of the actin cytoskeleton to initiate, sustain and spread infections. The molecular biology of such interactions continues to engage virologists in their quest to understand viral replication and informs cell biologists about the role of the cytoskeleton in the uninfected cell.

N1L is an ectromelia virus virulence factor and essential for in vivo spread upon respiratory infection

The emergence of zoonotic orthopoxvirus infections and the threat of possible intentional release of pathogenic orthopoxviruses have stimulated renewed interest in understanding orthopoxvirus infections and the resulting diseases. Ectromelia virus (ECTV), the causative agent of mousepox, offers an excellent model system to study an orthopoxvirus infection in its natural host. Here, we investigated the role of the vaccinia virus ortholog N1L in ECTV infection. Respiratory infection of mice with an N1L deletion mutant virus (ECTVΔN1L) demonstrated profound attenuation of the mutant virus, confirming N1 as an orthopoxvirus virulence factor. Upon analysis of virus dissemination in vivo, we observed a striking deficiency of ECTVΔN1L spreading from the lungs to the livers or spleens of infected mice. Investigating the immunological mechanism controlling ECTVΔN1L infection, we found the attenuated phenotype to be unaltered in mice deficient in Toll-like receptor (TLR) or RIG-I-like RNA helicase (RLH) signaling as well as in those missing the type I interferon receptor or lacking B cells. However, in RAG-1(-/-) mice lacking mature B and T cells, ECTVΔN1L regained virulence, as shown by increasing morbidity and virus spread to the liver and spleen. Moreover, T cell depletion experiments revealed that ECTVΔN1L attenuation was reversed only by removing both CD4(+) and CD8(+) T cells, so the presence of either cell subset was still sufficient to control the infection. Thus, the orthopoxvirus virulence factor N1 may allow efficient ECTV infection in mice by interfering with host T cell function.

The taking of the cytoskeleton one two three: how viruses utilize the cytoskeleton during egress

The final assembly of nonlytic envelope viruses requires the coordinated transport of either subviral particles or fully formed virions to the plasma membrane for release from the cell. Recent research has delved into the mechanisms viruses employ to hijack the host cell's cytoskeletal system for active transport to the site of final assembly and release. This review will look at recent findings that relate to the transport of virions to the cell periphery and out of the cell.

Identification of host factors involved in coronavirus replication by quantitative proteomics analysis

In this study, we applied a quantitative proteomic approach, based on SILAC, to investigate the interactions of coronaviruses with the secretory pathway of the host cell, with the aim to identify host factors involved in coronavirus replication. Comparison of the protein profiles of Golgi‐enriched fractions of cells that were either mock infected or infected with mouse hepatitis virus revealed the significant depletion or enrichment of 116 proteins. Although ribosomal/nucleic acid binding proteins were enriched in the Golgi‐fractions of mouse hepatitis virus‐infected cells, proteins annotated to localize to several organelles of the secretory pathway were overrepresented among the proteins that were depleted from these fractions upon infection. We hypothesized that proteins, of which the abundance or distribution is affected by infection, are likely to be involved in the virus life cycle. Indeed, depletion of a small subset of the affected proteins by using small interfering RNAs identified several host factors involved in coronavirus infection. Transfection of small interfering RNAs targeting either C11orf59 or Golgi apparatus glycoprotein 1 resulted in increased virus replication, whereas depletion of vesicle‐trafficking protein vesicle‐trafficking protein sec22b enhanced the release of infectious progeny virus. Overexpression of these proteins, on the other hand, had a negative effect on virus replication. Overall, our study shows that the SILAC approach is a suitable tool to study host–pathogen interactions and to identify host proteins involved in virus replication.

Introduction of the six major genomic deletions of modified vaccinia virus Ankara (MVA) into the parental vaccinia virus is not sufficient to reproduce an MVA-like phenotype in cell culture and in mice

Modified vaccinia virus Ankara (MVA) has a highly restricted host range in cell culture and is apathogenic in vivo. MVA was derived from the parental chorioallantois vaccinia virus Ankara (CVA) by more than 570 passages in chicken embryo fibroblast (CEF) cells. During CEF cell passaging, six major deletions comprising 24,668 nucleotides occurred in the CVA genome. We have cloned both the MVA and the parental CVA genome as bacterial artificial chromosomes (BACs) and have sequentially introduced the six major MVA deletions into the cloned CVA genome. Reconstituted mutant CVA viruses containing up to six major MVA deletions showed no detectable replication restriction in 12 of 14 mammalian cell lines tested; the exceptions were rabbit cell lines RK13 and SIRC. In mice, CVA mutants with up to three deletions showed slightly enhanced virulence, suggesting that gene deletion in replicating vaccinia virus (VACV) can result in gain of fitness in vivo. CVA mutants containing five or all six deletions were still pathogenic, with a moderate degree of attenuation. Deletion V was mainly responsible for the attenuated phenotype of these mutants. In conclusion, loss or truncation of all 31 open reading frames in the six major deletions is not sufficient to reproduce the specific MVA phenotype of strong attenuation and highly restricted host range. Mutations in viral genes outside or in association with the six major deletions appear to contribute significantly to this phenotype. Host range restriction and avirulence of MVA are most likely a cooperative effect of gene deletions and mutations involving the major deletions.