Protein B5 is required on extracellular enveloped vaccinia virus for repulsion of superinfecting virions

Comparative Analysis of 2022 Outbreak MPXV and Previous Clade II MPXV

The 2022-2024 outbreak of MPOX is an important worldwide public health issue that has triggered significant concerns in the scientific community. MPOX is caused by monkeypox virus (MPXV) belonging to the Poxviridae family. The study of MPXV presents a multifaceted challenge due to the diverse viral formThis study was supported by ISIDORe consortium and Agencia Estatal de Investigación.s produced by this pathogen. Notably the intracellular mature viruses (MVs) primarily contribute to localized lesions and host-to-host transmission, while the extracellular enveloped viruses (EVs) are associated with systemic infection. Clinically, MPOX manifests as a vesiculopustular rash that initially emerges on the face and trunk, subsequently spreading throughout the body, with heightened severity observed in immunocompromised individuals. Results obtained in this manuscript indicate that the 2022 outbreak MPXV has a significantly slower viral cycle compared with previous Clade II strains, with WRAIR 7-61 being more intermediate and USA 2003 producing highest viral titers. Additionally, proteomic and phospho-proteomic analysis displays differences in protein expression between these three strains. These findings highlight key differences between the current Lineage B.1 MPXV and previous strains. Further studies will be undertaken to demonstrate if these differences are important for the apparent increased human-to-human transmission mechanisms observed in the Clade IIb MPXV outbreak.

Enhanced Immune Responses in Mice by Combining the Mpox Virus B6R-Protein and Aluminum Hydroxide-CpG Vaccine Adjuvants

Novel adjuvants and innovative combinations of adjuvants (Adjuvant Systems) have facilitated the development of enhanced and new vaccines against re-emerging and challenging pathogenic microorganisms. Nonetheless, the efficacy of adjuvants is influenced by various factors, and the same adjuvant may generate entirely different immune responses when paired with different antigens. Herein, we combined the MPXV-B6R antigen with BC02, a novel adjuvant with proprietary technology, to assess its capability to induce both cellular and humoral immunity in mouse models. Mice received two intramuscular injections of B6R-BC02, which resulted in the production of MPXV-specific IgG, IgG1, and IgG2a antibodies. Additionally, it elicited strong MPXV-specific Th1-oriented cellular immunity and persistent effector memory B-cell responses. The advantages of BC02 were further validated, including rapid initiation of the immune response, robust recall memory, and sustained immune response induction. Although the potential of immunized mice to produce serum-neutralizing antibodies against the vaccinia virus requires further improvement, the exceptional performance of BC02 as an adjuvant for the MPXV-B6R antigen has been consistently demonstrated.

Two noncompeting human neutralizing antibodies targeting MPXV B6 show protective effects against orthopoxvirus infections

The recent outbreak of mpox epidemic, caused by monkeypox virus (MPXV), poses a new threat to global public health. Here, we initially assessed the preexisting antibody level to the MPXV B6 protein in vaccinia vaccinees born before the end of the immunization program and then identified two monoclonal antibodies (MAbs), hMB621 and hMB668, targeting distinct epitopes on B6, from one vaccinee. Binding assays demonstrate that both MAbs exhibit broad binding abilities to B6 and its orthologs in vaccinia (VACV), variola (VARV) and cowpox viruses (CPXV). Neutralizing assays reveal that the two MAbs showed potent neutralization against VACV. Animal experiments using a BALB/c female mouse model indicate that the two MAbs showed effective protection against VACV via intraperitoneal injection. Additionally, we determined the complex structure of B6 and hMB668, revealing the structural feature of B6 and the epitope of hMB668. Collectively, our study provides two promising antibody candidates for the treatment of orthopoxvirus infections, including mpox.

In silico development of a novel anti-mutation, multi-epitope mRNA vaccine against MPXV variants of emerging lineage and sub-lineages by using immunoinformatics approaches

Over the past year, an unexpected surge in human monkeypox (hMPX) cases has been observed. This outbreak differs from previous ones, displaying distinct epidemiological characteristics and transmission patterns, believed to be influenced by a newly emerging monkeypox virus (MPXV) lineage. Notably, this emerging MPXV lineage has exhibited several non-synonymous mutations, some of which are linked to immunomodulatory activities and antigenic characteristics that aid in host detection. However, specific treatments or vaccines for human monkeypox are currently lacking. Hence, we aim to develop a multi-epitope mRNA vaccine by using immunoinformatics approaches against the MPXV, particularly its emerging variants. Six proteins (A29L, A35R, B6R, M1R, H3L, and E8L) were chosen for epitope and mutation site identification. Seventeen top-performing epitopes and eight epitopes containing mutation sites were selected and combined with adjuvants, the PADRE sequence, and linkers for vaccine development. The molecular and physical properties of the designed vaccine (WLmpx) were favorable. Immunological characteristics of WLmpx were assessed through molecular docking, molecular dynamics (MD) simulations, and immune simulations. Finally, the vaccine sequence was utilized to formulate an mRNA-based vaccine. The informatics-based predicted results indicated that the designed vaccine exhibits significant potential in eliciting high-level humoral and cellular immune responses, but further validation through in vivo and vitro studies is warranted.Communicated by Ramaswamy H. Sarma.

Immunogenic proteins and potential delivery platforms for mpox virus vaccine development: A rapid review

Since May 2022, the mpox virus (MPXV) has spread worldwide and become a potential threat to global public health. Vaccines are important tools for preventing MPXV transmission and infection in the population. However, there are still no available potent and applicable vaccines specifically for MPXV. Herein, we highlight several potential vaccine targets for MPVX and emphasize potent immunogens, such as M1R, E8L, H3L, A29L, A35R, and B6R proteins. These proteins can be integrated into diverse vaccine platforms to elicit powerful B-cell and T-cell responses, thereby providing protective immunity against MPXV infection. Overall, research on the MPXV vaccine targets would provide valuable information for developing timely effective MPXV-specific vaccines.

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.

Proteomic assessment of humoral immune responses in smallpox vaccine recipients

The availability of effective smallpox vaccines was a critical element of the successful eradication of smallpox in 1980. Antibody responses play a primary role in protective immunity and neutralizing antibody is an established correlate of protection against smallpox. In this study we used a poxvirus proteome array to assess the antibody response to individual viral proteins in a cohort of 1,037 smallpox vaccine recipients. Several statistically significant differences were observed in the antibody response to immunodominant proteins between men and women, including B5R-a major target of neutralizing antibody in vaccinia immune globulin, and the membrane proteins D8L and A27L, both of which have been used as vaccine antigens providing protection in animal models. We also noted differences across racial/ethnic groups. In this cohort, which consisted of both ACAM2000 and Dryvax recipients, we noted minute differences in the antibody responses to a restricted number of viral proteins, providing additional support for the use of ACAM2000 as a replacement smallpox vaccine. Furthermore, our data indicate that poxvirus proteome microarrays can be valuable for screening and monitoring smallpox vaccine-induced humoral immune responses in large-scale serologic surveillance studies and prove useful in the guidance of developing novel smallpox candidate vaccines.

Short-term and longer-term protective immune responses generated by subunit vaccination with smallpox A33, B5, L1 or A27 proteins adjuvanted with aluminum hydroxide and CpG in mice challenged with vaccinia virus

Smallpox, a contagious and deadly disease caused by variola virus, was eradicated by a strategy that included vaccination with vaccinia virus, a live-virus vaccine. Because the threat of bioterrorism with smallpox persists and infections with zoonotic poxvirus infections like monkeypox continue, and there may be a time when an alternative vaccine platform is needed, recombinant-subunit vaccine strategies for poxviruses have been pursued. Our prior work focused on understanding the immune responses generated to vaccine-formulations containing the virus protein L1. In this work, we examine vaccine-formulations with additional key protein targets: A33 and B5 (components of the extracellular virus) and another protein on the mature virus (A27) adjuvanted with aluminum hydroxide (AH) with and without CpG- oligonucleotide. Each vaccine was formulated to allow either adsorption or non-adsorption of the protein (and CpG) to AH. Mice given a prime and single boost produced long-lasting antibody responses. A second boost (given ~5-months after the first) further increased antibody titers. Similar to our prior findings with L1 vaccine-formulations, the most protective A33 vaccine-formulations included CpG, resulted in the generation of IgG2a-antibody responses. Unlike the prior findings with L1 (where formulations that adsorbed both the protein and the CpG to AH resulted in 100% survival after challenge and minimal weight loss), the AH-adsorption status of A33 and CpG did not play as important a role, since both AH-adsorbed and non-adsorbed groups lost weight after challenge and had similar survival. Vaccination with B5-formulations gave different results. While CpG-containing formulations were the only ones that generated IgG2a-antibody responses, the vaccine-formulation that adsorbed B5 to AH (without CpG) was as equally effective in protecting mice after challenge. These results indicate that the mechanism of how antibodies against A33 and B5 protect differ. The data also show the complexity of designing optimized vaccine-formulations containing multiple adjuvants and recombinant protein-based antigens.

Particle-specific neutralizing activity of a monoclonal antibody targeting the poxvirus A33 protein reveals differences between cell associated and extracellular enveloped virions

Only a small subset of the hundreds of proteins encoded by the poxvirus genome have been shown to be effective as vaccine and/or therapeutic targets. One of these proteins is A33. Here we assess and dissect the ability of an anti-A33 humanized monoclonal antibody, c6C, to affect vaccinia virus infection in vitro. Enveloped virions (EV) released from infected cells can be sensitive or resistant to neutralization by c6C indicating there are different types of EV particles, extracellular enveloped virions (EEV) and released cellular-associated virions (rCEV), that are biologically distinct. Through a combination of plaque phenotype, confocal imaging, and neutralization assays, we found that c6C differentially affects EV from two different virus strains, IHD-J and WR. Evidence for an anti-A33 resistant EV particle, and strain differences in this phenotype, provides a logical answer as to why certain functional assays in the literature have been unable to detect anti-viral effects of anti-A33 antibodies.

Host Src controls gallid alpha herpesvirus 1 intercellular spread in a cellular fatty acid metabolism-dependent manner

Viral spread is considered a promising target for antiviral therapeutics, but the associated mechanisms remain unclear for gallid alpha herpesvirus 1 (ILTV). We previously identified proto-oncogene tyrosine-protein kinase Src (Src) as a crucial host determinant of ILTV infection. The present study revealed accelerated spread of ILTV upon Src inhibition. This phenomenon was independent of either viral replication or the proliferation of infected cells and could not be compromised by neutralizing antibody. Neither extracellular vesicles nor the direct cytosol-to-cytosol connections between adjacent cells contributed to the enhanced spread of ILTV upon Src inhibition. Further genome-wide transcriptional profile analyses in combination with functional validation identified fatty acid metabolism as an essential molecular event during modulation of the intercellular spread and subsequent cytopathic effect of ILTV by Src. Overall, these data suggest that Src controls the cell-to-cell spread of ILTV in a cellular fatty acid metabolism-dependent manner, which determines the virus's cytopathic effect.

High-Content Analyses of Vaccinia Plaque Formation

Vaccinia virus plaque assays are employed for quantification of virus titer through serial dilution of virus on a monolayer of cells. Once the virus titer is diluted enough to allow for only few cells of the monolayer to be infected, clonal spread of infection can be detected by observing the lesion in the cell monolayer or using virus-specific staining methods. Beyond simple titration, plaque formation bares priceless underlying information about subtle virus-host interactions and their impact on virus spread during multiple rounds of infection. These include virus infectivity, mode of virus spread, virus replication rate, and spatiotemporal spread efficacy. How this underlying information can be harnessed using a high-content imaging setup is discussed here.

Partial Deletion of Glycoprotein B5R Enhances Vaccinia Virus Neutralization Escape while Preserving Oncolytic Function

Vaccinia virus (VV) has been utilized in oncolytic virotherapy, but it risks a host antiviral immune response. VV has an extracellular enveloped virus (EEV) form consisting of a normal virion covered with a host-derived outer membrane that enables its spread via circulation while evading host immune mechanisms. However, the immune resistance of EEV is only partial, owing to expression of the surface protein B5R, which has four short consensus repeat (SCR) domains that are targeted by host immune factors. To engineer a more effective virus for oncolytic virotherapy, we developed an enhanced immune-evading oncolytic VV by removing the SCRs from the attenuated strain LC16mO. Although deletion of only the SCRs preserved viral replication, progeny production, and oncolytic activity, deletion of whole B5R led to attenuation of the virus. Importantly, SCR-deleted EEV had higher neutralization resistance than did B5R-wild-type EEV against VV-immunized animal serum; moreover, it retained oncolytic function, thereby prolonging the survival of tumor-bearing mice treated with anti-VV antibody. These results demonstrate that partial SCR deletion increases neutralization escape without affecting the oncolytic potency of VV, making it useful for the treatment of tumors under the anti-virus antibody existence.

Vaccinia virus-mediated cancer immunotherapy: cancer vaccines and oncolytics

Cancer vaccines and oncolytic immunotherapy are promising treatment strategies with potential to provide greater clinical benefit to patients with advanced-stage cancer. In particular, recombinant vaccinia viruses (VV) hold great promise as interventional agents. In this article, we first summarize the current understanding of virus biology and viral genes involved in host-virus interactions to further improve the utility of these agents in therapeutic applications. We then discuss recent findings from basic and clinical studies using VV as cancer vaccines and oncolytic immunotherapies. Despite encouraging results gleaned from translational studies in animal models, clinical trials implementing VV vectors alone as cancer vaccines have yielded largely disappointing results. However, the combination of VV vaccines with alternate forms of standard therapies has resulted in superior clinical efficacy. For instance, combination regimens using TG4010 (MVA-MUC1-IL2) with first-line chemotherapy in advanced-stage non-small cell lung cancer or combining PANVAC with docetaxel in the setting of metastatic breast cancer have clearly provided enhanced clinical benefits to patients. Another novel cancer vaccine approach is to stimulate anti-tumor immunity via STING activation in Batf3-dependent dendritic cells (DC) through the use of replication-attenuated VV vectors. Oncolytic VVs have now been engineered for improved safety and superior therapeutic efficacy by arming them with immune-stimulatory genes or pro-apoptotic molecules to facilitate tumor immunogenic cell death, leading to enhanced DC-mediated cross-priming of T cells recognizing tumor antigens, including neoantigens. Encouraging translational and early phase clinical results with Pexa-Vec have matured into an ongoing global phase III trial for patients with hepatocellular carcinoma. Combinatorial approaches, most notably those using immune checkpoint blockade, have produced exciting pre-clinical results and warrant the development of innovative clinical studies. Finally, we discuss major hurdles that remain in the field and offer some perspectives regarding the development of next generation VV vectors for use as cancer therapeutics.

Exploitation of Cytoskeletal Networks during Early Viral Infection

Being dependent upon host transport systems to navigate the cytoplasm, viruses have evolved various strategies to manipulate cytoskeletal functions. Generally, viruses use the actin cytoskeleton to control entry and short-range transport at the cell periphery and exploit microtubules (MTs) for longer-range cytosolic transport, in some cases to reach the nucleus. While earlier studies established the fundamental importance of these networks to successful infection, the mechanistic details and true extent to which viruses usurp highly specialized host cytoskeletal regulators and motor adaptors is only beginning to emerge. This review outlines our current understanding of how cytoskeletal regulation contributes specifically to the early stages of viral infection, with a primary focus on retroviruses and herpesviruses as examples of recent advances in this area.

Loss of Actin-Based Motility Impairs Ectromelia Virus Release In Vitro but Is Not Critical to Spread In Vivo

Ectromelia virus (ECTV) is an orthopoxvirus and the causative agent of mousepox. Like other poxviruses such as variola virus (agent of smallpox), monkeypox virus and vaccinia virus (the live vaccine for smallpox), ECTV promotes actin-nucleation at the surface of infected cells during virus release. Homologs of the viral protein A36 mediate this function through phosphorylation of one or two tyrosine residues that ultimately recruit the cellular Arp2/3 actin-nucleating complex. A36 also functions in the intracellular trafficking of virus mediated by kinesin-1. Here, we describe the generation of a recombinant ECTV that is specifically disrupted in actin-based motility allowing us to examine the role of this transport step in vivo for the first time. We show that actin-based motility has a critical role in promoting the release of virus from infected cells in vitro but plays a minor role in virus spread in vivo. It is likely that loss of microtubule-dependent transport is a major factor for the attenuation observed when A36R is deleted.

How Does Vaccinia Virus Interfere With Interferon?

Interferons (IFNs) are secreted glycoproteins that are produced by cells in response to virus infection and other stimuli and induce an antiviral state in cells bearing IFN receptors. In this way, IFNs restrict virus replication and spread before an adaptive immune response is developed. Viruses are very sensitive to the effects of IFNs and consequently have evolved many strategies to interfere with interferon. This is particularly well illustrated by poxviruses, which have large dsDNA genomes and encode hundreds of proteins. Vaccinia virus is the prototypic poxvirus and expresses many proteins that interfere with IFN and are considered in this review. These proteins act either inside or outside the cell and within the cytoplasm or nucleus. They function by restricting the production of IFN by blocking the signaling pathways leading to transcription of IFN genes, stopping IFNs binding to their receptors, blocking IFN-induced signal transduction leading to expression of interferon-stimulated genes (ISGs), or inhibiting the antiviral activity of ISG products.

Combined Proteomics/Genomics Approach Reveals Proteomic Changes of Mature Virions as a Novel Poxvirus Adaptation Mechanism

DNA viruses, like poxviruses, possess a highly stable genome, suggesting that adaptation of virus particles to specific cell types is not restricted to genomic changes. Cowpox viruses are zoonotic poxviruses with an extraordinarily broad host range, demonstrating their adaptive potential in vivo. To elucidate adaptation mechanisms of poxviruses, we isolated cowpox virus particles from a rat and passaged them five times in a human and a rat cell line. Subsequently, we analyzed the proteome and genome of the non-passaged virions and each passage. While the overall viral genome sequence was stable during passaging, proteomics revealed multiple changes in the virion composition. Interestingly, an increased viral fitness in human cells was observed in the presence of increased immunomodulatory protein amounts. As the only minor variant with increasing frequency during passaging was located in a viral RNA polymerase subunit and, moreover, most minor variants were found in transcription-associated genes, protein amounts were presumably regulated at transcription level. This study is the first comparative proteome analysis of virus particles before and after cell culture propagation, revealing proteomic changes as a novel poxvirus adaptation mechanism.

Tagging of the vaccinia virus protein F13 with mCherry causes aberrant virion morphogenesis

Vaccinia virus produces two distinct infectious virions; the single-enveloped intracellular mature virus (IMV), which remains in the cell until cell lysis, and the double-enveloped extracellular enveloped virus (EEV), which mediates virus spread. The latter is derived from a triple-enveloped intracellular enveloped virus (IEV) precursor, which is transported to the cell periphery by the kinesin-1 motor complex. This transport involves the viral protein A36 as well as F12 and E2. A36 is an integral membrane protein associated with the outer virus envelope and is the only known direct link between virion and kinesin-1 complex. Yet in the absence of A36 virion egress still occurs on microtubules, albeit at reduced efficiency. In this paper double-fluorescent labelling of the capsid protein A5 and outer-envelope protein F13 was exploited to visualize IEV transport by live-cell imaging in the absence of either A36 or F12. During the generation of recombinant viruses expressing both A5-GFP and F13-mCherry a plaque size defect was identified that was particularly severe in viruses lacking A36. Electron microscopy showed that this phenotype was caused by abnormal wrapping of IMV to form IEV, and this resulted in reduced virus egress to the cell surface. The aberrant wrapping phenotype suggests that the fluorescent fusion protein interferes with an interaction of F13 with the IMV surface that is required for tight association between IMVs and wrapping membranes. The severity of this defect suggests that these viruses are imperfect tools for characterizing virus egress.

Vaccinia virus egress mediated by virus protein A36 is reliant on the F12 protein

Egress of vaccinia virus from its host cell is mediated by the microtubule-associated motor kinesin-1, and three viral proteins, A36 and the F12/E2 complex, have been implicated in this process. Deletion of F12 expression causes a more severe reduction in egress than deletion of A36 but whether these proteins are involved in the same or different mechanisms of kinesin-1 recruitment is unknown. Here it is shown that a virus lacking both proteins forms a smaller plaque than mutants lacking either gene alone, indicating non-redundant functions. A36 not only links virions directly to kinesin-1 but also nucleates actin polymerization to propel surface virions away from the host cell. To address the relative importance of these functions for virus spread, a panel of recombinant viruses was constructed in which the ability of A36 to bind kinesin-1 or to nucleate actin polymerization was abrogated individually or together, in the presence or absence of F12 expression. Analysis of these viruses revealed that in the presence of the F12 protein, loss of kinesin-1 interaction made a greater contribution to plaque size than did the formation of actin tails. However in the absence of F12, the ability of A36 to promote egress was abrogated. Therefore, the ability of A36 to promote egress by kinesin-1 is reliant on the F12 protein.

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.

Cloak and Dagger: Alternative Immune Evasion and Modulation Strategies of Poxviruses

As all viruses rely on cellular factors throughout their replication cycle, to be successful they must evolve strategies to evade and/or manipulate the defence mechanisms employed by the host cell. In addition to their expression of a wide array of host modulatory factors, several recent studies have suggested that poxviruses may have evolved unique mechanisms to shunt or evade host detection. These potential mechanisms include mimicry of apoptotic bodies by mature virions (MVs), the use of viral sub-structures termed lateral bodies for the packaging and delivery of host modulators, and the formation of a second, “cloaked” form of infectious extracellular virus (EVs). Here we discuss these various strategies and how they may facilitate poxvirus immune evasion. Finally we propose a model for the exploitation of the cellular exosome pathway for the formation of EVs.

Vaccinia virus protein complex F12/E2 interacts with kinesin light chain isoform 2 to engage the kinesin-1 motor complex

During vaccinia virus morphogenesis, intracellular mature virus (IMV) particles are wrapped by a double lipid bilayer to form triple enveloped virions called intracellular enveloped virus (IEV). IEV are then transported to the cell surface where the outer IEV membrane fuses with the cell membrane to expose a double enveloped virion outside the cell. The F12, E2 and A36 proteins are involved in transport of IEVs to the cell surface. Deletion of the F12L or E2L genes causes a severe inhibition of IEV transport and a tiny plaque size. Deletion of the A36R gene leads to a smaller reduction in plaque size and less severe inhibition of IEV egress. The A36 protein is present in the outer membrane of IEVs, and over-expressed fragments of this protein interact with kinesin light chain (KLC). However, no interaction of F12 or E2 with the kinesin complex has been reported hitherto. Here the F12/E2 complex is shown to associate with kinesin-1 through an interaction of E2 with the C-terminal tail of KLC isoform 2, which varies considerably between different KLC isoforms. siRNA-mediated knockdown of KLC isoform 1 increased IEV transport to the cell surface and virus plaque size, suggesting interaction with KLC isoform 1 is somehow inhibitory of IEV transport. In contrast, knockdown of KLC isoform 2 did not affect IEV egress or plaque formation, indicating redundancy in virion egress pathways. Lastly, the enhancement of plaque size resulting from loss of KLC isoform 1 was abrogated by removal of KLC isoforms 1 and 2 simultaneously. These observations suggest redundancy in the mechanisms used for IEV egress, with involvement of KLC isoforms 1 and 2, and provide evidence of interaction of F12/E2 complex with the kinesin-1 complex.

Viruses often hijack the cellular transport systems to facilitate their movement within and between cells. Vaccinia virus (VACV), the smallpox vaccine, is very adept at this and exploits cellular transport machinery at several stages during its life cycle. For instance, during transport of new virus particles to the cell surface VACV interacts with a protein motor complex called kinesin-1 that moves cargo on microtubules. However, details of the cellular and viral components needed and the molecular mechanisms involved remain poorly understood. Hitherto, only the VACV protein A36 has been shown to interact with kinesin-1, however viruses lacking A36 still reach the cell surface, albeit at reduced efficiency, indicating other factors are involved. Here we describe an interaction between kinesin-1 and a complex of VACV proteins F12 and E2, which are both needed for virus transport. The F12/E2 complex associates with a subset of kinesin-1 molecules (kinesin light chain isoform 2) with a region thought to be involved in modulation of cargo binding and kinesin-1 motor activity. Further study of this interaction will enhance understanding of the VACV life cycle and of the roles of different kinesin-1 subtypes in cellular processes and the mechanisms that regulate them.

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.

Arp2/3-mediated actin-based motility: a tail of pathogen abuse

Intracellular pathogens have developed elaborate mechanisms to exploit the different cellular systems of their unwilling hosts to facilitate their entry, replication, and survival. In particular, a diverse range of bacteria and viruses have evolved unique strategies to harness the power of Arp2/3-mediated actin polymerization to enhance their cell-to-cell spread. In this review, we discuss how studying these pathogens has revolutionized our molecular understanding of Arp2/3-dependent actin assembly and revealed key signaling pathways regulating actin assembly in cells. Future analyses of microbe-host interactions are likely to continue uncovering new mechanisms regulating actin assembly and dynamics, as well as unexpected cellular functions for actin. Further, studies with known and newly emerging pathogens will also undoubtedly continue to enhance our understanding of the role of the actin cytoskeleton during pathogenesis and potentially highlight future therapeutic approaches.

A36-dependent actin filament nucleation promotes release of vaccinia virus

Cell-to-cell transmission of vaccinia virus can be mediated by enveloped virions that remain attached to the outer surface of the cell or those released into the medium. During egress, the outer membrane of the double-enveloped virus fuses with the plasma membrane leaving extracellular virus attached to the cell surface via viral envelope proteins. Here we report that F-actin nucleation by the viral protein A36 promotes the disengagement of virus attachment and release of enveloped virus. Cells infected with the A36(YdF) virus, which has mutations at two critical tyrosine residues abrogating localised actin nucleation, displayed a 10-fold reduction in virus release. We examined A36(YdF) infected cells by transmission electron microscopy and observed that during release, virus appeared trapped in small invaginations at the plasma membrane. To further characterise the mechanism by which actin nucleation drives the dissociation of enveloped virus from the cell surface, we examined recombinant viruses by super-resolution microscopy. Fluorescently-tagged A36 was visualised at sub-viral resolution to image cell-virus attachment in mutant and parental backgrounds. We confirmed that A36(YdF) extracellular virus remained closely associated to the plasma membrane in small membrane pits. Virus-induced actin nucleation reduced the extent of association, thereby promoting the untethering of virus from the cell surface. Virus release can be enhanced via a point mutation in the luminal region of B5 (P189S), another virus envelope protein. We found that the B5(P189S) mutation led to reduced contact between extracellular virus and the host membrane during release, even in the absence of virus-induced actin nucleation. Our results posit that during release virus is tightly tethered to the host cell through interactions mediated by viral envelope proteins. Untethering of virus into the surrounding extracellular space requires these interactions be relieved, either through the force of actin nucleation or by mutations in luminal proteins that weaken these interactions.

Unusual features of vaccinia virus extracellular virion form neutralization resistance revealed in human antibody responses to the smallpox vaccine

The extracellular virion form (EV) of vaccinia virus (VACV) is essential for viral pathogenesis and is difficult to neutralize with antibodies. Why this is the case and how the smallpox vaccine overcomes this challenge remain incompletely understood. We previously showed that high concentrations of anti-B5 antibodies are insufficient to directly neutralize EV (M. R. Benhnia, et al., J. Virol. 83:1201-1215, 2009). This allowed for at least two possible interpretations: covering the EV surface is insufficient for neutralization, or there are insufficient copies of B5 to allow anti-B5 IgG to cover the whole surface of EV and another viral receptor protein remains active. We endeavored to test these possibilities, focusing on the antibody responses elicited by immunization against smallpox. We tested whether human monoclonal antibodies (MAbs) against the three major EV antigens, B5, A33, and A56, could individually or together neutralize EV. While anti-B5 or anti-A33 (but not anti-A56) MAbs of appropriate isotypes were capable of neutralizing EV in the presence of complement, a mixture of anti-B5, anti-A33, and anti-A56 MAbs was incapable of directly neutralizing EV, even at high concentrations. This remained true when neutralizing the IHD-J strain, which lacks a functional version of the fourth and final known EV surface protein, A34. These immunological data are consistent with the possibility that viral proteins may not be the active component of the EV surface for target cell binding and infectivity. We conclude that the protection afforded by the smallpox vaccine anti-EV response is predominantly mediated not by direct neutralization but by isotype-dependent effector functions, such as complement recruitment for antibodies targeting B5 and A33.