The vaccinia virus B5 protein requires A34 for efficient intracellular trafficking from the endoplasmic reticulum to the site of wrapping and incorporation into progeny virions

The amount of Nck rather than N-WASP correlates with the rate of actin-based motility of Vaccinia virus

IMPORTANCE

Vaccinia virus is a large double-stranded DNA virus and a close relative of Mpox and Variola virus, the causative agent of smallpox. During infection, Vaccinia hijacks its host's transport systems and promotes its spread into neighboring cells by recruiting a signaling network that stimulates actin polymerization. Over the years, Vaccinia has provided a powerful model to understand how signaling networks regulate actin polymerization. Nevertheless, we still lack important quantitative information about the system, including the precise number of viral and host molecules required to induce actin polymerization. Using quantitative fluorescence microscopy techniques, we have determined the number of viral and host signaling proteins accumulating on virions during their egress. Our analysis has uncovered two unexpected new aspects of this process: the number of viral proteins in the virion is not fixed and the velocity of virus movement depends on the level of a single adaptor within the signaling network.

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.

An increase in glycoprotein concentration on extracellular virions dramatically alters vaccinia virus infectivity and pathogenesis without impacting immunogenicity

The extracellular virion (EV) form of Orthopoxviruses is required for cell-to-cell spread and pathogenesis, and is the target of neutralizing antibodies in the protective immune response. EV have a double envelope that contains several unique proteins that are involved in its intracellular envelopment and/or subsequent infectivity. One of these, F13, is involved in both EV formation and infectivity. Here, we report that replacement of vaccinia virus F13L with the molluscum contagiosum virus homolog, MC021L, results in the production of EV particles with significantly increased levels of EV glycoproteins, which correlate with a small plaque phenotype. Using a novel fluorescence-activated virion sorting assay to isolate EV populations based on glycoprotein content we determine that EV containing either higher or lower levels of glycoproteins are less infectious, suggesting that there is an optimal concentration of glycoproteins in the outer envelope that is required for maximal infectivity of EV. This optimal glycoprotein concentration was required for lethality and induction of pathology in a cutaneous model of animal infection, but was not required for induction of a protective immune response. Therefore, our results demonstrate that there is a sensitive balance between glycoprotein incorporation, infectivity, and pathogenesis, and that manipulation of EV glycoprotein levels can produce vaccine vectors in which pathologic side effects are attenuated without a marked diminution in induction of protective immunity.

Viral glycoproteins are critical determinants of host cell tropism, immunity, and pathogenesis. Vaccinia virus was used for the most successful immunization program in history, and poxviruses continue to be used as vaccine vectors. Here, we report that vaccinia virus extracellular virion (EV) protein F13 plays an important, previously unappreciated, role in controlling glycoprotein incorporation, and that there is a direct relationship between glycoprotein concentrations and subsequent infectivity. Crucially, manipulation of the EV glycoprotein concentrations altered pathogenesis and lethality in an in vivo infection model, but did not markedly alter the induced immune response. These results have important implications that inform the design of safer and more efficacious poxvirus-based vaccine vectors by altering glycoprotein content.

[Mutations in the A34R gene increase the immunogenicity of vaccinia virus]

Самым простым и надежным способом защиты от вирусных инфекций является вакцинопрофилактика. При этом наибольшей протективной эффективностью обладают живые вакцины, в основе которых используют слабовирулентные для человека вирусы, близкородственные патогенным, или аттенуированные (ослабленные за счет мутаций/делеций в вирусном геноме) варианты патогенного для человека вируса. Вакцинация против оспы с использованием живого вируса осповакцины (vaccinia virus, VACV), близкородственного вирусу натуральной оспы, сыграла важнейшую роль в успехе программы глобальной ликвидации оспы, которая осуществлялась под эгидой Всемирной организации здравоохранения. Прекращение после 1980 г. противооспенной вакцинации привело к тому, что огромная часть населения Земли в настоящее время не имеет иммунитета не только к оспе, но и любым другим зоонозным ортопоксвирусным инфекциям. Это создает возможность циркуляции зоонозных ортопоксвирусов в человеческой популяции и, как следствие, приводит к изменению экологии и круга чувствительных хозяев для разных видов ортопоксвирусов. При этом использование классической живой вакцины на основе VACV для защиты от этих инфекций в настоящее время не приемлемо, так как она может обусловливать тяжелые побочные реакции. В связи с этим все более актуальной становится разработка новых безопасных вакцин против ортопоксвирусных инфекций человека и животных. Аттенуация (ослабление вирулентности) VACV достигается в результате направленной инактивации определенных генов вируса и обычно приводит к уменьшению эффективности размножения VACV in vivo. Следствием этого может быть снижение иммунного ответа при введении аттенуированного вируса пациентам в стандартных дозах. Часто используемым для встройки/инактивации в геноме VACV является ген тимидинкиназы, нарушение которого приводит к аттенуации вируса. В данной работе изучено, как введение двух точечных мутаций в ген A34R аттенуированного штамма LIVP-GFP (ТК-), увеличивающих выход внеклеточных оболочечных вирионов (EEV), влияет на свойства пато- и иммуногенности варианта VACV LIVP-GFP-A34R при интраназальном заражении лабораторных мышей. Показано, что увеличение продукции EEV рекомбинантным штаммом VACV LIVP-GFP-A34R не меняет аттенуированный фенотип, характерный для родительского штамма LIVP-GFP, но приводит к существенно большей продукции VACV-специфичных антител. Ключевые слова: вирус осповакцины; направленные мутации; аттенуация; иммуногенность.

Enhancing the Protective Immune Response to Administration of a LIVP-GFP Live Attenuated Vaccinia Virus to Mice

Following the WHO announcement of smallpox eradication, discontinuation of smallpox vaccination with vaccinia virus (VACV) was recommended. However, interest in VACV was soon renewed due to the opportunity of genetic engineering of the viral genome by directed insertion of foreign genes or introduction of mutations or deletions into selected viral genes. This genomic technology enabled production of stable attenuated VACV strains producing antigens of various infectious agents. Due to an increasing threat of human orthopoxvirus re-emergence, the development of safe highly immunogenic live orthopoxvirus vaccines using genetic engineering methods has been the challenge in recent years. In this study, we investigated an attenuated VACV LIVP-GFP (TK^-) strain having an insertion of the green fluorescent protein gene into the viral thymidine kinase gene, which was generated on the basis of the LIVP (Lister-Institute for Viral Preparations) strain used in Russia as the first generation smallpox vaccine. We studied the effect of A34R gene modification and A35R gene deletion on the immunogenic and protective properties of the LIVP-GFP strain. The obtained data demonstrate that intradermal inoculation of the studied viruses induces higher production of VACV-specific antibodies compared to their levels after intranasal administration. Introduction of two point mutations into the A34R gene, which increase the yield of extracellular enveloped virions, and deletion of the A35R gene, the protein product of which inhibits presentation of antigens by MHC II, enhances protective potency of the created LIVP-TK^--A34R*-dA35R virus against secondary lethal orthopoxvirus infection of BALB/c mice even at an intradermal dose as low as 10³ plaque forming units (PFU)/mouse. This virus may be considered not only as a candidate attenuated live vaccine against smallpox and other human orthopoxvirus infections but also as a vector platform for development of safe multivalent live vaccines against other infectious diseases using genetic engineering methods.

The Influence of an Elevated Production of Extracellular Enveloped Virions of the Vaccinia Virus on Its Properties in Infected Mice

The modern approach to developing attenuated smallpox vaccines usually consists in targeted inactivation of vaccinia virus (VACV) virulence genes. In this work, we studied how an elevated production of extracellular enveloped virions (EEVs) and the route of mouse infection can influence the virulence and immunogenicity of VACV. The research subject was the LIVP strain, which is used in Russia for smallpox vaccination. Two point mutations causing an elevated production of EEVs compared with the parental LIVP strain were inserted into the sequence of the VACV A34R gene. The created mutant LIVP-A34R strain showed lower neurovirulence in an intracerebral injection test and elevated antibody production in the intradermal injection method. This VACV variant can be a promising platform for developing an attenuated, highly immunogenic vaccine against smallpox and other orthopoxvirus infections. It can also be used as a vector for designing live-attenuated recombinant polyvalent vaccines against various infectious diseases.

The Molluscum Contagiosum Gene MC021L Partially Compensates for the Loss of Its Vaccinia Virus Homolog, F13L

Orthopoxviruses produce two antigenically distinct infectious enveloped virions termed intracellular mature virions and extracellular virions (EV). EV have an additional membrane compared to intracellular mature virions due to a wrapping process at the trans-Golgi network and are required for cell-to-cell spread and pathogenesis. Specific to the EV membrane are a number of proteins highly conserved among orthopoxviruses, including F13, which is required for the efficient wrapping of intracellular mature virions to produce EV and which plays a role in EV entry. The distantly related molluscipoxvirus, molluscum contagiosum virus, is predicted to encode several vaccinia virus homologs of EV-specific proteins, including the homolog of F13L, MC021L. To study the function of MC021, we replaced the F13L open reading frame in vaccinia virus with an epitope-tagged version of MC021L. The resulting virus (vMC021L-HA) had a small-plaque phenotype compared to vF13L-HA but larger than vΔF13L. The localization of MC021-HA was markedly different from that of F13-HA in infected cells, but MC021-HA was still incorporated in the EV membrane. Similar to F13-HA, MC021-HA was capable of interacting with both A33 and B5. Although MC021-HA expression did not fully restore plaque size, vMC021L-HA produced amounts of EV similar to those produced by vF13L-HA, suggesting that MC021 retained some of the functionality of F13. Further analysis revealed that EV produced from vMC021L-HA exhibit a marked reduction in target cell binding and an increase in dissolution, both of which correlated with a small-plaque phenotype.IMPORTANCE The vaccinia virus extracellular virion protein F13 is required for the production and release of infectious extracellular virus, which in turn is essential for the subsequent spread and pathogenesis of orthopoxviruses. Molluscum contagiosum virus infects millions of people worldwide each year, but it is unknown whether EV are produced during infection for spread. Molluscum contagiosum virus contains a homolog of F13L termed MC021L. To study the potential function of this homolog during infection, we utilized vaccinia virus as a surrogate and showed that a vaccinia virus expressing MC021L-HA in place of F13L-HA exhibits a small-plaque phenotype but produces similar levels of EV. These results suggest that MC021-HA can compensate for the loss of F13-HA by facilitating wrapping to produce EV and further delineates the dual role of F13 during infection.

Vaccinia Virus Glycoproteins A33, A34, and B5 Form a Complex for Efficient Endoplasmic Reticulum to trans-Golgi Network Transport

Orthopoxviruses produce two, antigenically distinct, infectious enveloped virions termed intracellular mature virions and extracellular virions. Extracellular virions are required for cell-to-cell spread and pathogenesis. Specific to the extracellular virion membrane, glycoproteins A33, A34, and B5 are highly conserved among orthopoxviruses and have roles during extracellular virion formation and subsequent infection. B5 is dependent on an interaction with either A33 or A34 for localization to the site of intracellular envelopment and incorporation into the envelope of released extracellular virions. In this report we show that an interaction between A33 and A34 can be detected in infected cells. Furthermore, we show that a three-protein complex between A33, A34, and B5 forms in the endoplasmic reticulum (ER) that disassociates post ER export. Finally, immunofluorescence reveals that coexpression of all three glycoproteins results in their localization to a juxtanuclear region that is presumably the site of intracellular envelopment. These results demonstrate the existence of two previously unidentified interactions: one between A33 and A34 and another simultaneous interaction between all three of the glycoproteins. Furthermore, these results indicate that interactions among A33, A34, and B5 are vital for proper intracellular trafficking and subcellular localization.IMPORTANCE The secondary intracellular envelopment of poxviruses at the trans-Golgi network to release infectious extracellular virus (EV) is essential for their spread and pathogenesis. Viral glycoproteins A33, A34, and B5 are critical for the efficient production of infectious EV and interactions among these proteins are important for their localization and incorporation into the outer extracellular virion membrane. We have uncovered a novel interaction between glycoproteins A33 and A34. Furthermore, we show that B5 can interact with the A33-A34 complex. Our analysis indicates that the three-protein complex has a role in ER exit and proper localization of the three glycoproteins to the intracellular site of wrapping. These results show that a complex set of interactions occur in the secretory pathway of infected cells to ensure proper glycoprotein trafficking and envelope content, which is important for the release of infectious poxvirus virions.

Sequestration of Late Antigens Within Viral Factories Impairs MVA Vector-Induced Protective Memory CTL Responses

Cytotoxic CD8+ T cell (CTL) responses play an essential role in antiviral immunity. Here, we focused on the activation of CTL which recognize epitopes derived from viral or recombinant antigens with either early or late expression kinetics after infection with Modified Vaccinia Virus Ankara (MVA). Late antigens but not early antigens failed to efficiently stimulate murine CTL lines in vitro and were unable to activate and expand protective memory T cell responses in mice in vivo. The reduced or absent presentation of late antigens was not due to impaired antigen presentation or delayed protein synthesis, but was caused by sequestration of late antigens within viral factories (VFs). Additionally, the trapping of late antigens in VFs conflicts with antigen processing and presentation as proteasomal activity was strongly reduced or absent in VFs, suggesting inefficient antigen degradation. This study gives for the first time a mechanistic explanation for the weak immunogenicity of late viral antigens for memory CTL activation. Since MVA is preferentially used as a boost vector in heterologous prime/boost vaccinations, this is an important information for future vaccine design.

The Ectodomain of the Vaccinia Virus Glycoprotein A34 Is Required for Cell Binding by Extracellular Virions and Contains a Large Region Capable of Interaction with Glycoprotein B5

An interaction between the orthopoxvirus glycoproteins A34 and B5 has been reported. The transmembrane and ectodomain of A34 are sufficient for interaction with B5, localization of B5 to the site of intracellular wrapping, and subsequent incorporation into the envelope of released extracellular virions. Several mutagenic approaches were undertaken to better define the B5 interaction domain on A34. A set of C-terminal truncations in A34 identified residues 1 to 80 as sufficient for interaction with B5. Additional truncations identified residues 80 to 130 of A34 as sufficient for interaction with B5. To better understand the function of this region, a set of recombinant viruses expressing A34 with the full, partial, or no B5 interaction site (residues 1 to 130, 1 to 100, and 1 to 70, respectively) was constructed. All the recombinants expressing truncations of A34 incorporated B5 into extracellular virions but had a small-plaque phenotype similar to that of a virus with the A34R gene deleted (vΔA34R). Further characterization indicated that the small-plaque phenotype exhibited by these viruses is due to a combination of abrogated actin tail formation, reduced cell binding, and a defect in polyanion-induced nonfusogenic dissolution. Taken together, these results suggest that residues 80 to 130 of A34 are not necessary for the proper localization and incorporation of B5 into extracellular virions and, furthermore, that the C-terminal residues of A34 are involved in cell binding and dissolution.IMPORTANCE Previous studies have shown that the vaccinia virus glycoproteins A34 and B5 interact, and in the absence of A34, B5 is mislocalized and not incorporated into extracellular virions. Here, using a transient-transfection assay, residues 80 to 130 of the ectodomain of A34 were determined to be sufficient for interaction with B5. Recombinant viruses expressing A34 with a full, partial, or no B5 interaction site were constructed and characterized. All of the A34 truncations interacted with B5 as predicted by the transient-transfection studies but had a small-plaque phenotype. Further analysis revealed that all of the recombinants incorporated detectable levels of B5 into released virions but were defective in cell binding and extracellular virion (EV) dissolution. This study is the first to directly demonstrate that A34 is involved in cell binding and implicate the ectodomain in this role.

Vaccinia Virus Phospholipase Protein F13 Promotes Rapid Entry of Extracellular Virions into Cells

The vaccinia virus protein F13, encoded by the F13L gene, is conserved across the subfamily Chordopoxvirinae and is critical among orthopoxviruses to produce the wrapped form of virus that is required for cell-to-cell spread. F13 is the major envelope protein on the membrane of extracellular forms of virus; however, it is not known if F13 is required in steps postwrapping. In this report, we utilize two temperature-sensitive vaccinia virus mutants from the Condit collection of temperature-sensitive viruses whose small plaque phenotypes have been mapped to the F13L gene. Despite the drastic reduction in plaque size, the temperature-sensitive viruses were found to produce levels of extracellular virions similar to those of the parental strain, Western Reserve (WR), at the permissive and nonpermissive temperatures, suggesting that they are not defective in extracellular virion formation. Analyses of extracellular virions produced by one temperature-sensitive mutant found that those produced at the nonpermissive temperature had undetectable levels of F13 and bound cells with efficiency similar to that of WR but displayed delayed cell entry kinetics. Additionally, low-pH treatment of cells bound by extracellular virions produced at the nonpermissive temperature by the temperature-sensitive reporter virus was unable to overcome a block in infection by bafilomycin A1, suggesting that these virions display increased resistance to dissolution of the extracellular virion envelope. Taken together, our results suggest that F13 plays a role both in the formation of extracellular virions and in the promotion of their rapid entry into cells by enhancing the sensitivity of the membrane to acid-induced dissolution.IMPORTANCE Vaccinia virus (VACV) is an orthopoxvirus and produces two infectious forms, mature virions (MV) and extracellular virions (EV). EV are derived from MV and contain an additional membrane that must first be removed prior to cell entry. F13 is critical for the formation of EV, but a postenvelopment role has not been described. Here, two temperature-sensitive VACV mutants whose deficiencies were previously mapped to the F13L locus are characterized. Both viruses produced EV at the nonpermissive temperature at levels similar to those of a virus that has F13L, yet they had a small plaque phenotype and rate of spread similar to that of an F13L deletion virus. F13 was undetectable on the EV membrane at the nonpermissive temperature, and these EV exhibited delayed cell entry kinetics compared to EV containing F13. This study is the first to conclusively demonstrate a novel role for F13 in cell entry of the EV form of the virus.

Modified vaccinia virus Ankara-infected dendritic cells present CD4+ T-cell epitopes by endogenous major histocompatibility complex class II presentation pathways

CD4(+) T lymphocytes play a central role in the immune system and mediate their function after recognition of their respective antigens presented on major histocompatibility complex II (MHCII) molecules on antigen-presenting cells (APCs). Conventionally, phagocytosed antigens are loaded on MHCII for stimulation of CD4(+) T cells. Certain epitopes, however, can be processed directly from intracellular antigens and are presented on MHCII (endogenous MHCII presentation). Here we characterized the MHCII antigen presentation pathways that are possibly involved in the immune response upon vaccination with modified vaccinia virus Ankara (MVA), a promising live viral vaccine vector. We established CD4(+) T-cell lines specific for MVA-derived epitopes as tools for in vitro analysis of MHCII antigen processing and presentation in MVA-infected APCs. We provide evidence that infected APCs are able to directly transfer endogenous viral proteins into the MHCII pathway to efficiently activate CD4(+) T cells. By using knockout mice and chemical inhibitory compounds, we further elucidated the molecular basis, showing that among the various subcellular pathways investigated, proteasomes and autophagy are key players in the endogenous MHCII presentation during MVA infection. Interestingly, although proteasomal processing plays an important role, neither TAP nor LAMP-2 was found to be involved in the peptide transport. Defining the molecular mechanism of MHCII presentation during MVA infection provides a basis for improving MVA-based vaccination strategies by aiming for enhanced CD4(+) T-cell activation by directing antigens into the responsible pathways.

IMPORTANCE

This work contributes significantly to our understanding of the immunogenic properties of pathogens by deciphering antigen processing pathways contributing to efficient activation of antigen-specific CD4(+) T cells. We identified autophagosome formation, proteasomal activity, and lysosomal integrity as being crucial for endogenous CD4(+) T-cell activation. Since poxvirus vectors such as MVA are already used in clinical trials as recombinant vaccines, the data provide important information for the future design of optimized poxviral vaccines for the study of advanced immunotherapy options.

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.

Attenuated and replication-competent vaccinia virus strains M65 and M101 with distinct biology and immunogenicity as potential vaccine candidates against pathogens

Replication-competent poxvirus vectors with an attenuation phenotype and with a high immunogenic capacity of the foreign expressed antigen are being pursued as novel vaccine vectors against different pathogens. In this investigation, we have examined the replication and immunogenic characteristics of two vaccinia virus (VACV) mutants, M65 and M101. These mutants were generated after 65 and 101 serial passages of persistently infected Friend erythroleukemia (FEL) cells. In cultured cells of different origins, the mutants are replication competent and have growth kinetics similar to or slightly reduced in comparison with those of the parental Western Reserve (WR) virus strain. In normal and immune-suppressed infected mice, the mutants showed different levels of attenuation and pathogenicity in comparison with WR and modified vaccinia Ankara (MVA) strains. Wide genome analysis after deep sequencing revealed selected genomic deletions and mutations in a number of viral open reading frames (ORFs). Mice immunized in a DNA prime/mutant boost regimen with viral vectors expressing the LACK (Leishmania homologue for receptors of activated C kinase) antigen of Leishmania infantum showed protection or a delay in the onset of cutaneous leishmaniasis. Protection was similar to that triggered by MVA-LACK. In immunized mice, both polyfunctional CD4(+) and CD8(+) T cells with an effector memory phenotype were activated by the two mutants, but the DNA-LACK/M65-LACK protocol preferentially induced CD4(+) whereas DNA-LACK/M101-LACK preferentially induced CD8(+) T cell responses. Altogether, our findings showed the adaptive changes of the WR genome during long-term virus-host cell interaction and how the replication competency of M65 and M101 mutants confers distinct biological properties and immunogenicity in mice compared to those of the MVA strain. These mutants could have applicability for understanding VACV biology and as potential vaccine vectors against pathogens and tumors.

Transport and stability of the vaccinia virus A34 protein is affected by the A33 protein

Vaccinia virus (VACV) has two infectious forms called intracellular mature virus and extracellular enveloped virus (EEV). Two of the seven viral proteins in the EEV outer envelope, A33 and A34, are type II membrane glycoproteins that each interact with another EEV protein called B5; however, evidence for direct A33–A34 interaction is lacking. The localization and stability of A34 is affected by B5 and here data are presented showing that A34 is also affected by A33. In the absence of A33, just as without B5, the level, localization and glycosylation profile of A34 was altered. However, the glycosylation profile of A34 without A33 is different to that observed in the absence of B5, and A34 accumulates in the Golgi apparatus rather than in the endoplasmic reticulum. Thus, A34 requires more than one other EEV protein for its processing and cellular transport.

Increased interaction between vaccinia virus proteins A33 and B5 is detrimental to infectious extracellular enveloped virion production

Two mechanisms exist for the incorporation of B5 into extracellular virions, one of which is dependent on A33. In the companion to this paper (W. M. Chan and B. M. Ward, J. Virol. 86:8210-8220, 2012), we show that the lumenal domain of A33 is sufficient for interaction with the coiled-coil domain of B5 and capable of directing B5-green fluorescent protein (GFP) into extracellular virions. Here, we have created a panel of charge-to-alanine mutations in the lumenal domain of A33 to map the B5 interaction site. While none of these mutations abolished the interaction with B5, a subset displayed an increased interaction with both B5 and B5-GFP. Both B5 and B5-GFP recombinant viruses expressing these mutant proteins in place of normal A33 had a small-plaque phenotype. The increased interaction of the mutant proteins was detected during infection, suggesting that normally the interaction is either weak or transient. In addition, the increased A33-B5 interaction was detected on virions produced by recombinant viruses and correlated with reduced target cell binding. Taken together, these results show that both B5 and B5-GFP interact with A33 during infection and that the duration of this interaction needs to be regulated for the production of fully infectious extracellular virions.

The A33-dependent incorporation of B5 into extracellular enveloped vaccinia virions is mediated through an interaction between their lumenal domains

There are two mechanisms for the incorporation of B5 into the envelope of extracellular virions produced by orthopoxviruses, one that requires A33 and one that does not. We have hypothesized that the A33-dependent mechanism requires a direct interaction between A33 and B5. In this study, chimeric constructs of A33 and B5/B5-green fluorescent protein (GFP) were used to show that the two proteins interact through their lumenal domains and that the coiled-coil domain of B5 is sufficient for an interaction with A33. Furthermore, our experiments reveal that a transmembrane domain, not necessarily its own, is requisite for the lumenal domain of B5 to interact with A33. In contrast, the lumenal domain of A33 is sufficient for interaction with B5. Furthermore, the lumenal domain of A33 is sufficient to restore the proper localization of B5-GFP in infected cells. Taken together, our results demonstrate that the lumenal domains of A33 and B5 interact and that the interaction is required for the incorporation of B5-GFP into extracellular virions, whereas the incorporation of A33 is independent of B5. These results suggest that viral protein incorporation into extracellular virions is an active process requiring specific protein-protein interactions.

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

Vaccinia virus (VACV) spreads across cell monolayers fourfold faster than predicted from its replication kinetics. Early after infection, infected cells repulse some superinfecting extracellular enveloped virus (EEV) particles by the formation of actin tails from the cell surface, thereby causing accelerated spread to uninfected cells. This strategy requires the expression of two viral proteins, A33 and A36, on the surface of infected cells and upon contact with EEV this complex induces actin polymerization. Here we have studied this phenomenon further and investigated whether A33 and A36 expression in cell lines causes an increase in VACV plaque size, whether these proteins are able to block superinfection by EEV, and which protein(s) on the EEV surface are required to initiate the formation of actin tails from infected cells. Data presented show that VACV plaque size was not increased by expression of A33 and A36, and these proteins did not block entry of the majority of EEV binding to these cells. In contrast, expression of proteins A56 and K2 inhibited entry of both EEV and intracellular mature virus. Lastly, VACV protein B5 was required on EEV to induce the formation of actin tails at the surface of cells expressing A33 and A36, and B5 short consensus repeat 4 is critical for this induction.

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.

Vaccinia virus A6 is essential for virion membrane biogenesis and localization of virion membrane proteins to sites of virion assembly

Poxvirus acquires its primary envelope through a process that is distinct from those of other enveloped viruses. The molecular mechanism of this process is poorly understood, but several poxvirus proteins essential for the process have been identified in studies of vaccinia virus (VACV), the prototypical poxvirus. Previously, we identified VACV A6 as an essential factor for virion morphogenesis by studying a temperature-sensitive mutant with a lesion in A6. Here, we further studied A6 by constructing and characterizing an inducible virus (iA6) that could more stringently repress A6 expression. When A6 expression was induced by the inducer isopropyl-β-D-thiogalactoside (IPTG), iA6 replicated normally, and membrane proteins of mature virions (MVs) predominantly localized in viral factories where virions were assembled. However, when A6 expression was repressed, electron microscopy of infected cells showed the accumulation of large viroplasm inclusions containing virion core proteins but no viral membranes. Immunofluorescence and cell fractionation studies showed that the major MV membrane proteins A13, A14, D8, and H3 did not localize to viral factories but instead accumulated in the secretory compartments, including the endoplasmic reticulum. Overall, our results show that A6 is an additional VACV protein that participates in an early step of virion membrane biogenesis. Furthermore, A6 is required for MV membrane protein localization to sites of virion assembly, suggesting that MV membrane proteins or precursors of MV membranes are trafficked to sites of virion assembly through an active, virus-mediated process that requires A6.

Glomulin: a permissivity factor for vaccinia virus infection

In earlier studies we provided evidence that vaccinia virus (VACV) phosphorylation-activation of host cell signaling effectors is critical for subsequent viral replication. In this report, using mass spectrometry-based proteomics, we have identified 387 host cell proteins that co-immunoprecipitate with VACV in infected, permissive PM1.CCR5 human T cells. Among these, glomulin was distinguishable based on its known interaction with a tyrosine kinase receptor, c-Met, its ability to become tyrosine-phosphorylated, and its association with signaling effectors. siRNA knockdown of glomulin expression in PM1.CCR5 T cells reduces VACV infection. Glomulin interacts with the inactive, nonphosphorylated form of c-MET. We demonstrate that treatment of PM1.CCR5 T cells with a c-Met phosphorylation inhibitor leads to a significant reduction in VACV infectivity. Additionally, inhibition of phosphorylation of c-Met abrogates VACV-inducible phosphorylation of Erk 1/2 and IRS-2, signaling effectors identified as critical for VACV infection. These data identify glomulin as a permissivity factor for VACV infection and as a potential therapeutic target for inhibition of VACV infection.

Vaccinia extracellular virions enter cells by macropinocytosis and acid-activated membrane rupture

Double membrane-bound vaccinia extracellular virions actively promote their own macropinocytosis. This, followed by acid-induced rupture of the outer membrane in endocytic vesicles, exposes the inner membrane for fusion with the endocytic membrane and release into the cytosol.

Vaccinia virus (VACV), the model poxvirus, produces two types of infectious particles: mature virions (MVs) and extracellular virions (EVs). EV particles possess two membranes and therefore require an unusual cellular entry mechanism. By a combination of fluorescence and electron microscopy as well as flow cytometry, we investigated the cellular processes that EVs required to infect HeLa cells. We found that EV particles were endocytosed, and that internalization and infection depended on actin rearrangements, activity of Na⁺/H⁺ exchangers, and signalling events typical for the macropinocytic mechanism of endocytosis. To promote their internalization, EVs were capable of actively triggering macropinocytosis. EV infection also required vacuolar acidification, and acid exposure in endocytic vacuoles was needed to disrupt the outer EV membrane. Once exposed, the underlying MV-like particle presumably fused its single membrane with the limiting vacuolar membrane. Release of the viral core into the host cell cytosol allowed for productive infection.

The inability of vaccinia virus A33R protein to form intermolecular disulfide-bonded homodimers does not affect the production of infectious extracellular virus

The orthopoxvirus protein A33 forms a disulfide-bonded high molecular weight species that could be either a homodimer or a heteromultimer. The protein is a major target for neutralizing antibodies and the majority of antibodies raised against A33 only recognize the disulfide-bonded form. Here, we report that A33 is present as a disulfide-bonded homodimer during infection. Additionally, we examined the function of intermolecular disulfide bonding in A33 homodimerization during infection. We show that the cysteine at amino acid 62 is required for intermolecular disulfide bonding, but not dimerization as this mutant was still able to form homodimers. To investigate the role of disulfide-bonded homodimers during viral morphogenesis, recombinant viruses that express an A33R with cysteine 62 mutated to serine were generated. The recombinant viruses had growth characteristics similar to their parental viruses, indicating that intermolecular disulfide-bonded homodimerization of A33 is not required for its function.

Multiple phosphatidylinositol 3-kinases regulate vaccinia virus morphogenesis

Poxvirus morphogenesis is a complex process that involves the successive wrapping of the virus in host cell membranes. We screened by plaque assay a focused library of kinase inhibitors for those that caused a reduction in viral growth and identified several compounds that selectively inhibit phosphatidylinositol 3-kinase (PI3K). Previous studies demonstrated that PI3Ks mediate poxviral entry. Using growth curves and electron microscopy in conjunction with inhibitors, we show that that PI3Ks additionally regulate morphogenesis at two distinct steps: immature to mature virion (IMV) transition, and IMV envelopment to form intracellular enveloped virions (IEV). Cells derived from animals lacking the p85 regulatory subunit of Type I PI3Ks (p85α^−/−β^−/−) presented phenotypes similar to those observed with PI3K inhibitors. In addition, VV appear to redundantly use PI3Ks, as PI3K inhibitors further reduce plaque size and number in p85α^−/−β^−/− cells. Together, these data provide evidence for a novel regulatory mechanism for virion morphogenesis involving phosphatidylinositol dynamics and may represent a new therapeutic target to contain poxviruses.

There is an A33-dependent mechanism for the incorporation of B5-GFP into vaccinia virus extracellular enveloped virions

Orthopoxviruses produce two, antigenically distinct, infectious virions, intracellular mature virions and extracellular virions (EV). A33 and B5 are found on EV but not on intracellular mature virions. To investigate the function of A33, a recombinant virus that has A33R deleted and expresses B5R-GFP (vB5R-GFP/DeltaA33R) was generated. A comparison of vB5R-GFP/DeltaA33R to an analogous virus (vDeltaA33R) revealed an additional defect in infectious EV production that was not apparent when A33R was present. Characterization of these recombinants revealed that EV produced in the absence of A33 had undetectable levels of B5-GFP. Both recombinants released similar amounts of EV but there were differences in their infectivity. Approximately equal numbers of virions produced by these recombinants were able to bind cells even though EV produced by vB5R-GFP/DeltaA33R do not contain B5. These results suggest that in the absence of A33, the cytoplasmic tail of B5 contributes to its incorporation into the envelope of progeny virions.

Vaccinia virus B5 protein affects the glycosylation, localization and stability of the A34 protein

Vaccinia virus has two infectious forms, the intracellular mature virus, which has a single envelope, and the extracellular enveloped virus (EEV), which is surrounded by two lipid bilayers. The outer membrane of the EEV contains at least six viral proteins. Among them A34, a type II membrane glycoprotein, and B5, a type I membrane glycoprotein, form a complex and are involved in processes such as morphogenesis and EEV entry. A34 is required for normal incorporation of B5 into the EEV membrane. Here, we used a virus lacking B5 and viruses with mutations in the B5 membrane-proximal stalk region and looked at the effect of those modifications on A34. Data presented show that B5 is required for the correct glycosylation, trafficking and stability of A34, emphasizing the complex interactions and mutual dependence of these vaccinia EEV proteins.

Poxvirus proteomics and virus-host protein interactions

Studies of the functional proteins encoded by the poxvirus genome provide information about the composition of the virus as well as individual virus-virus protein and virus-host protein interactions, which provides insight into viral pathogenesis and drug discovery. Widely used proteomic techniques to identify and characterize specific protein-protein interactions include yeast two-hybrid studies and coimmunoprecipitations. Recently, various mass spectrometry techniques have been employed to identify viral protein components of larger complexes. These methods, combined with structural studies, can provide new information about the putative functions of viral proteins as well as insights into virus-host interaction dynamics. For viral proteins of unknown function, identification of either viral or host binding partners provides clues about their putative function. In this review, we discuss poxvirus proteomics, including the use of proteomic methodologies to identify viral components and virus-host protein interactions. High-throughput global protein expression studies using protein chip technology as well as new methods for validating putative protein-protein interactions are also discussed.

Acidic residues in the membrane-proximal stalk region of vaccinia virus protein B5 are required for glycosaminoglycan-mediated disruption of the extracellular enveloped virus outer membrane

The extracellular enveloped virus (EEV) form of vaccinia virus (VACV) is surrounded by two lipid envelopes. This presents a topological problem for virus entry into cells, because a classical fusion event would only release a virion surrounded by a single envelope into the cell. Recently, we described a mechanism in which the EEV outer membrane is disrupted following interaction with glycosaminoglycans (GAGs) on the cell surface and thus allowing fusion of the inner membrane with the plasma membrane and penetration of a naked core into the cytosol. Here we show that both the B5 and A34 viral glycoproteins are required for this process. A34 is required to recruit B5 into the EEV membrane and B5 acts as a molecular switch to control EEV membrane rupture upon exposure to GAGs. Analysis of VACV strains expressing mutated B5 proteins demonstrated that the acidic stalk region between the transmembrane anchor sequence and the fourth short consensus repeat of B5 are critical for GAG-induced membrane rupture. Furthermore, the interaction between B5 and A34 can be disrupted by the addition of polyanions (GAGs) and polycations, but only the former induce membrane rupture. Based on these data we propose a revised model for EEV entry.

Vaccinia virus protein F12 associates with intracellular enveloped virions through an interaction with A36

Vaccinia virus is the prototypical member of the family Poxviridae. Three morphologically distinct forms are produced during infection: intracellular mature virions (IMV), intracellular enveloped virions (IEV), and extracellular enveloped virions (EEV). Two viral proteins, F12 and A36, are found exclusively on IEV but not on IMV and EEV. Analysis of membranes from infected cells showed that F12 was only associated with membranes and is not an integral membrane protein. A yeast two-hybrid assay revealed an interaction between amino acids 351 to 458 of F12 and amino acids 91 to 111 of A36. We generated a recombinant vaccinia virus that expresses an F12, which lacks residues 351 to 458. Characterization of this recombinant revealed a small-plaque phenotype and a subsequent defect in virus release similar to a recombinant virus that had F12L deleted. In addition, F12 lacking residues 351 to 458 was unable to associate with membranes in infected cells. These results suggest that F12 associates with IEV through an interaction with A36 and that this interaction is critical for the function of F12 during viral egress.

Vaccinia virus extracellular enveloped virion neutralization in vitro and protection in vivo depend on complement

Antibody neutralization is an important component of protective immunity against vaccinia virus (VACV). Two distinct virion forms, mature virion and enveloped virion (MV and EV, respectively), possess separate functions and nonoverlapping immunological properties. In this study we examined the mechanics of EV neutralization, focusing on EV protein B5 (also called B5R). We show that neutralization of EV is predominantly complement dependent. From a panel of high-affinity anti-B5 monoclonal antibodies (MAbs), the only potent neutralizer in vitro (90% at 535 ng/ml) was an immunoglobulin G2a (IgG2a), and neutralization was complement mediated. This MAb was the most protective in vivo against lethal intranasal VACV challenge. Further studies demonstrated that in vivo depletion of complement caused a >50% loss of anti-B5 IgG2a protection, directly establishing the importance of complement for protection against the EV form. However, the mechanism of protection is not sterilizing immunity via elimination of the inoculum as the viral inoculum consisted of a purified MV form. The prevention of illness in vivo indicated rapid control of infection. We further demonstrate that antibody-mediated killing of VACV-infected cells expressing surface B5 is a second protective mechanism provided by complement-fixing anti-B5 IgG. Cell killing was very efficient, and this effector function was highly isotype specific. These results indicate that anti-B5 antibody-directed cell lysis via complement is a powerful mechanism for clearance of infected cells, keeping poxvirus-infected cells from being invisible to humoral immune responses. These findings highlight the importance of multiple mechanisms of antibody-mediated protection against VACV and point to key immunobiological differences between MVs and EVs that impact the outcome of infection.

Vaccinia virus morphogenesis and dissemination

Vaccinia virus is the smallpox vaccine. It is the most intensively studied poxvirus, and its study has provided important insights about virus replication in general and the interactions of viruses with the host cell and immune system. Here, the entry, morphogenesis and dissemination of vaccinia virus are considered. These processes are complicated by the existence of two infectious vaccinia virus particles, called intracellular mature virus (IMV) and extracellular enveloped virus (EEV). The IMV particle is surrounded by one membrane, and the EEV particle comprises an IMV particle enclosed within a second lipid membrane containing several viral antigens. Consequently, these virions have different biological properties and play different roles in the virus life cycle.