Morphogenesis and release of fowlpox virus

Pros and Cons on Use of Live Viral Vaccines in Commercial Chicken Flocks

The poultry industry is the largest source of meat and eggs for the growing human population worldwide. Key concerns in poultry farming are nutrition, management, flock health, and biosecurity measures. As part of the flock health, use of live viral vaccines plays a vital role in the prevention of economically important and common viral diseases. This includes diseases and production losses caused by Newcastle disease virus, infectious bronchitis virus, infectious laryngotracheitis virus, infectious bursal disease virus, Marek's disease virus, chicken infectious anemia virus, avian encephalomyelitis virus, fowlpox virus, and avian metapneumovirus. These viruses cause direct and indirect harms, such as financial losses worth millions of dollars, loss of protein sources, and threats to animal welfare. Flock losses vary by type of poultry, age of affected animals, co-infections, immune status, and environmental factors. Losses in broiler birds can consist of high mortality, poor body weight gain, high feed conversion ratio, and increased carcass condemnation. In commercial layers and breeder flocks, losses include higher than normal mortality rate, poor flock uniformity, drops in egg production and quality, poor hatchability, and poor day-old-chick quality. Despite the emergence of technology-based vaccines, such as inactivated, subunit, vector-based, DNA or RNA, and others, the attenuated live vaccines remain as important as before. Live vaccines are preferred in the global veterinary vaccine market, accounting for 24.3% of the global market share in 2022. The remaining 75% includes inactivated, DNA, subunit, conjugate, recombinant, and toxoid vaccines. The main reason for this is that live vaccines can induce innate, mucosal, cellular, and humoral immunities by single or multiple applications. Some live vaccine combinations provide higher and broader protection against several diseases or strains of viruses. This review aimed to explore insights on the pros and cons of attenuated live vaccines commonly used against major viral infections of the global chicken industry, and the future road map for improvement.

Characterization of Fowlpox Virus

The complex cytoplasmic DNA virus known as the fowlpox virus (FWPV) is a member of the avipoxvirus genus, Subfamily Chordopoxvirinae, and Family Poxviridae. The large genome size of FWPV makes it a potential vector for the creation of vaccines against a range of serious veterinary and human ailments. It also allows for multiple gene insertion and the generation of abortive infection in mammalian cells. The virus, which causes fowlpox in chickens and turkeys, is mainly transmitted to poultry through aerosols or biting insects. Fowlpox is a highly contagious disease that affects both domestic and wild birds, causing cutaneous and/or diphtheritic illnesses. To control the illness, strict hygiene practices and immunization with FWPV attenuated strains or antigenically similar pigeon pox virus vaccines are employed. Recent years have seen an increase in fowlpox outbreaks in chicken flocks, primarily due to the introduction of novel forms of FWPV. It is believed that the pathogenic characteristics of these strains are enhanced by the integration of reticuloendotheliosis virus sequences of variable lengths into the FWPV genome. The standard laboratory diagnosis of FPV involves histopathological analysis, electron microscopy, virus isolation on chorioallantoic membrane (CAM) of embryonated chicken eggs or cell cultures, and serologic techniques. For quick and consistent diagnosis, polymerase chain reaction (PCR) has proven to be the most sensitive method. PCR is used in concert with restriction endonuclease enzyme analysis (REA) to identify, differentiate, and characterize the molecular makeup of isolates of the fowlpox virus. Sequencing of the amplified fragments is then done.

Ultrastructural analysis of SARS-CoV-2 interactions with the host cell via high resolution scanning electron microscopy

SARS-CoV-2 is the cause of the ongoing COVID-19 pandemic. Here, we investigated the interaction of this new coronavirus with Vero cells using high resolution scanning electron microscopy. Surface morphology, the interior of infected cells and the distribution of viral particles in both environments were observed 2 and 48 h after infection. We showed areas of viral processing, details of vacuole contents, and viral interactions with the cell surface. Intercellular connections were also approached, and viral particles were adhered to these extensions suggesting direct cell-to-cell transmission of SARS-CoV-2.

Discrete Virus Factories Form in the Cytoplasm of Cells Coinfected with Two Replication-Competent Tagged Reporter Birnaviruses That Subsequently Coalesce over Time

The Birnaviridae family, responsible for major economic losses to poultry and aquaculture, is composed of nonenveloped viruses with a segmented double-stranded RNA (dsRNA) genome that replicate in discrete cytoplasmic virus factories (VFs). Reassortment is common; however, the underlying mechanism remains unknown given that VFs may act as a barrier to genome mixing. In order to provide new information on VF trafficking during dsRNA virus coinfection, we rescued two recombinant infectious bursal disease viruses (IBDVs) of strain PBG98 containing either a split GFP11 or a tetracysteine (TC) tag fused to the VP1 polymerase (PBG98-VP1-GFP11 and PBG98-VP1-TC). DF-1 cells transfected with GFP1-10 prior to PBG98-VP1-GFP11 infection or stained with a biarsenical derivative of the red fluorophore resorufin (ReAsH) following PBG98-VP1-TC infection, had green or red foci in the cytoplasm, respectively, that colocalized with VP3 and dsRNA, consistent with VFs. The average number of VFs decreased from a mean of 60 to 5 per cell between 10 and 24 h postinfection (hpi) (P < 0.0001), while the average area increased from 1.24 to 45.01 μm² (P < 0.0001), and live cell imaging revealed that the VFs were highly dynamic structures that coalesced in the cytoplasm. Small VFs moved faster than large (average 0.57 μm/s at 16 hpi compared to 0.22 μm/s at 22 hpi), and VF coalescence was dependent on an intact microtubule network and actin cytoskeleton. During coinfection with PBG98-VP1-GFP11 and PBG98-VP1-TC viruses, discrete VFs initially formed from each input virus that subsequently coalesced 10 to 16 hpi, and we speculate that Birnaviridae reassortment requires VF coalescence.IMPORTANCE Reassortment is common in viruses with segmented double-stranded RNA (dsRNA) genomes. However, these viruses typically replicate within discrete cytoplasmic virus factories (VFs) that may represent a barrier to genome mixing. We generated the first replication competent tagged reporter birnaviruses, infectious bursal disease viruses (IBDVs) containing a split GFP11 or tetracysteine (TC) tag and used the viruses to track the location and movement of IBDV VFs, in order to better understand the intracellular dynamics of VFs during a coinfection. Discrete VFs initially formed from each virus that subsequently coalesced from 10 h postinfection. We hypothesize that VF coalescence is required for the reassortment of the Birnaviridae This study provides new information that adds to our understanding of dsRNA virus VF trafficking.

Poxvirus Infection in a Colony of Laboratory Pigeons (Columba livia)

Pigeons (Columba livia) are used in biomedical research for studies of vision, cognition, neuronal pathways, and spatial orientation. Because there are few commercial laboratory sources, research pigeons are typically acquired from local fancier breeders or bred onsite. For acquired pigeons, the health and vaccine status is often unknown. A juvenile pigeon, born onsite and living in an enclosed outdoor loft, presented with small, bleeding, wart-like lesions on the medial aspects of digits 1 and 4. Topical treatment was initiated. Within a week, 4 fledglings were reported for small, dark papular lesions on the face, head, neck, and beak, and shortly thereafter, 2 additional juvenile pigeons developed similar lesions. The fledglings were euthanized, and histologic examination revealed numerous intralesional eosinophilic cytoplasmic viral inclusions (Bollinger bodies) confirming a diagnosis of poxvirus infection, likely pigeon pox. Although usually self-limiting, pigeon pox can cause moderate to severe lesions in fledgling and juvenile birds. Vaccination with a modified live poxvirus labeled for chickens was used to create herd immunity to pigeon poxvirus. Since vaccination of our entire flock and implementation of more stringent health protocols, all lesions have resolved, and no new lesions have been noted.

Brefeldin A and Cytochalasin B reduce dengue virus replication in cell cultures but do not protect mice against viral challenge

BACKGROUND

Dengue is an emerging arboviral disease caused by dengue virus (DENV). DENV belongs to the family Flaviviridae and genus Flavivirus. No specific anti-DENV drugs are currently available.

METHODS

We investigated the antiviral activity of Brefeldin A (BFA) and Cytochalasin B (CB) against this infection. The drugs BFA and CB were used in the in vitro treatment of dengue-2 virus (DENV-2) infections in Vero cell cultures and in protection from lethality by post-challenge administration in Swiss mice. Viral load was quantified by qRT-PCR and plaque assay in Vero cell cultures, post-infection, treated or not with the drugs. Post-challenge drug levels were evaluated by survival analysis.

RESULTS

Our results indicate that doses of 5 µg ml^-1 of BFA and 10 µg ml^-1 of CB are not toxic to the cells and induce a statistically significant inhibition of DENV-2 replication in Vero cells when compared to control. No BFA- or CB-treated mice survived the challenge with DENV-2.

CONCLUSION

These data suggest that BFA and CB have an antiviral action against DENV-2 replication in Vero cell culture, but do not alter infected mice mortality.

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.

Poxvirus membrane biogenesis

Poxviruses differ from most DNA viruses by replicating entirely within the cytoplasm. The first discernible viral structures are crescents and spherical immature virions containing a single lipoprotein membrane bilayer with an external honeycomb lattice. Because this viral membrane displays no obvious continuity with a cellular organelle, a de novo origin was suggested. Nevertheless, transient connections between viral and cellular membranes could be difficult to resolve. Despite the absence of direct evidence, the intermediate compartment (ERGIC) between the endoplasmic reticulum (ER) and Golgi apparatus and the ER itself were considered possible sources of crescent membranes. A break-through in understanding poxvirus membrane biogenesis has come from recent studies of the abortive replication of several vaccinia virus null mutants. Novel images showing continuity between viral crescents and the ER and the accumulation of immature virions in the expanded ER lumen provide the first direct evidence for a cellular origin of this poxvirus membrane.

Fowlpox virus as a recombinant vaccine vector for use in mammals and poultry

Live vaccines against fowlpox virus, which causes moderate pathology in poultry and is the type species of the Avipoxvirus genus, were developed in the 1920s. Development of recombinant fowlpox virus vector vaccines began in the 1980s, for use not only in poultry, but also in mammals including humans. In common with other avipoxviruses, such as canarypox virus, fowlpox virus enters mammalian cells and expresses proteins, but replicates abortively. The use of fowlpox virus as a safe vehicle for expression of foreign antigens and host immunomodulators, is being evaluated in numerous clinical trials of vaccines against cancer, malaria, tuberculosis and AIDS, notably in heterologous prime-boost regimens. In this article, technical approaches to, and issues surrounding, the use of fowlpox virus as a recombinant vaccine vector in poultry and mammals are reviewed.

Avian pox virus infection in a common barn owl (Tyto alba) in southern Brazil

A young common barn owl (Tyto alba) was referred to the Núcleo de Reabilitação da Fauna Silvestre (Nurfs), Federal University of Pelotas (UFPel), after been found in a barn of a brick factory in the urban area of Pelotas, Rio Grande do Sul, Brazil. The bird was apathic, weak and with crusty lesions in the featherless areas (eyes, beak, legs), and died soon after arrival at Nurfs. Necropsy and histopathological examination of the lesions were carried out. The hyperplasia and hypertrophy of the cutaneous lesions, several eosinophilic intracyto-plasmic inclusion bodies in epithelial cells (Bollinger bodies), as well as particles characteristic of poxvirus, observed by electronic microscopy, confirmed the infection by avian poxvirus, what highlights the importance of Tyto alba as carrier of the virus in the wild.

Avipoxviruses: infection biology and their use as vaccine vectors

Avipoxviruses (APVs) belong to the Chordopoxvirinae subfamily of the Poxviridae family. APVs are distributed worldwide and cause disease in domestic, pet and wild birds of many species. APVs are transmitted by aerosols and biting insects, particularly mosquitoes and arthropods and are usually named after the bird species from which they were originally isolated. The virus species Fowlpox virus (FWPV) causes disease in poultry and associated mortality is usually low, but in flocks under stress (other diseases, high production) mortality can reach up to 50%. APVs are also major players in viral vaccine vector development for diseases in human and veterinary medicine. Abortive infection in mammalian cells (no production of progeny viruses) and their ability to accommodate multiple gene inserts are some of the characteristics that make APVs promising vaccine vectors. Although abortive infection in mammalian cells conceivably represents a major vaccine bio-safety advantage, molecular mechanisms restricting APVs to certain hosts are not yet fully understood. This review summarizes the current knowledge relating to APVs, including classification, morphogenesis, host-virus interactions, diagnostics and disease, and also highlights the use of APVs as recombinant vaccine vectors.

Interaction of poxvirus intracellular mature virion proteins with the TPR domain of kinesin light chain in live infected cells revealed by two-photon-induced fluorescence resonance energy transfer fluorescence lifetime imaging microscopy

Using two-photon-induced fluorescence lifetime imaging microscopy, we corroborate an interaction (previously demonstrated by yeast two-hybrid domain analysis) of full-length vaccinia virus (VACV; an orthopoxvirus) A36 protein with the cellular microtubule motor protein kinesin. Quenching of enhanced green fluorescent protein (EGFP), fused to the C terminus of VACV A36, by monomeric red fluorescent protein (mDsRed), fused to the tetratricopeptide repeat (TPR) domain of kinesin, was observed in live chicken embryo fibroblasts infected with either modified vaccinia virus Ankara (MVA) or wild-type fowlpox virus (FWPV; an avipoxvirus), and the excited-state fluorescence lifetime of EGFP was reduced from 2.5 ± 0.1 ns to 2.1 ± 0.1 ns due to resonance energy transfer to mDsRed. FWPV does not encode an equivalent of intracellular enveloped virion surface protein A36, yet it is likely that this virus too must interact with kinesin to facilitate intracellular virion transport. To investigate possible interactions between innate FWPV proteins and kinesin, recombinant FWPVs expressing EGFP fused to the N termini of FWPV structural proteins Fpv140, Fpv168, Fpv191, and Fpv198 (equivalent to VACV H3, A4, p4c, and A34, respectively) were generated. EGFP fusions of intracellular mature virion (IMV) surface protein Fpv140 and type II membrane protein Fpv198 were quenched by mDsRed-TPR in recombinant FWPV-infected cells, indicating that these virion proteins are found within 10 nm of mDsRed-TPR. In contrast, and as expected, EGFP fusions of the IMV core protein Fpv168 did not show any quenching. Interestingly, the p4c-like protein Fpv191, which demonstrates late association with preassembled IMV, also did not show any quenching.

Rice dwarf virus is engulfed into and released via vesicular compartments in cultured insect vector cells

Vector insect cells infected with Rice dwarf virus had vesicular compartments containing viral particles located adjacent to the viroplasm when examined by transmission electron and confocal microscopy. Such compartments were often at the periphery of infected cells. Inhibitors of vesicular transport, brefeldin A and monensin, and an inhibitor of myosin motor activity, butanedione monoxime, abolished the formation of such vesicles and prevented the release of viral particles from infected cells without significant effects on virus multiplication. Furthermore, the actin-depolymerizing drug, cytochalasin D, inhibited the formation of actin filaments without significantly interfering with formation of vesicular compartments and the release of viruses from treated cells. These results together revealed intracellular vesicular compartments as a mode for viral transport in and release from insect vector cells infected with a plant-infecting reovirus.

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.

Avipoxvirus in blackcaps (Sylvia atricapilla)

From July to September 2005, 1075 wild birds of 37 species were mist-netted at a location in the north-eastern part of the Czech Republic. The birds were examined for the presence of avipoxvirus lesions. This was demonstrated by electron microscopy in skin lesions in nine of 244 blackcaps (Sylvia atricapilla) examined (4% prevalence). Blackcaps skin bioptates were processed using the ultrathin section method. In skin bioptates, avipoxviruses were demonstrated in intracytoplasmic inclusions where, in addition to mature viruses, lipids and filamentous structures concentrated into large circular formations were found. The so-called additional inclusions were also found. These did not contain any virus components, and they served as the precursor of A-type intracytoplasmic inclusions. Blackcap avipoxvirus was isolated by passage on the chorioallantoic membrane of 9-day-old chicken embryos. The virus was successfully adapted after 11 passages (each passage lasted 5 to 7 days), at which time a marked changes in the form of tiny nodules 2 to 3 mm in diameter were observed on the chorioallantoic membrane. Further identification of field isolates and of the cultured virus was carried out using polymerase chain reaction and sequencing. Sequences were compared with consensus sequences of both canarypoxviruses and fowlpoxviruses. Our sequence was found to be 98.8% identical to the canarypox consensus sequence, but only 63% identical to the fowlpox consensus sequence. Our avipoxvirus sequence was proven to be significantly more closely related to canarypoxviruses than to fowlpoxviruses also by phylogenetic analysis.

Comparative study of pathological changes in the mouse viscera in infection caused by two orthopoxviruses

The mechanisms of infection development in intraperitoneal inoculation of mice by ectromelia virus strain K-1 and cowpox strain EP-2 were studied. Ultrastructural parameters of virus assembly and maturation are described. Differences in the types of cells replicating the viruses and in the type of visceral injuries were detected. The studies showed a local type of strain EP-2 cowpox infection and dissemination of ectromelia strain K-1.

Porcine reproductive and respiratory syndrome virus (PRRSV) infection spreads by cell-to-cell transfer in cultured MARC-145 cells, is dependent on an intact cytoskeleton, and is suppressed by drug-targeting of cell permissiveness to virus infection

Background

Porcine reproductive and respiratory syndrome virus (PRRSV) is the etiologic agent of PRRS, causing widespread chronic infections which are largely uncontrolled by currently available vaccines or other antiviral measures. Cultured monkey kidney (MARC-145) cells provide an important tool for the study of PRRSV replication. For the present study, flow cytometric and fluorescence antibody (FA) analyses of PRRSV infection of cultured MARC-145 cells were carried out in experiments designed to clarify viral dynamics and the mechanism of viral spread. The roles of viral permissiveness and the cytoskeleton in PRRSV infection and transmission were examined in conjunction with antiviral and cytotoxic drugs.

Results

Flow cytometric and FA analyses of PRRSV antigen expression revealed distinct primary and secondary phases of MARC-145 cell infection. PRRSV antigen was randomly expressed in a few percent of cells during the primary phase of infection (up to about 20–22 h p.i.), but the logarithmic infection phase (days 2–3 p.i.), was characterized by secondary spread to clusters of infected cells. The formation of secondary clusters of PRRSV-infected cells preceded the development of CPE in MARC-145 cells, and both primary and secondary PRRSV infection were inhibited by colchicine and cytochalasin D, demonstrating a critical role of the cytoskeleton in viral permissiveness as well as cell-to-cell transmission from a subpopulation of cells permissive for free virus to secondary targets. Cellular expression of actin also appeared to correlate with PRRSV resistance, suggesting a second role of the actin cytoskeleton as a potential barrier to cell-to-cell transmission. PRRSV infection and cell-to-cell transmission were efficiently suppressed by interferon-γ (IFN-γ), as well as the more-potent experimental antiviral agent AK-2.

Conclusion

The results demonstrate two distinct mechanisms of PRRSV infection: primary infection of a relatively small subpopulation of innately PRRSV-permissive cells, and secondary cell-to-cell transmission to contiguous cells which appear non-permissive to free virus. The results also indicate that an intact cytoskeleton is critical for PRRSV infection, and that viral permissiveness is a highly efficient drug target to control PRRSV infection. The data from this experimental system have important implications for the mechanisms of PRRSV persistence and pathology, as well as for a better understanding of arterivirus regulation.

The potential role of fowlpox virus in rational vaccine design

The design of optimal vaccines requires detailed knowledge of how protective immune responses are generated in vivo under normal circumstances. This approach to vaccine development, where the immune correlates of protection are defined and vaccines are designed to elicit the same response, is called rational vaccine design. Poxviruses are attractive candidates for inclusion in such design strategies owing to their large genome, which allows for the inclusion of multiple heterologous genes, including those encoding antigens, co-stimulatory molecules and cytokines. Fowlpox virus, the prototypical member of the Avipoxvirus genus, is particularly suitable, as it is also incapable of replicating in mammalian cells. The potential of recombinant fowlpox virus as a safe vaccine vector is being evaluated currently in a number of clinical trials for diseases, including HIV, malaria and various types of cancer. Despite their promise, intricate details regarding how fowlpox virus interacts with the host immune system have not been resolved. In this review, the issues surrounding the use of fowlpox virus as a vaccine vector and possible strategies for enhancing its efficacy are discussed.

Packaging of actin into Ebola virus VLPs

The actin cytoskeleton has been implicated in playing an important role assembly and budding of several RNA virus families including retroviruses and paramyxoviruses. In this report, we sought to determine whether actin is incorporated into Ebola VLPs, and thus may play a role in assembly and/or budding of Ebola virus. Our results indicated that actin and Ebola virus VP40 strongly co-localized in transfected cells as determined by confocal microscopy. In addition, actin was packaged into budding VP40 VLPs as determined by a functional budding assay and protease protection assay. Co-expression of a membrane-anchored form of Ebola virus GP enhanced the release of both VP40 and actin in VLPs. Lastly, disruption of the actin cytoskeleton with latrunculin-A suggests that actin may play a functional role in budding of VP40/GP VLPs. These data suggest that VP40 may interact with cellular actin, and that actin may play a role in assembly and/or budding of Ebola VLPs.

Vaccinia virus intracellular enveloped virions move to the cell periphery on microtubules in the absence of the A36R protein

Vaccinia virus (VACV) intracellular enveloped virus (IEV) particles are transported to the cell periphery on microtubules where they fuse with the plasma membrane to form cell-associated enveloped virus (CEV). Two IEV-specific proteins, F12L and A36R, are implicated in mediating transport of IEV. Without F12L, virus morphogenesis halts after formation of IEV, and CEV is not formed, whereas without A36R, IEV was reported not to be transported, yet CEV was formed, To address the roles of A36R and F12L in IEV transport, viruses with deletions of either F12L (vΔF12L) or A36R (vΔA36R) were labelled with enhanced green fluorescent protein (EGFP) fused to the core protein A5L, and used to follow CEV production with time. Without F12L, CEV production was inhibited by >99 %, whereas without A36R, CEV were produced at ∼60 % of wild-type levels at 24 h post-infection. Depolymerization of microtubules, but not actin, inhibited CEV formation in vΔA36R-infected cells. Moreover, vΔA36R IEV labelled with EGFP fused to the B5R protein co-localized with microtubules, showing that the A36R protein is not required for the interaction of IEV with microtubules. Time-lapse confocal microscopy confirmed that both wild-type and vΔA36R IEV moved in a stop–start manner at speeds consistent with microtubular movement, although the mean length of vΔA36R IEV movement was shorter. These data demonstrate that VACV IEV is transported to the cell surface using microtubules in the absence of A36R, and therefore IEV must attach to microtubule motors using at least one protein other than A36R.

Topographic changes in SARS coronavirus-infected cells at late stages of infection

Scanning electron and atomic force microscopy was used for the first time to view the maturation of SARS-CoV at the cell surface.

Scanning electron and atomic force microscopy was used for the first time to view the maturation of the severe acute respiratory syndrome–associated coronavirus at the cell surface. The surface form of the cells at advanced infection displayed prolific pseudopodia that, in addition to the rest of the plasma membrane, were also active sites of virus release. High magnification of the maturing virus particles showed a rosette appearance with short knoblike spikes under both the scanning electron and atomic force microscopes. The final expulsion step of the maturing virus particles seemed to result in some disruptions to the plasma membrane. The cytoskeletal network along the edge of the infected cells was enhanced and could be involved in transporting and expelling the progeny virus particles. Thickening of the actin filaments at the cell edge provided the bending force to extrude the virus particles.

Avipoxvirus multiplication in a mammalian cell line

Avipoxviruses have many advantages and are being increasingly employed as recombinant vaccine vectors. One attractive feature is that while inserted transgenes are expressed in immunologically favourable ways, avipoxvirus infections of mammalian cells are believed to be abortive. The experimental evidence supporting this belief is, however, based on a limited number of mammalian cell-types and a few avipoxvirus species. We evaluated two avian and eight mammalian cell lines for permissivity to three avipoxvirus strains, one reference fowlpoxvirus and two newly isolated strains from sparrow and pigeon, respectively. Both avian cell lines were, as expected, permissive for all three avipoxvirus strains. However, by multiplication assays, we found to our surprise that Syrian baby hamster kidney (BHK-21) cells were equally permissive to all virus strains. Results from electron microscopy of infected BHK-21 cells revealed viral morphogenesis proceeding to various forms of infectious viruses. These results were supported by the demonstration of avipoxvirus specific late gene expression and avipoxvirus specific DNA restriction pattern in BHK-21 infected cells.

The exit of vaccinia virus from infected cells

Vaccinia virus (VACV) is the prototypic member of the Poxviridae a group of large DNA viruses that replicate in the cell cytoplasm. The entry and exit of VACV are complicated by the existence of two distinct forms of virus, intracellular mature virus (IMV) and extracellular enveloped virus (EEV), that are surrounded by different numbers of lipid membranes and have different surface proteins. Here the mechanisms used by these different forms of VACV to leave the infected cell are reviewed. Whereas some enveloped viruses complete virus assembly by budding through the plasma membrane, infectious poxvirus particles (IMV) are produced within the cytoplasm. These particles are either further enveloped by intracellular membranes to form intracellular enveloped virus (IEV) that are transported to the cell surface on microtubules and exposed on the cell surface by exocytosis, or are released after cell lysis. If the enveloped virion remains attached to the cell surface it is called cell-associated enveloped virus (CEV) and is propelled into surrounding cells by growing actin tails beneath the plasma membrane. Alternatively, the surface virion may be released as EEV.

Virus factories: associations of cell organelles for viral replication and morphogenesis

Genome replication and assembly of viruses often takes place in specific intracellular compartments where viral components concentrate, thereby increasing the efficiency of the processes. For a number of viruses the formation of 'factories' has been described, which consist of perinuclear or cytoplasmic foci that mostly exclude host proteins and organelles but recruit specific cell organelles, building a unique structure. The formation of the viral factory involves a number of complex interactions and signalling events between viral and cell factors. Mitochondria, cytoplasmic membranes and cytoskeletal components frequently participate in the formation of viral factories, supplying basic and common needs for key steps in the viral replication cycle.

Vaccinia virus A36R membrane protein provides a direct link between intracellular enveloped virions and the microtubule motor kinesin

Previous work demonstrated that intracellular enveloped vaccinia virus virions use microtubules to move from the site of membrane wrapping to the cell periphery. The mechanism and direction of intracellular virion movement predicted that viral proteins directly or indirectly interact with the microtubule motor protein kinesin. The yeast two-hybrid assay was used to test for interactions between the light chain of kinesin and the cytoplasmic tails from five viral envelope proteins. We found that the N-terminal tetratricopeptide repeat region of the kinesin light chain (KLC-TPR) interacted with the cytoplasmic tail of the viral A36R protein. A series of C- and N-terminal truncations of A36R further defined a region from residues 81 to 111 that was sufficient for interaction with KLC-TPR. Interactions were confirmed by using pull-down assays with purified glutathione S-transferase (GST)-A36R and (35)S-labeled KLC-TPR. The defined region on A36R for interaction with kinesin overlaps the recently defined region (residues 91 to 111) for interaction with the A33R envelope protein. The yeast three-hybrid system was used to demonstrate that expression of A33R interrupted the interaction between A36R and KLC-TPR, indicating that the binding of A36R is mutually exclusive to either A33R or kinesin. Pull-down assays with purified GST-A36R and (35)S-labeled KLC-TPR in the presence of competing A33R corroborated these findings. Collectively, these results demonstrated that the viral A36R protein interacts directly with the microtubule motor protein kinesin and that the viral protein A33R may regulate this interaction.

Comparison of the genome sequence of FP9, an attenuated, tissue culture-adapted European strain of Fowlpox virus, with those of virulent American and European viruses

The 266 kbp genome sequence of plaque-purified, tissue culture-adapted, attenuated EuropeanFowlpox virusFP9 has been determined and compared with the 288 kbp sequence of a pathogenic US strain (FPVUS). FP9 carries 244 of the 260 reported FPVUS ORFs (both viruses also have an unreported orthologue of conserved poxvirus gene A14.5L). Relative to FPVUS, FP9 differed by 118 mutations (26 deletions, 15 insertions and 77 base substitutions), affecting FP9 equivalents of 71 FPVUS ORFs. To help to identify mutations involved in adaptation and attenuation, the virulent parent of FP9, HP1, was sequenced at positions where FP9 differed from FPVUS. At 68 positions, FP9 and HP1 sequences were identical, reflecting differences between American and European lineages. Mutations at the remaining 50 positions in FP9 relative to FPVUS and HP1, involving 46 ORFs, therefore accounted for adaptation and attenuation. ORFs deleted during passage included those encoding members of multigene families: 12 ankyrin repeat proteins, three C-type lectin-like proteins, two C4L/C10L-like proteins, one G-protein coupled receptor protein, one V-type Ig domain protein, two N1R/p28 proteins and one EFc family protein. Tandem ORFs encodingVariola virusB22R orthologues were fused by a 5·8 kbp deletion. Single-copy genes disrupted or deleted during passage included those encoding a homologue of murine T10, a conserved DNA/pantothenate metabolism flavoprotein, photolyase, the A-type inclusion protein and an orthologue of vaccinia A47L. Gene assignments have been updated for DNase II/DLAD, binding proteins for IL-18 and interferon-γ, phospholipid hydroperoxide glutathione peroxidase (PHGPX/GPX-4) and for a highly conserved homologue of ELOVL4.

The genome of canarypox virus

Here we present the genomic sequence, with analysis, of a canarypox virus (CNPV). The 365-kbp CNPV genome contains 328 potential genes in a central region and in 6.5-kbp inverted terminal repeats. Comparison with the previously characterized fowlpox virus (FWPV) genome revealed avipoxvirus-specific genomic features, including large genomic rearrangements relative to other chordopoxviruses and novel cellular homologues and gene families. CNPV also contains many genomic differences with FWPV, including over 75 kbp of additional sequence, 39 genes lacking FWPV homologues, and an average of 47% amino acid divergence between homologues. Differences occur primarily in terminal and, notably, localized internal genomic regions and suggest significant genomic diversity among avipoxviruses. Divergent regions contain gene families, which overall comprise over 49% of the CNPV genome and include genes encoding 51 proteins containing ankyrin repeats, 26 N1R/p28-like proteins, and potential immunomodulatory proteins, including those similar to transforming growth factor beta and beta-nerve growth factor. CNPV genes lacking homologues in FWPV encode proteins similar to ubiquitin, interleukin-10-like proteins, tumor necrosis factor receptor, PIR1 RNA phosphatase, thioredoxin binding protein, MyD116 domain proteins, circovirus Rep proteins, and the nucleotide metabolism proteins thymidylate kinase and ribonucleotide reductase small subunit. These data reveal genomic differences likely affecting differences in avipoxvirus virulence and host range, and they will likely be useful for the design of improved vaccine vectors.

Actin filaments participate in West Nile (Sarafend) virus maturation process

West Nile (Sarafend) virus has previously been shown to egress by budding at the plasma membrane of infected cells, but relatively little is known about the mechanism involved in this mode of release. During the course of this study, it was discovered that actin filaments take part in the virus maturation process. Using dual-labeled immunofluorescence and immunoelectron microscopy at late infection (10 hr p.i.), co-localization of viral structural (envelope and capsid) proteins with actin filaments was confirmed. The virus structural proteins were also immunoprecipitated with anti-actin antibody, further demonstrating the strong association between the two components. Perturbation of actin filaments by cytochalasin B strongly inhibited the release of West Nile virus (approximately 10,000-fold inhibition) when compared with the untreated cells. Infectious virus particles were recovered after the removal of cytochalasin B. Further confirmation was obtained when nucleocapsid particles were found associated with disrupted actin filaments at the periphery of cytochalasin B-treated cells. Together, these results showed that actin filaments do indeed have a key role in the release of West Nile (Sarafend) virions.

Plasma membrane budding as an alternative release mechanism of the extracellular enveloped form of vaccinia virus from HeLa cells

In HeLa cells the assembly of modified vaccinia virus Ankara (MVA), an attenuated vaccinia virus (VV) strain, is blocked. No intracellular mature viruses (IMVs) are made and instead, immature viruses accumulate, some of which undergo condensation and are released from the cell. The condensed particles may undergo wrapping by membranes of the trans-Golgi network and fusion with the plasma membrane prior to their release (M. W. Carroll and B. Moss, Virology 238:198-211, 1997). The present study shows by electron microscopy (EM), however, that the dense particles made in HeLa cells are also released by a budding process at the plasma membrane. By labeling the plasma membrane with antibodies to B5R, a membrane protein of the extracellular enveloped virus, we show that budding occurs at sites that concentrate this protein. EM quantitation revealed that the cell surface around a budding profile was as strongly labeled with anti-B5R antibody as were the extracellular particles, whereas the remainder of the plasma membrane was significantly less labeled. To test whether budding was a characteristic of MVA infection, HeLa cells were infected with the replication competent VV strains Western Reserve strain (WR) and International Health Department strain-J (IHD-J) and also prepared for EM. EM analyses, surprisingly, revealed for both virus strains IMVs that evidently budded at the cell surface at sites that were significantly labeled with anti-B5R. EM also indicated that budding of MVA dense particles was more efficient than budding of IMVs from WR- or IHD-J-infected cells. This was confirmed by semipurifying [(35)S]methionine-labeled dense particles or extracellular enveloped virus (EEVs) from the culture supernatant of MVA- or IHD-J-infected HeLa cells, respectively, showing that threefold more labeled dense particles were secreted than EEVs. Finally, although the released MVA dense particles contain some DNA, they are not infectious, as assessed by plaque assays.

Comparison of virus production in chicken embryo fibroblasts infected with the WR, IHD-J and MVA strains of vaccinia virus: IHD-J is most efficient in trans-Golgi network wrapping and extracellular enveloped virus release

Modified vaccinia virus Ankara (MVA) is an attenuated strain derived from vaccinia virus (VV) Ankara that grows efficiently in primary chicken embryo fibroblasts (CEFs) and baby hamster kidney cells only. MVA produces significantly more of the enveloped forms of VV in infected CEFs compared with VV strain Copenhagen. In the present study, production of the different infectious forms of VV was compared in CEFs infected with MVA or with two well-characterized replication-competent VV strains, WR and IHD-J. In a time-course experiment, the infectivity associated with the extracellular enveloped virus (EEV), the cell-associated enveloped virus (CEV) and intracellular mature and enveloped viruses was determined. Further, the production of the different viral forms was quantified by electron microscopy (EM). The data collectively indicate that IHD-J is most efficient in producing all of the trans-Golgi network-wrapped forms and releases the highest titres of EEVs into the extracellular medium, with WR being least efficient. MVA initially replicated with faster kinetics, resulting in more intracellular virus and CEVs between 8 and 24 h post-infection (p.i.). As assessed by EM, the faster growth kinetics of MVA resulted in 3.5-fold more CEVs at the cell surface at 24 h p.i., compared with both WR and IHD-J. Accordingly, we found that despite the presence of two in-frame deletions in the A36R gene of MVA, this virus was able to make actin tails in CEFs.

Identification and characterization of three immunodominant structural proteins of fowlpox virus

Genes encoding fowlpox virus (FWPV) structural proteins have been identified mainly by sequence homology with those from vaccinia virus (VACV), but little is known about the encoded proteins. Production of monoclonal antibodies (MAbs) against Poxine and HP1-440 (Munich) clone FP9 allowed the identification of three immunodominant FWPV proteins: the 39-kDa core protein (encoded by FPV168, homologous to VACV A4L), a 30- and 35-kDa protein doublet, and an abundant 63-kDa protein. The 30- and 35-kDa proteins are nonglycosylated, antigenically related proteins present in the intracellular mature virus membrane and localizing closely with the viral factories. N-terminal sequencing identified the 35-kDa protein as encoded by FPV140 (the FWPV homolog of VACV H3L). The 63-kDa protein forms covalently linked dimers and oligomers. It remained mainly insoluble upon detergent treatment of purified virus but did not localize closely with the viral factory. N-terminal sequencing was unsuccessful, suggesting N-terminal blocking. CNBr digestion generated a peptide encoded by FPV191, predicted to encode one of two FWPV A-type inclusion (ATI) proteins. The characteristics of the 63-kDa protein were inconsistent with published observations on cowpox or VACV ATI proteins (it appears to be essential). The 63-kDa protein, however, shares characteristics with both VACV p4c virus occlusion and 14-kDa fusion proteins. Gene assignment at the poxvirus ATI locus (between VACV A24R and A28L) is complicated by sequence redundancies and variations, often due to deletions and multiple frameshift mutations. The identity of FPV191 in relation to genes at this locus is discussed.

Antibody-sensitive and antibody-resistant cell-to-cell spread by vaccinia virus: role of the A33R protein in antibody-resistant spread

The roles of vaccinia virus (VV) intracellular mature virus (IMV), intracellular enveloped virus (IEV), cell-associated enveloped virus (CEV) and extracellular enveloped virus (EEV) and their associated proteins in virus spread were investigated. The plaques made by VV mutants lacking individual IEV- or EEV-specific proteins (vDeltaA33R, vDeltaA34R, vDeltaA36R, vDeltaA56R, vDeltaB5R, vDeltaF12L and vDeltaF13L) were compared in the presence of IMV- or EEV-neutralizing antibodies (Ab). Data presented show that for long-range spread, the comet-shaped plaques of VV were caused by the unidirectional spread of EEV probably by convection currents, and for cell-to-cell spread, VV uses a combination of Ab-resistant and Ab-sensitive pathways. Actin tails play a major role in the Ab-resistant pathway, but mutants such as vDeltaA34R and vDeltaA36R that do not make actin tails still spread from cell to cell in the presence of Ab. Most strikingly, the Ab-resistant pathway was abolished when the A33R gene was deleted. This effect was not due to alterations in the efficiency of neutralization of EEV made by this mutant, nor due to a deficiency in IMV wrapping to form IEV, which was indispensable for EEV formation by vDeltaA33R and vDeltaA34R. We suggest a role for A33R in promoting Ab-resistant cell-to-cell spread of virus. The roles of the different virus forms in the VV life-cycle are discussed.

Viral transport and the cytoskeleton

In the past decade, studies into the way in which intracellular bacterial pathogens hijack and subvert their hosts have provided many important insights into regulation of the actin cytoskeleton and cell motility, in addition to increasing our understanding of the infection process. Viral pathogens, however, may ultimately unlock more cellular secrets as they are even more dependent on their hosts during their life cycle.