High-speed mass transit for poxviruses on microtubules

ROCK1/MLC2 inhibition induces decay of viral mRNA in BPXV infected cells

Rho-associated coiled-coil containing protein kinase 1 (ROCK1) intracellular cell signaling pathway regulates cell morphology, polarity, and cytoskeletal remodeling. We observed the activation of ROCK1/myosin light chain (MLC2) signaling pathway in buffalopox virus (BPXV) infected Vero cells. ROCK1 depletion by siRNA and specific small molecule chemical inhibitors (Thiazovivin and Y27632) resulted in a reduced BPXV replication, as evidenced by reductions in viral mRNA/protein synthesis, genome copy numbers and progeny virus particles. Further, we demonstrated that ROCK1 inhibition promotes deadenylation of viral mRNA (mRNA decay), mediated via inhibiting interaction with PABP [(poly(A)-binding protein] and enhancing the expression of CCR4-NOT (a multi-protein complex that plays an important role in deadenylation of mRNA). In addition, ROCK1/MLC2 mediated cell contraction, and perinuclear accumulation of p-MLC2 was shown to positively correlate with viral mRNA/protein synthesis. Finally, it was demonstrated that the long-term sequential passage (P = 50) of BPXV in the presence of Thiazovivin does not select for any drug-resistant virus variants. In conclusion, ROCK1/MLC2 cell signaling pathway facilitates BPXV replication by preventing viral mRNA decay and that the inhibitors targeting this pathway may have novel therapeutic effects against buffalopox.

The EB66® cell line as a valuable cell substrate for MVA-based vaccines production

The selection of a cell substrate is a critical step for the development and manufacturing of a viral vaccine candidate. Several parameters such as cell susceptibility and permissiveness to the viral pathogens but also performance in terms of viral antigens quality and production yields are important considerations when identifying the ideal match between a viral vaccine and cell substrate. The modified vaccinia virus Ankara (MVA) is a replication-deficient viral vector that holds great promise as a vaccine platform, however only limited cell substrates have been tested or are available for industrialization. Here we evaluate the duck embryo-derived EB66® cell line as potential cell substrate for MVA production. To this end, we used two recombinant MVA constructs and demonstrated that EB66® cells are propagating the tested MVA viruses very efficiently, while preserving viral attenuation and transgene expression for up to 20 serial passages. Furthermore we developed upstream and downstream processes that enable industrialization of the virus production. In conclusion, we showed that EB66® cells can be used as potent cell substrate for MVA-based vaccines and represent therefore an attractive alternative for vaccine production.

The cellular antiviral restriction factor tetherin does not inhibit poxviral replication

Interferon-stimulated genes fulfill innate antiviral effector functions. Among them, tetherin (THN) blocks the release of many enveloped viruses from infected cells. Vaccinia virus (VACV) encodes immune modulators interfering with antiviral host responses. Therefore, it was tempting to study a potential VACV-THN interaction. Remarkably, THN expression did not inhibit VACV release and replication. VACV infection did not diminish THN surface levels or impair its function on retroviral release. This suggests that THN is unable to restrict VACV replication.

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

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

Sequestration of free tubulin molecules by the viral protein NSP2 induces microtubule depolymerization during rotavirus infection

Microtubules, components of the cell cytoskeleton, play a central role in cellular trafficking. Here we show that rotavirus infection leads to a remodeling of the microtubule network together with the formation of tubulin granules. While most microtubules surrounding the nucleus depolymerize, others appear packed at the cell periphery. In microtubule depolymerization areas, tubulin granules are observed; they colocalize with viroplasms, viral compartments formed by interactions between rotavirus proteins NSP2 and NSP5. With purified proteins, we show that tubulin directly interacts in vitro with NSP2 but not with NSP5. The binding of NSP2 to tubulin is independent of its phosphatase activity. The comparison of three-dimensional (3-D) reconstructions of NSP2 octamers alone or associated with tubulin reveals electron densities in the positively charged grooves of NSP2 that we attribute to tubulin. Site-directed mutagenesis of NSP2 and competition assays between RNA and tubulin for NSP2 binding confirm that tubulin binds to these charged grooves of NSP2. Although the tubulin position within NSP2 grooves cannot be precisely determined, the tubulin C-terminal H12 alpha-helix could be involved in the interaction. NSP2 overexpression and rotavirus infection produce similar effects on the microtubule network. NSP2 depolymerizes microtubules and leads to tubulin granule formation. Our results demonstrate that tubulin is a viroplasm component and reveal an original mechanism. Tubulin sequestration by NSP2 induces microtubule depolymerization. This depolymerization probably reroutes the cell machinery by inhibiting trafficking and functions potentially involved in defenses to viral infections.

Inhibition of vaccinia virus replication by two small interfering RNAs targeting B1R and G7L genes and their synergistic combination with cidofovir

In view of the threat of the potential use of variola virus in a terrorist attack, considerable efforts have been performed to develop new antiviral strategies against orthopoxviruses. Here we report on the use of RNA interference, either alone or in combination with cidofovir, as an approach to inhibit orthopoxvirus replication. Two selected small interfering RNAs (siRNAs), named siB1R-2 and siG7L-1, and a previously reported siRNA, i.e., siD5R-2 (which targets the viral D5R mRNA), were evaluated for antiviral activity against vaccinia virus (VACV) by plaque reduction and virus yield assays. siB1R-2 and siG7L-1, administered before or after viral infection, reduced VACV replication by more than 90%. Also, these two siRNAs decreased monkeypox virus replication by 95% at a concentration of 1 nM. siB1R-2 and siG7L-1 were demonstrated to specifically silence their corresponding transcripts, i.e., B1R and G7L mRNAs, without induction of a beta interferon response. Strong synergistic effects were observed when siB1R-2, siG7L-1, or siD5R-2 was combined with cidofovir. In addition, the antiviral activities of these three siRNAs were evaluated against VACV resistant to cidofovir and other acyclic nucleoside phosphonates. siG7L-1 and siD5R-2 remained active against four of five VACV mutants, while siB1R-2 showed activity against only one of the mutants. Our results showed that siRNAs are potent inhibitory agents in vitro, not only against wild-type VACV but also against several cidofovir-resistant VACV. Furthermore, we showed that a combined therapy using siRNA and cidofovir may be useful in the treatment of poxvirus infections.

Expression of the A56 and K2 proteins is sufficient to inhibit vaccinia virus entry and cell fusion

Many animal viruses induce cells to fuse and form syncytia. For vaccinia virus, this phenomenon is associated with mutations affecting the A56 and K2 proteins, which form a multimer (A56/K2) on the surface of infected cells. Recent evidence that A56/K2 interacts with the entry/fusion complex (EFC) and that the EFC is necessary for syncytium formation furnishes a strong connection between virus entry and cell fusion. Among the important remaining questions are whether A56/K2 can prevent virus entry as well as cell-cell fusion and whether these two viral proteins are sufficient as well as necessary for this. To answer these questions, we transiently and stably expressed A56 and K2 in uninfected cells. Uninfected cells expressing A56 and K2 exhibited resistance to fusing with A56 mutant virus-infected cells, whereas expression of A56 or K2 alone induced little or no resistance, which fits with the need for both proteins to bind the EFC. Furthermore, transient or stable expression of A56/K2 interfered with virus entry and replication as determined by inhibition of early expression of a luciferase reporter gene, virus production, and plaque formation. The specificity of this effect was demonstrated by restoring entry after enzymatically removing a chimeric glycophosphatidylinositol-anchored A56/K2 or by binding a monoclonal antibody to A56. Importantly, the antibody disrupted the interaction between A56/K2 and the EFC without disrupting the A56-K2 interaction itself. Thus, we have shown that A56/K2 is sufficient to prevent virus entry and fusion as well as formation of syncytia through interaction with the EFC.

Vaccinia virus l1 protein is required for cell entry and membrane fusion

Genetic and biochemical studies have provided evidence for an entry/fusion complex (EFC) comprised of at least eight viral proteins (A16, A21, A28, G3, G9, H2, J5, and L5) that together with an associated protein (F9) participates in entry of vaccinia virus (VACV) into cells. The genes encoding these proteins are conserved in all poxviruses, are expressed late in infection, and are components of the mature virion membrane but are not required for viral morphogenesis. In addition, all but one component has intramolecular disulfides that are formed by the poxvirus cytoplasmic redox system. The L1 protein has each of the characteristics enumerated above except that it has been reported to be essential for virus assembly. To further investigate the role of L1, we constructed a recombinant VACV (vL1Ri) that inducibly expresses L1. In the absence of inducer, L1 synthesis was repressed and vL1Ri was unable to form plaques or produce infectious progeny. Unexpectedly, assembly and morphogenesis appeared normal and the noninfectious virus particles were indistinguishable from wild-type VACV as determined by transmission electron microscopy and analysis of the component polypeptides. Notably, the L1-deficient virions were able to attach to cells but the cores failed to penetrate into the cytoplasm. In addition, cells infected with vL1Ri in the absence of inducer did not form syncytia following brief low-pH treatment even though extracellular virus was produced. Coimmunoprecipitation experiments demonstrated that L1 interacted with the EFC and indirectly with F9, suggesting that L1 is an additional component of the viral entry apparatus.

The vaccinia virus F11L gene product facilitates cell detachment and promotes migration

We previously showed that infection with vaccinia virus (VV) induces cell motility, characterized by contractility and directed migration. Motility is temporally regulated because cells are motile immediately after infection, whereas late in infection motility ceases and cells resettle. Motility and its cessation are accompanied by temporal rearrangements of both the microtubule and the actin networks. Because the F11L gene has previously been implicated in VV-induced migration, we now explore the role of F11L in contractility, migration, the cessation of motility and the cytoskeletal rearrangements. By live cell imaging using a VV that lacks an intact F11L gene, we show that F11L facilitates cell detachment and is required for migration but not for contractility. By light microscopy, F11L expression induces a remodeling of the actin, but not the microtubule, network. The lack of migration correlates with smaller plaques, indicating that this process facilitates cell-to-cell spreading of VV. Late in infection, when motility ceases, cells re-establish cell-to-cell contacts in an F11L-independent manner. We finally show that VV-induced motility and its cessation correlate with a temporal regulation of the guanosine triphosphatase RhoA as well as the expression levels of F11L during the infectious cycle.

Resistance of a vaccinia virus A34R deletion mutant to spontaneous rupture of the outer membrane of progeny virions on the surface of infected cells

The extracellular form of vaccinia virus is referred to as an enveloped virion (EV) because it contains an additional lipoprotein membrane surrounding the infectious mature virion (MV) that must be discarded prior to cell fusion and entry. Most EVs adhere to the surface of the parent cell and mediate spread of the infection to adjacent cells. Here we show that some attached EVs have ruptured envelopes. Rupture was detected by fluorescence microscopy of unfixed and unpermeabilized cells using antibodies to the F13 and L1 proteins, which line the inner side of the EV membrane and the outer side of the MV membrane, respectively. The presence of ruptured EV membranes was confirmed by immunogold transmission electron microscopy. EVs with broken membranes were present on several cell lines examined including one deficient in glycosaminoglycans, which are thought to play a role in breakage of the EV membrane prior to fusion of the MV. No correlation was found between EVs with ruptured membranes and actin tail formation. Studies with several mutant viruses indicated that EV membranes lacking the A34 protein were unbroken. This result was consistent with other properties of A34R deletion mutants including resistance of the EV membrane to polyanions, small plaque formation and low infectivity that can be increased by disruption of the EV membrane by freezing and thawing.

Vaccinia virus entry into cells via a low-pH-dependent endosomal pathway

Previous studies established that vaccinia virus could enter cells by fusion with the plasma membrane at neutral pH. However, low pH triggers fusion of vaccinia virus-infected cells, a hallmark of viruses that enter by the endosomal route. Here, we demonstrate that entry of mature vaccinia virions is accelerated by brief low-pH treatment and severely reduced by inhibitors of endosomal acidification, providing evidence for a predominant low-pH-dependent endosomal pathway. Entry of vaccinia virus cores into the cytoplasm, measured by expression of firefly luciferase, was increased more than 10-fold by exposure to a pH of 4.0 to 5.5. Furthermore, the inhibitors of endosomal acidification bafilomycin A1, concanamycin A, and monensin each lowered virus entry by more than 70%. This reduction was largely overcome by low-pH-induced entry through the plasma membrane, confirming the specificities of the drugs. Entry of vaccinia virus cores with or without brief low-pH treatment was visualized by electron microscopy of thin sections of immunogold-stained cells. Although some virus particles fused with the plasma membrane at neutral pH, 30 times more fusions and a greater number of cytoplasmic cores were seen within minutes after low-pH treatment. Without low-pH exposure, the number of released cores lagged behind the number of virions in vesicles until 30 min posttreatment, when they became approximately equal, perhaps reflecting the time of endosome acidification and virus fusion. The choice of two distinct pathways may contribute to the ability of vaccinia virus to enter a wide range of cells.

Vaccinia-virus-induced cellular contractility facilitates the subcellular localization of the viral replication sites

Poxviruses, such as vaccinia virus (VV), replicate their DNA in endoplasmic-reticulum-enclosed cytoplasmic sites. Here, we compare the dynamics of the VV replication sites with those of the attenuated strain, modified VV Ankara (MVA). By live-cell imaging, small, early replication sites of both viruses undergo motility typical of microtubule (MT)-motor-mediated movement. Over time, growing replication sites of VV collect around the nucleus in a MT-dependent fashion, whereas those of MVA remain mostly scattered in the cytoplasm. Surprisingly, blocking the dynein function does not impair the perinuclear accumulation of large VV replication sites. Live-cell imaging demonstrates that in contrast to small replication sites, large sites do not display MT-motor-mediated motility. Instead, VV infection induces cellular contractility that facilitates the collection of growing replication sites around the nucleus. In a subset of cells (30-40%), this VV-induced contractility is alternated by phases of directed cell migration, suggesting that the two processes may be linked. The MVA-infected cells do not display contractility or cell migration, supporting the idea that these cellular activities facilitate the efficient accumulation of the VV replication sites around the nucleus. We propose that the recently described cytoskeletal rearrangements induced by VV are a prerequisite for the observed cell contractility and migration activities that apparently contribute to the organization of the complex cytoplasmic life cycle of VV.

Ultrastructural study of myxoma virus morphogenesis

Poxviruses are among the largest and most complex viruses known. Vaccinia virus, the prototype of the family Poxviridae, has been studied much more than myxoma virus. The aim of this work was to have a better knowledge about myxoma virus morphogenesis. The characterization of the main stages of MV morphogenesis was achieved by ultrastructural and immunological analysis. Specific antibodies were raised against M022L and M071L, two envelope proteins of extracellular enveloped virus and intracellular mature virus, respectively. The main stages of assembly were similar to those seen with other poxviruses, and the duration of the whole replication cycle was estimated to be around 16 h, longer than what was described for vaccinia virus. Morphological changes of infected cells were associated with the development of long cellular projections and enlarged microvilli. Intracellular enveloped viruses are associated with the cytoskeleton to move through the cell. Unlike earlier studies, as many cell-associated enveloped viruses as intracellular enveloped viruses were observed in relation with specialized microvilli, although these structures were rarely noticed. Finally, an unusual spreading process was observed, which uses cytoplasmic corridors.

Vaccinia virus-induced microtubule-dependent cellular rearrangements

Although infection with vaccinia virus (VV) is known to affect the cytoskeleton, it is not known how this affects the cellular architecture or whether the attenuated modified VV ankara (MVA) behaves similar to wild-type VV (wtVV). In the present study, we therefore compared effects of wtVV and MVA infection on the cellular architecture. WtVV-infection induces cell rounding early in infection, which coincides with the retraction of microtubules (MTs) and intermediate filaments from the cellular periphery, whereas mitochondria and late endosomes cluster around the nucleus. Nocodazole treatment demonstrates that cell rounding and organelle clustering require intact MTs. At the onset of virus assembly late in infection, cells reflatten, a process that coincides with the regrowth of MTs into the cellular periphery. We find that the actin network undergoes several rearrangements that occur sequentially in time and that closely follow the cell-shape changes. Unexpectedly, these actin changes are blocked or reversed upon nocodazole treatment, indicating that intact MTs are also responsible for the wtVV-induced actin rearrangements. Finally, MVA infection does not induce any of these cellular changes. Because this virus lacks a substantial number of VV genes, MVA opens up a system to search for the molecules involved in wtVV-induced cellular changes; in particular, those that may regulate actin/MT interactions.

Poxvirus entry and membrane fusion

The study of poxvirus entry and membrane fusion has been invigorated by new biochemical and microscopic findings that lead to the following conclusions: (1) the surface of the mature virion (MV), whether isolated from an infected cell or by disruption of the membrane wrapper of an extracellular virion, is comprised of a single lipid membrane embedded with non-glycosylated viral proteins; (2) the MV membrane fuses with the cell membrane, allowing the core to enter the cytoplasm and initiate gene expression; (3) fusion is mediated by a newly recognized group of viral protein components of the MV membrane, which are conserved in all members of the poxvirus family; (4) the latter MV entry/fusion proteins are required for cell to cell spread necessitating the disruption of the membrane wrapper of extracellular virions prior to fusion; and furthermore (5) the same group of MV entry/fusion proteins are required for virus-induced cell-cell fusion. Future research priorities include delineation of the roles of individual entry/fusion proteins and identification of cell receptors.

Poxvirus multiprotein entry-fusion complex

Poxviruses have evolved elaborate mechanisms for cell entry, assembly, and exocytosis. Recently, four vaccinia virus membrane proteins, namely A21, A28, H2 and L5, were reported to be necessary for cell entry and virus-induced cell-cell fusion but not for virion morphogenesis or attachment of virus particles to cells. Using immunoaffinity purification followed by mass spectrometry, we now show that these four proteins as well as four additional previously uncharacterized putative membrane proteins (A16, G3, G9, and J5) form a stable complex. These proteins fall into two groups: A21, A28, G3, H2, and L5 have an N-terminal transmembrane domain, 0-2 intramolecular disulfide bonds, and no sequence similarity, whereas A16, G9, and J5 have a C-terminal transmembrane domain and 4-10 predicted disulfide bonds and are homologous. Studies with conditional-lethal null mutants indicated that the viral membrane was crucial for assembly of the complex and that the absence of individual polypeptide components profoundly decreased complex formation or stability, suggesting a complicated interaction network. Analysis of purified virions, however, demonstrated that the polypeptides of the complex trafficked independently to the viral membrane even under conditions in which the complex itself could not be isolated. All eight proteins comprising the entry-fusion complex are conserved in all poxviruses, suggesting that they have nonredundant functions and that the basic entry mechanism evolved before the division between vertebrate and invertebrate poxvirus species.

Cytoplasmic organization of POXvirus DNA replication

Poxviruses, a family of large DNA viruses, are unique among DNA viruses, because they carry out DNA replication in the cytoplasm rather than the nucleus. This process does not occur randomly, but instead, these viruses create cytoplasmic 'mini-nuclei', distinct sites that are surrounded by membranes derived from the rough endoplasmic reticulum (ER) that support viral replication. This review summarizes how distinct steps preceding cytoplasmic DNA replication, as well as replication itself, operate in the host cell. The collective data point to an important role for both the rough ER and the microtubules and indicate that these cellular structures help to co-ordinate the virus life cycle to ensure that individual steps occur at the right time and place. In a broader sense, they emphasize how viruses have evolved sophisticated ways to use host cells to optimize their life cycles to ensure efficient production of infectious progeny.

Cytoskeletal rearrangements and cell extensions induced by the US3 kinase of an alphaherpesvirus are associated with enhanced spread

The US3 protein is a viral kinase that is conserved among the Alphaherpesvirinae. Here, we show that US3 of the swine alphaherpesvirus pseudorabies virus causes dramatic alterations in the cytoskeleton, resulting in the formation of long actin- and microtubule-containing cell projections in infected and transfected cells. Analysis with a GFP-labeled virus showed that multiple virus particles move inside the projections toward the tip. GFP-labeled virus could also be found in the cytoplasm of neighboring cells that were in contact with the projections. In addition, projection formation could be inhibited by using the actin-stabilizing drug jasplakinolide and could be induced by using the Rho kinase inhibitor Y27632. Analyzing the effect of these drugs on intercellular virus spread indicated that the observed US3-induced alterations in the host cytoskeleton are associated with enhanced intercellular virus spread, thereby suggesting a previously undescribed aspect of alphaherpesvirus spread.

Association of ebola virus matrix protein VP40 with microtubules

Viruses exploit a variety of cellular components to complete their life cycles, and it has become increasingly clear that use of host cell microtubules is a vital part of the infection process for many viruses. A variety of viral proteins have been identified that interact with microtubules, either directly or via a microtubule-associated motor protein. Here, we report that Ebola virus associates with microtubules via the matrix protein VP40. When transfected into mammalian cells, a fraction of VP40 colocalized with microtubule bundles and VP40 coimmunoprecipitated with tubulin. The degree of colocalization and microtubule bundling in cells was markedly intensified by truncation of the C terminus to a length of 317 amino acids. Further truncation to 308 or fewer amino acids abolished the association with microtubules. Both the full-length and the 317-amino-acid truncation mutant stabilized microtubules against depolymerization with nocodazole. Direct physical interaction between purified VP40 and tubulin proteins was demonstrated in vitro. A region of moderate homology to the tubulin binding motif of the microtubule-associated protein MAP2 was identified in VP40. Deleting this region resulted in loss of microtubule stabilization against drug-induced depolymerization. The presence of VP40-associated microtubules in cells continuously treated with nocodazole suggested that VP40 promotes tubulin polymerization. Using an in vitro polymerization assay, we demonstrated that VP40 directly enhances tubulin polymerization without any cellular mediators. These results suggest that microtubules may play an important role in the Ebola virus life cycle and potentially provide a novel target for therapeutic intervention against this highly pathogenic virus.

Vaccinia virus H2 protein is an essential component of a complex involved in virus entry and cell-cell fusion

The vaccinia virus H2R gene (VACWR 100) is conserved in all sequenced members of the poxvirus family and encodes a protein with a predicted transmembrane domain and four invariant cysteines. A recombinant vaccinia virus, in which expression of the H2 protein is stringently regulated, was unable to replicate without inducer. However, under nonpermissive conditions, all stages of virus morphogenesis appeared normal and extracellular virions were detected at the tips of actin tails. Nevertheless, virus did not spread to neighboring cells nor did syncytia form after low-pH treatment. Purified -H2 and +H2 virions from cells infected in the absence or presence of inducer, respectively, were indistinguishable in microscopic appearance and contained the same complement of major proteins, though only +H2 virions were infectious. The -H2 virions bound to cells, but their cores did not penetrate into the cytoplasm. In addition, exogenously added -H2 virions were unable to mediate the formation of syncytia after low-pH treatment. In contrast, virions lacking the A27 (p14) protein, which was previously considered to have an essential role in fusion, penetrated cells and induced extensive syncytia. The properties of H2, however, are very similar to those recently reported for the A28 protein. Moreover, coimmunoprecipitation experiments indicated an interaction between H2 and A28. Therefore, H2 and A28 are the only proteins presently known to be specifically required for vaccinia virus entry and are likely components of a fusion complex.

The evolving field of biodefence: therapeutic developments and diagnostics

Bioweapons are a clear threat to both military and civilian populations. Here, the latest advances in the pursuit of inhibitors against biothreat threat toxins, current therapeutic strategies for treating biodefence related pathogens, and strategies for improving detection and exposure survivability are covered.

There are numerous lead therapeutics that have emerged from drug discovery efforts. However, many of these are toxic and/or fail to possess conventional drug-like properties. One clear advantage of small (non-peptidic) molecules is that they possess scaffolds that are inherently more likely to evolve into real therapeutics.

One of the major obstacles impeding the translation of these lead therapeutics into viable drugs is the lack of involvement of the pharmaceutical industry, which has been discovering leads and translating them into drugs for decades. The expertise of the pharmaceutical industry therefore needs to be more effectively engaged in developing drugs against biothreat agents.

New methods for rapidly detecting and diagnosing biothreat agents are also in development. The detection and diagnosis of biothreats is inherently linked with treatment. The means for detecting the release of bioweapons are being deployed, and new technologies are shortening the timeframe between initial sample collection and conclusive agent determination. However, the organization of this process is imperfect.

At present, a unifying entity that orchestrates the biodefence response is clearly needed to reduce the time-to-drug process and redundancies in drug development efforts. Such a central entity could formulate and implement plans to coordinate all participants, including academic institutions, government agencies and the private sector. This could accelerate the development of countermeasures against high probability biothreat agents.

The threat of bioterrorism and the potential use of biological weapons against both military and civilian populations has become a major concern for governments around the world. For example, in 2001 anthrax-tainted letters resulted in several deaths, caused widespread public panic and exerted a heavy economic toll. If such a small-scale act of bioterrorism could have such a huge impact, then the effects of a large-scale attack would be catastrophic. This review covers recent progress in developing therapeutic countermeasures against, and diagnostics for, such agents.

Characterization of West Nile viral replication and maturation in peripheral neurons in culture

The North American West Nile virus (WNV), New York 1999 strain, appears to be highly neurotropic, and its neuroinvasiveness is an important aspect of human disease. The authors have developed an in vitro model to study WNV replication and protein processing in neurons. They compared WNV infection of the dorsal root ganglion (DRG) neurons (sensory neurons) and PC-12 cells (sympathetic neurons) to WNV infection of the mosquito cell line, C6/36, and Vero cells. WNV infection of both neuronal cell types and C6/36 cells was not cytopathic up to 30 days post infection, and continual viral shedding was observed during this period. However, WNV infection of Vero cells was lytic. Interestingly, WNV infection of neurons was not efficient, requiring a high multiplicity of infection of > or = 10. Indirect immunofluorescence assays using normal and confocal microscopy with flavivirus-reactive antibodies and WNV-infected neurons demonstrated viral antigen mostly associated with the plasma membrane and in the neurite processes. Treatment of WNV-infected C6/36, PC-12, or DRG cells with brefeldin A (BFA; a trans-Golgi inhibitor) or nocadazole (a beta-tubulin inhibitor) had little effect on viral maturation and secretion. Treatment of WNV-infected Vero cells with BFA resulted in a 1000-fold decrease in viral titer, but nocodazole had no effect. Our studies suggest that even though PC-12 and DRG neurons are mammalian cells, viral protein processing and maturation in these cells more closely resembles replication in C6/36 insect cells than in mammalian Vero cells.

Protective immunity to vaccinia virus induced by vaccination with multiple recombinant outer membrane proteins of intracellular and extracellular virions

Infectious intracellular and extracellular forms of vaccinia virus have different outer membrane proteins, presenting multiple targets to the immune system. We investigated the immunogenicity of soluble forms of L1, an outer membrane protein of the intracellular mature virus, and of A33 and B5, outer membrane proteins of the extracellular enveloped virus. The recombinant proteins, in 10-microg amounts mixed with a Ribi- or saponin-type adjuvant, were administered subcutaneously to mice. Antibody titers to each protein rose sharply after the first and second boosts, reaching levels that surpassed those induced by percutaneous immunization with live vaccinia virus. Immunoglobulin G1 (IgG1) antibody predominated after the protein immunizations, indicative of a T-helper cell type 2 response, whereas live vaccinia virus induced mainly IgG2a, indicative of a T-helper cell type 1 response. Mice immunized with any one of the recombinant proteins survived an intranasal challenge with 5 times the 50% lethal dose of the pathogenic WR strain of vaccinia virus. Measurements of weight loss indicated that the A33 immunization most effectively prevented disease. The superiority of protein combinations was demonstrated when the challenge virus dose was increased 20-fold. The best protection was obtained with a vaccine made by combining recombinant proteins of the outer membranes of intracellular and extracellular virus. Indeed, mice immunized with A33 plus B5 plus L1 or with A33 plus L1 were better protected than mice immunized with live vaccinia virus. Three immunizations with the three-protein combination were necessary and sufficient for complete protection. These studies suggest the feasibility of a multiprotein smallpox vaccine.

Role of the cytoskeleton during leukocyte responses

The cytoskeleton is a cellular network of structural, adaptor and signalling molecules that regulates most cellular functions that are related to the immune response, including migration, extravasation, antien recognition, activation and phagocytosis by different subsets of leukocytes. Recently, a large number of regulatory elements and structural constituents of the leukocyte cytoskeleton have been identified. In this review, we discuss the composition and regulation of the different cytoskeletal elements and their role in immune responses.

Macromolecular Transport and Signaling Through Plasmodesmata

Plasmodesmata (Pd) are channels in the plant cell wall that in conjunction with associated phloem form an intercellular communication network that supports the cell-to-cell and long-distance trafficking of a wide spectrum of endogenous proteins and ribonucleoprotein complexes. The trafficking of such macromolecules is of importance in the orchestration of non-cell autonomous developmental and physiological processes. Plant viruses encode movement proteins (MPs) that subvert this communication network to facilitate the spread of infection. These viral proteins thus represent excellent experimental keys for exploring the mechanisms involved in intercellular trafficking and communication via Pd.

Evidence against an essential role of COPII-mediated cargo transport to the endoplasmic reticulum-Golgi intermediate compartment in the formation of the primary membrane of vaccinia virus

Vaccinia virus assembles two distinct lipoprotein membranes. The primary membrane contains nonglycosylated proteins, appears as crescents in the cytoplasm, and delimits immature and mature intracellular virions. The secondary or wrapping membrane contains glycoproteins, is derived from virus-modified trans-Golgi or endosomal cisternae, forms a loose coat around some intracellular mature virions, and becomes the envelope of extracellular virions. Although the mode of formation of the wrapping membrane is partially understood, we know less about the primary membrane. Recent reports posit that the primary membrane originates from the endoplasmic reticulum-Golgi intermediate compartment (ERGIC). According to this model, viral primary membrane proteins are cotranslationally inserted into the ER and accumulate in the ERGIC. To test the ERGIC model, we employed Sar1(H79G), a dominant negative form of the Sar1 protein, which is an essential component of coatomer protein II (COPII)-mediated cargo transport from the ER to the ERGIC and other post-ER compartments. Overexpression of Sar1(H79G) by transfection or by a novel recombinant vaccinia virus with an inducible Sar1(H79G) gene resulted in retention of ERGIC 53 in the ER but did not interfere with localization of viral primary membrane proteins in factory regions or with formation of viral crescent membranes and infectious intracellular mature virions. Wrapping of intracellular mature virions and formation of extracellular virions did not occur, however, because some proteins that are essential for the secondary membrane were retained in the ER as a consequence of Sar1(H79G) overexpression. Our data argue against an essential role of COPII-mediated cargo transport and the ERGIC in the formation of the viral primary membrane. Instead, viral membranes may be derived directly from the ER or by a novel mechanism.

Differences in virus-induced cell morphology and in virus maturation between MVA and other strains (WR, Ankara, and NYCBH) of vaccinia virus in infected human cells

Live recombinants based on attenuated modified vaccinia virus Ankara (MVA) are potential vaccine candidates against a broad spectrum of diseases and tumors. To better understand the efficacy of MVA as a human vaccine, we analyzed by confocal and electron microscopy approaches MVA-induced morphological changes and morphogenetic stages during infection of human HeLa cells in comparison to other strains of vaccinia virus (VV): the wild-type Western Reserve (WR), Ankara, and the New York City Board of Health (NYCBH) strains. Confocal microscopy studies revealed that MVA infection alters the cytoskeleton producing elongated cells (bipolar), which do not form the characteristic actin tails. Few virions are detected in the projections connecting neighboring cells. In contrast, cells infected with the WR, Ankara, and NYCBH strains exhibit a stellated (multipolar) or rounded morphology with actin tails. A detailed transmission electron microscopy analysis of HeLa cells infected with MVA showed important differences in fine ultrastructure and amounts of the viral intermediates compared to cells infected with the other VV strains. In HeLa cells infected with MVA, the most abundant viral forms are intracellular immature virus, with few intermediates reaching the intracellular mature virus (IMV) form, at various stages of maturation, which exhibit a more rounded shape than IMVs from cells infected with the other VV strains. The "IMVs" from MVA-infected cells have an abnormal internal structure ("atypical" viruses) with potential alterations in the core-envelope interactions and are unable to significantly acquire the additional double envelope to render intracellular envelope virus. The presence of potential cell-associated envelope virus is very scarce. Our findings revealed that MVA in human cells promotes characteristic morphological changes to the cells and is able to reach the IMV stage, but these virions were not structurally normal and the subsequent steps in the morphogenetic pathway are blocked.

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.

Intracellular trafficking of a palmitoylated membrane-associated protein component of enveloped vaccinia virus

The F13L protein of vaccinia virus, an essential and abundant palmitoylated peripheral membrane component of intra- and extracellular enveloped virions, associates with Golgi, endosomal, and plasma membranes in the presence or absence of other viral proteins. In the present study, the trafficking of a fully functional F13L-green fluorescent protein (GFP) chimera in transfected and productively infected cells was analyzed using specific markers and inhibitors. We found that Sar1(H79G), a trans-dominant-negative protein inhibitor of cargo transport from the endoplasmic reticulum, had no apparent effect on the intracellular distribution of F13L-GFP, suggesting that the initial membrane localization occurs at a downstream compartment of the secretory pathway. Recycling of F13L-GFP from the plasma membrane was demonstrated by partial colocalization with FM4-64, a fluorescent membrane marker of endocytosis. Punctate F13L-GFP fluorescence overlapped with clathrin and Texas red-conjugated transferrin, suggesting that endocytosis occurred via clathrin-coated pits. The inhibitory effects of chlorpromazine and trans-dominant-negative forms of dynamin and Eps15 protein on the recycling of F13L-GFP provided further evidence for clathrin-mediated endocytosis. In addition, the F13L protein was specifically coimmunoprecipitated with alpha-adaptin, a component of the AP-2 complex that interacts with Eps15. Nocodazole and wortmannin perturbed the intracellular trafficking of F13L-GFP, consistent with its entry into late and early endosomes through the secretory and endocytic pathways, respectively. The recycling pathway described here provides a mechanism for the reutilization of the F13L protein following its deposition in the plasma membrane during the exocytosis of enveloped virions.

Investigation of structural and functional motifs within the vaccinia virus A14 phosphoprotein, an essential component of the virion membrane

We have previously reported the construction and characterization of an inducible recombinant virus in which expression of the vaccinia virus membrane protein A14 is experimentally regulated using the tetracycline operator-repressor system. Repression of A14, which results in a 1,000-fold reduction in viral yield, leads to an early block in viral morphogenesis characterized by the accumulation of large virosomes, empty "crescents" that fail to contact these virosomes, and, most strikingly, large numbers of aberrant 25-nm vesicles. Here we report the establishment of a transient-complementation system for the structure-function analysis of A14. We have constructed numerous mutant alleles of A14 designed to identify and test the importance of key structural and sequence motifs within A14, including sites of posttranslational modification, such as glycosylation, phosphorylation, and dimerization. From these studies we have determined that robust complementation ability requires an intact N terminus and two regions flanking the first membrane-spanning domain of A14. We show that A14 is modified by N-linked glycosylation both in vitro and in vivo. However, only a minority of A14 molecules are glycosylated in vivo and these are not encapsidated. In this report we also identify the sole phosphorylated serine residue of A14 as lying within the NHS(85) motif that undergoes glycosylation. Additionally, we show that the Cys(71) residue is required for intermolecular disulfide bond formation and describe the properties of a virus expressing an allele of A14 that cannot form disulfide-linked dimers.

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.

Break ins and break outs: viral interactions with the cytoskeleton of Mammalian cells

The host cytoskeleton plays important roles in the entry, replication, and egress of viruses. An assortment of viruses hijack cellular motor proteins to move on microtubules toward the cell interior during the entry process; others reverse this transport during egress to move assembling virus particles toward the plasma membrane. Polymerization of actin filaments is sometimes used to propel viruses from cell to cell, while many viruses induce the destruction of select cytoskeletal filaments apparently to effect efficient egress. Indeed, the tactics used by any given virus to achieve its infectious life cycle are certain to involve multiple cytoskeletal interactions. Understanding these interactions, and their orchestration during viral infections, is providing unexpected insights into basic virology, viral pathogenesis, and the biology of the cytoskeleton.

Microbial pathogenesis and cytoskeletal function

Pathogenic microbes subvert normal host-cell processes to create a specialized niche, which enhances their survival. A common and recurring target of pathogens is the host cell's cytoskeleton, which is utilized by these microbes for purposes that include attachment, entry into cells, movement within and between cells, vacuole formation and remodelling, and avoidance of phagocytosis. Our increased understanding of these processes in recent years has not only contributed to a greater comprehension of the molecular causes of infectious diseases, but has also revealed fundamental insights into normal functions of the cytoskeleton. From the use of bacterial toxins to investigate Rho family GTPases to in vitro studies of actin polymerization using Listeria and Shigella, the study of pathogenesis has provided important tools to probe cytoskeletal function.

Interaction of L2 with beta-actin directs intracellular transport of papillomavirus and infection

Viruses that replicate in the nucleus, including the primary causative agent of cervical cancer, human papillomavirus type 16 (HPV16), must first cross the cytoplasm. We compared the uptake of HPV16 virus-like particles (VLPs) either with or without the minor capsid protein L2. Whereas VLPs containing only the major capsid protein L1 were diffusely distributed within the cytoplasm even 6 h post-infection, VLPs comprising both L1 and L2 exhibited a radial distribution in the cytoplasm and accumulated in the perinuclear region of BPHE-1 cells within 2 h. L2 of HPV16 or bovine papillomavirus was shown to bind to a 43-kDa cellular protein that was subsequently identified as beta-actin by matrix-assisted laser desorption ionization time-of-flight analysis. A conserved domain comprising residues 25-45 of HPV16 L2 was sufficient for interaction with beta-actin. HPV16 L2 residues 25-45 fused to green fluorescent protein, but not green fluorescent protein alone, colocalized with actin and caused cell retraction and disruption of the microfilament network. Finally, wild-type L2, but not L2 with residues 25-45 deleted, facilitated HPV16 pseudovirion infection. Thus, binding of beta-actin by L2 residues 25-45 facilitates transport of HPV16 across the cytoplasm during infection, and blockade of this novel interaction may be useful for prophylaxis.

Identification of second-site mutations that enhance release and spread of vaccinia virus

The spread of most strains of vaccinia virus in cell monolayers occurs predominantly via extracellular enveloped virions that adhere to the tips of actin-containing microvilli and to a lesser extent via diffusion of released virions. The mechanism by which virions adhere to the cell surface is unknown, although several viral proteins may be involved. The present investigation was initiated with the following premise: spontaneous mutations that increase virus release will be naturally selected by propagating a virus unable to spread by means of actin tails. Starting with an A36R deletion mutant that forms small, round plaques, five independent virus clones with enhanced spread due to the formation of comet or satellite plaques were isolated. The viral membrane glycoprotein genes of the isolates were sequenced; four had mutations causing C-terminal truncations of the A33R protein, and one had a serine replacing proline 189 of the B5R protein. The comet-forming phenotype was specifically reproduced or reversed by homologous recombination using DNA containing the mutated or natural sequence, respectively. Considerably more extracellular enveloped virus was released into the medium by the second-site mutants than by the parental A36R deletion mutant, explaining their selection in tissue culture as well as their comet-forming phenotype. The data suggest that the B5R protein and the C-terminal region of the A33R protein are involved in adherence of cell-associated enveloped virions to cells. In spite of their selective advantage in cultured cells, the second-site mutants were not detectably more virulent than the A36R deletion mutant when administered to mice by the intranasal route.

The role of the cytoskeleton in the life cycle of viruses and intracellular bacteria: tracks, motors, and polymerization machines

Recent advances in microbiology implicate the cytoskeleton in the life cycle of some pathogens, such as intracellular bacteria, Rickettsia and viruses. The cellular cytoskeleton provides the basis for intracellular movements such as those that transport the pathogen to and from the cell surface to the nuclear region, or those that produce cortical protrusions that project the pathogen outwards from the cell surface towards an adjacent cell. Transport in both directions within the neuron is required for pathogens such as the herpesviruses to travel to and from the nucleus and perinuclear region where replication takes place. This trafficking is likely to depend on cellular motors moving on a combination of microtubule and actin filament tracks. Recently, Bearer et al. reconstituted retrograde transport of herpes simplex virus (HSV) in the giant axon of the squid. These studies identified the tegument proteins as the viral proteins most likely to recruit retrograde motors for the transport of HSV to the neuronal nucleus. Similar microtubule-based intracellular movements are part of the biological behavior of vaccinia, a poxvirus, and of adenovirus. Pathogen-induced surface projections and motility within the cortical cytoplasm also play a role in the life cycle of intracellular pathogens. Such motility is driven by pathogen-mediated actin polymerization. Virulence depends on this actin-based motility, since virulence is reduced in Listeria ActA mutants that lack the ability to recruit Arp2/3 and polymerize actin and in vaccinia virus mutants that cannot stimulate actin polymerization. Inhibition of intracellular movements provides a potential strategy to limit pathogenicity. The host cell motors and tracks, as well as the pathogen factors that interact with them, are potential targets for novel antimicrobial therapy.

The block in assembly of modified vaccinia virus Ankara in HeLa cells reveals new insights into vaccinia virus morphogenesis

It has previously been shown that upon infection of HeLa cells with modified vaccinia virus Ankara (MVA), assembly is blocked at a late stage of infection and immature virions (IVs) accumulate (G. Sutter and B. Moss, Proc. Natl. Acad. Sci. USA 89:10847-10851, 1992). In the present study the morphogenesis of MVA in HeLa cells was studied in more detail and compared to that under two conditions that permit the production of infectious particles: infection of HeLa cells with the WR strain of vaccinia virus (VV) and infection of BHK cells with MVA. Using several quantitative and qualitative assays, we show that early in infection, MVA in HeLa cells behaves in a manner identical to that under the permissive conditions. By immunofluorescence microscopy (IF) at late times of infection, the labelings for an abundant membrane protein of the intracellular mature virus, p16/A14L, and the viral DNA colocalize under permissive conditions, whereas in HeLa cells infected with MVA these two structures do not colocalize to the same extent. In both permissive and nonpermissive infection, p16-labeled IVs first appear at 5 h postinfection. In HeLa cells infected with MVA, IVs accumulated predominantly outside the DNA regions, whereas under permissive conditions they were associated with the viral DNA. At 4 h 30 min, the earliest time at which p16 is detected, the p16 labeling was found predominantly in a small number of distinct puncta by IF, which were distinct from the sites of DNA in both permissive and nonpermissive infection. By electron microscopy, no crescents or IVs were found at this time, and the p16-labeled structures were found to consist of membrane-rich vesicles that were in continuity with the cellular endoplasmic reticulum. Over the next 30 min of infection, a large number of p16-labeled crescents and IVs appeared abruptly under both permissive and nonpermissive conditions. Under permissive conditions, these IVs were in close association with the sites of DNA, and a significant amount of these IVs engulfed the viral DNA. In contrast, under nonpermissive conditions, the IVs and DNA were mostly in separate locations and relatively few IVs acquired DNA. Our data show that in HeLa cells MVA forms normal DNA replication sites and normal viral precursor membranes but the transport between these two structures is inhibited.