The Mammalian Cell Cycle Regulates Parvovirus Nuclear Capsid Assembly

It is unknown whether the mammalian cell cycle could impact the assembly of viruses maturing in the nucleus. We addressed this question using MVM, a reference member of the icosahedral ssDNA nuclear parvoviruses, which requires cell proliferation to infect by mechanisms partly understood. Constitutively expressed MVM capsid subunits (VPs) accumulated in the cytoplasm of mouse and human fibroblasts synchronized at G0, G1, and G1/S transition. Upon arrest release, VPs translocated to the nucleus as cells entered S phase, at efficiencies relying on cell origin and arrest method, and immediately assembled into capsids. In synchronously infected cells, the consecutive virus life cycle steps (gene expression, proteins nuclear translocation, capsid assembly, genome replication and encapsidation) proceeded tightly coupled to cell cycle progression from G0/G1 through S into G2 phase. However, a DNA synthesis stress caused by thymidine irreversibly disrupted virus life cycle, as VPs became increasingly retained in the cytoplasm hours post-stress, forming empty capsids in mouse fibroblasts, thereby impairing encapsidation of the nuclear viral DNA replicative intermediates. Synchronously infected cells subjected to density-arrest signals while traversing early S phase also blocked VPs transport, resulting in a similar misplaced cytoplasmic capsid assembly in mouse fibroblasts. In contrast, thymidine and density arrest signals deregulating virus assembly neither perturbed nuclear translocation of the NS1 protein nor viral genome replication occurring under S/G2 cycle arrest. An underlying mechanism of cell cycle control was identified in the nuclear translocation of phosphorylated VPs trimeric assembly intermediates, which accessed a non-conserved route distinct from the importin α2/β1 and transportin pathways. The exquisite cell cycle-dependence of parvovirus nuclear capsid assembly conforms a novel paradigm of time and functional coupling between cellular and virus life cycles. This junction may determine the characteristic parvovirus tropism for proliferative and cancer cells, and its disturbance could critically contribute to persistence in host tissues.     

Diminished Reovirus Capsid Stability Alters Disease Pathogenesis and Littermate Transmission

Reovirus is a nonenveloped mammalian virus that provides a useful model system for studies of viral infections in the young. Following internalization into host cells, the outermost capsid of reovirus virions is removed by endosomal cathepsin proteases. Determinants of capsid disassembly kinetics reside in the viral σ3 protein. However, the contribution of capsid stability to reovirus-induced disease is unknown. In this study, we found that mice inoculated intramuscularly with a serotype 3 reovirus containing σ3-Y354H, a mutation that reduces viral capsid stability, succumbed at a higher rate than those infected with wild-type virus. At early times after inoculation, σ3-Y354H virus reached higher titers than wild-type virus at several sites within the host. Animals inoculated perorally with a serotype 1 reassortant reovirus containing σ3-Y354H developed exaggerated myocarditis accompanied by elaboration of pro-inflammatory cytokines. Surprisingly, unchallenged littermates of mice infected with σ3-Y354H virus displayed higher titers in the intestine, heart, and brain than littermates of mice inoculated with wild-type virus. Together, these findings suggest that diminished capsid stability enhances reovirus replication, dissemination, lethality, and host-to-host spread, establishing a new virulence determinant for nonenveloped viruses.     

An ITAM in a Nonenveloped Virus Regulates Activation of NF-κB,Induction of Beta Interferon,and Viral Spread

Immunoreceptor tyrosine-based activation motifs (ITAMs) are signaling domains located within the cytoplasmic tails of many transmembrane receptors and associated adaptor proteins that mediate immune cell activation. ITAMs also have been identified in the cytoplasmic tails of some enveloped virus glycoproteins. Here, we identified ITAM sequences in three mammalian reovirus proteins: μ2, σ2, and λ2. We demonstrate for the first time that μ2 is phosphorylated, contains a functional ITAM, and activates NF-κB. Specifically, μ2 and μNS recruit the ITAM-signaling intermediate Syk to cytoplasmic viral factories and this recruitment requires the μ2 ITAM. Moreover, both the μ2 ITAM and Syk are required for maximal μ2 activation of NF-κB. A mutant virus lacking the μ2 ITAM activates NF-κB less efficiently and induces lower levels of the downstream antiviral cytokine beta interferon (IFN-β) than does wild-type virus despite similar replication. Notably, the consequences of these μ2 ITAM effects are cell type specific. In fibroblasts where NF-κB is required for reovirus-induced apoptosis, the μ2 ITAM is advantageous for viral spread and enhances viral fitness. Conversely, in cardiac myocytes where the IFN response is critical for antiviral protection and NF-κB is not required for apoptosis, the μ2 ITAM stimulates cellular defense mechanisms and diminishes viral fitness. Together, these results suggest that the cell type-specific effect of the μ2 ITAM on viral spread reflects the cell type-specific effects of NF-κB and IFN-β. This first demonstration of a functional ITAM in a nonenveloped virus presents a new mechanism for viral ITAM-mediated signaling with likely organ-specific consequences in the host.     

Alphavirus-Specific Cytotoxic T Lymphocytes Recognize a Cross-Reactive Epitope from the Capsid Protein and Can Eliminate Virus from Persistently Infected Macrophages

Persistent alphavirus infections in synovial and neural tissues are believed to be associated with chronic arthritis and encephalitis, respectively, and represent likely targets for CD8⁺ αβ cytotoxic T lymphocytes (CTL). Here we show that the capsid protein is a dominant target for alphavirus-specific CTL in BALB/c mice and that capsid-specific CTL from these mice recognize an H-2K^d restricted epitope, QYSGGRFTI. This epitope lies in the highly conserved region of the capsid protein, and QYSGGRFTI-specific CTL were cross reactive across a range of Old World alphaviruses. In vivo the acute primary viraemia of these highly cytopathic viruses was unaffected by QYSGGRFTI-specific CTL. However, in vitro these CTL were able to completely clear virus from macrophages persistently and productively infected with the arthrogenic alphavirus Ross River virus.     

Capsid Structure of Simian Cytomegalovirus from Cryoelectron Microscopy: Evidence for Tegument Attachment Sites

We have used cryoelectron microscopy and image reconstruction to study B-capsids recovered from both the nuclear and the cytoplasmic fractions of cells infected with simian cytomegalovirus (SCMV). SCMV, a representative betaherpesvirus, could thus be compared with the previously described B-capsids of the alphaherpesviruses, herpes simplex virus type 1 (HSV-1) and equine herpesvirus 1 (EHV-1), and of channel catfish virus, an evolutionarily remote herpesvirus. Nuclear B-capsid architecture is generally conserved with SCMV, but it is 4% larger in inner radius than HSV-1, implying that its ∼30% larger genome should be packed more tightly. Isolated SCMV B-capsids retain a relatively well preserved inner shell (or “small core”) of scaffolding-assembly protein, whose radial-density profile indicates that this protein is ∼16-nm long and consists of two domains connected by a low-density linker. As with HSV-1, the hexons but not the pentons of the major capsid protein (151 kDa) bind the smallest capsid protein (∼8 kDa). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed cytoplasmic B-capsid preparations to contain proteins similar in molecular weight to the basic phosphoprotein (∼119 kDa) and the matrix proteins (65 to 70 kDa). Micrographs revealed that these particles had variable amounts of surface-adherent material not present on nuclear B-capsids that we take to be tegument proteins. Cytoplasmic B-capsids were classified accordingly as lightly, moderately, or heavily tegumented. By comparing the three corresponding density maps with each other and with the nuclear B-capsid, two interactions were identified between putative tegument proteins and the capsid surface. One is between the major capsid protein and a protein estimated by electron microscopy to be 50 to 60 kDa; the other involves an elongated molecule estimated to be 100 to 120 kDa that is anchored on the triplexes, most likely on its dimer subunits. Candidates for the proteins bound at these sites are discussed. This first visualization of such linkages makes a step towards understanding the organization and functional rationale of the herpesvirus tegument.Herpesvirus form an extensive family of DNA viruses, whose host range encompasses much of the animal kingdom. They are classified into three subfamilies alpha-, beta-, and gammaherpesviruses, on the basis of biological properties [53]. Despite this diversity, their assembly pathway is closely conserved: the nucleocapsid is formed in the nucleus and follows a pathway that bears a marked resemblance to those of DNA bacteriophages (8, 14). First, a procapsid is assembled, which then releases its morphogenic internal scaffolding protein and becomes filled with DNA, concomitant with a major conformational transition of the capsid shell. Subsequent events, however, are not phage-like. The nucleocapsid exits the nucleus and acquires its tegument (53) and lipoprotein envelope. The latter events remain a focus of active research and some controversy (see, for example, references 13, 26, 51, and 63).Although virion assembly is a continuous sequential process, several kinds of capsids have been identified as representing stable endpoints or long-lived states. They include A-capsids, empty shells thought to arise from abortive attempts to package DNA or its aberrant release; C-capsids, which are filled with DNA; and B-capsids, which contain internal proteins but little or no DNA (for reviews, see references 28, 49 and 62). “B-capsid” refers either to an intranuclear capsid visualized in situ that contains internal material with different staining properties from the DNA in C-capsids (which we interpret as mainly scaffolding-assembly protein) or to a capsid isolated from the nuclear fraction that contains scaffolding proteins but little or no DNA. Here, we relax the requirement for nuclear provenance and refer to nuclear and cytoplasmic B-capsids, respectively. It now appears that there are several kinds of B-capsids. “Large-cored” B-capsids correspond to the normally short-lived and infrequently observed procapsid (42, 47). “Small-cored” B-capsids (49) derive from large-cored B-capsids and have a mature surface shell. It remains unclear whether these particles represent an assembly intermediate or an abortive byproduct (56) or how many subclasses they may comprise.As noted above, herpesvirus capsid assembly is a conservative process. The current paradigm is that the surface shell is a T=16 icosahedron containing three essential proteins. Nine hundred sixty copies of the major capsid protein (M_r ≃ 120,000 to 155,000, depending on the virus) make up 150 hexons and 12 pentons. The other two proteins form the triplexes, complexes that are present on the outer surface at the 320 sites of local threefold symmetry and have an α₂β stoichiometry (43). Typically, the α-protein has a mass of 33 to 35 kDa, and the mass of the β-protein is either similar or in the 50-kDa range (10). Most herpesvirus capsids also contain an additional low-molecular-weight protein that binds around the rims of hexons but not to pentons (11, 65, 72, 73).The above account owes most to observations relating to herpes simplex virus type 1 (HSV-1), the archetypal alphaherpesvirus. However, one property of herpesviruses that does not suggest an immutable capsid structure is their variation in genome size, which ranges from ∼120 to ∼230 kbp (18). Betaherpesviruses account for three of the eight herpesviruses known to cause disease in humans and include the cytomegaloviruses (CMVs), which have the largest of all known herpesvirus genomes (4). Although the overall structure of the herpesvirus capsid appears to be conserved, it has not been clear whether there are features specific to each subfamily. To investigate this possibility, we have used cryoelectron microscopy to study isolated capsids of simian CMV (SCMV), a close and experimentally more tractable relative of human CMV (HCMV) [24].A second motivation for this study was that it might offer insight into tegumentation. Nonenveloped capsids have been observed in significant numbers in the cytoplasms of cells infected with CMV (57) and appear to be at least partly tegumented (24, 30). We have noticed that, late in the infection of cultured cells with SCMV, B-capsids appear in the cytoplasmic fraction, and these cytoplasmic B-capsids differ from nuclear B-capsids in several respects, including their state of tegumentation. Here, we characterize the structures of nuclear and cytoplasmic B-capsids at a resolution of ∼2.2 nm and, by quantitative comparison between them, identify sites of tegument attachment.     

Structural Studies of the Sputnik Virophage

The virophage Sputnik is a satellite virus of the giant mimivirus and is the only satellite virus reported to date whose propagation adversely affects its host virus'' production. Genome sequence analysis showed that Sputnik has genes related to viruses infecting all three domains of life. Here, we report structural studies of Sputnik, which show that it is about 740 Å in diameter, has a T=27 icosahedral capsid, and has a lipid membrane inside the protein shell. Structural analyses suggest that the major capsid protein of Sputnik is likely to have a double jelly-roll fold, although sequence alignments do not show any detectable similarity with other viral double jelly-roll capsid proteins. Hence, the origin of Sputnik''s capsid might have been derived from other viruses prior to its association with mimivirus.Mimivirus is the largest virus known to date and has a 1.2-Mbp genome. It was discovered in a British water tower while searching for the cause of a hospital-acquired pneumonia outbreak (14). Although mimivirus'' natural host is amoeba, it could be a potential human pathogen (2, 7, 8, 15). Recently, a smaller virus named Sputnik was isolated from amoeba infected with mamavirus, a new strain of mimivirus (10). Sputnik utilizes the virus factory formed by mamavirus for replication and cannot reproduce on its own in amoeba. Furthermore, the coinfection of Sputnik with mamavirus reduces the yield of mamavirus by about 70% and causes the formation of many types of defective mamavirus virions.Sputnik has an 18-kbp, double-stranded, circular, highly AT-rich genome, which is predicted to encode 21 proteins ranging from 88 to 779 amino acids in size. Of these 21 proteins, 13 do not have detectable homologues in current sequence databases. The other eight genes have homologues in viruses whose hosts are from all three domains of life, the Eukarya, Archaea, and Bacteria. The chimeric characteristics of the Sputnik genome implies that it is involved in lateral gene transfer between viruses. It was proposed that Sputnik represents a new family of viruses termed virophage (10).Because of the mosaic nature of viral genomes and the lack of 16S rRNA for traditional phylogenetic tree analysis, the classification of viruses has been difficult. Recent structural studies of viral capsid proteins have led to the idea of structure-based viral lineages, which classify viruses based on the organization and structure of the viral capsids (3, 9). One of these lineages is the PRD1-adenovirus lineage, which is comprised of icosahedral double-stranded DNA (dsDNA) viruses including adenovirus, bacteriophage PRD1, Sulfolobus turreted icosahedral virus, the marine bacteriophage PM2, and the nucleocytoplasmic large DNA viruses (NCLDVs) such as mimivirus and Paramecium bursaria Chlorella virus 1 (PBCV-1). All these viruses have major capsid proteins (MCPs) whose polypeptides have a similar fold and in some cases, such as the NCLDVs, have significant sequence similarity. The MCP structures of the above-mentioned viruses, excluding mimivirus, have been determined to atomic resolution and were shown to have two consecutive “jelly-roll” domains (double jelly-roll fold) (1, 4, 13, 16, 17). A jelly-roll domain is an antiparallel β barrel consisting of eight β strands named B, C, …, I. The MCPs are organized into “capsomers” that are arranged into hexagonal arrays. Each viral capsomer contains three monomers that have a double jelly-roll fold, resulting in a pseudohexameric shape at the base, appropriate for packing into the hexagonal arrays. However, there are often large insertions in the loops between β strands D and E as well as between strands F and G of each jelly-roll fold (loops “DE” and “FG”). This gives the capsomers a triangular appearance on the surface. The thickness of capsomers is about 75 Å, and the diameter varies between 74 Å and 85 Å.Here, we report the cryo-electron microscopy (cryoEM) three-dimensional (3D) reconstruction of Sputnik to 10.7-Å resolution. We show that the MCP is organized into a hexagonal surface lattice characterized by a T=27 triangulation number. We also show that the capsomer structure in Sputnik is trimeric and that the MCP structure of PBCV-1 can be fitted into the cryoEM map of Sputnik. Thus, the MCP of Sputnik is probably a double jelly-roll fold as in viruses belonging to the PRD1-adenovirus lineage. However, there is no significant sequence similarity between the MCP of Sputnik and other members of the PRD1-adenovirus lineage, suggesting that Sputnik is a member of a separate branch from the NCLDVs (mimivirus and PBCV-1, etc.).     

Dengue Virus Capsid Protein Usurps Lipid Droplets for Viral Particle Formation

Dengue virus is responsible for the highest rates of disease and mortality among the members of the Flavivirus genus. Dengue epidemics are still occurring around the world, indicating an urgent need of prophylactic vaccines and antivirals. In recent years, a great deal has been learned about the mechanisms of dengue virus genome amplification. However, little is known about the process by which the capsid protein recruits the viral genome during encapsidation. Here, we found that the mature capsid protein in the cytoplasm of dengue virus infected cells accumulates on the surface of ER-derived organelles named lipid droplets. Mutagenesis analysis using infectious dengue virus clones has identified specific hydrophobic amino acids, located in the center of the capsid protein, as key elements for lipid droplet association. Substitutions of amino acid L50 or L54 in the capsid protein disrupted lipid droplet targeting and impaired viral particle formation. We also report that dengue virus infection increases the number of lipid droplets per cell, suggesting a link between lipid droplet metabolism and viral replication. In this regard, we found that pharmacological manipulation of the amount of lipid droplets in the cell can be a means to control dengue virus replication. In addition, we developed a novel genetic system to dissociate cis-acting RNA replication elements from the capsid coding sequence. Using this system, we found that mislocalization of a mutated capsid protein decreased viral RNA amplification. We propose that lipid droplets play multiple roles during the viral life cycle; they could sequester the viral capsid protein early during infection and provide a scaffold for genome encapsidation.     

The Reovirus Mutant tsA279 L2 Gene Is Associated with Generation of a Spikeless Core Particle: Implications for Capsid Assembly

Previous studies which used intertypic reassortants of the wild-type reovirus serotype 1 Lang and the temperature-sensitive (ts) serotype 3 mutant clone tsA279 identified two ts lesions; one lesion, in the M2 gene segment, was associated with defective transmembrane transport of restrictively assembled virions (P. R. Hazelton and K. M. Coombs, Virology 207:46–58, 1995). In the present study we show that the second lesion, in the L2 gene segment, which encodes the λ2 protein, is associated with the accumulation of a core-like particle defective for the λ2 pentameric spike. Physicochemical, biochemical, and immunological studies showed that these structures were deficient for genomic double-stranded RNA, the core spike protein λ2, and the minor core protein μ2. Core particles with the λ2 spike structure accumulated after temperature shift-down from a restrictive to a permissive temperature in the presence of cycloheximide. These data suggest the spike-deficient, core-like particle is an assembly intermediate in reovirus morphogenesis. The existence of this naturally occurring primary core structure suggests that the core proteins λ1, λ3, and ς2 interact to initiate the process of virion capsid assembly through a dodecahedral mechanism. The next step in the proposed capsid assembly model would be the association of the minor core protein μ2, either preceding or collateral to the condensation of the λ2 pentameric spike at the apices of the primary core structure. The assembly pathway of the reovirus double capsid is further elaborated when these observations are combined with structures identified in other studies.     

Localization of herpes simplex virus type 1 UL37 in the Golgi complex requires UL36 but not capsid structures

The herpes simplex virus type 1 (HSV-1) UL37 gene encodes a 120-kDa polypeptide which resides in the tegument structure of the virion and is important for morphogenesis. The goal of this study was to use green fluorescent protein (GFP) to follow the fate of UL37 within cells during the normal course of virus replication. GFP was inserted in frame at the C terminus of UL37 to generate a fluorescent-protein-tagged UL37 polypeptide. A virus designated K37eGFP, which replicated normally on Vero cells, was isolated and was shown to express the fusion polypeptide. When cells infected with this virus were examined by confocal microscopy, the fluorescence was observed to be predominantly cytoplasmic. As the infection progressed, fluorescence began to accumulate in a juxtanuclear structure. Mannosidase II and giantin were observed to colocalize with UL37eGFP at these structures, as judged by immunofluorescence assays. Therefore, UL37 traffics to the Golgi complex during infection. A VP26mRFP marker (red fluorescent protein fused to VP26) was recombined into K37eGFP, and when cells infected with this “dual-color” virus were examined, colocalization of the red (capsid) and green (UL37) fluorescence in the Golgi structure was observed. Null mutations in VP5 (ΔVP5), which abolished capsid assembly, and in UL36 (Δ36) were recombined into the K37eGFP virus genome. In cells infected with K37eGFP/ΔVP5, localization of UL37eGFP to the Golgi complex was similar to that for the parental virus (K37eGFP), indicating that trafficking of UL37eGFP to the Golgi complex did not require capsid structures. Confocal analysis of cells infected with K37eGFP/Δ36 showed that, in the absence of UL36, accumulation of UL37eGFP at the Golgi complex was not evident. This indicates an interaction between these two proteins that is important for localization of UL37 in the Golgi complex and thus possibly for cytoplasmic envelopment of the capsid. This is the first demonstration of a functional role for UL36:UL37 interaction in HSV-1-infected cells.     

The Globoside Receptor Triggers Structural Changes in the B19 Virus Capsid That Facilitate Virus Internalization

Globoside (Gb4Cer), Ku80 autoantigen, and α5β1 integrin have been identified as cell receptors/coreceptors for human parvovirus B19 (B19V), but their role and mechanism of interaction with the virus are largely unknown. In UT7/Epo cells, expression of Gb4Cer and CD49e (integrin alpha-5) was high, but expression of Ku80 was insignificant. B19V colocalized with Gb4Cer and, to a lesser extent, with CD49e. However, only anti-Gb4Cer antibodies could disturb virus attachment. Only a small proportion of cell-bound viruses were internalized, while the majority became detached from the receptor. When added to uninfected cells, the receptor-detached virus showed superior cell binding capacity and infectivity. Attachment of B19V to cells triggered conformational changes in the capsid leading to the accessibility of the N terminus of VP1 (VP1u) to antibodies, which was maintained in the receptor-detached virus. VP1u became similarly accessible to antibodies following incubation of B19V particles with increasing concentrations of purified Gb4Cer. The receptor-mediated exposure of VP1u is critical for virus internalization, since capsids lacking VP1 could bind to cells but were not internalized. Moreover, an antibody against the N terminus of VP1u disturbed virus internalization, but only when present during and not after virus attachment, indicating the involvement of this region in binding events required for internalization. These results suggest that Gb4Cer is not only the primary receptor for B19V attachment but also the mediator of capsid rearrangements required for subsequent interactions leading to virus internalization. The capacity of the virus to detach and reattach again would enhance the probability of productive infections.Human parvovirus B19 (B19V) belongs to the Erythrovirus genus of the Parvoviridae family. The virus has a worldwide distribution and typically causes a mild childhood febrile illness known as erythema infectiosum or fifth disease. In patients with underlying immunologic and hematologic disorders, B19V has been associated with more severe manifestations, such as arthropathies, aplastic anemia, hydrops fetalis, and fetal death (4).B19V has a single-stranded DNA genome encapsidated in a T=1 nonenveloped icosahedral capsid. The capsid is assembled from two structural proteins, VP1 (83 kDa) and VP2 (58 kDa). VP1 is identical to VP2, with the exception of 227 amino acids (aa) in the N-terminal part, the so-called VP1 unique region (VP1u) (9, 26). Despite VP1u being the minor component of the capsid, the dominant immune response against B19V is elicited by the VP1u region, which harbors strong neutralizing epitopes (2, 31, 41). A secreted phospholipase A₂ (PLA₂)-like activity has been located in the VP1 unique region of B19V (12), which is required for infection (13, 17, 40). Despite all these properties, we recently showed that VP1u is not accessible to antibodies. However, brief exposure to mild temperatures or low pH can render this region accessible (30). In this sense, B19V is similar to other parvoviruses in which VP1u is not accessible but can become exposed in vitro by mild heat or low-pH treatment (10, 21). In all parvoviruses tested so far, VP1u becomes exposed during the intracellular trafficking of the virus (18, 23, 28, 32, 33). However, B19V VP1u harbors strong neutralizing epitopes, meaning that its accessibility to antibody binding should occur prior to uptake by cells. In line with this hypothesis, we have demonstrated that incubation of B19V with red blood cells, which allow virus binding but not virus internalization, can trigger the externalization of VP1u in a proportion of the bound particles (3).The glycosphingolipid globoside (globotetraosylceramide [Gb4Cer]) is the cellular receptor of B19V (5, 6). Gb4Cer is largely expressed in human erythroid progenitor cells in the bone marrow, which are the main target cells for the virus. However, the pathogenicity and tropism of B19V cannot be explained if Gb4Cer is the only receptor. Previous studies have suggested that Gb4Cer is necessary for B19V to bind to cells but is not sufficient for cell entry (35). Subsequently, α5β1 integrin (36, 37) and the Ku80 autoantigen (25) were identified as coreceptors for B19V infection. While Ku80 might assist in virus attachment (25), α5β1 integrin is thought to be required for internalization (36, 37). In line with a complex mechanism of internalization based on multiple receptors is the observation that B19V does not stably bind membrane-associated globoside in vitro (20), indicating that B19V probably binds globoside jointly with other molecular structures present on cell membranes.In the present studies, the interaction of B19V with cell surface receptors and the implication of this interaction for the capsid structure were examined. The cells chosen for this study were of the erythropoietin (Epo)-dependent bone marrow megakaryoblastic leukemia UT7/Epo cell line, which is commonly used to study B19V infection. UT7/Epo cells support viral DNA replication and protein expression; however, intracellular factors limit the production of infectious progeny virus. The results indicate that B19V interacts dynamically with cell surface receptors and that internalization is a complex process involving sequential steps. B19V binds initially to Gb4Cer, which triggers the externalization of VP1u. The modified capsid is then ready for a subsequent interaction. Whenever the second interaction is not possible, the virus detaches from Gb4Cer and is ready for another attempt. This mechanism of “detachment-reattachment” is repeated until the required second interaction occurs, after which the virus is internalized.     

AIC649 Induces a Bi-Phasic Treatment Response in the Woodchuck Model of Chronic Hepatitis B

AIC649 has been shown to directly address the antigen presenting cell arm of the host immune defense leading to a regulated cytokine release and activation of T cell responses. In the present study we analyzed the antiviral efficacy of AIC649 as well as its potential to induce functional cure in animal models for chronic hepatitis B. Hepatitis B virus transgenic mice and chronically woodchuck hepatitis virus (WHV) infected woodchucks were treated with AIC649, respectively. In the mouse system AIC649 decreased the hepatitis B virus titer as effective as the “gold standard”, Tenofovir. Interestingly, AIC649-treated chronically WHV infected woodchucks displayed a bi-phasic pattern of response: The marker for functional cure—hepatitis surface antigen—first increased but subsequently decreased even after cessation of treatment to significantly reduced levels. We hypothesize that the observed bi-phasic response pattern to AIC649 treatment reflects a physiologically “concerted”, reconstituted immune response against WHV and therefore may indicate a potential for inducing functional cure in HBV-infected patients.     

Identical Strength of the T Cell Responses against E2, nsP1 and Capsid CHIKV Proteins in Recovered and Chronic Patients after the Epidemics of 2005-2006 in La Reunion Island

To characterize the immunity developed by patients infected by chikungunya virus (CHIKV), we studied the intensity and specificity of CHIKV-specific T cells mediated responses in chronic and recovered patients at 12 to 24 months post-infection. T cells were challenged in vitro against CHIKV synthetic peptides covering the length of three viral proteins, capsid, E2 and nsP1 proteins as well as all inactivated virus particles. Cytokine production was assessed by ELISPOT and intracellular labeling. T cells producing IFN-γ were detected against CHIKV in 85% patient’s cells either by direct ELISPOT assay (69% of patients) or after expansion of memory T cells allowing the detection of both CD4 and CD8 specific-T cells in 16% additional cases. The IFN-γ response was mainly engaged in response to nsP1 or E2 (52% and 46% cases, respectively) but in only 27% cases against the capsid. The anti-E2 response represented half the magnitude of the total CHIKV IFN-γ production and was mainly directed against the C-terminal half part of the protein. Almost all patients had conserved a T cell specific response against CHIKV with a clear hierarchy of T cell responses (CD8 > CD4) engaged against E2 > nsP1 > capsid. More importantly, the intensity of responses was not significantly different between recovered and chronic patients. These findings constitute key elements to a better understanding of patient T cell immunoreactivity against CHIKV and argue against a possible defect of T cell immunoresponse in the chronicity post-CHIKV infection.     

Effect of Ultraviolet Irradiation and Actinomycin D on Polyoma Virus Replication in Mouse Embryo Cell Cultures

Ultraviolet irradiation and actinomycin D impair the capacity of mouse embryo (ME) cells to support the replication of polyoma virus, but not of encephalomyocarditis (EMC) virus. The loss in capacity for polyoma virus synthesis was an “all-or-none” effect and followed closely upon the loss in cellular capacity for clone formation. Cells treated with either agent produced polyoma “T” antigen, but did not synthesize polyoma structural protein. Infection of untreated ME cells with polyoma virus produced marked stimulation of both deoxyribonucleic acid (DNA) synthesis and ribonucleic acid (RNA) synthesis. ME cell cultures irradiated with ultraviolet for 30 sec at 60 μw/cm² or treated with actinomycin D at 0.1 μg/ml for 6 hr prior to infection were incapable of synthesizing DNA or RNA, even after infection with polyoma virus. Irradiation of cells during infection produced cessation of synthesis of both RNA and DNA. Addition of actinomycin D during infection did not inhibit DNA synthesis but abolished RNA synthesis and reduced the yield of polyoma virus to 10% of that in untreated infected cultures. Both agents lost the ability to prevent replication of a full yield of polyoma virus when administered 30 hr after infection or later. The period after which neither agent inhibited polyoma replication corresponded with the period at which maximal RNA synthesis in untreated infected cultures had subsided. It can be concluded on the basis of the data presented that the functional integrity of the mouse embryo cell genome is required for the replication of polyoma virus, but not for EMC virus. Whereas the requirement for cellular DNA-dependent RNA synthesis for polyoma virus replication has been demonstrated, the exact nature of the host-cell function remains to be elucidated.     

Incorporation of the Green Fluorescent Protein into the Herpes Simplex Virus Type 1 Capsid

The herpes simplex virus type 1 (HSV-1) UL35 open reading frame (ORF) encodes a 12-kDa capsid protein designated VP26. VP26 is located on the outer surface of the capsid specifically on the tips of the hexons that constitute the capsid shell. The bioluminescent jellyfish (Aequorea victoria) green fluorescent protein (GFP) was fused in frame with the UL35 ORF to generate a VP26-GFP fusion protein. This fusion protein was fluorescent and localized to distinct regions within the nuclei of transfected cells following infection with wild-type virus. The VP26-GFP marker was introduced into the HSV-1 (KOS) genome resulting in recombinant plaques that were fluorescent. A virus, designated K26GFP, was isolated and purified and was shown to grow as well as the wild-type virus in cell culture. An analysis of the intranuclear capsids formed in K26GFP-infected cells revealed that the fusion protein was incorporated into A, B, and C capsids. Furthermore, the fusion protein incorporated into the virion particle was fluorescent as judged by fluorescence-activated cell sorter (FACS) analysis of infected cells in the absence of de novo protein synthesis. Cells infected with K26GFP exhibited a punctate nuclear fluorescence at early times in the replication cycle. At later times during infection a generalized cytoplasmic and nuclear fluorescence, including fluorescence at the cell membranes, was observed, confirming visually that the fusion protein was incorporated into intranuclear capsids and mature virions.     

Cytomegalovirus Assembly Protein Precursor and Proteinase Precursor Contain Two Nuclear Localization Signals That Mediate Their Own Nuclear Translocation and That of the Major Capsid Protein

The cytomegalovirus (CMV) assembly protein precursor (pAP) interacts with the major capsid protein (MCP), and this interaction is required for nuclear translocation of the MCP, which otherwise remains in the cytoplasm of transfected cells (L. J. Wood et al., J. Virol. 71:179–190, 1997). We have interpreted this finding to indicate that the CMV MCP lacks its own nuclear localization signal (NLS) and utilizes the pAP as an NLS-bearing escort into the nucleus. The CMV pAP amino acid sequence has two clusters of basic residues (e.g., KRRRER [NLS1] and KARKRLK [NLS2], for simian CMV) that resemble the simian virus 40 large-T-antigen NLS (D. Kalderon et al., Cell 39:499–509, 1984) and one of these (NLS1) has a counterpart in the pAP homologs of other herpesviruses. The work described here establishes that NLS1 and NLS2 are mutually independent NLS that can act (i) in cis to translocate pAP and the related proteinase precursor (pNP1) into the nucleus and (ii) in trans to transport MCP into the nucleus. By using combinations of NLS mutants and carboxy-terminal deletion constructs, we demonstrated a self-interaction of pAP and cytoplasmic interactions of pAP with pNP1 and of pNP1 with itself. The relevance of these findings to early steps in capsid assembly, the mechanism of MCP nuclear transport, and the possible cytoplasmic formation of protocapsomeric substructures is discussed.     

Increased Early RNA Replication by Chimeric West Nile Virus W956IC Leads to IPS-1-Mediated Activation of NF-κB and Insufficient Virus-Mediated Counteraction of the Resulting Canonical Type I Interferon Signaling

Although infections with “natural” West Nile virus (WNV) and the chimeric W956IC WNV infectious clone virus produce comparable peak virus yields in type I interferon (IFN) response-deficient BHK cells, W956IC infection produces higher levels of “unprotected” viral RNA at early times after infection. Analysis of infections with these two viruses in IFN-competent cells showed that W956IC activated NF-κB, induced higher levels of IFN-β, and produced lower virus yields than WNV strain Eg101. IPS-1 was required for both increased induction of IFN-β and decreased yields of W956IC. In Eg101-infected cells, phospho-STAT1/STAT2 nuclear translocation was blocked at all times analyzed, while some phospho-STAT1/STAT2 nuclear translocation was still detected at 8 h after infection in W956IC-infected mouse embryonic fibroblasts (MEFs), and early viral protein levels were lower in these cells. A set of additional chimeras was made by replacing various W956IC gene regions with the Eg101 equivalents. As reported previously, for three of these chimeras, the low early RNA phenotype of Eg101 was restored in BHK cells. Analysis of infections with two of these chimeric viruses in MEFs detected lower early viral RNA levels, higher early viral protein levels, lower early IFN-β levels, and higher virus yields similar to those seen after Eg101 infection. The data suggest that replicase protein interactions directly or indirectly regulate genome switching between replication and translation at early times in favor of translation to minimize NF-κB activation and IFN induction by decreasing the amount of unprotected viral RNA, to produce sufficient viral protein to block canonical type I IFN signaling, and to efficiently remodel cell membranes for exponential genome amplification.     

The Capsid Proteins of a Large, Icosahedral dsDNA Virus

Chilo iridescent virus (CIV) is a large (∼ 1850 Å diameter) insect virus with an icosahedral, T = 147 capsid, a double-stranded DNA (dsDNA) genome, and an internal lipid membrane. The structure of CIV was determined to 13 Å resolution by means of cryoelectron microscopy (cryoEM) and three-dimensional image reconstruction. A homology model of P50, the CIV major capsid protein (MCP), was built based on its amino acid sequence and the structure of the homologous Paramecium bursaria chlorella virus 1 Vp54 MCP. This model was fitted into the cryoEM density for each of the 25 trimeric CIV capsomers per icosahedral asymmetric unit. A difference map, in which the fitted CIV MCP capsomers were subtracted from the CIV cryoEM reconstruction, showed that there are at least three different types of minor capsid proteins associated with the capsomers outside the lipid membrane. “Finger” proteins are situated at many, but not all, of the spaces between three adjacent capsomers within each trisymmetron, and “zip” proteins are situated between sets of three adjacent capsomers at the boundary between neighboring trisymmetrons and pentasymmetrons. Based on the results of segmentation and density correlations, there are at least eight finger proteins and three dimeric and two monomeric zip proteins in one asymmetric unit of the CIV capsid. These minor proteins appear to stabilize the virus by acting as intercapsomer cross-links. One transmembrane “anchor” protein per icosahedral asymmetric unit, which extends from beneath one of the capsomers in the pentasymmetron to the internal leaflet of the lipid membrane, may provide additional stabilization for the capsid. These results are consistent with the observations for other large, icosahedral dsDNA viruses that also utilize minor capsid proteins for stabilization and for determining their assembly.     

Induction of Humoral and Cell-Mediated Immune Responses by Hepatitis B Virus Epitope Displayed on the Virus-Like Particles of Prawn Nodavirus

Hepatitis B virus (HBV) is a deadly pathogen that has killed countless people worldwide. Saccharomyces cerevisiae-derived HBV vaccines based upon hepatitis B surface antigen (HBsAg) is highly effective. However, the emergence of vaccine escape mutants due to mutations on the HBsAg and polymerase genes has produced a continuous need for the development of new HBV vaccines. In this study, the “a” determinant within HBsAg was displayed on the recombinant capsid protein of Macrobrachium rosenbergii nodavirus (MrNV), which can be purified easily in a single step through immobilized metal affinity chromatography (IMAC). The purified protein self-assembled into virus-like particles (VLPs) when observed under a transmission electron microscope (TEM). Immunization of BALB/c mice with this chimeric protein induced specific antibodies against the “a” determinant. In addition, it induced significantly more natural killer and cytotoxic T cells, as well as an increase in interferon gamma (IFN-γ) secretion, which are vital for virus clearance. Collectively, these findings demonstrated that the MrNV capsid protein is a potential carrier for the HBV “a” determinant, which can be further extended to display other foreign epitopes. This paper is the first to report the application of MrNV VLPs as a novel platform to display foreign epitopes.     

The TRIM5alpha B-box 2 domain promotes cooperative binding to the retroviral capsid by mediating higher-order self-association

The retroviral restriction factor, TRIM5α, blocks infection of a spectrum of retroviruses soon after virus entry into the cell. TRIM5α consists of RING, B-box 2, coiled-coil, and B30.2(SPRY) domains. The B-box 2 domain is essential for retrovirus restriction by TRIM5α, but its specific function is unknown. We show here that the B-box 2 domain mediates higher-order self-association of TRIM5α_rh oligomers. This self-association increases the efficiency of TRIM5α binding to the retroviral capsid, thus potentiating restriction of retroviral infection. The contribution of the B-box 2 domain to cooperative TRIM5α association with the retroviral capsid explains the conditional nature of the restriction phenotype exhibited by some B-box 2 TRIM5α mutants; the potentiation of capsid binding that results from B-box 2-mediated self-association is essential for restriction when B30.2(SPRY) domain-mediated interactions with the retroviral capsid are weak. Thus, B-box 2-dependent higher-order self-association and B30.2(SPRY)-dependent capsid binding represent complementary mechanisms whereby sufficiently dense arrays of capsid-bound TRIM5α proteins can be achieved.