Sequential events in the morphogenesis of japanese Encephalitis virus

Roles of NS1 Protein in Flavivirus Pathogenesis

Flaviviruses such as dengue, Zika, and West Nile viruses are highly concerning pathogens that pose significant risks to public health. The NS1 protein is conserved among flaviviruses and is synthesized as a part of the flavivirus polyprotein. It plays a critical role in viral replication, disease progression, and immune evasion. Post-translational modifications influence NS1's stability, secretion, antigenicity, and interactions with host factors. NS1 protein forms extensive interactions with host cellular proteins allowing it to affect vital processes such as RNA processing, gene expression regulation, and cellular homeostasis, which in turn influence viral replication, disease pathogenesis, and immune responses. NS1 acts as an immune evasion factor by delaying complement-dependent lysis of infected cells and contributes to disease pathogenesis by inducing endothelial cell damage and vascular leakage and triggering autoimmune responses. Anti-NS1 antibodies have been shown to cross-react with host endothelial cells and platelets, causing autoimmune destruction that is hypothesized to contribute to disease pathogenesis. However, in contrast, immunization of animal models with the NS1 protein confers protection against lethal challenges from flaviviruses such as dengue and Zika viruses. Understanding the multifaceted roles of NS1 in flavivirus pathogenesis is crucial for effective disease management and control. Therefore, further research into NS1 biology, including its host protein interactions and additional roles in disease pathology, is imperative for the development of strategies and therapeutics to combat flavivirus infections successfully. This Review provides an in-depth exploration of the current available knowledge on the multifaceted roles of the NS1 protein in the pathogenesis of flaviviruses.

Molecular Insights into the Flavivirus Replication Complex

Flaviviruses are vector-borne RNA viruses, many of which are clinically relevant human viral pathogens, such as dengue, Zika, Japanese encephalitis, West Nile and yellow fever viruses. Millions of people are infected with these viruses around the world each year. Vaccines are only available for some members of this large virus family, and there are no effective antiviral drugs to treat flavivirus infections. The unmet need for vaccines and therapies against these flaviviral infections drives research towards a better understanding of the epidemiology, biology and immunology of flaviviruses. In this review, we discuss the basic biology of the flavivirus replication process and focus on the molecular aspects of viral genome replication. Within the virus-induced intracellular membranous compartments, flaviviral RNA genome replication takes place, starting from viral poly protein expression and processing to the assembly of the virus RNA replication complex, followed by the delivery of the progeny viral RNA to the viral particle assembly sites. We attempt to update the latest understanding of the key molecular events during this process and highlight knowledge gaps for future studies.

Nucleocytoplasmic shuttling of the West Nile virus RNA-dependent RNA polymerase NS5 is critical to infection

West Nile virus (WNV) is a single-stranded, positive sense RNA virus of the family Flaviviridae and is a significant pathogen of global medical importance. Flavivirus replication is known to be exclusively cytoplasmic, but we show here for the first time that access to the nucleus of the WNV strain Kunjin (WNV_KUN ) RNA-dependent RNA polymerase (protein NS5) is central to WNV_KUN virus production. We show that treatment of cells with the specific nuclear export inhibitor leptomycin B (LMB) results in increased NS5 nuclear accumulation in WNV_KUN -infected cells and NS5-transfected cells, indicative of nucleocytoplasmic shuttling under normal conditions. We used site-directed mutagenesis to identify the nuclear localisation sequence (NLS) responsible for WNV_KUN NS5 nuclear targeting, observing that mutation of this NLS resulted in exclusively cytoplasmic accumulation of NS5 even in the presence of leptomycin B. Introduction of NS5 NLS mutations into FLSDX, an infectious clone of WNV_KUN , resulted in lethality, suggesting that the ability of NS5 to traffic into the nucleus in integral to WNV_KUN replication. This study thus shows for the first time that NLS-dependent trafficking into the nucleus during infection of WNV_KUN NS5 is critical for viral replication. Excitingly, specific inhibitors of NS5 nuclear import reduce WNV_KUN virus production, proving the principle that inhibition of WNV_KUN NS5 nuclear import is a viable therapeutic avenue for antiviral drug development in the future.

The IMPORTance of the Nucleus during Flavivirus Replication

Flaviviruses are a large group of arboviruses of significant medical concern worldwide. With outbreaks a common occurrence, the need for efficient viral control is required more than ever. It is well understood that flaviviruses modulate the composition and structure of membranes in the cytoplasm that are crucial for efficient replication and evading immune detection. As the flavivirus genome consists of positive sense RNA, replication can occur wholly within the cytoplasm. What is becoming more evident is that some viral proteins also have the ability to translocate to the nucleus, with potential roles in replication and immune system perturbation. In this review, we discuss the current understanding of flavivirus nuclear localisation, and the function it has during flavivirus infection. We also describe-while closely related-the functional differences between similar viral proteins in their nuclear translocation.

The dengue virus conceals double-stranded RNA in the intracellular membrane to escape from an interferon response

The dengue virus (DENV) circulates between humans and mosquitoes and requires no other mammals or birds for its maintenance in nature. The virus is well-adapted to humans, as reflected by high-level viraemia in patients. To investigate its high adaptability, the DENV induction of host type-I interferon (IFN) was assessed in vitro in human-derived HeLa cells and compared with that induced by the Japanese encephalitis virus (JEV), a closely related arbovirus that generally exhibits low viraemia in humans. A sustained viral spread with a poor IFN induction was observed in the DENV-infected cells, whereas the JEV infection resulted in a self-limiting and abortive infection with a high IFN induction. There was no difference between DENV and JEV double-stranded RNA (dsRNA) as IFN inducers. Instead, the dsRNA was poorly exposed in the cytosol as late as 48 h post-infection (p.i.), despite the high level of DENV replication in the infected cells. In contrast, the JEV-derived dsRNA appeared in the cytosol as early as 24 h p.i. Our results provided evidence for the first time in DENV, that concealing dsRNA in the intracellular membrane diminishes the effect of the host defence mechanism, a strategy that differs from an active suppression of IFN activity.

Attenuated West Nile virus mutant NS1130-132QQA/175A/207A exhibits virus-induced ultrastructural changes and accumulation of protein in the endoplasmic reticulum

We have previously shown that ablation of the three N-linked glycosylation sites in the West Nile virus NS1 protein completely attenuates mouse neuroinvasiveness (≥1,000,000 PFU). Here, we compared the replication of the NS1130-132QQA/175A/207A mutant to that of the parental NY99 strain in monkey kidney Vero cells. The results suggest that the mechanism of attenuation is a lack of NS1 glycosylation, which blocks efficient replication, maturation, and NS1 secretion from the endoplasmic reticulum and results in changes to the virus-induced ultrastructure.

A three-dimensional comparison of tick-borne flavivirus infection in mammalian and tick cell lines

Tick-borne flaviviruses (TBFV) are sustained in nature through cycling between mammalian and tick hosts. In this study, we used African green monkey kidney cells (Vero) and Ixodes scapularis tick cells (ISE6) to compare virus-induced changes in mammalian and arthropod cells. Using confocal microscopy, transmission electron microscopy (TEM), and electron tomography (ET), we examined viral protein distribution and the ultrastructural changes that occur during TBFV infection. Within host cells, flaviviruses cause complex rearrangement of cellular membranes for the purpose of virus replication. Virus infection was accompanied by a marked expansion in endoplasmic reticulum (ER) staining and markers for TBFV replication were localized mainly to the ER in both cell lines. TEM of Vero cells showed membrane-bound vesicles enclosed in a network of dilated, anastomosing ER cisternae. Virions were seen within the ER and were sometimes in paracrystalline arrays. Tubular structures or elongated vesicles were occasionally noted. In acutely and persistently infected ISE6 cells, membrane proliferation and vesicles were also noted; however, the extent of membrane expansion and the abundance of vesicles were lower and no viral particles were observed. Tubular profiles were far more prevalent in persistently infected ISE6 cells than in acutely infected cells. By ET, tubular profiles, in persistently infected tick cells, had a cross-sectional diameter of 60–100 nm, reached up to 800 nm in length, were closed at the ends, and were often arranged in fascicle-like bundles, shrouded with ER membrane. Our experiments provide analysis of viral protein localization within the context of both mammalian and arthropod cell lines as well as both acute and persistent arthropod cell infection. Additionally, we show for the first time 3D flavivirus infection in a vector cell line and the first ET of persistent flavivirus infection.

A novel coding-region RNA element modulates infectious dengue virus particle production in both mammalian and mosquito cells and regulates viral replication in Aedes aegypti mosquitoes

Dengue virus (DENV) is an enveloped flavivirus with a positive-sense RNA genome transmitted by Aedes mosquitoes, causing the most important arthropod-borne viral disease affecting humans. Relatively few cis-acting RNA regulatory elements have been described in the DENV coding-region. Here, by introducing silent mutations into a DENV-2 infectious clone, we identify the conserved capsid-coding region 1 (CCR1), an RNA sequence element that regulates viral replication in mammalian cells and to a greater extent in Ae. albopictus mosquito cells. These defects were confirmed in vivo, resulting in decreased replication in Ae. aegypti mosquito bodies and dissemination to the salivary glands. Furthermore, CCR1 does not regulate translation, RNA synthesis or virion retention but likely modulates assembly, as mutations resulted in the release of non-infectious viral particles from both cell types. Understanding the role of CCR1 could help characterize the poorly-defined stage of assembly in the DENV life cycle and uncover novel anti-viral targets.

Morphological changes in human neural cells following tick-borne encephalitis virus infection

Tick-borne encephalitis (TBE) is one of the leading and most dangerous human viral neuroinfections in Europe and north-eastern Asia. The clinical manifestations include asymptomatic infections, fevers and debilitating encephalitis that might progress into chronic disease or fatal infection. To understand TBE pathology further in host nervous systems, three human neural cell lines, neuroblastoma, medulloblastoma and glioblastoma, were infected with TBE virus (TBEV). The susceptibility and virus-mediated cytopathic effect, including ultrastructural and apoptotic changes of the cells, were examined. All the neural cell lines tested were susceptible to TBEV infection. Interestingly, the neural cells produced about 100- to 10 000-fold higher virus titres than the conventional cell lines of extraneural origin, indicating the highly susceptible nature of neural cells to TBEV infection. The infection of medulloblastoma and glioblastoma cells was associated with a number of major morphological changes, including proliferation of membranes of the rough endoplasmic reticulum and extensive rearrangement of cytoskeletal structures. The TBEV-infected cells exhibited either necrotic or apoptotic morphological features. We observed ultrastructural apoptotic signs (condensation, margination and fragmentation of chromatin) and other alterations, such as vacuolation of the cytoplasm, dilatation of the endoplasmic reticulum cisternae and shrinkage of cells, accompanied by a high density of the cytoplasm. On the other hand, infected neuroblastoma cells did not exhibit proliferation of membranous structures. The virions were present in both the endoplasmic reticulum and the cytoplasm. Cells were dying preferentially by necrotic mechanisms rather than apoptosis. The neuropathological significance of these observations is discussed.

Dengue virus serotype infection specifies the activation of the unfolded protein response

Background

Dengue and Dengue hemorrhagic fever have emerged as some of the most important mosquito-borne viral diseases in the tropics. The mechanisms of pathogenesis of Dengue remain elusive. Recently, virus-induced apoptosis mediated by the Unfolded Protein Response (UPR) has been hypothesised to represent a crucial pathogenic event in viral infection. In an attempt to evaluate the contribution of the UPR to virus replication, we have characterized each component of this signalling pathway following Dengue virus infection.

Results

We find that upon Dengue virus infection, A549 cells elicit an UPR which is observed at the level of translation attenuation (as visualized by the phosphorylation of eIF2alpha) and activation of specific pathways such as nuclear translocation of ATF-6 and splicing of XBP-1. Interestingly, we find that specific serotype of virus modulate the UPR with different selectivity. In addition, we demonstrate that perturbation of the UPR by preventing the dephosphorylation of the translation initiation factor eIF2alpha using Salubrinal considerably alters virus infectivity.

Conclusion

This report provides evidence that Dengue infection induces and regulates the three branches of the UPR signaling cascades. This is a basis for our understanding of the viral regulation and conditions beneficial to the viral infection. Furthermore, modulators of UPR such as Salubrinal that inhibit Dengue replication may open up an avenue toward cell-protective agents that target the endoplasmic reticulum for anti-viral therapy.

The non-structural protein 4A of dengue virus is an integral membrane protein inducing membrane alterations in a 2K-regulated manner

Dengue virus (DV) is a positive sense RNA virus replicating in the cytoplasm in membranous compartments that are induced by viral infection. The non-structural protein (NS) 4A is one of the least characterized DV proteins. It is highly hydrophobic with its C-terminal region (designated 2K fragment) serving as a signal sequence for the translocation of the adjacent NS4B into the endoplasmic reticulum (ER) lumen. In this report, we demonstrate that NS4A associates with membranes via 4 internal hydrophobic regions, which are all able to mediate membrane targeting of a cytosolic reporter protein. We also developed a model for the membrane topology of NS4A in which the N-terminal third of NS4A localizes to the cytoplasm, while the remaining part contains three transmembrane segments, with the C-terminal end localized in the ER lumen. Subcellular localization experiments in DV-infected cells revealed that NS4A resides primarily in ER-derived cytoplasmic dot-like structures that also contain dsRNA and other DV proteins, suggesting that NS4A is a component of the membrane-bound viral replication complex (RC). Interestingly, the individual expression of DV NS4A lacking the 2K fragment resulted in the induction of cytoplasmic membrane alterations resembling virus-induced structures, whereas expression of full-length NS4A does not induce comparable membrane alterations. Thus, proteolytic removal of the 2K peptide appears to be important for induction of membrane alterations that may harbor the viral RC. These results shed new light on the role of NS4A in the DV replication cycle and provide a model of how this protein induces membrane rearrangements and how this property may be regulated.

Mosquito cells infected with Japanese encephalitis virus release slowly-sedimenting hemagglutinin particles in association with intracellular formation of smooth membrane structures

Arthropod-borne flaviviruses can grow in both arthropod and mammalian cells. Virion morphogenesis, though well studied in mammalian cells, is still unclear in arthropod cells. Here, we compared a mosquito cell line C6/36 and a mammalian cell line Vero in extracellular virus particles and intracellular ultrastructures triggered by infection with Japanese encephalitis virus (JEV). Sedimentation analyses of virion and slowly-sedimenting hemagglutinin (SHA) particles released by infection with the Nakayama strain revealed that C6/36 cells produced higher envelope (E) antigen levels in the SHA than the virion fraction in contrast to Vero cells that showed the opposite pattern. Specific infectivities per ng of E were similar in both cells, whereas specific hemagglutinating activities in the SHA fraction were lower in C6/36 than Vero cells. The precursor membrane protein was less efficiently cleaved to the membrane protein in SHA particles released from C6/36 than Vero cells. Ultrastructural studies showed more remarkable production of smooth membrane structures (SMSs) in C6/36 than in Vero cells. The differences in sedimentation patterns of extracellular virus particles between Nakayama-infected C6/36 and Vero cells were consistently observed in 5 other strains (Beijing P1, Beijing P3, JaTH-160, KE-093 and JaGAr-O1), except for KE-093-infected C6/36 cells which exhibited the Vero-type sedimentation profile under conditions of open cultivation. By electron microscopy, the production of SMSs from KE-093-infected C6/36 cells under open conditions was markedly less than that under closed conditions where the cells exhibited the C6/36-type sedimentation profile. Thus, intracellular SMS formations were associated with extracellular SHA production in JEV-infected mosquito cells.

Subcellular localization and membrane topology of the Dengue virus type 2 Non-structural protein 4B

Dengue virus (DV) is a member of the family Flaviviridae. These positive strand RNA viruses encode a polyprotein that is processed in case of DV into 10 proteins. Although for most of these proteins distinct functions have been defined, this is less clear for the highly hydrophobic non-structural protein (NS) 4B. Despite its possible role as an antagonist of the interferon-induced antiviral response, this protein may play an additional more direct role for viral replication. In this study we determined the subcellular localization, membrane association, and membrane topology of DV NS4B. We found that NS4B resides primarily in cytoplasmic foci originating from the endoplasmic reticulum. NS4B colocalizes with NS3 and double-stranded RNA, an intermediate of viral replication, arguing that NS4B is part of the membrane-bound viral replication complex. Biochemical analysis revealed that NS4B is an integral membrane protein, and that its preceding 2K signal sequence is not required for this integration. We identified three membrane-spanning segments in the COOH-terminal part of NS4B that are sufficient to target a cytosolic marker protein to intracellular membranes. Furthermore, we established a membrane topology model of NS4B in which the NH2-terminal part of the protein is localized in the endoplasmic reticulum lumen, whereas the COOH-terminal part is composed of three trans-membrane domains with the COOH-terminal tail localized in the cytoplasm. This topology model provides a good starting point for a detailed investigation of the function of NS4B in the DV life cycle.

Identifying the region influencing the cis-mode of maturation of West Nile (Sarafend) virus using chimeric infectious clones

West Nile (Sarafend) virus [WN(S)V] has been shown to egress by budding at the plasma membrane of infected cells. However, the region influencing this mode of virus release remains to be deciphered. In this study, we have constructed three chimeric clones in which specific regions of West Nile (Wengler) virus [WN(W)V] were replaced for the corresponding regions of WN(S)V in the full-length infectious clone of WN(S)V to define the region responsible for the cis-mode of WN(S)V maturation. The WN(W)V matures by the trans-mode. All of the resulting chimeric viruses were found to be infective. Transmission electron microscopy analyses performed in Vero cells infected with these chimeric viruses disclosed that the 5' end of the WN(S)V genome plays a major role in influencing the process of maturation at the plasma membrane.

Ultrastructural study of West Nile virus pathogenesis in Culex pipiens quinquefasciatus (Diptera: Culicidae)

The ultrastructural features of West Nile virus (WNV) replication and dissemination in orally infected Culex pipiens quinquefasciatus Say were analyzed over a 25-d infection period. To investigate the effects of virus replication on membrane induction, cellular organization, and cell viability in midgut and salivary gland tissues, midguts were dissected on days 3, 7, 14, and 21, and salivary glands were collected on days 7, 14, 21, and 25 postinfection (d.p.i.) for examination by transmission electron microscopy (TEM). Whole mosquito heads were embedded for TEM analysis 14 d.p.i. to localize WNV particles and to investigate the effects of replication on nervous tissues of the brain. Membrane proliferation was induced by WNV in the midgut epithelium, midgut muscles, and salivary glands, although extensive endoplasmic reticulum swelling was a unique feature of salivary gland infection. TEM revealed WNV-induced pathology in salivary glands at 14, 21, and 25 d.p.i., and we hypothesize that long-term virus infection of this tissue results in severe cellular degeneration and apoptotic-like cell death. This finding indicates that the efficiency of WNV transmission may decrease with mosquito age postinfection.

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

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

Comparisons of physical separation methods of Kunjin virus-induced membranes

The two sets of connected membranes induced in Kunjin virus-infected cells are characterized by the presence of NS3 helicase/protease in both, and by RNA-dependent RNA polymerase (RdRp) activity plus the associated double-stranded RNA (dsRNA) template in vesicle packets (VP), or by the absence of both the VP-specific markers in the convoluted membranes/paracrystalline arrays (CM/PC). Attempts were made to separate flavivirus-induced membranes by sedimentation or flotation analyses in density gradients of sucrose or iodixanol, respectively, after treatment of cell lysates by sonication, osmotic shock, or tryptic digestion. Only osmotic shock treatment provided suggestive evidence of separation. This was explored by flow cytometry analysis (FCA) of RdRp active membrane fractions from a sucrose gradient, using dual fluorescent labelling via antibodies to NS3 and dsRNA. FCA revealed the presence of a dual labelled membrane population indicative of VP, and in a faster sedimenting fraction a membrane population able to be labelled only in NS3, representative of CM/PC and associated (R)ER. It was postulated that osmotic shock ruptured the bounding membrane of the VP, releasing the enclosed small vesicles associated with the Kunjin virus replication complex characterized previously. Notably, the presence of the full spectrum of nonstructural proteins in some membrane fractions was not a reliable marker for RdRp activity. These experiments may provide the opportunity for isolation of relatively pure flavivirus replication complexes in their native membrane-associated state by fluorescence-activated cell sorting.

Kunjin RNA replication and applications of Kunjin replicons

The Kunjin virus (KUNV) has provided a useful laboratory model for Flavivirus RNA replication. The synthesis of progeny RNA(+) strands occurs via asymmetric and semiconservative replication on a template of recycling double-stranded RNA (dsRna) or replicative form (RF). Kinetics of viral RNA synthesis indicated a cycle period of about 15 min during which, on average, a single nascent RNA (+) strand displaces the pre-existing RNA(+) strand in the replicative intermediate. Data on the composition of the replication complex (RC) in KUNV-infected cells were obtained from several sources, including analyses of the partially-purified still active RC, immunogold labeling of cryosections using monospecific antibodies to the nonstructural proteins and to the dsRNA, radioimmunoprecipitations of cell lysates using antibodies to dsRNA and to an RC-associated cell marker, and pull-down assays of cell lysates using fusion proteins GST-NS2A and GST-NS4A. These results yeilded a consensus composition of NS1, NS2A, NS3, NS4A, and NS5 strongly associated with the dsRNA template. The RC was located in induced membranes described as vesicle packets. The RNA-dependent RNA polymerase activity late in infection did not require continuing protein synthesis. Replication of genomic RNA was completely dependent on the presence of conserved complementary or cyclization sequences near the 5' and 3' ends. Assembly of the RC during translation in cis and the relationships, particularly those of NS1 and NS5 among the components, were deduced from an extensive set of complementation experiments in trans involving mutations/deletions in all the nonstructural proteins and use of KUN or alphahavirus replicons as helpers. The KUN replicon has found useful applications also as a noncytopathic vector for the continuing expression of foreign genes, delivered either as packaged RNA or as plasmid DNA.

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

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

Association of Japanese encephalitis virus NS3 protein with microtubules and tumour susceptibility gene 101 (TSG101) protein

Previously reported findings by our group showed that non-structural protein 3 (NS3) of Japanese encephalitis virus (JEV) was localized mainly in the JEV-induced convoluted membrane (CM), which has been proposed to originate from rough endoplasmic reticulum (rER), Golgi apparatus or the trans-Golgi network (TGN), and serves as a reservoir for viral proteins during virus assembly. Earlier findings indicated that NS3 of Kunjin virus interacts with microtubules. In addition, one of the Golgi-associated proteins, tumour susceptibility protein 101 (TSG101), associates with microtubules and is required for budding of retroviral particles. To clarify the association of NS3 with microtubules or with TSG101 during JEV assembly, we applied immunofluorescence, co-immunoprecipitation and immunoelectron microscopic methods. Virus infection, as well as transfection with an NS2B-NS3 expression plasmid, induced microtubule rearrangement. When cells were treated with colchicine, which interferes with microtubule polymerization, NS3 still associated with tubulin and TSG101. Furthermore, tubulin and TSG101 were co-localized with NS3 in the CM by immunogold labelling. Our observations indicate that microtubules and TSG101 associate with NS3, which is incorporated into the JEV-induced structure during JEV replication.

MHC class I up-regulation by flaviviruses: Immune interaction with unknown advantage to host or pathogen

In contrast to many other viruses that escape from cytotoxic T cell recognition by down-regulating major histocompatibility complex class I-restricted antigen presentation, flavivirus infection of mammalian cells up-regulates cell surface expression of major histocompatibility complex class I molecules. Two putative mechanisms for flavivirus-induced major histocompatibility complex class I up-regulation, one via activation of the transcription factor NF-kappaB, the second by augmentation of peptide import into the lumen of the endoplasmic reticulum, are reviewed, and the biological effect of the flavivirus-mediated phenomenon on target cell recognition by natural killer and cytotoxic T cells is addressed. Finally, we speculate on the physiological role of flavivirus-mediated modulation of major histocompatibility complex class I antigen presentation in the context of the biology of flavivirus transmission between the vertebrate host and arthropod vector and suggest that it may represent a strategy for immune evasion from the natural killer cell response or, alternatively, that up-regulation of major histocompatibility complex class I is a by-product of flavivirus replication without significance for virus growth.

Assembly and maturation of the flavivirus Kunjin virus appear to occur in the rough endoplasmic reticulum and along the secretory pathway, respectively

The intracellular assembly site for flaviviruses in currently not known but is presumed to be located within the lumen of the rough endoplasmic reticulum (RER). Building on previous studies involving immunofluorescence (IF) and cryoimmunoelectron microscopy of Kunjin virus (KUN)-infected cells, we sought to identify the steps involved in the assembly and maturation of KUN. Thus, using antibodies directed against envelope protein E in IF analysis, we found the accumulation of E within regions coincident with the RER and endosomal compartments. Immunogold labeling of cryosections of infected cells indicated that E and minor envelope protein prM were localized to reticulum membranes continuous with KUN-induced convoluted membranes (CM) or paracrystalline arrays (PC) and that sometimes the RER contained immunogold-labeled virus particles. Both proteins were also observed to be labeled in membranes at the periphery of the induced CM or PC structures, but the latter were very seldom labeled internally. Utilizing drugs that inhibit protein and/or membrane traffic throughout the cell, we found that the secretion of KUN particles late in infection was significantly affected in the presence of brefeldin A and that the infectivity of secreted particles was severely affected in the presence of monensin and N-nonyl-deoxynojirimycin. Nocodazole did not appear to affect maturation, suggesting that microtubules play no role in assembly or maturation processes. Subsequently, we showed that the exit of intact virions from the RER involves the transport of individual virions within individual vesicles en route to the Golgi apparatus. The results suggest that the assembly of virions occurs within the lumen of the RER and that subsequent maturation occurs via the secretory pathway.

Pathology of fatal West Nile virus infections in native and exotic birds during the 1999 outbreak in New York City, New York

West Nile fever caused fatal disease in humans, horses, and birds in the northeastern United States during 1999. We studied birds from two wildlife facilities in New York City, New York, that died or were euthanatized and were suspected to have West Nile virus infections. Using standard histologic and ultrastructural methods, virus isolation, immunohistochemistry, in situ hybridization and reverse-transcriptase polymerase chain reaction, we identified West Nile virus as the cause of clinical disease, severe pathologic changes, and death in 27 birds representing eight orders and 14 species. Virus was detected in 23/26 brains (88%), 24/ 25 hearts (96%), 15/18 spleens (83%), 14/20 livers (70%), 20/20 kidneys (100%), 10/13 adrenals (77%), 13/ 14 intestines (93%), 10/12 pancreata (83%), 5/12 lungs (42%), and 4/8 ovaries (50%) by one or more methods. Cellular targets included neurons and glial cells in the brain, spinal cord, and peripheral ganglia; myocardial fibers; macrophages and blood monocytes; renal tubular epithelium; adrenal cortical cells; pancreatic acinar cells and islet cells; intestinal crypt epithelium; oocytes; and fibroblasts and smooth muscle cells. Purkinje cells were especially targeted, except in crows and magpies. Gross hemorrhage of the brain, splenomegaly, meningoencephalitis, and myocarditis were the most prominent lesions. Immunohistochemistry was an efficient and reliable method for identifying infected cases, but the polyclonal antibody cross-reacted with St. Louis encephalitis virus and other flaviviruses. In contrast, the in situ hybridization probe pWNV-E (WN-USAMRIID99) reacted only with West Nile virus. These methods should aid diagnosticians faced with the emergence of West Nile virus in the United States.

Markers for trans-Golgi membranes and the intermediate compartment localize to induced membranes with distinct replication functions in flavivirus-infected cells

Replication of the flavivirus Kunjin virus is associated with virus-induced membrane structures within the cytoplasm of infected cells; these membranes appear as packets of vesicles associated with the sites of viral RNA synthesis and as convoluted membranes (CM) and paracrystalline arrays (PC) containing the components of the virus-specified protease (E. G. Westaway, J. M. Mackenzie, M. T. Kenney, M. K. Jones, and A. A. Khromykh, J. Virol. 71:6650-6661, 1997). To determine the cellular origins of these membrane structures, we compared the immunolabelling patterns of several cell markers in relation to these sites by immunofluorescence and immunoelectron microscopy. A marker for the trans-Golgi membranes and the trans-Golgi network, 1,4-galactosyltransferase (GalT), was redistributed to large foci in the cytoplasm of Kunjin virus-infected cells, partially coincident with immunofluorescent foci associated with the putative sites of viral RNA synthesis. As determined by immunoelectron microscopy, the induced vesicle packets contained GalT, whereas the CM and PC contained a specific protein marker for the intermediate compartment (ERGIC53). A further indicator of the role of cellular organelles in their biogenesis was the observation that the Golgi apparatus-disrupting agent brefeldin A prevented further development of immunofluorescent foci of induced membranes if added before the end of the latent period but that once formed, these membrane foci were resistant to brefeldin A dispersion. Reticulum membranes emanating from the induced CM and PC were also labelled with the rough endoplasmic reticulum marker anti-protein disulfide isomerase and were obviously redistributed during infection. This is the first report identifying trans-Golgi membranes and the intermediate compartment as the apparent sources of the flavivirus-induced membranes involved in events of replication.

Subcellular localization and some biochemical properties of the flavivirus Kunjin nonstructural proteins NS2A and NS4A

In a previous study on the replication of Kunjin virus using immunoelectron microscopy (E. G. Westaway, J. M. Mackenzie, M. T. Kenney, M. K. Jones, and A. A. Khromykh, 1997, J. Virol. 71, 6650-6661), NS1 and NS3 were found associated with double-stranded RNA (dsRNA) within vesicle packets (VP) in infected Vero cells, suggesting that these induced membrane structures may be the cytoplasmic sites of RNA replication. NS2B and NS3 (comprising the virus-encoded protease) were colocalized within distinct paracrystalline (PC) or convoluted membranes (CM), also induced in the cytoplasm, suggesting that these membranes are the sites of proteolytic cleavage. In this study we found by immunofluorescence (IF) that the small hydrophobic nonstructural proteins NS2A and NS4A were located in discrete foci in the cytoplasm of infected cells at both 16 and 24 h postinfection, partially coincident with dsRNA foci. In cryosections of infected cells at 24 h, NS2A was located by immunogold labeling primarily within VP, associated with labeled dsRNA. NS2A fused to glutathione S-transferase (GST) bound strongly to the 3' untranslated region of Kunjin RNA and also to the proposed replicase components NS3 and NS5 in cell lysates. NS4A was localized by immunogold labeling within a majority of the virus-induced membranes, including VP, CM, and PC. GST-NS4A bound weakly to the 3' untranslated region of Kunjin RNA but was bound to NS4A strongly and to most of the other viral nonstructural proteins, including NS3 and NS5. Taken together the results indicate that the flavivirus replication complex includes NS2A and NS4A in the VP in addition to the previously identified NS1 and NS3.

Ultrastructure of Kunjin virus-infected cells: colocalization of NS1 and NS3 with double-stranded RNA, and of NS2B with NS3, in virus-induced membrane structures

The subcellular location of the nonstructural proteins NS1, NS2B, and NS3 in Vero cells infected with the flavivirus Kunjin was investigated using indirect immunofluorescence and cryoimmunoelectron microscopy with monospecific antibodies. Comparisons were also made by dual immunolabelling using antibodies to double-stranded RNA (dsRNA), the putative template in the flavivirus replication complex. At 8 h postinfection, the immunofluorescent patterns showed NS1, NS2B, NS3, and dsRNA located in a perinuclear rim with extensions into the peripheral cytoplasm. By 16 h, at the end of the latent period, all patterns had changed to some discrete perinuclear foci associated with a thick cytoplasmic reticulum. By 24 h, this localization in perinuclear foci was more apparent and some foci were dual labelled with antibodies to dsRNA. In immuno-gold-labelled cryosections of infected cells at 24 h, all antibodies were associated with clusters of induced membrane structures in the perinuclear region. Two important and novel observations were made. First, one set of induced membranes comprised vesicle packets of smooth membranes dual labelled with anti-dsRNA and anti-NS1 or anti-NS3 antibodies. Second, adjacent masses of paracrystalline arrays or of convoluted smooth membranes, which appeared to be structurally related, were strongly labelled only with anti-NS2B and anti-NS3 antibodies. Paired membranes similar in appearance to the rough endoplasmic reticulum were also labelled, but less strongly, with antibodies to the three nonstructural proteins. Other paired membranes adjacent to the structures discussed above enclosed accumulated virus particles but were not labelled with any of the four antibodies. The collection of induced membranes may represent virus factories in which translation, RNA synthesis, and virus assembly occur.

Proteins C and NS4B of the flavivirus Kunjin translocate independently into the nucleus

The subcellular locations in infected Vero cells of Kunjin (KUN) virus core protein C and NS4B were analyzed by immunofluorescence (IF) and by immunoelectron microscopy using monospecific antibodies. Selection of appropriate fixation methods for IF showed that both proteins were associated at all times with perinuclear membranes spreading outward in a reticular pattern and they entered the nucleus late during the latent period. Subsequently NS4B was also dispersed through the nucleoplasm, while C appeared in the nucleolus and the nucleoplasm. These nuclear locations were confirmed by immunogold labeling of cryosections of infected cells at 24 hr postinfection. Labeling of NS4B in cryosections was especially enriched in the perinuclear membranes of the endoplasmic reticulum. When C and NS4B were each expressed separately in stably transformed cell lines, both cytoplasmic and nuclear localization was observed by IF and confirmed by immunoelectron microscopy. Thus the two proteins translocated to the nucleus independently of each other and of other viral proteins. Dual IF with antibodies to double-stranded RNA showed that cytoplasmic locations of C and NS4B were apparently associated in part with the sites of viral RNA synthesis which were resistant to solubilization by Triton X-100.

Improved membrane preservation of flavivirus-infected cells with cryosectioning

Ultra-cryomicrotomy and electron microscopy were used to investigate membranous structures in dengue virus-infected mammalian and insect cells. The cryo-sectioned samples displayed ultrastructure comparable to their resin-embedded counterparts with all previously identified virus-induced structures being observed. Structures not previously identified were also found. In particular, membrane-bound packets of vesicles, 100-200 nm in diameter were seen distributed throughout areas of virus-induced membrane proliferation. These packets were clearly distinct from virion arrays. Small smooth membrane vesicles, previously found to contain thread-like enclosures (M.L. Ng, J. Gen. Virol. 68 (1987) 577-582), were frequently observed to contain dense staining material, however the exact nature of this material remains unclear. Virus-induced modification of golgi-like and/or ER membranes was also observed and may represent early events in the generation of the smooth membrane vesicles seen during infection. We suggest that cryosectioning is the method of choice to investigate membrane rearrangement induced by this family of viruses and that a diamond knife and modified staining techniques, as utilised in this report, be employed to enhance morphology and section preservation.

Virion-like structures in HeLa G cells transfected with the full-length sequence of the hepatitis C virus genome

BACKGROUND & AIMS

The process and the site of hepatitis C virus (HCV) particle formation in cells after infection remain unknown. The aim of this study was to create an in vitro model for the study of HCV particle formation.

METHODS

HeLa G cells were transfected with the full-length sequence of the HCV genome. Viral protein expression was analyzed using immunoblotting. The cells were examined using immunoelectron and conventional electron microscopy.

RESULTS

Core, E2, NS3, NS5a, and NS5b proteins were identified using immunoblotting. Immunoelectron microscopy showed that the core antigen was located along the membrane of the endoplasmic reticulum (ER) and occasionally in its cisternae. Core antigen-positive particles of 30 nm in diameter were found in the cytosol and in the cisternae of the ER. The particles in the cisternae were coated with an outer membrane that was connected to the ER membrane. Conventional electron microscopy revealed particles of 45 nm in diameter with electron-dense cores in the cisternae of the ER. The outer membrane of the particles was occasionally connected to the ER membrane.

CONCLUSIONS

The findings suggest that HCV core proteins are synthesized and assembled into particles in the cytosol and that they bud into the cisternae of the ER to form coated particles.

Cryosubstitution technique reveals new morphology of flavivirus-induced structures

Cryotechniques in combination with electron microscopy were used in an attempt to obtain more precise morphological details of flavivirus-induced structures. From conventional chemical fixation procedure, proliferation of endoplasmic reticulum, formation of microtubule paracrystals and clusters of smooth membrane vesicles (with 'thread-like' enclosures) were observed. These induced changes are typical for flavivirus infections. The images obtained from cryosections were disappointing as the structures were not well preserved. On the other hand, cryosubstituted-infected cells gave revealing images of the virus-induced structures. The most obvious difference between the cryosubstituted and chemical fixed processes was on the morphology of the 'thread-like' structure. The 'thread-like' structures instead appeared as dense cores. The morphology of the virus particles was also better defined. The envelope of the virus appeared clearly differentiated from the nucleocapsid. The most important finding was that the cryosubstituted technique was able to preserve the structures of the flavivirus nucleocapsids which so far has not been convincing reported with chemical processing.

Flavivirus West Nile (Sarafend) egress at the plasma membrane

West Nile (Sarafend) virus was distinctly observed to bud from the plasma membrane rather than mature intracellularly. This has been observed with transmission electron microscopy. Using conventional scanning electron microscopy, budding at the plasma membrane especially at the filopodia was clearly illustrated. Immunogold labelling against the virus envelope protein was also performed to confirm this mode of exit. The gold particles were observed to be located at the sites where virus budding was seen under the field emission scanning electron microscope.

Inhibition of African swine fever virus binding and infectivity by purified recombinant virus attachment protein p12

The African swine fever virus protein p12, involved in virus attachment to the host cell, has an apparent molecular mass of 17 kDa in sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions. We have also identified 12- and 10-kDa forms of the p12 protein in infected Vero cells and found that the mature 17-kDa protein is the only form present in virus particles. The p12 protein has been produced in large amounts in Spodoptera frugiperda insect cells infected with a recombinant baculovirus. A 17-kDa protein that possessed the biological properties of the viral protein was produced, since it bound to susceptible Vero cells and not to receptor-negative L cells, which do not support virus replication. The binding of the baculovirus-expressed protein p12 to Vero cells was specifically blocked by virus particles. In addition, the recombinant protein purified by immunoaffinity chromatography blocked the specific binding of virus particles to susceptible cells and prevented infection, demonstrating that the p12 protein mediates the attachment of virions to specific receptors and indicating that blocking the p12-mediated interaction between African swine fever virus and receptors in Vero cells can inhibit infection. However, although antibodies specific for protein p12 are induced in natural infections and in animals inoculated with inactivated virus or recombinant protein p12, these antisera did not inhibit virus binding to the host cell or neutralize virus infectivity.

Comparison of replication rates and pathogenicities between the SA14 parent and SA14-14-2 vaccine strains of Japanese encephalitis virus in mouse brain neurons

The replication rates and pathogenicities of the SA 14 parent and SA 14-14-2 vaccine strains of Japanese encephalitis (JE) virus in neurons of the mouse brain following intracerebral inoculation were compared. All the mice inoculated with the SA 14 parent strain died within one week postinoculation (p.i.), whereas all the mice inoculated with the SA 14-14-2 vaccine strains survived without showing any signs of central nervous system (CNS) involvement. The virus titers of the mouse brains inoculated with the SA 14 strain reached progressively higher levels until day 5 when the animals died. On the other hand, the virus titers of the mouse brains inoculated with the SA 14-14-2 strain persisted at low levels for several days and could not be detected after 10 days. In the routine electron microscopical study, a majority of neurons in the mouse brains inoculated with the SA 14 strain contained virions and showed characteristic cytopathological changes in connection with viral replication. In the brains inoculated with the SA 14-14-2 strain, however, we failed to find neurons containing virions or showing characteristic cytopathological changes. In the alkaline phosphatase immunostaining of paraffin-embedded sections, a majority of neurons in the brains of mice inoculated with the SA 14 strain stained positively on day 5 p.i., but only a small number of neurons in scattered small foci stained positively in the brains inoculated with the SA 14-14-2 strain. The immunogold staining of Vibratome sections also revealed the identical patterns; moreover, electron microscopical examination of the immunopositive foci of the brain inoculated with the vaccine strain revealed neurons that contained virions in dilated cisternae of rough endoplasmic reticulum (RER), indicating that the SA 14-14-2 strain also replicated, albeit poorly, in neurons. The present results showed that upon intracerebral inoculation into mice the SA 14 parent strain of JE virus grew vigorously in a large number of neurons, killing the animals, while the SA 14-14-2 vaccine strain grew poorly only in a small number of neurons without causing mortality. Possible mechanisms involved in the alteration of pathogenicity between the SA 14 parent virus and the SA 14-14-2 vaccine virus are discussed.

Cytopathology of PC12 cells infected with Japanese encephalitis virus

Infection of a clonal rat pheochromocytoma cell line, PC12, with Japanese encephalitis (JE) virus produced successively higher titers of virus in the culture fluid during the 72-h experimental period. In electron microscopical observation, JE virus entered PC12 cells by direct penetration through the plasma membrane at 2 min postinoculation (p.i.) and caused marked cellular hypertrophy and extensive proliferation of the cellular secretory system including rough endoplasmic reticulum (RER) and Golgi complexes starting 24 h p.i. The proliferating RER of the virally infected cells contained progeny virions and characteristic endoplasmic reticulum vesicles in its cisternae, and the proliferating Golgi complexes contained virions in their saccules. These findings indicated that the proliferation of the cellular secretory system occurred in association with viral replication and maturation in the system. Seventy-two hours p.i., the cellular secretory system of infected PC12 cells showed degenerative changes with vesiculation, disorganization, and dispersion of the Golgi complexes and fragmentation, focal cystic dilation, and dissolution of the RER in the same manner as those seen in the secretory system of JE-virus-infected neurons in the mouse brain. Thus, JE-virus-infected PC12 cells seem to be a suitable neurogenic cell line for the study of the pathogenic mechanism of JE virus. At the same time, the virally infected cells seem to offer an interesting cell model for the study of the morphogenesis of the cellular secretory system.

Characterization of the hepatitis C virus E2/NS1 gene product expressed in mammalian cells

Truncated and full-length versions of the hepatitis C virus protein domain encoding a presumptive envelope glycoprotein designated E2/NS1 were stably expressed in CHO cell lines. Characterization of the processing events involved in the maturation of E2/NS1 revealed that a high-mannose form resident in the endoplasmic reticulum was the most abundant form detected intracellularly. The ionophore carboxyl cyanide m-chlorophenyl-hydrazone was used to show that the E2/NS1 glycoprotein resided in the endoplasmic reticulum. The full-length form of E2/NS1 appeared to be cell-associated and could not be detected as a secreted product. C-terminal truncated molecules could be detected in the extracellular media as fully processed glycoproteins containing terminal sialic acid additions. These truncated glycoproteins are predicted to be biologically relevant targets of the host immune response and are therefore potential subunit vaccine candidates.

Comparison of centrifugation methods for molecular and morphological analysis of membranes associated with RNA replication of the flavivirus Kunjin

Kunjin virus-infected cells were lysed and the cytoplasmic extract was subjected to sedimentation analysis. After centrifugation at 16,000 x g for 10 min about 70% of the original RNA-dependent RNA polymerase (RDRP) was recovered in the pellet; most of this enzymic activity was recovered in the soluble fraction after treatment with NP40 detergent. Membrane fractions were prepared from cytoplasmic extracts by centrifugation in discontinuous density gradients comprising w/w or w/v sucrose solutions, either for 3 h (top-loaded on 4 ml 20-60% sucrose) or for 19 h (centre-loaded in 37 ml 0-60% sucrose). Similar separations of bands of light membranes were obtained in all gradients. Multi-layered heavy membrane bands obtained with w/w sucrose gradients were resolved into two well-separated bands (F4 and F5) using w/v sucrose gradients. Thin-section electron microscopy of embedded membrane fractions, gel analysis of intracellular RNA, and RDRP assays showed that the w/w centre loading method and the w/v top-loading (short spin) method produced similar recoveries and distributions of smooth and rough membranes, intact virus particles and RDRP activity. The distribution of intracellular viral RNA and proteins was coincident with the RDRP, all being located in the F4 and F5 bands which contained the characteristic membrane structures induced during flavivirus infection. Significant advantages of the preferred method (w/v sucrose, top loading and short spin) were its rapidity, good preservation of membranes and RDRP, and the concentrations of RDRP achieved in the small volume fractions collected from a total of 4.5 ml.

Molecular and ultrastructural analysis of heavy membrane fractions associated with the replication of Kunjin virus RNA

In subcellular extracts of Kunjin virus-infected cells prepared by lysis and differential centrifugation, the viral RNA polymerase, RNA and proteins were associated mainly with cytoplasm. When the cytoplasmic extract (500 g supernate) of infected cells labelled for 3 h from 24 h post-infection was further fractionated by rapid centrifugation through a sucrose density gradient, all viral products were located only in dense or "heavy membrane" fractions, which contained three types of virus-induced morphologically distinct membrane structures. These dense fractions were treated with 0.5% NP40 and the soluble material was again centrifuged through a sucrose gradient for analyses as before. Viral RNA polymerase activity was retained and was associated with replicative intermediate RNA and some replicative form RNA in the peak enzyme fractions sedimenting at 20S to 40S. Enrichment of NS3 and of the small nonstructural proteins NS2A and NS2B/NS4A was apparent in these fractions which were well separated from the slow sedimenting structural proteins. No detergent-resistant structures in the "heavy membrane" fractions other than ribosome-like particles were visible. The data show that the RNA polymerase complex cosedimented with virus-induced membrane structures and remained associated with specific nonstructural proteins and replicative intermediate RNA after detergent treatment.

Golgi complex localization of the Punta Toro virus G2 protein requires its association with the G1 protein

The glycoproteins of bunyaviruses accumulate in membranes of the Golgi complex, where virus maturation occurs by budding. In this study we have constructed a series of full length or truncated mutants of the G2 glycoprotein of Punta Toro virus (PTV), a member of the Phlebovirus genus of the Bunyaviridae, and investigated their transport properties. The results indicate that the hydrophobic domain preceding the G2 glycoprotein can function as a translocational signal peptide, and that the hydrophobic domain near the C-terminus serves as a membrane anchor. A G2 glycoprotein construct with an extra hydrophobic sequence derived from the N-terminal NSM region was stably retained in the ER, and was unable to be transported to the Golgi complex. The full-length G2 glycoprotein, when expressed on its own, was transported out of the ER and expressed on the cell surface, whereas the G1 and G2 proteins when expressed together are retained in the Golgi complex. A truncated anchor-minus form of the G2 glycoprotein was found to be secreted into the culture medium, but was retained in the Golgi complex when coexpressed with the G1 glycoprotein. These results indicate that the G2 membrane glycoprotein is a class I membrane protein which does not contain a signal sufficient for Golgi retention, and suggest that its Golgi localization is a result of association with the G1 glycoprotein.

Assembly and polarized release of Punta Toro virus and effects of brefeldin A

Punta Toro virus (PTV), a member of the sandfly fever group of bunyaviruses, is assembled by budding at intracellular membranes of the Golgi complex. We have examined PTV glycoprotein transport, assembly, and release and the effects of brefeldin A (BFA) on these processes. Both the G1 and G2 proteins were transported out of the endoplasmic reticulum (ER) and retained in the Golgi complex in a stable structure, either during PTV infection or when expressed from a vaccinia virus recombinant. BFA treatment causes a rapid and dramatic change in the distribution of the G1 and G2 proteins, from a Golgi pattern to an ER pattern. The G1 and G2 proteins were found to be modified by medial but not trans Golgi network enzymes, in the presence or absence of BFA. We found that BFA blocks PTV release from cells but does not interfere with the intracellular assembly of infectious virions. Further, the BFA block of virus release is fully reversible, with high levels of virus release occurring upon removal of the inhibitor. It was also found that the release of PTV virions is polarized, occurring exclusively from the basolateral surfaces of the polarized Vero C1008 epithelial cell line.

The dengue viruses

Dengue, a major public health problem throughout subtropical and tropical regions, is an acute infectious disease characterized by biphasic fever, headache, pain in various parts of the body, prostration, rash, lymphadenopathy, and leukopenia. In more severe or complicated dengue, patients present with a severe febrile illness characterized by abnormalities of hemostasis and increased vascular permeability, which in some instances results in a hypovolemic shock. Four distinct serotypes of the dengue virus (dengue-1, dengue-2, dengue-3, and dengue-4) exist, with numerous virus strains found worldwide. Molecular cloning methods have led to a greater understanding of the structure of the RNA genome and definition of virus-specific structural and nonstructural proteins. Progress towards producing safe, effective dengue virus vaccines, a goal for over 45 years, has been made.

Membrane association and secretion of the Japanese encephalitis virus NS1 protein from cells expressing NS1 cDNA

The biosynthesis of the flavivirus nonstructural glycoprotein, NS1, has important implications for (1) vaccine production, since NS1 immunity can protect animals from flavivirus infection; and (2) virion maturation, since NS1 is coretained with immature forms of the structural glycoprotein E in the endoplasmic reticulum (ER) of infected cells. To examine the molecular basis for NS1 retention within the ER we have expressed fragments of Japanese encephalitis virus (JEV) cDNA encoding NS1 in BHK cells. These transient expression studies showed that the JEV NS1 protein was faithfully processed when expressed in isolation, and have revealed that NS1 expressed in the absence of any other viral genes behaves like the NS1 protein found in JEV-infected cells with respect to retention in the ER, secretion, glycosylation, membrane association, and dimerization.

Maturation of Japanese encephalitis virus glycoproteins produced by infected mammalian and mosquito cells

The Japanese encephalitis virus (JE) structural glycoprotein (E) and two nonstructural glycoproteins (NS1 and NS1') were processed differently by JE-infected vertebrate and invertebrate cell lines. All three proteins were released slowly (t1/2 greater than 6 hr) from JE-infected monkey cells (Vero cells). Mosquito cell lines released E at a similar rate (t1/2 greater than 8 hr), while NS1 and NS1' were retained in an undegraded form in the cell layer. The proteolytic processing of the three proteins appeared identical in both cell types, but some differences in N-linked glycosylation were observed. E, NS1, and NS1' found within the infected cells of both types contained high-mannose oligosaccharide groups for more than 8 hr after synthesis. Additional sugar residues were added to the single E protein oligosaccharide group prior to release from Vero cells, while sugar residues were trimmed from the E protein oligosaccharide group prior to release from mosquito cells. The forms of NS1 and NS1' found in the culture fluid of infected Vero cells contained one complex and one high-mannose oligosaccharide. All three glycoproteins released from JE-infected Vero cells were associated with extracellular particles, the virion in the case of E and a low density particle in the case of and NS1' exhibited amphipathic properties in Triton X-114 extraction experiments. Taken together, these results suggest that both the structural (E) and nonstructural (NS1 and NS1') glycoproteins were pathway of the infected Vero cells, assembled into particles, and then released into the extracellular fluid.

Flavivirus infection: essential ultrastructural changes and association of Kunjin virus NS3 protein with microtubules

Virus-induced vesicles evolved early in the Kunjin virus replication cycle around 9 to 10 h p.i. just before the end of the latent period in infected Vero cells. About 2 h following the appearance of the vesicles, microtubule paracrystals were also formed. These two virus-induced structures seemed interlinked and have essential roles in Kunjin virus replication. A viral protein NS3 was found to be associated with the microtubule component of the cells. When vinblastine sulphate was added to the cells immediately after infection, formation of the paracrystals was delayed by two hours, and the affiliation of NS3 protein was also observed to be rearranged.

Intracellular accumulation of Punta Toro virus glycoproteins expressed from cloned cDNA

The Punta Toro virus (PTV) middle size (M) RNA encodes two glycoproteins, G1 and G2, and possibly a nonstructural protein, NSM. A partial cDNA clone of the M segment which contains G1 and G2 glycoprotein coding sequences but lacks most of the NSM sequences was inserted into the genome of vaccinia virus under the control of an early vaccinia promoter. Cells infected with the recombinant virus were found to synthesize two polypeptides with molecular weights of 65,000 (G1) and 55,000 (G2) that reacted specifically with antibody against PTV. Studies using indirect immunofluorescence microscopy revealed that these proteins accumulated intracellularly in the perinuclear region. The results of endoglycosidase H digestion of these glycoproteins suggested that both G1 and G2 glycoproteins were transported from the RER to the Golgi complex. These proteins were not chased out from the Golgi region during a 6-hr incubation in the presence of cycloheximide. Surface immune precipitation and 125I-protein A binding assays also demonstrated that the majority of the G1 and G2 glycoproteins are retained intracellularly. These results indicate that the PTV glycoproteins contain the necessary information for retention in the Golgi apparatus.

Maturation process of Japanese encephalitis virus in cultured mosquito cells in vitro and mouse brain cells in vivo

The maturation process of Japanese encephalitis (JE) virus in C6/36 cells in vitro and in mouse brain cells in vivo was studied by electron microscopy. In the C6/36 cell infection, 500 to 2250 virions per cell were released into the medium during the period of study; yet, no virus budding process was observed at the host cell membranes. JE virions at various maturation stages appeared within the cisternae of rough endoplasmic reticulum (RER) of infected cells at 24 hours p.i.; and, although C6/36 cells did not show a well-developed Golgi apparatus, the virions appeared to be carried to the cell surface within host-cell secretory vesicles for extracellular release as early as 24 hours p.i. The occurrence of a secretory-type intracellular transport of maturing JE virus particles was well recognizable in brain cells of infected mice, in which JE virus particles were found almost exclusively in the cisternae of RER, in the Golgi apparatus, and in various vesicles, including coated vesicles, in the vicinity of the Golgi apparatus. Our previous study of dengue-2 virus morphogenesis and our present study of JE virus morphogenesis differed substantially at various stages of maturation. Possible mechanisms which explain these differences were discussed.

Variation in distribution of the three flavivirus-specified glycoproteins detected by immunofluorescence in infected Vero cells

Indirect immunofluorescence with rabbit antisera was used to probe the intracellular locations of the antigens of envelope, prM (precursor to structural protein M) and the nonstructural glycoproteins NS 1 (formerly described as NV 3 or SCF) specified by the flaviviruses dengue-2 and Kunjin. Perinuclear staining in various types of foci was prominent for all antigens, and the distribution was influenced by whether cells were fixed with acetone or formaldehyde. Staining of Golgi-like masses or inclusions by anti-envelope sera occurred regularly and prominently in cells infected and stained with homologous anti-envelope antibodies; in the cross reactions, such staining was largely absent, especially in dengue-2 infected cells in which it was replaced by many small circular foci scattered throughout the cytoplasm. Anti-NS 1 also stained large perinuclear inclusions and small cytoplasmic foci, but the distribution of these was dissimilar to that observed with anti-envelope sera. Anti-prM appeared to contain a mixture of antibodies of different specificities, evident at different dilutions, possibly because of different cytoplasmic locations of prM and its cleavage products. All antisera produced small discontinuous foci on the plasma membrane of unfixed infected cells; antigens of NS 1 were sometimes prominent on the surface of acetone-fixed cells.