Linkage of an alphavirus host-range restriction to the carbohydrate-processing phenotypes of the host cell

Mutation of N-glycosylation Sites in Salmonid Alphavirus (SAV) Envelope Proteins Attenuate the Virus in Cell Culture

Salmonid alphavirus (SAV) is the cause of pancreas disease and sleeping disease in farmed salmonid fish in Europe. The spread of these diseases has been difficult to control with biosecurity and current vaccination strategies, and increased understanding of the viral pathogenesis could be beneficial for the development of novel vaccine strategies. N-glycosylation of viral envelope proteins may be crucial for viral virulence and a possible target for its purposed attenuation. In this study, we mutated the N-glycosylation consensus motifs of the E1 and E2 glycoproteins of a SAV3 infectious clone using site-directed mutagenesis. Mutation of the glycosylation motif in E1 gave a complete inactivation of the virus as no viral replication could be detected in cell culture and infectious particles could not be rescued. In contrast, infectious virus particles could be recovered from the SAV3 E2 mutants (E2319Q, E2319A), but not if they were accompanied by lack of N-glycosylation in E1. Compared to the non-mutated infectious clone, the SAV3-E2319Q and SAV3-E2319A recombinant viruses produced less cytopathic effects in cell culture and lower amounts of infectious viral particles. In conclusion, the substitution in the N-linked glycosylation site in E2 attenuated SAV3 in cell culture. The findings could be useful for immunization strategies using live attenuated vaccines and testing in fish will be desirable to study the clone’s properties in vivo.

An influenza A virus vaccine based on an M2e-modified alphavirus

The 23-residue external domain of the influenza A virus M2 protein (M2e) has significant potential as a vaccine antigen. Here, we describe the construction and characterization of an M2e-modified Sindbis virus designated E2S1-M2e. E2S1-M2e virions contain M2e as an N-terminal extension of the E2 glycoprotein and therefore express 240 copies of the M2e peptide on their surface. The E2S1-M2e virus expressed M2e in an accessible and immunogenic form and induced M2e-specific antibodies when administered to mice. Mice that received an intranasal vaccination with E2S1-M2e were protected against a lethal challenge with a virulent, mouse-adapted strain of influenza A virus.

Encapsidation of host-derived factors correlates with enhanced infectivity of Sindbis virus

The genus Alphavirus consists of a group of enveloped, single-stranded RNA viruses, many of which are transmitted by arthropods to a wide range of vertebrate host species. Here we report that Sindbis virus (SINV) produced from a representative mammalian cell line consists of at least two unique particle subpopulations, separable on the basis of virion density. In contrast, mosquito-derived SINV consists of a homogeneous population of particles. Our findings indicate that the denser particle subpopulation, SINV(Heavy), is more infectious on a per-particle basis than SINV(Light). SINV produced in mosquito cell lines (SINV(C6/36)) exhibited particle-to-PFU ratios similar to those observed for SINV(Heavy). In mammalian cells, viral RNA was synthesized and accumulated more rapidly following infection with SINV(Heavy) or SINV(C6/36) than following infection with SINV(Light), due partly to enhanced translation of viral genomic RNA early in infection. Analysis of the individual particle subpopulations indicated that SINV(Heavy) and SINV(C6/36) contain host-derived factors whose presence correlates with the enhanced translation, RNA synthesis, and infectivity observed for these particles.

An arthropod enzyme, Dfurin1, and a vertebrate furin homolog display distinct cleavage site sequence preferences for a shared viral proprotein substrate

Alphaviruses replicate in vertebrate and arthropod cells and utilize a cellular enzyme called furin to process the PE2 glycoprotein precursor during virus replication in both cell types. Furin cleaves PE2 at a site immediately following a highly conserved four residue cleavage signal. Prior studies demonstrated that the amino acid immediately adjacent to the cleavage site influenced PE2 cleavage differently in vertebrate and mosquito cells (HW Heidner et al. 1996 . Journal of Virology 70: 2069-2073.). This finding was tentatively attributed to potential differences in the substrate specificities of the vertebrate and arthropod furin enzymes or to differences in the carbohydrate processing phenotypes of arthropod and vertebrate cells. To further address this issue, we evaluated Sindbis virus replication and PE2 cleavage in the Chinese hamster, Cricetulus griseus Milne-Edwards (Rodentia: Cricetidae) ovary cells (CHO-K1) and in a CHO-K1-derived furin-negative cell line (RPE.40) engineered to stably express the Dfurin1 enzyme of Drosophila melanogaster Meigen (Diptera: Drosophilidae). Expression of Dfurin1 enhanced Sindbis virus titers in RPE.40 cells by a factor of 10(2)-10(3), and this increase correlated with efficient cleavage of PE2. The PE2-cleavage phenotypes of viruses containing different amino acid substitutions adjacent to the furin cleavage site were compared in mosquito (C6/36), CHO-K1, and Dfurin1-expressing RPE.40 cells. This analysis confirmed that the substrate specificities of Dfurin1 and the putative mosquito furin homolog present in C6/36 cells are similar and suggested that the alternative PE2 cleavage phenotypes observed in vertebrate and arthropod cells were due to differences in substrate specificity between the arthropod and vertebrate furin enzymes and not to differences in host cell glycoprotein processing pathways.

Biological transmission of arboviruses: reexamination of and new insights into components, mechanisms, and unique traits as well as their evolutionary trends

Among animal viruses, arboviruses are unique in that they depend on arthropod vectors for transmission. Field research and laboratory investigations related to the three components of this unique mode of transmission, virus, vector, and vertebrate host, have produced an enormous amount of valuable information that may be found in numerous publications. However, despite many reviews on specific viruses, diseases, or interests, a systematic approach to organizing the available information on all facets of biological transmission and then to interpret it in the context of the evolutionary process has not been attempted before. Such an attempt in this review clearly demonstrates tremendous progress made worldwide to characterize the viruses, to comprehend disease transmission and pathogenesis, and to understand the biology of vectors and their role in transmission. The rapid progress in molecular biologic techniques also helped resolve many virologic puzzles and yielded highly valuable data hitherto unavailable, such as characterization of virus receptors, the genetic basis of vertebrate resistance to viral infection, and phylogenetic evidence of the history of host range shifts in arboviruses. However, glaring gaps in knowledge of many critical subjects, such as the mechanism of viral persistence and the existence of vertebrate reservoirs, are still evident. Furthermore, with the accumulated data, new questions were raised, such as evolutionary directions of virus virulence and of host range. Although many fundamental questions on the evolution of this unique mode of transmission remained unresolved in the absence of a fossil record, available observations for arboviruses and the information derived from studies in other fields of the biological sciences suggested convergent evolution as a plausible process. Overall, discussion of the diverse range of theories proposed and observations made by many investigators was found to be highly valuable for sorting out the possible mechanism(s) of the emergence of arboviral diseases.

Targeting Sindbis virus-based vectors to Fc receptor-positive cell types

Some viruses display enhanced infection for Fc receptor (FcR)-positive cell types when complexed with virus-specific immunoglobulin (Ig). This process has been termed antibody-dependent enhancement of viral infection (ADE). We reasoned that the mechanism of ADE could be exploited and adapted to target alphavirus-based vectors to FcR-positive cell types. Towards this goal, recombinant Sindbis viruses were constructed that express 1 to 4 immunoglobulin-binding domains of protein L (PpL) as N-terminal extensions of the E2 glycoprotein. PpL is a bacterial protein that binds the variable region of antibody kappa light chains from a range of mammalian species. The recombinant viruses incorporated PpL/E2 fusion proteins into the virion structure and recapitulated the species-specific Ig-binding phenotypes of native PpL. Virions reacted with non-immune serum or purified IgG displayed enhanced binding and ADE for several species-matched FcR-positive murine and human cell lines. ADE required virus expression of a functional PpL Ig-binding domain, and appeared to be FcgammaR-mediated. Specifically, ADE did not occur with FcgammaR-negative cells, did not require active complement proteins, and did not occur on FcgammaR-positive murine cell lines when virions were bound by murine IgG-derived F(ab')2 fragments.

Attenuation of Sindbis virus variants incorporating uncleaved PE2 glycoprotein is correlated with attachment to cell-surface heparan sulfate

Sindbis virus virions incorporating uncleaved precursor envelope protein PE2 bind efficiently to cell-surface heparan sulfate (HS) because the furin cleavage site (a consensus HS-binding domain) is retained in the mature virus particle. However, they are essentially nonviable. Resuscitating mutations selected in the E3 or E2 protein preserve the PE2 noncleaving phenotype and HS binding, but facilitate fusion, and thereby restore wild-type infectivity on cultured cells. Here, we have demonstrated that the resuscitated PE2 noncleaving virus was almost avirulent in vivo, but mutated during the infection. Mutants had increased virulence and cleavage of PE2, with reduced HS binding capacity. We hypothesize that HS binding leads to sequestration of PE2 noncleaving virus particles and suppression of serum viremia, thereby selecting for evolution of the virus into a PE2-cleaving, low HS-binding phenotype.

Future vaccines against emerging encephalitides

The vaccine strategies available for control of emerging encephalitides range in a continuum from traditional approaches to those utilizing new technologies. In this report, we explore the use of live attenuated vaccines where the attenuating mutations have been selected in a rational way to improve attenuation without sacrificing effectiveness. A strategy for paired lethal and resuscitating mutations is presented that will greatly reduce the possibility of reversion to virulence. Finally, we describe an example of a vaccine vector system that could be rapidly adapted for use against these virus diseases as they emerge.

A trade-off in replication in mosquito versus mammalian systems conferred by a point mutation in the NS4B protein of dengue virus type 4

An acceptable live-attenuated dengue virus vaccine candidate should have low potential for transmission by mosquitoes. We have identified and characterized a mutation in dengue virus type 4 (DEN4) that decreases the ability of the virus to infect mosquitoes. A panel of 1248 mutagenized virus clones generated previously by chemical mutagenesis was screened for decreased replication in mosquito C6/36 cells but efficient replication in simian Vero cells. One virus met these criteria and contained a single coding mutation: a C-to-U mutation at nucleotide 7129 resulting in a Pro-to-Leu change in amino acid 101 of the nonstructural 4B gene (NS4B P101L). This mutation results in decreased replication in C6/36 cells relative to wild-type DEN4, decreased infectivity for mosquitoes, enhanced replication in Vero and human HuH-7 cells, and enhanced replication in SCID mice implanted with HuH-7 cells (SCID-HuH-7 mice). A recombinant DEN4 virus (rDEN4) bearing this mutation exhibited the same set of phenotypes. Addition of the NS4B P101L mutation to rDEN4 bearing a 30 nucleotide deletion (Delta30) decreased the ability of the double-mutant virus to infect mosquitoes but increased its ability to replicate in SCID-HuH-7 mice. Although the NS4B P101L mutation decreases infectivity of DEN4 for mosquitoes, its ability to enhance replication in SCID-HuH-7 mice suggests that it might not be advantageous to include this specific mutation in an rDEN4 vaccine. The opposing effects of the NS4B P101L mutation in mosquito and vertebrate systems suggest that the NS4B protein is involved in maintaining the balance between efficient replication in the mosquito vector and the human host.

Restriction of a Sindbis virus mutant in BHK cells and relief of the restriction by the addition of adenosine

SV(PZF) is a mutant of Sindbis virus (SV) which we selected on the basis of its ability to replicate in mosquito cells treated with pyrazofurin (PZF), a drug which inhibits pyrimidine nucleotide biosynthesis (Lin et al., 2000, Virology 272, 61-71). Three mutations, A6627U, A7543U, and C7593A, were identified in the nsP4 (the viral RNA polymerase) coding region, which were required for the PZF-resistant phenotype. We report here that SV(PZF) has a second phenotype. Its replication in BHK cells is severely restricted; yields of SV(PZF) from BHK cells are 100- to 1000-fold lower than the yields of standard SV (SV(STD)). However, addition of adenosine to the SV(PZF)-infected cultures completely relieves this restriction and results in yields comparable to those observed with SV(STD). Adenosine has no effect on the yield of SV(STD) from BHK cells. Synthesis of the viral structural proteins is markedly depressed in SV(PZF)-infected BHK cells, as is synthesis of the viral subgenomic (SG) RNA from which these proteins are translated. In contrast, normal amounts of genomic RNA are made. Experiments with mutagenized viruses indicated that the SV(PZF) mutation, C7593A, by itself, was sufficient to produce the restriction phenotype. However, this mutation not only changes Pro 609 of nsP4 to Thr, it also changes the nucleotide at the minus sign5 position of the SG promoter. To evaluate the relative contributions of the change in nsP4 and the change in the SG promoter to the restriction phenotype, we made use of double SG viruses, in which nsP4 and the promoter for the SG RNA which encodes the structural proteins can be changed independent of each other. Our results indicated that both the change in nsP4 and the change in the SG promoter were required to produce the full restriction phenotype. We suggest that the changes in nsP4 and the SG promoter destabilize the RNA initiation complex assembled at the SG promoter and that since ATP is the initiating nucleotide in the SG RNA transcript, the increased level of ATP resulting from the addition of adenosine is able to compensate for this destabilization and restore the synthesis of SG RNA to normal levels.

The host range phenotype displayed by a Sindbis virus glycoprotein variant results from virion aggregation and retention on the surface of mosquito cells

The Sindbis virus variant NE2G216 is a PE2-containing host range mutant that is growth restricted in cultured mosquito cells (C6/36) due to inefficient release of virions from this cell type. The maturation defect of NE2G216 has been linked to the structures of N-linked oligosaccharides synthesized by arthropod cells. Analysis of C6/36 cells infected with NE2G216 by transmission electron microscopy revealed the presence of dense virus aggregates within cytoplasmic vacuoles and virus aggregates adhered to the cell surface. The virus aggregation phenotype of NE2G216 was reproduced in vertebrate cells (Pro-5) by the addition of 1-deoxymannojirimycin, an inhibitor of carbohydrate processing which limits the processing of N-linked oligosaccharides to structures that are structurally similar, albeit not identical, to those synthesized in C6/36 cells. We conclude that defective maturation of NE2G216 in mosquito cells is due to virion aggregation and retention on the cell surface and that this phenotype is directly linked to the carbohydrate-processing properties of these cells.