Point mutations within the betaG-betaH loop of foot-and-mouth disease virus O1K affect virus attachment to target cells

Tropism of Coxsackie virus A9 depends on the +1 position of the RGD (arginine- glycine- aspartic acid) motif found at the C' terminus of its VP1 capsid protein

An understanding of how viruses interact with their receptors is vital as this step is a major determinant of host susceptibility and disease. The enterovirus coxsackievirus A9 (CVA9) is an important pathogen responsible for respiratory infections, myocarditis, infections of the central nervous system, chronic dilated cardiomyopathy and possibly type I diabetes. CVA9 harbours an integrin- recognition motif, RGD (Arg-Gly-Asp), in the capsid protein VP1 and this motif is believed to be primarily responsible for binding to integrins αvβ6 and/or αvβ3 during cell entry. Despite the consistent conservation of RGD-flanking amino acids in multiple RGD-containing picornaviruses, the significance of these amino acids to cell tropism has not been thoroughly investigated. In this study we used 10 CVA9 mutants and a panel of cells to analyse cell tropism. We showed that CVA9 infection proceeds by either an RGD- dependent or an apparently RGD- independent pathway. Differences in the amino acid found at the +1 position of the RGD motif affect the cell tropism of CVA9 when an RGD- dependent pathway is used. Naturally occurring CVA9 isolates have either the sequence RGDM and RGDL and we found that the corresponding viruses in our panel infected cells most efficiently. There was also a strong selection pressure for RGDL in adaptation experiments. However, there was also an unexpected selection of an RGDL variant in an apparently RGD- independent cell line. There was also no simple relationship between infection of cells and expression of integrins αvβ3 and αvβ6. The results obtained have greatly improved our understanding of how CVA9 infects cells. This will be useful in the design of antivirus drugs and also gives a framework for the modification of CVA9 or other RGD containing picornaviruses for specific targeting of cancer cells for oncolytic therapy.

Foot-and-mouth disease virus VP1 target the MAVS to inhibit type-I interferon signaling and VP1 E83K mutation results in virus attenuation

VP1, a pivotal capsid protein encoded by the foot-and-mouth disease virus (FMDV), plays an important role in receptor-mediated attachment and humoral immune responses. Previous studies show that amino acid changes in the VP1 protein of cell culture-adapted strains of FMDV alter the properties of the virus. In addition, FMDV VP1 modulates host IFN signal transduction. Here, we examined the ability of cell culture-adapted FMDV VP1(83K) and wild-type FMDV VP1(83E) to evade host immunity by blocking mitochondrial antiviral signaling protein (MAVS)/TNF Receptor Associated Factor 3 (TRAF3) mediated cellular innate responses. Wild-type FMDV VP1(83E) interacted specifically with C-terminal TRAF3-binding site within MAVS and this interaction inhibited binding of TRAF3 to MAVS, thereby suppressing interferon-mediated responses. This was not observed for cell culture-adapted FMDV VP1(83K). Finally, chimeric FMDV harboring VP1(83K) showed very low pathogenicity in pigs. Collectively, these data highlight a critical role of VP1 with respect to suppression of type-I IFN pathway and attenuation of FMDV by the E83K mutation in VP1.

Foot-and-Mouth disease (FMD), a highly contagious viral disease of cloven-hoofed animals, causes huge economic losses. To generate a FMD vaccine, cell culture-adapted strains of FMDV that show improved growth properties and allow repeated passage are needed. Generally, adaptation of field-isolated FMDV is accompanied by changes in viral properties, including amino acid mutations. A VP1 E83K mutation in cell culture-adapted FMDV was identified previously; here, we examined the impact of VP1 E83K on virus pathogenicity and type-I IFN pathway. Cell culture-adapted FMDV O1 Manisa, and highly virulent strain of O/Andong/SKR/2010, acquired the E83K mutation in the VP1 protein, which attenuated the virus via disposing VP1 mediate negative regulation ability of host cellular IFN responses. The data suggest a rational approach to viral propagation in cell culture and virus attenuation, which could be utilized for future development of FMDV vaccines.

Symmetrical arrangement of positively charged residues around the 5-fold axes of SAT type foot-and-mouth disease virus enhances cell culture of field viruses

Field isolates of foot-and-mouth disease viruses (FMDVs) utilize integrin-mediated cell entry but many, including Southern African Territories (SAT) viruses, are difficult to adapt to BHK-21 cells, thus hampering large-scale propagation of vaccine antigen. However, FMDVs acquire the ability to bind to cell surface heparan sulphate proteoglycans, following serial cytolytic infections in cell culture, likely by the selection of rapidly replicating FMDV variants. In this study, fourteen SAT1 and SAT2 viruses, serially passaged in BHK-21 cells, were virulent in CHO-K1 cells and displayed enhanced affinity for heparan, as opposed to their low-passage counterparts. Comparative sequence analysis revealed the fixation of positively charged residues clustered close to the icosahedral 5-fold axes of the virus, at amino acid positions 83-85 in the βD-βE loop and 110-112 in the βF-βG loop of VP1 upon adaptation to cultured cells. Molecular docking simulations confirmed enhanced binding of heparan sulphate to a model of the adapted SAT1 virus, with the region around VP1 arginine 112 contributing the most to binding. Using this information, eight chimeric field strain mutant viruses were constructed with additional positive charges in repeated clusters on the virion surface. Five of these bound heparan sulphate with expanded cell tropism, which should facilitate large-scale propagation. However, only positively charged residues at position 110-112 of VP1 enhanced infectivity of BHK-21 cells. The symmetrical arrangement of even a single amino acid residue in the FMD virion is a powerful strategy enabling the virus to generate novel receptor binding and alternative host-cell interactions.

Foot-and-mouth disease virus type O specific mutations determine RNA-dependent RNA polymerase fidelity and virus attenuation

Previous studies have shown that the FMDV Asia1/YS/CHA/05 high-fidelity mutagen-resistant variants are attenuated (Zeng et al., 2014). Here, we introduced the same single or multiple-amino-acid substitutions responsible for increased 3D^pol fidelity of type Asia1 FMDV into the type O FMDV O/YS/CHA/05 infectious clone. The rescued viruses O-DA and O-DAMM are lower replication fidelity mutants and showed an attenuated phenotype. These results demonstrated that the same amino acid substitution of 3D^pol in different serotypes of FMDV strains had different effects on viral fidelity. In addition, nucleoside analogues were used to select high-fidelity mutagen-resistant type O FMDV variants. The rescued mutagen-resistant type O FMDV high-fidelity variants exhibited significantly attenuated fitness and a reduced virulence phenotype. These results have important implications for understanding the molecular mechanism of FMDV evolution and pathogenicity, especially in developing a safer modified live-attenuated vaccine against FMDV.

The VP1 S154D mutation of type Asia1 foot-and-mouth disease virus enhances viral replication and pathogenicity

One of the proteins encoded by the foot-and-mouth disease virus (FMDV), the VP1 protein, a capsid protein, plays an important role in integrin receptor attachment and humoral immunity-mediated host responses. The integrin receptor recognition motif and an important antigenic epitope exist within the G-H loop, which is comprised of amino acids 134-160 of the VP1 protein. FMDV strain, Asia1/HN/CHA/06, isolated from a pig, was passaged four times in suckling mice and sequenced. Sequencing analyses showed that there was a mutation of the integrin receptor recognition motif Arg-Gly-Asp/Arg-Asp-Asp (RGD/RDD, VP1 143-145) and a VP1 154 serine/Asp (VP1 S154D) mutation in the G-H loop of the VP1 protein. The influence of the RGD/RDD mutation on Asia1 FMDV disease phenotype has been previously studied. In this study, to determine the influence of the VP1 S154D mutation on FMDV Asia1 replication and pathogenicity, two recombinant FMDVs with different residues only at the VP1 154 site were rescued by reverse genetics techniques and their infectious potential in host cells and pathogenicity in pigs were compared. Our data indicates that the VP1 S154D mutation increases the replication level of FMDV Asia1/HN/CHA/06 in BHK-21, IB-RS-2, and PK-15 cells and enhances pathogenicity in pigs. Through the transient transfection-infection assay to compare integrin receptor usage of two recombinant viruses, the result shows that the VP1 S154D mutation markedly increases the ability of type Asia1 FMDV to use the integrin receptors αυβ6 and αυβ8 from pig. This study identifies a key research target for illuminating the role of residues located at G-H loop in FMDV pathogenicity.

Roles of the putative integrin-binding motif of the human metapneumovirus fusion (f) protein in cell-cell fusion, viral infectivity, and pathogenesis

Human metapneumovirus (hMPV) is a relatively recently identified paramyxovirus that causes acute upper and lower respiratory tract infection. Entry of hMPV is unusual among the paramyxoviruses, in that fusion is accomplished by the fusion (F) protein without the attachment glycoprotein (G protein). It has been suggested that hMPV F protein utilizes integrin αvβ1 as a cellular receptor. Consistent with this, the F proteins of all known hMPV strains possess an integrin-binding motif ((329)RGD(331)). The role of this motif in viral entry, infectivity, and pathogenesis is poorly understood. Here, we show that α5β1 and αv integrins are essential for cell-cell fusion and hMPV infection. Mutational analysis found that residues R329 and G330 in the (329)RGD(331) motif are essential for cell-cell fusion, whereas mutations at D331 did not significantly impact fusion activity. Furthermore, fusion-defective RGD mutations were either lethal to the virus or resulted in recombinant hMPVs that had defects in viral replication in cell culture. In cotton rats, recombinant hMPV with the R329K mutation in the F protein (rhMPV-R329K) and rhMPV-D331A exhibited significant defects in viral replication in nasal turbinates and lungs. Importantly, inoculation of cotton rats with these mutants triggered a high level of neutralizing antibodies and protected against hMPV challenge. Taken together, our data indicate that (i) α5β1 and αv integrins are essential for cell-cell fusion and viral replication, (ii) the first two residues in the RGD motif are essential for fusion activity, and (iii) inhibition of the interaction of the integrin-RGD motif may serve as a new target to rationally attenuate hMPV for the development of live attenuated vaccines.

IMPORTANCE

Human metapneumovirus (hMPV) is one of the major causative agents of acute respiratory disease in humans. Currently, there is no vaccine or antiviral drug for hMPV. hMPV enters host cells via a unique mechanism, in that viral fusion (F) protein mediates both attachment and fusion activity. Recently, it was suggested that hMPV F protein utilizes integrins as receptors for entry via a poorly understood mechanism. Here, we show that α5β1 and αv integrins are essential for hMPV infectivity and F protein-mediated cell-cell fusion and that the integrin-binding motif in the F protein plays a crucial role in these functions. Our results also identify the integrin-binding motif to be a new, attenuating target for the development of a live vaccine for hMPV. These findings not only will facilitate the development of antiviral drugs targeting viral entry steps but also will lead to the development new live attenuated vaccine candidates for hMPV.

The KGD motif of Epstein-Barr virus gH/gL is bifunctional, orchestrating infection of B cells and epithelial cells

Epstein-Barr virus (EBV), a member of the herpesvirus family, is the causative agent of common human infections and specific malignancies. EBV entry into target cells, including B cells and epithelial cells, requires the interaction of multiple virus-encoded glycoproteins. Glycoproteins H and L (gH/gL) cooperate with glycoprotein B (gB) to mediate fusion of the viral envelope with target cell membranes. Both the gH/gL complex and gB are required for fusion, whereas glycoprotein 42 (gp42) acts as a tropism switch and is required for B cell infection and inhibits epithelial cell infection. Our previous studies identified a prominent KGD motif located on the surface of gH/gL. In the current study, we found that this motif serves as a bifunctional domain on the surface of gH/gL that directs EBV fusion of B cells and epithelial cells. Mutation of the KGD motif to AAA decreased fusion with both epithelial and B cells and reduced the binding of gH/gL to epithelial cells and to gp42. We also demonstrate that deletion of amino acids 62 to 66 of gp42 selectively reduces binding to wild-type gH/gL, but not the KGD mutant, suggesting that the KGD motif of gH/gL interacts with the N-terminal amino acids 62 to 66 of gp42.

Epithelial and B cells are the major targets of Epstein-Barr virus (EBV) infection in the human host. EBV utilizes different glycoprotein complexes to enter these cell types. For B cell fusion, EBV uses complexes containing gp42, gH/gL, and gB, whereas just gH/gL and gB are required for epithelial cell fusion. In the current study, a bifunctional domain consisting of a prominent KGD motif on the surface of the gH/gL structure was identified; this domain affects interactions with gp42 or epithelial receptors, ultimately dictating with which cell type virus-induced fusion can occur. These studies will lead to a better understanding of the mechanism of EBV-induced membrane fusion and herpesvirus-induced membrane fusion in general.

In-vitro and in-vivo phenotype of type Asia 1 foot-and-mouth disease viruses utilizing two non-RGD receptor recognition sites

Background

Foot-and-mouth disease virus (FMDV) uses a highly conserved Arg-Gly-Asp (RGD) triplet for attachment to host cells and this motif is believed to be essential for virus viability. Previous sequence analyses of the 1D-encoding region of an FMDV field isolate (Asia1/JS/CHA/05) and its two derivatives indicated that two viruses, which contained an Arg-Asp-Asp (RDD) or an Arg-Ser-Asp (RSD) triplet instead of the RGD integrin recognition motif, were generated serendipitously upon short-term evolution of field isolate in different biological environments. To examine the influence of single amino acid substitutions in the receptor binding site of the RDD-containing FMD viral genome on virus viability and the ability of non-RGD FMDVs to cause disease in susceptible animals, we constructed an RDD-containing FMDV full-length cDNA clone and derived mutant molecules with RGD or RSD receptor recognition motifs. Following transfection of BSR cells with the full-length genome plasmids, the genetically engineered viruses were examined for their infectious potential in cell culture and susceptible animals.

Results

Amino acid sequence analysis of the 1D-coding region of different derivatives derived from the Asia1/JS/CHA/05 field isolate revealed that the RDD mutants became dominant or achieved population equilibrium with coexistence of the RGD and RSD subpopulations at an early phase of type Asia1 FMDV quasispecies evolution. Furthermore, the RDD and RSD sequences remained genetically stable for at least 20 passages. Using reverse genetics, the RDD-, RSD-, and RGD-containing FMD viruses were rescued from full-length cDNA clones, and single amino acid substitution in RDD-containing FMD viral genome did not affect virus viability. The genetically engineered viruses replicated stably in BHK-21 cells and had similar growth properties to the parental virus. The RDD parental virus and two non-RGD recombinant viruses were virulent to pigs and bovines that developed typical clinical disease and viremia.

Conclusions

FMDV quasispecies evolving in a different biological environment gained the capability of selecting different receptor recognition site. The RDD-containing FMD viral genome can accommodate substitutions in the receptor binding site without additional changes in the capsid. The viruses expressing non-RGD receptor binding sites can replicate stably in vitro and produce typical FMD clinical disease in susceptible animals.

Mapping of amino acid residues responsible for adhesion of cell culture-adapted foot-and-mouth disease SAT type viruses

Foot-and-mouth disease virus (FMDV) infects host cells by adhering to the alpha(V) subgroup of the integrin family of cellular receptors in a Arg-Gly-Asp (RGD) dependent manner. FMD viruses, propagated in non-host cell cultures are reported to acquire the ability to enter cells via alternative cell surface molecules. Sequencing analysis of SAT1 and SAT2 cell culture-adapted variants showed acquisition of positively charged amino acid residues within surface-exposed loops of the outer capsid structural proteins. The fixation of positively charged residues at position 110-112 in the beta F-beta G loop of VP1 of SAT1 isolates is thought to correlate with the acquisition of the ability to utilise alternative glycosaminoglycan (GAG) molecules for cell entry. Similarly, two SAT2 viruses that adapted readily to BHK-21 cells accumulated positively charged residues at positions 83 and 85 of the beta D-beta E loop of VP1. Both regions surround the fivefold axis of the virion. Recombinant viruses containing positively charged residues at position 110 and 112 of VP1 were able to infect CHO-K1 cells (that expresses GAG) and demonstrated increased infectivity in BHK-21 cells. Therefore, recombinant SAT viruses engineered to express substitutions that induce GAG-binding could be exploited in the rational design of vaccine seed stocks with improved growth properties in cell cultures.

Studies of Echovirus 5 interactions with the cell surface: heparan sulfate mediates attachment to the host cell

Infections caused by Echovirus 5 (E5), an enterovirus of the Picornaviridae family, have been associated with fever, rashes and sporadic cases of aseptic meningitis. To elucidate the receptor usage of this virus, the significance of a previously proposed integrin binding arginine-glycine-aspartic acid (RGD) motif found in the VP3 capsid protein was investigated, as well as the capacity of E5 to interact with heparan sulfate on the cell surface. Using the prototype strain E5 Noyce (E5N), an E5N mutant where the aspartic acid of the RGD motif has been substituted to a glutamic acid and clinical E5 isolates, the RGD motif of VP3 was found to be non-essential and hence not involved in integrin receptor binding. However, E5N and clinical E5 isolates interact with heparan sulfate at the cell surface, as demonstrated by virus replication inhibition assays using heparin and heparinase III, and studies of E5 interactions at the cell surface measured by real-time PCR analysis. In conclusion, E5 utilizes heparan sulfate as a cellular receptor, but the RGD motif of VP3 is not essential for E5 infectivity.

Engineering infectious foot-and-mouth disease virus in vivo from a full-length genomic cDNA clone of the A/AKT/58 strain

Two full-length genomic cDNA clones, pTA/FMDV and pCA/FMDV, were constructed that contained three point-mutants [A174G and A308G (not present in pTA/FMDV); T1029G] in the genome compared with the wild type A/AKT/58 strain of foot-and-mouth disease virus. These two viruses were rescued by co-transfection of pCA/FMDV with pCT7RNAP, which can express T7 RNA polymerase in BHK-21 cell-lines, or by transfection of the in vitro transcribed RNA. Their biological properties were analyzed for their antigenicity, virulence in suckling-mice (LD50) and growth kinetics in BHK-21 cells. The in vivo rescued viruses showed high pathogenicity for 3-day-old unweaned mice (LD50=10(-7.5)). However, the in vitro transcribed RNA derived from pTA/FMDV had lower pathogenicity for suckling-mice (LD50=10(-6)), and the in vivo transcribed RNA recovered from pCA/FMDV co-transfected with pCT7RNAP showed no significant differences from the wild type virus. These data showed that recovery of the infectious foot-and-mouth disease virus directly from the use of in vivo techniques was better than from in vitro methods. Furthermore, the reverse genetic procedure technique was simplified to a faster one-step procedure based on co-transfection with pCT7RNAP. These results suggest that in vivo RNA transcripts may be more valuable for engineering recombinant foot-and-mouth disease virus than in vitro RNA transcripts, and may contribute to further understanding of the biological properties, such as replication, maturation and quasispecies, of the foot-and-mouth disease virus.

Tolerance to mutations in the foot-and-mouth disease virus integrin-binding RGD region is different in cultured cells and in vivo and depends on the capsid sequence context

Engineered RNAs carrying substitutions in the integrin receptor-binding Arg-Gly-Asp (RGD) region of foot-and-mouth disease virus (FMDV) were constructed (aa 141-147 of VP1 capsid protein) and their infectivity was assayed in cultured cells and suckling mice. The effect of these changes was studied in the capsid proteins of two FMDVs, C-S8c1, which enters cells through integrins, and 213hs(-), a derivative highly adapted to cell culture whose ability to infect cells using the glycosaminoglycan heparan sulfate (HS) as receptor, acquired by multiple passage on BHK-21 cells, has been abolished. The capsid sequence context determined infectivity in cultured cells and directed the selection of additional replacements in structural proteins. Interestingly, a viral population derived from a C-S8c1/L144A mutant, carrying only three substitutions in the capsid, was able to expand tropism to wild-type (wt) and mutant (mt) glycosaminoglycan-deficient CHO cells. In contrast, the 213hs(-) capsid tolerated all substitutions analysed with no additional mutations, and the viruses recovered maintained the ability of the 213hs(-) parental virus to infect wt and mt CHO cells. Viruses derived from C-S8c1 with atypical RGD regions were virulent and transmissible for mice with no other changes in the capsid. Substitution of Asp143 for Ala in the C-S8c1 capsid eliminated infectivity in cultured cells and mice. Co-inoculation with a neutralizing monoclonal antibody directed against the type C FMDV RGD region abolished infectivity of C-S8c1 virus on suckling mice, suggesting that FMDV can infect mice using integrins. Sequence requirements imposed for viral entry in vitro and in vivo are discussed.

Heparan sulfate-binding foot-and-mouth disease virus enters cells via caveola-mediated endocytosis

Foot-and-mouth disease virus (FMDV) utilizes different cell surface macromolecules to facilitate infection of cultured cells. Virus, which is virulent for susceptible animals, infects cells via four members of the alpha(V) subclass of cellular integrins. In contrast, tissue culture adaptation of some FMDV serotypes results in the loss of viral virulence in the animal, accompanied by the loss of virus' ability to use integrins as receptors. These avirulent viral variants acquire positively charged amino acids on surface-exposed structural proteins, resulting in the utilization of cell surface heparan sulfate (HS) molecules as receptors. We have recently shown that FMDV serotypes utilizing integrin receptors enter cells via a clathrin-mediated mechanism into early endosomes. Acidification within the endosome results in a breakdown of the viral capsid, releasing the RNA, which enters the cytoplasm by a still undefined mechanism. Since there is evidence that HS internalizes bound ligands via a caveola-mediated mechanism, it was of interest to analyze the entry of FMDV by cell-surface HS. Using a genetically engineered variant of type O(1)Campos (O(1)C3056R) which can utilize both integrins and HS as receptors and a second variant (O(1)C3056R-KGE) which can utilize only HS as a receptor, we followed viral entry using confocal microscopy. After virus bound to cells at 4 degrees C, followed by a temperature shift to 37 degrees C, type O(1)C3056R-KGE colocalized with caveolin-1, while O(1)C3056R colocalized with both clathrin and caveolin-1. Compounds which either disrupt or inhibit the formation of lipid rafts inhibited the replication of O(1)C3056R-KGE. Furthermore, a caveolin-1 knockdown by RNA interference also considerably reduced the efficiency of O(1)C3056R-KGE infection. These results indicate that HS-binding FMDV enters the cells via the caveola-mediated endocytosis pathway and that caveolae can associate and traffic with endosomes. In addition, these results further suggest that the route of FMDV entry into cells is a function solely of the viral receptor.

Complete nucleotide sequence of a Chinese serotype Asia1 vaccine strain of foot-and-mouth disease virus

The full-length nucleotide sequence of the Chinese vaccine strain Asia1/YNBS/58 of foot-and-mouth disease virus (FMDV) was determined. The results showed that the complete genome of YNBS/58 is 8,164 nucleotides (nt) in length including a 1,061-nt 5' untranslated region (UTR), a 6,990-nt open reading frame (ORF), and a 113-nt 3' UTR. Genome sequences of Asia1/YNBS/58 and other known FMDV strains were compared. The homology analysis indicated that non-structural proteins are more conserved than structural proteins in FMDV and that the 5' UTR is more conserved than the 3' UTR. Phylogenetic analysis revealed that Asia1/YNBS/58 is clustered in the Asia1 serotype and is linked to three other isolates, Asia1/IND/63/72, Asia1/3kimron/63, and Asia1/2ISRL/63, suggesting that they have a close genetic relationship. The VP1-, VP2-, and VP3-based phylogenetic trees divided into distinct clusters according to the different serotypes, while other gene-based phylogenetic trees exhibited some degree of intercrossing among serotypes. According to the nucleotide similarities between Asia1/YNBS/58 and two recent Chinese Asia1 isolates, Asia1/HKN/05, and Asia1/JS/05, each forms a distinct genotype. This study is the first description of the full-length genomic sequence of FMDV Chinese serotype Asia1.

Genetic variation of foot-and-mouth disease virus isolates recovered from persistently infected water buffalo (Bubalus bubalis)

Genetic variation of foot-and-mouth disease virus (FMDV) isolates, serotype O, recovered serially over a 1-year period from persistently infected buffalos was assessed. The persistent state was established experimentally with plaque-purified FMDV, strain O(1)Campos, in five buffalos (Bubalus bubalis). Viral isolates collected from esophageal-pharyngeal (EP) fluids for up to 71 weeks after infection were analyzed at different times by nucleotide sequencing and T(1) RNase oligonucleotide fingerprinting to assess variability in the VP1-coding region and in the complete genome, respectively. Genetic variation increased, although irregularly, with time after infection. The highest values observed for the VP1-coding region and for the whole genome were 2.5% and 1.8%, respectively. High rates of fixation of mutations were observed using both methodologies, reaching values of 0.65 substitutions per nucleotide per year (s/nt/y) and 0.44s/nt/y for nucleotide sequencing and oligonucleotide fingerprinting, respectively, when selected samples recovered at close time periods were analyzed. The data herein indicate that complex mixtures of genotypes may arise during FMDV type O persistent infection in water buffalos, which can act as viral reservoirs and also represent a potential source of viral variants. These results fit within the quasi-species dynamics described for FMDV, in which viral populations are constituted by related, non-identical genomes that evolve independently from each other, and may predominate at a given time.

Specificity of the VP1 GH loop of Foot-and-Mouth Disease virus for alphav integrins

Foot-and-mouth disease virus (FMDV) can use a number of integrins as receptors to initiate infection. Attachment to the integrin is mediated by a highly conserved arginine-glycine-aspartic acid (RGD) tripeptide located on the GH loop of VP1. Other residues of this loop are also conserved and may contribute to integrin binding. In this study we have used a 17-mer peptide, whose sequence corresponds to the GH loop of VP1 of type O FMDV, as a competitor of integrin-mediated virus binding and infection. Alanine substitution through this peptide identified the leucines at the first and fourth positions following RGD (RGD+1 and RGD+4 sites) as key for inhibition of virus binding and infection mediated by alphavbeta6 or alphavbeta8 but not for inhibition of virus binding to alphavbeta3. We also show that FMDV peptides containing either methionine or arginine at the RGD+1 site, which reflects the natural sequence variation seen across the FMDV serotypes, are effective inhibitors for alphavbeta6. In contrast, although RGDM-containing peptides were effective for alphavbeta8, RGDR-containing peptides were not. These observations were confirmed by showing that a virus containing an RGDR motif uses alphavbeta8 less efficiently than alphavbeta6 as a receptor for infection. Finally, evidence is presented that shows alphavbeta3 to be a poor receptor for infection by type O FMDV. Taken together, our data suggest that the integrin binding loop of FMDV has most likely evolved for binding to alphavbeta6 with a higher affinity than to alphavbeta3 and alphavbeta8.

Analysis of a foot-and-mouth disease virus type A24 isolate containing an SGD receptor recognition site in vitro and its pathogenesis in cattle

Foot-and-mouth disease virus (FMDV) initiates infection by binding to integrin receptors via an Arg-Gly-Asp (RGD) sequence found in the G-H loop of the structural protein VP1. Following serial passages of a type A(24) Cruzeiro virus (A(24)Cru) in bovine, via tongue inoculation, a virus was generated which contained an SGD sequence in the cell receptor-binding site and expressed a turbid plaque phenotype in BHK-21 cells. Propagation of this virus in these cells resulted in the rapid selection of viruses that grew to higher titers, produced clear plaques, and now contained an RGD sequence in place of the original SGD. To study the role of the SGD sequence in FMDV receptor recognition and bovine virulence, we assembled an infectious cDNA clone of an RGD-containing A(24)Cru and derived mutant clones containing either SGD with a single nucleotide substitution in the R(144) codon or double substitutions at this position to prevent mutation of the S to an R. The SGD viruses grew poorly in BHK-21 cells and stably maintained the sequence during propagation in BHK-21 cells expressing the bovine alpha(V)beta(6) integrin (BHK3-alpha(V)beta(6)), as well as in experimentally infected and contact steers. While all the SGD-containing viruses used only the bovine alpha(V)beta(6) integrin as a cellular receptor with relatively high efficiency, the revertant RGD viruses utilized either the alpha(V)beta(1) or alpha(V)beta(3) bovine integrins with higher efficiency than alpha(V)beta(6) and grew well in BHK-21 cells. Replacing the R at the -1 SGD position with either K or E showed that this residue did not contribute to integrin utilization in vitro. These results illustrate the rapid evolution of FMDV with alteration in receptor specificity and suggest that viruses with sequences other than RGD, but closely related to it, can still infect via integrin receptors and induce and transmit the disease to susceptible animals.

Analysis of foot-and-mouth disease virus internalization events in cultured cells

It has been demonstrated that foot-and-mouth disease virus (FMDV) can utilize at least four members of the alpha(V) subgroup of the integrin family of receptors in vitro. The virus interacts with these receptors via a highly conserved arginine-glycine-aspartic acid amino acid sequence motif located within the betaG-betaH loop of VP1. While there have been extensive studies of virus-receptor interactions at the cell surface, our understanding of the events during viral entry into the infected cell is still not clear. We have utilized confocal microscopy to analyze the entry of two FMDV serotypes (types A and O) after interaction with integrin receptors at the cell surface. In cell cultures expressing both the alphaVbeta3 and alphaVbeta6 integrins, virus adsorbed to the cells at 4 degrees C appears to colocalize almost exclusively with the alphaVbeta6 integrin. Upon shifting the infected cells to 37 degrees C, FMDV capsid proteins were detected within 15 min after the temperature shift, in association with the integrin in vesicular structures that were positive for a marker of clathrin-mediated endocytosis. In contrast, virus did not colocalize with a marker for caveola-mediated endocytosis. Virus remained associated with the integrin until about 1 h after the temperature shift, when viral proteins appeared around the perinuclear region of the cell. By 15 min after the temperature shift, viral proteins were seen colocalizing with a marker for early endosomes, while no colocalization with late endosomal markers was observed. In the presence of monensin, which raises the pH of endocytic vesicles and has been shown to inhibit FMDV replication, viral proteins were not released from the recycling endosome structures. Viral proteins were not observed associated with the endoplasmic reticulum or the Golgi. These data indicate that FMDV utilizes the clathrin-mediated endocytosis pathway to infect the cells and that viral replication begins due to acidification of endocytic vesicles, causing the breakdown of the viral capsid structure and release of the genome by an as-yet-unidentified mechanism.

Interactions of foot-and-mouth disease virus with soluble bovine alphaVbeta3 and alphaVbeta6 integrins

At least four members of the integrin family of receptors, alphaVbeta1, alphaVbeta3, alphaVbeta6, and alphaVbeta8, have been identified as receptors for foot-and-mouth disease virus (FMDV) in vitro. Our investigators have recently shown that the efficiency of receptor usage appears to be related to the viral serotype and may be influenced by structural differences on the viral surface (H. Duque and B. Baxt, J. Virol. 77:2500-2511, 2003). To further examine these differences, we generated soluble alphaVbeta3 and alphaVbeta6 integrins. cDNA plasmids encoding the individual complete integrin alphaV, beta3, and beta6 subunits were used to amplify sequences encoding the subunits' signal peptide and ectodomain, resulting in subunits lacking transmembrane and cytoplasmic domains. COS-1 cells were transfected with plasmids encoding the soluble alphaV subunit and either the soluble beta3 or beta6 subunit and labeled with [35S]methionine-cysteine. Complete subunit heterodimeric integrins were secreted into the medium, as determined by radioimmunoprecipitation with specific monoclonal and polyclonal antibodies. For the examination of the integrins' biological activities, stable cell lines producing the soluble integrins were generated in HEK 293A cells. In the presence of divalent cations, soluble alphaVbeta6 bound to representatives of type A or O viruses, immobilized on plastic dishes, and significantly inhibited viral replication, as determined by plaque reduction assays. In contrast, soluble alphaVbeta3 was unable to bind to immobilized virus of either serotype; however, virus bound to the immobilized integrin, suggesting that FMDV binding to alphaVbeta3 is a low-affinity interaction. In addition, soluble alphaVbeta3 did not neutralize virus infectivity. Incubation of soluble alphaVbeta6 with labeled type A12 or O1 resulted in a significant inhibition of virus adsorption to BHK cells, while soluble alphaVbeta3 caused a low (20 to 30%), but consistent, inhibition of virus adsorption. Virus incubated with soluble alphaVbeta6 had a lower sedimentation rate than native virus on sucrose density gradients, but the particles retained all of their structural proteins and still contained bound integrin and, therefore, were not exhibiting characteristics of a picornavirus A particle.

Integrin alpha v beta 6 is an RGD-dependent receptor for coxsackievirus A9

Coxsackievirus A9 (CAV9), a member of the Enterovirus genus of Picornaviridae, is a common human pathogen and is one of a significant number of viruses containing a functional arginine-glycine-aspartic acid (RGD) motif in one of their capsid proteins. Previous studies identified the RGD-recognizing integrin alpha(v)beta(3) as its cellular receptor. However, integrin alpha(v)beta(6) has been shown to be an efficient receptor for another RGD-containing picornavirus, foot-and-mouth disease virus (FMDV). In view of the similarity in sequence context of the RGD motifs in CAV9 and FMDV, we investigated whether alpha(v)beta(6) can also serve as a receptor for CAV9. We found that CAV9 can bind to purified alpha(v)beta(6) and also to SW480 cells transfected with beta(6) cDNA, allowing expression of alpha(v)beta(6) on their surface, but it cannot bind to mock-transfected cells. In addition, a higher yield of CAV9 was obtained in beta(6)-expressing cells than in mock-transfected cells. There was no similar enhancement in infection with an RGD-less CAV9 mutant. We also found beta(6) on the surface of GMK cells, a cell line which CAV9 infects efficiently by an RGD-dependent mechanism. Significantly, this infection is blocked by an antibody to alpha(v)beta(6), while this antibody did not block the low level of infection by the RGD-less mutant. Thus, integrin alpha(v)beta(6) is an RGD-dependent receptor for CAV9 and may be important in natural CAV9 infections.

Foot-and-mouth disease

Foot-and-mouth disease (FMD) is a highly contagious disease of cloven-hoofed animals. The disease was initially described in the 16th century and was the first animal pathogen identified as a virus. Recent FMD outbreaks in developed countries and their significant economic impact have increased the concern of governments worldwide. This review describes the reemergence of FMD in developed countries that had been disease free for many years and the effect that this has had on disease control strategies. The etiologic agent, FMD virus (FMDV), a member of the Picornaviridae family, is examined in detail at the genetic, structural, and biochemical levels and in terms of its antigenic diversity. The virus replication cycle, including virus-receptor interactions as well as unique aspects of virus translation and shutoff of host macromolecular synthesis, is discussed. This information has been the basis for the development of improved protocols to rapidly identify disease outbreaks, to differentiate vaccinated from infected animals, and to begin to identify and test novel vaccine candidates. Furthermore, this knowledge, coupled with the ability to manipulate FMDV genomes at the molecular level, has provided the framework for examination of disease pathogenesis and the development of a more complete understanding of the virus and host factors involved.

Retrospective genetic analysis of SAT-1 type foot-and-mouth disease outbreaks in West Africa (1975-1981)

The complete 1D genome region encoding the immunogenic and phylogenetically informative VP1 gene was genetically characterized for 23 South African Territories (SAT)-1 viruses causing foot-and-mouth (FMD) disease outbreaks in the West African region between 1975 and 1981. The results indicate that two independent outbreaks occurred, the first involved two West African countries, namely Niger and Nigeria, whilst the second affected Nigeria alone. In the former epizootic, virus circulation spanned a period of 2 years, whilst in the latter virus was recovered from the field over a 3 year period. Comparison of the West African viruses with SAT-1 viruses from other regions on the continent revealed that the two West African lineages identified in this study are regionally distinct. Furthermore, variation in VP1 gene length was identified in SAT-1 viruses for the first time, further emphasizing the uniqueness of these pathogens in West Africa. This first retrospective analysis in which the molecular epidemiology of SAT-1 viruses in West Africa is reported, provides a useful measure of the regional variation of these viruses and is an essential first step in the establishment of a West African sequence database that will be a useful reference for future outbreak eventualities.

Phenotypic mixing and hiding may contribute to memory in viral quasispecies

BACKGROUND

In a number of recent experiments with food-and-mouth disease virus, a deleterious mutant, RED, was found to avoid extinction and remain in the population for long periods of time. Since RED characterizes the past evolutionary history of the population, this observation was called quasispecies memory. While the quasispecies theory predicts the existence of these memory genomes, there is a disagreement between the expected and observed mutant frequency values. Therefore, the origin of quasispecies memory is not fully understood.

RESULTS

We propose and analyze a simple model of complementation between the wild type virus and a mutant that has an impaired ability of cell entry, the likely cause of fitness differences between wild type and RED mutants. The mutant will go extinct unless it is recreated from the wild type through mutations. However, under phenotypic mixing-and-hiding as a mechanism of complementation, the time to extinction in the absence of mutations increases with increasing multiplicity of infection (m.o.i.). If the RED mutant is constantly recreated by mutations, then its frequency at equilibrium under selection-mutation balance also increases with increasing m.o.i. At high m.o.i., a large fraction of mutant genomes are encapsidated with wild-type protein, which enables them to infect cells as efficiently as the wild type virions, and thus increases their fitness to the wild-type level. Moreover, even at low m.o.i. the equilibrium frequency of the mutant is higher than predicted by the standard quasispecies model, because a fraction of mutant virions generated from wild-type parents will also be encapsidated by wild-type protein.

CONCLUSIONS

Our model predicts that phenotypic hiding will strongly influence the population dynamics of viruses, particularly at high m.o.i., and will also have important effects on the mutation-selection balance at low m.o.i. The delay in mutant extinction and increase in mutant frequencies at equilibrium may, at least in part, explain memory in quasispecies populations.

Evaluation of genetically engineered derivatives of a Chinese strain of foot-and-mouth disease virus reveals a novel cell-binding site which functions in cell culture and in animals

Adaptation of field isolates of foot-and-mouth disease virus (FMDV) to grow in cells in culture can result in changes in viral properties that include acquisition of the ability to bind to cell surface heparan sulfate (HS). After 13 passages on BHK cells to produce a vaccine, a Cathay topotype isolate of FMDV serotype O from China (O/CHA/90) extended its cell culture host range and bound to heparin-Sepharose, although it did not require cell surface HS as a receptor molecule. To understand these phenomena, we constructed chimeric viruses by using a type A(12) infectious cDNA and the capsid protein-coding regions of O/CHA/90 and its cell culture-adapted derivative (vac-O/CHA/90). Using a set of viruses derived from these chimeras by exchanging portions of the capsid-coding regions, we discovered that a group of amino acid residues that surround the fivefold axis of the icosahedral virion determine host range in cell culture and influence pathogenicity in pigs. These residues included aromatic amino acids at positions 108 and 174 and positively charged residues at positions 83 and 172 in protein 1D. To test if these residues participated in non-integrin-dependent cell binding, the integrin-binding RGD sequence in protein 1D was changed to KGE in two different chimeras. Evaluation of these KGE viruses indicated that growth in cell culture was not dependent on HS. One of these viruses was tested in pigs, where it produced a mild disease and maintained its KGE sequence. These results are discussed in terms of receptor utilization and pathogenesis of this important pathogen.

Foot-and-mouth disease virus receptors: comparison of bovine alpha(V) integrin utilization by type A and O viruses

Three members of the alpha(V) integrin family of cellular receptors, alpha(V)beta(1), alpha(V)beta(3), and alpha(V)beta(6), have been identified as receptors for foot-and-mouth disease virus (FMDV) in vitro. The virus interacts with these receptors via a highly conserved arginine-glycine-aspartic acid (RGD) amino acid sequence motif located within the betaG-betaH (G-H) loop of VP1. Other alpha(V) integrins, as well as several other integrins, recognize and bind to RGD motifs on their natural ligands and also may be candidate receptors for FMDV. To analyze the roles of the alpha(V) integrins from a susceptible species as viral receptors, we molecularly cloned the bovine beta(1), beta(5), and beta(6) integrin subunits. Using these subunits, along with previously cloned bovine alpha(V) and beta(3) subunits, in a transient expression assay system, we compared the efficiencies of infection mediated by alpha(V)beta(1), alpha(V)beta(3), alpha(V)beta(5), and alpha(V)beta(6) among three strains of FMDV serotype A and two strains of serotype O. While all the viruses could infect cells expressing these integrins, they exhibited different efficiencies of integrin utilization. All the type A viruses used alpha(V)beta(3) and alpha(V)beta(6) with relatively high efficiency, while only one virus utilized alpha(V)beta(1) with moderate efficiency. In contrast, both type O viruses utilized alpha(V)beta(6) and alpha(V)beta(1) with higher efficiency than alpha(V)beta(3). Only low levels of viral replication were detected in alpha(V)beta(5)-expressing cells infected with either serotype. Experiments in which the ligand-binding domains among the beta subunits were exchanged indicated that this region of the integrin subunit appears to contribute to the differences in integrin utilizations among strains. In contrast, the G-H loops of the different viruses do not appear to be involved in this phenomenon. Thus, the ability of the virus to utilize multiple integrins in vitro may be a reflection of the use of multiple receptors during the course of infection within the susceptible host.

Molecular basis of pathogenesis of FMDV

Current understanding of the molecular basis of pathogenesis of foot-and-mouth disease (FMD) has been achieved through over 100 years of study into the biology of the etiologic agent, FMDV. Over the last 40 years, classical biochemical and physical analyses of FMDV grown in cell culture have helped to reveal the structure and function of the viral proteins, while knowledge gained by the study of the virus' genetic diversity has helped define structures that are essential for replication and production of disease. More recently, the availability of genetic engineering methodology has permitted the direct testing of hypotheses formulated concerning the role of individual RNA structures, coding regions and polypeptides in viral replication and disease. All of these approaches have been aided by the simultaneous study of other picornavirus pathogens of animals and man, most notably poliovirus. Although many questions of how FMDV causes its devastating disease remain, the following review provides a summary of the current state of knowledge into the molecular basis of the virus' interaction with its host that produces one of the most contagious and frightening diseases of animals or man.

Macrophage phagocytosis of foot-and-mouth disease virus may create infectious carriers

Macrophages play critical roles in innate defences against virus infections, particularly pertinent to the rapid immune response required following emergency vaccination against foot-and-mouth disease virus (FMDV). Consequently, macrophage-FMDV interaction was studied in vitro, in the absence of specific antibodies, to mimic the animal early postvaccination. A gradual loss of infectivity and viral antigen was observed over 48 hr, and no evidence of productive virus replication was found. From the pathological viewpoint, an important observation was that the majority of macrophages carried infectious virus for at least 10 hr. Pronase and mild acid treatments showed the virus to be primarily on the cell surface during the first 4 hr. Thereafter, it became internalized (pronase- and pH resistant), but remained infectious for 10-24 hr. The internalization process was dependent on microfilament activity, while the survival of infectious virus related to live virus-dependent inhibition of macrophage protein synthesis. Infectious centre assays demonstrated that this infectious virus - whether on the cell surface or internalized - was actually being released from the cells. This is interesting considering that FMDV is highly pH labile. Together, these characteristics suggest that the virus had been internalized by a process such as macropinocytosis, and fusion with endosomes was delayed or impaired. This mechanism whereby the virus could 'piggyback' on or in the macrophage, becoming internalized but not degraded for at least 10 hr, are important considerations in FMD pathogenesis. Such 'virus-transporting' macrophages would be in a position to carry infectious FMDV to different sites in the body, where it could be released to infect other cells for replication.

Arginine-glycine-aspartic acid motif is critical for human parechovirus 1 entry

The human parechovirus 1 RGD motif in VP1 was studied by mutagenesis. An RGD-to-RGE change gave only revertant viruses with a restored RGD, while deletion of GD was lethal and nonrevertable. Mutations at the +1 and +2 positions had some effect on growth properties and a +1 M-to-P change was lethal. These studies indicate that the RGD motif plays a critical role in infectivity, presumably by interacting with integrins, and that downstream amino acids can have an influence on function.

Foot-and-mouth disease virus lacking the VP1 G-H loop: the mutant spectrum uncovers interactions among antigenic sites for fitness gain

The Arg-Gly-Asp (RGD) triplet found in the G-H loop of capsid protein VP1 of foot-and-mouth disease virus (FMDV) is critically involved in the interaction of FMDV with integrin receptors and with neutralizing antibodies. Multiplication of FMDV C-S8c1 in baby hamster kidney 21 (BHK-21) cells selected variant viruses exploiting alternative mechanisms of cell recognition that rendered the RGD integrin-binding triplet dispensable for infectivity. By constructing chimeric viruses, we show that dispensability of the RGD in these variant FMDVs can be extended to surrounding amino acid residues. Replacement of eight amino acid residues within the G-H loop of VP1 by an unrelated FLAG marker yielded infectious virus. Evolution of FLAG-containing viruses in BHK-21 cells generated complex quasispecies in which individual mutants included amino acid replacements at other antigenic sites of FMDV. Inclusion of such replacements in the parental FLAG clone resulted in an increase of relative fitness of the viruses. These results suggest structural or functional connections between antigenic sites of FMDV and underscore the value of mutant spectrum analysis for the identification of fitness-promoting genetic modifications in viral populations. The possibility of producing viable viruses lacking antigenic site A may find application in the design of new anti-FMD vaccines.

Foot-and-mouth disease virus can utilize the C-terminal extension of coxsackievirus A9 VP1 for cell infection

Foot-and-mouth disease virus (FMDV) is known to employ the conserved Arg-Gly-Asp (RGD) tripeptide located on the variable betaG-betaH loop of the VP1 capsid protein for binding to cells. Coxsackievirus A9 (CAV9) also carries an RGD sequence, but on a short C-terminal extension of its VP1 and in a different amino acid context. This apparent relationship raised the question of whether insertion of the heterologous CAV9 sequence into FMDV would influence infection by the genetically modified FMDV. Four VP1 mutants were generated by PCR mutagenesis of a full-length FMDV cDNA plasmid. After transfection of BHK-21 cells, viral protein synthesis and virus particle formation could be detected. Two of the four mutants, mV9b and mV9d, could be propagated in BHK-21 cells, but not in CV-1 cells. Both of these mutants contained 17 amino acids of the C terminus of CAV9 VP1. Infection of BHK cells could be specifically inhibited by rabbit immune serum raised against a synthetic peptide representing the amino acid sequence of the C-terminal extension of CAV9 VP1. This demonstrated the direct involvement of the inserted sequence in cell infection. In fact, genetically modified FMDV O(1)K was capable of employing the VP1 C-terminal RGD region of CAV9 for infection of BHK cells. In addition, these results show that, even in cell culture-adapted viruses, the RGD-containing betaG-betaH loop plays an important role in virus infectivity.

High affinity interactions of Coxsackievirus A9 with integrin alphavbeta3 (CD51/61) require the CYDMKTTC sequence of beta3, but do not require the RGD sequence of the CAV-9 VP1 protein

Integrins are transmembrane molecules involved in numerous cell matrix, cell-cell adhesion phenomena and also utilised as viral receptors. These interactions with integrins are mediated by brief oligopeptide recognition sequences. The Arg-Gly-Asp sequence (RGD), is recognized by many integrins, including integrin alphavbeta3 (CD51/61). Coxsackievirus A9 (CAV-9), a human pathogen that has an Arg-Gly-Asp sequence in the VP1 capsid protein, has been known to be one of the many viruses that utilise integrin alphavbeta3 as a receptor. In order to determine important binding sites of CAV-9 on integrin alphavbeta3, we performed binding studies of CAV-9 on CHO-alphavbeta3, CHO-alphavbeta1 and CHO-alphavbeta1-3-1 mutant cell line, in the presence of function blocking mAb specific for integrin alphavbeta3 and natural ligand vitronectin. Our experiments show that the CYDMKTTC sequence (187-193 residue) of integrin beta3, which has been shown to be involved in ligand specificity, is an important binding site for CAV-9. We also report that an RGD-less Coxsackievirus A9 mutant can bind efficiently on the ligand binding site of integrin alphavbeta3. Thus documenting the capability of this RNA virus to interact with integrin alphavbeta3, without the presence of an Arg-Gly-Asp sequence.

Cell recognition by foot-and-mouth disease virus that lacks the RGD integrin-binding motif: flexibility in aphthovirus receptor usage

Cell surface molecules that can act as virus receptors may exert an important selective pressure on RNA viral quasispecies. Large population passages of foot-and-mouth disease virus (FMDV) in cell culture select for mutant viruses that render dispensable a highly conserved Arg-Gly-Asp (RGD) motif responsible for integrin receptor recognition. Here, we provide evidence that viability of recombinant FMDVs including a Asp-143-->Gly change at the RGD motif was conditioned by a number of capsid substitutions selected upon FMDV evolution in cell culture. Multiply passaged FMDVs acquired the ability to infect human K-562 cells, which do not express integrin alpha(v)beta(3). In contrast to previously described cell culture-adapted FMDVs, the RGD-independent infection did not require binding to the surface glycosaminoglycan heparan sulfate (HS). Viruses which do not bind HS and lack the RGD integrin-binding motif replicate efficiently in BHK-21 cells. Interestingly, FMDV mutants selected from the quasispecies for the inability to bind heparin regained sensitivity to inhibition by a synthetic peptide that represents the G-H loop of VP1. Thus, a single amino acid replacement leading to loss of HS recognition can shift preferential receptor usage of FMDV from HS to integrin. These results indicate at least three different mechanisms for cell recognition by FMDV and suggest a potential for this virus to use multiple, alternative receptors for entry even into the same cell type.

Involvement of beta2-microglobulin and integrin alphavbeta3 molecules in the coxsackievirus A9 infectious cycle

It is becoming apparent that many viruses employ more than one cell surface molecule for their attachment and cell entry. In this study, we have tested the role of integrin alpha(v)beta3 and MHC class I molecules in the coxsackievirus A9 (CAV-9) infectious cycle. Binding experiments utilizing CHO cells transfected and expressing human integrin alpha(v)beta3, revealed that CAV-9 particles were able to bind to cells, but did not initiate a productive cell infection. Antibodies specific for integrin alpha(v)beta3 molecules significantly reduced CAV-9 infection in susceptible cell lines. Moreover, MAbs specific for beta2-microglobulin (beta2-m) and MHC class I molecules completely inhibited CAV-9 infection. To assess the effect of these antibodies on virus binding, we analysed CAV-9 binding by flow cytometry in the presence of alpha2-m- or integrin alpha(v)beta3-specific antibodies. The results showed a reduction in CAV-9 binding in the presence of integrin alpha(v)beta3-specific antibodies while there was no reduction in the presence of beta2-m-specific MAb. Taken together, these data suggest that integrin alphavbeta3 is required for CAV-9 attachment but is not sufficient for cell entry, while beta2-m, although not directly involved in CAV-9 binding, plays a post-attachment role in the CAV-9 infectious process, possibly being involved in virus entry.

Antigenic properties and population stability of a foot-and-mouth disease virus with an altered Arg-Gly-Asp receptor-recognition motif

The antigenic properties and genetic stability of a multiply passaged foot-and-mouth disease virus (FMDV) clone C-S8c1 with an Arg-Gly-Gly triplet (RGG) instead of the Arg-Gly-Asp (RGD) integrin-recognition motif at positions 141 to 143 of capsid protein VP1 are described. Clear antigenic differences between FMDV RGG and clone C-S8c1 have been documented in ELISA, enzyme-linked immunoelectrotransfer (Western) blot and neutralization assays using site A-specific monoclonal antibodies and anti-FMDV polyclonal antibodies from swine and guinea pigs. The results validate with a live virus the role of the RGD (in particular Asp-143) in recognition of (and neutralization by) antibodies, a role previously suggested by immunochemical and structural studies with synthetic peptides. The FMDV RGG was genetically stable in a large proportion of serial infections of BHK-21 cells. However, a revertant virus with RGD was generated in one out of six passage series. Interestingly, this revertant FMDV did not reach dominance but established an equilibrium with its parental FMDV RGG, accompanied by an increase of quasispecies complexity at the sequences around the RGG triplet. FMDV RGG exhibited a selective disadvantage relative to other RGD-containing clones isolated from the same parental FMDV population. The results suggest that large antigenic variations can be prompted by replacements at critical capsid sites, including those involved in receptor recognition. These critical replacements may yield viruses whose stability allows them to replicate efficiently and to expand the sequence repertoire of an antigenic site.

Nucleotide sequence of UK and Australian isolates of feline calicivirus (FCV) and phylogenetic analysis of FCVs

We have determined the first complete genome sequence and capsid gene sequences of feline calicivirus (FCV) isolates from the UK and Australia. These were compared with other previously published sequences. The viruses used in the comparisons were isolated between 1957 and 1995 from various geographical locations and obtained from cats showing a range of clinical signs. Despite these diverse origins, comparisons between all strains showed a similar degree of sequence variation within both ORF1 (non-structural polyprotein) and ORF2 (major capsid protein) (amino acid distances of 7.7-13.0% and 8.8-18.6%, respectively). In contrast, ORF3 (putative minor structural protein) sequences indicated a more heterogenous distribution of FCV relatedness (amino acid distances of 1.9-17.9%). Phylogenetic analysis suggested that, unlike some other caliciviruses, FCV isolates within the current data set fall into one diverse genogroup. Within this group, there was an overall lack of geographic or temporal clustering which may be related to the epidemiology of FCV infection in cats. Analysis of regions of variability in the genome has shown that, as well as the previously identified variable regions in ORF2, similar domains exist within ORFs 1 and 3 also, although to a lesser extent. In ORF1, these variable domains largely fall between the putative non-structural protein functional domains.

Population dynamics in the evolution of RNA viruses

RNA virus quasispecies are subjected to processes of positive Darwinian selection, to a very active and continuous negative selection and to random genetic drift. The course of RNA virus evolution is often unpredictable, and recent results suggest that even highly conserved motifs, once regarded as essential for infectivity, may be rendered dispensable by singular evolutionary events. An immediate consequence of the quasispecies genetic organization of RNA viruses is a surprising ability to gain fitness once a minimal replication ability is established in a biological environment. The unique features of RNA genetics should not be underestimated since they are at the basis of the emergence of new viral diseases and of the current difficulties to control many diseases associated variable viruses.

Foot-and-mouth disease virus virulent for cattle utilizes the integrin alpha(v)beta3 as its receptor

Adsorption and plaque formation of foot-and-mouth disease virus (FMDV) serotype A12 are inhibited by antibodies to the integrin alpha(v)beta3 (A. Berinstein et al., J. Virol. 69:2664-2666, 1995). A human cell line, K562, which does not normally express alpha(v)beta3 cannot replicate this serotype unless cells are transfected with cDNAs encoding this integrin (K562-alpha(v)beta3 cells). In contrast, we found that a tissue culture-propagated FMDV, type O1BFS, was able to replicate in nontransfected K562 cells, and replication was not inhibited by antibodies to the endogenously expressed integrin alpha5beta1. A recent report indicating that cell surface heparan sulfate (HS) was required for efficient infection of type O1 (T. Jackson et al., J. Virol. 70:5282-5287, 1996) led us to examine the role of HS and alpha(v)beta3 in FMDV infection. We transfected normal CHO cells, which express HS but not alpha(v)beta3, and two HS-deficient CHO cell lines with cDNAs encoding human alpha(v)beta3, producing a panel of cells that expressed one or both receptors. In these cells, type A12 replication was dependent on expression of alpha(v)beta3, whereas type O1BFS replicated to high titer in normal CHO cells but could not replicate in HS-deficient cells even when they expressed alpha(v)beta3. We have also analyzed two genetically engineered variants of type O1Campos, vCRM4, which has greatly reduced virulence in cattle and can bind to heparin-Sepharose columns, and vCRM8, which is highly virulent in cattle and cannot bind to heparin-Sepharose. vCRM4 replicated in wild-type K562 cells and normal, nontransfected CHO (HS+ alpha(v)beta3-) cells, whereas vCRM8 replicated only in K562 and CHO cells transfected with alpha(v)beta3 cDNAs. A similar result was also obtained in assays using a vCRM4 virus with an engineered RGD-->KGE mutation. These results indicate that virulent FMDV utilizes the alpha(v)beta3 integrin as a primary receptor for infection and that adaptation of type O1 virus to cell culture results in the ability of the virus to utilize HS as a receptor and a concomitant loss of virulence.

Arginine-glycine-aspartic acid-specific binding by foot-and-mouth disease viruses to the purified integrin alpha(v)beta3 in vitro

The integrin alpha(v)beta3 has been shown to act as the receptor for internalization of foot-and-mouth disease virus (FMDV) (A12), with attachment being through a highly conserved RGD motif located on the G-H loop of viral capsid protein VP1. In addition, however, we have recently shown that efficient infection of culture-grown cells by FMDV (O1BFS) requires binding to cell surface heparan sulfate. In this study, we have used a solid-phase receptor binding assay to characterize the binding by FMDV to purified alpha(v)beta3 in the absence of heparan sulfate and other cell surface components. In this assay, FMDV (O1BFS) successfully replicated authentic ligand binding by cellular alpha(v)beta3 in terms of its high affinity, dependence on divalent cations, and activation by manganese ions. Virus binding to this preparation of alpha(v)beta3 was exquisitely sensitive to competition by short RGD-containing peptides (50% inhibition at < 10(-8) M peptide), and this inhibition was highly sequence specific, with the equivalent RGE peptide being at least 10(4) fold less effective as a competitor. Representative viruses of the other six serotypes of FMDV bound to alpha(v)beta3 in a similar RGD-specific manner, although significant differences in sensitivity to RGD peptides suggest that the affinity of the different FMDV serotypes for alpha(v)beta3 is influenced, in part, by the variable amino acid residues in the VP1 G-H loop on either side of the RGD.