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

Bovine Rhinitis B Virus Variant as the Putative Cause of Bronchitis in Goat Kids

A diagnostic investigation into an outbreak of fatal respiratory disease among young goats in Iowa, USA revealed bronchitis lesions of unknown etiology and secondary bacterial bronchopneumonia. Hypothesis-free metagenomics identified a previously unreported picornavirus (USA/IA26017/2023), and further phylogenetic analysis classified USA/IA26017/2023 as an aphthovirus related to bovine rhinitis B virus. Viral nucleic acid was localized to lesions of bronchitis using in situ hybridization. This marks the first report of a picornavirus putatively causing respiratory disease in goats and highlights the potential for cross-species transmission of aphthoviruses.

Establishing an In Vitro System to Assess How Specific Antibodies Drive the Evolution of Foot-and-Mouth Disease Virus

Viruses can evolve to respond to immune pressures conferred by specific antibodies generated after vaccination and/or infection. In this study, an in vitro system was developed to investigate the impact of serum-neutralising antibodies upon the evolution of a foot-and-mouth disease virus (FMDV) isolate. The presence of sub-neutralising dilutions of specific antisera delayed the onset of virus-induced cytopathic effect (CPE) by up to 44 h compared to the untreated control cultures. Continued virus passage with sub-neutralising dilutions of these sera resulted in a decrease in time to complete CPE, suggesting that FMDV in these cultures adapted to escape immune pressure. These phenotypic changes were associated with three separate consensus-level non-synonymous mutations that accrued in the viral RNA-encoding amino acids at positions VP2⁶⁶, VP2⁸⁰ and VP1¹⁵⁵, corresponding to known epitope sites. High-throughput sequencing also identified further nucleotide substitutions within the regions encoding the leader (L^pro), VP4, VP2 and VP3 proteins. While association of the later mutations with the adaptation to immune pressure must be further verified, these results highlight the multiple routes by which FMDV populations can escape neutralising antibodies and support the application of a simple in vitro approach to assess the impact of the humoral immune system on the evolution of FMDV and potentially other viruses.

Insights into Jumonji C-domain containing protein 6 (JMJD6): a multifactorial role in foot-and-mouth disease virus replication in cells

The Jumonji C-domain containing protein 6 (JMJD6) has had a convoluted history, and recent reports indicating a multifactorial role in foot-and-mouth disease virus (FMDV) infection have further complicated the functionality of this protein. It was first identified as the phosphatidylserine receptor on the cell surface responsible for recognizing phosphatidylserine on the surface of apoptotic cells resulting in their engulfment by phagocytic cells. Subsequent study revealed a nuclear subcellular localization, where JMJD6 participated in lysine hydroxylation and arginine demethylation of histone proteins and other non-histone proteins. Interestingly, to date, JMDJ6 remains the only known arginine demethylase with a growing list of known substrate molecules. These conflicting associations rendered the subcellular localization of JMJD6 to be quite nebulous. Further muddying this area, two different groups illustrated that JMJD6 could be induced to redistribute from the cell surface to the nucleus of a cell. More recently, JMJD6 was demonstrated to be a host factor contributing to the FMDV life cycle, where it was not only exploited for its arginine demethylase activity, but also served as an alternative virus receptor. This review attempts to coalesce these divergent roles for a single protein into one cohesive account. Given the diverse functionalities already characterized for JMJD6, it is likely to continue to be a confounding protein resulting in much contention going into the near future.

Truncated Bovine Integrin Alpha-v/Beta-6 as a Universal Capture Ligand for FMD Diagnosis

Foot-and-mouth disease (FMD) is endemic in many regions of the world and is one of the most prevalent epizootic animal diseases. FMD affects livestock, such as cattle, sheep, goats and pigs, and causes enormous economic losses due to reduced productivity and trade restrictions. Preparedness and early diagnosis are essential for effective control of FMD. Many diagnostic assays are dependent on raising high-affinity, anti-FMD virus (FMDV) serotype-specific antibodies in small animals (rabbits and guinea pigs) that give broad virus coverage. Here we show that soluble, truncated forms of bovine αvβ6 bind FMDV in an authentic RGD and divalent cation dependent interaction and can be used as the trapping reagent in a FMDV sandwich ELISA. In addition, inclusion of FLAG or His tags facilitates simple purification without the loss of virus binding. We also provide evidence that when combined with a guinea pig polyclonal serum, or serotype-specific monoclonal antibodies, the integrin can be used to detect viruses representative of all FMDV serotypes. We also show that recombinant FMDV empty capsids, with stabilising disulphide bonds, can serve as an antigen in the ELISA and can therefore replace inactivated virus antigen as a positive control for the assay. Our results demonstrate the potential use of bovine αvβ6 and FMDV empty capsids in FMD diagnostic assays.

Myocarditis Associated with Foot-and-Mouth Disease Virus Type O in Lambs

The present study describes the pathogenetic mechanisms of myocarditis in 9 lambs that died in a foot-and-mouth disease outbreak in Samsun, Turkey. In all the heart samples tested, ELISA and sequencing for phylogenetic analyses showed that the virus, namely O/TUR/Samsun/05, was associated with the PanAsia pandemic strain of foot-and-mouth disease virus (FMDV) type O. The lambs had myocardial lesions but no typical vesicular lesions. In situ reverse transcription showed that many cardiomyocytes and some interstitial cells were positive for FMDV type O. Inflammatory infiltration, hyaline degeneration, and necrosis of sheets of myocytes were observed. The cellular infiltrates were mononuclear cells, including many lymphocytes, macrophages, a few plasma cells, and neutrophils. Major histocompatibility complex Class II+ dendritic and mononuclear cells, γδ T cells, CD172A+ and CD14+ macrophages and monocytes, and IgM+ B cells were detected mainly in the infected hearts. Inducible nitric oxide synthetase (iNOS) was seen mostly in areas of inflammation infiltrated by large numbers of cells. Of the 2 α-subunits of integrin known to be used as receptors by FMDV in epithelial tissues, CD49e (integrin α5) was detected in the membranes of cardiac myocytes with intercalated discs, but CD51 (integrin αV) was not detected in cardiac myocytes from infected or normal lambs. Interstitial and inflammatory cells were positive for both integrin subunits. The terminal deoxynucleotidyl transferase mediated dUTP nick-end labeling (TUNEL)-positive signal was detected in the nuclei of both cardiac myocytes and interstitial cells from infected lambs. These findings suggest that the iNOS expressed by inflammatory cells in lesions may have a deleterious effect on cardiac myocytes in these lesions.

Pathogenesis and micro-anatomic characterization of a cell-adapted mutant foot-and-mouth disease virus in cattle: Impact of the Jumonji C-domain containing protein 6 (JMJD6) and route of inoculation

A companion study reported Jumonji-C domain containing protein 6 (JMJD6) is involved in an integrin- and HS-independent pathway of FMDV infection in CHO cells. JMJD6 localization was investigated in animal tissues from cattle infected with either wild type A24-FMDV (A24-WT) or mutant FMDV (JMJD6-FMDV) carrying E95K/S96L and RGD to KGE mutations in VP1. Additionally, pathogenesis of mutant JMJD6-FMDV was investigated in cattle through aerosol and intraepithelial lingual (IEL) inoculation. Interestingly, JMJD6-FMDV pathogenesis was equivalent to A24-WT administered by IEL route. In contrast, JMJD6-FMDV aerosol-infected cattle did not manifest signs of FMD and animals showed no detectable viremia. Immunofluorescent microscopy of post-mortem tissue revealed JMJD6-FMDV exclusively co-localized with JMJD6(+) cells while A24-WT was occasionally found in JMJD6(+) cells. In vitro, chemical uptake inhibitors demonstrated JMJD6-FMDV entered cells via clathrin-coated pit endocytosis. In vivo, JMJD6-FMDV exhibited preference for JMJD6(+) cells, but availability of this alternative receptor likely depends on route of inoculation.

Role of Jumonji C-domain containing protein 6 (JMJD6) in infectivity of foot-and-mouth disease virus

Foot-and-mouth disease virus (FMDV) utilizes four integrins (αvβ1, αvβ3, αvβ6, and αvβ8) as its primary cell receptor. During cell culture propagation, FMDV frequently adapts to use heparan sulfate (HS), and rarely utilizes an unidentified third receptor. Capsid mutations acquired by a soluble integrin resistant FMDV cause (i) adaptation to CHO-677 cells (ii) increased affinity to membrane-bound Jumonji C-domain containing protein 6 (JMJD6) (iii) induced JMJD6 re-localization from the cell surface and cytoplasm to the nucleus. Interestingly, pre-treatment of cells with N- and C-terminal JMJD6 antibodies or by simultaneous incubation of mutant virus with soluble JMJD6 (but not by treatment with HS or αvβ6) impaired virus infectivity in cultured cells. JMJD6 and mutant virus co-purified by reciprocal co-immunoprecipitation. Molecular docking predictions suggested JMJD6 C-terminus interacts with mutated VP1 capsid protein. We conclude when specific VP1 mutations are displayed, JMJD6 contributes to FMDV infectivity and may be a previously unidentified FMDV receptor.

How foot-and-mouth disease virus receptor mediates foot-and-mouth disease virus infection

This study reviews the FMDV receptor-binding domain, integrin receptors, and heparan sulfate receptors to provide references for studies regarding the mechanisms underlying FMDV infection.

Virus-host interactions in persistently FMDV-infected cells derived from bovine pharynx

Foot-and-mouth disease virus (FMDV) produces a disease in cattle characterized by vesicular lesions and a persistent infection with asymptomatic low-level production of virus in pharyngeal tissues. Here we describe the establishment of a persistently infected primary cell culture derived from bovine pharynx tissue (PBPT) infected with FMDV serotype O1 Manisa, where surviving cells were serially passed until a persistently infected culture was generated. Characterization of the persistent virus demonstrated changes in its plaque size, ability to grow in different cell lines, and change in the use of integrins as receptors, when compared with the parental virus. These results demonstrate the establishment of persistently infected PBPT cell cultures where co-adaptation has taken place between the virus and host cells. This in vitro model for FMDV persistence may help further understanding of the molecular mechanisms of the cattle carrier state.

Development of a tetra-primer ARMS PCR-based assay for detection of a novel single-nucleotide polymorphism in the 5' untranslated region of the bovine ITGB6 receptor gene associated with foot-and-mouth disease susceptibility in cattle

ITGB6 is known to be one of the major receptor components involved in host tropism of foot-and-mouth disease (FMD) virus in cattle. A competitive PCR technique called ARMS PCR was adapted to identify a single-nucleotide polymorphism (SNP), G29A, db SNP Id: rs109075046, in the 5' untranslated region (5'UTR) of the bovine ITGB6 gene. Genotype profiling identified three kinds of genetic variation within the targeted SNP among Frieswal crossbred cattle. The occurrence of FMD in the three genotypes was further evaluated, revealing a clear role in the incidence of FMD in the studied population.

Analysis of SAT type foot-and-mouth disease virus capsid proteins and the identification of putative amino acid residues affecting virus stability

Foot-and-mouth disease virus (FMDV) initiates infection by adhering to integrin receptors on target cells, followed by cell entry and disassembly of the virion through acidification within endosomes. Mild heating of the virions also leads to irreversible dissociation into pentamers, a characteristic linked to reduced vaccine efficacy. In this study, the structural stability of intra- and inter-serotype chimeric SAT2 and SAT3 virus particles to various conditions including low pH, mild temperatures or high ionic strength, was compared. Our results demonstrated that while both the SAT2 and SAT3 infectious capsids displayed different sensitivities in a series of low pH buffers, their stability profiles were comparable at high temperatures or high ionic strength conditions. Recombinant vSAT2 and intra-serotype chimeric viruses were used to map the amino acid differences in the capsid proteins of viruses with disparate low pH stabilities. Four His residues at the inter-pentamer interface were identified that change protonation states at pH 6.0. Of these, the H145 of VP3 appears to be involved in interactions with A141 in VP3 and K63 in VP2, and may be involved in orientating H142 of VP3 for interaction at the inter-pentamer interfaces.

Comparative analysis of cloned cDNAs encoding Chinese yellow cattle and Gansu black swine integrin receptors for foot-and-mouth disease virus

To analyze foot-and-mouth disease virus tropism and host range with respect to the integrin receptor, we cloned cDNAs encoding the integrin αν, β1, β3, β6 and β8 subunits from Chinese yellow cattle and Gansu black swine and carried out comparative analysis of their molecular characteristics. The lengths of the mature proteins and the functional domains of the four integrin β subunits were the same between bovine and swine; however, the number of putative N-linked glycosylation sites and cysteine residues and their arrangement varied. Homology analysis of the nucleotide and amino acid sequences showed that FMDV integrin receptors of Chinese yellow cattle and Gansu black swine are highly conserved. Phylogenetic analysis showed that all FMDV integrin receptor subunits of cattle and swine are clustered into the Artiodactyla group; however, Chinese yellow cattle are phylogenetically closer to sheep than to Gansu black swine. We postulate that the host tropism of FMDV may, in part, be related to the divergence of integrin subunits among different species.

Induction of partial protection against foot and mouth disease virus in guinea pigs by neutralization with the integrin β6-1 subunit

The mechanism by which the foot-and-mouth disease virus (FMDV) initiates infection of cells is thought to involve the attachment of the viral capsid to host integrins on the surface of target cells. However, the role of integrins in FMDV infection still needs to be fully understood, although it has been demonstrated that integrin αvβ6 interferes with FMDV in vitro and results in neutralization of its infectivity. In the present study, we describe the cloning and sequencing of suckling mouse integrin β6 and the subsequent expression of two segments of integrin β6 extracellular domains: β6-1 (which contains the ligand-binding domain) and β6-2. Sequencing of the mouse integrin β6 subunit revealed close homology (~90%) with its human counterpart. When recombinant integrin extracellular domains β6-1 and β6-2 formulated with adjuvant were inoculated into guinea pigs, anti-integrin antibody expression was high before FMDV challenge. Interestingly, guinea pigs (50%) inoculated with integrin β6-1 were protected from FMDV infection; in contrast, none of the animals inoculated with integrin β6-2 were protected. This result indicates that an integrin blockade may be able to interfere with FMDV infection in vivo, which raises the possibility that targeting integrin in vivo may be the basis for a new strategy to control FMDV infection.

A continuous bovine kidney cell line constitutively expressing bovine αvβ6 integrin has increased susceptibility to foot-and-mouth disease virus

Foot-and-mouth disease (FMD) is a worldwide problem limiting the trade of animals and their products from affected countries. The rapid isolation, serotyping, and vaccine matching of FMD virus from disease outbreaks is critical for enabling the implementation of effective vaccination programs and to stop the spread of infection during outbreaks. Some primary cells have been shown to be highly susceptible to most strains of FMD virus (FMDV) but are difficult and expensive to prepare and maintain. Since the αVβ6 integrin is a principal receptor for FMDV, we transduced a bovine kidney cell line to stably express both the αV and β6 bovine integrin subunits. This stable cell line (LFBK-αVβ6) showed β6 expression and enhanced susceptibility to FMDV infection for ≥ 100 cell passages. LFBK-αVβ6 cells were highly sensitive for detecting all serotypes of FMDV from experimentally infected animals, including the porcinophilic FMDV strain O/TAW/97. In comparison to other cell types that are currently used for virus isolation, LFBK-αVβ6 cells were more effective at detecting FMDV in clinical samples, supporting their use as a more sensitive tool for virus isolation.

Examination of soluble integrin resistant mutants of foot-and-mouth disease virus

Background

Foot-and-mouth disease virus (FMDV) initiates infection via recognition of one of at least four cell-surface integrin molecules α_vβ₁, α_vβ₃, α_vβ₆, or α_vβ₈ by a highly conserved Arg-Gly-Asp (RGD) amino acid sequence motif located in the G-H loop of VP1. Within the animal host, the α_vβ₆ interaction is believed to be the most relevant. Sub-neutralizing levels of soluble secreted α_vβ₆ (ssα_vβ₆) was used as a selective pressure during passages in vitro to explore the plasticity of that interaction.

Results

Genetically stable soluble integrin resistant (SIR) FMDV mutants derived from A24 Cruzeiro were selected after just 3 passages in cell culture in the presence of sub-neutralizing levels of ssα_vβ₆. SIR mutants were characterized by: replication on selective cell lines, plaque morphology, relative sensitivity to ssα_vβ₆ neutralization, relative ability to utilize α_vβ₆ for infection, as well as sequence and structural changes. All SIR mutants maintained an affinity for α_vβ₆. Some developed the ability to attach to cells expressing heparan sulfate (HS) proteoglycan, while others appear to have developed affinity for a still unknown third receptor. Two classes of SIR mutants were selected that were highly or moderately resistant to neutralization by ssα_vβ₆. Highly resistant mutants displayed a G145D substitution (RGD to RDD), while moderately resistant viruses exhibited a L150P/R substitution at the conserved RGD + 4 position. VP1 G-H loop homology models for the A-type SIR mutants illustrated potential structural changes within the integrin-binding motif by these 2 groups of mutations. Treatment of O1 Campos with ssα_vβ₆ resulted in 3 SIR mutants with a positively charged VP3 mutation allowing for HS binding.

Conclusions

These findings illustrate how FMDV particles rapidly gain resistance to soluble receptor prophylactic measures in vitro. Two different serotypes developed distinct capsid mutations to circumvent the presence of sub-neutralizing levels of the soluble cognate receptor, all of which resulted in a modified receptor tropism that expanded the cell types susceptible to FMDV. The identification of some of these adaptive mutations in known FMDV isolates suggests these findings have implications beyond the cell culture system explored in these studies.

Direct contact transmission of three different foot-and-mouth disease virus strains in swine demonstrates important strain-specific differences

A novel direct contact transmission model for the study of foot-and-mouth disease virus (FMDV) infection of swine was utilized to investigate transmission characteristics of three FMDV strains belonging to serotypes A, O and Asia1. Each strain demonstrated distinct transmission characteristics and required different exposure times to achieve successful contact transmission. While a 4h exposure was sufficient for strain A24 Cruzeiro (A24Cru), both O1 Manisa and Asia1 Shamir transmission required 18 h or more. Viral excretion levels from donors (for all three strains) and virus present in room air (for A24Cru and O1 Manisa) were evaluated and associated with clinical signs and observed transmission pattern. Although all directly inoculated donor animals showed acute FMD, A24Cru had the highest levels of viral shedding in saliva and nasal swabs followed by O1 Manisa and Asia1 Shamir. Virus levels in room air were higher and were detected longer for A24Cru than for O1 Manisa. These results provide direct evidence for important strain-specific variation in transmission characteristics and emphasize the need for thorough evaluation of different FMDV viral strains using a well defined contact transmission methodology. This information is critical for vaccine and biotherapeutic efficacy testing, pathogenesis and disease modeling of FMDV transmission.

Introduction of tag epitopes in the inter-AUG region of foot and mouth disease virus: effect on the L protein

Foot-and-mouth disease virus (FMDV) initiates translation from two in-frame AUG codons producing two forms of the leader (L) proteinase, Lab (starting at the first AUG) and Lb (starting at second AUG). In a previous study, we have demonstrated that a cDNA-derived mutant FMDV (A24-L1123) containing a 57-nucleotide transposon (tn) insertion between the two AUG initiation codons (inter-AUG region) was completely attenuated in cattle, suggesting that this region is involved in viral pathogenesis. To investigate the potential role of the Lab protein in attenuation, we have introduced two epitope tags (Flag: DYKDDDK and HA: YPYDVPDYA) or a small tetracysteine motif (tc: CCGPCC) into the pA24-L1123 infectious DNA clone. Mutant viruses with a small plaque phenotype similar to the parental A24-L1123 were recovered after transfection of constructs encoding the Flag tag and the tc motif. However, expression of the Flag- or tc-tagged Lab protein was abolished or greatly diminished in these viruses. Interestingly, the A24-L1123/Flag virus acquired an extra base in the inter-AUG region that resulted in new AUG codons in-frame with the second AUG, and produced a larger Lb protein. This N terminal extension of the Lb protein in mutant A24-L1123/Flag did not affect virus viability or L functions in cell culture.

Domain disruptions of individual 3B proteins of foot-and-mouth disease virus do not alter growth in cell culture or virulence in cattle

Picornavirus RNA replication is initiated by a small viral protein primer, 3B (also known as VPg), that is covalently linked to the 5' terminus of the viral genome. In contrast to other picornaviruses that encode a single copy of 3B, foot-and-mouth disease virus (FMDV) encodes three copies of 3B. Viruses containing disrupted native sequence or deletion of one of their three 3B proteins were derived from a FMDV A24 Cruzeiro full-length cDNA infectious clone. Mutant viruses had growth characteristics similar to the parental virus in cells. RNA synthesis and protein cleavage processes were not significantly affected in these mutant viruses. Cattle infected by aerosol exposure with mutant viruses developed clinical disease similar to that caused by the parental A24 Cruzeiro. Therefore, severe domain disruption or deletion of individual 3B proteins in FMDV do not affect the virus' ability to replicate in vitro and cause clinical disease in cattle.

Identification of selection signatures in cattle breeds selected for dairy production

The genomics revolution has spurred the undertaking of HapMap studies of numerous species, allowing for population genomics to increase the understanding of how selection has created genetic differences between subspecies populations. The objectives of this study were to (1) develop an approach to detect signatures of selection in subsets of phenotypically similar breeds of livestock by comparing single nucleotide polymorphism (SNP) diversity between the subset and a larger population, (2) verify this method in breeds selected for simply inherited traits, and (3) apply this method to the dairy breeds in the International Bovine HapMap (IBHM) study. The data consisted of genotypes for 32,689 SNPs of 497 animals from 19 breeds. For a given subset of breeds, the test statistic was the parametric composite log likelihood (CLL) of the differences in allelic frequencies between the subset and the IBHM for a sliding window of SNPs. The null distribution was obtained by calculating CLL for 50,000 random subsets (per chromosome) of individuals. The validity of this approach was confirmed by obtaining extremely large CLLs at the sites of causative variation for polled (BTA1) and black-coat-color (BTA18) phenotypes. Across the 30 bovine chromosomes, 699 putative selection signatures were detected. The largest CLL was on BTA6 and corresponded to KIT, which is responsible for the piebald phenotype present in four of the five dairy breeds. Potassium channel-related genes were at the site of the largest CLL on three chromosomes (BTA14, -16, and -25) whereas integrins (BTA18 and -19) and serine/arginine rich splicing factors (BTA20 and -23) each had the largest CLL on two chromosomes. On the basis of the results of this study, the application of population genomics to farm animals seems quite promising. Comparisons between breed groups have the potential to identify genomic regions influencing complex traits with no need for complex equipment and the collection of extensive phenotypic records and can contribute to the identification of candidate genes and to the understanding of the biological mechanisms controlling complex traits.

Alphav and beta1 integrins regulate dynamic compression-induced proteoglycan synthesis in 3D gel culture by distinct complementary pathways

OBJECTIVE

Our goal was to test the hypothesis that specific integrin receptors regulate chondrocyte biosynthetic response to dynamic compression at early times in 3D gel culture, during initial evolution of the pericellular matrix, but prior to significant accumulation of further-removed matrix. The study was motivated by increased use of dynamic loading, in vitro, for early stimulation of tissue engineered cartilage, and the need to understand the effects of loading, in vivo, at early times after implantation of constructs.

METHODS

Bovine articular chondrocytes were seeded in 2% agarose gels (15x10(6)cells/mL) and incubated for 18 h with and without the presence of specific integrin blockers (small-molecule peptidomimetics, function-blocking antibodies, and RGD-containing disintegrins). Samples were then subjected to a 24-h dynamic compression regime found previously to stimulate chondrocyte biosynthesis in 3D gel as well as cartilage explant culture (1 Hz, 2.5% dynamic strain amplitude, 7% static offset strain). At the end of loading, proteoglycan (PG) synthesis ((35)S-sulfate incorporation), protein synthesis ((3)H-proline incorporation), DNA content (Hoechst dye 33258) and total glycosaminoglycan (GAG) content (dimethyl methylene blue (DMMB) dye binding) were assessed.

RESULTS

Consistent with previous studies, dynamic compression increased PG synthesis and total GAG accumulation compared to free-swelling controls. Blocking alphavbeta3 abolished this response, independent of effects on controls, while blocking beta1 abolished the relative changes in synthesis when changes in free-swelling synthesis rates were observed.

CONCLUSIONS

This study suggests that both alphavbeta3 and beta1 play a role in pathways that regulate stimulation of PG synthesis and accumulation by dynamic compression, but through distinct complementary mechanisms.

Picornaviruses

The picornavirus family consists of a large number of small RNA viruses, many of which are significant pathogens of humans and livestock. They are amongst the simplest of vertebrate viruses comprising a single stranded positive sense RNA genome within a T = 1 (quasi T = 3) icosahedral protein capsid of approximately 30 nm diameter. The structures of a number of picornaviruses have been determined at close to atomic resolution by X-ray crystallography. The structures of cell entry intermediate particles and complexes of virus particles with receptor molecules or antibodies have also been obtained by X-ray crystallography or at a lower resolution by cryo-electron microscopy. Many of the receptors used by different picornaviruses have been identified, and it is becoming increasingly apparent that many use co-receptors and alternative receptors to bind to and infect cells. However, the mechanisms by which these viruses release their genomes and transport them across a cellular membrane to gain access to the cytoplasm are still poorly understood. Indeed, detailed studies of cell entry mechanisms have been made only on a few members of the family, and it is yet to be established how broadly the results of these are applicable across the full spectrum of picornaviruses. Working models of the cell entry process are being developed for the best studied picornaviruses, the enteroviruses. These viruses maintain particle integrity throughout the infection process and function as genome delivery modules. However, there is currently no model to explain how viruses such as cardio- and aphthoviruses that appear to simply dissociate into subunits during uncoating deliver their genomes into the cytoplasm.

Fusion of epithelial cells by Epstein-Barr virus proteins is triggered by binding of viral glycoproteins gHgL to integrins alphavbeta6 or alphavbeta8

Epstein-Barr virus (EBV) is a ubiquitous human herpesvirus that is causally implicated in the development of lymphoid and epithelial tumors. Entry of virus requires fusion of virus envelopes and cell membranes. Fusion with B lymphocytes requires virus glycoprotein gB and a 3-part complex of glycoproteins, gHgLgp42. It is triggered by interactions between glycoprotein 42 (gp42) and HLA class II. However, fusion with epithelial cells is impeded by gp42 and instead is triggered by interactions between an unknown epithelial protein and a 2-part complex of gHgL. We report here that gHgL binds with high affinity to epithelial cells and that affinity of binding is increased by 3 orders of magnitude in the presence of Mn(2+). Binding and infection can be reduced by fibronectin and vitronectin, by down-regulation of integrin alphav, or by a peptide corresponding to 13 aa of gH which include a KGDE motif. Fusion of cells expressing gB and gHgL can be blocked by vitronectin or triggered by addition of soluble truncated integrins alphavbeta6 and alphavbeta8. We conclude that the direct interaction between EBV gHgL and integrins alphavbeta6 and alphavbeta8 can provide the trigger for fusion of EBV with an epithelial cell.

Identification of cellular genes affecting the infectivity of foot-and-mouth disease virus

Foot-and-mouth disease virus (FMDV) produces one of the most infectious of all livestock diseases, causing extensive economic loss in areas of breakout. Like other viral pathogens, FMDV recruits proteins encoded by host cell genes to accomplish the entry, replication, and release of infectious viral particles. To identify such host-encoded proteins, we employed an antisense RNA strategy and a lentivirus-based library containing approximately 40,000 human expressed sequence tags (ESTs) to randomly inactivate chromosomal genes in a bovine kidney cell line (LF-BK) that is highly susceptible to FMDV infection and then isolated clones that survived multiple rounds of exposure to the virus. Here, we report the identification of ESTs whose expression in antisense orientation limited host cell killing by FMDV and restricted viral propagation. The role of one such EST, that of ectonucleoside triphosphate diphosphohydrolase 6 (NTPDase6; also known as CD39L2), a membrane-associated ectonucleoside triphosphate diphosphohydrolase that previously was not suspected of involvement in the propagation of viral pathogens and which we now show is required for normal synthesis of FMDV RNA and proteins, is described in this report.

Integrin alphaVbeta6 is a high-affinity receptor for coxsackievirus A9

Coxsackievirus A9 (CAV9), a member of the genus Enterovirus in the family Picornaviridae, possesses an integrin-binding arginine-glycine-aspartic acid (RGD) motif in the C terminus of VP1 capsid protein. CAV9 has been shown to utilize integrins alphaVbeta3 and alphaVbeta6 as primary receptors for cell attachment. While CAV9 RGD-mutants (RGE and RGDdel) are capable of infecting rhabdomyosarcoma (RD) cell line, they grow very poorly in an epithelial lung carcinoma cell line (A549). In this study, the relationships between CAV9 infectivity in A549 and RD cells, receptor expression and integrin binding were analysed. A549 cells were shown to express both integrins alphaVbeta3 and alphaVbeta6, whereas alphaVbeta6 expression was not detected on the RD cells. Native CAV9 but not RGE and RGDdel mutants bound efficiently to immobilized alphaVbeta3 and alphaVbeta6. Adhesion of CAV9 but not RGE/RGDdel to A549 cells was also significantly higher than to RD cells. In contrast, no affinity or adhesion of bacterially produced VP1 proteins to the integrins or to the cells was detected. Function-blocking antibodies against alphaV-integrins blocked CAV9 but not CAV9-RGDdel infectivity, indicating that the viruses use different internalization routes; this may explain the differential infection kinetics of CAV9 and RGDdel. In an affinity assay, soluble alphaVbeta6, but not alphaVbeta3, bound to immobilized CAV9. Similarly, only soluble alphaVbeta6 blocked virus infectivity. These data suggest that CAV9 binding to alphaVbeta6 is a high-affinity interaction, which may indicate its importance in clinical infections; this remains to be determined.

Molecular decoys: antidotes, therapeutics and immunomodulators

Receptor–ligand interactions are fundamental to the regulation of cell physiology, enabling the communication between cells and their environment via signal transduction. Receptors are also exploited by toxins and infectious agents to mediate pathogenesis. Over the past 20 years, however, this bi-partite paradigm for cellular regulation, that is, receptors and their ligands, has been revised to include an unforeseen participant namely, soluble receptors or molecular decoys. Decoys function as nature's modifiers of potent responses such as inflammation, stimulation of cell proliferation and triggering apoptosis. Decoys not only provide the means to fine tune the regulation of these phenomena; they also serve as potential leads for the development of recombinant anti-toxins, anti-viral agents and novel therapeutics for combating cancer and inflammatory disease.

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.

Early events in the pathogenesis of foot-and-mouth disease in cattle after controlled aerosol exposure

The goal of this study was to identify the primary sites of replication of foot-and-mouth disease virus (FMDV) in cattle subsequent to aerogenous inoculation. A novel aerosol inoculation method was developed to simulate natural, airborne transmission and thereby allow the identification of early replication sites. Virus distribution after aerosol inoculation was compared at 24h post inoculation with simple nasal instillation. Aerosol inoculation of FMDV consistently resulted in virus detection by real-time reverse transcriptase-polymerase chain reaction and viral isolation in the soft palate, pharynx, and lungs. Viral antigen was also detected in each of these tissues by immunohistochemistry. Aerosol exposure resulted in typical clinical signs of FMD when animals were kept alive long enough to develop disease. This aerosol infection method is highly reproducible regarding inoculum dose and volume, and allowed the detailed study of early events in FMDV-infected cattle. Extensive postmortem sampling and trimodal virus detection system allows a more precise determination of FMDV localization than previously reported.

Engineering a single-chain Fv antibody to alpha v beta 6 integrin using the specificity-determining loop of a foot-and-mouth disease virus

The alpha v beta 6 integrin is a promising target for cancer therapy. Its expression is up-regulated de novo on many types of carcinoma where it may activate transforming growth factor-beta1 and transforming growth factor-beta 3, interact with the specific extracellular matrix proteins and promote migration and invasion of tumor cells. The viral protein 1 (VP1) coat protein of the O(1) British field strain serotype of foot-and-mouth disease virus is a high-affinity ligand for alpha v beta 6, and we recently reported that a peptide derived from VP1 exhibited alpha v beta 6-specific binding in vitro and in vivo. We hypothesized that this peptide could confer binding specificity of an antibody to alpha v beta 6. A 17-mer peptide of VP1 was inserted into the complementarity-determining region H3 loop of MFE-23, a murine single-chain Fv (scFv) antibody reactive with carcinoembryonic antigen (CEA). The resultant scFv (B6-1) bound to alpha v beta 6 but retained residual reactivity with CEA. This was eliminated by point mutation (Y100bP) in the variable heavy-chain domain to create an scFv (B6-2) that was as structurally stable as MFE-23 and reacted specifically with alpha v beta 6 but not with alpha 5 beta 1, alpha v beta 3, alpha v beta 5, alpha v beta 8 or CEA. B6-2 was internalized into alpha v beta 6-expressing cells and inhibited alpha v beta 6-dependent migration of carcinoma cells. B6-2 was subsequently humanized. The humanized form (B6-3) was obtained as a non-covalent dimer from secretion in Pichia pastoris (115 mg/l) and was a potent inhibitor of alpha v beta 6-mediated cell adhesion. Thus, we have used a rational stepwise approach to create a humanized scFv with therapeutic potential to block alpha v beta 6-mediated cancer cell invasion or to deliver and internalize toxins specifically to alpha v beta 6-expressing tumors.

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.

Dendritic cell internalization of foot-and-mouth disease virus: influence of heparan sulfate binding on virus uptake and induction of the immune response

Dendritic cells (DC), which are essential for inducing and regulating immune defenses and responses, represent the critical target for vaccines against pathogens such as foot-and-mouth disease virus (FMDV). Although it is clear that FMDV enters epithelial cells via integrins, little is known about FMDV interaction with DC. Accordingly, DC internalization of FMDV antigen was analyzed by comparing vaccine virus dominated by heparan sulfate (HS)-binding variants with FMDV lacking HS-binding capacity. The internalization was most efficient with the HS-binding virus, employing diverse endocytic pathways. Moreover, internalization relied primarily on HS binding. Uptake of non-HS-binding virus by DC was considerably less efficient, so much so that it was often difficult to detect virus interacting with the DC. The HS-binding FMDV replicated in DC, albeit transiently, which was demonstrable by its sensitivity to cycloheximide treatment and the short duration of infectious virus production. There was no evidence that the non-HS-binding virus replicated in the DC. These observations on virus replication may be explained by the activities of viral RNA in the DC. When DC were transfected with infectious RNA, only 1% of the translated viral proteins were detected. Nevertheless, the transfected cells, and DC which had internalized live virus, did present antigen to lymphocytes, inducing an FMDV-specific immunoglobulin G response. These results demonstrate that DC internalization of FMDV is most efficient for vaccine virus with HS-binding capacity, but HS binding is not an exclusive requirement. Both non-HS-binding virus and infectious RNA interacting with DC induce specific immune responses, albeit less efficiently than HS-binding virus.

The role of integrins in cancer and the development of anti-integrin therapeutic agents for cancer therapy

Integrins have been reported to mediate cell survival, proliferation, differentiation, and migration programs. For this reason, the past few years have seen an increased interest in the implications of integrin receptors in cancer biology and tumor cell aggression. This review considers the potential role of integrins in cancer and also addresses why integrins are present attractive targets for drug design. It discusses of the several properties of the integrin-based chemotherapeutic agents currently under consideration clinically and provides an insight into cancer drug development using integrin as a target.

The Role of Integrins in Cancer and the Development of Anti-Integrin Therapeutic Agents for Cancer Therapy

Integrins have been reported to mediate cell survival, proliferation, differentiation, and migration programs. For this reason, the past few years have seen an increased interest in the implications of integrin receptors in cancer biology and tumor cell aggression. This review considers the potential role of integrins in cancer and also addresses why integrins are present attractive targets for drug design. It discusses of the several properties of the integrin-based chemotherapeutic agents currently under consideration clinically and provides an insight into cancer drug development using integrin as a target.

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.

Early events in integrin alphavbeta6-mediated cell entry of foot-and-mouth disease virus

We have shown that foot-and-mouth disease virus (FMDV) infection mediated by the integrin alphavbeta6 takes place through clathrin-dependent endocytosis but not caveolae or other endocytic pathways that depend on lipid rafts. Inhibition of clathrin-dependent endocytosis by sucrose treatment or expression of a dominant-negative version of AP180 inhibited virus entry and infection. Similarly, inhibition of endosomal acidification inhibited an early step in infection. Blocking endosomal acidification did not interfere with surface expression of alphavbeta6, virus binding to the cells, uptake of the virus into endosomes, or cytoplasmic virus replication, suggesting that the low pH within endosomes is a prerequisite for delivery of viral RNA into the cytosol. Using immunofluorescence confocal microscopy, FMDV colocalized with alphavbeta6 at the cell surface but not with the B subunit of cholera toxin, a marker for lipid rafts. At 37 degrees C, virus was rapidly taken up into the cells and colocalized with markers for early and recycling endosomes but not with a marker for lysosomes, suggesting that infection occurs from within the early or recycling endosomal compartments. This conclusion was supported by the observation that FMDV infection is not inhibited by nocodazole, a reagent that inhibits vesicular trafficking between early and late endosomes (and hence trafficking to lysosomes). The integrin alphavbeta6 was also seen to accumulate in early and recycling endosomes on virus entry, suggesting that the integrin serves not only as an attachment receptor but also to deliver the virus to the acidic endosomes. These findings are all consistent with FMDV infection proceeding via clathrin-dependent endocytosis.

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.

An arthritogenic alphavirus uses the α1β1 integrin collagen receptor

Ross River (RR) virus is an alphavirus endemic to Australia and New Guinea and is the aetiological agent of epidemic polyarthritis or RR virus disease. Here we provide evidence that RR virus uses the collagen-binding alpha1beta1 integrin as a cellular receptor. Infection could be inhibited by collagen IV and antibodies specific for the beta1 and alpha1 integrin proteins, and fibroblasts from alpha1-integrin-/- mice were less efficiently infected than wild-type fibroblasts. Soluble alpha1beta1 integrin bound immobilized RR virus, and peptides representing the alpha1beta1 integrin binding-site on collagen IV inhibited virus binding to cells. We speculate that two highly conserved regions within the cell-receptor binding domain of E2 mimic collagen and provide access to cellular collagen-binding receptors.