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

Foot-and-Mouth Disease Vaccines by Design; Production of Capsid-Modified Foot-and-Mouth Disease Viruses with Improved Cell Culture Growth

BACKGROUND/OBJECTIVES

Vaccination is important for controlling foot-and-mouth disease (FMD) in endemic regions and to lessen the effects of outbreaks in FMD-free countries. The adaptation of FMD virus to BHK cells is a necessary but time-consuming and costly step in vaccine production and can prove problematic for some isolates. Adaptation is, in part, driven by receptor availability and selects variants with altered receptor specificity that result from amino acid substitutions in the capsid proteins.

METHODS

To bypass the need for cell culture adaptation, we generated chimeric viruses with field-strain capsids and introduced amino acid substitutions associated with cell culture adaptation. We targeted two sites on the capsid: the canonical heparan sulphate binding site and the icosahedral 5-fold symmetry axes.

RESULTS

Our results show that some viruses with unmodified wild-type (wt) capsids grew well in BHK cells (suspension and adherent), whereas others showed poor growth. For viruses that showed good growth, the introduction of amino acid changes associated with cell culture adaptation improved the rate of growth but not virus titres or yields of 146S particles, whereas growth and 146S yields for viruses that grew poorly in BHK cells were greatly enhanced by some of the amino acid changes. For the latter viruses, the introduced changes did not appear to adversely affect virion stability or antigenicity.

CONCLUSIONS

For FMD viruses that grow poorly in BHK cells, this approach could be a viable alternative to protracted adaptation by serial passage and could expedite the production of a new vaccine strain from a field virus.

A Review of the Utility of Established Cell Lines for Isolation and Propagation of the Southern African Territories Serotypes of Foot-and-Mouth Disease Virus

Cell culture underpins virus isolation and virus neutralisation tests, which are both gold-standard diagnostic methods for foot-and-mouth disease (FMD). Cell culture is also crucial for the propagation of inactivated foot-and-mouth disease virus (FMDV) vaccines. Both primary cells and cell lines are utilised in FMDV isolation and propagation. Widely used cell lines for FMDV and isolation and propagation include baby hamster kidney cells (BHK-21), swine kidney cells (IB-RS-2), foetal goat tongue (ZZ-R 127), foetal porcine kidney cells (LFBKvB6), bovine kidney cells (BK), human telomerase reverse transcriptase bovine thyroid (hTERT-BTY) and porcine kidney-originating PK-15 or SK 6 cell lines. This review highlights how different receptors and molecules-integrins, heparan sulphate (HS), and the Jumonji C-domain containing Protein 6 (JMJD6)-found on the surface of different cell types contribute to differences experienced with susceptibility and sensitivity of the cells to infection with different serotypes of FMDV. This review specifically focuses on Southern African territory (SAT) serotypes, which are unique to the Southern African context and are often under-investigated in cell line development for FMDV isolation and propagation.

The prevalence of foot-and-mouth disease in Asia

Foot-and-mouth disease (FMD) is listed among the highly contagious diseases in animals and is endemic throughout the Asian continent. The disease is caused by the Foot-and-mouth disease virus (FMDV) and affects a wide variety of domesticated animals as well as wild ungulates. Clinically, the disease is described as a vesicular lesion on the tongue, muzzle, lips, gum, dental pad, interdigital cleft, coronary band, and heel of the foot. Sometimes these lesions give rise to lameness. Mastitis is also caused due to teat lesions. A biochemical test reveals that during FMD infection, there are elevated levels of interleukin-1 (IL-1), tumor necrosis factor-alpha, interferon-gamma (IFN-γ), interleukin-6, serum amyloid A protein, lactoferrin, mannose-binding lectin, and monocytes chemo-attractant protein-1 in the serum of infected animals. There is no specific treatment for FMD although some antivirals are given as prophylaxis and antibiotics are given to prevent secondary bacterial infection. This review presents comprehensive data on the prevalence of FMD and serotypes of FMDV that are attributable to the cause of FMD from a regional point of view. It also explains the worldwide dynamics of the seven serotypes of FMD and tries to identify epidemiological clusters of FMD in various geographical areas. Furthermore, the pathology associated with the foot and mouth disease virus along with the pathophysiology is discussed. The continent-wide prevalence and diversity patterns of FMD suggest that there is a need for stringent policies and legislation implementation regarding research and development aimed at manufacturing strain-specific vaccination, infection prevention, and control of the disease.

Viruses Binding to Host Receptors Interacts with Autophagy

Viruses must cross the plasma membrane to infect cells, making them eager to overcome this barrier in order to replicate in hosts. They bind to cell surface receptors as the first step of initiating entry. Viruses can use several surface molecules that allow them to evade defense mechanisms. Various mechanisms are stimulated to defend against viruses upon their entry into cells. Autophagy, one of the defense systems, degrades cellular components to maintain homeostasis. The presence of viruses in the cytosol regulates autophagy; however, the mechanisms by which viral binding to receptors regulates autophagy have not yet been fully established. This review discusses recent findings on autophagy induced by interactions between viruses and receptors. It provides novel perspectives on the mechanism of autophagy as regulated by viruses.

Molecular detection and phylogenetic analysis of newly emerging foot-and-mouth disease virus type A, Lineage EURO-SA in Egypt in 2022

A newly emerging and exotic foot-and-mouth disease virus (FMDV) caused a recent outbreak of serotype A in Egypt in 2022, which affected cattle and water buffalo. Previous phylogenetic studies on FMDV circulating in Egypt have mainly focused on genomic regions encoding the structural proteins which determine FMDV serotype. No study has yet determined structural proteins sequences of the newly emerging Europe-South America (EURO-SA) lineage which was recently isolated from Egypt during a routine surveillance in 2022. The objective of the current study was to analyze the structural proteins of the Venezuelan type which belongs to EURO-SA. The new isolate was related to serotype A lineage Euro-South America. Phylogentic analyses have reveled that the newly isolated lineage samples were closely related to reported sequences that have been identified in Venzuela and Colombia. Analysis of structural protein sequences revealed the recent isolates belong to prototype strain A24 Cruzeiro. Notably, nucleotide sequences of the Egyptian isolate was related to Venezuelan, Brazilian, and Colombian strains with identity not exceeding 90%. The divergence which appears in the genetic identity of the Egyptian A/EURO-SA lineage from other related strains may be attributed to the absence of Euro-SA lineage sequence from Egypt. The present study is the first report on the detection of EURO-SA lineage in Egypt. The recent detection of the EURO-SA lineage samples may be explained due to imported animals from Colombia or Brazil which share geographical borders with Venezuela. The findings of the present study highlight the significance of continuous monitoring of FMDV in Egypt for newly emerging FMDVs.

Evolution of SLiM-mediated hijack functions in intrinsically disordered viral proteins

Viruses and their hosts are involved in an ‘arms race’ where they continually evolve mechanisms to overcome each other. It has long been proposed that intrinsic disorder provides a substrate for the evolution of viral hijack functions and that short linear motifs (SLiMs) are important players in this process. Here, we review evidence in support of this tenet from two model systems: the papillomavirus E7 protein and the adenovirus E1A protein. Phylogenetic reconstructions reveal that SLiMs appear and disappear multiple times across evolution, providing evidence of convergent evolution within individual viral phylogenies. Multiple functionally related SLiMs show strong coevolution signals that persist across long distances in the primary sequence and occur in unrelated viral proteins. Moreover, changes in SLiMs are associated with changes in phenotypic traits such as host range and tropism. Tracking viral evolutionary events reveals that host switch events are associated with the loss of several SLiMs, suggesting that SLiMs are under functional selection and that changes in SLiMs support viral adaptation. Fine-tuning of viral SLiM sequences can improve affinity, allowing them to outcompete host counterparts. However, viral SLiMs are not always competitive by themselves, and tethering of two suboptimal SLiMs by a disordered linker may instead enable viral hijack. Coevolution between the SLiMs and the linker indicates that the evolution of disordered regions may be more constrained than previously thought. In summary, experimental and computational studies support a role for SLiMs and intrinsic disorder in viral hijack functions and in viral adaptive evolution.

The endocytosis of foot-and mouth disease virus requires clathrin and caveolin and is dependent on the existence of Rab5 and Rab7 in CHO-677 cells

Foot-and-mouth disease virus (FMDV) is a highly contagious virus that causes severe vesicular disease of cloven-hoofed animals. Various endocytosis mechanisms are involved in the entry of FMDV after binding to the integrin and heparan sulfate (HS) receptors. However, the mechanism of FMDV using other unknown receptors to enter the cells remains unclear. Here, we reported that the endocytosis and endosomal pathways are employed by FMDV to invade the Chinese hamster ovary cell line (CHO-677) without the integrin and HS receptors. We demonstrated that the internalization of FMDV into CHO-677 cells was abrogated by chlorpromazine, an inhibitor of clathrin-mediated endocytosis. Knockdown of the clathrin heavy chain decreased the viral protein abundance. Incubation of the CHO-677 cells with the inhibitors of caveolae-mediated endocytosis or transfection by caveolin-1 siRNA also limited FMDV replication. In addition, we determined that the acidic environment and the existence of dynamin were essential for FMDV infection in CHO-677 cells. The endosomal proteins Rab5 (early endosome) and Rab7 (late endosome), but not Rab11 (recycling endosome), were utilized by FMDV during infection. These data provide a new entry model of FMDV by unknown receptors which will help to better understand the pathogenesis mediated by FMDV.

The Use of Distinctive Monoclonal Antibodies in FMD VLP- and P1-Based Blocking ELISA for the Seromonitoring of Vaccinated Swine

The serum neutralization (SN) test has been regarded as the “gold standard” for seroconversion following foot-and-mouth disease virus (FMDV) vaccination, although a high-level biosafety laboratory is necessary. ELISA is one alternative, and its format is constantly being improved. For instance, standard polyclonal antisera have been replaced by monoclonal antibodies (MAbs) for catching and detecting antibodies, and inactive viruses have been replaced by virus-like particles (VLPs). To the best of current knowledge, however, no researchers have evaluated the performances of different MAbs as tracers. In previous studies, we successfully identified site 1 and site 2 MAbs Q10E and P11A. In this study, following the established screening platform, the VLPs of putative escape mutants from sites 1 to 5 were expressed and used to demonstrate that S11B is a site 3 MAb. Additionally, the vulnerability of VLPs prompted us to assess another diagnostic antigen: unprocessed polyprotein P1. Therefore, we established and evaluated the performance of blocking ELISA (bELISA) systems based on VLPs and P1, pairing them with Q10E, P11A, S11B, and the non-neutralizing TSG MAb as tracers. The results indicated that the VLP paired with S11B demonstrated the highest correlation with the SN titers (R² = 0.8071, n = 63). Excluding weakly positive serum samples (SN = 16–32, n = 14), the sensitivity and specificity were 95.65% and 96.15% (kappa = 0.92), respectively. Additionally, the P1 pairing with Q10E also demonstrated a high correlation (R² = 0.768). We also discovered that these four antibodies had steric effects on one another to varying degrees, despite recognizing distinct antigenic sites. This finding indicated that MAbs as tracers could not accurately detect specific antibodies, possibly because MAbs are bulky compared to a protomeric unit. However, our results still provide convincing support for the application of two pairs of bELISA systems: VLP:S11B-HRP and P1:Q10E-HRP.

Adherent and suspension baby hamster kidney cells have a different cytoskeleton and surface receptor repertoire

Animal cell culture, with single cells growing in suspension, ideally in a chemically defined environment, is a mainstay of biopharmaceutical production. The synthetic environment lacks exogenous growth factors and usually requires a time-consuming adaptation process to select cell clones that proliferate in suspension to high cell numbers. The molecular mechanisms that facilitate the adaptation and that take place inside the cell are largely unknown. Especially for cell lines that are used for virus antigen production such as baby hamster kidney (BHK) cells, the restriction of virus growth through the evolution of undesired cell characteristics is highly unwanted. The comparison between adherently growing BHK cells and suspension cells with different susceptibility to foot-and-mouth disease virus revealed differences in the expression of cellular receptors such as integrins and heparan sulfates, and in the organization of the actin cytoskeleton. Transcriptome analyses and growth kinetics demonstrated the diversity of BHK cell lines and confirmed the importance of well-characterized parental cell clones and mindful screening to make sure that essential cellular features do not get lost during adaptation.

An adventitious agent-free clonal cell line that is highly susceptible to foot -and-mouth disease virus

Porcine LFBKαVβ6 cells have been successfully used for diagnostics and propagation of all FMDV serotypes/subtypes. Unfortunately, after initial characterization, these cells showed contamination with bovine viral diarrhea virus (BVDV), a non-cytopathic adventitious agent. Persistent infection with BVDV could interfere with diagnostic tests and, also prevent consideration for other uses, i.e., vaccine production. In this study, we developed a three-prong methodology to completely remove BVDV from LFBKαVβ6 cells. Combined treatment with siRNA against BVDV NS5A, porcine interferon alpha and ribavirin resulted in the elimination of BVDV, as determined by immunohistochemistry analysis, quantitative RT-PCR and RNA sequencing. Importantly, elimination of BVDV from LFBKαVβ6 did not affect FMDV growth and plaque phenotype from different serotypes isolated and propagated in the clean cell line, newly named MGPK αVβ6-C5. Additionally, isolation of FMDV from field oro-pharyngeal samples, was successful at the same sensitivity as in BVDV-contaminated LFBKαVβ6 cells. Our results identified a direct method to efficiently eliminate BVDV from porcine cells without altering FMDV permissiveness, diagnostic value, or potential for use in vaccine production. Furthermore, these cells may provide an improved platform for diagnostics and propagation of other viruses of interest in the veterinary field and the virology community at large.

A Vaccine Strain of the A/ASIA/Sea-97 Lineage of Foot-and-Mouth Disease Virus with a Single Amino Acid Substitution in the P1 Region That Is Adapted to Suspension Culture Provides High Immunogenicity

There are seven viral serotypes of foot-and-mouth disease virus (FMDV): A, O, C, Asia 1, and Southern African Territories 1, 2, and 3 (SAT 1–3). Unlike serotype O FMDV vaccine strains, vaccine strains of serotype A FMDV do not provide broad-range cross-reactivity in serological matching tests with field isolates. Therefore, the topotype/lineage vaccine strain circulating in many countries and a highly immunogenic strain might be advantageous to control serotype A FMDV. We developed a new vaccine strain, A/SKR/Yeoncheon/2017 (A-1), which belongs to the A/ASIA/Sea-97 lineage that frequently occurs in Asian countries. Using virus plaque purification, we selected a vaccine virus with high antigen productivity and the lowest numbers of P1 mutations among cell-adapted virus populations. The A/SKR/Yeoncheon/2017 (A-1) vaccine strain has a single amino acid mutation, VP2 E82K, in the P1 region, and it is perfectly adapted to suspension culture. The A/SKR/Yeoncheon/2017 (A-1) experimental vaccine conferred high immunogenicity in pigs. The vaccine strain was serologically matched with various field isolates in two-dimensional virus neutralization tests using bovine serum. Vaccinated mice were protected against an A/MAY/97 virus that was serologically mismatched with the vaccine strain. Thus, A/SKR/Yeoncheon/2017 (A-1) might be a promising vaccine candidate for protection against the emerging FMDV serotype A in Asia.

Virus-Host Interactions in Foot-and-Mouth Disease Virus Infection

Foot-and-mouth disease (FMD) is a highly contagious disease of cloven-hoofed animals, which has been regarded as a persistent challenge for the livestock industry in many countries. Foot-and-mouth disease virus (FMDV) is the etiological agent of FMD that can spread rapidly by direct and indirect transmission. FMDV is internalized into host cell by the interaction between FMDV capsid proteins and cellular receptors. When the virus invades into the cells, the host antiviral system is quickly activated to suppress the replication of the virus and remove the virus. To retain fitness and host adaptation, various viruses have evolved multiple elegant strategies to manipulate host machine and circumvent the host antiviral responses. Therefore, identification of virus-host interactions is critical for understanding the host defense against virus infections and the pathogenesis of the viral infectious diseases. This review elaborates on the virus-host interactions during FMDV infection to summarize the pathogenic mechanisms of FMD, and we hope it can provide insights for designing effective vaccines or drugs to prevent and control the spread of FMD and other diseases caused by picornaviruses.

Biological and Clinical Consequences of Integrin Binding via a Rogue RGD Motif in the SARS CoV-2 Spike Protein

Although ACE2 (angiotensin converting enzyme 2) is considered the primary receptor for CoV-2 cell entry, recent reports suggest that alternative pathways may contribute. This paper considers the hypothesis that viral binding to cell-surface integrins may contribute to the high infectivity and widespread extra-pulmonary impacts of the SARS-CoV-2 virus. This potential is suggested on the basis of the emergence of an RGD (arginine-glycine-aspartate) sequence in the receptor-binding domain of the spike protein. RGD is a motif commonly used by viruses to bind cell-surface integrins. Numerous signaling pathways are mediated by integrins and virion binding could lead to dysregulation of these pathways, with consequent tissue damage. Integrins on the surfaces of pneumocytes, endothelial cells and platelets may be vulnerable to CoV-2 virion binding. For instance, binding of intact virions to integrins on alveolar cells could enhance viral entry. Binding of virions to integrins on endothelial cells could activate angiogenic cell signaling pathways; dysregulate integrin-mediated signaling pathways controlling developmental processes; and precipitate endothelial activation to initiate blood clotting. Such a procoagulant state, perhaps together with enhancement of platelet aggregation through virions binding to integrins on platelets, could amplify the production of microthrombi that pose the threat of pulmonary thrombosis and embolism, strokes and other thrombotic consequences. The susceptibility of different tissues to virion–integrin interactions may be modulated by a host of factors, including the conformation of relevant integrins and the impact of the tissue microenvironment on spike protein conformation. Patient-specific differences in these factors may contribute to the high variability of clinical presentation. There is danger that the emergence of receptor-binding domain mutations that increase infectivity may also enhance access of the RGD motif for integrin binding, resulting in viral strains with ACE2 independent routes of cell entry and novel integrin-mediated biological and clinical impacts. The highly infectious variant, B.1.1.7 (or VUI 202012/01), includes a receptor-binding domain amino acid replacement, N501Y, that could potentially provide the RGD motif with enhanced access to cell-surface integrins, with consequent clinical impacts.

Single Amino Acid Substitutions Surrounding the Icosahedral Fivefold Symmetry Axis Are Critical for Alternative Receptor Usage of Foot-and-Mouth Disease Virus

The integrins function as the primary receptor molecules for the pathogenic infection of foot-and-mouth disease virus (FMDV) in vivo, while the acquisition of a high affinity for heparan sulfate (HS) of some FMDV variants could be privileged to facilitate viral infection and expanded cell tropism in vitro. Here, we noted that a BHK-adapted Cathay topotype derivative (O/HN/CHA/93tc) but not its genetically engineered virus (rHN), was able to infect HS-positive CHO-K1 cells and mutant pgsD-677 cells. There were one or three residue changes in the capsid proteins of O/HN/CHA/93tc and rHN, as compared with that of their tissue-originated isolate (O/HN/CHA/93wt). The phenotypic properties of a set of site-directed mutants of rHN revealed that E83K of VP1 surrounding the fivefold symmetry axis was necessary for the integrin-independent infection of O/HN/CHA/93tc. L80 in VP2 was essential for the occurrence of E83K in VP1 during the adaptation of O/HN/CHA/93wt to BHK-21 cells. L80M in VP2 and D138G in VP1 of rHN was deleterious, which could be compensated by K83R of VP1 for restoring an efficient infection of integrin-negative CHO cell lines. These might have important implications for understanding the molecular and evolutionary mechanisms of the recognition and binding of FMDV with alternative cellular receptors.

Analysis of Amino Acid Mutations of the Foot-and-Mouth Disease Virus Serotype O Using both Heparan Sulfate and JMJD6 Receptors

Foot-and-mouth disease (FMD) is an economically devastating animal disease. Adapting the field virus to cells is critical to the vaccine production of FMD viruses (FMDV), and heparan sulfate (HS) and Jumonji C-domain-containing protein 6 (JMJD6) are alternative receptors of cell-adapted FMDV. We performed serial passages of FMDV O/SKR/Andong/2010, classified as the O/Mya-98 topotype/lineage and known as a highly virulent strain, to develop a vaccine seed virus. We traced changes in the amino acid sequences of the P1 region, plaque phenotypes, and the receptor usage of the viruses, and then structurally analyzed the mutations. VP3 H56R and D60G mutations were observed in viruses using the HS receptor and led to changes in the hydrogen bonding between VP3 56 and 60. A VP1 P208L mutation was observed in the virus using the JMJD6 receptor during cell adaptation, enabling the interaction with JMJD6 through the formation of a new hydrogen bond with JMJD6 residue 300. Furthermore, VP1 208 was near the VP1 95/96 amino acids, previously reported as critical mutations for JMJD6 receptor interactions. Thus, the mutation at VP1 208 could be critical for cell adaptation related to the JMJD6 receptor and may serve as a basis for mechanism studies on FMDV cell adaptation.

Diagnostic and Epitope Mapping Potential of Single-Chain Antibody Fragments Against Foot-and-Mouth Disease Virus Serotypes A, SAT1, and SAT3

Foot-and-mouth disease (FMD) affects cloven-hoofed domestic and wildlife animals and an outbreak can cause severe losses in milk production, reduction in meat production and death amongst young animals. Several parts of Asia, most of Africa, and the Middle East remain endemic, thus emphasis on improved FMD vaccines, diagnostic assays, and control measures are key research areas. FMD virus (FMDV) populations are quasispecies, which pose serious implications in vaccine design and efficacy where an effective vaccine should include multiple independent neutralizing epitopes to elicit an adequate immune response. Further investigation of the residues that comprise the antigenic determinants of the virus will allow the identification of mutations in outbreak strains that potentially lessen the efficacy of a vaccine. Additionally, of utmost importance in endemic regions, is the accurate diagnosis of FMDV infection for the control and eradication of the disease. To this end, a phage display library was explored to identify FMDV epitopes for recombinant vaccines and for the generation of reagents for improved diagnostic FMD enzyme-linked immunosorbent assays (ELISAs). A naïve semi-synthetic chicken single chain variable fragment (scFv) phage display library i.e., the Nkuku ^® library was used for bio-panning against FMD Southern-African Territories (SAT) 1, SAT3, and serotype A viruses. Biopanning yielded one unique scFv against SAT1, two for SAT3, and nine for A22. SAT1 and SAT3 specific scFvs were exploited as capturing and detecting reagents to develop an improved diagnostic ELISA for FMDV. The SAT1 soluble scFv showed potential as a detecting reagent in the liquid phase blocking ELISA (LPBE) as it reacted specifically with a panel of SAT1 viruses, albeit with different ELISA absorbance signals. The SAT1svFv1 had little or no change on its paratope when coated on polystyrene plates whilst the SAT3scFv's paratope may have changed. SAT1 and SAT3 soluble scFvs did not neutralize the SAT1 and SAT3 viruses; however, three of the nine A22 binders i.e., A22scFv1, A22scFv2, and A22scFv8 were able to neutralize A22 virus. Following the generation of virus escape mutants through successive virus passage under scFv pressure, FMDV epitopes were postulated i.e., RGD+3 and +4 positions respectively, proving the epitope mapping potential of scFvs.

Host receptors: the key to establishing cells with broad viral tropism for vaccine production

Cell culture-based vaccine technology is a flexible and convenient approach for vaccine production that requires adaptation of the vaccine strains to the new cells. Driven by the motivation to develop a broadly permissive cell line for infection with a wide range of viruses, we identified a set of the most relevant host receptors involved in viral attachment and entry. This identification was done through a review of different viral entry pathways and host cell lines, and in the context of the Baltimore classification of viruses. In addition, we indicated the potential technical problems and proposed some solutions regarding how to modify the host cell genome in order to meet industrial requirements for mass production of antiviral vaccines. Our work contributes to a finer understanding of the importance of breaking the host–virus recognition specificities for the possibility of creating a cell line feasible for the production of vaccines against a broad spectrum of viruses.

Pathogenesis, biophysical stability and phenotypic variance of SAT2 foot-and-mouth disease virus

Foot-and-mouth disease (FMD) is a highly contagious vesicular disease of cloven-hoofed animals, which severely decreases livestock productivity. FMD virus (FMDV), the causative agent, initiates infection by interaction with integrin cellular receptors on pharyngeal epithelium cells, causing clinical signs one to four days after transmission to a susceptible host. However, some Southern African Territories (SAT) viruses have been reported to cause mild or subclinical infections that may go undiagnosed in field conditions and are likely to be more common than previously expected. The studies presented here demonstrate that not all SAT2 viruses are equally virulent in cattle. The two SAT2 viruses, ZIM/5/83 and ZIM/7/83, were both highly attenuated in cattle, as evidenced by the mild clinical signs observed after needle challenge, while two incongruent SAT2 viruses showed significantly different clinical signs in challenged cattle. We then explored the ability of the SAT2 viruses to infect different cell types with defined receptors that are utilised by FMDV and found differences in their ability to lyse cells in culture and to compete in a controlled cell culture environment. The population sequence variation between ZIM/5/83 and ZIM/7/83 revealed multiple sites of single nucleotide variants of low frequency between the predominant virus populations, as could be expected from the genome of an RNA virus. An assessment of the biophysical stability of SAT2 virions during acidification indicated that the SAT2 virus EGY/09/12 was more resilient to acidification than the ZIM/5/83 and ZIM/7/83 viruses; however, whether this difference relates to differences in virulence in vivo is unclear. This study is a consolidated view of the key findings of SAT2 viruses studied over a 14-year period involving many different experiments.

Cell culture propagation of foot-and-mouth disease virus: adaptive amino acid substitutions in structural proteins and their functional implications

Foot-and-mouth disease is endemic in livestock in large parts of Africa and Asia, where it is an important driver of food insecurity and a major obstacle to agricultural development and the international trade in animal products. Virtually all commercially available vaccines are inactivated whole-virus vaccines produced in cell culture, but the adaptation of a field isolate of the virus to growth in culture is laborious and time-consuming. This is of particular concern for the development of vaccines to newly emerging virus lineages, where long lead times from virus isolate to vaccine can delay the implementation of effective control programs. High antigen yields in production cells are also necessary to make vaccines affordable for less developed countries in endemic areas. Therefore, a rational approach to cell culture adaptation that combines prior knowledge of common adaptive mutations and reverse genetics techniques is urgently required. This review provides an overview of amino acid exchanges in the viral capsid proteins in the context of adaptation to cell culture.

Engineering Responses to Amino Acid Substitutions in the VP0- and VP3-Coding Regions of PanAsia-1 Strains of Foot-and-Mouth Disease Virus Serotype O

The presence of sequence divergence through adaptive mutations in the major capsid protein VP1, and also in VP0 (VP4 and VP2) and VP3, of foot-and-mouth disease virus (FMDV) is relevant to a broad range of viral characteristics. To explore the potential role of isolate-specific residues in the VP0 and VP3 coding regions of PanAsia-1 strains in genetic and phenotypic properties of FMDV, a series of recombinant full-length genomic clones were constructed using Cathay topotype infectious cDNA as the original backbone. The deleterious and compensatory effects of individual amino acid substitutions at positions 4008 and 3060 and in several different domains of VP2 illustrated that the chain-based spatial interaction patterns of VP1, VP2, and VP3 (VP1-3), as well as between the internal VP4 and the three external capsid proteins of FMDV, might contribute to the assembly of eventually viable viruses. The Y2079H site-directed mutants dramatically induced a decrease in plaque size on BHK-21 cells and viral pathogenicity in suckling mice. Remarkably, the 2079H-encoding viruses displayed a moderate increase in acid sensitivity correlated with NH4Cl resistance compared to the Y2079-encoding viruses. Interestingly, none of all the 16 rescued viruses were able to infect heparan sulfate-expressing CHO-K1 cells. However, viral infection in BHK-21 cells was facilitated by utilizing non-integrin-dependent, heparin-sensitive receptor(s) and replacements of four uncharged amino acids at position 3174 in VP3 of FMDV had no apparent influence on heparin affinity. These results provide particular insights into the correlation of evolutionary biology with genetic diversity in adapting populations of FMDV.IMPORTANCE The sequence variation within the capsid proteins occurs frequently in the infection of susceptible tissue cultures, reflecting the high levels of genetic diversity of FMDV. A systematic study for the functional significance of isolate-specific residues in VP0 and VP3 of FMDV PanAsia-1 strains suggested that the interaction of amino acid side chains between the N terminus of VP4 and several potential domains of VP1-3 had cascading effects on the viability and developmental characteristics of progeny viruses. Y2079H in VP0 of the indicated FMDVs could affect plaque size and pathogenicity, as well as acid sensitivity correlated with NH4Cl resistance, whereas there was no inevitable correlation in viral plaque and acid-sensitive phenotypes. The high affinity of non-integrin-dependent FMDVs for heparin might be explained by the differences in structures of heparan sulfate proteoglycans on the surfaces of different cell lines. These results may contribute to our understanding of the distinct phenotypic properties of FMDV in vitro and in vivo.

Recombination of host cell mRNA with the Asia 1 foot-and-mouth disease virus genome in cell suspension culture

Foot-and-mouth disease virus (FMDV) exhibits high mutation rates during replication. In this study, an isolate of FMDV serotype Asia-1 was serially passaged in a BHK-21 cell monolayer and then adapted to serum-free BHK-21 cell suspension culture to produce a seed virus for production of an inactivated vaccine. Analysis of the sequence encoding the structural proteins of the virus at various passages showed the presence of overlapping peaks in sequencing electropherograms after nucleotide 619 of VP1 in viruses recovered from the fourth passage in suspension culture, suggesting the possible introduction of an insertion or deletion into this portion of the viral genome of our seed virus stock. To evaluate this phenomenon, a virus designated "Vac-Asia1-VDLV", was isolated by plaque purification from the tenth passage in suspension culture. Sequencing results showed that a 12-nt-long exogenous sequence was inserted into the 3' end of the VP1 coding region at the position where the original overlapping peaks were identified. Analysis of the host cell transcriptome showed that the 12-nt sequence was identical to a highly expressed sequence in BHK-21 cells, strongly suggesting that recombination between the FMDV genome and host cell mRNA produced the recombinant virus. A growth curve showed that the virus with the 12-nt insertion reached a peak earlier than the parental strain and that this virus had acquired the ability to bind to the cell surface by a mechanism that was not dependent on integrin or the heparan sulfate receptor. This novel pathogen-host cell recombination event is discussed in terms of the mechanism of viral RNA replication and the phenotypic constraints of FMDV biology and evolution.

Influence of cell type and cell culture media on the propagation of foot-and-mouth disease virus with regard to vaccine quality

BACKGROUND

Suspension culture of BHK cells allows large-scale virus propagation and cost-efficient vaccine production, while the shift to animal-component-free cell culture media without serum is beneficial for the quality and downstream processing of the product. Foot-and-mouth disease virus is still endemic in many parts of the world and high-quality vaccines are essential for the eradication of this highly contagious and economically devastating disease.

METHODS

Changes to the viral genome sequence during passaging in an adherent and a suspension cell culture system were compared and the impact of amino acid substitutions on receptor tropism, antigenicity and particle stability was examined. Virus production in suspension cells in animal-component-free media and in serum-containing media as well as in adherent cells in serum-containing media was compared. Infection kinetics were determined and the yield of intact viral particles was estimated in all systems using sucrose density gradient centrifugation.

RESULTS

Capsid protein sequence alterations were serotype-specific, but varied between cell lines. But The A24-2P virus variant had expanded its receptor tropism, but virus neutralization tests found no changes in the antigenic profile in comparison to the original viruses. There were no differences in viral titer between a suspension and an adherent cell culture system, independent of the type of media used. Also, the usage of a serum-free suspension culture system promoted viral growth and allowed an earlier harvest. For serotype O isolates, no differences were seen in the yield of 146S particles. Serotype A preparations revealed a decreased yield of 146S particles in suspension cells independent of the culture media.

CONCLUSION

The selective pressure of the available surface receptors in different cell culture systems may be responsible for alterations in the capsid coding sequence of culture-grown virus. Important vaccine potency characteristics such as viral titer and the neutralization profile were unaffected, but the 146S particle yield differed for one of the tested serotypes.

A replication-competent foot-and-mouth disease virus expressing a luciferase reporter

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We have generated a replication-competent foot-and-mouth disease virus expressing Nanoluciferase, designated as Nano-FMDV.

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Nano-FMDV is genetically stable.

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The replication of Nano-FMDV can be monitored by bioluminescent methods.

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This reporter virus has potential applications in real-time monitoring of FMDV infection in vitro and in vivo, and in screening of antivirals and antibodies.

Bioluminescence is a powerful tool in the study of viral infection both in vivo and in vitro. Foot-and-mouth disease virus (FMDV) has a small RNA genome with a limited tolerance to foreign RNA entities. There has been no success in making a reporter FMDV expressing a luciferase in infected cell culture supernatants. We report here for the first time a replication-competent FMDV encoding Nanoluciferase, named as Nano-FMDV. Nano-FMDV is genetically stable during serial passages in cells and exhibits growth kinetics and plaque morphology similar to its parental virus. There are applications for the use of Nano-FMDV such as real-time monitoring of FMDV replication in vitro and in vivo.

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.

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.

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.

Beyond RGD: virus interactions with integrins

Viruses successfully infect host cells by initially binding to the surfaces of the cells, followed by an intricate entry process. As multifunctional heterodimeric cell-surface receptor molecules, integrins have been shown to usefully serve as entry receptors for a plethora of viruses. However, the exact role(s) of integrins in viral pathogen internalization has yet to be elaborately described. Notably, several viruses harbor integrin-recognition motifs displayed on viral envelope/capsid-associated proteins. The most common of these motifs is the minimal peptide sequence for binding integrins, RGD (Arg-Gly-Asp), which is known for its role in virus infection via its ability to interact with over half of the more than 20 known integrins. Not all virus-integrin interactions are RGD-dependent, however. Non-RGD-binding integrins have also been shown to effectively promote virus entry and infection as well. Such virus-integrin binding is shown to facilitate adhesion, cytoskeleton rearrangement, integrin activation, and increased intracellular signaling. Also, we have attempted to discuss the role of carbohydrate moieties in virus interactions with receptor-like host cell surface integrins that drive the process of internalization. As much as possible, this article examines the published literature regarding the role of integrins in terms of virus infection and virus-encoded glycosylated proteins that mediate interactions with integrins, and it explores the idea of targeting these receptors as a therapeutic treatment option.

Cell culture adaptation mutations in foot-and-mouth disease virus serotype A capsid proteins: implications for receptor interactions

In this study we describe the adaptive changes fixed on the capsid of several foot-and-mouth disease virus serotype A strains during propagation in cell monolayers. Viruses passaged extensively in three cell lines (BHK-21, LFBK and IB-RS-2) consistently gained positively charged amino acids in the putative heparin-sulfate-binding pocket (VP2 βE-βF loop, VP1 C-terminus and VP3 β-B knob) surrounding the fivefold symmetry axis (VP1 βF-βG loop) and at other discrete sites on the capsid (VP3 βG-βH loop, VP1 C-terminus, VP2 βC strand and VP1 βG-βH loop). A lysine insertion in the VP1 βF-βG loop of two of the BHK-21-adapted viruses supports the biological advantage of positively charged residues acquired in cell culture. The charge transitions occurred irrespective of cell line, suggesting their possible role in ionic interaction with ubiquitous negatively charged cell-surface molecules such as glycosaminoglycans (GAG). This was supported by the ability of the cell-culture-adapted variants to replicate in the integrin-deficient, GAG-positive CHO-K1 cells and their superior fitness in competition assays compared with the lower passage viruses with WT genotypes. Substitutions fixed in the VP1 βG-βH loop (-3, -2 and +2 'RGD' positions) or in the structural element known to be juxtaposed against that loop (VP1 βB-βC loop) suggest their possible role in modulating the efficiency and specificity of interaction of the 'RGD' motif with αv-integrin receptors. The nature and location of the substitutions described in this study could be applied in the rapid cell culture adaptation of viral strains for vaccine production.

Effects of two amino acid substitutions in the capsid proteins on the interaction of two cell-adapted PanAsia-1 strains of foot-and-mouth disease virus serotype O with heparan sulfate receptor

Background

Some cell-adapted strains of foot-and-mouth disease virus (FMDV) can utilize heparan sulfate (HS) as a receptor to facilitate viral infection in cultured cells. A number of independent sites on the capsid that might be involved in FMDV-HS interaction have been studied. However, the previously reported residues do not adequately explain HS-dependent infection of two cell-adapted PanAsia-1 strains (O/Tibet/CHA/6/99tc and O/Fujian/CHA/9/99tc) of FMDV serotype O.

To identify the molecular determinant(s) for the interaction of O/Tibet/CHA/6/99tc and O/Fujian/CHA/9/99tc with HS receptor, several chimeric viruses and site-directed mutants were generated by using an infectious cDNA of a non-HS-utilizing rescued virus (Cathay topotype) as the genomic backbone. Phenotypic properties of these viruses were determined by plaque assays and virus adsorption and penetration assays in cultured cells.

Results

Only two of the rescued viruses encoding VP0 of O/Tibet/CHA/6/99tc or VP1 of O/Fujian/CHA/9/99tc formed plaques on wild-type Chinese hamster ovary (WT-CHO; HS+) cells, but not on HS-negative pgsD-677 cells. The formation of plaques by these two chimeric viruses on WT-CHO cells could be abolished by the introduction of single amino acid mutations Gln-2080 → Leu in VP2 of O/Tibet/CHA/6/99tc and Lys-1083 → Glu in VP1 of O/Fujian/CHA/9/99tc, respectively. Nonetheless, the introduced mutation Leu-2080 → Gln in VP2 of O/Fujian/CHA/9/99tc for the construction of expectant recombinant plasmid led to non-infectious progeny virus in baby hamster kidney 21 (BHK-21) cells, and the site-directed mutant encoding Glu-1083 → Lys in VP1 of O/Tibet/CHA/6/99tc did not acquire the ability to produce plaques on WT-CHO cells.

Significant differences in the inhibition of the infectivity of four HS-utilizing viruses by heparin and RGD-containing peptide were observed in BHK-21 cells.

Interestingly, the chimeric virus encoding VP0 of O/Fujian/CHA/9/99tc, and the site-directed mutant encoding Gln-2080 → Leu in VP2 of O/Tibet/CHA/6/99tc could bind to HS, but there was no expression of the 3A protein of these two viruses in WT-CHO cells.

Conclusion

The results suggest that the cooperation of certain specific amino acid residues in the capsid proteins of these two cell-adapted PanAsia-1 strains is essential for viral infectivity, the heparin affinity and the capability on FMDV-HS interaction.

Redistribution of demethylated RNA helicase A during foot-and-mouth disease virus infection: role of Jumonji C-domain containing protein 6 in RHA demethylation

Previously, RNA helicase A (RHA) re-localization from the nucleus to the cytoplasm in foot-and-mouth disease virus (FMDV) infected cells was shown to coincide with loss of RHA methylated arginine residues at its C-terminus. The potential interaction between RHA and Jumonji C-domain (JmjC) protein 6 (JMJD6) arginine demethylase in infected cells was investigated. Treatment with N-oxalylglycine (NOG) inhibitor of JmjC demethylases prevented FMDV-induced RHA demethylation and re-localization, and also decreased viral protein synthesis and virus titers. Physical interaction between JMJD6 and RHA was demonstrated via reciprocal co-immunoprecipitation, where RHA preferentially bound JMJD6 monomers. Nuclear efflux of demethylated RHA (DM-RHA) coincided with nuclear influx of JMJD6, which was not observed using another picornavirus. A modified biochemical assay demonstrated JMJD6 induced dose-dependent demethylation of RHA and two RHA-derived isoforms, which could be inhibited by NOG. We propose a role for JMJD6 in RHA demethylation stimulated by FMDV, that appears to facilitate virus replication.

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.