An increase in acid resistance of foot-and-mouth disease virus capsid is mediated by a tyrosine replacement of the VP2 histidine previously associated with VP0 cleavage

Calcium Chloride as a Novel Stabilizer for Foot-and-Mouth Disease Virus and Its Application in the Vaccine Formulation

The thermal stability of the in-house-developed foot-and-mouth disease (FMD) type O and A viruses was evaluated, and the O Jincheon virus was found to exhibit the lowest thermal stability. To overcome this instability, we proposed a novel stabilizer, calcium chloride. The thermal stability of FMDVs increased up to a CaCl2 concentration of 10 mM, and it had a decreasing trend at >30 mM. The O Jincheon virus showed a significant decrease in the amount of antigen over time at 4 °C. In contrast, the samples treated with CaCl2 showed stable preservation of the virus without significant antigen loss. After the CaCl2-formulated vaccine was administered twice to pigs, the virus neutralization titer reached approximately 1:1000, suggesting that the vaccine could protect pigs against the FMDV challenge. In summary, the O Jincheon virus is difficult to utilize as a vaccine given its low stability during storage after antigen production. However, following its treatment with CaCl2, it can be easily utilized as a vaccine. This study evaluated CaCl2 as a novel stabilizer in FMD vaccines and may contribute to the development of stable vaccine formulations, especially for inherently unstable FMDV strains.

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.

Oil-in-ionic liquid nanoemulsion-based adjuvant simultaneously enhances the stability and immune responses of inactivated foot-and-mouth disease virus

Maintaining structural integrity and enhancing stability of inactivated foot-and-mouth disease virus (iFMDV) antigen in adjuvants is crucial to ensure the vaccine potency. Unfortunately, formulation with most reported adjuvants leads to the accelerated dissociation of iFMDV into inactive pentamers. Here, an ionic liquid, i.e., choline and niacin ([Cho][Nic]), which was found to stabilize iFMDV against the acid- and thermo- induced dissociation in buffer solution, was applied to construct a novel oil-in-ionic liquid (o/IL) nanoemulsion adjuvant composed of [Cho][Nic], squalene, and Tween 80. The o/IL nanoemulsion formulated with iFMDV has a monodisperse diameter of 135.8 ± 40.4 nm. The thermostability and long-term stability of iFMDV were remarkably enhanced in o/IL nanoemulsion compared with that in the o/w emulsion without [Cho][Nic] and in the commercial Montanide ISA 206 adjuvant. The o/IL nanoemulsion exerted its adjuvant effects by improving the humoral immune responses. Immunization of o/IL nanoemulsion adjuvanted iFMDV induced specific IgG titers similar to that adjuvanted by Montanide ISA 206 and about 4-fold higher than the un-adjuvanted iFMDV, also promoted the activation of B lymphocytes and the secretion of interleukin-4 in the mice model. This [Cho][Nic]-based o/IL nanoemulsion can serve as a promising adjuvant platform for the foot-and-mouth disease vaccine.

Foot-and-mouth disease virus VP1 promotes viral replication by regulating the expression of chemokines and GBP1

Foot-and-mouth disease virus (FMDV) is an acute, highly contagious, and economically destructive pathogen of vesicular disease that affects domestic and wild cloven-hoofed animals. The FMDV VP1 protein is an important part of the nucleocapsid and plays a significant role during FMDV infection. However, the signal pathways mediated by VP1 in the life cycle of FMDV and the related mechanisms are not yet fully understood. Here, we performed RNA-seq to compare gene expression profiles between pCAGGS-HA-VP1 transfected PK-15 cells and pCAGGS-HA (empty vector) transfected PK-15 cells. The results showed 5,571 genes with significantly different expression levels, of which 2,981 were up-regulated and 2,590 were down-regulated. GO enrichment analysis showed that 51 GO terms were significantly enriched in cell components including protein complex, membrane and organelle part. KEGG enrichment analysis showed 11 KEGG pathways were significantly enriched which were mainly related to the immune system, infectious viral disease, and signal transduction. Among the up-regulated genes, the chemokines such as CCL5, CXCL8, and CXCL10 in turn promoted FMDV replication. In contrast, GBP1, an interferon-stimulated gene that was suppressed by VP1 and FMDV, could effectively inhibit FMDV replication. Our research provides a comprehensive overview of the response of host cells to VP1 protein and a basis for further research to understand the roles of VP1 in FMDV infection including the genes involved in FMDV replication.

Adaptive value of foot-and-mouth disease virus capsid substitutions with opposite effects on particle acid stability

Foot-and-mouth disease virus (FMDV) is a picornavirus that exhibits an extremely acid sensitive capsid. This acid lability is directly related to its mechanism of uncoating triggered by acidification inside cellular endosomes. Using a collection of FMDV mutants we have systematically analyzed the relationship between acid stability and the requirement for acidic endosomes using ammonium chloride (NH₄Cl), an inhibitor of endosome acidification. A FMDV mutant carrying two substitutions with opposite effects on acid-stability (VP3 A116V that reduces acid stability, and VP1 N17D that increases acid stability) displayed a rapid shift towards acid lability that resulted in increased resistance to NH₄Cl as well as to concanamicyn A, a different lysosomotropic agent. This resistance could be explained by a higher ability of the mutant populations to produce NH₄Cl-resistant variants, as supported by their tendency to accumulate mutations related to NH₄Cl-resistance that was higher than that of the WT populations. Competition experiments also indicated that the combination of both amino acid substitutions promoted an increase of viral fitness that likely contributed to NH₄Cl resistance. This study provides novel evidences supporting that the combination of mutations in a viral capsid can result in compensatory effects that lead to fitness gain, and facilitate space to an inhibitor of acid-dependent uncoating. Thus, although drug-resistant variants usually exhibit a reduction in viral fitness, our results indicate that compensatory mutations that restore this reduction in fitness can promote emergence of resistance mutants.

The rescue and selection of thermally stable type O vaccine candidate strains of foot-and-mouth disease virus

Inactivated foot-and-mouth disease virus (FMDV) vaccines have been used widely to control foot-and-mouth disease (FMD). However, the virions (146S) of this virus are easily dissociated into pentamer subunits (12S), which limits the immune protective efficacy of inactivated vaccines when the temperature is higher than 30 °C. A cold-chain system can maintain the quality of the vaccines, but such systems are usually not reliable in limited-resource settings. Thus, it is imperative to improve the thermostability of vaccine strains to guarantee the quality of the vaccines. In this study, four recombinant FMDV strains containing single or multiple amino acid substitutions in the structural proteins were rescued using a previously constructed FMDV type O full-length infectious clone (pO/DY-VP1). We found that single or multiple amino acid substitutions in the structural proteins affected viral replication to different degrees. Furthermore, the heat and acid stability of the recombinant viruses was significantly increased when compared with the parental virus. Three thermally stable recombinant viruses (rHN/DY-VP1^Y2098F, rHN/DY-VP1^{V2090A-S2093H}, and rHN/DY-VP1^{V2090A-S2093H-Y2098F}) were prepared as inactivated vaccines to immunize pigs. Blood samples were collected every week to prepare sera, and a virus neutralization test showed that the substitutions S2093H and Y2098F, separately or in combination, did not affect the immunogenicity of the virus, but the Y2098F mutation increased the thermostability significantly (p < 0.05). Therefore, the rHN/DY-VP1^Y2098F mutant should be considered for use in future vaccines.

Possible Action of Transition Divalent Metal Ions at the Inter-Pentameric Interface of Inactivated Foot-and-Mouth Disease Virus Provide A Simple but Effective Approach to Enhance Stability

The structural instability of inactivated foot-and-mouth disease virus (FMDV) hinders the development of vaccine industry. Here we found that some transition metal ions like Cu²⁺ and Ni²⁺ could specifically bind to FMDV capsids at capacities about 7089 and 3448 metal ions per capsid, respectively. These values are about 33- and 16-folds of the binding capacity of non-transition metal ion Ca²⁺ (about 214 per capsid). Further thermodynamic studies indicated that all these three metal ions bound to the capsids in spontaneous enthalpy driving manners (ΔG<0, ΔH<0, ΔS<0), and the Cu²⁺ binding had the highest affinity. The binding of Cu²⁺ and Ni²⁺ could enhance both the thermostability and acid-resistant stability of capsids, while the binding of Ca²⁺ was helpful only to the thermostability of the capsids. Animal experiments showed that the immunization of FMDV bound with Cu²⁺ induced the highest specific antibody titers in mice. Coincidently, the FMDV bound with Cu²⁺ exhibited significantly enhanced affinities to integrin β6 and heparin sulfate, both of which are important cell surface receptors for FMDV attaching. Finally, the specific interaction between capsids and Cu²⁺ or Ni²⁺ was applied to direct purification of FMDV from crude cell culture feedstock by the immobilized metal affinity chromatography. Based on our new findings and structural analysis of the FMDV capsid, a "transition metal ion bridges" mechanism that describes linkage between adjacent histidine and other amino acids at the inter-pentameric interface of the capsids by transition metal ions coordination action was proposed to explain their stabilizing effect imposed on the capsid.IMPORTANCE How to stabilize the inactivated FMDV without affecting virus infectivity and immunogenicity is a big challenge in vaccine industry. The electrostatic repulsion induced by protonation of a large amount of histidine residues at the inter-pentameric interface of viral capsids is one of the major mechanisms causing the dissociation of capsids. In the present work, this structural disadvantage inspired us to stabilize the capsids through coordinating transition metal ions with the adjacent histidine residues in FMDV capsid, instead of removing or substituting them. This approach was proved effective to enhance not only the stability of FMDV, but also enhance the specific antibody responses; thus, providing a new guideline for designing an easy-to-use strategy suitable for large-scale production of FMDV vaccine antigen.

Modification of the second translation initiation site restricts the replication of foot-and-mouth disease virus in PK-15 cells

Abstract

The translation initiation of foot-and-mouth disease virus (FMDV) occurs at two alternative initiation sites (Lab AUG and Lb AUG). Usually, the Lb AUG is more favorably used to initiate protein synthesis than the Lab AUG. To explore the effect of Lb AUG on FMDV replication and obtain FMDV with restricted replication, this initiation codon was mutated to a variety of non-AUG codons (UGG, AUC, CUG, and AAA). Fortunately, the modifications did not prevent viral viability but influenced replication characteristics of some FMDV mutants in a cell-specific manner, as was shown by the similar replication in BHK-21 cells and delayed growth kinetics in PK-15 cells. This attenuated phenotype of FMDV mutants in PK-15 cells was found to be correlated with reduced abilities to cleave eIF4GI and suppress interference (IFN) expression. As leader (L) protein was reported to be responsible for eIF4GI cleavage and inhibition of IFN expression, the in vivo L protein synthesis was examined during the infection of FMDV mutants. Our results showed that not only the total yield of L proteins was severely influenced but also the individual yield of L protein was seen to be affected, which implied that both the relative usage of the two initiation sites and overall translation efficiency were changed by Lb AUG modifications. In addition, the in vitro translation activity was also negatively regulated by Lb AUG mutations. Collectively, these findings suggested that the restricted replications of Lb AUG-modified FMDVs were related to the delayed eIF4GI cleavage and decreased ability to block IFN expression but were mainly determined by the inefficient translation initiation. FMDVs precisely with modifications of Lb AUG initiation codon may represent safer seed viruses for vaccine production.

Key points

• The polyprotein translation of FMDV initiates at two alternative initiation sites (Lab AUG and Lb AUG). In order to explore the effect of Lb AUG on FMDV replication and obtain FMDV with restricted replication, the Lb initiation AUG was mutated to a variety of non-AUG codons (UGG, AUC, CUG, and AAA), and four FMDV mutants with Lb AUG modification were generated.

• We found that partial FMDV mutants grew almost as well as WT virus in BHK-21 cells, a typical cell line used for FMD vaccine production, but displayed impaired replication in IFN-competent PK-15 cells.

• The attenuation of mutant FMDVs in PK-15 cells was found to be correlated with delayed eIF4GI cleavage and decreased ability to block IFN expression.

• We proved that the attenuated phenotype of Lb AUG-modified FMDVs was mainly determined by the inefficient translation initiation, as demonstrated by the decrease of total yield of L proteins and individual production of L protein.

• We successfully generated genetically engineered FMDV with attenuated phenotype. The approach of precise engineering of FMDV with the modification of initiation codon provides a safe platform to produce inactivated antigen vaccines.

Negatively charged amino acids at the foot-and-mouth disease virus capsid reduce the virion-destabilizing effect of viral RNA at acidic pH

Elucidation of the molecular basis of the stability of foot-and-mouth disease virus (FMDV) particles is relevant to understand key aspects of the virus cycle. Residue N17D in VP1, located at the capsid inner surface, modulates the resistance of FMDV virion to dissociation and inactivation at acidic pH. Here we have studied whether the virion-stabilizing effect of amino acid substitution VP1 N17D may be mediated by the alteration of electrostatic charge at this position and/or the presence of the viral RNA. Substitutions that either introduced a positive charge (R,K) or preserved neutrality (A) at position VP1 17 led to increased sensitivity of virions to inactivation at acidic pH, while replacement by negatively charged residues (D,E) increased the resistance of virions to acidic pH. The role in virion stability of viral RNA was addressed using FMDV empty capsids that have a virtually unchanged structure compared to the capsid in the RNA-filled virion, but that are considerably more resistant to acidic pH than WT virions, supporting a virion-destabilizing effect of the RNA. Remarkably, no differences were observed in the resistance to dissociation at acidic pH between the WT empty capsids and those harboring replacement N17D. Thus, the virion-destabilizing effect of viral RNA at acidic pH can be partially restored by introducing negatively charged residues at position VP1 N17.

Engineering viable foot-and-mouth disease viruses with increased acid stability facilitate the development of improved vaccines

Foot-and-mouth disease virus (FMDV), the most acid-unstable virus among picornaviruses, tends to disassemble into pentamers at pH values slightly below neutrality. However, the structural integrity of intact virion is one of the most important factors that influence the induction of a protective antibody response. Thus, improving the acid stability of FMDV is required for the efficacy of vaccine preparations. According to the previous studies, a single substitution or double amino acid substitutions (VP1 N17D, VP2 H145Y, VP2 D86H, VP3 H142D, VP3 H142G, and VP1 N17D + VP2 H145Y) in the capsid were introduced into the full-length infectious clone of type O FMDV vaccine strain O/HN/CHN/93 to develop seed FMDV with improved acid stability. After the transfection into BSR/T7 cells of constructed plasmids, substitution VP1 N17D or VP2 D86H resulted in viable and genetically stable FMDVs, respectively. However, substitution VP2 H145Y or VP1 N17D + VP2 H145Y showed reverse mutation and additional mutations, and substitution VP3 H141G or VP3 H141D prevented viral viability. We found that substitution VP1 N17D or VP2 D86H could confer increased acid resistance, alkali stability, and thermostability on FMDV O/HN/CHN/93, whereas substitution VP1 N17D was observed to lead to a decreased replication ability in BHK-21 cells and mildly impaired virulence in suckling mice. In contrast, substitution VP2 D86H had no negative effect on viral infectivity. These results indicated that the mutant rD86H carrying substitution VP2 D86H firstly reported by us could be more adequate for the development of inactivated FMD vaccines with enhanced acid stability.

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.

Immunogenicity and protective efficacy of a novel foot-and-mouth disease virus empty-capsid-like particle with improved acid stability

Foot-and-mouth disease virus (FMDV) is the etiological agent of a highly contagious disease that affects cloven-hoofed animal species. The FMDV capsid is highly acid labile and viral particles lose their immunogenicity when they disassemble at mildly acidic pHs. The viral capsid of FMDV serotype O is more sensitive than those of other serotypes, making it more difficult to acquire enough empty-capsid-like particles in the acidic insect cell environment for research. In this study, novel FMDV mutants with increased acid resistance were isolated using BHK-21 cell cultured under low-pH conditions. Amino acid substitutions Q25R, K41E, and N85A in the VP1 capsid protein and K154Q in the VP3 capsid protein were detected in all six mutants. Based on these amino acid replacements, empty-capsid-like particles of FMDV serotype O, which were resistant to the acid-induced dissociation of the capsid into pentameric subunits, were produced in insect cells. We characterized the protective immunity induced by these acid-resistant empty capsid particles. Significant humoral and cellular immune responses were elicited in mice after immunization with the acid-resistant empty capsid particles. The acid-resistant empty-capsid-like particles also induced strong neutralizing antibodies in guinea pigs and protected all the guinea pigs from FMDV challenge. Our results suggest that these acid-resistant empty-capsid-like particles have potential utility as a vaccine against serotype O FMDV infection.

Transient expression of heat- and acid-resistant foot-and-mouth disease virus P1-2A mutants in Nicotiana benthamiana

Recombinant foot-and-mouth disease virus-like particles (VLPs) can be expressed in a number of expression systems including plants. However, yields in plants have formerly been shown to be low, possibly due to their acid and/or heat lability, previously shown to affect VLP yields produced in other systems. This work describes the introduction of mutations into the FMDV structural protein-encoding gene (P1-2A) which have been previously shown to increase acid and thermostability. VLPs expressed in plants using the mutant constructs had negative rather than positive effects on yield and temperature and acid stability compared to the control.

The pH stability of foot-and-mouth disease virus

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This review summarized the molecular determinants of the acid stability of FMDV in order to explore the uncoating mechanism of FMDV and improve the acid stability of vaccines.

Background

The foot-and-mouth disease virus (FMDV) capsid is highly acid labile and tends to dissociate into pentameric subunits at acidic condition to release viral RNA for initiating virus replication. However, the acid stability of virus capsid is greatly required for the maintenance of intact virion during the process of virus culture and vaccine production. The conflict between the acid lability in vivo and acid stability in vitro of FMDV capsid promotes the selection of a series of amino acid substitutions which can confer resistance to acid-induced FMDV inactivation. In order to explore the uncoating activity of FMDV and enhance the acid stability of vaccines, we summarized the available works about the pH stability of FMDV.

Main body of the abstract

In this review, we analyzed the intrinsic reasons for the acid instability of FMDV from the structural and functional aspects. We also listed all substitutions obtained by different research methods and showed them in the partial capsid of FMDV. We found that a quadrangle region in the viral capsid was the place where a great many pH-sensitive residues were distributed. As the uncoating event of FMDV is dependent on the pH-sensitive amino acid residues in the capsid, this most pH-sensitive position indicates a potential candidate location for RNA delivery triggered by the acid-induced coat disassociation.

Short conclusion

This review provided an overview of the pH stability of FMDV. The study of pH stability of FMDV not only contributes to the exploration of molecule and mechanism information for FMDV uncoating, but also enlightens the development of FMDV vaccines, including the traditionally inactivated vaccines and the new VLP (virus-like particle) vaccines.

Global Foot-and-Mouth Disease Research Update and Gap Analysis: 7 - Pathogenesis and Molecular Biology

We assessed research knowledge gaps in the fields of FMDV (foot-and-mouth disease virus) pathogenesis and molecular biology by performing a literature review (2011-15) and collecting research updates (2014) from 33 institutes from across the world. Findings were used to identify priority areas for future research. There have been important advances in FMDV pathogenesis; FMDV remains in lymph nodes of many recovered animals that otherwise do not appear persistently infected, even in species previously not associated with the carrier state. Whether virus retention helps maintain host immunity and/or virus survival is not known. Studies of FMDV pathogenesis in wildlife have provided insights into disease epidemiology, in endemic and epidemic settings. Many aspects of FMDV infection and virus entry remain unknown; however, at the cellular level, we know that expression level and availability of integrins (that permit viral entry), rate of clearance of infected cells and strength of anti-viral type I IFN (interferon) response are key determinants of tissue tropism. Extending findings to improved understanding of transmission requires a standardized approach and adoption of natural routes of infection during experimental study. There has been recognition of the importance of autophagosomes for FMDV entry into the cytoplasm following cell surface receptor binding, and that distinct internal cellular membranes are exploited for viral replication and immune evasion. New roles for viral proteins in blocking type I IFN production and downstream signalling have been identified facilitating research in anti-viral therapeutics. We know more about how infection affects cell protein expression, and research into molecular determinants of capsid stability has aided the development of stable vaccines. We have an expanding knowledge of viral and host molecular determinates of virulence and infectiousness, and of how phylogenetics may be used to estimate vaccine match and strain distribution. With ongoing advances, these areas could translate into significantly improved disease control.

Equine Rhinitis A Virus Mutants with Altered Acid Resistance Unveil a Key Role of VP3 and Intrasubunit Interactions in the Control of the pH Stability of the Aphthovirus Capsid

Equine rhinitis A virus (ERAV) is a picornavirus associated with respiratory disease in horses and is genetically closely related to foot-and-mouth disease virus (FMDV), the prototype aphthovirus. ERAV has recently gained interest as an FMDV alternative for the study of aphthovirus biology, including cell entry and uncoating or antiviral testing. As described for FMDV, current data support that acidic pH inside cellular endosomes triggers ERAV uncoating. In order to provide further insights into aphthovirus uncoating mechanism, we have isolated a panel of ERAV mutants with altered acid sensitivity and that differed on their degree of sensitivity to the inhibition of endosome acidification. These results provide functional evidence of the involvement of acidic pH on ERAV uncoating within endosomes. Remarkably, all amino acid substitutions found in acid-labile or acid-resistant ERAVs were located in the capsid protein VP3, indicating that this protein plays a pivotal role for the control of pH stability of the ERAV capsid. Moreover, all amino acid substitutions mapped at the intraprotomer interface between VP3 and VP2 or between VP3 and the N terminus of VP1. These results expand our knowledge on the regions that regulate the acid stability of aphthovirus capsid and should be taken into account when using ERAV as a surrogate of FMDV.

IMPORTANCE

The viral capsid constitutes a sort of dynamic nanomachine that protects the viral genome against environmental assaults while accomplishing important functions such as receptor attachment for viral entry or genome release. We have explored the molecular determinants of aphthovirus capsid stability by isolating and characterizing a panel of equine rhinitis A virus mutants that differed on their acid sensitivity. All the mutations were located within a specific region of the capsid, the intraprotomer interface among capsid proteins, thus providing new insights into the regions that control the acid stability of aphthovirus capsid. These findings could positively contribute to the development of antiviral approaches targeting aphthovirus uncoating or the refinement of vaccine strategies based on capsid stabilization.

The pH Stability of Foot-and-Mouth Disease Virus Particles Is Modulated by Residues Located at the Pentameric Interface and in the N Terminus of VP1

The picornavirus foot-and-mouth disease virus (FMDV) is the etiological agent of a highly contagious disease that affects important livestock species. The FMDV capsid is highly acid labile, and viral particles lose infectivity due to their disassembly at pH values slightly below neutrality. This acid sensitivity is related to the mechanism of viral uncoating and genome penetration from endosomes. In this study, we have analyzed the molecular basis of FMDV acid-induced disassembly by isolating and characterizing a panel of novel FMDV mutants differing in acid sensitivity. Amino acid replacements altering virion stability were preferentially distributed in two different regions of the capsid: the N terminus of VP1 and the pentameric interface. Even more, the acid labile phenotype induced by a mutation located at the pentameric interface in VP3 could be compensated by introduction of an amino acid substitution in the N terminus of VP1. These results indicate that the acid sensitivity of FMDV can be considered a multifactorial trait and that virion stability is the fine-tuned product of the interaction between residues from different capsid proteins, in particular those located within the N terminus of VP1 or close to the pentameric interface.

IMPORTANCE

The viral capsid protects the viral genome from environmental factors and contributes to virus dissemination and infection. Thus, understanding of the molecular mechanisms that modulate capsid stability is of interest for the basic knowledge of the biology of viruses and as a tool to improve the stability of conventional vaccines based on inactivated virions or empty capsids. Using foot-and-mouth disease virus (FMDV), which displays a capsid with extreme acid sensitivity, we have performed a genetic study to identify the molecular determinants involved in capsid stability. A panel of FMDV mutants with differential sensitivity to acidic pH was generated and characterized, and the results showed that two different regions of FMDV capsid contribute to modulating viral particle stability. These results provide new insights into the molecular mechanisms of acid-mediated FMDV uncoating.

Single amino acid substitution of VP1 N17D or VP2 H145Y confers acid-resistant phenotype of type Asia1 foot-and-mouth disease virus

Infection by foot-and-mouth disease virus (FMDV) is triggered by the acidic pH in endosomes after virus uptake by receptor-mediated endocytosis. However, dissociation of the FMDV 146S particle in mildly acidic conditions renders inactivated foot-and-mouth disease (FMD) vaccines much less effective. Type Asia1 FMDV mutants with increased resistance to acid inactivation were selected to study the molecular basis of viral resistance to acid-induced disassembly and improve the acid stability of FMDV. Sequencing of capsid-coding regions revealed four amino acid replacements (VP1 N17D, VP2 H145Y, VP2 G192D, and VP3 K153E) in the viral population of the acid-selected 10th passage. We performed single or combined mutagenesis using a reverse genetic system, and our results provide direct experimental evidence that VP2 H145Y or VP1 N17D substitution confers an acid-resistant phenotype to type Asia1 FMDV.