Antigenic variation of Foot and Mouth Disease Virus-An Overview

Foot and mouth disease (FMD) is a highly contagious viral diseases of cloven-hoofed animals, caused by FMD virus, a single molecule of linear positive sense, with single stranded RNA of size 7.2– 8.4 kb. Antigenic variation is one of the striking characters of FMD virus. It is a process by which an infectious organism alters its surface proteins in order to evade a host immune response and is associated with mutation leading to amino acid replacement. These changes may result either in the field or in the laboratory leading to the development of a new strain of virus which totally differs from the circulating field strain challenging the vaccine strains use for controlling the diseases. The high sequence variability found in VP1 region of this virus accounts for the low cross-reactivity observed among different serotypes of FMDV and also due to high degree of antigenic variation may be attributed to different reasons like high rate of mutation, genetic recombination, quasispecies nature of the virus and continuous circulation of the virus in the field, which is a main loophole for severe economic loss in livestock productions.


Introduction
The Foot and Mouth Disease Virus Classification : Foot and mouth disease is group under list-A of (1898), is the first animal pathogen identified as a Genus -Aphthovirus virus.Since, then it has been most contagious viral Species -FMDV disease of cloven-hoofed animal.
Virus have seven distinct serotype.They are Still being major global animal health problem serotype O, A, C, Asia 1, SAT 1, SAT 2 and SAT 3. and with its recent outbreak in developed countries of Virion structure: The structure of the FMDV particle their significant economic impact have increased the was first resolved for serotype O1BFS 1860 by x-ray concern of Government worldwide.About 2500 diffraction analysis.Since then, other serotype of outbreaks are reported annually in India.The direct FMDV have been crystallized and analysed.They are loss due to FMD in India is roughly estimated as 2250 non-enveloped, 27 nm in diameter approximately crores of rupees, and indirect loss is much more, L. spherical and relatively smooth with capsid consisting Mathew and D. G Menon (2008).So, knowing every of a densely packed icosahedral arrangement of 60 copies detail of the virus and the disease is a necessity to go each consisting of four polypeptides VP 1, 2, 3 and 4.
Genomic RNA is a single molecule of linear for controlling as early as possible.The main controlling method adopted is either slaughter or vaccination but positive sense, single stranded RNA with 7.2 -8.4 kb in country like India and many other developing in size.It is infectious in nature and has polycytidylic countries which vaccination is the only option.
acid (poly C tract) and is polyadexylated at its 3' end.However, in this method regular follow up of Have VPg (virus protein genome linked) protein, vaccination is not the crucial point but monitoring linked covalently to its 5' end.whether the vaccine in used have the same strain as Structural Proteins of FMDV compare to the circulating field strain or whether it is giving appropriate immunity, which it should give.All The four main structural proteins of FMDV are these problems are the consequences of antigenic VP1, VP2, VP3 and VP4.VP1-3 have a molecular weight approximately 24 KDa each and VP4 approxivariation of the FMD virus, which avail even within mately 8.5 KDa.VP1-3 have surface component and the serotype.
VP4 is internal, buried within the capsid and has a resulting in antigenic variants.This site also contains myristyl group covalently attached to its N terminus.
the RGD (Arg-Gly-Asp) motif receptor binding recog-VP1, VP2 and VP3 are structurally rather similar nition sequence and this is the major antigenic site.to one another, each being composed of a wedge-Other important antigenic site which have been shaped, eight stranded Beta-barrel and differing identified in several FMDV serotypes are the Cprimarily in the size and conformation of the loops that terminal stretch of VP1 (which together with the GHoccur between the strands and also in the extensions of loop, defines the main antigenic site 1 in serotype O), their amino and carboxy termini.VP1 proteins are or site involving different loops from the three located around the 5-fold axes of icosohedral accessible viral protein.(McCahon and Crowther, symmetry, and VP2 and VP3 alternate around the two-1989).Another is the site D which involves at least part and three fold axes, the amino terminal extension of of the C-terminal region of VP1, the B-B knob of VP3 these three proteins form an intricate network on the and the B-C loop of VP2 located near the capsid inner surface of the protein shell.The small, threefold axis.myristylated protein VP4, is located entirely at the Antigenic Variation inner surface of the capsid, probably in contact with the RNA.
Antigenic variation is the process by which an infectious organism alters its surface proteins in order Non-Structural Proteins of FMDV to evade a host immune response and is associated There are about seven to nine non-structural with mutation leading to amino acid replacement.This (non-capsid) proteins, viz., L (Lab and Lb) and 2A change in antigenic profile may occur as the pathogen proteins; 2B, 2C and their precursor (2BC); 3A, 3AB, passes through a host population (also called antigenic and 3D (VPg), 3ABC.Both Lab and Lb of FMDV are diversity) or may take place in the originally infected Cpro proteases with multiple activities.3 is a protease host.The strategy is particularly important for Dpro and an RNA-binding protein, whereas 3 is an RNA organism that target long-lived host, repeatedly infect polymerase and is involved in proteolysis in the form a single host, and are easily transmitted.Pathogens Cpro of 3CD.2A pro and B not only participate in the that express these characteristics and undergo virus replication but also appear to inhibit essential antigenic variation have a selective advantage over host function (RNA transcription and protein their more genetically stable counterparts.synthesis) and VPg (3B) plays a role in the initiation of RNA viruses are characterized by an error-prone viral RNA synthesis (Lawson, M.A.A, and B.L. RNA replication, which gives them great potential for Semler, 1991).
variation (Domingo et al., 1990 and1992).Antigenic variants result from the high mutation rates during

Antigenic Structure of FMDV
RNA replication which allow FMD viruses to Antigenic site on the surface of the FMD virion continuously evolve and adapt to new environments.have been identified for five of the seven serotypes of Although most mutations will be detrimental and the virus (SAT1 and SAT 3 being the only exception).eliminated by natural selection (negative selection), Serological studies showed that different serotypes of others can be of value under the particular conditions FMDV shared a highly variable region of VP1, where the virus is replicating and are therefore comprising residue 135 to 155, as one of the major selected (positive selection) (Domingo et al.,1990 ; antigenic sites of the virus.Several overlapping Beta-Fry et al., 1999; Novella et al., 1996; Baranowski et al., cell epitopes are located within this region and are able 1998).Antigenic variants have been isolated under to induce both neutralizing and non-neutralizing variable conditions, such as in partially immune antibody responses.(Strohmaier et al., 1982).The animals, persistently infected cattle4 and in cell high sequence variability found in this region accounts culture (Diez et al., 1990;Piatti et al., 1995), in the for the low cross-reactivity observed among different latter case both in the presence or the absence of serotypes (Cheung et al., 1983).
immune pressure (Borrego et al., 1993 and Holguin et This immunodominant region was seen to al.,1997).This antigenic diversity has serious correspond to the loop which connects Beta-sheet G implications in vaccine design since synthetic and H of the VP1 Beta-barrel, named the GH loop vaccines should include multiple independent (Acharya et al., 1989).Within this loop antigenic site epitopes in order to decrease the probability of A is present and very likely to face substitution selection of FMD viruses resistant to the immune because of high mutation rates during RNA replication response.

Types of Antigenic Variation
steeply with increasing secondary structure in the region encoding non-structural proteins compared

Antigenic variation in the field
with the structural protein-coding region of the Antigenic variations in the field increases with genome.time and most probably result from immunogenic 3. Quasispecies nature of the virus pressure placed on the virus by either the infected or FMD virus being quasispecies in nature, high vaccinated host species and to the selective pressure variability of FMDV population is manifested and exerted in the field by virus replication in the animal when mutation lead to codon change it result in a having inadequate level of neutralizing antibodies change in the viral phenotype.And when, variant (Domingo et al., 2003).Genetic variants also arise population are selected from the quasispecies pool of during persistent infection of animal (Gebauer et  to circulate in the field, continues to cause infection of As virus is propagated in cell culture and as susceptible host and remained persistence in their replication increase it increased every possibility of host, then with due course of time or probably due to producing antigenic variant.Say, for example, in the immunological pressure placed on the virus either production of vaccine where a number of tissue culture by the infected or vaccinated host or either by the passages are required, antigenic variant do developed challenge in the environment and also to the selective which becomes a constraint in vaccine production pressure exerted in the field by virus replication in (Sobrins et al., 1983;Bolwell et al., 1989).
animals having inadequate level of neutralizing antibodies had somehow lead to the antigenic

Causes of Antigenic Variation
variation giving rise to virus population different from

High rate of mutation
those that initiated the infection.FMD virus have very high mutation rate in the range of 10-3 to 10-5 per nucleotide site per genome Antigenic Variation in absence of any replication, due to the lack of error correction detectable immune selection mechanisms i.e. proof reading during RNA replication Antigenic variation of viruses may occur upon (Domingo and Holland. 1988; Drake and Holland.virus replication in the absence of any detectable 1999).This high error rate leads to differences of FMD immune selection.The observation has been virus replicated genome from the original parental explained by two possible models.genome and thus resulting in antigenic variation.
First, residue at antigenic site may be involved in

Genetic recombination
viral function unrelated to the evoking of an immune Compare to DNA viruses, the RNA viruses are response.Such residues may be replaced as a result of uncommon to recombination (there are probably no selective forces unconnected to immune pressure, but host enzymes for RNA recombination).But, picorna the replacement may fortuitously contribute to viruses show a form of very low efficiency recombiantigenic variation.The second model termed the nation and study has shown that recombination was random change model, suggest that fluctuation in the more likely to occur within the 3 half region of the genomic distribution of a quasispecies-triggered by genome coding for the NSP, and has not been any selective force other than immune selection, or by demonstrated in the capsid-coding genes of FMDV random sampling event may mediate the hitch-hiking (King et al., 1985, King, 1988).However, a more of mutation occurring at those genomic site where recent study has suggested, that RNA recombination mutation are more tolerated.within the capsid-coding PI region of the genome may contribute to the genetic diversity in FMDV isolated In case of a possible deliberate introduction of the livestock producers, the veterinary field services vius, it will also have forensic value in tracking and the diagnosis, production and control laboratories, and must likewise maintain among them active, the source.
continuous information regarding the characteristics The great variability of the FMD virus continually of the viruses active in the field, the performance of the produces new strains that must be classified according vaccine and the epidemiological field situation.to pre-established standards to avoid confusion.And Additionally, FMD control programs must put forth so, the knowledge of the antigenic and immunogenic every effort to implement new techniques to study the characteristics of the vaccine strains and field strains, antigenic and immunogenic characteristics of the and the importance of the latter, had been the virus and the changes in its nucleic acid for its better fundamental elements for classifying the new strains.
identification.Thus will the process of eradicating the To study any new strain availing in the field and to disease be accelerated.classify it, become an important element in knowing and controlling the disease.In country, like India were References the disease is controlled by means of vaccination only,

Genome-
Single stranded positive sense RNA OIE classification of diseases, initially describe in the th Order -Nidovirales 16 century and recognized by Loeffler and Frosch Family -Picornaviridae

Continuous circulation of the virus in the field
Holguin et al., 1997)n the presence(Borrego et al., 1993(Borrego et al.,  )  1988;; Salt, 1993; Malirat et al., Woodbury, 1995).orabsence of immune pressure (Doming et al., 1993; Which give rise to virus population different fromHolguin et al., 1997)a new antigenic variant developed those that initiated the infection.This process cause which is different from its origin.changes in the sequence of the nucleotides of the 4. amino acids and among other aspect changing theThe rise of new variants is inevitably caused by antigenic and immunogenic properties of the viral continued circulation of the virus in the field and the antigenic determinants.quasispeciesnature of the RNA genome(Domingo et al., 2003;Haydon et al., 2001)propagation in cellal., 2003;Haydon et al., 2001).As the virus continues culture