Journal of Antivirals & Antiretrovirals
Open Access

Antiviral Immunity Evoked Post Foot-and-Mouth Disease Virus (FMDV) Infection and Vaccination

Author info »

Introduction

Foot-and-Mouth Disease (FMD) is a highly contagious and economically important disease of domestic and wild cloven hoofed animals. The disease is caused by foot-and-mouth disease virus (FMDV) which belongs to genus Aphthovirus of the family Picornaviridae [1].

The mechanism of spread of FMDV is primarily in the form of aerosolised droplets, saliva or through indirect contact by personnel or contaminated surfaces [2]. However, infection can also happen via skin or mucous membranes, but these are inefficient means of entry, unless abrasions or cuts are present [3,4]. Cattle sheep and goats can exhale up to 5.2 log 10 TCID50 virus/day and pigs up to 8.6 log 10 TCID50 virus/day which shows that infected pigs are bigger source of infection than infected ruminants [2,5]. In comparison to pigs, ruminants excrete less number of viruses in breath but are more susceptible than pigs to infection through respiratory route. Ruminants can be infected experimentally by airborne exposure of 10 TCID50 whereas pigs get infected by 103 TCID50 viruses [5,6]. However, the infective experimental oral doses for pigs are 104-105 TCID50 viruses and for ruminants 105-106TCID50 viruses [7] which shows that they are relatively insensitive to experimental infection by oral route. The incubation period for FMDV is from 2 to 14 days.

FMDV enters cells through binding to surface receptors of host cells. The major receptors for FMDV are integrins and heparan sulphate (HS) proteoglycans [8]. The integrin binding is mediated by a highly conserved arginine-glycine aspartic acid (RGD) motif located in GH-loop of VP1. The principal receptor used by field strains of FMDV to initiate infection is αvβ6 integrin which is due to its epithelial cell restricted expression [9-12]. The other 3 integrins recognized as receptors for field strains of FMDV are αvβ 1, αvβ 3 and αvβ 8 [13-15]. The role of these 3 integrins in pathogenesis is not clear and αvβ3 has been found as a poor receptor for FMDV [16]. However, cell culture adapted viruses often use heparansulphate (HS) as receptors and can initiate infection via an integrin-independent process [8,17-19].

After binding to the receptor, virus is endocytosed by clathrin dependent endocytosis into endosomes and then acidic pH of endosomes dissociates the capsid to release viral RNA [20]. The hydrophobic regions of capsid fuse with the endosomal lipid bilayer leading to a pore formation and then subsequently release of viral RNA into cytoplasm [21]. The viral RNA is later on translated into viral structural and non-structural proteins [22]. After FMDV infection, primary replication occurs in the mucosaassociated lymphoid tissue (MALT) of the nasopharynx and thereafter in the pulmonary alveolar septa. As viraemia approaches in 24-48 hours [23], the replication increases in the lungs and decreases in the nasopharynx [9,10,23]. Then lesions appear in the mouth and feet of susceptible animals. The viraemic phase is defined by vesiculation and erosion of epithelia of mouth, feet, teats, prepuce and ruminal pillars [24,25]. It has been shown that cattle are non-infectious until 0.5 days after appearance of clinical signs and the infectious period is usually shorter (1.7 days) [26]. After clearance of the virus from lesion sites, some ruminants may develop chronic asymptomatic infection called the carrier state [5,27].

Animals from which live FMDV can be recovered after 28 days of infection are referred as carriers [28,29]. Carriers have been recorded in cattle [30], African buffalo [31], sheep [32], and goats but not in pigs [33]. Carriers have been recognized for all serotypes of FMDV and in both experimentally and naturally infected animals [33]. The maximum duration observed for the carrier state in African buffalo, cattle, sheep and goats is 5 years, 3.5 years, 9 months and 4 months, respectively [3,34]. 45%- 50% of FMD-infected ruminants may turn into virus carriers. Although pigs can clear infection and do not turn into carriers [3], however, in some studies live virus was been isolated till 14 days of infection from tissues like tonsil, spleen, thymus and lymphnodes [35]. FMDV RNA was detected in the nasopharynx [36], dorsal palate [36] and tonsils [37] of FMDV infected pigs up to 33 days, 48 days and 36 days post infection, respectively.

Apart from oropharynx, FMDV may persist in other organs like mammary gland, testicles, pituitary, pancreas and thyroid [38-42]. Tonsil is the prominent site of persistence of FMDV in sheep [43,44].

FMDV structural proteins have been located till 38 days post infection in the germinal centres of lymphoid tissue of cattle [45] and these FMDV particles located in the lymphoid tissue could be a possible source of infectious material encountered during probang sampling of infected cattle [45].

There is no experimental evidence of virus transmission from carrier cattle or sheep to uninfected animals [46]. The only evidence available for virus transmission from a carrier to a susceptible animal is from African buffalo to cattle during the outbreaks in Zimbabwe in 1989 and 1991 [47]. Furthermore, it has been reported that FMD transmission can occur sexually from infected buffalo harbouring virus in oesophageal-pharyngeal (OP) fluids to uninfected cattle [48].

Immune Response

Immune defense against FMDV has been related to circulating antibody titres and humoral antibody responses are the most important factor which protects animal against FMD [49-51]. The antibodies are produced by B lymphocytes by their interaction with Th lymphocytes. Hence, both humoral and cell mediated immunity play important role in providing protection. In addition, the innate immune response also protect animals during early FMDV infection [52].

Innate Immune Response

The main components of the innate immune system are macrophages and dendritic cells. Macrophages play an important role in early phases of FMDV infection. Macrophages infected with FMDV have been observed to play role in acute infection [53] acting as infectious carriers and disseminating virus to other parts of body. Infection in these virus infected macrophages can be cleared within 10-14 hours in vitro. Macrophages are considered to play role in immune responses through opsonisation and subsequently destruction of virus.

Natural killer (NK) cells obtained from FMDV restimulated bovine PBMC (peripheral blood mononuclear cells) of vaccinated cattle, have been shown to be cytotoxic against FMDV-infected target cells [54]. In addition, γ δ T-cells have been shown to proliferate and produce cytokines in response to FMDV antigen [55].

Dendritic cells (DCs) are the major antigen presenting cells in mammals and plasmacytoid DCs (pDCs) produce significant amounts of IFN-α early after infection [56]. Studies in humans and mice have demonstrated that DCs have roles in linking innate and adaptive immune responses [57]. Various subsets of porcine dendritic cells including plasmacytoid dendritic cells (PDC), monocyte-derived dendritic cells and bone marrow-derived dendritic cells have shown to be susceptible to infection [58].

FMDV is highly sensitive to type 1 IFNs in vitro [59] and FMDV Leader proteinase (Lpro) and 3C block the expression of type 1 IFN in vitro [60-62]. This blocking of interferon may help the virus to replicate and disseminate effectively [63]. It has been demonstrated that FMDV replication in cell culture is inhibited by IFN-α and delivery of IFN-α protects swine from subsequent FMDV challenge [59,64]. IFN-α and/or IFN-β mRNA production after experimental infection has been reported in skin [65], nasal associated lymphoid tissue [66], mononuclear cells of lung [67], and epithelial cells of bovine tongue, coronary band and dorsal soft palate [66]. Recently, Segundo et al., reported that Ad5- boIFN-λ 3 induced systemic antiviral activity and boIFN-λ3 has potential as a biotherapeutic candidate to inhibit FMDV or other viruses in cattle.

Adaptive Immune Response

Adaptive (acquired) immunity includes both humoral and cell mediated immunity (CMI). It involves antigen presenting cells (APCs) such as macrophages and dendritic cells, the activation and proliferation of antigen specific B cells and T cells and the production of antibody molecules, cytotoxic T lymphocytes (CTLs) and cytokines.

Humoral immunity

There is a strong correlation between the circulating humoral antibody titre against FMDV and protection against the virus [51,68]. The first neutralising antibody is IgM, appearing after 3-4 days of infection/vaccination, peaking after 10-14 days and thereafter declining [69,70]. IgG appears 4-7 days post infection/ vaccination, and becomes the major neutralising antibody after 2 weeks [71,72]. The titre of IgG1 is reported to be higher than IgG2 [34]. IgM followed by IgA and IgG are the major antibody subclasses found in the upper respiratory tract [34]. IgM and IgA mediated neutralising activity has been observed in the pharyngeal fluid after 7 days of virus exposure [73]. However the presence of IgA antibody in early stage of infection was considered to be the leakage of tissue fluid and serum whereas presence of IgA antibody in the pharyngeal fluid after 20 days of infection was reported to be synthesized at mucosal surface rather than serum transudation. Salt et al. [72] considered the possibility of mucosal IgA detection in the oropharyngeal fluids of persistently FMDV infected animals and later on developed an IgA assay to detect the persistently infected (carrier) cattle and demonstrated that IgA is the indicator of oropharyngeal replication of FMDV.

The production of antibody after FMDV infection has been demonstrated to be T cell independent in both mice [74] and cattle [74,75]. The induction of IgG after FMDV immunization has been shown to be T-cell-dependent in a murine experimental model [71,76]. However, T cells have been shown to play a role in induction of antibody responses in ruminants which is demonstrated by FMDV specific CD4 T-cell proliferation after infection/vaccination [77-79]. The NSPs of FMDV have many T-cell epitopes [77,79] which might elicit prolonged immune response in infected animals [80]. The presence of non-replicating FMDV antigen in the light zone of the germinal centre of mandibular lymph node may elicit long-term antibody response in infected animals [45]. Pega et al. [81] demonstrated a rapid local antibody response on aerosol exposure to FMDV, where all the infected animals developed antibody secreting cells after 4 days of infection in all the lymphoid organs along the respiratory tract. Recently the role of CD4+ cells in induction of neutralizing antibodies against FMDV infection and isotype switching has been demonstrated by Carr et al. [82]. They found that after depletion of CD4+ cells in vaccinated animals there was significant decrease in the titre of neutralizing antibodies and delay in the isotype switching from IgM to IgG suggesting the possible role of CD4+ in induction of humoral immunity against FMDV.

Cell-mediated immunity

The cellular immune response is evoked by CD8+ cytotoxic T-cells and CD4+ helper T-cells [83]. The Th1 secrete interferon gamma (IFNγ), IL-2 and tumour necrosis factor beta (TNFβ) which stimulates phagocytosis to remove the intracellular microbes [84]. It has been reported in mice that Th1 cells promote an IgG2 antibody response [84]. The Th2 cell identifies the antigen presented by B lymphocytes and secrete IL-4, IL-5, IL-6, IL-10 and IL-13 which stimulates production of IgG1 antibody [85].

Although it is a well-known fact that protection against FMDV infection is provided by virus neutralizing antibodies, however, specific antibody response does not always provide clinical protection against FMDV infection [49]. Intriguingly, in many cases animals are found protected even in the absence of humoral response against FMDV [86]. Hence it has been suggested that a cell mediated immune response is required for protection against FMDV infection [87]. Both CD8+ and CD4+ antiviral responses have been observed after FMDV infection, however, the role of these responses in providing protection is unclear [88,89].

The T cell response to FMDV is cross reactive between FMDV serotypes hence it forms an important tool for the vaccine design [90-93]. It has been shown that the CD4+ cell mediated T cell response is required for providing protection against FMDV infection by the production of antiviral antibodies [88,89,94]. Hence, IFN-ϒ has been used for measurement of antigen-specific T cell activation [95]. The proliferation and production of IFN-ϒ from lymphocytes derived from FMDV infected animals have been demonstrated after restimulating with vaccine antigen [79,82,96]. Moreover, a memory CD4+ T-cell population has been detected in vaccinated cattle [97,98]. Recently a positive correlation has been found between IFN-ϒ and vaccine induced protection [68]. It was found that CD4+ T cells are the major proliferating cells and produce interferon gamma in stimulated cells of vaccinated and subsequently infected animals [68].

The FMDV specific MHC-I restricted CD8+ T cells response has been detected in cattle using a sensitive IFN-restimulation ELISpot assay [99]. Recently a highly conserved putative cross reactive cytotoxic CD8+ T-cell epitope was detected in the VP1 protein of FMDV [100]. The cell-mediated immune response against FMDV has been demonstrated by induction of IFN-γ [94]. Also, induction of FMDV-specific CD8+ cytotoxic T lymphocytes killing of MHC matched target cells in an antigen specific manner has been demonstrated [101]. Recently, Zhang et al. [102] reported that IL-2 and GM-CSF can be used as adjuvants with VP1 to enhance both humoral and cell mediated immune responses. Proliferation of CD8+ cells has been demonstrated starting from 10-14 days post vaccination up to 3-4 weeks following FMDV infection [87]. However, the role of CD8+ immune responses could be limited due to the rapid decrease in MHC 1 expression that occurs with FMDV infection. This has been demonstrated in vitro with MHC-1 expression down to about 53% that of normal at 6 hours post infection [103].

During FMDV infection, impairment of T-cell function has been observed. During acute phase of infection transient lymphopenia has been observed in swine [104] with T cell function returning to normal after 4-7 days of infection [105]. It has been demonstrated that incorporation of T cell epitope along with B cell epitope in a peptide based vaccine followed by vaccination and subsequent challenge leads to significant reduction in virus excretion and clinical score [106]. Moraes et al. [107] found that inclusion of 2B peptide in adenovirus vectored based vaccine increased CD8+ and CD4+ response which correlates with protection.

Conclusion

Thus, both innate and adaptive immunity play an important role in providing anti-viral immunity against FMDV. The humoral anti-FMDV response is T cell dependent and in some of the instances in the absence of humoral antibody response the cell mediated immune response has been found to be active.

References

1. Belsham GJ. Distinctive features of foot-and-mouth disease virus, a member of the picornavirus family; aspects of virus protein synthesis, protein processing and structure. Prog Biophys Mol Bio. 1993;60:241-60.
2. Alexandersen S, Zhang Z, Reid SM, Hutchings GH, Donaldson AI. Quantities of infectious virus and viral RNA recovered from sheep and cattle experimentally infected with foot-and-mouth disease virus O UK 2001. J Gen Virol, 2002;83:1915-23.
3. Alexandersen S, Zhang Z, Donaldson AI, Garland AJ. The pathogenesis and diagnosis of foot-and-mouth disease. J Comp Pathol. 2003;129:1-36.
4. Donaldson AI, Gibson CF, Oliver R, Hamblin C, Kitching RP. Infection of cattle by airborne foot-and-mouth disease virus: minimal doses with O1 and SAT 2 strains. Res Vet Sci. 1987;43:339-46.
5. Alexandersen S, Brotherhood I, Donaldson AI. Natural aerosol transmission of foot-and-mouth disease virus to pigs: minimal infectious dose for strain O1 Lausanne. Epidemiol Infect. 2002a;128:301-12.
6. Alexandersen S, Donaldson AI. Further studies to quantify the dose of natural aerosols of foot-and-mouth disease virus for pigs. Epidemiol Infect. 2002;128:313-23.
7. Sellers RF, Herniman KA Donaldson AI. The effects of killing or removal of animals affected with foot-and-mouth disease on the amounts of airborne virus present in looseboxes. Br Vet J. 1971;127: 358-65.
8. Jackson T, Ellard FM, Ghazaleh RA, Brookes SM, Blakemore WE, Corteyn AH, et al. Efficient infection of cells in culture by type O foot-and-mouth disease virus requires binding to cell surface heparan sulfate. J Virol. 1996;70: 5282-87.
9. Arzt J, Baxt B, Grubman MJ, Jackson T, Juleff N, Rhyan J, et al. The pathogenesis of foot-and-mouth disease II: Viral pathways in swine, small ruminants, and wildlife; myotropism, chronic syndromes, and molecular virus-host interactions. Transbound Emerg Dis. 2011;58:305-26.
10. Arzt J, Pacheco JM, Rodriguez LL. The early pathogenesis of foot-and-mouth disease in cattle after aerosol inoculation. Identification of the nasopharynx as the primary site of infection. Vet Patho. 2010;47:1048-63.
11. Jackson T, Sheppard D, Denyer M, Blakemore W, King AM. The epithelial integrin alphavbeta6 is a receptor for foot-and-mouth disease virus. J Virol. 2000;74:4949-56.
12. Monaghan P, Gold S, Simpson J, Zhang Z, Weinreb PH, Violette SM, et al. The alpha(v)beta6 integrin receptor for Foot-and-mouth disease virus is expressed constitutively on the epithelial cells targeted in cattle. J Gen Virol. 2005;86:2769-80.
13. Berinstein A, Roivainen M, Hovi T, Mason PW, Baxt B. Antibodies to the vitronectin receptor (integrin alpha V beta 3) inhibit binding and infection of foot-and-mouth disease virus to cultured cells. J Virol. 1995;69:2664-66.
14. Jackson T, Clark S, Berryman S, Burman A, Cambier S, Mu D, et al. Integrin alphavbeta8 functions as a receptor for foot-and-mouth disease virus: role of the beta-chain cytodomain in integrin-mediated infection. J Virol. 2004;78: 4533-40.
15. Jackson T, Mould AP, Sheppard D, King AM. Integrin alphavbeta1 is a receptor for foot-and-mouth disease virus. J Virol. 2002;76:935-41.
16. Burman A, Clark S, Abrescia NG, Fry EE, Stuart DI, Jackson T. Specificity of the VP1 GH loop of Foot-and-Mouth Disease virus for alphav integrins. J Virol. 2006;80: 9798-810.
17. Fry EE, Newman JW, Curry S, Najjam S, Jackson T, Blakemore W, et al. Structure of Foot-and-mouth disease virus serotype A10 61 alone and complexed with oligosaccharide receptor: Receptor conservation in the face of antigenic variation. J Gen Viro. 2005;86:1909-20.
18. Maree FF, Blignaut BDE, Beer TA, Visser N, Rieder EA. Mapping of amino acid residues responsible for adhesion of cell culture-adapted foot-and-mouth disease SAT type viruses. Virus Res. 2010;153:82-91.
19. Maree FF, Blignaut B, Aschenbrenner L, Burrage T, Rieder E. Analysis of SAT1 type foot-and-mouth disease virus capsid proteins: Influence of receptor usage on the properties of virus particles. Virus Res. 2011;155:462-72.
20. Berryman S, Clark S, Monaghan P, Jackson T. Early events in integrin alphavbeta6-mediated cell entry of foot-and-mouth disease virus. J Virol, 2005;79:8519-34.
21. O'Donnell V, Larocco M, Duque H, Baxt B. Analysis of foot-and-mouth disease virus internalization events in cultured cells. J Virol. 2005;79:8506-18.
22. Belsham GJ. Translation and replication of FMDV RNA. Curr Top Microbiol Immunol. 2005;288:43-70.
23. Pacheco JM, Arzt J, Rodriguez LL. Early events in the pathogenesis of foot-and-mouth disease in cattle after controlled aerosol exposure. Vet J. 2010;183:46-53.
24. Alexandersen S, Mowat N. Foot-and-mouth disease: host range and pathogenesis. Curr Top Microbiol Immunol. 2005;288:9-42.
25. Arzt J, Gregg DA, Clavijo A, Rodriguez LL. Optimization of immunohistochemical and fluorescent antibody techniques for localization of Foot-and-mouth disease virus in animal tissues. J Vet Diagn Invest. 2009;21:779-92.
26. Charleston B, Bankowski BM, Gubbins S, Chase-Topping ME, Schley D, Howey R, et al. Relationship between clinical signs and transmission of an infectious disease and the implications for control. Science. 2011;33: 726-29.
27. Salt JS (2004) Persistent of foot-and-mouth disease virus. In: Sobrino F, Domingo E (eds) Foot and Mouth Disease. Current Perspectives.
28. Alexandersen S, Zhang Z, Donaldson AI. Aspects of the persistence of foot-and-mouth disease virus in animals--the carrier problem. Microbes Infect. 2002b; 4:1099-110.
29. Salt JS, Barnett PV, Dani P, Williams L. Emergency vaccination of pigs against foot-and-mouth disease: Protection against disease and reduction in contact transmission. Vaccine. 1998;16:746-54.
30. Burrows R. Studies on the carrier state of cattle exposed to foot-and-mouth disease virus. J Hyg (Lond). 1966;64:81-90.
31. Hedger RS, Condy JB. Transmission of foot-and-mouth disease from African buffalo virus carriers to bovines. Vet Rec. 1985;117:205.
32. Burrows R. The persistence of foot-and mouth disease virus in sheep. J Hyg. 1968;66:633-40.
33. Hedger RS. The isolation and characterization of foot-and-mouth disease virus from clinically normal herds of cattle in Botswana. J Hyg (Lond). 1968;66:27-36.
34. Salt JS. The carrier state in foot and mouth disease an immunological review. Br Vet J. 1993;149:207-23.
35. Rodriguez CT, Diaz-San SF, Sanz RM, Sevilla N. A replication analysis of foot-and-mouth disease virus in swine lymphoid tissue might indicate a putative carrier stage in pigs. Vet Res. 2011;42:22.
36. Parida S, Fleming L, Oh Y, Mahapatra M, Hamblin P, Gloster J, et al. Reduction of foot-and-mouth disease (FMD) virus load in nasal excretions, saliva and exhaled air of vaccinated pigs following direct contact challenge. Vaccine. 2007b;25:7806-17.
37. Mohamed F, Swafford S, Petrowski H, Bracht A, Schmit B, Fabian A, et al. Foot-and-mouth disease in feral swine: susceptibility and transmission. Transbound Emerg Dis. 2011;58:358-71.
38. Burrows R, Mann JA, Greig A, Chapman WG, Goodridge D. The growth and persistence of foot-and-mouth disease virus in the bovine mammary gland. J Hyg (Lond). 1971;69:307-21.
39. Cottral GE, Gailiunas P, Cox BF. Foot-and-mouth disease virus in semen of bulls and its transmission by artificial insemination. Arch Gesamte Virusforsch. 1968;23:362-77.
40. Jones AL. Growth of foot and mouth disease virus in organ cultures of mouse pancreas. Nature. 1965;207:665-66.
41. Sellers RF, Burrows R, Garland AJ, Greig A, Parker J. Exposure of vaccinated bulls and steers to airborne infection with foot-and-mouth disease. Vet Rec. 1969;85:198-99.
42. Sellers RF, Burrows R, Mann JA, Dawe P. Recovery of virus from bulls affected with foot-and-mouth disease. Vet Rec. 1968;83:303.
43. Burrows R. The persistence of foot-and mouth disease virus in sheep. J Hyg (Lond). 1968;66:633-40
44. Ryan E, Horsington J, Durand S, Brooks H, Alexandersen S, Brownlie J, et al. Foot-and-mouth disease virus infection in young lambs: pathogenesis and tissue tropism. Vet Microbiol. 2008;127:258-74.
45. Juleff N, Windsor M, Reid E, Seago J, Zhang Z, Monaghan P, et al. Foot-and-mouth disease virus persists in the light zone of germinal centres. PLoS One. 2008; 3:e3434.
46. Kitching RP. Future research on foot and mouth disease. Rev Sci Tech. 2002a;21:885-9.
47. Dawe PS, Flanagan FO, Madekurozwa RL, Sorensen KJ, Anderson EC, Foggin CM, et al. Natural transmission of foot-and-mouth disease virus from African buffalo (Syncerus caffer) to cattle in a wildlife area of Zimbabwe. Vet Rec. 1994a;134:30-32.
48. Dawe PS, Sorensen K, Ferris NP, Barnett IT, Armstrong RM, Knowles NJ. Experimental transmission of foot-and-mouth disease virus from carrier African buffalo (Syncerus caffer) to cattle in Zimbabwe. Vet Rec. 1994b;134:211-15.
49. Mccullough KC, Bruckner L, Schaffner R, Fraefel W, Muller HK, Kihm U. Relationship between the anti-FMD virus antibody reaction as measured by different assays, and protection in vivo against challenge infection. Vet Microbiol. 1992a;30:99-112.
50. Mccullough KC, Simone F, Brocchi E, Capucci L, Crowther JR, Kihm U. Protective immune response against foot-and-mouth disease. J Virol. 1992b;66:1835-40.
51. Pay TW, Hingley PJ. Correlation of 140S antigen dose with the serum neutralizing antibody response and the level of protection induced in cattle by foot-and-mouth disease vaccines. Vaccine. 1987;5:60-4.
52. Summerfield A, Guzylack PL, Harwood L, Mccullough KC. Innate immune responses against foot-and-mouth disease virus: Current understanding and future directions. Vet Immunol Immunopathol. 2009;128:205-10.
53. Rigden RC, Carrasco CP, Summerfield A, Kc MC. Macrophage phagocytosis of foot-and-mouth disease virus may create infectious carriers. Immunology. 2002;106:537-48.
54. Amadori M, Archetti IL, Verardi R, Berneri C. Isolation of mononuclear cytotoxic cells from cattle vaccinated against foot-and-mouth disease. Arch Virol. 1992;122:293-306.
55. Takamatsu HH, Denyer MS, Stirling C, Cox S, Aggarwal N, Dash P, et al. Porcine gammadelta T cells: Possible roles on the innate and adaptive immune responses following virus infection. Vet Immunol Immunopathol. 2006;112:49-61.
56. Colonna M, Krug A, Cella M. Interferon-producing cells: On the front line in immune responses against pathogens. Curr Opin Immunol. 2002;14:373-79.
57. Banchereau J, Steinman RM. Dendritic cells and the control of immunity. Nature. 1998;392:245-52.
58. Guzylack PL, Bergamin F, Gerber M, Mccullough KC, Summerfield A. Plasmacytoid dendritic cell activation by foot-and-mouth disease virus requires immune complexes. Eur J Immunol. 2006;36:1674-83.
59. Chinsangaram J, Koster M, Grubman MJ. Inhibition of L-deleted foot-and-mouth disease virus replication by alpha/beta interferon involves double-stranded RNA-dependent protein kinase. J Virol. 2001;75:5498-503.
60. De Los ST, De Avila BS, Weiblen R, Grubman MJ. The leader proteinase of foot-and-mouth disease virus inhibits the induction of beta interferon mRNA and blocks the host innate immune response. J Virol. 2006;80:1906-14.
61. Wang D, Fang L, Liu L, Zhong H, Chen Q, Luo R, et al. Foot-and-mouth disease virus (FMDV) leader proteinase negatively regulates the porcine interferon-lambda1 pathway. Mol Immunol. 2011;49:407-12.
62. Wang D, Fang L, Li K, Zhong H, Fan J, Ouyang C, et al. Foot-and-mouth disease virus 3C protease cleaves NEMO to impair innate immune signaling. J Virol. 2012; 86:9311-22.
63. Nfon CK, Toka FN, Kenney M, Pacheco JM, Golde WT. Loss of plasmacytoid dendritic cell function coincides with lymphopenia and viremia during foot-and-mouth disease virus infection. Viral Immunol. 2010;23: 29-41.
64. Moraes MP, Chinsangaram J, Brum MC, Grubman MJ. Immediate protection of swine from foot-and-mouth disease: a combination of adenoviruses expressing interferon alpha and a foot-and-mouth disease virus subunit vaccine. Vaccine. 2003;22:268-79.
65. Bautista EM, Ferman GS, Gregg D, Brum MC, Grubman MJ, Golde WT. Constitutive expression of alpha interferon by skin dendritic cells confers resistance to infection by foot-and-mouth disease virus. J Virol. 2005;79:4838-47.
66. Zhang Z, Bashiruddin JB, Doel C, Horsington J, Durand S, Alexandersen S. Cytokine and Toll-like receptor mRNAs in the nasal-associated lymphoid tissues of cattle during foot-and-mouth disease virus infection. J Comp Pathol. 2006;134:56-62.
67. Brown CC, Chinsangaram J, Grubman MJ. Type I interferon production in cattle infected with 2 strains of foot-and-mouth disease virus, as determined by in situ hybridization. Can J Vet Res. 2000;64:130-3.
68. Oh Y, Fleming L, Statham B, Hamblin P, Barnett P, Paton DJ, et al. Interferon-gamma induced by in vitro re-stimulation of CD4+ T-cells correlates with in vivo FMD vaccine induced protection of cattle against disease and persistent infection. PLoS One. 2012;7: e44365.
69. Golde WT, Nfon CK, Toka FN. Immune evasion during foot-and-mouth disease virus infection of swine. Immunol Rev. 2008;225:85-95.
70. Sobrino F, Saiz M, Jimenez CM, Nunez JI, Rosas MF, Baranowski E, et al. Foot-and-mouth disease virus: A long known virus, but a current threat. Vet Res. 2001;32:1-30.
71. Collen T, Pullen L, Doel TR. T cell-dependent induction of antibody against foot-and-mouth disease virus in a mouse model. J Gen Virol. 1989;70:395-403.
72. Salt JS, Mulcahy G, Kitching RP. Isotype-specific antibody responses to foot-and-mouth disease virus in sera and secretions of "carrier' and "non-carrier' cattle. Epidemiol Infect. 1996;117:349-60.
73. Francis MJ, Ouldridge EJ, Black L. Antibody response in bovine pharyngeal fluid following foot-and-mouth disease vaccination and, or, exposure to live virus. Res Vet Sci. 1983;35: 206-10.
74. Borca MV, Fernandez FM, Sadir AM, Braun M, Schudel AA. Immune response to foot-and-mouth disease virus in a murine experimental model: effective thymus-independent primary and secondary reaction. Immunology. 1986; 59:261-67.
75. Juleff N, Windsor M, Lefevre EA, Gubbins S, Hamblin P, Reid E, et al. Foot-and-mouth disease virus can induce a specific and rapid CD4+ T-cell-independent neutralizing and isotype class-switched antibody response in naive cattle. J Virol. 2009; 83:3626-36.
76. Ostrowski M, Vermeulen M, Zabal O, Zamorano PI, Sadir AM, Geffner JR. The early protective thymus-independent antibody response to foot-and-mouth disease virus is mediated by splenic CD9+ B lymphocytes. J Virol. 2007;81:9357-67.
77. Blanco E, Garcia BM, Sanz PA, Gomes P, De Oliveira E, Valero ML, et al. Identification of T-cell epitopes in nonstructural proteins of foot-and-mouth disease virus. J Virol. 2001;75: 3164-74.
78. Collen T, Doel TR. Heterotypic recognition of foot-and-mouth disease virus by cattle lymphocytes. J Gen Virol. 1990;71:309-15.
79. Gerner W, Carr BV, Wiesmuller KH, Pfaff E, Saalmuller A, Charleston B. Identification of a novel foot-and-mouth disease virus specific T-cell epitope with immunodominant characteristics in cattle with MHC serotype A31. Vet Res. 2007; 38:565-72.
80. Parida S. Vaccination against foot-and-mouth disease virus: Strategies and effectiveness. Expert Rev Vaccines. 2009;8:347-65.
81. Pega J, Bucafusco D, Giacomo S, Schammas J, Malacari D, Capozzo A, et al. Early adaptive immune responses in the respiratory tract of foot and mouth disease-infected cattle. J Virol. 2012.
82. Carr BV, Lefevre EA, Windsor MA, Inghese C, Gubbins S, Prentice H, et al. CD4+ T-cell responses to foot-and-mouth disease virus in vaccinated cattle. J Gen Virol. 2013;94:97-107.
83. Zajac AJ. Immune response to viruses: Cell-Mediated immunity. Encyclopedia of Virol. 2008; 3:70-77.
84. Desmedt M, Rottiers P, Dooms H, Fiers W, Grooten J. Macrophages induce cellular immunity by activating Th1 cell responses and suppressing Th2 cell responses. J Immunol. 1998;160:5300-08.
85. Abbas AK, Litchman AH, Pober JS. Cellular and Molecular Immunology (4th edn). WB Saunders. 2000.
86. Sanz-Parra A, Jimenez-Clavero MA, Garcia-Briones MM, Blanco E, Sobrino F, Ley V. Recombinant viruses expressing the foot-and-mouth disease virus capsid precursor polypeptide (P1) induce cellular but not humoral antiviral immunity and partial protection in pigs. Virol. 1999;259:129-34.
87. Becker Y. Need for cellular and humoral immune responses in bovines to ensure protection from foot-and-mouth disease virus (FMDV): A point of view. Virus Genes. 1994;8:199-214.
88. Childerstone AJ, Cedillo-Baron L, Foster-Cuevas M, Parkhouse RM. Demonstration of bovine CD8+ T-cell responses to foot-and-mouth disease virus. J Gen Virol. 1999;80:663-9.
89. Van MJ, Wagenaar JP, Van Noort JM, Hensen EJ. Sequences derived from the highly antigenic VP1 region 140 to 160 of foot-and-mouth disease virus do not prime for a bovine T-cell response against intact virus. J Virol. 1995;69:4511-14.
90. Blanco E, Garcia-Briones M, Sanz-Parra A, Gomes P, De Oliveira E, Valero ML, et al. Identification of T-cell epitopes in nonstructural proteins of foot-and-mouth disease virus. J Virol. 2001;75:3164-74.
91. Collen T, Baron J, Childerstone A, Corteyn A, Doel TR, Flint M, et al. Heterotypic recognition of recombinant FMDV proteins by bovine T-cells: The polymerase (P3Dpol) as an immunodominant T-cell immunogen. Virus Res. 1998;56:125-33.
92. Cox SJ, Carr BV, Parida S, Hamblin PA, Prentice H, Charleston B, et al. Longevity of protection in cattle following immunisation with emergency FMD A22 serotype vaccine from the UK strategic reserve. Vaccine. 2010;28:2318-22.
93. Parida S, Oh Y, Reid SM, Cox SJ, Statham RJ, Mahapatra M, et al. Interferon-gamma production in vitro from whole blood of foot-and-mouth disease virus (FMDV) vaccinated and infected cattle after incubation with inactivated FMDV. Vaccine. 2006b;24:964-69.
94. Bautista EM, Ferman GS, Golde WT. Induction of lymphopenia and inhibition of T cell function during acute infection of swine with foot and mouth disease virus (FMDV). Vet Immunol Immunopathol. 2003;92:61-73.
95. Tassignon J, Burny W, Dahmani S, Zhou L, Stordeur P, Byl B, et al. Monitoring of cellular responses after vaccination against tetanus toxoid: Comparison of the measurement of IFN-gamma production by ELISA, ELISPOT, flow cytometry and real-time PCR. J Immunol Methods. 2005;305:188-98.
96. Van Lierop MJ, Van Maanen K, Meloen RH, Rutten VP, De Jong MA, Hensen EJ. Proliferative lymphocyte responses to foot-and-mouth disease virus and three FMDV peptides after vaccination or immunization with these peptides in cattle. Immunol. 1992;75:406-13.
97. Glass EJ, Millar P. Induction of effective cross-reactive immunity by FMDV peptides is critically dependent upon specific MHC-peptide-T cell interactions. Immunol. 1994;82:1-8.
98. Naessens J, Scheerlinck JP, De Buysscher EV, Kennedy D, Sileghem M. Effective in vivo depletion of T cell subpopulations and loss of memory cells in cattle using mouse monoclonal antibodies. Vet Immunol Immunopathol. 1998;64:219-34.
99.  Guzman E, Taylor G, Charleston B, Skinner MA, Ellis SA. An MHC-restricted CD8+ T-cell response is induced in cattle by foot-and-mouth disease virus (FMDV) infection and also following vaccination with inactivated FMDV. J Gen Virol. 2008;89: 667-75.
100. Guzman E, Taylor G, Charleston B, Ellis SA. Induction of a cross-reactive CD8(+) T cell response following foot-and-mouth disease virus vaccination. J Virol. 2010;84: 12375-84.
101. Patch JR, Pedersen LE, Toka FN, Moraes M, Grubman MJ, Nielsen M, et al. Induction of foot-and-mouth disease virus-specific cytotoxic T cell killing by vaccination. Clin Vaccine Immunol. 2011;18: 280-88.
102. Zhang C, Wang B, Wang M. GM-CSF and IL-2 as adjuvant enhance the immune effect of protein vaccine against foot-and-mouth disease. Virol J. 2011;8:7.
103. Sanz-Parra A, Sobrino F, Ley V. Infection with foot-and-mouth disease virus results in a rapid reduction of MHC class I surface expression. J Gen Virol. 1998;79:433-36.
104. Diaz-San SF, Weiss M, Perez-Martin E, Koster MJ, Zhu J, Grubman MJ, et al. Antiviral activity of bovine type III interferon against foot-and-mouth disease virus. Virol. 2011;413:283-92.
105. Diaz-San SF, Salguero FJ, De Avila A, De Marco MM, Sanchez-Martin MA, Sevilla N. Selective lymphocyte depletion during the early stage of the immune response to foot-and-mouth disease virus infection in swine. J Virol. 2006;80:2369-79.
106. Cubillos C, De La TB, Barcena J, Andreu D, Sobrino F, Blanco E. Inclusion of a specific T cell epitope increases the protection conferred against foot-and-mouth disease virus in pigs by a linear peptide containing an immunodominant B cell site. Virol J. 2012;9:66.
107. Moraes MP, Segundo FD, Dias CC, Pena L, Grubman MJ. Increased efficacy of an adenovirus-vectored foot-and-mouth disease capsid subunit vaccine expressing nonstructural protein 2B is associated with a specific T cell response. Vaccine. 2011;29: 9431-40.