Induction of foot-and-mouth disease virus-specific cytotoxic T cell killing by vaccination

Foot-and-mouth disease: genomic and proteomic structure, antigenic sites, serotype relationships, immune evasion, recent vaccine development strategies, and future perspectives

Foot-and-mouth disease (FMD) is a highly contagious and transmissible disease that can have significant economic and trade repercussions during outbreaks. In Egypt, despite efforts to mitigate FMD through mandatory immunization, the disease continues to pose a threat due to the high genetic variability and quasi-species nature of the FMD virus (FMDV). Vaccines have been crucial in preventing and managing FMD, and ongoing research focusses on developing next-generation vaccines that could provide universal protection against all FMDV serotypes. This review thoroughly examines the genetic structure of FMDV, including its polyprotein cleavage process and the roles of its structural and non-structural proteins in immune evasion. Additionally, it explores topics such as antigenic sites, specific mutations, and serotype relationships from Egypt and Ethiopia, as well as the structural changes in FMDV serotypes for vaccine development. The review also addresses the challenges associated with creating effective vaccines for controlling FMD, particularly focusing on the epitope-based vaccine. Overall, this review offers valuable insights for researchers seeking to develop effective strategies and vaccines for controlling FMD.

A comprehensive perspective of traditional Arabic or Islamic medicinal plants as an adjuvant therapy against COVID-19

COVID-19 is a pulmonary disease caused by SARS-CoV-2. More than 200 million individuals are infected by this globally. Pyrexia, coughing, shortness of breath, headaches, diarrhoea, sore throats, and body aches are among the typical symptoms of COVID-19. The virus enters into the host body by interacting with the ACE2 receptor. Despite many SARS-CoV-2 vaccines manufactured by distinct strategies but any evidence-based particular medication to combat COVID-19 is not available yet. However, further research is required to determine the safety and effectiveness profile of the present therapeutic approaches. In this study, we provide a summary of Traditional Arabic or Islamic medicinal (TAIM) plants' historical use and their present role as adjuvant therapy for COVID-19. Herein, six medicinal plants Aloe barbadensis Miller, Olea europaea, Trigonella foenum-graecum, Nigella sativa, Cassia angustifolia, and Ficus carica have been studied based upon their pharmacological activities against viral infections. These plants include phytochemicals that have antiviral, immunomodulatory, antiasthmatic, antipyretic, and antitussive properties. These bioactive substances could be employed to control symptoms and enhance the development of a possible COVID-19 medicinal synthesis. To determine whether or if these TAIMs may be used as adjuvant therapy and are appropriate, a detailed evaluation is advised.

Evaluation of Potency and Duration of Immunity Elicited by a Multivalent FMD Vaccine for Use in South Africa

South Africa (SA) experiences sporadic foot and mouth disease (FMD) outbreaks irrespective of routine prophylactic vaccinations of cattle using imported commercial vaccines. The problem could be mitigated by preparation of vaccines from local virus strains related to those circulating in the endemically infected buffalo populations in the Kruger National Park (KNP). This study demonstrates the individual number of protective doses (PD) of five vaccine candidate strains after homologous virus challenge, as well as the vaccines safety and onset of humoral immunity in naïve cattle. Furthermore, the duration of post-vaccination immunity over a 12-month period is shown, when a multivalent vaccine prepared from the five strains is administered as a primary dose with or without booster vaccinations. The five monovalent vaccines were shown to contain a 50% PD between 4 and 32, elicit humoral immunity with antibody titers ≥2.0 log10 from day 7 post-vaccination, and cause no adverse reactions. Meanwhile, the multivalent vaccine elicited antibody titers ≥2.0 log10 and clinical protection up to 12 months when one or two booster vaccinations were administered within 6 months of the primary vaccination. An insignificant difference between the application of one or two booster vaccinations was revealed. Owing to the number of PDs, we anticipate that the multivalent vaccine could be used successfully for prophylactic and emergency vaccinations without adjustment of the antigen payloads. Furthermore, a prophylactic vaccination regimen comprising primary vaccination of naïve cattle followed by two booster vaccinations 1.5 and 6 months later could potentially maintain herd immunity over a period of 12 months.

Local and Systemic T Cell Immunity in Fighting Pig Viral and Bacterial Infections

T cells are an essential component of the adaptive immune system. Over the last 15 years, a constantly growing toolbox with which to study T cell biology in pigs has allowed detailed investigations on these cells in various viral and bacterial infections. This review provides an overview on porcine CD4, CD8, and γδ T cells and the current knowledge on the differentiation of these cells following antigen encounter. Where available, the responses of these cells to viral infections like porcine reproductive and respiratory syndrome virus, classical swine fever virus, swine influenza A virus, and African swine fever virus are outlined. In addition, knowledge on the porcine T cell response to bacterial infections like Actinobacillus pleuropneumoniae and Salmonella Typhimurium is reviewed. For CD4 T cells, the response to the outlined infections is reflected toward the Th1/Th2/Th17/Tfh/Treg paradigm for functional differentiation. Expected final online publication date for the Annual Review of Animal Biosciences, Volume 10 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Foot and Mouth Disease Vaccine Development and Challenges in Inducing Long-Lasting Immunity: Trends and Current Perspectives

Foot and mouth disease (FMD) is an extremely contagious viral disease of livestock caused by foot and mouse disease virus genus: Aphthovirus, which causes a serious economic impact on both individual farmers and the national economy. Many attempts to advance a vaccine for FMD have failed to induce sterile immunity. The classical methods of vaccine production were due to selective accumulation of mutations around antigenic and binding sites. Reversion of the agent by positive selection and quasi-species swarm, use of this method is inapplicable for use in non-endemic areas. Chemical attenuation using binary ethyleneimine (BEI) protected the capsid integrity and produced a pronounced immunity against the challenge strain. Viral antigens which have been chemically synthesized or expressed in viruses, plasmid, or plants were tried in the vaccination of animals. DNA vaccines expressing either structural or nonstructural protein antigens have been tried to immunize animals. Using interleukins as a genetic adjuvant for DNA vaccines have a promising effect. While the challenges of inducing sterile immunity lies on non-structural (NS) proteins of FMDV which are responsible for apoptosis of dendritic cells and have negative effects on lympho-proliferative responses which lead to transient immunosuppression. Furthermore, destruction of host protein trafficking by nonstructural proteins suppressed CD₈⁺ T-cell proliferation. In this review, it tried to address multiple approaches for vaccine development trials and bottle necks of producing sterile immunity.

Efficacy of SAT2 Foot-and-Mouth Disease Vaccines Formulated with Montanide ISA 206B and Quil-A Saponin Adjuvants

The effective control of foot-and-mouth disease (FMD) relies strongly on the separation of susceptible and infected livestock or susceptible livestock and persistently infected wildlife, vaccination, and veterinary sanitary measures. Vaccines affording protection against multiple serotypes for longer than six months and that are less reliant on the cold chain during handling are urgently needed for the effective control of FMD in endemic regions. Although much effort has been devoted to improving the immune responses elicited through the use of modern adjuvants, their efficacy is dependent on the formulation recipe, target species and administration route. Here we compared and evaluated the efficacy of two adjuvant formulations in combination with a structurally stabilized SAT2 vaccine antigen, designed to have improved thermostability, antigen shelf-life and longevity of antibody response. Protection mediated by the Montanide ISA 206B-adjuvanted or Quil-A Saponin-adjuvanted SAT2 vaccines were comparable. The Montanide ISA 206B-adjuvanted vaccine elicited a higher SAT2 neutralizing antibody response and three times higher levels of systemic IFN-γ responses at 14- and 28-days post-vaccination (dpv) were observed compared to the Quil-A Saponin-adjuvanted vaccine group. Interestingly, serum antibodies from the immunized animals reacted similarly to the parental vaccine virus and viruses containing mutations in the VP2 protein that simulate antigenic drift in nature.

An Adenovirus Vector Expressing FMDV RNA Polymerase Combined with a Chimeric VLP Harboring a Neutralizing Epitope as a Prime Boost Strategy to Induce FMDV-Specific Humoral and Cellular Responses

Foot and mouth disease is a highly contagious disease affecting cattle, sheep, and swine among other cloven-hoofed animals that imposes serious economic burden by its direct effects on farm productivity as well as on commerce of farmed produce. Vaccination using inactivated viral strains of the different serotypes is an effective protective measure, but has several drawbacks including a lack of cross protection and the perils associated with the large-scale growth of infectious virus. We have previously developed chimeric virus-like particles (VLPs) bearing an FMDV epitope which induced strong specific humoral responses in vaccinated pigs but conferred only partial protection against homologous challenge. While this and other FMD vaccines under development mostly rely on the induction of neutralizing responses, it is thought that induction of specific T-cell responses might improve both cross protective efficacy as well as duration of immunity. Therefore, we here describe the development of a recombinant adenovirus expressing the highly conserved nonstructural FMDV 3D protein as well as its capacity to induce specific T-cell responses in a murine model. We further describe the generation of an FMDV serotype C-specific chimeric VLP and analyze the immunogenicity of two different prime-boost strategies combining both elements in mice. This combination can effectively induce both humoral and cellular FMDV-specific responses eliciting high titers of ELISA and neutralizing antibodies anti-FMDV as well as a high frequency of IFNγ-secreting cells. These results provide the basis for further testing of this anti FMD vaccination strategy in cattle or pig, two of the most relevant natural host of this pathogen.

Peptide-Based Vaccines: Foot-and-Mouth Disease Virus, a Paradigm in Animal Health

Vaccines are considered one of the greatest global health achievements, improving the welfare of society by saving lives and substantially reducing the burden of infectious diseases. However, few vaccines are fully effective, for reasons ranging from intrinsic limitations to more contingent shortcomings related, e.g., to cold chain transport, handling and storage. In this context, subunit vaccines where the essential antigenic traits (but not the entire pathogen) are presented in rationally designed fashion have emerged as an attractive alternative to conventional ones. In particular, this includes the option of fully synthetic peptide vaccines able to mimic well-defined B- and T-cell epitopes from the infectious agent and to induce protection against it. Although, in general, linear peptides have been associated to low immunogenicity and partial protection, there are several strategies to address such issues. In this review, we report the progress towards the development of peptide-based vaccines against foot-and-mouth disease (FMD) a highly transmissible, economically devastating animal disease. Starting from preliminary experiments using single linear B-cell epitopes, recent research has led to more complex and successful second-generation vaccines featuring peptide dendrimers containing multiple copies of B- and T-cell epitopes against FMD virus or classical swine fever virus (CSFV). The usefulness of this strategy to prevent other animal and human diseases is discussed.

Swine T-Cells and Specific Antibodies Evoked by Peptide Dendrimers Displaying Different FMDV T-Cell Epitopes

Dendrimeric peptide constructs based on a lysine core that comprises both B- and T-cell epitopes of foot-and-mouth disease virus (FMDV) have proven a successful strategy for the development of FMD vaccines. Specifically, B₂T dendrimers displaying two copies of the major type O FMDV antigenic B-cell epitope located on the virus capsid [VP1 (140–158)], covalently linked to a heterotypic T-cell epitope from either non-structural protein 3A [3A (21–35)] or 3D [3D (56–70)], named B₂T-3A and B₂T-3D, respectively, elicit high levels of neutralizing antibodies (nAbs) and IFN-γ-producing cells in pigs. To assess whether the inclusion and orientation of T-3A and T-3D T-cell epitopes in a single molecule could modulate immunogenicity, dendrimers with T epitopes juxtaposed in both possible orientations, i.e., constructs B₂TT-3A3D and B₂TT-3D3A, were made and tested in pigs. Both dendrimers elicited high nAbs titers that broadly neutralized type O FMDVs, although B₂TT-3D3A did not respond to boosting, and induced lower IgGs titers, in particular IgG2, than B₂TT-3A3D. Pigs immunized with B_2, a control dendrimer displaying two B-cell epitope copies and no T-cell epitope, gave no nABs, confirming T-3A and T-3D as T helper epitopes. The T-3D peptide was found to be an immunodominant, as it produced more IFN-γ expressing cells than T-3A in the in vitro recall assay. Besides, in pigs immunized with the different dendrimeric peptides, CD4⁺ T-cells were the major subset contributing to IFN-γ expression upon in vitro recall, and depletion of CD4⁺ cells from PBMCs abolished the production of this cytokine. Most CD4⁺IFN-γ⁺ cells showed a memory (CD4⁺2E3⁻) and a multifunctional phenotype, as they expressed both IFN-γ and TNF-α, suggesting that the peptides induced a potent Th1 pro-inflammatory response. Furthermore, not only the presence, but also the orientation of T-cell epitopes influenced the T-cell response, as B₂TT-3D3A and B₂ groups had fewer cells expressing both cytokines. These results help understand how B₂T-type dendrimers triggers T-cell populations, highlighting their potential as next-generation FMD vaccines.

Designing Functionally Versatile, Highly Immunogenic Peptide-Based Multiepitopic Vaccines against Foot-and-Mouth Disease Virus

A broadly protective and biosafe vaccine against foot-and-mouth disease virus (FMDV) remains an unmet need in the animal health sector. We have previously reported solid protection against serotype O FMDV afforded by dendrimeric peptide structures harboring virus-specific B- and T-cell epitopes, and also shown such type of multivalent presentations to be advantageous over simple B-T-epitope linear juxtaposition. Chemically, our vaccine platforms are modular constructions readily made from specified B- and T-cell epitope precursor peptides that are conjugated in solution. With the aim of developing an improved version of our formulations to be used for on-demand vaccine applications, we evaluate in this study a novel design for epitope presentation to the immune system based on a multiple antigen peptide (MAP) containing six immunologically relevant motifs arranged in dendrimeric fashion (named B₂T-TB₂). Interestingly, two B₂T units fused tail-to-tail into a single homodimer platform elicited higher B- and T-cell specific responses than former candidates, with immunization scores remaining stable even after 4 months. Moreover, this macromolecular assembly shows consistent immune response in swine, the natural FMDV host, at reduced dose. Thus, our versatile, immunogenic prototype can find application in the development of peptide-based vaccine candidates for various therapeutic uses using safer and more efficacious vaccination regimens.

A novel multiple emulsion enhanced immunity via its biomimetic delivery approach

A special emulsion with biomimetic structural dynamic properties was fabricated, inducing efficient vaccine–cell interaction and robust immunity.

Chemokine CCL20 plasmid improves protective efficacy of the Montanide ISA™ 206 adjuvanted foot-and-mouth disease vaccine in mice model

This study aimed to investigate the chemokine CCL20, a macrophage inflammatory protein-3 alpha, for adjuvant potential in inactivated foot-and-mouth disease (FMD) vaccine. Groups of mice were injected intramuscularly with either murine CCL20 DNA or CCL20 protein two days ahead of the immunization with Montanide ISA206 adjuvanted inactivated FMD vaccine and humoral and cellular immune responses were measured in post-vaccinal sera. We demonstrated that the mice immunized with CCL20 plasmid plus FMD vaccine showed earlier and significantly (p < 0.05) higher neutralizing antibody responses compared to the mice vaccinated with CCL20 protein plus FMD vaccine. In fact, CCL20 as a protein did not show any adjuvant effect and the immune responses induced in this group were comparable to that of the mice vaccinated with FMD vaccine alone. All the vaccination groups showed serum IgG1 and IgG2 antibody responses; however, the mice vaccinated with CCL20 plasmid plus FMD vaccine showed significantly (p < 0.05) higher IgG1 and IgG2 responses and the responses remained high at all-time points post vaccination, although not always statistically significant. Upon restimulation of the vaccinated splenocytes with the inactivated FMD viral antigen, significantly (p < 0.05) higher IFN-γ and IL-2 levels in culture supernatants were found in animals vaccinated with the CCL20 plasmid plus FMD vaccine, which is indicative of the T_H1 type of cellular immunity. On challenge with the homologous FMD virus on 28th day post immunization, CCL20 plasmid plus FMD vaccine showed complete protection (100%) while animals immunized with CCL20 protein plus FMD vaccine or FMD vaccine alone showed 66% protection. In summary, we show that prior injection of CCL20 plasmid improved protective efficacy of the inactivated FMD vaccine and thus offers a valuable strategy to modulate the efficacy and polarization of specific immunity against inactivated vaccines.

Identification of two novel foot-and-mouth disease virus cytotoxic T lymphocyte epitopes that can bind six SLA-I proteins

Currently available vaccines from inactivated foot-and-mouth disease virus (FMDV) only protect animals by inducing neutralizing antibodies. A vaccine that contains cytotoxic T lymphocytes (CTL) epitopes to induce strong CTL responses might protect animals more effectively. Herein, we used swine leukocyte antigen class I (SLAI) proteins derived from six different strains of domestic pigs to screen and identify shared FMDV CTL epitopes. Four potential FMDV CTL epitopes (Q01, Q02, AS3, and QA4) were confirmed by mass spectrometry. We also determined the antigenicity of these epitopes to elicit cell-mediated immunoresponse by the ELISPOT and CTL assays. Among the four peptides, Q01 and QA4 were found to bind all six SLA-I proteins with strong affinity and elicit significant activity of CTL (P < 0.01). We conclude that Q01 and QA4 peptides are novel shared epitopes that can be recognized by all six SLA-I molecules on representative CTLs.

Interplay of foot and mouth disease virus with cell-mediated and humoral immunity of host

Foot and mouth disease virus (FMDV) causes a communicable disease of cloven hoofed animals, resulting in major economic losses during disease outbreaks. Like other members of the Picornaviridae FMDV has a relatively short infectious cycle; initiation of infection and dissemination, with production of infectious virions occurs in less than a week. The components of innate immunity as well as cell-mediated and humoral immunity play a crucial role in control of FMDV. However, it has been shown in vitro using a mouse model that FMDV has evolved certain mechanisms to counteract host immune responses ensuring its survival and spread. The viral leader proteinase, L pro, deters interferon beta (IFN-β) mRNA synthesis, thus, inhibiting host cell translation. Another viral proteinase, 3C pro, disrupts host cell transcription by cleaving histone H3. A transient lymphopenia in swine as a consequence of FMDV infection has also been observed, but the mechanism involved and viral protein(s) associated with this process are not clearly understood. In this review, we have covered the interaction of FMDV with different immune cells including lymphocytes and antigen presenting cells and their consequences.

Enhanced sensitivity in detection of antiviral antibody responses using biotinylation of foot-and-mouth disease virus (FMDV) capsids

Analysis of the immune response to infection of livestock by foot-and-mouth disease virus (FMDV) is most often reported as the serum antibody response to the virus. While measurement of neutralizing antibody has been sensitive and specific, measurements of the quality of the antibody response are less robust. Determining the immunoglobulin (Ig) isotype of the serum antibody response provides a deeper understanding of the biology of the response and more sensitive methods for these assays will facilitate analyses of B cell mediated immunity. We tested the hypothesis that using the virus as the molecular probe could be achieved by adding tags to the surface of the FMDV capsid, and that would enhance sensitivity in assays for anti-FMDV antibody responses. The use of a FLAG-tagged virus in these assays failed to yield improvement whereas chemically biotinylating the virus capsid resulted in significant enhancement of the signal. Here we describe methods using biotinylated virus for measuring anti-viral antibody in serum and antibody secreting cells (ASCs) in blood that are sensitive and specific. Finally, we describe using the biotinylated virus in flow cytometry where such assays should greatly enhance the analysis of anti-virus antibody producing B cells, allowing the investigator to focus on only the FMDV specific B cells when analyzing the development of the B cell response to either infection or vaccination.

Adenovirus-vectored novel African Swine Fever Virus antigens elicit robust immune responses in swine

African Swine Fever Virus (ASFV) is a high-consequence transboundary animal pathogen that often causes hemorrhagic disease in swine with a case fatality rate close to 100%. Lack of treatment or vaccine for the disease makes it imperative that safe and efficacious vaccines are developed to safeguard the swine industry. In this study, we evaluated the immunogenicity of seven adenovirus-vectored novel ASFV antigens, namely A151R, B119L, B602L, EP402RΔPRR, B438L, K205R and A104R. Immunization of commercial swine with a cocktail of the recombinant adenoviruses formulated in adjuvant primed strong ASFV antigen-specific IgG responses that underwent rapid recall upon boost. Notably, most vaccinees mounted robust IgG responses against all the antigens in the cocktail. Most importantly and relevant to vaccine development, the induced antibodies recognized viral proteins from Georgia 2007/1 ASFV-infected cells by IFA and by western blot analysis. The recombinant adenovirus cocktail also induced ASFV-specific IFN-γ-secreting cells that were recalled upon boosting. Evaluation of local and systemic effects of the recombinant adenovirus cocktail post-priming and post-boosting in the immunized animals showed that the immunogen was well tolerated and no serious negative effects were observed. Taken together, these outcomes showed that the adenovirus-vectored novel ASFV antigen cocktail was capable of safely inducing strong antibody and IFN-γ⁺ cell responses in commercial swine. The data will be used for selection of antigens for inclusion in a multi-antigen prototype vaccine to be evaluated for protective efficacy.

Cross reactivity of immune responses to porcine reproductive and respiratory syndrome virus infection

Because porcine reproductive and respiratory syndrome virus (PRRSV) exhibits extensive genetic variation among field isolates, characterizing the extent of cross reactivity of immune responses, and most importantly cell-mediated immunity (CMI), could help in the development of broadly cross-protective vaccines. We infected 12 PRRSV-naïve pigs with PRRSV strain FL12 and determined the number of interferon (IFN)-γ secreting cells (SC) by ELISpot assay using ten type 2 and one type 1 PRRSV isolates as recall antigens. The number of IFN-γ SC was extremely variable among animals, and with exceptions, late to appear. Cross reactivity of IFN-γ SC among type 2 isolates was broad, and we found no evidence of an association between increased genetic distance among isolates and the intensity of the CMI response. Comparable to IFN-γ SC, total antibodies evaluated by indirect immunofluorescence assay (IFA) were cross reactive, however, neutralizing antibody titers could only be detected against the strain used for infection. Finally, we observed a moderate association between homologous IFN-γ SC and neutralizing antibodies.

Challenges of Generating and Maintaining Protective Vaccine-Induced Immune Responses for Foot-and-Mouth Disease Virus in Pigs

Vaccination can play a central role in the control of outbreaks of foot-and-mouth disease (FMD) by reducing both the impact of clinical disease and the extent of virus transmission between susceptible animals. Recent incursions of exotic FMD virus lineages into several East Asian countries have highlighted the difficulties of generating and maintaining an adequate immune response in vaccinated pigs. Factors that impact vaccine performance include (i) the potency, antigenic payload, and formulation of a vaccine; (ii) the antigenic match between the vaccine and the heterologous circulating field strain; and (iii) the regime (timing, frequency, and herd-level coverage) used to administer the vaccine. This review collates data from studies that have evaluated the performance of foot-and-mouth disease virus vaccines at the individual and population level in pigs and identifies research priorities that could provide new insights to improve vaccination in the future.

Systemic immune response and virus persistence after foot-and-mouth disease virus infection of naïve cattle and cattle vaccinated with a homologous adenovirus-vectored vaccine

Background

In order to investigate host factors associated with the establishment of persistent foot-and-mouth disease virus (FMDV) infection, the systemic response to vaccination and challenge was studied in 47 steers. Eighteen steers that had received a recombinant FMDV A vaccine 2 weeks earlier and 29 non-vaccinated steers were challenged by intra-nasopharyngeal deposition of FMDV A24. For up to 35 days after challenge, host factors including complete blood counts with T lymphocyte subsets, type I/III interferon (IFN) activity, neutralizing and total FMDV-specific antibody titers in serum, as well as antibody-secreting cells (in 6 non-vaccinated animals) were characterized in the context of viral infection dynamics.

Results

Vaccination generally induced a strong antibody response. There was a transient peak of FMDV-specific serum IgM in non-vaccinated animals after challenge, while IgM levels in vaccinated animals did not increase further. Both groups had a lasting increase of specific IgG and neutralizing antibody after challenge.

Substantial systemic IFN activity in non-vaccinated animals coincided with viremia, and no IFN or viremia was detected in vaccinated animals. After challenge, circulating lymphocytes decreased in non-vaccinated animals, coincident with viremia, IFN activity, and clinical disease, whereas lymphocyte and monocyte counts in vaccinated animals were unaffected by vaccination but transiently increased after challenge. The CD4⁺/CD8⁺ T cell ratio in non-vaccinated animals increased during acute infection, driven by an absolute decrease of CD8⁺ cells.

Conclusions

The incidence of FMDV persistence was 61.5 % in non-vaccinated and 54.5 % in vaccinated animals. Overall, the systemic factors examined were not associated with the FMDV carrier/non-carrier divergence; however, significant differences were identified between responses of non-vaccinated and vaccinated cattle.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-016-0838-x) contains supplementary material, which is available to authorized users.

Expanding specificity of class I restricted CD8+ T cells for viral epitopes following multiple inoculations of swine with a human adenovirus vectored foot-and-mouth disease virus (FMDV) vaccine

The immune response to the highly acute foot-and-mouth disease virus (FMDV) is routinely reported as a measure of serum antibody. However, a critical effector function of immune responses combating viral infection of mammals is the cytotoxic T lymphocyte (CTL) response mediated by virus specific CD8 expressing T cells. This immune mechanism arrests viral spread by killing virus infected cells before new, mature virus can develop. We have previously shown that infection of swine by FMDV results in a measurable CTL response and have correlated CTL killing of virus-infected cells with specific class I major histocompatibility complex (MHC) tetramer staining. We also showed that a modified replication defective human adenovirus 5 vector expressing the FMDV structural proteins (Ad5-FMDV-T vaccine) targets the induction of a CD8⁺ CTL response with a minimal humoral response. In this report, we show that the specificity of the CD8⁺ T cell response to Ad5-FMDV-T varies between cohorts of genetically identical animals. Further, we demonstrate epitope specificity of CD8⁺ T cells expands following multiple immunizations with this vaccine.

A modern approach for epitope prediction: identification of foot-and-mouth disease virus peptides binding bovine leukocyte antigen (BoLA) class I molecules

Major histocompatibility complex (MHC) class Imolecules regulate adaptive immune responses through the presentation of antigenic peptides to CD8+ T cells. Polymorphisms in the peptide binding region of class I molecules determine peptide binding affinity and stability during antigen presentation, and different antigen peptide motifs are associated with specific genetic sequences of class I molecules. Understanding bovine leukocyte antigen (BoLA), peptide-MHC class I binding specificities may facilitate development of vaccines or reagents for quantifying the adaptive immune response to intracellular pathogens, such as foot-and-mouth disease virus (FMDV). Six synthetic BoLA class I (BoLA-I) molecules were produced, and the peptide binding motif was generated for five of the six molecules using a combined approach of positional scanning combinatorial peptide libraries (PSCPLs) and neural network-based predictions (NetMHCpan). The updated NetMHCpan server was used to predict BoLA-I binding peptides within the P1 structural polyprotein sequence of FMDV (strain A24 Cruzeiro) for Bo-LA-1*01901, BoLA-2*00801, BoLA-2*01201, and BoLA-4*02401. Peptide binding affinity and stability were determined for these BoLA-I molecules using the luminescent oxygen channeling immunoassay (LOCI) and scintillation proximity assay (SPA). The functional diversity of known BoLA alleles was predicted using theMHCcluster tool, and functional predictions for peptide motifs were compared to observed data from this and prior studies. The results of these analyses showed that BoLA alleles cluster into three distinct groups with the potential to define BBoLA supertypes.^ This streamlined approach identifies potential T cell epitopes from pathogens, such as FMDV, and provides insight into T cell immunity following infection or vaccination.

Porcine reproductive and respiratory syndrome (PRRS): an immune dysregulatory pandemic

Porcine reproductive and respiratory disease syndrome (PRRS) is a viral pandemic that especially affects neonates within the “critical window” of immunological development. PRRS was recognized in 1987 and within a few years became pandemic causing an estimated yearly $600,000 economic loss in the USA with comparative losses in most other countries. The causative agent is a single-stranded, positive-sense enveloped arterivirus (PRRSV) that infects macrophages and plasmacytoid dendritic cells. Despite the discovery of PRRSV in 1991 and the publication of >2,000 articles, the control of PRRS is problematic. Despite the large volume of literature on this disease, the cellular and molecular mechanisms describing how PRRSV dysregulates the host immune system are poorly understood. We know that PRRSV suppresses innate immunity and causes abnormal B cell proliferation and repertoire development, often lymphopenia and thymic atrophy. The PRRSV genome is highly diverse, rapidly evolving but amenable to the generation of many mutants and chimeric viruses for experimental studies. PRRSV only replicates in swine which adds to the experimental difficulty since no inbred well-defined animal models are available. In this article, we summarize current knowledge and apply it toward developing a series of provocative and testable hypotheses to explain how PRRSV immunomodulates the porcine immune system with the goal of adding new perspectives on this disease.

Phenotypic and functional differentiation of porcine αβ T cells: current knowledge and available tools

Domestic pigs are considered as a valuable large animal model because of their close relation to humans in regard to anatomy, genetics and physiology. This includes their potential use as organ donors in xenotransplantation but also studies on various zoonotic infections affecting pigs and humans. Such work also requires a thorough understanding of the porcine immune system which was partially hampered in the past by restrictions on available immunological tools compared to rodent models. However, progress has been made during recent years in the study of both, the innate and the adaptive immune system of pigs. In this review we will summarize the current knowledge on porcine αβ T cells, which comprise two major lymphocyte subsets of the adaptive immune system: CD4(+) T cells with important immunoregulatory functions and CD8(+) T cells, also designated as cytolytic T cells. Aspects on their functional and phenotypic differentiation are presented. In addition, we summarize currently available tools to study these subsets which may support a more widespread use of swine as a large animal model.

Swine Leukocyte Antigen (SLA) class I allele typing of Danish swine herds and identification of commonly occurring haplotypes using sequence specific low and high resolution primers

The swine major histocompatibility complex (MHC) genomic region (SLA) is extremely polymorphic comprising high numbers of different alleles, many encoding a distinct MHC class I molecule, which binds and presents endogenous peptides to circulating T cells of the immune system. Upon recognition of such peptide-MHC complexes (pMHC) naïve T cells can become activated and respond to a given pathogen leading to its elimination and the generation of memory cells. Hence SLA plays a crucial role in maintaining overall adaptive immunologic resistance to pathogens. Knowing which SLA alleles that are commonly occurring can be of great importance in regard to future vaccine development and the establishment of immune protection in swine through broad coverage, highly specific, subunit based vaccination against viruses such as swine influenza, porcine reproductive and respiratory syndrome virus, vesicular stomatitis virus, foot-and-mouth-disease virus and others. Here we present the use of low- and high-resolution PCR-based typing methods to identify individual and commonly occurring SLA class I alleles in Danish swine. A total of 101 animals from seven different herds were tested, and by low resolution typing the top four most frequent SLA class I alleles were those of the allele groups SLA-3*04XX, SLA-1*08XX, SLA-2*02XX, and SLA-1*07XX, respectively. Customised high resolution primers were used to identify specific alleles within the above mentioned allele groups as well as within the SLA-2*05XX allele group. Our studies also suggest the most common haplotype in Danish pigs to be Lr-4.0 expressing the SLA-1*04XX, SLA-2*04XX, and SLA-3*04XX allele combination.

Identification of swine influenza virus epitopes and analysis of multiple specificities expressed by cytotoxic T cell subsets

BACKGROUND

Major histocompatibility complex (MHC) class I peptide binding and presentation are essential for antigen-specific activation of cytotoxic T lymphocytes (CTLs) and swine MHC class I molecules, also termed swine leukocyte antigens (SLA), thus play a crucial role in the process that leads to elimination of viruses such as swine influenza virus (SwIV). This study describes the identification of SLA-presented peptide epitopes that are targets for a swine CTL response, and further analyses multiple specificities expressed by SwIV activated CTL subsets.

FINDINGS

Four SwIV derived peptides were identified as T cell epitopes using fluorescent influenza:SLA tetramers. In addition, multiple CTL specificities were analyzed using peptide sequence substitutions in two of the four epitope candidates analyzed. Interestingly both conserved and substituted peptides were found to stain the CD4-CD8+ T cell subsets indicating multiple specificities.

CONCLUSIONS

This study describes a timely and cost-effective approach for viral epitope identification in livestock animals. Analysis of T cell subsets showed multiple specificities suggesting SLA-bound epitope recognition of different conformations.

Association between Genetic Polymorphism in the Swine Leukocyte Antigen-DRA Gene and Piglet Diarrhea in Three Chinese Pig Breeds

The swine leukocyte antigen (SLA)-DRA locus is noteworthy among other SLA class II loci for its limited variation and has not been investigated in depth. This study was investigated to detect polymorphisms of four exons of SLA-DRA gene and its association with piglet diarrhea in Landrace, Large White and Duroc pigs. No polymorphisms were detected in exon 3, while 2 SNPs (c.178G>A and c.211T>C), 2 SNPs (c.3093A>C and c.3104C>T) and 5 SNPs (c.4167A>G, c.4184A>G, c.4194A>G, c.4246A>G and c.4293G>A) were detected in exon 1, exon 2 and exon 4 respectively, and 1 SNP (c.4081T>C) in intron 3. Statistical results showed that genotype had significant effect on piglet diarrhea, individuals with genotype BC had a higher diarrhea score when compared with the genotypes AA, AB, AC and CC. Futhermore, genotype AC had a higher diarrhea score than the genotype CC in exon 1 (p<0.05); diarrhea scores of genotype AA and BB were higher than those of genotypes AC and CC in exon 2 (p<0.05); individuals with genotype AA had a higher diarrhea score than individuals with genotype AB and BB in exon 4 (p<0.05). Fourteen common haplotypes were founded by haplotype constructing of all SNPs in the three exons, its association with piglet diarrhea appeared that Hap2, 5, 8, 10, and 14 may be the susceptible haplotypes and Hap9 may be the resistant haplotype to piglet diarrhea. The genetic variations identified of the SLA-DRA gene may potentially be functional mutations related to piglet diarrhea.

Infection with foot-and-mouth disease virus (FMDV) induces a natural killer (NK) cell response in cattle that is lacking following vaccination

Natural killer (NK) cells play a role in innate antiviral immunity by directly lysing virus-infected cells and producing antiviral cytokines such as interferon gamma (IFN-γ). We developed a system for characterizing the bovine NK response to foot-and-mouth disease virus (FMDV), which causes a disease of cloven-hoofed animals and remains a threat to livestock industries throughout the world. IL-2 stimulation of PBMC resulted in poor killing of human K562 cells, which are often used as NK target cells, while lysis of the bovine BL3.1 cell line was readily detected. Depletion of NKp46-expressing cells revealed that 80% of the killing induced by IL-2 could be attributed to NKp46(+) cells. In order to characterize the response of NK cells against FMDV in vivo, we infected groups of cattle with three different strains of the virus (A24 Cruzeiro, O1 Manisa, O Hong Kong) and evaluated the cytolytic ability of NK cells through the course of infection. We consistently observed a transient increase in cytolysis, although there was variation in magnitude and kinetics. This increase in cytolysis remained when CD3(+) cells were removed from the preparation of lymphocytes, indicating that cytolysis was independent of MHC-T cell receptor interaction or γδ T cell activation. In contrast, animals monitored following vaccination against FMDV did not exhibit any increase in NK killing. These data suggest that NK cells play a role in the host immune response of cattle against FMDV, and contrast with the suppression of NK activity previously observed in swine infected with FMDV.

Swine Leukocyte Antigen-DQA Gene Variation and Its Association with Piglet Diarrhea in Large White, Landrace and Duroc

The swine leukocyte antigen class II molecules are possibly associated with the induction of protective immunity. The study described here was to investigate the relationship between polymorphisms in exon 2 of the swine DQA gene and piglet diarrhea. This study was carried out on 425 suckling piglets from three purebred pig strains (Large White, Landrace and Duroc). The genetic diversity of exon 2 in swine DQA was detected by PCR-SSCP and sequencing analysis, eight unique SSCP patterns (AB, BB, BC, CC, CD, BD, BE and DD) representing five specific allele (A to E) sequences were detected. Sequence analysis revealed 21 nucleotide variable sites and resulting in 12 amino acid substitutions in the populations. A moderate level polymorphism and significant deviations from Hardy-Weinberg equilibrium of the genotypes distribution were observed in the populations (p<0.01). The association analysis indicated that there was a statistically significant difference in the score of piglet diarrhea between different genotypes, individuals with genotype CC showed a lower diarrhea score than genotypes AB (0.98±0.09), BB (0.85±0.77) and BC (1.25±0.23) (p<0.05), and significantly low than genotype BE (1.19±0.19) (p<0.01), CC genotype may be a most resistance genotype for piglet diarrhea.

Advances in swine immunology help move vaccine technology forward

In veterinary animal species, vaccines are the primary tool for disease prevention, a key tool for treatment of infection, and essential for helping maintain animal welfare and productivity. Traditional vaccine development by trial-and-error has achieved many successes. However, effective vaccines that provide solid cross-protective immunity with excellent safety are still needed for many diseases. The path to development of vaccines against difficult pathogens requires recognition of uniquely evolved immunological interactions of individual animal hosts and their specific pathogens. Here, general principles that currently guide veterinary immunology and vaccinology research are reviewed, with an emphasis on examples from swine. Advances in genomics and proteomics now provide the community with powerful tools for elucidation of regulatory and effector mechanisms of protective immunity that provide new opportunities for successful translation of immunological discoveries into safe and effective vaccines.

Proteome-wide screening reveals immunodominance in the CD8 T cell response against classical swine fever virus with antigen-specificity dependent on MHC class I haplotype expression

Vaccination with live attenuated classical swine fever virus (CSFV) vaccines induces a rapid onset of protection which has been associated with virus-specific CD8 T cell IFN-γ responses. In this study, we assessed the specificity of this response, by screening a peptide library spanning the CSFV C-strain vaccine polyprotein to identify and characterise CD8 T cell epitopes. Synthetic peptides were pooled to represent each of the 12 CSFV proteins and used to stimulate PBMC from four pigs rendered immune to CSFV by C-strain vaccination and subsequently challenged with the virulent Brescia strain. Significant IFN-γ expression by CD8 T cells, assessed by flow cytometry, was induced by peptide pools representing the core, E2, NS2, NS3 and NS5A proteins. Dissection of these antigenic peptide pools indicated that, in each instance, a single discrete antigenic peptide or pair of overlapping peptides was responsible for the IFN-γ induction. Screening and titration of antigenic peptides or truncated derivatives identified the following antigenic regions: core₂₄₁₋₂₅₅ PESRKKLEKALLAWA and NS3₁₉₀₂₋₁₉₁₂ VEYSFIFLDEY, or minimal length antigenic peptides: E2₉₉₆₋₁₀₀₃ YEPRDSYF, NS2₁₂₂₃₋₁₂₃₀ STVTGIFL and NS5A₃₀₇₀₋₃₀₇₈ RVDNALLKF. The epitopes are highly conserved across CSFV strains and variable sequence divergence was observed with related pestiviruses. Characterisation of epitope-specific CD8 T cells revealed evidence of cytotoxicity, as determined by CD107a mobilisation, and a significant proportion expressed TNF-α in addition to IFN-γ. Finally, the variability in the antigen-specificity of these immunodominant CD8 T cell responses was confirmed to be associated with expression of distinct MHC class I haplotypes. Moreover, recognition of NS₁₂₂₃₋₁₂₃₀ STVTGIFL and NS3₁₉₀₂₋₁₉₁₂ VEYSFIFLDEY by a larger group of C-strain vaccinated animals showed that these peptides could be restricted by additional haplotypes. Thus the antigenic regions and epitopes identified represent attractive targets for evaluation of their vaccine potential against CSFV.

Identification of cytotoxic T lymphocyte epitopes on swine viruses: multi-epitope design for universal T cell vaccine

Classical swine fever (CSF), foot-and-mouth disease (FMD) and porcine reproductive and respiratory syndrome (PRRS) are the primary diseases affecting the pig industry globally. Vaccine induced CD8⁺ T cell-mediated immune response might be long-lived and cross-serotype and thus deserve further attention. Although large panels of synthetic overlapping peptides spanning the entire length of the polyproteins of a virus facilitate the detection of cytotoxic T lymphocyte (CTL) epitopes, it is an exceedingly costly and cumbersome approach. Alternatively, computational predictions have been proven to be of satisfactory accuracy and are easily performed. Such a method enables the systematic identification of genome-wide CTL epitopes by incorporating epitope prediction tools in analyzing large numbers of viral sequences. In this study, we have implemented an integrated bioinformatics pipeline for the identification of CTL epitopes of swine viruses including the CSF virus (CSFV), FMD virus (FMDV) and PRRS virus (PRRSV) and assembled these epitopes on a web resource to facilitate vaccine design. Identification of epitopes for cross protections to different subtypes of virus are also reported in this study and may be useful for the development of a universal vaccine against such viral infections among the swine population. The CTL epitopes identified in this study have been evaluated in silico and possibly provide more and wider protection in compared to traditional single-reference vaccine design. The web resource is free and open to all users through http://sb.nhri.org.tw/ICES.

Assessment of the phenotype and functionality of porcine CD8 T cell responses following vaccination with live attenuated classical swine fever virus (CSFV) and virulent CSFV challenge

Vaccination with live attenuated classical swine fever virus (CSFV) induces solid protection after only 5 days, which has been associated with virus-specific T cell gamma interferon (IFN-γ) responses. In this study, we employed flow cytometry to characterize T cell responses following vaccination and subsequent challenge infections with virulent CSFV. The CD3(+) CD4(-) CD8(hi) T cell population was the first and major source of CSFV-specific IFN-γ. A proportion of these cells showed evidence for cytotoxicity, as evidenced by CD107a mobilization, and coexpressed tumor necrosis factor alpha (TNF-α). To assess the durability and recall of these responses, a second experiment was conducted where vaccinated animals were challenged with virulent CSFV after 5 days and again after a further 28 days. While virus-specific CD4 T cell (CD3(+) CD4(+) CD8α(+)) responses were detected, the dominant response was again from the CD8 T cell population, with the highest numbers of these cells being detected 14 and 7 days after the primary and secondary challenges, respectively. These CD8 T cells were further characterized as CD44(hi) CD62L(-) and expressed variable levels of CD25 and CD27, indicative of a mixed effector and effector memory phenotype. The majority of virus-specific IFN-γ(+) CD8 T cells isolated at the peaks of the response after each challenge displayed CD107a on their surface, and subpopulations that coexpressed TNF-α and interleukin 2 (IL-2) were identified. While it is hoped that these data will aid the rational design and/or evaluation of next-generation marker CSFV vaccines, the novel flow cytometric panels developed should also be of value in the study of porcine T cell responses to other pathogens/vaccines.

Co-administration of flagellin augments immune responses to inactivated foot-and-mouth disease virus (FMDV) antigen

Foot-and-mouth disease virus (FMDV) is one of the most contagious animal virus known that affects livestock health and production. This study aimed to investigate the effect of flagellin, a toll-like receptor 5 agonist, on the immune responses to inactivated FMDV antigen in guinea pig model. Our results showed that the co-administration of flagellin with FMDV antigen through intradermal route induces earlier and higher anti-FMDV neutralizing antibody responses as compared to FMDV antigen alone. Both IgG1 and IgG2 antibody-isotype responses were enhanced, but the IgG1/IgG2 ratios were relatively low, indicative of TH1 type of immune activation. On live viral challenge, flagellin+FMDV immunized guinea pigs showed 70% (7 out of 10) protection rate as compared to 40% (4 out of 10) in FMDV alone immunized guinea pigs. The results demonstrate that the co-administration of flagellin augments immune responses (preferably TH1 type) and protective efficacy against FMDV in guinea pigs.

Characterization of cytotoxic T lymphocyte function after foot-and-mouth disease virus infection and vaccination

The induction of neutralizing antibodies specific for foot-and-mouth disease virus (FMDV) has been the central goal of vaccination efforts against this economically important disease of cloven-hoofed animals. Although these efforts have yielded much success, challenges remain, including little cross-serotype protection and inadequate duration of immunity. Commonly, viral infections are characterized by induction of cytotoxic T lymphocytes (CTL), yet the function of CTL in FMDV immunity is poorly defined. We developed an assay for detection of CTL specific for FMDV and reported that a modified adenovirus-vectored FMDV vaccine could induce CTL activity. This allowed us to determine whether FMDV-specific CTL responses are induced during infection and to test further whether vaccine-induced CTL could protect against challenge with FMDV. We now show the induction of antigen-specific CTL responses after infection of swine with FMDV strain A24 Cruizero. In addition, we developed a vaccination strategy that induces FMDV-specific CTL in the absence of significant neutralizing antibody. Animals vaccinated using this protocol showed delayed clinical disease and significantly suppressed viremia compared to control animals, suggesting a role for CTLs in the control of virus shedding. These results provide new insights showing induction of CTL responses to FMDV following infection or vaccination, and create the potential for improving vaccine performance by targeting cellular immunity.

Montanide ISA™ 201 adjuvanted FMD vaccine induces improved immune responses and protection in cattle

Despite significant advancements in modern vaccinology, inactivated whole virus vaccines for foot-and-mouth disease (FMD) remain the mainstay for prophylactic and emergency uses. Many efforts are currently devoted to improve the immune responses and protective efficacy of these vaccines. Adjuvants, which are often used to potentiate immune responses, provide an excellent mean to improve the efficacy of FMD vaccines. This study aimed to evaluate three oil adjuvants namely: Montanide ISA-201, ISA-206 (SEPPIC, France) and GAHOL (an in-house developed oil-adjuvant) for adjuvant potential in inactivated FMD vaccine. Groups of cattle (n=6) were immunized once intramuscularly with monovalent FMDV 'O' vaccine formulated in these adjuvants, and humoral (serum neutralizing antibody, IgG1 and IgG2) and cellular (lymphoproliferation) responses were measured. Montanide ISA-201 adjuvanted vaccine induced earlier and higher neutralizing antibody responses as compared to the two other adjuvants. All the adjuvants induced mainly serum IgG1 isotype antibody responses against FMDV. However, Montanide ISA-201 induced relatively higher IgG2 responses than the other two adjuvants. Lymphoproliferative responses to recall FMDV antigen were relatively higher with Montanide ISA-201, although not always statistically significant. On homologous FMDV challenge at 30 days post-vaccination, 100% (6/6) of the cattle immunized with Montanide-201 adjuvanted vaccine were protected, which was superior to those immunized with ISA-206 (66.6%, 4/6) or GAHOL adjuvanted vaccine (50%, 3/6). Virus replication following challenge infection, as determined by presence of the viral genome in oropharynx and non-structural protein serology, was lowest with Montanide ISA-201 adjuvant. Collectively, these results indicate that the Montanide ISA-201 adjuvanted FMD vaccine induces enhanced immune responses and protective efficacy in cattle.

Identification of peptides from foot-and-mouth disease virus structural proteins bound by class I swine leukocyte antigen (SLA) alleles, SLA-1*0401 and SLA-2*0401

Characterization of the peptide-binding specificity of swine leukocyte antigen (SLA) class I and II molecules is critical to the understanding of adaptive immune responses of swine toward infectious pathogens. Here, we describe the complete binding motif of the SLA-2*0401 molecule based on a positional scanning combinatorial peptide library approach. By combining this binding motif with data achieved by applying the NetMHCpan peptide prediction algorithm to both SLA-1*0401 and SLA-2*0401, we identified high-affinity binding peptides. A total of 727 different 9mer and 726 different 10mer peptides within the structural proteins of foot-and-mouth disease virus (FMDV), strain A24 were analyzed as candidate T-cell epitopes. Peptides predicted by the NetMHCpan were tested in ELISA for binding to the SLA-1*0401 and SLA-2*0401 major histocompatibility complex class I proteins. Four of the 10 predicted FMDV peptides bound to SLA-2*0401, whereas five of the nine predicted FMDV peptides bound to SLA-1*0401. These methods provide the characterization of T-cell epitopes in response to pathogens in more detail. The development of such approaches to analyze vaccine performance will contribute to a more accelerated improvement of livestock vaccines by virtue of identifying and focusing analysis on bona fide T-cell epitopes.

Use of replication-defective adenoviruses to develop vaccines and biotherapeutics against foot-and-mouth disease

We have developed a replication-defective human adenovirus (Ad5) vectored foot-and-mouth disease (FMD) vaccine platform that protects both swine and cattle from subsequent challenge with homologous virus after a single immunization. This Ad5-FMD vaccine has undergone testing following the requirements of the Center for Veterinary Biologics of the Animal Plant and Health Inspection Service, US Department of Agriculture, and has recently been granted a conditional license for inclusion of the vaccine in the US National Veterinary Vaccine Stockpile. In this review, we will describe the approaches we have taken to improve the potency and efficacy of this vaccine platform. Furthermore, we will discuss the development of Ad5 vector-based biotherapeutics to generate rapid protection against FMD virus prior to vaccine-induced adaptive immunity and describe the use of a combination of these approaches to stimulate both fast and long-lasting immunity.

Immune markers and correlates of protection for vaccine induced immune responses

Vaccines have been a major innovation in the history of mankind and still have the potential to address the challenges posed by chronic intracellular infections including tuberculosis, HIV and malaria which are leading causes of high morbidity and mortality across the world. Markers of an appropriate humoral response currently remain the best validated correlates of protective immunity after vaccination. Despite advancements in the field of immunology over the past few decades currently there are, however, no sufficiently validated immune correlates of vaccine induced protection against chronic infections in neither human nor veterinary medicine. Technological and conceptual advancements within cell-mediated immunology have led to a number of new immunological read-outs with the potential to emerge as correlates of vaccine induced protection. For T(H)1 type responses, antigen-specific production of interferon-gamma (IFN-γ) has been promoted as a quantitative marker of protective cell-mediated immune responses over the past couple of decades. More recently, however, evidence from several infections has pointed towards the quality of the immune response, measured through increased levels of antigen-specific polyfunctional T cells capable of producing a triad of relevant cytokines, as a better correlate of sustained protective immunity against this type of infections. Also the possibilities to measure antigen-specific cytotoxic T cells (CTL) during infection or in response to vaccination, through recombinant major histocompatibility complex (MHC) class I tetramers loaded with relevant peptides, has opened a new vista to include CTL responses in the evaluation of protective immune responses. Here, we review different immune markers and new candidates for correlates of a protective vaccine induced immune response against chronic infections and how successful they have been in defining the protective immunity in human and veterinary medicine.

Designing bovine T cell vaccines via reverse immunology

T cell responses contribute to immunity against many intracellular infections. There is, for example, strong evidence that major histocompatibility complex (MHC) class I-restricted cytotoxic T lymphocytes (CTLs) play an essential role in mediating immunity to East Coast fever (ECF), a fatal lymphoproliferative disease of cattle prevalent in sub-Saharan Africa and caused by Theileria parva. To complement the more traditional approaches to CTL antigen identification and vaccine development that we have previously undertaken we propose a use of immunoinformatics to predict CTL peptide epitopes followed by experimental verification of T cell specificity to candidate epitopes using peptide-MHC (pMHC) tetramers. This system, adapted from human and rodent studies, is in the process of being developed for cattle. Briefly, we have used an artificial neural network called NetMHCpan, which has been trained mainly on existing human, mouse, and non-human primate MHC-peptide binding data in an attempt to predict the peptide-binding specificity of bovine MHC class I molecules. Our data indicate that this algorithm needs to be further optimized by incorporation of bovine MHC-peptide binding data. When retrained, NetMHCpan may be used to predict parasite peptide epitopes by scanning the predicted T. parva proteome and known parasite CTL antigens. A range of pMHC tetramers, made "on-demand", will then be used to assay cattle that are immune to ECF or in vaccine trials to determine if CTLs of the predicted epitope specificity are present or not. Thus, pMHC tetramers can be used in one step to identify candidate CTL antigens and to map CTL epitopes. Our current research focuses on 9 different BoLA class I molecules. By expanding this repertoire to include the most common bovine MHCs, these methods could be used as generic assays to predict and measure bovine T cell immune responses to any pathogen.

Inoculation of swine with foot-and-mouth disease SAP-mutant virus induces early protection against disease

Foot-and-mouth disease virus (FMDV) leader proteinase (L(pro)) cleaves itself from the viral polyprotein and cleaves the translation initiation factor eIF4G. As a result, host cell translation is inhibited, affecting the host innate immune response. We have demonstrated that L(pro) is also associated with degradation of nuclear factor κB (NF-κB), a process that requires L(pro) nuclear localization. Additionally, we reported that disruption of a conserved protein domain within the L(pro) coding sequence, SAP mutation, prevented L(pro) nuclear retention and degradation of NF-κB, resulting in in vitro attenuation. Here we report that inoculation of swine with this SAP-mutant virus does not cause clinical signs of disease, viremia, or virus shedding even when inoculated at doses 100-fold higher than those required to cause disease with wild-type (WT) virus. Remarkably, SAP-mutant virus-inoculated animals developed a strong neutralizing antibody response and were completely protected against challenge with WT FMDV as early as 2 days postinoculation and for at least 21 days postinoculation. Early protection correlated with a distinct pattern in the serum levels of proinflammatory cytokines in comparison to the levels detected in animals inoculated with WT FMDV that developed disease. In addition, animals inoculated with the FMDV SAP mutant displayed a memory T cell response that resembled infection with WT virus. Our results suggest that L(pro) plays a pivotal role in modulating several pathways of the immune response. Furthermore, manipulation of the L(pro) coding region may serve as a viable strategy to derive live attenuated strains with potential for development as effective vaccines against foot-and-mouth disease.

Porcine major histocompatibility complex (MHC) class I molecules and analysis of their peptide-binding specificities

In all vertebrate animals, CD8⁺ cytotoxic T lymphocytes (CTLs) are controlled by major histocompatibility complex class I (MHC-I) molecules. These are highly polymorphic peptide receptors selecting and presenting endogenously derived epitopes to circulating CTLs. The polymorphism of the MHC effectively individualizes the immune response of each member of the species. We have recently developed efficient methods to generate recombinant human MHC-I (also known as human leukocyte antigen class I, HLA-I) molecules, accompanying peptide-binding assays and predictors, and HLA tetramers for specific CTL staining and manipulation. This has enabled a complete mapping of all HLA-I specificities (“the Human MHC Project”). Here, we demonstrate that these approaches can be applied to other species. We systematically transferred domains of the frequently expressed swine MHC-I molecule, SLA-1*0401, onto a HLA-I molecule (HLA-A*11:01), thereby generating recombinant human/swine chimeric MHC-I molecules as well as the intact SLA-1*0401 molecule. Biochemical peptide-binding assays and positional scanning combinatorial peptide libraries were used to analyze the peptide-binding motifs of these molecules. A pan-specific predictor of peptide–MHC-I binding, NetMHCpan, which was originally developed to cover the binding specificities of all known HLA-I molecules, was successfully used to predict the specificities of the SLA-1*0401 molecule as well as the porcine/human chimeric MHC-I molecules. These data indicate that it is possible to extend the biochemical and bioinformatics tools of the Human MHC Project to other vertebrate species.