Polymorphism and peptide-binding specificities of porcine major histocompatibility complex (MHC) class I molecules

Identification of potential SLA-I-specific T-cell epitopes within the structural proteins of porcine deltacoronavirus

Porcine deltacoronavirus (PDCoV) is an emerging swine enteropathogenic coronavirus that mainly threatens newborn piglets and poses a potential broad cross-species transmission risk. The antigenic epitopes of PDCoV are currently unidentified, and no information about T cell epitopes is available. Here, T-cell epitopes of PDCoV structural proteins were predicted using computational methods. 17 epitope peptides were synthesized and then screened using ELIspot, intracellular cytokine staining (ICS), and RT-qPCR detection of IFN-γ mRNA to evaluate their ability to elicit interferon-gamma (IFN-γ) responses in peripheral blood mononuclear cells (PBMCs) from PDCoV-challenged pigs. Five peptides (M1, M2, M3, N6, and S4) elicited high levels of IFN-γ and were investigated further as potential T-cell epitope candidates. All five peptides were cytotoxic T lymphocyte (CTL) epitopes, and two peptides (M3, N6) were recognized simultaneously by CD8 + and CD4 + T cells. A multi-epitope peptide combining the five epitopes (designated "5T") was synthesized and its immune response and protection efficacy was evaluated in a piglet model. ELISpot assay results indicated that 5T induces robust epitope-specific cellular immune responses. Four epitopes (M1, M2, N6, S4) elicited IFN-γ responses in 5T-vaccinated piglets. No obvious protection efficacy was detected in piglets vaccinated with 5T alone. Our results provide valuable information concerning PDCoV-related antigenic epitopes and will be useful in the design of epitope-based vaccines.

Using a novel structure/function approach to select diverse swine major histocompatibility complex 1 alleles to predict epitopes for vaccine development

MOTIVATION

Swine leukocyte antigens (SLAs) (i.e. swine major histocompatibility complex proteins) conduct a fundamental role in swine immunity. To generate a protective vaccine across an outbred species, such as pigs, it is critical that epitopes that bind to diverse SLA alleles are used in the vaccine development process. We introduced a new strategy for epitope prediction.

RESULTS

We employed molecular dynamics simulation to identify key amino acids for interactions with epitopes. We developed an algorithm wherein each SLA-1 is compared to a crystalized reference allele with unique weighting for non-conserved amino acids based on R group and position. We then performed homology modeling and electrostatic contact mapping to visualize how relatively small changes in sequences impacted the charge distribution in the binding site. We selected eight diverse SLA-1 alleles and performed homology modeling followed, by protein-peptide docking and binding affinity analyses, to identify porcine reproductive and respiratory syndrome virus matrix protein epitopes that bind with high affinity to these alleles. We also performed docking analysis on the epitopes identified as strong binders using NetMHCpan 4.1. Epitopes predicted to bind to our eight SLA-1 alleles had equivalent or higher energetic interactions than those predicted to bind to the NetMHCpan 4.1 allele repertoire. This approach of selecting diverse SLA-1 alleles, followed by homology modeling, and docking simulations, can be used as a novel strategy for epitope prediction that complements other available tools and is especially useful when available tools do not offer a prediction for SLAs/major histocompatibility complex.

AVAILABILITY AND IMPLEMENTATION

The data underlying this article are available in the online Supplementary Material.

Newly identified variability of the antigen binding site coding sequences of the equine major histocompatibility complex class I and class II genes

The major histocompatibility complex (MHC) with its class I and II genes plays a crucial role in the immune response to pathogens by presenting oligopeptide antigens to various immune response effector cells. In order to counteract the vast variability of infectious agents, MHC class I and II genes usually retain high levels of SNPs mainly concentrated in the exons encoding the antigen binding sites. The aim of the study was to reveal new variability of selected MHC genes with a special focus on MHC class I physical haplotypes. Long-range NGS to was used to identify exon 2-exon 3 alleles in three genetically distinct horse breeds. A total of 116 allelic variants were found in the MHC class I genes Eqca-1, Eqca-2, Eqca-7 and Eqca-Ψ, 112 of which were novel. The MHC class II DRA locus was confirmed to comprise five exon 2 alleles, and no new sequences were observed. Additional variability in terms of 15 novel exon 2 alleles was identified in the DQA1 locus. Extensive overall variability across the entire MHC region was confirmed by an analysis of MHC-linked microsatellite loci. Both diversifying and purifying selection were detected within the MHC class I and II loci analyzed.

Comparative Analysis of SLA-1, SLA-2, and DQB1 Genetic Diversity in Locally-Adapted Kenyan Pigs and Their Wild Relatives, Warthogs

Swine leukocyte antigen (SLA) plays a central role in controlling the immune response by discriminating self and foreign antigens and initiating an immune response. Studies on SLA polymorphism have demonstrated associations between SLA allelic variants, immune response, and disease resistance. The SLA polymorphism is due to host-pathogen co-evolution resulting in improved adaptation to diverse environments making SLA a crucial genomic region for comparative diversity studies. Although locally-adapted African pigs have small body sizes, they possess increased resilience under harsh environmental conditions and robust immune systems with reported tolerance to some diseases, including African swine fever. However, data on the SLA diversity in these pigs are not available. We characterized the SLA of unrelated locally-adapted domestic pigs from Homa Bay, Kenya, alongside exotic pigs and warthogs. We undertook SLA comparative diversity of the functionally expressed SLA class I (SLA-1, SLA-2) and II (DQB1) repertoires in these three suids using the reverse transcription polymerase chain reaction (RT-PCR) sequence-based typing (SBT) method. Our data revealed higher genetic diversity in the locally-adapted pigs and warthogs compared to the exotic pigs. The nucleotide substitution rates were higher in the peptide-binding regions of the SLA-1, SLA-2, and DQB1 loci, indicative of adaptive evolution. We obtained high allele frequencies in the three SLA loci, including some breed-specific private alleles, which could guide breeders to increase their frequency through selection if confirmed to be associated with enhanced resilience. Our study contributes to the growing body of knowledge on genetic diversity in free-ranging animal populations in their natural environment, availing the first DQB1 gene data from locally-adapted Kenyan pigs.

Identification of potential SLA-I-restricted CTL epitopes within the M protein of porcine reproductive and respiratory syndrome virus

CD8+ cytotoxic T lymphocytes (CTLs), are essential for clearance of porcine reproductive and respiratory syndrome virus (PRRSV) infection and regulation of host immune responses. Identification of SLA I-restricted CD8+ CTL epitopes would facilitate PRRSV vaccine development. Here, cells isolated from peripheral blood mononuclear cells (PBMCs) of PRRSV-immunized Large White pigs (JXA1-R strain) were screened for immunodominant PRRSV-2 M protein T cell epitopes via ELISPOT assay. Of nine immunodominant epitopes detected, eight elicited significant IFN-γ secretion responses that varied among individual pigs and according to epitope. To predict which epitopes harbored potential CTL epitopes, swine leukocyte antigen (SLA) class I genes of Large White pigs were cloned and sequenced, yielding fourteen distinct SLA class I gene sequences. Based on ELISPOT and SLA genotyping results, SLA-restricted binding of the fourteen predicted class I proteins to peptides derived from the eight immunodominant epitopes were predicted in-silico. After evaluation of 42 pET-peptide-SLA-I-β2m complexes containing predicted restricted peptides, extracellular SLA class I domains and β2m, ELISA testing of 33 peptide-SLA-I-β2m complexes detected four complexed peptides. These four peptides were evaluated using in vitro complex refolding assays that confirmed that M_2-5 and M_6-1 peptides each formed complexes with SLA-2*0502 and sβ2m, while M_9-1 formed a complex with SLA-2*1201 and sβ2m. ELISPOT results confirmed these three 9-mer potential CTL epitopes efficiently stimulated IFN-γ secretion when presented by SLA class I molecules specified here. This study describes effective CTL epitope identification methods for use in future investigations of swine cellular immunity toward T cell-based vaccine development.

Peptidomes and Structures Illustrate Two Distinguishing Mechanisms of Alternating the Peptide Plasticity Caused by Swine MHC Class I Micropolymorphism

The micropolymorphism of major histocompatibility complex class I (MHC-I) can greatly alter the plasticity of peptide presentation, but elucidating the underlying mechanism remains a challenge. Here we investigated the impact of the micropolymorphism on peptide presentation of swine MHC-I (termed swine leukocyte antigen class I, SLA-I) molecules via immunopeptidomes that were determined by our newly developed random peptide library combined with the mass spectrometry (MS) de novo sequencing method (termed RPLD–MS) and the corresponding crystal structures. The immunopeptidomes of SLA-1*04:01, SLA-1*13:01, and their mutants showed that mutations of residues 156 and 99 could expand and narrow the ranges of peptides presented by SLA-I molecules, respectively. R156A mutation of SLA-1*04:01 altered the charge properties and enlarged the volume size of pocket D, which eliminated the harsh restriction to accommodate the third (P3) anchor residue of the peptide and expanded the peptide binding scope. Compared with 99^Tyr of SLA-1*0401, 99^Phe of SLA-1*13:01 could not form a conservative hydrogen bond with the backbone of the P3 residues, leading to fewer changes in the pocket properties but a significant decrease in quantitative of immunopeptidomes. This absent force could be compensated by the salt bridge formed by P1-E and 170^Arg. These data illustrate two distinguishing manners that show how micropolymorphism alters the peptide-binding plasticity of SLA-I alleles, verifying the sensitivity and accuracy of the RPLD-MS method for determining the peptide binding characteristics of MHC-I in vitro and helping to more accurately predict and identify MHC-I restricted epitopes.

Illumination of PRRSV Cytotoxic T Lymphocyte Epitopes by the Three-Dimensional Structure and Peptidome of Swine Lymphocyte Antigen Class I (SLA-I)

To investigate CTL epitope applications in swine, SLA-1*1502-restricted peptide epitopes matching porcine reproductive and respiratory syndrome virus (PRRSV) strains were explored by crystallography, biochemistry, and the specific pathogen-free (SPF) swine experiments. First, nine predicted PRRSV peptides were tested by assembly of the peptide-SLA-1*1502 (pSLA-1*1502) complexes, and the crystal structure of the SLA-1*1502 complex with one peptide (NSP9-TMP9) was determined. The NSP9-TMP9 peptide conformation presented by pSLA-1*1502 is different from that of the peptides presented by the known pSLA-1*0401 and pSLA-3*hs0202 complexes. Two consecutive Pro residues make the turn between P3 and P4 of NSP9-TMP9 much sharper. The D pocket of pSLA-1*1502 is unique and is important for peptide binding. Next, the potential SLA-1*1502-restricted peptide epitopes matching four typical genetic PRRSV strains were identified based on the peptide-binding motif of SLA-1*1502 determined by structural analysis and alanine scanning of the NSP9-TMP9 peptide. The tetrameric complex of SLA-1*1502 and NSP9-TMP9 was constructed and examined. Finally, taking NSP9-TMP9 as an example, the CTL immunogenicity of the identified PRRSV peptide epitope was evaluated. The SPF swine expressing the SLA-1*1502 alleles were divided into three groups: modified live vaccine (MLV), MLV+NSP9-TMP9, and the blank control group. NSP9-TMP9 was determined as a PRRSV CTL epitope with strong immunogenicity by flow cytometry and IFN-γ expression. Our study developed an integrated approach to identify SLA-I-restricted CTL epitopes from various important viruses and is helpful in designing and applying effective peptide-based vaccines for swine.

Importance of the Major Histocompatibility Complex (Swine Leukocyte Antigen) in Swine Health and Biomedical Research

In pigs, the major histocompatibility complex (MHC), or swine leukocyte antigen (SLA) complex, maps to Sus scrofa chromosome 7. It consists of three regions, the class I and class III regions mapping to 7p1.1 and the class II region mapping to 7q1.1. The swine MHC is divided by the centromere, which is unique among mammals studied to date. The SLA complexspans between 2.4 and 2.7 Mb, depending on haplotype, and encodes approximately 150 loci, with at least 120 genes predicted to be functional. Here we update the whole SLA complex based on the Sscrofa11.1 build and annotate the organization for all recognized SLA genes and their allelic sequences. We present SLA nomenclature and typing methods and discuss the expression of SLA proteins, as well as their role in antigen presentation and immune, disease, and vaccine responses. Finally, we explore the role of SLA genes in transplantation and xenotransplantation and their importance in swine biomedical models.

Molecular genetic characterization and haplotype diversity of swine leukocyte antigen in Chinese Rongshui miniature pigs

The Rongshui miniature pig is an important model animal for studying livestock disease prevention and control in China. The highly polymorphic swine leukocyte antigen (SLA) has been the focus of considerable interest because of the strong, reproducible associations between particular SLA haplotypes and infectious diseases. In this study, we identified 42 alleles at eight polymorphic SLA loci (SLA-1, SLA-3, SLA-2, SLA-6, DRA, DRB1, DQA, and DQB1) representing seven class I and six class II haplotypes using reverse transcription-polymerase chain reaction (RT-PCR) sequence-based typing and PCR-sequence specific primers in Rongshui miniature pigs. The official names were designated by the SLA Nomenclature Committee of the International Society for Animal Genetics. Seven class I haplotypes, Hp-5b.0, 86.0, 87.0, 88.0, 89.0, 90.0 and 91.0, and four class II haplotypes, Hp-0.18b, 0.19c, 0.41 and 0.47, had not previously been reported in other pig breeds. We also comprehensively analyzed the molecular genetic characterization and phylogenies of the identified alleles and the SLA haplotype diversity in Rongshui miniature pigs. SLA-1 and SLA-6 genes were under positive selection, while SLA-2 was under neutral selection, and the other five genes were under purifying selection. The highly polymorphic new alleles may be derived by nucleotide mutations, insertions and deletions, and fragment recombination, and alleles segregated based on sequence differences and peptide-binding motifs, rather than on pig breed. SLA haplotype diversity was generated by allele/gene conversion and recombination. These results will be helpful for elucidating the molecular genetic mechanisms influencing differential disease resistance among pigs with different SLA haplotypes.

Distribution of ancient α1 and α2 domain lineages between two classical MHC class I genes and their alleles in grass carp

Major histocompatibility complex (MHC) class I molecules play a crucial role in the immune response by binding and presenting pathogen-derived peptides to specific CD8⁺ T cells. From cDNA of 20 individuals of wild grass carp (Ctenopharyngodon idellus), we could amplify one or two alleles each of classical MHC class I genes Ctid-UAA and Ctid-UBA. In total, 27 and 22 unique alleles of Ctid-UAA and Ctid-UBA were found. The leader, α1, transmembrane and cytoplasmic regions distinguish between Ctid-UAA and Ctid-UBA, and their encoded α1 domain sequences belong to the ancient lineages α1-V and α1-II, respectively, which separated several hundred million years ago. However, Ctid-UAA and Ctid-UBA share allelic lineage variation in their α2 and α3 sequences, in a pattern suggestive of past interlocus recombination events that transferred α2+α3 fragments. The allelic Ctid-UAA and Ctid-UBA variation involves ancient variation between domain lineages α2-I and α2-II, which in the present study was dated back to before the ancestral separation of teleost fish and spotted gar (> 300 million years ago). This is the first report with compelling evidence that recombination events combining different ancient α1 and α2 domain lineages had a major impact on the allelic variation of two different classical MHC class I genes within the same species.

Polymorphism of duck MHC class molecules

Major histocompatibility complex class I (MHC I) molecules are critically involved in defense against pathogens, and their high polymorphism is advantageous to a range of immune responses, especially in duck displaying biased expression of one MHC I gene. Here, we examined MHC I polymorphism in two duck (Anas platyrhynchos) breeds from China: Shaoxing (SX) and Jinding (JD). Twenty-seven unique UAA alleles identified from the MHC I genes of these breeds were analyzed concerning amino acid composition, homology, and phylogenetic relationships. Based on amino acid sequence homology, allelic groups of Anas platyrhynchos MHC I (Anpl-MHC I) were established and their distribution was analyzed. Then, highly variable sites (HVSs) in peptide-binding domains (PBD) were estimated and located in the three-dimensional structure of Anpl-MHC I. The UAA alleles identified showed high polymorphism, based on full-length sequence homology. By adding the alleles found here to known Anpl-MHC I genes from domestic ducks, they could be divided into 17 groups and four novel groups were revealed for SX and JD ducks. The UAA alleles of the two breeds were not divergent from the MHC I of other duck breeds, and HVSs were mostly located in the peptide-binding groove (PBG), suggesting that they might determine peptide-binding characteristics and subsequently influence peptide presentation and recognition. The results from the present study enrich Anpl-MHC I polymorphism data and clarify the distribution of alleles with different peptide-binding specificities, which might also accelerate effective vaccine development and help control various infections in ducks.

Analysis of the affinity of influenza A virus protein epitopes for swine MHC I by a modified in vitro refolding method indicated cross-reactivity between swine and human MHC I specificities

In vitro refolding assays can be used to investigate the affinity and stability of the binding of epitope peptides to major histocompatibility complex (MHC) class I molecules, which are key factors in the presentation of peptides to cytotoxic T lymphocytes (CTLs). The recognition of peptide epitopes by CTLs is crucial for protection against influenza A virus (IAV) infection. The peptide-binding motif of the swine SLA-3*hs0202 molecule has been previously reported and partly overlaps with the binding motif of the most abundant human MHC allele, HLA-A*0201. In this study, we screened all the protein sequences of the swine-origin epidemic IAV strain A/Beijing/01/2009 (H1N1), and a total of 73 9-mer epitope peptides were predicted to fit the consensus motif of the swine SLA-3*hs0202 or HLA-A*0201 molecule. Then, 14 peptides were selected, and their affinities to SLA-3*hs0202 were tested by a modified in vitro refolding assay. Our results show that ten epitopes could tolerate gel filtration, indicating that these epitopes formed stable or partly stable complexes with SLA-3*hs0202. Eight out of the ten epitopes have been previously reported as HLA-A2-restricted epitopes, which implied cross-reactivity between swine and human MHC I specificities. Furthermore, the modified mini-system refolding method could be applied for the screening of peptides because the refolding efficiency remained almost unchanged with the positive peptide (HA-KMN9) subjected to size-exclusion chromatography and Resource Q anion-exchange chromatography. The results presented here provide new insight into the development of epitope-based vaccines to control IAV and increase our understanding of swine molecular immunology.

Major histocompatibility complex I of swine respiratory cells presents conserved regions of influenza proteins

Influenza A virus in swine (IAV-S) is a prevalent respiratory pathogen in pigs that has deleterious consequences to animal and human health. Pigs represent an important reservoir for influenza and potential mixing vessel for novel gene reassortments. Despite the central role of pigs in recent influenza outbreaks, much remains unknown about the impact of swine immunity on IAV-S transmission, pathogenesis, and evolution. An incomplete understanding of interactions between the porcine immune system and IAV-S has hindered development of new diagnostic tools and vaccines. In order to address this gap in knowledge, we identified swine leukocyte antigen (SLA) restricted IAV-S peptides presented by porcine airway epithelial cells using an immunoproteomics approach. The majority of MHC-associated peptides belonged to matrix 1, nucleoprotein and nonstructural 1 proteins. Future investigation of the potential cross-reactive nature of these peptides is needed to confirm antigen recognition by cytotoxic T lymphocytes and their utility as vaccine candidates.