Use of a Pan-Vbeta primer permits the amplification and sequencing of TCRbeta chains expressed by bovine T-cell clones following a single semi-nested PCR reaction

An Ad/MVA vectored Theileria parva antigen induces schizont-specific CD8+ central memory T cells and confers partial protection against a lethal challenge

The parasite Theileria parva is the causative agent of East Coast fever (ECF), one of the most serious cattle diseases in sub-Saharan Africa, and directly impacts smallholder farmers’ livelihoods. There is an efficient live-parasite vaccine, but issues with transmission of vaccine strains, need of a cold chain, and antibiotics limit its utilization. This has fostered research towards subunit vaccination. Cytotoxic T lymphocytes (CTL) are crucial in combating the infection by lysing T. parva-infected cells. Tp1 is an immunodominant CTL antigen, which induces Tp1-specific responses in 70–80% of cattle of the A18 or A18v haplotype during vaccination with the live vaccine. In this study, human adenovirus serotype 5 (HAd5) and modified vaccinia Ankara (MVA) were assessed for their ability to induce Tp1-specific immunity. Both viral vectors expressing the Tp1 antigen were inoculated in cattle by a heterologous prime-boost vaccination regimen. All 15 animals responded to Tp1 as determined by ELISpot. Of these, 14 reacted to the known Tp1 epitope, assayed by ELISpot and tetramer analyses, with CTL peaking 1-week post-MVA boost. Eleven animals developed CTL with specific cytotoxic activity towards peripheral blood mononuclear cells (PBMC) pulsed with the Tp1 epitope. Moreover, 36% of the animals with a Tp1 epitope-specific response survived a lethal challenge with T. parva 5 weeks post-MVA boost. Reduction of the parasitemia correlated with increased percentages of central memory lymphocytes in the Tp1 epitope-specific CD8⁺ populations. These results indicate that Tp1 is a promising antigen to include in a subunit vaccine and central memory cells are crucial for clearing the parasite.

A vaccine expressing parasitic proteins offers more convenient East Coast fever prophylaxis. Current vaccination for the cattle disease, caused by the parasite Theileria parva and a detriment to sub-Saharan African farmers, involves inconvenient injection with live parasites before antibiotic treatment (ITM). A collaboration led by Nicholas Svitek, of the Kenyan International Livestock Research Institute, designed a candidate to provoke cellular immune responses against the parasitic antigen Tp1—an ITM vaccine candidate. In tests on cattle, 93% created Tp1-targeting T cells, and 33% survived a lethal dose of T. parva. The East Coast fever reduction seen in animals in this research outperformed a recent study and was able to generate the same immune memory cells that ITM inspires to provide long-lasting protection. Future research might integrate more antigens with this Tp1 vaccine to provide more comprehensive protection.

CD8 T-cell responses against the immunodominant Theileria parva peptide Tp249-59 are composed of two distinct populations specific for overlapping 11-mer and 10-mer epitopes

Immunity against Theileria parva is associated with CD8 T-cell responses that exhibit immunodominance, focusing the response against limited numbers of epitopes. As candidates for inclusion in vaccines, characterization of responses against immunodominant epitopes is a key component in novel vaccine development. We have previously demonstrated that the Tp249-59 and Tp1214-224 epitopes dominate CD8 T-cell responses in BoLA-A10 and BoLA-18 MHC I homozygous animals, respectively. In this study, peptide-MHC I tetramers for these epitopes, and a subdominant BoLA-A10-restricted epitope (Tp298-106 ), were generated to facilitate accurate and rapid enumeration of epitope-specific CD8 T cells. During validation of these tetramers a substantial proportion of Tp249-59 -reactive T cells failed to bind the tetramer, suggesting that this population was heterogeneous with respect to the recognized epitope. We demonstrate that Tp250-59 represents a distinct epitope and that tetramers produced with Tp50-59 and Tp49-59 show no cross-reactivity. The Tp249-59 and Tp250-59 epitopes use different serine residues as the N-terminal anchor for binding to the presenting MHC I molecule. Molecular dynamic modelling predicts that the two peptide-MHC I complexes adopt structurally different conformations and Tcell receptor β sequence analysis showed that Tp249-59 and Tp250-59 are recognized by non-overlapping T-cell receptor repertoires. Together these data demonstrate that although differing by only a single residue, Tp249-59 and Tp250-59 epitopes form distinct ligands for T-cell receptor recognition. Tetramer analysis of T. parva-specific CD8 T-cell lines confirmed the immunodominance of Tp1214-224 in BoLA-A18 animals and showed in BoLA-A10 animals that the Tp249-59 epitope response was generally more dominant than the Tp250-59 response and confirmed that the Tp298-106 response was subdominant.

Cattle NK Cell Heterogeneity and the Influence of MHC Class I

Primate and rodent NK cells form highly heterogeneous lymphocyte populations owing to the differential expression of germline-encoded receptors. Many of these receptors are polymorphic and recognize equally polymorphic determinants of MHC class I. This diversity can lead to individuals carrying NK cells with different specificities. Cattle have an unusually diverse repertoire of NK cell receptor genes predicted to encode receptors that recognize MHC class I. To begin to examine whether this genetic diversity leads to a diverse NK cell population, we isolated peripheral NK cells from cattle with different MHC homozygous genotypes. Cytokine stimulation differentially influenced the transcription of five receptors at the cell population level. Using dilution cultures, we found that a further seven receptors were differentially transcribed, including five predicted to recognize MHC class I. Moreover, there was a statistically significant reduction in killer cell lectin-like receptor mRNA expression between cultures with different CD2 phenotypes and from animals with different MHC class I haplotypes. This finding confirms that cattle NK cells are a heterogeneous population and reveals that the receptors creating this diversity are influenced by the MHC. The importance of this heterogeneity will become clear as we learn more about the role of NK cells in cattle disease resistance and vaccination.

Escape from CD8+ T cell response by natural variants of an immunodominant epitope from Theileria parva is predominantly due to loss of TCR recognition

Polymorphism of immunodominant CD8(+) T cell epitopes can facilitate escape from immune recognition of pathogens, leading to strain-specific immunity. In this study, we examined the TCR β-chain (TRB) diversity of the CD8(+) T cell responses of cattle against two immunodominant epitopes from Theileria parva (Tp1(214-224) and Tp2(49-59)) and investigated the role of TCR recognition and MHC binding in determining differential recognition of a series of natural variants of the highly polymorphic Tp2(49-59) epitope by CD8(+) T cell clones of defined TRB genotype. Our results show that both Tp1(214-224) and Tp2(49-59) elicited CD8(+) T cell responses using diverse TRB repertoires that showed a high level of stability following repeated pathogenic challenge over a 3-y period. Analysis of single-alanine substituted versions of the Tp2(49-59) peptide demonstrated that Tp2(49-59)-specific clonotypes had a broad range of fine specificities for the epitope. Despite this diversity, all natural variants exhibited partial or total escape from immune recognition, which was predominantly due to abrogation of TCR recognition, with mutation resulting in loss of the lysine residue at P8, playing a particularly dominant role in escape. The levels of heterozygosity in individual Tp2(49-59) residues correlated closely with loss of immune recognition, suggesting that immune selection has contributed to epitope polymorphism.

Genomic analysis reveals extensive gene duplication within the bovine TRB locus

Background

Diverse TR and IG repertoires are generated by V(D)J somatic recombination. Genomic studies have been pivotal in cataloguing the V, D, J and C genes present in the various TR/IG loci and describing how duplication events have expanded the number of these genes. Such studies have also provided insights into the evolution of these loci and the complex mechanisms that regulate TR/IG expression. In this study we analyze the sequence of the third bovine genome assembly to characterize the germline repertoire of bovine TRB genes and compare the organization, evolution and regulatory structure of the bovine TRB locus with that of humans and mice.

Results

The TRB locus in the third bovine genome assembly is distributed over 5 scaffolds, extending to ~730 Kb. The available sequence contains 134 TRBV genes, assigned to 24 subgroups, and 3 clusters of DJC genes, each comprising a single TRBD gene, 5–7 TRBJ genes and a single TRBC gene. Seventy-nine of the TRBV genes are predicted to be functional. Comparison with the human and murine TRB loci shows that the gene order, as well as the sequences of non-coding elements that regulate TRB expression, are highly conserved in the bovine. Dot-plot analyses demonstrate that expansion of the genomic TRBV repertoire has occurred via a complex and extensive series of duplications, predominantly involving DNA blocks containing multiple genes. These duplication events have resulted in massive expansion of several TRBV subgroups, most notably TRBV6, 9 and 21 which contain 40, 35 and 16 members respectively. Similarly, duplication has lead to the generation of a third DJC cluster. Analyses of cDNA data confirms the diversity of the TRBV genes and, in addition, identifies a substantial number of TRBV genes, predominantly from the larger subgroups, which are still absent from the genome assembly. The observed gene duplication within the bovine TRB locus has created a repertoire of phylogenetically diverse functional TRBV genes, which is substantially larger than that described for humans and mice.

Conclusion

The analyses completed in this study reveal that, although the gene content and organization of the bovine TRB locus are broadly similar to that of humans and mice, multiple duplication events have led to a marked expansion in the number of TRB genes. Similar expansions in other ruminant TR loci suggest strong evolutionary pressures in this lineage have selected for the development of enlarged sets of TR genes that can contribute to diverse TR repertoires.