Genes encoding two <i>Theileria parva</i> antigens recognized by CD8⁺ T-cells exhibit sequence diversity in South Sudanese cattle populations but the majority of alleles are similar to the Muguga component of the live vaccine cocktail

East Coast fever (ECF), caused by Theileria parva infection, is a frequently fatal disease of cattle in eastern, central and southern Africa, and an emerging disease in South Sudan. Immunization using the infection and treatment method (ITM) is increasingly being used for control in countries affected by ECF, but not yet in South Sudan. It has been reported that CD8+ T-cell lymphocytes specific for parasitized cells play a central role in the immunity induced by ITM and a number of T. parva antigens recognized by parasite-specific CD8+ T-cells have been identified. In this study we determined the sequence diversity among two of these antigens, Tp1 and Tp2, which are under evaluation as candidates for inclusion in a sub-unit vaccine. T. parva samples (n = 81) obtained from cattle in four geographical regions of South Sudan were studied for sequence polymorphism in partial sequences of the Tp1 and Tp2 genes. Eight positions (1.97%) in Tp1 and 78 positions (15.48%) in Tp2 were shown to be polymorphic, giving rise to four and 14 antigen variants in Tp1 and Tp2, respectively. The overall nucleotide diversity in the Tp1 and Tp2 genes was π = 1.65% and π = 4.76%, respectively. The parasites were sampled from regions approximately 300 km apart, but there was limited evidence for genetic differentiation between populations. Analyses of the sequences revealed limited numbers of amino acid polymorphisms both overall and in residues within the mapped CD8+ T-cell epitopes. Although novel epitopes were identified in the samples from South Sudan, a large number of the samples harboured several epitopes in both antigens that were similar to those in the T. parva Muguga reference stock, which is a key component in the widely used live vaccine cocktail.东海岸热（East Coast fever, ECF）是由泰勒虫（Theileria parva）感染引发的牛类高致死性疾病，在非洲东部、中部与南部地区广泛流行，同时在南苏丹呈现为新兴疫病。目前，感染与治疗法（infection and treatment method, ITM）免疫已在受ECF影响的国家中愈发广泛地用于疫病防控，但南苏丹尚未应用该方案。已有研究证实，针对感染宿主细胞的CD8+ T淋巴细胞（CD8+ T-cell lymphocytes）在感染与治疗法诱导的免疫反应中发挥核心作用，目前已鉴定出多种可被寄生虫特异性CD8+ T细胞识别的泰勒虫抗原。本研究针对两款候选亚单位疫苗（sub-unit vaccine）靶点抗原Tp1与Tp2的序列多样性展开分析。我们对取自南苏丹四个地理区域牛只的81份泰勒虫样本（n = 81）进行研究，分析Tp1与Tp2基因部分序列的多态性。结果显示，Tp1基因内存在8个多态性位点（占比1.97%），Tp2基因内存在78个多态性位点（占比15.48%），分别衍生出4种和14种抗原变异体。Tp1与Tp2基因的整体核苷酸多样性（nucleotide diversity）分别为π=1.65%和π=4.76%。本次采样的寄生虫分布区域间隔约300公里，但种群间的遗传分化证据有限。序列分析表明，无论是整体水平还是已定位的CD8+ T细胞表位（CD8+ T-cell epitopes）区域内，氨基酸多态性位点的数量均较少。尽管在南苏丹的样本中鉴定出了新型表位，但大量样本的两种抗原中均包含多个与广泛使用的活疫苗混合物（live vaccine cocktail）核心组分——泰勒虫Muguga参考株（Muguga reference stock）表位相似的表位。