Characterisation of loci that encode immunoprotective antigens of Eimeria maxima identified through genetic linkage analyses

原生艾美耳球虫是普遍存在的寄生虫，它们以宿主特有的方式感染所有牲畜（即感染鸡的7种原虫不寄生于其他宿主）。许多种类的牲畜都暴露于艾美耳虫感染，最显著的是集约化饲养的家禽，如鸡和火鸡。在缺乏有效控制的情况下，感染可导致球虫病，其特征是嗜睡、表现不佳和死亡率高达50%。目前的控制主要是基于药物的使用（自2000年以来，每年在英国销售超过230吨的抗球虫化合物），但由于耐药性的上升和对食物链污染的担忧，这种控制已经受到损害。其直接后果是立法变得越来越严格，大大减少了可用化学品的数量。主要的替代控制策略是通过接触活寄生虫感染进行疫苗接种。不幸的是，尽管球虫病估计成本的98.1%是由肉鸡部门造成的，但与生产复杂的寄生虫活疫苗（包括多达8种寄生虫品系）相关的成本，在很大程度上限制了疫苗接种在饲养员和蛋鸡部门的使用。需要一种有效的、经济上可行的控制艾美耳亚种的方法来改善家禽生产和福利。自然感染艾美耳球虫引起宿主强烈的保护性免疫反应。获得刺激保护性免疫的寄生虫成分将为开发具有保护性的亚基疫苗或重组疫苗提供合理的基础。然而，尽管已经对许多蛋白质进行了大量的试验，但没有一个提供了成功的前景，关键的保护性抗原尚未被发现。这些抗原的鉴定以前一直受到阻碍，因为我们无法区分刺激免疫反应的抗原，但对那些刺激保护性免疫反应的抗原却没有提供保护。利用两种不同的最大艾美耳球虫菌株作为杂交群体的亲本，一种经典的遗传作图策略已经被开发出来，以确定被菌株特异性免疫选择消除的寄生虫基因组区域。通过这种方式，我们将研究重点放在了大肠杆菌基因组的三个区域上，这些区域预计代表了6000 - 8000个基因库中的大约80个基因。在该策略的下一阶段，我们将对大马大肠杆菌基因组的已识别区域进行测序，并预测它们所代表的基因。候选基因最有可能编码保护性抗原，根据标准选择，包括亲本菌株之间的多态性和刺激已知由艾美耳虫感染在宿主中刺激的免疫机制的能力，将用于疫苗接种试验。

The protozoan Eimeria species are ubiquitous parasites that infect all livestock in a host-specific manner (i.e. the seven species that infect the chicken do not parasitise other hosts). Many species of livestock are exposed to eimerian infection, most notably intensively-reared poultry such as chickens and turkeys. In the absence of effective control, infection can result in the disease coccidiosis, characterised by lethargy, poor performance and mortality rates as high as 50%. Current control is based primarily on the use of medication (more than 230 tonnes of coccidiostatic compounds have been sold in the UK every year since 2000), but this has been compromised by the rise of drug resistance and concerns about food chain contamination. As a direct consequence legislation is becoming increasingly restrictive, dramatically reducing the number of chemicals available. The leading alternative control strategy is vaccination by exposure to infection with live parasites. Unfortunately, the costs associated with producing complex live parasite vaccines, comprising as many as eight parasite lines, has mostly limited the use of vaccination to the breeder and egg-laying sectors even though 98.1% of the estimated cost of coccidiosis has been attributed to the broiler sector. An effective, economically viable means of controlling the Eimeria species is required to improve poultry production and welfare. Natural infection with parasites of the Eimeria species induces a strong protective immune response in the host. Access to the components of the parasite that stimulate protective immunity will provide a rational basis for the development of a subunit or recombinant vaccine that is protective. However, whilst numerous trials with many proteins have been undertaken not one has offered the prospect of success and the key protective antigens have yet to be discovered. The identification of these antigens has previously been hampered by our inability to differentiate antigens that stimulate an immune response but provide no protection from those that stimulate a protective immune response. A classical genetic mapping strategy that utilises two distinct strains of Eimeria maxima as the parents of a hybrid population has been developed to identify regions of the parasite genome eliminated by strain-specific immune selection. In this manner we have focused the search for genes that encode protective antigens on just three regions of the E. maxima genome, predicted to represent approximately 80 genes from a total pool of 6,000-8,000. In the forthcoming phases of this strategy we will sequence the identified regions of the E. maxima genome and predict the genes that they represent. The candidate genes most likely to encode protective antigens, selected based upon criteria including polymorphism between the parental strains and ability to stimulate immune mechanisms known to be stimulated in the host by eimerian infection will be used in vaccination trials.