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

原生动物艾美球虫属是普遍存在的寄生虫，以宿主特异性方式感染所有牲畜（即感染鸡的七种物种不寄生其他宿主）。许多种类的牲畜都暴露于艾美耳球虫感染，最明显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，感染可导致疾病球虫病，其特征是嗜睡，表现不佳，死亡率高达50%。目前的控制主要基于药物的使用（自2000年以来，英国每年销售超过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，这一控制受到了影响。其直接后果是，立法的限制性越来越强，大大减少了可获得的化学品的数量。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，与生产复杂的活寄生虫疫苗相关的成本，包括多达八个寄生虫系，主要限制了对育种和产蛋部门的疫苗接种，即使98.1%的估计球虫病成本已归因于肉鸡部门。需要一种有效的、经济上可行的控制艾美耳球虫的方法来改善家禽的生产和福利。自然感染艾美耳球虫属寄生虫可在宿主体内诱导强烈的保护性免疫反应。获得激发保护性免疫的寄生虫成分将为开发具有保护性的亚单位或重组疫苗提供合理的基础。然而，尽管已经进行了许多蛋白质的试验，但没有一个提供了成功的前景，并且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应答但不提供保护的抗原与刺激保护性免疫应答的抗原。一个经典的遗传作图策略，利用两个不同的株的巨型艾美耳球虫作为杂交种群的父母已经开发出识别区域的寄生虫基因组消除株特异性免疫选择。通过这种方式，我们将寻找编码保护性抗原的基因集中在E.最大基因组，预测代表来自6，000 - 8，000个总库的大约80个基因。在该策略的下一阶段，我们将对E. maxima基因组并预测它们所代表的基因。最有可能编码保护性抗原的候选基因将用于疫苗接种试验，这些候选基因的选择基于包括亲本菌株之间的多态性和刺激已知在宿主中由艾美耳球虫感染刺激的免疫机制的能力的标准。

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.