Anticoccidial vaccine development: the importance of genetic diversity and delivery strategy

鸡是数量最多的家畜，每年饲养500亿英镑。它们是世界上最贫穷的人最广泛饲养的牲畜物种，既提供动物来源的膳食蛋白，也为当地的营销和贸易提供货币。在许多国家，包括印度的许多地区，妇女在后院和传统家禽生产系统中的作用至关重要，危害家禽健康或生存的疾病影响到社会上许多最贫穷的成员。在印度，球虫病一直被列为禽类死亡的前三大原因，但在有组织的部门很少使用疫苗接种，在无组织的部门几乎没有采取任何形式的控制措施。球虫病是一种由艾美耳球虫属原生动物寄生虫引起的家禽破坏性疾病，估计每年造成全球超过15亿GB的损失，根据对南亚穷人的影响，球虫病是十大牲畜疾病之一。随着免疫保护性抗原的识别和转基因技术的发展，经济高效的多价重组疫苗正成为一种现实的前景。然而，抗球虫药物的遗传耐药性可能会在现场迅速出现，类似的命运可能会降临到依赖于少量抗原的新型抗球虫疫苗身上，如果这些抗原是多态的和可选择的。为了预测这类疫苗在现场的可能效力和寿命，重要的是了解自然发生的遗传(抗原)多样性的流行率、多重感染率和寄生虫之间的基因交换频率。此外，还必须建立强大的地理网络，以便收集寄生虫进行实验室研究，并确保向世界上最贫穷的农村经济体有效传播、提供和支持传统和新的诊断和治疗方法(药物和疫苗)。已经在两种艾美耳球虫中发现了一小群免疫保护性寄生虫抗原。建议确定在其他经济上重要的艾美耳球虫物种中编码同等抗原的基因，并从在印度各地收集的一组野外菌株以及从世界其他地区收集的菌株中对这些基因进行测序(可通过IAH广泛的全球联系网络获得)。总共将对每个艾美耳球虫野外菌株的10个基因组区域进行测序，其中7个被假设为在免疫选择下编码抗原，3个‘管家’或‘中性’基因座。利用这些信息，将调查遗传多样性和艾美耳球虫田间种群结构，为建立数学模型预测自然感染期间交叉受精的发生和相关性提供数据。针对每个艾美耳球虫田间种群中不同遗传亚群的定量聚合酶链式反应将用于测试和开发早期模型。识别有效的疫苗抗原只是成功疫苗的一步。使用遗传上不同的艾美耳球虫菌株，感染诱导的保护性免疫将与通过以重组蛋白、DNA疫苗或转基因艾美耳球虫株为载体接种一个或多个测试抗原而刺激的免疫反应进行比较。将描述同源和异源挑战系统的特点。产生的数据将有助于预测现场艾美耳球虫种群对引进新疫苗的可能反应，以确保对所有艾美耳球虫物种的可持续效力。新的廉价抗球虫疫苗的开发将对商业家禽生产产生巨大影响，并有可能被发展中国家的政府用作将传染病对家禽的影响降至最低的工具。向最贫穷的社会阶层提供免费疫苗接种可对减贫产生巨大影响。

Chickens are the most numerous livestock animals, with >50 billion reared annually. They are the livestock species most widely kept by the poorest people in the world, providing both dietary protein of animal origin and a currency for local marketing and trade. In many countries, including many parts of India, the role of women in backyard and traditional poultry production systems is crucial and diseases that compromise the health or survival of poultry impact on many of the poorest members of society. In India, coccidiosis is consistently ranked in the top three causes of bird mortality but vaccination is only rarely used in the organised sector and little control of any sort is applied in the unorganised sector. Coccidiosis is a devastating disease of poultry caused by protozoan parasites of the genus Eimeria, estimated to incur global costs in excess of £1.5 billion per annum and ranked in the top ten diseases of livestock based upon impact on the poor in South Asia. Cost-effective, multivalent recombinant vaccines are becoming a realistic prospect due to the identification of immunoprotective antigens and development of transfection technologies. However, genetic resistance to anticoccidial drugs can occur rapidly in the field and a similar fate could befall novel anticoccidial vaccines that rely on a small number of antigens if these are polymorphic and selectable. To predict the likely efficacy and longevity of such vaccines in the field it is important to know the prevalence of naturally-occurring genetic (antigenic) diversity, rate of multiple infections and frequency of genetic exchange between parasites. It is crucial also to have strong geographical networks in place, both for collecting parasites for laboratory studies and to ensure efficient dissemination, delivery and support of traditional and new diagnostics and therapeutics (drugs and vaccines) to the world's poorest rural economies. A small panel of immunoprotective parasite antigens has been identified in two Eimeria species. It is proposed to identify genes coding for equivalent antigens in other economically important Eimeria species and to sequence these genes from a panel of field strains collected across India, and from strains collected from other parts of the world (available through an extensive global network of contacts at IAH). In total, ten genomic regions will be sequenced from each Eimeria field strain, seven of which are hypothesised to encode antigens under immune selection and three 'housekeeping' or 'neutral' loci. Using this information genetic diversity and Eimeria field population structure will be investigated, providing data for the construction of mathematical models to predict the occurrence and relevance of cross-fertilisation during natural infection. Quantitative PCR targeting genetically distinct sub-populations within each Eimeria field population will be used to test and develop the early models. The identification of effective vaccine antigens is just one step towards a successful vaccine. Using genetically distinct Eimeria strains, protective immunity induced by infection will be compared with immune responses stimulated through vaccination with one or more test antigens delivered as a recombinant protein, DNA vaccine, or using a transgenic Eimeria line as a vehicle. Homologous and heterologous challenge systems will be characterised. Data generated will help predict the likely responses of Eimeria field populations to the introduction of novel vaccines with a view towards ensuring sustainable efficacy against all Eimeria species. The development of new cheaper anticoccidial vaccines will have a huge impact on commercial poultry production and has the potential to be used as a tool by governments in developing countries to minimise the impact of infectious diseases on poultry. The provision of free vaccination to the poorest sectors of society can have a massive impact on the alleviation of poverty.