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

鸡是数量最多的牲畜，每年饲养超过500亿只。它们是世界上最贫穷的人饲养最广泛的牲畜品种，既提供动物来源的膳食蛋白，又为当地的销售和贸易提供货币。在许多国家，包括印度许多地区，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许多最贫穷的社会成员。在印度，球虫病一直排在鸟类死亡的前三位，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任何控制措施。球虫病是由艾美耳球虫属的原生动物寄生虫引起的家禽的毁灭性疾病，估计每年引起超过15亿英镑的全球成本，并且基于对南亚穷人的影响而被列为十大牲畜疾病。由于免疫保护性抗原的鉴定和转染技术的发展，经济有效的多价重组疫苗正成为一个现实的前景。然而，抗球虫药物的遗传耐药性可以在该领域迅速发生，并且如果这些抗原是多态性的和可选择的，则依赖于少量抗原的新型抗球虫疫苗可能会遭遇类似的命运。为了预测这种疫苗在该领域的可能效力和寿命，重要的是要知道自然发生的遗传（抗原）多样性的流行率，多重感染率和寄生虫之间的遗传交换频率。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）。在两种艾美耳球虫中鉴定出一小组免疫保护性寄生虫抗原。建议确定编码其他经济上重要的艾美耳球虫属物种中等效抗原的基因，并从印度各地收集的一组现场菌株和从世界其他地区收集的菌株（可通过IAH广泛的全球联系网络获得）中对这些基因进行测序。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个假设编码免疫选择下的抗原和3个“管家”或“中性”基因座。利用这些信息的遗传多样性和艾美耳球虫领域的人口结构将进行调查，提供数据的数学模型的建设，以预测自然感染过程中的交叉受精的发生和相关性。定量PCR靶向每个艾美耳球虫田间种群内遗传上不同的亚群将用于测试和开发早期模型。有效疫苗抗原的鉴定只是迈向成功疫苗的一步。使用遗传上不同的艾美耳球虫菌株，将感染诱导的保护性免疫与通过接种一种或多种作为重组蛋白、DNA疫苗递送的试验抗原或使用转基因艾美耳球虫品系作为媒介物刺激的免疫应答进行比较。将对同源和异源攻毒系统进行表征。所产生的数据将有助于预测艾美耳球虫田间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。新的更便宜的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工具，以尽量减少传染病对家禽的影响。向社会最贫穷阶层提供免费疫苗接种可对减轻贫穷产生巨大影响。

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.