目前主流观点认为禽类球虫病疫苗的免疫保护机制主要是其激发的细胞免疫应答，而体液免疫应答发挥的作用则争议很大，一方认为其作用很小，另一方则认为其作用不可忽略。艾美耳球虫是不感染专业抗原递呈细胞的病原却能激发产生很好的细胞免疫应答，表明球虫抗原是通过交叉递呈途径而激发CD8+T细胞免疫应答的，但是这种交叉递呈的机制尚不清楚。在前期研究基础上，本申请提出了特异性抗体拮抗艾美耳球虫感染的新假说：球虫诱导产生的抗体与球虫抗原形成抗原抗体复合物，复合物上抗体Fc片段与其受体相互作用介导球虫抗原交叉递呈，增强保护性细胞免疫应答。我们拟通过建立抗原抗体复合物增强细胞免疫应答的体内研究模型、构建基于转基因球虫研究Fc片段介导交叉递呈的模型和建立交叉递呈机制研究的体外模型来验证此假说。本申请的意义还在于为研制广泛应用于肉鸡的活卵囊疫苗提供基础研究支持，也为防控其它胞内病原如疟原虫的疫苗研制提供新思路。

Eimeria infection induces strong humoral and cell-mediated immunity in chickens. Cell-mediated immunity plays a key role in protection, while role of humoral immunity in protection is unclear. As Eimeria parasites do not infect professional antigen-presentation cells (pAPCs), parasitic antigens are proposed to be cross-presented to MHC I molecules, eliciting cell-mediated immunity. In this proposal, we hypothesize that Eimeria-specific antibodies play an important role in resisting coccidial infection through their mediating cross-presentation of antigens. Eimeria-specific antibodies and parasitic antigens form immune complexes (ICs). ICs are phagocytosed and cross-presented by pAPCs, mediated by their surface Fc receptors binding Fc fragement of ICs. Our hypothesis will be tested by 1）establishing an in vivo B cell-deletion model to confirm cell-mediated immunity is enhanced by Eimeria-specific antibodies; 2) constructing transgenic Eimeria parasites expressing surface Fc fragement or a model antigen HA fused with Fc fragement to further confirm that enhanced cellular immunity is mediated by Fc fragment; 3) conducting in vitro confirmation experiments to demonstrate cross-presentation is mediated by Fc receptor bingding with Fc fragement of ICs. Our results have important implication for the development of higher effective anticoccidial vaccines for broilers and effective vaccines for malaria caused by close relatives of Eimeria.