新城疫是由新城疫病毒（NDV）引起的禽烈性传染病，疫苗免疫是防控该病的重要手段。鸡胚免疫具有填补免疫空白期、提高生产效率等优点，在我国应用前景广阔。但由于NDV传统疫苗株对18日龄鸡胚接种是高度致病的，导致其鸡胚免疫疫苗株资源稀缺。本项目前期选育得到毒力超弱化、可鸡胚免疫的NDV TS09-C株，发现该毒株含有稀有型F蛋白裂解位点（FCS），并可调控NDV毒力的超弱化。拟进一步研究其分子机理：①应用反向遗传操作技术，构建系列稀有型FCS突变的重组病毒，分析其致病力，系统确定可调控NDV毒力超弱化的稀有FCS种类；②利用差异表达分析、蛋白互作等技术，筛选并确定特异切割稀有FCS的宿主蛋白酶及切割效率；③在细胞和胚体水平分析蛋白酶切割稀有FCS对病毒复制、组织嗜性及毒力超弱化的影响，以阐明NDV毒力超弱化的分子机理。为NDV鸡胚免疫疫苗研发提供理论支撑，也有助于深入解读NDV的毒力因子。

Newcastle disease virus (NDV) causes a highly contagious disease in chickens, and poses a considerable threat to poultry industry worldwide. Newcastle disease (ND) has been classified into class-I animal disease in China. Live vaccine immunization is the important method to control ND. Although most of the commercial NDV avirulent vaccine strains are safe for the hatched chickens, they are highly pathogenic for 18-day-old chicken embryos, and could not be used as in-ovo vaccine. Previously we obtained the highly avirulent NDV strain TS09-C, which could be used safely as in-ovo vaccine. We found that the TS09-C virus possessed the rare F cleavage site (FCS), which is the key determinant of the high avirulence of NDV. In this project, we will used the reverse genetic operation technology to confirm the relationship between all rare FCS and high avirulence of NDV. Next, the protease that participated in the cleavage of rare FCS will be identified, by using the differential expression and protein interaction assays. The effect of FCS cleavage by protease on the viral replication, tissue tropism and high avirulence will be further studied. Finally, the mechanism by which the rare FCS regulating the high avirulence of NDV will be elucidated. The project will help us to understand deeply the virulent factor of NDV, and lay the foundation for the further development of ND in-ovo vaccine.