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Abstract：
Chagas disease, a chronic disabling disease caused by the protozoan Trypanosoma cruzi, has no standardized treatment or preventative vaccine. The infective trypomastigote form of T. cruzi is highly resistant to killing by the complement immune system. Factor H (FH), a negative regulator of the alternative pathway (AP) of complement on cell surfaces and in blood, contains 20 short consensus repeat domains. The four N-terminal domains of FH inactivate the AP, while the other domains interact with C3b/d and glycan markers on cell surfaces. Various pathogens bind FH to inactivate the AP. T. cruzi uses its trans-sialidase enzyme to transfer host sialic acids to its own surface, which could be one of the approaches it uses to bind FH. Previous studies have shown that FH binds to complement-opsonized T. cruzi and parasite desialylation increases complement-mediated lysis of trypomastigotes. However, the molecular basis of FH binding to T. cruzi remain unknown. Only trypomastigotes, but not epimastigotes (non-infective, complement susceptible) bound FH directly, independent of C3 deposition, in a dose-dependent manner. Domain mapping experiments using 3-5 FH domain fragments showed that domains 5-8 competitively inhibited FH binding to the trypomastigotes by ~35% but did not decrease survival in complement. FH-Fc or mutant FH-Fc fusion proteins (3-11 contiguous FH domains fused to the IgG Fc) also did not kill trypomastigotes. FH-related protein-5, whose domains bear significant sequence identity to all known polyanion-binding FH domains (6-7, 10-14, 19-20), fully inhibited FH binding to trypomastigotes and reduced trypomastigote survival to < 24% in the presence of serum. In conclusion, we have elucidated the role of FH in complement resistance of trypomastigotes.
摘要：
查加斯病,一种由原生动物克氏锥虫引起的慢性致残疾病，没有标准化的治疗或预防性疫苗。克氏锥虫的感染性锥虫形式对补体免疫系统的杀伤具有高度抗性。系数H(FH),细胞表面和血液中补体替代途径（AP）的负调节因子，包含20个短的共有重复结构域。FH的四个N端结构域使AP失活，而其他结构域与细胞表面的C3b/d和聚糖标记相互作用。各种病原体结合FH以灭活AP。克氏杆菌使用其反式唾液酸酶酶将宿主唾液酸转移到其自身表面，这可能是它用来绑定FH的方法之一。先前的研究表明，FH与补体调理的T.cruzi结合，并且寄生虫的去唾液酸化增加了补体介导的色素动物的裂解。然而,FH与克氏虫结合的分子基础仍然未知。只有锥虫，但不是epimastigotes（非感染性，补体敏感)直接结合FH，独立于C3沉积，以剂量依赖的方式。使用3-5个FH结构域片段的结构域作图实验表明，结构域5-8竞争性抑制FH与色素动物的结合约35％，但不会降低补体中的存活率。FH-Fc或突变型FH-Fc融合蛋白(与IgGFc融合的3-11个连续FH结构域)也不杀死色素动物。FH相关蛋白5，其结构域与所有已知的聚阴离子结合FH结构域（6-7、10-14、19-20）具有显著的序列同一性，在血清存在下，完全抑制FH与色素动物的结合，并将色素动物的存活率降低至<24％。总之,我们已经阐明了FH在色素动物补体抗性中的作用。
