Cook Alexander D, Carrington Mark, Higgins Matthew K
Department of Biochemistry, University of Oxford, Oxford, United Kingdom.
Kavli Institute for Nanoscience Discovery, Dorothy Crowfoot Hodgkin Building, University of Oxford, Oxford, United Kingdom.
Elife. 2024 Apr 24;12:RP88960. doi: 10.7554/eLife.88960.
African trypanosomes replicate within infected mammals where they are exposed to the complement system. This system centres around complement C3, which is present in a soluble form in serum but becomes covalently deposited onto the surfaces of pathogens after proteolytic cleavage to C3b. Membrane-associated C3b triggers different complement-mediated effectors which promote pathogen clearance. To counter complement-mediated clearance, African trypanosomes have a cell surface receptor, ISG65, which binds to C3b and which decreases the rate of trypanosome clearance in an infection model. However, the mechanism by which ISG65 reduces C3b function has not been determined. We reveal through cryogenic electron microscopy that ISG65 has two distinct binding sites for C3b, only one of which is available in C3 and C3d. We show that ISG65 does not block the formation of C3b or the function of the C3 convertase which catalyses the surface deposition of C3b. However, we show that ISG65 forms a specific conjugate with C3b, perhaps acting as a decoy. ISG65 also occludes the binding sites for complement receptors 2 and 3, which may disrupt recruitment of immune cells, including B cells, phagocytes, and granulocytes. This suggests that ISG65 protects trypanosomes by combining multiple approaches to dampen the complement cascade.
非洲锥虫在受感染的哺乳动物体内进行复制，在此过程中它们会接触到补体系统。该系统以补体C3为核心，C3以可溶性形式存在于血清中，但在蛋白水解裂解为C3b后会共价沉积在病原体表面。膜结合的C3b会触发不同的补体介导效应器，促进病原体清除。为了对抗补体介导的清除作用，非洲锥虫具有一种细胞表面受体ISG65，它能与C3b结合，并在感染模型中降低锥虫的清除率。然而，ISG65降低C3b功能的机制尚未确定。我们通过低温电子显微镜发现，ISG65对C3b有两个不同的结合位点，其中只有一个在C3和C3d中可用。我们发现ISG65不会阻止C3b的形成或催化C3b表面沉积的C3转化酶的功能。然而，我们发现ISG65与C3b形成了一种特定的复合物，可能起到了诱饵的作用。ISG65还会封闭补体受体2和3的结合位点，这可能会干扰包括B细胞、吞噬细胞和粒细胞在内的免疫细胞的募集。这表明ISG65通过多种方式抑制补体级联反应来保护锥虫。
