Molecular mechanism of complement inhibition by the trypanosome receptor ISG65

Abstract

African trypanosomes replicate within infected mammals where they are constantly exposed to the molecules of the complement system. This system centres around complement factor C3, which is present in a soluble form in serum but can become covalently deposited onto the surfaces of pathogens after proteolytic cleavage to C3b. Membrane-associated C3b triggers different complement-mediated effectors which promote pathogen clearance, including complement receptor-mediated stimulation of immune cells or recruitment of components of the pore-forming membrane attack complex. 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 had 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, ensuring that ISG65 preferentially binds to active C3b. 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 will 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.