How does ISG65 help trypanosomes resist complement?

The complement system is an important component of our defence against pathogens. It centres around deposition of the complement C3b molecule on the surface of a pathogen, triggering a variety of downstream responses which result in pathogen destruction. African trypanosomes, as pathogens which live within our blood, are constantly at risk from complement. How do they resist being destroyed?

In 2022, we published a paper in which we reported the discovery, made by Olivia Macleod and Mark Carrington, that the trypanosome surface protein ISG65 acts as a receptor for C3b. Alex Cook then showed how ISG65 binds to a part of C3b known as the TED domain. Despite these advances, this study left us with questions. ISG65 binds with lower affinity to the TED domain than to the whole C3b molecule, showing that we had not observed the full interaction. What did the rest of the binding interface look like and how does ISG65 block C3b function?

In this second study, Alex was able to show how ISG65 binds to C3b, using cryogenic electron microscopy. This revealed that, in addition to binding to the TED domain, ISG65 also binds to the CUB domain of C3b. C3b is formed when the serum molecule, C3, is cleaved, allowing C3b to attach to a pathogen surface. It is therefore important that trypanosomes do not attract C3 towards themselves but instead selective bind to and inhibit surface bound C3b. The second interaction between ISG65 and C3b does not occur with C3, allowing ISG65 to preferentially bind to the dangerous C3b molecule.

The structure also provided clues about how ISG65 might function. In particular, C3b can recruit deadly cells of the immune system to pathogen surfaces, including B cells which generate antibodies, and macrophages which can engulf pathogens. These recognition and recruitment events occur due to receptors on immune cells, known as complement receptors, which bind to pathogen-associated C3b. Alex could see that ISG65 will block complement receptors 2 and 3 from binding to C3b, providing a clue about how ISG65 might dampen the function of the immune system by hiding trypanosomes from recognition by the deadly cells of the immune system.

Cook, A.D., Carrington, M.^* and Higgins, M.K.^* (2023) Molecular mechanism of complement inhibition by the trypanosome receptor ISG65. eLife doi.org/10/7554/eLife.88960.1