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S C I E N T I F I C H I G H L I G H T S

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Structural insights into innate immune evasion in human-infective African trypanosomes

Cryo-electron microscopy and small-angle X-ray scattering at beamline BM29 have been used to determine the structures and conformations of a surface protein that helps human-infective trypanosomes to evade the alternative pathway of the innate immune response.

Researchers have made a significant breakthrough in understanding how the parasite Trypanosoma brucei gambiense, which causes African sleeping sickness, evades the human innate immune system. A new study has revealed the structure and conformational dynamics of the parasite s invariant surface glycoprotein 65 (ISG65) in complex with human complement factors C3 and C3b, key components of the immune system s alternative pathway.

African sleeping sickness is caused by the parasite Trypanosoma brucei gambiense, which is transmitted by the tsetse fly. The disease can be fatal if left untreated, and currently there are no effective vaccines or drugs available to prevent or treat the disease. One of the ways the parasite is able to evade the immune system is by expressing a dense surface layer of variant surface glycoproteins (VSGs) that undergo constant antigenic variation, preventing the host from mounting a long- lasting immune response. However, another class of surface protein, the invariant surface glycoproteins (ISGs), remain relatively constant and do not trigger immune detection of the parasite. The biology of these proteins remains poorly understood, but they are believed to play a crucial role in the parasite s ability to evade the immune system.

In this study, researchers used cryo-electron microscopy (cryo-EM) at CEITEC (Czech Republic) and SOLARIS (Poland) to capture detailed images of ISG65 bound to human complement C3 and C3b (Figure 23). They found that ISG65 is able to specifically inhibit the C5 convertase of the alternative complement pathway, thereby preventing the immune system from attacking the parasite at an early stage of infection.

Using small-angle X-ray scattering (SAXS) at beamline BM29, the researchers also showed that ISG65 can exist in two different states: one where it would extend away from the cell surface of the parasite and bind complement factor C3b, and another where it would be embedded in the protective surface layer of the parasite (Figure 24).

Fig. 23: Cryo-EM structures of T. brucei gambiense ISG65 in complex with native C3 and C3b. Cryo-EM density maps showing side views of ISG65:C3 (left), and ISG65:C3b (right) at two different angles. ISG65 is represented by map regions coloured in green. Interacting domains in C3 and C3b are depicted in blue. In both C3 conformations, TED provides the primary interface. ISG65 and the thioester domain TED (C3d, when liberated) show a high degree of shape complementarity. Smaller, secondary interfaces are

located in ANA (native C3) and CUB (C3b). The remaining scaffold (C3c, grey) shows no additional contact points.