Computational Design of a Bicyclic Peptide Inhibitor Targeting the ICOS/ICOS-L Protein–Protein Interaction

The interaction between the inducible T-cell costimulatory molecule (ICOS) and its ligand (ICOS-L) is a critical pathway in T-cell activation and immune regulation. We computationally designed a bicyclic peptide (CP5) that inhibits the ICOS/ICOS-L protein–protein interaction (PPI). Using the structural insights derived from the ICOS/ICOS-L co-crystal structure (PDB: 6X4G) and bias-exchange metadynamics simulations (BE-META), we first designed monocyclic peptide candidates containing the β-strand (residues 51–55 ⁵¹YVYWQ⁵⁵) of ICOS-L that interact with ICOS. Using Rosetta's flex ddG calculations and further disulfide-bond restraint, we arrived at CP5 (cyclo-RVY[CQPGWC]WVLpG) as a potential ICOS/ICOS-L inhibitor. Using dynamic light scattering (DLS), we examined the interaction between CP5 and ICOS. Importantly, we validated the ICOS/ICOS-L inhibitory activity of CP5 using both TR-FRET assay and ELISA. Notably, CP5 demonstrated satisfactory in vitro pharmacokinetic properties, such as metabolic stability and lipophilicity, positioning it as a promising candidate for further drug development. Our findings provide a foundation for future drug discovery efforts aiming to develop cyclic peptides that specifically target the ICOS/ICOS-L interaction.