Molecular mechanistic insights into the <scp>OX40</scp> – <scp>OX40L</scp> complex from biophysical and computational analyses

OX40 and OX40L belong to the tumor necrosis factor receptor superfamily (TNFRSF) and tumor necrosis factor superfamily (TNFSF), respectively. Protein-protein interactions between OX40 and OX40L facilitate T cell responses, triggering various immunological and pathophysiological events. Excessive activation frequently contributes to the onset of autoimmune and allergic diseases. Therefore, the OX40/OX40L system is considered a promising target for drug discovery. Given that the structure of the OX40-OX40L complex exhibits some unique features compared to other members of these protein super families, it is reasonable to assume that this tandem possesses distinct interaction mechanisms. However, detailed interaction analysis using quantitative parameters such as binding kinetics or thermodynamics, with remains to be performed for OX40/OX40L. In this study, we identified several hot spot residues from the OX40 cysteine-rich domains (CRDs) 1 to 3 by alanine scanning. Kinetic and thermodynamic analysis combined with molecular dynamics simulations highlighted the characteristics of a hot spot from CRD3 due to its indirect influence on those from CRD1 and CRD2, providing insights into the interaction mechanism and a strategy for drug discovery targeting this interaction.