Mechanical force regulates the inhibitory function of PD-1.

The immune checkpoint molecule, programmed cell death 1 (PD-1), critically regulates T-cell activation upon binding PD-L1 or PD-L2, making it a key target in cancer immunotherapy. Although extensively studied, the molecular mechanism of the inhibitory function of PD-1 remains incompletely understood. Using the biomembrane force probe (BFP), we measure catch-slip bond behavior between PD-1 and PD-L1/PD-L2 under force. Steered molecular dynamics (SMD) simulation reveals a force-induced bound state distinct from the force-free state observed in solved complex structures. Disrupting interactions that stabilize either state weakens the catch bond, and diminishes the inhibitory function of PD-1. Interestingly, soluble forms of PD-L1/PD-L2 compete with their surface-bound counterparts and attenuate PD-1-mediated T-cell inhibition, suggesting that soluble PD-1 ligands could potentially serve as anti-PD-1 drugs. Tumor growth studies using a gain of function mutant based on the catch-bond mechanism confirm the anti-cancer activity of soluble PD-L1. Our findings highlight that mechanical force governs the inhibitory function of PD-1 and suggest that PD-1 acts as a mechanical sensor in T-cell suppression. Thus, mechanical regulation should be considered when designing PD-1 blocking therapies.