Fab Dimerization Crosslinks and Activates the γδ T-Cell Receptor

Abstract

The gamma delta T-cell receptor ({gamma}{delta} TCR) mediates {gamma}{delta} T-cell responses that are crucial for immune defense, tissue surveillance, and homeostasis. While recent studies suggest that {gamma}{delta} TCRs engage ligands multivalently, how multivalent engagement activates the {gamma}{delta} TCR remains elusive. Here, we present the structures of the V{gamma}9V{delta}2 (clone G115) TCR in both the inactive and the agonist-crosslinked active states. In the {gamma}{delta} TCR lipoprotein complex, cholesterol and phospholipids form a "barrier" that isolates CD3{zeta}{zeta} from the other subunits and likely prevents autoactivation. As an agonist, the Fab fragment of the UCHT1 antibody dimerizes upon binding the V{gamma}9V{delta}2 TCR, thereby crosslinking and activating the receptor. Cryo-EM analysis of Fab-crosslinked V{gamma}9V{delta}2 TCR identifies three Fab dimer interfaces, one between the two light chains and the other two between light and heavy chains. These Fab-Fab interfaces, together with the Fab-CD3{epsilon} interface, mediate the assembly of the V{gamma}9V{delta}2 TCR and UCHT1 Fab into 2:2 tetramers. Upon activation, the V{gamma}9V{delta}2 TCR becomes conformationally plastic and loses enriched lipids, including cholesterol and phosphatidylethanolamine. Our findings reveal a dimerization-driven activation mechanism, inform the design of rigid TCR-crosslinking agonists, and suggest multivalency-induced clustering as a general paradigm for {gamma}{delta} TCR activation.