Abstract
The gamma delta T-cell receptor (γδ TCR) mediates γδ T-cell responses that are crucial for immune defense, tissue surveillance, and homeostasis. While recent studies suggest that γδ TCRs engage ligands multivalently, how multivalent engagement activates the γδ TCR remains elusive. Here, we present the structures of the Vγ9Vδ2 (clone G115) TCR in both the inactive and the agonist-crosslinked active states. In the γδ TCR lipoprotein complex, cholesterol and phospholipids form a "barrier" that isolates CD3ζζ from the other subunits and likely prevents autoactivation. As an agonist, the Fab fragment of the UCHT1 antibody dimerizes upon binding the Vγ9Vδ2 TCR, thereby crosslinking and activating the receptor. Cryo-EM analysis of Fab-crosslinked Vγ9Vδ2 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-CD3ϵ interface, mediate the assembly of the Vγ9Vδ2 TCR and UCHT1 Fab into 2:2 tetramers. Upon activation, the Vγ9Vδ2 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 γδ TCR activation.