A Structural Change in Butyrophilin upon Phosphoantigen Binding Underlies Phosphoantigen-Mediated Vγ9Vδ2 T Cell Activation

Highlights

• The crystal structure of HMBPP-bound intracellular BTN3A1 was determined at 1.97 Å

• HMBPP forms hydrogen bonds with H 351 for efficient Vγ9Vδ2 T cell activation

• An asymmetric intracellular dimer is involved in HMBPP-mediated γδ T cell activation

• HMBPP doubles the binding force between extracellular BTN3A and Vγ9Vδ2 TCR

Summary

Human Vγ9Vδ2 T cells respond to microbial infections and malignancy by sensing diphosphate-containing metabolites called phosphoantigens, which bind to the intracellular domain of butyrophilin 3A1, triggering extracellular interactions with the Vγ9Vδ2 T cell receptor (TCR). Here, we examined the molecular basis of this “inside-out” triggering mechanism. Crystal structures of intracellular butyrophilin 3A proteins alone or in complex with the potent microbial phosphoantigen HMBPP or a synthetic analog revealed key features of phosphoantigens and butyrophilins required for γδ T cell activation. Analyses with chemical probes and molecular dynamic simulations demonstrated that dimerized intracellular proteins cooperate in sensing HMBPP to enhance the efficiency of γδ T cell activation. HMBPP binding to butyrophilin doubled the binding force between a γδ T cell and a target cell during “outside” signaling, as measured by single-cell force microscopy. Our findings provide insight into the “inside-out” triggering of Vγ9Vδ2 T cell activation by phosphoantigen-bound butyrophilin, facilitating immunotherapeutic drug design.