Vgamma9Vdelta2 T cells recognize butyrophilin 2A1 and 3A1 heteromers
Butyrophilin (BTN) molecules are emerging as key regulators of T cell immunity, however, how they trigger cell-mediated responses is poorly understood. Here, the crystal structure of a gamma-delta T cell receptor ({gamma}{delta}TCR) in complex with BTN member 2A1 (BTN2A1) revealed that BTN2A1 engages the side of the {gamma}{delta}TCR, leaving the apical TCR surface bioavailable. We reveal that BTN3A1 is a second {gamma}{delta}TCR ligand, that co-engages {gamma}{delta}TCR via binding to this accessible apical surface. BTN2A1 and BTN3A1 also directly interact with each other in cis, and structural analysis revealed formation of W-shaped heteromeric multimers. This BTN2A1-BTN3A1 interaction involved the same epitopes that BTN2A1 and BTN3A1 each use to engage {gamma}{delta}TCR; indeed, either forced separation or locking together of BTN2A1 and BTN3A1 resulted in enhanced or abrogated {gamma}{delta}TCR interaction, respectively. Our findings reveal a new paradigm in immune activation, whereby {gamma}{delta}TCRs recognize dual epitopes on BTN2A1 and BTN3A1 complexes.