A distinct topology of BTN3A IgV and B30.2 domains controlled by juxtamembrane regions favors optimal human æ T cell phosphoantigen sensing

Butyrophilin (BTN)-3A and BTN2A1 molecules control the activation of human V gamma 9V delta 2 T cells during T cell receptor (TCR)-mediated sensing of phosphoantigens (PAg) derived from microbes and tumors. However, the molecular rules governing PAg sensing remain largely unknown. Here, we establish three mechanistic principles of PAg-mediated gamma delta T cell activation. First, in humans, following PAg binding to the intracellular BTN3A1-B30.2 domain, V gamma 9V delta 2 TCR triggering involves the extracellular V-domain of BTN3A2/BTN3A3. Moreover, the localization of both protein domains on different chains of the BTN3A homo-or heteromers is essential for efficient PAg-mediated activation. Second, the formation of BTN3A homo-or heteromers, which differ in intracellular trafficking and conformation, is controlled by molecular interactions between the juxtamembrane regions of the BTN3A chains. Finally, the ability of PAg not simply to bind BTN3A-B30.2, but to promote its subsequent interaction with the BTN2A1-B30.2 domain, is essential for T-cell activation. Defining these determinants of cooperation and the division of labor in BTN proteins improves our understanding of PAg sensing and elucidates a mode of action that may apply to other BTN family members. gamma delta T cells are known to recognize phosphoantigen in the context of butyrophilins (BTN), yet the molecular rules underlying this phenomenon are unclear. Here, the authors demonstrate that a distinct topology of B30.2 and IgV domains within BTN3A complexes promotes antigen sensing and that the juxtamembrane region controls BTN complex formation.