γ9δ2T cell diversity and the receptor interface with tumor cells.

γ9δ2T cells play a major role in cancer immune surveillance, yet the clinical translation of their in vitro promise remains challenging. To address limitations of previous clinical attempts utilizing expanded γ9δ2T cells, we explored the clonal diversity of γ9δ2T cell repertoires and characterized their target. We demonstrated that only a fraction of expanded γ9δ2T cells is active against cancer cells, and that activity of the parental clone, or functional avidity of selected γ9δ2TCRs does not associate with clonal frequency. We also analyzed the target-receptor-interface and provided a two-receptor, three-ligand model. Activation is initiated by binding of the γ9δ2TCR to BTN2A1 through the regions between CDR2 and CDR3 of the TCR γ chain, and modulated by the affinity of the CDR3 region of the TCR δ chain, which is phosphoantigen (pAg)-independent and does not depend on CD277. CD277 is secondary, serving as mandatory co-activating ligand. Binding of CD277 to its putative ligand does not depend on the presence of γ9δ2TCR, does depend on usage of the intracellular CD277, creates pAg-dependent proximity to BTN2A1, enhances cell-cell conjugate formation and stabilizes the immunological synapse. This process critically depends on the affinity of the γ9δ2TCR and requires membrane flexibility of the γ9δ2TCR and CD277, facilitating their polarization and high-density recruitment during immunological synapse formation.