Phosphoantigen sensing combines TCR-dependent recognition of the BTN3A IgV domain and germline interaction with BTN2A1

• DOI: 10.1016/j.celrep.2023.112321
• Publication Date: 2023-03-28T22:53:21Z
• Authors: Carrie R. Willcox; Mahboob Salim; Charlotte R. Begley; Mohindar M. Karun...; Emily J. Easton; Carlotta von Klop...; Katie A. Berwick; Thomas Herrmann; Fiyaz Mohammed; Mark Jeeves; Benjamin E. Willcox
• Keywords: Germ Cells; QH301-705.5; T-Lymphocytes; CP: Immunology; Receptors, Antigen, T-Cell, gamma-delta; Biology (General); Lymphocyte Activation; Ligands

Vγ9Vδ2 T cells play critical roles in microbial immunity by detecting target cells exposed to pathogen-derived phosphoantigens (P-Ags). Target cell expression of BTN3A1, the "P-Ag sensor," and BTN2A1, a direct ligand for T cell receptor (TCR) Vγ9, is essential for this process; however, the molecular mechanisms involved are unclear. Here, we characterize BTN2A1 interactions with Vγ9Vδ2 TCR and BTN3A1. Nuclear magnetic resonance (NMR), modeling, and mutagenesis establish a BTN2A1-immunoglobulin V (IgV)/BTN3A1-IgV structural model compatible with their cell-surface association in cis. However, TCR and BTN3A1-IgV binding to BTN2A1-IgV is mutually exclusive, owing to binding site proximity and overlap. Moreover, mutagenesis indicates that the BTN2A1-IgV/BTN3A1-IgV interaction is non-essential for recognition but instead identifies a molecular surface on BTN3A1-IgV essential to P-Ag sensing. These results establish a critical role for BTN3A-IgV in P-Ag sensing, in mediating direct or indirect interactions with the γδ-TCR. They support a composite-ligand model whereby intracellular P-Ag detection coordinates weak extracellular germline TCR/BTN2A1 and clonotypically influenced TCR/BTN3A-mediated interactions to initiate Vγ9Vδ2 TCR triggering.