Butyrophilin 3A1 Plays an Essential Role in Prenyl Pyrophosphate Stimulation of Human V gamma 2V delta 2 T Cells

Most human gamma delta T cells express V gamma 2V delta 2 TCRs and play important roles in microbial and tumor immunity. V gamma 2V delta 2 T cells are stimulated by self- and foreign prenyl pyrophosphate intermediates in isoprenoid synthesis. However, little is known about the molecular basis for this stimulation. We find that a mAb specific for butyrophilin 3 (BTN3)/CD277 Ig superfamily proteins mimics prenyl pyrophosphates. The 20.1 mAb stimulated V gamma 2V delta 2 T cell clones regardless of their functional phenotype or developmental origin and selectively expanded blood V gamma 2V delta 2 T cells. The gamma delta TCR mediates 20.1 mAb stimulation because IL-2 is released by beta(-) Jurkat cells transfected with V gamma 2V delta 2 TCRs. 20.1 stimulation was not due to isopentenyl pyrophosphate (IPP) accumulation because 20.1 treatment of APC did not increase IPP levels. In addition, stimulation was not inhibited by statin treatment, which blocks IPP production. Importantly, small interfering RNA knockdown of BTN3A1 abolished stimulation by IPP that could be restored by re-expression of BTN3A1 but not by BTN3A2 or BTN3A3. Rhesus monkey and baboon APC presented HMBPP and 20.1 to human V gamma 2V delta 2 T cells despite amino acid differences in BTN3A1 that localize to its outer surface. This suggests that the conserved inner and/or top surfaces of BTN3A1 interact with its counterreceptor. Although no binding site exists on the BTN3A1 extracellular domains, a model of the intracellular B30.2 domain predicts a basic pocket on its binding surface. However, BTN3A1 did not preferentially bind a photoaffinity prenyl pyrophosphate. Thus, BTN3A1 is required for stimulation by prenyl pyrophosphates but does not bind the intermediates with high affinity. The Journal of Immunology, 2013, 191: 1029-1042.