The intracellular tails of PD-1 and BTLA dictate their specificities. (A) Cartoon depicting a Raji-Jurkat co-culture assay in which Jurkat (PD-1^FF–mGFP⁺), Jurkat (PD-1^WT–mGFP⁺), or Jurkat (PD-1^ECTO-TMD–BTLA^INT-mGFP⁺) cells were stimulated with SEE-loaded Raji (PD-L1–mCherry⁺) cells. (B) Flow cytometry histograms showing PD-1 levels on indicated Jurkat T cells. (C) Representative IBs of phosphotyrosine (pY IB), GFP (GFP IB), and the coprecipitated SHP1 and SHP2 of immunoprecipitated GFP-tagged receptors. (D–F) Same as A–C, except replacing Raji (PD-L1–mCherry⁺), Jurkat (PD-1^FF–mGFP⁺), Jurkat (PD-1^WT–mGFP⁺), and Jurkat (PD-1^ECTO-TMD–BTLA^INT-mGFP⁺) cells with Raji (HVEM-mRuby⁺), Jurkat (BTLA^FFFF-mGFP⁺), Jurkat (BTLA^WT-mGFP⁺), and Jurkat (BTLA^ECTO-TMD–PD-1^INT–mGFP⁺) cells, respectively. WCL, whole cell lysate.