The 8-kDa p12^I protein down-regulates TCR signaling and is recruited to the immunologic synapse. (A) Jurkat T cells expressing the wild-type p12^I, p12^IG29S, or p12^IΔ29 deletion mutants were cocultivated with SEE prepulsed B cells for 15 minutes. Cells were costained with anti-CD3 and anti-HA antibodies. The arrows indicate the IS found between the B cell and the T cell. (B-D) Jurkat T cells were cotransfected with the p12^I knockout proviral clone, pAB-GTG, p12^I, the p12^I mutants, and the HTLV-LTR–driven luciferase reporter plasmid. Cells were collected 24 hours later, resuspended in fresh media, and stimulated with or without anti-CD3 antibody. Tax-driven luciferase activity (B), p19 Gag production in the supernatant (C), and p12^I expression (D) at 48 hours are shown.