Tetherin cell surface expression on effector cells inhibits viral cell-to-cell transfer and transmission and is counteracted by Vpu. Tetherinpos or tetherinneg Sup-T1 effector cells, infected with wt or Δvpu virus, were co-cultured with tetherinpos or tetherinneg Sup-T1 target cells. Target cells were assessed for viral transfer and viral transmission by flow cytometry analysis for CA p24 and GFP expression, respectively. Data are derived from three independent experiments; error bars represent SEM. A & B. Viral transfer (CA p24) of Δvpu (A) and wt (B) virus from tetherinpos and tetherinneg effector cells. Data are normalized to the number of infected tetherinneg effector cells. C & D. Viral transfer (CA p24) of wt and Δvpu virus from tetherinpos (C) and tetherinneg (D) effector cells. Data are normalized for Δvpu infections in each case. E & F. Viral transmission (GFP) of wt and Δvpu virus from tetherinpos (E) and tetherinneg (F) effector cells. Data are normalized for Δvpu infections in each case.