Tetherin cell surface expression on target cells promotes viral cell-to-cell transfer and transmission. 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 normalized for tetherinpos target cells in each case. Data are derived from three independent experiments; error bars represent SEM. (A & B). Viral transfer (CA p24) of wt and Δvpu virus from tetherinpos (A) and tetherinneg (B) effector cells. (C & D). Viral transmission (GFP) of wt and Δvpu virus from tetherinpos (C) and tetherinneg (D) effector cells.