Tat specifically enhances gp120-LV entry. (A) Increase in gp120-LV entry into C8166 cells following coculture with HIV-1–infected U937 cells. During coculture, C8166 cells were either untreated (▦) or pretreated with NT3 2D1 anti-Tat mAb (▪) or unrelated IgG (□) as a control (*P ≤ .01 within day 6 and day 3 after infection; §P ≤ .01 within anti-Tat–treated and control cells). (B) Increase in gp120-LV entry into C8166 cells following coculture with U937/Tat cells. Cells were either untreated (▦), pretreated with Hep III (□), or incubated with NT3 2D1 anti-Tat mAb (▪) or unrelated IgG (▨) as a control (n = 8; F = 624.99; *P ≤ .05 within treated and control cells). (C) Dose-dependent increase in gp120-LV entry into C8166 cells incubated with rTat (n = 3; F = 258.54; *§P ≤ .05 within rTat-treated and control cells). (D) Time-course effect of coculture with U937/Tat cells on gp120-LV (HIV-1, ▦) and VSV-G–LV (▪) entry into C8166 cells (n = 3; F = 107.4). (E) Effect of 2 hours of coculture with different Tat-expressing clonal lines on gp120-LV and VSV-G–LV entry into C8166 cells (n = 3; F = 937.74). Tat concentrations in these experiments were as follows: U937/Tat, 2.5 nM; clone 1, 5 nM; clone 2, 1.5 nM; clone 3, 3.5 nM. In panels A, B, D, and E, values are shown as percent variation of transduced cells compared with transduction of C8166 cocultured either with uninfected PBMCs or U937/PINCO cells. In panel C, values are shown as percent variation of transduced cells compared with a control transduction of rTat-untreated C8166 cells. Each value indicates mean ± SD of indicated number of experiments performed at least in duplicate.