Transgenic expression of hCD4 and hCCR5 efficiently overcomes the HIV-1 entry block in primary rat T-lymphocytes. Fusion of HIV-1YU-2 virions carrying BlaM-Vpr was analyzed in primary T-cells from humans or hCD4/hCCR5-transgenic rats by multi-parameter flow cytometry [22,25]. (A) Representative FACS dot plots for the detection of cleaved CCF2 substrate, reflecting HIV-1 entry. T-cells from humans (upper panels) and double-transgenic rats (lower panels) were either mock-infected (left panels) or infected with HIV-1YU-2 (50 ng HIV-1 p24 CA per 2–3 × 106 cells), either without (middle panels) or with (right panels) the fusion inhibitor T20 (50 μM). (B) Results from virion-fusion assays with T-cells from 5–9 different donors per species. Were indicated, the CCR5 antagonist TAK-779, the CXCR4 antagonist AMD3100 (both 1 μM), or T20 (50 μM) were added 15–30 min before virus challenge. Symbols indicate arithmetic means of triplicates from one virion-fusion experiment; horizontal bars depict the arithmetic mean ± S.E.M. of all experiments (n.s. = not significant; p = 0.66; * p ≤ 0.02) (C) Titration of HIV-1R7/3YU-2 Env GFP carrying BlaM-Vpr in virion-fusion assays on primary T-cells from both species. Where indicated (filled triangle) the anti-hCCR5 mAb 2D7 (50 μg/ml) were added to cells 15–30 min before virus challenge.