Evidence for a spreading infection of a recombinant HIV-1 GFP reporter virus in rat T-cells carrying the HIV receptor complex. Activated T-cells derived from two hCD4/hCCR5-tg rats and two human donors were challenged with either replication-competent HIV-1R7/3 YU-2 Env GFP virus or single-round YU-2 Env pseudotyped HIV-1NL4-3 E- GFP virus (each 500 ng p24 per 2 × 106 cells) and analyzed at the indicated time points p.i. for (A) percentage of GFP-positive cells by flow cytometry and (B) the p24 concentration in culture supernatants by antigen ELISA until day 12 p.i. (C) The data obtained on day 3 under (A) were also analyzed for the MFI of the GFP-expressing cells as a surrogate for early HIV-1 gene expression, analogous to the procedure described in the legend to Fig. 2. Data shown are arithmetic means ± SD of triplicates of one experiment, which is representative for two to five independent experiments. For the data in (C) a Student's t-test was performed and significance is indicated. n.s. = not significant.