Assessment of susceptibility of epithelial cells to HTLV-1 infection. (A-D) Immunofluorescence observations of a coculture between Caco-2 cells and HTLV-1–infected lymphocytes (confocal microscopy at apical and basal levels of the cells). Nuclei are stained with DAPI (blue) and the p24 capsid was detected by immunofluorescence (red). In panels A and B, lymphocytes were stained (day 0) in green with the cell tracker CMFDA. Original magnification, 250×. Observation was with a Zeiss AxioVision fluorescence microscope equipped with an Apotome device. Image acquisition was with a Zeiss Axiocam camera. (A) Top view on the infected lymphocytes (apical level) of the coculture 3 days after contact. (C) Basal view on the Caco-2 cell level at day 3 after contact. (B) Top view on the level of the infected lymphocytes on day 7 after coculture. (D) Basal view of the culture showing the absence of immunoreactivity within Caco-2 cells on day 7 after coculture. (E) Production of p19 up to 21 days after coculture with irradiated HTLV-1–infected lymphocytes monitored by ELISA assay. Three different human enterocytic cell lines were tested for viral p19 production: Caco-2 (blue line), HT29 (red line), and T84 (green line) cells. For the 3 cell lines, viral protein p19 production was shown to decrease in a constant manner up to day 21.