Microscopic observations of tight junctions between Caco-2 cells cocultured with HTLV-1–infected or uninfected lymphocytes. (A) Experimental setting to study the viral passage after coculture. Caco-2 cells were cultured on Transwell filters before the addition of HTLV-1–infected (or uninfected) lymphocytes to the apical compartment. Two days later, cocultures were fixed and processed for observations by transmission electron microscopy or immunofluorescence microscopy. (B-C) Transmission electron microscopy showing tight junctions (indicated by black arrows) between epithelial cells cocultured with uninfected lymphocytes (B) and with HTLV-1–infected lymphocytes (C). Scale bar indicates 1 μm. Observation was with a JEOL 1010 transmission electron microscope operating at 80 kV. Image acquisition was with a Nikon CCD camera. (D-G) Detection of tight-junction proteins by immunofluorescence. Immunoreactivity for occludin and ZO-1 (D-E and F-G, respectively) could be detected according to the same pattern between epithelial cells cocultured with uninfected lymphocytes (D,F) or with HTLV-1–infected lymphocytes (E,G). Original magnification, 200×. Observation was with a Leica DMRB microscope equipped for fluorescence. Image acquisition was with a Nikon Coolpix 8400 camera.