A: The expressions of CD81 proteins in U937-Mock and -CD81 stable transfectant cells were examined by Western blot. The levels of tubulin proteins are used as loading control. B: The U937 transfectant cells were fixed, thin-sectioned, stained, and analyzed with TEM as described earlier (28). Images represent the results from one out of three individual experiments. Representative images of whole cells and microvilli from the U937 transfectants expressing either control construct or CD81 are presented. Scale bar, 2.0 µm. Arrow: microvillus. C. Quantitative analysis. The numbers of microvilli were counted from individual cells, the radii of microvillar tips were assessed using the transverse cross section of microvilli, and the lengths of microvilli were measured using the longitudinal cross section of microvilli. n=15~17 cells for Mock group and =17~20 cells for CD81 group. *, P<0.05; **, P< or =0.01. D: The localization of CD81 in U937 transfectant cells was examined using immunofluorescence under a confocal microscope. Arrows indicate the microvilli positive in CD81. Scale bar, 5.0 µm.