Immunoelectron microscopy analysis of CD63 association with the chlamydial inclusion. HEp-2 cells were infected with C. trachomatis E for 48 hours and analyzed by cryo immunoelectron microscopy. Infected cells immunolabeled with anti-CD63 antibody (anti-mouse-18 nm colloidal gold) revealed CD63 within the chlamydial inclusion and in compartments adjacent to the inclusion that morphologically resembled MVBs (arrows). CD63-labeling was also evident along the inclusion membrane (upper panel, arrowhead; enlarged in right panel) and in small vesicles within the inclusion lumen (lower panel arrowheads; enlarged in right panel). C, Chlamydia; m; mitochondria. Bar, 0.5 μm. For quantification of anti-CD63-labeling, the density of colloidal-gold particles was determined per cross-sectional area of the compartments indicated (particles/μm2). The density was also expressed as fold over background, based on the average background-labeling in mitochondria and nuclei.