Stimulation of human DC maturation by Ab-opsonized C. burnetii. Human monocyte-derived DC were infected for 24 h with C. burnetii (Cb), C. burnetii that had been incubated with naïve human serum (Cb + naïve), or C. burnetii that had been incubated with immune human serum (Cb + immune). Treatment of DC with E. coli LPS served as a positive control for maturation. DC maturation was indicated by increased expression of the maturation markers CD86, CD80 and CD83 as determined by flow cytometry. Blue histograms represent untreated or uninfected control DC. Red histograms represent LPS treated or infected DC. Black histograms represent isotype controls. The mean fluorescence intensity (MFI) of untreated or uninfected DC and LPS-treated or infected DC are shown. The results shown are from one experiment and representative of three independent experiments.