Dose-dependent effect of LPS on intracellular IL-12 (p40/p70) expression. Flow cytometric analysis of stimulated human monocytes (IFN-γ 300 U/mL, LPS 50 ng/mL) pretreated for 24 hours with varying concentrations of LPS. Intracellular cytokine staining for IL-12 (p40/p70) and IL-6 is shown. The quadrant markers for the dot plot were set based on the negative staining controls using isotype-matched Ig antibodies. The percentages of cells in the corresponding quadrants are given. On the horizontal axis, forward scatter is depicted, and on the vertical axis, binding of PE-labeled anti–IL-6 or anti–IL-12 (p40/p70) antibody is shown. The corresponding results for IL-12 and IL-6 are given. (Top) Negative control (autofluorescence of monocytes); (first two lines left panel) stimulation with IFN-γ/LPS for 24 hours without LPS preincubation; (middle) preincubation with 50 ng/mL LPS before stimulation; (right) preincubation with 10 ng/mL LPS; (last two lines left panel) 1 ng/mL LPS; middle: 0.1 ng/mL LPS; right: 0.01 ng/mL LPS.