Dose-dependent effect of LPS on the secretion of IL-12p70 and p40 by stimulated monocytes. Monocytes were pretreated with medium alone or varying concentrations of LPS for 24 hours. After washing, cells were challenged with IFN-γ (300 U/mL) and LPS (50 ng/mL) for 24 hours, culture supernatants were harvested, and assayed for IL-12p70 and p40 accumulation by ELISA. A stimulation index was calculated using LPS/IFN-γ–stimulated cells without pretreatment as positive control (100%; mean of p70 positive control: 35.4 pg/mL, mean of p40 positive control: 1,331 pg/mL). Mean values of 5 independent experiments ± SEM are shown. The downmodulation of IL-12p40 and p70 was significant (P < .05) for all LPS concentrations.