Kinetics of γ-IFN down-regulation of LPS-induced MIP-1α expression. Macrophages (1 × 106 cells) were incubated with medium alone or with γ-IFN (200 units/ml) for the times indicated on the left axis, and at time zero (to) LPS was added to a final concentration of 100 ng/ml. Supernatant fluids were collected 24 hr later and assayed for MIP-1 peptides by separate ELISAs. Data are from a representative experiment (of 2) and are expressed as the mean ± SD of values obtained from duplicate macrophage stimulations. As is evident from this figure, pretreatment of macrophages for as little as 1 hour leads to a marked drop in the amount of MIP-1α released in response to LPS stimulation, with less affect on MIP-1β.