Effect of MAP kinase inhibitors on PF-4/TNF-induced lactoferrin release in human neutrophils. Neutrophils were preincubated for 15 minutes in the absence or presence of increasing concentrations of (A) SB203580 (p38 inhibitor), or (B) PD098059 (MEK inhibitor), followed by stimulation with a combination of PF-4 (4 μM) and TNF (9 ng/mL) for 30 minutes at 37°C. Cells that received stimulus alone and were left without inhibitor served as controls. Lactoferrin release was determined in cell supernatants by the use of a sandwich ELISA system. Assay backgrounds were determined in samples of unstimulated cells run in parallel were subtracted (A, 1.4% ± 1.1%; B, 1.8% ± 0.6%). Data represent mean ± SD of 5 (A) or 3 (B) independent experiments, each performed in duplicate. Asterisk (*) indicates significant differences (P < .006) between inhibitor-treated and untreated samples based on the data from 5 or 3 individual experiments.