Dual effects of formylpeptides on the adhesion of endotoxin-primed human neutrophils

Neutrophils, treated with sequential additions of bacterial products such as endotoxin (E. Coli lipopolysaccharide, LPS) and the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP), undergo to metabolic activation and express membrane-anchoring proteins that promote adhesion to serum-coated culture wells. By investigating the dose–response relationships of these phenomena, we have found that: (a) resting neutrophils do not produce a significant amount of superoxide (O) and show only minimal adhesion to serum-coated plastic surfaces; (b) fully activatory doeses (> 5 × 10−8M) of fMLP induce the release of O and a significant increase of the cell adhesion; (c) pretreatment of the cells for 1 h with LPS augments cell adhesion to serum-coated culture wells in the absence of further stimulation and primes the neutrophils to enhanced fMLP-dependent O release; (d) addition of low, substimulatory doses of fMLP (from 10−10M to 5 × 10−9M) inhibits and reverses the adhesion of LPS-treated cells, (e) high fMLP doses (> 10−7M) are additive to LPS in promoting adhesion. Phorbol-myristate acetate (> 10−9M) increased adhesion in both normal and LPS-treated neutrophils, but low doses of this stimulant did not inhibit adhesion. Low doses (10−9M) of fMLP increased intracellular cyclic AMP in both normal and LPS-treated neutrophils, suggesting that stimulus-induced rises in cAMP may be the negative signal responsible for down-modulation of adhesion. Low (5 × 10−9M) and high (5 × 10−7M) fMLP doses induced the same increase of expression of CD11/CD18 integrins, indicating that the inhibition of adhesion caused by low doses is not due to quantitative down-regulation of integrins. These findings may provide an in vitro model of the complex biological events involved in the regulation of neutrophil adhesion.