Homologous priming in chemotactic peptide-stimulated neutrophils

The kinetics and dose-dependence of activation of human neutrophils exposed to sequential additions of the chemotactic peptide n-formyl-methionyl-leucyl-phenylalanine (fMLP) have been investigated by multiwell microplate assays. Treatment of neutrophils with medium-high doses (from 10(-8) to 5 x 10(-7) M) of fMLP caused activation of superoxide anion (O2-) production, but prevented further activation by a subsequent addition of an optimal dose (from 10(-7) M to 5 x 10(-7) M) of fMLP. These findings represent an example of cell desensitization, or adaptation. However, neutrophils treated with low, sub-stimulatory doses (from 10(-10) to 5 x 10(-9) M) of the peptide and then treated with optimal doses of fMLP exhibited an O2- production that was two to three-fold higher than that induced by the same optimal doses on untreated cells. A similar phenomenon of homologous priming of the oxidative metabolism of neutrophil has not previously been described or characterized. Priming was maximal after about 30 min of incubation with fMLP, which differed from desensitization, which required only a few minutes. Homologous priming was not confined to O2- production, but was also observed with the release of the granule enzyme, lysozyme. Low doses of fMLP were also capable of triggering an increase of intracellular free Ca2+ and of fMLP membrane receptors, which are possible mechanisms responsible for priming.