Neutrophil chemotaxis in moving gradients of fMLP.

In this study the fluid gradient chamber, a modified version of the Boyden chamber that enables mobile gradients, was used to study the migration of human granulocytes in gradients of fMLP. Temporal chemotactic gradients were created by moving density-stabilized spatial gradients at different velocities in relation to migrating cells. Random and directed cell migration was quantified by applying a theoretical population distribution model to experimental cell distributions obtained from cell counts at different depths in the filters. Rates of random and directed migration generally increased with gradient velocity. At negative gradient velocities, i.e., when the gradients were moved in a direction opposite to that of cell migration to decrease fMLP concentration over time, random and directed migration was inhibited. At positive gradient velocities, migration rates were not significantly different from those seen in immobile gradients. The fact that the rate of directed migration was smaller at negative gradient velocities indicates that negative temporal gradients reduced the average speed and/or orientation of the chemotactically migrating cells. In immobile gradients, the cells generated a small concentration increase over time when they migrated in the up-gradient direction. Consequently, a positive temporal gradient as perceived by the cells may act as a positive feedback signal to maintain chemotactic migration.