Cellular memory: neutrophil orientation reverses during temporally decreasing chemoattractant concentrations

Cell directional orientation or shape polarization is the first cellular step in neutrophil locomotion. To better understand how chemoattractants interact with cells, we studied neutrophil polarization (or shape changes) during exposure to a temporally decreasing chemoattractant signal of N-formyl-methionyl-leucyl-phenylalanine (FMLP) in the absence of a spatial concentration gradient. To accomplish this objective, we used a manifold of differing FMLP concentrations attached to a stopped-flow microscope chamber. Spatial gradients of a fluorescent chemotactic peptide could not be detected in the chamber by using microfluorometry. When FMLP was injected at continually increasing concentrations at 10-s intervals, the shape and relative direction of the neutrophil persisted. However, when temporally decreasing FMLP concentrations were injected, approximately 80% of the cells changed their direction with 44% of the total cells swinging about to 180 degrees +/- 15 degrees. Most of these directional changes involved dissolution of both the lamellipodium and uropod and reformation of these structures 180 degrees from their original positions. This research suggests that neutrophils reverse their morphological polarity when exposed to temporally decreasing ligand concentrations by "remembering" their ligand exposure history and relative direction.