COMMENTARY Gradient perception by neutrophil leucocytes, continued

The mechanism by which neutrophil leucocytes exhibit chemotaxis continues to be the subject of debate. In recent articles in this journal (Vicker et al. 1986; Ilaston & Wilkinson, 1987) the question of whether neutrophils can perceive spatial attractant concentration gradients has been discussed. The purpose of this brief note is to give attention to some information that should help clarify some of the points raised by these articles. Vicker et al. (1986) reported that, in order for neutrophils to exhibit directional orientation bias, they need to encounter a chemoattractant concentration that increases with respect to time, and may not be able to detect a stable spatial concentration gradient. This conclusion was based on comparison of cell movement in gradients possessing a temporally increasing component to movement in gradients lacking a significant temporally increasing component. It is not difficult to show, however, that their results could be explained equally well by the spatial gradients present in these experiments. Calculations based on analysis of molecular diffusion processes (e.g. see Lauffenburger & Zigmond, 1981) showed that the magnitudes of the spatial gradients present in the experiments with a temporally increasing concentration were great enough to stimulate an orientation response (cf. Zigmond, 1977), while the magnitudes of the spatial gradients present in the experiments lacking a temporal increase in concentration were too small to stimulate orientation. That is, roughly 1 % concentration change over about 10 jitm distance is needed to induce a significant level of orientation at optimal peptidc chemoattractant concentrations. In the filter experiments performed by Vicker et al., however, we calculate that the gradient present after the lOmin incubation at 0°C is only about 0-6% over 10/.{in and decreases to less than 0-3% at 60min. Similarly, in the experiments under agarose that they present, we calculate that there is less than a 0 1 % gradient present over 10 jitm after the 2-h incubation at 0°C. In contrast, the spatial gradients present during the initial incubation periods were calculated to