A novel experimental method of producing and observing the active motion of polymorphonuclear leukocytes (PMNs) using a micropipette technique has been recently developed (Usami et al., 1992). The present paper develops a quantitative theory for the chemoattractant gradients and cell locomotion observed in these experiments. In previous experimental methods (e.g., the Boyden chamber, the Zygmond chamber and the Dunn chamber) for study chemotaxis of leukocytes, fibroblasts, and PMNs, the exact nature of the concentration gradient of the chemoattractant is unknown. The cells may themselves modify the local gradient of the chemoattractant. In experiments using the micropipette, an internal source of chemoattractant provides well-defined boundary and initial conditions which allow the computation of the chemoattractant concentration gradient during the active locomotion of the PMNs. Since the cell completely fills the pipette lumen, convection is limited to the motion of the cells themselves. In coordinates moving with cell, it is assumed that diffusion is the only mechanism of mass transport of the chemoattractant (fMLP). Computations of the fMLP concentration during locomotion of the cell were carried out for a range of rates of fMLP binding by the receptors expressed on the front face of the cell membrane. The results show that the front face of the cell is subjected to increasing fMLP concentration during the cell motion. The sequence of events involve receptor binding of fMLP, signal transduction, polymerization of the cell cytoskeleton at the membrane of the front face, spatially dependent adhesion to the pipette wall, and localized contraction of the cytoskeleton. This sequence of events leads to the steady locomotion of the leukocytes in the micropipette. The computation of the distribution of the fMLP concentration during cell locomotion with constant velocity in micropipette experiments shows that the cell is exposed to increasing concentration of fMLP. This suggests that chemotaxis maybe induced by temporal gradient of an attractant.

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