Mechanics of neutrophil phagocytosis: behavior of the cortical tension

The mechanical implementation of phagocytosis requires a well-coordinated deployment of cytoplasm and membrane during the creation of a phagosome. We follow the time course of this process in initially round passive neutrophils presented with antibody-coated beads of radii 1.1 to 5.5 µm. In particular, we monitor the cortical tension as the apparent cellular surface area increases due to cell-driven deformations induced by phagocytosis. The behavior of the tension is then compared with conditions of similar area expansion caused by externally imposed deformations during cell aspiration into a micropipette. Whereas the resting tension remains low for an area expansion of up to only 30% during aspiration, it remains low even after an area expansion of up to 80% in phagocytosis. This is probably the result of membrane insertion from inner stores by exocytosis. We further find that the onset of viscous tension, proportional to the rate of area expansion and caused by the unfurling of plasma membrane wrinkles, is significantly delayed in phagocytosis compared with aspiration. We propose that this is the result of phagocytosis-triggered enzymatic activity that releases spare plasma membrane normally sequestered by velcro-like bonds in a reservoir of surface folds and villi.