Dynamic traction force measurements of migrating immune cells in 3D matrices

Immune cells such as natural killer (NK) cells migrate with high speeds of several {micro}m/min through dense tissue, but the traction forces are unknown. We present a method to measure dynamic traction forces of fast migrating cells in non-linear biopolymer matrices. The method accounts for the mechanical non-linearity of the 3D tissue matrix and can be applied to time series of confocal or bright-field image stacks. The method is highly sensitive over a large range of forces and object sizes, from [~]1 nN for axon growth cones up to [~]10 {micro}N for mouse intestinal organoids. We find that NK cells display bursts of large traction forces that increase with matrix stiffness and facilitate migration through tight constrictions.