Invited Talk

Three-dimensional force microscopy of immune cells in biopolymer networks

FAU Erlangen-Nuremberg, Department of Physics, Biophysics Group, Henkestraße 91, 91052 Erlangen

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To reach targets outside the bloodstream, immune cells such as macrophages, dendritic cells, B cells, T cells and Natural Killer (NK) cells can extravasate and migrate through connective tissue. In contrast to migratory mesenchymal cells, however, the importance of focal adhesion contact formation and traction force generation for the migration of immune cells are not well understood. We employ time-lapse confocal reflection microscopy in combination with Bayesian filtering and high-throughput 3D migration assays to obtain simultaneous measurements of migration speed, directional persistence, cell morphology, and cell contractility. While we confirm that immune cells employ a non-contractile amoeboid migration mode by default, we also find that NK92 cells as well as ex-vivo expanded primary NK cells exert substantial forces on the ECM in the form of short contractile phases that reach ~25nN. Even non-activated primary B-, T-, NK-cells, Neutrophils, Monocytes all show this burst-like contractile behavior, albeit with weaker forces ranging between 5-25nN. Importantly, we show that cell speed and directional persistence of NK cells increase during and following these contractile phases, indicating that the cells actively use traction forces to overcome steric hindrance and avoid getting stuck in narrow pores of the ECM. Consequently, inhibiting cell adhesion to the ECM (and thus traction force generation) by using integrin b1 antibodies, by adding EDTA (to reduce the adhesive strength of integrins), by adding Pluronic (to reduce adhesion to collagen fibers), or by placing the cells in a non-adhesive Carbomer hydrogel, reduces the fraction of motile cells and their directional persistence, while the remaining motile cells retain their full cell speed. We conclude that steric hindrance can induce a switch in the migration mode of immune cells, from a non-adhesive amoeboid migration mode to a highly contractile migration mode that very much resembles the gliding motion of motile mesenchymal cells.