Cell mechanics during phagocytosis studied by optical tweezers-based microscopy

Phagocytosis, which is a central cellular mechanism in the mammalian immune system, was so far - in the context of cell-biological studies - mainly investigated by conventional light and electron microscopies. However, the mechanical properties of this process, like the forces and energies involved were barely known up to now. Here, optical tweezers-based microscopy in combination with coated beads as bacterial model system was applied to investigate the mechanical properties of phagocytosis. The binding, the uptake into the cell and the intracellular transport of single optically trapped beads was induced by the optical trap and measured in 3D with nanometer precision by an interferometric tracking technique. Thereby, a novel cellular behaviour was discovered: A few seconds after binding, filopodia and ruffles retracted and pulled the bound beads towards the cell. The observation of discrete F-actin dependent 33-nanometer steps during retraction led to the hypothesis that a processive myosin motor plays an important role in the retraction. Force-velocity measurements revealed the mechanical properties of this putative motor. A model for the force-dependent motor kinetics confirming these results was developed. In order to perform these measurements, the microscope’s optical trapping and tracking properties were significantly improved and shown to be in very good agreement with theoretical predictions based on Fourieroptics and extendedMie-scattering theory.