The biological function of filopodia has been extensively studied while only little work has been done on their mechanical properties. In the present study, we apply magnetic microbeads to explore the capturing and initial step of phagocytosis of pathogens by macrophages through filopodia. Microbeads were covered by the bacterial coat protein invasin which is known to trigger the invasion of the intestine by the bacteria Yersinia enterocolitica. These mimetics of bacteria were placed in the vicinity of J774 mouse macrophages exhibiting long filopodia. The specific adhesion of beads to the tip of a filopodium induced the retraction of the protrusion resulting in the dragging of the bead towards the cell body. The dynamics of the retraction process was analyzed by following the in-plane motion of the bead. We estimated the minimal force developed by filopodia and compared the results with previous magnetic tweezer studies of mechanical force induced growth of protrusions (Vonna et al. 2003). We show that very thin filopodia can generate astonishingly large retraction forces over large distances (>10 microm) and can act as an efficient mechanical tool to detach pathogens adhering on surfaces.
