Combined Atomic Force Microscopy and Light sheet Microscopy Characterize the Mechanobiology of Phagocytosis

Phagocytosis is a force-driven cellular process that requires fine spatial and temporal control of the actin cytoskeleton to effectively ingest targets of engulfment. To dislodge disease-causing pathogens embedded in tissues or attached to surfaces, macrophages must overcome the adhesive force and disrupt the contact between the target and the surface. The majority of phagocytosis assays are limited to observing the internalization of freely diffusing targets, ignoring the obstacles immune cells face in native tissue.  Here, I present a method to challenge individual macrophage cells to engulf and detach opsonized polystyrene beads from atomic force microscope (AFM) cantilevers. With the aid of line-Bessel light sheet microscopy (LS), our system allows for high resolution sideview imaging of the events leading up to the bead detachment event simultaneous with sub-nanonewton level force sensitivity to quantify the boundaries of the progressing, actin-filled phagocytic cup as well as quantify the engulfment force. To better understand the force generating mechanisms leading up to engulfment, I have considerd several models for how phagocytic cup height implicitly scales with force and test their validity by fitting them to the experimental data. I have found that the observed force behavior can be broadly categorized into cells which exert an upward push before downward engulfment forces or only downward forces. This is consistent with a model for engulfment in which the dominating contributions to the engulfment force originate from a contractile ring and are proportional to the cell-target contact area. To further analyze the causal relationship between the force generated on the target and the phagocytic cup, I have also cross-correlated the AFM force data with the cup height.  Finally, I report the maximum engulfment force exerted by the cell on the target and argue that bead detachment is achieved via a peeling mechanism. With this, I had provided new insight into the mechanistic engulfment strategies macrophage cells use to navigate host tissues.

Megan Kern’s Thesis Defense

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