Mechanics of Phagocytosis, and a method to quantify both viscoelasticity and surface tension of cells
Julien Husson (LadHyX, Ecole polytechnique)
Phagocytic cells form the first line of defense of the organism, engulfing microbial pathogens. Phagocytosis involves cell mechanical changes that are not yet well understood. Understanding these mechanical modifications promises to shed light on the immune processes that trigger pathological complications. Previous research revealed that phagocytes go through a sequence that includes spreading around their target, followed by an increase in cell tension. Other studies observed an increase in cell tension concomitant with membrane expansion. In the first part of the presentation, I will explain how we solved the apparent contradiction by performing microindentation of phagocytes engulfing targets of different sizes.
In the second part of this presentation, I will present a microindentation method that we are developing in order to measure both the tension of cells and their viscoelastic properties. Key features used in cell mechanical models are indeed cell tension and cell viscoelasticity (representing the average of the cell interior), or a combination of both. It is not clear which of these features is the most relevant or if both should be included. To clarify this, we performed microindentation experiments on cells with microindenters of varying tip radii, including micron-size microneedles. We derived a model predicting that the contact stiffness of the cell should be an affine function of the contact radius and that at vanishing contact radius, the cell stiffness should be equal to the cell tension multiplied by a constant. We tested this model on T cells and both adherent and trypsinized endothelial cells. We draw conclusions on how indenting cells with sharp tips requires careful analysis but is a good way to directly measure surface tension.