Integrative experimental/computational approach establishes active cellular protrusion as the primary driving force of phagocytic spreading by immune cells

Abstract

The dynamic interplay between cell adhesion and protrusion is a critical determinant of many forms of cell motility. When modeling cell spreading on adhesive surfaces, traditional mathematical treatments often consider passive cell adhesion as the primary, if not exclusive, mechanistic driving force of this cellular motion. To better assess the contribution of active cytoskeletal protrusion to immune-cell spreading during phagocytosis, we here develop a computational framework that allows us to optionally investigate both purely adhesive spreading (“Brownian zipper hypothesis”) as well as protrusion-dominated spreading (“protrusive zipper hypothesis”). We model the cell as an axisymmetric body of highly viscous fluid surrounded by a cortex with uniform surface tension and incorporate as potential driving forces of cell spreading an attractive stress due to receptor-ligand binding and an outward normal stress representing cytoskeletal protrusion, both acting on the cell boundary. We leverage various model predictions against the results of a directly related experimental companion study of human neutrophil phagocytic spreading on substrates coated with different densities of antibodies. We find that the concept of adhesion-driven spreading is incompatible with experimental results such as the independence of the cell-spreading speed on the density of immobilized antibodies. In contrast, the protrusive zipper model agrees well with experimental findings and, when adapted to simulate cell spreading on discrete adhesion sites, it also reproduces the observed positive correlation between antibody density and maximum cell-substrate contact area. Together, our integrative experimental/computational approach shows that phagocytic spreading is driven by cellular protrusion, and that the extent of spreading is limited by the density of adhesion sites.

A theoretical model of efficient phagocytosis driven by curved membrane proteins and active cytoskeleton forces

“…11 Phagocytosis is, however, an active process which is known to involve the forces of actin polymerization that push the engulfing membrane forward. 17,18 In subsequent models, a detailed description of active forces [18][19][20][21] and also the bending and elastic energies of the membrane and cortex are considered. [22][23][24][25][26] The cytoskeletal components of phagocytosis were investigated during the engulfment of artificially produced particles 14,[27][28][29] or during bacterial engulfment by the immune cells.…”

Mechanisms of frustrated phagocytic spreading of human neutrophils on antibody-coated surfaces

“…The comparison of experimental results with theoretical predictions is an essential approach to test mechanistic hypotheses and decide whether observed correlations are coincidental or causative. In a companion study, we have translated different hypotheses about the driving force of phagocytic spreading into mathematical models and examined the mechanical ingredients that such models require to reproduce the cellular behavior ( 28 ). This section provides a broadly accessible summary of our comparison between the measured dynamics of the growth of the cell-substrate contact area and theoretical predictions based on the Brownian zipper and the protrusive zipper models, respectively.…”

“…Details of our mathematical models are provided in the companion paper ( 28 ). In short, we consider an immune cell as an axisymmetric body with uniform surface tension that is filled with a highly viscous, incompressible fluid ( 36 , 37 ).…”

“…Details of our computer model, including all equations and parameter values, are presented in our companion paper ( 28 ). In short, we represent the cell interior as a highly viscous Newtonian fluid and assume mechanical equilibrium.…”

“…In addition to the spreading speed and maximum contact area, this analysis enables us to further characterize the effects of IgG density on phagocytic spreading by quantifying parameters, such as the spreading probability, contour roundness, and centroid motion. In an accompanying theoretical study, we leverage these results against computational models of cell-spreading mechanics ( 28 ). Overall, we find that, although the strength of adhesion can modulate phagocytic spreading, it is the cellular protrusion that sets the rate of spreading, supporting the protrusive zipper hypothesis.…”