Experimental and computational studies of receptor-mediated cell detachment in shear flow

Specific, receptor-mediated adhesion of cells to ligand-coated surfaces in viscous shear flow is a central phenomenon in many physiological and biotechnological processes. Mechanical strength of specific adhesion is generally presumed to be related to chemical affinity of receptor/ligand bonds, but no experimental study has been previously directed toward this issue. I investigate the dependence of receptor/ligand adhesion strength on bond affinity using a radial fluid flow chamber assay to measure the force needed to detach polystyrene beads covalently coated with immunoglobulin G (IgG) from glass surfaces covalently coated with protein A (SpA). A spectrum of animal species sources for IgG permits examination of three decades of SpA/IgG binding affinity. The results for the model cell system demonstrate that bond strength varies with the logarithm of the binding affinity, providing the first experimental support for theoretical treatments of this relationship.In addition, I simulate the detachment of antibody-coated hard spheres from a ligand-coated substrate using a dynamic, probabilistic single-cell model. The antibody-ligand bonds are treated as adhesive springs. The simulation method is an extension of the cell attachment and rolling simulation developed by Hammer and Apte (1992). The distribution of receptors on the sphere and the forward and reverse reactions between receptor and ligand are simulated using random number sampling of appropriate probability functions. After attachment of the sphere to the substrate, flow is initiated, and detachment is measured by the significant displacement of previously bound particles. Displacement results when bonds break and too few bonds remain to withstand the external hydrodynamic stresses. The model simulates the effects of many parameters on cell detachment, including hydrodynamic stresses, receptor number, ligand density, reaction rates between receptor and ligand, and stiffness and reactive compliance of the adhesive springs. The simulations accurately recreate our experimental data relating measured bead adhesion strength to molecular properties of the adhesion molecules. In addition, the model predicts that a critical modulating parameter of detachment is the fractional spring slippage, which relates the strain of a bond to its rate of breakage.U of I OnlyETDs are only available to UIUC Users without author permissio