Receptor-mediated adhesion phenomena: Theoretical analysis and experimental model studies with the radial-flow detachment assay

Receptor-mediated cell adhesion to ligand-coated surfaces plays a key role in many physiological and biotechnology-related processes. Our goal is to combine theoretical models with well-defined experiments to single out the role of certain key parameters in cell adhesion. Our experimental studies use a simple model system that consists of receptor-coated latex beads (prototype cells) and a Radial-Flow Detachment Assay (RFDA) with a ligand-coated surface. The RFDA produces a laminar axisymmetric flow field, characterized by a radial decrease in the surface shear stress. The RFDA is designed for direct observation of cell detachment over a range of shear stresses with quantitative measurement of the adhesive force. The receptors and ligands used in this work are complementary antibodies. The beads enable us to examine several aspects of adhesion with particles possessing comparatively uniform properties that can be varied systematically in a range typical for receptor-mediated cell adhesion. Model cell data are obtained under both equilibrium and transient conditions, yielding equilibrium detachment data as well as time-dependent attachment and detachment data. Our equilibrium detachment experiments focus on the effects of receptor and ligand densities and pH and ionic strength of the medium. We develop a deterministic model for the analysis of these data. We demonstrate experimental validation of a theoretical expression for the critical shear stress for particle detachment and show that it is consistent with reasonable estimates for the receptor-ligand affinity constant. Our transient experiments focus on the effects of receptor and ligand densities and shear stress. A deterministic model of receptor-ligand bond dynamics cannot account for the transient behavior because it does not predict temporally-continuous attachment or detachment. We develop a probabilistic model to account for deviations from ideal, deterministic behavior, and show that our data can be readily explained with this model. This is not surprising because fluctuations are inherent in chemical reactions, particularly when the number of reacting molecules is small (as with adhesion receptors). In addition, we use our equilibrium detachment and transient probabilistic analyses to examine cell adhesion data reported in the literature