Increased numbers of bonds stabilize adhesion with multiple tethers between endothelium and monocytes

This investigation explores the hypothesis that increased receptor density promotes the formation of tethers. Micropipet aspiration was used to produce defined normal forces during contact and detachment between monocytes and human umbilical vein endothelial cells (HUVEC). Receptor densities were altered by incubation of HUVEC with TNF-α or TNF-α in the presence of cycloheximide. Receptor numbers were quantified using calibrated flow cytometry. Increased receptor densities resulted in a significant increase in the probability of adhesion between cells and the percent of multiple tethers formed. For a given adhesion probability (P_a), the frequency of forming multiple tethers coincided with the theoretical frequency of forming multiple bonds (P_n>1). There was a lag period prior to tether formation that increased with TNF-α stimulation of HUVEC. Cycloheximide significantly reduced the density of ICAM-1 receptors, the percent of multiple tethers, and the characteristic lag time before tether formation. These results imply that elevated receptor densities in the contact region stabilize adhesion through the formation of multiple attachment points. Multiple tethers may provide a mechanism for stabilizing firm adhesion and resisting detachment in the relatively high shear stress environment found in the arterial regions prone to monocyte accumulation at the onset of atherosclerosis.