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Specific Recognition of Macroscopic Objects by the Cell Surface: Evidence for a Receptor Density Threshold Revealed by Micrometric Particle Binding Characteristics

Stéphanie Sarda ^*, David Pointu , Frédéric Pincet  and Nelly Henry ^* 

^* Laboratoire Chimie Bioinorganique Médicale, Institut Universitaire Technologique Paul Sabatier, Castres, France; Institut Curie, Laboratoire Physico Chimie Curie, Centre National de la Recherche Scientifique, Unité Mixte de Recherche 168, Paris, France; and Laboratoire de Physique Statistique–Ecole Normale Supérieure, Paris, France

Correspondence: Address reprint requests to Nelly Henry, Institut Curie, CNRS, UMR 168, Physico Chimie Curie, 11 rue P. et M. Curie, Paris, France 75005. Tel.: 33-01-42-34-6495; E-mail: nelly.henry{at}curie.fr.

The establishment of specific molecular bonds between a cell and a facing surface is involved in many physiological and technological situations. Using micrometric magnetic particles, we have explored the formation of specific molecular bonds between the cell and surfaces bearing complementary ligands under passive conditions. Streptavidin-coated particles were targeted to the cell surface of a B-cell line through a specific biotinylated antibody against the CD19 receptor. Flow cytometry, optical microscopy, and micropipette experimental techniques have been used. Main findings have been that cell surface receptor density acted like a switch for particle capture with a threshold value found here equal to 1.6 x 10³ receptor/µm². This led to exclusion from binding of the cells of lowest receptor density. The density threshold was modulated by the length of the binding link and the physics of the cell/particle collision. We suggest that the shear stress is one of the main determinants of the characteristics of binding. We also show that several thousand receptors were involved in the cell particle contact at the end of the binding process, although only eight bonds are required for the initial capture of a particle. A passive binding inhibition process due to link concentration by the initial contact was proposed to account for the small number of particles per cell.