Hyaluronan Binding by Cell Surface CD44

Abstract

CD44 is the primary cell surface receptor for the extracellular matrix glycosaminoglycan hyaluronan. Here we determined the relative avidities of unlabeled hyaluronan preparations for cell surface CD44 by their ability to block the binding of fluorescein-conjugated hyaluronan to a variety of cells. We show that hyaluronan binding at the cell surface is a complex interplay of multivalent binding events affected by the size of the multivalent hyaluronan ligand, the quantity and density of cell surface CD44, and the activation state of CD44 as determined by cell-specific factors and/or treatment with CD44-specific monoclonal antibody (mAb). Using low <i>M</i> _r hyaluronan oligomers of defined sizes, we observed monovalent binding between 6 and 18 sugars. At ∼20 to ∼38 sugars, there was an increase in avidity (∼3×), suggesting that divalent binding was occurring. In the presence of the inducing mAb IRAWB14, monovalent binding avidity was similar to that of noninduced CD44, but beginning at ∼20 residues, there was a dramatic and progressive increase in avidity with increasing oligomer size (∼22 < 26 < 30 < 34 < 38 sugars). Kinetic studies of binding and dissociation of fluorescein-conjugated hyaluronan indicated that inducing mAb treatment had little effect on the binding kinetics, but dissociation from the cell surface was greatly delayed by inducing mAb.