Affinity Modulation of Integrin t 5/31: Regulation of the Functional Response by Soluble Fibronectin

We report that a E1 integrin (o~5fll) can exist in different affinity states for its soluble ligand, fibronectin. The ot5fll expressed by the erythroleu- kemic cell line K562 binds soluble fibronectin with low affinity (Ka > 1 #M), but is induced to bind it with 20-fold higher affinity (Kd-54 nM) in the presence of the anti-ill mAb 8A2. This activation seems to be due to direct antibody-induced change in the receptor that does not require intracellular signaling, and is a plausible basis for the 8A2-induced enhancement of ill-dependent adhesion to fibronectin and other im- mobilized ligands (Kovach, N. L., T. M. Carlos,