Integrins play important roles in immune response, lymphocyte targeting, and regulation of gene expression. Recently, alpha3 and alpha4 chains of integrins VLA3 and 4 were cloned and sequenced. VLA3 is a fibronectin (FN)/collagen/laminin receptor. VLA4 is a) a FN receptor, b) receptor for vascular cell adhesion molecule-1 (VCAM-1) which is expressed on an activated endothelial cell surface, and c) a homing receptor of lymphocytes for Peyer's patch. VLA5 is a classical FN receptor which recognizes the RGD sequence of a FN cell binding domain. Each integrin plays a role in lymphocyte function, and therefore is a potential diagnostic and therapeutic target in immunologic diseases. To elucidate the mechanism of VLA3-5/ligands interaction, it is proposed 1) to identify the VLA3 binding sites in collagen, laminin and FN molecules by using the recombinant VLA3 expressed in transfected cells and ligand fragments or synthetic peptides, 2) to identify candidate ligand binding sites on VLA3-5 molecules by chemical cross-linking of the labeled synthetic peptides from the identified binding sites in the ligand molecules (e.g. CS-1 peptide for VLA4, RGD containing peptide for VLA5) to integrins, and 3) to examine a) the effects of the mutation of the identified ligand binding sites, b) the effects of the introduction of the function inhibiting mutations in beta2 or beta3 integrin in patients [e.g. Glanzmann's thrombasthenia (Asp 119 to Tyr in beta3 chain)] into the corresponding site of beta1 integrin on the functions of the VLA3-5. In order to accomplish the proposed work the stably transfected CHO cell lines which express a large quantity of recombinant VLA3 or VLA4 species on the surface will be established (VLA5 overproducing cells are already available). These experiments will analyze the common and specific mechanisms of VLA3- 5/ligand interaction. Peptides which modify these interactions might modulate the targeting and function of lymphocytes, monocytes and other cells involved in the pathogenesis of the immunologic diseases.