SummaryThe heterodimeric ol4 integrins c~4~7 lymphocyte Peyer's patch adhesion molecule ([LPAM]-I) and ol431 (very late antigen-4) are cell surface adhesion molecules involved in lymphocyte trafficking and lymphocyte-cell and matrix interactions. Known cellular ligands include vascular cell adhesion molecule (VCAM)-I, which binds to ~431 and ol437, and the mucosal addressin cell adhesion molecule (MAdCAM)-I, which binds to ol437. Here we show that the o~4 chain of these integrins can itself serve as a ligand. The c~4 chain, immunoaffinity purified and immobilized on glass slides, binds thymocytes and T lymphocytes. Binding exhibits divalent cation requirements and temperature sensitivity which are characteristic of integrin-mediated interactions, and is specifically inhibited by anti-cx4 integrin antibodies, which exert their effect at the cell surface. Cells expressing exclusively c~4/~7 (TK-1) or c~431 (L1-2) both bound avidly, whereas ol4-negative cells did not. A soluble 34-kD ol4 chain fragment retained binding activity, and it inhibited lymphocyte adhesion to ~4 ligands. It has been shown that c~4 integrin binding to fibronectin involves an leucine-aspartic acid-valine (LDV) motif in the HeplI/IIICS region of fibronectin (CS-1 peptide), and homologous sequences are important in binding to VCAM-1 and MAdCAM-1. Three conserved LDV motifs occur in the extracellular sequence of c~4. A synthetic LDV-containing c~4-derived oligopeptide supports c~4-integrin-dependent lymphocyte adhesion and blocks binding to the 34-kD ol4 chain fragment. Our results suggest that r and r integrins may be able to bind to the ol4 subunit on adjacent cells, providing a novel mechanism for ol4 integrin-mediated and activation-regulated lymphocyte interactions during immune responses.