MUCOSAL ADDRESSIN CELL-ADHESION MOLECULE-1 - A STRUCTURAL AND FUNCTIONAL-ANALYSIS DEMARCATES THE INTEGRIN BINDING MOTIF

The leukocyte integrin receptor, alpha(4) beta(7), and the mucosal add ressin cell adhesion molecule-1 (MAdCAM-1) are postulated to be import ant in regulating lymphocyte trafficking to normal intestine. Here we provide the first description of MAdCAM-1 expression in inflamed intes tine. Using mouse models of experimentally induced colitis, we show a concordant increase in MAdCAM-1 expression associated with increased c ellular infiltrates in areas of intestinal inflammation. To understand more of the molecular nature of the interactions between MAdCAM-1 and its leukocyte ligand, the alpha(4) beta(7) integrin receptor, we have analyzed the structural and functional properties of chimeric recombi nant MAdCAM-1 proteins in vitro. Using site-directed mutagenesis and m olecular modeling, we demarcate the alpha(4) beta(7) binding mofif as three linear residues within the C-D loop in the first domain of MAdCA M-1. Mutation of residue L40, D41, or T42 in the first domain complete ly abrogates alpha(4) beta(7)(+) cell binding and cellular activation. Mutagenesis of other residues in the first domain do not impact these functions. We have modeled peptides based on the predicted structure of the alpha(4) beta(7) integrin binding motif on MAdCAM-1 and are abl e to show specific and selective blocking of cell binding. These obser vations suggest that the amino acid residues LDT on MAdCAM-1 play a ro le in the interaction with alpha(4) beta(7) in cell adherence and fell activation.