An Antibody to the Lutheran Glycoprotein Recognizing the LU4 Blood Type Variant Inhibits Cell Adhesion to Laminin [alpha]5

Background The Lutheran blood group glycoprotein (Lu), an Ig superfamily (IgSF) transmembrane receptor, is also known as basal cell adhesion molecule (B-CAM). Lu/B-CAM is a specific receptor for laminin [alpha]5, a major component of basement membranes in various tissues. Previous reports have shown that Lu/B-CAM binding to laminin [alpha]5 contributes to sickle cell vaso-occlusion. However, as there are no useful tools such as function-blocking antibodies or drugs, it is unclear how epithelial and sickled red blood cells adhere to laminin [alpha]5 via Lu/B-CAM. Methodology/Principal Findings In this study, we discovered a function-blocking antibody that inhibits Lu binding to laminin [alpha]5 using a unique binding assay on tissue sections. To characterize the function-blocking antibody, we identified the site on Lu/B-CAM recognized by this antibody. The extracellular domain of Lu/B-CAM contains five IgSF domains, D1-D2-D3-D4-D5. The antibody epitope was localized to D2, but not to the D3 domain containing the major part of the laminin [alpha]5 binding site. Furthermore, mutagenesis studies showed that Arg.sup.175, the LU4 blood group antigenic site, was crucial for forming the epitope and the antibody bound sufficiently close to sterically hinder the interaction with [alpha]5. Cell adhesion assay using the antibody also showed that Lu/B-CAM serves as a secondary receptor for the adhesion of carcinoma cells to laminin [alpha]5. Conclusion/Significance This function-blocking antibody against Lu/B-CAM should be useful for not only investigating cell adhesion to laminin [alpha]5 but also for developing drugs to inhibit sickle cell vaso-occlusion.