T-Cell Activation Molecule 4-1BB Binds to Extracellular Matrix Proteins

The recently isolated 4-1BB cDNA clone encodes a cell surface protein expressed by activated T cells. Its extracellular domain is homologous to members of the nerve growth factor receptor super family and its cytoplasmic domain contains a sequence homologous to the binding site for the T-cell-specific tyrosine kinase p56lckfound in the cytoplasmic domains of CD4 and CD8α. At present the function of 4-1BB is not known. We prepared a 4-1BB-immunoglobulin fusion protein (4-1BB Rg). This protein was used in immunohistochemical studies to identify tissues that express the 4-1BB ligand. 4-1BB Rg bound to virtually all tissues examined, suggesting that extracellular components might function as its ligands. To explore this possibility, 4-1BB was expressed in COS cells and found to mediate the binding of fibronectin, vitronectin, laminin, and collagen VI but not of collagen I. The binding of extracellular matrix proteins to 4-1BB was not mediated by Arg-Gly-Asp (RGD) or CS-1 amino acid sequences. Experiments with overlapping proteolytic fragments of fibronectin showed that 4-1BB interacts with multiple regions of fibronectin. The interaction between extracellular matrix proteins and 4-1BB was completely blocked by the anionic carbohydrate polymer fucoidan and was partially blocked by the anionic carbohydrate polymer dextran sulfate and the glycosaminoglycan heparin sulfate but was unaffected by desulfated heparin. These results suggest that carbohydrates may play a role in mediating the 4-1BB-extracellular matrix protein adhesion.