ANALYSIS OF 4-1BBL AND LAMININ-BINDING TO MURINE 4-1BB, A MEMBER OF THE TUMOR-NECROSIS-FACTOR RECEPTOR SUPERFAMILY, AND COMPARISON WITH HUMAN 4-1BB

The T cell activation antigen 4-1BB (CDw137) is a distantly related me mber of the tumor necrosis factor receptor family of cell surface rece ptors, We previously reported that murine 4-1BB (m4-1BB) bound to extr acellular matrix (ECM) proteins, Recently, a tumor necrosis factor-lik e ligand of m4-1BB, m4-1BBL, as well as the human counterparts of 4-1B B (ILA) and 4-1BBL (h4-1BB and h4-1BBL, respectively) have been cloned , No information is currently available on how binding of m4-1BB to EC M proteins affects its binding to m4-1BBL and vice versa and if the ab ility of m4-1BB to bind ECM proteins is conserved across species, We r eport that binding of m4-1BBL to m4-1BB blocked its ability to bind la minin (LN), while binding of m4-1BB to LN did not block its ability to bind m4-1BBL, Furthermore, binding of m4-1BBL to the m4-1BB LN comple x did not displace LN, These findings suggest the two ligands bind to proximal but distinct sites on m4-1BB, This is supported by the observ ation that six of eight anti-m4-1BB monoclonal antibodies blocked the interaction between 4-1BB and 4-1BBL, while seven blocked LN binding, Ligand and monoclonal antibody binding studies with a truncated protei n lacking the amino-terminal LN-homologous domain of m4-1BB demonstrat ed that regions downstream of the LN-homologous domain participate in LN binding and that the intact protein is required for m4-1BBL binding , Studies with h4-1BB showed that h4-1BB only bound h4-1BBL, indicatin g that the ECM binding activity of 4-1BB is not conserved across speci es, This finding allowed the construction of murine/human 4-1BB chimer as, which permitted further dissection of the regions of 4-1BB involve d in LN and 4-1BBL binding and suggests that sequence differences in t he LN-homologous domain of h4-1BB in part account for the inability of h4-1BB to bind ECM proteins.