There is provided an isolated monoclonal antibody or antigen-binding fragment thereof that competes with the monoclonal antibody produced by the hybridoma deposited with the American Type Culture Collection having the ATCC Patent Deposit Designation PTA-6815 for binding to a polypeptide consisting of amino acid residues 27-164 of SEQ ID NO:2 wherein the monoclonal antibody has a human IgG4 heavy chain immunoglobulin constant domain or wherein the antigen-binding fragment further comprises a human IgG4 heavy chain immunoglobulin constant domain and wherein the monoclonal antibody or antigen-binding fragment inhibits signal transduction induced by the polypeptide through its heterodimeric receptor.
