A polypeptide (8F4 molecule) with a T-cell costimulating biological activity is disclosed, as well as monoclonal antibodies against the 8F4 molecule and hybridoma cells which produce the monoclonal antibodies, the use as medicaments of substances which inhibit the biological activity of the 8F4 polypeptide, in particular monoclonal antibodies, natural or synthetic ligands, agonists or antagonists, in particular for preventing or treating diseases which involve the immune system, the use of the 8F4 molecule or cells containing the 8F4 molecule as medicaments, in particular for preventing or treating diseases which involve the immune system, and the use of substances which specifically recognize the 8F4 polypeptide, in particular monoclonal antibodies, natural or synthetic ligands, agonists or antagonists, for diagnosing diseases which involve the immune system.