In another embodiment, the nucleic acid fragment encodes a peptide including an extracellular portion of the human B7-2 antigen (e.g., approximately amino acid residues 24-245 of the sequence provided in FIG. 8 (SEQ ID NO:2)) which can be used to bind CTLA4 and/or CD28 and, in monovalent form, inhibit costimulation, or in multivalent form, induce or enhance costimulation.