Transfection of tumor cells with the gene encoding the costimulatory molecule B7-1 (CD80), the ligand for CD28 and cytotoxic T lymphocye antigen-4 on T cells, has been shown to result in potent T-cell-mediated antitumor immunity. As an alternative approach, this study analyzed the costimulatory capacity of a human B7-1 immunoglobulin G (IgG) fusion protein targeted to the cell membrane of human acute myeloid leukemia (AML) blasts. Flow cytometric analysis revealed a low constitutive expression of B7-1 on human AML blasts (on average, 3.0 +/- 4.3%; n = 50). In contrast, the expression of B7-2 (CD86) was highly heterogeneous and higher in AML blasts of French-American-British classification types M4 and M5 (P <.0001). The B7-1 IgG fusion protein used in this study efficiently costimulated the proliferation of resting and preactivated T cells when immobilized on plastic. After preincubation with B7-1 IgG, specific binding of the fusion protein to the high-affinity Fcgammareceptor I (CD64) on leukemic cells was demonstrated and was found to increase the proliferation of both allogeneic and autologous T cells in costimulation experiments. Furthermore, targeting of B7-1 IgG to the tumor membrane resulted in increased proliferation of autologous remission T cells and had the potential to generate an enhanced redirected cytotoxic T-cell response against autologous AML blasts. In summary, the targeting of B7-1 IgG fusion protein described in this study represents a strategy alternative to gene therapy to restore the expression of the costimulatory molecule B7-1 on human AML blasts, thereby enhancing their immunogenicity for autologous T cells. This new approach may have implications for T-cell-mediated immunotherapy in AML.