A molecular and biological study of the role of B7-1 and B7-2 antigens in the immunostimulatory properties of myeloid leukaemic cells

T lymphocytes require two signals for their optimal activation and subsequent proliferation. One of these signals is provided by antigen presented in the relevant HLA groove binding to a reciprocal T cell receptor and the second by a costimulatory molecule interacting with its ligand on the T cell. If antigen is presented to the T cell in the absence of a costimulatory signal the T cell enters into a state of unresponsiveness, termed anergy. The lack of expression of co-stimulatory molecules by tumour cells has been suggested to be a reason why the immune response fails to eradicate the tumour. The major co-stimulatory molecules are B7-1 (CD80) and B7-2 (CD86) which bind to CD28 on the T cell and induction of either of these molecules on tumour cells has resulted in the development of effective anti-tumour immunity in syngeneic murine models. Phenotyping studies of AML demonstrated B7-1 was infrequently expressed, whereas greater than 50% of the samples analysed expressed 87-2. The duration of first remission was longer in those patients in whom a higher percentage of blasts expressed B7-2 than those that expressed less, suggesting that, in these patients, an anti-leukaemic immune response may be accounting for the longer duration of remission. B7-2+ve AML blasts were better able to stimulate allogeneic T cells to proliferate and produce interleukin-2 (IL-2) when compared to B7-2-ve AML blasts. This suggests that the B7-2 was functional and that expression of B7-2 by the AML blasts may induce an anti-leukaemic immune response, if a suitable immunogenic peptide was expressed. Transfectants expressing B7-1 or B7-2 were generated in the "293" cell line to compare their co-stimulatory ability. The transfectants were used in a costimulation assay to stimulate allogeneic T cells to show the molecules were functional. In a cytotoxicity assay greatest killing was seen with the B7-1 293 cells as stimulators, followed by the B7-2 293 cells and finally the control 293 cells, suggesting B7-1 is the preferred co-stimulatory molecule for tumour vaccination strategies. To overcome lack of costimulation by the AML blasts leading to anergy in potential tumour reactive T cells, AML cells were transduced with recombinant AAV containing an expression cassette for B7-1 or B7-2. Compared to control AML blasts, the B7-1 or B7-2 expressing AML blasts induced greater proliferation in allogeneic T cells. This suggests that the strategy of inducing expression of costimulatory molecules on AML cells may be effective in the induction of an anti-leukaemic immune response. It is hoped that in the future these modified leukaemic cells will be used to generate autologous anti-leukaemic T cells ex vivo, which may, after expansion be returned to the patient as specific anti-leukaemic cellular therapy