ENHANCED IMMUNE COSTIMULATORY ACTIVITY OF PRIMARY ACUTE MYELOID-LEUKEMIA BLASTS AFTER RETROVIRUS-MEDIATED GENE-TRANSFER OF B7.1

Gene modification of malignant cells to express immune stimulators (cy tokines and immune costimulators) has provided he basis for a novel fo rm of immunotherapy. Using a MPSV-based retroviral vector with hygromy cin resistance gene as a selectable marker, we have studied retrovirus -meditated gene transfer of an immune costimulator, B7.1, into primary human acute myeloid leukaemia (AML) cells and the subsequent inductio n of immune costimulatory function. AML blasts from 10 patients were t ransduced by co-culture for 48 h with or without haemopoietic growth f actors (HGFs). In the absence of HGFs, transduction efficiency (TE), a s judged by % B7.1 expressing cells, was low, varying from 0.3 to 8.2% (median 1.5%). Addition of HGFs increased the median TE 1.8-fold with stem cell factor alone and 2.6-fold with SCF, interleukin-3 and GM-CS F. Effects on cell cycling alone could not explain this difference sug gesting other factors such as virus binding and promoter activity, are also involved. CFU-AL assays indicated a higher transduction efficien cy of clonogenic cells, which was not improved by growth factors. Limi ted duration I of cell growth prevented significant expansion of trans duced populations by culture in the presence of hygromycin. Although n or increasing transduction efficiency, CD34 enrichment enhanced drug s election, by targeting cells with the greatest self-renewal capacity. Immunoselection of B7.1 expressing cells produced transduced populatio ns with 30-60% expressing B7.1. in an allogeneic mixed leukaemic cell/ T lymphocyte reaction (MLLR), transduced AML cells enriched by immunos election were able to stimulate allogeneic T cells (CD4 and CD8 positi ve), which could be inhibited by a solubilised B7 receptor, CTLA4.lg. Our results demonstrate that using a replication incompetent retrovira l vector, if is possible to introduce the immune costimulator B7.1 int o primary AML blasts and, by immunoselection, enrich the transduced ce lls, which may be used for subsequent administration as an autologous cellular vaccine.