TRANSDUCTION OF MURINE BONE-MARROW CELLS WITH AN MDR1 VECTOR ENABLES EX-VIVO STEM-CELL EXPANSION, BUT THESE EXPANDED GRAFTS CAUSE A MYELOPROLIFERATIVE SYNDROME IN TRANSPLANTED MICE

Attempts to expand repopulating hematopoietic cells ex vivo have yield ed only modest amplification in stem cell numbers. We now report that expression of an exogenous human multi-drug resistance 1 (MDR1) gene e nables dramatic ex vivo stem cell expansion in the presence of early a cting hematopoietic cytokines, Bone marrow cells were transduced with retroviral vectors expressing either the MDR1 gene or a variant of hum an dihydrofolate reductase (DHFR), and then expanded for 12 days in th e presence of interleukin-3 (IL-3), IL-6, and stem cell factor. When t hese cells were injected into nonirradiated mice, high levels of long- term engraftment were only seen with MDR1-transduced grafts. To verify that expansion of MDR1-transduced repopulating cells had occurred, co mpetitive repopulation assays were performed using MDR1 expanded graft s. These experiments showed progressive expansion of MDR1-transduced r epopulating cells over the expansion period, with a 13-fold overall in crease in stem cells after 12 days. In all of the experiments, mice tr ansplanted with expanded MDR1-transduced stem cells developed a myelop roliferative disorder characterized by high peripheral white blood cel l counts and splenomegaly. These results show that MDR1-transduced ste m cells can be expanded in vitro using hematopoietic cytokines without any drug selection, but enforced stem cell self-renewal divisions can have adverse consequences, (C) 1998 by The American Society of Hemato logy.