PERSISTENCE OF RESIDUAL TUMOR-CELLS AFTER CYTOKINE-MEDIATED EX-VIVO EXPANSION OF MOBILIZED CD34(-CELLS IN MULTIPLE-MYELOMA() BLOOD)

Mobilized CD34(+) blood cells were immunomagnetically enriched from le ukapheresis products in five multiple myeloma (MM) patients, Thawed sa mples of selected CD34(+) cells were cultured for up to 21 d in a liqu id and stroma-free culture system with different combinations of recom binant cytokines. The most successful cell expansion was obtained when a combination of rh-IL-1 beta, rh-IL-3, rh-IL-6, rh-SCF, rh-G-CSF and rh-GM-CSF was used. After 14 d this mixture gave a 120-187-fold overa ll increase of total nuclear cells and a 4-8-fold overall increase of early CFU-GM numbers. In four patients a very sensitive patient-specif ic PCR analysis showed the presence of monoclonal cells in the initial leukapheresis products. After immunomagnetic separation a tumour cell depletion of 2-4 logs was observed, although all samples still contai ned malignant cells. Cell suspensions that were cultured with the most potent cytokine combination showed tumour contamination in two-thirds of evaluable cases at the moment of maximal CFU-GM output. Serial cDN A dilution experiments indicated that the positive PCR results at day 14 reflected the persistence of pre-culture tumour cells rather than i n vitro expansion of tumour cells in two cases. This study demonstrate s that ex vivo expansion of myeloid precursor cells from mobilized CD3 4(+) cells in MM patients does not always result in an effective purgi ng of residual tumour cells. On the other hand, our culture conditions do not seem to favour in vitro expansion of malignant cells, despite the use of a cytokine cocktail that includes potential myeloma growth factors.