AN IN-VITRO MODEL FOR CYTOGENETIC CONVERSION IN CML - INTERFERON-ALPHA PREFERENTIALLY INHIBITS THE OUTGROWTH OF MALIGNANT STEM-CELLS PRESERVED IN LONG-TERM CULTURE

IFN-alpha has been shown to prolong survival in chronic myeloid leukem ia patients, but its mechanism of action is still not understood. The human cobblestone area-forming cell (CAFC) assay allows for the measur ement of the concentration of normal as well as malignant stem cells, while their progeny can be measured in parallel long-term culture (LTC ) in flasks. Using CAFC and LTC assays, we have examined direct effect s of IFN-alpha (500; 5,000 IU/ml) on the maintenance and outgrowth of CD34-enriched normal and malignant stem cells, obtained from six patie nts with an established major cytogenetic response to IFN-alpha and fr om four nonresponding patients, CAFC concentrations were not affected by IFN-alpha. In contrast, IFN-alpha strongly inhibited the clonogenic output in flask LTC, Nucleated cells (NC) produced in LTC were evalua ted by fluorescent in situ hybridization (FISH) for the presence of th e Philadelphia (Ph) translocation. After 8 wk of LTC, the percentage o f Ph+ NCs produced was significantly more inhibited by IFN-alpha in re sponding patients than in nonresponders. Control LTC without IFN-alpha showed no significant differences of Ph+ NC production between respon ders and nonresponders. These findings provide the first in vitro mode l for cytogenetic conversion and suggest that direct antiproliferative effects of IFN-alpha account for the cytogenetic response observed cl inically.