Efficient long-term maintenance of chronic myeloid leukemic cobblestone area forming cells on a murine stromal cell line

Abstract

Stroma-supported long-term cultures (LTC) of chronic myeloid leukemia (CML) progenitor cells have previously revealed differences between normal and malignant stem cells with respect to their maintenance and adhesive properties. Using the cobblestone area forming cell (CAFC) assay and LTC, we have examined the frequencies of stem cell subsets, their ability for long-term progenitor cell production and the relative frequencies of malignant and normal progenitor cells before and after a 5–6 week culture period. Cells were obtained from bone marrow (BM) and peripheral blood (PB) samples of patients in chronic phase CML. CD34-enriched cells were sorted by FACS on the basis of CD34 and CD38 expression and overlaid on confluent stromal layers of murine FBMD-1 cells. The presence of the bcr/abl chimeric gene was detected by fluorescent in situ hybridization (FISH) using differently labelled bcr and abl- specific probes. In the CD34^pos/CD38^pos subset of CML-PB, representing 64–95% of CD34^pos cells, CAFC frequencies at week 1 (wk-1) were much higher than those of CAFC wk-5 (1·10⁴/10⁵ cells vs 1·10³/10⁵). In contrast, in the CD34^pos/CD38^neg subset, representing 2–3% of CD34^pos cells, the frequency of CAFC wk-1 was only 1·10²/10⁵ cells, but a high CAFC frequency (10³–10⁴/10⁵) was detected after 5 weeks of culture. CAFC frequencies in the CD34^pos subset obtained from CML-BM were 10- to 100-fold lower than those from CML-PB, but displayed a similar distribution over CD38^pos, CD38^dim and CD38^neg cells. Analysis of the percentage of Philadelphia chromosome-positive (Ph⁺) and Ph⁻ cells by FISH on freshly sorted cells revealed that normal cells were not enriched in any CD34^pos/CD38 subset. In addition, Ph⁻ as well as Ph⁺ cells were maintained with similar efficiency throughout 5 week LTC. These results demonstrate that immature normal and malignant stem cells in CML have a comparable distribution on the basis of CD34 and CD38 expression. The ability to maintain immature normal and malignant hemopoietic cells with similar efficiency in LTC provides a model enabling a direct comparison of differential effects of cytokines or drugs on either normal or malignant immature stem cells in CML.