Induction of stem cell features in all cells by microenvironmental factors

Drug resistance remains a major obstacle to the successful treatment of a significant number of children and the majority of adult patients with acute lymphoblastic leukemia (ALL). Small numbers of cells that survive chemotherapy result in disease relapse in ∼25% of children and 65% of adults following treatment. Side-population (SP) cells represent stem/progenitor cells in normal and malignant tissues and have greater resistance to radiation and chemotherapy, likely due to their quiescent nature. We have identified leukemic SP cells in the bone marrow of patients with ALL. Hematopoietic stem cell markers do not distinguish these cells from the bulk population. In serial samples collected during induction protocols, enrichment of the SP cells within the ALL population was observed in most but not all patients, consistent with a relative resistance to chemotherapy. There was no correlation between minimal residual disease (MRD) results and the percentage of SP cells detected at day 15 or 33, suggesting the percentage SP cells is independent from MRD results, potentially providing additional prognostic information. ALL cells xenografted into mice also developed an SP fraction in vivo, which were sorted for microarray analysis. The SP cells were enriched for chemokine and circadian gene signatures while the bulk population displayed cell cycle and division gene signatures. Among bone marrow stromal cells, we found mesenchymal cells best able to induce SP cells in vitro, followed by osteoblasts, while macrophages, bone marrow endothelial cells and megakaryocytes had little capacity. The surface protein expression of SP cells produced in culture revealed an increase in the chemokine receptor CXCR4 and reduction of the integrins α4, α5 and the cytokine receptor VEGFR1. Surprisingly we did not detect increased expression of P-glycoprotein or ABCG2. The monitoring of SP populations may have prognostic value permitting a greater refinement of treatment options and targeting these cells may improve treatment outcomes in ALL.