Long term maintenance of myeloid leukemic stem cells cultured with unrelated human mesenchymal stromal cells

Mesenchymal stromal cells (MSCs) support the growth and differentiation of normal hematopoietic stem cells (HSCs). Here we studied the ability of MSCs to support the growth and survival of leukemic stem cells (LSCs) in vitro. Primary leukemic blasts isolated from the peripheral blood of 8 patients with acute myeloid leukemia (AML) were co-cultured with equal numbers of irradiated MSCs derived from unrelated donor bone marrow, with or without cytokines for up to 6weeks. Four samples showed CD34⁺CD38^- predominance, and four were predominantly CD34⁺CD38⁺. CD34⁺ CD38^- predominant leukemia cells maintained the CD34⁺ CD38^- phenotype and were viable for 6weeks when co-cultured with MSCs compared to co-cultures with cytokines or medium only, which showed rapid differentiation and loss of the LSC phenotype. In contrast, CD34⁺ CD38⁺ predominant leukemic cells maintained the CD34⁺CD38⁺ phenotype when co-cultured with MSCs alone, but no culture conditions supported survival beyond 4weeks. Cell cycle analysis showed that MSCs maintained a higher proportion of CD34⁺ blasts in G0 than leukemic cells cultured with cytokines. AML blasts maintained in culture with MSCs for up to 6weeks engrafted NSG mice with the same efficiency as their non-cultured counterparts, and the original karyotype persisted after co-culture. Chemosensitivity and transwell assays suggest that MSCs provide pro-survival benefits to leukemic blasts through cell-cell contact. We conclude that MSCs support long-term maintenance of LSCs in vitro. This simple and inexpensive approach will facilitate basic investigation of LSCs and enable screening of novel therapeutic agents targeting LSCs.