A clinically suitable ex vivo expansion culture system for LTC-IC and CFC using stroma-conditioned medium

FACS-selected CD34¹HLA-DR cells (DR cells) may provide a source of benign stem cells suitable for autografting in chronic myelogenous leukemia (CML) and other hematological malignancies. However, DR^- cell selection depletes the majority of committed hematopoietic progenitors, which may be important for early engagement. Furthermore, only a small number of DR cells may be selectable in certain patients. These impediments to the use of DR cells for autografting may be overcome through the development of ex vivo culture systems that support expansion and initial differentiation of primitive progenitors. Because 2-week culture of DR cells in a stroma 'noncontact' system supplemented with interleukin-3 (IL-3) and macrophage inflammatory protein 1-α (MIP-1α) expands both long-term culture-initiating cells (LTC-ICs) and colony-forming cells (CFCs), we adapted this system to a clinically applicable method for expanding LTC-ICs and CFCs ex vivo. In initial small-scale studies, DR cells were grown in stroma-conditioned medium (SCM) supplemented with IL-3 with or without additional growth-promoting cytokines and the chemokines PF-4 and BB10010, all approved for clinical use. An IL-3 dose-dependent expansion of committed progenitors and LTC-ICs was observed when DR cells were cultured in tissue culture plates in SCM+IL-3 for 2 weeks. Similar CFC expansion along with increased (5-fold) LTC-IC expansion was observed following addition of PF-4 to SCM+IL-3 cultures. The addition of stem cell factor (SCF), but not of IL-6, IL-11, granulocyte colony- stimulating factor (G-CSF), granulocyte-macrophage (GM)-CSF, IL-1, and IL-7, increased CFC and LTC-IC expansion beyond the levels observed with SCM+IL-3 alone. We next evaluated the suitability of this culture system for scale- up. Culture of 2-6x10⁵ DR cells in gas-permeable bags with SCM+IL-3 resulted in similar CFC and LTC-IC expansion as seen in small-scale cultures. In addition, we observed that progenitors capable of differentiating to natural killer (NK)-cells were maintained under these conditions. Finally, we found that BCR/ABL mRNA-negative CFCs and LTC-ICs present in DR^- cells selected from steady-state CML marrow could be expanded in large-scale SCM+IL-3 cultures. We conclude that culture of DR cells for 2 weeks in SCM+IL-3 culture, with or without PF-4 or SCF, results in significant CFC and LTC-IC expansion and lymphoid NK progenitor maintenance. This culture system is readily adaptable to the expansion of primitive progenitors for autotransplantation.