The myeloid-lymphoid initiating cell (ML-IC) assay assesses the fate of multipotent human progenitors in vitro

Hematopoietic stem cells (HSC) are cells with self-renewing multilineage di fferentiation potential. Although engraftment in xenogeneic recipients can be used to measure human HSC, these assays do not allow assessment of indiv idual progenitors. We developed an in vitro assay that allows the identific ation of a single human bone marrow progenitor closely related to HSC, whic h we termed "Myeloid-Lymphoid Initiating Cell," or ML-IC, because it is cap able of generating multiple secondary progenitors that can reinitiate long- term myeloid and lymphoid hematopoiesis in vitro. The assay is done in cont act with murine AFT024 fetal liver stromal cells and with Flt3-Ligand, stem cell factor, and interleukin-7. In this assay, 0.2% to 1.7% of Lin (-)/34( +)/DRdim cells could generate 1 to 3 long-term culture initiating cells (LT C-IC) as well as 1 to 4 NK-IC after 4 to 6 weeks. In addition, this assay m easures contribution of net-progenitor conservation and net-progenitor prol iferation over time, providing insight in the fate of individual LTC-IC and NK-IC. This assay will prove useful to enumerate the number of very primit ive human progenitors with multilineage differentiation potential, as well as to evaluate future ex vivo culture conditions. (C) 1999 by The American Society of Hematology.