Identification of a New Population of Human Cord Blood Cells with Lymphoid-Restricted Reconstituting Potential in Sublethally Irradiated Immunodeficient Mice.

Throughout adult life, human hematopoiesis is sustained by the activity of a small compartment of pluripotent stem cells with extensive self-renewal potential. Available evidence suggests that these cells undergo a process of progressive lineage restriction similar to that described for murine hematopoietic cells, although many of the intermediate stages of human hematopoiesis have not yet been characterized. In human hematopoietic tissues, cells with short-term (<4 months) as well as long term (>4 months) repopulating activity (termed STRCs and LTRCs, respectively) are distinguished by their differential ability to engraft sublethally irradiated NOD/SCID-β2microglobulin null mice as well as their transient versus sustained output of differentiated cells. In previous studies, both a myeloid-restricted type of human STRC (STRC-M) and a type of STRC with lymphoid as well as myeloid potential (STRC-ML) have been identified. STRC-Ms are CD34+CD38+ and produce mainly erythroid progeny for the first 3–4 weeks post-transplant. In contrast, STRC-MLs are CD34+CD38− and produce progeny only between weeks 5 and 12 post-transplant which consist mainly of B-lymphoid cells plus some granulopoietic cells. We show here that both STRC-MLs and STRC-Ms are similarly distributed among lin- cord blood cells with intermediate to high levels of aldehyde dehydrogenase activity (ALDH-int/hi) as evidenced by staining with the fluorescent dye BAAA. In addition, BAAA-staining has allowed a previously undescribed primitive cell with low ALDH activity (ALDH-lo) and lymphoid-restricted repopulating activity to be identified. Assessment of NOD/SCID-β2microglobulin null mice transplanted with various subsets of cord blood cells further demonstrated that these “STRC-Ls” are CD38− and 10-fold more prevalent in the CD133+ subset of the low-density SSC-low ALDH-lo/neg population but, numerically, are equally distributed between the CD133+ and CD133− fractions because of the proportionately larger size of the CD133− subpopulation. Phenotype analysis of CD34+CD38− cord blood cells revealed a small and distinct ALDH-lo subset that expressed 10-fold higher levels of CD7 than any other CD34+CD38− cells. However, transplantation of this small CD7++ subset into NOD/SCID- β2microglobulin null mice revealed that they accounted for very few of the ALDH-lo STRC-Ls. The discovery of a CD38− ALDH-lo population of lymphoid-restricted human cells with in vivo reconstituting activity identifies a key step in the process of human hematopoietic cell lineage determination and the ability to prospectively isolate these progenitors separately from other types of short- and long-term repopulating cells present in normal human hematopoietic tissues should greatly facilitate future analysis of the mechanisms regulating their normal differentiation or malignant transformation.