Identification of distinct elements of the stromal microenvironment that control human hematopoietic stem/progenitor cell growth and differentiation.

Using a novel collection of conditionally immortalized mouse stromal cell clones, we evaluated the role of distinct elements of the hematopoietic microenvironment in supporting and regulating the growth, division, and differentiation of a candidate human stem cell population (CD34+/CD38-). We found functional diversity in the capacity of different stromal cell clones to support the growth of primitive (CD34+/CD38-) and committed (CD34+/CD38+) hematopoietic progenitors and their differentiation into mature hematopoietic cells (CD34-/CD45+). Among the stromal cell clones that supported long-term hematopoiesis, we identified two clones that induced expansion of CD34+ progenitor/stem cells during the first 4 weeks of coculture and that supported the maintenance of this CD34+ population for up to 10 weeks in vitro. However, these two clones appeared to represent two different microenvironments with regard to the signals they provide to the different CD34+ progenitor subpopulations: One stromal clone preserved a pool of undifferentiated, relatively quiescent (CD34+/CD38-) progenitor cells, allowing their differentiation at a low rate into more committed (CD34+/CD38+) progenitors; the other fostered a more extensive and rapid differentiation of all CD34+/CD38- progenitors into CD34+/CD38+ cells, preferentially maintaining this committed population at a higher rate of cell division. These stromal cell clones were also able to support the proliferation and differentiation of CD34+/CD38- cells in conditions in which progenitor-stroma contact was prevented. This collection of stromal cell clones may represent a unique tool for the study of stromal regulators of hematopoiesis as well as for the support of gene transfer into hematopoietic progenitor cells.