Hematopoietic stem cells in co-culture with mesenchymal stromal cells -modeling the niche compartments in vitro

Abstract

Background: Hematopoietic stem cells located in the bone marrow interact with a specific microenvironment referred to as the stem cell niche. Data derived from ex vivo co-culture systems using mes-enchymal stromal cells as a feeder cell layer suggest that cell-to-cell contact has a significant impact on the expansion, migratory potential and 'stemness' of hematopoietic stem cells. Here we investigated in detail the spatial relationship between hematopoietic stem cells and mes-enchymal stromal cells during ex vivo expansion. Design and Methods: In the co-culture system, we defined three distinct localizations of hematopoietic stem cells relative to the mesenchymal stromal cell layer: (i) those in supernatant (non-adherent cells); (ii) those adhering to the surface of mesenchymal stromal cells (phase-bright cells) and (iii) those beneath the mesenchymal stromal cells (phase-dim cells). Cell cycle, proliferation, cell division and immunophenotype of these three cell fractions were evaluated from day 1 to 7. Results: Phase-bright cells contained the highest proportion of cycling progenitors during co-culture. In contrast, phase-dim cells divided much more slowly and retained a more immature phenotype compared to the other cell fractions. The phase-dim compartment was soon enriched for CD34+/CD38-cells. Migration beneath the mesenchymal stromal cell layer could be hampered by inhibiting integrin ß1 or CXCR4. Conclusions: Our data suggest that the mesenchymal stromal cell surface is the predominant site of prolifer-ation of hematopoietic stem cells, whereas the compartment beneath the mesenchymal stro-mal cell layer seems to mimic the stem cell niche for more immature cells. The SDF-1/CXCR4 interaction and integrin-mediated cell adhesion play important roles in the distribution of hematopoietic stem cells in the co-culture system. © 2010 Ferrata Storti Foundation.