Mesenchymal Stem Cells Regulate the Cell Division Symmetry of Hematopoietic Stem Cells by an Integrin Dependent Mechanism.

Asymmetric cell division is the basal assumption for the generation of a hematpoietic system. On the basis of the murine stromal cell line AFT024 we could demonstrate that cell division symmetry is governed by intercellular contacts between hematopoietic stem cells (HSC) and niche cells. In the adult bone marrow the niche is primarily represented by osteoblasts, whereas during fetal hematopoiesis and pathological extramedullary hematopoiesis in the adulthood mesenchymal stem cells (MSC) are thought to hold this function. Nevertheless data demonstrating an alteration of cell division symmetry by MSC as well as an determination of the type of cellular interaction regulating this process are lacking so far. Therefore the cell division symmetry of individual human PKH-26 labelled CD34⁺/133⁺ HSC in contact and non-contact cultures with MSC was monitored for 7d by means of time lapse fluorescence microscopy. For comparison, cultures without feeder layer were monitored. The experiments were performed both, in the absence and presence of early acting cytokines (LIF, IL-6, MIP-1α, G-CSF, TPO, SCF, Flt-3L) promoting the maintenance of primitive progenitors. We found that the vitality and number of dividing cells was exclusively dependent on soluble factors with the highest vitality and number of dividing cells in cytokine augmented MSC cultures (89.9 ± 6.4% vital cells and 98.8 ± 2.7% dividing cells) and the lowest in cultures with neither stromal nor cytokine support (67.9 ± 0.7% vital cells and 41.8 ± 2.5% dividing cells). Nevertheless the percentage of asymmetrically dividing cells was twice as much in MSC contact cultures (22.7 ± 0.7%) than in all other culture conditions (9.1 ± 3.4 to 12.2 ± 2.3%). This demonstrates for the first time that MSC can alter cell division symmetry by a contact dependent mechanism. To identify the type of cellular interactions between HSC and MSC regulating the cell division symmetry, we monitored the latter in prescence of a function blocking β₁-integrin antibody that reduced the percentage of asymmetrically dividing cells in MSC contact cultures from 22.7 ± 0.7% to 8.9 ± 4.2%. This was accompanied by a reduction of adherent HSC from 31.6 ± 6.1% to 9.7 ± 4.1% and a loss of functionally primitive cells as determined by a LTC-IC assay. Taken together these data demonstrate for the first time that MSC can alter the cell division symmetry of HSC by a mechanism that is partially dependent on β₁-integrin mediated cell contacts.