Adhesion of Hematopoietic Progenitor Cells to Human Mesenchymal Stromal Cells as a Model for Interaction between Stem Cells and Their Niche.

Objective: The significant role of direct contact between hematopoietic progenitor cells (HPC) and the cellular microenvironment for maintaining “stemness” has been demonstrated. Human mesenchymal stromal cell (MSC) feeder layers represent a surrogate model for this interaction. The molecular composition of this heterotypic cell-cell contact is yet unknown.

Methods: To define this cell-cell contact between HPC and MSC, we have studied adhesion of various fractions of HPC with different preparations of MSC by using a novel assay based on gravitational force upon inversion. Adherent and non-adherent cells were then separated. Gene expression analysis by microarray (GeneChip Human Genome U133_Plus_2.0, Affymetrix) of the two populations was performed and the relationship to long-term hematopoietic culture initiating cell (LTC-IC) frequency examined.

Results: HPC subsets with higher self-renewing capacity demonstrated significantly higher adherence to MSC from human bone marrow (CD34⁺vs. CD34⁻, CD34⁺/CD38⁻vs. CD34⁺/CD38⁺, slow dividing fraction vs. fast dividing fraction). LTC-IC frequency was significantly higher in the adherent fraction than in the non-adherent CD34+ cells, thus providing evidence for specific adhesive interaction of primitive HPC with MSC. Genes coding for adhesion proteins and extracellular matrix were highly expressed in the adherent fraction compared to non-adherent CD34+ cells. These genes included fibronectin1 (FN1), cadherin11, VCAM1, connexin43 and ITGBL1. Furthermore, affinity of CD34+ cells was analyzed on human MSC isolated from bone marrow (BM), adipose tissue (AT) and cord blood (CB). Affinity to BM-MSC was significantly higher compared to AT-MSC and CB-MSC. Gene expression in different MSC preparations (BM-MSC, AT-MSC and CB-MSC) correlated in various adhesion proteins with the differences observed in affinity of HPC (including cadherin11, VCAM1, N-cadherin, ITGB1, ITGA1, ITGA5, SDF-1 and osteopontin). Western blot analysis also confirmed higher protein expression of FN1, cadherin11, N-cadherin and ITGB1 in BM-MSC compared to AT-MSC and CB-MSC.

Conclusion: MSC represent a model for the human hematopoietic niche. Primitive subsets of HPC have significantly higher affinity to BM-MSC. The essential role of specific junction proteins (cadherin11, VCAM1, N-cadherin) for stabilization of cell-cell contact is indicated by their significant higher expression on both sides of the heterotypic interaction.