Role of Integrins in Adhesion of Hematopoietic Progenitor Cells.

The hematopoietic stem cell niche is the set of soluble growth factors, cell-cell and cell-matrix interactions that contribute to stem cell self renewal in the bone marrow. While cytokines and cell-cell interactions have been well documented, cell-matrix interactions in the niche are less understood. Integrins are a class of highly conserved cell adhesion molecules that are important in hematopoietic development and homing. However the specific role of integrins in mediating adhesion to extracellular matrix in the hematopoietic stem cell niche is unknown. The terminal stages of erythropoiesis in the fetal liver provide a good model system with which to develop several of the assays to be used with HSCs. Using flow cytometry, murine fetal liver erythroid progenitors can be separated at four distinct stages of development based on expression of CD71 and Ter119. Further FACS and quantitative PCR analysis revealed that α4β1 integrin is significantly downregulated over the course of erythroid differentiation. Using a centrifugation assay, we determined that this change is accompanied by a loss of adhesion to fibronectin, and that adhesion to fibronectin is blocked by addition of anti-integrin antibodies. Finally, fetal liver progenitor cells adhered to comb co-polymer surfaces engineered to present peptides specifically recognized by α4β1 integrins. By determining the integrin profile expressed by hematopoietic stem cells and measuring stem cell adhesion to ECM in a similar manner, we can begin to understand how these specific interactions present developmental cues important to maintaining the stem cell phenotype in vivo, in addition to leading to design parameters for ex vivo culture systems.