The hematopoietic microenvironment is a complex structure in which stem cells, progenitor cells, stromal cells, growth factors, and extracellular matrix (ECM) molecules each interact to direct the coordinate regulation of blood cell development. While much is known concerning the individual components of this microenvironment, little is understood of the interactions among these various components or, in particular, the nature of those interactions responsible for the regional localization of specific developmental signals. We hypothesized that cytokines act together with ECM molecules to anchor stem cells within the microenvironment, thus modulating their function. In order to analyze matrix-cytokine-stem cell interactions, we developed an ECM model system in which purified stem cell populations and plastic-immobilized individual proteins are used to assess the role of various matrix molecules and/or cytokines in human hematopoietic cell development. Analysis of these interactions revealed that a single ECM protein, thrombospondin, in conjunction with a single cytokine (e.g., c-kit ligand), constitutes a developmental signal that synergistically modulates hematopoietic stem cell function.
M W Long, R Briddell, A W Walter, E Bruno, R Hoffman