Self-renewal, maturation, and differentiation of the rat myelomonocytic hematopoietic stem cell

Hematopoiesis is viewed as a differentiating system emanating from a pluripotent hematopoietic stem cell capable of both self-renewal and differentiation. By identifying and characterizing a novel and highly specific in vitro mitogenic response to the N-acetyl glucosamyl/sialic acid specific, stem cell-binding lectin wheat germ agglutinin (WGA), we demonstrate the existance of a rare (0.1%), plastic adherent precursor in rat bone marrow capable of proliferation (two to seven divisions) in response to WGA. Stimulated cells possess a lineage (lin)low/- immunophenotype and immature blastoid morphology (WGA blasts). A subsequent proliferative response to stem cell factor (SCF), the ligand for the proto-oncogene receptor tyrosine kinase c-kit, is characterized by an initial maturation in immunophenotype and subsequent self-renewal of cells (SCF blasts) without differentiation for at least 50 generations. Although granulocyte colony-stimulating factor (G-CSF), interleukin (IL) -6, IL-7, and IL-11 synergize with SCF to increase blast colony formation, cytokines such as granulocyte-macrophage CSF or IL-3 are without significant effect. At all time points in culture, however, cells rapidly differentiate to mature neutrophils with dexamethasone or to mainly monocytes/macrophages in the presence of 1alpha,25-dihydroxyvitamin D3, characterized by cell morphology and cytochemistry. Removal of SCF during blast maturation, self-renewal, or induction of differentiation phases results in apoptotic cell death. Data indicate a pivotal role for SCF/c-kit interaction during antigenic maturation, self-renewal, and apoptotic protection of these lineage-restricted progenitors during non-CSF-mediated induction of differentiation. This approach provides a source of many normal, proliferating myelomonocytic precursor cells, and introduces possible clinical applications of ex vivo expanded myeloid stem cells.