Normal differentiation of hematopoietic stem cells (HSCs) is critical for maintenance of physiological levels of all blood cell lineages. During differentiation, in a coordinated response to extracellular cues including growth factors and cytokines, HSCs first give rise to multipotent progenitor (MPP) cells that have lympho-myeloid potential1,2,3,4 (Figure 1). MPPs give rise to common myeloid progenitors (CMPs) and common lymphoid progenitors (CLPs) that are lineage-restricted. CLPs differentiate into the lymphoid lineages comprised of B, T, and natural killer cells. CMPs generate the myeloid lineages through two more restricted progenitor populations, megakaryocyte erythroid progenitors (MEPs), and granulocyte monocyte progenitors (GMPs). MEPs give rise to megakaryocytes and erythrocytes, whereas GMPs give rise to granulocytes and monocytes. In addition to arising through CMPs, megakaryocytes have been reported to also arise directly from HSCs or early MPPs via non-canonical pathways5,6.
Hematopoietic stem and progenitor cells (HSPCs) are characterized by the surface marker CD34 and the lack of lineage specific markers (Lin-). Other surface markers that are commonly employed to distinguish HSCs and myeloid progenitor populations include CD38, CD45RA, and CD1232 (Figure 1). HSCs and MPPs are Lin-/CD34+/CD38- and Lin-/CD34+/CD38+, respectively. Myeloid committed progenitor populations are distinguished by the presence or absence of CD45RA and CD123. CMPs are Lin-/CD34+/CD38+/CD45RA-/CD123lo, GMPs are Lin-/CD34+/CD38+/CD45RA+/CD123lo, and MEPs are Lin-/CD34+/CD38+/CD45RA-/CD123-.
The total population of CD34+ stem and progenitor cells can be obtained from human umbilical cord blood (UCB), bone marrow, and peripheral blood. CD34+ cells constitute 0.02% to 1.46% of total mononuclear cells (MNCs) in human UCB, whereas their percentage varies between 0.5% and 5.3% in bone marrow and is much lower at ~0.01% in peripheral blood7,8,9. The proliferative capacity and differentiation potential of UCB derived CD34+ cells is significantly higher than that of bone marrow or peripheral blood cells1,10, thereby offering a distinct advantage for obtaining sufficient material for molecular analyses in combination with performing immunophenotypic and morphological characterization of the cells during differentiation.
Ex vivo differentiation of umbilical cord blood derived CD34+ HSPCs is a widely applied model for investigating normal hematopoiesis and hematopoietic disease mechanisms. When cultured with the appropriate cytokines, the UCB CD34+ HSPCs can be induced to differentiate along the myeloid or lymphoid lineages11,12,13,14,15,16. Here, we describe protocols for isolation and immunophenotypic characterization of the CD34+ HSPCs from human UCB, and for their differentiation to myeloid lineage cells. This culture system is based on cytokine-induced differentiation of HSPCs in the presence of MS-5 stromal cells to mimic the microenvironment in bone marrow. The culture conditions cause an initial expansion of the CD34+ cells, followed by their differentiation to cells that express markers for the four myeloid lineage cells, namely granulocytes (CD66b), monocytes (CD14), megakaryocytes (CD41), and erythrocytes (CD235a). Applications of the CD34+ cell differentiation protocol include studies on molecular mechanisms regulating hematopoiesis, and investigations of the impact of myeloid disease associated mutations and small molecules on self-renewal and differentiation of HSPCs.