Hematopoietic Stem Cells (HSCs) are defined by two characteristics: multipotency and self renewal. Multipotency refers to ability of HSCs to differentiate into any hematopoietic cell types without restriction. Self renewal is defined as the ability of an HSC to divide and produce an identical multipotent progeny cell. While the defining characteristics of stem cells are well known, there is no known cell surface protein unique to stem cells, making it difficult to sort them out of the larger, more differentiated population. Work is currently underway using a variety of cell surface antigens to better define the HSC population. For the purpose of therapeutic hematopoietic reconstitution, however, the transmembrane phosphoglycoprotein CD34 as a marker for a diverse group of non-differentiated stem and progenitor cells, many of which are from the hematopoietic compartment (Ivanovic, 2010). CD34 was first identified in 1984, and was long considered to be a marker for hematopoietic stem and progenitor cells, although subsequent characterization indicated that it was also expressed in multipotent mesenchymal stromal cells, interstitial dendritic cells, epithelial progenitors and several other cell types. Despite its expression across different cell subsets, no common function for CD34 has been found (Sidney, 2014). Its most commonly reported ligand is L-Selectin, but it has also been shown to bind the adaptor protein CrkL, which regulates adhesion. The cell type-specific functions of CD34 have been primarily investigated in hematopoietic cells, where CD34 appears to play a role in cytoadhesion, regulation of differentiation and proliferation, and trafficking of hematopoietic stem cells (HSCs) to niches within the bone marrow (Sidney, 2014).
Reconstitution of the hematopoietic compartment requires the extraction and purification of hematopoietic stem and progenitor cells. While reconstitution is theoretically possible using a single hematopoietic stem cell (HSC), there is a strong correlation between number of stem and progenitor cells infused and the time required for engraftment and reconstitution of the hematopoietic compartment. Therapeutic hematopoietic reconstitution has helped define the hierarchy within the hematopoietic lineage. Sorted CD34+ cells contain not only the quiescent HSCs that allow long term maintenance of the reconstituted hematopoietic compartment, but also the multipotent progenitor cells that have lost the ability for self renewal, as well as the oligopotent progenitor cells that are lineage restricted (Figure 1). Progenitor cells allow short term reconstitution of the immune compartment while HSCs allow its long term maintenance. The frequency of various types of stem and progenitor cells within the CD34+ subset is constant enough that CD34+ cell number can be used as an accurate predictor of time to hematopoietic engraftment (Chao, 2004). Research has shown that 5×10^6 CD34+ cells per kilogram of patient weight is the optimal number for hematopoietic reconstitution.
Because there is no phenotypic assay to directly measure the number of CD34+ multipotent stem cells in a given sample, researchers rely on functional assays to determine the number of HSCs in a sample. One such assay is the Long Term Culture Initiating Cell (LTC-IC) assay, which determines the number of primitive cells based on the ability to produce myeloid progeny over at least five weeks in culture. LTC-IC assays can be used to measure single cell proliferative potential (generative capacity) after sorting cells using FACS and plating one cell per well (Liu, 2013). Such an assay actually determines the number of intermediate progenitor cells in the sample; researchers have shown that these intermediate progenitors play a very small role in long term reconstitution of the hematopoietic compartment. However given the consistent inverse correlation between CD34+ cell number and time to engraftment, it is likely that the ratio of intermediate progenitor cells to primitive HSCs is maintained between samples, and CD34+ cell numbers are an accurate reflection of total HSCs infused during the transplant process (Chao, 2004).
This paper will compare the phenotypes, frequency, and function of CD34+ hematopoietic stem and progenitor cells found in the cord blood, bone marrow and peripheral blood.