Welcome back to our cell of the month series. This time we’re talking about CD34+ cells, a type of undifferentiated multipotent hematopoetic stem cell (HSC) with the potential to differentiate into almost any other blood cell type under specific conditions. As stem cells, CD34+ cells naturally have the capacity for self-renewal, allowing them to divide and replicate indefinitely, making them a highly valuable source of hematopoetic cells in research and clinical settings. However, the CD34+ cell population in blood is extremely small, and is estimated to represent less than 0.5% of all other blood cell types.

So What Is CD34?

CD34 (cluster of differentiation 34) is a highly glycosylated single pass membrane protein that adorns the surface of most HSCs, as well as certain non-hematopoietic cell types including mesenchymal stem cells (MSCs), muscle satellite cells, corneal keratocytes, interstitial cells, epithelial progenitors, and vascular endothelial progenitors.

In the clinic, CD34 has been widely used as a marker for the identification and isolation of HSCs and their progenitors ahead of bone marrow transplantation. Despite the wide and long-standing usage of CD34 in this area, its usefulness as a HSC marker has been challenged by a number of observations, including the ex vivo generation of CD34+ cells from CD34 hematopoietic cells (1). Today, CD34 is also recognized as a marker for vascular endothelial progenitors and embryonic fibroblasts (2), and it has also gained traction as a marker for other tissue-specific stem cells, including muscle satellite cells and epidermal precursors. However, despite these applications, the exact biological function of CD34 has been difficult to elucidate for a number of reasons:

  • It is subjected to many distinct tissue-specific post-transcriptional and post-translational modifications that are expected to impact its biological function significantly
  • Genetic attempts to interrogate its function (either though gain- or loss-of-function experiments have been complicated by the presence and overlapping expression of closely related proteins such as podocalyxin and endoglycan)
  • Robust in vitro or in vivo functional assays that permit structural and functional analysis of CD34 and related proteins are lacking.

While we don’t yet know the exact biological function of CD34, it is believed to be involved in maintaining progenitor cells in a phenotypically undifferentiated state. Recent data also suggests that CD34 is an adhesion molecule with a role in early hematopoiesis through mediating the attachment of stem cells to the bone marrow extracellular matrix or directly to stromal cells. Analyzing and comparing the characteristics of the many cell types that express CD34 may also offer further insights into its biological function.

CD34+ Cells

CD34+ cells originate as embryonic stem cells from the inner cell mass of blastocysts during early embryonic development. In the clinic, CD34+ cells are used therapeutically to restore bone marrow function, e.g., during the replacement of leukemic bone marrow with healthy marrow. CD34+ cells have also been used to treat various other diseases including liver cirrhosis and cardiac and vascular disease (3, 4). In research, CD34+ cells make an important contribution to our understanding of how cell lineages originate and develop. Research data suggests that CD34+ HSC and progenitor cells actually have the ability to differentiate in vivo into other lineages, including respiratory epithelial cells, hepatocytes, and cardiomyocytes, but so far the properties of CD34+ HSCs have not been directly linked to the properties of these CD34+ non-HSCs (See Ref. 2 for more details).

CD34+ cells may be obtained from many distinct bodily locations. For research purposes, these cells are generally procured from four distinct sources; peripheral blood, bone marrow, cord blood and more recently iPSCs (see Table 1 for a brief overview).

Table 1: Sources of CD34+ Cells
CD34+ Cells from Peripheral Blood

Peripheral blood is the most accessible source of CD34+ cells, and the collection process is straightforward and non-invasive. However, the downside is that peripheral blood won’t leave you with large numbers of CD34+ cells since HSCs are not naturally found in circulating blood, so this isn’t the way to go if your research demands many CD34+ cells. To obtain peripheral blood with higher CD34+ cell counts, cytokine mobilization might be a useful option. Here, the blood donor (e.g., a mouse) is treated with a cytokine to stimulate migration of HSCs from the bone marrow to the peripheral blood, and these can then be easily isolated. This process is claimed to yield the highest number of CD34+ cells possible in a single peripheral blood lot.

CD34+ Cells From Bone Marrow

While bone marrow is a natural source of CD34+ cells, isolating cells from this source is both tedious and invasive. Researchers tend therefore to look for CD34+ cells from more easily accessible blood sources if isolating cells in-house.

CD34+ Cells from Cord Blood

HSCs obtained from umbilical cord blood exhibit a higher cell density, superior capacity for cell division, and longer lifespan than CD34+ cells obtained from adult bone marrow. So with cord blood stem cells, you can expect many cells and more rounds of cell division, so this may be the right source if you plan to use the cells in larger and/or long-term experiments.

CD34+ Cells from Human iPSCs

The possibility to obtain multipotent HSCs from iPSCs creates vast opportunities within cell therapy, disease modeling, and drug screening, a potential that has prompted intense research efforts to develop efficient protocols to this end. Previously, the generation of iPSC-derived HSCs with non-biased multilineage differentiation potential proved challenging even with co-expression of the transcription factors known to be important for HSC function. Recently, additional methods have been developed to increase the manufacturing yield of iPS-derived CD34+ HSCs. Thus, iPSC generated CD34+ cells will become useful models soon.

(UPDATE: After this blog post went LIVE, human iPSC-derived CD14+ monocytes and iPSC-derived phagocytes became available via Tempo Bioscience. Check out the announcement here!)

Are you working with CD34+ cells, or do you have an interesting fact to share? Get in touch with us!

References

  1. Y. Nakamura et al., Ex vivo generation of CD34(+) cells from CD34(-) hematopoietic cells. Blood 94, 4053-4059 (1999).
  2. L. E. Sidney, M. J. Branch, S. E. Dunphy, H. S. Dua, A. Hopkinson, Concise review: evidence for CD34 as a common marker for diverse progenitors. Stem Cells 32, 1380-1389 (2014).
  3. M. Pai et al., Autologous infusion of expanded mobilized adult bone marrow-derived CD34+ cells into patients with alcoholic liver cirrhosis. Am J Gastroenterol 103, 1952-1958 (2008).
  4. A. R. Mackie, D. W. Losordo, CD34-positive stem cells: in the treatment of heart and vascular disease in human beings. Tex Heart Inst J 38, 474-485 (2011).
  5. Y. T. Tan et al., Respecifying human iPSC-derived blood cells into highly engraftable hematopoietic stem and progenitor cells with a single factor. Proc Natl Acad Sci U S A 115, 2180-2185 (2018).

Article by Karen O’Hanlon Cohrt PhD. Contact Karen at karen@tempobioscience.com.

Karen O’Hanlon Cohrt is a Science Writer with a PhD in biotechnology from Maynooth University, Ireland (2011). After her PhD, Karen moved to Denmark and held postdoctoral positions in mycology and later in human cell cycle regulation, before moving to the world of drug discovery. Her broad research background provides the technical know-how to support scientists in diverse areas, and this in combination with her passion for writing helps her to keep abreast of exciting research developments as they unfold. Follow Karen on Twitter @KarenOHCohrt. Karen has been a science writer since 2014; you can find her other work on her portfolio.