Hematopoietic progenitor and stem cells, which express CD34 antigen, are widely used clinically in the treatment of a number of conditions and diseases such as vascular disease, spinal cord injury, multiple myeloma, and more. However, emerging research data indicates that CD34 stems cells are not exclusively hematopoietic. CD34 antigen is expressed on a number of other cell types including keratocytes, multipotent mesenchymal stromal cells, and interstitial dendritic cells, among others.
Essentially, CD34 is a transmembrane glycoprotein that plays roles in cell adhesion, proliferation, and differentiation; however, the full range of CD34 function is not fully known. The expression of CD34 is associated with having a strong and durable proliferative capacity. Since CD34 stem cells have been considered hematopoietic only for many years, CD34 antigen has been used extensively as a marker for hematopoietic stem and progenitor cells. So much so that researchers working with mesenchymal stromal cells believed that their cultures were contaminated when CD34 expression was detected. It is now known that these cells can express CD34.
The other cell types that express CD34 include hematopoietic, endothelial, epithelial, and stromal. The diversity of cells that express CD34 have progenitor and stem cell activity as common characteristics. Despite this, these different types of CD34 stem cells do not have exactly the same molecular or functional properties. This suggests that the expression of one antigen class is not sufficient to characterize a specific cell type. A given stem or progenitor cell would have a specific profile based on a family of markers, function, and other factors.
CD34 can be used as a marker to detect or isolate CD34 stem cell subpopulations from various tissue types. Therefore, it is now necessary to change the thinking regarding CD34 and the types of cells that can express it. Further study of the diversity of CD34 stem cells will lead to valuable information to develop different regenerative medicine approaches and therapies.
Reference: Sidney LE, Branch MJ, Dunphy SE, Dua HS, Hopkinson A. Concise Review: Evidence for CD34 as a Common Marker for Diverse Progenitors. Stem Cells (Dayton, Ohio). 2014;32(6):1380-1389. doi:10.1002/stem.1661.