The cluster of differentiation methodology used to identify cell surface antigens is where CD34 gets its name. Civin et al. and Tindle et al. simultaneously described CD34 for the first time on hematopoietic stem cells as a cell surface glycoprotein that serves as a cell-cell adhesion factor. Additionally, it might facilitate the bonding of hematopoietic stem cells with stromal cells or the extracellular matrix of bone marrow. In terms of medicine, it relates to the choice and enhancement of hematopoietic stem cells for bone marrow transplants. Although CD34 expression is actually seen on many other cell types, it is typically always associated with hematopoietic cells because of these historical and clinical links.

Functions of CD 34

The CD34 protein is a member of a family of single-pass transmembrane sialomucin proteins that show the expression on early haematopoietic and vascular-associated progenitor cells. However, little is known about its exact function.

CD34 is also an important adhesion molecule and is required for T cells to enter lymph nodes. It is expressed on lymph node endothelial, whereas the L-selectin to which it binds is on the T cell. Conversely, under other circumstances, CD34 has been shown to act as molecular “Teflon” and block mast cell, eosinophil and dendritic cell precursor adhesion, and facilitate the opening of vascular lumina. Finally, recent data suggest CD34 may also play a more selective role in the chemokine-dependent migration of eosinophils and dendritic cell precursors. Regardless of its mode of action, under all circumstances, CD34, and its relative podocalyxin and endoglycan, facilitates cell migration.

Distribution of Tissues

Cells expressing CD34 (CD34+ cell) are normally found in the umbilical cord and bone marrow as haematopoietic cells, or in endothelial progenitor cells, endothelial cells of blood vessels but not lymphatics (except pleural lymphatics), mast cells, the subpopulation of dendritic cells (which are factor XIIIa-negative) in the interstitium and around the adnexa of the dermis of the skin, as well as cells in soft tissue tumours like DFSP, GIST, SFT, HPC, and to some degree in MPNSTs, etc. The presence of CD34 on non-hematopoietic cells in various tissues has been linked to progenitor and adult stem cell phenotypes.

It is important to mention that Long-Term Haematopoietic Stem Cells (LT-HSCs) in mice and humans are the haematopoietic cells with the greatest self-renewal capacity and were shown to be CD34+ and CD38− cell fraction within the lineage-depleted cell population (LIn−). Human HSCs express the CD34 marker. Later studies have reported that low rhodamine retention identifies LT-HSCs within the Lin−CD34+CD38− population.

CD34 is expressed in roughly 20% of murine haematopoietic stem cells and can be stimulated and reversed.

Applications of CD34

CD34+ is often used clinically to quantify the number of haemopoietic stem cells for use in haemopoietic stem cell transplantation. This is generally a useful marker for cell dosing although there is some evidence that the CD34+ quantification may not be reliable in some circumstances. CD34+ cells may be isolated from blood samples using immunomagnetic techniques and used for CD34+ transplants, which have lower rates of graft-versus-host disease.

Antibodies are used to quantify and purify hematopoietic progenitor stem cells for research and for clinical bone marrow transplantation. However, counting CD34+ mononuclear cells may overestimate myeloid blasts in bone marrow smears due to hematogenous (B lymphocyte precursors) and CD34+ megakaryocytes.