Hematopoietic Stem Cells (HSCs) are the source for all mature blood cells, including red cells, white cells, and platelets, through a continuous process called hematopoiesis. These cells are rare, representing only about one in every ten thousand nucleated cells found in the bone marrow, the primary site of blood production in adults. Studying HSCs is challenging because they are morphologically similar to surrounding cells, making them impossible to distinguish visually under a microscope. Scientists overcame this barrier by using specific protein markers expressed on the cell surface, which act like unique cellular fingerprints allowing researchers to isolate and study their function.
Understanding Cluster of Differentiation (CD) Markers
The system used to categorize these surface proteins is known as the Cluster of Differentiation, or CD system. This nomenclature provides a standardized way to classify the many different antigens found on the surface of leukocytes (white blood cells). Each CD number corresponds to a specific protein molecule recognized by a group of antibodies, ensuring consistent reporting across different laboratories. These CD proteins perform diverse functions, often acting as receptors for external signals, ligands for cell-to-cell communication, or molecules involved in cell adhesion. The presence or absence of a specific marker is denoted by a plus (+) or minus (-) sign, which defines cell populations, such as “CD34+”.
Identifying the Core Stem Cell Population
The identification of the most primitive, self-renewing HSC population relies on a specific combination of positive and negative markers. The first distinction involves the concept of “Lineage Negative” (Lin-), meaning the cell lacks all markers associated with mature blood cell types. This negative selection process excludes the vast majority of mature cells, leaving behind a small population of primitive stem and progenitor cells. Within this Lin- fraction, the most commonly used positive marker for human hematopoietic stem cells is CD34. CD34 serves as a general indicator of primitive status and is the standard marker used clinically to determine the stem cell dose in grafts.
To further narrow the population to true stem cells, the marker CD38 is used for negative selection. The phenotype Lin-/CD34+/CD38- defines a highly enriched population of primitive hematopoietic cells, including Long-Term HSCs capable of lifelong blood system reconstitution. The absence of CD38 expression indicates a less differentiated, more primitive state with greater self-renewal capacity. For greater purity, researchers often add markers like the presence of CD90 (Thy-1) and the absence of CD45RA.
Tracking Lineage Commitment
As the primitive HSC differentiates, its surface marker profile changes to reflect commitment to a specific blood cell lineage. The initial division leads to two major branches: the Common Myeloid Progenitor (CMP) and the Common Lymphoid Progenitor (CLP).
Common Myeloid Progenitors are defined by the presence of CD34 and CD38, along with markers like CD135 (Flt-3) and intermediate levels of CD123 (IL-3Rα). CMPs are the precursors for the myeloid lineage, which includes red blood cells, platelets, monocytes, and granulocytes like neutrophils. The acquisition of these new markers signals their commitment toward producing these short-lived, high-volume cells.
In contrast, Common Lymphoid Progenitors are characterized by a Lin-/CD34+/CD38-/low/CD45RA+ phenotype, indicating commitment to the lymphoid fate. CLPs are the precursors to the immune system’s T-cells, B-cells, and Natural Killer (NK) cells. Further down the pathway, mature cells acquire highly specific markers, such as CD3 for T-cells and CD19 and CD20 for B-cells, defining their specialized roles.
Clinical Use of HSC Markers
The precise identification provided by CD markers has practical applications in modern medicine, particularly in hematology. In stem cell transplantation, the ability to selectively isolate HSCs is essential for the procedure’s success. Donor grafts are routinely processed to enrich for stem cells, focusing on the positive selection of the CD34+ population. Monitoring the number of CD34+ cells in a donor’s blood or bone marrow is the standard method for determining the yield and quality of the graft, as a higher count correlates with faster engraftment.
Beyond transplantation, CD markers are fundamental tools for the diagnosis and monitoring of hematological malignancies, such as leukemias and lymphomas. Cancer cells often display aberrant combinations of CD markers that reflect their lineage of origin and stage of arrested development. For example, the presence of the myeloid marker CD33 is used in classifying acute myeloid leukemia. Analyzing a cell’s CD marker profile (immunophenotyping) allows clinicians to accurately classify the cancer subtype, choose appropriate targeted therapy, and track treatment effectiveness.