Acute Myeloid Leukemia (AML) is a rapidly progressing cancer originating in the bone marrow, characterized by the uncontrolled growth of immature white blood cells called myeloid blasts. These abnormal cells interfere with the production of normal blood components, leading to complications like infection and bleeding. To accurately diagnose and classify AML, oncologists identify specific protein tags on the surface of the leukemic cells, known as Cluster of Differentiation (CD) markers. The unique profile of CD markers on a patient’s blasts is essential for directing the appropriate course of treatment.
Identifying Leukemia Cells
The primary laboratory technique used to analyze CD markers on blood or bone marrow cells is multiparameter flow cytometry. This sophisticated method involves mixing a cell sample with fluorescently tagged antibodies, each designed to bind to a specific CD marker. The cells are then passed one by one through a narrow tube and illuminated by multiple lasers.
As each cell passes through the laser beam, the light scatter and the fluorescence emitted by the attached antibodies are measured. Forward scatter indicates the cell’s size, while side scatter reflects its internal complexity, such as granularity. This combination of physical properties and fluorescent signals allows the instrument to rapidly analyze tens of thousands of cells.
The resulting data generates a distinct plot, or immunophenotype, which serves as a molecular fingerprint for the cells. Clinicians use this marker pattern to distinguish cancerous myeloid blasts from normal, developing blood cells found in the bone marrow. This immunophenotype is also used to definitively differentiate AML from other blood cancers, such as Acute Lymphoblastic Leukemia (ALL).
Defining the precise pattern of markers expressed confirms the AML diagnosis and helps determine its specific subtype. The detailed lineage assignment provided by flow cytometry is a prerequisite for all subsequent prognostic and therapeutic decisions.
Common Marker Groups in AML
AML diagnosis depends on the presence of a specific collection of myeloid markers on the blast cells. The two most common defining myeloid lineage markers are CD13 and CD33, found on the majority of AML cases. CD33 expression is noted in a high percentage of patients, with positivity rates often ranging from 73% to nearly 90% across various subtypes.
CD13 is another frequently expressed antigen, showing positivity in around two-thirds of AML cases, often co-expressed with CD33. A third common myeloid-associated marker is CD117, also known as c-Kit, which is expressed by approximately 67% of AML blasts. CD117 is a receptor tyrosine kinase typically found on early hematopoietic progenitors, and its presence helps confirm the myeloid origin of the cancerous cells.
Markers associated with immaturity, such as CD34, are also analyzed because they indicate an early stage of cell development. CD34 is a hematopoietic stem cell marker expressed by many AML blasts, with prevalence recorded between 42% and 62% in non-M3 AML subtypes. However, CD34 expression is often absent in the more differentiated forms of AML.
To classify specific AML subtypes, particularly those with monocytic differentiation, markers like CD14 and CD64 are examined. CD64, a receptor for immunoglobulin G, is expressed by about 55% of AML cells and is strongly associated with monocytic-lineage differentiation. The simultaneous expression of CD14 and CD64 points toward an acute myelomonocytic or monocytic leukemia subtype.
The absence of lymphoid markers is equally important, as this helps confirm the myeloid lineage and exclude ALL. A definitive AML diagnosis requires the lack of B-cell markers like CD19 or T-cell markers like surface CD3. However, leukemic blasts sometimes show aberrant expression of lymphoid-associated markers, with CD7 being the most frequently observed non-myeloid antigen.
Markers and Targeted Treatment Selection
Beyond initial diagnosis and classification, the CD marker profile holds significant implications for selecting targeted therapies. Identifying specific surface antigens allows for the use of drugs designed to selectively attack malignant cells while minimizing damage to healthy tissue. This therapeutic strategy moves beyond conventional chemotherapy, which broadly kills rapidly dividing cells.
The CD33 marker serves as a primary example of a therapeutic target due to its high and widespread expression on AML blasts. Gemtuzumab ozogamicin, an antibody-drug conjugate, was specifically developed to exploit the presence of CD33. This drug consists of a monoclonal antibody linked to a potent chemotherapy agent, calicheamicin.
The antibody component binds precisely to the CD33 protein on the cell surface. Once bound, the entire complex is internalized into the leukemic cell, where the toxic calicheamicin is released. This mechanism causes double-strand breaks in the cell’s DNA, leading to programmed cell death within the malignant population.
The expression level of certain markers also influences the overall prognosis and guides risk stratification. For example, high expression of leukemic stem cell markers, such as CD123, is associated with more aggressive disease. CD123 is currently being explored as another target for novel monoclonal antibody therapies.
Ultimately, immunophenotyping results inform the intensity of the initial treatment plan. Patients whose marker profile suggests a higher risk of relapse may be directed toward intensive chemotherapy or allogeneic stem cell transplantation earlier in their treatment course. Thus, the CD marker analysis is a continuous process that not only diagnoses the disease but also dictates the most appropriate, personalized therapeutic strategy.