What is CD56?
CD56 is a neural cell adhesion molecule (NCAM) that serves as a cell surface glycoprotein marker expressed primarily on natural killer (NK) cells and various other immune cells, and is widely used in diagnostic immunohistochemistry and flow cytometry to identify neuroendocrine tumors and specific lymphoid malignancies.
Molecular Characteristics
CD56 is a 200-220 kD glycoprotein that represents an isoform of the neural cell adhesion molecule 1. The NCAM-120 molecule is the main isoform expressed by immune cells 2. This molecule functions as an adhesion molecule involved in cell-to-cell interactions 3.
Expression on Normal Immune Cells
Primary Expression
- Natural killer cells: CD56 is the archetypal phenotypic marker of NK cells 4
- T cell subsets: Including both alpha-beta and gamma-delta T cells 4
- Dendritic cells and monocytes 4
Functional Significance
CD56-expressing immune cells share common features including strong immunostimulatory effector functions, T helper 1 cytokine production, and efficient cytotoxic capacity 4. CD56 participates directly in tumor cell lysis and may serve as a co-stimulatory molecule in immune cell interactions 2.
Diagnostic Applications in Hematologic Malignancies
Multiple Myeloma and Plasma Cell Disorders
In plasma cell neoplasms, CD56 expression patterns have important diagnostic and prognostic implications:
- Normal plasma cells: Typically express CD56 5
- Plasma cell leukemia: Shows characteristically lower CD56 expression compared to multiple myeloma 5
- Prognostic marker: CD56 expression on abnormal plasma cells serves as a prognostic marker in myeloma 6
The lack of CD56 antigen is a neural cell adhesion molecule important in anchoring plasma cells to bone marrow stroma, and its absence likely impairs circulation to peripheral blood and migration to extramedullary sites 5.
Lymphoproliferative Disorders
CD56 helps differentiate specific lymphoid malignancies:
- Peripheral T-cell lymphomas: CD56 positivity can be seen in certain subtypes 7
- Natural killer/T-cell lymphoma: CD56+ is characteristic 7
- Hepatosplenic T-cell lymphoma: CD56+/− 7
- Enteropathy-associated T-cell lymphoma type 2: CD56+ (versus type 1 which is CD56−) 7
Neuroendocrine Tumors
CD56 is a neuroendocrine marker used alongside chromogranin, synaptophysin, neuron-specific enolase (NSE), and neurofilament 1, 8, 1:
- Merkel cell carcinoma: CD56 is found in most MCC tumors and may be used in addition to CK-20 and TTF-1 to exclude other diagnostic considerations 1
- Small cell lung cancer: CD56 helps identify neuroendocrine differentiation 8
Clinical Utility in Diagnostic Panels
Recommended Use
When evaluating neuroendocrine tumors, CD56 should be used as part of a panel rather than in isolation, as its specificity is not high when used alone 1. When combined with other markers, it helps identify tumors that are CK20 negative or have other features making diagnosis difficult 1.
Flow Cytometry Applications
In resource-limited settings, CD56 is not typically included in basic lymphoproliferative disorder panels, which prioritize CD19, CD5, kappa/lambda, CD20, CD10, CD45, and CD23 9. However, it remains valuable for specific diagnostic scenarios requiring differentiation of NK-cell or T-cell malignancies.
Pathophysiologic Role
CD56 functions as an adhesion molecule through homophilic interactions (CD56-CD56 binding) between cells 2, 3. This interaction is involved in NK cell-target cell binding and cytotoxicity 3. The molecule participates in the third pathway of cell adhesion distinct from CD2/LFA-3 and LFA-1/ICAM-1 interactions 3.
Disease Associations Beyond Malignancy
Numerical and functional deficiencies and phenotypic alterations of CD56+ immune cells have been reported in patients with infectious, autoimmune, and malignant diseases 4. The CD56-CD16+ NK cell subset has been specifically associated with chronic virus infections including HIV-1 10.