Immunohistochemical Markers for Confirming Plasma Cell Clonality in Multiple Myeloma
For confirming clonality of plasma cells in multiple myeloma on bone marrow biopsy blocks, a minimal IHC panel should include CD19 and CD56, while a preferred panel would also include CD20, CD117, CD28, CD27, and most importantly, kappa and lambda light chain assessment to demonstrate light chain restriction. 1
Essential Markers for Plasma Cell Identification
- CD38, CD138, and CD45 should all be included in at least one tube for plasma cell identification and enumeration 1
- The primary gating for plasma cell identification should be based on CD38 vs. CD138 expression 1
- These markers allow for the unequivocal identification of plasma cells among other hematopoietic cells 1
Core Markers for Clonality Assessment
- Cytoplasmic kappa and lambda light chain expression is critical to demonstrate clonality of plasma cells 1
- Light chain restriction is the gold standard for establishing clonality, with kappa/lambda ratio cut-offs of ≤1/7 or ≥9 yielding the highest diagnostic accuracy 2
- Assessment of cytoplasmic κ/λ expression is important at presentation and appropriate for assessment of stringent complete remission according to IMWG criteria 1
Minimal Panel for Detecting Abnormal Plasma Cells
- CD19 and CD56 form the minimal panel for detection of abnormal plasma cells 1
- CD19 negativity is a hallmark of neoplastic plasma cells (normal plasma cells are typically CD19+) 1, 3
- CD56 expression is seen in approximately 63% of myeloma cases while being negative in reactive plasmacytosis 4
- CD56 expression correlates significantly with the presence of lytic bone lesions 4
Preferred Extended Panel
- CD20, CD117, CD28, and CD27 should be included in a preferred panel 1
- CD28 is frequently gained in malignant plasma cells 3
- CD27 is typically expressed in normal plasma cells but may be lost in malignant ones 3
- CD117 (c-kit) expression is common in myeloma cells but absent in normal plasma cells 1, 3
- CD20 is typically negative in plasma cells but may be expressed in a subset of myeloma cases 1, 3
Technical Considerations
- For light chain assessment, bone marrow samples should be washed twice in buffered saline solution prior to assessment of cytoplasmic immunoglobulin expression to remove cytophilic immunoglobulin 1
- Standard commercial fixation and permeabilization kits are suitable for assessment of cytoplasmic kappa/lambda detection 1
- At least 100 neoplastic plasma cell events should be acquired for accurate enumeration when using flow cytometry to assess light chain expression 5
- For bone marrow biopsies, harsh decalcification processes may render protein epitopes unsuitable for IHC detection, so proper processing techniques are essential 6
Sample Quality Assessment
- The sample is suitable for quantitative analysis if normal plasma cells (CD19+CD56- and/or polyclonal) are detectable 1
- If normal plasma cells are not detected, sample quality should be assessed by morphological evaluation or additional flow cytometry for normal marrow elements 1
- Discrepancies between the percentage of plasma cells detected by flow cytometry and morphology are primarily related to sample quality 1
Pitfalls and Caveats
- Rare cases of dual expression of kappa and lambda light chains in the same myeloma cells have been reported, which can lead to false negative results for clonality 7
- In such cases, additional investigations such as cyclin D1 expression and FISH studies may be necessary 7
- The demonstration of phenotypically abnormal plasma cells is more sensitive and specific for detecting residual disease than clonality assessment alone 1
- No single marker can systematically differentiate neoplastic plasma cells from their normal counterparts, necessitating a panel approach 1
By applying this comprehensive panel of markers, pathologists can accurately identify and confirm the clonality of plasma cells in bone marrow biopsies, which is essential for the diagnosis, risk stratification, and monitoring of multiple myeloma.