Flow Cytometric Detection of Minimal Residual Disease in Multiple Myeloma Patients

Background: Multiple myeloma (MM) is a hematological malignancy characterized by expansion of neoplastic plasma cells (PCs). Despite of advances in treatment, achieved in past ten years, many patients relapse due to minimal residual disease (MRD) existence.

Aim: Monitoring of number and phenotype of plasma cells in transplanted MM patients by multiparametric flow cytometry.

Patients and Methods: Bone marrow of 24 MM patients (13 male, 11 female) after autologous stem cells transplantation was used. Analyses were done in 1^st month and 1^st year after transplantation. Relapsed patients were analyzed earlier. Plasma cells were identified as CD138⁺CD38⁺ cells, CD19 and CD56 markers were used for discrimination of normal polyclonal (N-PCs) and abnormal monoclonal (A-PCs) plasma cells, respectively.

Results: Median number of PCs in whole group of patients in 1^st month after transplantation was 0.1% (range 0.0–2.5%), median number of CD19⁺ N-PC 39.6% (5.0–79.2%), CD56⁺ A-PC 31.7% (3.5–90.9%) and N-PC/A-PC ratio was 1.1 (0.1–22.6). There were identified 8 relapsed patients during 1^st year from transplantation, all with detectable monoclonal immunoglobulin, and with median number of PCs 10.6% (range 0.1–76.6%), N-PC 0.2% (0.0–56.2%), A-PC 60.1% (0.8–99.9%) and N-PC/A-PC ratio 0.0 (0.0–3.0). In a group of non-relapsed patients in 1^st year after transplantation was median number of PCs 0.2% (range 0.0–1.9%), N-PC 64.2% (6.3–90.0%), A-PC 21.7% (3.0–93.2%) and N-PC/A-PC ratio 3.4 (0.1–25.0). There is an evidence that relapse is relate to loss of CD19 expression on PCs and to higher number of CD56⁺ PC.

Conclusion: Flow cytometric analysis of PCs phenotype profile is relatively quick and sufficiently sensitive method that can be used for monitoring of MRD. Detailed analysis of higher number of patients and comparison of their data with other parameters (immunofixation etc.) are necessary for verification of this method and for its application in routine diagnostics.