Immunophenotypic Heterogeneity of Polytypic Plasma Cells and the Impact on Myeloma Minimal Residual Disease Detection by Multiparameter Flow Cytometry

Background Flow cytometry is widely used for minimal residual disease (MRD) detection in plasma cell myeloma (PCM). Recently, an increasing number of studies have demonstrated that polytypic plasma cells (PPCs) display greater immunophenotypic variation than previously appreciated. Our aim was to further characterize the immunophenotype of PPC in this setting. Methods PPC in 102 bone marrow specimens (93 MRD-negative post-treatment PCM and 9 negative initial evaluations for plasma cell neoplasm) were evaluated by 10-color flow cytometry. Frequency of CD19, CD27, CD28, CD56, CD117, and CD138 expression was assessed. Results All cases showed CD27 and CD19 positivity. CD117 was uniformly negative. Percentage of CD138 expression was variable with one negative case (<20% expression). Percentage of CD28 and CD56 expression was variable and included 11 CD28+ cases as well as 38 CD56+ cases. Forty-two percent (43 of 102) cases showed atypical expression of at least one marker with 36 cases (35%) showing atypical expression of a single marker and 7 cases (7%) showing dual atypical marker expression (CD56+/CD28+). Conclusions Considerable immunophenotypic variation exists in PPC. The assessment of cytoplasmic light chain distribution, in conjunction with surface marker expression, is recommended to avoid diagnostic inaccuracy in MRD evaluation. (c) 2019 International Clinical Cytometry Society