Immunophenotypic heterogeneity of normal plasma cells: comparison with minimal residual plasma cell myeloma

Plasma cell myeloma (PCM) exhibits immunophenotypic aberrancies that can be used for minimal residual disease (MRD) detection after therapy. The authors sought to determine whether non-neoplastic plasma cells, especially in the bone marrow (BM) post various therapies, would exhibit immunophenotypic variations interfering PCM MRD detection. The authors studied the flow cytometric immunophenotypes of non-neoplastic plasma cells from 50 BM specimens, including 12 untreated BM and 38 BM specimens from patients with non-plasmacytic haematological malignancies undergoing various therapies, and compared with 59 BM specimens positive for PCM MRD. Non-neoplastic plasma cells showed heterogeneous expressions of CD45 (78% (41-100)) and CD19 (80% (52-97)), and were negative for CD20 and CD117. CD56 was observed in a small subset (6% (0-37)) and CD28 in a larger subset (15% (0-59)) of non-neoplastic plasma cells, with the latter more frequently expressed in post-treatment BMs (p=0.01). However, despite a partial immunophenotypic overlap, PCM cells could be reliably discriminated from non-neoplastic plasma cells by the presence of a higher number of aberrancies (3 (1-6) vs 0 (0-2)) and stronger intensity and uniformity of aberrant expression (p<0.001 in each marker using a cut-off value). In addition, simultaneous assessment of cytoplasmic κ/λ with surface markers detected light chain restriction in all 59 PCM cases. In conclusion, non-neoplastic plasma cells in BM are more immunophenotypically heterogeneous than previously understood; however, these immunophenotypic variations differ from those of PCM. With advances in multicolour flow cytometry and application of recently validated markers, PCM MRD may still be reliably distinguished from non-neoplastic plasma cells.