Phenotyping studies of clonotypic B lymphocytes from patients with multiple myeloma by flow cytometry

Context. - Clonotypic B lymphocytes, monoclonal B lymphocytes sharing identical, rearranged IGH-CDR3 sequences with the patient's myeloma cells, have been detected in the peripheral blood of patients with multiple myeloma. These cells have been postulated to act as a therapy-resistant tumor reservoir that drives recurrence. Objective. - To characterize clonotypic B lymphocytes for future investigation of their role in myeloma pathogenesis. Design.-Harvests of cryopreserved peripheral blood stem-cells from 20 myeloma patients were enriched for clonotypic B lymphocytes. Cytoplasmic immunoglobulin light chain and surface immunophenotype were analyzed by flow cytometry. IGH-CDR3 gene-rearrangement pattern was performed to determine clonality. Posttransplant remission rate was compared with the percentage of clonotypic B lymphocytes. Results. - Clonotypic B lymphocytes expressing CD34^±, CD38⁺, CD184⁺, CD31^±, CD50 ^±, CD138^-, CD19^-, CD20^-, and the same immunoglobulin light chain as the patients' known myeloma cells were identified in 12 of 20 patients (60%). Progenitor B lymphocytes expressing similar surface immunophenotype but opposite light chains were identified in the same patients. Polymerase chain reaction for IGH rearrangement showed clonal rearrangement pattern in clonotypic lymphocytes but not in B lymphocytes expressing light chains opposite to myeloma cells. There was no statistically significant correlation between the percentage of clonotypic B lymphocytes and response to autologous transplant. Conclusions. - Clonotypic B lymphocytes expressing CD34, but not CD19, were identified in stem cell harvests from patients with myeloma and could represent progenitor cells of neoplastic plasma cells. However, the same or similar immunophenotyping can be detected in both clonotypic B lymphocytes and benign progenitor B cells, suggesting clonality analysis might be needed to determine clonotypic B lymphocytes in patients with myeloma. Further studies are warranted to study the role of clonotypic B lymphocytes in the pathogenesis of myeloma.