Background: Autologous stem cell transplantation (ASCT) remains the standard treatment for patients with multiple myeloma (MM), yet autologous hematopoietic cell grafts are often contaminated by abnormal plasma cells after CD34+ cell mobilization. Previous studies have shown that positive MRD in grafts is associated with poor progression-free survival and overall survival. However, there is still a significant research gap in whether autografts for plasma cell diseases can be tested for MRD and clinical reinfusion.

Objective: To enhance the purity of autologous stem cell collections and further explore the clinical significance of circulating plasma cell (CPC) and MRD in MM patients. This center conducted a single-center retrospective study. Meanwhile, the efficacy and safety of in vitro purification of autologous stem cells via co-incubation with CD3/BCMA bispecific antibodies were explored.

Methods: This study included 86 MM patients diagnosed between January 2017 and June 2025 who met the transplantation criteria and underwent stem cell mobilization. All patients received standard induction therapy, followed by CD34+ cell mobilization, hematopoietic cell graft leukocyte apheresis, and ASCT. MRD was assessed in MM patients at the following time points (bone marrow within +1 month and peripheral blood within 1 day before stem cell collection, stem cell collection and bone marrow + 100 days after transplantation)using 10-color MFC technique. Two cases of MRD+ peripheral blood stem cells from MM autologous grafts were selected. A certain proportion of the cells were co-incubated with CD3/BCMA bispecific antibodies for 2 hours and then filtered. The amount of CD34+ cells, apoptosis of CD34+ cells, MRD and cytokines in the collected materials before and after incubation were detected by flow cytometry.

Result: Sufficient hematopoietic stem cells were successfully collected from all 86 patients. The median plasma cell content in BM before stem cell collection was M = 0.17% (P25 = 0.05%, P75 = 1.29%), among which 23/86 cases (26.74%) were positive for BM-MRD before collection. After G-CSF mobilization, the proportion of CPC in peripheral blood was M = 0.01% (P25 = 0.01%, P75 = 0.05%), among which 6/86 cases (6.98%) had positive MRD in the collected samples. Pairing post-mobilization CPC with the MRD of the collected materials revealed a high degree of clinical correlation. The peripheral blood MRD of 6 cases with positive collected samples was positive, with CPC > 0.05 %. 64/86 cases (74.41%) completed autologous transplantation (including 2 MRD+ ≤ 0.10%). 10/64 cases (15.8%) were assessed as positive for bone marrow MRD at +3 months after transplantation. The MRD status was compared with the paired BM-MRD evaluated before transplantation (n=86) and after transplantation (n=64), and it was found to be clinically highly correlated. Among the 16 cases of bone marrow MRD+ before transplantation, 5 cases had MRD+ 3 months after transplantation, and 4 MM patients with MRD+ > 0.1% gave up transplantation. Among the 45 cases with negative bone marrow MRD before transplantation, 4 cases had bone marrow MRD+ 3 months after transplantation. Two MRD-positive graft collected samples were selected for purification treatment, and the plasma cell counts were 0.04% and 1.26% respectively. There were no significant changes in the cell count of stem cell collections, placental rejection rate, the number of CD34+ cells and apoptosis before and after incubation. After co-incubation with the bispecific antibodies, the IFN-γ in the collected materials increased slightly compared with before. After filtration through the filter membrane of the blood transfusion device, neither collection showed cell aggregation or adhesion, and the MRD was negative.

Conclusion: The detection of CPC in peripheral blood after G-CSF mobilization can predict the contamination of autologous stem cell collection in MM patients in advance and improve the clean collection rate of autologous stem cell collections. Co-incubation of CD3/BCMA bispecific antibodies and grafts can reduce the contamination rate of abnormal plasma cells in autologous stem cells of MM without affecting the quality of hematopoietic stem cells.

Keywords: Multiple myeloma, circulating plasma cells, autologous transplantation stem cell, CD3/BCMA bispecific antibodies, minimal residual disease

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2025
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