Different Patterns of Antigen Expression in Bone Marrow and Circulating Tumor Plasma Cells in Patients with MGUS and Smoldering Multiple Myeloma

Introduction: Circulating tumor plasma cell (CTPC) is widely recognized as an important prognostic factor in multiple myeloma (MM). Recent studies have demonstrated that the presence of CTPCs is associated with an increased risk of progression in monoclonal gammopathy of undetermined significance (MGUS) and smoldering multiple myeloma (SMM). CTPCs in MM have been highlighted for their unique phenotypic features, with reports indicating downregulation of integrins, adhesion, and activation molecules. However, the understanding of CTPCs in MGUS and SMM remains to be established, with a particular focus on phenotypic differences compared to BM clonal PCs and their association with chromosomal abnormalities. Despite their importance in understanding progression risk and elucidating the mechanisms of dissemination, these findings have not yet been reported. Therefore, we aimed to investigate the phenotypic and cytogenetic characteristics of CTPCs in MGUS and SMM.

Methods: Patients diagnosed with MGUS or SMM at Kameda Medical Center between April 2004 and January 2024 and whose CTCs were examined by multicolor flow cytometry (MFC) before treatment were retrospectively reviewed. Either an 8-color panel using the DuraClone RE PC kit (CD138-APC, CD38-PB, CD56-APC-A750, CD19-PC5.5, CD45-KrO, CD200-PC7, CD81–FITC, and CD27-PE) or a 10-color panel with additional CD28-A700 and CD117-ECD was used. Data collection was performed on a Navios flow cytometer using Kaluza software (Beckman Coulter, CA). Abnormal PCs were identified and the mean fluorescence intensity (MFI) was quantified in accordance with the 2016 guidelines of the International Clinical Cytometry Society. The median total cell count in peripheral blood FCM analysis was 2,765,773, and the lower limit of quantification (LLOQ) was set at 0.0025%, establishing a cutoff for CTPC positivity at ≥0.0025%.

Results: A total of 186 patients were enrolled, including 106 with MGUS and 80 with SMM. CTPCs were detected in 15.1% of MGUS cases and 30.0% of SMM cases, significantly higher in SMM (p = 0.019). There were no significant differences in age, gender, light chain, or heavy chain between CTPC-positive and CTPC-negative cases. According to the Mayo Clinic criteria, there was no significant difference in risk scores between CTPC-positive and CTPC-negative cases. With a median follow-up of 37 months, progression rates did not show significant differences between CTPC-positive and CTPC-negative cases, possibly because of the limited follow-up period. CTPC-positive cases in both MGUS and SMM had a significantly higher percentage of BM clonal PCs among total nucleated cells compared to CTPC-negative cases (MGUS: median 1.42% vs. 0.47%, P=0.011; SMM: 2.48% vs. 1.99%, P=0.018). The MFI comparison between CTPCs and paired BM clonal PCs revealed significantly lower expression of CD28, CD38, CD56, and CD200 in CTPCs (CTPC/BM PC MFI ratio: CD28, 95% CI 0.24-0.77, P=0.004; CD38, 95% CI 0.35-0.73, P<0.001; CD56, 95% CI 0.39-0.83, P<0.001; CD200, 95% CI 0.25-0.49, P<0.001). In contrast, CD27, CD81, CD117, and CD138 did not show significant differences in MFI. Fluorescence in situ hybridization (FISH) analysis showed that the frequency of del13q14 was significantly higher in CTPC-positive cases than in CTPC-negative cases (42.9% vs. 13.9%, P=0.002), but no difference was observed for other abnormalities such as t(4;14), t(11;14), t(14;16), 1q21 gain, and del17p.

Conclusion: This study is the first report on the association between CTPCs and cytogenetics in MGUS and SMM, as well as the differences in MFI compared to paired BM. Reduced expression of several antigens, including the adhesion molecule CD56, was observed in CTPC-positive cases, consistent with previous reports in MM. CTPC positivity was significantly associated with a higher prevalence of del13q14, suggesting a correlation between CTPCs and specific cytogenetic characteristics.