Distinct immunophenotypic profiles of circulating tumor plasma cells in MGUS and smoldering multiple myeloma

This study characterized the phenotypic and cytogenetic features of circulating tumor plasma cells (CTPCs) in patients with monoclonal gammopathy of undetermined significance (MGUS) and smoldering multiple myeloma (SMM). A total of 120 patients were analyzed (MGUS, n = 70; SMM, n = 50). CTPCs were detected in 38.2% of MGUS and 61.8% of SMM. Immunophenotypic analysis based on mean fluorescence intensity revealed that CD200, CD117, CD38, CD56, and CD28 were significantly downregulated and CD45 was significantly overexpressed in CTPCs compared with BMPCs, whereas CD138 and CD81 expression levels did not differ. Cytogenetic analysis demonstrated higher prevalence of del(13q) and high-risk chromosomal abnormalities in CTPC-positive patients than CTPC-negative patients (24.0% vs. 4.5%, p = 0.048; 26.5% vs. 7.0%, p = 0.011, respectively). No significant differences were observed in other cytogenetic abnormalities including 1q gain/amplification, t(4;14), t(11;14), t(14;16), or del(17p). Serum soluble B cell maturation antigen levels did not differ between patients with or without detectable CTPCs in MGUS or SMM. These findings highlight the distinct phenotypic and cytogenetic characteristics of CTPCs in MGUS and SMM, and provide insights regarding their biological features and pathogenesis.