Detection of Clonal Plasma Cells in POEMS Syndrome Using Multiparameter Flow Cytometry

Introduction

POEMS syndrome is a rare systemic disorder characterized by various symptoms, including polyneuropathy, organomegaly, endocrinopathy, monoclonal gammopathy (M-protein), and skin changes, which is based on the presence of monoclonal plasma cells (PC). We previously reported the analysis of the clonal immunoglobulin λ light-chain variable region (IGLV) gene rearrangements in POEMS syndrome using next-generation sequencing (NGS), and the clone identification rate was 36.7% (Kawajiri-Manako C, Am J Hematol. 2018;93(9):1161). EuroFlow-based next-generation flow (EuroFlow-NGF) has also been standardized for detecting minimal residual disease in multiple myeloma (MM). However, multiparameter flow cytometry (MFC) utility in detecting clonal PC of POEMS syndrome remains unknown. Therefore, to clarify the feasibility of detecting the clonality with MFC, we analyzed PC of bone marrow samples in patients with POEMS syndrome by MFC and validated gating strategies of PC in POEMS syndrome.

Methods

Patients with newly diagnosed or relapsed POEMS syndrome (n=25) at Chiba University Hospital from April 2019 to July 2021 were included in this study. Primary bone marrow aspirations were performed after obtaining informed consent following the Declaration of Helsinki. The collected bone marrow samples were analyzed by the SRL-Flow protocols (single-tube 8-color: CD138/CD27/CD38/CD56/CD45/CD19/CyIgκ/CyIgλ), which is highly correlated with EuroFlow-NGF (Takamatsu H, Int J Hematol. 2019;109(4): 377). Furthermore, 21 cases for which data files were available from SRL Inc. were reanalyzed by gating PC with lower expression of CD45 regarding the previous study (Dao LN, Blood. 2011;117(24):6438). This study was approved by the ethics committee of Chiba University Graduate School of Medicine.

Results

Among 25 patients, the median age was 59 (25-77) years, and 13 (52.0%) were men. The types of M-proteins identified by immunofixation electrophoresis (IFE) were IgA-λ in 12 (48.0%), IgG-λ in 9 (36.0%), IgA-κ in 1 (4.0%), IgG-κ in 1 (4.0%), and negative in 2 patients (8.0%), respectively. The median serum VEGF level was 3,240 (747-9,060) pg/ml. SRL-Flow identified the clonal PC in 9 of 25 (36.0%, λ in 7 and κ in 2): 7 of 23 IFE-positive cases (30.4%) and 2 of 2 IFE-negative cases (100%). The median clone size was 0.03 (0.01-3.78)%, smaller than MM. Interestingly, the clone size was not correlated with the level of serum VEGF (r = -0.17). Aberrant expression of clonal PC in POEMS syndrome was similar to MM: CD19-56+ in 5 (55.6%), CD19-56- in 2 (22.2%), CD19dim56- in 1 (11.1%), and CD19-56dim in 1 (11.1%) patient, respectively. No clone was detected in PC with CD19+56-. The clone identification rate by single-tube 8-color SRL-Flow protocols was lower than expected. Therefore, we reanalyzed the data by gating PC with lower expression of CD45 and CD38. In the reanalysis, we gated broadly for CD38 dim-to-bright and CD138 dim-to-bright cells and narrowly for CD38 dim and CD45 negative-to-dim cells. As a result, we identified the clonal PC in 15 of 21 cases (71.4%, λ in 13 and κ in 2): 13 of 19 IFE-positive cases (68.4%) and 2 of 2 IFE-negative cases (100%)). Of the 15 cases in which monoclonal PC were identified by reanalysis, 8 were negative by SRL-Flow. Of these, 6 were identified by selective gating of CD45- and CD38 dim, and 2 were identified by selective gating of CD45- and CD38+. The median clone size was 0.008 (0.001-3.27)%, smaller than that of MM, and the clone size was not correlated with the level of serum VEGF (r = -0.15). The aberrant expression of the clonal PC in POEMS remained similar to MM for CD19 and CD56 expression: CD19-56+ in 3 (20.0%), CD19-56- in 7 (46.7%), CD19-56dim in 4 (26.7%), and CD19dim 56- in 1 (6.7%) patient, respectively. No clone was detected in PC with CD19+56-.

Conclusions

EuroFlow-based SRL-Flow protocols detected the clonal PC in about one-third of POEMS patients. Aberrant immunophenotype of clonal PC in POEMS syndrome was similar to MM for CD19 and CD56 expression; however, CD45 and CD38 expression of POEMS PC tended to be downregulated. Selective gating of PC with CD38dim and CD45 negative-to-dim detected clonal PC in 71.4% of cases. This novel gating strategy is more accessible and might improve the identification rate of clonal PC in POEMS syndrome.