[Correlation between Morphological Typing and Monoclonality of Bone Marrow Plasma Cells and Its Diagnostic Value for High-Risk Smoldering Multiple Myeloma].

Objective: To investigate the correlation between morphological typing and monoclonality of bone marrow plasma cells, and explore the diagnostic value of plasma cell morphological typing for high-risk smoldering multiple myeloma(HR-SMM).

Methods: The correlation between the morphological characteristics and the monoclonality of bone marrow plasma cells was analyzed in 84 patients with HR-SMM who treated in our hospital. The consistency of morphologically abnormal bone marrow plasma cells with serum free light chain (sFLC) ratio, next-generation sequencing (NGS) detection results, and its correlation with monoclonal plasma cells detected by flow cytometry (FCM) were further verified. The immunoglobulin types and levels of non-involved immunoglobulins in serum of the patients were detected, and the distribution of plasma cell clusters in patients with different disease was observed.

Results: The mean percentage of mature plasma cells were decreased successively in the order of reactive plasmacytosis (RP) group, monoclonal gammopathy of undetermined significance (MGUS) group, smoldering multiple myeloma (SMM) group, HR-SMM group and multiple myeloma (MM) group; while the mean percentage of immature, primitive, reticular and flaming plasma cells were increased successively in the order of RP group, MGUS group, SMM group, and HR-SMM group, and the difference between any two groups was statistically significant (P < 0.05).The average proportion of abnormal plasma cells in the bone marrow of HR-SMM patients was 96.2% of the total plasma cells. The proportion of abnormal plasma cells were in good agreement with the sFLC ratio and the results of NGS detection in HR-SMM patients (kappa=0.879 and kappa=0.891, both >0.75)，and showed good correlation with the monoclonal plasma cells with immunophenotype of CD45^-/CD38⁺/CD138⁺/CD56⁺/CD19^-( γ=0.825). The levels of non-involved immunoglobulin in IgG, IgA and IgM type HR-SMM patients were all decreased by more than 25% compared with the normal reference range, and the differences were statistically significant (P < 0.05). There was no significant difference in the distribution ratio of plasma cell clusters among different disease groups (P >0.05).

Conclusion: In HR-SMM patients, the immature, primitive, reticular and flaming plasma cells in bone marrow are considered as abnormal plasma cells, and they are correlated with monoclonal plasma cells. The proportion of abnormal plasma cells in total plasma cells of bone marrow and the reduction extent of non-involved immunoglobulin level in patients have certain reference value for the diagnosis of HR-SMM.