Measurement of the Proliferation of Clonal Plasma Cells By Multiparametric Flow Cytometry Is a Clinically Useful Tool in Multiple Myeloma

Background: Proliferation rate of plasma cells (PC) is a powerful tool in evaluating all types of monoclonal gammopathies, predicting for progression as well as survival in all phases of the disease. A flow cytometry (FC)-based method to delineate the cell cycle of clonotypic PCs and determine the proportion in S-phase was developed. This method replaces a labor-intensive slide based methodology, but the prognostic power of this FC approach has not been clearly elucidated.

Methods: A total of 1061 patients who had bone marrow (BM) FC PC proliferation analysis at Mayo Clinic, Rochester from May 2012 to December 2013 were studied. These included MGUS/ SMM (n=134), newly diagnosed (NMM; n=88), plateau in response (>=PR; n=544), and stable disease (n=38), and relapsed disease (RMM; n=257). PC proliferation was measured by 8-color FC using antibodies to CD19, CD38, CD138, CD45, and cytoplasmic kappa and lambda Ig light chains, and DAPI DNA staining. The DNA content of the abnormal, clonotypic PCs was specifically analyzed and the %S-phase was determined by measuring the proportion of cells between G0/G1 and G2/M. All studies collected 5x10⁵ events on BD FACSCanto II instruments and analysis was performed with BD FACSDiva software. The CV of the DAPI staining of G0/G1 lymphocytes as an internal control population was always less than 3.5. Patients with fewer than 300 PCs for assessment of %SP were designated as 0%.

Results: The median follow up from testing for the entire cohort was 13 months (95% CI; 12.7, 13.3); 156 (15%) had died. The proportion of patients with adequate numbers of plasma cells available for %SP estimation and the median (IQR) %SP for the different groups are as shown in table. As expected, patients with MGUS/SMM and those in a response state had slightly lower proportions of patients with sufficient PCs to estimate %SP. The clinical features and outcomes were further analyzed using two cutoffs, %SP ≥1 and ≥2, in the different patient groups. Among most of the disease groups, especially the newly diagnosed MM and relapsed disease, %SP was prognostic for OS using both cutoffs (Table). Among the NMM in univariate analysis, %SP ≥2, B2microglobulin >5.5, age >70 and creatinine > 2 mg/dl were all significant, but only B2microglobulin >5.5 retained significance in multivariable analysis. Among the RMM, in univariate analysis, %SP ≥2, B2microglobulin >5.5, HR FISH (t4; 14, t14;16, t14;20 & del17p) and LDH >221 were all significant, but only %SP ≥2 was significant in multivariable analysis.

Conclusion: The %S-phase of clonal PCs determined by FC is of prognostic value in patients with MM, in all disease phases. This is particularly relevant among patients with relapsed disease, where it is independent of some of the other known poor prognostic factors such as high risk FISH. This is in sharp contrast with recent studies that have shown no prognostic value for slide based labeling index in patients treated with novel agents.

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