The role of multiparameter flow cytometry in the work-up of IGM-monoclonal gammopathies

Background:IgM-Monoclonal Gammopathy of Undetermined Significance(MGUS) accounts for 15 to 20% of all MGUS cases and it poses a uniquediagnostic challenge as it can be associated with a broad spectrum of patho-logical processes including both B-cell lymphoproliferative disorders (LPD),and monoclonal-IgM related-disorders (IgM-RD). Among all, IgM-MGUSmostly progresses into Waldenström’s macroglobulinemia (WM). Accordingto the II International Workshop on WM (IWWM), IgM-MGUS and WMcan be differentiated by absenceversus presence of marrow infiltration bymalignant B-cells, while smoldering WM (sWM) from sympomatic WMare differentiated by presenceversus absence of clinical findings due to bonemarrow (BM) infiltration.Aims:Based on previous literature data, we aimed to evaluate the contributeof multiparameter flow cytometry (MFC) to discriminate between IgM-MGUS and WM by searching for clonal B-lymphocytes in BM and PB sam-ples.Methods:A total of 102 patients (64/38, M/F) with a median age of 70 yrs(36-89) were investigated. They were selected among patients with an IgMmonoclonal gammopathy not associated with B-cell LPD other than WM.Median serum monoclonal (M)-protein level was 1.0 g/dl (0.2-3.9) andlight-chain was kappa in 75% of cases. According to the II IWWM criteria,52 patients were diagnosed as having MGUS, 23 sWM, and 27 WM. Fifty-one BM aspirates and 85 PB samples were immunophenotyped for evalua-tion of lymphocyte subsets and detection of clonal restriction by Ig Κandλlight chain analyses on B-cell surface, irrespectively from B-cell proportion;all PB samples resulted positive for clonal restriction, and all BM aspirateswere further analyzed with a large panel of MoAbs; data acquisition wasperformed on a Beckman Coulter Navios flow cytometer and analyses wereperformed by using Kaluza software.Results:The median percentage of mature B-cells was found 1.8 (0-50) inPB, and 16 (1.3-50) in BM samples. Overall, in 56 out of 104 patients,clonal B-cells were detected; in particular, clonal restriction was demon-strated in 79% of BM and 40% of PB samples. The FCM results are sum-marized in the following table with regard to the different diagnosis. Ofnote, the identification of clonal populations required accurate gating strate-gies in 5 PB and 1 BM samples because of the presence of low clonal B-cellnumber within total B-lymphocytes.Summary/Conclusion:Our results suggest a pivotal role of MFC in thediagnostic work-up of patients with IgM monoclonal gammopathies.Despite in most cases malignant cells display a non-specific phenotype,accurate gating strategies can enable to identify low-sized clonal populationswithin normal B-cell background in BM as well as in PB samples. In addi-tion, further studies could establish a possible role of PB FCM studies inthe management of patients with IgM monoclonal gammopathies.