P819: contribution of immunophenotyping of bone marrow b cells and plasma cells in the diagnostic classification of individuals presenting with a serum m-protein

Topic: 13. Myeloma and other monoclonal gammopathies - Biology & Translational Research Background: Monoclonal gammopathies (MG) consist of neoplastic disorders characterized by the accumulation of clonal (c) plasma cells (PC) and/or mature B-cells which are associated with detectable serum/urine M-protein. Premalignant conditions known as MG of undetermined significance (MGUS) and smoldering multiple myeloma (SMM), as well as smoldering Waldenstrom’s macroglobulinemia (SWM), can progress to overt (malignant) diseases such as multiple myeloma (MM) and Waldenstrom’s macroglobulinemia (WM), respectively. Aims: Here we investigated the contribution of immunophenotyping by next-generation flow cytometry (NGF) in the diagnostic classification of individuals from the general population presenting with a serum M-protein. Methods: A total of 164 paired BM (n=82) and PB (n=82) samples from 82 Icelandic donors aged ≥40 years recruited in the iSTOPMM study and preselected based on the presence of a serum M-protein by electrophoresis were investigated by NGF using the EuroFlow MM minimal residual disease (MRD) NGF and lymphoid screening tube (LST) antibody combinations for detection of cPC and cB-cells. A median of 107 (0.2-1.5x107) and 1.3x106 (0.2-5x106) cells were acquired in the NGF and LST tubes, in FACSCantoTM II (BD) and FACSLyricTM (BD) cytometers, respectively. Results: Based on the IMWG diagnostic criteria, donors were classified as MGUS (55/82, 67%), SMM (12/82, 15%), MM (1/82, 1%), SWM (8/82, 10%), plasmacytoma (1/82, 1%) and CLL (1/82, 1%). Presence of a serum M-protein could not be confirmed after initial assessment in 4 donors. In BM, clonal cells were detected in 71/78 (91%) cases, including cPC only in 65/78 (83%), cB-cells in 35/78 (45%), and both in 29/78 (37%) cases. No clonal cells were detected in 7/78 (9%) subjects. Interestingly, in 35/78 (49%) donors clonal cells were also detected in PB, corresponding to ≈40% of BM-positive cases. A significant correlation (rho= 0.46; p<0.001) was found between the percentage of cPC and/or cB-cells in BM and their number in PB, particularly among cases who had cB-cells only (rho= 0.96; p<0.001). The immunophenotypic profile of BM clonal cells matched the diagnosis by conventional IMWG/WHO criteria in 65% (36/55) of MGUS, 92% (12/13) of SMM+MM, 75% (6/8) of SWM and in the single plasmacytoma and CLL cases. The 19 discordant MGUS cases included: i) 8/19 (42%) subjects with coexisting cPC and cB-cells with a typical lymphoplasmacytic immunophenotypic profile and an IgM M-protein, identical to SWM; ii) 5/19 (26%) in whom only CLL-like MBL cB-cells were detected in the absence of cPC; and iii) 6/19 (32%) who showed no clonal cells. Likewise, the discrepant SMM+MM case showed coexisting cPC and cB-cells together with an IgM M-protein, consistent with SWM. In turn, of the 2 discrepant SWM cases, one presented with unrelated cPC and cB-cells, with no lymphoplasmacytic-like immunophenotypic characteristics suggesting independent disorders, i.e, MGUS and CLL-like MBL, whereas the remaining case showed no detectable clonal cells. Summary/Conclusion: Our results highlight the contribution of flow cytometry immunophenotyping for more accurate classification of subjects presenting with a serum M-protein and an early diagnosis of MG. In addition, we demonstrate the utility of performing high-sensitive flow cytometry in PB in the initial evaluation of patients with MG, which might help better schedule subsequent BM investigations in half of the cases. Keywords: Monoclonal gammopathy, Flow cytometry, Diagnosis, Immunophenotype