High Frequency of Aberrant Phenotype in Multiple Myeloma Patients Detected by Multiparametric Flow Cytometry.

Multiple Myeloma (MM) is a B cell lymphoproliferative disease with clonal plasma cell accumulation in bone marrow. Multiparametric flow cytometry (MFC) is an usefull tool to distinguish MM cells from normal plasma cells. Normal plasma cells are characterized by the expression of CD19+, CD45++, CD38++, CD138++, cytoplasmic immunoglobulin light chains (κ and λ) and CD56- while most MM plasma cells lose CD19, CD45 and gain CD56. In addition, many other antigens may be expressed by myeloma cells such as myeloid or lymphoid lineage associated antigens and these abnormal antigen expression is known as aberrant phenotype (AP). We studied 29 MM patients at diagnosis, in attempt to evaluate AP, it’s frequency and relation to prognostic parameters. The following monoclonal antibodies were used: CD45, CD38, CD138, CD56, CD19, CD20, CD22, CD10, CD13, CD14, CD33, CD117, CD28 and CD40, conjugated to FITC, PE, PerCP and APC) and acquisition / analysis were done through flow cytometer (FACS calibur, BD, San Jose) using CELL QUEST software (BD). Plasma cells were identified by the expression of CD38, CD138 and CD45 and the monoclonality confirmed by immunoglobulin light chain restriction. Our results showed presence of at least 2 AP in all cases : 2 AP (7 patients), 3 AP (12 patients), 4 AP( 5 cases), 5 AP (4 cases) and 8 AP in one case. The most frequent APs were CD45⁻, CD56⁺, CD117⁺, CD13⁺, CD33⁺, and were observed in 88% of patients. The most frequent AP association was CD56⁺/CD45⁻ (40%), followed by myeloid antigen associated phenotypes (CD117, CD33, CD13). The lymphoid antigens expression was more observed in patients with large number of AP (>4 AP). CD56- patients presented serum β₂-microglobulin and ionic calcium labeling levels higher than CD56+ patients (p=0,02) showing the usefulness of this antigen as prognostic marker. Morphological analysis showed that the majority (55%) of plasmablastic cases expressed >2 myeloid antigens against 18% of mature plasma cell morphology cases. These results allow us to conclude that MM express high frequency of AP, highlighting the importance of CD56 as a prognostic factor. MFC may be useful to the immunological detection of minimal residual disease in a great majority of MM patients and we suggest the panel CD45, CD56, CD117, CD33 and CD13 for this purpose, in addition to CD38 and CD138.