Phenotypic Analysis of Plasma Cells in Monoclonal Gammopathy and Multiple Myeloma Subjects.

Introduction: Multiple myeloma (MM) is a B-cell neoplasia caused by the proliferation of clonal plasma cells (PCs). MM and benign monoclonal gammopathy of unknown significance (MGUS) are routinely distinguished on the basis of paraprotein concentration, level of PCs infiltration and the presence or absence of other clinical features. Flowcytometric detection of PCs according to the expression of CD38 and CD138 has limitation in discrimination between normal and abnormal PCs, but this is possible in multicolor phenotypic analysis. The aim of this study is to compare the numbers of normal and abnormal PCs in MGUS and MM subjects and to find some parameter useful for evaluation PCs distribution.

Materials and methods: 51 newly diagnosed untreated MM patients (63,9±10,0 years old) and 31 non-treated MGUS subjects (64,2±13,8 years old) were analysed. Lysed whole bone marrows were analysed by flowcytometric immunophenotyping and PCs were indentified by expression of CD38, CD138, CD45 and also CD56 and CD19.

Results: Discrimination between normal CD19⁺ PCs and abnormal CD56⁺ PCs was done on CD38⁺CD138⁺ population. Ratio of normal PCs count and abnormal PCs counts (normal/abnormal=N/A) was used to describe a distribution of PCs. Subjects with MGUS had 0,97±1,65% (average±SD) of CD38⁺CD138⁺ cells (median 0,45; range 0,03–7,47%) with average N/A ratio 2,91±3,94 (median 1,36; range 0,01–18,44). Newly diagnosed MM patients had 4,96±9,94% of CD38⁺CD138⁺ cells (median 0,85; range 0,02–41,80%) with average N/A ratio 1,70±3,03 (median 0,13; range 0–11,5). In 9,7% MGUS subjects evaluation of distibution of PCs showed transformation of MGUS into MM. In some newly diagnosed MM patients (31,4%) CD38⁺CD138⁺ plasma cells were positive for CD19 although they were aberrant and these PCs were mostly CD45⁺. In some patients (9,7% of MGUS; 17,6% of MM) PC did express neither CD19 nor CD56 and this fact may complicate further evaluation of PCs.

Conclusions: Results confirmed that in MGUS subjects we can find lower numbers of PCs which are mostly CD45⁺CD19⁺. A majority of plasma cells in newly diagnosed MM patients are abnormal CD56⁺ PCs and these plasma cells are usually CD45⁻. Further follow-up of patients can confirm the N/A ratio as a predictive factor for transformation of MGUS into MM and its value for evidence of relapse or progression to active myeloma.