CIRCULATING PLASMA-CELLS IN MULTIPLE-MYELOMA - CHARACTERIZATION AND CORRELATION WITH DISEASE STAGE

The aim of this study was to develop a now cytometric test to quantita te low levels of circulating myeloma plasma cells, and to determine th e relationship of these cells with disease stage. Cells were character ized using five-parameter now cytometric analysis with a panel of anti bodies, and results were evaluated by comparison with fluorescent cons ensus-primer IgH-PCR. Bone marrow myeloma plasma cells, defined by hig h CD38 and Syndecan-1 expression, did not express CD10, 23, 30, 34 or 45RO, and demonstrated weak expression of CD37 and CD45. 65% of patien ts had CD19(-)56(+) plasma cells, 30% CD19(-)56(low), and 5% CD19(+)56 (+), and these two antigens discriminated myeloma from normal plasma c ells, which were all CD19(+)56(low). Peripheral blood myeloma plasma c ells had the same composite phenotype, but expressed significantly low er levels of CD56 and Syndecan-1, and were detected in 75% (38/51) of patients al presentation, 92% (11/12) of patients in relapse, and 40% (4/10) of stem cell harvests. Circulating plasma cells were not detect able in patients in CR (n = 9) or normals (n = 10), at a sensitivity o f up to 1 in 10 000 cells. There was good correlation between the now cytometric test and IgH-PCR results: myeloma plasma cells were detecta ble by flow cytometry in all PCR positive samples, and samples with no detectable myeloma plasma cells were PCR negative. Absolute numbers d ecreased in patients responding to treatment, remained elevated in pat ients with refractory disease, and increased in patients undergoing re lapse. We conclude that now cytometry can provide an effective aternat ive to IgH-PCR that will allow quantitative assessment of low levels o f residual disease.