EXPRESSION OF THE CD117 ANTIGEN (C-KIT) ON NORMAL AND MYELOMATOUS PLASMA-CELLS

The surface expression of CD117 on plasma cells (PCs) from normal indi viduals and patients with multiple myeloma (MM) has been analysed usin g triple-stained cells for now cytometry. In addition, the clinical si gnificance of CD117 expression in MM patients and its possible value f or the evaluation of minimal residual disease was explored, A total of 11 healthy volunteers and 56 untreated MM patients were included in t he study. The expression of CD117 was analysed by now cytometry, using simultaneous staining with the MAbs BB4, CD117 and CD38, Cell acquisi tion was performed in two consecutive steps using a live gate drawn on SSC/CD38(+++) cells and a total of 300 000 events were acquired. For data analysis, the Paint-a-Gate Plus software (Becton Dickinson) was u sed, PCs were identified according to their strong reactivity for CD38 and their positivity for BB4, as well as by their light scatter distr ibution, Dilution experiments of CD 117(+) myelomatous PCs with normal bone marrow (BM) cells were performed in order to assess the sensitiv ity level of the technique for detection of CD117(+) residual PCs. Non e of the PCs from normal BM samples showed reactivity for the CD117 an tigen, In contrast, CD117 antigen was present in 18/56 MM patients (32 %), the proportion of positive cells in these cases being as high as 9 2.1 +/- 9%. Therefore, within PC lineage the c-Kit antigen would be re stricted to the myelomatous population and thus could be considered as a 'tumour-associated marker' for monitoring minimal residual disease in about one third of MM patients. Dilution experiments indicate that the detection limit with this marker would be 10(-4) (one myelomatous PC/10(4) normal BM cells). Upon comparing the clinical and haematologi cal disease characteristics of CD117-positive and CD117-negative cases , no significant differences were found.