Intensity of expression of the CD117 antigen (c-kit) on myelomatous plasma cells

Abstract
Background: We have previously shown the expression of c-kit-a receptor with tyrosine kinase activity on plasma cells (PCs) of some multiple myeloma (MM) patients. The aim of the present study was to evaluate the frequency and intensity of c-kit expression on bone marrow (BM) myelomatous PCs.
Material and methods: The study group consisted of 109 MM patients (8 at stage I, 22-II, 79-III acc. to D.S. with monoclonal protein: IgG-69, IgA-25, IgM-1, BJ-12 and non-secretory-2). The control group was 10 healthy subjects. Immunophenotyping was performed on freshly collected BM samples using triple staining combination of the CD138/CD117/CD38 monoclonal antibodies analysed by flow cytometry (Cytoron Absolute and FACSCalibur – Becton Dickinson). PCs were indentified acc. to their strong reactivity for CD38, CD138 (syndecan-1) and their typical light scatter distribution. The antigen expression intensity was calculated as relative fluorescence intensity (RFI) and for direct quantitative analysis the QuantiBRITE test (Becton Dickinson) was applied. The mean channels of phycoerythrin fluorescence were defined and antibody bounding capacity (ABC) was then calculated using QuantiCALC software.
Results: In 37 patients (=33%) myeloma PCs showed CD117 expression. Out of all nucleated BM cells the mean proportion of PCs with CD117 expression was 25.7±20.3%, median 21%. Values of RFI ranged from 8.4 to 20 in particular MM patients (12.4±3.2 median 11.6) and the number of CD117 binding sites (ABC) on MM plasma cells ranged from 1443 to 6217 (3262±1410, median 3109). A correlation was found between RFI and ABC values (r=0.87). In 72 patients myeloma cells did not express CD117 and mean proportion of all BM cells with CD117 expression was 3±2%, median 2.7%. Normal PCs did not express CD117. In BM of healthy persons the mean proportion of CD117+ cells was 2.4±0.7, median 1.8%. Morphological analysis of MGG stained BM aspirate smears revealed that the percentage of BM PCs in MM patients with PCs positivity for CD117 was 36±24%, median 35%, while a corresponding value in MM patients with PCs negative for CD117 was 41±23%, median 38%. In patients with CD117+ myeloma a positive correlation was found between proportion of CD117+ bone marrow cells and percentage of PCs in BM smears (r=0.95). No differences were seen in occurrence of CD117 expression depending on stage of disease and monoclonal protein isotype.
Conclusions: In one third of MM patients CD117 antigen could be considered as a „tumour associated marker” and it may be of value for the identification of the malignant clone in minimal residual disease. The intensity of c-kit expression on PCs varies among particular c-kit positive MM patients and the differences in expression level may be as big as many times. It may be rationale to consider usefulness of therapy with tyrosine kinase inhibitors in the management of MM patients with c-kit positive plasma cell proliferation.