ANALYSIS OF THE B-CELL COMPARTMENT IN PLASMA-CELL LEUKEMIA AND MULTIPLE-MYELOMA - IMMUNOGLOBULIN GENE REARRANGEMENT OF EBV-INFECTED B-CELL LINES

Multiple myeloma (MM) is defined as a tumoral expansion of plasma cell s occurring in the bone marrow and sometimes in the peripheral blood ( plasma-cell leukemia, PCL). Many reports have demonstrated a clonal ex pansion of B cells bearing the same idiotypic determinants as the myel oma protein (idiotypic B cells) in MM, suggesting that they could belo ng to the malignant clone. In order to investigate whether the B-cell population is a malignant component or not, either in the peripheral b lood of patients with PCL or in the bone marrow of patients with MM, w e derived B-cell lines by infecting, with the Epstein-Barr virus (EBV) , cultures in limiting dilution of mononuclear cells from six patients . A limiting dilution culture was used to prevent the elimination of s lowly proliferating clones by the more rapidly dividing ones, and thus to get the most exact representation of the B-cell repertoire of thes e patients. The cloning efficiency of the EBV-infected cells was simil ar in patients and healthy individuals (range: 1 in 100 to 1 in 1650 B cells). All of the clones obtained from a single patient exhibited di fferent clonal immunoglobulin gene rearrangements (IGR), proving the v alidity of our cloning technique. No tumoral clones (61 clones analyse d) showed the IGR pattern specific of autologous myeloma cells. These results indicate that malignant plasma cells cannot be immortalized wi th EBV. These results show that, if malignant B cells (pre-switch or p ost-switch) exist, they could be present only in a minor population, a nd the corollary of this is that there is a major population of non-ma lignant B cells in the sites of tumoral proliferation of patients with MM. This is remarkable in view of numerous reports showing a profound defect of the polyclonal B lymphopoiesis in these patients, and even an absence of B lymphocytes. Thus, these results challenge the existen ce of a major compartment of malignant idiotypic B cells and favor the hypothesis of non-malignant B cells sharing cross-reactive idiotypes with the autologous myeloma protein.