Phenotype and immunoglobulin gene configuration of blood B cells from patients with multiple myeloma

Abstract

We have studied the phenotype and the immunoglobulin gene configuration of blood B cells from 15 patients with stage III multiple myeloma (MM) at diagnosis. Highly purified B cells (greater than 90% CD20 positive cells) were obtained after L-leucine methyl ester monocyte depletion and elimination of T cells by rosetting. The percentage of B cells with surface immunoglobulin (sIg) featuring the same light and heavy chain isotype as the serum monoclonal immunoglobulin was very low, except in one patient, in whom 25-30% of B cells displayed surface and cytoplasmic immunoglobulin (cIg) sharing idiotypic determinants with the serum monoclonal IgG kappa. In all cases but one the percentage of circulating plasma cells accounted for less than 2% of the enriched B cell preparations. In one patient purified B cell population contained 30% of plasma cells and the immunoglobulin gene study revealed a rearranged JH hybridizing fragment identical in bone marrow and blood B cell DNA samples. In the other 14 cases no rearranged fragment was detected although we used a technique allowing the detection of at least 2% clonal cells. The absence of clonal cells in the patient whose B cells contained a high percentage of cells featuring surface IgG molecules was confirmed on purified sIgG-positive cells. In addition CD20-positive cells from this patient did not contain gamma mRNA. Therefore the IgG molecules were clearly extrinsic. Although the existence of clonal B lymphocytes or of myeloma idiotype related B cells cannot be ruled out, they escape detection by sensitive genetic studies of immunoglobulin genes.