S‐Phase cells of the lymphoplasmocytic compartment in hyperdiploid multiple myeloma are diploid cells

Abstract

In vivo S-phase cell labeling with iododeoxyuridine (IdUrd) was performed in six multiple myeloma (MM) patients. Myeloma cells from four patients were hyperploid. In three out of four patients, DNNIdUrd flow cytometry revealed that most of the labeled cells, which had divided during the period, elapsed between flash labeling and sampling, had returned to the diploid GO/Gl compartment and not to the hyperdiploid peak. To eliminate contaminating cells belonging to the normal hematopoiesis, plasmocytic and lymphocytic cells were fractionated and analyzed separately. Cell enrichment was performed with use of murine monoclonal antibodies (MoAbs) against plasmocytic and lymphocytic cell markers and subsequent magnetic activated cell sorting with immunobeads, i.e., polysterene magnetic particles coated with sheep anti-mouse IgG. The purified plasmocytic cells were mainly hyperdiploid and largely unlabeled. The lymphocytic cells appeared to belong chiefly to the diploid cell population. The IdUrd-labeled cells were predominantly lymphocytic cells, returning after mitosis to the diploid GO/G1 peak. Although this pattern of S-phase cells in hyperdiploid MM, belonging to the diploid cell compartment, was observed in three out of four hyperploid cases and although the number of observations is small, S-phase cells may demonstrate an aspect of tumor cell kinetics in hyperploid MM, which has been debated for many years and which indicates the existence of a non-plasmocytic stem cell compartment that feeds the plasmocytoma. The behavior of the labeled cells as observed in a few cases of MM provides another, hitherto undescribed, argument that, at least in some MM patients, a part of the proliferating tumor cells may be diploid lymphocytic (precursor) cells. These findings should be considered when targeting and monitoring treatment of MM and also in purging procedures of bone marrow in patients to be treated by ablative cytotoxic therapy and autologous bone marrow transplantation. 0 1995 Wiley-Liss, Inc.Key terms: Multiple myeloma, precursor cells, iododeoxyuridine (IdUrd), in vivo IdUrd labeling, immunomagnetic cell separation, DNA/IdUrd flow cytometryIn the majority of patients with multiple myeloma (MM), the tumor cells, or at least a large part of them, are aneuploid (1 1,34,35,53). Evaluation of cellular DNA content by flow cytometry (FCM) has revealed the existence of an aneuploid plasma cell population in 65-75% of the MM bone marrow samples with a median DNA index of 15% above normal (2,3,9,11,34,35,53). Most aneuploid tumor cells show a cell marker that is consistent with differentiated B cells, but subpopulations of tumor cells have been observed that express pre-B, early-B, or T lymphocytic, and even myelomonocytic or erythroid, features (16,17,22,23). Differentiation and lineage infedilities of plasmocytic cells in MM suggest that the malignant transformation involves a more pluripotent lymphoplasmacytoid precursor cell with a flexible pattern of gene expression.In 1974, Pileri and Tarocco (45) demonstrated the non...