Bone marrow kinetic studies on three patients with myelomatosis. Indications for malignant proliferation within both the plasma cell and lymphoid cell compartments.

The proliferative activity has been studied in lymphocyte and plasma cell populations of purified bone marrow from three myeloma patients. In one of them, proliferation was also recorded in blood lymphocytes. Immunofluorescence studies, using idiotypic antisera against the M-component, were performed to identify lymphocytes and plasma cells belonging to the malignant cell clone. Pulse incubations with 3H-thymidine (3H-TdR) were made in vitro. Autoradiographic analyses revealed a higher overall 3H-TdR labelling index in the myeloma cell populations than in normal control cell populations. A few labelled plasma cells were observed in the myeloma cases (2.5-5%) but the major fraction (11.5-14%) and intensity of labelled cells were found among the lymphoid cells. This indicates that the proliferation of malignant cells in myeloma occurs not only within the plasma cell but also in the lymphoid cell population. The results provide additional support for the assumption that B-lymphocytes are part of the malignant cell clone in myelomatosis and most likely precursors to the myeloma plasma cells.