Characterization of the monoclonal blood and bone marrow B lymphocytes in Waldenström's macroglobulinaemia.

Blood lymphocytes were studied in sixteen patients with Waldenström's macroglobulinaemia. In four of them bone marrow lymphocytes were studied simultaneously. In all four a monoclonal lymphocyte plasma cell fraction carrying the light chain isotype of the serum M-component was identified by direct immunofluorescence. The cell-bound immunoglobulins had idiotypic structures in common with the serum M-component. The monoclonal cell component was pleomorphic, varying from small lymphocytes carrying surface IgM/IgD or IgM only to plasmacytoid cells with both surface and cytoplasmic IgM to mature plasma cells with only cytoplasmic IgM. A differentiating B-cell clone was further suggested by the sequential loss of complement and Fc receptors during maturation to plasma cells. In nine patients monoclonal lymphocytes were increased also in blood. However, the monoclonal fraction was small in all except two patients with advanced disease. The results support the assumption that a clone of B lymphocytes differentiates into immunoglobulin-secreting cells in Waldenström's macroglobulinaemia. The pattern of cell differentiation in bone marrow was the same in patients with and without a monoclonal blood cell fraction. Progression of the disease is probably the main factor determining the appearance of large numbers of tumour cells in blood.