Biological significance of heterogeneity in human myeloma cells

B cells differentiate into plasma cells which produce antibodies in the bone marrow (BM). Multiple myeloma (MM) is a hematologic malignancy in human plasma cells, and myeloma cells grow mainly in BM. According to phenotypic differences, such as expression of adhesion molecules, human myeloma cells as well as normal plasma cells can be classified into several differentiation stages. We have found that cells strongly expressing CD38 antigens (CD38++(+)) in BM are all plasma cells, and that there also are no plasma cells in either CD38- cell fraction or fraction of cells weakly expressing CD38 antigens (CD38+). Myeloma cells in BM consist of CD38++(+)MPC-1-CD49e (VLA-5)-immature and CD38++(+)MPC-1+CD49e+ mature myeloma cells. Immature myeloma cells proliferate markedly in vitro and respond to interleukin-6 (IL-6), a growth factor for myeloma cells, whereas mature myeloma cells show very low proliferative activities and show no response to IL-6. Immature myeloma cells expressing CD21 molecules on their surface seem to attach to stromal cells in BM through binding to CD23 molecules. Thus, there is a heterogeneity in human myeloma cells, and immature myeloma cells appear to proliferate in response to IL-6.