[Mechanisms of multiple myeloma cell growth]

The mechanism of human multiple myeloma cell growth was studied utilizing eleven myeloma cell lines established in vitro or in vivo (Scid mouse). Four bone marrow derived cell lines grew dependently on IL-6 or bone marrow stromal cells. Seven extramedullary lesion derived cell lines grew spontaneously and additively proliferated in response to IL-6. All cell lines expressed the IL-6 receptor (IL-6R) and IL-6RmRNA, but none expressed IL-6mRNA. No IL-6 activity was detected in the myeloma cell culture supernatant. Both the anti-IL-6 antibody and anti-IL-6R antibody neutralized IL-6-induced proliferation, but did not inhibit spontaneous proliferation of extramedullary lesion derived cell lines. While establishing cell lines, it was found that the proliferating fraction was primarily included in a fraction which was non-adherent to stromal cells and composed of undifferentiated plasmablasts. Undifferentiated plasmablasts proliferated in response to IL-6, in contrast to the adherent, mature form of myeloma cells which did not proliferate in response to IL-6. Innoculation of myeloma cells into Scid mice induced subcutaneous tumor formation. These tumors were composed of undifferentiated plasmablasts, which also proliferated in response to IL-6. These results imply that the growth of bone marrow derived myeloma cell lines are dependent on the IL-6 paracrine mechanism and that the growth of extramedullary lesion derived cell lines primarily autonomous and additively dependent on the IL-6 paracrine mechanism.