CD40 EXPRESSION IN MALIGNANT PLASMA-CELLS - ROLE IN STIMULATION OF AUTOCRINE IL-6 SECRETION BY A HUMAN MYELOMA CELL-LINE

Myeloma is a neoplasia characterized by the accumulation of malignant plasma cells in the bone marrow. In these studies, we have demonstrate d that CD40 is expressed in human myeloma cells and have used a recent ly established IL-6-dependent myeloma cell line, ANBL-6, to examine th e potential function of CD40 expression in myeloma cells. In addition to its expression on the ANBL-6 cells, we show that CD40 is expressed on.freshly isolated myeloma cells from seven of seven patients tested. To address the role of CD40 expression in myeloma cells, we have exam ined the responsiveness of the ANBL-6 cell line to a CD40-specific mAb , G28-5. This cell line has previously been shown to proliferate only in response to IL-6. Of interest in this study, G28-5 also induced pro liferation of the ANBL-6 cells. This proliferation was substantially i nhibited by an IL-6-neutralizing mAb. Analysis of ANBL-6 cell culture supernatants by ELISA demonstrated that G28-5-stimulated cells secrete d significant levels of IL-6, whereas unstimulated cell culture supern atants contained undetectable levels of IL-6. Furthermore, CV-1/EBNA c ells expressing the human CD40 ligand also induced the proliferation o f the ANBL-6 cell line, an effect that was inhibited by the anti-IL-6 mAb. Lastly, RNA blot analysis demonstrated an increase in IL-6 messag e in G28-5-stimulated ANBL-6 cells over unstimulated cells. These resu lts indicate that the primary mechanism of anti-CD40-stimulated prolif eration of the ANBL-6 cells is the induction of autocrine IL-6 product ion. Moreover, these data suggest that the expression of CD40 in malig nant plasma cells may play a role in tumor cell expansion, possibly by stimulating the induction of autocrine IL-6 secretion.