ANTI-CD40 ANTIBODY-BINDING MODULATES HUMAN MULTIPLE-MYELOMA CLONOGENICITY IN-VITRO

Ligand binding of the B-cell lineage antigen CD40 enhances growth and interleukin-d (IL-6) secretion in human B cells (the CD40/IL-6 loop). IL-6 has an autocrine and paracrine role in human multiple myeloma (MM ) cell growth. With the use of the CD40 monoclonal antibody (MoAb) G28 -5, we examined CD40 expression and the effect of CD40 binding on MM c lonogenic colony (MCC) formation to characterize the IL-6/CD40 loop ac tivity in NIM. CD40 was expressed on plasmacytoid cells in 21 of 28 pl asma cell dyscrasia (PCD) bone marrow (BM) biopsies tested (10 of 14 M M, 2 of 2 Waldenstrom's macroglobulinemia [WM], 2 of 2 plasma cell leu kemia [PCL], 6 of 8 monoclonal gammopathy of undetermined significance [MGUS], and 1 of 2 primary amyloidosis [AL]). G28-5 binding increased MCCs by 35% to 150% in 11 of 17 CD40(+) PCD BM cultures, but did not affect MCC formation in CD40(-) specimens or normal BM colony forming units (CFU-GEMM, CFU-GM, BFU-E). Responsive cultures originated from B M of patients with MM (2 of 5 cases tested), WM (2 of 2), PCL (2 of 2) , and MGUS (5 of 6). CD4O-responsiveness was not significantly inhibit ed by the presence of an anti-IL-6 MoAb (2 of 2 MGUS cultures tested), and did not correlate with the capacity to respond to IL-6 stimulatio n (n = 17, P > .05) or a detectable level of endogenous IL-6 (n = 15, P > .05). Additional studies were performed with PCD cell lines to cha racterize the interrelationship of CD40 activation and IL-6 production . Fifty percent to greater than 95% of cells from the RPMI 8226 and AR H77 lines expressed CD40, whereas 6% of U266 cells were CD40(+). For R PMI 8226, ARH-77, and U266 cells, the increased MCC formation after an ti-CD40 stimulation was not affected by the presence of an anti-IL-6 n eutralizing MoAb and was not accompanied by detectable IL-6 secretion. There was no apparent increase in IL-6 mRNA transcription following G 28-5 treatment of U266 or RPMI 8226 cells. Our observations indicate t hat CD40 is expressed in a subset of human myeloma cells present in va rious PCDs. Cell-line studies suggest that the CD40(+) myeloma cell ma y regulate MM clonogenic colony formation without activating the IL-6 pathway. (C) 1994 by The American Society of Hematology.