Autocrine interleukin-6 production and highly malignant multiple myeloma: relation with resistance to drug-induced apoptosis

In this study, flow cytometry was used to evaluate interleukin-6 (IL-6) production by bone marrow mononuclear cells from 47 patients with multiple myeloma (MM) in different clinical stages and 15 patients with monoclonal gammopathy of undetermined significance. In patients with MM, autocrine IL-6 production paralleled the clinical disease stage. The largest proportion of syndecan-1(+)/IL-6(+) cells was detected in patients with resistant relapse or primary refractory disease, suggesting that tumor progression involves expansion of myeloma cells producing IL-6. The authors assessed autocrine IL-6 production and in vitro proliferation and apoptosis of myeloma cells in 6 myeloma cell clones (MCCs) and in 2 myeloma cell lines, namely IM-9 and U-266-1970, which showed different sensitivities to the addition of exogenous IL-6. Autocrine IL-6 production was observed in IL-6-independent MCC-2, MCC-3, and MCC-5 cloned from patients with aggressive disease and in the IM-9 cell line. In contrast, IL-6-dependent MCC-1, MCC-4, and MCC-6 were syndecan-1(+) and IL-6(-). Blocking experiments with anti-IL-6 monoclonal antibody from clone AH65, which binds IL-6-IL-6Ralpha complexes, prevented cell proliferation of IL-6(+) MCCs. Flow cytometry evaluations after propidium iodide staining revealed different susceptibilities of MCCs to cell death. IL-6-producing MCCs showed minimal spontaneous and dexamethasone-induced apoptosis, whereas a regular amplitude of apoptosis occurred in the IL-6(-) MCCs. These data provide evidence that autocrine IL-6 reflects a highly malignant phenotype of myeloma cells. In fact, autocrine IL-6 production and deregulated apoptosis may induce expansion of selective IL-6(+) myeloma cells resistant to spontaneous and drug-induced cell death.