4-1BBL Reverse Signaling Enhances Proliferation of Human Multiple Myeloma Cell Line U266

Abstract Abstract 5074 Multiple myeloma is an incurable plasma cell malignancy. Unraveling the intricate pathobiology and exploitation of novel therapeutic approaches are needed. 4-1BBL, a member of the tumor necrosis factor family, cross-links its receptor, 4-1BB, and co-stimulates T cell activities. Recent studies confirm that 4-1BB-4-1BBL interaction generates bi-directional signals. 4-1BBL has been detected on many tumor cells, and many tumor microenvironmental cells express 4-1BB molecule. However, whether the reverse signaling through 4-1BBL participates the pathology of multiple myeloma is unknown. In this study, we identify the constitutive expression of 4-1BBL on U266, LP1, MY5 and OPM2 MM cell lines as 89.2%, 57.9%, 77.9% and 78.7%. Then we select U266 cell to explore the biological effect of 4-1BBL reverse signaling and its underlying mechanism. Triggered by anti-4-1BBL mAb (1F1), cell counting and MTT assay both demonstrated that U266 cell proliferation increased dramatically. Intracellular cytokine staining showed that treatment of cells with 1F1 also increased the production of IL-6. When neutralizing anti-IL-6 mAb was added to the culture medium, 1F1-induced U266 cell proliferation was effectively inhibited. These effects of 4-1BBL require immobilization of 1F1 but not soluble form. Taken together, 4-1BBL reverse signaling promotes U266 cell proliferation remarkably while increasing the production of IL-6. Moreover, neutralizing anti-IL-6 mAb inhibits U266 cell proliferation stimulated by mAb 1F1, supporting that 4-1BBL reverse signaling promotes the proliferation of U266 cell through increasing the production of IL-6. Disclosures: No relevant conflicts of interest to declare.