A Toll-like receptor 7/8 ligand secreted by a myeloma cell line induces primary bone marrow stromal cells to produce survival promoting cytokines
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Multiple myeloma (MM) is a cancer of plasma cells, which accumulate in the bone marrow (BM). The BM milieu provides MM cells with survival and proliferation signals. Toll-like receptors (TLRs) are immune receptors that mediate inflammatory cytokines upon recognition of molecules associated with infection, danger or stress. Tumor cells have been reported to secrete TLR-activating components, activating immune cells to release inflammatory cytokines, which in turn drives tumorigenesis. This study sought to investigate if myeloma cell lines release TLR-activating components that stimulate TLR-expressing cells in the BM to produce inflammatory cytokines that promote MM survival. Conditioned medium (CM) from MM cells was assayed for TLR activating ability using a TLR driven NF-κB reporter system. CM from the MM cell line U266 was found to activate TLR7 and TLR8. TLR7 and TLR8 are known to recognize RNA, however, RNase treatment of the U266 CM did not abolish the TLR7/8 activation. Extracellular vesicles (ECVs) were isolated from U266 to determine if the RNA ligand may be protected from RNase in ECV. U266-derived ECVs were found to mediate some TLR7 activation. Bone marrow stromal cells (BMSCs) were further found to up-regulate CCL3, CXCL1 and CXCL5 in response to TLR8 ligands, but not TLR7, indicating presence of TLR8 in these cells, which was confirmed at the mRNA level. Macrophages had a similar cytokine profile as BMSCs when stimulated with TLR ligands. However, only 3% of the BMSCs stained positive for the macrophage marker CD14. The results indicate that CD14- cells, in addition to CD14+ cells, in BMSCs respond to TLR8 stimulation. Furthermore, BMSCs treated with U266 CM resulted in induction of CXCL1 and CXCL5. Collectively, U266 secreted a TLR7/8-activating component able to induce cytokine response in primary BMSCs. Taken together, this study provides further insight into the interplay between MM and BMSCs and a potential role for TLR8 to promote MM.