Abstract 164: Bone marrow myeloid cells promote multiple myeloma chemoresistance by DNA mediated signaling

Bone marrow (BM) niche provides a sanctuary for multiple myeloma (MM) cells allowing them to escape chemotherapy. Previous in vitro studies have implicated some accessory cells present in BM microenvironment including stromal cells, macrophages, osteoclasts and plasmacytoid dendritic cells in MM chemoresistance. However, these cells comprise only a small fraction of BM niche. The role of the major cellular component of BM represented by immature myeloid cells, neutrophils, and monocytes in MM survival from chemotherapy remains unknown. To determine whether myeloid cells are involved in regulation of MM growth and chemosensitivity, we initially employed in vivo mouse models that allow for the modulation of the number of myeloid cells in BM. Using the CD11b-DTR model, we demonstrated that depletion of myeloid CD11b-expressing cells from BM led to a significant reduction of MM tumor growth in these mice and improved the antitumor effect of doxorubicin. Conversely, GM-CSF-induced expansion of myeloid cells in the BM of MM-bearing mice resulted in reduced survival and increased resistance to chemotherapy. CD11b+Gr1+ myeloid cells isolated from BM of MM-bearing or control tumor-free mice demonstrated chemoprotective effects on mouse MM cells. Human CD33+CD14-CD11b+ myeloid cells isolated from BM of patients with MM or healthy donors were able to protect human MM cells from doxorubicin and melphalan-induced apoptosis in vitro. The addition of supernatant collected from myeloid cells to MM cells was sufficient to induce chemoprotection in mice and human cells. The protection was completely lost when myeloid cells were separated from MM cells by transwell insert. Our further studies demonstrated that myeloid cells, but not other BM accessory cells, were able to release substantial amount of DNA. Importantly, treatment of these cells with DNAse I abrogated the chemoprotective effect of myeloid cells on MM. The addition of genomic DNA isolated from myeloid cells promoted chemoresistance of MM cells. Further, Stimulation of TLR9 with its ligand CpG oligonucleotides mimicked the chemoprotective effect of DNA on MM cells. Cell-free DNA up-regulated spliced XBP-1, a transcription factor involved in unfolded protein response. Pharmacological inhibition of IRE-1α, a kinase required for XBP-1 splicing, reversed myeloid cell induced MM chemoprotection. Finally, in vivo depletion of myeloid cells in BM of MM-bearing mice resulted in decreased expression of spliced XBP-1 in MM cells and improved response to chemotherapy. Taken together, our data demonstrated a critical role played by myeloid cells in growth and chemosensitivity of MM cells and identified a novel mechanism that mediates this effect. Our results suggest that targeting myeloid cells or TLR9 in MM may reduce tumor growth and improve response to the chemotherapy in this disease. Citation Format: Indu Ramachandran, Thomas Condamine, Dmitry Gabrilovich, Yulia Nefedova. Bone marrow myeloid cells promote multiple myeloma chemoresistance by DNA mediated signaling. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 164. doi:10.1158/1538-7445.AM2014-164