Abstract A12: Osteocyte apoptosis induces an immunosuppressive microenvironment in bone marrow and promotes myeloma chemoresistance
Authors: Hao Xiaoxiao , Zhang Chao , Trotter Timothy N , Gowda Pramod S , Yang Yang 
Published: December 2022

Abstract
Multiple myeloma (MM) is a plasma-cell malignancy that grows in bone marrow (BM). MM resistance to chemotherapy is a major cause of patient mortality. Osteocytes are the most abundant cell type in bone. We have recently demonstrated that MM cells induce osteocyte apoptosis. While the mechanisms responsible for MM-induced osteocyte apoptosis have been described, it is unknown whether apoptotic osteocytes provide feedback that regulates MM cells. To study this, we developed a unique syngenic animal model of murine MM in which osteocyte apoptosis can be induced through targeted expression of human diphtheria toxin receptor (DTR) in osteocytes and subsequent injection of DT. In our current study, we used this DTR mouse model to access the impact of osteocyte apoptosis on MM cell response to the treatments of bortezomib (BTZ) and dexamethasone (DEX), two frontline anti-MM drugs. Eight-week-old DTR mice were i.p. injected with either PBS as control or DT (12.5µg/kg) to induce apoptosis in osteocytes. One week later, DTR+PBS and DTR+DT mice were injected with 106 5TGM1-luc MM cells via tail vein (n=21/group) and after 4 days, randomly assigned to treatment for 4 weeks with PBS or DEX (i.p. 1mg/kg/day) or BTZ (i.p. 0.5mg/kg/3 days) (n=7/group). Bioluminescence imaging and serum IgG2bκ (a soluble marker of 5TGM1-luc MM cells) ELISA, performed after 4-week treatment, demonstrated that osteocyte apoptosis promotes tumor growth in BM; DEX or BTZ treatment significantly inhibited tumor growth in DTR+PBS mice but not in DTR+DT mice. Flow cytometry analysis of 5TGM1-Luc MM cells in the BM further revealed that DEX and BTZ failed to induce MM cell apoptosis/death in DTR+DT mice, in contrast to the effectiveness observed in DTR+PBS mice. Analyses of BM cells harvested from these mice showed that compared with tumor-bearing DTR+PBS+PBS mice, tumor-bearing DTR+DT+PBS mice had significantly higher concentration of immunosuppressive myeloid-derived suppressor cells (MDSCs) and fewer effective CD8+ T cells in the BM. These CD8+ T cells also have a lower expression of the cytotoxic molecules granzyme B and IFN-γ. Importantly, DEX or BTZ treatment improved anti-MM immunity in the BM of tumor-bearing DTR+PBS mice but failed in tumor-bearing OB-Runx2-/- mice. Gemcitabine (GEM) is a FDA-approved anti-cancer agent and MDSC inhibitor used in the treatment of solid tumors but not commonly used in MM therapy. To confirm that MDSCs indeed promote osteocyte-apoptosis-induced MM resistance to DEX/BTZ and to determine if depletion of MDSCs can overcome this resistance, we treated 5TGM1-Luc tumor-bearing DTR+DT mice with PBS, GEM (i.p. 30 mg/kg/week), GEM+DEX or GEM+BTZ for 4 weeks. Bioluminescence imaging and serum IgG2bκ ELISA demonstrated that GEM+DEX or GEM+BTZ reversed MM resistance to DEX and BTZ and resulted in the greatest reduction in tumor burden among all treatments. In conclusion, our findings demonstrate that osteocyte apoptosis promotes MM cell resistance to DEX and BTZ and BM immunosuppression induced by osteocyte apoptosis plays a role in this event.
Citation Format: Xiaoxiao Hao, Chao Zhang, Timothy N Trotter, Pramod S Gowda, Yang Yang. Osteocyte apoptosis induces an immunosuppressive microenvironment in bone marrow and promotes myeloma chemoresistance [abstract]. In: Proceedings of the AACR Special Conference: Tumor Immunology and Immunotherapy; 2022 Oct 21-24; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2022;10(12 Suppl):Abstract nr A12.