Mature Osteoblasts Are Critical Bone Marrow Niche Cells Regulating Multiple Myeloma Cell Growth, at Least in Part, By Affecting the Local Immune Microenvironment

Despite high response rates to new therapies, most multiple myeloma (MM) patients relapse and become refractory to treatment. This is likely because MM cells enter dormancy and reactivate only under permissive conditions. Cellular dormancy is ubiquitous in normal tissues as well as in malignant, non-malignant, and metastasizing tumors. Dormant tumor cells (DTC) are protected from therapies which can reactivate later at the time of disease progression, resulting in relapsed refractory MM (RRMM). The tumor microenvironment (TME) is a key determinant of tumor dormancy and reactivation. We have previously shown that inhibition of Activin A in SCID mice transplanted with human MM.1S increased osteoblast numbers and resulted in inhibition of MM growth (Vallet et al., PNAS). We hypothesize that MM-DTCs reside in osteoblastic niche in the BM TME. Our long-term goal is to identify factors regulating DTCs in MM and develop strategies for long term disease control.

We have generated mice in which mature osteoblasts can be postnatally deleted in an inducible and reversible manner. To achieve this, mice carrying floxed diphtheria toxin receptor (DTR) alleles were mated with mice expressing Cre-recombinase driven by the osteocalcin promoter (OC-Cre). This led to the expression of DTR in mature osteoblasts (OC-Cre/iDTR) only. The control mice were littermates lacking the OC-Cre allele (iDTR). Until the treatment with diphtheria toxin (DT), the OC-Cre/iDTR mice were immunocompetent compared to iDTR mice, as they did not show any differences in immune cell populations by flow cytometry, and did not show any skeletal phenotype, as measured by micro-computed CT (micro-CT).

We chose to induce mature osteoblast-deficiency at 8-weeks of age to allow skeletal maturation which is assumed to be completed by 6-7weeks of age. To induce postnatal deletion of mature osteoblasts, the OC-Cre/iDTR and control mice both were treated with 50 µg/kg DT once a week, beginning at 8-weeks of age. Micro-CT analysis showed a significant increase in cortical porosity within 1-week after DT injection. 8-weeks of DT treatment significantly reduced trabecular bone fraction (BV/TV), trabecular numbers (Tb.N), and bone mineral density (BMD) with a significant increase in trabecular spacing (Tb.Sp). Immunohistochemistry for osteocalcin showed rapid loss of mature and endosteal osteoblasts (N.Ob/T.Ar). This was accompanied with a marked decreased in serum sclerostin and serum osteocalcin levels suggesting reduced osteocytes and mature osteoblasts, respectively. Importantly, serum CTX levels or numbers of osteoclasts (N.Oc/T.Ar) were unchanged. To study MM engraftment and progression, 3x10 ⁶ 5TGM1 luciferase tdTomato positive (5TGM1-Luc-Tom) MM cells were injected into the tibia of OC-Cre/iDTR and control iDTR mice followed by weekly injection of DT for 8-weeks. Flow cytometry analysis showed a 4-fold increase in the 5TGM1-Tom cells in the BM from OC-Cre/iDTR mice compared to controls. Bioluminescence imaging (BLI) for 5TGM1-Luc-Tom cells at 4-weeks showed dramatic increase in the OC-Cre/iDTR mice compared to controls. Interestingly, by 8-weeks, the BLI imaging showed 5TGM1-Luc-Tom cells in other long-bones of OC-Cre/iDTR mice but not the controls. To identify local BM TME changes, we assessed the levels of 200 cytokines and growth factors in the BM from these mice. The levels of immune regulatory cytokines, such as IL-17F and IL-21, were found to be decreased in the OC-Cre/iDTR mice compared to control iDTR mice, both after 4-weeks from intratibial 5TGM1 cell injection. This was accompanied by significant decrease in the fraction of CD4+ T-helper and Th17+ cells in the OC-Cre/iDTR mice compared to controls, as assessed by flow cytometry.

These data show that MM cells engraft and proliferate rapidly within the BM in the absence of mature osteoblasts and have a propensity to migrate to other long bones with time. This may be partially mediated by the altered immune cell profile in the BM in the absence of mature osteoblasts. These data further suggest that expanding the mature osteoblast niche may provide novel therapeutic avenues and reduce disease burden creating an environment for long term tumor control. Importantly, this model will allow us to understand the osteoblastic niche for MM and the mechanisms of activation and dormancy, at least in part, via a role of the immune cells.