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    Myeloma Genomics and Microenvironment and immune profiling

    Category: Myeloma Genomics and Microenvironment and immune profiling

    Immunosuppressive bone marrow microenvironment promotes multiple myeloma proliferation in adult mice with induced osteoblast and osteocyte deficiency

    (PA-253) Immunosuppressive Bone Marrow Microenvironment Promotes Multiple Myeloma Proliferation in Adult Mice with Induced Osteoblast and Osteocyte Deficiency

    Wednesday, September 17, 2025
    Introduction: Most multiple myeloma (MM) patients relapse and become refractory to treatment. This may be because MM cells are protected in the tumor microenvironment (TME) and reactivate only under permissive conditions. Osteoblasts are versatile regulators of hematopoietic cells. We previously showed that inhibition of Activin A in SCID mice transplanted with human MM.1S increased osteoblast numbers and inhibited MM growth (Vallet et al., PNAS). Here, we hypothesize that losing mature osteoblasts promotes MM progression.

    Methods: We generated immunocompetent mice with inducible deletion of mature osteoblasts by mating mice carrying floxed diphtheria toxin receptor (DTR) alleles 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. Littermates lacking the OC-Cre allele (iDTR) were used as controls. Until diphtheria toxin (DT) treatment, the OC-Cre/iDTR mice were immunocompetent and indistinguishable from iDTR control mice as shown by flow cytometry and micro-computed CT (micro-CT). Mice reach skeletal maturity by 8 weeks of age. Beginning at 8 weeks of age, the OC-Cre/iDTR and iDTR control mice were treated with 25 µg/kg DT once a week to induce mature osteoblast deficiency. At 9 weeks of age, 1x106 5TGM1 luciferase tdTomato positive (5TGM1-Luc-Tom) MM cells were injected into the tibia. MM expansion, immune profiling, and molecular mechanisms were assessed by bioluminescence imaging (BLI), flow cytometry, cytokine array, ELISA, micro-CT, and immunohistochemistry up to 8 weeks post MM cell inoculation.

    Results: DT injections induced more than 80% loss of mature osteoblasts and osteocytes beginning after 1 week as assessed by micro-CT, histomorphometry, immunohistochemistry for osteocalcin, and ELISA for serum osteocalcin and sclerostin. Notably, the number of osteoclasts per trabecular area and serum CTX levels were unchanged, indicating normal osteoclast numbers and activity. One week after the 5TGM1-Luc-Tom MM intratibial inoculation, both OC-Cre/iDTR and iDTR control mice showed similar BLI flux; however, it was rapidly lost in the iDTR control mice and multiplied 30-50-fold in OC-Cre/iDTR mice, indicating proliferation of the MM cells. Assessing 200 cytokines and growth factors in the BM of these mice showed altered levels of immune regulatory cytokines such as IL-4, IL-13, and CCL2. Flow cytometric analysis of the bone marrow (BM) constituents from the MM-cell-injected tibia revealed immunosuppressive BM TME characterized by increased Tregs and MDSCs in OC-Cre/iDTR mice compared to the iDTR controls.

    Conclusions:

    These data show that MM cells expand rapidly within BM in the absence of mature osteoblasts and osteocytes in adult mice, at least in part, due to the immunosuppressive BM TME. Expanding the mature osteoblast niche may provide novel therapeutic avenues and reduce disease burden, creating an environment for long-term tumor control.