Enhanced bone destruction is a hallmark of various carcinomas such as breast cancer, where osteolytic bone metastasis is associated with increased morbidity and mortality. Immune cells contribute to osteolysis in cancer growth but the factors contributing to aggressive bone destruction are not well understood. In this study, we demonstrate the importance of myeloid-derived suppressor cells (MDSC) in this process at bone metastatic sites. Since MDSC originate from the same myeloid lineage as macrophages, which are osteoclast precursors, we hypothesized that MDSC may undergo osteoclast differentiation and contribute to enhanced bone destruction and tumor growth. Using an immunocompetent mouse model of breast cancer bone metastasis, we confirmed that MDSC isolated from the tumor-bone microenvironment differentiated into functional osteoclasts both in vitro and in vivo. Mechanistic investigations revealed that nitric oxide signaling was critical for differentiation of MDSC into osteoclasts. Remarkably, osteoclast differentiation did not occur in MDSC isolated from control or tumor-bearing mice that lacked bone metastasis, signifying the essential cross-talk between tumor cells and myeloid progenitors in the bone microenvironment as a requirement for osteoclast differentiation of MDSC. Overall, our results identify a wholly new facet to the multifunctionality of MDSC in driving tumor progression, in this case as a novel osteoclast progenitor that specifically drives bone metastasis during cancer progression.

Elevated levels of plasmacytoid dendritic cells (pDC) have been reported in breast cancer patients, the significance of which remains undefined. Using three different immunocompetent mouse models of BCa bone metastasis, we identified a key role for pDC in facilitating tumor growth through immunosuppression and aggressive osteolysis. Following infiltration of macrophages upon breast cancer dissemination, there was a steady increase of pDC within the bone, which resulted in a sustained Th2 response along with elevated levels of Treg and MDSC. Subsequently, pDC and CD4+ T cells, producing osteolytic cytokines, increased with tumor burden causing severe bone damage. Micro-CT and histology analyses of bone showed destruction of femur and tibia. Therapeutic significance of this finding was confirmed by depletion of pDC, which resulted in decreased tumor burden and bone loss by activating tumor-specific cytolytic CD8+ T cells and decreasing suppressor cell populations. Thus, pDC depletion may offer a novel adjuvant strategy to therapeutically influence breast cancer bone metastasis.

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Chemoresistance due to heterogeneity of the tumor microenvironment (TME) hampers the long-term efficacy of frontline therapies for lung cancer. Current combination therapies for lung cancer provide only modest improvement in survival, implicating necessity for novel approaches that suppress malignant growth and stimulate long-term anti-tumor immunity. Oxidative stress in the TME promotes immunosuppression by tumor infiltrating myeloid-derived suppressor cells (MDSC), which inhibit host protective anti-tumor immunity. Using a murine model of lung cancer, we demonstrate that a combination treatment with gemcitabine and a superoxide dismutase mimetic targets immunosuppressive MDSC in the TME and enhances the quantity and quality of both effector and memory CD8+ T cell responses. At the effector cell function level, the unique combination therapy targeting MDSC and redox signaling greatly enhanced cytolytic CD8+ T cell response and further decreased T regulatory cell infiltration. For long-term anti-tumor effects, this therapy altered the metabolism of memory cells with self-renewing phenotype and provided a preferential advantage for survival of memory subsets with long-term efficacy and persistence. Adoptive transfer of memory cells from this combination therapy prolonged survival of tumor-bearing recipients. Furthermore, the adoptively-transferred memory cells responded to tumor re-challenge exerting long-term persistence. This approach offers a new paradigm to inhibit immunosuppression by direct targeting of MDSC function, generate effector and persistent memory cells for tumor eradication, and prevent lung cancer relapse.

Immune cells and their secreted growth factors play major roles in tumor growth and metastasis. Interplay between the growing tumor and infiltrating immune cells decides the nature of immune response and ultimately, tumor fate. Increased infiltration of pro-tumorigenic immune cells promotes tumor growth as well as dissemination to distant sites. These cells induce immunosuppression that inhibits proliferation and functions of cells of anti-tumor immune response. One population of immunosuppressive cells that is increasingly gaining attention is myeloid-derived suppressor cells (MDSCs). MDSCs are immature myeloid progenitors that suppress T cell effector functions and promote angiogenesis. MDSC numbers are elevated at both the primary tumor and metastatic sites including bone. In addition to immunosuppressive functions of MDSCs, we and other have recently discovered a novel function for MDSCs as osteoclast progenitors. Osteolysis is a common complication in the carcinomas of breast, lung, prostate and multiple myeloma with poor prognosis. Therefore, targeting MDSCs functions may exert dual therapeutic effects on immunosuppression and bone pathology.