Therapeutic interventions to harness the immune system against tumor cells have provided mixed results in the past for several solid tumors and hematologic malignancies. However, immunotherapy has advanced considerably over the last decade and is becoming an integral combination for treating patients with advanced solid tumors. In particular, prostate cancer immunotherapy has shown modest efficacy for patients in the past. With several key discoveries on immune mechanisms and advanced molecular diagnostic platforms recently, immunotherapy is re-emerging as a viable option for prostate cancer, especially castration-resistant prostate cancer (CRPC), to stimulate antitumor immunity. Combination of patient-tailored immunotherapy and immune checkpoint blockers with conventional cytotoxic agents and androgen receptor–targeted therapies should move the field forward. With a recent adaptation that the application of immune checkpoint inhibitors has been successful in the treatment of more than a dozen solid tumors, including melanoma, lymphoma, liver, cervical, gastrointestinal, and breast cancers, it is a timely endeavor to harness immunotherapy for prostate cancer. Here, we provide an account on the progression of immunotherapy with new discoveries and precision approaches for tumors, in particular CRPC, from mechanistic standpoint to emerging limitations and future directions.

Elevated levels of plasmacytoid dendritic cells (pDC) have been reported in breast cancer patients, but the significance remains undefined. Using three immunocompetent mouse models of breast cancer 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 in pDC within the bone, which resulted in a sustained Th2 response along with elevated levels of regulatory T cells and myeloid-derived suppressor cells. Subsequently, pDC and CD4⁺ T cells, producing osteolytic cytokines, increased with tumor burden, causing severe bone damage. Microcomputed tomography and histology analyses of bone showed destruction of femur and tibia. The 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.

Cell-based therapy for cancer is a promising new field. Among cell types that can be used for this purpose, mesenchymal stem cells (MSCs) appear to hold great advantage for reasons including easier propagation in culture, possible genetic modification to express therapeutic proteins and preferential homing to sites of cancer growth upon in vivo transfer. The present study evaluated the potential of genetically modified MSC, constitutively expressing interferon (IFN)-β, in an immunocompetent mouse model of prostate cancer lung metastasis. A recombinant adeno-associated virus (rAAV) encoding mouse IFN-β was constructed and initially tested in vitro for high-level expression and bioactivity of the transgenic protein. MSCs were transduced by the rAAV-IFN-β or green fluorescent protein ex vivo and used as cellular vehicles to target lung metastasis of TRAMP-C2 prostate cancer cells in a therapy model. Cohorts of mice were killed on days 30 and 75 to determine the effect of therapy by measurement of tumor volume, histology, immunohistochemistry, enzyme-linked immunosorbent assay and flow cytometry. Results indicated a significant reduction in tumor volume in lungs following IFN-β-expressing MSC therapy. Immunohistochemistry of the lung demonstrated increased tumor cell apoptosis and decreased tumor cell proliferation and blood vessel counts. A significant increase in the natural kill cell activity was observed following IFN-β therapy correlating the antitumor effect. Systemic level of IFN-β was not significantly elevated from this targeted cell therapy. These data demonstrate the potential of MSC-based IFN-β therapy for prostate cancer lung metastasis.

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 show the importance of myeloid-derived suppressor cells (MDSC) in this process at bone metastatic sites. Because 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. Cancer Res; 73(2); 672–82. ©2012 AACR.

Adult stem cells represent a potential source for cell-based therapy of cancer. The present study evaluated the potential of bone marrow-derived mesenchymal stem cells (MSC), genetically modified to express interferon (IFN)-α, for the treatment of lung metastasis in an immunocompetent mouse model of metastatic melanoma. A recombinant adeno-associated virus (rAAV) 6 vector encoding IFN-α was used to transduce mouse bone marrow-derived MSC ex vivo. Expression and bioactivity of the transgenic protein from rAAV-transduced MSC were confirmed prior to in vivo studies. A lung metastasis model of melanoma was developed by i.v. injection of B16F10 cells into 8-week-old C57BL/6 mice. Ten days later, MSC transduced with rAAV-IFN-α or green fluorescent protein were intravenously injected. One cohort of mice was sacrificed to determine the effects of the therapy at an earlier time point, and another cohort was observed for long-term survival. Results indicated that systemic administration of MSC producing IFN-α reduced the growth of B16F10 melanoma cells and significantly prolonged survival. Immunohistochemistry analysis of the tumors from MSC-IFN-α-treated animals indicated an increase in apoptosis and a decrease in proliferation and blood vasculature. These data demonstrate the potential of adult MSC constitutively producing IFN-α to reduce the growth of lung metastasis in melanoma. Disclosure of potential conflicts of interest is found at the end of this article.

The potential of mesenchymal stem cells (MSC) in tissue regeneration is increasingly gaining attention. There is now accumulating evidence that MSC make an important contribution to postnatal vasculogenesis. During bone development and fracture healing, vascularization is observed before bone formation. The present study determined the potential of MSC, transduced ex vivo with a recombinant adeno-associated virus 6 (rAAV6) encoding bone morphogenetic protein 2 (BMP2) and vascular endothelial growth factor (VEGF) in a mouse model of segmental bone defect created in the tibiae of athymic nude mice. Mouse MSC that were mock-transduced or transduced with rAAV6-BMP2:VEGF were systemically transplanted following radiographic confirmation of the osteotomy. Effects of the therapy were determined by enzyme-linked immunosorbent assay measurements for BMP2 and VEGF, dual-energy X-ray absorptiometry (DXA) for bone density, three-dimensional microcomputed tomography (µCT) for bone and capillary architecture, and histomorphometry for bone remodeling. Results of these analyses indicated enhanced bone formation in the group that received BMP2+VEGF-expressing MSC compared to other groups. The therapeutic effects were accompanied by increased vascularity and osteoblastogenesis, indicating its potential for effective use while treating difficult nonunion bone defects in humans.