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INTRODUCTION: Melanoma patients that develop brain metastasis have a median survival of 6 months. Radiation, a standard of care for local control of melanoma, has been reported to have immune stimulatory capacities. Therefore, we investigated the combinatorial effect of inhibitors of the signal transducer and activator of transcript 3 (STAT3) pathway, a potent mediator of immune suppression, and whole-brain radiation in a murine brain metastasis model. METHODS: C57BL/6 mice were intracerebrally implanted with B16 melanoma, irradiated with 7.5 Gy, and treated with an advanced lipid nanoparticle system that contained miR-124 (LNP-124) that inhibits the STAT3 pathway, or WP1066, a small molecule inhibitor of STAT3. Mice surviving 130+ days after tumor implantation were subcutaneously rechallenged with tumor. RESULTS: Monotherapy (LNP-124, WP1066, or radiation alone) demonstrated longer survival relative to untreated control mice; combination therapies further extended survival compared to single treatment agents. Median survival for mice receiving radiation combined with LNP124 or WP1066 was 65 days and 45.5 days, respectively, compared to 15 days for untreated mice and 19 - 40 days for monotherapy. While no untreated mice (control) survived over 20 days, the survival rate at day 130 for mice receiving combination therapy was almost 40%. Flow cytometry analysis showed higher IL-2 production in T cells from tumor-draining lymph nodes in mice treated with radiation plus STAT3 inhibitor. SUMMARY: These results indicate the combination of STAT3 inhibition and radiation enhances the therapeutic effect against established murine brain metastasis. The implications for this combination on antitumor immunity, immunologic memory and protection from recurrence will also be discussed.

INTRODUCTION: Exosomes secreted by cancer cells have pleiotropic functions, and can promote autocrine signaling to distant cells. Elucidating the mechanistic modulation of the immune system by these exosomes provides insight into potential biomarkers for detection, recurrence, and response, and identifies potential new therapeutic targets. METHODS: Exosomes were isolated from human glioblastoma stem cells (GSC) and fibroblasts (control) using differential centrifugation. Fluorescent-labeled exosomes were co-cultured with human peripheral blood mononuclear cells (PBMCs). Flow cytometry and confocal microscopy methods were utilized to determine the ability of immune cells to uptake exosomes and to evaluate subsequent intracellular trafficking. The exosomal protein and RNA content was analyzed by mass spectrometry and Nanostring Counter System, respectively. The phenotypic and functional skewing of the monocyte lineage was analyzed after exposing these cells to exosomes. The cytokine array was used to analyze the cytokines generated following treatment of human normal monocytes with exosomes. RESULTS: The GSC-secreted exosomes were preferentially absorbed by CD14+ monocytes and Gr-1+ derived myeloid cells isolated from healthy volunteers and/or glioblastoma patients. When activated, CD4+ and CD8+ T cells could also uptake GSC-secreted exosomes. Confocal microscopy revealed that only monocytes could internalize GSC-secreted exosomes but not fibroblast-secreted exosomes. The exposure to GSC-secreted exosomes induces a phenotypic change in monocytes and prevents them from undergoing apoptosis. GSC-secreted exosomes, but not the fibroblast-secreted exosomes, increased expression of CD163, CD206, and decreased expression of MHC class II. Monocytes treated with GSC-secreted exosomes release IL-6, IL-1RA, CCL3, and CCL4 when compared to cells exposed to fibroblast exosomes. GSC-secreted exosomes contained distinct protein composition in contrast to fibroblast-secreted exosomes that may affect the anti-tumor function of monocyte-derived macrophages. CONCLUSIONS: Monocytes demonstrated a preferential uptake of GSC-secreted exosomes which then induced a glioma-supportive M2 phenotype. GSC-secreted exosomes can be a contributing factor in the M2 skewing within glioma microenvironment.