Tumor Antigen-Primed Dendritic Cell-Derived Exosome Synergizes with Colony Stimulating Factor-1 Receptor Inhibitor by Modulating the Tumor Microenvironment and Systemic Immunity

Dendritic cell-derived exosomes (Dex) have overcome the disadvantages associated with dendritic cell (DC) vaccines, such as cost effectiveness, stability, and sensitivity to the systemic microenvironment. However, in clinical trials, Dex failed to provide satisfactory results because of many reasons, including inadequate maturation of DC as well as the immunosuppressive tumor microenvironment (TME). Hence, culturing DCs in the presence of a maturation cocktail showed an induced expression of MHCs and co-stimulatory molecules. Additionally, targeting the colony stimulating factor-1 (CSF-1)/CSF-1 receptor (CSF-1R) signaling pathway by a CSF-1R inhibitor could deplete tumor-associated macrophages (TAMs) and myeloid-derived suppressor cells (MDSCs) which are responsible for immunosuppressive TME. Hence, in this study, mDex^TA were isolated from bone marrow-derived DC cultured in the presence of a novel maturation cocktail and tumor antigen. mDex^TA showed elevated expression of major histocompatibility complexes (MHCs) and co-stimulatory molecules and was found capable of activating naïve DC and T cells in vitro more efficiently when compared to imDex^TA isolated from immature DCs. In addition, PLX-3397, a small molecule inhibitor of CSF-1/CSF-1R, was used in combination to enhance the antitumor efficacy of mDex^TA. PLX-3397 showed dose-dependent toxicity against bone marrow-derived macrophages (BMDMs). In the B16-F10 murine melanoma model, we found that the combination treatment delayed tumor growth and improved survival compared to the mice treated with mDex^TA alone by enhancing the CD8 T cells infiltration in TME. mDex^TA when combined with PLX-3397 modulated the TME by shifting the Th1/Th2 toward a dominant Th1 population and depleting the TAMs and MDSCs. Interestingly, PLX-3397-induced FoxP3 expression was diminished when it was used in combination with mDex^TA. Combination treatment also induced favorable systemic antitumor immunity in the spleen and lymph node. In conclusion, our findings provide insights into the synergy between mDex^TA-based immunotherapy and PLX-3397 as the combination overcame the disadvantages associated with monotherapy and offer a therapeutic strategy for the treatment of solid tumors including melanoma.