Abstract 3666: Chemotherapeutic sensitivity of myeloid-derived suppressor cells during cancer therapy is dictated by selective expression of clusterin
Authors: Tu Nhan , Trinh Thu Le , Zhou Jun-Min , Gilvary Danielle L. , Coppola Domenico , Wei Sheng , Djeu Julie Y. 
Published: July 2017

Diverse chemotherapeutic agents, including Docetaxel, have been reported by us and others to specifically target myeloid-derived suppressor cells (MDSCs) which mediate tumor-associated immune repression. However, the mechanism is unknown. To find a less toxic anticancer drug, we focused on Curcumin which exhibits natural anti-oxidant and antitumor activities with low toxicity to normal cells. Here, we demonstrate that curcumin, like Docetaxel, promotes cytotoxic function against tumor cells and depletes MDSC populations in vivo and in vitro. In 4T1 mammary tumor-bearing BALB/c mice treated with curcumin, we observed dose-dependent tumor reductions in mice treated with curcumin compared to DMSO controls, indicating that curcumin can be an effective antitumor agent. To elucidate the mechanism, we embarked on examining different populations of myeloid-derived suppressor cells (MDSCs). Flow cytometric analysis of splenic cells from tumor bearers indicated that the antitumor effect by curcumin is due to depletion of immunosuppressive Ly6G+ granulocytic (G)-MDSC lineage with selective promotion of Ly6C+ monocytic (M)-MDSC towards the CCR7+F4/80+ M1 subset. Furthermore, the increase in M1 MDSCs correlated with greater T-cell adaptive immune response and cytotoxicity against 4T1 tumor cells. Since clearance of G-MDSCs was mediated by apoptosis as evidenced by our Annexin V staining, we hypothesized that clusterin (sCLU), a potent anti-apoptotic protein, unrelated to the Bcl-2 family, may be the underlying mechanism by which M-MDSCs and M1 cells are selected out following treatment of curcumin. Western blot analysis revealed that sCLU was, indeed, differentially expressed in M1-MDSCs but not in G-MDSCs. Antisense-CLU transfection into M-MDSCs reduced chemoresistance not only to curcumin but also to Docetaxel, confirming the critical involvement of CLU in selective survival of M-MDSCs. Using both curcumin and Docetaxel we confirmed that sCLU works by binding Bax, preventing its translation to the mitochondria to initiate apoptosis. For clinical relevance, we examined human breast cancer samples and noted that the tumor microenvironment is replete with immature CD33+ MDSCs which lack sCLU expression, unlike infiltrating mature CD68+ macrophages which possess high sCLU expression. Our results taken together indicate that selective survival of M-MDSCs under chemotherapy dictate their survival advantage, allowing for maturation into M1 cells and promotion of adaptive T cell immunity against cancer.

Citation Format: Nhan Tu, Thu Le Trinh, Jun-Min Zhou, Danielle L. Gilvary, Domenico Coppola, Sheng Wei, Julie Y. Djeu. Chemotherapeutic sensitivity of myeloid-derived suppressor cells during cancer therapy is dictated by selective expression of clusterin [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3666. doi:10.1158/1538-7445.AM2017-3666