Abstract 2364: 1α, 25 dihydroxyvitamin D (1,25(OH)2D) inhibits the T cell suppressive function of myeloid derived suppressor sells (MDSC)
Authors: Fleet James C. , Burcham Grant N. , Calvert Ryan , Ratliff Timothy L. 
Published: August 2019

Abstract
MDSC within the tumor microenvironment are antagonists to immune surveillance and T-cell based immunotherapies because they can suppresses the ability of cytotoxic T cells to attack and clear tumor cells from the body. The active, hormonal form of vitamin D, 1,25(OH)2D, regulates many components of the immune system and previous research shows that in mouse models 1,25(OH)2D reduces the number of CD34+ cells, an MDSC precursor, in tumors and can reduce metastasis when coupled to adoptive immunity. We tested whether MDSC are vitamin D target cells by examining granulocytic- (G-MDSC) and monocytic (M-MDSC) MDSC from tumors, spleen, and bone marrow. Messenger RNA levels of the vitamin D receptor (VDR), the mediator of 1,25(OH)2D molecular action, are low in MDSC from marrow and spleen but are 20-fold higher in tumor MDSC. In addition, at all sites the M- subtype cells have 4-fold higher VDR mRNA expression than G- subtype cells. Next, we tested whether MDSC are responsive to changes in 1,25(OH)2D signaling. Bone marrow MDSC were isolated by flow cytometry and induced to differentiate into tumor MDSC-like cells using treatment with a cytokine cocktail of IFN-γ, IL-13, and GM-CSF for up to 72 h. This treatment significantly elevated Arg1 and Nos2 levels (48 h) and activated the ability of the bone marrow MDSC to suppress T cell proliferation in a 16 h assay. 48 h of cytokine treatment also increased VDR expression 100-fold and made the bone marrow MDSC responsive to 1,25(OH)2D treatment i.e. 10 nM for 24 h increased CYP24A1 and VDR mRNA in wild-type (WT) cells. Importantly, 1,25(OH)2D treatment reduced the T cell suppressive capacity of cytokine-induced total MDSC and M-MDSC by ≥70% in a 16 h assay. We next examined the impact of VDR deletion on the development of tumor M- and G-MDSC function. WT and VDR KO mice were injected ip with 106 RM-1 cells and tumor MDSC were isolated by FACS 7 days later. As expected, the T cell suppressive function of WT M-MDSC was 4X greater than G-MDSC. However, T cell suppression was significantly greater in both VDR KO M-MDSC (2X vs WT at 1:4 MDSC:T-cell ratio) and G-MDSC (4X vs WT, 1:4 ratio). The major finding of this study is that MDSC are novel targets of 1,25(OH)2D. The effect of 1,25(OH)2D is to decrease the immunosuppressive capability of MDSC. In addition, expression of VDR is necessary to attenuate the immunosuppressive capabilities of MDSC that are recruited to the tumor microenvironment. Collectively, this data suggests that activation of vitamin D signaling could be used to suppress MDSC function and release a constraint on T-cell mediated clearance of tumor cells.
Citation Format: James C. Fleet, Grant N. Burcham, Ryan Calvert, Timothy L. Ratliff. 1α, 25 dihydroxyvitamin D (1,25(OH)2D) inhibits the T cell suppressive function of myeloid derived suppressor sells (MDSC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2364.