Abstract LB-186: The rexinoid LG100268 decreases immunosuppressive immune cells and activates T cells in two preclinical models of breast cancer

Breast cancer (BC) was considered an immunologically silent tumor, however recent findings suggest that immune cells present in the tumor microenvironment play important roles in tumor growth. Retinoid X receptors (RXRs) are a subclass of nuclear receptors that act as ligand-dependent transcription factors that regulate a variety of cellular processes including proliferation and differentiation. Rexinoids are synthetic molecules that bind and activate RXRs. Bexarotene (Targretin®) is the only rexinoid approved by the FDA for the treatment of refractory cutaneous T-cell lymphoma. Other more potent rexinoids have been synthesized, such as LG100268 (LG268) and LG101506. Because recent studies have suggested that RXRs are involved in immune regulation, we studied the effects of LG268 as an immune modulator in a preclinical model of breast cancer. MMTV-Neu mice with tumors approximately 32 mm3 in volume were treated with control diet or LG268 (100 mg/kg diet) for 5 days. Rexinoid treatment significantly (p=0.0047) reduced tumor weight from 3.2% of body weight in the control group to 1.1% in the treated group. Bexarotene (100 mg/kg diet), however, did not reduce tumor weight even after 10 days of treatment. Analysis of the tumor by flow cytometry showed a significant (p=0.0013) reduction in the percentage of myeloid derived suppressor cells (MDSCs) in the treated mice (3.4% vs 1.6%). MDSCs are associated with a lack of immune surveillance against tumor cells, and in BC, are correlated with higher metastatic burden and poor survival. The reduction in MDSCs was accompanied by a 80% reduction in p-STAT1 (p=0.02). The percentage of activated CD4 T cells (CD45, CD3, CD4, CD25) also decreased when mice were treated with LG268 (27.5% vs 13%; p=0.012). Moreover, the ratio of CD8/CD4, CD25 cells was increased in the tumors of treated mice (1.58 vs 3.07; p=0.04), suggesting increased activation of cytotoxic T cells. Higher CD8/CD4, CD25 ratios correlate with better outcomes in patients with several solid tumors. To further elucidate the effects of LG268 on immunosuppressive T cells, CD4 cells were isolated from spleen and stimulated with CD28, IL-2 and TGFβ, skewing the cells towards regulatory T cells. CD4 Treg FOXP3 expressing cells contribute to the immunosuppressive milieu in tumors. Treatment with LG268 at 100 nM reduced the mRNA expression of FOXP3 by 40% (p=0.03) when compared with untreated CD4 T cells. In the MMTV-PyMT model of triple negative BC, LG268 (100 mg/kg diet) significantly (p=0.018) extended survival by an average of 29 days when compared with the controls. Histopathological analysis revealed that tumors treated with LG268 had higher infiltration of CD8 cytotoxic T cells, which correlated with increased cleaved caspase 3. These results show that RXR agonists can be used to modulate immune cell populations present in the tumor microenvironment, and reduce immunosuppressive MDSC cells and CD4 T regulatory cell populations in breast cancer models. We thank the Breast Cancer Research Foundation for support. Citation Format: Ana Sofia Leal, Sarah Carapellucci, Kayla ZydecK, Michael B. Sporn, Karen T. Liby. The rexinoid LG100268 decreases immunosuppressive immune cells and activates T cells in two preclinical models of breast cancer [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 LB-186.