Abstract LB-344: The rexinoid LG100268 modulates immune cell populations in mammary gland tumors of MMTV-neu mice
Authors: Leal Ana Sofia , Carapellucci Sarah , Zydeck Kayla , Raychaudhuri Nupur , Sporn Michael B. , Liby Karen T. 
Published: August 2018

Abstract
Despite numerous therapeutic advances in the past decade, breast cancer is expected to cause around 41,000 deaths in 2018. Breast cancer 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 and LG101506. The rexinoid LG100268 delays the development of mammary tumors in MMTV-Neu mice. Because recent findings suggested an involvement of RXRs in immune regulation, we studied the effects of LG100268 as an immune modulator in preclinical breast cancer. MMTV-Neu mice with tumors approximately 32 mm3 in volume were treated with control diet or LG100268 (100 mg/kg diet) for 5 days. Rexinoid treatment significantly (p&lt;0.05) reduced tumor weight from 3.2% of body weight in the control group to 1.1% in the treated group. Analysis of the tumor by flow cytometry showed a significant (p&lt;0.05) reduction in the percentage of myeloid derived suppressor cells (MDSCs) in the treated mice (3.4% vs 1.6%). MDSCs associate with a lack of immune surveillance against tumor cells, and in breast cancer, are associated with higher metastatic burden and poor survival. The percentage of activated CD4 T cells (CD45, CD3, CD4, CD25) also decreased when mice were treated with LG100268 (27.5% vs 13%; p&lt;0.05). Moreover, the ratio of CD8/CD4, CD25 cells was increased in the tumors of treated mice (1.58 vs 3.07), suggesting more activation of cytotoxic T cells. Higher CD8/CD4, CD25 ratios correlate with better outcomes in patients with several solid tumors. Tumors treated with LG100268 also showed decreased expression of CD31, indicating reduced vascularization. To further elucidate the effects of LG100268 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 LG100268 at 100 nM reduced the mRNA expression of FOXP3 by 45% when compared with untreated CD4 T cells. These results show that RXR agonists can be used to modulate the immune populations present in the tumor microenvironment in breast cancer, and reduce immunosuppressive MDSC cells and CD4 T regulatory cell populations.
We thank the Breast Cancer Research Foundation for supporting this research.
Citation Format: Ana Sofia Leal, Sarah Carapellucci, Kayla Zydeck, Nupur Raychaudhuri, Michael B. Sporn, Karen T. Liby. The rexinoid LG100268 modulates immune cell populations in mammary gland tumors of MMTV-neu mice [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr LB-344.