Elevated cholesterol has been identified as a major risk factor of breast cancer onset and recurrence, while cholesterol-lowering drugs are associated with a good prognosis. Previous work in murine models has found that cholesterol increases breast cancer metastasis. However, the prometastatic effects of cholesterol were due to its primary metabolite, 27-hydroxycholesterol (27HC). 27HC is a ligand of both the estrogen receptor (ER) and liver X receptor (LXR). Intriguingly, the prometastatic effects of 27HC require the presence of myeloid-immune cells. This cell type has been implicated in suppressing acquired immunity, allowing cancer cells to escape immune surveillance. Therefore, we hypothesize that 27HC acts on myeloid cells to facilitate immune escape of cancer cells. To characterize the immunomodulatory capacity of 27HC, we cocultured vehicle- or 27HC-treated bone marrow-derived macrophages (BMDMs) with T cells. In support of our hypothesis, we found that 27HC-treated BMDMs inhibited T cell expansion in a dose-dependent manner. Additionally, cocultured BMDMs treated with 27HC resulted in decreased granzyme B expression and secretion in CD8+ cytotoxic T cells. Subsequent experiments showed that T cells activated by 27HC-treated macrophages had significantly reduced ability to eliminate cancer cells. Although 27HC modulated the expression of MHCII and CD80/86, the immunosuppressive effect of 27HC-treated BMDMs persisted even without a direct contact between BMDMs and T cells, suggesting that the T cell inhibition is not limited to cell-cell interaction. To determine the mechanisms by which 27HC-treated BMDMs suppress T-cell activity, we first evaluated the relative contributions of the two receptors known to bind 27HC: the ERs and LXRs. To this end, BMDMs were exposed to pharmacologic antagonists of, or siRNA against the ERs or LXRs. These BMDMs were subsequently cocultured with activated T cells to study their impact on T-cell proliferation. Interestingly, 27HC’s immunosuppressive capacity on preactivated T cells required LXR in BMDMs, but not ER. The involvement of LXR was further validated by surveying a panel of related oxysterols and LXR ligands, as the inhibition of T-cell expansion is directly proportional to their ability to activate LXR. Therefore, our results suggest that 27HC affects the microenvironment of breast cancer by altering the activity of antigen-presenting cells at least in part through the modulation of LXR. This in turn reduces the expansion and function of T cells, ultimately resulting in immune escape and tumor progression. Our ongoing work is aimed at elucidating the 27HC-induced signaling pathway in BMDMs. Collectively, these data provide further support to target the cholesterol-27HC axis as an adjuvant with other immune therapy. This is especially relevant given the prevalence of hypercholesterolemia as well as metastatic breast cancer. This work was supported by the grants from the NCI and DOD-BCRP to ERN (R00CA172357, R01CA234025, BC171214).

Citation Format: Liqian Ma, Chaeyeon Han, Lawrence Wang, Amy E. Baek, David J. Shapiro, Som G. Nanjappa, Erik R. Nelson. 27-hydroxycholesterol acts on myeloid cells to inhibit both T cell expansion and cytotoxic activity [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2019 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(3 Suppl):Abstract nr B86.