Abstract 2791: Semaphorin7A and estrogen work in concert to promote mammary tumor growth and alter the immune tumor microenvironment
Authors: Elder Alan , Crump Lyndsey , Stoller Alexander , Lyons Traci 
Published: July 2021

Abstract
Breast cancer (BC) is the 2nd leading cause of cancer-related death in women. Increased lymphatic vessel density (LVD), lymphovascular invasion (LVI), and lymph node positivity (LN+) are associated with increased metastasis in BC patients. LVD, LVI, and LN+ are promoted by angiogenesis and lymphangiogenesis, which can be induced by tumor-associated macrophages (TAMs). TAMs are implicated in creating a pro-tumor tumor microenvironment (TME) in various cancers. We have shown that semaphorin7A (SEMA7A)—a signaling molecule that activates integrin-mediated signaling cascades—is associated with decreased survival and increased metastasis in BC patients. In mouse models, SEMA7A expression is associated with increased LVD and TAM presence. SEMA7A can also induce expression of lymphatic endothelial cell (LEC)-associated proteins, such as podoplanin (PDPN), on macrophages to polarize them into “PoEMs” (PDPN-expressing macrophages). Furthermore, PoEMs intercalate into lymphatic vessels to form PoEM-LEC chimeric vessels; tumor cells are found within these vessels at the PoEM-LEC junctions. We have also shown that SEMA7A promotes programmed death ligand-1 (PD-L1) expression on TAMs, LECs, PoEMs, and BC cells to inhibit anti-tumor immune responses and promote a pro-tumor TME. In neuronal cells, SEMA7A expression is regulated by steroid hormones in a B1-integrin mediated manner. Given the link between SEMA7A and hormones, we hypothesized that SEMA7A expression in tumors could be induced by estrogen (E2) and that SEMA7A and E2 may cooperate to create a pro-tumor immune TME (iTME). To test this, we orthotopically implanted E0771-DDK (Ctrl) and E0771-DDK-SEMA7A (SEMA7A overexpressing [OE]) cells into female mice with or without E2 supplementation. E0771 cells have recently been characterized as the luminal B subtype—which is ER+ and highly proliferative—and E0771 cells are dependent upon E2 supplementation for growth in vivo. SEMA7A OE tumors without E2 overcame the need for E2 and grew as well as the Ctrl+E2 tumors, but SEMA7A+E2 significantly accelerated tumor growth. Tumors were harvested for flow cytometry (FC) to reveal that E2 increased SEMA7A expression in Ctrl tumors to similar levels observed in the SEMA7A OE and SEMA7A+E2 significantly increased expression of PD-L1 expression on tumor cells. Additionally, SEMA7A+E2 was the only condition to increase the presence of intratumoral LECs; however, E2, SEMA7A, and SEMA7A+E2 all induced PD-L1 expression on LECs. Additionally, SEMA7A and SEMA7A+E2 groups had more TAMs and PoEMs compared to controls, and significantly more of those TAMs/PoEMs were PD-L1+. Lastly, we treated Ctrl and SEMA7A OE tumors (in the presence of E2) with anti-PD1 or anti-PD-L1 therapies and only the SEMA7A OE tumors responded. These results indicate that E2 can induce SEMA7A expression in tumors and that SEMA7A can cooperate with E2 to inhibit anti-tumor immunity.
Citation Format: Alan Elder, Lyndsey Crump, Alexander Stoller, Traci Lyons. Semaphorin7A and estrogen work in concert to promote mammary tumor growth and alter the immune tumor microenvironment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2791.