LAUSR

Rationale: Tetraspanins (TSPANs) have emerged as key players in regulating tumor cell migration, invasion, and metastasis. Whilst we know TSPANs are expressed by macrophages very little is known regarding their role in Tumor Associated Macrophages (TAMs). We have shown that all 33 TSPAN members are differentially expressed by macrophages; however, following exposure to tumor-derived factors we see a significant increase in TSPAN5. We hypothesize that TSPAN5 plays a role in supporting the pro-tumor activities of TAMs and targeting tetraspanins in cancer may be a novel therapeutic approach. Objectives: To determine TSPAN5 expression by macrophages in vitro and in situ in human breast cancer tissue and investigate the role of TSPAN5 in regulating the pro-tumor phenotype of TAMs. Methodology: Expression of TSPAN5 was confirmed at the gene and protein level by qPCR, flow cytometry, western blotting, immunocytochemistry and by immunohistochemistry in formalin fixed breast cancer tissue. Accell siRNA knockdown was performed to determine the role of TSPAN5 in macrophage phenotype and function. NextSeq500 was performed to identify how TSPAN5 influences gene expression in BMDMs prepared from C57Bl/6 wild-type (WT) and TSPAN5 knockout (KO) mice. The role of TSPAN5 in a mammary carcinoma model in C57BL/6 WT and TSPAN5KO mice was also determined. Findings: TSPAN5 was up-regulated by tumor-conditioned macrophages both in vitro and in vivo. This expression was also evident in TAMs located in patient derived breast cancer tissue (n=30). Interestingly, 64% of TAMs expressed TSPAN5 and these cells were the main source of TSPAN5 in human breast tumors (77%). Knockdown of TSPAN5 promoted a pro-inflammatory macrophage phenotype and NextSeq500 (n=3) revealed significant changes in genes involved in cell adhesion (Fn1, Cdh11, Lamb1, Cyr61, Postn and Wisp2) and immune regulation (PTGS2, GPX8 and Crip2) in BMDMs from TSPAN5KO mice compared to WT mice. Genetic and antibody ablation of TSPAN5 in C57BL/6 significantly reduced primary EO771 tumors and enhanced the survival of tumor bearing mice. Interestingly, clodronate depletion of macrophages in TSPAN5KO mice promoted mammary tumor growth. Conclusion: Our study demonstrates for the first-time expression of TSPAN5 by TAMs in breast cancers and the pro-tumor properties of TSPAN5 in primary and metastatic mammary tumor growth. Citation Format: Mohammed Ridha Moamin, Peter Monk, Angela Cox, Penelope Ottewell, Julien Saint-Pol, Eric Rubinstein, Munitta Muthana. Tetraspanin 5 expressing macrophages promote mammary tumor growth [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-036.