Ethnopharmacological relevance: Triple-negative breast cancer (TNBC) is the most aggressive and the worst prognosis breast cancer with limited treatment options. Taraxacum mongolicum (also called dandelion) is a traditional Chinese medicine has been used to treat mastitis, breast abscess, and hyperplasia of mammary glands since ancient times. In modern pharmacological research, dandelion has been proven with anti-breast cancer activities. We previously reported that dandelion extract could induce apoptosis in TNBC cells. However, its anti-tumor effects and mechanisms in the tumor microenvironment have not yet been elucidated.

Aim of the study: Tumor-associated macrophages (TAMs) play an important role in regulating the interaction between tumor cells and the immune system. The present study aimed to investigate the effects and mechanisms of dandelion extract on TNBC cells under the microenvironment of TAMs, as well as its influence on the polarization of M2 macrophages.

Materials and methods: M2 macrophages were induced by phorbol-12-myristate 13-acetate (PMA) and interleukin 4 (IL-4), and verified by flow cytometry, quantitative RT-PCR (qRT-PCR), Western blotting, and ELISA. MDA-MB-231 and MDA-MB-468 TNBC cells were co-cultured with the supernatant of M2 macrophage which providing the TAMs microenvironment. The antitumor activity of dandelion extract in TNBC cells was evaluated by MTT assay. The invasive and migratory capacity of TNBC cells was measured by transwell assays. The expression of protein and gene was assessed by Western blotting and qRT-PCR, respectively.

Results: TAMs microenvironment promoted the proliferation, migration, and invasion of TNBC cells. However, dandelion extract inhibited the malignant property of MDA-MB-231 and MDA-MB-468 cells induced by TAMs. Both of TAMs and IL-10 caused STAT3 activation and PD-L1 higher expression, the immunosuppressive molecules in TNBC cells, and this effect can be attenuated by IL-10 neutralizing antibody. Dandelion extract exerted inhibition on STAT3 and PD-L1 in TNBC cells under TAMs microenvironment. Furthermore, in M2 macrophages, dandelion extract remarkably promoted the expression of M1-like marker TNF-α, IL-8, and iNOS, but reduced M2-like marker IL-10, CD206, Arginase-1, and TGF-β.

Conclusion: Dandelion extract inhibited the proliferation, migration and invasion of TNBC cells in TAMs microenvironment through suppressing IL-10/STAT3/PD-L1 immunosuppressive signaling pathway. Furthermore, dandelion extract promoted the polarization of macrophages from M2 to M1 phenotype. Thus, our results indicated that dandelion may serve as a promising therapeutic strategy for TNBC by modulating tumor immune microenvironment.