e12517 Background: Estrogen receptor (ER)-positive breast cancer is the most frequent subtype accounting for about 70% of all cases. Tamoxifen (TAM) is still the most effective drug for treating ER+/PR+ breast cancer. However, recurrence and metastasis still occur in 30-50% of patients due to TAM resistance. Furthermore, studies have shown that breast cancer stem cells (BCSCs) play an important role in TAM resistance, and exosomes regulate dynamic changes of BCSCs in tumor microenvironment. Additionally, the extract of Huangqi and Ezhu (EHE) has been shown to inhibit multiple TAM-associated resistance signaling pathways. In the current study, we investigated the effect of EHE to reverse TAM-resistance in LCC9 cells via crosstalk of distinct types of exosomes. Methods: Exosomes were extracted from LCC9 cells by ultracentrifugation and characterized by particle size detection, transmission electron microscopy and western blot analysis. Subsequently, isolated LCC9 exosomes were used to treat bone marrow stem cells (BMSCs) with EHE only, TAM only, EHE and TAM (EHETAM), and no treatment (negative control, NC). After treatment, exosomes of BMSCs were isolated and incubated further with LCC9 cells. CCK-8 assay and flow cytometry analysis were performed, and protein expression in treated LCC9 cells was tested by western blot analysis. Results: Data from CCK-8 assay showed that compared with the NC group, proliferation of LCC9 cells was significantly reduced in the presence of exosomes of BMSCs treated with TAM (82.9%, 77.1% , 75.4%), EHE (73.2 %, 69.5% , 68.2 %) and EHETAM (68.9 %, 52.3%，54.1% ) after 24h, 48h, and 72 h, respectively (p < 0.001). EHE and EHETAM groups have more effective inhibition on cell proliferation than TAM only group on the different time separately(p < 0.001).Flow cytometry analysis demonstrated that compared with the NC group(6.1%), increased apoptosis was observed for groups of TAM (8.1%), EHE (12.5%) and EHETAM (12.2%) (p < 0.001). Compared with TAM only group, EHE and EHETAM groups also have significantly higher cell death rate (p < 0.001). Lower CD44 and HER2 expression and higher ERα expression were observed in EHE, TAM, and EHETAM than those of NC group by western blot analysis of treated LCC9 cells. Conclusions: EHE treatment can reverse TAM-resistance by interfering with the crosstalk between BMSCs and breast cancer cells exosomes to induce more active apoptosis and reduce TAM-resistant CD44+ cancer stem cells.