The secretion of TβRII by extracellular vesicles was investigated both in murine models of breast cancer as well as in breast cancer patients. In mice, blood levels of TβRII+ crEV positively correlated with tumor burden, increased proportionally with tumor size, and were associated with metastasis. In the sera of healthy human donors, crEVs exhibited very low baseline TβRII positivity, which was much higher in 89% of breast cancer patients. Additionally, the amount of TβRII in crEVs was the highest in TNBC patients compared to HER2+ and luminal patients. Patients with distant metastasis also exhibited higher amounts of TβRII in crEVs than those without metastasis.
The authors determined that the percentage of TβRII+ crEVs could be a classifier for breast cancer, as they were able to differentiate between breast cancer patients and healthy donors with 92% accuracy, 93% sensitivity, and 90% specificity. Furthermore, patients with lower levels of TβRII+ crEVs also exhibited improved overall survival and metastasis-free survival compared to those with higher TβRII+ crEVs levels. Results from these experiments indicate that TβRII is a noninvasive biomarker for metastatic breast cancer with both diagnostic and prognostic potential.
This paper also reports several mechanistic studies that examine the effects of TβRII+ crEVs on metastatic tumor outgrowth and anti-tumor immunity. The authors determined that TβRII transferred by TEVs induces TGF-β/SMAD activity, and that TβRII+ extracellular vesicles play a critical role in cancer stemness and metastasis. Furthermore, breast cancer cells pre-treated with TβRII+ extracellular vesicles were found to be more resistant to the chemotherapy drugs paclitaxel and doxorubicin. The authors also found an inverse correlation with the amount of TβRII+ crEVs and interferon-γ production, suggesting that TβRII+ secretion from tumor cells may negatively affect anti-tumor immunity.
Lastly, the authors investigated the relationship between TβRII and CD8+ T cell exhaustion. TβRII depletion in tumors was associated with higher frequencies of effector cytokines, higher levels of inhibitory receptors, and decreased proliferating capacity of CD8+ T cells, suggesting that TβRII from TEVs may induce T cell exhaustion and suppress anti-tumor immunity. Mechanistically, the authors determined that SMAD3 partners with T cell factor-1 (TCF1) to induce CD8+ T cell exhaustion (Figure 2).