CSCs are considered as the root of cancer initiation, metastasis and relapse [16–17]. Studies showed that CD105 are well-accepted as renal CSCs markers [2–5, 18, 19]. Our development of a method for enrichment of CD105+ CSCs through primary RCC cell lines showed the higher expression CSC markers such as Nanog, Pax2, Vimentin, Oct4 and tumorigenicity compared with other RCC cell lines, displayed the characteristic of CSCs in vitro and vivo.
Solid tumors such as RCC possess an immunosuppressive microenvironment that can reduce the antitumor function of CD8+T cells [20, 21]. Studies showed that TGF-β, as an immunosuppressive cytokine in the tumor microenviroment, has been associated with poor prognosis short overall survival and recurrence [22, 23]. We also found that the high expression of TGF-β1 in RCC tissues significantly correlated with poor prognosis. Furthermore, when cultured with TGF-β1, CD105+ CSCs have acquired EMT alteration including downregulation of E-cad expression, upregulation of Vim and N-cad expression, and increased sphere formation population. These results indicated that TGF-β play important role in RCC and promote the EMT in CSCs, which related with recurrence and metastasis. Accumulating evidences have shown the connection between the EMT and CSCs in human cancers, and TGF-β plays pivotal role not only in the stemness of CSCs but also in the immune cells in tumor microenvironment [24–26]. TGF-β can control adaptive immunity by promoting the expansion of Treg cells directly, regulating the regulatory CD4+ T cell response, controlling the function of effector T cells. Therefore, interfering with TGF-β signaling in immune cells is a rational strategy for cancer immunotherapy [27–30].
Gorelik et al reported that blockade of TGFβ pathway in T lymphocytes could elicit a potent anti-tumoral response [31]. A strategy that has been evaluated in the context of T-cell engineering is expression of a dominant-negative TGF-β receptor that led to improved efficacy of prostate-specific membrane antigen-specific CAR-T cells in murine xenograft model [33]. In previous studies, we demonstrated that the TGF-β insensitive CD8+ T cells show the potent antitumor effect in RCC. However, in CD105+ CSCs, due to the tumor heterogeneity, whether this strategy still work remains unknown. Therefore, we focused on CSCs as a target of cancer immunotherapy to achieve more efficient anti-tumor responses. In vitro experiment, we found that TβRIIDN CD8+ T cells show the specific tumor-killing activity against CD105+CSCs. In vivo experiment showed that TβRIIDN CD8+ T cells could inhibit CD105+ CSC growth, reduce tumor burden and pulmonary metastasis in vivo, further experiment revealed that this effect was mainly achieved by blockade the EMT mechanism in CSCs.
Redirect and reprogram T cells to enhance the T cell toxicity ability is also important in the process of tumor rejection. Eyquem J et al reported that using the CRISPR/Cas9 genome editing technique targeting the CAR to the TRAC locus in T cell could avert tonic CAR signaling and establish effective internalization and re-expression of the CAR following delaying effector T-cell differentiation and exhaustion [34]. Zheng F et al reported that combined α-CTLA-4/α-PD-L1 and CSC-DC vaccine could enhance the number of circulating functional CD8+ T cells, enhanced CSC-DC vaccine-induced host immune responses [35]. However, in this study, we did not perform the experiment about the effect of PD/L-1 drugs against CD105 + CSCs. We also didn’t quantify the affection of the absolute number of CD8 + T cells in blood circulation and the tumor parenchyma. Although it will take time and effort to illuminate this issue, it maybe provides new insights into the mechanisms of this immunotherapy.
In conclusion, our findings demonstrate that the TGF-β insensitive CD8+ T cells show the tumor-specific antitumor effect including reduce tumor burden, inhibit tumor growth and pulmonary metastasis by blockade the EMT mechanism existed in CD105+CSCs. This study may provide a new perspective for the immunotherapy in RCC.