Breast cancer cell–derived IL-35 promotes tumor progression via induction of IL-35-producing induced regulatory T cells

Interleukin 35 (IL-35) is a potent immunosuppressive cytokine, consisting of an Epstein–Barr virus-induced gene 3 (EBI3) subunit and a p35 subunit. IL-35 is mainly produced by regulatory T and regulatory B cells, and plays a crucial role in the development and prevention of infectious and autoimmune diseases. However, the effect of IL-35 in malignant disease is not well understood. In this study, we demonstrated that breast cancer cells (BCCs) also expressed and secreted IL-35 and higher level of IL-35 in BCCs was closely associated with poor prognosis of patients and was an independent unfavorable prognostic factor for breast cancer. Subsequent study revealed that BCC-derived IL-35 inhibited conventional T (Tconv) cell proliferation and further induced suppressed Tconv cells into IL-35-producing induced regulatory T (iTr35) cells. Furthermore, BCC-derived IL-35 promoted the secretion of inhibitory cytokine IL-10 and obviously decreased the secretion of Th1-type cytokine IFN-γ and Th17-type cytokine IL-17 in Tconv cells. Meanwhile, the expression of inhibitory receptor CD73 was also elevated on the surface of Tconv cells following the BCCs' supernatant treatment. Mechanistically, BCC-derived IL-35 exhausted Tconv cells and induced iTr35 by activating transcription factor STAT1/STAT3. Hence, our results indicate functions of BCC-derived IL-35 in promoting tumor progression through proliferation inhibition of tumor-infiltrating Tconv cells and induction of iTr35 cells in tumor microenvironment. This study highlights that IL-35 produced by BCCs are a potential therapeutic target for breast cancer.