T cell-intrinsic STING signaling promotes regulatory T cell induction and immunosuppression by upregulating FOXP3 transcription in cervical cancer

Stimulator of interferon genes (STING) is an innate immune sensor of cytoplasmic double-stranded DNA originating from microorganisms and host cells. The activation of cytosolic DNA-STING pathway in tumor microenvironments is usually linked to more robust adaptive immune responses to tumors, however the intracellular function of STING in regulatory T cells is largely unknown. In the present study, we aimed to explore the contribution of intracellular STING activation to regulatory T cell induction (iTreg) in cervical cancer (CC) microenvironments.Blood samples and tumor specimens were obtained from patients with CC. The intratumoral STING, CCL22, CD8 and forkhead box P3 (FOXP3) expression levels were measured by immunohistochemistry. T cell-specific STING conditional knockout mice (CD4-Cre/STINGflox/flox, TKO) were generated, and syngeneic TC-1 tumor model were investigated. The differentiation and molecular regulatory pathway of human and murine iTreg under different treatments were investigated by ex vivo assays, immunoblotting and quantitative PCR. Tumor-associated exosomes (T-EXO) were isolated from CC cell lines and exosomal contents were identified by ELISA and Western blot analysis. The impact of T-EXO on T cell differentiation was tested in in vitro cell culture.Increased STING, CCL22 level, FOXP3+ cells but decreased CD8+ cells in tumor tissues predicted poor survival. Tumor-bearing CD4-Cre-STINGflox/flox (TKO) mice displayed slower tumor growth tendencies as well as fewer FOXP3+ cells but higher CD8+ cell proportion in tumor tissues than wild-type (WT) mice. Activating of STING signaling cooperated with T cell receptor, interleukin-2 receptor and transforming growth factor-beta (TGF-β) signals to promote CD4+CD25highFOXP3+ iTreg differentiation from both human and murine CD4+-naïve T cells from WT and IFNAR-/- mice but not TKO or IRF3-/- mice in vitro. Ectopic STING, TBK1 or IRF3 expression promoted iTreg differentiation from human CD4+-naïve T cells. T cell-intrinsic STING activation induced FOXP3 transcription through TBK1-IRF3-mediated SMAD3 and STAT5 phosphorylation independent of interferon-β. In CC, tumor-derived exosomes activated STING signaling in tumor-infiltrated T cells by exosomal TGF-β, cyclic GMP-AMP synthase and 2'-3'-cGAMP, leading to iTreg expansion.These findings highlight a novel mechanism for iTreg expansion mediated by tumor-derived exosome-activated T cell-intrinsic STING signal, and provide a rationale for developing immunotherapeutic strategies targeting STING signal in CC.