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FOXP3+ regulatory T Cell perturbation mediated by the IFNγ-STAT1-IFITM3 feedback loop is essential for anti-tumor immunity

ABSTRACT: Targeting tumor-infiltrating Treg cells is an efficient way to evoke an anti-tumor immune response. How Treg cells fragility and stability are regulated remains largely unknown. IFITM3 and STAT1 are interferon-induced genes that play a positive role in the progression of tumors. Here, we showed that IFITM3-deficient Tregs blunted tumor growth by strengthening the tumor-killing response and displayed the Th1-like Treg phenotype with higher secretion of IFNγ. Mechanistically, depletion of IFITM3 enhances the translation and phosphorylation of STAT1. Conversely, the decreased IFITM3 protein and mRNA levels present in STAT1-deficient Treg cells indicate that STAT1 conversely regulates the expression of IFITM3 to form a feedback loop. Blocking the inflammatory cytokine IFNγ or directly depleting STAT1-IFITM3 axis phenocopies the restored suppressive function of TI-Tregs in the tumor model. Overall, our study demonstrates that the perturbation of TI-Tregs through IFNγ-IFITM3-STAT1 feedback is essential for anti-tumor immunity and constitutes a targetable vulnerability of cancer immunotherapy.

ORGANISM(S): Mus musculus

PROVIDER: GSE248223 | GEO | 2023/11/21

REPOSITORIES: GEO

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Project description:Ifitm3 (interferon-inducible transmembrane protein 3) was previously identified as an endosomal antiviral effector protein that blocks viral infection1-4. In analyses of gene expression data from patients with B-cell leukemia and lymphoma, we identified Ifitm3 as one of the strongest predictors of poor clinical outcome. In normal resting B-cells, Ifitm3 was minimally expressed and mainly localized in endosomes. However, engagement of the B-cell receptor (BCR) and PI3K-activation strongly induced expression of Ifitm3 and phosphorylation of its N-terminus at Y20, resulting in accumulation at the cell surface. In B-cell leukemia, oncogenic kinases induced phosphorylation at Y20, resulting in constitutive plasma membrane localization of Ifitm3. Ifitm3¯ / ¯ naïve B-cells developed at normal numbers, however, B-cell activation upon antigen encounter, germinal center formation and production of antigen-specific antibodies were compromised. Likewise, oncogenes that typically induce development of leukemia and lymphoma failed to transform Ifitm3¯ / ¯ B-cells. Conversely, a phosphomimetic mutation of Y20 induced oncogenic PI3K-signaling and initiated transformation of premalignant B-cells into overt leukemia. Functional experiments revealed a previously unrecognized function of Ifitm3 as PIP3-scaffold and central amplifier of PI3K signaling downstream of the BCR and adhesion receptors. PI3K signal-amplification depends on binding of Ifitm3 to PIP3 via two lysine residues (K83 and K104) in its conserved intracellular loop. In Ifitm3¯ / ¯ B-cells, lipid rafts were depleted of PIP3, resulting in defective expression of >60 lipid raft-associated surface receptors, which impaired BCR-signaling and cellular adhesion. We conclude that phosphorylation of IFITM3 upon Bcell antigen-encounter induces a dynamic switch from antiviral effector functions in endosomes to a PI3K-amplification loop at the cell surface. IFITM3-dependent amplification of PI3K-signaling downstream of the BCR and adhesion receptors is critical to enable rapid expansion of B-cells with high affinity to antigen. In addition, multiple oncogenes depend on IFITM3 to assemble PIP3- dependent signaling complexes and amplify PI3K-signaling for malignant transformation.

Project description:Interferon-induced transmembrane protein (IFITM1) plays a dual role in restriction of RNA viruses and in metastatic cancer cell growth. The signal transduction events that are orchestrated by IFITM1 are not well defined. We set out to identify IFITM1 interacting proteins to begin to define its mechanism of action. Affinity purification of SBP-tagged IFITM1, coupled to SWATH-mass spectrometry, identified significantly higher confidence interacting proteins from IFN- treated cells, relative to non-treated cells. This promoted an examination of the protein synthetic machinery in order to determine whether IFITM1 can impact on ribosomal proteome from IFN- treated cells. Isogenic ifitm1 null and ifitm1-ifitm3 null-SiHa cell panels were generated using CRISPR gRNAs to define signaling events that are linked to IFN--dependent protein synthesis. Ultracentrifugation sedimentation of cell lysates from interferon gamma treated wt and ifitm1-ifitm3 null-SiHa cells was carried out to isolate ribosomal constituents. Although SWATH-mass spectrometry of the 40S, 60S, and 80S fractions demonstrated changes in selected protein content, a significant reduction in A254 (RNA) in the 80S ribosomal fractions suggested a relatively select defect in 80S ribosomal biogenesis in ifitm1-ifitm3 null cells. The localization of IFITM1 to ribosomal protein components prompted an analysis of IFN--dependent protein synthesis using pulse SILAC. STAT1 and B2M were two dominant proteins whose synthesis increased equivalently in wt or ifitm1-ifitm3 null cells. However, MHC class I molecules and ISG15 were the most highly suppressed IFN--responsive proteins in the ifitm1-ifitm3 double null cells and this was confirmed using ifitm1 single null cells. Transient depletion of IFITM1 using targeted siRNA also depleted MHC class I molecules as well as IFITM3, STAT1, B2M, and ISG15. These data have implications for the function of IFITM1 in mediating IFN--stimulated protein synthesis and associated antigen presentation during either oncogenic or anti-viral signalling.

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FOXP3+ regulatory T Cell perturbation mediated by the IFNγ-STAT1-IFITM3 feedback loop is essential for anti-tumor immunity

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