STING-Dependent FasL Upregulation Converts Senescent Fibroblasts from Regenerative Negotiators to Immune-Suppressive Hubs

Chronic senescent fibroblasts acquire a pathogenic phenotype marked by surface FasL expression that kills infiltrating immune cells and blocks surveillance. We hypothesize that this shift is driven by persistent cytosolic DNA sensing through the cGAS-STING pathway, which triggers type I IFN signaling and subsequently upregulates FasL as a feedback mechanism to limit immune infiltration. In young, transient senescence, STING activity is low or transient, allowing beneficial SASP factors such as PDGF-AA and osteopontin to promote tissue repair without engaging immunosuppressive checkpoints. When STING signaling becomes constitutively active in aged fibroblasts, FasL dominates the secretome, turning the hostage negotiator into an immune‑evading barrier.

This hypothesis generates several testable predictions. First, fibroblast‑specific deletion of STING in aged mice will reduce FasL levels on senescent cells without diminishing PDGF‑AA or osteopontin secretion. Second, blocking STING pharmacologically (e.g., with C‑176) will restore macrophage and T‑cell infiltration into senescent‑rich lesions, as measured by flow cytometry and immunohistochemistry. Third, preserving the regenerative SASP while removing FasL should improve functional outcomes: wound closure rates in aged mice will increase relative to controls, and tumor surveillance in models of chemically induced skin carcinogenesis will be enhanced, evidenced by lower incidence of malignant transformation. Fourth, the protective effect of STING inhibition will be lost if FasL is simultaneously overexpressed, confirming FasL as the critical downstream effector.

To test these ideas, we will generate p16‑3MR;Fsp1‑Cre‑ERT2;Sting1^fl/fl mice, induce senescence with low‑dose irradiation, and administer tamoxifen to delete STING specifically in p16^positive fibroblasts. Control groups will receive vehicle or a senolytic (navitoclax) to compare broad removal versus pathway‑specific modulation. We will quantify SASP components by ELISA (PDGF‑AA, osteopontin, FasL, IL‑6), assess immune cell populations (F4/80^+ macrophages, CD8^+ T cells) in tissue sections, and measure functional readouts: epithelial migration in excisional wound assays and tumor burden in DMBA/TPA protocols. RNA‑seq of isolated fibroblasts will verify that STING loss attenuates interferon‑stimulated genes while leaving regenerative transcripts intact.

If the data support the hypothesis, it will redefine senolytics not as blunt eliminators but as precision tools that uncouple detrimental SASP arms (FasL‑mediated immunosuppression) from beneficial ones. It also provides a mechanistic bridge between the observed vicious cycle of immune evasion and senescent accumulation, offering a clear intervention point: targeting STING to restore the negotiator’s original function without triggering the fallout of wholesale senescent cell removal.