¹⁸F-FDG PET visualizes systemic STING agonist-induced lymphocyte activation in preclinical models

Abstract

Rationale: Stimulator of Interferon Genes (STING) is an essential mediator of innate immune recognition of cytosolic DNA, which plays important roles in cancer, cytotoxic therapies, and infections with certain pathogens. While pharmacological STING activation stimulates potent anti-tumor immune responses in animal models, clinically applicable pharmacodynamic biomarkers that inform on the magnitude, duration, and location of immune activation elicited by systemic STING agonists are yet to be described. We investigated whether systemic STING activation induces metabolic alterations in immune cells that can be visualized by Positron Emission Tomography (PET) imaging. Methods: C57BL/6 mice were treated with systemic STING agonists and imaged with [¹⁸F]fluorodeoxyglucose ([¹⁸F]FDG) PET 24 hrs later. Splenocytes were harvested 6 hrs after STING agonist administration and analyzed by single-cell RNA sequencing and flow cytometry. [¹⁸F]FDG uptake in total splenocytes and immunomagnetically-enriched splenic B and T lymphocytes from STING agonist-treated mice was measured by gamma counting. In mice bearing prostate or pancreas cancer tumors, the effects of STING agonist treatment on [¹⁸F]FDG uptake, T lymphocyte activation marker levels, and tumor growth were evaluated. Results: Systemic delivery of structurally distinct STING agonists in mice significantly increased [¹⁸F]FDG uptake in the spleen. The average spleen SUV_max in control mice was 1.90 (range 1.56–2.34) compared to 4.55 (range 3.35–6.2) in STING agonist-treated mice (P < 0.0001). Single-cell transcriptional and flow cytometry analyses of immune cells from systemic STING agonist-treated mice revealed enrichment of a glycolytic transcriptional signature in both T and B lymphocytes which correlated with the induction of immune cell activation markers. In tumor-bearing mice, STING agonist administration significantly delayed tumor growth and increased [¹⁸F]FDG uptake in secondary lymphoid organs. Conclusion: These findings reveal hitherto unknown functional links between STING signaling and immunometabolism and suggest that [¹⁸F]FDG PET could provide a widely applicable approach to measure the pharmacodynamic effects of systemic STING agonists at a whole-body level and guide their clinical development.