18F-FDG PET Visualizes Systemic STING Agonist-Induced Lymphocyte Activation in Preclinical Models

Abstract

Stimulator of interferon genes (STING) is a mediator of immune recognition of cytosolic DNA, which plays important roles in cancer, cytotoxic therapies, and infections with certain pathogens. Although pharmacologic STING activation stimulates potent antitumor immune responses in animal models, clinically applicable pharmacodynamic biomarkers that inform of 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 PET imaging. Methods: C57BL/6 mice were treated with systemic STING agonists and imaged with ¹⁸F-FDG PET after 24 h. Splenocytes were harvested 6 h 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 γ-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 with 4.55 (range, 3.35-6.20) 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 that 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 may provide a widely applicable approach toward measuring the pharmacodynamic effects of systemic STING agonists at a whole-body level and guiding their clinical development.

Keywords: 18F-FDG PET; STING agonists; immune activation; immunometabolism; lymphocytes.