PET Imaging of Diabetes-Induced Alterations in Metabolism and Immune Activation.

Introduction: Obesity and type 2 diabetes (T2D) influence the tumor microenvironment by altering glucose metabolism, which has been shown to decrease immune cell infiltration and activation. Positron emission tomography (PET) imaging provides a non-invasive method to detect molecular markers of immune populations in the tumor microenvironment and systemic organs. The goal of this study is to utilize advanced molecular imaging to quantify differences in innate and adaptive immune responses in diabetic obese mice systemically and within the tumor microenvironment.

Methods: 5-6-week-old female C57BL6/J mice were placed on a high-fat diet (HFD) composed of 60% kcal fat or control low-fat diet with 10% kcal fat. Animals were treated with subsequent low doses of streptozotocin to induce T2D and blood glucose was monitored. Following induction of diabetes, E0771-luc + cells were implanted into the 4th mammary fat pad and allowed to grow to a tumor volume of 100mm³. PET imaging was acquired over the course of 5 days with the following tracers: [¹⁸F]-FDG PET for glucose metabolism, [⁶⁸Ga]Ga-RP832c (CD206) PET for M2 macrophages, and [⁶⁸Ga]Ga-GZP PET for granzyme B, an indicator of effector cell activation, and [¹⁸F]-DPA-714 PET for neuroinflammation. Regions of interest were identified for the tumor, brain, kidneys, heart, muscle, brown adipose tissue (BAT), to characterize differences in important organs and tumor tissue. Metrics of standardized uptake value (SUV) were extracted from imaging data including mean, max, peak, and tumor-to-background ratios. Following the final imaging timepoint, tumors were extracted for biological characterization via flow cytometry.

Results: Diabetic obese mice have no difference in tumor glucose metabolism, but have decreased FDG uptake in the brain and BAT compared to controls. Obesity and T2D systemically affect innate and adaptive immune infiltration and activation including significantly increased RP832c and GZP in muscle, heart, brain, and BAT. Hyperglycemic tumors had trending decreases in GZP SUV_mean and increased RP832c SUV_mean. Flow cytometry shows diabetic obese tumors have a significant increase in CD206 + macrophages and no significant difference in GZB + CD8 + T cells compared to controls.

Conclusion: PET imaging reveals that obesity and T2D alter glucose metabolism and immune activation while suppressing tumor-immune activation in diabetic obese mice both within the tumor microenvironment and systemically.