Deciphering sources of PET signals in the tumor microenvironment of glioblastoma at cellular resolution.

Various cellular sources hamper interpretation of positron emission tomography (PET) biomarkers in the tumor microenvironment (TME). We developed an approach of immunomagnetic cell sorting after in vivo radiotracer injection (scRadiotracing) with three-dimensional (3D) histology to dissect the cellular allocation of PET signals in the TME. In mice with implanted glioblastoma, translocator protein (TSPO) radiotracer uptake per tumor cell was higher compared to tumor-associated microglia/macrophages (TAMs), validated by protein levels. Translation of in vitro scRadiotracing to patients with glioma immediately after tumor resection confirmed higher single-cell TSPO tracer uptake of tumor cells compared to immune cells. Across species, cellular radiotracer uptake explained the heterogeneity of individual TSPO-PET signals. In consideration of cellular tracer uptake and cell type abundance, tumor cells were the main contributor to TSPO enrichment in glioblastoma; however, proteomics identified potential PET targets highly specific for TAMs. Combining cellular tracer uptake measures with 3D histology facilitates precise allocation of PET signals and serves to validate emerging novel TAM-specific radioligands.

Single-Cell Radiotracer Allocation via Immunomagnetic Sorting to Disentangle PET Signals at Cellular Resolution.

With great interest, our independent groups of scientists located in Korea and Germany recognized the use of a very similar methodologic approach to quantify the uptake of radioactive glucose (18F-FDG) at the cellular level. The focus of our investigations was to disentangle microglial 18F-FDG uptake. To do so, CD11b immunomagnetic cell sorting was applied to isolate microglia cells after in vivo 18F-FDG injection, to allow simple quantification via a γ-counter. Importantly, this technique reveals a snapshot of cellular glucose uptake in living mice at the time of injection since 18F-FDG is trapped by hexokinase phosphorylation without a further opportunity to be metabolized. Both studies indicated high 18F-FDG uptake of single CD11b-positive microglia cells and a significant increase in microglial 18F-FDG uptake when this cell type is activated in the presence of amyloid pathology. Furthermore, another study noticed that immunomagnetic cell sorting after tracer injection facilitated determination of high 18F-FDG uptake in myeloid cells in a range of tumor models. Here, we aim to discuss the rationale for single-cell radiotracer allocation via immunomagnetic cell sorting (scRadiotracing) by providing examples of promising applications of this innovative technology in neuroscience, oncology, and radiochemistry.