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

The 18 kDa translocator protein (TSPO) emerges as an important PET biomarker to assess the tumor microenvironment (TME) in glioblastoma. However, various cellular sources hamper interpretation and biological understanding of TSPO and other immune biomarkers in the TME. Thus, we established a novel method, combining immunomagnetic cell sorting after radiotracer injection (scRadiotracing) with 3D histology via light sheet microscopy and proteomics to dissect cellular allocation of TSPO enrichment in glioblastoma. Single tumor cells of implanted SB28 glioblastoma mice indicated 1.37-fold higher TSPO tracer uptake and 1.46-fold higher TSPO protein expression levels when compared to tumor associated microglia/macrophages (TAMs). Using proteomics, we compared the proteome of tumor associated microglia/macrophages (TAMs), Tumor tissue (TT) and control microglia from WT mice without glioblastoma. This analysis identified TAM specific targets for PET radioligand development with additional potential to monitor diverse TAM subpopulations in vivo. In summary, our data indicate that tumor cells need to be acknowledged as the main contributor to TSPO as a biomarker in glioblastoma. Combining cellular tracer uptake measures with 3D histology facilitates precise allocation of complex PET signal sources and will serve to validate novel TAM specific radioligands.