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

Abstract

Various cellular sources hamper interpretation of positron-emission-tomography (PET) biomarkers in the tumor microenvironment (TME). We developed immunomagnetic cell sorting after in vivo radiotracer injection (scRadiotracing) in combination with 3D-histology via tissue clearing to dissect the cellular allocation of PET signals in the TME. In SB28 glioblastoma mice, translocator protein (TSPO) radiotracer uptake per tumor cell was higher compared to tumor-associated microglia/macrophages (TAMs). Cellular radiotracer uptake was validated by proteomics and confirmed for in vitro samples of patients with glioblastoma. Regional agreement between PET signals and single cell tracer uptake predicted the individual cell distribution in 3D-histology. 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 complex PET signal sources and will serve to validate novel TAM-specific radioligands.

Remote Neuroinflammation in Newly Diagnosed Glioblastoma Correlates with Unfavorable Clinical Outcome

“…We note that human TSPO-PET signal changes in tissue distant of the tumor could still be influenced by infiltrating tumor cells, which even exceeded the cellular uptake of the same TSPO tracer compared to myeloid cells (31). To minimize this potential impact of tumor cell invasion on TSPO expression, we focused on the contralateral hemisphere and used only tumors without bilateral manifestation.…”

Neuronal and oligodendroglial but not astroglial tau translates to in vivo tau-PET signals in primary tauopathies

“…reactive astrocytes. As a major novelty, we applied cell sorting after in vivo tracer injection (22)(23)(24) to a tau radiotracer. Here we found that i) tau tracer binding was tremendously higher in neurons when compared to astrocytes, ii) neurons of PS19 mice had nearly 2-fold higher tracer uptake when compared to neurons of wild-type mice, and iii) single cell tracer uptake determined the in vivo tau-PET signal.…”

“…scRadiotracing data in PS19 mice were in line with immunohistochemistry which also determined neurons as the major tau-positive cell type. Future work could directly correlate cellular radiotracer binding with the cellular amount of tau protein (22).…”

“…Five PS19 and five WT mice underwent scRadiotracing (22)(23)(24) immediately after the tau-PET scan. Adult Brain Dissociation Kit (mouse and rat) (Miltenyi Biotec, 130-107-677) was used for brain dissociation according to the manufacturer's instructions.…”

“…The cell pellets were collected and resuspended with 1 ml cold red blood cell removal solution followed by 10 minutes incubation. Cell pellets were collected for astrocyte and subsequent neuron isolation via magnetic activated cell sorting (MACS) (22)(23)(24).…”