Transport vehicle–mediated TREM2 PET maps myeloid cells in the glioblastoma microenvironment

Glioblastomas exhibit low immunogenicity and a highly immunosuppressive tumor microenvironment (TME) mediated by both tumor cells and tumor associated microglia/macrophages (TAMs). However, biomarkers that allow for specific detection, monitoring and therapy response assessment of the TAM phenotype in glioblastoma are still lacking. To address this gap, we characterized the dynamics of TAM clusters in the TME of experimental SB28 glioblastoma using single-cell RNA sequencing, immunofluorescence, and molecular imaging and identified Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) as a dynamic TAM-specific target. Consequently, we investigated a novel ⁶⁴ Cu-labeled TREM2 radiotracer with a blood-brain barrier transport vehicle for translational mapping of TAMs in the TME of glioblastoma. We found strong and highly myeloid cell-specific PET signals that followed the trajectory of TREM2 gene expression from early- towards late-stage tumors. Transient myeloid cell depletion via CSF1R inhibition showed a reduction of TREM2 PET signals compared to placebo and prolonged survival in SB28 glioblastoma mice, while TREM2 stimulation led to increased TREM2 PET signals. Translation into a pilot cohort of patients with glioblastoma revealed strong TREM2 PET signals predominantly at lesion edges and high heterogeneity across individuals, matching with immunohistochemistry. In summary, TREM2 PET imaging has strong potential as an immune cell-specific biomarker for personalized assessment of the TAM phenotype in the TME of patients with glioblastoma.