A pair of congenic mice for imaging of transplants by positron emission tomography using anti-transferrin receptor nanobodies.

Two anti-transferrin receptor (TfR) nanobodies, VH123 specific for mouse TfR and VH188 specific for human TfR, were used to track transplants non-invasively by PET/CT in mouse models, without the need for genetic modification of the transferred cells. We provide a comparison of the specificity and kinetics of the PET signals acquired when using nanobodies radiolabeled with Zr, Cu, and F, and find that the chelation of the Zr and Cu radioisotopes to anti-TfR nanobodies results in radioisotope release upon endocytosis of the radiolabeled nanobodies. We used a knock-in mouse that expresses a TfR with a human ectodomain (Tfrc) as a source of bone marrow for transplants into C57BL/6 recipients and show that VH188 detects such transplants by PET/CT. Conversely, C57BL/6 bone marrow and B16.F10 melanoma cell line transplanted into Tfrc recipients can be imaged with VH123. In C57BL/6 mice impregnated by Tfrc males, we saw an intense VH188 signal in the placenta, showing that TfR-specific VHs accumulate at the placental barrier but do not enter the fetal tissue. We were unable to observe accumulation of the anti-TfR radiotracers in the central nervous system (CNS) by PET/CT but showed evidence of CNS accumulation by radiospectrometry. The model presented here can be used to track many transplanted cell types by PET/CT, provided cells express TfR, as is typically the case for proliferating cells such as tumor lines.