Tracking Innate Immune Activation in a Mouse Model of Parkinson’s Disease Using TREM1 and TSPO PET Tracers

Abstract

Parkinson's disease (PD) is associated with aberrant innate immune responses, including microglial activation and infiltration of peripheral myeloid cells into the central nervous system (CNS). Methods to investigate innate immune activation in PD are limited and have not yet elucidated key interactions between neuroinflammation and peripheral inflammation. Translocator protein 18 kDa (TSPO)-positron emission tomography (PET) is a widely evaluated imaging approach for studying activated microglia and peripheral myeloid lineage cells in vivo, however it is yet to be fully explored in PD. Herein we investigate the utility of TSPO-PET, in addition to PET imaging of triggering receptor expressed on myeloid cells 1 (TREM1) -a novel biomarker of proinflammatory innate immune cells -for detecting innate immune responses in the 6-hydroxydopamine (6-OHDA) mouse model of dopaminergic neuron degeneration. Methods: C57/BL6J and TREM1-knockout mice were stereotaxically injected with 6-OHDA in the left striatum; control mice were saline-injected. At day 7 or 14 post-surgery, mice were administered 18 F-GE-180, 64 Cu-TREM1-mAb or 64 Cu-Isotype control-mAb and imaged by PET/CT. Ex vivo autoradiography (ARG) was performed to obtain high resolution images of tracer binding within the brain. Immunohistochemistry was conducted to verify myeloid cell activation and dopaminergic cell death and quantitative PCR and flow cytometry were completed to assess levels of target in the brain. Results: PET/CT images of both tracers showed elevated signal within the striatum of 6-OHDA-injected mice compared to those injected with saline. ARG afforded higher resolution brain images and revealed significant TSPO and TREM1 tracer binding within the ipsilateral striatum of 6-OHDA-compared to salineinjected mice at both 7-and 14-days post-toxin. Interestingly, 18 F-GE-180 enabled detection of inflammation in the brain and peripheral tissues (blood and spleen) of 6-OHDA mice, whereas 64 Cu-TREM1-mAb appeared to be more sensitive and specific for detecting neuroinflammation, in particular infiltrating myeloid cells, in these mice, as demonstrated by flow cytometry findings and higher tracer binding signal-to-background ratios in brain. Conclusion: TSPO-and TREM1-PET tracers are promising tools for investigating different cell types involved in innate immune activation in the context of dopaminergic neurodegeneration, thus warranting further investigation in other PD rodent models and human postmortem tissue to assess their clinical potential.

The associations between plasma soluble Trem1 and neurological diseases: a Mendelian randomization study

“…In an ischemic stroke model, mTrem1 activated the NF-κB signaling pathway and NLRP3 inflammasome via its interaction with SYK in microglia [ 11 ]. Besides, the protein could be adopted as a reliable PET probe to visualize the presence of early immune activation in PD and MS [ 58 , 59 ]. Therefore, neuroinflammation as the thread connecting the above diseases may explain the null results in our study.…”

PET imaging in animal models of Parkinson’s disease

“…Other studies have utilized PET to directly detect changes occurring in the periphery. Lucot et al recently reported whole-body imaging of innate immunity in 6-OHDA rats using the third-generation TSPO tracer [ 18 F]GE-180 combined with [ 64 Cu]TREM1-mAb, which targets the inflammation amplifier triggering receptor expressed on myeloid cells 1 (TREM1) [147]. In addition, the cholinergic tracers [ 11 C]donepezil and [ 18 F]fluoroethoxy-benzovesamicol (FEOBV) have been used to assess the levels of acetylcholinesterase density in the gastrointestinal tract and gastrointestinal parasympathetic innervation in PD patients [148,149].…”

Applications of radiolabeled antibodies in neuroscience and neuro‐oncology

“…In some cases, radiolabeled antibodies can enter the brain by attachment to (immune) cells that become privy to the CNS. 104 Alternatively, CNS targets can sometimes be imaged with antibodies via a disrupted or "leaky" BBB, as with some cancers 82,105 (Table 2), neuroinflammation, 104 and multiple sclerosis. 81 However, to expand the use of antibodies in diagnosis and treatment of CNS conditions, reliable engineered methods of delivering antibodies across an intact BBB are critical.…”

“…Despite this, there are some methods by which antibodies inadvertently gain access to the CNS, and a number of reports exist of PET imaging of CNS targets using antibody platforms. In some cases, radiolabeled antibodies can enter the brain by attachment to (immune) cells that become privy to the CNS 104 . Alternatively, CNS targets can sometimes be imaged with antibodies via a disrupted or “leaky” BBB, as with some cancers 82,105 (Table 2), neuroinflammation, 104 and multiple sclerosis 81 .…”