monitoring the immune cell infiltration in the LPS-induced neuroinflammation region with simultaneous small animal PET/MR imaging using a TSPO-targeting probe

Objectives: Recently, activated immune cell infiltration has been reported to involve in neuronal damages. Trafficking adoptive-transferred immune cells may provide valuable information to understand of various neuroinflammatory diseases. Here, we asked whether acute brain regional inflammation have any potential to recruit adoptively transferred splenocytes using simultaneous small animal PET/MR.

Methods: Lipopolysaccharide (LPS) was used to induce regional inflammation in the right striatum of a mouse brain (anteroposterior, +1.8; mediolateral, +2.0; dorsoventral,-3.0) using a Hamilton syringe at a rate of 0.25 ul/ min using a robotic stereotacxic device (Stoelting, IL, USA). Saline was used as a control treatment. Splenocytes (2 x 10⁷) were isolated from luciferase expressing transgenic mice (B6.Luc^Tg), and adoptively transferred to recipient B6 (C57BL/6) mouse by intravenously injection for visualizing immune cell localization with bioluminescence imaging. To evaluate the inflammation, simultaneous PET/MR scans were acquired using simultaneous small animal PET insert combined with 1T MRI (SimPET, Aspect imaging, Israel). ¹⁸F-CB251 probewas used to detect the 18-kDa translocator protein (TSPO, a marker for activated immune cells in the inflammatory region) activity at a dose ranging from 9.25 to 11.1 MBq per mouse. In vitro uptake assay was also performed for confirming the correlation between TSPO expression level and uptake of ¹⁸F-CB251 probe.

Results: Uptake experiments of ¹⁸F-CB251 probe were measured in the cells with differential TSPO expression such as TSPO-overexpressed 293FT cells and activated-macrophage Raw 264.7 cells. Higher uptake of ¹⁸F-CB251 was found in cells with higher TSPO expression. PET signals from ¹⁸F-CB251 showed 2.58 times (n=5) higher uptake in right striatum (LPS injection site) than in left striatum. MR scan, which is taken simultaneously, also showed slightly increased T2-weigthed signals in the right striatum of LPS-induced inflammatory region. On the other hand, saline injected mouse showed no significant difference (1.14 times) between the right striatum and left striatum. In addition, luciferase-expressing splenocytes were localized at the LPS-treated site in right striatum of mouse brain. Though BLI signals in LPS-treated region were 5.02 times (n=5) higher than that of saline-treated region after skin removal, ¹⁸F-CB251 PET signals generate more quantitative and non-invasive images for whole body scan.

Conclusion: We successfully visualized adoptively-transferred immune cell recruitment in the inflammatory region of brain using three different imaging modalities. Simultaneous scanning from simultaneous small animal PET/MR provides more accurate spatiotemporal information of immune cell localization in the neuro-inflammation site.