Peripheral inflammation is associated with reduced influx of TSPO PET tracers into the brain: insights from a non-invasive mapping methodology

<underline>Introduction</underline> Neuroinflammation is a hallmark of various brain disorders, including neuropsychiatric conditions like major depressive disorder and schizophrenia. Increasing evidence suggests that both peripheral and central inflammation play a critical role in central nervous system dysfunction associated with such disorders. There is evidence, particularly in preclinical models, of a mediating role of the blood-brain barrier in the relationship between peripheral and central immunity. However, this relationship is poorly studied in human cohorts and, importantly, little is known about its effect on the quantification of radioligands used to monitor neuroinflammation in-vivo. <underline>Methods</underline> A recently developed non-invasive method for estimating the blood-to-brain influx rate constant (K₁) (Maccioni et al., 2024) was applied to TSPO PET imaging, a proposed marker of neuroinflammation. In total, 358 dynamic TSPO PET scans from three different radiotracers ([¹¹C]-PK11195, [¹⁸F]-DPA714, and [¹¹C]-PBR28) were reanalyzed using data from healthy controls, as well as patients with depression and schizophrenia. The relationship between brain-wide K₁ estimates, peripheral inflammatory marker C-reactive protein (CRP), sex, anthropometric measures, and disease states was systematically evaluated. <underline>Results</underline> A brain-wide negative correlation between peripheral inflammation and K₁ was observed (range: [-0.33, -0.25]). Notably, this association was not influenced by diagnostic labels. Additionally, significant effects of body weight (or body mass index) and sex on K₁ were identified. The findings were consistent at both regional and voxel levels, as well as across the three radiotracers. Imaging transcriptomics analyses revealed that the effect of CRP on K₁ was associated with the expression of genes involved in transport and homeostasis. <underline>Discussion</underline> The study confirms that increased peripheral inflammation is associated with reduced blood-to-brain transport of TSPO tracers, suggesting a role for blood-brain barrier permeability in the observed effect. This finding supports the emerging model of peripheral-to-central immune interactions via brain barriers by Turkheimer and colleagues (2023). By considering variables such as body mass, sex, and peripheral inflammatory status, this research provides crucial insights into the use of TSPO PET in psychiatry and the interpretability of its findings.