Increased serum peripheral C-reactive protein is associated with reduced small-molecule brain perfusion in healthy volunteers and subjects with major depressive disorder

The relationship between peripheral and central immunity and how these ultimately may cause depressed behaviour has been the focus of a number of imaging studies conducted with Positron Emission Tomography (PET). These studies aimed at testing the immune-mediated model of depression that proposes a direct effect of peripheral cytokines and immune cells on the brain to elicit a neuroinflammatory response via a leaky blood-brain barrier and ultimately depressive behaviour. However, studies conducted so far using PET radioligands targeting the neuroinflammatory marker 18 kDa translocator protein (TSPO) in patient cohorts with depression have demonstrated mild inflammatory brain status but no correlation between central and peripheral immunity. To gain a better insight into the relationship between heightened peripheral immunity and neuroinflammation, we estimated blood-to-brain and blood-to-CSF perfusion rates for two TSPO radiotracers collected in two separate studies, one large cross-sectional study of neuroinflammation in normal and depressed cohorts and a second study where peripheral inflammation in healthy controls was induced via subcutaneous injection of interferon (IFN)-. In both studies we observed a consistent negative association between peripheral inflammation, measured with c-reactive protein P (CRP), and radiotracer perfusion into and from the brain parenchyma and CSF. Importantly, there was no association of this effect with the marker of BBB leakage S100 {beta}, that was unchanged, but there was an association between the reduction of tracer perfusion in volunteers injected with interferon (IFN)- and VEGF, a potent vascular permeability factor. These results support a different model of peripheral-to-central immunity interaction whereas peripheral inflammation causes a stiffening of the healthy BBB with consequent reduction of small molecule trafficking to and from the blood into the brain and CSF. This effect, on the long term, is likely to disrupt brain homeostasis and induce depressive behavioural symptoms. Moreover, given the molecular similarity between the TSPO ligands and antidepressant, this phenomenon may underlie treatment resistance in depressive cohorts with heightened peripheral status.