Abstract
The interaction between immune cells in the periphery and brain in humans is poorly understood, partly due to the lack of in vivo studies. Here, we examined 32 healthy individuals using positron emission tomography (PET) and [11C]PBR28, a radioligand for the 18-kDa translocator protein (TSPO) which is expressed in immune cells both in brain and blood. In 26 individuals, two measurements were performed. In a subgroup of 19 individuals, of which 12 had repeat examinations, leukocyte numbers in blood was measured on each day of PET measurements. We assessed TSPO binding expressed as total distribution volume of [11C]PBR28 in brain and in blood cells. TSPO binding in brain was strongly and positively correlated to binding in blood cells at baseline (r = .85, p = 2.1 × 10−9, corrected for TSPO genotype). A correlation was also observed when analyzing change in TSPO levels in brain and blood between two PET examinations (r = .60, p = .002). Finally, there was a significant correlation between change of leukocyte numbers and change in brain TSPO binding, and a trend-level correlation to TSPO change in blood cells (r = .63, p = .038; r = .6, p = .052). These findings indicate a functional association between immunological cells in blood and brain at physiological conditions, such as interchange of peripherally derived cells or a common regulatory mechanism.
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