Does seasonal variation affect the neuroimmune system? A retrospective [11C]PBR28 PET study in healthy individuals

• Published in: Neuroscience letters Vol. 828; p. 137766
• Main Authors: Ibrahim, Waleed, An, Jeonghyun, Yang, Yanghong, Cosgrove, Kelly P., Matuskey, David
• Format: Journal Article
• Language: English
• Published: Ireland Elsevier B.V 01.04.2024
• Subjects: [11C]PBR28; Brain - metabolism; Carrier Proteins - metabolism; Humans; Neuroinflammation; PET; Positron-Emission Tomography - methods; Receptors, GABA - metabolism; Retrospective Studies; Season; Seasons; Translocator-protein; TSPO; TSPO; Neuroinflammation; [11C]PBR28; Translocator-protein; Season; PET
• ISSN: 0304-3940; 1872-7972; 1872-7972
• DOI: 10.1016/j.neulet.2024.137766

•The neuroimmune system has an established role in various neuropsychiatric disorders. Our study addresses whether neuro-immunity might be affected by seasonal changes. It also adds to the ongoing discourse regarding the efficacy of the marker TSPO and seeks to understand its limitations. The neuroimmune system performs a wide range of functions in the brain and the central nervous system. The microglial translocator protein (TSPO) has an established role as a cell marker in identification of the neuroimmune system. Previously, human studies have shown TSPO differences in neuropsychiatric disorders. Seasonal variability has also been demonstrated in multiple systems of healthy individuals. Therefore, in this study, we attempt to understand whether seasonal changes affect brain TSPO levels using [11C]PBR28 positron emission tomography (PET) imaging. 46 healthy subjects (mean age ± SD = 32.5 ± 10); sex (M/F) = 32/14)) underwent PET imaging with [11C]PBR28 in a retrospectively conducted analysis. All PET scans were performed on the HRRT scanner. Volume of distribution (VT) values were generated for cortical and subcortical regions and the cerebellum. Spring/summer months were defined as March to August while fall/winter months were defined as September to February and were compared through 2-tailed t-tests (SciPy library v.1.10.1 and Pinguoin library on Python v.3.8.8). Average daylight hours and temperature in New Haven, CT were obtained online (www.wunderground.com) and compared to VT with Spearman’s correlations. There were no significant differences observed between the TSPO levels of spring/summer and fall/winter months in the brain (t = 0.52, p = 0.61). Additional analysis on all individual brain regions also indicated non-significance. Likewise, no significant correlations were found between TSPO levels in the whole brain and brain regions against daylight hours (ρ= 0.05, p = 0.74), temperature (ρ = 0.04, p = 0.81), or month (ρ = 0.08, p = 0.60). Controlling TSPO gene polymorphisms and other variables had no significant effect on the outcome. To the best of our knowledge, this is the first human study to investigate seasonal changes in TSPO expression. Our results can be interpreted as the lack of seasonal variability in the neuroimmune system, but important limitations include high interindividual variability, test–retest variability, specificity of the tracer, and a limited sample size. Limitations notwithstanding, our results conclude that TSPO levels in the brain are not impacted by light and temperature changes in different seasons.