Background and objectives: Recent imaging studies have suggested a possible involvement of choroid plexus (CP) in multiple sclerosis (MS). Here, we investigated whether CP changes are already detectable at the earliest stage of MS, preceding symptom onset. Methods: The study is a retrospective analysis of 27 pre-symptomatic MS subjects, 97 clinically definite MS (CDMS) patients and 53 healthy controls (HC) who underwent a crosssectional 3T-MRI acquisition; of which, 22 MS, 19 HC and one pre-symptomatic MS (evaluated 8 months before conversion to CDMS) also underwent translocator protein (TSPO) 18 F-DPA-714 PET and were included in the analysis. CPs were manually segmented on 3DT1-weighted images for volumetric analysis. CP 18 F-DPA-714 uptake, reflecting inflammation, was calculated as the average standardized uptake value (SUV). Multivariable regressions adjusted for age, sex, ventricular and brain volume were fitted to test CP volume differences between pre-symptomatic subjects and MS or HC. For the pre-symptomatic case who also had 18 F-DPA-714-PET, CP SUV differences with MS and HC were assessed through Crawford-Howell tests. To provide further insight into the interpretation of 18 F-DPA-714-PET uptake at the CP level, a post-mortem analysis of CPs in MS versus HC was performed to characterize the cellular localization of TSPO expression. Results: Compared with HC, pre-symptomatic MS subjects had 32% larger CPs (ß=0.38, p=0.001), which were not dissimilar to MS CPs (p=0.69). Moreover, in the baseline scan of the pre-symptomatic case who later on developed MS, TSPO PET showed 33% greater CP inflammation versus HC (p=0.04), while no differences in 18 F-DPA-714 uptake were found in parenchymal regions versus controls. CP post-mortem analysis identified a population of CD163+ mononuclear phagocytes expressing TSPO in MS, possibly contributing to the increased in 18 F-DPA-714 uptake. Discussion: We identified an imaging signature in CPs at the pre-symptomatic MS stage using MRI; additionally, we found an increased CP inflammation with PET in a single presymptomatic patient. These findings suggest a role of CP imaging as an early biomarker and argue for the involvement of the blood-CSF barrier dysfunction in disease development.