Compartmentalized meningeal inflammation is thought to represent one of the key players in the pathogenesis of cortical demyelination in multiple sclerosis. Positron emission tomography targeting the 18 kDa mitochondrial Translocator Protein (TSPO) is a molecular-specific approach to quantify immune cell-mediated density in the cortico-meningeal tissue compartment in vivo. The aim of this study was to characterize cortical and meningeal TSPO expression in a heterogeneous cohort of multiple sclerosis cases using in vivo simultaneous MR-PET with ¹¹C-PBR28, a second-generation TSPO radioligand, and ex vivo immunohistochemistry. Forty-nine multiple sclerosis patients (21 with secondary progressive and 28 with relapsing-remitting multiple sclerosis) with mixed or high affinity binding for ¹¹C-PBR28 underwent 90-min ¹¹C-PBR28 simultaneous MR-PET. Tracer binding was measured using 60-90 min normalized standardized uptake value ratio values sampled at mid-cortical depth and ∼3 mm above the pial surface. Data in multiple sclerosis patients were compared to 21 age-matched healthy controls. To characterize the nature of ¹¹C-PBR28 PET uptake, the meningeal and cortical lesion cellular expression of TSPO was further described in post-mortem brain tissue from 20 cases with secondary progressive multiple sclerosis and five age-matched healthy donors. Relative to healthy controls, patients with multiple sclerosis exhibited abnormally increased TSPO signal in the cortex and meningeal tissue, diffusively in progressive disease and more localized in relapsing-remitting multiple sclerosis. In multiple sclerosis, increased meningeal TSPO levels were associated with increased Expanded Disability Status Scale scores (p = 0.007, by linear regression). Immunohistochemistry, validated using in-situ sequencing analysis, revealed increased TSPO expression in the meninges and adjacent subpial cortical lesions of post-mortem secondary progressive multiple sclerosis cases relative to control tissue. In these cases, increased TSPO expression was related to meningeal inflammation. Translocator Protein immunostaining was detected on meningeal major histocompatibility complex (MHC)-class II + macrophages and cortical activated MHC-class II + transmembrane protein (TMEM)119+ microglia. In vivo arterial blood data and neuropathology showed that endothelial binding did not significantly account for increased TSPO cortico-meningeal expression in multiple sclerosis. Our findings support the use of TSPO-PET in multiple sclerosis for imaging in vivo inflammation in the cortico-meningeal brain tissue compartment and provide in vivo evidence implicating meningeal inflammation in the pathogenesis of the disease.

Background: Activated microglia, which can be detected in vivo by ¹¹C-PBR28 positron emission tomography (PET), represent a main component of MS pathology in the brain. Their role in the cerebellum is still unexplored, although cerebellar involvement in MS is frequent and accounts for disability progression. Objectives: We aimed at characterizing cerebellar neuroinflammation in MS patients compared to healthy subjects by combining ¹¹C-PBR28 MRI-Positron Emission Tomography (MR-PET) with 7 Tesla (T) MRI and assessing its relationship with brain neuroinflammation and clinical outcome measures. Methods: Twenty-eight MS patients and 16 healthy controls underwent ¹¹C-PBR28 MR-PET to measure microglia activation in normal appearing cerebellum and lesions segmented from 7 T scans. Patients were evaluated using the Expanded Disability Status Scale and Symbol Digit Modalities Test. ¹¹C-PBR28 binding was assessed in regions of interest using 60–90 minutes standardized uptake values normalized by a pseudo-reference region in the brain normal appearing white matter. Multilinear regression was used to compare tracer uptake in MS and healthy controls and assess correlations with clinical scores. Results: In all cerebellar regions examined, MS patients showed abnormally increased tracer uptake, which correlated with cognitive and neurological disability. Conclusion: Neuroinflammation is widespread in the cerebellum of patients with MS and related to neurological disability and cognitive impairment.

Cortical demyelination occurs early in multiple sclerosis (MS) and relates to disease outcome. The brain cortex has endogenous propensity for remyelination as proven from histopathology study. In this study, we aimed at characterizing cortical microstructural abnormalities related to myelin content by applying a novel quantitative MRI technique in early MS. A combined myelin estimation (CME) cortical map was obtained from quantitative 7-Tesla (7T) T2* and T₁ acquisitions in 25 patients with early MS and 19 healthy volunteers. Cortical lesions in MS patients were classified based on their myelin content by comparison with CME values in healthy controls as demyelinated, partially demyelinated, or non-demyelinated. At follow-up, we registered changes in cortical lesions as increased, decreased, or stable CME. Vertex-wise analysis compared cortical CME in the normal-appearing cortex in 25 MS patients vs. 19 healthy controls at baseline and investigated longitudinal changes at 1 year in 10 MS patients. Measurements from the neurite orientation dispersion and density imaging (NODDI) diffusion model were obtained to account for cortical neurite/dendrite loss at baseline and follow-up. Finally, CME maps were correlated with clinical metrics. CME was overall low in cortical lesions (p = 0.03) and several normal-appearing cortical areas (p < 0.05) in the absence of NODDI abnormalities. Individual cortical lesion analysis revealed, however, heterogeneous CME patterns from extensive to partial or absent demyelination. At follow-up, CME overall decreased in cortical lesions and non-lesioned cortex, with few areas showing an increase (p < 0.05). Cortical CME maps correlated with processing speed in several areas across the cortex. In conclusion, CME allows detection of cortical microstructural changes related to coexisting demyelination and remyelination since the early phases of MS, and shows to be more sensitive than NODDI and relates to cognitive performance.

Background: Paramagnetic rim white matter (WM) lesions (PRL) are thought to be a main driver of non-relapsing multiple sclerosis (MS) progression. It is unknown whether cerebrospinal fluid (CSF)-soluble factors diffusing from the ventricles contribute to PRL formation. Objective: To investigate the distribution of PRL and non-rim brain WM lesions as a function of distance from ventricular CSF, their relationship with cortical lesions, the contribution of lesion phenotype, and localization to neurological disability. Methods: Lesion count and volume of PRL, non-rim WM, leukocortical lesion (LCL), and subpial/intracortical lesions were obtained at 7-T. The brain WM was divided into 1-mm-thick concentric rings radiating from the ventricles to extract PRL and non-rim WM lesion volume from each ring. Results: In total, 61 MS patients with ⩾1 PRL were included in the study. Both PRL and non-rim WM lesion volumes were the highest in the periventricular WM and declined with increasing distance from ventricles. A CSF distance-independent association was found between non-rim WM lesions, PRL, and LCL, but not subpial/intracortical lesions. Periventricular non-rim WM lesion volume was the strongest predictor of neurological disability. Conclusions: Non-rim and PRL share a gradient of distribution from the ventricles toward the cortex, suggesting that CSF proximity equally impacts the prevalence of both lesion phenotypes.

Background and Purpose: Corpus callosum (CC) atrophy is a strong predictor of multiple sclerosis (MS) disability but the contributing pathological mechanisms remain uncertain. We aimed to apply advanced MRI to explore what drives the often nonuniform callosal atrophy. Methods: Prospective brain 7 Tesla and 3 Tesla Human Connectom Scanner MRI were performed in 92 MS patients. White matter, leukocortical, and intracortical lesions were manually segmented. FreeSurfer was used to segment the CC and topographically classify lesions per lobe or as deep white matter lesions. Regression models were calculated to predict focal CC atrophy. Results: The frontal and parietal lobes contained the majority (≥80%) of all lesion classifications in both relapsing‐remitting and secondary progressive MS subtypes. The anterior subsection of the CC had the smallest proportional volume difference between subtypes (11%). Deep, temporal, and occipital white matter lesions, and occipital intracortical lesions were the strongest predictors of middle‐posterior callosal atrophy (adjusted R ² = .54‐.39, P < .01). Conclusions: Both white matter and cortical lesions contribute to regional corpus callosal atrophy. The lobe‐specific lesion topology does not fully explain the inhomogeneous CC atrophy.

The skull bone marrow contributes to brain immune homeostasis via recently discovered skull-meningeal channels, enabling the bidirectional trafficking of immune cells between skull bone and underlying dura mater. In multiple sclerosis, autoreactive T cells migrate to the bone marrow and shift its hematopoietic output toward myeloid differentiation, contributing to disease progression. However, the role of the skull bone marrow in multiple sclerosis pathophysiology, and its relationship to brain damage and clinical disability remain largely unexplored. We utilized simultaneous MR-PET with the second-generation radioligand ¹¹C-PBR28 to characterize within the skull bone of multiple sclerosis patients the in vivo expression of the translocator protein (TSPO), an 18-kilodalton mitochondrial membrane protein largely expressed by microglia, astrocytes, and peripheral myeloid cells. Sixty-five multiple sclerosis subjects (46 relapsing-remitting, 19 secondary progressive) and 26 healthy controls underwent ¹¹C-PBR28 MR-PET to acquire 60–90-minute post-injection standardized uptake value maps and anatomical scans for brain volumetrics. Voxel-wise analyses of skull TSPO signal were conducted to assess group differences and associations with demographic, clinical, and brain volumetrics. Voxel-wise analyses revealed a divergent association between age and skull TSPO signal, with a negative correlation in healthy controls in bilateral frontal and right parietal regions (r=-0.67, p<0.001), and a positive correlation in multiple sclerosis patients in bilateral parietal and occipital skull bone regions (r=0.44, p<0.001). Compared to both healthy controls and relapsing-remitting multiple sclerosis, patients with secondary progressive multiple sclerosis showed widespread elevation in skull TSPO signal, in frontal, parietal, temporal, occipital, and skull base regions. No significant differences were detected between relapsing-remitting multiple sclerosis and healthy controls. Elevated skull TSPO signal was also observed in patients with more severe neurological disability irrespective of clinical phenotype. Widespread skull TSPO expression was observed to be positively correlated with EDSS scores (ρ=0.49, p<0.001) while a negative association was observed with white matter volume (r=-0.45, p<0.001) and SDMT z-scores (r=-0.48, p0.550). We provide in vivo evidence of skull TSPO overexpression in multiple sclerosis, observed in progressive disease and associated with clinical disability and structural brain damage. Overall, these findings suggest a role for the skull bone marrow in disease-related processes and highlight its potential as a novel radiological marker and therapeutic target.