Studies from our lab have reported extensive presence of myelin particles in the leptomeninges of multiple sclerosis (MS) cases, hinting at a potential role of this debris in instigating MS-related immunological responses.
Here, to gain further insight into this possibility we closely looked at meningeal myelin fragments for the presence of posttranslational modifications (citrullination) which render myelin debris more immunogenic. We also tested whether citrullinated myelin is associated with antigen presenting cells relevant to induction of autoimmunity in MS, i.e B cells.
We performed immunohistochemistry experiments on post-mortem meningeal material coming from 8 SPMS patients (age: 61.29±4.66, disease duration: 30±4.40 years) and 7 non-neurological controls (age: 55.88±5.32 years). In addition, using confocal colocalization analysis tools, we investigated the presence of citrullinated myelin basic protein (MBP) in CD19+ B lymphocytes in MS meninges.
Compared with tissue from non-neurological controls, meninges of MS patients presented with a larger surface proportion of citrullinated proteins (p=0.03). In MS material, this effect was accompanied by the presence of meningeal CD19+ B-cells. Almost all the analyzed CD19+ B-cells showed high citrulline content (n=144, 99% citrulline positive cells; mean % citrulline inside B-cells= 50.81±3.86 %) and 95.13% of these cells were also positive for myelin basic protein (MBP; median percent MBP which colocalizes with CD19= 42.05%). Interestingly, Mander´s coefficient analysis to colocalize MBP and citrulline inside B-cells underlined that almost half of the lymphocytic cellular surface was occupied by citrullinated MBP (40.06±3.23 %).
Our results are indicative of a B cell-mediated uptake mechanism that operates in MS meninges to clear up highly immunogenic myelin fragments. Although the question remains on whether myelin citrullination happens in the absence of a primary immunological response, studies in cuprizone mouse models of MS have highlighted that MBP citrullination already occurs before demyelination via intramyelinic Ca2+-dependent activation of protein arginine deiminases. This evidence combined with previous studies showing abundant citrulline content in both MS normal appearing white matter and white matter lesion support the concept that a primary cytodegenerative process instigates demyelination, which elicits secondary immune reactions in MS.