Blood‐brain barrier (BBB) breakdown, demonstrable in vivo by enhanced MRI is characteristic of new and expanding inflammatory lesions in relapsing‐remitting and chronic progressive multiple sclerosis (MS). Subtle leakage may also occur in primary progressive MS. However, the anatomical route(s) of BBB leakage have not been demonstrated. We investigated the possible involvement of interendothelial tight junctions (TJ) by examining the expression of TJ proteins (occludin and ZO‐1) in blood vessels in active MS lesions from 8 cases of MS and in normal‐appearing white (NAWM) matter from 6 cases. Blood vessels (10–50 per frozen section) were scanned using confocal laser scanning microscopy to acquire datasets for analysis. TJ abnormalities manifested as beading, interruption, absence or diffuse cytoplasmic localization of fluorescence, or separation of junctions (putative opening) were frequent (affecting 40% of vessels) in oil‐red‐O‐positive active plaques but less frequent in NAWM (15%), and in normal (<2%) and neurological controls (6%). Putatively “open” junctions were seen in vessels in active lesions and in microscopically inflamed vessels in NAWM. Dual fluorescence revealed abnormal TJs in vessels with pre‐mortem serum protein leakage. Abnormal or open TJs, associated with inflammation may contribute to BBB leakage in enhancing MRI lesions and may also be involved in subtle leakage in non‐enhancing focal and diffuse lesions in NAWM. BBB disruption due to tight junctional pathology should be regarded as a significant form of tissue injury in MS, alongside demyelination and axonopathy.

Epithelial and endothelial tight junctions are pathologically altered in infectious, inflammatory, neoplastic and other diseases. Previously, we described such abnormalities, associated with serum protein leak, in tight junctions of the blood–brain barrier endothelium, in lesional and normal‐appearing white matter (NAWM) in secondary progressive (SP) and acute multiple sclerosis (MS). This work is extended here to lesions and NAWM in primary progressive multiple sclerosis (PPMS) and to cortical grey matter in PPMS and SPMS. Immunocytochemistry and semiquantitative confocal microscopy for the tight junction protein zonula occludens 1 (ZO‐1) was performed on snap‐frozen sections from PPMS (n = 6) and controls (n = 5). Data on 2103 blood vessels were acquired from active lesions (n = 10), inactive lesions (n = 15), NAWM (n = 42) and controls (n = 20). Data on 1218 vessels were acquired from normal‐appearing grey matter (PPMS, 5; SPMS, 6; controls, 5). In PPMS abnormal ZO‐1 expression in active white matter lesions and NAWM, was found in 42% and 13% of blood vessels, respectively, comparable to previous data from acute and SPMS. In chronic white matter plaques, however, abnormalities were considerably more frequent (37%) in PPMS than in SPMS. Abnormality was also more frequent in normal‐appearing grey matter in SPMS (23%) than in PPMS (10%). In summary, abnormal tight junctions in both SPMS and PPMS are most frequent in active white matter lesions but persist in inactive lesions, particularly in PPMS. Abnormal tight junctions are also common in normal‐appearing grey matter in SPMS. Persistent endothelial abnormality with leak (PEAL) is therefore widespread but variably expressed in MS and may contribute to disease progression.