CLDN5 and secondary progressive multiple sclerosis: In the present study, we first showed that fingolimod restores the BBB dysfunction induced by sera from MS patients; specifically, pretreatment with fingolimod-phosphate restored the decreased expression levels of claudin-5 protein/mRNA and TEER values in the BMECs following exposure to sera derived from RRMS and SPMS patients, thus suggesting that fingolimod-phosphate is able to restore BBB damage and make the BBB less permeable in both the relapse and progressive stages of MS.