We recently reported that sera derived from patients in the relapse phase of RRMS (RRMS-R) or secondary progressive MS (SPMS) decrease the claudin-5 protein levels and transendothelial electrical resistance (TEER) values in BMECs, while that derived from patients with RRMS-R, stable phase of RRMS (RRMS-S) and SPMS increases the VCAM-1 protein levels in BMECs [9]. Here, CLDN5 is linked to secondary progressive multiple sclerosis.