Recent studies have revealed that the disruption of the blood–brain barrier (BBB) might contribute to the induction of neurodegeneration in the progressive stage of multiple sclerosis (MS).

We investigated a potential target for the serum auto-antibodies responsible for the BBB impairment in patients with secondary progressive MS (SPMS).

We identified undetermined target antigens in human brain microvascular endothelial cells (BMECs) that reacted with auto-antibodies in sera from SPMS patients using a proteomic approach. In addition, we examined how the identified auto-antibodies compromise the BBB integrity.

We found that 10 of 11 SPMS sera had auto-antibodies against galectin-3, although the patients with other neurological diseases did not have these antibodies. Downregulation of galectin-3 led to elevated intercellular adhesion molecule-1 (ICAM-1) and phospho-nuclear factor-kappa (NF) B p65 expression in the BMECs. Exposure to SPMS patients’ sera also increased the protein levels of ICAM-1 and phospho-NFB p65 in BMECs, but these effects induced by anti-galectin-3 immunoreactivity were canceled by the downregulation of galectin-3.

Galectin-3 is a possible immunological target molecule of the pathogenic auto-antibodies and contributes to the persistent BBB breakdown in patients with SPMS. These antibodies may also serve as a novel biomarker for SPMS.