Kallikrein 6 secreted by oligodendrocytes regulates the progression of experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system (CNS), and experimental autoimmune encephalomyelitis (EAE) is a well-established animal model of the disease. Here, we examined the pathophysiological role of Kallikrein 6 (Klk6), a serine protease produced by oligodendrocytes (OLs), in EAE using Klk6 knockout (Klk6-/-) mice. Compared with Klk6+/+ (wild-type) mice, Klk6-/- mice showed milder EAE symptoms, including delayed onset and milder paralysis. Loss of Klk6 suppressed matrix metalloprotease-9 expression and diminished the infiltration of peripheral inflammatory cells into the CNS by decreasing blood-brain barrier (BBB) permeability and reducing expression levels of inflammatory cytokines, chemokines and their receptors. Scanning electron microscopic analysis revealed demyelination characterized by myelin detachment from the axons in the early phase of EAE progression (days 3-7) in Klk6+/+ mice but not in Klk6-/- mice. Interestingly, anti-MOG (myelin oligodendrocyte glycoprotein) autoantibody was also detected in the cerebrospinal fluid (CSF) and spinal cord on day 3 after MOG immunization. Furthermore, treatment of primary cultured OLs with anti-MOG autoantibody induced oligodendroglial morphological changes and increases in myelin basic protein and Klk6 expression. We also developed a novel enzyme-linked immunoabsorbent assay method for detecting activated KLK6 in human CSF. In human autopsy brain samples, expression of active KLK6 was detected in OLs using an antibody that specifically recognizes the protein's activated form. Taken together, our findings demonstrate that Klk6 secreted by OLs plays a critical role in the pathogenesis of EAE/MS and that it might serve as a potential therapeutic target for MS.