CHI3L1 and neuromyelitis optica: Strikingly, CHI3L1 alone without AQP4-IgG caused moderate yet significant demyelination pathology, and coadministration of CHI3L1 with AQP4-IgG exacerbated the demyelination pathology (Figure 1B), suggesting that CHI3L1 actively contributes to demyelination in NMO.