TNFSF13 and neuromyelitis optica: A proliferation-inducing ligand (APRIL), also known as the tumour necrosis factor superfamily member 13 (TNFSF13), is a peculiar member in the TNFSF, since it binds to heparan sulphate proteoglycans (HSPGs) in addition to canonical receptors from the TNFRSF.16 We recently demonstrated that APRIL also binds to CSPGs upregulated in multiple sclerosis (MS) and neuromyelitis optica (NMO) lesions.17,18 In the present study, we analysed functionally the effect of APRIL binding to CSPGs.