CXCL13 and secondary progressive multiple sclerosis: Moreover, the detection of ectopic lymphoid structures (ELS) containing B cells and CXCL13-producing stromal cells in the leptomeninges of secondary progressive multiple sclerosis (MS) patients and CXCL13-immunoreactivity in actively demyelinating lesions [7–9] pointed at a role of CXCL13 also in the structural organization and localization of intrathecal B cell-related immune activities [10].