CXCL13 and primary progressive multiple sclerosis: Indeed, sections from post-mortem brains and spinal cords of secondary progressive MS (SPMS) patients led to the identification of eLFs with B, T, plasma cells, and a network of FDCs producing CXCL13, although they were not described in relapsing-remitting MS (RRMS) and only in a lesser defined state in primary progressive MS (PPMS) (31–34).