C3 and secondary progressive multiple sclerosis: Levels of C3 and NFL, an established marker of neurodegeneration in MS [31–33], were analyzed in CSF obtained from a cohort of patients (n = 48) with relapsing-remitting (RRMS) or progressive (SPMS/PPMS) disease, as well as in controls with other non-inflammatory neurological/psychiatric diseases (OND; n = 18) (Fig 1A and 1B).