Terminal component of complement (C9) in cerebrospinal fluid of patients with multiple sclerosis.

An immunoradiometric assay was used to measure the concentration of the terminal component of complement (C9) in cerebrospinal fluid (CSF) and plasma from 35 patients with multiple sclerosis and 55 controls with other neurological diseases. There was a highly significant reduction in cerebrospinal fluid C9 concentration in patients with multiple sclerosis (0.26 +/- 0.02 microgram/ml) compared with controls (1.52 +/- 0.20 micrograms/ml; p less than 0.0005). As a single protein measurement C9 seemed to be more useful as an aid to clinical diagnosis than CSF IgG; the C9 index was also a better discriminator than IgG index between the two groups of patients. Reduced CSF C9 concentration in patients with multiple sclerosis implies C9 consumption due to formation of membrane attack complexes, which could mediate myelin damage and cause more widespread but reversible loss of function, accounting for the transient symptoms characteristic of the disease.