Complement activation has been shown to occur in patients with juvenile rheumatoid arthritis (JRA). Since the two pathways of complement are activated by different stimuli (the alternate pathway by microbial products and IgA, and the classical pathway by immune complexes), we decided to study the relative contribution of the two pathways of complement activation in patients with JRA.
In 56 patients with JRA, plasma levels of C3 and C4 were measured by turbidimetric assays, and those of C4d, factor Bb and sC5-9 complex by solid-phase enzyme immunoassays. Levels beyond the mean +/- 2 S.D. of normal were considered abnormal.
Plasma C3 and C4 levels were decreased in one patient each. The C4d values were increased in 17 patients, whereas levels of factor Bb were elevated in 42 patients and levels of sC5-9 complex were elevated in 51 patients. The values of factor Bb and sC5-9 had a linear correlation (r = 0.75), but there was no significant correlation between C4d and sC5-9 levels (r = 0.36).
Complement activation in JRA is initiated predominantly by the alternate pathway and culminates in the formation of terminal membrane attack complex.