To investigate terminal complement activation and lysis inhibitors in rheumatoid arthritis (RA).
C5a, vitronectin and clusterin were quantitated by enzyme immunoassays in plasma and synovial fluid (SF) in RA (n = 30) and osteoarthritis (OA) (n = 11).
In RA the concentration of C5a was 3-fold increased in plasma (21.9 vs 7.2 micrograms/l) and 5-fold increased in SF (7.8 vs 1.7 micrograms/l) compared to OA. The SF/plasma ratios for C5a, vitronectin and clusterin were 0.35, 0.36 and 0.23, respectively, not significantly different in the 2 diseases.
SF terminal pathway activation in RA combined with low local levels of lysis inhibitors might allow lytic or sublytic attacks on local cells, resulting in inflammation and cell damage.