C3 Activation by the Classical and Alternative Pathways by Sera and Synovial Fluids from Patients with Rheumatoid Arthritis (R A)

Low levels of C4 in comparison to C3 and presence of complement-fixing material in certain rheumatoid arthritis (RA) sera and synovial fluids have suggested activation of C in RA mainly via classical pathway. We examined consumption of hemolytic C3 via the two pathways by six RA sera and six synovial fluids. For human serum (HS) we compared C3 depletion in presence (EGTA-HS) and absence (NHS) of Mg EGTA and for guinea pig serum in normal (NGPS) and C4-deficient serum (def-GPS). The six RA sera reduced the C2 sera titer in NHS by 32% to 76% (mean, 51%). The same sera reduced C3 titers of EGTA-HS-by 31% to 62% (mean, 45%). Similarly, the six sera reduced the C3 titer in NGPS by 20% to 98%, (mean, 54%) and in def-GPS by 19% to 82% (mean, 45%). Thus, there is little difference between ability of NHS to activate C3 in presence and absence of classical components.