[Circulating immune complexes-like material and complement in rheumatoid arthritis and ankylosing spondylitis...

Circulating immune complexes (IC) in 52 rheumatoid arthritis (RA) seropositive, 25 RA seronegative and 32 ankylosing spondylitis (AS) patients were assayed by two methods: 1) precipitation with a final 2% polyethylene glycol (PEG) concentration with immunoglobulins (Igs) quantitation in the precipitate and 2) the ability of IC to consume complement, evaluating conversion products from human C3 by bidimensional rocket immunoelectrophoresis. CH50, C3, C4, C3PA and circulating C3 conversion products were quantitated. IC precipitates with 2% PEG contained three classes of Igs: IgG, IgM and IgA, the latter being the most rare. In the control group Igs level precipitated with 2% PEG were: 2% PEG-IgG 2.5 mg/100 ml +/- 2; 2% PEG-IgM 0.72 mg/100 ml +/- 1.09; 2% PEG-IgA 0.04 mg/100 ml +/- 0.18. With 2% PEG IC-like material was detected in 58.5% of RA seropositive patients but none generated C3 conversion products. With neither methods was IC-like material detected in the RA seronegative group and none with 2% PEG in AS, either. However, in the latter, 21.8% generated C3 conversion products and 18.7% circulating C3 conversión products. In the RA seropositive patients no correlation was observed between the levels of 2% PEG-IgG and 2% PEG-IgM and the titre of antinuclear antibodies (ANA) nor between 2% PEG-IgG and that of IgG in serum. However, there was a low correlation between 2% PEG-IgM and that of IgM in serum and between 2% PEG-IgG and 2% PEG-IgM with rheumatoid factor (RF). It should be stressed that the value of the PEG method in the detection of IC-like material must be reconsidered, since using a 2% final concentration and a specific evaluation of Igs and complement components in the precipitate, it may be a method easy to perform and useful for clinical purposes.