Correlation between anti-C1q and immune conglutinin levels, but not between levels of antibodies to the structurally related autoantigens C1q and type II collagen in SLE or RA.

The simultaneous appearance of autoantibodies with either a functional or structural relationship to anti-C1q antibodies (anti-C1q) was investigated in 39 systemic lupus erythematosus (SLE) patients and in 28 rheumatoid arthritis (RA) patients, in both cross-sectional and longitudinal design. Levels of anti-C1q showed an isotype-specific correlation to levels of immune con-glutinin (IK) in SLE patients, whereas no correlation was evident to levels of antibodies to the structurally related antigen type II collagen (anti-CII) in SLE or RA patients. IgG anti-C1q levels correlated with serum levels of the terminal complement complex (sC5b-9) in SLE patients. In two longi-tudinally followed patients, the IK response preceded the anti-C1q response. Possibilities for regulation of the humoral anti-complement response are discussed.