Cold-Insoluble Complexes and Complement Levels in Systemic Lupus Erythematosus

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Abstract
Circulating cold-insoluble complexes, observed in 11 of 31 patients with systemic lupus erythematosus (SLE), consisted mainly of immunoglobulin G and the C′lq component of complement and did not contain DNA or selective amounts of antinuclear antibody. In 10 of 11 (91 per cent) of these patients, the presence of complexes was associated with a decrease in the serum level of C′3 (β1c-β1a) as measured by a radial diffusion method. Of 25 patients without cold-insoluble complexes in the serum, the level of C′3 was reduced in only three (12 per cent). Evidence that the decrease in the levels of C′3 was related to the presence of cold-insoluble complexes was provided by the observation that interaction of the IgG and C′lq constituents of a cryoprotein isolated from the serum of a patient with SLE produced a 79 per cent reduction in hemolytic activity of human complement in vitro.