IgG Subclasses of PEG Precipitable IgG in Systemic Lupus Erythematosus Sera

Polyethylene glycol (PEG, 4%)-precipitated macromolecular IgG isolated from the sera of 20 patients with systemic lupus erythematosus (SLE) and 15 control subjects was analyzed for its IgG isotype concentration by single radial immunodiffusion. PEG precipitates from SLE sera had higher mean levels of IgG1, IgG2 and IgG3 and lower IgG4 than PEG precipitates isolated from normal sera although only the IgG2 levels were significantly different. Using an anti-complementary assay there was a significant correlation between the ability of parent sera to fix complement and the absolute levels of PEG precipitable IgG1, IgG2 and IgG3. These data suggest that the ability of immune complexes in the sera of patients with SLE to fix complement is dependent on their IgG subclass composition.