POLYETHYLENE-GLYCOL PRECIPITATES OF SERUM CONTAIN A LARGE PROPORTION OF UNCOMPLEXED IMMUNOGLOBULINS AND C3

High pressure liquid chromatography (HPLC) was used to fractionate red issolved polyethylene glycol (PEG) precipitates isolated from the sera of normal volunteers and from patients with IgA nephropathy (IgAN) an d systemic lupus erythematosus (SLE), 2 diseases characterized by elev ated levels of circulating immune complexes. The individual fractions were analyzed by solid phase ELISA for IgA, IgM, C3, IgG, and complexe s of IgG-IgA and IgG-C3. Although PEG precipitates were enriched for h igh molecular weight IgA and IgG (presumably bound within CIC), signif icant amounts of IgM, unbound IgG and C3 were also present. The quanti ties of the PEG-precipitable proteins did not correlate with their ser um concentrations. IgG-IgA and IgG-C3 complexes were found in all prec ipitates examined, but the levels of complexes were higher in both pat ient groups. These results indicate that PEG precipitates a considerab le quantity of proteins not bound in immune complexes. There appeared to be greater protein precipitation from sera of the patient groups co mpared to the amount precipitated from the normal sera. These results suggest that an understanding of the mechanism of PEG precipitation ma y be important in defining abnormalities in IgAN, SLE and perhaps othe r diseases characterized by elevated levels of CIC. In addition, the p ossibility of undetected CIC in PEG precipitable material must be cons idered.