Circulating immune complexes and rheumatoid arthritis: the induction of immune complex formation between rheumatoid factor and IgG by polyethylene glycol.

Circulating immune complexes (CIC) as detected by the C1q binding assay (C1qBA) in sera from patients with rheumatoid arthritis (RA) were not demonstrable in these sera with the indirect granulocyte phagocytosis test (IGPT). This discrepancy could be explained by the finding that polyethylene glycol (PEG), used in the C1qBA, enhanced the binding of rheumatoid factor IgM (RFIgM) and IgG resulting in immune complex (IC) formation. The addition of PEG to RA sera and subsequent testing of these sera in the IGPT revealed increased uptake of IC by these cells, dependent on the PEG dose. Addition of purified RFIgM to a normal human serum generated a positive IGPT in a dose dependent way. We conclude that in RA sera PEG induces IC between RFIgM and IgG. Therefore, assays devised to measure CIC in RA sera which are based on PEG (like the C1qBA) overestimate the amounts of IC present in the circulation in vivo.