Methods for measuring circulating immune complexes
The Nature of CIC Immune complexes are antibodies bound to antigens. Circulating immune complexes (CIC) are those found in serum or plasma. Endogenous CIC would be expected to bind complement. The size, structure, and composition of CIC are heterogeneous and depend on the binding characteristics of the components, which include antigens, immunoglobulins, complement, and other serum proteins. On first appearance, one might hypothesize that the complexity of such molecules would prevent effective measurement. Poor correlations between patient sera tested by different assay methods and the failure of CIC levels to correspond to the postulated pathological role in many patients have caused much controversy, and would seem to corroborate this hypothesis (14). A major reason for this controversy may be unreliable information obtained from ineffective assays. In addition to poor precision inherent in many assays, interference from monomeric immunoglobulins has been an important factor in causing misinformation. Failure to understand some pertinent aspects regarding the nature of CIC may be yet another factor. In this article, we critically examine the problems of interference and suggest alternatives to eliminate them. We also suggest a model of CIC that explains many observations. A number of distinct features regarding the nature of immune complexes is known: a) Endogenous immune complexes in serum and synovial fluid are composed mainly of immunoglobulins and other serum proteins (18, 19). Only small amounts of DNA that could not contribute significantly to the structure have been found (5, 26). b) Rheumatoid factors (RF) have been shown to be a significant component in CIC. c) In some infectious diseases, very sensitive methods indicate that organism-specific antigens are present in small amounts in CIC. Although these antigens may contribute only minimally to the overall CIC structure, they may be the seed that initiated the production of serum proteins, which comprise the bulk of the immune complex. In autoimmune disease, specific foreign antigens have not been identified in CIC. In autoimmune disease, the size of CIC has been shown to vary from 7S to greater than 19S (27). It is thought that smaller CIC do not contribute significantly to the disease process. Moreover, the range of sizes may be artifactually broad because methods such as gel filtration chromatography and sucrose gradient centrifugation used to analyze size may foster dissociation among many components of CIC, such that small complexes may increase (6, 13). Apparent size differences may actually represent differences in the internal binding forces of the aggregates. According to Jerne, the large majority of antibodies are idiotypic, forming networks with activity against other antibodies (9). If so, one might expect interactions among immunoglobulins to cause CIC in healthy persons. How, then, do we reconcile the finding that in healthy persons less than 0.5% of IgG is found in CIC? The binding forces between idiotypic antibodies and idiotopes may be weak; the networks may fragment easily with minor perturbations. In sick people, increases in RF, alterations in idiotypic antibody binding, and production of other proteins may facilitate adhesiveness like a glue and alter the intrinsic binding forces within these networks. Thus, CIC appear to be large aggregates with varying degrees of internal binding that depend on the reactivity of the components shown in Table 1.