A simple radiolabelled rheumatoid factor binding assay for the measurement of circulating immune complexes

Revisiting immune complexes: Key to understanding immune-related diseases

Immune complexes (ICs) formed by foreign or self-antigens and antibodies in biological fluids affect various tissues and are thought to cause several diseases. Biological and physical properties of IC, abnormal IC amounts, IC deposition and their relationships with disease pathogenesis had been studied. However, the relationship between ICs and each disease is not well understood and little is known of what determined ICs deposition in particular organ and why different organs are affected in different diseases. Recent technological advance enables identification of ICs in particular its antigens in tissues and body fluids, which may provide a key to discover an important trigger for immunological abnormality occurrence. Further identification of their epitopes, that are the exact origin of antigenicity, is developing and may be useful for diagnosis, elucidation of pathogenesis and treatment against IC-induced diseases. Here, we first make an overview of clearance of ICs, IC-induced pathogenesis and biological properties of ICs. Then, we introduce various methods developed to recover ICs from biological fluids or to identify antigens incorporated into ICs. Furthermore, several methods that can be used in epitope mapping for IC antigens are also documented.

Complement-mediated inhibition of immune precipitation in rheumatoid vasculitis

Complement-mediated inhibition of immune precipitation (CMIP) was measured in patients with rheumatoid arthritis (RA), rheumatoid vasculitis (RA VASC), patients with skin vasculitis not associated with a systemic connective tissue disease and normal healthy controls. CMIP was impaired in 100% (14/14) of the RA vasculitic patients, 60% (12/20) of the RA patients and 22% (2/9) of the dermovasculitic patients. The degree of impairment of CMIP was significantly greater in the RA vasculitic patients compared to the non-vasculitic patients. This difference was due to the significantly lower complement levels and the presence of higher concentrations of an inhibitor of CMIP in the RA vasculitic sera. The levels of this inhibitory activity correlated significantly with IgM rheumatoid factor concentration. Serial studies in three patients with RA vasculitis treated with corticosteroids and immunosuppressive drugs showed significant clinical improvement in two patients, which was associated with improvement in CMIP, reduction in circulating immune complex levels and reduction in IgM rheumatoid factor concentrations.

Absence of circulating IgG immune complexes in minor recurrent aphthous ulceration

A monoclonal rheumatoid factor binding assay has been employed to detect IgG type immune complexes in sera from 21 patients with minor recurrent aphthous ulceration and 31 control subjects. In only one of the ulcer group patients was there a marginal elevation of immune complexes above the normal range, with no statistically significant difference between the patient and control groups. Assays of intrinsic serum rheumatoid factor and serum IgG, IgM and IgA levels also showed no significant differences between patient and control groups, with all mean values falling within the normal ranges.

Characterization of the soluble immune complexes that are detected by three different techniques

Three different tests which are based on different principles were used for the detection of soluble immune complexes (IC): (i) a PEG precipitation test, which is based on the solubility characteristics of IC; (ii) a solid-phase C1q binding assay, which is based on the complement binding property of IC, and uses peroxidase-linked anti-human IgG to detect the bound IC (C1q-ELISA); and (iii) the indirect granulocyte phagocytosis test (IGPT) which is based on the Fc R- and possibly the C3 R-binding of IC. Using heat-aggregated IgG (A-IgG) as a model for soluble IC all three tests showed a linear relation with the amount of A-IgG. The C1q-ELISA and the IGPT had a detection limit of less than 1 microgram/ml while the PEG test only detected quantities of more than 10 micrograms/ml. However, when using artificially produced soluble IC, which were prepared from human antibodies (ab) of different specificities and their respective antigens (ag) i.e., (i) tetanus toxoid, (ii) Helix pomatia hemocyanin (HPH), and (iii) dsDNA, and which consisted of the two components in a wide range of ag/ab ratios, distinct results were obtained with the three tests. Thus demonstrating that results obtained with A-IgG as a model for soluble IC can not simply be extrapolated to the behavior of real complexes in IC detection assays. No matter which ag was used, the composition of the IC, i.e., the ratio in which ag and ab were present, appeared to be the crucial factor for detectability in the different tests. The C1q-ELISA can detect IC over a wide range of ag/ab ratios, while it is particularly sensitive for IC formed in slight ag excess. The IGPT in contrast primarily detects, and is highly sensitive for, IC formed in ab excess. The PEG test appears to detect IC with freshly bound complement only. Another interesting finding has to be mentioned here: when increasing amounts of dsDNA were added to a SLE serum containing anti-DNA ab, the IC that had been detectable in the native serum with the IGPT completely disappeared, thus demonstrating that these complexes did consist of DNA and anti-DNA.

Circulating immune complexes in cutaneous and systemic vasculitis: clinicopathological correlations

A high incidence of raised levels of circulating immune complexes has found in a group of patients with idiopathic vasculitis. Serial measurements in 8 patients with systemic vasculitis showed a good correlation between clinical status and circulating immune complex levels. Some correlation existed between the constituents of circulating and tissue-bound complexes in 6 patients so studied. However, there was no correlation between apparent organ involvement and hypocomplementaemia and the levels of size of circulating immune complexes. It is concluded that the measurement of circulating immune complexes may be clinically useful in vasculitis, but that no direct evidence is yet available that they are pathogenetically related to those which are tissue-bound.

Measurement of antigen-antibody complexes in mouse sera by conglutinin, C1q and rheumatoid factor solid phase binding assays

The application of three solid phase tests for the detection of antigen-antibody complexes in the sera of inbred and selectively-bred mice with experimentally induced chronic antigen-antibody complex disease is described. Two of the tests used--the C1q binding assay (C1qBA) and the conglutinin binding assay (KBA)--were previously described methods which we have modified for the detection of murine complexes. The third test is a new solid phase polyclonal rheumatoid factor binding assay (RFBA). The results demonstrate that the correlation of positivity with the three methods varied during the course of a chronic disease but in general results with the KBA showed a better correlation with the RFBA than with the C1qBA. Furthermore, the KBA and RFBA preferentially bound complexes of lower molecular weight than did the C1qBA. It is suggested that the parallel use of these three tests during the study of murine antigen-antibody complex disease will provide valuable information concerning the nature of the complexes involved; since the three tests, when run in parallel, will detect large complement fixing complexes (C1q), small complement fixing complexes (KBA) and small non-complement fixing complexes (RFBA).

A modified C1q deviation radioimmunoassay for use in a routine clinical laboratory

Current clinical interest in circulating immune complexes defines a need to develop immune complex assays suitable for use in routine clinical immunology laboratories. We describe one such assay based on the principle of immune complexes binding to radiolabelled C1q, using commercially available IgG-coated latex particles for competitive binding. The assay is simple, rapid, cheap, reproducible and sensitive (5 micrograms/ml). Specificity for both antibody and antigen was demonstrated and clinical value was proven with defined clinical groups.