Detection of IgA-class circulating immune complexes (CIC) in sera from patients with IgA nephropathy using a solid-phase anti-C3 Facb enzyme immunoassay (EIA)

The detection of circulating immune complexes (CIC) in sera from patients with IgA nephropathy is described. A solid-phase anti-C3 Facb enzyme immunoassay (EIA) was employed for detection of IgA-, IgG- and IgM-CIC in sera. The C1q-binding enzyme assay was also used for the detection of CIC in sera from these patients and healthy adults. Twenty-two patients with IgA nephropathy, 14 patients with other glomerular diseases and 19 healthy adults were examined by anti-C3 Facb EIA. The levels of IgA-CIC in sera from patients with IgA nephropathy were significantly higher than those in sera from patients with other glomerular diseases and healthy adults. CIC measured by the C1q-binding enzyme assay was detected in some patients with IgA nephropathy. The levels of serum IgA in patients with IgA nephropathy were significantly higher than those in patients with other glomerular diseases and healthy adults. However, there was no significant correlation between the levels of IgA-CIC in sera and those of serum IgA in patients with IgA nephropathy. There was also no significant correlation between the levels of IgA-CIC in sera and the degree of histopathological injuries in the patients. It is concluded that the solid-phase anti-C3 Facb EIA is useful for the detection of IgA-CIC in sera from patients with IgA nephropathy.     

Detection of IgA class circulating immune complexes bound to anti-C3d antibody in patients with IgA nephropathy

IgA class circulating immune complexes (CIC) were detected by solid-phase fluorescent enzyme immunoassay of F(ab')2 anti-C3d antibody in the serum of 52 patients with IgA nephropathy. Conglutinin (Kg) binding IgA class CIC were also measured, and results by these assays were compared. Kg binding IgA class CIC and anti-C3d binding IgA class CIC were detected in 27% and 44%, respectively, of the patients with IgA nephropathy. Either or both of the two were found in 65% of the patients. There was no significant correlation between IgA class CIC detected by these methods and serum IgA. Although all samples with a very high level of anti-C3d binding IgA class CIC did not also have a very high level of Kg binding IgA class CIC, there was a slight quantitative correlation between the 2 assays. Ultracentrifugation analysis showed that anti-C3d binding IgA class CIC were of various sizes between polymeric (21 S) and monomeric IgA (7 S), whereas Kg binding IgA class CIC were mostly monomeric IgA (8 S) with a minor component of heavy fractions (14 S). Both IgA class CIC fixed iC3b and IgA class CIC fixed C3d are present in IgA nephropathy. These observations suggest that the different types of complement bound to IgA class CIC have different roles in IgA nephropathy.     

Current situation and problems in the estimation of circulating immune complexes]

Some methods employing murine monoclonal antibodies have been developed for the estimation of circulating immune complexes (ICs). In the assays using monoclonal antibodies to C1q and C3d, ICs attached by reaction of C1q or C3d with the corresponding antibodies are detected by enzyme-labelled anti-IgG antibody. The murine monoclonal rheumatoid factor (RF) of IgG class is employed in the assay for detection of ICs. ICs reacted with the RF on the solid phase are further detected by the reaction with the second anti-IgG antibody labelled with the enzyme. The anti-C1q antibody in the sera as well as ICs produces positive reactions in the solid phase C1q assay, the assays using monoclonal antibodies are recommended for use in the detection of circulating ICs. In the pretreatment of serum samples, heating at 56 degrees C induces aggregation of IgG to produce a positive reaction by these sensitive assays, and the addition of EDTA-Na2 increases free C1q detached from C1 to induce increased binding to IgG. Reactions of aggregated IgG with RF and C1q in the fluid phase inhibit the following binding of monoclonal RF and anti-C1q antibody on the solid phase. Sera of patients with SLE were examined for CH50, anti-DNA antibody and ICs. The levels of ICs determined by the anti-C1q and C3d antibody assay did not correlate with other parameters. Positivity of ICs was unexpectedly lower in SLE sera. To evaluate the significance of the estimation of ICs, more data must be analyzed by these methods.     

Characterization of immune complexes in acute lymphoblastic leukaemia.

Sera from 120 children and young adults with acute leukaemia (59), various other tumours (53) and histiocytosis X (eight) were studied for the presence and characteristics of circulating immune complexes (CIC). Serial and parallel testing was performed using: C1q binding (solid phase), Raji cell radioimmunoassay and anti-C3 (solid phase). CIC were detected in 36 of 56 (64%) patients with acute lymphoblastic leukaemia (ALL) and in 62% of other tumour subjects. In the ALL sera, the mean positive C1q binding was 5.4 s.d., Raji cell 4.2 s.d. and anti-C3 4.4 s.d. In 12 ALL sera CIC were characterized for molecular size by sucrose gradient centrifugation. Most samples showed high molecular weight (19S) complexes but intermediate (11-14S) and smaller (8-9S) complexes were also detected. There was no apparent relationship between the presence, amount or physical size of the detectable CICs and clinical course of the patients studied; 12 patients with ALL in long term remission showed presence of CIC at some time during their course. Immune complexes precipitated from leukaemic sera were also examined for the presence of common ALL antigen (cALL) and Ia(DR) antigens utilizing rabbit antisera and mouse monoclonal antibodies. Experiments with isolated immune complexes from ALL sera provided no positive evidence for the presence of cALL antigen or Ia antigen within immune complex materials from ALL patients.     

Enzyme immunoassay detection of circulating immune complexes by monoclonal antibodies to C3 neoepitopes with special reference to IgG concentration and to interfering anti-immunoglobulin antibodies

A sensitive solid-phase anti-C3 enzyme immunoassay for detection of circulating immune complexes (CIC) is described. A mixture of the monoclonal antibodies (MoAbs) bH6 and Clone 9 specific for neoepitopes on C3 activation products was used as capture reagent. MoAb bH6 recognized C3b, iC3b and C3c, and Clone 9 recognized iC3b and C3dg. Detection antibody was a polyclonal peroxidase-conjugated rabbit anti-human Ig antiserum. A quantitative assay was constructed using serum incubated with heat aggregated IgG (HAG) as standard. The lower detection limit was 5 micrograms/ml of HAG. Interassay and intra-assay coefficient of variation was 15% and 5%, respectively. Anti-animal immunoglobulin antibodies were detected both in normal and pathological sera. This activity was efficiently absorbed by nonimmune immunoglobulins added to the samples. The present assay was compared with a polyethylene glycol precipitation assay for CIC determination. The latter assay was strongly influenced by the IgG concentration (rs = 0.78; P = 0.006), whereas no such correlation was seen for the anti-C3 immune complex assay (rs = -0.30; P = 0.20).     

Analysis of antigen involved in circulating immune complexes in patients with idiopathic thrombocytopenic purpura.

Circulating immune complexes (CIC) were frequently observed in patients with idiopathic thrombocytopenic purpura (ITP). To analyse the pathogenic role of CIC, we studied the correlation between the disease activity and the CIC level, and whether platelet antigens were involved in CIC from ITP sera. Elevated CIC levels, measured by Clq, anti-C3 and spermatozoa micro-enzyme-linked immunosorbent assay, were found in 37%, 54% and 52% of the patients, respectively. However, there were no significant correlations (r = -0.11, -0.03 and -0.04, respectively) between the platelet count and the CIC level. Platelet antigens in CIC were detected with rabbit anti-human platelet serum. The CIC from 18%, 25% and 25%, respectively, of ITP sera contained platelet antigens, but the CIC from only 3%, 9% and 6% of SLE sera and from 3%, 2% and 5% of the sera of alloimmunized patients, respectively, contained these antigens. There were significant correlations (r = 0.51, 0.80 and 0.65, respectively) between the amount of platelet antigens and the CIC level in ITP sera. However, there were no correlations (r = -0.26, -0.29 and 0.08, respectively) between the platelet count and the amount of platelet antigens in the CIC. We detected platelet antigens in CIC from ITP sera, but surmise that these CIC perhaps do not play an important role in platelet destruction.     

Detection of circulating immune complexes with polyethylene glycol precipitation F(ab')2 anti-C3 ELISA

A polyethylene glycol (PEG) precipitation F(ab')2 anti-C3 ELISA for the detection of complement-fixing IgG circulating immune complexes (CIC) is described. For this assay, test sera were treated with 3.5% PEG and then measured with F(ab')2 anti-C3 ELISA. The lower detection limit was 4 micrograms/ml of heat aggregated human IgG (HAHG). Intra-assay coefficient of variation (CV) was 4.9-8.3%. High levels of CIC are found in the sera of patients with systemic lupus erythematosus (SLE), hepatitis B and stomach cancer.     

Complement-mediated solubilization of immune complexes in systemic lupus erythematosus.

The capacity of complement-mediated solubilization of immune complexes (complex releasing activity: CRA) was studied in 63 sera from eight systemic lupus erythematosus (SLE) patients. CRA in sera of active SLE (35 +/- 17.%) was significantly lower than that of inactive SLE (64.1 +/- 24.1%, P less than 0.001). In addition, 20 of 23 sera collected during active diseases demonstrated CRA values less than 50% of the control pooled serum. On the other hand, CRA of 29 of 40 sera from inactive disease exceeded the 50% level. CRA in SLE sera correlated with complement component levels and in particular with the CH50. Serial determination of CRA and of levels of circulating immune complexes (CIC), C4 and C3 in two active patients indicated that the correlation between CRA and the complement components was positive, while that between CRA and CIC was negative. These studies provide evidence that CRA may be useful for following the activity of SLE and that CRA reflects the levels of the complement components of both classical and alternative pathways. The possibility that CIC may be solubilized and opsonized by complement and cleared by the reticuloendothelial system was discussed.     

Detection of bovine serum albumin in the circulating IgA immune complexes of patients with IgA nephropathy

Alimentary antigenic challenge has been postulated to have a role in the genesis of IgA circulating immune complexes (CIC), resulting in mesangial IgA disease. In this study, we examined the relationship between bovine serum albumin (BSA) and IgA CIC in patients with IgA nephropathy. Of the 47 patients studied, elevated IgA CIC levels were found in 32% by the F(ab')2 anti-C3 and Raji cell enzyme immunoassays (EIA). Elevated IgA anti-BSA antibody levels were found in 9 patients, and there was a positive correlation between these levels and IgA CIC as measured in the Raji cell EIA (R = 0.60, P less than 0.001). In 4 patients with elevation of both IgA CIC and IgA anti-BSA antibody levels, solubilization experiments were done to demonstrate the presence of BSA antigen in the IgA CIC. Using the Raji cell EIA, the IgA CIC levels decreased significantly after preincubating the sera with serial concentrations of excess BSA. No corresponding effect was seen with human serum albumin used as control. Hence, BSA may be the antigenic stimulus in the formation of IgA CIC in selected patients with IgA nephropathy. The pathogenic capacity of these IgA-BSA CIC remains to be determined.     

Immunopathological correlation between mesangial C3d-deposition and C3d-fixing circulating immune complexes in lupus nephritis

By a direct immunofluorescent technique, glomerular C3d deposition was examined in a total of 50 renal biopsy specimens from patients with lupus nephritis. C3d deposition was then compared with disease activity, glomerular IgG and C3c deposition, and the levels of circulating immune complexes (CIC) measured by a solid-phase anti-C3d assay. There was a good correlation between disease activity and the positivity of glomerular C3d deposits (P less than 0.001), as well as C3c deposits (P less than 0.001). Even in clinically inactive patients, a relatively high percentage (59%) of C3d deposits were positive compared with C3c deposits (17%). Mesangial C3d deposition correlated with clinical disease activity more significantly (P less than 0.005) than capillary wall C3d deposition (P less than 0.025). C3d deposits were detected in all of the 30 cases with positive C3c deposits, and moreover, in 15 of the 20 (75%) cases with negative C3c deposits. Glomerular IgG deposits were almost always associated with C3d deposits, both in mesangial areas and along capillary walls, with statistical significance (P less than 0.005, P less than 0.001, respectively). The serum levels of C3d-fixing immune complexes (IC) were significantly correlated with the positivity and intensity of mesangial C3d deposits. This study demonstrates glomerular deposition of C3d in patients with lupus nephritis and reveals a significant correlation between mesangial C3d deposition and disease activity.     

Serum complement levels in patients with rheumatoid arthritis and vasculitis

It is known that serum complement levels are decreased in patients with rheumatoid arthritis associated with vasculitis, also called malignant rheumatoid arthritis (MRA). The complement profiles in patients with MRA were compared with those in uncomplicated rheumatoid arthritis (RA) and in those with systemic lupus erythematosus (SLE). In MRA patients, serum CH50, C4 and C3 levels were all decreased as in SLE patients; and C3d, an activation product of C3, and the C3d/C3 ratio as a C3 activation index were increased. Serum B level was also increased as in RA patients, but AH50 level as the haemolytic activity of the alternative pathway was within the normal range. The regulatory proteins, H and I, were elevated in sera of MRA patients. Circulating immune complexes (CIC) detected by a solid phase C1q binding method were increased, whereas the serum activity to solubilize a performed immune complex (CRA) was decreased in MRA sera. It was suggested that increased consumption of complements, mainly through the classical pathway may be responsible for the complement profiles in MRA patients: a markedly consumed classical pathway and relatively preserved alternative pathway.     

Quantitative immunoassay for IgA class circulating immune complexes using solid phase Facb fragment of anti-C3

A quantitative assay of IgA class circulating immune complexes (IgA-CIC) by a solid phase anti-C3 enzyme immunoassay (anti-C3 EIA) is described. A stable and reproducible standard for determination of IgA-CIC was prepared successfully by chemical binding of complement C3 to human serum IgA. Two of 27 sera from patients with systemic lupus erythematosus (SLE), however, contained high concentrations of IgA class anti-F(ab')2 antibodies that caused false positive results when the F(ab')2 of anti-C3 was used for EIA. Solid phase Facb of anti-C3 was found to eliminate the false positive results caused by IgA class anti-F(ab')2 and IgA class rheumatoid factor. Good reproducibility and recovery were observed with this Facb anti-C3 EIA using the IgA-C3, a stable standard material, and so this method should be useful clinically in elucidating the role of IgA-CIC.     

Comparative study of circulating immune complexes quantity detection by three assays--CIF-ELISA, C1q-ELISA and anti-C3 ELISA

The assessment of the soluble immune complexes (IC) in human sera is traditionally performed by the C1q binding assay. In the present study, a novel method for the quantity of immune complexes was reported. The methodology was based on measuring their deposition on solid-phase C3 binding glycoprotein (CIF), using an enzyme-linked immunosorbent assay. We also used ELISA that employed anti-C3 antibodies to determined the quantity of immune complexes. The three assays were evaluated for their performance characteristics on the same specially prepared samples: 55 normal sera, 99 sera from RA, 88 sera from SLE, and 27 sera from PSS. The results were compared by reference to a common standard-heat aggregated IgG that possesses many activities of immune complexes. Three of the tests used displayed almost the same specificity (over 95%), while their relative sensitivity varied depending on the disease sera tested. The sensitivity of the assays used was recorded highest for C1q ELISA-28.97% of positive sera, followed by CIF-ELISA-19.63% and lowest for anti-C3 ELISA-17.29%. A well-expressed correlation was found between CIF-ELISA and anti-C3 ELISA data (r=0.42), and a week correlation was noted when comparing CIF-ELISA and C1q ELISA IC levels detected (r=0.28). When the correlation coefficients were calculated individually for each disease category, they were clearly different, and that reflected indirectly in different sensitivities of the test for various disease categories. We also found that the results from the simultaneous performance of the tests demonstrated low percentage positive results when three or two assays were used. This is most probably due to the different assay abilities to detect IC with different sizes and composition, which shows that a small part of IC in the tested sera can be detected simultaneously by more than one assay. On the basis of the results obtained, we concluded that optimal screening for IC could be achieved by parallel application of several different methods.     

New ELISA kits using C3 binding glycoprotein from Cuscuta europea detect mainly IgM CIC in rheumatoid arthritis and progressive systemic sclerosis, but not in systemic lupus erythematosus

Elevated levels of circulating immune complexes (CIC), containing IgG, IgM or IgA antibodies were detected in the sera of patients with autoimmune diseases. This might indicate a different biological meaning of the three isotypes of immunoglobulin (Ig) in the CIC. Each CIC assay detected only certain classes and subclasses of Ig in CIC material or fixed complement protein. In this study, a new method based on C3binding glycoprotein named CIF-ELISA and a well-known method ANTI-C3 ELISA, were used for quantitative assessment of IgM-CIC, IgG-CIC and IgA-CIC levels in human sera. A modified CIF-ELISA and ANTI-C3 ELISA for simultaneous detection of CIC, containing IgG, IgM and IgA, (stCIC), were also performed. The assays were evaluated on the same specially prepared samples: 55 normal sera, 99 sera from rheumatoid arthritis (RA), 88 sera from systemic lupus erythematosus (SLE), and 27 sera from progressive systemic sclerosis (PSS). We found that the sensitivity of the tests used varied depending on the diseases studied. CIF-ELISA displayed higher sensitivity of IgM-CIC when compared to ANTI-C3 ELISA in RA patients (40.0 and 20.95%, respectively) and PSS (44.43 and 37.04%, respectively). Results for the sensitivity of IgA-CIC were in adverse direction in the RA group (14.28 and 19.05%) and PSS (14.81 and 25.93%) by both methods. It was also established that the concordance of IgM-CIC positives by both methods was 48.84% in RA and 46.67% in PSS, while in SLE it was 18.78%. These results are most probably due to the different assay abilities to detect antibody isotype of the CIC material and help to explain what specific role each Ig isotype in CIC has in the course of the disease.     

Circulating antigens, immune complexes and C3d levels in human schistosomiasis: relationship with Schistosoma mansoni egg output.

Circulating schistosome antigens (CSA), circulating immune complexes (CIC) and C3 breakdown product - C3d - were investigated in human schistosomiasis in comparison to the S. mansoni egg count. A close relationship was observed between the mean number of eggs/g of stool and the detection of CSA (evaluated by the radioimmunoprecipitation-PEG assay - Ripega), CIC (Clq-binding test) and C3d levels (quantitated by radial immunodiffusion). All the patients with more than 500 S. mansoni eggs/g of stool also presented antigen '4', specific of the genus Schistosoma, in the serum. A significant correlation was noticed between levels of CSA and CIC. This suggests the involvement of several schistosome antigens in the detected CIC. No relationship was noted between CIC and C3d levels. In contrast, there was a highly significant correlation between levels of CSA and C3d. The interaction between certain schistosome antigens and the complement system is discussed.     

Polyethylene Glycol Precipitates of Serum Contain a Large Proportion of Uncomplexed Immunoglobulins and C3

High pressure liquid chromatography (HPLC) was used to fractionate redissolved polyethylene glycol (PEG) precipitates isolated from the sera of normal volunteers and from patients with IgA nephropathy (IgAN) and systemic lupus erythematosus (SLE), 2 diseases characterized by elevated levels of circulating immune complexes. The individual fractions were analyzed by solid phase ELISA for IgA, IgM, C3, IgG, and complexes of IgG-IgA and IgG-C3. Although PEG precipitates were enriched for high molecular weight IgA and IgG (presumably bound within CIC), significant amounts of IgM, unbound IgG and C3 were also present. The quantities of the PEG-precipitable proteins did not correlate with their serum concentrations. IgG-IgA and IgG-C3 complexes were found in all precipitates examined, but the levels of complexes were higher in both patient groups. These results indicate that PEG precipitates a considerable quantity of proteins not bound in immune complexes. There appeared to be greater protein precipitation from sera of the patient groups compared to the amount precipitated from the normal sera. These results suggest that an understanding of the mechanism of PEG precipitation may be important in defining abnormalities in IgAN, SLE and perhaps other diseases characterized by elevated levels of CIC. In addition, the possibility of undetected CIC in PEG precipitable material must be considered.     

Polyethylene Glycol Precipitates of Serum Contain a Large Proportion of Uncomplexed Immunoglobulins and C3

High pressure liquid chromatography (HPLC) was used to fractionate redissolved polyethylene glycol (PEG) precipitates isolated from the sera of normal volunteers and from patients with IgA nephropathy (IgAN) and systemic lupus erythematosus (SLE), 2 diseases characterized by elevated levels of circulating immune complexes. The individual fractions were analyzed by solid phase ELISA for IgA, IgM, C3, IgG, and complexes of IgG-IgA and IgG-C3. Although PEG precipitates were enriched for high molecular weight IgA and IgG (presumably bound within CIC), significant amounts of IgM, unbound IgG and C3 were also present. The quantities of the PEG-precipitable proteins did not correlate with their serum concentrations. IgG-IgA and IgG-C3 complexes were found in all precipitates examined, but the levels of complexes were higher in both patient groups. These results indicate that PEG precipitates a considerable quantity of proteins not bound in immune complexes. There appeared to be greater protein precipitation from sera of the patient groups compared to the amount precipitated from the normal sera. These results suggest that an understanding of the mechanism of PEG precipitation may be important in defining abnormalities in IgAN, SLE and perhaps other diseases characterized by elevated levels of CIC. In addition, the possibility of undetected CIC in PEG precipitable material must be considered.     

Determination of the second component of complement (C2) by electroimmunoassay in sera from patients with systemic lupus erythematosus

The concentration of C2 was determined by electroimmunoassay in sera from healthy controls, patients with systemic lupus erythematosus (SLE), their relatives and patients with other diseases. Monospecific anti-human C2 serum was obtained by immunizing rabbits with purified human C2 and then absorbing the rabbit serum with inactivated normal human serum that was made insoluble. In addition, it was shown that human C2 could be purified by means of affinity chromatography on anti-C2 antibody coupled Sepharose. The serum concentration of C2 was 37.8 +/- 5.0 (s.d.) micrograms/ml in healthy controls (n = 133). In patients with SLE, the values were below normal in the active phase and were within normal limits in the inactive phase, showing good correlations with other complement parameters such as CH50, C4, C3 and factor B. C2 concentration was well correlated with C2 haemolytic activity in the inactive phase of SLE, but there was no relationship between the two in the active phase. The mean value of C2 concentration in the relatives of patients with SLE showed no significant difference from that in healthy controls. C2 concentration tended to be high in patients with scleroderma, polymyositis, rheumatoid arthritis, Behçet's disease and aortitis syndrome. However, the values were often low in patients with chronic liver diseases, suggesting a decrease of C2 production in the liver.     

Human Autoantibodies Against C1q: Lack of Cross-Reactivity with the Collectins Mannan-Binding Protein, Lung Surfactant Protein A and Bovine Conglutinin

The collectins, a group of humoral C-type lectins, have globular and collagen-like regions and share structural features with the complement protein C1q. The question was asked if autoantibodies to the collagen-like region of C1q (anti-C1qCLR) might cross-react with collectins, such as mannan-binding protein (MBP), lung surfactant protein A (SP-A) and bovine conglutinin (BK). Anti-C1qCLR antibodies of the systemic lupus erythematosus (SLE) type and anti-C1qCLR antibodies of the hypocomplementemic urticarial vasculitis syndrome (HUVS) type were investigated. Cross-absorption and elution experiments combined with antibody detection by enzyme-linked immunosorbent assay (ELISA) and immunoblot analysis gave no evidence of cross-reactive anti-C1qCLR antibodies. However, one serum with HUVS type anti-C1qCLR antibodies contained anti-MBP antibodies that were cross-reactive with SP-A. Judging from results of ELISA inhibition experiments and immunoblot analysis, four SLE sera contained antibodies to native BK, while two sera with HUVS type anti-C1qCLR antibodies contained antibodies to epitopes of denatured BK. This might imply that autoimmunity to collagen-like structures is not restricted to C1qCLR in HUVS and HUVS/SLE overlap syndromes.     

Detection and quantitation of circulating immune complexes in arterial blood of patients with rheumatic disease

We developed antigen-nonspecific enzyme-linked immunoassays (ELISA) to quantitate IgG-C3- and IgM-C3-containing circulating immune complexes (CIC) in venous and arterial blood from rheumatic disease patients. Standards were diethylaminoethyl (DEAE)-purified, heat-aggregated IgG incubated with fresh human serum (for IgG-C3 CIC) and IgM rheumatoid factor-rich serum incubated with reduced, alkylated IgG and then with fresh human serum (for IgM-IgG-C3 CIC). Venous serum and plasma IgG-C3 and IgM-C3 CIC correlated closely (P less than 0.01). Rheumatoid arthritis (RA) and systemic lupus erythematous (SLE) patients had elevated levels of venous IgM-C3 CIC (P less than 0.0001) but not IgG-C3 CIC; patients with vasculitis, inflammatory rheumatic diseases, or noninflammatory rheumatic diseases had mean values similar to normal individuals. Venous IgG-C3 and IgM-C3 CIC did not correlate. Paired venous and arterial samples from 16 rheumatic disease patients averaged comparable amounts of IgG-C3 and IgM-C3 CIC, respectively; venous and arterial IgM-C3 CIC levels in patients significantly exceeded normals (P less than 0.05). Venous and arterial IgG-C3 CIC levels correlated closely (P less than 0.01) as did venous and arterial IgM-C3 levels (P less than 0.05). Thus, arterial CIC offered no advantage over venous determinations for rheumatic disease patients. IgM-C3 CIC were elevated in patients with RA and SLE when IgG-C3 CIC were not. Ig isotype-specific CIC quantitation may be useful for certain rheumatic diseases.