Circulating immune complex assays in patients with lupus and membranous glomerulonephritis

Complement and systemic lupus erythematosus

Complement is implicated in the pathogenesis of systemic lupus erythematosus (SLE) in several ways and may act as both friend and foe. Homozygous deficiency of any of the proteins of the classical pathway is causally associated with susceptibility to the development of SLE, especially deficiency of the earliest proteins of the activation pathway. However, complement is also implicated in the effector inflammatory phase of the autoimmune response that characterizes the disease. Complement proteins are deposited in inflamed tissues and, in experimental models, inhibition of C5 ameliorates disease in a murine model. As a further twist to the associations between the complement system and SLE, autoantibodies to some complement proteins, especially to C1q, develop as part of the autoantibody response. The presence of anti-C1q autoantibodies is associated with severe illness, including glomerulonephritis. In this chapter the role of the complement system in SLE is reviewed and hypotheses are advanced to explain the complex relationships between complement and lupus.

C1q as antigen in humoral autoimmune responses

The observation of anti-C1q antibodies (C1qAb) in patients with various autoimmune diseases has led to the establishment of a strong correlation between these antibodies and renal involvement in patients with systemic lupus erythematosus (SLE). The measurement of anti C1q antibodies requires detailed insight in the reactivity of C1q with immune complexes and the methods to detect C1q as an antigen by solid phase assays. In this overview we describe the pitfulls of the anti C1qAb assay and its use in the measurement of C1qAb. Further we discuss the relevance of C1qAb in the pathogenesis of SLE and especially in relation to lupus nephritis.

C1q and systemic lupus erythematosus

In this chapter we review the association between SLE and C1q. In the first part of the chapter we discuss the clinical associations of C1q deficiency, and tabulate the available information in the literature relating to C1q deficiency and autoimmune disease. Other clinical associations of C1q deficiency are then considered, and we mention briefly the association between other genetically determined complement deficiencies and lupus. In the review we explore the relationship between C1q consumption and lupus and we discuss the occurrence of low molecular weight (7S) C1q in lupus, which raises the possibility that increased C1q turnover in the disease may result in unbalanced chain synthesis of the molecule. Anti-C1q antibodies are also strongly associated with severe SLE affecting the kidney, and with hypocomplementaemic urticarial vasculitis, and these associations are also examined. We address the question of how C1q deficiency may cause SLE, discussing the possibility that this may be due to abnormalities of immune complex processing, which have been well characterised in a umber of different human models. There is clear evidence that immune complex processing is abnormal in patients with hypocomplementaemia, and this is compatible with the hypothesis that ineffective immune complex clearance could cause tissue injury, and this may in turn stimulate an autoantibody response. We have also considered the possibility that C1q-C1q receptor interactions are critical in the regulation of apoptosis, and we explore the hypothesis that dysregulation of apoptosis could explain important features in the development of autoimmune disease associated with C1q deficiency. An abnormally high rate of apoptosis, or defective clearance of apoptotic cells, could promote the accumulation of abnormal cellular products that might drive an autoimmune response. Anti-C1q antibodies have been described in a number of murine models of lupus, and these are also briefly discussed. We focus on the recently developed C1q "knockout" mice, which have been developed in our laboratory. Amongst the C1q deficient mice of a mixed genetic background high titres of antinuclear antibodies were detected in approximately half the animals, and around 25% of the mice, aged eight months had evidence of a glomerulonephritis with immune deposits. Large numbers of apoptotic bodies were also present in diseased glomeruli, and this supports the hypothesis that C1q may have a critical role to play in the physiological clearance of apoptotic cells.

Predictive value of IgG autoantibodies against C1q for nephritis in systemic lupus erythematosus

OBJECTIVES

Antibodies against C1q (C1qAb) have been demonstrated in the serum of patients with several immune complex diseases. Patients, particularly those with lupus nephritis, were found to have increased serum titres of IgG C1qAb in a cross-sectional analysis. In the present prospective study correlations were sought between serum titres of IgG C1qAb and clinical as well as laboratory parameters of disease activity in patients with systemic lupus erythematosus (SLE).

METHODS

Titres of IgG C1qAb in the serum of 68 SLE patients were measured serially during a three year period. At the same time clinical and laboratory parameters of disease activity were assessed.

RESULTS

Increased titres of IgG C1qAb were found in the serum of 56% of SLE patients during the study. Significant correlations were found between increased titres of IgG C1qAb and renal involvement. Clinical signs of renal involvement were found to be associated with significant increases of serum titres of IgG C1qAb in the six months preceding this appearance. Fifty per cent of the increases in serum titres of IgG C1qAb were followed by the development of renal involvement. Elevated serum titres of IgG C1qAb were especially related to proliferative forms of glomerulonephritis. Furthermore, significant correlations were found between serum titres of IgG C1qAb and serum levels of immune complexes, levels of complement components, and titres of antibodies to DNA.

CONCLUSIONS

The results suggest that IgG C1qAb play a pathogenic role in the development of lupus nephritis and that serial measurement of serum titres of IgG C1qAb is useful in the management of SLE patients.

IgG autoantibodies to C1q do not detectably influence complement activation in vivo and in vitro in systemic lupus erythematosus

The influence of IgG antibodies to C1q (C1qAb) on activation of the classical pathway of the complement system was investigated in patients with systemic lupus erythematosus (SLE). In in vivo experiments, a prototype for immune complexes was administered intravenously to 14 patients and 9 healthy controls. Eight SLE patients had increased C1qAb titers. The increase of C3a levels, which was measured as a parameter of C1 activation, was significantly lower in SLE patients than in the healthy controls (p = 0.01). No correlation was found between C3a increases and C1qAb titers. In in vitro experiments the influence on C1 activation of monomeric IgG isolated from serum of 11 SLE patients, 7 of whom had increased C1qAb titers, was measured in a C4 consumption assay. The presence of C1qAb did not influence C4 consumption. The results demonstrate that C1qAb do not influence C1 activation by immune complexes in SLE patients.

Detection of DNA-bound immunoglobulins in patients with lupus nephritis, using monoclonal anti-DNA antibody

In order to investigate the relationship between renal histopathology and the characteristics of circulating immune complexes (CICs) in patients with lupus nephritis (LN), we measured the sizes of CICs, DNA-bound immunoglobulins in patients with systemic lupus erythematosus (SLE) and different histopathological forms of nephritis. Sera were obtained from nine patients: four with diffuse proliferative LN (DPLN), four with membranous LN (MLN), and one with mesangial LN, who fulfilled the criteria of the American Rheumatism Association for SLE. The DNA-bound immunoglobulins were measured by ELISA, in which ELISA plates were coated with mouse monoclonal anti-DNA antibodies. The sizes of CICs were analysed by sucrose density gradient ultracentrifugation. Large (larger than 19S), intermediate (19-7S) and small (nearly 7S) sized DNA-bound immunoglobulins (high peaks of IgG and IgA, but low IgM peaks) were found in the patients with DPLN. By contrast, in patients with MLN, the sizes of ICs; DNA-bound IgG, IgA were in general slightly larger than 7S. In one patient with DPLN, at the onset, various sized DNA-bound IgG, IgA and IgM were found. After the methylprednisolone pulse therapy, CICs became smaller and gradually disappeared. We conclude that the characteristics of DNA-anti-DNA IgG, IgA complexes may determine the localization of ICs in the glomeruli and suggest that CICs play an important role in the pathogenesis of LN.

Antibodies to the collagen-like region of C1q in sera of patients with autoimmune rheumatic diseases

Antibodies to the collagen-like region of C1q have recently been observed in sera of patients with systemic lupus erythematosus (SLE). In this study, we documented that these antibodies were present in 47.3% of SLE patient sera, whereas they were uncommon in sera from patients with rheumatoid arthritis (2.8%) and Sjögren's syndrome (12.8%), as well as in normal sera (6.4%). Markedly elevated antibody levels (greater than 4 SD above the normal mean) were observed almost exclusively in sera of patients with SLE. Levels of antibodies to the collagen-like region correlated highly with levels of solid-phase C1q-binding IgG when analyzed by the C1q solid-phase assay for immune complexes (r = 0.87). We previously found that, after sucrose density gradient ultracentrifugation, a predominance of the solid-phase C1q-binding IgG in SLE sera sediments as monomeric IgG. These findings, together with the present data, indicate that reactivity of SLE patients' sera in the C1q solid-phase assay reflects primarily the presence of antibodies to the collagen-like region, and not the presence of immune complexes.

Low-molecular weight C1q-binding immunoglobulin G in patients with systemic lupus erythematosus consists of autoantibodies to the collagen-like region of C1q

The majority of C1q-binding IgG in sera of some patients with systemic lupus erythematosus (SLE) cosediments with monomeric IgG. This study was undertaken to provide definitive proof that the low-molecular weight C1q-binding IgG consists of autoantibodies to C1q. Monomeric C1q-binding IgG was isolated from five SLE plasmas by C1q affinity chromatography and gel filtration. All C1q-binding IgG preparations and their F(ab')2 fragments bound to both C1q and the collagen-like region of C1q by an ELISA. To rule out the possibility that small DNA-antiDNA immune complexes caused this binding activity, Fab' fragments of the C1q-binding IgG preparations were digested with DNase I to degrade any DNA. The Fab' fragments continued to bind to C1q and its collagen-like region after this treatment. C1q-binding IgG was heterogenous on isoelectric focusing. Interaction of C1q-binding IgG with solid-phase C1q was retained in 1 M NaCl, whereas the binding of DNA or heat-aggregated IgG to solid-phase C1q was abrogated or markedly diminished. The association constant of C1q-binding IgG with solid-phase C1q was 2.7 X 10(7) M-1. We conclude that low-molecular weight C1q-binding IgG in the studied patients with SLE consists of autoantibodies to the collagen-like region of C1q.

Airways disease in rheumatoid arthritis patients. One element of a general exocrine dysfunction

Airflow limitation is a frequent finding in patients with rheumatic diseases. We have previously suggested that it is associated with autoimmune exocrinopathy in Sjögren's syndrome. To compare clinical features of patients with and without airways dysfunction and to further test the hypothesis of a link between airways disease and exocrinopathy, we prospectively studied 2 groups of 15 lifetime nonsmoker female patients with seropositive rheumatoid arthritis (RA). The 2 groups were similar in their clinical and immunologic features, but differed in terms of airways function. Salivary, lacrimal, and sweat gland dysfunction were significantly more prevalent or severe in the group with airways disease. Antinuclear antibodies were also more prominent in the patients with airways disease, but antibodies against RNP, SS-A, SS-B, and double-stranded DNA were not present in these patients. HLA-DR4 was found in 80% of the RA patients with airways disease and in 57% of those without airways disease. HLA-B8 and DR3 were equivalently distributed in both groups. This prospective study further documents the existence of small airways disease in RA and supports the view that autoimmune exocrinopathy predisposes to its expression.

Eosinophilic synovitis: clinical observations on a newly recognized subset of patients with dermatographism

Eosinophilia of synovial fluid is uncommon. Using the identification of Charcot-Leyden crystals to alert for the presence of eosinophils, we have increased by a factor of 5 the detection rate of synovial fluid eosinophilia. We describe here our clinical and laboratory findings in 7 patients with this feature. We believe they constitute a defined syndrome. Typically, the patients were young (ages 18-51) and had a personal and family history of allergy. They developed an acute, painless monarthritis after a minor trauma, and had no concurrent allergic symptoms. Each episode resolved in 1-2 weeks without therapy, and 3 patients had recurrences. All had pronounced dermatographism. The synovial fluid was mildly inflammatory: 10,850 +/- 3,665 white blood cells/mm3, with 41 +/- 5% eosinophils (mean +/- SEM). The cellularity and chemistry of the peripheral blood was unremarkable, except for a mild elevation of IgE levels (370 +/- 104 IU/ml). The exact pathophysiologic mechanism underlying this benign entity is not clear, but we suspect a nonimmunologic triggering event is operant, i.e., synovial trauma which mimics the cutaneous dermatographism.

Differences in immunochemical characteristics of cryoglobulins in rheumatoid arthritis and systemic lupus erythematosus and their complement binding properties

Cryoglobulins isolated from sera of patients with rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) were analysed for their immunoglobulin, antibody, and complement components. In both disease categories the cryoglobulins contained predominantly IgG with lesser amounts of IgM and IgA, but relative to serum more IgM was concentrated in the cryoglobulins. IgM rheumatoid factor was found in 65% of RA cryoglobulins but in only 17% of SLE cryoglobulins (p less than 0.02), whereas SLE cryoglobulins contained more DNA binding activity than RA cryoglobulins (p less than 0.01). C1q binding activity was detectable in the majority of SLE and RA sera and SLE cryoglobulins. Paradoxically only two out of 34 RA cryoglobulins bound C1q, although rheumatoid factor activity was present in both cryoglobulins and sera. When isolated from serum the rheumatoid factor fraction strongly bound C1q. Both RA and SLE cryoglobulins contained similar small amounts of C3 and C4. Differences in antibody composition and complement binding activity of cryoglobulins from RA and SLE sera may reflect properties of immune complexes which affect their tissue localisation and pathogenicity.

Autoantibodies in juvenile arthritis

Sera from 104 children with JA with different onset-types of disease were evaluated for 19S IgM RF by the LFT , hidden 19S IgM RF by the hemolytic assay, ANA by HEp-2 cell substrate, and levels of IC by the C1qSPA . Their relationship to active disease was determined. Classical 19S IgM RF were detected by the LFT in only seven patients. All were late-onset polyarticular females. Hidden 19S IgM RF were detected by the hemolytic assay in the separated IgM-containing fraction in 55 patients of all onset-types. Clinical activity correlated with the presence of hidden 19S IgM RF in 82% of cases. ANA, using the HEp-2 cell substrate, were found in 61 patients, the majority showing a speckled, immunofluorescent pattern. ANA were noted in all RF positive patients and in nine of 10 patients with iridocyclitis. IC were found in 39 patients, and correlation with clinical activity occurred in 54% of cases. A search for positive associations among the four parameters showed no statistically significant correlations except for the concordance of ANA positivity in all seven RF positive patients. The presence of hidden RF correlated more closely with disease activity (P less than 0.001) than did that of ANA or IC. The significance of these data and previous studies remains to be determined. We have demonstrated that in the average JA population 7% have 19S IgM RF and about 60% have hidden RF, ANA, or elevated levels of IC. The present findings of 98 of 104 patients with at least one of the abnormal immunoproteins , the association of ANA in patients with iridocyclitis or with RF positivity, of hidden RF with disease activity, and the presence of 19S IgM RF in isolated IC suggest a possible immunologic etiology for JA.

Membranous and vascular choroidopathy: two patterns of immune deposits in systemic lupus erythematosus

Two patterns of immune aggregate localization were demonstrated by immunofluorescence and electron microscopy in the choroid plexus of four young women with fatal systemic lupus erythematosus. The two patients with granular immune aggregates localized to the basement membrane of the choroid epithelium (membranous choroidopathy) had subepithelial and intramembranous electron-dense deposits and membranous glomerulopathy in their kidneys. The two patients with immune aggregates in the walls of choroidal blood vessels (vascular choroidopathy) had subendothelial electron-dense deposits and proliferative glomerulonephritis. Vascular deposits in the choroid plexus were associated with capillary thrombi and extravasation of fibrinoid material, while isolated membranous choroidopathy had no histopathologic evidence of inflammation. The clinical presentation and serological studies of blood and cerebrospinal fluid were compared in an effort to discriminate between patients with membranous and vascular choroidopathy. All patients had variable neuropsychiatric symptoms and major motor seizures. While those with vascular choroidopathy had more evidence of disease activity in their sera, both groups demonstrated elevated titers of immune-complexed antinuclear antibodies in cerebrospinal fluid. Although both patterns of choroidal localization of immune aggregates were associated with neuropsychiatric dysfunction, we were unable to identify discrete clinical-symptom complexes which differentiated patients with membranous and vascular choroidopathy. These contrasting patterns of choroid plexus immunopathology suggest that factors responsible for differential localization of immune aggregates are not restricted to the renal glomerulus.

Differential characteristics of immune-bound antibodies in diffuse proliferative and membranous forms of lupus glomerulonephritis

Diffuse proliferative (PGN) and membranous (MGN) glomerulonephritis represent contrasting histologic lesions in systemic lupus erythematosus (SLE). Serum, cryoglobulins, and renal biopsies in 8 SLE patients with PGN and 8 with MGN were studied in order to determine whether variations in the properties of immune-bound antibodies correlate with the pattern of glomerular involvement. Several immunologic parameters suggested differences in complement activation between the two groups. PGN cryoglobulins demonstrated immunoglobulin G (IgG) anti-native DNA (nDNA) subclass heterogeneity with highest titers of IgG3. These findings contrasted with the observation that MGN was characterized by a predominance of IgG4 in cryoglobulins. The major glomerular IgG subclasses in PGN were IgG3 and IgG1, while MGN biopsies demonstrated IgG4 in largest amount. Serum C1q was lower in PGN than in MGN. Serum anti-nDNA antibodies, solid-phase C1q-binding IgG immune complexes, and cryoglobulin protein concentrations were higher in PGN sera. Cryoglobulin complement component and control protein concentrations were greater in PGN than in MGN, while cryoglobulin Ig and immune-bound anti-nDNA were not different. In vitro C3 fixation by cryoglobulin anti-nDNA was greater in PGN than in MGN. Glomerular C1q, C4-binding protein (C4bp), and C3c were present in comparable amounts to IgG deposits in PGN biopsies, while in MGN IgG was demonstrable in greater quantities than C1q, C4bp, and C3c. In contrast, glomerular C3d (alpha 2d), C5, C6, P, and H were comparable in the two groups. It was concluded that immune-bound antibodies in cryoglobulins and in glomerular immune deposits in SLE PGN appear to activate complement via the classical and alternative pathways, while complement activation in MGN appears to occur predominantly via the alternative pathway. These differences in IgG subclass composition may account for the differential complement activation and may explain the contrasting histologic expression of immune aggregate localization in glomerular capillaries in these variants of lupus nephritis.

Cold-precipitable immune complexes in collagen diseases: evidence for the coexistence of multiple types of circulating complexes in the same serum

In patients with systemic lupus erythematosus, mixed cryoglobulinemia, and rheumatoid arthritis, the presence of cold-precipitable immune complexes (IC) was investigated by means of two different methods, i.e., the Clq-binding activity (ClqBA) and a competitive enzyme immunoassay, based on solid-phase bovine conglutinin (K). Cold precipitability of IC ranged between 0 and 100% with K and between 0 and 71% with ClqBA. No correlation existed either between the levels or the cold precipitability of the IC measured by the two systems in the same sera. On the whole, cold-precipitable IC were better determined by the K method than by ClqBA and in mixed cryoglobulinemia cryocrit levels correlated with IC levels determined with K, but not ClqBA. These data provide direct evidence of the coexistence of several types of circulating IC in the same serum and that the two methods recognize, at least in part, different IC in the same specimen. It might be hypothesized that different IC present in a serum may have a distinct biological significance.

Relationship of clinical findings in systemic lupus erythematosus to seroreactivity

We have characterized 52 consecutive patients fulfilling 4 or more of the American Rheumatism Association criteria for systemic lupus erythematosus in order to provide, for the first time, a homogeneous sample for statistical comparison of antinuclear antibody (ANA)-positive and ANA-negative groups. Ten patients (19%) were seronegative. There was no significant difference in age, disease activity, organ system involvement, erythrocyte sedimentation rate, immune complex levels, or C3 levels. The ANA-negative group showed a higher incidence of involvement for whites and men. Leukopenia, lower levels of antibody to DNA, and higher C4 levels were also characteristic of the ANA-negative group.

Solid-phase Clq-binding fluorescence immunoassay for detection of circulating immune complexes

A fluorescence immunoassay for detection of immune complexes bound to solid-phase C1q was developed. The method was standardized by using human aggregated immunoglobulin G (IgG) to simulate immune complexes. A linear relationship existed between the concentrations of the aggregated IgG standards and the resulting fluorescent intensity. The method was found to be reproducible and capable of detecting as little as 10 micrograms of aggregated IgG per ml of heat-inactivated human serum. Antigen-antibody complexes prepared in vitro were detectable from equivalence to moderate antigen excess. Endogenous serum C1q inhibited the binding of aggregated IgG to solid-phase C1q. Pretreatment of test sera with EDTA was ineffective in eliminating this competitive effect. Heating the sera at 56 degrees C alleviated, but did not abolish, interference of endogenous C1q. Elevated levels of immune complexes were detectable in sera fro seven of nine patients wit systemic lupus erythematosus, provided the samples were heat inactivated before testing. Heparin and DNA were also found to interfere with the detection of aggregated IgG added to human serum. Assay values were falsely decreased due to competitive inhibition by these anions. Lipopolysaccharides from a variety of bacterial preparations produced no detectable interference. A comparative study was conducted on samples that had previously been tested by fluid-phase C1q-binding radioimmunoassay. The two methods were concordant in assigning normal or elevated levels of immune complexes in 70% of the samples tested. This solid-phase fluorescence immunoassay is proposed as a possible alternative to radioimmunoassay for the detection of circulating immune complexes.

Circulating immune complexes in idiopathic glomerular disease

Circulating immune complexes (CIC) could be found in the majority of 271 sera from 131 patients with idiopathic minimal change, membranous and mesangial proliferative glomerulonephritis when a combination of CIC assays detecting different properties of CIC were used. In neither individual patients nor in any of the three groups as a whole did CIC levels reflect the state of the renal lesion. No correlation was found between the class of immunoglobulin in the CIC and that deposited in the kidney. With the exception of minimal change disease in which non-C1q binding IgG CIC predominated, a range of CIC was found in the patients examined. The pattern of CIC detected did not allow different forms of renal disease to be distinguished. IgA CIC could be found in mesangial proliferative glomerulonephritis both with and without IgA deposition and in some patients with membranous and minimal change disease, as well as in a high proportion of sera from 12 patients with the Henoch-Schönlein syndrome. CIC size was estimated in six patients, but only in one did a specific size of complex predominate. The CIC which may be found in the majority of sera from patients with idiopathic glomerulonepohritis provide little information of clinical value; no direct relationship can be demonstrated between the CIC found and the renal lesion.

Therapeutic plasmapheresis in systemic lupus erythematosus. Effect on immune complexes and antibodies to DNA

The effect of plasmapheresis in 8 patients with systemic lupus erythematosus (SLE) was investigated. Drug treatment was maintained at a constant level for at least 4 weeks before plasmapheresis. Levels of immune complexes were measured by a Raji cell radioimmunoassay, and by a solid-phase C1q-binding assay. Antibodies to ds-DNA and ss-DNA were measured by the Farr assay. In all cases, immune complexes and antibodies were lowered by plasmapheresis. In 5 patients, plasmapheresis was followed by a rapid rebound of complexes and antibody to pretreatment levels. In 3 in whom plasmapheresis was followed by treatment with cyclophosphamide for 1 month, a sustained immunochemical and clinical improvement followed, lasting in 2 cases for up to 3 years.

Is autoimmunity a common denominator in immune complex diseases?

In normal circumstances, antibodies reactive with native DNA appear in the plasma during the course of many clinical conditions associated with inflammation. These antibodies seem to be elaborated in response to the release of exceptional amounts of DNA by nucleated cells. As a result, DNA/anti-DNA complexes can be demonstrated in the cryoprecipitable fraction of plasma from patients with various inflammatory diseases. A significant proportion of these immune complexes contain low-molecular-weight polynucleotide antigens. These polynucleotides are derived from DNA which has been degraded by plasma DNAase. Because of the digestion of DNA in the plasma a spectrum of antigen/antibody complexes forms. While large, relatively insoluble complexes would be expected to be rapidly cleared by the reticuloendothelial system, low-molecular-weight complexes are removed more slowly. It is proposed that the action of plasma DNAase upon both free and immune bound-DNA can lead to a preponderance of small, soluble, polynucleotide/anti-DNA complexes. Under appropriate conditions of vascular permeability, these soluble complexes may be deposited in vessel walls. Hence, regardless of the initiating infectious or inflammatory agent, polynucleotide antigen/anti-DNA antibody complexes form and can result in immune-mediated inflammatory phenomena in diverse disease states.