The specific detection of IgA in immune complexes

IgA vasculitis with nephritis: update of pathogenesis with clinical implications

IgA vasculitis with nephritis (IgAVN) shares many pathogenetic features with IgA nephropathy (IgAN). The purpose of this review is to describe our current understanding of the pathogenesis of pediatric IgAVN, particularly as it relates to the four-hit hypothesis for IgAN. These individual steps, i.e., hits, in the pathogenesis of IgAN are (1) elevated production of IgA1 glycoforms with some O-glycans deficient in galactose (galactose-deficient IgA1; Gd-IgA1), (2) generation of circulating IgG autoantibodies specific for Gd-IgA1, (3) formation of pathogenic circulating Gd-IgA1-containing immune complexes, and (4) kidney deposition of the Gd-IgA1-IgG immune complexes from the circulation and induction of glomerular injury. Evidence supporting the four-hit hypothesis in the pathogenesis of pediatric IgAVN is detailed. The genetics, pediatric outcomes, and kidney histopathologic features and the impact of these findings on future treatment and potential biomarkers are discussed. In summary, the evidence points to the critical roles of Gd-IgA1-IgG immune complexes and complement activation in the pathogenesis of IgAVN. Future studies are needed to characterize the features of the immune and autoimmune responses that enable progression of IgA vasculitis to IgAVN.

Henoch-Schonlein purpura nephritis: pathophysiology, treatment, and future strategy

Henoch-Schönlein purpura nephritis is a rare kidney disease leading to chronic kidney disease in a non-negligible percentage of patients. Although retrospective studies suggest beneficial effects of some therapies, prospective randomized clinical trials proving treatment efficacy are still lacking. The dilemma of spontaneous recovery even in patients with severe clinical and histologic presentation and of late evolution to chronic kidney disease in patients with mild initial symptoms renders it difficult for clinicians to expose patients to treatment protocols that are not evidence-based. A better understanding of the pathophysiology of progression to chronic kidney disease in Henoch-Schönlein purpura patients could be achieved by designing prospective international multicenter studies looking at determinants of clinical and histopathological evolution as well as possible circulating and urinary markers of progression. Such studies should be supported by a database available on the web and a new histologic classification of kidney lesions. This paper reports clinical, pathologic, and experimental data to be used for this strategy and to assist clinicians and clinical trial designers to reach therapeutic decisions.

Pathogenesis of Henoch-Schönlein purpura nephritis

The severity of renal involvement is the major factor determining the long-term outcome of children with Henoch-Schönlein purpura (HSP) nephritis (HSPN). Approximately 40% children with HSP develop nephritis, usually within 4 to 6 weeks after the initial onset of the typical purpuric rashes. Although the pathogenetic mechanisms are still not fully delineated, several studies suggest that galactose-deficient IgA1 (Gd-IgA1) is recognized by anti-glycan antibodies, leading to the formation of the circulating immune complexes and their mesangial deposition that induce renal injury in HSPN.

What is the difference between IgA nephropathy and Henoch-Schönlein purpura nephritis?

Henoch-Schönlein purpura nephritis (HSPN) and IgA nephropathy (IgAN) are considered to be related diseases since both can be encountered consecutively in the same patient, they have been described in twins, and bear identical pathological and biological abnormalities. Apart from the presence of extrarenal clinical signs found only in HSPN, other differences are noticed between the two diseases. The peak age ranges between 15 and 30 years for a diagnosis of IgAN, whereas HSPN is mainly seen in childhood. Nephritic and/or nephrotic syndromes are more often seen at presentation in HSPN. In contrast to IgAN, HSPN has been described in association with hypersensitivity. Endocapillary and extracapillary inflammations as well as fibrin deposits in the glomerulus are more frequent in HSPN. No major biological differences have been found between the two illnesses, except for a larger size of circulating IgA-containing complexes (IgA-CC) and a greater incidence of increased plasma IgE levels in HSPN. As tissue infiltration by leukocytes is a major feature of HSPN vasculitis, a possible role of a more potent activation of the latter cells by IgA-CC and/or circulating chemokines in HSPN should be considered. Further studies are required to elucidate this possible mechanism as well as the role of hypersensitivity in HSPN.

Is there IgA from gut mucosal origin in the serum of children with Henoch-Schönlein purpura?

Thirty-two children with Henoch-Schönlein purpura with or without renal symptoms were studied to characterize the IgA hyperglobulinemia observed in the serum of most patients. It was shown that only the IgA1 subclass concentration was increased. Secretory IgA and IgA to gliadin levels were frequently increased in serum, with a good correlation between them. Circulating IgA immune complexes were detected often and contained high activity to gliadin. In contrast, IgA activity to tetanus toxoid did not change. We failed to show any differences in renal involvement. These data suggest that elevation of serum IgA in Henoch-Schönlein purpura is due in part to a disturbance of the gut mucosal immune system, and the presence of circulating IgA immune complexes with dietary antigens can be postulated but cannot explain the occurrence of urinary symptoms.

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.

Glomerulonephritis in Henoch-Schöenlein purpura without mesangial IgA deposition

Ten patients with Henoch-Schöenlein purpura (HSP) were selected for study because their early renal biopsies showed focal and segmental hypercellularity, with IgA present only in deposits at the periphery of the lobules. Mesangial deposits of IgA were absent. All had laboratory evidence of nephrotic syndrome and/or renal compromise. The glomerular hypercellularity was largely the result of the infiltration of monocytes whose cytoplasm often contained tubular lysosomes and wrapping lysosomal membranes, evidence of monocyte activation. Mean levels of C3 were normal but those of C4 and properdin significantly depressed. This complement profile, as well as a glomerular monocytic infiltrate, are also seen in essential cryoglobulinemia in the adult. Of follow-up biopsies in six patients, the glomeruli were normal in three, with no IgA deposition. In the other three, mesangial deposits of IgA typical of HSP were present. The initial focal-segmental glomerulitis of these patients appeared to be the benign first phase of a disease which had the potential to culminate in mesangial IgA deposition. Patients like the three who escaped mesangial IgA would be among those responsible for the observed dissociation between severity of the initial illness and ultimate prognosis.

Evidence that the interaction between circulating IgA and fibronectin is a normal process enhanced in primary IgA nephropathy

A solid-phase ELISA was set up to measure the direct binding capacity (BC) of different, commercially available, purified human IgA preparations to plates coated with human fibronectin (FN). It was found that secretory, polymeric, and, to a much lesser extent, monomeric IgA exhibited elevated FN-BC as compared to their BC to plates coated with bovine serum albumin. This binding was specific since not observed with human IgG or IgM antibodies. In addition, we noted that this interaction was dose dependent, Ca2+ dependent, saturable, and not covalent, was inhibited by soluble FN, but not by a prior incubation of FN-coated plates with anti-human fibronectin antibodies, and appeared to involve on the dimeric FN other structures than its heparin-binding, collagen-binding, or C1q-binding domains. Similar experiments conducted with normal plasma indicated that plasma IgA, but not plasma IgG or IgM, was also capable of significant binding to FN-coated plates. In contrast, serum IgA did not significantly bind to those plates under otherwise identical experimental conditions. Thus, the coagulation process induces a strong decrease in the FN-BC of circulating IgA, which implies the necessity of using plasma rather than serum to study such interactions. The apparent molecular weight of plasma IgA interacting with FN-coated plates ranged between 450 and 900 kd, and its major binding characteristics were quite similar to those observed with purified polymeric IgA. The FN-BC of plasma IgA was then measured by the same ELISA in 30 patients with primary IgA nephropathy (IgAN) and in 23 healthy controls. The mean FN-BC of plasma IgA was significantly higher in patients than in normal controls. This enhancement was due mainly to the augmentation in the concentration of circulating "macromolecular" IgA and was significantly correlated with the plasma levels of IgA-FN complexes. However, the pathogenetic role of these findings was probably not determinant since similar observations were made in alcoholic liver cirrhosis without urinary abnormalities and since the FN-BC of plasma IgA or the plasma levels of IgA-FN complexes were not correlated with the various biological parameters of evolutivity of primary IgAN. In conclusion, these studies suggest that the ability of polymeric IgA to directly bind to FN is involved in the formation of circulating IgA-FN complexes and that this normal binding process, although enhanced in IgAN, is probably not responsible for kidney injury, at least in the patients studied.

Evaluation of the efficiency of two IgA immune complex assays

Two IgA immune complex assays, namely the anti-IgA inhibition of binding assay (a-IgA-InhBA) and the IgA polyethylene glycol assay (IgA-PEG assay) were evaluated using IgA aggregates (AIgA) of well defined sizes. AIgA was prepared by heat treatment. After ultracentrifugation the size of AIgA was found to be in a range between 7 S and 64 S. Five AIgA pools (greater than 64 S, 44-64 S, 24-43 S, 20-24 S and 9-19 S) and a pool of monomeric IgA were prepared to test the efficiency of the two assays. The IgA-PEG assay exclusively detected very large sized AIgA (greater than 64 S), whereas the a-IgA-InhBA detected also AIgA of intermediate size (9-19 S). The sensitivity of the latter assay was found to be size dependent. After ultracentrifugation of serum samples from patients with IgA nephropathy, IgA of large and intermediate macromolecular size was detected. It is concluded that the a-IgA-InhBA is useful for the detection of circulating IgA-containing immune complexes in IgA nephropathy.

The rheumatic poison: a survey of some published investigations of the immunopathogenesis of Henoch-Schönlein purpura

Laboratory studies of the pathophysiology of Henoch-Schönlein purpura (HSP) have become more numerous in recent years with the recognition of the disease's links with the mucosal immune system in general and IgA nephropathy in particular. There are weak genetic associations with C4 null phenotypes and with HLA B35 and DR4. Studies of plasma proteins in HSP patients show an increased IgA concentration, activation of the alternative pathway of complement and consumption of factor XIII. High molecular weight (polymeric) IgA has been detected in affected individuals, which some investigators have called "immune complexes". Many patients synthesise an IgA rheumatoid factor in the acute phase, but other autoantibodies are largely absent. In vitro studies of lymphocytes from HSP patients have demonstrated an increased number of IgA-bearing and secreting B-cells, with altered T-cell regulation of antibody synthesis. While these observations point to immune dysregulation--primarily of IgA production--as a consistent feature of acute HSP, there is as yet insufficient information available to allow a consistent theory of pathogenesis to be formulated.

Inflammatory bowel disease and ankylosing spondylitis associated with cutaneous vasculitis, glomerulonephritis, and circulating IgA immune complexes

Two patients both with inflammatory bowel disease (IBD) and ankylosing spondylitis (AS) developed leucocytoclastic vasculitis of the skin and nephropathy. Immunofluorescence studies showed that there was perivascular deposition of immunoglobulin A in the skin biopsy specimens of both patients and in the renal mesangium of one patient. Serum samples of the two patients contained IgA immune complexes. The absence of previous reports on such a combination of symptoms in IBD or AS suggests that these patients may have a disease entity which is distinct from uncomplicated IBD or AS, and which may combine the immunopathological features of both underlying disorders.

C3-containing serum immune complexes in patients with systemic lupus erythematosus: correlation to disease activity and comparison with other rheumatic diseases

Although testing for circulating immune complexes (CIC) is regarded as a useful, complementary, laboratory parameter in the differential diagnosis and management of immune complex-induced vasculitis syndromes, there is still an uncertainty with regard to assay systems used for the demonstrated of soluble immune complexes. This is partly due to difficulties in the reproducibility, handling and principle limitations of available test systems for the assessment of soluble immune complexes in body fluids. In the present communication a modification of the anti-C3 test for the determination of CIC was developed using nitrocellulose as a solid phase matrix. IgG-, IgA- and IgM-containing CIC were determined and quantified using standard immune complex preparations. When 39 sera of SLE patients, 12 sera of patients with vasculitis syndromes, 10 sera of rheumatoid arthritis patients and 11 sera of patients with ankylosing spondylitis were tested, predominantly IgG-containing CIC could be demonstrated. Only in SLE patients was a significant amount of other immunoglobulin isotypes detected in CIC. In these patients a significant difference of IgG-containing CIC levels was found with regard to patients with high and low disease activity (P less than 0.0001). A significant correlation was also established between IgG-containing CIC and anti-dsDNA antibodies (P less than 0.001). In a longitudinal study the isotypes in the isolated CIC were found to be constant.

Immune complex assays: diagnostic and clinical application

The presence of circulating immune complexes have been described in many different human disease states but the significance of their presence has always been a subject for debate. Improvements in the methods of detecting immune complexes have demonstrated a wide degree of heterogeneity, which accounts for the difficulty in obtaining accurate and reproducible measurements, even in the same individual. Techniques for isolating individual complexes, characterizing their pathophysiological properties, and biochemically analyzing the nature of the complexed antigen are now being used to provide data that is helping to clarify the role of immune complexes in the pathogenesis of disease. In addition, such studies are also providing data which is proving that immune complexes have a potential role in immune regulation.

Occurrence of polymeric IgA1 rheumatoid factor in the acquired immune deficiency syndrome

The sera of 34 acquired immune deficiency syndrome (AIDS) patients and 20 healthy male homosexuals were examined for the presence of elevated levels of IgA and IgM rheumatoid factor (RF) and compared with results obtained with sera from 23 healthy laboratory volunteers. IgA RF levels were elevated (greater than 3 standard deviation units) in 9 of 34 (26%) patients with AIDS as compared to the panel of laboratory controls. Levels of IgM RF did not differ significantly in the AIDS patients and in the controls. There were no differences in levels of either IgA RF or IgM RF when the homosexual controls were compared with the laboratory volunteers. Sucrose-gradient ultracentrifugation experiments and assays using monoclonal reagents specific for IgA subclasses indicated that the IgA RF was predominantly of the polymeric configuration and restricted to the IgA1 subclass, respectively. Polyethylene glycol (PEG) precipitates of serum enriched for circulating immune complexes (CIC) were also assayed for the presence of IgA RF and IgM RF. Although levels of IgA RF in serum and in PEG precipitates did not correlate with levels of IgA- or IgA/IgG-containing CIC in AIDS patients, levels of IgA RF in both serum and CIC-enriched material were significantly elevated in the AIDS population when compared with the control panel. In contrast, levels of IgM RF in both serum and CIC-enriched material were low and not significantly different from those in healthy controls. These results indicate that both IgA-containing CIC and IgA RF occur in many AIDS patients and raise the possibility that IgA RF may contribute significantly to the formation of immune complexes in this disease.

Circulating IgA immune complexes and skin IgA deposits in liver disease. Relation to liver histopathology

Alcoholic liver disease (ALD) is characterized by elevated serum IgA concentrations, the presence of circulating immune complexes containing IgA, and IgA deposits along sinusoids in the liver. When combined with the presupposed IgA-clearance function of the liver, a causal association between IgA abnormalities and the liver disease in ALD can be suggested. This prompted us to study the presence and concentration of circulating IgA-containing immune complexes (IgA-CIC) in 41 patients with ALD and 41 patients with other nonalcoholic liver diseases having comparable serum IgA levels. We searched for relationships among IgA-CIC and history of alcohol abuse, liver histopathology, and IgA deposits in the liver. Using an anti-IgA inhibition binding assay, 56% of the patients exhibited IgA-CIC in polyethylene glycol precipitate of serum and 38% showed IgA-CIC in whole serum. The prevalence and concentration of IgA-CIC was lowest in cases with nonspecific changes or steatosis in the liver biopsy and highest in cases with hepatitis or cirrhosis (P less than 0.01). The occurrence of IgA-CIC was not related to a history of alcohol abuse or to the presence of IgA deposits along hepatic sinusoids (which occurred in 78% of ALD and 20% of non-ALD cases). A skin biopsy was available from 34 patients (19 with ALD and 15 with non-ALD). In 68% of these biopsies, IgA deposits were observed in superficial blood capillaries.(ABSTRACT TRUNCATED AT 250 WORDS)

IgA1 is the major immunoglobulin component of immune complexes in the acquired immune deficiency syndrome

Compared to a panel of healthy controls, sera from 13 of 23 (57%) patients with the acquired immune deficiency syndrome (AIDS) were shown to have elevated levels of circulating immune complexes (CIC) containing IgA. Levels of IgG-containing CIC were increased in seven patients (30%); no patients had elevated levels of IgM-containing CIC. Additional experiments showed that in all instances in which IgG CIC were demonstrable, IgA was also present; however, IgA CIC could be found that did not contain IgG. The IgA in the CIC was restricted to the IgA1 subclass. These data suggest selective abnormalities of IgA regulation in AIDS and raise questions as to the role in this disease of the immunoglobulin isotype usually thought to possess different protective mechanisms from those attributed to other isotypes.

Composition of IgA immune complexes precipitated with polyethylene glycol. A model for isolation and analysis of immune complexes

Covalently cross-linked large and intermediate-sized IgA oligomers, prepared with IgA anti-dinitrophenyl (DNP) and bis-DNP-pimelic acid ester, were used to examine the ability of different concentrations (3.5%, 5% and 7%, w/v) of polyethylene glycol (PEG) to precipitate IgA immune complexes (IgA-IC). The size of the IgA-IC precipitated with PEG was determined by gradient polyacrylamide gel electrophoresis and quantitative autoradiography. The standard concentration of 3.5% PEG precipitated only a minor fraction (20%) of the IgA-IC. In contrast, 5% and 7% PEG precipitated 45% and 79% of the complexes, respectively. To test the influence of the antigen on the PEG assay. IgA-IC prepared with IgA anti-DNP and DNP conjugates of either bovine serum albumin or Ficoll were also used. Approximately 38% of these IgA-IC were precipitated with 3.5%, PEG. By comparison, the concentration of 5% and 7% PEG precipitated 60% and 76% of the IgA-IC, respectively. Distilled water rather than the standard borate-buffered saline was shown to be the optimal solvent for resolubilization of the PEG precipitates. Serum samples from 22 IgA nephropathy patients and 12 normal donors were tested with 3.5%, 5% and 7% PEG. Only the 7% PEG assay showed a significant difference between patients and controls (P less than 0.001) in the IgA levels of precipitates. Thus, the use of 7% PEG is recommended for the detection, isolation and analysis of large- and intermediate-sized IgA-IC.

Anti-alpha-galactosyl antibodies and immune complexes in children with Henoch-Schönlein purpura or IgA nephropathy

Episodes of hematuria in IgA nephropathy or Henoch-Schönlein purpura are frequently associated with microbial infections. Some of those infectious agents bear alpha-galactosyl residues on their cell surface. These observations prompted us to determine, by passive hemagglutination, the titers of natural anti-galactosyl antibodies in the serum of children presenting with Henoch-Schönlein purpura (10 cases) or IgA nephropathy (7 cases). Antibody titers of normal subjects (103 cases), children with a pharyngitis of unknown etiology (7 cases), and children exhibiting mesangial IgA deposits but no hematuria at the time of testing (6 cases) ranged from 1:20 to 1:80. Elevated titers (greater than 1:80) were observed in nine of 11 patients with mesangial IgA deposits and micro- or macroscopic hematuria, in nine of 19 children with other evolutive glomerular diseases (5 cases of acute glomerulonephritis and 4 cases of minimal change disease), and in most subjects presenting with a M. pneumoniae (4/5 cases) or a E. Coli (4/5 cases) infection. Antibody titers decreased after incubation of normal and pathological sera with D-galactose (10 mM) or with alpha-galactosyl-glucoside (10 mM), but not with D-glucose (10 mM). The anti-alpha-galactosyl antibodies purified, by affinity chromatography, from sera of 10 normal children, 10 pathological controls and four children with mesangial IgA deposits without hematuria belonged to IgG class. In contrast, both IgG and IgA anti-alpha-galactosyl antibodies were detected in six of six patients with mesangial IgA deposits and hematuria. The IgA content of immune complexes detected in those patients decreased after incubation of sera with alpha-galactosyl-glucoside, but not with D-glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

Circulating immune complexes and immunoglobulin A rheumatoid factor in patients with mesangial immunoglobulin A nephropathies

Circulating immune complexes (CIC) containing IgA and C3 were elevated in 48% of IgA nephropathy patients; IgA1 was the predominant subclass. IgA1-IgG CIC were detected in 44%, IgA2-IgG CIC in 7%, and IgM-IgA1 CIC in 16% of the patients. No IgM-IgA2 CIC were detectable. Sucrose gradient ultracentrifugation indicated that IgG-IgA1 CIC were predominantly of intermediate (13-19S) size whereas IgA1-C3 CIC sedimented from 11S to 19S. At acid pH, isolated CIC revealed the presence of substantial amounts of 7S IgA. One third of the patients had elevated serum IgA rheumatoid factor (RF) of both polymeric and monomeric forms despite normal levels of IgM-RF; 87% of patients with elevated IgA-RF had IgA1-IgG CIC. These results indicate that the IgA1 component of CIC in patients with IgA nephropathy is not necessarily of mucosal origin and suggest that a portion of these CIC consists of IgA RF immunologically complexed with autologous IgG.

Circulating and mesangial secretory component-binding IgA-1 in primary IgA nephropathy

In a prospective study of 38 patients who presented with hematuria of renal origin, 15 patients were found to have primary IgA nephropathy and 23 had other renal disorders. Sera and renal biopsy specimens of these patients were studied for the presence of macromolecular IgA1 and IgA2 using monoclonal antibodies, and the presence of J-chain as demonstrated either by immunofluorescence or its capacity to bind free secretory component. Circulating macromolecular IgA was found exclusively in the sera of patients (80%) with primary IgA nephropathy. In these sera the polymer/monomer ratio for IgA1 (0.64 +/- 0.13) was significantly higher than for normal human serum (0.39 +/- 0.01) (P less than 0.001), while no differences were found for IgA2. The polymeric IgA1 was isolated from serum by gel chromatography and was shown to have the capacity to bind free secretory component. Direct two-color immunofluorescence studies revealed the presence of only IgA1 in the mesangial deposits and also its capacity to bind free secretory component. We conclude (1) that demonstration of circulating macromolecular IgA in patients with renal hematuria is of diagnostic value and (2) that antigenetic similarities between the circulating and the mesangial macromolecular IgA suggest that dimeric IgA1 is deposited in the mesangium of patients with primary IgA nephropathy.

Immunoelectron microscopy of IgA nephropathy

The distribution of IgA, IgG, IgM, C3, and albumin in kidney biopsy specimens from 11 children and adults with recurrent gross and microscopic hematuria and IgA nephropathy and 7 control specimens were evaluated by the direct peroxidase-labeled antibody method and electron microscopy. Granular masses of reaction product (RP), representing IgA, IgG, IgM, and C3, were observed within the mesangial matrix of glomeruli from all patients with IgA nephropathy. Occasional smaller masses of IgA-RP and C3-RP were noted along the peripheral glomerular capillary loops, the tubular basement membranes, and within the interstitial matrix of some patients. Large amounts of IgA-RP and C3-RP were present within the walls of small renal arterioles of several patients. These observations support the concept that immune-complex deposits are involved in the pathogenesis of IgA nephropathy and suggest that vascular deposits may have a more important role in the progression of the disease in some patients.

Presence of circulating macromolecular IgA in patients with hematuria due to primary IgA nephropathy

The relation between renal histologic features and the presence of circulating immune complexes was studied in 50 patients with hematuria. Primary IgA nephropathy was found in 25 patients, and various other forms of glomerulopathy were seen in the remaining 25 patients. Circulating immune complexes were detected with the 125I-C1q-binding assay, the conglutinin-binding assay, and the anti-IgA inhibition binding assay, the latter detecting specifically IgA-containing immune complex-like material. The 125I-C1q-binding assay gave negative findings for all patients except one. With the conglutinin-binding assay, immune complexes were found in a similar frequency for patients with and without IgA nephropathy. However, the anti-IgA inhibition binding assay gave positive results only in patients with primary IgA nephropathy (68 percent) and in none of the other patients. Sucrose density ultracentrifugation, as well as experiments in which the anti-IgA inhibition binding assay was performed with and without pretreatment of serum with polyethylene glycol, showed the presumed IgA immune complexes to have intermediate sedimentation coefficients (11 to 21S). The presence and level of this macromolecular IgA in the circulation correlated significantly (p less than 0.001) with the presence of hematuria in patients who had this clinical manifestation intermittently. Furthermore, a significant correlation (r = 0.69, p less than 0.0001) was found between the degree of hematuria and the degree of positive findings of the anti-IgA inhibition binding assay. This study shows that in patients presenting with hematuria, a positive finding on the anti-IgA inhibition binding assay is restricted to patients with primary IgA nephropathy and therefore could be of diagnostic value.