Immunopathogenesis of experimental IgA nephropathy

Conclusions The ultimate purpose of this review of experimental IgA nephropathy has been to identify the important immunopathogenetic factors that need to be elucidated en route to clear understanding of clinical IgA nephropathy. Summarizing from the discussion above, key experimental findings include the following.All the experimental models propound glomerular IgA-IC deposits as an end product of IC formation. The molecular form of IgA plays an important role in causing IC formation. Monomer IgA can only form small-sized complexes without any propensity for glomerular deposition. In contrast, polymeric IgA can form large-sized complexes that get entrapped in the mesangium. Free antigenic determinants in the deposited polymeric IgA complexes may serve as a binding site for circulating monomer IgA. Hence IgA-IC deposits may represent a continuous process of entrapped circulating macromolecules and in situ formed complexes.The liver has a major responsibility of maintaining the circulation free of IgA-IC. Under normal conditions, IgA-specific receptor on sinusoidal cells removes rapidly large-sized complexes. Hepatobiliary dysfunction or saturation of the elimination process may induce or contribute to glomerular IgA-IC deposition. Exacerbating factors under these conditions may also include heightened IgA immune response to alimentary and microbial antigens due to celiac disease or alcohol consumption.Macromolecules of purified human or murine IgA, free in solution or insolublized as renal deposits, lack the ability to activate the complement system. Glomerular C3 deposits that parallel the IgA pattern are due to complement activation by the antigen component of the IgA-IC deposits. Experimental and clinical evidence does not support the assumption that the complement system is essential for renal injury.The search for the inductor of tissue injury in the glomerular IgA-IC deposits revealed the antigen as the main proponent of damage. Comparative and correlative studies demonstrated the antigen as an elicitor of mesangial cell proliferation, inflammatory cell infiltration, fibrin deposition, and proteinuria production. This inflammatory process may also culminate from a cascade of engendered cytokines and eicosanoids.Although the extrapolation of experimental findings to IgA nephropathy is still in a developmental stage, astonishing progress has been made. In some instances, experimental investigations blazed the way for human studies that mutually reciprocated with seminal clinical observations providing essential clues and guides for the research to follow. Comparative analysis of the clinical and experimental literature on IgA nephropathy reveals parallel and complementary tracks that are converging on the preeminent objective of understanding and treatment of human renal disease.     

Immunohistochemical study of the membrane attack complex of complement in IgA nephropathy

Summary The localization of the membrane attack complex of complement (MAC) was examined in the normal human kidneys and in biopsy specimens from patients with primary IgA nephropathy by immunofluorescent and immunoelectron microscopies. Immunofluorescent staining for MAC was significantly more intense than in the normal kidneys, and was observed in the mesangium and occasionally along the glomerular capillary walls of 22 of 30 patients with IgA nephropathy. By dualstaining, the MAC deposits were generally concordant with the deposits of IgA, C3, C5 and C9, or of IgG, when present. C1_q or C4 was infrequently observed in the glomeruli. Immunoelectron microscopy revealed various staining patterns of glomerular MAC deposition; homogeneous fine-granular staining beneath the glomerular basement membrane (GBM) in the paramesangial zone, patchy staining within the mesangial electron dense deposits (EDD), and ring-shaped or ribbon-like staining, associated with the striated membrane structures (SMS), in the matrix of the mesangium, GBM and tubular basement membrane (TBM). This study suggests that the terminal complement system is activated, mainly by an alternative complement pathway mechanism, in the mesangium of IgA nephropathy, and is associated with the paramesangial lesion and EDD. MAC deposition in glomerular SMS may also result from in situ activation rather than trapping from the circulation. There was little correlation between glomerular MAC deposition and proteinuria or renal histology of patients with IgA nephropathy.     

Relationship between complement activation and renal deposition of immune complexes made with IgG2a monoclonal antibodies

Previous studies identified a single murine monoclonal IgG2a anti-dinitrophenyl antibody that, when combined with the antigen, formed immune complexes (IC) that were preferentially deposited in glomeruli. The present study examined the clearance and organ localization in Balb/c mice of expanded panels of radiolabeled IC containing murine monoclonal antibodies. The results identified a second IgG2a antibody that formed IC with a predilection for renal deposition. IC made with the two IgG2a antibodies that were preferentially deposited in the kidney were the least efficient binders of human C1q or homologous murine C3b and C4b within the IgG2a panel. These observations suggest a new model of IC-mediated renal disease initiation in which relatively weak complement activation leads to inefficient IC clearance by complement receptor-bearing circulating cells and consequent IC deposition in tissues susceptible to IC-mediated injury.     

The role of mesangial complement in the hematuria of experimental IgA nephropathy

We sought to determine if codeposits of IgG and IgM and glomerular complement, observed in most cases of human IgA nephropathy, might be important for inducing hematuria. All combinations of three binary variables, the protein immunogen, the duration of oral immunization, and the protein used for intravenous challenge, were accommodated by eight groups of BALB/c mice in an active model of IgA nephropathy. Mice drank 0.1% solutions of either of two proteins for either 6 or 14 weeks, and then were challenged intravenously with either the same protein or the alternate protein. After 6 weeks, all mice had significant increases of serum IgA, IgG, and IgM antibody to the oral immunogen. At 14 weeks, IgG and IgM antibodies were reduced, presumably due to the onset of oral tolerance, but IgA titers persisted. Nearly all mice had mesangial deposits of IgA and oral immunogen. However, only mice immunized for 6 weeks and challenged with the same protein had significant IgG and IgM deposits (100%), C3 deposits (76%), and significant microhematuria. To distinguish between the role of IgG/IgM codeposits and C3 in the pathogenesis of the hematuria, we induced passive IgA nephropathy with immune complexes of monoclonal IgA anti-dinitrophenyl antibody, dinitrophenyl-bovine albumin as antigen, and one of two monoclonal IgG antibodies specific for dinitrophenyl; one of the IgGs fixes complement, the other does not. Despite comparable mesangial deposits of IgA, IgG, and antigen, only mice given immune complexes containing the complement-fixing IgG had glomerular C3 and hematuria. Furthermore, when mice depleted of serum complement via cobra venom factor were given immune complexes containing the complement-fixing IgG, no glomerular complement was observed and no hematuria ensued. We conclude that IgG/IgM codeposits in murine IgA nephropathy do not directly cause hematuria but do induce the deposition of complement, which is in turn required for glomerular injury.     

Glomerular deposition of IgA in experimental hepatic cirrhosis

Wistar rats rendered cirrhotic with carbon tetrachloride excreted significant proteinuria and hematuria. Serum levels of IgA and IgG were significantly elevated in cirrhotic animals. They showed mild mesangial proliferation and immunofluorescent studies revealed deposits of IgA and IgG predominantly in mesangial areas and along capillary walls. These findings were very similar to those seen in patients with hepatic cirrhosis or IgA nephropathy. The deposits of IgA were also found in hepatic tissue from cirrhotic animals. The intensity and distribution of glomerular IgA deposits were not diminished after treatment with acid buffer. These results suggest that glomerular IgA are IgA polymers and decreased hepatic clearance of hepatic IgA polymers may be responsible for the glomerular deposition of IgA.     

GLOMERULAR DEPOSITION OF IgA IN EXPERIMENTAL HEPATIC CIRRHOSIS

Wistar rats rendered cirrhotic with carbon tetrachloride excreted significant proteinuria and hematuria. Serum levels of IgA and IgG were significantly elevated in cirrhotic animals. They showed mild mesangial proliferation and immunofiuorescent studies revealed deposits of IgA and IgG predominantly in mesangial areas and along capillary walls. These findings were very similar to those seen in patients with hepatic cirrhosis or IgA nephropathy. The deposits of IgA were also found in hepatic tissue from cirrhotic animals. The intensity and distribution of glomerular IgA deposits were not diminished after treatment with acid buffer. These results suggest that glomerular IgA are IgA polymers and decreased hepatic clearance of hepatic IgA polymers may be responsible for the glomerular deposition of IgA. ACTA PATHOL. JPN. 35: 561–567, 1985.     

Mechanisms of clearance of immune complexes from peritubular capillaries in the rat.

These experiments evaluated extraglomerular sites of renal immune complex (IC) deposition and specific features of host capability to remove these IC. Ex vivo perfusion of rat kidneys with the endothelium binding lectin concanavalin A (con A) followed by rabbit anti con A IgG results in a subendothelial IC nephritis in glomerular capillaries (GC) and diffuse IC formation with complement (C3) deposition in peritubular capillaries (PC). Histologic, immunofluorescence, and ultrastructural studies were performed at 10 minutes and 1, 4, and 24 hours after perfusion. At 10 minutes, strong linear binding of con A, rabbit IgG, and rat C3 to the endothelium was detected by immunofluorescence in both GC and PC. In GC this was followed by endothelial cell swelling and denudation (1 hour) with platelet and neutrophil infiltration and formation of subendothelial IC deposits which persisted at 4 and 24 hours. In contrast, some PC endothelial swelling was also present at 10 minutes and 1 hour, but ICs (IgG, con A, C3) were capped and shed into capillary lumina at 1 to 2 hours with complete clearance of IC by 4 hours. Selective neutrophil depletion, by antisera and irradiation, and complement depletion with cobra venom factor, delayed clearance of PC IC by several hours but complete clearance of IC with restored structural integrity of PC was still achieved by 24 hours. Platelet depletion had no effect on PC IC clearance. These studies demonstrate a model for study of PC IC. Such a model may aid our understanding of lupus nephritis in which extensive GC IC deposits associated with severe inflammatory injury may coexist with PC deposits. Efficient clearance of IC in PC compared with GC may be due to differences in hemodynamic forces, amounts of IC formed in each of these sites, differences in binding of IC to subendothelial basement membrane, or phenotypic specialization of the endothelium lining these two different capillary beds.     

Immunology of membranous nephropathy: from animal models to humans

Membranous nephropathy (MN), the leading cause of nephrotic syndrome in adults, is characterized by the deposition of subepithelial immune deposits that consist mainly of immunoglobulin (Ig)G and complement. Most of the cases are primary or idiopathic (iMN), while only approximately 25% of the cases are secondary to some known disease such as systemic lupus erythematosus, hepatitis B, drugs and malignancies. Most of our knowledge on the pathogenesis of iMN has relied upon old experimental models (i.e. Heymann nephritis) that have shown that immune deposits are formed in situ by the reaction of autoantibodies against the respective podocyte antigen. Recent findings indicate that podocyte proteins also act as an autoantigen in human iMN. The M‐type phospholipase A2 receptor (PLA2R) has been identified as the main target antigen, as it can be found in approximately 70% of iMN patients but only rarely in other glomerulonephritides. Podocytes damage in the experimental model of Heymann nephritis is complement‐mediated. In humans, the presence of complement within the subepithelial deposits is well established, but IgG4, which does not activate complement by classical or alternative pathways, represents the predominant subclass of IgG anti‐PLA2R. Some evidence suggests that IgG4 anti‐PLA2R autoantibodies can bind mannan‐binding lectin (MBL) and activate the lectin complement pathway. A genetic background for iMN has been demonstrated by genome‐wide association studies that have shown highly significant associations of the PLA2R1 and the human leucocyte antigen (HLA)‐DQA1 loci with iMN. In addition to their diagnostic value, anti‐PLA2R antibodies may be useful to monitor disease activity and predict response to treatment.     

Deposition of complement C3c, immunoglobulin (Ig)G4 and IgG at the basement membrane of pancreatic ducts and acini in autoimmune pancreatitis

Detlefsen S, Bräsen J H, Zamboni G, Capelli P & Klöppel G
(2010) Histopathology 57, 825–835 Deposition of complement C3c, immunoglobulin (Ig)G4 and IgG at the basement membrane of pancreatic ducts and acini in autoimmune pancreatitis Aims: Autoimmune pancreatitis (AIP) is a type of pancreatitis whose immunopathogenesis is still unknown. It has been reported that renal biopsy specimens from patients diagnosed with both AIP and tubulointerstitial nephritis reveal deposits containing complement C3, immunoglobulin (Ig)G and IgG4 at the tubular basement membranes (BMs). The aim was to investigate the deposition of complement and immunoglobulins in pancreatic tissue from AIP patients compared to non‐AIP patients. Methods: Double immunofluorescence microscopy for C3c, IgG4 and IgG together with CK7, trypsin, collagen IV, CD31 and CD79a, as well as immunofluorescence microscopy for C1q, IgA and IgM, were performed on frozen pancreatic tissue from AIP and alcoholic chronic pancreatitis (ACP) patients. Results: In AIP patients, complement C3c, IgG4 and IgG were deposited at the collagen IV‐positive BMs of pancreatic and bile ducts and of acini. In a minority of the ACP patients, weak C3c‐positive BM deposits were detected, but no IgG4‐ or IgG‐positive BM deposits were present. Conclusion: The deposition of C3c, IgG4 and IgG at the BM of small‐ and medium‐sized ducts and acini of the pancreas is characteristic of AIP. This suggests that immune complex‐mediated destruction of ducts and acini play a role in the pathogenesis of AIP.     

Depletion of C6 prevents development of proteinuria in experimental membranous nephropathy in rats.

To study the possible role of the complement membrane attack complex, C5b-9, in an experimental rat model that is morphologically indistinguishable from membranous nephropathy in man (passive Heymann nephritis [PHN]), an antibody to rat C6 was used to deplete C6 levels to less than 5% of pretreatment values (C6D) during disease development. C3, C7, C8, and C9 levels were not different in C6D and control rats. After injection of nephritogenic quantities of 125I-anti-Fx1A antibody, the kinetics of disappearance of labeled IgG from the blood were identical in the complement deficient and sufficient groups, and glomerular deposition of 125I-antibody was the same in both groups at 5 days. Glomerular deposits of sheep IgG and C3 were also similar in C6D and controls, but glomerular deposits of C6 and C5b-9 neoantigens were markedly reduced or absent in C6 depleted rats. However, despite equivalent antibody deposits, proteinuria was abolished in C6D rats compared with normocomplementemic controls. Similar results were obtained when F(ab')2 anti-rat C6 IgG was used to deplete C6 during development of PHN. These results demonstrate that C6 is required for the development of the increased glomerular permeability that occurs in PHN, presumably because C6 is required for formation of C5b-9. We conclude that glomerular injury in the PHN model of membranous nephropathy in the rat is mediated by C5b-9.     

Solubilization of intraglomerular deposits of IgG immune complexes by human sera or gamma-globulin in patients with lupus nephritis.

A study of the solubilization of glomerular deposition of IgG immune complexes by sera from patients with lupus nephritis is described. Renal biopsy specimens were obtained from 11 patients with lupus nephritis, five patients with IgA nephropathy and one patient with minimal change nephrotic syndrome. These renal specimens were incubated with fresh, stored or heated sera from the same patients or healthy adults and human gamma-globulins at 37 degrees C for 1 h in plastic test tubes. The sections were stained with FITC conjugated heavy chain specific anti-human IgG or C3 antisera and then examined with a fluorescent microscope. The sections were also stained with FITC conjugated human gamma-globulins and rhodamine conjugated anti-human IgG, IgM or IgA antisera and then examined by double exposure under a fluorescent microscope. It was demonstrated that fresh human sera or gamma-globulins significantly solubilize glomerular immune deposits in patients with lupus nephritis in vitro. It was indicated that the solubilization of IgG glomerular deposits from patients with lupus nephritis does not depend on complement. It is postulated that solubilization of immune deposits in glomeruli requires the excess amounts of antigenic substances in patients with lupus nephritis.     

Membranoproliferative glomerulonephritis. Localization of early components of complement in glomerular deposits.

A body of evidence suggests that in membranoproliferative glomerulonephritis (MPGN), complement is activated by the alternate pathway. Therefore, deposition of early components of complement should not be expected in glomeruli. The renal tissues of 16 patients--13 with classic MPGN and 3 with dense deposit disease, a variant of MPGN--were studied by light and electron microscopy and by means of elution and immunofluorescence for the localization of complement (C1q, C4, and C3), immunoglobulins (1gG, IgM, and 1gA), and other serum proteins. Variable amounts of C3, C4 and/or C1q, and IgM were detected in the glomeruli of all patients, whereas IgG and IgA were present, respectively, in 15 of 16 and 6 of 16 patients. Deposits were localized in mesangium and in peripheral capillary loops in a typical lobular distribution. The specificity of each antiserum was verified by immunodiffusion, immunoelectrophoresis, and blocking experiments utilizing unlabeled antibody. Glomerular-bound IgG was eluted with acid citrate buffer, suggesting that IgG might be complexed with antigen(s) in glomerular deposits. By light microscopy, lesions ranged from focal proliferation and lobulation to more severe involvement with typical splitting of glomerular basement membranes, sclerosis, and less frequently, crescent formation. Ultrastructurally, all patients with classic MPGN exhibited mesangial and subendothelial deposits, and in 5 of these patients, subepithelial deposits were demonstrated. With the exception of ultrastructural lesions, patients with the dense deposit variant lacked distinguishable features when compared with those with classic MPGN. The significance of these findings is discussed in relation to a) activation of complement and the possible role of an immune complex mechanism and b) the variability of the morphologic expression.     

Rheumatoid factor and glomerulonephritis

It is presently unknown whether rheumatoid factors have a pathogenic role in the development of various types of glomerulonephritis with immune deposits. Three isotypes of rheumatoid factors (RFs), which are autoantibodies to IgG, were measured using the solid-phase fluorescence immunoassay in sera from patients with diffuse proliferative lupus nephritis (DPLN), membranous lupus nephritis (MLN), IgA nephropathy (IgAN) and idiopathic membranous nephropathy (MN). RF activity of immunoglobulins deposited in the glomeruli from these patients was also studied by examining the binding of the FITC-conjugated human IgG and Fc portion of IgG to the glomeruli of renal biopsy specimens. IgG, IgA and IgM RFs were significantly increased in sera from patients with DPLN, and the increase was significantly lower in patients with MLN, IgAN and MN. Human IgG bound to immunoglobulin on the glomeruli only in DPLN, but not in MLN, IgAN or MN. The Fc portion of IgG was demonstrated to be involved in this reaction. It was suggested that RFs and IgG may play a major role in immune deposits on the glomeruli in DPLN and may be involved in the development of DPLN; however, this is not likely in MLN, IgAN or MN.     

Histologic localization of terminal complement complexes in renal diseases. An immunohistochemical study

Histologic localizations of terminal complement complexes (TCCs) were examined and compared with clinical findings in 154 patients with various renal diseases. Immunohistochemical demonstration of TCCs was carried out on ethanol-fixed paraffin-embedded renal biopsy specimens by indirect immunoperoxidase technique. In glomerular diseases that are thought to be immune-complex glomerulonephritis (IC-GN), such as IgA-nephropathy, membranous nephropathy, and systemic lupus erythematosus (SLE), TCCs were demonstrated in a pattern similar to that of immunoglobulins and C3, indicating that TCCs were induced by immune complexes. The intensity of TCC deposition was correlated with the morphologic destruction of glomeruli or serum creatinine levels in IgA-nephropathy, with urine protein in membranous nephropathy, and with serum C4 in SLE. TCC deposits without IC were also observed in tissue damages without disease specificity such as glomerular or vascular sclerosis and tubulointerstitial lesions. These findings suggested the existence of various roles of TCCs in renal injury, according to IC-mediated or non-IC-mediated mechanism acting in individual diseases.     

免疫球蛋白沉积类型在IgA肾病中的意义

３１例小儿ＩｇＡ肾病报道结果表明：免疫球蛋白沉积类型与临床表现及组织损伤程度有一定关系。单纯系膜区ＩｇＡ沉积，组织损伤程度轻，临床多表现为单纯性血尿，预后较好，ＩｇＡ合并ＩｇＧ及／ＩｇＭ沉积者，易合并补体的沉积，组织损伤程度重，临床表现以肾病综合征及血尿合并蛋白尿发生率高。免疫复合物不仅沉积在系膜区，也可沉积在内皮下，并有相应的基底膜损伤。     

Characterization of circulating idiotypes containing immune complexes and their presence in the glomerular mesangium in patients with IgA nephropathy.

The possible pathogenic role for idiotype-anti-idiotype interactions in kidney diseases has recently been suggested. Since patients with IgA nephropathy often present antibodies against alimentary antigens, like bovine serum albumin (BSA), we isolated an idiotypic antibody with BSA specificity from one of these patients. By means of a specific anti-idiotypic antibody raised in rabbits, we have studied the participation of these idiotypes in circulating and renal deposited immune complexes (IC) in patients with IgA nephropathy. On indirect immunofluorescence, the presence of cross-reactive idiotypes was detected in the glomeruli of 12 out of 42 (28%) patients with IgA nephropathy, but in none of 15 membranous or mesangiocapillary nephritis examined. The staining was located within mesangial and paramesangial areas, with a similar, but less intensive, pattern distribution than IgA. Previous adsorption of rabbit anti-idiotype antibodies on an idiotype-Sepharose column completely abolished that staining. A close relationship was found between the presence of cross-reactive idiotypes on mesangial immunoglobulins and the existence of increased levels of serum idiotypes and idiotype-containing IC. Serum analytical ultracentrifugation showed that circulating IC containing idiotypes have chiefly a large (greater than 19 S) and intermediate (13 S-19 S) size, while those containing anti-BSA antibodies were only between 7 S-13 S fractions, or absent. Our results suggest that in patients with IgA nephropathy, shared idiotypes participate in the formation of circulating and renal deposited IC. It is possible that the apposition of free anti-idiotype to idiotype already bound to glomeruli, and vice versa, could contribute to increasing the amount and size of mesangial immune deposits, and, therefore, facilitate or perpetuate tissue injury.     

Structural analysis of IgG2A monoclonal antibodies in relation to complement deposition and renal immune complex deposition

This study explores the structural features of murine monoclonal IgG2a anti-dinitrophenyl (DNP) antibodies that were previously shown to form immune complexes (IC) differing in their capacity to bind complement, their clearance from the circulation and their deposition in the kidney. Interestingly, the sequence of one of these antibodies has a missing stretch of 14 amino acids within FR3. Molecular modeling suggests that this sequence deletion corresponds to the loss of beta-pleated sheet structure for two beta-strands (designated 4-3 and 4-4) on the external surface of the V(H) domain. Despite this sequence and conformational abnormality, the antibody retains affinity for DNP comparable to other IgG2a antibodies. Data presented here identify monoclonal IgG2a antibodies that form IC with varying propensity for both complement binding and renal deposition and yet have similar V(H) domain sequences. In fact, in the case of two IgG2a antibodies that form IC with very different renal tropisms and complement binding capacity, sequence variation within V(H) was observed only at three clustered residues within FR2, a single residue within FR3 and nine clustered residues spanning CDR3 and FR4. Sequence and modeling analysis also yielded the paradoxical finding that an antibody forming IC with a relatively high capacity to serve as a target for complement binding displays a relatively low number of solvent exposed acceptor residues for C4b and C3b. These data underscore the complex relationship between V domain structure, complement activation and renal deposition of model IC.     

Glomerular deposition of immune complexes prepared with monomeric or polymeric IgA.

Clearance kinetics and renal deposition of soluble IgA immune complexes (IgA IC) were examined to determine the nephritogenic potential of IgA molecular form in mediating experimental IgA nephropathy. The immune complexes were prepared by mixing purified radiolabelled monomeric (mIgA) or polymeric (pIgA) IgA anti-dinitrophenyl (DNP), derived from MOPC-315 myeloma, with DNP conjugated Ficoll (DNP8-Ficoll). Clearance of IgAIC from the circulation was curve fitted by two exponential components. The first component was similar for both mIgA IC and pIgA IC. The second component was slightly more rapid for mIgA IC than for pIgA IC. Immunofluorescence studies, however, showed that only pIgA IC deposited in the kidneys. Analysis of IgA IC by gradient polyacrylamide gel electrophoresis indicated that mIgA formed only small latticed complexes. The critical role of IgA IC lattice size in renal deposition was confirmed by demonstrating that large latticed mIgA IC, prepared by covalent cross-linking mIgA with a specific affinity labelling antigen, deposited in the kidneys in a pattern similar to pIgA IC. Our results suggest that the monovalency of mIgA is responsible for its inability to form a large latticed nephritogenic complexes.     

Defective hepatic handling of IgA immune aggregates by mice with experimental IgA nephropathy.

In an experimental model of IgA nephropathy induced in mice by chronic immunization with dextran, we tested the hypothesis that a defect in the hepatic handling of IgA could be an important determinant in the deposition of IgA in the mesangium. In mice injected with 1-16 doses of 1 mg of dextran (after a preimmunization period of 21 days) the blood clearance of IgA immune aggregates was significantly delayed in relation to control animals, becoming normal at 24 injections. This alteration seems specific since the clearance of IgG aggregates was normal. The percentage of isolated hepatocytes with Fc receptors for IgA decreased significantly over the whole period of dextran immunization. The binding rate of 125I-IgA aggregates to hepatocytes of mice with 24 dextran injections was twice lower than that of control animals. By contrast, the percentage of Kupffer cells with IgA receptors increased over ensuing dextran injections. A progressive increase in the IgA blood levels and in the percentage of mice with mesangial IgA deposits was seen along the period of study. At 24 injections most animals presented moderate to intense mesangial proliferation and abundant electron-dense deposits. On the whole, these data suggest that the early impairment in the liver IgA clearance capacity observed in these animals could facilitate the presence of circulating immune complexes (IC) and their deposition in the mesangium. The increase in serum IgA, seen thereafter, together with the normalization of the IgA clearance capacity, suggest that other pathophysiological mechanism(s) (e.g. in situ IC formation or IgA polymers deposition) must also be involved in this model of experimental IgA nephropathy.     

Antibodies to glycolipids activate complement and promote proteinuria in passive Heymann nephritis.

Passive Heymann nephritis is an experimental rat model of human membranous nephropathy induced by injection of antisera against crude renal cortical fractions such as Fx1A or rat tubular microvilli. This results in the formation of subepithelial immune deposits, the activation of the C5b-9 membrane attack complex of complement, and severe proteinuria. While the formation of immune deposits is attributed to in situ immune complex formation with antibodies specific for the gp330-Heymann nephritis antigenic complex (HNAC), activation of complement and proteinuria appear to be caused by at least one additional antibody species present in anti-Fx1A sera. We have separated by affinity absorption polyspecific antisera against Fx1A and rat microvilli into one IgG fraction directed specifically against microvillar proteins (anti-Fx1A-prot) and another IgG fraction specific for glycolipids (ant-Fx1A-lip) of tubular microvilli. When injected into rats, the anti-Fx1A-prot fraction induced immune deposits but failed to activate complement or produce proteinuria, similar to results obtained with affinity-purified anti-gp330 IgG. When the antibodies of the anti-Fx1A-lip fraction were injected alone they did not bind to glomeruli. By contrast, when the IgGs specific for the Fx1A-prot fraction (or for gp330-HNAC) were combined with those directed against the Fx1A-lip glycolipid preparation, immune deposits were formed, in situ complement activation was observed, and also proteinuria was induced. It is concluded that within anti-Fx1A and anti-microvillar sera there are at least two IgG fractions of relevance for the development of PHN: one directed against the gp330-HNAC complex which is responsible for the development of immune deposits, and a second specific for glycolipid antigen(s) which activate(s) the complement cascade.