Comparative study of in situ immune deposit formation in active and passive Heymann nephritis

[Clinical significance of the determination of antibodies to thrombospondin type 1 containing domain 7A (THSD7A) in membranous nephropathy]

BACKGROUND

Membranous nephropathy (MN) is an immunocomplex glomerular disease, which is the most common cause of nephrotic syndrome in adults. Numerous studies have established that autoantibodies against the target podocyte autoantigens, including the thrombospondin type 1 domain containing 7A (THSD7A), play a leading role in the development of idiopathic MN.

AIM

To evaluate the prevalence of anti-THSD7A autoantibodies (anti-THSD7A AB) in a group of Russian patients with MN.

MATERIALS AND METHODS

Serum titers of anti-THSD7A AB were tested in 61 patients with biopsy-proven MN and 12 healthy controls.

RESULTS

The prevalence of anti-THSD7A AB was not differing significantly in patients with MN and in the control group (110.9 [71.63; 210.62] and 159.25 [125.64; 231.97] pg/ml, respectively; p=0.111). When comparing subgroups of anti-PLA2R-negative patients and patients who did not receive immunosuppressive therapy with the control group, there were also no statistically significant differences in the Anti-THSD7A AB levels (p>0.05). In the MN group, 1 (1.6%) patient was anti-THSD7A-positive: a 60-year-old man with anti-PLA2R-negative MN and the presence of hormonally inactive adenomas of both adrenal glands and colon polyps (villous adenoma with focal moderate dysplasia, tubulo-villous and tubular adenoma with focal moderate severe dysplasia).

CONCLUSION

THSD7-associated MN is a rare variant of MN and is usually detected in PLA2R-negative patients. Screening for malignancies in THSD7A-positive MN patients is proposed.

How Times Have Changed! A Cornucopia of Antigens for Membranous Nephropathy

The identification of the major target antigen phospholipase A2 receptor (PLA2R) in the majority of primary (idiopathic) cases of membranous nephropathy (MN) has been followed by the rapid identification of numerous minor antigens that appear to define phenotypically distinct forms of disease. This article serves to review all the known antigens that have been shown to localize to subepithelial deposits in MN, as well as the distinctive characteristics associated with each subtype of MN. We will also shed light on the novel proteomic approaches that have allowed identification of the most recent antigens. The paradigm of an antigen normally expressed on the podocyte cell surface leading to in-situ immune complex formation, complement activation, and subsequent podocyte injury will be discussed and challenged in light of the current repertoire of multiple MN antigens. Since disease phenotypes associated with each individual target antigens can often blur the distinction between primary and secondary disease, we encourage the use of antigen-based classification of membranous nephropathy.

Membranous nephropathy and cerebellar degeneration with anti-GAD antibodies in type 2 diabetes mellitus

AIMS

To study the potential pathogenic significance of the coexistence of membranous nephropathy, cerebellar degeneration and anti-glutamic acid decarboxylase (GAD) autoantibodies in patients with diabetes.

METHODS

We performed a direct immunocytochemistry on human kidney slides, electron microscopy on human kidney biopsy, direct immunofluorescence on human kidney biopsy. Baboon and rat kidney cell lines were fractionated and subjected to western blotting with antibodies to GAD.

RESULTS

In this patient we demonstrate the presence of autoantibodies to GAD, which is highly enriched in podocytes plasma membrane and tubular cells of the kidney as well as sub-endothelial IgG and complement C3 deposits in the glomerular basement membrane (GBM).

CONCLUSIONS

We hypothesize the existence in this patient of a common autoimmune pathogenic mechanism with GAD as the autoantigenic determinant, underlying cerebellar degeneration and membranous nephropathy.

Production of heterologous IgG antibody against Heymann nephritis antigen by injections of immune complexes

Heterologous IgG antibody (ab) can be produced against Heymann nephritis (HN) antigen (ag) in rabbits by administering it in Freund's complete adjuvant. The developing abs reacted at high titre with rat kidney brush border (BB) regions of the renal proximal tubules in an indirect fluorescence ab test. A single IV injection of the heterologous ab into a susceptible strain of rat resulted in the localization of IgG ab to glomerular fixed ags, producing immune complex glomerular nephritis. The injected ab also reacted with the BB region of the renal proximal tubules. The aim of this experiment was to find out whether heterologous IgG ab against the HN ag can also be produced in recipient rabbits by injecting immune complexes (ICs) composed of a rat kidney tubular preparation [rat kidney fraction 3 (rKF3)] and donor rabbit-derived rabbit anti-rKF3 IgG ab. We found that anti-rKF3 IgG ab--against the BB region of the renal proximal tubules--could be induced in rabbits injected with ICs, and the resulting ab was able to initiate passive HN in rats. This was the first time a pathogenic IgG ab was produced against HN ag in rabbits without the use of adjuvant. Ab responses in recipient rabbits were achieved by ab information transfer. Recipient rabbits injected with the IC produced the same class of immunoglobulin with the same specificity against the target ag rKF3, as was present in the innoculum, namely rabbit anti-rKF3 IgG ab.

Discrepancies in glomerular and tubulointerstitial/vascular immune complex IgG subclasses in lupus nephritis

Background and objectives: Lupus nephritis is characterized by glomerular and extraglomerular immune complex deposition in the kidney. It is unclear whether the same circulating immune complexes deposit in the glomeruli and in extraglomerular structures, or whether they are pathogenetically different. Differences in the IgG subclass composition may point towards different pathways in the formation of glomerular and extraglomerular immune complexes. Therefore we investigated IgG subclass distribution in the immune complex deposits at these anatomic sites.

Design: A total of 84 biopsies diagnosed as lupus nephritis and classified according to the International Society of Nephrology/Renal Pathology Society (ISN/RPS) 2003 classification, were examined by direct immunofluorescence staining for IgG subclasses. The IgG subclass composition in the glomerular, tubular basement membrane (TBM) and vascular wall deposits was compared. We also correlated the presence/absence of interstitial inflammation and IgG subclasses in the TBM and vascular deposits. Lastly, we looked for correlation between staining for IgG subclasses and complement C1q and C3 staining.

Results: IgG staining was present in the TBM in 52/84 biopsies, and in the vascular walls in 40/84 biopsies. IgG subclass distribution was discrepant between glomerular and TBM deposits in 36/52 biopsies, and between glomerular and vascular deposits in 27/40 biopsies. Interstitial inflammation did not correlate with the presence of IgG staining or distribution of IgG subclasses in the TBM. Interstitial inflammation was more common in biopsies of African–American patients than Caucasian patients. The IgG subclass staining correlated with C1q staining in all the three compartments.

Conclusions: The antibody composition of the glomerular and extraglomerular immune complex deposits appear to differ from each other. They may not represent the same preformed immune complexes from the circulation. It is likely that their pathogenesis and site of formation are different.

Mechanisms of immune-deposit formation and the mediation of immune renal injury

The passive trapping of preformed immune complexes is responsible for some forms of glomerulonephritis that are associated with mesangial or subendothelial deposits. The biochemical characteristics of circulating antigens play important roles in determining the biologic activity of immune complexes in these cases. Examples of circulating immune complex diseases include the classic acute and chronic serum sickness models in rabbits, and human lupus nephritis. Immune deposits also form "in situ". In situ immune deposit formation may occur at subepithelial, subendothelial, and mesangial sites. In situ immune-complex formation has been most frequently studied in the Heymann nephritis models of membranous nephropathy with subepithelial immune deposits. While the autoantigenic target in Heymann nephritis has been identified as megalin, the pathogenic antigenic target in human membranous nephropathy had been unknown until the recent identification of neutral endopeptidase as one target. It is likely that there is no universal antigen in human membranous nephropathy. Immune complexes can damage glomerular structures by attracting circulating inflammatory cells or activating resident glomerular cells to release vasoactive substances, cytokines, and activators of coagulation. However, the principal mediator of immune complex-mediated glomerular injury is the complement system, especially C5b-9 membrane attack complex formation. C5b-9 inserts in sublytic quantities into the membranes of glomerular cells, where it produces cell activation, converting normal cells into resident inflammatory effector cells that cause injury. Excessive activation of the complement system is normally prevented by a series of circulating and cell-bound complement regulatory proteins. Genetic deficiencies or mutations of these proteins can lead to the spontaneous development of glomerular disease. The identification of specific antigens in human disease may lead to the development of fundamental therapies. Particularly promising future therapeutic approaches include selective immunosuppression and interference in complement activation and C5b-9-mediated cell injury.

Cellular Response to Injury in Membranous Nephropathy

The pathogenesis of membranous nephropathy (MN) involves in situ formation of subepithelial immune deposits that produce glomerular injury by damaging and/or activating podocytes through complement-dependent processes. C5b-9 formation and insertion into podocyte cell membranes causes glomerular injury in MN. C5b-9 in sublytic quantities stimulates podocytes to produce proteases, oxidants, prostanoids, extracellular matrix components, and cytokines including TGF-beta. C5b-9 also causes alterations of the cytoskeleton that lead to abnormal distribution of slit diaphragm protein and detachment of viable podocytes that are shed into Bowman's space. These events result in disruption of the functional integrity of the glomerular basement membrane and the protein filtration barrier of podocytes with subsequent development of massive proteinuria. Complement components in proteinuric urine also induce tubular epithelial cell injury and mediate progressive interstitial disease in MN. Measurements of urinary C5b-9 or podocyte excretion in the urine may be useful in the diagnosis of MN and as measures of disease activity and response to therapy. Recent studies of cell-cycle proteins and DNA damage in podocytes have clarified why podocytes fail to proliferate in response to C5b-9-mediated injury and podocyte loss in MN, resulting in the development of glomerular sclerosis and renal failure. Improved understanding of the role of complement in the pathogenesis of MN and of the cellular response to C5b-9 attack creates several new opportunities for therapeutic intervention that may benefit patients with MN in the future.

Presence of immunoglobulin M antibodies around the glomerular capillaries and in the mesangium of normal and passive Heymann nephritis rats

Summary Diffuse distribution of small, faintly staining, beaded deposits of rat immunoglobulin M (IgM) around the glomerular capillary blood vessels, and a more intensely staining larger deposition in the mesangium, were observed on the kidney sections of normal rats. As glomerular-fixed nephritogenic antigens are known to be present on the epithelial aspect of the glomerular basement membrane (GBM), especially at the soles of foot processes and at the slit pores, it was assumed that the IgM antibodies were directed against these antigens. Investigation by immunofluorescent antibody double-staining techniques of rat kidney sections obtained from normal and rabbit anti-FX1A-injected rats stained for the nephritogenic antigen showed that a number of antigenic sites in the glomeruli and in the mesangium shared antibody hits by heterologous rabbit IgG and autologous rat IgM antibodies. Most sites in the glomeruli stained specifically for rat IgM or rabbit IgG, but preferentially for the latter. The intensely fluorescent mesangial deposits stained mainly for rat IgM, indicating that at these sites the antigenic material was virtually saturated, while areas at the entry to the mesangial space also stained for rabbit IgG, indicating that at these locations free nephritogenic epitopes were still available for reaction with the anti-FX1A antibody. Western blot analysis have shown that the rabbit anti-rat FX1A IgG and the rat anti-rat KF3 IgM antibodies are directed against the same renal tubular-derived antigen with a molecular weight of 70,000. These experimental findings collectively demonstrate that the heterologous IgG and autologous IgM antibodies are directed against the same nephritogenic antigen, which is found in the glomeruli, the mesangium and the proximal convoluted tubules. Thus, the IgM autoantibody has a possible physiological role but, in addition, there is evidence of active immunophagocytic events, manifested in a rapid and continuous entrapment and expulsion of macromolecules after their processing by the mesangial cells of normal and passive Heymann nephritis rats.

Production of a new model of slowly progressive Heymann nephritis

A slowly progressive autoimmune kidney disease was induced in Sprague Dawley rats by subcutaneous injection of a chemically modified kidney antigen (rKF3), incorporated into Alum and Distemper complex vaccine, followed by subcutaneous injections of an aqueous preparation of the same antigen. Pathogenic autoantibodies developed, which reacted with fixed glomerular nephritogenic antigen. Subsequently, immunopathological events lead to chronic progressive immune complex glomerulonephritis and proteinuria. The slowly developing disease was morphologically and functionally similar to Heymann nephritis (HN). The damage observed in the kidneys of experimental animals at 8 weeks and at the end of the experiment was examined by direct fluorescent antibody test, histology and electron microscopy. The changes were similar to the typical lesions found in HN rat kidneys, but less severe. Animals became proteinuric from 17 weeks onward (instead of the usual 4-8 weeks). By the end of the experiment, at 8 months, 100% of the rats were proteinuric. This new experimental model of autoimmune kidney disease, which is not complicated by intraperitoneal deposition and retention of Freund's complete adjuvant and renal tubular antigens, allowed us to investigate the pathogenesis of the disease processes from a different aspect, and promises to be a useful and improved model for the investigation of future treatment options.

Receptor-mediated cellular entry of nuclear localizing anti-DNA antibodies via myosin 1

A unique subset of anti-DNA antibodies enters living cells, interacts with DNase 1, and inhibits endonuclease activity, before their nuclear localization and subsequent attenuation of apoptosis. We now report that endocytosis of these immunoglobulins is mediated by cell surface binding to brush border myosin (myosin 1). Cellular entry and internalization via this unique receptor provides initial contact for entry and sorting these immunoglobulins to translocate to the nuclear pore and enter the nucleus, interact with DNase 1 within the cytoplasm, or recycle back to the cell surface. This internalization pathway provides clues to the translocation of large proteins across cell membranes and the functional effects of intracellular antibodies on cytopathology. This is the first demonstration that brush border myosin functions as a specific cell surface receptor for internalization of large proteins.

Active Heymann nephritis in complement component C6 deficient rats

The mechanisms of renal injury that result in proteinuria in active Heymann nephritis (AHN) remain unclear, though data suggest that in analogy of the passive form of the disease the membrane attack complex C5b-9 may be involved. AHN was induced in an inbred strain of PVG/c-rats that are totally deficient in the C6 component of complement and are unable to form the lytic C5b-9 complex, as well as in non-complement deficient PVG/c+ rats that are immunologic identical to the deficient strain. In both groups of animals comparably high titers of anti-Fx1A autoantibodies were found after three weeks and persisted at 40 weeks. Proteinuria was also similar in both groups, and was first evident at six weeks. High levels of urinary protein, ranging from 200 mg/24 hr to 500 mg/24 hr, were found after 10 weeks and persisted up to one year. Renal biopsy findings at various times post-immunization were identical in both groups, including immunofluorescence staining for Ig and C3 deposits, and also EM findings of subepithelial electron-dense deposits were not different. The injection of heterologous rabbit complement, that partially and temporarily restored the CH50 activity in PVG/c- rats did not alter or hasten the disease. Long-term follow-up showed that all rats in both groups continued to have severe proteinuria and that most animals died between 8 to 12 months after disease induction, without renal impairment. EM findings in serial biopsies demonstrated that the growth of the subepithelial deposits as measured by surface area occurred between weeks 4 and 12. A positive correlation (r = 0.94) between the size of the deposits and the level of proteinuria was found. These studies demonstrate that the membrane attack complex of complement does not play a major role in AHN. The relationship of the size of the immune deposits to the level of proteinuria suggests that the growth of the immune deposits on itself initiate secondary mechanisms that damage the permselective characteristics of the glomerular membrane.

A major pathogenic antigen of Heymann nephritis is present exclusively in the renal proximal tubule brush border--studies with a monoclonal antibody against pronase-digested tubular antigen

We have isolated a nephritogenic 120-kD antigen from rat renal tubule brush border that induces rat Heymann nephritis. A MoAb that recognized this antigen reacted exclusively with the brush border on indirect immunofluorescence and immunoelectron microscopy. Rabbit antiserum against this antigen also reacted exclusively with the brush border. With the injection of this antiserum, rabbit IgG became detectable along the glomerular basement membrane (GBM) after 3 days. Our 120-kD antigen was shown to have a close relationship with gp330 based on the following: (i) this antigen can induce active Heymann nephritis as gp330; (ii) our MoAb reacted with the immune deposits of nephritic kidneys induced not only by the 120-kD antigen but also by gp330, and conversely, rabbit antiserum against gp330 reacted with those induced by the 120-kD antigen as well as gp330; and (iii) by immunoblotting, polyclonal antibodies against the 120-kD antigen reacted with gp330 and polyclonal antibodies against gp330 reacted with the 120-kD antigen. These observations indicate that antigen present exclusively in the brush border can induce active Heymann nephritis, and the common antigenic determinants shared by brush border and the coated pits of glomerular epithelium may not be a prerequisite to induce nephritis. A more precise relationship between the 120-kD antigen and reported C14 fusion protein or 40-kD alpha 2MRAP remains to be established.

Altered glomerular extracellular matrix synthesis in experimental membranous nephropathy

Chronic progressive membranous nephropathy (MN) in humans is characterized by thickening of the glomerular basement membrane (GBM) with formation of spikes which contain laminin and other extracellular matrix (ECM) proteins. We have utilized two models of MN in the rat (active and passive Heymann nephritis, AICN, PHN) to define the sequential changes in composition of GBM as they relate to changes in glomerular gene expression for ECM components, altered permeability and morphological changes. Renal biopsies obtained during the course of AICN and PHN were immunostained for various ECM proteins and total glomerular RNA was hybridized with cDNA probes specific for laminin B2-chain, s-laminin, and types I and IV collagen. In addition, the ability of anti-glomerular epithelial cell (GEC) antibody and complement on rat GEC in culture to induce laminin release or laminin and s-laminin mRNA expression was determined. The results demonstrate that at weeks 12, 16, and 20 of AICN, immunostaining for laminin, s-laminin, fibronectin, entactin, and heparan sulfate proteoglycan increased in the GBM in a spike-like pattern. Concomitantly, glomerular mRNA levels of laminin B2-chain and of s-laminin increased. Type IV collagen protein and gene expression remained unchanged or decreased. No glomerular immunostaining for type I collagen occurred during AICN despite increased expression of mRNA for this collagen type. In contrast to AICN, in PHN no pronounced changes of the glomerular ECM occurred, except for transient expression of type I collagen mRNA in whole glomerular RNA and type I collagen protein the GEC cytoplasm. Stimulation of GEC in culture with anti-GEC antibody and complement also failed to induce transcription of laminin or s-laminin mRNA or the release of laminin protein. These findings suggest that the polyantigenic expansion of GBM which occurs in chronic experimental MN may be stimulated by factors different from the C5b-9 mediated processes that cause the initial proteinuria.

Anti-DNA antibodies form immune deposits at distinct glomerular and vascular sites

To investigate the capacity of lupus autoAb to produce glomerular immune deposits (ID) and nephritis, 24 murine monoclonal (m) anti-DNA antibodies (Ab), derived from either MRL-lpr/lpr, SNF1 or NZB lupus-prone mice and selected based on properties shared with nephritogenic Ig, were administered i.p. (as hybridomas) and i.v. (as purified Ig) to normal mice; at least four mice/mAb were evaluated. Three general patterns of immune deposit formation (IDF) were observed: extracellular ID within glomeruli (+/- blood vessels, N = 8); intranuclear ID (N = 5); or minimal or no ID (N = 11). The four MRL m anti-DNA Ab that produced significant extracellular ID demonstrated different disease profiles including: (a) mesangial and subendothelial ID with anti-basement membrane staining, associated with proliferative glomerulonephritis, PMN infiltration, and proteinuria; (b) diffuse fine granular mesangial and extraglomerular vascular ID, associated with proliferative glomerulonephritis and proteinuria; (c) dense intramembranous ID and intraluminal ID, associated with capillary wall thickening, mesangial interposition and expansion, aneurysmal dilatation and intraluminal occlusion of glomerular capillary loops, and heavy proteinuria; and (d) mesangial and extraglomerular vascular ID, associated with mild segmental mesangial expansion, without proteinuria. These MRL mAb were derived from four different mice, and they had variable pIs and isotypes. They all cross reacted with multiple autoantigens (autoAg), however, their autoAg binding profiles were distinguishable. Among the SNF1 derived mAb, four produced histologically and clinically indistinguishable disease characterized by diffuse mesangial and capillary wall ID, associated with cellular proliferation/infiltration and proteinuria. Three of the four mAb were derived from the same mouse and were clonally related; they were: IgG2b with SWR allotype, relatively cationic, highly cross reactive with similar Ag binding patterns, idiotypically related and encoded by identical VH and nearly identical VL sequences. We conclude that both the capacity of lupus autoAb to form ID and the location of IDF are dependent on properties unique to individual Ig. The results also indicate that the Ag binding region of the autoAb is influential in this process, and they suggest that multiple Ab-Ag interactions contribute to IDF in individuals with lupus nephritis. Furthermore, these observations raise the possibility that the pathologic and clinical abnormalities resulting from these interactions are influenced by the location of IDF, and that the dominant interaction, in a given individual, may be highly influential in the phenotypic expression of nephritis.

Glomerular localization of circulating single-stranded DNA in mice. Dependence on the molecular weight of DNA

Although it has been observed that DNA has a high binding affinity for the glomerular basement membrane (GBM) in vitro, glomerular localization of DNA has not been demonstrated in vivo. To evaluate this possibility, after injection of 125I ssDNA of varying molecular weight (mol. wt.) to normal mice, we measured glomerular levels of DNA in vivo. Following administration of 2 mg of 125I high mol. wt. purified single stranded(ss) DNA (2-6 kilobases; 0.7-2.0 x 10(6)D) to normal mice, DNA was not detected in glomeruli, despite measurable blood levels of DNA for 72 h. In contrast, after injection of 280 micrograms of low mol. wt. 125ssDNA (160-200 bases; mol. wt. = 5.3-6.6 x 10(4)D) to normal mice, glomerular localization was observed throughout the 24-h study period despite relatively low 125IssDNA blood levels. The results of these studies indicate that free circulating DNA can bind to sites within glomeruli in vivo, and that the size of DNA is crucial for this interaction. Since low mol. wt. DNA is present in the plasma of patients with active lupus, these findings raise the possibility that DNA may bind to glomeruli and serve as a planted antigen for in situ immune complex formation with circulating anti-DNA antibodies.

Charge selective properties of the glomerular capillary wall influence antibody binding in rat membranous nephropathy

IgG antibodies, eluted from kidneys of rats with Heymann nephritis were separated into cationic and anionic fractions, labeled with 125I and 131I, respectively, mixed in equal amounts, and then injected in incremental doses into 10 rats. Glomerular antibody binding was highly correlated with blood concentration of antibody at 24 hr, however, significantly more cationic antibody bound to glomeruli than did anionic antibody at all blood levels studied. The differences were not due to greater antibody content and/or avidity of the cationic preparation, as measured by binding to isolated glomeruli in vitro. These studies demonstrate the influence of glomerular permselectivity and antibody charge on subepithelial antibody deposition.

Influence of renal transplantation in rats on glomerular and tubular immune complexes

Kidneys were transplanted from rats with immune complex nephritis to normal rats to determine the role of circulating antibodies to renal antigens in the maintenance of immune complexes. Glomerular subepithelial immune complexes in nine proteinuric donor rats, actively or passively immunized with proximal tubular antigen Fx1A (active and passive Heymann nephritis), were studied at the time of transplantation and two to four and one-half months after transplantation to normal unilaterally nephrectomized recipient rats. These subepithelial IgG deposits and proteinuria persisted throughout the four-month period after transplantation. Tubular subepithelial immune complexes in the kidneys of nine rats, actively immunized with a distal renal tubular antigen, Tamm-Horsfall protein, were studied at the time of transplantation when donors had high circulating IgG antibody titers to Tamm-Horsfall protein and at one to eight weeks after transplantation. The abundant granular and nodular immune complexes of rat IgG and Tamm-Horsfall protein at the base of tubular cells were very rapidly cleared during the first two weeks after transplantation and were virtually absent by four weeks. The rapid clearance of tubular immune complexes after transplantation of kidneys from actively immunized rats followed a time course similar to that after passive immunization with antisera to Tamm-Horsfall protein. These findings support the concept that high titers of circulating antibodies to Tamm-Horsfall protein are required to prevent the rapid clearance of tubular immune complexes, a process that is facilitated by conditions of antigen excess at the surface of tubular cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Detection of terminal complement components in experimental immune glomerular injury

Complement mediates glomerulonephritis by inflammatory cell-dependent and non-inflammatory cell-independent effects on glomerular permeability. The latter may involve terminal components of the complement system. We examined several models of immunologic renal injury in the rat by immunofluorescence (IF) for terminal complement components C5, C6, C7, and C8 in glomeruli using antisera to human C5-8, which cross-react with the analogous rat complement components. Rats with the heterologous and autologous phases of passive Heymann nephritis (PHN) had proteinuria and 1 to 2+ capillary wall deposits of heterologous or rat IgG, rat C3, and C5-8. Complement depletion with cobra venom factor (CVF) significantly decreased proteinuria in both models and prevented deposition of all complement components. Rats with active Heymann nephritis had similar deposits of rat IgG and C5-8. Rats with anti-GBM nephritis and aminonucleoside nephrosis had severe proteinuria which was not affected by CVF treatment and deposits of C5-8 were absent. The presence of terminal complement components in immune deposits in experimental glomerular disease correlates with a functional role for complement in mediating glomerular injury. These data support the hypothesis that the terminal complement pathway may be a major mediator of some types of immune glomerular injury.

Spontaneous glomerulonephritis in rabbits: role of a glomerular capillary antigen

Overt glomerulonephritis, detected by abnormal proteinuria, occurred in 3.3% of young (2.5 kg) male New Zealand White (NZW) rabbits. Histologically, mild to moderate mixed membranous and proliferative glomerulonephritis was observed. Glomerular deposits of IgG and C3 and electron microscopic findings were not typical of circulating immune complex accumulation, nor did they suggest anti-glomerular basement membrane (GBM) antibody. Segmental and less intense glomerular deposits of IgG were found in up to 48% of nonproteinuric NZW rabbits of the same age; histologic changes were minimal. IgG antibodies in sera from proteinuric rabbits or eluted from their kidneys reacted by indirect immunofluorescence with antigens distributed (discontinuously) along the glomerular capillary walls, and in some cases within the walls of small arteries of normal rabbit kidney sections. By indirect immunoperoxidase electron microscopy, the reactive antigens were present at the surfaces of the epithelial cell foot processes where they abut the GBM. This spontaneous glomerulonephritis appears to involve fixation of antibodies to antigens distributed in a discontinuous pattern in the glomerular capillary wall. The mechanism would be much like that causing anti-GBM antibody glomerulonephritis, except that the glomerular antigen is different. Possibly some forms of glomerulonephritis develop in humans as a result of a similar process.

Effect of antibody charge and concentration on deposition of antibody to glomerular basement membrane

Fixed anionic sites within the glomerular capillary wall influence the permeation of serum proteins, the localization of various antigens, and the deposition of antibody in the subepithelial space. In anti-GBM nephritis antibody deposition occurs very rapidly to antigenic sites located relatively proximal in the glomerular capillary wall. We examined the influence of the glomerular charge barrier on anti-GBM antibody deposition by comparing the rate of deposition of antibodies with cationic and anionic isoelectric points. Purified sheep anti-rat GBM IgG was isolated from acid eluates of kidneys obtained 24 hr after rats were injected with sheep antiserum to rat GBM. Anti-GBM IgG was separated into cationic (pI 6.4-8.5) and anionic (pI 4.2-6.8) fractions, which were radiolabelled with 131I and 125I, respectively, shown to have equal antibody contents measured by in vitro binding to normal glomeruli, mixed in equal amounts, and injected in incremental doses to ten rats. At 1 hr the glomerular antibody binding of each fraction was directly related to the blood level (r = 0.95, r = 0.97) and delivery of antibody (r = 0.98, r = 0.98). Glomerular binding of cationic antibody was four times greater than anionic antibody over the entire range of deliveries studied (P less than 0.001). We conclude that glomerular deposition of anti-GBM antibody is directly related to blood concentration and delivery of antibody. Furthermore, the deposition of cationic antibodies to GBM antigens was significantly greater than the deposition of anionic antibodies. The charge-selective glomerular filtration barrier may be an important determinant of the quantity and subclass composition of anti-GBM IgG deposits in glomeruli, and therefore of the severity of tissue injury produced.