Kinetics of immune deposits in membranous nephropathy

Experimental Models of Membranous Nephropathy

Membranous nephropathy (MN) is one of the commonest glomerular diseases, typically presenting in older males with nephrotic syndrome. The development and characterization of animal models of MN, in particular, the passive Heymann nephritis model (PHN), has greatly advanced our understanding of this disease. In this review we discuss the different animal models of human MN that are available, with an emphasis on the PHN model, including technical issues, the typical disease course and its application to human disease.

Drug-induced glomerulopathies

Normal renal function depends upon an intact glomerular apparatus. Many drugs and chemicals are capable of damaging the glomerulus, causing its increased permeability to large molecules. Glomerular lesions are usually responsible for proteinuria and the nephrotic syndrome. This also holds true for the drug-induced glomerulopathies, of which membranous glomerulo-nephritis is the most frequent type of lesion encountered. Apart from this, several cases of different glomerular changes such as focal segmental glomerulosclerosis and crescentic glomerulonephritis have also been reported. The drug-induced glomerulopathies are probably immune mediated. This is, for instance, reflected in the fact that patients with drug-induced nephritic syndrome frequently have the HLA-B8 and DR3 antigens. In depth information is provided for the previously mentioned disorders.

Idiopathic membranous glomerulonephritis is associated with altered patterns of self-reactive IgM and IgG antibody repertoires

Idiopathic membranous glomerulonephritis (MGN) is an immune complex nephropathy characterized by the subepithelial deposition of immunoglobulin (Ig)G. The pathogenesis of the disease remains largely unknown, but recent evidence suggests that human MGN may involve an autoimmune component. In the present study, we have analyzed the IgM and IgG antibody repertoires of patients with MGN towards self- and nonself-antigens using a technique of quantitative immunoblotting on a panel of whole human tissue or solubilized bacterial cell extracts as sources of antigens. Data were compared by means of multiparametric statistical analysis. We demonstrate that the antibody repertoires of self-reactive IgM and IgG in plasma of patients with MGN exhibit significantly altered patterns of reactivity, as compared with those of healthy controls. In contrast, multiparametric statistical analysis does not discriminate the reactivity patterns of IgM and IgG in plasma of patients and healthy controls towards nonself antigens. These observations indicate that a failure in the regulation of physiological self-reactivity is associated with immune complex nephropathy in MGN.

Membranous glomerulonephritis induced in the pig by antibody to angiotensin-converting enzyme: considerations on its relevance to the pathogenesis of human idiopathic membranous glomerulonephritis

In the course of studies on the humoral consequences of swine to primate xenotransplantation, the investigators induced formation of glomerular subepithelial immune deposits and tubular lesions in pigs injected with heterologous antibody to angiotensin-converting enzyme. This study describes the morphology of the lesions, discusses their mechanism, explains their relevance for understanding the pathogenesis of human idiopathic membranous glomerulonephritis, and proposes future directions for investigations.

Induction of albuminuria in mice: synergistic effect of two monoclonal antibodies directed to different domains of aminopeptidase A

BACKGROUND

Aminopeptidase A is an enzyme that is present on podocytes and is involved in the degradation of angiotensin II. In previous studies in mice, we administered single monoclonal antibodies directed against aminopeptidase A. We observed that only monoclonal antibodies that inhibited aminopeptidase A enzyme activity caused albuminuria.

METHODS

In this study, the effects of the combined injections of two monoclonal anti-aminopeptidase A antibodies (mAbs) were studied, using a combination of anti-aminopeptidase A mAbs that were directed against two different domains involved in the aminopeptidase A enzyme activity (ASD-3 or ASD-37) and an anti-aminopeptidase A mAb not related to the enzyme active site (ASD-41).

RESULTS

An injection of the combinations ASD-3/37 (total 4 mg, 1:1 ratio) and ASD-37/41 (total 4 mg, 1:1 ratio) in doses that do not cause albuminuria when given alone (4 mg) induced massive albuminuria at day 1 after injection. The combination ASD-3/41 had no effect. This albuminuria was not dependent on systemic immune mediators of inflammation and could not merely be related to a blockade of aminopeptidase A enzyme activity. However, a correlation was observed between the induction of albuminuria and the aggregation of the mAbs injected and aminopeptidase A on the podocytes. An injection of the combinations ASD-3/37 or ASD-37/41 did not cause an increase in systemic blood pressure. The treatment with a combination of enalapril and losartan lowered blood pressure (53 +/- 10 vs. 90 +/- 3 mm Hg in untreated mice) and reduced the acute albuminuria by 55% (11,145 +/- 864 vs. 24,517 +/- 2448 micrograms albumin/18 hr in untreated mice). However, similar effects were observed using triple therapy. Therefore, the reduction of albuminuria by the combined treatment of enalapril/losartan seems to be the consequence of the reduction in the systemic blood pressure. These findings argue against a specific role for angiotensin II in this model.

CONCLUSIONS

The combined injection of two mAbs directed against different domains of aminopeptidase A induces a massive albuminuria in mice, which is not merely dependent on angiotensin II. We hypothesize that the direct binding of mAbs to at least two pathogenic domains on aminopeptidase A triggers the podocyte to release mediators that are involved in the observed albuminuria.

Membranous nephropathy: an IgG4-mediated disease

Membranous nephropathy is characterised by the deposition of immunoglobulin, predominantly of the IgG4 subclass, along the epithelial surface of the glomerular-basement membrane. Current models of pathogenesis usually assume in-situ immune-complex formation involving an as yet uncharacterised fixed glomerular antigen. I argue that the properties of IgG4 (inability to fix complement and therefore impaired clearance of IgG4-containing complexes; low affinity and therefore ability for IgG4-containing complexes to dissociate and traverse the glomerular-basement membrane) are compatible with a pathogenic mechanism that involves the deposition of circulating IgG4 immune complexes containing diverse antigens.

Increased urinary trypsin-inhibitory activity in mercuric chloride induced nephrotoxicity in Wistar rats

The relationship between trypsin-inhibitory activity (TIA) and the nephrotoxic effects of mercuric chloride (HgCl2)--as illustrated by proteinuria and by a drop in the glomerular filtration rate (GFR) measured by creatinine clearance test (CCT)--was investigated in Wistar rats. HgCl2, 150 or 250 micrograms/100 g BW per day was injected intraperitoneally three times a week for 2 weeks. Both groups showed a significant degree of proteinuria and urinary TIA. Group B (250 micrograms HgCl2/100 g BW) displayed a greater drop in GFR than group A (150 micrograms HgCl2/100 g BW). The urinary TIA was significantly correlated with proteinuria (group A: r = 0.87, group B: r = 0.84), but it was also significantly inversely correlated with the CCT (A: r = -0.96; B: r = -0.88).

IN CONCLUSION

these results suggest that increased urinary TIA may be involved in and indicative of the pathogenesis of mercuric chloride induced nephrotoxicity.

Proteinuria in passive Heymann nephritis is associated with lipid peroxidation and formation of adducts on type IV collagen

Passive Heymann nephritis (PHN) is a model of human membranous nephropathy that is characterized by formation of granular subepithelial immune deposits in the glomerular capillary wall which results in complement activation. This is causally related to damage of the filtration barrier and subsequent proteinuria. The local accumulation of injurious reactive oxygen species (ROS) is a major effector mechanism in PHN. ROS may induce tissue damage by initiating lipid peroxidation (LPO). In turn, this leads to adduct formation between breakdown products of LPO with structural proteins, such as formation of malondialdehyde (MDA) or 4-hydroxynonenal-lysine adducts. To examine the role of LPO in the development of proteinuria we have localized MDA and 4-hydroxynonenal-lysine adducts in glomeruli of PHN rats by immunofluorescence microscopy, using specific monoclonal antibodies. By immunogold electron microscopy, MDA adducts were localized to cytoplasmic vesicles and cell membranes of glomerular epithelial cells, to the glomerular basement membrane (GBM), and also to immune deposits. Type IV collagen was specifically identified as being modified by MDA adducts, using a variety of techniques. Collagenase pretreatment of GBM extracts indicated that the NC-1 domain of type IV collagen was a site of adduct formation. When LPO was inhibited by pretreatment of PHN rats with the antioxidant probucol, proteinuria was reduced by approximately 85%, and glomerular immunostaining for dialdehyde adducts was markedly reduced, even though the formation of immune deposits was not affected. By contrast, lowering of the serum cholesterol levels had no influence on the development of proteinuria. These findings are consistent with the premise that ROS-induced glomerular injury in PHN involves LPO and that this results not only in damage of cell membranes but in modification of type IV collagen in the GBM as well. The close temporal correlation of the occurrence of LPO with proteinuria and the ability of probucol to inhibit proteinuria support a causal role for LPO in the the alteration of the glomerular permselectivity which results in proteinuria.

Immune complex deposition in Bowman's capsule is associated with parietal podocytes

We have recently documented the presence of podocytes lining part of Bowman's capsule at the vascular pole, in adult human kidney. In this study, we describe the deposition of immune complexes in Bowman's capsule in association with these parietal podocytes. We examined 1 year's consecutive human renal biopsies (n = 170). Transmission electron microscopy (TEM) revealed 18 cases in which parietal podocytes were present. Of these 18, there were 11 cases of glomerulonephritis, in which immune complexes were demonstrated in the capillary tuft by both TEM and direct immunofluorescence microscopy. In seven of these 11 cases, TEM showed immune complex-type deposits in Bowman's capsule, always associated with parietal podocytes. These deposits were similar in size, appearance, and distribution to the deposits in the capillary tuft. By contrast, non-specific electron densities within Bowman's capsule were found beneath both squamous parietal cells and parietal podocytes. In four cases, Bowman's capsule also showed focal positive immunostaining for complement components and/or fibrinogen. Both parietal and visceral podocytes showed similar fusion of pedicles. We suggest that filtration through parietal podocytes may be responsible for immune complex deposition and subsequent damage to the vascular pole of the glomerulus in human renal disease.

Long-term beneficial effects of angiotensin-converting enzyme inhibition in patients with nephrotic proteinuria

Angiotensin-converting enzyme inhibitors (ACEI) can reduce proteinuria in diabetic and nondiabetic nephropathy. However, no studies have determined whether this antiproteinuric effect modifies the progression of renal insufficiency. We studied the evolution of 46 nondiabetic patients with nephrotic proteinuria treated with captopril for a minimum of 12 months. The follow-up period before captopril treatment was 12 to 18 months. At the end of follow-up, after captopril introduction (24.4 +/- 7.6 months), proteinuria had decreased from 6.3 +/- 2.5 to 3.9 +/- 3.1 g/24 h (P less than 0.001), with a mean decrease of 45% +/- 28%. The proteinuria decrease was higher in patients with reflux nephropathy, proteinuria associated with reduction of renal mass, inactive crescentic glomerulonephritis, nephroangiosclerosis, and IgA nephropathy, whereas patients with membranous glomerulonephritis and idiopathic focal glomerulosclerosis showed a poorer response. Patients were separated according to a proteinuria reduction greater (group A, 23 patients) or lower (group B, 23 patients) than 45% of the initial value. At the end of follow-up, renal function had not significantly changed in group A with respect to values at the start of treatment: serum creatinine (SCr) was 229 +/- 167 mumol/L (2.6 +/- 1.9 mg/dL) versus 203 +/- 97 mumol/L (2.3 +/- 1.1 mg/dL), and creatinine clearance (CrCl) was 0.80 +/- 0.52 mL/s (48 +/- 31 mL/min) versus 0.87 +/- 0.47 mL/s (52 +/- 28 mL/min). The slope of the reciprocal of Scr (1/SCr) showed a significantly beneficial change after captopril introduction.(ABSTRACT TRUNCATED AT 250 WORDS)

The contribution of antibody-mediated cytotoxicity and immune-complex formation to tubulointerstitial disease in passive Heymann nephritis

Passive Heymann nephritis (PHN), an experimental model of membranous nephropathy, is produced by Fx1A antiserum, which also reacts with antigens on the brush border (gp 330) and basolateral membrane (gp 90) of proximal tubules. We examined tubulointerstitial disease in PHN, identifying two distinct processes occurring on the luminal and basolateral membranes, respectively. Injected antibody bound diffusely to the tubular brush border from Day 1 to Day 7, followed by sloughing of microvilli and tubular-cell regeneration. Fine granular deposits of Fx1A antibody were present along the basolateral cell membrane by Day 1. These deposits rearranged in situ, enlarged, and became more focally distributed along tubular basement membranes (TBM). Interstitial inflammation, dominated by macrophages (Ia+, ED-1+) in association with a smaller number of T-cytotoxic cells (OX19+, OX8+) began by Day 3, reached peak intensity and persisted throughout the autologous phase (to Day 21). The distribution of focal clusters of interstitial macrophages predominately in association with TBM-immune deposits was demonstrated. Complement depletion prevented proteinuria but TBM deposits developed and the interstitial inflammation was unchanged. All aspects of the tubulointerstitial disease were amplified by a second injection of Fx1A antiserum. In vitro, Fx1A antibody bound to the surface of isolated proximal tubular epithelial cells and redistributed to form clusters of immune aggregates. Anti-Fx1A-induced cytotoxicity of tubular cells was demonstrated by prelabeling cells with 2'-7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. The degree of cytotoxicity was dependent on complement concentration and the duration of incubation at 37 degrees C. PHN induced by Fx1A antiserum causes tubular-cell injury following interactions with brush-border antigens and TBM immune-complex disease associated with interstitial inflammation. These findings may be relevant to the acute and chronic interstitial disease of human membranous nephropathy.

Molecular cloning of a cDNA encoding a major pathogenic domain of the Heymann nephritis antigen gp330

Heymann nephritis is an experimental autoimmune disease in rats that is characterized by accumulation of immune deposits (IDs) in kidney glomeruli. The disease is initiated by the binding of circulating antibodies to a membrane glycoprotein, gp330, which is a resident protein of clathrin-coated pits on glomerular epithelial cells (podocytes). We have defined a domain representing about 10% of gp330 that appears to be responsible for the formation of stable glomerular IDs. A cDNA clone (clone 14) was isolated from a rat kidney cDNA expression library by screening with IgG eluted from glomeruli of rats in early stages (3 days) of passive Heymann nephritis. The clone 14 cDNA contains an open reading frame encoding the C-terminal 319 amino acids of gp330. The predicted amino acid sequence contains four internal repeats of 11 amino acids, which are also found in the putative ligand-binding region of carbohydrate-binding lectin-like receptors. An antibody raised against the clone 14 fusion protein recognized gp330 by immunoblotting and induced formation of subepithelial IDs typical of passive Heymann nephritis when injected into normal rats. When the clone 14 fusion protein was used to immunize rats, subepithelial IDs of active Heymann nephritis were found after 12 weeks. No IDs were formed by active or passive immunization of rats with fusion proteins derived from other regions of gp330. These results demonstrate that clone 14 encodes a region of gp330 responsible for antibody binding and ID formation in vivo.