Mediation of proteinuria in membranous nephropathy due to a planted glomerular antigen

Treatment of idiopathic membranous nephropathy

Exciting progress recently has been made in our understanding of idiopathic membranous nephropathy, as well as treatment of this disease. Here, we review important advances regarding the pathogenesis of membranous nephropathy. We will also review the current approach to treatment and its limitations and will highlight new therapies that are currently being explored for this disease including Rituximab, mycophenolate mofetil, and adrenocorticotropic hormone, with an emphasis on results of the most recent clinical trials.

Distinct and separable roles of the complement system in factor H-deficient bone marrow chimeric mice with immune complex disease

Plasma complement factor H (Cfh) is a potent complement regulator, whereas Cfh on the surface of rodent platelets is responsible for immune complex processing. For dissection between the two, bone marrow chimeras between Cfh-deficient (Cfh(-/-)) and wild-type C57BL/6 mice were created. Platelet Cfh protein was tracked with the Cfh status of the bone marrow donor, indicating that platelet Cfh is of intrinsic origin. In an active model of immune complex disease, Cfh(-/-) mice that were reconstituted with wild-type bone marrow had levels of platelet-associated immune complexes comparable to those of wild-type mice and were protected against the excessive glomerular deposition of immune complexes seen in Cfh(-/-) mice, yet these mice still developed glomerular inflammation. In contrast, wild-type mice with Cfh(-/-) bone marrow had reduced platelet-associated immune complexes and extensive glomerular deposition of complement-activating immune complexes, but they did not develop glomerular pathology. The large quantities of glomerular C3 in wild-type mice with Cfh(-/-) bone marrow were in the form of iC3b and C3dg, whereas active C3b remained in Cfh(-/-) recipients of wild-type bone marrow. These data show that plasma Cfh limits complement activation in the circulation and other accessible sites such as the glomerulus, whereas platelet Cfh is responsible for immune complex processing.

Complement-induced phospholipase A2 activation in experimental membranous nephropathy

BACKGROUND

In the passive Heymann nephritis (PHN) model of membranous nephropathy, C5b-9 induces glomerular epithelial cell (GEC) injury and proteinuria, which is partially mediated by eicosanoids. By analogy, in cultured rat GEC, sublytic C5b-9 injures plasma membranes and releases arachidonic acid (AA) and eicosanoids, due to activation of phospholipase A2 (PLA2). This study addresses the mechanisms of PLA2 activation.

METHODS

PLA2 expression was assessed with the polymerase chain reaction or immunoblotting, and activity was determined using an in vitro assay or by measurement of free AA.

RESULTS

Under basal conditions, GEC in culture expressed a relatively low level of cytosolic PLA2 (cPLA2) protein, while mRNAs of groups IB, IIA and V secretory PLA2s (sPLA2) were not detectable. Incubation of GEC with sublytic C5b-9 induced 1.5- to 2.0-fold increases in free [3H]AA at 40 minutes, and three and 24 hours. C5b-9 did not increase cPLA2 protein, and did not induce group IB, IIA or V sPLA2 mRNAs. Stable overexpression of cPLA2 in GEC amplified the C5b-9-induced increases in free [3H]AA, while analogous overexpression of group IIA sPLA2 had no effect. PLA2 activity was increased in glomeruli of rats with PHN, and this enhanced activity was characterized as cPLA2. There were no differences in cPLA2 protein expression between PHN and control glomeruli.

CONCLUSIONS

Release of AA by C5b-9 in GEC in culture and in vivo is mediated by cPLA2, and the mechanism is consistent with post-translational regulation of cPLA2 activity. C5b-9 does not induce expression or stimulate activity of sPLA2 isoforms in GEC.

Complement C5b-9 induces receptor tyrosine kinase transactivation in glomerular epithelial cells

In the passive Heymann nephritis (PHN) model of membranous nephropathy, C5b-9 induces glomerular epithelial cell (GEC) injury and proteinuria, which is partially mediated via production of eicosanoids. Using rat GEC in culture, we demonstrated that sublytic C5b-9 induced tyrosine phosphorylation of the epidermal growth factor receptor (EGF-R), Neu, fibroblast growth factor receptor-2, and hepatocyte growth factor receptor. In addition, C5b-9 stimulated increases in tyrosine(204) phosphorylation of extracellular signal-regulated kinase-2 (ERK2), as well as free [(3)H]arachidonic acid (AA) and prostaglandin E(2) (PGE(2)). Phosphorylated EGF-R bound the adaptor protein, Grb2, and the EGF-R-selective tyrphostin, AG1478, blocked the C5b-9-induced ERK2 phosphorylation, [(3)H]AA release, and PGE(2) production by 45 to 65%, supporting a functional role for EGF-R kinase in mediating the activation of these pathways. Glomeruli isolated from rats with PHN demonstrated increases in ERK2 tyrosine(204) phosphorylation and PGE(2) production, as compared with glomeruli from control rats, and these increases were partially inhibited with AG1478. Thus, C5b-9 induces transactivation of receptor tyrosine kinases, in association with ERK2 activation, AA release, and PGE(2) production in cultured GEC and glomerulonephritis in vivo. Transactivated tyrosine kinases may serve as scaffolds for assembly and/or activation of proteins, which then lead to activation of the ERK2 cascade and AA metabolism.

Direct and indirect tests of pore size and charge selectivity in nephrotic syndrome

We studied direct and indirect methods of measuring membrane charge by detecting fixed anionic sites with polyethylenimine (PEI) on the glomerular basement membrane (GBM) and Alcian blue on red blood cell (RBC) membrane (ABRBC), respectively, in 40 children with nephrotic syndrome (NS). Size selectivity of the GBM was measured indirectly by fine analysis of urinary proteins with sodium dodecyl sulfate-polyacrylamide gel electrophoresis in 22 of these children. Correlation between ABRBC and PEI was strongest (r = 0.79; p = 0.0037) in 11 children with steroid-responsive NS (SRNS), moderate (r = 0.31) in 10 children with focal glomerulosclerosis (FGS), and absent in 14 children with hepatitis B antigen membranous nephropathy (MGN) and 5 with mesangioproliferative glomerulonephritis (MPGN). ABRBC and PEI were reduced in the group as a whole as compared with their controls (ABRBC: 44.53 +/- 9.81 vs 71.54 +/- 12.14, p < 0.05; PEI: 16.31 +/- 4.34 vs 33.3 +/- 1.09, p < 0.005). This reduction was greater in SRNS (26.35 +/- 7.15 p = 0.004) but was also detected in the remainder of the group taken together (52.31 +/- 26.07, p < 0.001). Excretion of glomerular proteins was restricted by size (< or = 80 kd) in SRNS but unrestricted (< or = 80 kd plus > 80 kd) in FGS, MGN, and MPGN. The main cause of proteinuria is likely to be depletion of negative charge on the GBM in SRNS, and distortion of capillary pore size in MGN and MPGN, with probable overlap of these mechanisms in each disease, especially in FGS. Basement membrane injury appears widespread in SRNS but confined to the kidney in MGN and MPGN.

Antigen processing and presentation by glomerular visceral epithelium in vitro

Although macrophages are considered the prototype of antigen presenting cells (APC), recent studies have emphasized the potential role of several parenchymal and mesenchymal cells in this process. We have studied the capacity of cultured glomerular visceral epithelial cells (GEC) to act as effective APC and compared this capacity with that demonstrated by peritoneal macrophages. Affinity-purified and in vitro propagated rat GEC were exposed to hen egg lysozyme, keyhole limpet hemocyanin, and cationic ferritin. As effector cells, we used antigen-specific T cell hybridomas; the level of antigen presentation was assessed by determining the level of interleukin 2 (IL-2) present in tissue culture supernatants. Cytokine-treated GEC were capable of processing and presenting all antigens in a dose-dependent manner. Crucial for antigen presentation were intracellular processing of antigen and the presence of Ia on the cell surface. Our findings indicate that GEC can act as effective APC, and further suggest that this capacity may be relevant to cell-mediated immune injury at the level of the glomerular capillaries in vivo.

Sequential ultrastructural podocytic lesions and development of proteinuria in serum sickness nephritis in the rat

Serum sickness nephritis was induced in Fisher rats by immunization with egg albumin (EA) and correlations between immune complex deposition, alterations of podocytes and development of proteinuria were analysed. Immunoelectron microscopy showed that EA, rat IgG and C3 were confined to the electron-dense deposits (Ds). From 3 weeks, when significant proteinuria had developed, the subepithelial region was filled with large numbers of Ds on the peripheral capillary wall as well as in the paramesangium. The loss of slit diaphragms and detachment of foot processes overlying Ds were observed and the escape of Ds into urinary space was frequently detected. Morphometric evaluation showed that the volume of subepithelial Ds and the number of the sites of podocytic detachment correlate significantly with the amount of proteinuria. In addition, the native ferritin injected via the abdominal aorta was seen in large amounts in the urinary space near the areas devoid of epithelial covering. The development of podocytic detachment was clearly coincident with the appearance of proteins with a larger molecular weight in urine. From these results, it is suggested that the loss of slit diaphragms and the detachment of podocytes resulting from the progressive accumulation of Ds will allow the leakage of proteins of larger molecular weight across the capillary wall. These podocytic lesions may be one of the main aetiologies for the development of heavy proteinuria in this model.

Urinary excretion of C5b-9 reflects disease activity in passive Heymann nephritis

Passive Heymann nephritis (PHN) is a model of membranous nephropathy in rats in which glomerular injury is mediated by the terminal C5b-9 membrane attack complex of complement. This model has been shown to be associated with markedly elevated urinary excretion of C5b-9, compared to other experimental models of glomerulonephritis To determine if urinary C5b-9 excretion could serve as an index of disease activity by correlating with the formation and quantity of glomerular subepithelial immune deposits in PHN, we measured urinary excretion of C5b-9 in PHN under several experimental conditions. In the heterologous phase a direct correlation was demonstrated between levels of urinary C5b-9 excretion and the amount of anti-Fx1A IgG deposited in glomeruli (r = 0.85). In the autologous phase, C5b-9 excretion correlated with the amount of deposit forming antibody present in the serum and resolved when antibody disappeared, despite persistence of glomerular deposits of antigen, antibody, C5b-9 and heavy proteinuria. Glomerular C3 deposits paralleled urinary C5b-9 excretion. Re-initiation of active deposit formation by a second injection of anti-Fx1A produced new C3 deposits and a marked rise in C5b-9 excretion. Finally, complete abrogation of deposit formation by transplanting PHN kidneys into normal recipients also halted C5b-9 excretion. Our findings demonstrate that urinary excretion of C5b-9 is a sensitive index of on-going immunologic disease activity in the PHN model of membranous nephropathy.

Spontaneous immune complex orchitis in brown Norway rats

Immune complexes occur spontaneously in the testis of Brown-Norway (BN) inbred rats between the basal lamina of the seminiferous tubules and the outer lamina of the myoid testicular cells. The deposits can be detected immunohistologically (IgG; C3) and by electron microscopy. The immune complexes appear between the 8th and 12th weeks of life, increase in amount up to the 30th week and decrease thereafter. After about the 20th week, of life, 15% of the animals show destruction of the germinal epithelium accompanied by an infiltration of lymphocytes and plasma cells. The final stage of this disease, which initially shows no signs of inflammation, is characterized by diffuse tubular atrophy. However, up to the 70th week of life, 85% of the animals with immune complexes show no pathological alterations. Antibodies eluated from the testes react with spermatocytes I and structures close to the lumen of the seminiferous tubules, but not with mature sperms. Serum antibodies to sperms occur in about 25% of the BN rats, but the presence of these antibodies shows no correlation with the immunohistological findings. This newly described spontaneous immune complex orchitis is regarded as a further example of an in-situ-induced immune complex disease. The observations made here can be compared with those in (peri-) membraneous glomerulonephritis, another example of a disorder resulting from in-situ-formation of immune deposits.

Chronic serum sickness glomerulonephritis: modification of the immune response influences the rate of removal of mesangial electron-dense deposits

We have used a chronic serum sickness model of glomerulonephritis to investigate whether gross interference with the immune system can influence the rate of removal of antigen and established electron-dense deposits from the glomerulus. Radio-labelled cationized bovine serum albumin (BSA) was used as antigen. During the 2 weeks after the cessation of injections, the rate of removal of antigen from isolated glomeruli and from renal cortex, liver, spleen, and lung was measured. The rate of removal of mesangial and subepithelial deposits was assessed by point-counting. Urinary excretion of free and protein-bound isotope was also measured. Having quantified the rate of removal of antigen and deposits from the glomerulus, we attempted to influence the rate of removal by interfering with the immune response in the course of recovery. Contrary to our expectations, stimulation of the immune system with antigen in Freund's complete adjuvant, 4 days after the last injection of antigen, inhibited the removal of antigen and mesangial electron-dense deposits. Prednisolone had no detectable effect, but large doses of a non-nephritogenic form of the antigen (native BSA) enhanced removal. Removal of antigen and mesangial deposits was inversely correlated with the levels of circulating anti-BSA antibody, suggesting that specific antibody, circulating through the mesangium, inhibits the removal of antigen which is already trapped at that site. None of the forms of intervention applied during recovery produced a detectable change in the rate of removal of subepithelial deposits.

Determinants of immune complex-mediated glomerulonephritis

We have studied the influence of steric factors on the clinico-pathologic expression of immune complex-mediated glomerular diseases, utilizing ferritin as an exogenous antigen. The tracer was planted in the left kidney either in the subepithelial layer of the glomerular capillary wall or on the endothelium and lamina rara interna. Subepithelial immune complex formation resulted in non-inflammatory injury with heterologous and autologous proteinuric phases (115 +/- 16 mg/24 hrs on day 2; 183 +/- 16 mg/24 hrs on day 9) lasting four to five weeks. The glomerular filtration rate of the experimental left kidney was reduced by 19% at day 3, and was increased by 20% at day 12 over right kidney values. Immune complexes persisted for more than seven weeks in the lamina rara externa. In contrast, immune complex deposition on the endothelium and in the lamina rara interna led to acute transient anuria, with a 38% drop in glomerular filtration rate at one hour, massive platelet accumulation, followed by a strong inflammatory response. Proteinuria did not develop. Functional and structural integrity was restored within 24 hours, with complete clearing of immune deposits. We conclude that the distribution of exogenous antigens within the capillary wall determines the structural and functional expression of immune-mediated glomerular diseases.

Anti-Fx1A produces complement-dependent cytotoxicity of glomerular epithelial cells

Glomerular injury in passive Heymann nephritis (PHN) in rats is mediated by the C5b-9 membrane attack complex (MAC) and is associated with morphologic changes in glomerular visceral epithelial cells (GEC). We determined if the nephritogenic antibody of PHN (gamma 1 sheep anti-Fx1A IgG) directs insertion of the MAC into GEC plasma membranes with consequent cytotoxicity. Antibody-sensitized GEC were exposed to various sera serving as sources of complement. Loss of cell viability was determined by trypan blue uptake and/or by release of cellular lactate dehydrogenase (LDH). Incubation of antibody-sensitized primary and passaged GEC in fresh human serum (FHS) resulted in sigmoidal relationships between cytotoxicity and complement dose (r = 0.97 and 0.94, respectively) such that cytolysis approached 100% with FHS (10% vol/vol). Cytotoxicity was not evident if C8-deficient (C8D) plasma was substituted for FHS, but was restored in a dose-dependent manner by reconstitution with purified rat C8. Sublytic injury was demonstrated by wide separation between simultaneous release curves of cell-incorporated biscarboxyethyl carboxyfluorescein (BCECF; mol wt approximately equal to 520) and LDH at limiting doses of complement (at 2% FHS, BCECF release was 51.1 +/- 0.6% of maximum vs. 3.2 +/- 1.3% for LDH; N = 3) and by blebbing of the plasma membrane on electron microscopy. Thus, the pathogenic antibody of PHN produces complement-mediated sublytic as well as lytic cytotoxicity of GEC.

Inhibitory effect of lysozyme on the intraglomerular immune complex formation in lupus mice

The effect of lysozyme on intraglomerular immune complex deposition was examined in NZB/W F1 mice undergoing unilateral nephrectomy. Unilateral nephrectomy enhanced the glomerular immune complex deposition and glomerular lesions, which were suppressed by repeated intraperitoneal injections of lysozyme, in spite of unaltered serum anti-DNA antibody titers. DNA binding to the glomerular basement membrane (GBM) examined in vitro and that to glomeruli examined in vitro were also suppressed by lysozyme. An increased survival rate and decreased proteinuria were also induced by this basic protein. The mechanisms of the ameliorative effect were studied in vitro. DNA was bound to the GBM only in the presence of serum, plasma, or fibronectin. A similar inhibitory effect on DNA binding was also obtained by another polycation, hexadimethrine, in place of lysozyme. The in vitro findings suggest that DNA binding to the GBM is mediated by fibronectin, and that lysozyme electrostatically inhibits this binding, thereby possibly reducing the in situ DNA-anti-DNA complex formation in the GBM.

Role of abnormally high transmural pressure in the permselectivity defect of glomerular capillary wall: a study in early passive Heymann nephritis

To explore the mechanism of glomerular permselectivity defect in passive Heymann nephritis, an experimental model of human membranous glomerulopathy, Munich-Wistar rats were subjected to both micropuncture assessment of glomerular hemodynamics and whole kidney clearance measurements of graded size dextrans 10 days after injection of sheep anti-rat tubular antigen (anti-Fx1A). Compared with normal control rats, anti-Fx1A-treated animals were characterized by marked proteinuria (65 +/- 8 micrograms/min versus 6 +/- 1, p less than 0.001), markedly and significantly higher glomerular transcapillary hydraulic pressure difference (40 +/- 1 mm Hg versus 33 +/- 1, p less than 0.001), depressed ultrafiltration coefficient and impaired glomerular size-selective function as determined by fractional clearance of dextrans. Calculation of membrane parameters based on a recently defined heteroporous membrane model revealed abnormally high availability of non-size selective, large pore pathways in the glomerular capillary wall of the rats with passive Heymann nephritis. To ascertain the role of the altered hemodynamic pattern in the observed defect in the size-selective function of the glomerular capillary wall, glomerular transcapillary hydraulic pressure difference was manipulated experimentally in these proteinuric rats by intra-aortic infusion of acetylcholine or angiotensin II. These agents respectively suppressed and augmented glomerular transcapillary hydraulic pressure difference and brought about a decline of and a further rise in fractional clearance of larger dextrans along with parallel changes in both urine protein excretion rate and availability of nonselective channels. These results indicate that the permselectivity defect in passive Heymann nephritis is attributable, at least in part, to impaired size selectivity of the glomerular capillary wall caused by a prevailing abnormally high transcapillary hydraulic pressure difference.

Enhanced glomerular prostaglandin formation in experimental membranous nephropathy

To determine whether the induction of immune-mediated glomerular injury influences the formation of cyclooxygenase products by glomerular cells, we determined prostaglandin E2 (PGE2) and thromboxane B2 (TXB2) (as the stable metabolite of TXA2) formation in isolated glomeruli of rats with passive Heymann nephritis (PHN). PHN is a model of membranous nephropathy mediated by antibody and complement independent of inflammatory cells. Five days following induction of PHN by injection of heterologous antibody to rat proximal tubular brush border antigen (Fx1A) rats developed proteinuria 36.5 +/- 34 (controls 3.8 +/- 1 mg/day). Treatment with cobra venom factor, which depleted complement C3 levels to less than 10% of baseline, prevented the development of proteinuria (6.9 +/- 2 mg/day). The development of subepithelial, glomerular immune-complex deposits and proteinuria was associated with a significant stimulation of glomerular PGE2 (87%) and TXB2 (183%) formation. This increment in glomerular prostanoid biosynthesis was significantly inhibited (PGE2 increased 22%, TXB2 increased 75%) in animals that were complement depleted with cobra venom factor. Cobra venom factor had no effect on glomerular prostanoid formation in normal rats. In additional experiments we tested the hypothesis that TXA2 may contribute to mediation of proteinuria in PHN. We utilized a thromboxane synthetase inhibitor UK38485. UK38485 reduced glomerular TXB2 formation by 80% without influencing glomerular deposition of 125I-labeled antibody, and did not alter levels of urine protein excretion in rats with PHN (control 42 +/- 21, UK 38485, 39 +/- 24 mg/day, P greater than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Complement-induced glomerular epithelial cell injury. Role of the membrane attack complex in rat membranous nephropathy

In passive Heymann nephritis (PHN) in rats, antibody (anti-Fx1A) reacts in situ with a glomerular epithelial antigen and induces complement (C)-mediated cell-independent proteinuria. To assess the role of the membrane attack complex (MAC), we determined the need for C8 in the pathogenesis of proteinuria in an autologous-phase model of PHN. Isolated rat kidneys, containing nonnephritogenic, non-C-fixing gamma 2 sheep anti-Fx1A (planted antigen), when perfused in vitro with C-fixing guinea pig anti-sheep IgG and a source of C (fresh human plasma 50% vol/vol in buffer containing bovine serum albumin), developed marked proteinuria after 20 min (0.58 +/- 0.08 mg/min X g, n = 8) that increased further to 3.20 +/- 0.93 mg/min X g after 80 min. In contrast, identical kidneys perfused with antibody and heat-inactivated or C8-deficient human plasma and normal kidneys perfused with antibody and fresh plasma excreted only 0.27 +/- 0.03 (n = 6), 0.27 +/- 0.04 (n = 5), and 0.40 +/- 0.05 mg/min X g (n = 6) after 20 min, and 0.13 +/- 0.02, 0.22 +/- 0.03, and 0.32 +/- 0.05 mg/min X g after 80 min, respectively. When C8-deficient plasma was reconstituted with sources of C8 (n = 3), proteinuria was restored to the level observed with fresh normal plasma. Differences in protein excretion could not be explained by quantitative differences in glomerular antigen or antibody content. Extensive ultrastructural damage to glomerular visceral epithelial cells was exclusively seen in antigen-containing kidneys perfused with antibody and C8-replete plasma. Thus, glomerular injury in this model results from an antigen-specific, antibody-directed, C8-dependent reaction involving assembly of the MAC. The ultrastructural findings argue in favor of MAC-induced cytotoxicity of the glomerular visceral epithelial cells.

Complement membrane attack complex stimulates production of reactive oxygen metabolites by cultured rat mesangial cells

To explore possible mechanisms by which complement membrane attack complexes (MAC) that are deposited in the glomerular mesangium might be pathogenic, we stimulated rat glomerular mesangial cells grown in vitro with nascent MACs formed from the purified human complement components C5b6 and normal human serum and measured production of superoxide ion (O2-) and hydrogen peroxide (H2O2). Mesangial cells incubated with C5b6 + serum, which results in cell membrane interaction with the MAC, produce 0.9 +/- 0.15 nmol O2-/10(5) cells per 30 min, which was significantly greater than the amount produced by cells incubated with C5b6 alone, serum alone, or decayed MACs that can no longer interact with the cell membrane (0.3 +/- 0.2, 0.4 +/- 0.1, 0.3 +/- 0.2 nmol O2-/10(5) cells per 30 min, respectively; P less than 0.02). Production of O2- after stimulation with MACs increased during the first 20 min of incubation but then plateaued. Cells exposed to decayed MACs produced small amounts of O2-, which did not increase from 20 to 60 min. Production of H2O2 was also observed after stimulation with MACs, and continued to increase during 60 min of incubation (1.22 +/- 0.16 nmol H2O2/10(5) cells per 60 min), whereas H2O2 production could not be detected after exposure to decayed MACs. Cell viability was not adversely affected by exposure to nascent MACs as determined by trypan blue exclusion or chromium-51 release. These results demonstrate that glomerular mesangial cell membrane interaction with the MAC stimulates the production of the toxic oxygen metabolites O- and H2O2. Activation of the terminal complement pathway by mesangial immune deposits in vivo might lead to tissue injury by stimulation of local production of toxic oxygen-free radicals.

Antibody, antigen, and glomerular capillary wall charge interactions: influence of antigen location on in situ immune complex formation

These studies examined the charge interactions between the glomerular capillary wall, antibody and antigen at different sites in the glomerulus. Sheep IgG was separated into differently charged subclasses and immunologically placed in one of two glomerular locations (subepithelial or subendothelial) to serve as planted antigen. Single kidneys with planted antigen were transplanted into uninephrectomized recipients that received affinity-purified, cationic and anionic rat anti-sheep IgG labelled with 125I and 131I, respectively. Glomerular bound antibody was determined and corrected for antibody delivery. Specificity of antibody binding was confirmed by comparison of kidneys with or without planted antigen. The results indicate that the influence of charge on glomerular antibody binding depends on the site of the antigen. When antigen was planted in the subepithelial space, significantly more (15 to 25%) cationic than anionic antibody bound despite the fact that the antigen was cationic. Conversely, when the antigen was planted subendothelially, significantly more anionic (13 to 22%) antibody bound when the antigen was cationic, and significantly more cationic (7 to 16%) antibody bound when the antigen was anionic. Thus, the negatively-charged glomerular filtration barrier retards the permeation of anionic antibodies that complex with antigens located in the subepithelial space, but antigen-antibody charge interactions appear to predominate when the antigen is more proximally located.

Complement membrane attack complex stimulates production of reactive oxygen metabolites by cultured rat mesangial cells

To explore possible mechanisms by which complement membrane attack complexes (MAC) that are deposited in the glomerular mesangium might be pathogenic, we stimulated rat glomerular mesangial cells grown in vitro with nascent MACs formed from the purified human complement components C5b6 and normal human serum and measured production of superoxide ion (O2-) and hydrogen peroxide (H2O2). Mesangial cells incubated with C5b6 + serum, which results in cell membrane interaction with the MAC, produce 0.9 +/- 0.15 nmol O2-/10(5) cells per 30 min, which was significantly greater than the amount produced by cells incubated with C5b6 alone, serum alone, or decayed MACs that can no longer interact with the cell membrane (0.3 +/- 0.2, 0.4 +/- 0.1, 0.3 +/- 0.2 nmol O2-/10(5) cells per 30 min, respectively; P less than 0.02). Production of O2- after stimulation with MACs increased during the first 20 min of incubation but then plateaued. Cells exposed to decayed MACs produced small amounts of O2-, which did not increase from 20 to 60 min. Production of H2O2 was also observed after stimulation with MACs, and continued to increase during 60 min of incubation (1.22 +/- 0.16 nmol H2O2/10(5) cells per 60 min), whereas H2O2 production could not be detected after exposure to decayed MACs. Cell viability was not adversely affected by exposure to nascent MACs as determined by trypan blue exclusion or chromium-51 release. These results demonstrate that glomerular mesangial cell membrane interaction with the MAC stimulates the production of the toxic oxygen metabolites O- and H2O2. Activation of the terminal complement pathway by mesangial immune deposits in vivo might lead to tissue injury by stimulation of local production of toxic oxygen-free radicals.

Drug-associated glomerulopathies

The renal glomeruli are vulnerable to injury by a number of drugs and other toxic agents. These agents may lead to damage by one of two basic mechanisms: direct, dose-related toxic injury; indirect, immunologically mediated injury, largely dose-independent. Proteinuria is the simplest and most important functional indicator of glomerular injury. It occurs almost immediately in direct toxic injury, but there is a latent period of weeks to months with immunologically mediated processes. Of the two mechanisms, the second is by far the more common in clinical settings. The best studied experimental agent causing direct toxic injury is the aminonucleoside of puromycin. Clinically, perhaps the most important agent is Cyclosporine A. Although this agent is usually thought of primarily as a tubular toxin, it is capable of giving rise to a microangiopathic glomerular lesion similar to that in the hemolytic uremic syndrome. The classic model for immunologic glomerular lesion is Heymann nephritis, which produces a membranous glomerulopathy. Clinically, most drug mediated glomerulopathies also take the form of a membranous nephropathy, usually with a frank nephrotic syndrome. Among the more common offenders are penicillamine, gold salts used in rheumatoid arthritis, and captopril used in hypertension. The other common type of drug-related glomerulopathy occurs as part of a lupus-like syndrome induced by a variety of drugs, including hydralazine, procainamide, and penicillamine. All of these give rise to a variety of antibodies, most prominently antinuclear antibodies, and in the more severe cases there may be lupus-like glomerular lesions as well.

Influence of antigen distribution on the mediation of immunological glomerular injury

To determine if the site of immune reaction could influence the mediation and morphological expression of glomerular injury in experimental anti-glomerular basement membrane (anti-GBM) nephritis and membranous nephropathy, we studied the events that followed the in situ reaction of rat antibody with antigen planted in either the GBM (especially the lamina rara interna) or in the subepithelial space (SE). Non-nephritogenic amounts of noncomplement-fixing sheep anti-GBM or anti-tubular brushborder antibody were injected into separate groups of rats to plant sheep IgG in the GBM and SE, respectively. Kidneys containing sheep IgG were then transplanted into naive recipients that were passively immunized with rat anti-sheep IgG. There was marked proteinuria after 2 days (antigen in GBM: 226 +/- 50.7; antigen in SE: 69 +/- 50.7 mg/24 hr) that was abrogated by prior depletion of complement in both groups (antigen in GBM: 10.2 +/- 1.7; antigen in SE: 14.3 +/- 8.7 mg/24 hr). When antigen was planted in SE, inflammatory-cell depletion with either anti-neutrophil (PMN) serum or lethal irradiation had no effect on proteinuria. In contrast, anti-PMN abolished proteinuria (12.0 +/- 5.6 mg/24 hr) and irradiation reduced it by 60% when antigen was in GBM. Glomeruli of kidneys with antigen in GBM were significantly larger and more hypercellular than those with antigen in SE after transplantation into immunized recipients. Endothelial cell injury and adherence of inflammatory cells to denuded GBM were prominent in the former (antigen in GBM), while glomeruli with antigen in SE showed only subepithelial deposits, adjacent slit-diaphragm displacement, and epithelial cell foot-process effacement. Thus, the reaction of antigen and antibody in glomeruli produced complement-mediated injury which was cell-independent when complex formation occurred on the outer aspect of the GBM but was cell-dependent when the same reagents reacted more proximally to the circulation. We therefore conclude that antigen distribution can critically influence the mediation and morphologic expression of immune glomerular injury and may, in part, account for variations in the clinical and histological manifestations of antibody-induced glomerular disease in humans.

Hepatitis B surface antigenemia in North American children with membranous glomerulonephropathy. Southwest Pediatric Nephrology Study Group

We describe 11 North American children with hepatitis B-associated membranous glomerulonephropathy (MGN). The children are predominantly black boys (seven of 11) and younger (mean age 5.3 years) than children with idiopathic MGN (10.6 years) or lupus-associated MGN (13.4 years). Most of the patients (10 of 11) had nephrotic syndrome, and although clinical evidence of liver disease was present in only one child, eight of 10 patients had elevated aspartate amino transferase levels. Liver biopsies performed in three children revealed chronic persistent hepatitis in two and chronic active hepatitis in one. Hepatitis B surface antigen was found in the serum of one of the parents of five patients and in the brother of one patient. Low serum C3 values were observed in all 11 children at some stage of their illness. Renal biopsy specimens revealed stage II or III glomerular capillary wall changes, and immunofluorescence studies revealed three or more glomerular immunoreactants in 10 of 11 biopsies. One patient developed end-stage renal disease 9 years after presentation of nephrotic syndrome; the duration of follow-up of the other 10 patients is limited at this time. We conclude from these data that hepatitis B-associated MGN is more frequent in North American children than previous reports have suggested, is most commonly seen in young black boys, and is characterized by low serum C3 levels. It appears, therefore, to be a distinct clinicopathologic entity.

Role of terminal complement pathway in the heterologous phase of antiglomerular basement membrane nephritis

Terminal complement components, including the membrane attack complex, have been demonstrated in glomeruli of patients with immune complex and anti-GBM nephritis. We recently demonstrated the functional significance of C6 in the mediation of experimental membranous nephropathy in rabbits. In the present study, the role of C6 was examined in the heterologous phase of rabbit anti-GBM nephritis by studying normal and C6-deficient (C6D) rabbits. In C6D rabbits, C6 hemolytic activity was less than 0.01% of control. All control rabbits became heavily proteinuric in the first 24 hr following injection of a standard dose of sheep anti-rabbit GBM antibody (mean, 42.0 +/- 26.3; range, 18.4 to 83.5 mg protein/mg creatinine, N = 5). In contrast, C6D rabbits excreted a mean of only 5.1 +/- 5.5 mg/mg creatinine (range, 0.06 to 14.4, N = 6, P = 0.002). Protein excretion in normal rabbits was less than 0.06 mg/mg creatinine. Both control and C6D rabbits had similar deposits of sheep anti-rabbit GBM IgG in glomeruli when measured by radiolabeling techniques (control 15.8 +/- 2.71, N = 5; C6D 18.7 +/- 1.99 micrograms of sheep IgG/10(4) glomeruli, N = 6, P greater than 0.05). Control rabbits had a greater rise in serum creatinine in the first 24 hr (1.74 +/- 1.15 vs. 0.53 +/- 0.44 mg/dl, P less than 0.05). Both groups had similar deposits of sheep IgG and rabbit C3 by IF. By light microscopy at 4 and 24 hr, both groups had qualitatively similar proliferative changes and similar numbers of neutrophils infiltrating glomeruli.(ABSTRACT TRUNCATED AT 250 WORDS)

Anti-GBM nephritis in the mouse: severe proteinuria in the heterologous phase

Highly reproducible anti glomerular basement membrane (GBM) nephritis has been induced in the mouse after a single injection of rabbit or goat antibody against purified homologous GBM. The severity of albuminuria was closely related to the amount of antibody given. With doses of 4 mg or more, low serum albumin concentrations, sometimes accompanied by ascites and oedema, were observed after 1 week. Glomerular injury was characterized by an initial accumulation of polymorphonuclear granulocytes followed by thrombosis and necrosis, the extent of which defined the outcome of the glomerulonephritis. With high doses of antibody the exudative lesions entered a chronic phase, while at doses lower than 2 mg remission of the lesions occurred. Immunofluorescence studies showed prompt linear fixation of the injected antibodies to the glomerular capillary wall, accompanied by immediate binding of C3 in a fine granular pattern. Fibrin deposits appeared at 2 h in some glomeruli, increased thereafter, and were present after one day in more than 90% of the glomeruli in mice that had received 4 mg of antibody. This new reproducible model in the mouse is suited for the study of the relationship between activation of mediator systems, histological lesions, and proteinuria.

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.

Factors influencing susceptibility of LEW rats to Heymann nephritis

Although most LEW rats develop the proteinuria of Heymann nephritis (HN) within 2 months after immunization with Fx1A, protein excretion of some animals remains normal. We have compared nonproteinuric rats with those that developed HN in order to identify factors that influence susceptibility to immunologically medicated kidney disease. In the primary response to Fx1A, immunofluorescence tests showed that antibrush border titers in serum and immunoglobulin deposition in vivo were similar in all rats. However, complement was detected only in rats with proteinuria. Reimmunization with Fx1A at 30 weeks stimulated anamnestic antibody responses in all rats. Following reimmunization, 60% of nonproteinuric rats developed severe HN with an unusually rapid (1 week) onset. Once again, complement was present only in glomeruli of rats with proteinuria. It appears that titers of antibodies to brush border, measured by immunofluorescence tests, are not an index of the pathogenicity of the immune response to Fx1A. Immunological memory, leading to rapid expression of autoimmune disease upon reexposure to antigen, can be established by a primary immunization that does not produce clinical symptoms. Abnormal urine protein composition may provide a clue to subclinical immunopathology of the kidney.