Nephritogenicity of proteoglycans. II. A model of immune complex nephritis

Murine membranous nephropathy: immunization with α3(IV) collagen fragment induces subepithelial immune complexes and FcγR-independent nephrotic syndrome

Membranous nephropathy (MN) is a leading cause of nephrotic syndrome in adults and a significant cause of end-stage renal disease, yet current therapies are nonspecific, toxic, and often ineffective. The development of novel targeted therapies requires a detailed understanding of the pathogenic mechanisms, but progress is hampered by the lack of a robust mouse model of disease. We report that DBA/1 mice as well as congenic FcγRIII(-/-) and FcRγ(-/-) mice immunized with a fragment of α3(IV) collagen developed massive albuminuria and nephrotic syndrome, because of subepithelial deposits of mouse IgG and C3 with corresponding basement membrane reaction and podocyte foot process effacement. The clinical presentation and histopathologic findings were characteristic of MN. Although immunized mice produced genuine anti-α3NC1 autoantibodies that bound to kidney and lung basement membranes, neither crescentic glomerulonephritis nor alveolitis ensued, likely because of the predominance of mouse IgG1 over IgG2a and IgG2b autoantibodies. The ablation of activating IgG Fc receptors did not ameliorate injury, implicating subepithelial deposition of immune complexes and consequent complement activation as a major effector pathway. We have thus established an active model of murine MN. This model, leveraged by the availability of genetically engineered mice and mouse-specific reagents, will be instrumental in studying the pathogenesis of MN and evaluating the efficacy of novel experimental therapies.

Type 1 plasminogen activator inhibitor deficiency aggravates the course of experimental glomerulonephritis through overactivation of transforming growth factor beta

Type 1 plasminogen activator inhibitor (PAI-1) is the primary inhibitor of tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA). Whereas PAI-1 is not expressed in normal kidneys, it is strongly induced in glomerular diseases and thus could promote the local accumulation of fibrin. To study the role of PAI-1 in the development of inflammatory glomerular injury, passive antiglomerular basement membrane (GBM) glomerulonephritis (GN) was induced in PAI-1 knockout mice and in wild-type mice of the same genetic background. Unexpectedly, PAI-1 deficiency was associated with an early and severe exacerbation of glomerular injury: Infiltration by CD4 T cells, proportion of fibrinous crescents, and renal function impairment were significantly more pronounced in PAI-1 -/- mice. Interestingly, activation of transforming growth factor (TGF)- beta, which is known to be dependent on the PA/plasmin system in vitro, was dramatically enhanced in the kidneys in the absence of PAI-1. Moreover, administration of neutralizing antibodies against TGF-beta significantly attenuated the disease in PAI-1 -/- mice. This suggests that the poor outcome of GN in PAI-1 -/- mice is consecutive to an uncontrolled activation of TGF-beta and confers PAI-1 with a new, immunomodulatory role.

Matrix metalloproteinase 9 protects mice from anti-glomerular basement membrane nephritis through its fibrinolytic activity

Matrix metalloproteinase (MMP)9/gelatinase B is increased in various nephropathies. To investigate its role, we used a genetic approach. Adult MMP9-deficient (MMP9(-/)-) mice showed normal renal histology and function at 3 mo. We investigated the susceptibility of 3-mo-old mice to the accelerated model of anti-glomerular basement membrane nephritis, in which fibrin is an important mediator of glomerular injury and renal impairment. Unexpectedly, nephritis was more severe in MMP9(-/)- than in control mice, as attested by levels of serum creatinine and albuminuria, and the extent of crescents and fibrin deposits. Circulating or deposited immunoglobulin G, interleukin (IL)-1beta, or IL-10 were the same in MMP9(-/-) and MMP9(+/+) mice. However, we found that fibrin is a critical substrate for MMP9, and in its absence fibrin accumulated in the glomeruli. These data indicate that MMP9 is required for a novel protective effect on the development of fibrin-induced glomerular lesions.

alpha-galactosyl epitopes on glycoproteins of porcine renal extracellular matrix

BACKGROUND

The pig is the donor animal of choice for human xenotransplantation. In the most relevant pig-to-baboon model, pig organs transplanted into baboons are hyperacutely rejected by natural xenoantibodies, which mainly bind to alpha-galactosyl (alphaGal) epitopes expressed at the surface of endothelial cells. Recent advances in controlling hyperacute rejection have led to improved survival of these xenografts, and it is now important to identify alphaGal binding sites in other cells and tissues that may be subject to immunologic attack. To this end, we have studied whether alphaGal antibodies bind to glycated proteins of the extracellular matrix in the kidney and other organs most likely to be used for human xenotransplantation.

METHODS

High-titer anti-alphaGal antibodies, similar to human natural xenoantibodies, were prepared in baboons, and their reactivity with components of pig extracellular matrix was tested by serology and immunohistology.

RESULTS

The antibodies recognized epitopes of immobilized murine, bovine or porcine thyroglobulin, laminin, heparan sulfate proteoglycans, and fibronectin. In sections of pig tissue, the antibodies bound to endothelial and certain epithelial cells, as shown in previous studies, and also to mesenchymal cells, basement membranes, and extracellular matrices, in which they colocalized with matrix glycoproteins, especially laminin and heparan sulfate proteoglycans.

CONCLUSIONS

These results suggest that when pig xenografts can be made to survive for prolonged periods, the reactivity of alphaGal antibody with matrix molecules can induce basement membrane and matrix lesions similar to those induced in laboratory animals by antilaminin and antiheparan sulfate proteoglycans antibodies.

Interaction of baboon anti-alpha-galactosyl antibody with pig tissues

As barriers to xenotransplantation are surmounted, such as suppression of hyperacute rejection allowing improved graft survival, it becomes important to define longer-term host-xenograft interactions. To this end we have prepared in baboons high titer anti-alpha-Galactosyl (alphaGal) and anti-porcine aortic endothelial cell antibodies, similar to human natural xenoantibodies and reactive with epitopes of thyroglobulin, laminin, and heparan sulfate proteoglycans. When injected into pigs with a protocol similar to that used in the rat to show the nephritogenic potential of heterologous anti-laminin and anti-heparan sulfate proteoglycan antibodies, baboon immunoglobulins bound first to renal vascular endothelium, and later to interstitial cells, especially fibroblasts and macrophages, and to antigens in basement membranes and extracellular matrix, where they colocalized with laminin- and heparan sulfate proteoglycan-antibodies, and with bound Griffonia simplicifolia B4. A similar binding was observed in other organs. The pigs did not develop an acute complement-dependent inflammation, but rather chronic lesions of the basement membranes and the extracellular matrix. Incubation of renal fibroblasts with baboon anti-alpha-Galactosyl antibodies resulted in increased synthesis of transforming growth factor-beta and collagen, suggesting a possible basis for the fibrotic response. The results demonstrate that in this experimental model a consequence of alphaGal antibody interaction with porcine tissues, is immunoreactivity with alphaGal on matrix molecules and interstitial cells, priming mechanisms leading to fibrosis resembling that in chronic allograft rejection. The possibility that similar lesions may develop in long-surviving pig xenografts is discussed.

Anti-nuclear antibody production and immune-complex glomerulonephritis in BALB/c mice treated with pristane

The pathogenesis of systemic lupus erythematosus is thought to be primarily under genetic control, with environmental factors playing a secondary role. However, it has been shown recently that intraperitoneal injection of pristane (2,6,10,14-tetramethylpentadecane) induces autoantibodies typical of lupus in BALB/c mice, a strain not usually considered to be genetically susceptible to the disease. In this study, the induction of autoimmune disease by pristane was investigated. BALB/c mice receiving pristane were tested for autoantibody production and histopathological evidence of glomerulonephritis. Six of 11 mice developed IgM anti-single-stranded DNA antibodies shortly after receiving pristane and 4 developed IgM anti-histone antibodies, but anti-double-stranded DNA antibodies were absent. IgG anti-DNA and anti-histone antibodies were absent. In contrast, the lupus-associated anti-nuclear ribonucleoprotein/Sm and anti-Su autoantibodies produced by these mice were predominantly IgG. In addition to autoantibodies, most of the mice developed significant proteinuria. Light microscopy of the kidney showed segmental or diffuse proliferative glomerulonephritis. Electron microscopy showed subepithelial and mesangial immune-complex deposits and epithelial foot process effacement. Immunofluorescence revealed striking glomerular deposition of IgM, IgG, and C3 with a mesangial or mesangiocapillary distribution. Thus, pristane induces immune-complex glomerulonephritis in association with autoantibodies typical of lupus in BALB/c mice. These data support the idea that lupus is produced by an interplay of genetic and environmental factors and that unlike the MRL or (NZB x W)F1 mouse models, in which genetic susceptibility factors are of primary importance, environmental factors are of considerable importance in the autoimmune disease of pristane-treated BALB/c mice.

A monoclonal antibody against GBM heparan sulfate induces an acute selective proteinuria in rats

After immunization of mice with partially-purified heparan sulfate proteoglycan (HSPG) isolated from rat glomeruli, a monoclonal antibody (mAb JM-403) was obtained, which was directed against heparan sulfate (HS), the glycosaminoglycan side chain of HSPG. In ELISA it reacted with isolated human glomerular basement membrane (GBM) HSPG, HS and hyaluronic acid, but not with the core protein of human GBM HSPG, and not with chondroitin sulfate A and C, dermatan sulfate, keratan sulfate and heparin. Furthermore, it did not bind to laminin, collagen type IV or fibronectin. Specificity of JM-403 for HS was also suggested by results of inhibition studies, which found that intact HSPG and HS, but not the core protein, inhibited the binding of JM-403 to HS. In indirect immunofluorescence on cryostat sections of rat kidney, a fine granular to linear staining of the GBM was observed, along with a variable staining of the other renal basement membranes. Pretreatment of the sections with heparitinase completely prevented the binding of mAb JM-403, whereas pretreatment with chondroitinase ABC or hyaluronidase had no effect. The precise binding site of mAb JM-403 was investigated by indirect immunoelectron microscopy. It revealed a diffuse staining of the whole width of the GBM. One hour after intravenous injection of JM-403 into rats, the mAb was detected along the glomerular capillary wall in a fine granular pattern, which shifted towards a more mesangial localization after 24 hours. No binding was observed anymore by day 15. Intravenous injection induced a dose-dependent, transient and selective proteinuria that was maximal immediately after the injection.(ABSTRACT TRUNCATED AT 250 WORDS)

Human glomerular epithelial cell proteoglycans

Proteoglycans synthesized by cultures of human glomerular epithelial cells have been isolated and characterized. Three types of heparan sulfate were detected. Heparan sulfate proteoglycan I (HSPG-I; Kav 6B 0.04) was found in the cell layer and medium and accounted for 12% of the total proteoglycans synthesized. HSPG-II (Kav 6B 0.25) accounted for 18% of the proteoglycans and was located in the medium and cell layer. A third population (9% of the proteoglycan population), heparan sulfate glycosaminoglycan (HS-GAG; Kav 6B 0.4-0.8), had properties consistent with single glycosaminoglycan chains or their fragments and was found only in the cell layer. HSPG-I and HSPG-II from the cell layer had hydrophobic properties; they were released from the cell layer by mild trypsin treatment. HS-GAG lacked these properties, consisted of low-molecular-mass heparan sulfate oligosaccharides, and were intracellular. HSPG-I and -II released to the medium lacked hydrophobic properties. The cells also produced three distinct types of chondroitin sulfates. The major species, chondroitin sulfate proteoglycan I (CSPG-I) eluted in the excluded volume of a Sepharose CL-6B column, accounted for 30% of the proteoglycans detected, and was found in both the cell layer and medium. Cell layer CSPG-I bound to octyl-Sepharose. It was released from the cell layer by mild trypsin treatment. CSPG-II (Kav 6B 0.1-0.23) accounted for 10% of the total 35S-labeled macromolecules and was found predominantly in the culture medium. A small amount of CS-GAG (Kav 6B 0.25-0.6) is present in the cell extract and like HS-GAG is intracellular. Pulse-chase experiments indicated that HSPG-I and -II and CSPG-I and -II are lost from the cell layer either by direct release into the medium or by internalization where they are metabolized to single glycosaminoglycan chains and subsequently to inorganic sulfate.

Nephritogenicity of proteoglycans. III. Mechanism of immune deposit formation

Administration of antibody, directed against glomerular basement membrane (GBM) heparan sulfate-proteoglycan, into a presensitized rat results in the induction of membranous nephropathy with subepithelial immune-complex deposits. In this investigation, we examined the mechanisms responsible for the formation of subepithelial immune-complex deposits in the anti-HS-PG model. In initial experiments, the intravenously administered radioiodinated antibody was seen exclusively localized in the regions of the glomerular capillary wall where the subepithelial deposits were observed. To determine their exclusive localization in the subepithelial space, kinetics of movement of the intravenously administered antibody was investigated. The antibody localized in the inner layers of the GBM within a few minutes after its administration. It equilibrated in the inner and outer layers of the GBM in a matter of a few hours. Then, after 24 hours, it gradually disappeared from the inner layers of the GBM and persisted in the outer layers only. The ready clearance of the antibody from the inner layers may be related to the differential in the kinetics of lateral intrinsic plasma fluid currents within the GBM. The persistence of heterologous antibody exclusively in the outer layers and the availability of host autologous antibodies probably resulted in the development of immune complex deposits in the subepithelial space. The glomeruli devoid of plasma water currents showed no change in the concentration of the antibody in the inner and outer layers of the GBM or mesangial matrix. Also, no antibody binding was observed with the plasmalemma of either the foot processes or visceral epithelia. The data suggest that the biochemical-biophysical properties of the glomerular capillary wall, in concert with its intraglomerular hemodynamics, most likely played a significant role in the development of subepithelial immune-complex deposits in this model.