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Evans Department of Medicine and Clinical Research, Renal Section, Boston University Medical Center, Boston, Massachusetts 02118
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Key Words: complement regulation Heymann nephritis Crry autoantibodies glomerulus
Abbreviations used: CIA, collagen-induced arthritis; Crry, complement receptor 1-related protein/gene y; HN, Heymann nephritis; IF, immunofluorescence.
Heymann nephritis (HN)1 is a widely studied model of membranous nephropathy with many similarities to the human disease (1, 2). Active immunization of susceptible rat strains with fraction 1A (Fx1A), a crude renal tubular preparation, induces the production of IgG auto-Abs that accumulate in glomeruli. Within 8 wk of immunization with Fx1A,
Much effort has gone into identifying the antigen(s) contained within Fx1A that confer pathogenic potential to this immunogen. One well-described antigen was originally termed gp330 and is now known as megalin (8–10). Active immunization with megalin, or passive administration of anti-megalin Ab, leads to glomerular deposition of immune complexes. Yet despite the accumulation of IgG Ab in immune deposits, there is no glomerular C3 or C5b-9 deposition and animals do not develop abnormal proteinuria (2, 11).
Crry (complement receptor 1-related protein/gene y) is a rodent complement inhibitor that combines the functions of human decay-accelerating factor (CD55) and membrane cofactor protein (CD46). Although not a true genetic homologue, Crry is considered a functional analogue of these two proteins, and is widely expressed (12–17). Not surprisingly, given that Fx1A is a crude extract of renal tubules which contain Crry (18, 19), heterologous anti-Fx1A Abs contain reactivity towards Crry. These Abs inhibit the function of Crry in cultured glomerular epithelial cells leading to unrestricted complement activation (20, 21).
Since Crry is contained in Fx1A, we were interested in determining if autoAbs were generated to Crry and whether these were of pathogenic importance in HN. In this study, animals immunized with Fx1A lacking Crry did not develop abnormal proteinuria or glomerular C3 deposition despite the formation of anti-Fx1A Abs, which formed typical glomerular subepithelial deposits. Nephritogenicity of this preparation could be reconstituted by passive transfer of anti-Crry Abs or by inclusion of rCrry in the immunogen.
The five NH2-terminal short consensus repeats of Crry (22, 23) containing the active sites of Crry were produced as a recombinant soluble protein in Pichia pastoris. rCrry was purified by sequential Mono Q and Mono P chromatography (Amersham Pharmacia Biotech, Piscataway, NJ; reference 24).
Anti-Crry Abs.
Animal Studies.
To have controls with an actively produced autoimmune disease, collagen-induced arthritis (CIA) was induced contemporaneously in Lewis rats. These animals were immunized with 0.2 mg of bovine type II collagen (Sigma Chemical Co., St. Louis, MO) in incomplete Freund's adjuvant twice over 7 d (25). This resulted in a polyarticular arthritis, which peaked in severity by 4 wk after immunization and then resolved.
Passive administration of anti-Crry Abs was performed by injecting animals with 10 mg of anti-Crry IgG or F(ab')2 intravenously given in divided doses over 2 d. Control animals received 10 mg of preimmune IgG intravenously over 2 d.
Measures of Disease.
To measure glomerular anti-Fx1A and anti-Crry Abs, glomeruli were isolated from one kidney by sieving (6), after which Abs were eluted by sonication in 20 mM of citric acid, pH 3.2, in the presence of protease inhibitors (11). Glomerular protein content was determined by a bicinchoninic acid assay (Pierce Chemical Co., Rockford, IL).
ELISA.
Statistics.
The Fx1A preparation lacking Crry was further studied through the active immunization of 12 female Lewis rats. Animals were followed for 10 wk, during which time no significant proteinuria occurred (0.98 ± 0.12 and 2.14 ± 0.53 mg/d at 6 and 10 wk, respectively, after immunization). At this time, all animals underwent renal biopsy. By IF microscopy there was strongly positive granular staining for IgG outlining glomerular capillary walls in 10 out of 12 animals (Fig. 1 A), whereas the remaining 2 animals had only faintly apparent IgG deposits. In all animals, characteristic deposits of C3 were absent (Fig. 1 B).
60% of animals develop proteinuria, which is associated with glomerular deposition of C3 and C5b-9 (3, 4). As shown in passive HN, heterologous anti-Fx1A, when injected into rats, reacts with antigens on the glomerular epithelial cell surface and activates complement. In this model, the assembly of C5b-9 on glomerular epithelial cell membranes is directly pathogenic (5–7). Similar mechanisms are presumed to be operative in active HN.
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Materials and Methods
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Abstract
Materials and Methods
Results
Discussion
References
Fx1A and rCrry.
Fx1A was prepared from normal Sprague-Dawley rat kidneys by standard techniques (1). One batch was made from freshly isolated Sprague-Dawley rat kidneys, which led to typical HN upon active immunization of Lewis rats. The second Fx1A preparation was isolated from commercially obtained Sprague-Dawley rat kidneys (Pel-Freez Biologicals, Rogers, AR). This failed to induce classic disease manifestations of HN, the reasons for which are studied here.
Serum was obtained from a single sheep hyperimmunized with rCrry. IgG was isolated by protein G affinity chromatography (Amersham Pharmacia Biotech). Preimmune IgG from the same sheep was used as a control in these studies. Anti-Crry F(ab')2 was generated by pepsin treatment followed by size exclusion chromatography on a Sephacryl S-100HR column. No intact heavy chains were present by SDS-PAGE under reducing conditions.
HN was induced by immunizing 150-g female Lewis rats with Fx1A. 10 mg of Fx1A was mixed with 4 mg of Mycobacterium butyricum in 0.1 ml of PBS and emulsified with an equivalent volume of complete Freund's adjuvant (Difco, Detroit, MI), which was injected in equally divided doses into both hind footpads. Starting 6 wk after immunization, animals were housed biweekly in metabolic cages for urine collection. Sera were obtained at death by cardiac puncture. Renal tissue was obtained by survival renal biopsy and at death.
Urinary protein excretion was measured by a sulfosalicyclic acid precipitation method (26). Proteinuria was considered abnormal when excretion was
6 mg/d (1). Direct immunofluorescence (IF) was performed on cryostat sections for rat IgG and C3 (26). Semiquantitative scores for IF staining intensity were assigned to coded sections (27).
ELISAs for anti-Fx1A and anti-Crry Abs were performed by coating polystyrene plates with 10 µg/ml detergent-solubilized Fx1A or rCrry. Diluted sera or glomerular eluates were added, which were detected with peroxidase-conjugated anti-rat IgG (Sigma Chemical Co.), followed by development with o-phenylene-diamine. Measurements were made at OD450, and data are presented as OD450 U/ml. When comparisons were made, samples were run in duplicates on a single ELISA plate. For measurements of Crry, polystyrene plates were coated with 20 µg/ml of anti-Crry IgG, after which serial dilutions of rCrry or detergent-solubilized Fx1A were added. Bound Crry was detected with biotinylated anti-Crry, followed by streptavidin– horseradish peroxidase. The ELISA was sensitive to 10 ng/ml Crry.
Statistical analyses were performed with Minitab software (College Park, MD). Data are expressed as mean ± SEM and were analyzed by Student's t test when two groups were compared or by analysis of variance for multiple groups.
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Results
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Abstract
Materials and Methods
Results
Discussion
References
Two preparations of Fx1A were compared in our studies. These preparations differed in their capacity to induce classic HN, as defined by the development of abnormal proteinuria in the majority of animals starting 6–8 wk after immunization. The nephritogenic preparation of Fx1A isolated from freshly obtained kidneys contained 3.1% Crry (wt/wt), whereas the Fx1A that was prepared from freeze-thawed kidneys failed to induce disease manifestations of HN and lacked Crry by a sensitive ELISA. These results suggested that Crry was an important component in nephritogenic Fx1A.
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These results suggested that generation of an immune response to Crry is necessary for the development of proteinuria in HN. The next set of studies directly tested whether reconstituting Fx1A lacking Crry with rCrry could restore nephritogenicity. Therefore, female Lewis rats were immunized with Fx1A lacking Crry (n = 20) or with this same Fx1A preparation to which 0.5 mg of rCrry was added (n = 15). Animals were observed for 14 wk to ensure that the potential development of proteinuria was not missed. As in the previous studies, animals immunized with Fx1A lacking Crry did not develop proteinuria (Fig. 3). In contrast, in animals immunized with Fx1A reconstituted with rCrry there was progressive development of proteinuria, such that by 14 wk after immunization, 8 out of 15 animals had significant proteinuria. This proportion of proteinuric rats is very similar to that typically seen in active HN (1). At death, anti-Fx1A Ab levels were comparable in both groups (2.17 ± 0.23 and 2.17 ± 0.28 OD450 U/ml).
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In our past studies, we found that heterologous anti-Fx1A Abs reacted with glomerular epithelial cell antigens and could activate complement on the surface of these cells, leading to cellular injury (6). Subsequently, we observed that nephritogenic anti-Fx1A impaired intrinsic complement regulation in glomerular epithelial cells. Such an effect occurred at the level of complement C3/C5 convertases formed from either the classical or alternative pathways (20). At that time, the antigen responsible for these effects of anti-Fx1A was unknown. With the isolation of rat Crry as the antigen reactive with mAb 5I2 (28), it was possible to prove that anti-Fx1A reacted with rat Crry, consistent with its inhibiting intrinsic regulation of C3/C5 convertases (21). That heterologous anti-Fx1A Abs contains reactivity towards Crry is not surprising, given the widespread distribution of Crry, including in renal tubules (18, 19). Interestingly, we have also shown that heterologous anti-Fx1A has reactivity towards CD59, a regulator of C5b-9 formation, which can similarly be attributed to the presence of CD59 in renal tubular cells (18). The role for reactivity of heterologous anti-Fx1A with Crry or CD59 in passive HN is a subject of ongoing study.
Despite the identification of megalin as an unquestionably important immunogen within Fx1A, it is very clear that megalin is not the only nephritogenic component contained within Fx1A. Thus, either passive administration of antimegalin Ab or active immunization with megalin is insufficient to recapitulate the full disease of HN. This has stimulated a continuing search for additional antigenic component(s). A number of candidate antigens have been identified, including β1 integrins, dipeptidyl peptidase IV, aminopeptidase A, and an uncharacterized lipid antigen (29–31).
Although HN has many similarities to human membranous nephropathy, identifying target human antigens in this disease has proved elusive. For instance, as yet there is no identified analogue to megalin or the HN antigenic complex expressed in human glomeruli (8–10, 32). Human glomerular epithelial cells contain decay-accelerating factor and membrane cofactor protein (33, 34), which together contain the function of Crry. There are subepithelial IgG deposits in human membranous nephropathy, exactly as in HN. The evidence for the complement mediation of this disease in humans is strong. There is glomerular deposition of C3 and C5b-9, and the appearance of C5b-9 in the urine has prognostic importance (35–37). An intriguing possibility raised by these studies is that autoAbs to decay-accelerating factor and/or to membrane cofactor protein may form and accumulate in human membranous nephropathy. If so, it may be possible, with the use of recombinant complement regulators (38–42) to overcome the effects of these autoAbs and thereby prevent glomerular epithelial cell injury and proteinuria.
| Acknowledgments |
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This work was supported by National Institutes of Health grants DK30932 (to D.J. Salant) and DK41873 (to R.J. Quigg), and by a grant-in-aid from the National Kidney Foundation, Illinois Chapter (to R.J. Quigg). J.J. Alexander was supported by training grant DK07510 from the National Institutes of Health.
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