The ability to take up substances from the surrounding environment not only provides cells with vital nutrients, but also enables the selective transport of substances from one compartment to another. Megalin and cubilin are two structurally different endocytic receptors that interact to serve such functions. Evidence has accumulated in recent years to indicate that these receptors have important functions in both normal physiology and pathology.

Megalin and cubilin: multifunctional endocytic receptors

Daily injections of any one of several foreign serum proteins produced in rabbits functional and morphological alterations similar to those seen in acute, subacute, and chronic human glomerulonephritis. The critical factor determining whether a rabbit would develop renal disease and the type of disease developed was the amount of antibody the rabbit formed. Those responding with much antibody were likely to develop an acute, self-limited glomerulonephritis and to be subsequently immune to further renal damage. Those responding with antibody barely sufficient to neutralize the antigen injected developed subacute and chronic glomerulonephritis. In the circulation of the rabbits with chronic glomerulonephritis, there was a daily recurring antigen-antibody reaction in the region of near antigen excess to near antibody excess which presumably led to the disease. Antigen apparently in the form of antigen-antibody complexes was deposited along the renal capillary basement membranes coincident with the development of subacute and chronic glomerulonephritis. Once developed, the morphologic stigmata of chronic glomerulonephritis persisted even after injections of antigen were stopped. However, in milder instances the renal function recovered in part after stopping antigen. This experimental model has several implications: first, the renal injury is precipitated by antigens with no known affinity for, or immunologic relationship to, kidney; second, antigen antibody complexes localize in the kidney, apparently on the basis of non-immunologic factors, and may be an etiologic agent of renal injury; third, severe hypersensitivity disorders can be related specifically to relatively poor as well as to good antibody responses; and finally, the pathogenesis suggested here offers an alternative to that of nephrotoxic serum nephritis for the experimental approach to the study of human glomerulonephritis.

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Experimental glomerulonephritis. The pathogenesis of a laboratory model resembling the spectrum of human glomerulonephritis.

Adjuvants--a balance between toxicity and adjuvanticity.

Publisher Summary No experimental model has provided greater insight into the mechanism of immune complex disease than the experimental serum sickness. The morphological, immunohistological, and serological features of the laboratory models have provided a basis for understanding the pathogenic mechanisms responsible for human glomerulonephritis, vasculitis, and a variety of systemic connective tissue diseases. The subject of experimental acute and chronic immune complex disease produced by nonliving antigens has received extensive review in this series. This chapter summarizes the data presented in these two reviews and discusses in detail more recent studies. Experiments to study acute immune complex disease (serum sickness) have been performed in rabbits almost exclusively. Experimental chronic immune complex disease has proved to be a most useful model in understanding human glomerulonephritis. When injected daily with heterologous serum protein antigens, rabbits with strong antibody responses develop chronic membranous glomerulonephritis in about 5 weeks.

Immune Complex Disease in Experimental Animals and Man

The nephritogenic antigen of Heymann's nephritis (HN) was previously purified from tubular brush-border fractions of rat kidney and found to be a 330,000- mol-wt glycoprotein (gp330). This study was conducted to determine whether gp330 is also present in the rat glomerulus, and, if so, to establish where in the glomerulus it is located. Rabbit polyclonal and mouse monoclonal antibodies were raised against purified gp330, which specifically immunoprecipitated gp330 from solubilized brush-border fractions and specifically stained microvilli and coated invaginations (located at the base of the microvilli) of proximal tubule cells. Accordingly, they were used to localize gp330 by immunoprecipitation and immunocytochemistry in glomeruli of normal Lewis rats. 

For immunoprecipitation, purified glomerular fractions were prepared from [(35)S]-methionine-labeled kidneys, extracted with Triton X-100, and the extract was used for immunoprecipitation with affinity-purified rabbit polyclonal, or mouse monoclonal, anti-gp330 IgG. Analysis of immunoprecipitates by sodium dodecyl sulfate-polyacrylamide gel electrophoresis fluorography indicated that a band corresponding in mobility to gp330 was specifically precipitated. 

When unfixed cryostat sections were incubated for indirect immunofluorescence with monoclonal or affinity-purified polyclonal IgG, a fine granular fluorescent staining was seen throughout the glomerulus. When aldehyde-fixed cryostat sections were incubated for indirect immunoperoxidase, reaction product was detected only in the epithelial cells and was not seen in the GBM, endothelium, or mesangium. Within the epithelium it was localized to the endoplasmic reticulum, occasional Golgi elements, multivesicular bodies, and coated pits at the cell surface. The reactive coated pits were distributed all along the cell membrane, including the sides and base of the foot processes. Reaction product was detected in the latter location only in sections that had been digested with neuraminidase before antibody incubation. When rats were given rabbit anti-gp330 IgG by intravenous injection and their kidneys stained for direct immunoperoxidase 3 d later, rabbit IgG was seen to be deposited beneath the slit diaphragms and in the coated pits at the base of the foot processes. 

The immunocytochemical and immunoprecipitation data indicate, in confirmation of the results of others, that the nephritogenic HN antigen is present in renal glomeruli as well as in proximal tubular brush borders. The immunocytochemical results further demonstrate that gp330 is an epithelial, rather than a glomerular basement membrane, antigen. It appears to be synthesized by glomerular epithelial cells and subsequently becomes concentrated in coated pits. As both the endogenous antigen (gp330) and exogenously administered anti-gp330 antibody were localized to coated pits, it seems likely that coated pits located at the base of the foot processes are the sites where the HN antigen (gp330) and circulating antibodies directed against gp330 meet and where immune complexes are formed.

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Immunocytochemical localization of the Heymann nephritis antigen (GP330) in glomerular epithelial cells of normal Lewis rats

Summary1) A severe nephrotic syndrome was noted in 20 rats given Freund's adjuvants and a supernate of rat kidney suspension by intraperitoneal injection. When kidney tissue was replaced by liver, only 3 of 21 animals developed a much milder renal disease. Lung or muscle suspensions failed to induce proteinuria in 10 rats. 2) In large doses (0.5 ml) Freund's adjuvants alone produced a mild nephrotic disease in 3 of 10 rats. Even though the smaller amounts of the adjuvants usually used (0.25 ml) never produced proteinuria, slight histological alterations of the glomerular structure were frequently noted. 3) In contradistinction to other tissue suspensions addition of rat kidney supernate enhanced the effect of adjuvants markedly and regularly. 4) The small amounts of rat kidney proteins used, support the interpretation that an isoand autosensitization mechanism is at play.

Production of nephrotic syndrome in rats by Freund's adjuvants and rat kidney suspensions.

Summary 1.The importance of Freund9s adjuvants for the production of experimental autoimmune nephrosis was studied. The frequency of disease production depended on the amounts as well as on the frequency with which they were used. Incomplete adjuvants or replacement of Mycobacterium tuberculosis by Mycobacterium butyricum , did not produce renal disease. The virulence of the organism or the method by which Mycobacterium tuberculosis was killed did not affect disease production. 2.Production of a less severe, sterile peritonitis by replacement of Freund9s adjuvants by talcum suspensions containing rat kidney antigen, failed to produce renal disease. 3.Splenectomy and partial removal of the omentum before treatment did not affect incidence of ensuing renal disease. 4.When rat kidney cortex suspension was used as antigenic material, the disease incidence was 81%. With material obtained from renal medulla it was 25%. 5.ACTH and cortisone prevented the development of renal disease. Nitrogen mustard decreased the incidence from 87% to 30%. 6.Guinea pigs did not tolerate the i.p. injections and died after 2 to 4 injections. Six dogs received up to 10 injections. The degree of resulting peritonitis was less severe and was only localized around injection sites. No renal disease was noted and precipitins to dog kidney antigen could not be detected in their sera. The severity of peritonitis was more marked in 16 treated rabbits. Bands of precipitation to rabbit kidney suspensions were detected in their sera, but no evidence of renal disease was obtained.

Experimental auto-immune nephrosis in rats: III.

Experimental Autoimmune Renal Disease in Rats

Summary 1.Twenty-nine rats were injected intraperitoneally 6–10 times at 2-week intervals with kidney supernatant emulsified in Freund9s adjuvant. Their sera were tested against rat kidney antigen by Ouchterlony9s double diffusion agar plate technique. Bands of precipitation were found with 43 of 187 sera tested. They appeared from 55 days before the onset of disease and persisted up to 64 days after the onset of proteinuria. 2.Twelve of the sera from these 29 rats were also tested against rat liver, lung and muscle antigens. Four sera produced a band of precipitation with liver antigen. This band showed a reaction of identity with that formed by the same serum against kidney antigen. There were no bands formed against lung and muscle antigens. 3.Twenty-two sera from eight rats injected with adjuvant and liver supernatant were tested against rat kidney antigen. Four of these sera produced bands. Seventeen sera were also tested against rat liver, lung and muscle antigens. Three of these produced a band of precipitation against liver antigen. This band gave a reaction of identity with that produced by the same serum and rat kidney antigen. 4.Five sera from rats injected with rat kidney supernatant emulsified in adjuvant were tested against rat, rabbit and guinea pig kidney antigens. Two produced cross-reacting bands to the three antigens. 5.Hemagglutination titers were estimated on 84 sera from 13 rats injected with kidney supernatant in adjuvant obtained throughout the injection period. Positive titers were found in only four sera. Fifteen sera from 13 rats injected with liver supernatant in adjuvant and 15 sera from 13 rats injected with adjuvant alone gave negative hemagglutination results. 6.Serum complement values were determined serially in 98 sera of eight rats injected with kidney supernatant in adjuvant. A decrease was noted in five of these animals. The decrease correlated more closely with the onset of proteinuria than with the appearance or disappearance of precipitating antibodies. 7.Passive cutaneous anaphylaxis was tested using sera from four rats that had received injections of adjuvant and kidney with negative results. 8.Skin tests performed on eight animals treated with kidney antigen in adjuvant were also negative.

Nephrotic Syndrome in Rats Produced by Sensitization to Rat Kidney Proteins: Immunologic Studies

Doses of 38 to 57 mg of trimethadione (Tridione®) were given by stomach tube daily for 4 to 18 months to 22 rats. Eleven of these developed a nephrotic syndrome characterized by proteinuria (without hematuria), hypoproteinemia, hyperlipemia and histologic lesions in the kidneys. The disease began usually 5 to 6 months after trimethadione administration was started. It was mild in 3 rats, moderately severe in 6, and marked in 2. Renal failure was noted in three animals after proteinuria had been noted for 4 to 9 months. Spontaneous remissions after the administration of trimethadione had been discontinued were not noted. The administration of cortisone in two animals was without detectable beneficial effect. Complement activity in sera of 13 animals was normal, and no agglutinating antibodies could be detected in 10 sera obtained at the time of death. In 11 rats given trimethadione for 6 to 17 months without development of proteinuria, less marked, but definite, histologic alterations were noted. Four of these had some azotemia and in five, slight degrees of hyperlipemia were noted. These findings make it imperative to evaluate the renal status of human patients receiving dione derivatives as long-term treatment for petit mal seizures.

Janet L. P. Hunter

Papers

Production of nephrotic syndrome in rats by Freund's adjuvants and rat kidney suspensions.

Experimental auto-immune nephrosis in rats: III.

Experimental Autoimmune Renal Disease in Rats

Nephrotic Syndrome in Rats Produced by Sensitization to Rat Kidney Proteins: Immunologic Studies

Trimethadione (tridione) nephrosis in rats.