In vivo interaction of antibodies with cell surface antigens. A mechanism responsible for in situ formation of immune deposits in the zona pellucida of rabbit oocytes

Target antigens and nephritogenic antibodies in membranous nephropathy: of rats and men

Membranous nephropathy, a disease characterized by an accumulation of immune deposits on the outer aspect of the glomerular basement membrane, is the most common cause of idiopathic nephrotic syndrome in white adults. In the rat model of Heymann nephritis, the target antigen of antibodies is megalin, a multiligand receptor expressed at the podocyte cell surface. This review summarizes key findings provided by this experimental model and by our discovery of neutral endopeptidase being the alloantigen involved in neonatal cases of membranous nephropathy. We discuss the role of alloimmunization as a new mechanism of renal disease and the approach that we use to identify new podocyte antigens. We also summarize current knowledge on the mechanism of proteinuria, with special emphasis on the role of complement. In conclusion, substantial progresses have been made in understanding molecular mechanisms of membranous nephropathy, which should lead to novel therapeutic approaches.

Podocyte antigens and glomerular disease

BACKGROUND

Membranous nephropathy (MN), a major cause of nephrotic syndrome in the adult, is an immune-mediated disease characterized by the accumulation of subepithelial immune deposits leading to complement activation and podocyte injury. However, the target antigens of circulating antibodies are unknown. Current treatments for patients with MN are entirely empirical, and concept-driven therapies are dramatically lacking.

METHODS

Specificity of circulating antibodies and composition of glomerular deposits were analyzed in Heymann nephritis (HN), a faithful rat model of MN, and in a subset of patients with antenatal MN.

RESULTS

20 years after the identification of megalin as the podocyte target antigen of nephritogenic antibodies in HN, we identified the human counterpart of megalin, the enzymatic podocyte antigen neutral endopeptidase (NEP). Antibodies to megalin or NEP induce formation of subepithelial immune deposits and of C5b-9, the membrane attack complex of complement.

CONCLUSION

It is likely that antigens involved in idiopathic MN are expressed at the podocyte membrane. Their identification together with that of immunodominant epitopes may lead to specific antigen/ epitope-based immunotherapy aimed at inducing specific tolerance.

New insights into the pathogenesis of membranous glomerulonephritis

PURPOSE OF REVIEW

Membranous nephropathy is one of the most common glomerulopathies. Current treatments are entirely empirical, and concept-driven therapies are dramatically lacking. This review focuses on new pathophysiologic aspects of the disease, with special emphasis on the antigenic targets of pathogenic antibodies.

RECENT FINDINGS

Neutral endopeptidase - a podocyte antigen that can digest biologically active peptides - was recently identified as the target antigen of antibodies deposited in the subepithelial space of glomeruli in a subset of patients with antenatal membranous nephropathy. The mothers became immunized because they are deficient in neutral endopeptidase due to truncating mutations in the gene. Membranous nephropathy could be transferred to the rabbit by injection of mothers' immunoglobulin. Development of the renal disease was associated with anti-neutral endopeptidase IgG1.

SUMMARY

Membranous nephropathy most likely is a heterogeneous disease, although a common denominator may be that podocytes provide antigenic targets for in-situ formation of glomerular immune deposits. Identification of neutral endopeptidase and additional (podocyte) antigens and characterization of their epitopes should make it possible to design more effective and better tolerated therapies. Fetomaternal alloimmunization is a novel mechanism of renal disease that may apply to other organs as well.

Molecular pathomechanisms of membranous nephropathy: from Heymann nephritis to alloimmunization

Membranous nephropathy (MN), the most common cause of idiopathic nephrotic syndrome in white adults, is characterized by an accumulation of immune deposits on the outer aspect of the glomerular basement membrane. In Heymann nephritis, the rat experimental model for MN, megalin--the target antigen of the nephritogenic antibodies--is expressed on the surface of podocytes, where immune complexes are formed, leading to complement activation and nephrotic-range proteinuria. However, megalin cannot be held responsible for human MN because it has not been found in human podocytes or detected in subepithelial immune deposits in patients with MN. Several potential antigens have been identified in so-called secondary forms of MN, but there is no real proof that these antigens are pathogenic. In a subgroup of infants with antenatal MN, neutral endopeptidase (NEP) has been identified as the first protein target on human podocytes of nephritogenic antibodies. The infants' mothers became immunized during pregnancy against NEP expressed on syncytiotrophoblastic cells because they were NEP deficient as a result of truncating mutations in the MME gene. Severity of neonatal renal disease was determined by the mothers' IgG response that led to the formation of the membrane attack complex of complement in the subepithelial deposits. Alloimmunization against NEP is a novel pathomechanism of MN that might also account for some cases of MN after renal or bone marrow transplantation. Other types of alloimmunization should be investigated in MN but also in other renal and nonrenal diseases, particularly those that affect the pediatric age.

Antisperm antibodies detection by flow cytometry is affected by aggregation of antigen-antibody complexes on the surface of spermatozoa

Flow cytometry (FCM) analysis of live antibody-coated spermatozoa subjected to immunofluorescence staining (FCM test) is considered an objective method for the quantitative detection of antisperm antibodies (ASA). But the cross-linking of cell surface antigen (Ag) with bivalent antibodies and/or antigen-antibody (Ag-Ab) complexes with second antibodies may induce the reorganization of surface components (patching and capping) and result in their shedding from the sperm surface. The present study estimates the relationship between aggregation of Ag-Ab complexes on the sperm surface and the results of indirect FCM test. Swim-up spermatozoa of normozoospermic men were incubated with ASA-positive sera from infertile patients and with second antibodies fluorescein isothiocyanate (FITC)-labelled goat anti-human IgG polyclonal antiserum under different conditions and then analysed by FCM and fluorescence microscopy. It was shown that low temperature, cytochalasin B, excess or lack of the primary and/or secondary antibodies and sperm fixation by paraformaldehyde may inhibit aggregation and shedding of Ag-Ab complexes and dramatically increase ASA quantity determined on the sperm surface. However, inhibition of aggregation on the live sperm surface was observed only in a minority of ASA-positive samples and was poorly reproducible using semen of different donors. A high probability of Ag-Ab complex shedding from the sperm surface during experimental manipulation limits the use of indirect FCM test for quantitative ASA determination.

Effect of angiotensin converting enzyme (ACE) and angiotensins on human sperm functions

The presence of components of the renin angiotensin system (RAS) and specific receptors of angiotensin II in the female and male reproductive tract supports the hypothesis that reproductive functions may be controlled by RAS. Therefore, the present study investigated the influence of ACE and angiotensins on sperm functions and the sperm-egg interaction. The experiments did not indicate direct effects of ACE on the capacitation process or acrosome reaction. Release of ACE from human spermatozoa during capacitation was not related to their ability to undergo acrosome reaction after stimulation with ionophore. Therefore, ACE release does not seem to be a useful clinical marker for human sperm capacitation. However, decreased binding of human spermatozoa to the oolemma of zonafree hamster oocytes after inhibition of ACE by captopril indicates that kininase II is involved in sperm-egg interactions. In contrast to other studies, incubation with captopril had no influence on sperm binding to the zona pellucida. Because effects of ACE on sperm-egg interactions but not on capacitation or acrosome reaction were observed, several experiments were performed to study the influence of substrates and products on the acrosome reaction. Angiotensin II induced the acrosome reaction dose-dependently, whereas angiotensin I had no effect on the acrosome reaction. The effect of angiotensin II on acrosome reaction seems to be calcium-dependent and mediated by protein kinases. Since a specific type 2 angiotensin II receptor inhibits the acrosome reaction induced by angiotensin II, this subtype of receptors may be present at the surface of sperm heads. Another clue for the presence of type 2 receptors on human spermatozoa is the finding that pertussis toxin did not inhibit the angiotensin II induced acrosome reaction. In contrast to type 1 angiotensin II receptors, type 2 receptors are known to be G-protein independent.

The ovarian renin-angiotensin system in reproductive physiology

The identification of the presence of prorenin, renin, angiotensinogen, angiotensin-converting enzyme, angiotensin II (Ang II), and Ang II receptors in the ovary suggests that there is a functional ovarian renin-angiotensin system (RAS). It could play a significant role in such areas of ovarian physiology as follicular development, steroidogenesis, oocyte maturation, ovulation, and follicle atresia. Expression of the ovarian RAS is regulated by gonadotropins. Ang II, a bioactive octapeptide of RAS, has important effects as a paracrine/autocrine regulator at different stages of the reproductive cycle. Ang II modulates ovarian steroidogenesis and formation of the corpus luteum and also stimulates oocyte maturation and ovulation via Ang II receptors on granulosa cells. In addition, increasing evidence demonstrates that Ang II is a major factor in regulating the function of atretic follicles. In any physiologic system, aberrations result in the development of pathologic states. Disturbances in the ovarian RAS can be the cause or the result of such reproductive disorders as polycystic ovary syndrome, ovarian hyperstimulation syndrome, ovarian tumors, and ectopic pregnancy. Data support the concept of an active and regulated RAS in ovarian follicles. Species differences observed in the expression of ovarian RAS suggest varying functional roles among species with respect to ovarian physiology.

Neonatal injection of an ovarian peptide induces autoimmune ovarian disease in female mice: requirement of endogenous neonatal ovaries

Neonatal female mice injected with the self ZP3 peptide are not tolerant to the peptide; they develop autoimmune ovarian disease (AOD) and autoantibody response 5 weeks later. ZP3 challenge leads to severe AOD and ZP3-specific T cell and antibody responses. In contrast, neonatal tolerance to foreign ZP3 peptide is established in male mice: ZP3 peptide-specific T cell proliferative response is reduced and AOD is absent in ovarian grafts. Tolerance is associated with a Th2-dominant T cell cytokine and antibody isotype profiles. As controls, neonatal tolerance to foreign peptides, with Th2 deviation, was induced in both male and female mice. Endogenous ZP3 is important for the gender difference. Ablation of ovaries in female mice on days 2 and 5, but not on day 7 or 14, switches the ZP3 autoimmune response to a tolerogenic response with a concomitant change in cytokine profile. Thus, neonatal self ZP3 peptide, supported by endogenous ovaries within a neonatal time window, evokes a pathogenic autoimmune response.

Cellular mechanisms of adaptation of grafts to antibody

New, more effective, strategies of immunosuppression, including those recently designed to induce durable T cell tolerance (by grafting allogeneic or xenogeneic haematopoietic cells into T lymphocyte-depleted recipients), leave humoral rejection as the main barrier to transplantation of vascularized organs between different species. Recent experimental work indicates that hyperacute rejection can be prevented by manipulations of antibodies and complement. In this paper, we review the mechanisms governing the interaction of antibodies with cell surface antigens in vitro and in vivo, and their cellular consequences. Evidence is presented that, in appropriate conditions, antibodies can protect by effecting modification of graft antigenicity (adaptation or accommodation).

Interaction of antibody with Forssman antigen in guinea pigs. A mechanism of adaptation to antibody- and complement-mediated injury

Forssman antigen is a glycosphingolipid with antigenic specificity determined by extra-membrane haptenic sugars similar to blood group antigens and antigens that are the main barrier to xenogeneic organ transplantation. Herein, we describe the localization of Forssman antigen in guinea pig lungs and kidneys and the consequences of its interaction with antibodies in vitro and in vivo (Forssman reaction). Exposure of cultured guinea pig aortic endothelial cells to Forssman antibodies induced rapid redistribution of antigen-antibody complexes at the cell surface, followed by shedding that occurred by blebbing of plasma membrane as vesicles or fragments, and was associated with disappearance of antigen from the cell surface (antigenic modulation). Guinea pigs surviving frequent intravenous infections of increasing amounts of antibodies, for a total of 20 to 40 lethal doses, developed a partial or complete adaptation to generalized Forssman reaction, and adaptation was associated with partial or complete modulation of Forssman antigen at the surface of the pulmonary and, in minor degree, renal endothelial and epithelial cells. These findings support the hypothesis that modulation of endothelial carbohydrate antigens contributes to adaptation of highly vascularized organs exposed to tolerable levels of allo- or xenoantibodies.

Quantitative studies of tubular immune complex formation and clearance in rats

Tubular antibody deposition and clearance was quantitatively studied using affinity-purified rabbit antibodies to rat Tamm-Horsfall protein (TH), a surface membrane glycoprotein of the tubular cells of the thick ascending limb of the loop of Henle. Immune complexes are formed in situ at the base of these cells in rats injected with antisera to TH. The renal binding of I125-anti-TH was determined in pair label studies. Kidneys and other organs were removed from groups of rats for isotope counting at four hours to 14 days after an injection of I125-anti-TH and I131-normal rabbit IgG. The greatest total renal anti-TH binding after injection of 500 micrograms of anti-TH was observed at 24 hours in normal rats (18.55 +/- 1.6 micrograms). During the period of most rapid clearance (day 2 to day 7) the half life of renal anti-TH binding (84.2 hours) and the half life of anti-TH in the serum (68.5 hours) were shorter than that of IgG in the serum (117.8 hours). There was no substantial uptake of anti-TH by other organs. A close relationship between serum levels and renal uptake of anti-TH at 24 hours was also observed in rats given from 50 to 6000 micrograms of anti-TH; renal saturation was evident only at the highest dose. This close relationship was also present during the clearance phase in rats injected with 3700 micrograms of anti-TH; the half life of anti-TH was 96.2 hours in kidneys and 110 hours in serum while the half life of rabbit IgG in serum was 151.8 hours. Markedly increased renal uptake of anti-TH was observed in protein-uric rats with passive Heymann nephritis. In very proteinuric rats, 14.1% of the injected dose was bound to kidneys at 24 hours. In these rats, serum anti-TH levels decreased very rapidly to 4% of control serum levels by five days. Throughout the period of study, the serum levels of anti-TH determined by direct radiometric assay corresponded very closely to those obtained by enzyme-linked immunosorbent assay (ELISA). Urinary excretion was a major mechanism for the clearance of anti-TH in proteinuric rats; more than 10% of the injected I125-anti-TH was recovered intact (that is, protein bound) during the first day after injection. During the clearance phase for renal deposits, urinary clearance of anti-TH exceeded urinary clearance of IgG due to release of renal bound antibody into urine.(ABSTRACT TRUNCATED AT 400 WORDS)

Experimental glomerulonephritis induced in rats by a lectin and its antibodies

The aim of the present study was to investigate the events following interaction of antibody with an antigen planted on the surface of glomerular endothelial cell (GEN). A lectin, Helix pomatia agglutinin (HPA), was planted at the surface of rat GEN by the perfusion of the isolated left kidney with neuraminidase (NRD) and HPA. A subsequent perfusion with IgG fraction, but not with Fab fragments, of rabbit anti-HPA serum induced formation of granular immune deposits (IDs) on the surface of GEN. When the left kidney was revascularized after initial formation of IDs, acute glomerulonephritis ensued. Fifteen minutes after revascularization, granular IDs were observed at the subendothelial space. Two days later, there was decrease of subendothelial IDs with concomitant appearance of subepithelial IDs. At the seventh day, IDs were mainly localized in the subepithelial space. The results suggest that this model of experimental glomerulonephritis is characterized by: (1) initial formation of HPA immune complexes (ICs) at the surface of GEN; and by (2) subsequent movement of ICs from luminal side to subepithelial area with local formation of IDs. The results are relevant to the understanding of the kinetics of ICs resulting from interaction of antibodies with exogenous antigens "planted" in the glomerular capillary walls, and of the local formation of IDs.

The glomerular mesangium

The mesangium is more than simply a support structure for glomerular capillary circulation. Mesangial cells respond to various vasoactive mediators and probably contribute to the regulation of mesangial cell contractility and, consequently, glomerular perfusion. The surrounding extracellular matrix contains several glycoproteins and collagens that may affect the immune interactions of antigens and antibodies and may possibly influence the disposition of immune complexes. These properties may modulate the consequences of deposition of IgA-containing immune complexes in the mesangium in patients with IgA nephropathy and the consequent expression of clinical disease.

Quantitative and qualitative studies of antibody-induced mesangial cell damage in the rat

Intravenous administration of heterologous anti-rat thymocyte serum (ATS), which reacts with a Thy-1-like antigen present on rat glomerular mesangial cells, caused lytic (1 hr to 2 days), hypercellular (4 to 14 days), and sclerotic (2 to 3 months) mesangial lesions in Lewis rats. The normal control of 48.6 +/- 7.9 (mean +/- SD) glomerular nuclei on histologic section decreased significantly (P less than 0.001) to 39.8 +/- 6.1, 37.4 +/- 6.0, and 38.9 +/- 6.8 at one hour, four hours and two days after ATS administration, respectively. Thereafter glomerular nuclei increased to 54.7 +/- 11.5 (P less than 0.05) at four days, 62.5 +/- 9.6 (P less than 0.001) at one week and 64.1 +/- 14.2 (P less than 0.001) at two weeks, and normalized (P greater than 0.05) to 49.4 +/- 8.9 at one month and 50.6 +/- 9.0 at three months. By electron microscopy, glomerular damage in the lytic stage was restricted to mesangial cells and was manifested as hydropic degeneration or lysis. Rabbit IgG and rat C3 were found in the mesangium one hour after injection; they decreased at two days and were negligible at four days. By paired label isotope study, 11.6 micrograms of antibody bound per 7.6 X 10(4) glomeruli at one hour was needed to induce mesangial cell degeneration. No or only minimal changes in proteinuria and in serum creatinine were observed with the dosage used in this rat strain.(ABSTRACT TRUNCATED AT 250 WORDS)

An evaluation of the development of experimental membranous nephropathy

Heymann nephritis is a rat model of glomerulonephritis with morphologic manifestations of human membranous nephropathy. This model is generated by immunizing rats with Fx1A antigen. Passive Heymann's nephritis (PHN) can be produced by the administration of anti-Fx1A antibody (anti-Fx1A Ab) (with abnormal proteinuria appearing in 5 days). Studies were designed to examine the evolution of temporal changes in protein excretion, the glomerular ultrafiltration coefficient (LpA) and morphology of glomerular capillary three and five days after induction of PHN. Glomerular hemodynamic evaluation by micropuncture in euvolemic rats with PHN revealed normal values for nephron filtration rate (SNGFR), LpA and the glomerular hydrostatic pressure gradient (delta P) at day three, but by day five the whole kidney GFR and SNGFR were decreased, delta P increased and LpA significantly reduced. Glomerular binding of anti-Fx1A Ab increased from 38 micrograms/7.6 X 10(4) glomeruli on day three to 52 micrograms on day five. Immune complex deposits evaluated by immunofluorescence and electron microscopy appeared larger and were better defined on day five than on day three. Epithelial foot process fusion was more extensive on day five than day three. The onset of increased proteinuria correlated temporally with a reduction in LpA on day five, which in turn correlated with increased antibody binding, immune deposit accumulation and fusion of epithelial cell foot processes.

Lung injury mediated by antibodies to endothelium. II. Study of the effect of repeated antigen-antibody interactions in rabbits tolerant to heterologous antibody

The effect of repeated interactions of antibodies with cell surface antigens have been examined in in vitro, but not in in vivo systems. In this study are described the results of multiple antibody-cell surface antigen interactions in vivo. Rabbits were given repeated intravenous injections of goat antibodies to angiotensin converting enzyme (ACE), an antigen expressed on the surface of lung endothelial cells. For prevention of anaphylactic reactions, which would have been induced by multiple injections of heterologous immune or nonimmune IgG, the rabbits were made neonatally tolerant to goat IgG. Divalent immune IgG given daily for 21 days induced chronic antigenic modulation (antigen disappearance) with resistance to antibody-mediated inflammatory lesions. The rabbits, however, developed degenerative changes of alveolar endothelial and epithelial cells. Administration of immune IgG every other day for 43 days allowed partial reexpression of ACE and was associated with intravascular, but not interstitial, inflammatory changes. In contrast, repeated administration of monovalent immune Fab did not induce antigenic modulation but caused severe, lethal, interstitial pneumonitis. Thus, in this experimental model the development of acute interstitial inflammatory changes correlates with persistence of antigen and is abrogated by disappearance of antigen induced by divalent antibodies. Further, repeated endothelial antigen antibody interactions fail to induce chronic inflammatory or sclerosing lung lesions.

Renal tubular immune complex formation in mice immunized with Tamm-Horsfall protein

Mice were given injections of rat Tamm-Horsfall protein (TH) in order to study immune complex formation in the distal nephron. After immunization, all mice had high serum levels of antibodies to TH; immune deposits consisting of IgG antibodies to TH and TH formed at the base of cells of the thick ascending limb of the loop of Henle (ALH) and distal convoluted tubule. These basal complexes were maximal in number and size within the cortical ALH, where they often radiated toward the luminal surfaces. Although the highest anti-TH antibody titers were found in animals with the most extensive deposits, antibody levels were not directly proportional to IgG deposits or to the time after immunization. The larger immune deposits were visualized by light microscopy as PAS-positive deposits. The quantity of these deposits was proportional to the time after immunization and was directly related to anti-TH antibody levels. Electron microscopy showed that these immune deposits were present within the basal and lateral intercellular spaces of the cells of the ALH. The distribution and localization of TH within the normal mouse kidney was very similar to that in rats. However, the distribution of immune complexes within the distal nephron in mice differed from rats similarly treated with TH and indicates either species differences in the distribution and/or organization of TH associated with tubular cell surfaces or that the accessibility of distal tubules to antibodies is species-dependent.

Polyvalent antigen-antibody interactions are required for the formation of electron-dense immune deposits in passive Heymann's nephritis

In order to analyze the initial steps involved in the formation of subepithelial electron-dense immune deposits, the authors induced passive Heymann's nephritis in rats by intravenous injection of monoclonal or polyclonal antibodies to the relevant antigen, a 330,000-dalton brush border protein also expressed within coated pits of glomerular epithelial cells. Whereas both mono- and polyclonal antibodies induced deposits detectable by indirect immunofluorescence, electron-dense deposits were only found in glomerular capillary walls of rats injected with polyclonal antibodies. Immunoultrastructural analysis confirmed the subepithelial location of heterologous IgG: monoclonal antibodies were essentially confined to the coated pits of glomerular epithelial cells, whereas polyclonal antibodies were detected in coated pits and in the subepithelial electron-dense deposits. In the face of an excess of circulating antibody, gp 330 was not detectable in the deposits, but a striking increase in the number of endoplasmic reticula positive for gp 330 was found. The results indicate that highly cross-linked immune complexes formed by polyvalent antibodies on the epithelial cell surface are necessary for the induction of subepithelial electron-dense deposits.

Gametes contain angiotensin converting enzyme (kininase II)

The localization of angiotensin converting enzyme (ACE) in the gonads of the normal rabbit was studied by immunofluorescence and immunoelectron microscopy. The enzyme is present in the cytoplasm of testicular spermatids and of epididymal and ejaculated spermatozoa, and on the surface of follicular and tubal oocytes. These findings support the hypothesis that ACE has a role in gamete maturation and in fertilization.