Interaction of antibodies with renal cell surface antigens

On a Long and Winding Road: Alloantibodies in Organ Transplantation

Today we know that both the humoral and the cellular arm of the immune system are engaged in severe immunological challenges. A close interaction between B and T cells can be observed in most "natural" challenges, including infections, malignancies, and autoimmune diseases. The importance and power of humoral immunity are impressively demonstrated by the current coronavirus disease 2019 pandemic. Organ transplant rejection is a normal immune response to a completely "artificial" challenge. It took a long time before the multifaceted action of different immunological forces was recognized and a unified, generally accepted opinion could be formed. Here, we address prominent paradigms and paradigm shifts in the field of transplantation immunology. We identify several instances in which the transplant community missed a timely paradigm shift because essential, available knowledge was ignored. Moreover, we discuss key findings that critically contributed to our understanding of transplant immunology but sometimes developed with delay and in a roundabout way, as was the case with antibody-mediated rejection-a main focus of this article. These include the discovery of the molecular principles of histocompatibility, the recognition of the microcirculation as a key interface of immune damage, the refinement of alloantibody detection, the description of C4d as a footmark of endothelium-bound antibody, and last but not least, the developments in biopsy-based diagnostics beyond conventional morphology, which only now give us a glimpse of the enormous complexity and pathogenetic diversity of rejection.

Evaluation of renal injury and function biomarkers, including symmetric dimethylarginine (SDMA), in the rat passive Heymann nephritis (PHN) model

Symmetric dimethylarginine (SDMA) is a serum biomarker of excretory renal function which consistently correlates with glomerular filtration rate (GFR) across multiple species including rats, dogs, and humans. In human and veterinary clinical settings SDMA demonstrates enhanced sensitivity for detection of declining renal function as compared to other serum biomarkers, but application in preclinical study designs thus far has been limited. The purpose of this study was to determine the performance of serum SDMA in a rat passive Heyman nephritis model of glomerulopathy. In addition to SDMA other biomarkers of excretory renal function were measured including serum creatinine (sCr), blood urea nitrogen (BUN), and cystatin C along with creatinine clearance. Urinary renal biomarkers including microalbumin (μALB), clusterin (CLU), cystatin C, kidney injury marker-1 (KIM-1), neutrophil gelatinase-associated lipocalin (NGAL), and osteopontin (OPN) were also measured. PHN was induced using commercial sheep anti-Fx1A serum. Tissue, serum, and urine were collected from groups of control and anti-Fx1A-treated animals for biomarker evaluation, hematology, urinalysis, serum biochemistry, and histologic examination of kidney. Over the course of a 28-day study, concentrations of the urinary biomarkers μALB, CLU, cystatin C, NGAL, KIM-1 and the serum biomarker cystatin C increased significantly in anti-Fx1A-treated rats as compared to controls but no significant increase in serum SDMA, sCr, BUN, or creatinine clearance were noted in anti-Fx1A-treated rats. Given lack of direct GFR measurement or significant change in the renal function biomarkers sCr, BUN, and creatinine clearance, it is unclear if GFR differed significantly between control and anti-Fx1A-treated rats in this study, though urinary biomarkers and histopathologic findings supported renal injury in anti-Fx1A-treated rats over the time course investigated. This study is among the first to investigate serum SDMA in a rat model relevant to preclinical safety assessment and serves to inform future experimental designs and biomarker selection when evaluation of glomerular injury is of priority.

Immune response and histology of humoral rejection in kidney transplantation

The adaptive immune response forms the basis of allograft rejection. Its weapons are direct cellular cytotoxicity, identified from the beginning of organ transplantation, and/or antibodies, limited to hyperacute rejection by preformed antibodies and not as an allogenic response. This resulted in allogenic response being thought for decades to have just a cellular origin. But the experimental studies by Gorer demonstrating tissue damage in allografts due to antibodies secreted by B lymphocytes activated against polymorphic molecules were disregarded. The special coexistence of binding and unbinding between antibodies and antigens of the endothelial cell membranes has been the cause of the delay in demonstrating the humoral allogenic response. The endothelium, the target tissue of antibodies, has a high turnover, and antigen-antibody binding is non-covalent. If endothelial cells are attacked by the humoral response, immunoglobulins are rapidly removed from their surface by shedding and/or internalization, as well as degrading the components of the complement system by the action of MCP, DAF and CD59. Thus, the presence of complement proteins in the membrane of endothelial cells is transient. In fact, the acute form of antibody-mediated rejection was not demonstrated until C4d complement fragment deposition was identified, which is the only component that binds covalently to endothelial cells. This review examines the relationship between humoral immune response and the types of acute and chronic histological lesion shown on biopsy of the transplanted organ.

Nephrin and podocin dissociate at the onset of proteinuria in experimental membranous nephropathy

BACKGROUND

The slit diaphragm plays a critical role in maintaining the barrier function of the glomerular capillary wall. The pathogenic mechanism of proteinuria in membranous nephropathy remains uncertain. This study was undertaken to analyze the pathogenic role of slit diaphragm in proteinuria in experimental membranous nephropathy.

METHODS

The expression and the localization of slit diaphragm-associated molecules (nephrin, podocin, and CD2AP) and other podocyte-associated molecules (podocalyxin and alpha(3) integrin) in passive and active Heymann nephritis were analyzed by immunofluorescence and Western blot analysis. The interaction of slit diaphragm-associated molecules was investigated by the dual-labeling immunofluorescence method. The mRNA expression of these molecules was also analyzed.

RESULTS

Shifts in nephrin and podocin staining patterns, from linear to granular, were detected in the early stages of passive Heymann nephritis. These shifts were not parallel, and the dissociation of these molecules was detected by the dual-labeling immunofluorescence method in passive and active Heymann nephritis. Western blot analyses with sequentially solubilized materials indicated that the nephrin-rich fraction changed from being partly detergent-resistant to being predominantly detergent-soluble. This change did not occur with podocin. Nephrin excreted into urine was already detected in the early stages of passive Heymann nephritis. Decreased mRNA expression of nephrin and podocin was observed before the onset of proteinuria. By contrast, no extensive change in the expression of alpha(3) integrin was observed in this study.

CONCLUSION

Nephrin is dissociated from podocin and excreted into urine in the early stages of Heymann nephritis. The reduced expression of nephrin and podocin, along with their dissociation, may contribute to the development of proteinuria in Heymann nephritis.

Antibody-induced albuminuria and accelerated focal glomerulosclerosis in the Thy-1.1 transgenic mouse

BACKGROUND

Podocytes play an important role in the development of proteinuria and focal glomerulosclerosis. Previously we have demonstrated that a combination of two monoclonal antibodies (mAb) against aminopeptidase A (APA), an enzyme present on podocytes, induces a massive acute albuminuria in mice. The present study examined the relationship between the acute antibody-induced albuminuria and the development of focal glomerulosclerosis in the Thy-1.1 transgenic mouse. This mouse expresses a hybrid human-mouse Thy-1.1 antigen on the podocytes, and slowly but spontaneously develops albuminuria and focal glomerulosclerosis.

METHODS

Five-week-old non-albuminuric Thy-1.1 transgenic and non-transgenic control mice were injected with anti-APA and anti-Thy-1.1 mAb or saline. Albuminuria was measured at days 1, 7, 14 and 21. At day 21 kidneys were processed for light microscopy, immunofluorescence, and electron microscopy.

RESULTS

Injection of anti-APA and anti-Thy1.1 mAb in Thy-1.1 transgenic mice induced an albuminuria at day 1 that persisted at day 21. The acute albuminuria after injection of anti-APA mAb was more prominent but transient in non-transgenic mice. In non-trangenic mice no albuminuria could be induced with anti-Thy 1.1 mAb. Light microscopy revealed normal glomeruli at day 1 in all transgenic mice, however, at day 21 advanced glomerulosclerotic lesions were seen in mice injected with either anti-APA mAb (37+/-19% of glomeruli affected) or anti-Thy-1.1 mAb (71+/-5%). Non-transgenic mice did not reveal sclerotic lesions at any time investigated. In the transgenic mice the percentage of focal glomerulosclerosis at day 21 did not correlate with albuminuria at day 21. However, we found a highly significant correlation between percentage of focal glomerulosclerosis and the time-averaged albuminuria over the three-week study period (P < 0.001).

CONCLUSION

Injection of a combination of anti-APA or anti-Thy-1.1 mAb into one mo old, non-albuminuric Thy-1.1 transgenic mice induces an acute albuminuria at day 1 that is accompanied by an accelerated focal glomerulosclerosis at day 21. We suggest that the Thy-1.1 transgenic mouse is an excellent model to study specifically the relation between podocytic injury, albuminuria and the development of focal glomerulosclerosis.

DNA and heparin alter the internalization process of anti-DNA monoclonal antibodies according to patterns typical of both the charged molecule and the antibody

The internalization into CHO-K1 fibroblasts of three polyreactive monoclonal IgG2a anti-DNA autoantibodies (mAbs), F14.6, J20.8 and F4.1, isolated from the same unimmunized (NZBxNZW) F1 mouse, and synthetic peptides derived from F4.1 was studied using a technique which quantifies nuclear accumulation. The localization of the mAbs was intranuclear. We compared the influence of two negatively-charged molecules, DNA or heparin. At low concentrations, DNA had dual effects-inhibitory or stimulatory-depending on the mAb. Heparin was inhibitory or had no effect. The possibility that proteoglycans are 'receptors' recognized by anti-DNA mAbs which bind through heparin-sensitive reactions, was explored. Only F4.1 internalization was partly inhibited in glycosaminoglycan-deficient cells. We propose that the complex alterations of internalization patterns of these polyreactive mAbs by the two negatively charged molecules can be explained by (a) the potential of polyreactive mAbs to bind to various charge (or conformation-) dependent 'receptors', (b) the potential of a subclass of mAbs complexed with DNA to utilize additional 'receptor(s)'. Glycosaminoglycans were required for internalization of F4.1-derived peptides, which remained extranuclear, suggesting that nuclear internalization of mAb F4.1 is a multistep process that requires certain sequences present on the intact mAb.

Detection of humoral rejection in human cardiac allografts by assessing the capillary deposition of complement fragment C4d in endomyocardial biopsies

BACKGROUND

There are no well-established diagnostic criteria to detect humoral rejection in organ transplantation. The value of commonly used markers in immunohistochemistry, such as C1q, C3c, IgG, IgM and fibrinogen, is questioned by some groups. Complement fragment C4d is a more stable marker of complement activation as it is covalently bound to graft capillaries. C4d has been shown to identify clinically relevant, but otherwise undetectable humoral anti-graft reactions in human kidney transplants.

METHODS

Immunohistochemical techniques were used to evaluate 155 endomyocardial biopsies from 56 heart transplant recipients less than 3 months post transplantation for the presence of capillary C4d staining. In a subset of patients, C4d staining was compared with C1q, C3c, IgM and fibrin staining and was correlated with clinical outcome.

RESULTS

Within 3 months 9 of 56 patients died. Five of these nonsurvivors had prominent C4d staining (p < .05), whereas C1q, C3c and IgM showed no correlation with clinical outcome. Presence of fibrin correlated with clinical outcome and C4d staining (p < .05).

CONCLUSIONS

The capillary deposition of complement split product C4d in human endomyocardial biopsies was significantly associated with graft loss. Determination of fibrin deposition may yield additional information to establish a diagnosis of humoral rejection. The immunohistochemical assessment of capillary deposition of C4d and fibrin appears to be an appropriate tool for the identification of patients, who may require additional or alternative immunosuppressive therapy targeted against the humoral immune system.

Induction of albuminuria in mice: synergistic effect of two monoclonal antibodies directed to different domains of aminopeptidase A

BACKGROUND

Aminopeptidase A is an enzyme that is present on podocytes and is involved in the degradation of angiotensin II. In previous studies in mice, we administered single monoclonal antibodies directed against aminopeptidase A. We observed that only monoclonal antibodies that inhibited aminopeptidase A enzyme activity caused albuminuria.

METHODS

In this study, the effects of the combined injections of two monoclonal anti-aminopeptidase A antibodies (mAbs) were studied, using a combination of anti-aminopeptidase A mAbs that were directed against two different domains involved in the aminopeptidase A enzyme activity (ASD-3 or ASD-37) and an anti-aminopeptidase A mAb not related to the enzyme active site (ASD-41).

RESULTS

An injection of the combinations ASD-3/37 (total 4 mg, 1:1 ratio) and ASD-37/41 (total 4 mg, 1:1 ratio) in doses that do not cause albuminuria when given alone (4 mg) induced massive albuminuria at day 1 after injection. The combination ASD-3/41 had no effect. This albuminuria was not dependent on systemic immune mediators of inflammation and could not merely be related to a blockade of aminopeptidase A enzyme activity. However, a correlation was observed between the induction of albuminuria and the aggregation of the mAbs injected and aminopeptidase A on the podocytes. An injection of the combinations ASD-3/37 or ASD-37/41 did not cause an increase in systemic blood pressure. The treatment with a combination of enalapril and losartan lowered blood pressure (53 +/- 10 vs. 90 +/- 3 mm Hg in untreated mice) and reduced the acute albuminuria by 55% (11,145 +/- 864 vs. 24,517 +/- 2448 micrograms albumin/18 hr in untreated mice). However, similar effects were observed using triple therapy. Therefore, the reduction of albuminuria by the combined treatment of enalapril/losartan seems to be the consequence of the reduction in the systemic blood pressure. These findings argue against a specific role for angiotensin II in this model.

CONCLUSIONS

The combined injection of two mAbs directed against different domains of aminopeptidase A induces a massive albuminuria in mice, which is not merely dependent on angiotensin II. We hypothesize that the direct binding of mAbs to at least two pathogenic domains on aminopeptidase A triggers the podocyte to release mediators that are involved in the observed albuminuria.

Immune modulation of biologic systems in renal somatic cells

The various theories discussed here suggest that somatic renal cells are susceptible to biologic modulation by the immune system independent of an inflammatory effect. (1) The mode of repression of type IV collagen synthesis by novel, soluble antigen-binding proteins, the down-regulation of class II MHC expression with interruption of antigen presentation to epithelia after selective gene regulation by antibody, and the diverse interactions of antibody with renal glomerular cells producing functional disturbances in endocytosis and permselectivity; (2) modification of surface-antigen composition; (3) alteration of matrix deposition, remodeling and composition; (4) biophysical perturbation of cytoskeletal and cell membrane components; (5) and lastly, alterations in cell adhesion through cell-surface alterations, all lend testimony to the richness of the signal transduction pathways in somatic cells. Although the preceding examples represent only a small fraction of those which may take place within the glomerular and tubular microenvironments, these paradigms may nevertheless serve as new models upon which one can consider the multitude of potential communications between disparate biologic systems that connect in complex organisms.

Quantification of glomerular epithelial cell adhesion by using anti-DNA antibodies in ELISA

A sensitive and reproducible microassay is described for quantification of adhesion of cells to matrix-coated 96-wells plates under different experimental conditions. For this purpose glomerular visceral epithelial cells (GVEC) were used. Attached GVEC were fixed with methanol and incubated with a monoclonal anti-DNA antibody. Following standard procedures, the amount of bound antibody was quantified by ELISA. A positive linear relationship in the range of 800-5000 cells per well was found between OD values and cell numbers obtained by hand-counting (r = 0.94, p less than 0.001). The assay is 10 to 100 times more sensitive than most other adhesion assays. The applicability of the ELISA assay was demonstrated by manipulation of the temperature during adhesion and by using different concentrations of the matrix-molecules fibronectin, EHS-laminin and collagen type I. The ELISA assay was found to be unaffected by non-specific interaction of anti-DNA antibodies with the matrix molecules used for coating. The assay was neither affected by potential release of DNA from the GVEC under these different experimental conditions. In conclusion, this cell adhesion microassay is simple, reliable, sensitive, and cost-effective, since it requires small amounts of GVEC and reagents.

Antigenic specificities of glomerular-bound autoantibodies in membranous glomerulopathy induced by mercuric chloride

The present study describes the development of membranous glomerulopathy (MGP) with high proteinuria in DZB rats exposed to mercuric chloride (HgCl2). IgG1 and IgG2a antibodies, eluted from glomeruli with subepithelial immune deposits, bind to the interface of the GBM and epithelial cells. High reactivity to GBM was demonstrated by ELISA and Western blotting, which could be absorbed for 30% by laminin or laminin-associated extracellular matrix components. No reactivity was found with type IV collagen, fibronectin, heparan sulfate proteoglycans, or tubular brush border antigens. Absorption to GBM removed the reactivity to renal antigens. Passively transferred eluted antibodies bind in a predominantly linear pattern along the GBM, causing focal ultrastructural transformations of the podocytes. These results suggest that this type of HgCl2-induced MGP, associated with epithelial cell injury and proteinuria, is caused by autoantibodies to basement membrane components which are located at the epithelial cell-basement membrane interface and may be involved in cell-matrix binding.

A nephritogenic rat monoclonal antibody to mouse aminopeptidase A. Induction of massive albuminuria after a single intravenous injection

Antibodies directed against antigens present on renal epithelial cells can cause membranous glomerulonephritis in experimental animals, which closely resembles the human form of this disease. However, most antibodies produced so far fail to cause the persistent and severe proteinuria that is seen in humans. In our search for new antibodies of this kind, we have now produced a monoclonal antibody (mAb) against mouse aminopeptidase A, a hydrolase that is present in the mouse kidney. The mAb (ASD-4) was prepared by fusion of mouse myeloma cells with splenocytes of Lou rats immunized with brush border (BB) membranes from mouse kidneys. ASD-4 is of the IgG1 subclass and reacts with a 140-kD protein as demonstrated by immunoprecipitation on radiolabeled BB membranes. In indirect immunofluorescence and immunoelectronmicroscopy of normal mouse kidneys, ASD-4 was diffusely present on the BB of the S1 and S2 segments of the proximal tubules, and on the cell membranes of the glomerular visceral epithelia. It also bound to cell membranes of nonglomerular endothelia, smooth muscle cells of arteries, and juxtaglomerular cells. After injection of ASD-4 into normal mice, an immediate homogeneous binding to the capillary wall was seen that gradually changed into a fine granular pattern after 1 d. This glomerular binding was followed by binding to the BB and basolateral membranes of the convoluted proximal tubules. Immediately after injection of ASD-4, a dose-dependent albuminuria occurred that lasted for at least 16 d. ASD-4 is thus a new rat mAb against a well-defined renal epithelial antigen that causes not only membranous glomerulonephritis after a single injection in the mouse, but also severe albuminuria.

The contribution of antibody-mediated cytotoxicity and immune-complex formation to tubulointerstitial disease in passive Heymann nephritis

Passive Heymann nephritis (PHN), an experimental model of membranous nephropathy, is produced by Fx1A antiserum, which also reacts with antigens on the brush border (gp 330) and basolateral membrane (gp 90) of proximal tubules. We examined tubulointerstitial disease in PHN, identifying two distinct processes occurring on the luminal and basolateral membranes, respectively. Injected antibody bound diffusely to the tubular brush border from Day 1 to Day 7, followed by sloughing of microvilli and tubular-cell regeneration. Fine granular deposits of Fx1A antibody were present along the basolateral cell membrane by Day 1. These deposits rearranged in situ, enlarged, and became more focally distributed along tubular basement membranes (TBM). Interstitial inflammation, dominated by macrophages (Ia+, ED-1+) in association with a smaller number of T-cytotoxic cells (OX19+, OX8+) began by Day 3, reached peak intensity and persisted throughout the autologous phase (to Day 21). The distribution of focal clusters of interstitial macrophages predominately in association with TBM-immune deposits was demonstrated. Complement depletion prevented proteinuria but TBM deposits developed and the interstitial inflammation was unchanged. All aspects of the tubulointerstitial disease were amplified by a second injection of Fx1A antiserum. In vitro, Fx1A antibody bound to the surface of isolated proximal tubular epithelial cells and redistributed to form clusters of immune aggregates. Anti-Fx1A-induced cytotoxicity of tubular cells was demonstrated by prelabeling cells with 2'-7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. The degree of cytotoxicity was dependent on complement concentration and the duration of incubation at 37 degrees C. PHN induced by Fx1A antiserum causes tubular-cell injury following interactions with brush-border antigens and TBM immune-complex disease associated with interstitial inflammation. These findings may be relevant to the acute and chronic interstitial disease of human membranous nephropathy.

Mesangial cell autoantigens in immunoglobulin A nephropathy and Henoch-Schönlein purpura

The autoantigen(s) that we have previously described in human glomeruli, recognized in IgA nephropathy, has (have) been identified as mesangial cell in origin. Cultured mesangial cells expressed 48- and 55-kD components binding IgG isotype autoantibodies (IgG-MESCA) present in sera of patients with both IgA nephropathy and Henoch-Schönlein purpura (HSP). IgG-MESCA were not detected in sera of normals, or patients with other autoimmune-mediated glomerulonephritides: anti-glomerular basement membrane disease, Wegener's granulomatosis, or in IgM-mesangial proliferative disease. Binding specificity was proven by F(ab')2 studies in enzyme-linked immunosorbent assay (ELISA) and Western blotting, and there was no significant affinity of IgA or IgM immunoglobulins. Fluorescein isothiocyanate-conjugated IgG from ELISA-positive sera localized to the mesangium and peripheral capillary loops of glomeruli, supporting the belief that the antigen is expressed in normal human renal tissue. However, only about one third of mesangial cells in culture showed affinity for IgG from ELISA-positive sera, suggesting variable expression of the antigen(s) in vitro. The only autoantigen(s) present in glomeruli, and extractable from whole normal glomeruli by the techniques employed, localized on the mesangial cell. In both IgA nephropathy and HSP, autoimmunity was intermittently present, with fluctuating levels of IgG-MESCA detectable in sera. There were positive correlations with the degree of glomerular injury assessed by erythrocyturia and proteinuria in IgA nephropathy, but significance was reached with only the degree of hematuria in HSP. These findings suggest a contributing role in the pathogenesis of the mesangial proliferative lesions and demonstrate autoimmunity common to both IgA nephropathy and HSP.

Serum sickness and acute renal failure after streptokinase therapy for myocardial infarction

A patient developed serum sickness and acute renal failure following therapy with streptokinase for myocardial ischaemia. There was a previous history of a cellulitic infection of the leg, and antibodies to streptokinase were measurable in a serum sample taken from the patient before therapy. A cryoglobulin was detected at the time of presentation with serum sickness. This contained polyclonal IgG (with anti-streptokinase activity), streptokinase, and C3. Circulating immune complexes were demonstrated by C1q-binding assay. Deposition of C3 was observed in skin and renal biopsies, and bound to erythrocytes. Renal histology, however, showed acute tubular necrosis, with no vasculitis or inflammatory cell infiltrate. This case provides an unusual example of the characterization of an immune complex comprising a specific antibody and an exogenous antigen, and has clinical implications for the use of streptokinase.

Membranous nephropathy in childhood and its treatment

Membranous nephropathy is predominantly a disease of middle-aged and elderly individuals, and is thus rather an uncommon finding in proteinuric and nephrotic children. In children, it differs in several important respects from the disease as seen in adults: an apparent associated cause is more common, macroscopic haematuria is seen quite frequently, a relapsing course is more often noted, renal venous thrombosis is not found and evolution into renal failure is the exception. Nevertheless, a proportion of children with membranous nephropathy do evolve into renal failure, and their management is discussed with particular reference to recent papers on the treatment of membranous nephropathy in adults. An aggressive search for associated disease is worthwhile in children, and one should wait to see what the evolution or proteinuria and renal function may be. If a progressive course becomes evident, then a trial of treatment with corticosteroids is worthwhile, but if this is ineffective then a more aggressive approach involving the use of alkylating agents may be justified. It remains undetermined what the best regime in children and adolescents may be.

Experimental models of autoimmune diseases

Several experimental models of autoimmune diseases have been studied which often mimic the human situation. Autoreactive T cells that emerge either spontaneously or after immunization have been identified in several situations. The main lesson from these models is that these autoreactive T cells are negatively controlled in the normal situation and that a defect either inherited or acquired in this regulatory circuit is responsible for autoimmunity.