Rat C3 conversion by rat anti-2,4,dinitrophenyl (DNP) hapten IgA immune precipitates

Combined administration of IgA and IgG anti-Thy-1 antibodies enhances renal inflammation in rats

BACKGROUND

IgA nephropathy (IgAN) is the most common type of immunologically mediated glomerulonephritis (GN) and is characterized by deposition in the glomerular mesangium of IgA together with C3, C5b-9, and properdin. In patients, the codeposition of IgA together with IgG and/or IgM can lead to a more progressive course of disease. In Wistar rats, mesangial proliferative GN can be induced by the injection of mouse IgG anti-Thy-1 antibodies (ER4G). In contrast, the administration of mouse IgA anti-Thy-1 antibodies (ER4A) to rats results in isolated hematuria without detectable albuminuria and without detectable complement deposition.

METHODS

To investigate the effect of the combination of IgA and IgG on glomerular injury, Wistar rats were injected with a limiting dose of ER4G in the presence or absence of ER4A in a dose able to induce hematuria.

RESULTS

Although the limiting dose of ER4G or the dose of ER4A used did not induce significant albuminuria, the combination of ER4G and ER4A resulted in a synergistic increase in albuminuria. Microhematuria occurred in rats receiving either ER4A or ER4G alone or in combination. Although both ER4A or a limiting dose of ER4G induced minor increases in extracellular matrix expansion, the combination resulted in a pronounced, additive increased matrix expansion.

CONCLUSION

We conclude that in this model of IgA-mediated glomerulopathy, a selective complement-dependent synergistic renal injury is induced in Wistar rats by glomerular codeposition of mouse anti-Thy-1 monoclonal isotypes.

Induction of microhematuria by an IgA isotype switch variant of a monoclonal anti-Thy-1.1 antibody in the rat

IgA nephropathy (IgAN) is a chronic form of glomerulonephritis (GN) characterized by the deposition in the glomerular mesangium of mainly IgA. An experimental form of mesangial proliferative GN can be induced in rats by either polyclonal or monoclonal antibodies against Thy-1.1, a glycoprotein present on the surface of MC. The IgG-mediated renal inflammation is complement dependent and associated with influx of platelets and monocytes. In the present study we switched an IgG2a anti-Thy-1.1 (ER4G) producing hybridoma to an IgA anti-Thy-1.1 (ER4A) producing clone and analyzed the effects of IgA anti-Thy-1.1 in rats. FPLC analysis by gel filtration revealed that the IgA produced by the hybridoma cells was mainly dimeric and polymeric. Infusion of rats with purified ER4A (1 mg/kg) resulted in the deposition of IgA in a mesangial pattern in the glomeruli, similar to that found with ER4G. While administration of ER4G resulted in proteinuria, no significant urinary protein excretion was found in rats treated with ER4A. However, significant microhematuria was observed in rats receiving either ER4A or ER4G. Furthermore, the administration of ER4A was not accompanied by activation of complement, and no significant influx of monocytes or polymorphonuclear leukocytes was observed in contrast to the rats receiving ER4G. We conclude that microhematuria is selectively induced in Wistar rats by mouse IgA anti-Thy-1.1 without detectable complement-mediated injury to MC. These studies may be of importance in understanding the mechanisms leading to IgAN in patients.

Effect of thermal injury on transfer of IR22 IgA myeloma protein into bile in the rat

We previously observed a 75-90% decrease in concentration of biliary IgA after thermal injury to rat skin. Decrease in biliary IgA might result from an alteration in supply of polymeric IgA delivered to the hepatocyte or from an alteration in hepatocyte transfer of polymeric IgA into bile. In the present study, we examined the transfer of intravenously administered 125I-IgA into bile. Purified IR22 rat IgA myeloma protein consisting of both monomeric and polymeric IgA was labelled with 125I. Sprague-Dawley rats (140-180 g) received a 20-30% body surface area scald-burn or sham treatment. The bile duct was cannulated 18-24 h later and 125I-IgA preparations were injected into the tail vein. Bile was collected under light ether anesthesia for 3 h. In rats injected with 125I-IR22 IgA myeloma protein there were no significant differences in total, TCA-precipitable, or immunoprecipitable radioactivity in bile from burn-injured or sham-treated animals. On Bio-Gel A-1.5 m gel permeation, the radioactivity in bile from sham-treated animals eluted in the region of polymeric IgA as expected; the radioactivity in the bile from burn-injured animals eluted equally in the same regions as polymeric IgA and monomeric IgA. In sham-treated rats injected with isolated polymeric IgA only, bile contained primarily polymeric IgA. In burn-injured rats injected with polymeric IgA only, bile contained a mixture of polymeric IgA and monomeric IgA. These findings suggest that hepatocyte processing of polymeric IgA is altered after thermal injury, resulting in the transformation of some polymeric IgA into its monomeric form.

In vivo activation of complement by IgA in a rat model

In this study we investigated the capacity of rat IgA to activate complement (C) in vivo in a rat model. Rat monomeric (m-), dimeric (d-) and polymeric (p-) IgA MoAbs were injected intravenously and assessed for deposition of C3 and C4 on IgA. By ELISA it was shown that both d- and p-IgA bound C3 whereas no binding of C3 by m-IgA was observed. Polymeric IgA was more efficient in binding of C3 as compared with d-IgA. However, in haemolytic assays no consistent decrease of plasma complement levels was observed except for dimeric IgA which induced a marginal consumption of AP50. When rats were pre-treated with cobra venom factor (CVF) to deplete C3, no C3 deposition was found on m-, d- or p-IgA. Neither m- nor d- or p-IgA was able to bind C4 in vivo. In agreement with the results described above, large sized polymeric IgA was shown to be taken up by Kupffer cells (KC) together with C3. No C3 was detected when rats were depleted of C using CVF. Taken together, the experimental data suggest that d- and p-IgA are able to activate C via the alternative pathway in vivo.

Kupffer cell depletion in vivo results in clearance of large-sized IgA aggregates in rats by liver endothelial cells

We investigated the clearance kinetics and tissue distribution of different sized IgA in normal and macrophage-depleted rats. Rats were injected iv with liposomes containing dichloromethylene diphosphonate (DMDP). DMDP treatment resulted in complete depletion of liver macrophages 24-48 h after administration. Normal and macrophage depleted rats were injected intravenously with monomeric, dimeric, polymeric or aggregated polymeric IgA (AIgA) and assessed for blood clearance and tissue distribution. In normal rats, clearance of IgA was size dependent, i.e. a faster clearance with increasing size. No differences in clearance kinetics were observed of the different sized IgA between normal and DMDP-treated rats. TCA non-precipitable radioactivity, a measure for degradation of IgA, was found in the circulation of normal and DMDP-treated rats after AIgA administration. The liver was the main organ responsible for the clearance of IgA in normal and DMDP-treated rats. Immunofluorescence studies on liver biopsies indicated that AIgA was associated with Kupffer cells in normal rats. Electron microscopical studies revealed that the AIgA was internalized and located in vesicles in Kupffer cells. In DMDP-treated rats the AIgA was associated with endothelial cells and electron microscopy studies showed that this AIgA was taken up by endothelial cells. These data show that rat liver endothelial cells are able to bind, internalize and degrade AIgA in situations where Kupffer cells are absent, and that these cells may play an important role in the handling of AIgA and IgA-immune complexes.

Complement enhances the clearance of large-sized soluble IgA aggregates in rats

In the present study the involvement of the complement system (C) in the clearance of soluble IgA aggregates in the rat was studied. Monoclonal monomeric IgA (mIgA) antibody (which does not activate C) or aggregated polymeric IgA (aIgA; which activates C) were administered intravenously to phosphate-buffered saline-treated and complement-depleted [Cobra venom factor (CVF)-treated] rats and assessed for clearance from the circulation. In control rats, mIgA was cleared in a biphasic fashion with a first half-life (T1/2) of 29.5 +/- 14.2 min and a second T1/2 of 230 +/- 176 min. No differences were observed in clearance of mIgA in CVF-treated rats as compared to PBS-treated rats. In PBS-treated rats, aIgA with a size between 20 S and 150 S disappeared very rapidly from the circulation with a first T1/2 of 1.1 +/- 0.4 min and a second T1/2 of 23.2 +/- 11.3 min. In CVF-treated rats the clearance of aIgA was significantly delayed as compared to that in control rats, namely with a first T1/2 of 7.3 +/- 2.6 min and a second T1/2 of 64.2 +/- 19.4 min. Immunohistochemical studies of the liver (which is the main site of clearance of aIgA) revealed that Kupffer cells (KC) are mainly responsible for the uptake of aIgA. Furthermore, in PBS-treated rats aIgA deposition was accompanied by C3 deposition in the KC. In CVF-treated rats, the percentage of KC containing aIgA was significantly lower during the first 16 min after aIgA administration as compared to PBS treated rats. In addition no detectable C3 was found in KC of CVF-treated rats. These results indicate that KC play an important role in the clearance of large molecular weight IgA in rats and that C facilitates the clearance of these complexes from the circulation.

Rat polymeric IgA binds C1q, but does not activate C1

Immune complexes, prepared with monoclonal rat IgA antibodies directed against DNP, activate the alternative pathway of the complement system in rat serum. In this study, the interaction of these monoclonal IgA antibodies with the classical pathway of complement was investigated. Monoclonal polymeric IgA (p-IgA) was shown to inhibit the IgG2b-mediated classical pathway-dependent lysis of TNP-coated sheep red blood cells. In addition, the binding of C3 to solid phase IgG2b immune complexes was inhibited by p-IgA. Monoclonal monomeric IgA (m-IgA) was much less efficient in this respect. To further analyse the effect of p-IgA on the activation of the classical pathway by IgG2b immune complexes, the interaction of p-IgA with C1 was studied. It was found that p-IgA antibodies bind C1q. No species-specificity was observed, since both rat and human C1q were bound. Whereas binding of C1q in C1 to IgG2b resulted in activation of C1, binding to p-IgA did not. The binding of C1q to both p-IgA and IgG2b could be inhibited by monoclonal antibodies directed against the globular heads of C1q, but not by monoclonal antibodies directed against the collagen tail. The formation of insoluble p-IgA immune complexes was inhibited in the presence of rat serum or C1. These studies indicate that C1q binds to p-IgA by its globular heads, and thereby may modulate classical pathway-mediated reactions such as the inhibition of immune precipitate formation.

The involvement of Kupffer cells in the clearance of high molecular weight rat IgA aggregates in rats

In the present study the clearance kinetics and tissue distribution of aggregated 125I-labelled monoclonal rat IgA [( 125I] AIgA) of different sizes were studied in rats. Soluble [125I]AIgA disappeared from the circulation in a biphasic manner with an initial rapid distribution half-life (T1) and a second slower half-life (T2). T2 was directly related to the size of the aggregates. High molecular weight [125I]AIgA, containing 10-12 IgA molecules per aggregate [( IgA]10-12), was cleared much faster than low molecular weight aggregates. The main site of clearance was the liver. The larger the size of the AIgA, the more degradation products were found in the circulation. After injection of [IgA]10-12, non-parenchymal cells (NPC) contained three times more radioactivity than parenchymal cells (PC) (NPC:PC ratio 3.06 +/- 0.96). Ratios of 0.82 +/- 0.03 and 0.62 +/- 0.12 were observed when [IgA]5-6 and [IgA]2 were injected respectively, suggesting a greater role for Kupffer cells in the clearance of large-sized IgA aggregates. Kupffer cells were shown to be the main cells for localization of large-sized AIgA established by immunohistochemical staining on liver cryostat sections.

Anti-inflammatory activity of human IgA antibodies and their Fab alpha fragments: inhibition of IgG-mediated complement activation

The interaction of human IgA antibodies with the classical pathway of complement activation was investigated in a homologous human system, by means of two IgA1 and three IgG1 myeloma proteins having antibody activity against a defined antigen, staphylococcal alpha-toxin. In a solid-phase antigen-dependent C3b-binding ELISA system, the monoclonal IgG antibodies were previously shown to activate the classical complement pathway synergistically, resembling polyclonal IgG antibodies, whereas IgA antibodies were unable to activate complement by either pathway. In the present study, IgA antibodies were found to inhibit significantly the activation of complement initiated by antigen-bound polyclonal or mixed monoclonal IgG antibodies, in relation to the amount of IgA antibodies applied and bound to antigen. IgA1 myeloma proteins devoid of antigen-binding activity were without effect. Inhibition was independent of the ability of the IgA antibodies to compete against the IgG antibodies in binding to antigen, and was demonstrable with physiological concentrations of antibodies. Similar results were obtained with polyclonal serum IgA having antigen-binding activity. However, the binding of C1q to antigen-complexed IgG was inhibited only by a monoclonal IgA antibody that could compete against one of the three monoclonal IgG antibodies that bound C1q synergistically. This observation implied that at least two mechanisms were involved in the inhibition of C3b fixation. Fab alpha fragments of monoclonal IgA antibodies, obtained by cleavage with IgA1 protease from Haemophilus influenzae type b, were found to have a similar inhibitory effect on C3b fixation to the intact IgA1 antibodies. This observation supports the hypothesis that IgA1 proteases contribute to the invasive pathogenicity of certain mucosal bacteria, by cleaving secretory IgA1 antibodies to antigen-binding Fab alpha fragments, which are not only defective in mucosal defense properties, but which also protect the organisms from other immune effector systems, such as complement activation.

Complement-fixing properties of human IgA antibodies. Alternative pathway complement activation by plastic-bound, but not specific antigen-bound, IgA

The complement-fixing properties of human IgA antibodies bound to specific antigen, or coated directly on plastic surfaces, were examined in comparison with those of IgG antibodies. Use was made of antigen-binding (anti-staphylococcal alpha-toxin) IgA and IgG monoclonal antibodies and normal polyclonal IgA and IgG, purified greater than 99.9% by avoidance of denaturing processes. Complement-fixation ELISA was used, with a high density of biotin-conjugated staphylococcal alpha-toxin bound to avidin-coated plates for the efficient capture of antibodies, and conditions were adjusted for the assessment of classical and alternative pathways of complement activation. Although IgA coated directly on plastic surfaces activated the alternative complement pathway in a dose-dependent manner, IgA antibodies bound to antigen failed to fix complement by either classical or alternative pathways. In contrast, IgG antibodies, either bound to antigen or coated directly on plastic, activated complement mainly by the classical pathway. It was concluded that the complexation of IgA antibodies with antigen is insufficient to elicit complement activation: rather a degree of denaturation seems to play a part in the expression of alternative complement pathway-activating properties by IgA.

Chemotactic properties of rat immunoglobulins and immune complexes

The effect of rat immunoglobulins and immune complexes on the locomotor function of rat polymorphonuclear leukocytes (PMN) was investigated in vitro. Rat immunoglobulin G1 (IgG1), IgG2a, IgG2b, and IgA monoclonal antibodies specific for the dinitrophenyl hapten were used. Both monomeric and polymeric IgA showed chemotactic activity in a dose-dependent manner. IgG1 and IgG2b also induced a dose-dependent locomotor response of PMN, but the nature of the induced migration was chemokinetic (enhancing random migration). IgG2a was chemotactic and induced maximal migration at a relatively low concentration. IgG1- and IgG2b-immune complexes induced stronger migration than antibody alone; however, IgA- and IgG2a-immune complexes did not. IgA was shown to modify the chemotactic movement of PMN induced by N-formylmethionyl-leucyl-phenylalanine (FMLP). In the presence of both IgA and FMLP in the lower chamber, the migration towards suboptimal concentrations of FMLP was enhanced. By contrast, IgA in the upper chamber decreased migration towards the optimal or higher concentrations of FMLP. These findings suggest that IgA may work synergistically with luminal chemoattractants to mobilize PMN to the locus of infection on the mucosal surface. In addition, the intense activity of IgG2a alone and IgG1- or IgG2b-immune complexes in inducing PMN migration may play an important role in inflammatory processes. The data indicate that immunoglobulins have a direct effect on PMN mobility.

Acute experimental glomerulonephritis induced by the glomerular deposition of circulating polymeric IgA-concanavalin A complexes

The perfusion of polymeric or secretory IgA-Concanavalin A complexes into the aorta of rats led to a mannose-dependent binding of both IgA and lectin to the glomerular capillary wall, as shown by double immunolocalization experiments, by quantitative analysis of the amount of radiolabelled complexes bound per g of kidney, and by blocking experiments with the corresponding carbohydrate. Rats injected with amounts of those complexes as low as 500 micrograms developed, one hour later, a focal and segmental proliferative glomerulonephritis characterized by the deposition of injected complexes and of rat C3 and rat fibrin/fibrinogen in most glomeruli; focal thrombosis and small areas of necrosis in 10 to 15% of glomeruli, confined to the periphery of a single lobule of the tuft and segmental infiltration of these glomeruli by polymorphonuclear leucocytes and platelets. At the same time, many mesangial cells exhibited a hyperactive appearance, and red blood cells were noted in tubular lumens. In contrast, rats similarly injected with either monomeric IgA-ConA complexes, multimeric or secretory IgA-peanut agglutinin complexes or polymeric or monomeric IgA aggregates of comparable apparent molecular weight did not develop obvious glomerular lesions within one hour. The data indicate that performed polymeric IgA-ConA complexes can specifically bind to glomerular structures in vivo and trigger acute glomerular lesions locally, analogous to those observed in some glomerular diseases associated with a cryoglobulinaemia.

Activation of rat complement by soluble and insoluble rat IgA immune complexes

The ability of rat monoclonal IgA, specific for 2,4-dinitrophenyl (DNA), to activate the complement (C) system of the rat was investigated using aggregated IgA or IgA immune complexes (IC). IgA was coated onto a solid phase, and tested for its capacity to bind C3 upon incubation at 37 degrees C in normal rat serum (NRS) in the presence of Mg-EGTA. Binding of C3 was observed dependent on the dose of dimeric (d-), polymeric (p-) and secretory IgA tested. In contrast, little C3 fixation was observed in this system with monomeric (m-) rat IgA or with mouse m- and d-IgA (MOPC315). Soluble and insoluble rat IgA IC were prepared using dinitrophenylated rat serum albumin (DNP8RSA) as antigen (Ag), and assessed for C activation. It was shown that insoluble IC (immune precipitates; IP) containing m-, d- or pIgA of rat origin activate the alternative pathway of rat C, as demonstrated by their capacity to induce C consumption in NRS in the presence of Mg-EGTA. When p- and m-IgA IP were compared for their capacity to activate C, it was found that p-IgA activated C four times as efficiently as m-IgA IP (at 2 mg/ml). Soluble rat IgA IC were prepared in an excess of DNP8RSA, fractionated by gel filtration on Sepharose 6B, and analyzed for C activation and antibody (Ab)/Ag ratio. In contrast to m-IgA IP, soluble m-IgA did not activate C. On the other hand soluble d-IgA IC activated C dependent on their concentration and size: at a concentration of 0.1 mg/ml high-molecular weight d-IgA IC with a high Ab/Ag ratio were four times as efficient as low-molecular weight IC with a low Ab/Ag ratio, and twice as efficient as IP prepared at equivalence. To demonstrate the induction by IgA of the assembly of the terminal membrane attack complex, trinitrophenyl (TNP)-conjugated rat red blood cells (TNP-RRBC) coated with d- or p-IgA were shown to be lysed in NRS in the presence of Mg-EGTA. No lysis of m-IgA-coated TNP-RRBC was observed. The results in this study demonstrate that both soluble and insoluble rat IgA IC activate the alternative pathway of homologous rat C. Alternative pathway activation by soluble rate IgA IC is dependent on the size of the IC. The degree of polymerization of the IgA Ab itself also influences C activation.

Activation of complement by human serum IgA, secretory IgA and IgA1 fragments

Activation of the complement (C) system by human IgA was studied. Both subclasses of IgA, IgA1 and IgA2, and secretory IgA were shown to activate C, as determined by deposition of C3 on glutaraldehyde-activated microwells coated with IgA. The activation of the C system occurred in the presence of MgEGTA and not in D-deficient serum. In addition to C3, deposition of properdin (P) but not of C4 was detected. These results indicate that C activation, as determined by measuring deposition of C3 and P, occurred by the alternative pathway (AP). The data further show that the major part of the hinge region, which is deleted in IgA2 as compared with IgA1 and which forms the major structural difference between the two subclasses, is not involved in C activation. Reduction and alkylation destroyed the ability of IgA to activate C, as has also been demonstrated for IgG. In order to define the C activating region of the IgA molecule, several fragments of IgA1 were tested. The four-chain molecules F(ab')2 and F(abc)2 were shown to activate the AP. No activation was observed with the two-chain fragments Fab and Fc. The Fc fragment of IgA also did not activate the CP, as does the Fc fragment of IgG. This indicates that activation of the AP of C by IgA is dependent on the presence of the F(ab')2 fragment.

IN CONCLUSION

human IgA does activate C by the AP. This activation requires an intact F(ab')2 fragment.