The interactions of human complement with interfacially aggregated preparations of human secretory IgA

Is complement the main accomplice in IgA nephropathy? From initial observations to potential complement-targeted therapies

IgA Nephropathy (IgAN) is the main cause of primary glomerulonephritis, globally. This disease is associated with a wide range of clinical presentations, variable prognosis and a spectrum of histological findings. More than fifty years after its first description, this heterogeneity continues to complicate efforts to understand the pathogenesis. Nevertheless, involvement of the complement system in IgAN was identified early on. Dysfunction of the immunoglobulin A (IgA) system, the principal offender in this disease, including modification of isoforms and glycoforms of IgA1, the nature of immune complexes and autoantibodies to galactose deficient IgA1 might all be responsible for complement activation in IgAN. However, the specific mechanisms engaging complement are still under examination. Research in this domain should allow for identification of patients that may benefit from complement-targeted therapy, in the foreseeable future.

Biological Functions of IgA

Immunoglobulin A (IgA) is the most enigmatic of immunoglobulins. It is by far the most abundant of human Igs, being present in the blood plasma at concentrations approximating 2–3mg/mL, as well as the dominant isotype in most secretions where its output amounts to some 5–8g/day in adults. Furthermore, its evolutionary origins appear to precede the synapsid– diapsid divergence in tetrapod phylogeny (>300 million years ago) because it is present in both mammals and birds and therefore possibly also in reptiles (reviewed in Peppard et al., 2005); an IgA-like molecule has now been identified in a lizard (Deza et al., 2007).

Complement-coated antibody-transfer (CCAT); serum IgA1 antibodies intercept and transport C4 and C3 fragments and preserve IgG1 deployment (PGD)

In periodontal disease, IgG1 and IgA1 antibodies produced in situ deposit on antigens in the affected tissues. Thus, there is an interest in the effect of co-deposited IgA1 antibodies on complement activation by IgG1-immune complexes. In the present study, we first analyzed the effect of IgA1-immune complexes on complement using human IgA1 antibodies to dansyl (with dansylated human serum albumin serving as the immobilized antigen). It was observed that these IgA1-immune complexes when incubated for prolonged times with 33% human serum as a source of complement received C4b and C3b deposition. As C4b and C3b deposited on the IgA1 antibodies and on the antigenic surface, the complement-coated IgA1 antibodies departed. These fluid-phase complement-coated IgA1 antibodies were transferred to antigen-coated microtiter-ELISA plates, where they became bound to the antigens. Thus, the complement-coated IgA1 antibodies retained their antigen-binding function, especially as a proportion of their covalently bound C3b progressively degraded to iC3b and C3d. Genetically engineered carbohydrate-deficient mutant human IgA1 antibodies were used to assess the role of carbohydrate in accepting the C4b and C3b depositions, and these studies indicated that the carbohydrate on the Fc-region of IgA1 played a positive role. Another interesting finding generated by this study was that when IgA1 was co-deposited with IgG1 antibodies, and serum complement was added, the IgG1 antibodies tended to remain on the antigenic surface. The co-deposited IgA1 antibodies not only controlled (reduced) the rate of the consumption of the first component of complement (C1) and of classical complement pathway activation by IgG1-immune complexes (and therein reduced the rate of complement-mediated dissolution of the IgG1-immune complexes), but also the co-deposited IgA1 antibodies simultaneously intercepted/accepted C4b and C3b, then departed, as complement began to cover the antigenic surfaces. The process in which complement-coated IgA1 antibodies transferred to non-complement-coated antigens is termed complement-coated antibody-transfer/transport (CCAT). In this way, IgA1 antibodies extended the efficiency of the complement system by insuring the specific IgA1 antibody-mediated transport of the captured biologically active complement fragments to those antigens stimulating the IgA1 antibody response but not yet neutralized (completely coated) with complement. Simultaneously by impeding the rate of C1 consumption and by intercepting C4b and C3b, IgA1 antibodies slowed C4b and C3b deposition on the antigenic surface and on the co-deposited IgG1 antibodies. Thus, in the presence of ongoing complement activation, the deposition of serum IgA1 antibodies enabled the co-deposited IgG1 antibodies to better maintain their ability to interact with antigens. We termed this latter phenomenon, preservation of IgG antibody deployment (PGD). In summary, co-deposited IgA1 antibodies maximized the efficiency of the complement system, transported their covalently bound complement fragments to specific antigens and sustained the effective deployment of IgG1 antibodies directed to those same antigens.

Acylation of the lipid A region of a Klebsiella pneumoniae LPS controls the alternative pathway activation of human complement

Two mechanisms of direct activation of the complement system by LPS have been extensively documented: (i) activation of the alternative pathway (AP) by the polysaccharide region, and (ii) activation of the classical pathway (CP) by the lipid A region. Here we demonstrate that LPS from the Klebsiella pneumoniae I-145 strain activates the AP by a mechanism dependent on the acylation of the lipid A region. Cleavage of C3 by K. pneumoniae LPS in EGTA was blocked by polymyxin B. Two 34 kDa derivatives were prepared from a membrane extract of this K. pneumoniae strain: (i) an acyl-poly (1,3) galactoside containing two galactosamine-bound ester-linked and two amide-linked fatty acids (EFA-APG), and (ii) an acyl-poly (1,3) galactoside devoid of ester-linked fatty acids (APG). APG and EFA-APG share the structure of LPS molecules, with a long polysaccharidic chain, a core, and a lipid A region. The AP was activated by EFA-APG but not by APG nor by the isolated polygalactose chain GC-APG, indicating a critical role for ester-linked fatty acids in AP activation. Polymyxin B which binds to the lipid A region of LPS completely inhibited AP activation by EFA-APG. A small part of EFA-APG was shown to form aggregates in saline, but aggregation was not decreased by polymyxin B. Furthermore, APG formed aggregates of similar size although it was not able to activate AP. Therefore the role of lipid A acylation in triggering AP activation is not exclusively mediated by aggregation of the molecule. LPS from the rough strain of Salmonella minnesota (Sm Re LPS) directly activates the CP but not the AP. However, when mixed with the polygalactose chain GC-APG, Sm Re LPS activated the AP. The data demonstrate a cooperation between the lipid A region and the polysaccharidic chain in activation of the AP. Similar cooperation may occur with other LPS molecules.

Non-coordinated biosynthesis of early complement components in a deficiency of complement proteins C1r and C1s

We report on a 60-year-old woman with systemic lupus erythematosus and a total (95%) C1r and a partial (36%) C1s deficiency. The patient complained about cutaneous lesions on forearms and legs without other systemic involvement. Elevated anti-nuclear, anti-native DNA and anti-SSA antibodies were present. The finding of persistently depressed levels of haemolytic complement activity (CH50) on both serum and plasma, associated with normal levels of C3, C4 and C2 components, and normal alternative pathway haemolytic activity showed a deficiency of an early component of the classical pathway. Indeed C1r component was below the limits of detection whereas C1s component was lowered (36%). The depressed CH50 was only corrected by purified C1r. Biosynthesis of C1r and C1s by patient's monocytes was spontaneously normal but not up-regulated by interferon-gamma for C1r alone, whereas the biosynthesis of C1s, but also of interleukin-6, was increased, indicating a specific disregulation of C1r. The deficiency was associated with a lupus syndrome and a fatal assumed septic shock. This is in agreement with other reported cases.

All forms of human IgA antibodies bound to antigen interfere with complement (C3) fixation induced by IgG or by antigen alone

Polyclonal human secretory IgA1 and IgA2 antibodies to a bacterial protein antigen Streptococcus mutans AgI/II, and polyclonal human serum IgA1 and IgA2 antibodies to staphylococcal alpha-toxin, were found to interfere with antigen-mediated C3b fixation. In fluid phase, immune complexes of antigen and IgA failed to fix C3b, whereas antigen-IgG complexes did fix C3b. Partial removal of glycan chains with Streptococcus mitis SK96 glycosidases diminished the capacity of IgA antibodies to interfere with antigen-mediated C3b fixation by the alternative complement pathway. The authors conclude that native serum or secretory IgA antibodies suppress C3b fixation, and that the glycan chains play a significant role in maintaining this property.

High molecular weight non-immunoglobulin salivary agglutinins (NIA) bind C1Q globular heads and have the potential to activate the first complement component

Non-Immunoglobulin Salivary Agglutinins (NIA) which directly bind to microbes [including HIV] were studied for their potential to activate the first complement component (C1). It was determined that NIA had the same specific activity as heat aggregated IgG in binding to C1q and in activating C1. In order to determine the region of C1q which bound to NIA, C1q globular heads and C1q stems (collagen-like regions) were prepared and separated via a Western blot procedure. NIA bound principally to the globular heads of C1q and weakly to the collagen-like stem region. NIA were also studied for their potential to activate native C1 in normal human serum. Heat-aggregated IgG and cardiolipin served as positive controls. It was observed that incubation of isolated NIA with fresh normal human serum resulted in the formation of sodium dodecyl sulfate (SDS)-irreversible complexes of activated C1r-C1 inhibitor and activated C1s-C1 inhibitor and in activated C1s mediated C4 conversion. This indicated that isolated NIA had the potential to directly and effectively mediate classical complement pathway activation. Preincubation of NIA with C1q, blocked NIA mediated C1r and C1s activation and C4 conversion. The concn of NIA required to activate C1r and C1s was similar to that of heat-aggregated human IgG. In kinetic ELISA, NIA or aggregated IgG (positive controls) were first immobilized on microtiter plates, blocked with gelatin then incubated with fresh human serum as a source of complement. Depositions of C4b, C3b and iC3b substantiated that the complement system was effectively activated by immobilized NIA. The optimal relative NaCl concn for C4b deposition was 0.11 M. While pre-incubation of NIA with C1q blocked the subsequent C1 fixing potential of NIA, pre-incubation of NIA with rgp160 [HIV-1] or fibronectin did not interfere with the potential of NIA to fix C1.

Interaction of the envelope glycoprotein of human immunodeficiency virus with C1q and fibronectin under conditions present in human saliva

Human saliva has been shown to reduce the infectivity of human immunodeficiency virus (HIV) particles in vitro. The factors in human saliva involved in this inhibition of HIV infectivity are unknown, although the salivary sediment of normal individuals has the major HIV neutralizing activity. Interestingly, the first complement component (C1) has been detected on the surface of the salivary sediment in the whole saliva of normal individuals. At the relatively low ionic strength of saliva, we determined that purified human C1q bound with high affinity to the envelope glycoprotein of HIV. Normally, the interaction of the C1q globular heads with immune complexes causes C1 activation. However, direct interactions between C1 and rgp120 (or rgp160) did not lead to C1 fixation, as determined by hemolytic studies with rate-limiting levels of C1, nor did rgp120 cause C1 activation as determined by activated C1s-mediated C4 conversion in normal human serum. Using ELISA, it was observed that intact C1, with the C1r2C1s2 tetramer associated with the collagen-like stem of C1q, did not bind to immobilized rgp120, whereas free C1q did bind. In addition, digestion of the C1q stem portion with collagenase completely eliminated its binding to rgp120. These findings suggest that the collagen-like stem region of C1q, rather than the globular heads, may participate in the binding to the envelope glycoprotein of HIV. Fibronectin, which is present in submandibular saliva, appeared to bind to rgp120 and to enhance the interaction of C1q with rgp120. It is conceivable that C1q and fibronectin, in binding and sequestering HIV particles (i.e. to the salivary sediment), may play an important role in the reduction of HIV transmission via saliva. Further studies will be needed to test the latter speculation.

The interaction of salivary secretions with the human complement system--a model for the study of host defense systems on inflamed mucosal surfaces

When complement first contacts salivary secretions, as when gingival crevicular fluid first meets saliva at the gingival margin, complement function is enhanced. The immediate potentiation of the complement system at equal volume ratios of serum to saliva is due to several factors, including the lower ionic strength of saliva when compared with serum and the presence of certain salivary glyproteins such as the nonimmunoglobulin agglutinins that appear to simultaneously activate C1 and affect (sequester) certain complement control proteins, such as Factor H. This initial potentiation of the complement cascade by saliva may aid in defending the area immediately above the gingival crevice from oral microbiota that are being coated with a combination of serous exudate components and salivary components. As serum becomes much more diluted with saliva (i.e., crevicular fluid moves away from the supragingival area), the acidic proline-rich salivary proteins (APRP) begin to disrupt the unbound C1q-C1r2-C1s2 macromolecular complexes. Thus, the APRP along with other C1 fixing substances in saliva appear to restrict complement function, but only when the ratios of saliva to serum exceed 250:1. Since certain salivary glycoproteins bind to viruses, the potentiation of the complement system by saliva may also play a role in neutralizing certain viral infections on mucosal surfaces where tissue transudates containing complement begin to contact mucosal secretions such as saliva. Again, the ratio of serous fluid to mucosal secretion appears to be an important factor. This article also discusses some of our preliminary data and speculations concerning the binding of the self-associating high-molecular-weight nonimmunoglobulin salivary agglutinins (NIA) with the envelope of the human immunodeficiency virus (HIV) and the possible cooperative role of C1q and fibronectin in aiding neutralization of HIV infectivity.

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.

Lymphocyte infiltration and the synthesis of IgM and IgA rheumatoid factors by rheumatoid synovial membrane

IgM and IgA rheumatoid factor (RF) synthesis by synovial membrane mononuclear cells was measured in 14 patients with rheumatoid arthritis (RA). The results were compared with blood mononuclear cell cultures and correlated with the intensity of lymphocyte infiltration of the synovium. IgM RF was produced by all synovial cultures compared with 56% of blood cultures; IgA RF was produced by 86% of synovial cultures and by 21% of blood cultures. A correlation was observed between synovial IgM RF synthesis, but not IgA RF synthesis, and the intensity of T cell and B cell infiltration of the synovial membrane.

Cutaneous IgA deposits in bullous diseases function as ligands to mediate adherence of activated neutrophils

Linear IgA bullous dermatosis and dermatitis herpetiformis are inflammatory subepidermal blistering diseases characterized by IgA deposits at the cutaneous epithelial basement membrane and in dermal papillae, respectively. Inflammation in both disorders localizes to sites of IgA deposition and is characterized by a predominance of neutrophils. From these observations we postulate that IgA deposits in both diseases may contribute to the recruitment and/or localization of neutrophils. In this study we examined the ability of in vitro and in vivo bound IgA anti-basement membrane autoantibodies from patients with linear IgA bullous dermatosis and in vivo bound IgA deposits in dermal papillae from patients with dermatitis herpetiformis to mediate adherence of neutrophils stimulated by granulocyte macrophage colony-stimulating factor. The study showed that stimulated neutrophils adhered to basement membranes and dermal papillae containing IgA deposits. Adherence was IgA anti-basement membrane antibody concentration dependent and correlated with the immunofluorescence staining intensity of IgA deposits in dermal papillae. Adherence to IgA deposits but not IgG deposits could be inhibited by purified exogenous secretory IgA but not IgG and adherence to IgG deposits could be inhibited by purified exogenous IgG but not secretory IgA. These results provide direct experimental evidence that cutaneous IgA deposits in linear IgA bullous dermatosis and dermatitis herpetiformis can function as ligands for neutrophil adherence and have a role in the localization of inflammation in these disorders.

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 activation within the coeliac small intestine is localised to Brunner's glands

Complement activation may play an important role in the pathogenesis of coeliac disease. In the present study immunohistochemical localisation of C3 and of a neoantigen exposed only on the terminal C5b-9 complement complex has been performed on small intestinal biopsy sections from newly diagnosed untreated coeliac patients, from coeliac patients on long-term gluten-free diet and from disease controls. Levels of C3 were markedly increased in treated coeliac patients compared with controls. Staining of C3 was concentrated subepithelially and within the centre of the lamina propria. No staining was detected at these sites using antibody to the neoantigen, however, strongly suggesting that the increased levels of C3 seen in the coeliac patients was the result of increased extravasation of serum proteins rather than complement activation. Surprisingly, complement activation was detected within the glands of Brunner. Positive staining using anti-C5b-9 neoantigen was found in all coeliac patients, both treated and untreated. Three of the 13 disease controls also showed reactivity with this antibody. This novel finding suggests that Brunner's glands, hitherto largely neglected structures, may play an important role in the development of coeliac disease.

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.

Aggregated human colostral sIgA stimulates delayed, non-complement-dependent, NBT reduction by human neutrophils

Antibodies often alert polymorphonuclear neutrophils (PMNs) to the presence of pathogens. In a study to learn if secretory immunoglobulins can carry out this function, we observed that as little as 4 micrograms/ml of secreted human immunoglobulin A from colostrum (sIgA), in the absence of antigen, stimulated human PMNs to reduce nitroblue tetrazolium (NBT). NBT reduction was inhibited 71% by superoxide dismutase. Active complement pathways were not required since comparable activity was obtained in the presence of heat-inactivated serum. Aggregated forms of sIgA were much more stimulatory than nonaggregated dimeric sIgA. Such interaction between PMNs and sIgA could act in situ to enhance protection against infections of exposed body sites or could initiate inflammatory tissue damage.

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.

IgA1 is the major immunoglobulin component of immune complexes in the acquired immune deficiency syndrome

Compared to a panel of healthy controls, sera from 13 of 23 (57%) patients with the acquired immune deficiency syndrome (AIDS) were shown to have elevated levels of circulating immune complexes (CIC) containing IgA. Levels of IgG-containing CIC were increased in seven patients (30%); no patients had elevated levels of IgM-containing CIC. Additional experiments showed that in all instances in which IgG CIC were demonstrable, IgA was also present; however, IgA CIC could be found that did not contain IgG. The IgA in the CIC was restricted to the IgA1 subclass. These data suggest selective abnormalities of IgA regulation in AIDS and raise questions as to the role in this disease of the immunoglobulin isotype usually thought to possess different protective mechanisms from those attributed to other isotypes.

Complement activation in experimental IgA nephropathy: an antigen-mediated process

Complement activation associated with immune complex glomerular deposition plays an important role in renal injury. In the present studies we performed three series of experiments to identify how IgA immune complexes activate complement. The first series of experiments was designed to determine whether the presence of an antigen within a glomerular IgA immune deposit is required for complement activation. In these experiments, large-sized covalently cross-linked IgA oligomers (X-IgA) were prepared with purified IgA anti-dinitrophenyl (DNP) and a bivalent affinity-labeling antigen, bis-2,4-DNP-pimelic acid ester. These X-IgA oligomers have free antigen-binding sites that will bind DNP-conjugated antigens. Two groups of mice were treated with either X-IgA or X-IgA followed, after two hours, by an antigen DNP-Ficoll. Immunofluorescent examination of renal tissues, obtained six hours after the initial injection, revealed an equal intensity of IgA glomerular deposits in both groups of mice. Glomerular C3 deposits were only detectable in the renal tissues of mice that had DNP-Ficoll bound to X-IgA. In the second series of experiments, a pair of preformed IgA immune complexes, differing only in one antigenic structural feature (DNP), were used to examine the role of the antigen in inducing glomerular C3 deposits in two groups of mice. These pre-formed immune complexes were prepared with IgA anti-phosphorylcholine (PC) and either PC-conjugated to bovine serum albumin (PC-BSA) or PC-BSA which was further modified with DNP (PC/DNP-BSA). Although the IgA immunofluorescent intensity and pattern in the glomerular deposits were equivalent for both groups, intense C3 deposits were exclusively associated with the PC/DNP-BSA-containing immune complexes. Analysis of the relative conversion of normal human serum C3 to inactive C3b (iC3b) by X-IgA, various antigens and their respective IgA immune complexes was highly dependent on the nature of the antigen.(ABSTRACT TRUNCATED AT 250 WORDS)

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

Monomeric (m-) and polymeric (p-) anti-DNP monoclonal (MC) rat IgA antibodies (Ab) were tested for precipitation with DNP-bovine serum albumin (DNP-BSA) and C3 conversion in rat serum, with rat MC anti-DNP IgG2b used as reference. At equivalence, p-IgA rapidly precipitated DNP-BSA, with little antigen (Ag) left in the supernatant. In contrast, m-IgA at five-fold higher concentration precipitated Ag very slowly, with less than 50% of Ag precipitated at equivalence. The Ag/Ab weight ratio at equivalence was 0.13 for both m- and p-IgA, but the molar ratio was 0.3 for m-IgA and close to 1.0 for p-IgA, suggesting a higher avidity of p-IgA. Rat C3 conversion by rat IgA immune precipitates (IP) was about 20% with m-IgA and 40% with p-IgA. EGTA did not significantly affect these figures. Therefore, rat MC IgA IP activated the rat alternative C pathway. Neither rat nor mouse IgA anti-DNP IP activated C3 in normal human serum.

Complement activation in IgA nephropathy

Activation of alternative complement pathway is presumed to be important pathogenically in IgA nephropathy since renal biopsies usually exhibit glomerular deposition of C3 and P (properdin). Surprisingly, little is known about plasma complement activation in this disease, and the plasma C3 and C4 concentrations are usually normal or increased. We quantitated C3 activation in 202 plasmas from 81 patients with IgA nephropathy using a sensitive new assay that detects a neoantigen [iC3b-C3d neoantigen) which appears when C3b is inactivated to iC3b, C3dg, or C3d. This assay accurately quantitates small amounts of in vivo C3 activation. The concentration of iC3b-C3d neoantigen in plasma was significantly increased, indicating C3 activation in 37% of the pediatric and 57% of the adult plasmas assayed. When data from serial determinations in the patients were analyzed, 75% of the adult and 57% of the pediatric patients had C3 activation on at least one occasion. Classical pathway activation, quantitated by C4 activation was found in 20% of the adult and 5% of the pediatric plasmas. No association was found between elevated iC3b-C3d neoantigen concentration and history of macroscopic hematuria, chronic renal insufficiency or degree of proteinuria. These studies show that complement activation can frequently be detected in the plasma of IgA nephropathy patients. However, the pathophysiologic significance of this complement activation remains to be determined.

Activation of the alternative pathway of complement by human serum IgA

In order to study the activation of complement by soluble aggregates of human polyclonal serum IgA, lysis of sheep erythrocytes (E) coated with several IgA preparations was used as a model. A complement nonactivating monoclonal mouse IgG1 against IgA was used to coat the cells. IgA, isolated from normal human serum, was aggregated by either N-succinimidyl 3-(2-pyridyldithio)propionate (SPDP), glutaraldehyde, carbodiimide or heating. Depending on the size of the aggregates, and on the method of aggregation, E coated with aggregated IgA (E gamma 1.AIgA) could be lysed. The alternative pathway of complement appeared to mediate the lysis because the latter was observed in the presence of EGTA containing 5 mM Mg2+ (MgEGTA) and properdin (P) was deposited on the cells. Furthermore, no lysis was observed in C3-deficient serum. In the absence of AIgA the cells were not lysed, and no P deposition was observed. In another set of experiments E gamma 1.AIgA were first reacted with purified C3, B, D and P for 30 min at 30 degrees C, and subsequently in rat serum EDTA at 37 degrees C. Lysis occurred when E gamma 1.AIgA were prepared using SPDP-, glutaraldehyde- or carbodiimide-AIgA. Incubation of 100 micrograms/ml SPDP-AIgA with normal human serum for 30 min at 37 degrees C in the presence or absence of MgEGTA also induced consumption of total complement. The other soluble AIgA preparations were less effective in activating complement. These results suggest that polymeric serum IgA is capable of activating the alternative pathway of complement.

Neutrophil mediated and IgA dependent antibacterial immunity against enteropathogenic Escherichia coli in the porcine intestinal mucosa

The role of polymorphonuclear neutrophils (PMN) in the antibacterial immunity against enteropathogenic Escherichia coli (EEC) 0:149 in the porcine intestine was studied using intestinal Thiry-Vella loop (T-V loop) as a model. Intraluminal immunizations of T-V loops resulted in elevated levels of immunoglobulin A (IgA) anti-EEC 0:149 antibody in the loop secretions, an infiltration of PMN in the lumen of the loops and an increase in the concentrations of lactoferrin (LF), lysozyme (LY), cationic proteins (CP), and a specific bactericidal response in the immunized loops. PMN were observed by electron microscopy (EM) to be actively phagocytic in the lumen of the immune loops. EM observations of loop fluids as well as the abrogating effect of iron on the in vivo bactericidal response strongly suggest that the pMN played an important role in the bactericidal response in the loops against EEC. In addition to phagocytosis by PMN and subsequent intracellular killing, disintegration of PMN in the lumen of the loops and extracellular killing of EEC by the antibacterial products of PMN such as LF, LY and CP, with and/or without synergistic effect of IgA antibodies, also contribute to the bactericidal response of the immunized loops.

Molecular forms of IgA rheumatoid factor in serum and synovial fluid of patients with rheumatoid arthritis

The distribution of molecular forms of serum IgA rheumatoid factor (IgA-RF) in 42 patients with rheumatoid arthritis was examined by solid-phase radioimmunoassay following fractionation by gel chromatography or ultracentrifugation in acidic buffer. Analysis of the fractions using phosphate buffered saline indicated that the IgA-RF in each serum was mainly polymeric. However, monomeric IgA-RF was detected in sera from approximately two-thirds of the patients, after dilution of chromatographic or ultracentrifugal fractions in diluent containing mouse monoclonal anti-human alpha chain antibody. The levels of monomeric IgA-RF (mean +/- SD 38 +/- 86 micrograms/ml) and the ratios of monomeric to polymeric IgA-RF (mean +/- SD 0.29 +/- 0.41) varied over a wide range. Paired synovial fluids from 9 of the patients were also examined. Monomeric IgA-RF was detected in each, although 2 samples demonstrated only minimal quantities. Neither form of IgA-RF was detected in serum from healthy adults when analyzed under the same conditions. Thus, both monomeric and polymeric IgA-RF can occur in serum and synovial fluid from patients with rheumatoid arthritis, and their proportions vary widely among patients.

The effect of complement depletion on immunologically mediated middle ear effusion and inflammation

Immunologic injury during immune response to antigen in the middle ear cavity has been suggested to be a contributing factor in otitis media with effusion (OME). One of the mechanisms proposed involves the generation of immune complexes, with subsequent injury mediated by complement fixation. To evaluate the potential role of complement in OME, an animal model of immune-mediated middle ear effusion and inflammation was used. Antigenic challenge of the middle ear of normal, sensitized guinea pigs results in OME. This response was compared with that observed in guinea pigs whose sera were decomplemented by treatment with cobra venom factor. As compared to controls, decomplemented animals exhibited less effusion [130 (+/- 35) vs 83 (+/- 60) microliter, P less than 0.05] and inflammation in the middle ear following antigenic challenge. These results suggest that complement could play a role in the pathogenesis of OME. However, the fact that animals with severely depleted serum complement still displayed a moderate amount of effusion and inflammation indicates that other mechanisms are also involved in our model of immune-mediated OME.

Glycosaminoglycans enhance complement hemolytic efficiency: theoretical considerations for GAG-complement-saliva interactions

When human serum is diluted and pre-incubated at 37 degrees C in low ionic strength buffer (LIS, u = 0.07; made iso-osmotic with dextrose), a spontaneous activation of complement (C) is observed as determined by C4 and C3 electrophoretic conversion. In this paper it is postulated that most species of glycosaminoglycans (GAG) restricted non-specific fluid phase complement consumption induced by LIS, an effect which conserved complement and thereby enhanced the subsequent residual serum C mediated hemolytic activity. The capacity of glycosaminoglycans, to have modulated the hemolytic activity at low ionic strength, depended on the charge of the GAG species tested. In general, the GAG regulatory effects may have been due to GAG mediated restriction of spontaneous non-specific fluid phase C1 autoactivation, and/or restriction of activated C1 activity. Such effects would result in the subsequent reduction of the spontaneous fluid phase C4 and C3 consumption. Although the precise mechanisms responsible for the effects were not identified, it is speculated that the potentiation of C1 inhibitor function and direct effects on C1 might be involved. Overall, the relative specific activities of the glycosaminoglycans, on a weight basis, in mediating the fluid phase C regulatory effect were heparin greater than dermatan sulfate greater than chondroitin-6-sulfate greater than chondroitin-4-sulfate greater than hyaluronic acid and keratan sulfate. When much higher concns of heparin (greater than or equal 0.2 micrograms/ml) were used, complement mediated lysis of EA was inhibited, probably due to the direct inhibition of C1, even C1 which may have bound to the sensitized erythrocytes (EA). Results similar to that of heparin were obtained using greater than 1 mg/ml of dermatan sulfate or dextran sulfate. In contrast, pre-incubation of human serum in LIS with high concns (up to 10 mg/ml) of hyaluronic acid or chondroitin-4-sulfate, which are much less charged, continued to result only in the restriction of hemolytically non-specific (fluid phase) C consumption, resulting in a higher residual complement hemolytic activity. A theory is developed that the binding of polyionic GAG to C1 and to C1 INH may provide a charged local environment which simulates a relatively higher ionic strength. Chemical degradation of hyaluronic acid or chondroitin-4 or -6 sulfate resulted in lowering of this C modulating effect, indicative of the importance of the structural integrity of these charged glycosaminoglycans.(ABSTRACT TRUNCATED AT 400 WORDS)

The human IgA system: a reassessment

In healthy adults the total daily production of secretory and serum IgA exceeds that of other immunoglobulin classes. Secretory and serum IgA display features of mutual independence: they are represented by molecules with different physiochemical and immunochemical properties and antibody activities and are produced by cells with different organ distributions. Secretory and serum IgA also exhibit different effector functions: interaction of secretory IgA with environmental antigens results in prevention of the penetration of such antigens by a variety of mechanisms. Although the function of polymeric serum IgA antibodies in certain animal species involves elimination of antigenic substances by noninflammatory means, the primary function of serum IgA remains unknown. It is proposed that in humans monomeric serum IgA (which prevents activation of the complement systems, inhibits phagocytosis and antibody-dependent cellular cytotoxicity) may protect endogenous antigens expressed on various cells and tissues by preventing their interaction with humoral and cellular immune mechanisms that may lead to tissue damage.

Circulating immune complexes and immunoglobulin A rheumatoid factor in patients with mesangial immunoglobulin A nephropathies

Circulating immune complexes (CIC) containing IgA and C3 were elevated in 48% of IgA nephropathy patients; IgA1 was the predominant subclass. IgA1-IgG CIC were detected in 44%, IgA2-IgG CIC in 7%, and IgM-IgA1 CIC in 16% of the patients. No IgM-IgA2 CIC were detectable. Sucrose gradient ultracentrifugation indicated that IgG-IgA1 CIC were predominantly of intermediate (13-19S) size whereas IgA1-C3 CIC sedimented from 11S to 19S. At acid pH, isolated CIC revealed the presence of substantial amounts of 7S IgA. One third of the patients had elevated serum IgA rheumatoid factor (RF) of both polymeric and monomeric forms despite normal levels of IgM-RF; 87% of patients with elevated IgA-RF had IgA1-IgG CIC. These results indicate that the IgA1 component of CIC in patients with IgA nephropathy is not necessarily of mucosal origin and suggest that a portion of these CIC consists of IgA RF immunologically complexed with autologous IgG.

Hyaluronic acid-complement interactions--II. Role of divalent cations and gelatin

Native hyaluronic acid (HA) is reported to be a weak anticomplementary agent. However, the normal buffer systems used for complement tests incorporate gelatin, Ca2+ and Mg2+, which may bind to HA, influence its conformation and interfere with its anticomplementary reactions with complement components such as Cl. In this study, metal ions (Ca2+ and Mg2+), gelatin and fibronectin appeared to react with native HA preparations and block their anticomplementary effects on Cl. In previous studies, we obtained evidence for a relationship between reversible heat-induced HA conformational changes and a subsequent reversible increase in anticomplementary activity. The anticomplementary activity of heat-treated HA preparations was also reduced by gelatin.

Hyaluronic acid-complement interactions--I. Reversible heat-induced anticomplementary activity

The in vitro interaction of hyaluronic acid (HA) with complement (C) classical-pathway activity has been investigated. It was found that native HA, even at a high concn (greater than 3 mg/ml), has a relatively weak anticomplementary activity. However, we report here that native HA can be reversibly altered by heat treatment such that C-inhibitory properties are manifested. We have determined in this study that a potent C-inhibitory activity can be obtained if HA solutions are thermally treated (100 degrees C), and stabilized by prompt freezing with prompt thawing just prior to the interaction with human serum complement. Several investigators have proposed that the intermolecular-associated strands of HA undergo a reversible decoupling upon thermal treatment and this decoupled state of HA can be semi-stabilized by quickly cooling the sample. This heat-treated HA strongly inhibits C1 as well as classical-pathway-mediated C3 conversion. However, if heat-treated HA samples are not stabilized but, rather, slowly cooled after heating or if heated HA samples are snapfrozen and then slowly thawed, the anticomplementary activity is gradually lost. Interestingly, the activity for this same sample can be regenerated by retreatment of the same sample with heat followed by low-temp stabilization, indicating the reversibility of the physical state of HA responsible for the anticomplementary effect. Since no detectable molecular degradation of thermally-treated HA was found, it was assumed that a heat-induced physical transition of HA (decoupled state) was responsible for the C-inhibitory effect.

In vitro alternative and classical activation of complement by extracts of cotton mill dust: a possible mechanism in the pathogenesis of byssinosis

Extracts of cotton mill dust (CDE) were shown to activate complement by the classical and alternative pathways. Activation of the classical pathway, presented in this study for the first time, was verified by C1 consumption, C2 destruction, and C4 conversion tests. The component of cotton dust that causes complement activation precipitated in the presence of 20% saturated ammonium sulfate. The data presented suggest that endotoxin is not the principal complement-activating component, as complement activation could not be correlated to endotoxin concentrations of extracts of various parts of the cotton plant. Proteolytic enzymes were also eliminated as possible causative agents of complement cleavage since CDE did not cleave purified C3 in the absence of other complement components. Polyvinylpolypyrrolidone failed to remove the complement-activating component in CDE demonstrating that polyphenolic tannins are not the causative agents. Involvement of complement activation in the pathogenesis of byssinosis could explain in part the mechanism and symptoms of the acute byssinotic reaction.

Experimental IgA nephropathy in bile duct ligated rats

Polymeric IgA and polymeric IgA-containing immune complexes are transported from blood to bile through hepatocyte-bound secretory component. In order to investigate interruption of this transport and its effect on the glomerulus, Sprague-Dawley rats underwent bile duct ligation. Renal tissue obtained at the time of sacrifice was stained by immunofluorescent techniques with antibodies to IgG, IgM, and IgA, and secretory component (SC) and C3. A progressive selective increase in the staining intensity of the glomerular mesangium was observed for IgA, C3, and SC in bile duct ligated rats. These changes were paralleled by a rise in serum IgA and C3. These findings are consistent with the view that glomerular deposition of IgA, C3, and SC in bile duct ligated rats may result from impairment of normal handling of polymeric IgA and polymeric IgA-containing immune complexes.

Activation of the alternate complement pathway by peptidoglycan of Actinomyces viscosus, a potentially pathogenic oral bacterium

Peptidoglycans and cells walls from Actinomyces viscosus, Staphylococcus aureus, and group A streptococcus were compared for their relative abilities to activate the alternate complement pathway (ACP). On the dry-weight basis, the peptidoglycan from A. viscosus was 3.5 times more active than group A streptococcal peptidoglycan and 15.6 times more active than Staph. aureus peptidoglycan in activating the ACP. Consequently A. viscosus peptidoglycan is one of the most potent ACP-activators reported to date. For both A. viscosus and group A streptococcus, the peptidoglycan was a better ACP activator than cell walls from the same organism (125- and 52-fold, respectively) indicating that the peptidoglycan is probably the most important subcellular ACP-activator in these microorganisms. In contrast, cell walls from Staph, aureus were 9 times more active than peptidoglycan from Staph. aureus in activating the ACP, presumably because teichoic acids are the most important subcellular ACP activator in this microorganism.

Inhibition of zymosan-induced alternative complement pathway activation by concanavalin A

Zymosan, a polysaccharide composed primarily of glucan and mannan residues, activates the complement system through the alternative complement pathway. We showed that zymosan-induced complement activation is inhibited by zymosan-bound lectins with carbohydrate specificities for mannosyl and glycosyl residues. Lectins unable to bind mannosyl or glucosyl residues did not inhibit zymosan-induced complement activation.

A quantitative assay for IgA rheumatoid factor

We developed a solid-phase radioimmunoassay capable of detecting nanogram quantities of human IgA rheumatoid factor (RF) in biological fluids. Human IgM RF, IgG RF, IgG, IgA, IgM and whole serum did not significantly interfere with the IgA RF assay. Patients with sero-positive rheumatoid arthritis (RA) had significantly higher concentrations of IgA RF than sero-negative RA patients or healthy adult controls. Concentrations of IgA RF in paired sera and synovial fluids from sero-positive RA patients were comparable. Levels of IgA RF demonstrated a moderately good correlation with levels of IgM RF in sero-positive RA sera (r = 0.673). However, the ratio of IgA RF concentration to IgM RF concentration in sero-positive RA sera varied widely.

In vivo changes in complement induced with peptidoglycan-polysaccharide polymers from streptococcal cell walls

In rats injected with an arthropathogenic dose of streptococcal cell wall fragments, serum hemolytic activity decreased over the first 24 h and was then elevated from days 2 through 6 after injection. Hemolytic activity was again elevated at days 16 and 40. Levels of activity of alternative complement pathway, C3, and factor D were also altered.

Activation of the guinea pig alternative complement pathway by mouse IgA immune complexes

Activation of the complement system by IgA was investigated with immune complexes containing a mouse IgA myeloma protein with specificity for phosphorylcholine linked to bovine serum albumin (PC-BSA). These IgA anti-PC-BSA immune complexes activated the alternative complement pathway in mouse and guinea pig serum, while human complement was not affected. The activation proceeded with consumption of C3 but little or no consumption of C5. C3 did not bind to the IgA immune complexes during complement activation although it did bind covalently to IgG immune complexes. It is suggested that IgA immune complexes do not supply a suitable surface for C3 binding and effective alternative pathway convertase assembly; therefore, cleavage is limited and occurs primarily in the fluid phase. Without C3 binding, C5 cleavage does not occur nor can the alternative pathway activation proceed to the amplification step.

Disruption of complement-mediated reactions by insoluble dentifrice ingredients

It has been strongly suggested that the complement system plays a critical role in the pathological processes occurring in periodontal disease. Because individuals with this common disease are often instructed to brush their teeth more often and for longer periods of time, several commercial toothpastes were tested for their effects on complement-mediated reactions. In this study, six of the nine toothpastes tested activated the classical complement pathway, leading to C3 cleavage, as determined by crossed immunoelectrophoresis. The activating ingredients were contained in the insoluble fraction of the toothpastes. Of the toothpaste abrasives, Ca++ pyrophosphate was determined to have C3-converting activity. One of the toothpastes, which contains a particular type of amorphous silica abrasive, appeared to bind both native C3 and its conversion products. In general, disruption of complement-mediated functions in affected tissues could alter local immunological responses and interfere with healing. The in vitro studies described here suggest a need for clinical studies to ascertain the in vivo influences of dentifrice ingredients on complement-mediated reactions in periodontal disease.

Pathopysiological aspects of immune complex diseases. Part II. Phagocytosis, exocytosis, and pathogenic depositions

Elimination of IC by the phagocytic system occurs mainly by macrophages and contrarotates to the pathogenic effect. Decisive to prevent systemic IC disease is the capacity of the phagocytic system. In the case of its saturation, the danger of the occurrence of IC disease is greatly enhanced. Conclusive evidence seems to exist that IC of extremely small or extremely high lattice structure (precipitates) are less pathogenic than soluble IC of medium network. Small IC in extreme antigen and antibody excess or precipitates exhibit a reduced complement activating potency. Small IC in extreme antigen or antibody excess hardly interact in vitro with membrane receptors and do not induce IC disease when injected or formed in vivo. Highly lattices IC, especially precipitates, are eliminated extremely quickly from the circulation, mainly by macrophages and there deposition in the kidney is significantly reduced. Thus, lack of quality of the antibody to precipitate the antigen and a reduced capacity and effectivity of the phagocytic system to eliminate the IC may be extremely important in the generation of IC diseases. Facing the overwhelming and partly even inconsistant data of this topic, one may doubt whether IC diseases may be regarded to be a defined and coherent disease. Too many variables and questions exist concerning the nature of the antigen, especially in tumor and autoimmune diseases, concerning the quality of the antibody and the characteristics of the pathogenic IC and concerning localization and the elimination process. Nevertheless, common pathophysiological pathways of IC diseases may be recognized.

Prevention of bacterial infections of mucosal surfaces by immune secretory IgA

1) Colonization and diarrhea in the rabbit due to RDEC-1 strain E. coli elicits an immune response that prevents subsequent infection with the same bacterium. 2) RDEC-1 and InvEC strain colonization of the gut of pregnant rabbits is followed by the appearance, post-delivery, of anti-InvEC or RDEC-1 strain s-IgA in the milk, some of which was able to agglutinate as many as 3.4 X 10(11) bacteria per mg of s-IgA. 3) Immune s-IgA, given by the orogastric route to rabbits, is able to survive the digestive enzymes of the gastrointestinal tract to prevent the colonization and diarrhea that follows the inoculation of InvEC and RDEC-1 strains. One possible explanation for the protective effect of s-IgA is the in vivo agglutination of bacteria in the lumen of the gastrointestinal tract.