The effects of immunoglobulin isotype and antibody affinity on complement-mediated inhibition of immune precipitation and solubilization

Molecular bases of immune complex pathology

The binding of antigens with antibodies forms immune complexes in the body. Usually these complexes are eliminated by the system of mononuclear phagocytes without development of pathological changes. This review highlights principal mechanisms responsible for safe removal of immune complexes in primates and humans. Special attention is given to diseases known as "immune complex diseases", when antigen-antibody complexes induce inflammatory reactions. The review considers key experimental works that significantly contributed to current knowledge of etiology and pathogenesis of type III hypersensitivity. Some factors of the development of immune complex syndrome such as level of humoral immune response to antigen, isotype and affinity of forming antibodies, the amount of immune complexes, and the consequences of their interaction with the complement system and Fc-receptors are analyzed based on the molecular mechanisms involved. The review contains a retrospective analysis of the most significant scientific achievements in immune complex pathology investigation within the last 100 years.

The Complement—Fixing Activity of Immune Complexes Containing IgG Antibodies of Different Functional Affinities: Effects on Superoxide Production by Rabbit Neutrophils

When neutrophil phagocytes are stimulated by IgG containing immune complexes (IgG-IC), with or without the participation of the complement system, they show a sharp increase in oxygen uptake and begin to release large quantities of superoxide anions (O2-) and hydrogen peroxide (H2O2) into the surrounding medium. The aim of the present investigation was to provide insights into the production and release of O2- by rabbit neutrophils activated with immune complexes (IC) containing IgG antibodies of different functional affinity, opsonized and not opsonized by complement system components. For this purpose, two populations of polyclonal anti-ovalbumin (OVA) IgG antibodies with different functional affinity, 5 x 10(8) M(-1) and 2 x 10(7) M(-1), were prepared. The production of O2- was measured spectrophotometrically by a method using the superoxide dismutase-inhibited reduction of ferricytochrome C to the ferrous form. The activation of complement by different IgG-IC was determined by estimating the total residual haemolytic activity of the alternative and classical pathways in sera treated with different concentrations of anti-OVA IgG/ OVA immune complexes formed at equivalence. The results showed that: 1) antibody functional affinity influenced O2- production and the complement-fixing activity induced by the IC. In general, the higher functional affinity antibodies were more efficient in stimulating the respiratory burst of neutrophils and in activating complement by the classical and alternative pathways than the lower functional affinity antibodies at all IC concentrations tested; 2) complement components incorporated into the immune complex lattice caused an increase in the stimulatory activity of both IgG antibodies to produce O2- (approximately equal to 15% for the IC of IgG with Ka = 5 x 10(8) M(-1) and approximately equal to 7% for the IC of IgG with Ka = 2 x 10(7) M(-1)). This effect was dependent on antibody affinity and concentration; 3) there was a direct relationship between the overall level of complement activation, antibody affinity and superoxide production by neutrophils. Thus, we conclude that antibody affinity influences immune complex lattice formation, modulating its three-dimensional structure and the disposition of Fc fragments interfering with the antibody's biological properties. These results can help understand the precise role of antibody functional affinity in antigen-antibody complex diseases and define the immunochemical characteristics of pathogenic complexes.

Enzyme immunoassays for measuring complement-dependent prevention of immune precipitation (PIP) and solubilization of preformed antigen-antibody complexes (SOL)

We have developed simple and sensitive enzyme-based methods for evaluating the ability of serum complement to prevent immune complex precipitation (PIP) or to solubilize preformed immune complexes (SOL). Alkaline phosphatase, serving both as antigen and label, is added to goat IgG anti-alkaline phosphatase antibodies, with serum present throughout the assay (PIP), or added after immune complex formation (SOL). After incubation at 37 degrees C for 1 h followed by centrifugation, the enzyme activity of the supernatant, reflecting the amount of immune complexes in solution, is measured by colorimetry. Results are expressed with reference to a standard serum pool assigned 100 arbitrary units (AU). Intra- and inter-assay variabilities are within 10%. The normal ranges were 67-133 AU for PIP and 72-129 AU for SOL. These methods have been standardized for clinical use in relation to impaired complement function and immune complex disease, and adapted for measuring complement mediated binding of immune complexes to erythrocytes. They are sensitive, easy to perform and do not require expensive facilities. By measuring the interaction of complement with immune complexes, these methods may highlight aspects of the classical and the alternative pathway that are different from those detected using haemolysis as an endpoint.

Renal vascular lesions in lupus nephritis

We retrospectively studied the prevalence, histologic features, clinical correlations, and long-term outcome of the intrarenal vascular lesions of lupus nephritis (LN) in a series of 169 renal biopsies performed between 1980 and 1994 in 132 patients with systemic lupus erythematosus. The most common vascular lesions were nonspecific sclerotic changes, found in 37% of the biopsies (24% if only the cases with moderate to severe changes are considered). The other common vascular lesions were "immunoglobulin microvascular casts," found in 24% of the biopsies. Vasculitis and thrombotic microangiopathy were rare lesions and were seen in only 4 (2.4%) and 1 (0.6%) cases, respectively. Isolated sclerotic vascular changes were present in biopsies from older patients with a longer duration of LN, compared with the group with no vascular lesions, and were associated with a significantly higher prevalence of hypertension. Overall, however, the long-term renal and patient survival of this group did not differ significantly from that of the patients without vascular changes. Immunoglobulin microvascular casts (IMCs) ("lupus vasculopathy") were characterized by the presence of immunoglobulin deposition within the glomerular capillaries and small arterioles. In the present study we extensively investigated the morphologic and immunologic features of this lesion. The lesions were notable for the absence of endothelial or parietal vascular lesions and of fibrin, platelets, and leukocytes, which indicates that thrombosis is not involved in the vascular obstruction. According to our data immunoglobulin precipitation in the microvasculature seems to play a central role in the pathogenesis of this lesion, which is why we propose the term "immunoglobulin microvascular casts." In general, IMCs were associated with the most severe and active forms of diffuse proliferative lupus nephritis (World Health Organization [WHO] class IV). However our data show that, in contrast to previous studies, the long-term outcome of patients with IMCs is not worse than that of other patients with class IV LN. It may even be somewhat better, suggesting that this type of lesion may reverse with immunosuppressive therapy. In addition, we did not find any association between the presence of IMCs and the lupus anticoagulant, IgG anticardiolipin antibodies, or extrarenal vascular manifestations. Concerning vasculitis and thrombotic microangiopathy, our results confirm that their occurrence is quite rare in-lupus nephritis. The outcome of our 4 patients with vasculitis was not particularly poor, which could be related to early and/or aggressive treatment. Taken as a whole, our data confirm that the presence of active and severe forms of diffuse proliferative LN (WHO class IV) carries a worse prognosis compared with the other forms of LN. In our study, and in agreement with previous reports (23), the long-term renal survival of patients with class IV LN was significantly worse than that of patients with other forms of LN, with a 10-year renal survival of 70% compared with 85%, respectively. However our data do not support the conclusions of some previous studies that the presence of intrarenal vascular lesions is a marker of poor renal prognosis in lupus nephritis. More precisely, our data show that the somewhat poorer renal outcome observed in patients with IMCs is related to the fact that in most cases these lesions are associated with class IV lupus nephritis, and not related to the presence of the vascular lesion per se.

A systematic study of neutrophil degranulation and respiratory burst in vitro by defined immune complexes

Defined immune complexes (IC) were used to compare the effect of antibodies of different classes and subclasses on neutrophil respiratory burst and degranulation. IC were made from 5-iodo-4-hydroxy-3-nitrophenacetyl (NIP) conjugated to bovine serum albumin (BSA) and chimaeric mouse-human anti-NIP monoclonal antibodies including IgA2, IgE and all four IgG subclasses. The activation of neutrophils by IC depended on antibody class and subclass, on antigen epitope density, on antigen: antibody ratio and on the medium used. The ability to generate the respiratory burst showed a different pattern to the ability to give rise to degranulation. Compared with other IC, IgA2 IC provided the strongest stimulus for neutrophil activation. IgG1 IC, IgG2 IC and IgG4 IC activated neutrophils moderately or weakly IgG3 IC were unable to stimulate the respiratory burst, but could cause strong degranulation. IgE IC could hardly cause any neutrophil response. Neutrophil degranulation in response to IgG3 IC in serum-free medium or heat-inactivated serum was fast, and it quickly reached maximum. Degranulation caused by IgA IC was relatively slow, but gradually increased during incubation. The activity of IgG1 IC, IgG2 IC and IgG4 IC generated a respiratory burst increased with antibody excess and decreased with antigen excess. The activity of IgA2 IC, however, was not affected by change of antigen and antibody ratio. A specific role of serum, possibly due to complement, was found in enhancing degranulation, both temporally and quantitatively, by IgA2 IC.

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.

Differential susceptibility of immune complexes to release from the erythrocyte CR1 receptor by factor I

This study was designed to explore the role of Factor I in the release of immune complexes (IC) from human erythrocytes (E). The interactions between E and IC constructed with murine monoclonal antibodies were examined using, as a complement source, autologous plasma, plasma depleted of Factor I by > 90%, or Factor I-depleted plasma reconstituted with purified Factor I. Striking differences were observed in the interactions between E and different types of IC in Factor I-depleted plasma. The release of IC constructed with IgG1, IgG3, IgM or IgA antibodies was abolished by Factor I depletion whereas IC containing IgG2a or IgG2b antibodies were still released from E in Factor I-depleted plasma. Moreover, when IC containing IgG2a antibodies were incubated briefly in Factor I-depleted plasma, under conditions in which the IC were bound but not released, and then resuspended in the presence or absence of Factor I, as little as 5% of the normal physiologic level of Factor I released the IC from E. Thus, IC containing IgG2a antibodies appear to be exquisitely susceptible to release from E by Factor I. Additional differences in the susceptibility of IC containing IgG1, IgG3, IgM or IgA antibodies to release from E were revealed when Factor I-depleted plasma was reconstituted with Factor I. Under these conditions, the relative susceptibility of IC to release was: IC containing IgG1 or IgA antibodies > IC containing IgM antibodies > IC constructed with IgG3 antibodies. While isotype was critical in determining susceptibility to release, some clonotypic differences between isotype-matched pairs of IC were also evident. Differences in IC release from E by Factor I may reflect antibody matrix-mediated differential susceptibility of IC-bound C3b and/or C4b to cleavage by Factor I and may have implications for immunoregulation, host effector cell mechanisms and the pathophysiology of IC diseases.

The relative roles of C4A and C4B in prevention of immune precipitation, solubilisation and immune adherence

C4A and C4B levels were measured in serum from 246 normal individuals. Complement-mediated solubilisation, assayed using alkaline phosphatase anti-alkaline phosphatase immune complexes (IC), correlated with both C4A and C4B levels. However, C4A and C4B levels showed no correlation with solubilisation of bovine serum albumin (BSA) ICs, or with the prevention of immune precipitation of BSA or alkaline phosphatase ICs, nor with immune adherence assayed using thyroglobulin and BSA ICs.

Dissociation of primary antigen-antibody bonds is essential for complement mediated solubilization of immune precipitates

The role of dissociation of primary antigen-antibody bonds in the solubilization of immune complexes (IC) has been investigated using photo-affinity crosslinked IC comprising NAP15-BSA and murine monoclonal anti-DNP antibodies. Non-covalently linked IC were solubilized rapidly when incubated with normal human serum (NHS), whereas covalently-linked IC were solubilized poorly or not at all. The rate and extent of complement activation produced by incubating covalently-linked and non-covalently linked IC with NHS was similar as assessed by the production of the C1s:C1-inhibitor, C3:properdin and C5b-9 complexes and the anaphylatoxins C4a and C3a. Thus, the inability of serum to solubilize photo-affinity crosslinked IC must be due to failure of dissociation of primary antigen-antibody bonds.

The effect of antibody isotype on the activation of C3 and C4 by immune complexes formed in the presence of serum: correlation with the prevention of immune precipitation

Radioimmunoassays for C3a and C4a have been used to measure the activation of complement during the formation of immune complexes in human serum by the interaction of DNP-BSA and each of 11 mouse anti-DNP monoclonal antibodies of varied isotype and affinity. Those containing IgG2 or IgM were potent activators of C4, whilst IgG1 containing complexes were less efficient. C3 activation in normal serum was similar for complexes containing IgG1, IgG2a, IgG2b or IgM. IgA complexes did not activate C3 or C4. All complexes except those containing IgA precipitated more slowly in serum than in buffer. IgG2 antibodies were potent activators despite being very slow to precipitate in buffer. In serum containing EGTA activation of C4 was abolished and precipitation of complexes occurred at the same rate as in buffer. Nevertheless, C3 activation still occurred by the alternative pathway for all IgG and IgM complexes. Antibodies of the same isotype did not necessarily activate complement to the same extent. The ranking of the ability to activate complement was the same as that observed when performed complexes containing the same antibodies were added to serum. The levels of C4a generated were similar under both conditions but for most antibodies more C3a was generated by preformed complexes.

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.

Physiological and pathological aspects of circulating immune complexes

Complement participates in the elimination of IC in many circumstances. When antigen/antibody IC first form in the circulation, complement inhibits their aggregation because the covalent binding of C3b to the IC modifies their biophysical properties and they remain soluble. Such opsonized (C3b coated) IC attach to cells bearing C3b receptors (CR1) in the circulation, in particular to erythrocytes, since in humans 85 to 90% of CR1 in the blood is located on these cells. This immune adherence binding reaction appears to be a physiological system that allows IC to be transported through the circulation to the fixed macrophages of the MPS where they are safely eliminated. The deposition of circulating complement-fixing IC in various organs such as the kidney may be considered as a failure of this transport system. This is apparent in complement deficient and depleted states, and also for non-complement-fixing IC (IgA IC). The formation of insoluble IC (by definition immune deposits found in human pathology are insoluble) produces complement activation and inflammation at the site of the immune aggregate.

The activation of C3 and C4 in human serum by immune complexes containing mouse monoclonal antibodies of different isotype and affinity: effects on solubilisation

Radioimmunoassays for C3a and C4a have been used to measure the activation of complement in human serum by immune complexes containing DNP-BSA and each of 11 mouse anti-DNP monoclonal antibodies of varied isotype and affinity. When preformed complexes were added to serum, those containing IgG2 or IgM were potent activators of C4, whilst IgG1 complexes were less efficient. C3 activation in normal serum was similar for complexes containing IgG1, IgG2a, IgG2b or IgM. IgA complexes did not activate C3 or C4. Solubilisation of complexes was greatest for IgM and IgG2b and least for IgG2a and IgA. In serum containing Mg2+ EGTA C4 activation was abolished and the amount of C3 activation was lower for all IgG and IgM complexes. Antibodies of the same isotype did not necessarily activate complement to the same extent. Unexpectedly, three of the four IgMs activated C3 in EGTA. For IgMs, neither complement activation nor solubilisation correlated with affinity. For IgG1 antibodies, solubilisation was inversely proportional to affinity. C3 or C4 activation did not correlate with affinity.