The interaction of antibody/DNA immune complexes with complement. Influence of antibody class and DNA conformation

Concerted clearance of immune complexes bound to the human erythrocyte complement receptor: development of a heterologous mouse model

Experiments in primates have demonstrated that immune complexes (IC) bound to erythrocytes (E) via complement receptor 1 (CR1) are cleared to the liver in a process which removes CR1, but otherwise spares the E. Human E are stabilized for >1 h in the circulation of the mouse if the terminal complement pathway is blocked, and we used this paradigm to examine clearance in a mouse model. Human E were opsonized with an anti-CR1 mAb cross-linked to dsDNA (antigen-based heteropolymer, AHP), and then incubated with systemic lupus erythematosus (SLE) plasmas containing IgG anti-dsDNA to form IC in situ. These IC stably bind to E CR1 in the complete absence of complement, thus allowing analysis in a model which does not require human C3b to facilitate E binding. Dual label experiments, based on RIA, flow cytometry and fluorescence microscopy, were employed to monitor separately E and IC. When opsonized E-IC were injected into A/J mice, >90% of the IC were rapidly removed from the E coincident with loss of CR1. The E remained in the circulation while IC were localized to the liver, mainly to Kupffer cells. Preliminary experiments in NZB/W mice, which spontaneously develop IgG anti-dsDNA, indicated that infusion of E-AHP led to rapid binding of murine IgG to the E-AHP, followed by removal of the nascent IC from E, and loss of CR1 in a concerted reaction. These studies provide additional evidence that E CR1 functions as a privileged site for IC clearance, and that the key step in clearance requires removal of CR1 from E to release bound IC for uptake by acceptor macrophages. This model can be extended to genetically altered mice to investigate the role of specific Fc gamma receptors as well as complement receptors in IC clearance.

A bispecific dsDNAxmonoclonal antibody construct for clearance of anti-dsDNA IgG in systemic lupus erythematosus

High avidity anti-dsDNA IgG antibodies are believed to play an important role in the pathogenesis of the autoimmune disease systemic lupus erythematosus (SLE) and therefore attempts have been made to reduce the concentration of these antibodies in the bloodstream of SLE patients. Previously we reported the development of an antigen based heteropolymer (AHP), a bispecific complex prepared by using the avidin-biotin system to crosslink dsDNA to a mAb specific for the human erythrocyte (E) complement receptor. Our studies indicated that this AHP could bind anti-dsDNA antibodies to E and facilitate clearance of these autoantibodies from the circulation of a monkey without E destruction. Here we report an improved covalent crosslinking procedure and purification scheme in which the AHP construct is isolated by precipitation in 50% saturated ammonium sulfate. We used a dsDNA binding dye, PicoGreen, to demonstrate specificity of binding of dsDNA to E via the AHP. The efficacy of the AHP in binding IgG anti-dsDNA antibodies to E was demonstrated in a sensitive and quantitative assay, based on the time resolved fluorescence properties of europium-labeled anti-human IgG mAbs used to probe the E. We also used this assay to screen SLE patient and normal plasmas for levels of anti-dsDNA IgG. The results of this assay correlate very well with the Farr assay, and therefore this approach may be useful in the development of informative and specific assays for a variety of autoantibodies. Treatment of SLE plasmas with E-AHP under conditions close to physiological led to substantial reductions (> or = 90%) in anti-dsDNA titers. It should be possible to test these new AHP for their ability to target and safely remove IgG anti-dsDNA antibodies from the circulation in animal models.

Clearance of anti-double-stranded DNA antibodies: the natural immune complex clearance mechanism

OBJECTIVE

To develop an in vitro model for investigating the mechanism by which autoantibodies in immune complexes (ICs) that are bound to primate erythrocytes via antigen-based heteropolymers (AHPs) are cleared from the circulation and localized to the liver.

METHODS

IgG anti-double-stranded DNA (anti-dsDNA) antibodies in ICs with dsDNA were bound to human erythrocytes via complement receptor 1 (CR1) either by opsonization with normal human serum as a complement source or through the use of an AHP, which consists of an anti-CR1 monoclonal antibody (mAb) that is chemically crosslinked with dsDNA. We performed parallel investigations of the mechanism of transfer of both types of erythrocyte-bound ICs to a monocytic cell line (U937). Erythrocytes with CR1-bound ICs were incubated with U937 cells under a variety of conditions, and subsequently, the levels of IgG anti-dsDNA, CR1, AHP, or C3b on both erythrocytes and U937 cells were measured by flow cytometry with appropriate fluorescently labeled probes.

RESULTS

In the presence of U937 cells, both the AHP-anti-dsDNA and C3b-opsonized ICs were rapidly removed from the erythrocytes; at 37 degrees C, more than half of the complexes were removed in 2 minutes. Monomeric mouse IgG2a mAb blocked the transfer of both types of complexes by 75%, suggesting that Fcgamma receptor type I (FcgammaRI) is the main phagocyte receptor responsible for the removal of ICs from erythrocytes. Levels of CR1 on the erythrocyte surface were reduced during transfer of the AHP-anti-dsDNA ICs, suggesting that transfer involves a concomitant removal of CR1, presumably by proteolysis.

CONCLUSION

Transfer of AHP-anti-dsDNA ICs from erythrocyte CR1 to model phagocytes occurs by a mechanism that is similar to the natural mechanism of IC clearance, involving recognition by FcgammaRI and removal of erythrocyte CR1 as key steps.

Antigen-based heteropolymers. A potential therapy for binding and clearing autoantibodies via erythrocyte CR1

OBJECTIVE

To determine if complexes containing monoclonal antibodies to CR1 cross-linked with antigen (antigen-based heteropolymers [AHP]) can bind the corresponding autoantibody to primate erythrocyte CR1 and promote autoantibody clearance from the circulation.

METHODS

AHP were constructed by cross-linking double-stranded DNA (dsDNA) to monoclonal antibodies to CR1. The ability of AHP to facilitate binding of human anti-dsDNA antibodies to primate erythrocytes was studied in vitro using a variety of radioimmunoassays (including Farr assays), enzyme immunoassays, and fluorescence-activated cell sorting. In addition, we used a monkey model to study in vivo the AHP-mediated clearance of passively infused human anti-dsDNA antibodies.

RESULTS

Large amounts of lupus IgG anti-dsDNA antibodies can be specifically bound to human erythrocytes via the complexes, and studies in 2 rhesus monkeys indicate that the erythrocyte-bound antibodies are rapidly cleared from the circulation.

CONCLUSION

This methodology may allow for development of a new therapy to facilitate autoantibody clearance in autoimmune disease.

Recognition of oxidized DNA bases by sera of patients with inflammatory diseases

Chronic inflammatory conditions result from or contribute to many diseases. Prominent among them is systemic lupus erythematosus (SLE). Sera of SLE patients contain an array of various auto-antibodies (Ab), including antinuclear Ab of unknown etiologies. The most puzzling is formation of Ab directed against autologous DNA. Our hypothesis was that increased oxidant production causes oxidation of DNA bases, which provide antigenic determinants that elicit antioxidized DNA Ab. To test this hypothesis, we used oxidized DNA nucleoside (5-hydroxymethyl-2'-deoxyuridine [HMdU]) conjugated to bovine serum albumin (HMdU-BSA) as the antigen. The results of the enzyme-linked immunosorbent assay showed that these Abs are sensitively detectable in SLE sera and sera of various other inflammatory autoimmune diseases. The titers of anti-HMdU Ab were significantly higher (p < .01) than those present in the control sera. Anti-HMdU Ab were predominantly of the IgM isotype, with low levels of IgG and no IgA. Anti-HMdU Ab bound to the HMdU-BSA-coated wells in a concentration- and time-dependent manner. That binding was inhibited by HMdU-BSA and to a lesser extent by thymidine-BSA, a normal nucleoside conjugate. The specific binding appears to be inversely related to the age of the patients, but no significant differences were observed between the sexes of the same age.

Complement-opsonized IgG antibody/dsDNA immune complexes bind to CR1 clusters on isolated human erythrocytes

We used fluorescence microscopy, quantitative FACS analyses, and radioimmunoassays to examine the distribution of complement (C3b)-opsonized antibody/dsDNA immune complexes (IC) bound to normal human erythrocytes (RBCs) via immune adherence (IA). IC were detected with fluorescent anti-human IgG, and the RBC IA-receptor (CR1) was revealed with monoclonal antibodies (Mabs) to CR1 and fluorescent anti-mouse IgG. Under saturating conditions RBCs exhibit a large heterogeneity in binding; a significant fraction binds no IC. The positions of bound IC coincide with CR1 clusters on RBCs, as predicted by several investigators. FACS experiments indicate an excellent correlation between CR1 number and IC binding within an RBC population. The number of CR1 clusters able to bind IC is proportional to the number of CR1 per RBC. However, IC (fluorescent spots) detected per RBC (on average less than 10) are less than the average number of IC bound (20-30). This suggests that each fluorescent spot represents a small number of aggregated IC bound to a CR1 cluster. These "patches" of aggregated complexes may facilitate transfer of RBC-bound IC to cells of the mononuclear phagocytic system. Finally, not all the CR1 clusters bind IC, suggesting that proper geometric alignment of multiple C3b per IC with several CR1 in the cluster is required for IA.

Immune adherence and the processing of soluble complement-fixing antibody/DNA immune complexes in mice

We have examined in mice the immune adherence (IA) reactivity and the fate (clearance kinetics and organ distribution) of a variety of preformed and nascent soluble antibody/125I-DNA immune complexes prepared at antibody excess. Parallel studies of the clearance kinetics and organ deposition of free DNA were also conducted. Preformed immune complexes prepared with large dsDNA bound to mouse platelets in vivo and exhibited clearance kinetics in general agreement with our previous observations in rabbits. Very little kidney deposition was evident for these immune complexes, but three to four times as many counts were found in the kidneys when free DNA was injected. Nascent immune complexes did form in the circulation, but in contradistinction to our previous studies of nascent immune complexes in rabbits and rhesus monkeys, these immune complexes did not demonstrate IA and had clearance kinetics and organ distributions intermediate between the preformed immune complexes and free DNA. Thus, complement-fixing antibody/DNA immune complexes that could form in the circulation might be cleared both via immune complex recognition mechanisms and via DNA recognition mechanisms. The role of IA in modulating the organ deposition of complement-fixing immune complexes is also discussed.

Inefficient binding of IgM immune complexes to erythrocyte C3b-C4b receptors (CR1) and weak incorporation of C3b-iC3b into the complexes

The binding of soluble complement-reacted IgM immune complexes (IC) to erythrocyte (E) C3b-C4b receptors (CR1) and the incorporation of C3b-iC3b into solid phase IgM-IC was investigated. The optimal binding of liquid phase IgM-IC to E-CR1 was obtained with IC formed at moderate antibody excess, but the binding was low (2-3%) when compared to the binding of the corresponding IgG-IC (50-60%). Solid phase IC were prepared by coating microwells with heat-aggregated bovine serum albumin (BSA) followed by incubation with rabbit IgM anti-BSA antibody. The IC were reacted with human serum at 37 degrees C. The binding of C3b-iC3b was determined by use of biotinylated F(ab')2 antibodies to C3b-C3c and avidin-coupled alkaline phosphatase. The incorporation of C3b-iC3b into solid-phase IgM-IC increased when increasing amounts of IgM antibody were reacted with the antigen. The binding reaction was slow, reaching a maximum after about 2 h at 37 degrees C. The binding of C3b-iC3b to the IgM-IC was remarkably inefficient when compared to the incorporation into IgG-IC reacted with the same amounts of BSA-precipitating antibody.