Complement depletion accelerates the clearance of immune complexes from the circulation of primates

Immune adherence and clearance of hepatitis B surface Ag/Ab complexes is abnormal in patients with systemic lupus erythematosus (SLE)

Complement levels and complement receptor 1 (CR1) on erythrocytes (E) are reduced in systemic lupus erythematosus (SLE). To see whether these abnormalities are responsible for defective transport and elimination of immune complexes (IC) from the circulation, patients with active SLE (14) and normal volunteers (14) were injected with preformed IC (hepatitis B surface Ag/Ab). Two minutes after injection only 25.9 +/- 19.1% (mean +/- 1 s.d.) of the circulating IC were bound to E in the SLE patients as compared to 63 +/- 3.7% in the normal subjects (P = 0.0001). For SLE patients, the reduced immune adherence was best explained by a combination of complement depletion and low CR1 binding capacity (tau = 0.80, P = 0.0001). The disappearance of IC as estimated from the area under the elimination curve was faster in SLE than in controls (P = 0.02), and correlated with CR1 (tau = 0.54, P = 0.0001) and immune adherence observed in vivo (tau = 0.33, P = 0.013). Finally, immune adherence was absent and IC disappeared very rapidly in a patient with C2 deficiency and an SLE-like disease. These observations suggest that in SLE the defective immune adherence reaction might be responsible for the accelerated disappearance of IC from the circulation.

A flow cytometric assay for measuring complement receptor 1 (CR1) and the complement fragments C3d and C4d on erythrocytes

A flow cytometric assay (FCA) was developed to measure complement receptor 1 (CR1) and the complement fragments C3d and C4d on erythrocytes. It was possible to measure these parameters accurately with intra- and interassay coefficients of variation of 2.0% and 6.5% respectively. The method was able to discriminate between low and high levels of erythrocyte CR1, C3d and C4d. Comparison with a previously described RIA method gave excellent correlation coefficients with r2 values of 0.94, 0.93 and 0.91 for CR1, C3d and C4d respectively. The flow cytometric assay was used to measure CR1, C3d and C4d on the erythrocytes of 98 healthy individuals and the 95% upper limits for C3d and C4d were established. There was a wide distribution of CR1 levels amongst these individuals but their C3d and C4d levels were low and often not above background. The possible application of this method in clinical medicine is discussed.

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.

Elimination of soluble 123I-labeled aggregates of IgG in patients with systemic lupus erythematosus. Effect of serum IgG and numbers of erythrocyte complement receptor type 1

Using soluble 123I-labeled aggregates of human IgG (123I-AHIgG) as a probe, we examined the function of the mononuclear phagocyte system in 22 patients with systemic lupus erythematosus (SLE) and 12 healthy controls. In SLE patients, a decreased number of erythrocyte complement receptor type 1 was associated with less binding of 123I-AHIgG to erythrocytes and a faster initial rate of elimination of 123I-AHIgG (mean +/- SEM half-maximal clearance time 5.23 +/- 0.2 minutes, versus 6.58 +/- 0.2 minutes in the controls), with possible spillover of the material outside the mononuclear phagocyte system of the liver and spleen. However, multiple regression analysis showed that serum concentrations of IgG were the most important factor predicting the rate of 123I-AHIgG elimination. IgG concentration may thus reflect immune complex clearance, which in turn, would influence the inflammatory reaction, in SLE.

Experimental immune complex-mediated glomerulonephritis in the nonhuman primate

This study was undertaken to develop a model of immune complex (IC)-mediated glomerulonephritis (GN) in the nonhuman primate that could be used in subsequent studies to examine critically the role of the erythrocyte complement receptor (E-CR) in the pathogenesis of IC-mediated disease. Cynomolgus monkeys were chosen for study because they constitutively express E-CR levels that are either less than, equal to, or greater than that seen in normal man. After immunization with bovine gamma globulin (BGG), the GN induction protocol was begun in 10 cynomolgus by initiating daily i.v. administration of BGG in amounts sufficient to achieve or exceed antigen/antibody equivalence (assessed by the quantitative precipitin assay) for precipitating antibody present in the plasma volume. We found that within eight weeks of daily BGG administration of all the cynomolgus developed IC-mediated GN, irrespective of the initial E-CR level of the animals. However, the high E-CR cynomolgus tended to receive the higher BGG doses because of higher initial antibody levels to BGG. When the total number of glomerular deposits (determined by morphometric studies) per total BGG dose for each animal was plotted against the initial CR/E of that animal, there was a tendency for the animals with higher CR/E levels to have a lower number of glomerular deposits/BGG dose (r = 0.62, P = 0.06). Also, the total number of glomerular deposits correlated with the severity of the GN. During the early weeks of the GN induction protocol, the IC that formed in vivo (assessed by infusion of 125I-BGG) bound in large amounts to the circulating erythrocytes of the cynomolgus with medium or high E-CR levels. However, when tested after the onset of heavy proteinuria, which occurred between weeks 5 and 8 of daily BGG administration, the IC that formed in the circulation bound only poorly to circulating erythrocytes. By this time the E-CR levels had declined to 43 +/- 9% of initial values (P less than 0.01). This study demonstrates that: 1) A workable model of IC-mediated GN has been developed in the nonhuman primate. 2) During the induction of GN, CR/E and the ability of the erythrocyte to bind IC in vivo are decreased significantly. This suggests that an intact E-CR system could play a role in the protection against IC-mediated disease. However, further study will be needed to test that hypothesis critically. The present model should be useful in such studies.

Complement-mediated inhibition of immune precipitation in rheumatoid vasculitis

Complement-mediated inhibition of immune precipitation (CMIP) was measured in patients with rheumatoid arthritis (RA), rheumatoid vasculitis (RA VASC), patients with skin vasculitis not associated with a systemic connective tissue disease and normal healthy controls. CMIP was impaired in 100% (14/14) of the RA vasculitic patients, 60% (12/20) of the RA patients and 22% (2/9) of the dermovasculitic patients. The degree of impairment of CMIP was significantly greater in the RA vasculitic patients compared to the non-vasculitic patients. This difference was due to the significantly lower complement levels and the presence of higher concentrations of an inhibitor of CMIP in the RA vasculitic sera. The levels of this inhibitory activity correlated significantly with IgM rheumatoid factor concentration. Serial studies in three patients with RA vasculitis treated with corticosteroids and immunosuppressive drugs showed significant clinical improvement in two patients, which was associated with improvement in CMIP, reduction in circulating immune complex levels and reduction in IgM rheumatoid factor concentrations.

The human C3b receptor: function and role in human diseases

The human C3b receptor (CR1) is a polymorphic glycoprotein which functions regulating the complement system by inhibiting the activation of C3 and C5, through its effect on their convertases, and serving as cofactor for factor I in mediating the degradation of C3b to its inactive fragment C3bi and further to C3d-g. The latter are then ligands for their respective receptors on leukocytes, CR3 and CR2. Additionally, CR1 on erythrocytes endows these cells with the capacity to deliver immune complexes (IC) to the reticuloendothelial system, resulting in their clearance from the circulation. On phagocytes, this receptor participates in the process of endocytosis of foreign particles. There is a wide inherited variation of CR1 expression on erythrocytes (CR1/E) of different individuals. Patients with diseases which feature elevated levels of IC, such as systemic lupus erythematosus, leprosy, and AIDS, have a marked decrease of CR1/E, which may result in an altered clearance. This reduction appears to be related to disease activity, and the most probable site for CR1/E loss is during the transfer of IC to macrophages. Healthy neutrophils increase tenfold their expression of CR1 in response to the effect of chemoattractant peptides. Neutrophils from patients with AIDS display an altered response to stimulation. This defect may be of relevance in the process of endocytosis.

Immune complexes bind preferentially to specific subpopulations of human erythrocytes

Primate erythrocytes have complement receptors (CR1) that, both in vivo and in vitro, bind immune complexes (IC) opsonized with C3b. The present study was undertaken to determine whether the ability of human erythrocytes to bind IC is a characteristic shared by all erythrocytes. Binding of IC to erythrocytes probably involves the interaction of several C3b molecules with several CR1 clustered in small areas of the erythrocyte surface. To identify IC binding CR1 clusters, we first assessed the binding to erythrocytes of fluorescein-labeled polystyrene beads coated with monoclonal anti-CR1 antibodies (anti-CR1-beads) and second, performed IC. The binding of these ligands to erythrocytes was evaluated by immunofluorescence microscopy and flow cytometry. We found that only a fraction of erythrocytes from normal individuals bound anti-CR1-beads specifically and the percentage of erythrocytes able to bind beads increased with increasing numbers of CR1 per erythrocyte. However, the number of anti-CR1-beads bound per erythrocyte varied among cells from the same individual. We demonstrated further that the erythrocyte binding sites for anti-CR1-beads are also binding sites for opsonized IC. This was shown by demonstrating that anti-CR1-beads inhibited the binding of opsonized IC to erythrocytes and opsonized IC inhibited the binding of anti-CR1-beads to erythrocytes. Incubation of erythrocytes with opsonized IC, followed by FITC-labeled secondary antibodies, confirmed that indeed only a fraction of erythrocytes is capable of binding opsonized IC and that the binding sites for IC occupy small regions on the erythrocyte membrane. By contrast, we demonstrated that greater than 90% of erythrocytes express CR1. In conclusion, only some erythrocytes have the capacity to bind IC. Differences in the ability of erythrocytes to bind IC are probably related to differences in the clustering of CR1 in the erythrocyte membrane. Anti-CR1-beads identify erythrocyte binding sites for IC. These beads should prove useful to assess the changes that occur in the erythrocyte CR1 after exposure to IC in vivo.

Role of IgA in IgA nephropathy

IgA nephropathy (Berger disease) is defined by the dominant or codominant deposition of IgA in the renal mesangium. There is much evidence in vitro to suggest up-regulation of the IgA immune response in patients. Data from tonsillar and bone marrow-derived lymphocytes and from in vivo immunization studies indicate that the primary defect is an up-regulated systemic one, rather than mucosal IgA production. Several lines of evidence suggest that increased IgA production alone is inadequate to explain the pathogenesis of Berger disease. Murine models of IgA nephropathy indicate that local complement activation mediated by deposited IgG is essential for mesangial cell proliferation and subsequent renal injury. Circulating immune complexes from patients with Berger disease contain IgA and IgG within the same lattice. In vitro studies of model immune aggregates containing various mixtures of IgA and IgG indicate that the IgG is the site of complement activation and fixation. The IgA in the aggregate actually inhibits both complement activation and binding to erythrocyte complement receptor CR1. This effect of IgA may prevent effective immune complex clearance. In future studies, more emphasis should be placed on the roles of IgG and complement in the pathogenesis of IgA nephropathy.

Difference in the clustering of complement receptor type 1 (CR1) on polymorphonuclear leukocytes and erythrocytes: effect on immune adherence

Complement receptor type 1 (CR1) mediates the adherence of complement-reacted immune complexes (IC) to various blood cells. On the erythrocyte, CR1 are clustered, a distribution which favors efficient multivalent binding of C3b-coated IC. IC can also bind to CR1 expressed on polymorphonuclear (PMN) leukocytes. To evaluate the respective importance of these two cell types in immune adherence reactions, functional analysis of IC binding, as well as morphological studies of CR1 distribution at their surface were undertaken. At equal cell concentrations, resting PMN leukocytes bound the same percentage of IC as erythrocytes, despite expressing four times more CR1 at their surface. At equal CR1 concentrations, IC binding to resting or formyl-methionyl-leucyl-phenylalanine (fMLP)-stimulated PMN leukocytes was always lower than to erythrocytes. The morphological counterpart of these differences was studied by label-fracture immunoelectron microscopy. On erythrocytes, almost 50% of the CR1 were distributed in clusters of greater than or equal to 3 units, while less than 15% were grouped in such clusters on the surface of PMN leukocytes. Activation of PMN leukocytes by fMLP increased the surface density of CR1, but the proportion of clustered CR1 remained unchanged. These observations suggest that the low responsiveness of PMN leukocytes towards C3b-coated IC may be due to the unaggregated state of CR1. In the circulation, erythrocytes might function as a "buffer" for PMN leukocytes, which would otherwise engage too swiftly in reactions with IC.

Increased amounts of C4-containing immune complexes and inefficient activation of C3 and the terminal complement pathway in a patient with homozygous C2 deficiency and systemic lupus erythematosus

Plasma and serum samples from a patient with homozygous C2 deficiency and severe systemic lupus erythematosus who responded with full clinical remission after plasma infusions were examined for immune complexes (IC), C3 activation products, and the terminal complement complex (TCC). Plasma contained large amounts of C4-containing IC but no C3-containing IC or complement activation products. Classical pathway activation in vitro did not lead to C3 activation or TCC formation as seen in normal serum, but a very efficient binding of C1q and C4 was found. No disturbances in alternative pathway activation were observed. The results indicate an impaired formation of C3-containing IC and an inefficient clearance of C4-containing IC, supporting the idea of a causal relationship between the dysfunctional classical pathway, pathophysiology, and clinical manifestations in this patient.

Immune adherence of nascent hepatitis B surface antigen-antibody complexes in vivo in humans

Upon i.v. injection into humans, pre-formed immune complexes bind complement and adhere to complement receptor type I (CR1, CD35) on erythrocytes (immune adherence). However, in most circumstances antigen and antibody react in the presence of complement; such nascent immune complexes may have properties different from pre-formed immune complexes. To define whether nascent immune complexes would also adhere to erythrocytes in vivo in humans, we studied immune complexes that formed upon i.v. injection of radiolabelled hepatitis B surface antigen (HBsAg) into immunized volunteers (eight subjects with anti-HBsAb levels ranging from undetectable to 50 U/ml.; and three control non-immune individuals). Immune complexes formed immediately in the subjects with detectable levels of specific antibody, and the clearance rate of these immune complexes correlated with the anti-HBsAb level (r = 0.78, P < 0.01). A fraction of the circulating immune complexes bound to erythrocytes in the three individuals with the highest antibody level (8-15% at 10 min). The effect of CR1 number per erythrocytes was analysed in two subjects with similar antibody levels and immune complexes clearance rates: immune adherence was higher in the subject with more CR1 per erythrocytes. The same immune complexes model studied in vitro provided similar results: a fraction of nascent immune complexes bound to human erythrocytes; this immune adherence was observed only when immune complexes formed in the presence of antibody excess, and correlated with CR1 number per erythrocytes (r = 0.99, P < 0.01). Finally, adherence of nascent HBsAg-antibody immune complexes to platelets was demonstrated in rabbits. Although immune adherence involves only a small fraction of nascent immune complexes at any given time, it may be essential for the safe disposal of large nascent immune complexes.

Immune-complex vasculitis: role of complement and IgG-Fc receptor functions

Vasculitis contributes a major component to the pathogenesis of rheumatic diseases and glomerulonephritis. A common feature of these diseases is the presence of serum immune complexes (IC) which may be deposited in blood vessel walls. The modification of the size and solubility of IC by the classical and alternative complement pathways, and the recent demonstration of the role of cellular complement receptors and IgG-Fc receptors in the handling of IC, now allow a better understanding of the pathogenesis of the severe forms of vasculitis. When complement deficiencies are present, the handling of IC is impaired, and vasculitis results. New blood tests for Factor VIII-related antigen, alkaline ribonuclease, plasma thrombospondin, and anti-neutrophil cytoplasmic antibody correlate with the presence of selected types of vasculitis. In addition, tissue thromboplastin release after application of defined tourniquet pressure can also detect subtle blood vessel injury. These new tests may allow diagnosis without risky organ biopsies. Advances in the diagnosis and treatment of vasculitis will also be discussed.

In vivo handling of soluble complement fixing Ab/dsDNA immune complexes in chimpanzees

We used soluble, C-fixing antibody/dsDNA IC to investigate immune complex (IC) handling and erythrocyte (E)-to-phagocyte transfer in chimpanzees. IC bound efficiently to chimpanzee E in vitro and showed minimal release with further in vitro incubation in the presence of serum in EDTA (less than or equal to 15% within 1 h). These IC also bound rapidly to E in vivo (70-80% binding within 1 min) and did not show detectable release from E in the peripheral circulation after infusion in vivo (less than or equal to 2% within 1 h). Despite such slow C-mediated release of IC from E, IC were rapidly stripped from E by the mononuclear phagocyte system (T50 for E-IC1500 = 5 min) without sequestration of E. Treatment of the chimpanzees with the anti-Fc gamma RIII MAb 3G8 impaired the clearance of infused IC. This effect was most evident on the fraction of IC500 which did not bind to E and which presumably had captured less C3b (pre-MAb 3G8 T50: 45 min vs. post-MAb 3G8 T50: 180 min). With IC bound in vitro to E before infusion, anti-Fc gamma RIII MAb treatment led to significant amounts of non-E bound IC detectable in the circulation. Thus, the anti-Fc gamma RIII MAb appeared to interfere with the ability of fixed tissue mononuclear phagocytes to take up/or retain IC after their release from E. Both rapid stripping of IC from E, despite slow complement-mediated release of IC from E in the peripheral circulation, and blockade of IC clearance with anti-Fc gamma RIII MAb indicate that the interaction of IC with the fixed tissue phagocyte involves qualitatively different mechanisms than the interaction of IC with E. Fc gamma receptors appear to play an important role in the transfer and retention of IC by the phagocyte.

Elimination of soluble 123I-labelled aggregates of human immunoglobulin G in humans; the effect of splenectomy

To study the role of the spleen in the elimination of immune complexes we examined mononuclear phagocyte system function in eight healthy controls and eight splenectomized patients, with soluble 123I-labelled aggregates of human immunoglobulin G (AIgG). No differences were found between the two groups in elimination and degradation of AIgG. The loss of splenic function was compensated for by increased uptake of AIgG by the liver. With the dose of 123I-AIgG used in this study (10 micrograms/kg body weight), significant generation of C3a was observed. No correlation was found between erythrocyte CR1 number and the fraction of aggregates that bound to erythrocytes.

A kinetic study of erythrocyte-DNA/anti-DNA immune complex association and dissociation reactions in systemic lupus erythematosus

Decreased binding capacity of the erythrocyte complement receptor (RBC CR1) in systemic lupus erythematosus (SLE) may contribute to abnormal handling of circulating immune complexes in these patients. Decreased numbers of RBC CR1 have been reported in SLE, but, since binding is a function of both receptor number and receptor binding kinetics, we measured kinetic parameters for the interaction of complement (C) containing [3H]DNA:anti-DNA immune complexes (IC) with normal control (NC) and SLE RBC. Experiments were performed at five temperatures ranging from 7-37 degrees C. The parameters measured included: (1) the maximum quantity of DNA:anti-DNA:C which could bind per RBC, S; (2) the association rate constant, ka, for the binding of DNA:anti-DNA:C to RBC; (3) the dissociation rate constant, kd, for the dissociation of bound DNA:anti-DNA:C IC from RBC; (4) the steady-state constant, Kss (ka/kd); and (5) the energies of activation for association, Eaa, and dissociation, Ead. Although the relative amount of bound DNA:anti-DNA:C per RBC was significantly decreased in SLE patients compared to NC (P less than 0.001), the mean values for Kss, ka, kd, Eaa and Ead did not differ significantly between the two groups. These data suggest the following: (1) RBC CR1 binding and dissociation of DNA:anti-DNA:C are consecutive reactions resulting in steady-state concentrations of free and RBC-bound IC; (2) at steady-state times, the ratio of RBC bound to unbound DNA:anti-DNA:C are governed by kinetic factors; (3) since the binding kinetics of SLE and NC RBC are not significantly different, the decreased binding activity described by other investigators can only be due to a decreased number of CR1 per RBC; and (4) values for Eaa and Ead suggest that the rate-determining steps in IC association with and dissociation from RBC involves making and breaking of hydrogen bonds.

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 role of hypocomplementaemia and low erythrocyte complement receptor type 1 numbers in determining abnormal immune complex clearance in humans

Defective clearance of immune complexes (IC) may contribute to the pathogenesis of diseases such as SLE. We studied the effect of hypocomplementaemia and the influence of erythrocyte complement receptor type 1 (CR1, CD35) number on the clearance of radiolabelled tetanus toxoid (TT)-anti-TT IC from the circulation. These were injected intravenously into 9 normal subjects and 15 patients with diseases characterized by IC formation and/or hypocomplementemia, including 2 with hereditary complement deficiency. IC were found to bind to erythrocyte CR1 in a complement-dependent manner and their degree of uptake was directly correlated with CR1 numbers. Two phases of IC clearance were identified. The first was rapid, occurring within 1 min. Since this phase might represent inappropriate deposition of IC in target organs we called it trapping. It was seen predominantly in subjects with low CR1, low complement, and low binding of complexes to red cells. The second phase was monoexponential with a mean elimination rate of 14.1%/min; it was inversely correlated with CR1 numbers and binding of complexes to red cells. In a second study each individual was injected with IC bound to autologous erythrocytes in vitro using normal serum so that the effects of complement deficiency were eliminated. Up to 81.4% of these bound IC were released in vivo from erythrocytes in 1 min, and the proportion was inversely correlated with CR1 numbers. Only five patients showed trapping, and these had low CR1 numbers and high percentage release of IC. The second phase of elimination was inversely correlated with CR1 numbers and the proportion of IC remaining bound to red cells at 1 min. The two complement-deficient patients had normal CR1: when IC were injected, trapping and very fast clearance rates were observed; however complexes that had been opsonized and bound to erythrocytes were cleared at a slower rate without evidence for trapping. These studies show that complement and erythrocyte CR1 may determine the physiological clearance of certain types of IC and suggest that this system may function abnormally when CR1 number or complement function are reduced.

Immunogenetics of SLE and primary Sjögren's syndrome

SLE is a syndrome defined by clinical criteria and by the presence of autoantibodies reactive with nucleic acids and proteins concerned with transcription and translation. Breeding experiments in mice have illustrated the enormous genetic heterogeneity of this syndrome, of which the final common pathway is a widespread immune complex disease. The causes of SLE in humans are likely to be equally multifactorial. Family studies have demonstrated that genetic factors exist, but each factor appears to be a relatively weak disease-susceptibility gene. The major exceptions to this are the very rare complete deficiencies of classical pathway complement components, which are almost invariably accompanied by the development of SLE. Observations of these patients have led to the formulation of hypotheses relating complement and its receptor, CR1, to the defective removal of immune complexes from the circulation.

Disposition of IgA-containing circulating immune complexes

Two fundamentally different mechanisms may account for the glomerular immune deposits in IgA nephropathy (IgAN): (1) deposition of circulating immune complexes and (2) the in situ formation of immune complexes. In this review the experimental evidence for and against an important role of circulating IgA-containing immune complexes in the pathogenesis of IgAN is summarized. Several physical characteristics, including size, lattice composition, and electrical charge, may influence the deposition of immune complexes in the renal mesangium. Furthermore, the likelihood of deposition of circulating IgA-containing immune complexes in vulnerable locations (such as the kidney) may be increased because of their impaired removal from the circulation by macrophages of the liver and spleen and the erythrocyte-immune complex clearing mechanism. However, the relative contributions of these factors to the pathogenesis of IgAN remain speculative.

Clearance of cryoglobulins in man

From the foregoing it is evident that the defective clearance of immune complexes may contribute to tissue damage seen in patients with cryoglobulins. Hypocomplementaemia, decreased erythrocyte CR1, and the nature of the immune complexes may all contribute to decreased binding of immune complexes to erythrocytes with the potential consequence of increased availability for deposition and decreased processing of immune complexes. In addition, in type II cryoglobulinaemia the nature of the immune complex namely IgM RF/IgG complexes, has been shown in some circumstances to fix C3 and C4 inefficiently in spite of detectable fluid phase complement activation. The poor C3 fixation results not only in decreased transport by erythrocytes to the RES but also in inefficient removal of immune complexes by phagocytic cells, since the Fc receptor on these cells would be acting alone without the synergy of occupied complement receptors. Persistence of immune complexes in tissues in these circumstances has two potential consequences. First, the multivalency of IgM may contribute to enlargement of these immune complexes in situ by successive trapping of antigen and antibody complexes; this process may be aided by local factors such as low temperature at the peripheries and increased protein concentration in glomerular capillary loops, which favour "cryo" precipitation. The higher avidity of RF for IgG that is surface bound rather than monomeric would also favor immune complex formation in these circumstances. Second, although there is no opsonization of the immune complex complement activation results in bystander fixation of C4 and C3 to the surrounding tissues resulting in tissue injury.

The rate of loss of CR1 from ageing erythrocytes in vivo in normal subjects and SLE patients: no correlation with structural or numerical polymorphisms

The stability of CR1 (complement receptor type 1) on ageing erythrocytes in vivo was examined in a group of normal subjects who had been genotyped using a restriction fragment length polymorphism (detected using a cDNA probe for CR1) that correlates with the numerical expression of CR1 on normal erythrocytes (H = allele correlating with high expression, L = low). Erythrocytes were separated into 5 fractions of increasing age on discontinuous Percoll gradients. Mean CR1 numbers on erythrocytes fell from 636 molecules per cell in the first fraction to 384 in the fifth in the HH group and from 478 to 315 in the LL group. There was no difference in the rate of decline of CR1 numbers between the groups. A group of nine SLE patients was also studied in the same way; their genotypes were HH (four) and HL (five). Mean CR1 numbers amongst all of these patients fell from 477 to 232, a faster rate of decline than in a genotypically matched group of normal subjects. There was no difference in the prevalence of the different structural allotypes amongst 30 SLE patients compared with 21 normal subjects. These data provide further evidence that there are enhanced extracellular mechanisms for the removal of CR1 from erythrocytes of SLE patients and do not support the hypothesis that inherited variation in CR1 expression on erythrocytes increases disease susceptibility to SLE.

Immune complexes and erythrocyte CR1 (complement receptor type 1): effect of CR1 numbers on binding and release reactions

We performed experiments to investigate whether immune complexes opsonized with C3b and iC3b transferred from CR1 on one erythrocyte to CR1 on others, and studied the effect of variation in erythrocyte CR1 number on the transfer reaction. We used populations of cells of different blood groups to study this phenomenon which were separated by differential agglutination with monoclonal anti-group antibodies. The rate of transfer of immune complexes between erythrocytes was related to CR1 concentration of both donor and recipient cells; fastest transfer occurred from donor cells of low CR1 numbers to recipient cells of high CR1. These results were not explained by a difference in the binding constant of immune complexes to erythrocytes bearing different numbers of CR1. In the absence of factor I, complexes partitioned between erythrocytes according to their relative concentrations of CR1 with no release of complexes into solution. In serum, the proportion of complexes bound to donor and recipient erythrocytes was similarly related to their respective CR1 numbers with progressive release of complexes into solution. Erythrocyte CR1 may act as a dynamic buffering system which prevents immune complexes that have bound complement from fixing to vascular endothelium.

Monoclonal rheumatoid factor-IgG immune complexes. Poor fixation of opsonic C4 and C3 despite efficient complement activation

Monoclonal IgM rheumatoid factor forms complexes with IgG in essential mixed cryoglobulinemia. We demonstrate that such complexes fix C3 and C4 poorly, although efficient fluid-phase C3 conversion can occur. Fixation of small amounts of C4 may be sufficient to generate a C3 convertase, but may prevent subsequent fixation of C3 by competing for binding sites on the complex. These complexes bind inefficiently to normal erythrocyte complement receptor type 1 (CR1) in vitro, and are undetectable on erythrocytes of patients with essential mixed cryoglobulinemia in vivo. Clearance of such phlogistic complexes from tissues by CR1-bearing cells may be inefficient.

Role of paf-acether in the mediation of pathophysiological responses to aggregated immunoglobulins. Studies with the platelet-activating factor receptor antagonist BN 52021

Sprague-Dawley rats were challenged with an intravenous (i.v.) infusion of soluble aggregates of immunoglobulin G. Animals receiving a dose of aggregates of 40 mg/kg showed a significantly reduced time of lysis of diluted blood clot, which paralleled the appearance in plasma of tissue-type plasminogen activator. These changes occurred about 5-10 min after the challenge, which is a more protracted time-course than that observed in response to paf-acether. A significant increase in serum levels of N-acetylglucosaminidase was also observed in the animals several minutes after challenge. Blood neutrophil count showed a 50% reduction that reached its maximum at 10 min and was followed by an overshoot after 30 min. In experiments in rats previously depleted of circulating PMN by treatment with vinblastine, no significant differences were observed in N-acetylglucosaminidase release as compared to non-treated animals. Since prior evidence indicated that endogenously generated paf-acether could be a mediator responsible for these changes, at least to some extent, the compound BN 52021, a specific antagonist of the paf-acether receptor was given to these animals prior to the challenge with the complexes. All the above mentioned responses were significantly reduced by BN 52021, which is in keeping with the hypothesis involving endogenous paf-acether release in the mediation of these changes. By contrast, BN 52021 did not interfere with the clearance of the aggregates from the circulation, which seems to be a beneficial mechanism to reduce immune-mediated tissue injury. These data extend the number of paf-acether mediated pathophysiological changes that can be observed in response to immune aggregates.

Family study of the major histocompatibility complex in HLA DR3 negative patients with systemic lupus erythematosus

Susceptibility to systemic lupus erythematosus (SLE) is known to be governed by genes in the HLA region of the 6th chromosome. From previous studies it has not been possible to distinguish between the effects of null genes for the complement component C4 and HLA-DR3, because of the marked linkage disequilibrium between DR3 and a null allele of C4A (C4A QO) in caucasoid populations. We report here an immunogenetic study of 44 cases of SLE, selected because they were DR3 negative. Eighteen of the 30 Caucasoid cases (60%) had extended HLA haplotypes with a C4 null allele, compared with 22 of 60 (37%) of a control panel of 60 DR3 negative normal Caucasoid subjects. This difference is significant (chi 2 = 4.41; 0.05 greater than P greater than 0.01). Of 14 non-caucasoid patients analysed, 10 had a C4 null allele. It is concluded that the null alleles of the C4 A and B genes are themselves directly responsible for conferring susceptibility to SLE.

Inherited deficiency of erythrocyte complement receptor type 1 does not cause susceptibility to systemic lupus erythematosus

There is a deficiency of complement receptor type 1 (CR1) on the erythrocytes of patients with systemic lupus erythematosus (SLE). This receptor is involved in the processing of immune complexes. Whether the deficiency is inherited or acquired has been the subject of controversy. A restriction fragment length polymorphism (RFLP), identified using a complementary DNA probe for CR1, has been correlated with the numeric expression of CR1 on normal erythrocytes. The gene frequency for the 2 alleles defined by this RFLP was compared in 44 patients with SLE (from 42 families), 43 of their consanguineous relatives, and 50 nonrelated normal subjects. The gene frequency for the alleles correlating with high and low expression of CR1 was 0.73 and 0.27, respectively, in the normal subjects. The gene frequency was not significantly different in the SLE patients. However, the SLE patients expressed fewer CR1 molecules per erythrocyte within each genotype, compared with normal subjects and compared with their consanguineous relatives. The low allele for numeric expression of CR1 on erythrocytes is not a disease susceptibility gene for SLE.

Tetanus toxoid-anti-tetanus toxoid complexes: a potential model to study the complement transport system for immune complex in humans

Complement and its receptor on erythrocytes appears to play a physiological role in the elimination of large immune complexes (IC) in monkeys, and a similar system is likely to work in humans. Here we define a safe IC model which is suitable for clinical investigations. Soluble tetanus toxoid (TT)-human anti-TT (IgG) antibody complexes were prepared in large antibody excess. The size of the complexes was approximately 45 S. When incubated in normal human serum, 50% of the IC increased further in size, but remained soluble, and bound rapidly to human erythrocytes in vitro. This binding was shown to require intact classical pathway function. When injected into normal guinea-pigs a comparable proportion of IC bound immediately to blood cells (mainly to platelets). No platelet binding of IC occurred in C4-deficient guinea-pigs, but this binding was restored when C4 was supplied. Initial immune complex elimination was faster in C4 deficient than in C4-supplemented and normal guinea pigs. Thus classical pathway function appeared to be necessary for the normal processing, transport and elimination of TT-anti-TT complexes.

A lysine-binding protein in SLE sera inhibits the binding of immune complexes to normal erythrocyte CR1 (complement receptor type 1)

The binding of 125I-labelled immune complexes (IC) to normal human erythrocyte CR1 (complement receptor type 1) by sera from patients with SLE was found to be significantly decreased compared to normal sera. In 13/29 patients, there was an inhibitor which decreased the binding of opsonized IC in normal sera to normal erythrocytes. It was found in each of the nine patients who had clinically active disease. The inhibitor was shown to be a globulin that was labile at 56 degrees C and bound to lysine; low concentrations of tranexamic acid and of lysine abolished the effects of the inhibitor which suggests that it possesses lysine-binding sites: these may block the CR1-binding site on IC opsonized with complement. This inhibitor may decrease the efficiency of IC carriage by erythrocyte CR1.

Complement fixation by small, DNase-resistant DNA-anti-DNA immune complexes

125I-ds DNA-anti-DNA immune complexes (IC) formed at antibody excess and containing DNA of 300-350 base pairs (bp) fixed complement, incorporated C3b and bound to the C3b receptors (CR1) on human red blood cells (RBC). When the IC were treated with DNase to generate small, DNase-resistant IC, some of the IC incorporated C3b, but did not bind to RBC. In order to examine C3b incorporation and RBC binding by IC of specific sizes, the DNase treated IC were fractionated by sucrose density gradient (SDG) ultracentrifugation. Small IC containing one, two, three or four IgG molecules per fragment of 125I-ds DNA were identified by autoradiography after electrophoresis of the SDG fractions on 3-12% linear polyacrylamide gradient gels. The SDG fractions were tested for C3b incorporation and RBC binding ability. There was neither C3b incorporation nor RBC binding activity in fractions which corresponded to 9-11S (containing IC with one IgG/DNA). Fractions which corresponded to 12-22S (containing IC with up to four IgG/DNA fragment) demonstrated increased C3b incorporation with increased size, but did not show significant RBC binding activity. Fractions with IC containing four or more IgGs (22-24S) incorporated C3b and bound to RBC at approximately the same level. It is concluded that DNase digested IC which contain three-four IgG/DNA fragment are large enough to activate complement and incorporate C3b, but are too small to bind to RBC CR1. These IC could therefore escape rapid clearance from the circulation via the erythrocyte CR1 clearance mechanism. Such IC could persist in the circulation and potentially elicit pathogenic effects in patients with systemic lupus erythematosus.

The classical pathway of complement prevents the formation of insoluble antigen-antibody complexes: biological implications

There is now clear evidence for a complement dependent physiological system which is capable of processing immune complexes in and outside the vascular compartment so that they remain soluble, and transporting such complexes to the fixed macrophage system where they are safely eliminated. Defects in physiological immune complex disposal can occur at various stages described in this article, and it could well be that several of these stages could present subtle defects which are, however, additive so that under a given set of circumstances immune complexes end up in the wrong places, i.e. outside the fixed macrophage system.

Clearance of human antibody/DNA immune complexes and free DNA from the circulation of the nonhuman primate

The present study evaluated the participation of the primate erythrocyte immune complex (IC) clearing mechanism in the clearance and organ uptake of double-stranded DNA (dsDNA) and of soluble ICs formed with human anti-DNA antibodies and dsDNA (dsDNA-ICs). Five baboons received 51Cr-labeled autologous erythrocytes and after a period of equilibration received separate intraarterial injections of [125I]free dsDNA and [125I]dsDNA-IC. Four of these five baboons were studied on a second occasion. To assess clearance from the arterial circulation and organ uptake, multiple blood samples were obtained from aorta, hepatic vein, and renal vein after injection of each probe. Two minutes after injection, a mean of 85% of dsDNA-ICs were bound to erythrocytes. By contrast, free dsDNA did not bind significantly to blood cells. The clearance rate of dsDNA-ICs from the arterial circulation was significantly faster than that of free dsDNA in all animals but one. Erythrocyte-bound dsDNA-ICs were cleared at a rate similar to that of total dsDNA-ICs. The liver was the major site of uptake of free dsDNA and of dsDNA-ICs. The hepatic uptakes of free dsDNA (17 +/- 8%/5 min) and dsDNA-ICs (27 +/- 8%/5 min) were not significantly different. 51Cr-labeled erythrocytes were not sequestered in the liver. There was not detectable uptake of free dsDNA or dsDNA-ICs by the kidney but with one exception. Thus, the primate erythrocyte IC clearing mechanism is involved in the clearance of dsDNA-ICs from the circulation but not in the clearance of free dsDNA.

Altered erythrocyte C3b receptor expression, immune complexes, and complement activation in homosexual men in varying risk groups for acquired immune deficiency syndrome

We studied levels of erythrocyte C3b receptors (E-CR1) and correlated them to the level of circulating immune complexes (CIC) and complement activation in patients with or at risk for acquired immunodeficiency syndrome (AIDS). A significant reduction was found in patients with AIDS (185 +/- 93 CR1/cell), AIDS-related complex, and generalized lymphadenopathy, whereas healthy male homosexuals or normal controls had 434 +/- 193 and 509 +/- 140 CR1/cell, respectively (P less than 0.001). Family studies indicate that this defect is acquired. Reduction in E-CR1 was associated with increased levels of CIC when assayed by binding to Raji cells, but not when tested by C1q binding. Complement activation was assessed by levels of C3bi/C3d-g in plasma, measured with a monoclonal antibody specific for a neoantigen in C3d. AIDS patients had increased C3 activation (2.68 +/- 1.67%) when compared with normal controls (0.9 +/- 0.22%) (P less than 0.01). The decreased E-CR1, the presence of CIC, and C3 activation suggest that complement activation by immune complexes may play a role in the clinical expression of the disease.

Blockade of clearance of immune complexes by an anti-Fc gamma receptor monoclonal antibody

Clearance of immune complexes by the mononuclear phagocyte system is important for maintaining normal host defenses against bacterial and viral assault (1), but also contributes to the pathogenesis of a variety of immune- mediated diseases . For example, removal from the circulation of IgG-coated erythrocytes and platelets by the MPS is the sine qua non of immune-mediated cytopenias (2, 3). On the other hand, abnormally decreased removal by the MPS of smaller, soluble immune complexes may play a role in the pathogenesis of immune complex-mediated tissue damage found in such autoimmune diseases as SLE (4). Although the physicochemical nature and the size of immune complexes can influence rates of clearance and sites of deposition (reviewed in 5), interactions between immune complexes and the MPS in vivo are poorly understood. The inability to directly measure binding or internalization of immune complexes by cells in the liver and spleen has made the analysis of the molecular basis of immune complex clearance very difficult . Receptors for the Fc portion of IgG (FcgammaR) and for complement (CR) undoubtedly play a role in the removal of immune complexes, but the relative importance of these receptors is not known.

Complement in the pathophysiology and diagnosis of human diseases

Complement is a humoral effector system composed of 21 plasma proteins that was identified initially because of its cytolytic effects. In addition to cytolysis, complement has a number of different functions related to inflammatory and other host defense processes. The description of the reaction mechanism includes: (1) activation of the classical pathway through recognition of IgG and IgM antibodies by C1q, (2) activation of the alternative pathway which is usually achieved without participation of immunoglobulins, (3) generation of proteolytic enzymes composed of heteropolymers that cleave certain precursor proteins, (4) formation of the membrane attack complex (MAC), and (5) participation of control mechanisms. Methodologies for studying protein concentration and functional activities of complement components include not only the classical hemolytic techniques but also the extremely sensitive new radioimmunoassays and enzyme immunoassays for measuring the products of complement activation that are generated in vivo. Examples of genetically controlled complement deficiencies have been published for most complement components. The symptomatology of some of these patients serves to emphasize the protective role of complement. Acquired deficiencies are significant not only as laboratory aids in diagnosis and to evaluate the course of certain diseases, but also to indicate possible pathogenic disease mechanisms. Recently, it has been recognized that the complement proteins with genes located in the HLA region are polymorphic. Certain variants of proteins C2, C4, and factor B occur with higher frequencies in certain diseases than in the general population, which appears to be of great practical importance in laboratory medicine.

Inhibition of complement dependent experimental inflammation in human skin by different heparin fractions

The anticoagulant activity of heparin is dependent on its affinity for antithrombin III (AT III) and on its molecular weight. In contrast, heparin fractions differing in these respects are equally effective inhibitors of the human complement system in vitro. In this study we designed and evaluated a model to investigate the effects of different heparin fractions on a complement dependent inflammation. Locally administered heparin, in a dose-dependent manner, inhibited the flare, itch and wheal responses induced by intradermal injection of heat-aggregated human IgG (HAGG). These reactions were also inhibited by the antihistamine mepyramine, favouring the view that HAGG activates complement and that the observed inflammatory response is mediated by anaphylatoxin liberation of histamine. Similar cutaneous reactions induced by trypsin, which can generate C3a and C5a by proteolysis of C3 and C5, the histamine liberator compound 48/80 or histamine were inhibited by mepyramine but not by heparin. Thus it is strongly suggested that heparin inhibits the HAGG induced reactions by modulating the early pre-C3 steps of complement activation. On a weight basis heparin fractions differing in AT III-affinity or in average molecular weight (5,000 and 16,000 D) were equally potent modulators of the HAGG-induced inflammation. We conclude that heparin can inhibit an apparently complement-dependent inflammation irrespective of its AT III-affinity or of its size, and suggest that a heparin with low anticoagulant activity could be of value as a modulator of inflammation and should be useful in investigating the consequences of complement inhibition in inflammation.

Differential binding of immunoglobulin A and immunoglobulin G1 immune complexes to primate erythrocytes in vivo. Immunoglobulin A immune complexes bind less well to erythrocytes and are preferentially deposited in glomeruli

Primate erythrocytes appear to play a role in the clearance of potentially pathogenic immune complexes (IC) from the circulation. This study was undertaken to compare the clearance from the circulation and tissue uptake of two monoclonal IC probes: one of which, IgG1-IC, was bound well by erythrocytes, the other of which, IgA-IC, was bound relatively poorly by erythrocytes. The IC probes were labeled with different iodine isotopes and infused either concomitantly or sequentially into the arterial circulation. The results indicate that, compared with IgG1-IC, IgA-IC bind less well to primate erythrocytes, are cleared from the circulation more quickly despite their smaller size, and show increased uptake in kidney and lung but decreased uptake in liver and spleen. Evidence is presented which suggests that this pattern of clearance from the circulation and systemic uptake of IgA-IC is the result of decreased binding of IgA-IC to circulating erythrocytes. These findings support the hypothesis that the primate erythrocyte-IC clearing mechanism may be critically important for the safe removal of IC from the circulation.

Formation of soluble immune complexes by complement in sera of patients with various hypocomplementemic states. Difference between inhibition of immune precipitation and solubilization

To examine whether the ability of complement to form soluble immune complexes plays a role in preventing immune complex-mediated diseases, we analyzed the capacity of complement to inhibit immune precipitation (IIP) and to solubilize preformed immune aggregates (SOL) in 23 sera of patients with various hypocomplementemic states, and we correlated the results of these studies with the clinical syndromes found in the various patients. In sera with deficiency or depletion of early classical pathway components, IIP was profoundly altered, whereas SOL was delayed but in the normal range. In contrast, in sera with C3 depletion but intact classical pathway and in properdin-deficient serum, IIP was initially preserved, whereas SOL was abolished. Since the incidence of immune complex diseases in various hypocomplementemic states correlates with the severity of IIP defects, but not with reduced SOL, it is suggested that IIP is an essential biological function of complement that prevents the rapid formation of insoluble immune complexes in vivo.

Leprosy: altered complement receptors in disseminated disease

We have studied the expression of the C3b receptor (CR1) on erythrocytes of 55 patients with Hansen's disease. We developed a radioimmunoassay utilizing a monoclonal antibody that recognized an epitope different from the C3b binding site, which therefore enabled us to measure total number of CR1 regardless of receptor occupancy. We observed that patients in the lepromatous pole of the disease had a mean of 310 CR1/erythrocyte, whereas the ones in the tuberculoid pole showed a mean of 577 CR1/erythrocyte; 77 normal controls had a mean of 512 CR1/erythrocyte. The number of C3b receptors on the cells of lepromatous patients was significantly decreased (p less than .001) when compared to the normal population or tuberculoid patients. The presence of receptors for the C3b fragment of complement (CR1) on the surface of human erythrocytes enables these cells to participate in a number of immune functions including the clearance of circulating immune complexes. These findings could bear importance in the ability of the host to clear immune complexes from the circulation in patients with lepromatous leprosy.