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

Minimal role for the alternative pathway in complement activation by HIT immune complexes

BACKGROUND

Anti-platelet factor 4 (PF4)/heparin immune complexes that cause heparin-induced thrombocytopenia (HIT) activate complement via the classical pathway. Previous studies have shown that the alternative pathway of complement substantially amplifies the classical pathway of complement activation through the C3b feedback cycle.

OBJECTIVES

These studies sought to examine the contributions of the alternative pathway to complement activation by HIT antibodies.

METHODS

Using IgG monoclonal (KKO) and/or patient-derived HIT antibodies, we compared the effects of classical pathway (BBK32 and C1-esterase inhibitor [C1-INH]), alternative pathway (anti-factor B [fB] or factor D [fD] inhibitor) or combined classical and alternative pathway inhibition (soluble complement receptor 1 [sCR1]) in whole blood or plasma.

RESULTS

Classical pathway inhibitors BBK32 and C1-INH and the combined classical/alternative pathway inhibitor sCR1 prevented KKO/HIT immune complex-induced complement activation, including release of C3 and C5 activation products, binding of immune complexes to B cells, and neutrophil activation. The alternative pathway inhibitors fB and fD, however, did not affect complement activation by KKO/HIT immune complexes. Similarly, alternative pathway inhibition had no effect on complement activation by unrelated immune complexes consisting of anti-dinitrophenyl (DNP) antibody and the multivalent DNP--keyhole limpet hemocyanin antigen.

CONCLUSIONS

Collectively, these findings suggest the alternative pathway contributes little in support of complement activation by HIT immune complexes. Additional in vitro and in vivo studies are required to examine if this property is shared by most IgG-containing immune complexes or if predominance of the classic pathway is limited to immune complexes composed of multivalent antigens.

Current and Emerging Therapies for Lupus Nephritis

The introduction of corticosteroids and later, cyclophosphamide dramatically improved survival in patients with proliferative lupus nephritis, and combined administration of these agents became the standard-of-care treatment for this disease. However, treatment failures were still common and the rate of progression to ESRD remained unacceptably high. Additionally, treatment was associated with significant morbidity. Therefore, as patient survival improved, the goals for advancing lupus nephritis treatment shifted to identifying therapies that could improve long-term renal outcomes and minimize treatment-related toxicity. Unfortunately, progress has been slow and the current approaches to the management of lupus nephritis continue to rely on high-dose corticosteroids plus a broad-spectrum immunosuppressive agent. Over the past decade, an improved understanding of lupus nephritis pathogenesis fueled several clinical trials of novel drugs, but none have been found to be superior to the combination of a cytotoxic agent and corticosteroids. Despite these trial failures, efforts to translate mechanistic advances into new treatment approaches continue. In this review, we discuss current therapeutic strategies for lupus nephritis, briefly review recent advances in understanding the pathogenesis of this disease, and describe emerging approaches developed on the basis of these advances that promise to improve upon the standard-of-care lupus nephritis treatments.

The paradoxical roles of C1q and C3 in autoimmunity

In this review we will focus on the links between complement and autoimmune diseases and will highlight how animal models have provided insights into the manner by which C1q and C3 act to modulate both adaptive and innate immune responses. In particular we will highlight how C1q may not only act as initiator of the classical complement pathway, but can also mediate multiple immune responses in a complement activation independent manner.

The value of repeat kidney biopsy in quiescent Argentinian lupus nephritis patients

BACKGROUND

The duration of maintenance therapy after induction therapy for lupus nephritis has not been rigorously established. A common practice is to maintain immunosuppression for 1-2 years after complete remission, and longer for partial remission. The present work addresses whether a repeat kidney biopsy might be informative in deciding who should continue immunosuppression after complete or partial remission.

METHODS

The practice in a large Buenos Aires nephrology unit is to repeat a kidney biopsy before finalizing the decision to withdraw or continue immunosuppression. This work reports on a cohort of 25 Hispanic patients that had two or more kidney biopsies, the last occurring after at least 24 months of clinically quiescent disease.

RESULTS

Despite normalization of serum creatinine and reduction of proteinuria to <500 mg/d, 30% of patients still had significant activity at the last biopsy. Conversely, 60% of patients with ongoing proteinuria (500-1000 mg/d), or stable but abnormal serum creatinine, had no activity by biopsy. Univariate association analyses demonstrated that improvement in the activity index (AI) of the last biopsy was associated with choice of induction therapy (cyclophosphamide or mycophenolate), improvement in serum creatinine over the first six months of treatment, and improvement in complement component C4. By multivariate regression analyses, two AI prediction models emerged. Cyclophosphamide plus change in serum creatinine or cyclophosphamide plus change in C4 accounted for 50% of the improvement in AI.

CONCLUSION

These data suggest that a repeat biopsy may be useful in making the decision to withdraw or continue maintenance immunosuppression.

Linking complement and anti-dsDNA antibodies in the pathogenesis of systemic lupus erythematosus

Systemic lupus erythematosus is a severe autoimmune disease that affects multiple organ systems resulting in diverse symptoms and outcomes. It is characterized by antibody production to a variety of self-antigens, but it is specifically associated with those against anti-dsDNA. Anti-dsDNA antibodies are present before the onset of clinical disease and are associated with severe manifestations of lupus such as glomerulonephritis. Their levels fluctuate with changes in disease activity and, in combination with the levels of complement proteins C3 and C4, are strong indicators of disease flare and treatment response in patients with lupus. The decreased complement levels that are noted during flares of lupus activity are believed to be secondary to increased autoantibody production and immune complex formation that results in tissue damage; however, recent data suggest that complement activation can also drive development of these pathogenic autoantibodies. This review will explore the various roles of complement in the development and pathogenesis of anti-dsDNA antibodies.

C1q deficiency leads to the defective suppression of IFN-alpha in response to nucleoprotein containing immune complexes

Almost all humans with homozygous deficiency of C1q develop systemic lupus erythematosus (SLE). The precise cellular mechanism(s) by which C1q prevents the development of SLE remains unclear. In this study, we tested the role of C1q in the regulation of IFN-α induced by immune complexes (ICs) in vitro, as well as the consequences of lack of C1q in vivo. Our experiments revealed that C1q preferentially promotes the binding of SLE ICs to monocytes rather than plasmacytoid dendritic cells, but this inhibition was not due to the induction of inhibitory soluble factors. The presence of C1q also altered the trafficking of ICs within monocytes such that ICs persisted in early endosomes. In patients with C1q deficiency, serum and cerebrospinal fluid levels of IFN-α and IFN-γ-inducible protein-10 levels were elevated and strongly correlated with Ro autoantibodies, demonstrating the clinical significance of these observations. These studies therefore associate C1q deficiency with defective regulation of IFN-α and provide a better understanding of the cellular mechanisms by which C1q prevents the development of IC-stimulated autoimmunity.

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.

Clinical aspects and molecular basis of primary deficiencies of complement component C3 and its regulatory proteins factor I and factor H

The complement system participates in both innate and acquired immune responses. Deficiencies in any of the protein components of this system are generally uncommon and require specialized services for diagnosis. Consequently, complement deficiencies are clinically underscored and may be more common than is normally estimated. As C3 is the major complement component and participates in all three pathways of activation, it is fundamental to understand all the clinical consequences observed in patients for which this protein is below normal concentration or absent in the serum. C3 deficiencies are generally associated with higher susceptibility to severe infections and in some cases with autoimmune diseases such as systemic lupus erythematosus. Here, we review the main clinical aspects and the molecular basis of primary C3 deficiency as well as the mutations in the regulatory proteins factor I and factor H that result in secondary C3 deficiencies. We also discuss the use of animal models to study these deficiencies.

The Role of Complement in Cryoglobulin-Induced Immune Complex Glomerulonephritis

Many forms of glomerulonephritis are triggered by Ab localization in the glomerulus, but the mechanisms by which this induces glomerular inflammation are not fully understood. In this study we investigated the role of complement in a mouse model of cryoglobulin-induced immune complex glomerulonephritis. Several complement-deficient mice on a C57BL/6 and BALB/c genetic background were used and compared with strain-matched, wild-type controls. Cryoglobulinemia was induced by i.p. injection of 6-19 hybridoma cells producing an IgG3 cryoglobulin with rheumatoid factor activity against IgG2a of allotype a present in BALB/c, but not C57BL/6, mice. Thus, the cryoprecipitate in C57BL/6 mice consisted of the IgG3 cryoglobulin only (type I cryoglobulinemia) compared with IgG3-IgG2a complexes in BALB/c (type II cryoglobulinemia). The survival of mice was not affected by complement deficiency. Glomerular influx of neutrophils was significantly less in C3-, factor B-, and C5-deficient mice compared with wild-type and C1q-deficient mice. It did not correlate with C3 deposition, but did correlate with the amount of C6 deposited. Deficiency of CD59a, the membrane inhibitor of the membrane attack complex, did not induce an increase in neutrophil infiltration, suggesting that the generation of C5a accounts for the effects observed. There was no apparent difference between cryoglobulinemia types I and II regarding the role of complement. Our results suggest that in this model of cryoglobulin-induced glomerulonephritis the neutrophil influx was mediated by C5 activation with the alternative pathway playing a prominent role in its cleavage. Thus, blocking C5 is a potential therapeutic strategy for preventing renal injury in cryoglobulinemia.

The role of complement in the development of systemic lupus erythematosus

Complement has both beneficial and deleterious roles in the pathogenesis of systemic lupus erythematosus (SLE). On the one hand, patients with SLE present with decreased complement levels and with complement deposition in inflamed tissues, suggestive of a harmful role of complement in the effector phase of disease. On the other hand, homozygous deficiency of any of the classical pathway proteins is strongly associated with the development of SLE. There are two main hypotheses to explain these observations. The first invokes an important role for complement in the physiological waste-disposal mechanisms of dying cells and immune complexes. The second hypothesis is based around the role of complement in determining the activation thresholds of B and T lymphocytes, with the proposal that complement deficiency causes incomplete maintenance of peripheral tolerance. These two hypotheses are not mutually exclusive. In addition, there is evidence for a contribution from other genetic factors in determining the phenotype of disease in the absence of complement.

A small domain (6.5 kDa) of bacterial protein G inhibits C3 covalent binding to the Fc region of IgG immune complexes

Attachment of the complement component C3 to antigen-antibody (Ag-Ab) complexes (immune complexes, IC) is the key molecular event responsible for the elimination of many Ag in the form of Ag-Ab-C3b. The CH1 domain and the Fc region of the Ab, which have previously been involved in the binding of C3b, are also the targets of several bacterial IgG-binding proteins, particularly proteins G and A. Here we describe the ability of a small recombinant protein G domain (B2; 6.5 kDa) to inhibit the covalent binding of C3b to the Fc portion of IgG without affecting the binding to the Fab part. Protein G (B2 domain) produced a remarkable inhibition of covalent binding of C3b to IC formed with rabbit IgG, but none with the F(ab')2 fragment, indicating that B2 interferes with the C3b binding to the Fc region. A weak inhibition was observed with IC formed with mouse IgG2b which preferentially binds B2 domain on the CH1 domain of the Fab. To confirm these data, recombinant single-chain Ab devoid of CH1 domains (scAb), and including the rabbit or human Fc portion (hinge-CH2-CH3), were produced and used to form IC. Protein G-B2 domain inhibited C3b binding to IC formed with scAb of either human or rabbit constant regions, supporting the view of a specific blockade of C3b binding to the Fc region. A similar inhibition of C3b binding was observed using protein A instead of protein G B2 domain and the same set of IC. On the CH1 domain, C3b and B2 bind on opposite faces, and therefore do not interfere with each other in their binding. However, B2 domain bound to the inter-CH2-CH3 region impedes the C3b binding to the Fc. This inhibition clarifies the specificity of C3b for the different regions of IgG and explains how bacterial IgG-binding proteins provide the bacteria with a mechanism of evasion from the opsonizing action of complement and contribute to the virulence. This could be a general mechanism of escape because protein G binds the majority of mammalian Ig.

Binding of properdin to solid-phase immune complexes: critical role of the classical activation pathway of complement

The capacity of serum to support deposition of C3, properdin and factor B was studied by enzyme-linked immunosorbent assay using solid-phase immune complexes (IC) for activation of complement. Deposition of C3 and properdin occurred in fairly dilute normal human serum (NHS), but factor B uptake was hardly detectable. Alternative pathway-mediated deposition of C3 with slow kinetics was demonstrated in C2-deficient serum and in NHS depleted of C1q, factor D and properdin (C1qDP-depleted serum) after reconstitution with factor D and properdin. Efficient uptake of properdin required a functional classical pathway, in the presence of which C3 and properdin were rapidly deposited onto the IC. Judging from findings in C3-deficient serum, factor I-deficient serum, and C1qDPB-depleted serum, the uptake of properdin was strictly C3-dependent, and did not require the presence of factors B and D. Thus, C3b fixed to IC was the principal ligand for properdin in the assay. The findings could have biological implications relating to complement-mediated modification of immune complexes in disease.

Immune complex glomerulonephritis in C4- and C3-deficient mice

In this study, we examined the roles of C4 and C3 in immune complex glomerulonephritis by actively immunizing C4-deficient (C4 -/-), C3 deficient (C3 -/-) and wild-type mice with apoferritin. Wild-type animals with an intact complement system produced anti-apoferritin IgG and IgM antibodies, and developed mesangial proliferative glomerulonephritis characterized by hypercellularity, matrix expansion, deposition of IgG, IgM, IgA and C3, and the presence of electron dense deposits. In the majority of animals, the peripheral capillaries also contained IgG, C3 and subendothelial and subepithelial electron dense deposits. In contrast to wild-type animals, all apoferritin-immunized C4 -/- and C3 -/- mice had serum cryoprecipitates containing polyclonal IgM and the variable presence of polyclonal IgG. These animals also developed immune complex glomerulonephritis, but their disease manifestations were distinctly different from that of their wild-type littermates. In apoferritin-immunized C4 -/- and C3 -/- mice, IgG was either absent or present in reduced quantities in glomeruli, yet IgM and IgA were present in greater intensity in glomeruli. Capillary wall IgG deposits were absent in all C4 -/- and C3 -/- animals. C4 -/- animals also had significant glomerular C3 deposition, hypercellularity and neutrophil infiltration, which were not present in C3 -/- animals. These results illustrate the complex interplay between the effects of complement to process immune complexes and to lead to inflammation and tissue injury.

IgA nephropathy--human disease and animal model

IgA nephropathy is one of the most common chronic glomerulonephritides worldwide. Since the first publication on IgA nephropathy, a number of clinical and pathological investigations have revealed that the clinical course of patients with IgA nephropathy is extremely diverse, with approximately 10-20% of the patients developing end-stage chronic renal failure. Glomerular changes similar to IgA nephropathy have also been observed in patients with Schoenlein-Henoch purpura, and with other diseases such as liver cirrhosis and chronic inflammatory diseases of the lung. The broad spectrum of clinical and pathological features of IgA nephropathy encompasses a syndrome which includes both primary and secondary IgA nephropathy. The common etiology and pathogenesis of primary and secondary IgA nephropathy appear to be closely related to immunological abnormalities in the production of IgA induced by antigenic stimulation of the common mucosal immune system. IgA is one of the most important humoral factors of the mucosal immune defense system and functions as an antibody against various extrinsic and intrinsic substances. This review describes the Arthus type of IgA immune complex deposition in the glomeruli which can result from persistent or repeated increases in circulating IgA immune complexes. The latter occurs as a consequence of overproduction of IgA antibodies and/or impairment in clearance of IgA immune complexes by the mononuclear phagocytic system. The present review also focuses on the biology of the IgA-mediated immune system and on the etiology, pathogenesis, and animal models of IgA nephropathy.

Reciprocal changes in complement activity and immune-complex levels during plasma infusion in a C2-deficient SLE patient

Although systemic lupus erythematosus (SLE) is abnormally common in individuals with complement deficiency, conclusive evidence has been lacking for a direct causal relationship between disease manifestations and a missing complement component. A patient with C2 deficiency and SLE has been treated with 56 courses of fresh frozen plasma (FFP) infusions over a period of 8 years. Each infusion, involving a total of 12 units of FFP administered in equal doses over 4 consecutive days, has consistently resulted in a transient restoration of the classical pathway of complement, and a full clinical remission lasting 6-8 weeks. This report is concerned with changes in the levels of immune complexes, C2 and C3d during an infusion cycle. Four progressively rising peaks in C2 and C3d were observed during the 4 days of the plasma infusion, and these peaks coincided with four reciprocally descending troughs in the levels of immune complexes. Identical fluctuations have been consistent in all the plasma-infusion cycles that have so far been monitored, and their consistent association with clinical remissions indicates a causal relationship between the C2 restoration and clinical remissions in this C2-deficient SLE patient.

Normal human serum depleted of C1q, factor D and properdin: its use in studies of complement activation

Normal human sera were depleted of C1q, factor D (D) and properdin (P) by a simple and reproducible procedure providing reagents for analysis of complement-dependent functions. Classical pathway activity was restored with purified C1q, and alternative pathway activity with purified D and P. Since both activation pathways were abolished, antibodies and other components could be removed without loss of complement activity during immunoabsorption procedures. Synergism between the two pathways during haemolysis of rabbit erythrocytes was clearly demonstrated, and was also found on analysis of C3 cleavage in serum incubated with other alternative pathway activators such as zymosan and inulin. Experiments with a Neisseria meningitidis serogroup W-135 strain isolated from a patient with inherited P deficiency showed that both pathways were capable of supporting antibody-dependent killing of the bacteria in serum. The alternative pathway was possibly more efficient than the classical pathway in the assay system. In C1q,D,P-depleted serum with high concentrations of anticapsular IgG antibodies, the addition of D alone resulted in efficient alternative pathway-mediated killing. The alternative pathway was equally efficient in a C1q,D,P-depleted serum with low concentrations of anticapsular antibody, but in this case the reaction required both D and P.

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.

Inherited complement deficiency states: implications for immunity and immunological disease

The study of complement deficiency states and their influence on immune function has generated new insights and still provides a challenge to continued investigation. The association of classical pathway deficiencies (C1, C4, C2 or C3) with immunological diseases such as SLE and glomerulonephritis has contributed to current knowledge concerning complement-dependent immune complex handling and elimination. Susceptibility to systemic infection with encapsulated bacteria is encountered in most forms of inherited complement deficiency. Recurrent neisserial infection is the only clinical manifestation clearly associated with defects of the membranolytic sequence C5-C9, while deficiency of properdin, a component of the alternative activation pathway, appears to predispose to nonrecurrent meningococcal disease. Inherited complement deficiency is rare, but the perspective is widened by the more common occurence of acquired defects in immunological diseases, and the apparent requirement for efficient complement recruitment in host defense. Another aspect is the possibility that complement deficiency might alleviate or prevent inflammatory symptoms. Notably, complement deficiency has not been reported in classical rheumatoid arthritis. Considerations of this kind would be refuted or modified by findings of complement deficiency in single patients.

Substitution of a single amino acid (aspartic acid for histidine) converts the functional activity of human complement C4B to C4A

The C4B isotype of the fourth component of human complement (C4) displays 3- to 4-fold greater hemolytic activity than does its other isotype C4A. This correlates with differences in their covalent binding efficiencies to erythrocytes coated with antibody and complement C1. C4A binds to a greater extent when C1 is on IgG immune aggregates. The differences in covalent binding properties correlate only with amino acid changes between residues 1101 and 1106 (pro-C4 numbering)--namely, Pro-1101, Cys-1102, Leu-1105, and Asp-1106 in C4A and Leu-1101, Ser-1102, Ile-1105, and His-1106 in C4B, which are located in the C4d region of the alpha chain. To more precisely identify the residues that are important for the functional differences, C4A-C4B hybrid proteins were constructed by using recombinant DNA techniques. Comparison of these by hemolytic assay and binding to IgG aggregates showed that the single substitution of aspartic acid for histidine at position 1106 largely accounted for the change in functional activity and nature of the chemical bond formed (ester vs. amide). Surprisingly, substitution of a neutral residue, alanine, for histidine at position 1106 resulted in an increase in binding to immune aggregates without subsequent reduction in the hemolytic activity. This result strongly suggests that position 1106 is not "catalytic" as previously proposed but interacts sterically/electrostatically with potential acceptor sites and serves to "select" binding sites on potential acceptor molecules.

Combined complete C5 and partial C4 deficiency in humans: clinical consequences and complement-mediated functions in vitro

A family is described with two siblings who suffered at different times from a single episode of meningococcal meningitis by Neisseria meningitidis groups B and C, respectively. In the two subjects, hemolytically active fifth component of complement (C5) was not detectable and antigenic C5 was less than 0.05% and less than 0.7% of normal, respectively. Repletion of sera by purified human C5 (70 micrograms/ml) restored total complement hemolytic activities. The asymptomatic first degree family members had C5 levels compatible with a heterozygous state of C5 deficiency. C4 allotyping revealed an inherited partial deficiency (Q0) of C4A and C4B in the family with a combined C4AQ0 and C4BQ0 heterozygous condition in one and C4BQ0 heterozygosity in the other C5 deficient (C5D) subject. To our knowledge, this is the first human kindred with recognized combined C5 and C4 deficiency. No other defect of the humoral and cellular immune system was found in this family, including specific immune response to tetravalent meningococcal vaccine. The effect of partial C4 deficiency on classical pathway function was assessed by inhibition of immune precipitation (IIP) of forming bovine serum albumin (BSA)/anti-BSA immune complexes. Sera from all family members showed normal IIP values, with exception of the subject with combined partial deficiency in C4A, C4B, and complete deficiency in C5. Despite undetectable functional C5 in the C5D sera, the titration of the alternative pathway indicated intact but deficient hemolytic activities when rabbit erythrocytes (EC) were used as indicator cells in the presence of Mg2+ and EGTA in an end-point or kinetic assay. Preincubation of the two sera at 0 degrees C for 60 min with rabbit ECs reduced alternative pathway hemolytic activity by 24 and 100%, respectively. When rabbit ECs were replaced by guinea pig ECs no alternative pathway function could be measured. The results indicate that the apparent functional activity of the alternative pathway in C5D sera strongly depends on a factor(s) present in such serum and/or on the detection system used. We conclude that the two C5D individuals of the family reported here may not have sufficient C5 activity to provide efficient protection against Neisserial infections in conditions where complement functions beyond C3 opsonic activity are required in vivo.

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.

Familial properdin deficiency associated with chronic discoid lupus erythematosus

A large family comprised of 18 members is described. Four male members are properdin-deficient, all are healthy bar the index patient who presented with chronic discoid lupus erythematosus. Serum from properdin-deficient males had a reduced ability to lyse rabbit erythrocytes via the alternative pathway or solubilize pre-formed immune complexes. Addition of purified properdin restored these activities. Classical pathway activity was normal. Definite, probable and possible female carriers had normal classical and alternative pathway activities.

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.

Immune response to tetravalent meningococcal vaccine: opsonic and bactericidal functions of normal and properdin deficient sera

Neisseria meningitidis serogroup W-135 appears to be a fairly common cause of infection associated with properdin deficiency or dysfunction, and anticapsular antibodies might be protective in these patients. For this reason, bactericidal and opsonophagocytic activities for serogroup W-135 were investigated before and four weeks after vaccination of two properdin-deficient adults with tetravalent meningococcal vaccine. In addition, the response of IgM, IgG and IgA class antibodies to the serogroups A, C, Y and W-135 was determined by ELISA. There was no evidence of poor antibody responses in the properdin-deficient persons. Vaccination promoted classical pathway-mediated killing in serum and opsonization of serogroup W-135 to the same extent as that seen in vaccinated controls. The increase of alternative pathway-mediated killing in the properdin-deficient sera was moderate, but vaccination clearly enhanced alternative pathway-mediated opsonophagocytosis in the sera. It was also shown that vaccination markedly reduced the requirement for properdin in alternative pathway-mediated killing of the meningococci.

Influence of factor D concentrations on fluid phase C3 activation, lysis of rabbit erythrocytes and solubilization of immune complexes

Factor D depleted serum did not support hemolysis of rabbit erythrocytes and solubilization of performed immune complexes. Fluid phase C3 cleavage increased in a dose dependent manner when D protein was added to normal or to factor D depleted serum. During short incubation factor D concentrations were correlated with the capacity of serum to solubilize immune complexes and to lyse rabbit erythrocytes. With prolonged incubation, the hemolytic activity decreased in a factor D dose dependent manner. This was probably due to fluid phase breakdown of C3 and factor B in the presence of high factor D concentrations. Hypocomplementemic sera from patients with active systemic lupus erythematosus (SLE) did not support solubilization of bovine serum albumin (BSA) anti-BSA complexes when factor D was added in excess. Patients with polycystic kidney disease with reduced renal function and high factor D concentrations showed increased concentrations of circulating C3d/dg fragments. The possibility was considered that high factor D concentrations in uremia might promote fluid phase C3 degradation and thereby limit the in vivo efficiency of alternative pathway activation on target surfaces.

Immune complex assays: diagnostic and clinical application

The presence of circulating immune complexes have been described in many different human disease states but the significance of their presence has always been a subject for debate. Improvements in the methods of detecting immune complexes have demonstrated a wide degree of heterogeneity, which accounts for the difficulty in obtaining accurate and reproducible measurements, even in the same individual. Techniques for isolating individual complexes, characterizing their pathophysiological properties, and biochemically analyzing the nature of the complexed antigen are now being used to provide data that is helping to clarify the role of immune complexes in the pathogenesis of disease. In addition, such studies are also providing data which is proving that immune complexes have a potential role in immune regulation.

Complement mediated inhibition of immune precipitation in rheumatoid arthritis: studies on interaction of heat aggregated IgG with IgM rheumatoid factor

Serum samples from patients with seropositive rheumatoid arthritis contain an inhibitor of complement mediated inhibition of immune precipitation (CMIP). This inhibitory effect can be produced by the addition of either purified monoclonal or polyclonal IgM rheumatoid factor (RF) to human serum. The specificity of the rheumatoid factor influences the degree of inhibition, and when precipitation occurs the rheumatoid factor coprecipitates with the antigen-antibody complex. In rheumatoid sera there was a significant positive correlation between IgM RF concentration and inhibitory activity, though the range of inhibitory activity seen for the same concentration of rheumatoid factor was considerable. Small quantities of heat aggregated IgG (HAGG) had a much greater effect on the measurement in an enzyme linked immunosorbent assay (ELISA) of IgM RF than they did on the inhibitory activity of IgM RF in the CMIP assay. Larger quantities of HAGG initiated complement activation and increased the precipitation of immune complexes. IgM RF reduced the complement activating properties of HAGG by reducing the amount of Clq which bound to the aggregate. The mechanisms by which IgM RF overcomes CMIP in rheumatoid sera may involve its inhibitory effects on the binding of Cl to the antigen-antibody complex.

Solubilization of immune precipitates by complement in the absence of properdin or factor D

Various experiments have demonstrated that immune precipitates (IPs) are not solubilized by complement in the absence of alternative pathway function. To determine whether the characteristics of the IPs were responsible for these observations, we studied the solubilization (Sol) of IPs formed by bovine serum albumin (BSA)-rabbit antiBSA and tetanus toxoid (TT)-human antiTT. Sera deficient in properdin solubilized a fraction of BSA-antiBSA precipitates, although only when the IPs were formed in antibody excess. The same sera solubilized TT-antiTT precipitates with some delay but almost as efficiently as normal serum. Factor D-depleted serum solubilized a fraction of TT-antiTT precipitates too, indicating that Sol may proceed through activation of the classical pathway only. Thus, the requirements for complement-mediated Sol depend on the characteristics of the IPs and do not necessarily include alternative pathway function.

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.

Fast liver catabolism of C1q in patients with paraproteinaemia and depletion of the classical pathway of complement

The main clinical features in four patients with IgG1k paraproteinaemia and acquired complement deficiency included xanthomatous skin lesions (in three), panniculitis (in three) and hepatitis (in two). Hypocomplementaemia concerned the early classical pathway components--in particular C1q. Metabolic studies employing 125I-C1q revealed a much faster catabolism of this protein in the four patients than in five normal controls and three patients with cryoglobulinaemia (mean fractional catabolic rates respectively: 23.35%/h; 1.44%/h; 5.84%/h). Various experiments were designed to characterize the mechanism of the hypocomplementaemia: the patients' serum, purified paraprotein, blood cells, bone marrow cells, or xanthomatous skin lesions did not produce significant complement activation or C1q binding. When three of the patients (two with panniculitis and hepatitis) were injected with 123I-C1q, sequential gamma-camera imaging demonstrated rapid accumulation of the radionuclide in the liver, suggesting that complement activation takes place in the liver where it could produce damage.

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.

Impaired humoral immune response in complement C3-deficient guinea pigs: absence of secondary antibody response

A recently described genetically controlled C3 deficiency (C3D) in guinea pigs (GP) provided a unique model for studying the role of C3 in the afferent limb of the humoral immune response in a direct manner. These C3D animals, which have only 5-7% of normal serum C3 level, were immunized with the bacteriophage phi chi 174, a T cell-dependent antigen, followed by a booster injection after 4 weeks (1.5 X 10(9) plaque-forming units/kg). The formation of IgM and IgG antibody in the course of the primary and secondary response was determined and compared with a control group of inbred strain 2 GP. The C3D animals showed a markedly diminished antibody response to this antigen. Amplification of the antibody titer as well as regular isotype switching from IgM to IgG was absent in the secondary response. Increasing the amount of antigen to a high dose (1 X 10(10) plaque-forming units/kg) led to a normalization of the antibody response. The impairment in antibody formation resembles closely the impaired antibody response in C4-deficient or C2-deficient GP, which both have a block in activation of C3 via the classical pathway. However, in contrast to C4D GP or C2D GP the C3D GP do not exhibit serological characteristics of immune complex disease. They have normal levels of total serum IgM, of IgM anti-2,4-dinitrophenyl antibodies and of IgM rheumatoid factors.

Complement mediated inhibition of immune precipitation and solubilization generate different concentrations of complement anaphylatoxins (C4a, C3a, C5a)

Complement prevents the formation of insoluble immune complexes (inhibition of immune precipitation (IIP], and solubilizes preformed immune aggregates (solubilization (SOL]. Since the mechanism of complement activation differs in these two reactions, it is possible that they differ also in the amount of complement fragments released, in particular the anaphylatoxins C3a, C5a and C4a. We measured C4 and C3 consumption, and the formation of complement anaphylatoxins during IIP and SOL using two different immune complex models (BSA, rabbit anti-BSA; tetanus toxoid (TT), human anti-TT). At equal immune complex concentrations in both models, SOL was more efficient than IIP at cleaving C3, and more C3a and C5a was released. Comparing the two reactions, C3a formation was followed by more C5 cleavage (C5a) during SOL. Similarly C4a formation (classical pathway activation) was followed by more C3 cleavage (C3a: classical and alternative pathway activations), during SOL. It is suggested that in vivo SOL of insoluble complexes is rapidly accompanied by a damaging phlogistic reaction, whereas IIP produces less inflammation.