Lupus diseases associated with hereditary and acquired deficiencies of complement

Myasthenia gravis complement activity is independent of autoantibody titer and disease severity

Acetylcholine receptor (AChR) autoantibodies, found in patients with autoimmune myasthenia gravis (MG), can directly contribute to disease pathology through activation of the classical complement pathway. Activation of the complement pathway in autoimmune diseases can lead to a secondary complement deficiency resulting in reduced complement activity, due to consumption, during episodes of disease activity. It is not clear whether complement activity in MG patients associates with measurements of disease activity or the titer of circulating pathogenic AChR autoantibodies. To explore such associations, as a means to identify a candidate biomarker, we measured complement activity in AChR MG samples (N = 51) using a CH50 hemolysis assay, then tested associations between these values and both clinical status and AChR autoantibody titer. The majority of the study subjects (88.2%) had complement activity within the range defined by healthy controls, while six patients (11.8%) showed reduced activity. No significant association between complement activity and disease status or AChR autoantibody titer was observed.

Early Components of the Complement Classical Activation Pathway in Human Systemic Autoimmune Diseases

The complement system consists of effector proteins, regulators, and receptors that participate in host defense against pathogens. Activation of the complement system, via the classical pathway (CP), has long been recognized in immune complex-mediated tissue injury, most notably systemic lupus erythematosus (SLE). Paradoxically, a complete deficiency of an early component of the CP, as evidenced by homozygous genetic deficiencies reported in human, are strongly associated with the risk of developing SLE or a lupus-like disease. Similarly, isotype deficiency attributable to a gene copy-number (GCN) variation and/or the presence of autoantibodies directed against a CP component or a regulatory protein that result in an acquired deficiency are relatively common in SLE patients. Applying accurate assay methodologies with rigorous data validations, low GCNs of total C4, and heterozygous and homozygous deficiencies of C4A have been shown as medium to large effect size risk factors, while high copy numbers of total C4 or C4A as prevalent protective factors, of European and East-Asian SLE. Here, we summarize the current knowledge related to genetic deficiency and insufficiency, and acquired protein deficiencies for C1q, C1r, C1s, C4A/C4B, and C2 in disease pathogenesis and prognosis of SLE, and, briefly, for other systemic autoimmune diseases. As the complement system is increasingly found to be associated with autoimmune diseases and immune-mediated diseases, it has become an attractive therapeutic target. We highlight the recent developments and offer a balanced perspective concerning future investigations and therapeutic applications with a focus on early components of the CP in human systemic autoimmune diseases.

Classical pathway deficiencies - A short analytical review

Deficiencies in the classical pathway of complement activation have some common features but show also great differences. Deficiencies of each of the components (C1q, C1s, C1r, C4 and C2) imply increased susceptibility to bacterial infections. They are also associated with increased risk to develop systemic lupus erythematosus where deficiency of C1q is strongly associated to the disease while C4 less and C2 much less. Deficiency of C1q affects only activation of the classical pathway while deficiency of C4 and C2 also prevent activation of the lectin pathway. Bypass mechanisms may result in complement activation also in absence of C2 but not in absence of C1q or C4. The genes for C2 and C4 isotypes are closely located within the MHC class III region on chromosome 6p and the genes for the 3 C1q chains are on chromosome 1p. Deficiencies of C1q and of C4 show genetic heterogeneity while deficiency of C2 in the great majority of cases is caused by a specific deletion. The production of C4 and C2 is mainly by the hepatocytes in the liver while C1q is produced by monocytic bone marrow derived cells. This has implications for the possibility to treat the deficiency and hematopoietic stem cell transplantation has been tried in C1q deficiency.

Deficiency of the effector mechanisms of the immune response and autoimmunity

It has long been the conventional wisdom that most autoimmune responses represent a pathological aberration of the immune system and a great deal of effort has been devoted to investigating how such abnormal responses may be induced. It seems likely, however, particularly in the form of autoimmunity seen in diseases like systemic lupus erythematosus (LE) and associated with immune complex disease, that it is not the induction of the autoimmune response that is primarily abnormal, but its persistence, and that this abnormal persistence is a consequence of a failure of the proper functioning of the effector mechanisms of the immune response. The strongest reason for so believing is the striking incidence of these diseases in subjects with deficiencies of the early components of the classical complement pathway. Total, homozygous deficiencies are rare and account for only a small proportion of patients with systemic LE. However, partial, heterozygous deficiency of these components is much commoner and also carries an increased susceptibility to these diseases. An explanation for this association is given. In the presence of an adequately functioning complement system immune complexes remain soluble and of relatively small size. It is proposed that this is a result of the incorporation of C4 and C3 into the antigen-antibody lattice leading to a reduction in the effective valency of antigen and antibody. The Goldberg theory of immune precipitation predicts that a reduction in valency would inhibit precipitation and the formation of large complexes. In the absence of adequate complement function this mechanism will fail and large, potentially insoluble complexes with little C4 and C3 on them will be formed. These large immune complexes without sufficient C4 and C3 bound on them will also not be bound normally to erythrocyte CR1 and will therefore be transported in the (peripheral) plasma stream rather than in the (central) erythrocyte stream. It is proposed that this will result in the deposition of immune complexes in peripheral small blood vessels rather than in the sinusoids of the liver and spleen; and that this peripheral deposition gives rise to inflammation, with the release of autoantigens and the formation of further autoantibodies. The importance of CR1 in relation to these diseases is emphasized by the reduction in CR1 numbers that accompanies their active phase. This appears to be due to proteolysis of the receptor while the immune complex-bearing erythrocyte is sequestered in the reticuloendothelial system.

Antigen-induced B cell apoptosis is independent of complement C4

Deficiencies in early complement components are associated with the development of systemic lupus erythematosus (SLE) and therefore early complement components have been proposed to influence B lymphocyte activation and tolerance induction. A defect in apoptosis is a potential mechanism for breaking of peripheral B cell tolerance, and we hypothesized that the lack of the early complement component C4 could initiate autoimmunity through a defect in peripheral B lymphocyte apoptosis. Previous studies have shown that injection of a high dose of soluble antigen, during an established primary immune response, induces massive apoptotic death in germinal centre B cells. Here, we tested if the antigen-induced apoptosis within germinal centres is influenced by early complement components by comparing complement C4-deficient mice with C57BL/6 wild-type mice. We demonstrate that after the application of a high dose of soluble antigen in wild-type mice, antibody levels declined temporarily but were restored almost completely after a week. However, after antigen-induced apoptosis, B cell memory was severely limited. Interestingly, no difference was observed between wild-type and complement C4-deficient animals in the number of apoptotic cells, restoration of antibody levels and memory response.

Rheumatological manifestations, organ damage and autoimmunity in hereditary C2 deficiency

OBJECTIVE

To analyse rheumatological manifestations, organ damage and autoimmune responses in a large cohort of patients (n = 45) with homozygous C2 deficiency (C2D) and long-term follow-up.

METHODS

Medical records were reviewed and were supplemented with a mailed questionnaire for assessment of cardiovascular disease (CVD) risk factors. Organ damage was evaluated using the Systemic Lupus International Collaborative Clinics/American College of Rheumatology Damage Index (SLICC/ACR DI). Causes for disability pensions were investigated. Autoantibodies were determined with established methods.

RESULTS

Patients with rheumatological diseases had systemic lupus erythematosus (SLE, n = 12), undifferentiated connective tissue disease (n = 5) or vasculitis (n = 3). Judging from annual SLICC/ACR DI, C2D patients with SLE run a similar risk of development of severe disease as other patients with SLE. An increased rate of CVD was observed not explained by Framingham-related risk factors. Disability pensions were mainly related to rheumatological disease. The prevalence of anti-nuclear antibodies in C2D with SLE and of anti-SS-A was 25% while anti-RNP was found in 45%. Only one patient showed antibodies to dsDNA. Formation of anti-cardiolipin antibodies (aCL) appeared to be increased in C2D despite the absence of an anti-phospholipid syndrome. The prevalence of antibodies to the collagen-like region of C1q (C1qCLR) was also remarkably high and was not related to rheumatological manifestations.

CONCLUSIONS

Severity of SLE in C2D is similar to that of SLE in other patients. Conventional risk factors do not explain the occurrence of CVD in C2D. The high prevalence of aCL and anti-C1qCLR indicates mechanisms through which impaired complement function promotes formation of autoantibodies.

Defective prevention of immune precipitation in autoimmune diseases is independent of C4A*Q0

Increased prevalence of C4 null alleles is a common feature of autoimmune diseases. We have shown previously that complement-dependent prevention of immune precipitation (PIP) is defective in patients with systemic lupus erythematosus (SLE), and correlated this defect with C4A*Q0 and low levels of the C4A isotype. To further clarify the role of C4A in the aetiology of SLE, we now extend our studies to other diseases which have been associated with C4A*Q0. The frequency of C4A*Q0 was increased in Icelandic patients with coeliac disease (0.50; P < 0.001), Grave's disease (0.30; P = 0.002) and insulin-dependent diabetes mellitus (0.23; P = 0.04) and in British patients with dermatitis herpetiformis (0.42; P = 0.002) and this was reflected in low levels of C4A. In spite of this, PIP was normal in these patients, and in marked contrast to our previous observations on connective tissue diseases, PIP measurements in these patient groups correlated more strongly with levels of C4B (r = 0.51, P = 0.0000004) than C4A. Patients with increased levels of anti-C1q antibodies had significantly lower PIP than patients without such antibodies (P < 0.01) and a negative association of PIP with anti-C1q antibodies was also reflected in an increased prevalence (P = 0.006) and levels (P = 0.006) of anti-C1q antibodies in patients with subnormal PIP, as well as a negative correlation between PIP and anti-C1q antibodies (r = - 0.25, P = 0.02). These results show that the PIP defect cannot be explained by low levels of C4A alone and suggest that measurements of anti-C1q antibodies may be useful in future studies on the molecular cause of the PIP defect in autoimmune connective tissue disease.

Hereditary C2 deficiency in Sweden: frequent occurrence of invasive infection, atherosclerosis, and rheumatic disease

Although frequently asymptomatic, homozygous C2 deficiency (C2D) is known to be associated with severe infections and rheumatic disease. We describe the clinical findings in 40 persons with C2D from 33 families identified in Sweden over 25 years. Medical records covering 96% of the accumulated person-years were reviewed, giving a mean observation time of 39 years (range, 1-77 yr). Severe infection was the predominant clinical manifestation in the cohort: 23 patients had a past history of invasive infections, mainly septicemia or meningitis caused by Streptococcus pneumoniae, and 12 patients had repeated infections of this kind. Nineteen patients had at least 1 episode of pneumonia, and recurrent pneumonia was documented in 10 patients. Repeated infections occurred mainly during infancy and childhood. Systemic lupus erythematosus was found in 10 patients. Another 7 patients had undifferentiated connective tissue disease (n = 4) or vasculitis (n = 3). We found no correlation between susceptibility to invasive infection and rheumatologic disease. Cardiovascular disease occurred at a high rate, with a total of 10 acute myocardial infarctions and 5 cerebrovascular episodes in 6 patients. Causes of death among the C2D patients were infection (n = 5), acute myocardial infarction (n = 3), and cancer (n = 1). We suggest that severe infection may be the principal clinical manifestation of C2D. We also provide novel evidence for a possible role of C2D in the development of atherosclerosis consistent with findings in mannan-binding deficiency and experimental C3 deficiency. In addition, we confirm the well-known association between C2D and systemic lupus erythematosus.

Patients with systemic lupus erythematosus are deficient in complement-dependent prevention of immune precipitation

OBJECTIVE

A functional deficiency of complement has been implicated but not conclusively demonstrated in the pathogenesis of systemic lupus erythematosus (SLE). To test this, we studied several aspects of complement in 44 patients with SLE, 46 patients with rheumatoid arthritis and 102 blood donors.

METHODS

Prevention of immune precipitation (PIP) was measured by an enzyme immunoassay, levels of C1q, C4 and C3 by rocket immunoelectrophoresis, C4A, C4B and C3d by enzyme-linked immunosorbent assay (ELISA), complement haemolysis (CH50) by standard methods and C4 allotypes by high-voltage agarose electrophoresis and sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE).

RESULTS

PIP was significantly reduced in SLE (P<0.001); the defect was revealed by a sensitive assay measuring this function of complement but not by the other tests employed. The patients were clinically well at the time of study, and levels of C3d, which have been shown to correlate with disease activity, were normal. The defect was more common in patients with early disease (P = 0.009), supporting a role in aetiology or early pathophysiology. PIP was positively correlated with levels of C4 (P = 3 x 10(-5)) and in particular the C4A isotype (P = 9 x 10(-10)) whereas C4B was redundant.

CONCLUSIONS

Our results reveal a defect in prevention of immune precipitation in SLE that is apparent at an early stage in the disease and correlates with low levels of C4A. These results indicate that subtle deficiencies of complement may predispose to SLE.

Ocular manifestations of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a chronic, immunologic disorder that may affect multiple organ systems. Keratoconjunctivitis sicca is the most common ocular manifestation, but visual morbidity is usually due to retinal and neuro-ophthalmic manifestations of the disease. Ocular manifestations of lupus are a reflection of systemic disease. The presence of ocular manifestations should alert the clinician to the likely presence of disease activity elsewhere. Therefore, all patients with ocular lupus should be carefully evaluated for systemic involvement to detect potentially treatable and preventable complications of the disease. In addition, the ophthalmologist should include SLE in the differential diagnosis of many retinal vascular and neuro-ophthalmic disorders. The ophthalmologist may play an important role in the care of patients with SLE, since ocular inflammatory lesions may precede potentially serious extraocular disease.

Complement activation in patients with systemic lupus erythematosus without nephritis

OBJECTIVE

To study the association between disease activity and complement activation prospectively in patients with systemic lupus erythematosus (SLE).

PATIENTS AND METHODS

Twenty-one SLE patients were examined monthly for 1 yr. Disease activity, autoantibodies, conventional complement tests and the following complement activation products were investigated: C1rs-C1inh complexes, C4bc, Bb, C3a, C3bc, C5a and the terminal SC5b-9 complement complex (TCC).

RESULTS

Modest variation in disease activity was noted. None of the patients had nephritis. Flare was observed at 27 visits. Four patients had anti-C1q antibodies in conjunction with modestly low C1q concentrations. The complement parameters were rather constant during the observation period. Slightly to moderately decreased C4 (0.05-0.10 g/l) was found in 10 patients and severely decreased C4 (<0.05 g/l) in seven patients. Decreased C4 was not associated with increased complement activation. Complement activation products were either normal or slightly elevated. TCC was the only activation product correlating significantly with score for disease activity at flare. None of the variables tested predicted flares.

CONCLUSION

Complement tests are of limited importance in routine examination of SLE without nephritis, although TCC is suggested to be one of the most sensitive markers for disease activity.

Systemic lupus erythematosus and complement deficiency: clues to a novel role for the classical complement pathway in the maintenance of immune tolerance

Complete deficiency of C1q, the first component of the classical pathway of complement activation, is almost invariably associated with the development of systemic lupus erythematosus. Understanding why complement deficiency results in the specific autoimmune phenotype of SLE may provide valuable clues to the role of complement in the maintenance of immune tolerance. The following review will focus on the characteristics of complement-deficient SLE and the experimental evidence in support of our hypothesis that C1q may critically influence the immune response to self-antigen contained within surface blebs generated by apoptotic cells.

Upper airway obstruction in the rheumatic diseases

Upper airway obstruction is a potentially life-threatening event that can be managed effectively when promptly recognized and treated. For this reason, all patients who are suspected of having disease affecting the larynx or trachea should be regarded as having a potentially compromised airway until proven otherwise. Although clinically significant upper airway diseases occur infrequently in rheumatic disorders, these manifestations should be considered in such patients who present with upper airway symptomatology.

Association between circulating immune complexes, complement C4 null alleles, and myocardial infarction before age 45 years

One hundred patients who had survived a myocardial infarction before the age of 45 years and 90 age- and sex-matched healthy individuals were investigated for circulating immune complexes (CICs) and the presence of complement C4 null alleles (C4Q0). CICs were found in increased concentrations in 20% of patients and 6.7% of control subjects. Patients and control subjects had the same prevalence of C4Q0. CICs were present in all patients and in 36% of the control subjects homozygous for C4Q0. Patients and control subjects heterozygous for C4Q0 had CICs in 71% and 0%, respectively. The high prevalence and a high concentration of CICs were particularly associated with C4A*Q0. Patients homozygous for C4A*Q0 had concentrations of LDL that were lower than found in other patients. The increased concentration of CICs associated with genetic deficiencies of the complement factor C4 might thus be an additional etiological factor for the development of chronic vascular damage and premature myocardial infarction.

Mannose-binding protein gene polymorphism in systemic lupus erythematosus

OBJECTIVE

To determine whether an allelic form of mannose-binding protein (MBP) incapable of activating complement is associated with susceptibility to systemic lupus erythematosus (SLE).

METHODS

MBP allele frequencies were determined by amplification refractory mutation system-polymerase chain reaction in 102 white SLE patients and 136 controls.

RESULTS

The MBP allele that is unable to activate complement was present in 42 SLE patients (41%) and in 41 controls (30%) (P = 0.08, odds ratio [OR] = 1.6, 95% confidence interval [95% CI] 1.0-2.8). The gene frequency of this allele was 0.25 in SLE patients and 0.19 in controls (P = 0.08, OR = 1.5, 95% CI 1.0-2.3).

CONCLUSION

Our results suggest that this allele of the MBP gene represents a minor risk factor for SLE.

Down syndrome associated with systemic lupus erythematosus: a mere coincidence or a significant association?

An 8-year-old male, who had Down syndrome associated with systemic lupus erythematosus (SLE), is described. He also had a partial complement 4 deficiency. This case is a reminder that the physician should be aware of the possibility of an immune defect in a male presenting with SLE at a young age. The question of whether the association of Down syndrome with SLE is coincidental or whether there is a predilection for autoimmune disorders in Down syndrome is discussed.

Pediatric rheumatic diseases

The rheumatic diseases of childhood are a relatively common and extraordinarily diverse group of illnesses; nevertheless, they are at least distantly related by similarities of immunodysregulation. These pathophysiologic relationships are reflected in affected children in similarities of historical, physical, and laboratory data as well as therapeutic intervention.

Hajdu-Cheney syndrome

Images

Recurrent infections, pericarditis and renal disease in a patient with total C2 deficiency and decreased NK cell function consistent with acute rheumatic fever and systemic lupus erythematosus

Images

Protein S deficiency in lupus erythematosus secondary to hereditary angio-oedema

We report a child with lupus erythematosus secondary to type 2 hereditary angio-oedema (HAE). Reduced levels of free protein S were found. Total protein S, and C4 binding protein (C4bp) were normal, and there was no evidence of anticardiolipin antibodies or lupus anticoagulant. We postulate that reduced levels of C4, secondary to C1 inhibitor deficiency, resulted in increased binding of protein S to C4bp, leading to reduced levels of free protein S and thus functional deficiency of protein S.

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.

Association of C4B deficiency (C4B*Q0) with erythema nodosum in leprosy

A considerable number of studies have postulated significant associations between susceptibility to the different clinical manifestations of leprosy and the MHC. In this investigation, the association between the MHC class III complement proteins C2, BF, C4A and C4B and leprosy in a patient population of Southern Brazil was studied. A total of 109 non-related leprosy patients was investigated; 73 presented with lepromatous leprosy (LL), 46 of them had the immunopathological reaction of erythema nodosum (ENL), the remaining 36 were tuberculoid, borderline and indeterminate leprosy (TIBL) patients. The control group included 172 healthy individuals matched with the patients according to their ethnic and geographical origin. C2, BF, C4A and C4B allotypes were determined by standard technologies including Western blots for C2 and C4 variant alleles with monoclonal and polyclonal antibodies. Non-expressed ('silent') C4 alleles in hemizygously deficient individuals were estimated semiquantitatively on the basis of the C4A and C4B isotype ratio and by the MASC ('minimal chi-square') method. The results showed a significantly elevated presence of the non-expressed C4B allele (C4B*Q0) in the LL and ENL patient groups in comparison with the controls. The most significant difference was observed in the ENL group when compared with the controls. In addition, all patients who were homozygously C4B-deficient had ENL, and most of them had the BF*F1 allele. The comparison between LL patients with and without ENL also showed a statistically significant difference in the presence of C4B*Q0, indicating that C4B deficiency itself is associated with ENL. The relative risk of LL patients with the C4B*Q0 allele suffering from ENL was 5.3 compared with LL patients without C4B*Q0. Since immune complexes (IC) are considered to be the pathogenic cause of ENL, our findings indicate that C4B deficiency may play an important role in the abnormal immune response against Mycobacterium leprae and in the lack of IC clearance, leading to ENL reactions. Individuals with this allele seem to be at a higher risk of developing pathological immune reactivity in lepromatous leprosy.

HLA antigens and complement C4 allotypes in patients with chronic biologically false positive (CBFP) seroreactions for syphilis: a follow-up study of SLE patients and CBFP reactors

We report a follow-up of our previous study of HLA markers in 118 unrelated patients: 49 with definite systemic lupus erythematosus (SLE) (group 1), 32 with definite or probable SLE and chronic biologically false positive (CBFP) seroreactions for syphilis (group 2), and 37 CBFP reactors (group 3). Definite SLE was confirmed in 28 (90.3%) of the patients in group 2, equally in HLA B8- and HLA B7-positive patients. Three of the CBFP reactors developed SLE, two (40%) out of five HLA B8-positive as compared to one (6.6%) out of 15 HLA B7-positive CBFP reactors (P = 0.07). Fourteen patients died (groups 1 and 2). Eight of the 24 HLA B8-positive patients died in contrast to one of the 20 HLA B7-positive patients (P < 0.02). Of the CBFP reactors, 70.9% had complement C4 null alleles as compared to 47.9% in controls (P = 0.05) and 50% had C4A null alleles as compared to 17.8% in controls (P < 0.05). C4B null alleles were found in 28.6% (28.6% in controls, P is not significant). The null alleles for C4A were not solely in a linkage disequilibrium with the HLA B8 DR3 haplotype. CBFP reactors with C4A null alleles had a higher risk of developing SLE, lupus-like disease or symptoms such as photosensitivity, cutaneous vasculitis and/or autoantibodies than did those with no C4A null alleles (P < 0.02).

Complement and lupus: old concepts and new directions

In this review it is our intention to outline briefly the relevance of the complement system in systemic lupus erythematosus. Three main issues will be addressed: the role of complement in handling immune complexes (ICs), the association between complement deficiencies and IC diseases, and the value of measuring complement components and their conversion products in monitoring disease activity.

Western blot analysis of human IgG reactive with the collagenous portion of C1q: evidence of distinct binding specificities

An enzyme-linked immunosorbent assay (ELISA) with purified collagenous C1q fragments in the solid phase was used for detection of C1q-specific immunoglobulins in the sera of twelve patients with systemic lupus erythematosus (SLE) or the SLE-like disease hypocomplementemic urticarial vasculitis syndrome (HUVS). By clinical criteria, four patients had SLE, and three HUVS. Five patients had overlap syndromes. All patients demonstrated high concentrations of C1q-specific IgG and markedly low concentrations of circulating C1q. Detection of C1q-specific IgG in SLE sera was facilitated by employment of saturating concentrations of collagenous C1q fragments in the solid-phase ELISA. When added to SLE serum, immune complex-fixed C1q inhibited binding of IgG to the C1q fragments, whereas addition of C1q alone had limited inhibitory effects. Under similar conditions, using approximately equimolar amounts of C1q relative to solid-phase C1q fragments, no ELISA inhibition was obtained after addition of C1q or immune complex-fixed C1q to a HUVS serum. Even in large excess, purified C1q did not inhibit binding of HUVS-IgG to solid-phase C1q fragments. Thus, possible interactions between HUVS-IgG and native Clq are probably of low affinity. By Western blot analysis, IgG reactive with the B and C chains of C1q was found in the eight patients with evidence of HUVS, five of whom also showed IgG binding to C'-C' and A'-B' dimers of collagenous C1q fragments. Sera from SLE patients were negative by Western blot analysis. It seems likely that C1q-specific IgG in SLE primarily recognizes assembled C1q molecules or collagenous C1q fragments expressing conformational epitopes of bound C1q. Interestingly, patients with evidence of HUVS fairly consistently had zymogen (C1r-C1s)2 complexes in their serum, while patients with SLE showed high concentrations of complexes containing Cl inhibitor, C1r and C1s. Different binding specificities of C1q-reactive IgG could be of importance with regard to pathogenetic mechanisms in SLE and HUVS. There was no correlation between findings of C1q-specific IgG and a variety of autoantibodies associated with SLE and SLE-like disease.

The evolution of mouse and human complement C3-binding proteins: divergence of form but conservation of function

Despite the fact that the early components of the mouse and human complement cascades are very similar, there are marked differences between the two species in the structure of C3 receptors and the molecules that control homologous lysis. Here, Michael Holers, Taroh Kinoshita and Hector Molina compare and contrast the mouse and human RCA region products and conclude that the receptor and regulatory roles are conserved despite the structural variation.

Acquired C1 inhibitor deficiency associated with systemic lupus erythematosus affecting the central nervous system

A 22 year old woman with systemic lupus erythematosus affecting the central nervous system had acquired C1 inhibitor deficiency. She was admitted for treatment of psychotic behaviour, but showed no signs of angioedema. The serum complement profile of the patient showed normal C3 concentration and a depletion of C4, C2, C1 inhibitor, and C1q. Her parents had normal complement profiles. An extremely reduced C4 concentration may lead to involvement of the central nervous system in systemic lupus erythematosus.

The role of complement in the aetiopathogenesis of systemic lupus erythematosus

The role of classical pathway complement components in systemic lupus erythematosus (SLE) is reviewed. Their importance in maintaining immune complexes (IC) in soluble form and in enhancing clearance of IC through binding to red cell CR1 is such that deficiency, complete or partial, of these components or some of their controlling enzymes can lead to IC mediated disease like SLE. C2 and C4 are encoded within the class III region of the major histocompatibility complex (MHC). There are certain well described associations between class II MHC genes and the occurrence of SLE and the relative importance of the two sets of gene products and their potential interactions are discussed. Complement C4 plays a role in drug induced lupus as many of the lupus associated drugs bind to C4 and interfere with its protective functions. Classical genetic studies provide clear evidence that non MHC genes are important in the aetiopathogenesis of SLE. Non MHC encoded complement deficiencies and functional deficits may well represent some of these other genetic factors and is clearly a fertile area for future research.

Homozygous C4A deficiency in systemic lupus erythematosus: analysis of patients from a defined population

Homozygous C4A deficiency was found at a prevalence of 16% (13/80 patients) in systemic lupus erythematosus (SLE). The patients represented all diagnosed cases retrieved from a defined population in Southern Sweden, which minimizes the influence of patient selection. Photosensitivity was more common among C4A-deficient patients than among other SLE patients (p less than 0.05). Otherwise, clinical features were similar in the two groups. In addition, no differences were found with regard to presence of various autoantibodies (anti-dsDNA, anti-Sm, anti-RNP, anti-SSA, anti-SSB, rheumatoid factors and anti-cardiolipin). In patients expressing both C4A and C4B isotypes, C4B/C4A quotients were fairly stable in plasma irrespective of disease activity. This argues against preferential break-down of either isotype during complement activation in the disease. The increased photosensitivity of C4A-deficient patients partly resembles the findings in patients with complete deficiencies of classical pathway components.

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.

Uniparental isodisomy 6 associated with deficiency of the fourth component of complement

We identified an extremely rare condition, isolated complete deficiency of the fourth component of complement, in a child with systemic lupus erythematosus. The genes for C4 are located within the major histocompatibility complex (MHC) on the short arm of chromosome 6. The patient expressed only paternal phenotypes for proteins encoded by the MHC (HLA and GLO), yet was 46XX with no detectable 6p deletion. Genomic DNA from patient, parents, and sibling was digested with restriction enzymes, and blots were probed for five chromosome 6 markers. At all loci, maternal and paternal RFLPs could be distinguished, and the patient showed only paternal bands. RFLP analysis of markers from four other chromosomes showed maternal and paternal contribution. The data are consistent with uniparental isodisomy 6 (inheritance of two identical chromosome 6 haplotypes from the father and none from the mother). Direct analysis of genetic material from both parents, as well as detection of multiple protein polymorphisms encoded on chromosome 6, clearly demonstrates this novel mechanism for the expression of a recessive genetic condition.

Systemic lupus erythematosus in dogs: association to the major histocompatibility complex class I antigen DLA-A7

The DLA-A,B antigens and the allotypes of the fourth complement component have been determined in German shepherd dogs suffering from systemic lupus erythematosus. We have typed 26 unrelated affected dogs, 11 animals of a three generation family, and 16 dogs of a colony with a high frequency of the disease. The results obtained from the 26 unrelated diseased dogs were compared to those determined in the 23 unaffected German shepherds. The antigen DLA-A7 was found to be predominant in the diseased group with a c2 = 11.02, Pc = 0.02, and a relative risk for the carriers of 11.93. The antigens DLA-A1 and DLA-B5 were negatively associated to the disease (c2 = 14.95, Pc = 0.001, and c2 = 17.16, P = 0.0008 respectively) and thus may be of protective nature. These data were further substantiated by the typing of the three generation family and the colony.

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.

Classical pathway of complement activation in normal and diseased human glomeruli

Monoclonal antibodies (mAb) reactive against complement components involved in the classical activation pathway were applied in an indirect immunoperoxidase technique for the histological study of normal and diseased human renal tissues. Prominent staining with antibodies against the C4d fragment was seen in all glomeruli and some renal arteriolar walls. The C4d staining was mesangial with light microscopy, whereas the subendothelial site of the glomerular basement membrane (GBM) also appeared to be positive in immunoelectron microscopy. In similar localization, albeit with distinctly weaker intensity, IgM and C4 binding protein (C4bp) were detected. In kidney biopsies from patients with various types of glomerulonephritis, C4d reactive antibodies stained the glomerular structures in a strong, diffuse or granular pattern in contrast to the more segmental distribution and weaker staining intensity in normal kidney specimens. Increased amounts of C4d, occasionally also of C4b, were paralleled in diseased kidney tissues by distinct deposits of IgM and/or IgG in the presence of C4bp. This study suggests that the C4d fragment in normal human glomeruli is indicative of a continuous, local complement activation via the classical pathway induced by the physiological deposition of IgM-containing immune complexes.

C4A gene deletion and HLA associations in black Americans with systemic lupus erythematosus

In North America and European Caucasoids with systemic lupus erythematosus (SLE) there is an increased frequency of a C4A, CYP21A gene deletion, largely associated with the HLA-B8,DR3,C4A*QO extended haplotype. There have been no consistent HLA associations reported for SLE in blacks, although an increased frequency of serologically determined C4A null alleles has been reported in two studies. We studied 79 black American SLE patients and 68 black controls by restriction fragment length polymorphism analysis to determine if a C4A gene deletion was a genetic risk factor for SLE. Moreover, the nature of the deletion and any HLA phenotypic associations were sought. Nineteen of 79 (24%) patients compared to 5 of 68 (7.4%) controls had a phenotypic C4A,CYP21A gene deletion (P = .005; RR = 4). A homozygous deletion in four patients gave a genotypic frequency of 23/158 (14.5%) SLE patients vs 5/136 (3.7%) controls (P = .001; RR = 4.5). The deletion was associated with HLA-DR2 (P = .03) and HLA-DR3 (P = .03). Moreover, all subjects with the deletion had HLA-DR2 or DR3 (P = 7.7 x 10(-6). HLA-B44 was also associated with the deletion (P = .02), and eight of the nine HLA-B44 positives also carried HLA-DR2. HLA-B8 approached significance (P = .08) and was always accompanied by HLA-DR3. Finally, this black population demonstrated a unique C4B gene size polymorphism with 80% C4B "short" as compared to the 40% C4B "short" frequency reported in whites. We conclude that a large C4A,CYP21A gene deletion, particularly associated with the HLA-B44, -DR2, and -DR3 alleles, is the strongest genetic risk factor thus far identified for SLE susceptibility in black Americans. Furthermore, the unique preponderance of the C4B "short" gene form may be a factor in the actual formation of the deletion.