Homozygous deficiency of C4 in a child with a lupus erythematosus syndrome

Complement C4, Infections, and Autoimmune Diseases

Complement C4, a key molecule in the complement system that is one of chief constituents of innate immunity for immediate recognition and elimination of invading microbes, plays an essential role for the functions of both classical (CP) and lectin (LP) complement pathways. Complement C4 is the most polymorphic protein in complement system. A plethora of research data demonstrated that individuals with C4 deficiency are prone to microbial infections and autoimmune disorders. In this review, we will discuss the diversity of complement C4 proteins and its genetic structures. In addition, the current development of the regulation of complement C4 activation and its activation derivatives will be reviewed. Moreover, the review will provide the updates on the molecule interactions of complement C4 under the circumstances of bacterial and viral infections, as well as autoimmune diseases. Lastly, more evidence will be presented to support the paradigm that links microbial infections and autoimmune disorders under the condition of the deficiency of complement C4. We provide such an updated overview that would shed light on current research of complement C4. The newly identified targets of molecular interaction will not only lead to novel hypotheses on the study of complement C4 but also assist to propose new strategies for targeting microbial infections, as well as autoimmune disorders.

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.

Renal disease associated with inherited disorders of the complement system

The human complement system is vital for host defense and plays a role in a number of inflammatory disorders. Inherited deficiency or dysfunction of most of the individual complement components occurs uncommonly. The phenotype displayed by such patients varies with the specific component deficiency and ranges from recurrent infections to autoimmune diseases. Most of the latter are associated with glomerulonephritis. The onset of severe lupus erythematosus in a young child, with prominent cutaneous and renal manifestations, especially if a similar disorder is present in another family member, is a clue to the presence of a complement component deficiency. The distinguishing of acquired deficiencies from inherited deficiencies in complement components is sometimes difficult and may require sophisticated laboratory testing.

Use of serum or buffer-changed EDTA-plasma in a rapid, inexpensive, and easy-to-perform hemolytic complement assay for differential diagnosis of systemic lupus erythematosus and monitoring of patients with the disease

We previously described a simplified quantitative hemolytic assay for classical pathway (CP) hemolytic function in serum that has been shown to correlate with the 50% hemolytic complement (CH50) assay. In the present study, we used this assay to compare CP functions; plasma levels of C3, C4, and C3dg; and ratios of C3dg to C3 in healthy individuals and patients with systemic lupus erythematosus (SLE) or rheumatoid arthritis (RA) with different degrees of complement activation. A significant depression in CP function and levels of C4 and C3 and increased C3dg levels and C3dg/C3 ratios were observed in the SLE patients. In patients with RA, CP function was normal, whereas C3, C4, and C3dg levels and the C3dg/C3 ratio were elevated. The SLE results are compatible with systemic complement consumption, whereas the RA data suggest an acute-phase reaction with a normal C3 catabolic rate. To facilitate the handling of patient samples, we also developed a method to restore the hemolytic function of EDTA-plasma by transferring it to Veronal-buffered saline containing the thrombin inhibitor lepirudin. This process inhibits coagulation and enables complement activation, allowing a longer time lag between sample harvesting and testing. These results, combined with previous correlation studies, suggest that the CP hemolytic assay can effectively replace the CH50 assay for routine SLE differential diagnosis and monitoring of disease activity.

Characterization of non-expressed C4 genes in a case of complete C4 deficiency: identification of a novel point mutation leading to a premature stop codon

The genetic basis of complete C4 deficiency in a patient with SLE was investigated. Previous studies have demonstrated that this patient has two different major histocompatibility complex (MHC) haplotypes that each contain a major deletion and a non-expressed C4 gene. In the present study, non-expression of the C4 genes was explained by the finding of two distinct C4 gene mutations. A previously described two base pair insertion in exon 29 of the C4 gene was detected in the paternal MHC haplotype [HLA-A2, B40, SC00, DR6]. The maternal haplotype [HLA-A30, B18, F1C00, DR3] carried a C4 gene with a one base pair deletion in exon 20 generating a premature stop codon. This mutation was neither found in 10 individuals with known non-expressed C4 genes nor in 9 individuals homozygous for the complotype F1C30. The isotype and allotype specific regions of the patient's C4 genes were sequenced, and both contained C4A3a sequence. In conclusion, two different MHC haplotypes resembling the extended haplotypes [HLA-A2, B40, SC02, DR6] and [HLA-A30, B18, F1C30, DR3] both contained a non-expressed C4A gene that was due to either of two distinct mutations, demonstrating the heterogeneous genetic background of C4 deficiency.

New procedure for the detection of complement deficiency by ELISA. Analysis of activation pathways and circumvention of rheumatoid factor influence

A procedure using enzyme-linked immunosorbent assays for the assessment of complement function has been evaluated. The sera investigated were incubated in microtiter plates with solid-phase complement activators. Human polyclonal IgG or monoclonal IgM were used for classical activation pathway assays and Salmonella typhosa lipopolysaccharide (LPS) for alternative activation pathway assays. The analysis focussed on deposition of C9 and properdin as detected with enzyme-conjugated antibodies. In an attempt to avoid spurious results due to rheumatoid factors in patient sera, monoclonal mouse and chicken antibodies were unsuccessfully tested as indicator reagents in the assay with solid-phase IgG. However, the use of solid-phase IgM as an activator completely circumvented the influence of rheumatoid factors. With solid-phase IgG or IgM, properdin deposition occurred in the absence of factor D. A combination of assays is suggested for diagnostic purposes: IgM-coated plates with detection of bound C9 and properdin for the classical pathway and LPS-coated plates with detection of bound properdin for the alternative pathway. The procedure distinguished between defects of the classical activation pathway (C1, C4, C2), the alternative activation pathway (C3, factor B, factor D, properdin) and the terminal components (C5-C9). This analytical approach may be useful for detection of inherited complement deficiency and the assessment of complement function in acquired complement deficiency states.

Genetic deficiencies of the complement system and association with disease--early components

Genetic deficiency of one of the early components of the classical pathway of complement (C1q, C1r, C1s, C4 and C2) is often associated with clinical symptoms and immunochemical abnormalities common in idiopathic autoimmune diseases, such as lupus erythematosus, but also with an increased incidence of various, local and generalized infections. These observations are consistent with the current view of the complement system's role in handling immune complexes and combating microbial invasion. However, the absence of absolute correlations in these experiments of nature suggests that genetic defects of the classical pathway act only epistatically to other host factors and the primary etiologies of the associated diseases. In contrast, the strong association of properdin and factor D deficiency with serious infections caused by encapsulated Gram-negative bacteria suggests a more immediate involvement of the alternative pathway in a specific segment of immunity and its pathology. This concept is also supported by the primordial role of the alternative pathway in the evolution of the complement system and the apparent lethality of factor B deficiency. The gene structures of most of these early components have now been elucidated providing the basis for detailed analyses of the defective alleles, the determination of carrier status, and prenatal diagnosis.

Increased risk of neisserial infections in systemic lupus erythematosus

Survival in systemic lupus erythamatosus (SLE) continues to improve because of better ancillary care, earlier diagnosis, and earlier treatment. However, infection remains a leading cause of morbidity and mortality in this disease. Although corticosteroids and immunosuppresives increase the risk of opportunistic infection, the SLE patient is still most at risk from common bacterial pathogens. As the prototypic immune-complex disease, patients with active SLE have low circulating complement as well as a reticuloendothelial system (RES) saturated with immune complexes. It seems intuitive that SLE patients should be most at risk for organisms dependent for their removal on the RES or complement for opsonization or bacteriolysis. The current series presents four patients with SLE and disseminated neisseria infection and brings to 14 the number of patients in the literature with disseminated neisserial infection. They are typically young, female, with renal disease, and either congenital or acquired hypocomplementemia, and may present with all features of a lupus flare. Surprisingly, they are not all on corticosteroids or immunosuppressives and have some features that are unusual for non-SLE patients with these infections. There seems to be an over-representation of Nisseria meningitidis (despite potential reporting bias), and there ironically may be better tolerance with fewer fulminant complications in patients who have complement deficiencies. The best approach for the physician treating SLE is to immunize all SLE patients with available bacterial vaccines to N meningitidis and Streptococcus pneumonia, have a low threshold of suspicion for the diagnosis of disseminated neisserial or other encapsulated bacterial infection in the SLE patient who is sick, and to treat empirically with third generation cephalosporins after appropriate cultures.

Clinical aspects of systemic lupus erythematosus in childhood

Systemic lupus erythematosus (SLE) is an uncommon childhood illness that is characterized by the formation of autoantibodies and immune complexes, which mediate inflammatory responses in multiple organ systems. Children who develop SLE are frequently very ill at the time of presentation and need careful evaluation to determine which organ systems are involved and how severely, meticulous control of medications to suppress active disease, and close monitoring to avoid complications from both the disease and its treatment.

C4 allotypes and HLA-DR antigens in the family of a patient with C4 deficiency

Major histocompatibility complex (MHC) haplotypes, including HLA-A, -B, -C and -DR and complotypes (BF, C2, C4A and C4B) were determined in a large family with inherited C4 deficiency. The propositus, a 12-year-old girl with complete C4 deficiency and SLE, had the MHC haplotypes HLA-A2,Cw3,-B40,-DR6,BFS,C2C,C4AQO,C4ABQO inherited from her father and HLA-A30,-B8,-DR3,BFF1,C2C,C4AQO,C4BQO from her mother. These haplotypes were identified in several of the healthy relatives, who were thus shown to be carriers of C4 deficiency. Most of the other haplotypes found in the family were not considered to be unusual in the general population. The complete absence of C4 in the patient was confirmed by studies of Chido and Rodgers antigens in the plasma and on the erythrocytes, the absence of plasma C4d fragments and by studies of C4 chain antigens by immunoblotting technique. The results of the genetic analysis, together with the findings in other cases of C4 deficiency, supports the possibility that complete C4 deficiency in itself predisposes to development of SLE without contribution of other MHC gene products.

Immunoglobulin synthesis in a patient with C4 deficiency: functional immaturity of peripheral blood lymphocytes

Peripheral blood lymphocytes from a systemic lupus erythematosus patient with homozygous C4 deficiency were stimulated in vitro. Although T cell independent stimulation (Epstein-Barr virus) induced normal proportions of IgM secreting cells. IgG and IgA secreting cells were almost absent. Virtually no immunoglobulin secretion was obtained after T cell dependent stimulation (pokeweed mitogen). Cell fractionation experiments indicated that the low pokeweed mitogen response, at least with regard to IgG and IgA production, was mainly due to impaired B-lymphocyte function. The serum IgG subclass distribution pattern of specific antibodies against certain carbohydrate antigens demonstrated a relative lack of IgG2 antibodies. The findings were consistent with functional immaturity of B lymphocytes in the patient.

A family with three independent autosomal translocations associated with 7q32----7qter syndrome

Two persons within the same family were discovered to be trisomic for the segment 7qter. However, several features differed from those described in other patients with this syndrome, for example, normal birth weight and neck size, cleft palate, and beaked nose. In addition to the phenotypic variation, there were three independently segregating autosomal translocations in the pedigree: t(1;7)(q43;q32), t(1;6) (p22.3;q14.1), and t(3;10)(q26.1;p11.21). This is a finding that, to our knowledge, has not been previously reported.

Complement phenotypes in glomerulonephritis: increased frequency of homozygous null C4 phenotypes in IgA nephropathy and Henoch-Schönlein purpura

Polymorphism of three complement genes (C4A, C4B, and BF) located within the major histocompatibility complex was studied in 48 biopsy-proven IgA nephropathy patients and nineteen patients with Henoch-Schönlein purpura (HSP). Polymorphism was determined by immunoelectrophoretic techniques and functional activity using an overlay of sheep cells in agarose and C4 deficient sera. The subjects were divided into four large groups according to the presence or absence of a C4 null allele (a gene producing no identifiable gene product): group 1 (no null variants), group 2 (one C4A null variant), group 3 (one C4B null variant), and group 4 (two null variants at either the C4A or C4B locus, that is, homozygous null). Patients had a significantly increased frequency of group 4 phenotypes (homozygous null): (12 of 67 patients, 17.8%) as compared to controls (4 of 102 patients, 3.9%, P = 0.0031). Both IgA (P = 0.045) and HSP patients (P = 0.003) had a greater frequency of a C4 homozygous null phenotype. The serum C4 concentration was higher in patients than in controls (740 mg/ml and 576 micrograms/ml, respectively, P = less than 0.001) whether evaluated together or by C4 phenotypic group. The association between the presence of IgA nephropathy or HSP with a homozygous C4 null phenotype is of unknown significance but suggests a predisposition to development of HSP or IgA nephropathy for individuals with the C4 homozygous null phenotype.

Inherited incomplete deficiency of the fourth component of complement (C4) determined by a gene not linked to human histocompatibility leukocyte antigens

We have studied a family in which the proband had systemic lupus erythematosus and selective incomplete deficiency of the fourth component of complement (C4) (2-5% of the normal level). An additional six healthy family members also had low C4 levels (2.4-24.1% of normal) but no evidence of lupus. This form of inherited C4 deficiency differs from that in previously reported families in that inheritance was autosomal dominant (rather than recessive), C4 levels were markedly reduced (but not undetectable), and there was no linkage to HLA, BF, or C4 structural loci, all known to be within the major histocompatibility complex.

Anti-C4 in the serum of a transfused C4-deficient patient with systemic lupus erythematosus

A C4-deficient patient with systemic lupus erythematosus had been transfused on several occasions. His red cells reacted with a proportion of anti-Rg (Rodgers) and anti-Ch (Chido) reagents, but this was due to a separable antibody that did not have anti-Rg or anti-Ch specificity. Eluates of anti-Rg and anti-Ch indicate his red cell phenotype to be Rg-Ch-. Anti-C4 has been identified in his serum that exhibits neither anti-Rg nor anti-Ch specificity, but has similar serological characteristics in reacting with C4- and C4d-coated red cells.