Chido and Rodgers blood groups are distinct antigenic components of human complement C4

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.

RBCs as targets of infection

RBCs can be targets of infection directly or indirectly. When the microorganism enters the RBC directly, RBC damage becomes a fundamental aspect of the disease process. Malaria is the best example of an organism that directly targets the RBC, but others are Babesia and Bartonella. RBCs can also be indirect targets of infectious agents. This can occur when molecules are bound to the surface of the RBC, leading to immunologic clearance; when microorganism-produced toxins damage the RBC membrane, leading to hemolysis; when previous crypt-antigens are exposed, leading to accelerated removal; when microorganism-produced toxins alter RBC antigens to a different phenotype, or when microorganism suppression of erythropoiesis occurs due to specific binding to RBC precursors.

The impact of platelet additive solution apheresis platelets on allergic transfusion reactions and corrected count increment (CME)

BACKGROUND

Allergic transfusion reaction (ATR) incidence ranges from 1% to 3% of all transfusions. We evaluated the impact of InterSol platelet additive solution (PAS) apheresis platelets (APs) on the incidence of ATRs and the posttransfusion platelet (PLT) increment.

STUDY DESIGN AND METHODS

This retrospective study evaluated all ATRs among patients at a university hospital that maintained a mixed inventory of PAS APs and non-PAS APs (standard plasma-suspended PLTs). Corrected count increments (CCIs) were calculated for AP transfusions of individuals who received both a PAS and a non-PAS AP transfusion within a 7-day period. Hypothesis testing was performed with chi-square test for dichotomous variables and t tests for continuous variables.

RESULTS

The incidence of ATRs among the non-PAS APs was 1.85% (72 ATRs/3884 transfusions) and 1.01% (12 ATRs/1194 transfusions) for PAS APs (risk ratio [RR], 0.54; 95% confidence interval [CI]=0.30-0.99; p=0.04). However, there was no difference in the incidence of febrile nonhemolytic transfusion reactions between non-PAS APs (incidence, 0.70%; 27/3884) compared to PAS APs (incidence, 0.59%; 7/1194; p=0.69). Among 223 individuals with paired non-PAS and PAS AP transfusions, the mean CCI at 1 to 4 hours after transfusion was 4932 (95% CI, 4452-5412) for non-PAS APs and was lower for PAS APs (CCI, 3766; 95% CI, 3375-4158; p ≤ 0.001). However, there was no significant difference in mean CCI at 12 to 24 hours between non-PAS (CCI, 2135; 95% CI, 1696-2573) and PAS APs (CCI, 1745; 95% CI, 1272-2217; p=0.14).

CONCLUSIONS

PAS APs substantially reduce the number of ATRs. CCIs for PAS APs were lower immediately after transfusion, but not significantly different at 12 to 24 hours.

Initiation and regulation of complement during hemolytic transfusion reactions

Hemolytic transfusion reactions represent one of the most common causes of transfusion-related mortality. Although many factors influence hemolytic transfusion reactions, complement activation represents one of the most common features associated with fatality. In this paper we will focus on the role of complement in initiating and regulating hemolytic transfusion reactions and will discuss potential strategies aimed at mitigating or favorably modulating complement during incompatible red blood cell transfusions.

Relationships between the bovine major histocompatibility system and commonly recognized erythrocyte and serum polymorphisms

Linkage at a recombination frequency of 0.10 or less between the bovine major histocompatibility system and the B, C and L red blood cell groups and the albumin, haemoglobin and transferrin loci was excluded by Morton's lod score method. The white blood cell antigen CA19, which is independent of the bovine major histocompatibility system, is the J blood group.

Dancing with complement C4 and the RP-C4-CYP21-TNX (RCCX) modules of the major histocompatibility complex

The number of the complement component C4 genes varies from 2 to 8 in a diploid genome among different human individuals. Three quarters of the C4 genes in Caucasian populations have the endogenous retrovirus, HERV-K(C4), in the ninth intron. The remainder does not. The C4 serum proteins are highly polymorphic and their concentrations vary from 100 to approximately 1000 microg/ml. There are two distinct classes of C4 protein, C4A and C4B, which have diversified to fulfill (a) the opsonization/immunoclearance purposes and (b) the well-known complement function in the killing of microbes by lysis and neutralization, respectively. Many infectious and autoimmune diseases are associated with complete or partial deficiency of C4A and/or C4B. The adverse effects of high C4 gene dosages, however, are just emerging, as the concepts of human C4 genetics are revised and accurate techniques are applied to distinguish partial deficiencies from differential expression caused by unequal C4A and C4B gene dosages and gene sizes. This review attempts to dissect the sophisticated genetics of complement C4A and C4B. The emphases are on the qualitative and quantitative diversities of C4 genotypes and phenotypes. The many allotypic variants and the processed products of human and mouse C4 proteins are described. The modular variation of C4 genes together with the serine/threonine nuclear kinase gene RP, the steroid 21-hydroxylase CYP21, and extracellular matrix protein TNX (RCCX modules) are investigated for the effects on homogenization of C4 protein polymorphisms, and on the unequal genetic crossovers that knocked out the functions of CYP21 and/or TNX. Furthermore, the influence of the endogenous retrovirus HERV-K(C4) on C4 gene expression and the dispersal of HERV-K(C4) family members in the human genome are discussed.

The molecular basis of complete complement C4A and C4B deficiencies in a systemic lupus erythematosus patient with homozygous C4A and C4B mutant genes

The disease course of a complete C4-deficient patient in the U.S. was followed for 18 years. The patient experienced multiple episodes of infection, and he was diagnosed with systemic lupus erythematosus at age 9 years. The disease progressed to WHO class III mild lupus nephritis and to fatal CNS vasculitis at age 23 years. Immunochemical experiments showed that the patient and his sibling had complete absence of C4A and C4B proteins and were negative for the Rodgers and Chido blood group Ags. Segregation and definitive RFLP analyses demonstrated that the patient and his sibling inherited two identical haplotypes, HLA A2 B12 DR6, each of which carries a defective long C4A gene and a defective short C4B gene. PCR and DNA sequencing revealed that the mutant C4A contained a 2-bp insertion in exon 29 at the sequence for codon 1213. The identical mutation was absent in the mutant C4B. The C4B mutant gene was selectively amplified by long range PCR, and its 41 exons were completely sequenced. The C4B mutant had a novel single C nucleotide deletion at the sequence for codon 522 in exon 13, leading to frame-shift mutation and premature termination. Thus, a multiplex PCR is designed by which known mutations in C4A and C4B can be elucidated conveniently. Among the 28 individuals reported with complete C4 deficiency, 75-96% of the subjects (dependent on the inclusion criteria) were afflicted with autoimmune or immune complex disorders. Hence, complete C4 deficiency is one of the most penetrant genetic risk factors for human systemic lupus erythematosus.

Genetic, structural and functional diversities of human complement components C4A and C4B and their mouse homologues, Slp and C4

The complement protein C4 is a non-enzymatic component of the C3 and C5 convertases and thus essential for the propagation of the classical complement pathway. The covalent binding of C4 to immunoglobulins and immune complexes (IC) also enhances the solubilization of immune aggregates, and the clearance of IC through complement receptor one (CR1) on erythrocytes. Human C4 is the most polymorphic protein of the complement system. In this review, we summarize the current concepts on the 1-2-3 loci model of C4A and C4B genes in the population, factors affecting the expression levels of C4 transcripts and proteins, and the structural, functional and serological diversities of the C4A and C4B proteins. The diversities and polymorphisms of the mouse homologues Slp and C4 proteins are described and contrasted with their human homologues. The human C4 genes are located in the MHC class III region on chromosome 6. Each human C4 gene consists of 41 exons coding for a 5.4-kb transcript. The long gene is 20.6 kb and the short gene is 14.2 kb. In the Caucasian population 55% of the MHC haplotypes have the 2-locus, C4A-C4B configurations and 45% have an unequal number of C4A and C4B genes. Moreover, three-quarters of C4 genes harbor the 6.4 kb endogenous retrovirus HERV-K(C4) in the intron 9 of the long genes. Duplication of a C4 gene always concurs with its adjacent genes RP, CYP21 and TNX, which together form a genetic unit termed an RCCX module. Monomodular, bimodular and trimodular RCCX structures with 1, 2 and 3 complement C4 genes have frequencies of 17%, 69% and 14%, respectively. Partial deficiencies of C4A and C4B, primarily due to the presence of monomodular haplotypes and homo-expression of C4A proteins from bimodular structures, have a combined frequency of 31.6%. Multiple structural isoforms of each C4A and C4B allotype exist in the circulation because of the imperfect and incomplete proteolytic processing of the precursor protein to form the beta-alpha-gamma structures. Immunofixation experiments of C4A and C4B demonstrate > 41 allotypes in the two classes of proteins. A compilation of polymorphic sites from limited C4 sequences revealed the presence of 24 polymophic residues, mostly clustered C-terminal to the thioester bond within the C4d region of the alpha-chain. The covalent binding affinities of the thioester carbonyl group of C4A and C4B appear to be modulated by four isotypic residues at positions 1101, 1102, 1105 and 1106. Site directed mutagenesis experiments revealed that D1106 is responsible for the effective binding of C4A to form amide bonds with immune aggregates or protein antigens, and H1106 of C4B catalyzes the transacylation of the thioester carbonyl group to form ester bonds with carbohydrate antigens. The expression of C4 is inducible or enhanced by gamma-interferon. The liver is the main organ that synthesizes and secretes C4A and C4B to the circulation but there are many extra-hepatic sites producing moderate quantities of C4 for local defense. The plasma protein levels of C4A and C4B are mainly determined by the corresponding gene dosage. However, C4B proteins encoded by monomodular short genes may have relatively higher concentrations than those from long C4A genes. The 5' regulatory sequence of a C4 gene contains a Spl site, three E-boxes but no TATA box. The sequences beyond--1524 nt may be completely different as the C4 genes at RCCX module I have RPI-specific sequences, while those at Modules II, III and IV have TNXA-specific sequences. The remarkable genetic diversity of human C4A and C4B probably promotes the exchange of genetic information to create and maintain the quantitative and qualitative variations of C4A and C4B proteins in the population, as driven by the selection pressure against a great variety of microbes. An undesirable accompanying byproduct of this phenomenon is the inherent deleterious recombinations among the RCCX constituents leading to autoimmune and genetic disorders.

The phylogeny and evolution of the thioester bond-containing proteins C3, C4 and alpha 2-macroglobulin

The complement system is an effector of both the acquired and innate immune systems of the higher vertebrates. It has been traced back at least as far as the echinoderms and so predates the appearance of the antibodies, T-cell receptors and MHC molecules of adaptive immunity. Central to the function of complement is the reaction of the thioester bond located within the structure of complement components C3 and C4. The structural thioester first appeared in a protease inhibitor, alpha 2-macroglobulin, in which it is involved in the immobilisation and entrapment of proteases. An important development in the C3 molecule has been the acquisition of a catalytic His residue which greatly increases the rate of reaction of the thioester with hydroxyl groups and with water.

Genetic polymorphism of the fourth component of human complement: population study and proposal for a revised nomenclature based on genomic PCR typing of Rodgers and Chido determinants

The fourth component of human complement (C4) is coded for by two homologous genes, C4A and C4B, located in the class III region of the major histocompatibility complex (MHC). Genetic typing of C4A and B alleles is routinely carried out by high-voltage agarose gel electrophoresis. The electrophoretic C4 polymorphism can be further subdivided by the Rodgers (Rg) and Chido (Ch) blood groups, which are antigenic determinants of the C4A and B alpha-chains, respectively. We have used a recently described direct PCR typing method using sequence-specific primers (PCR-SSP) in combination with electrophoretic C4 typing as well as genomic RFLP analysis to determine the frequency of C4 allotypes, Rg/Ch subtypes and C4A-B haplotypes in a family study of the German population. As the current C4 allele designation does not provide any information about the presence or absence of Rodgers and Chido antigens, we have developed an extension to the existing C4 nomenclature. This revised allele designation combines the existing numerical allotypes defined by electrophoretic mobility with eight subtypes (01-08) based on Rg/Ch PCR genotyping results. Using this approach, most electrophoretic allotypes could be subdivided. Among the C4A allotypes, the most common allele was A*0301 (59.9%), and the most common subtype among all electrophoretic allotypes was 01 (85.1%; = Rg1,2-positive, Ch-negative). For C4B, the most common allele was B*0101 (64.3%), and the most common subtype was 01 (79.6%; = Ch1,2,3,4,5,6-positive, Rg-negative). The subtypes 03, 04, 07 and 08 of the C4A allotypes, and the subtypes 03, 07 and 08 of the C4B allotypes, were not detected in this study. The analysis of duplicated C4 alleles revealed considerable heterogeneity of their subtypes. The results demonstrate that all known C4 allotypes can now be assigned unambiguously, which facilitates the identification of MHC haplotypes relevant for transplantation and disease association studies.

Polymerase chain reaction analysis of the Xba I polymorphism of the human complement C4 genes provides evidence for strong haplotype conservation

The genes coding for the two isotypes of the fourth component of human complement, C4A and C4B, are located between the HLA-B and -DR loci of the MHC. We studied the linkage relationship of the previously described XbaI RFLP to obtain further insight into the evolution of the tandemly arranged C4 genes. Using exon-specific PCR amplification followed by restriction analysis and direct DNA sequencing, the polymorphic site could be located in exon 40 of the C4 gene (cDNA position 5095). The polymorphism does not change an amino acid residue. Using nested PCR amplification with isotype-specific primers to amplify either C4A or C4B alleles the haplotype arrangement of the XbaI sites in both isotypic C4 genes was analyzed independently. It was observed that the XbaI restriction site was either present or absent in both C4 genes of a given haplotype. In a study of 106 Caucasian haplotypes, only two different haplotypes could be identified carrying a C4A gene with and a C4B gene without the XbaI restriction site. Also, the XbaI site could only be detected in long C4 genes possessing the 6.5-kb insertion in intron 9. Our findings provide evidence that the mutation creating the XbaI polymorphism occurred in an ancestral C4 gene already carrying the long intron 9. The duplicating resulting in the presence of two isotypic genes, C4A and C4B, must have taken place subsequently giving rise to haplotypes with or without the XbaI site.

A new PCR-based typing of the Rodgers and Chido antigenic determinants of the fourth component of human complement

The Rodgers (Rg) and Chido (Ch) blood groups are antigenic determinants of the fourth component of human complement C4. They are associated with the two isotypes of C4, C4A and C4B, respectively. They serve as markers to distinguish C4A from C4B as well as for the definition of subtypes of common and rare allotypes. As an alternative to the serological typing method using human alloantisera, a PCR typing procedure with sequence-specific primers (PCR-SSP) was designed. The method was tested on selected DNA samples from individuals with well-defined C4 allotypes. No false-positive or false-negative typing results were obtained and all the determinant combinations could be distinguished. The PCR genotyping allowed the detection of all Rg/Ch sequence determinants of each isotype. Thus, reverse antigenicity could also be established in the presence of other C4 allotypes without a segregation study. To exclude the possibility that PCR-typed determinants originate from a non-expressed C4 null gene, a sequence-specific PCR was established detecting a 2-bp insertion in exon 29 described previously as a cause for C4A non-expression. PCR Rg/Ch genotyping provides a fast and efficient method for routine typing in HLA haplotype and disease association studies.

Antiglobulin testing for CR1-related (Knops/McCoy/Swain-Langley/York) blood group antigens: negative and weak reactions are caused by variable expression of CR1

The Knops, McCoy, Swain-Langley and York antigens have recently been identified as being on complement receptor type 1 (CR1, CD35, C3b/C4b receptor). We examined the relationship between CR1 expression and the reactivity of the CR1-related blood group antigens with their specific antibodies. RBC from donors of selected phenotypes were tested by hemagglutination using two monoclonal antibodies to CR1, as well as anti-Kna, -McCa, -S1a, -'Kn/McC' and -Yka. Monoclonal antibodies 3D9 and E11 required approximately 250 and approximately 400 CR1/RBC to obtain a positive reaction. Agglutination of antigen-positive cells by human polyclonal antisera was related to the CR1/RBC: thus, cells expressing 20-100 CR1/RBC were negative and included the previously designated null phenotypes for this collection, 100-150 were weak or negative, and greater than 200 were usually positive. One RBC sample carried Yka on the 190,000 dalton (A or F allele), but not the 220,000 dalton (B or S allele) variant of CR1, and gave inconsistent reactions with Yka antisera. These data provide an explanation for certain of the serologic characteristics of the CR1-related blood group antigen system.

HLA and disease

Many human diseases are associated with HLA class I, class II and class III antigens. It appears that the class III antigen disease associations can be explained by a direct defect operating at the level of either the class III gene or its gene product. The mechanism underlying class I and class II antigen disease associations is at present unknown. In this review we have considered thirty diseases which have been ranked according to their relative risk as defined by the frequency of a given HLA antigen in patient and control populations. The chronic inflammatory disorder, ankylosing spondylitis and its association with HLA B27 has been used as a model to study the HLA linked diseases. We have suggested that the disease may be caused by the Gram-negative microorganism Klebsiella which has antigenic similarity to HLA B27. It is proposed that some antibodies made against Klebsiella bind to HLA B27, thereby acting as autoantibodies leading to the pathological sequelae of chronic inflammatory arthritis. This is the crosstolerance hypothesis or molecular mimicry model and it has been compared to the receptor model. It is further suggested that the crosstolerance hypothesis can be utilised as a general theory to explain the association of other diseases with the class I and class II antigens, and offer a possible explanation for the polymorphism of HLA.

Complement

The complement system mediates a wide range of important biological functions. The use of modern techniques in protein chemistry and molecular biology has greatly facilitated our understanding of the interactions between the fluid phase and cell-bound components of the system. Structural and genetic analysis has shown that while many of these components are polymorphic, there are major similarities between many of the proteins serving enzymatic and regulatory roles in both the alternative and classical pathways. The regulation of complement activation and Class III genes, on chromosomes 1 and 6 respectively, encode nine of the major proteins in the system. The genetic basis of C4 and C3 polymorphisms is now well established, and further study may reveal functional differences between polymorphic variants of other components. The study of individuals with either genetic or acquired deficiencies of complement proteins and receptors has provided insight into the function of these components, leukocyte adherence deficiency (LAD) providing the best example. An appreciation of the genetics, structure and functions of the regulatory proteins decay-accelerating factor (DAF) and homologous restriction factor has enhanced our understanding of the pathogenesis of paroxysmal nocturnal haemoglobinuria. The full importance of CD59 glycoprotein, the newest member of the complement family, remains to be determined.

Complement allotypes in familial and sporadic Alzheimer's disease

To resolve conflicting findings on the association of complement allotypes with Alzheimer's disease (AD) we have studied the C4 phenotypes in 33 sporadic cases and in one family with familial AD. We found no association with complement alleles in familial or sporadic AD, even though a familial case had absence of the C4 null allele (C4BQ0). Our data do not suggest a role for complement genes in the pathogenesis of AD. It also seems that the C4B2 allele cannot be used as a marker for AD as has been suggested by others.

Use of DNA amplification (PCR) and direct DNA sequencing in the characterization of C4 alleles

A procedure for detailed characterization of individual C4 alleles has been developed. DNA containing the two polymorphic clusters of C4 was amplified in the polymerase chain reaction (PCR). Direct DNA sequencing of amplified DNA was then performed by a modification of previously described techniques. The results were confirmed by M13 sequencing. Single C4A3 and C4B1 allele sequences were in accordance with previous reports. An individual typed C4A3B1 revealed double bands in the autoradiogram in the positions corresponding to the polymorphic nucleotides. We did not find the reported thymine in position 3641 specific for the C4A4 allele in an individual typed C4A4B2.

Sandwich enzyme-linked immunosorbent assays for the quantification of the C4 isotypes (C4A and C4B) in human plasma

Sandwich enzyme-linked immunosorbent assays were developed to determine the concentration of the isotypes of the fourth component of human complement (C4A and C4B) in human plasma. In the case of C4A a monoclonal antibody against a common determinant of the alpha chain was used to capture the protein. The bound antigen was then detected with a biotinylated monoclonal antibody reacting exclusively with the C4A isotype, followed by peroxidase labeled avidin. For the quantification of C4B, C4B-specific monoclonal antibodies were coated onto a microtitration plate in order to capture the protein. Bound antigen was then detected with a biotinylated monoclonal antibody directed against C4 followed by peroxidase labeled avidin. The assays, which were rapid, selective and specific for C4A and C4B, respectively, provide an alternative to gel electrophoresis and blot procedures for the study of unexpressed alleles ('null alleles') at each of the C4 loci.

Heightened complement sensitivity of acquired immunodeficiency syndrome lymphocytes related to diminished expression of decay-accelerating factor

Although the human immunodeficiency virus can induce cytopathic changes in human lymphocytes in vitro, the mechanism(s) underlying progressive lymphopenia in patients with AIDS and AIDS-related complex has not been elucidated. To investigate this issue, peripheral blood lymphocytes of AIDS and AIDS-related complex patients and healthy control subjects were examined for their ability to resist homologous complement-mediated lysis. Upon sensitization with monoclonal antibodies to major histocompatibility complex class I antigen, as much as 48% lysis of patients' cells was observed in as little as a 1:32 dilution of human serum compared to 18 +/- 8% (mean +/- SD) lysis of controls' cells even in a 1:8 dilution of human serum. To investigate the mechanism of the abnormal complement sensitivity, AIDS and AIDS-related complex cells were analyzed for expression of decay-accelerating factor (DAF), a complement regulatory protein that functions intrinsically in blood cell membranes to prevent complement activation on their surfaces. Flow cytometric assays using anti-DAF monoclonal antibodies demonstrated that patients' lymphocytes and monocytes were DAF-deficient, in contrast to their polymorphonuclear leukocytes, which showed normal DAF levels. Expression of DAF was diminished on CD4+ as well as CD8+ T-lymphocyte subpopulations as opposed to expression of CD3, which was comparable in patients and controls. Incubation of normal lymphocytes with anti-DAF monoclonal antibodies or phosphatidylinositol-specific phospholipase C, an enzyme that cleaves DAF, enhanced lysis. Conversely, reconstitution of patients' cells with exogenous DAF reduced their lysis. The findings of heightened complement sensitivity and DAF deficiency of patients' lymphocytes in vitro suggest the possibility that the DAF deficit may contribute to the progressive lymphopenia of AIDS in vivo.

High-titer, low-avidity (HTLA) antibodies and antigens: a review

Antibodies that react to HTLA characteristics cause difficulties in serologic testing because of the weak reactions they produce in the indirect antiglobulin test. Those specificities that are more frequently encountered (anti-Yka, -McCa, -Kna, -Ch) are directed toward antigens of high incidence in both the white and black populations. They have not been shown to cause significant destruction of transfused antigen-positive red cells. The antibodies create problems in serologic tests because the reactions they produce interfere with the identification of reactions due to other, clinically significant antibodies.

The molecular genetics of components of the complement system

Rapid progress has been made recently on the elucidation of the structural components of the complement system by the application of recombinant DNA techniques. The derived amino acid sequences of most of the complement proteins are now available through cDNA cloning, and significant progress has been made in the discovery of the genetic organization of the corresponding genes. The linkage of some of the complement component genes has been established through the study of phenotypic genetics. Of particular interest has been the mapping of two clusters of genes which encode proteins involved in the activation of C3. C2, C4 and factor B, three of the structural components of the classical and alternative pathway C3 convertases, are encoded by genes which map to the MHC on human chromosome 6. The linkage of the genes with each other in a 100 kb segment of DNA has been established through the isolation of overlapping cosmid clones of genomic DNA, and PFGE has defined the molecular map position of these genes within the class III region of the MHC. The regulatory proteins factor H, C4BP, CR1 and DAF, which are involved in the control of C3 convertase activity, are encoded by closely-linked genes (termed the regulators of complement activation or RCA linkage group) that have been mapped to human chromosome 1. PFGE has defined the linkage of the CR1, C4BP and DAF genes, together with the CR2 gene in an 800 kb segment of DNA, and it is clear that this technique will eventually be applied to the molecular mapping of other complement genes in relation to their flanking loci. Polymorphism is a feature of many of the complement proteins, especially those encoded by genes in the MHC class III region. Of these, C4 is by far the most polymorphic, and differences in gene size and gene number, in addition to the functional and antigenic differences in the gene products, have been recognized. Null alleles at either of the C4 loci are rather common and may be important susceptibility factors in some HLA-associated diseases, particularly SLE. The molecular basis of complement deficiency states has begun to be elucidated. In many cases, the deficiency is not caused by a major gene deletion or rearrangement, and techniques which detect single point mutations in DNA (Cotton et al, 1988) will have to be applied to fully characterize the nature of the defect.

The antigenic determinants, Rg/Ch/WH, expressed by Japanese C4 allotypes

The expression of antigenic determinants, Rg/Ch/WH, on Japanese C4 allotypes has been studied. Although the Japanese C4 allotype frequencies are known to differ from Europeans, the antigenic expression of their C4 allotypes correlates with associations described previously. All 89 random donors and 17 selected donors were Rg:1,2 so neither Rg:1,-2 nor Rg:1,-2 was found. The frequency of Ch:1,-2,3 was elevated while that of Ch:1,2,3 was reduced, which was seen as a direct result of the higher frequency of B2 and B5 allotypes. None of the Japanese were Ch:1,2,-3, but this can be accounted for by the absence of the A*6,B*1 haplotype. The WH determinant, which has been associated completely with Rg:1,-2 in Caucasians, was found at a higher frequency, 32%, in association with an A*3,2,B*QO haplotype expressing Rg:1,2, which has not been described previously. Detailed investigation showed that the A3 allotype was Rg:1,2 whereas the A2 allotype only expressed Rg1 (Rg:1,-2 WH+).

An immunoblotting technique for direct visualization of Chido and Rodgers reactivity on C4 variants after electrophoresis

An immunoblotting technique allows direct visualization of Chido and Rodgers antigenic determinants on intact C4 proteins. C4 molecules separated by electrophoresis are selectively transferred to nitrocellulose membranes saturated with goat antiserum to human C4. The membranes are then incubated in alloanti-Chido or anti-Rodgers followed by enzyme-conjugated goat antihuman IgG. Molecules with Chido or Rodgers reactivity are visualized by incubation with an indicator substrate for the bound enzyme.

There are two C4 genetic loci and a null allele in the chimpanzee

Genetic polymorphism in C4 in the chimpanzee was studied by agarose gel electrophoresis of desialated plasma and development of patterns by immunofixation with antiserum to human C4 and by a C4-sensitive hemolytic overlay. In general, immunofixation patterns showed multiple partially overlapping bands of which only the most cathodal had strong hemolytic activity. In analogy to human C4, the latter were designated C4B, whereas those detected by immunofixation which had little hemolytic activity were designated C4A. Chimp C4A and C4B reacted with human and mouse (monoclonal) anti-C4B and human anti-Ch1 but neither reacted with monoclonal anti-C4A or human anti-Ch2, Ch3, Rg1, or Rg2. On sodium dodecyl sulfate polyacrylamide gel electrophoresis, the alpha chain of C4B showed a slightly lower apparent relative mass than that of C4A at around Mr 93,000. There were three C4A variants and two C4B variants inherited in families as autosomal codominant traits, as C4A-C4B cosegregating pairs with no detectable crossing-over. These pairs were inherited with chimpanzee leukocyte antigen types C2 and BF variants without detectable crossing-over. Half-null C4 haplotypes with C4B QO were observed in family studies. Nine BF, C2, C4A, C4B allelic haplotypic combinations (complotypes) were identified among presumably unrelated chimpanzees.

The chemical structure of the C4d fragment of the human complement component C4

The complete amino acid sequence of the C4d fragment (380 residues long) of the human complement component C4 is presented. Most of the sequence was determined by analysis of CNBr peptides and tryptic peptides obtained from S-carboxymethylated protein. The sequence of the amino terminal 88 residues [Campbell R. D., Gagnon J. and Porter R. R. (1981) Biochem. J. 199, 359-370] and a 106 residue polymorphic segment of C4d [Chakravarti D. N., Campbell R. D. and Gagon J. (1983) FEBS Lett. 154, 387-390] was extended. Some overlaps not provided by the protein sequence analysis were obtained from the amino acid sequence predicted by the nucleotide sequence [Belt K. T., Carroll M. C. and Porter R. R. (1984) Cell 36, 907-914]. The present protein sequence data provide information for the isolation of all the CNBr and succinylated tryptic peptides of C4d. In addition to the polymorphism previously described, two other sets of polymorphic amino acid residues at positions 153 (Ile/Ser) and 154 (Gln/Ala) have been identified. The major site of glycosylation has been shown to be an asparagine residue located in the sequence -Asn-Val-Thr- in the carboxy terminal end of C4d. A remarkable difference in the predicted secondary structure of C4d arising from one set of four polymorphic residues in a stretch of six residues and another single polymorphic residue suggests a structural basis for the origin of the different chemical reactivities of the C4 isotypes (C4A and C4B) and their serological difference in the expression of Rodgers or Chido blood group antigens. Possible non-covalent membrane attachment sites have been suggested from the hydropathy profile. Comparison of the C4d sequence with human C3, C5 and alpha 2-macroglobulin revealed extended stretches of sequence similarity (between 19 and 38% homology) with the corresponding regions of these proteins.

Definitive RFLPs to distinguish between the human complement C4A/C4B isotypes and the major Rodgers/Chido determinants: application to the study of C4 null alleles

Definitive restriction fragment length polymorphisms (RFLPs) representing the exact locations responsible for isotypicity between the human complement components C4A and C4B, and their generally associated major Rodgers (Rg1) and Chido (Ch1) antigenic determinants, have been designed. By means of C4d-specific genomic probe for Southern blot analysis, a C4A gene can be defined by the presence of the 276 bp and 191 bp N1a IV fragments, while a C4B gene can be defined by a single 467 bp N1aIV fragment. In addition, an Rg1-expressing C4 gene can be represented by a 565 bp EcoO 109 fragment, and a Ch1-expressing C4 gene by a 458 bp EcoO 109 fragment, under the same conditions. All these polymorphic restriction fragments can be unambiguously and conveniently detected. In combination with the Taq I polymorphic patterns specific for the C4 loci and for the neighboring 21-hydroxylase genes, the nature and structure of the tandem C4,21-hydroxylase gene complex can be elucidated. In this study, it is inferred that the null allele of the HLA haplotype B44 DR6 C4A3 C4BQO is not a C4B allele, but probably encodes another C4A 3 allotype at the second C4 locus.

Heterogeneity of steroid 21-hydroxylase genes in classical congenital adrenal hyperplasia

Careful genotyping of three families, each having a member with classical salt-losing steroid 21-hydroxylase deficiency, has allowed identification of carrier haplotypes. Digestion with TaqI or EcoRI and probing with a cDNA probe for the 21-hydroxylase genes (pC21/3c) revealed that all six affected haplotypes are abnormal with at least EcoRI. The data suggest that there is extreme polymorphism of the 21-hydroxylase genes and that dysfunction may result from several different abnormalities.

Allo-anti-Chido in a Ch-positive patient

Allo-anti-Chido (Ch) was detected in a patient whose red cells typed as Ch+. The C4 allotype of the patient was A4,B2 which associates strongly with the Ch phenotype Ch:1,-2,3,4,-5,6. Anti-Ch2 + Ch5 were the Ch specificities identified. Absence of only Ch2 and Ch5 determinants on the C4B protein allowed this unique immune response to blood transfusion.

Three Chido determinants detected on the B5Rg+ allotype of human C4: their expression in Ch-typed donors and families

A study was made of polyspecific human allo-anti-C4, anti-Chido (Ch), which reacts with determinants usually located on C4B protein. Some anti-Ch reagents are capable of reacting with Ch- red cells coated with C4 from Ch:-1,-2,-3 donors. A complex serologic pattern demonstrated three more Ch determinants, Ch4, Ch5, and Ch6, which were detected by haemagglutination-inhibition tests. All Ch:1,2,3 samples were Ch:4,5,6 but samples lacking one or more of the Ch1,Ch2,Ch3 series of determinants also lacked some of the new determinants. MHC typed families demonstrated the inheritance of the new determinants as part of the Ch haplotype, and associations with C4 allotypes and haplotypes have been established. Ch4 always associates with C4B protein. Ch5 and Ch6, normally detected on C4B protein, were detected in several individuals who lacked C4B (BQO allotypes) and were therefore presumed in these instances to be located on the accompanying C4A protein.

High resolution electrophoresis for the allotyping of human C4. Proposal of a relational nomenclature

Among the major histocompatibility complex (MHC)-linked complement genes, the loci for C4A and C4B exhibit the most extensive structural polymorphisms. Therefore the differentiation of variant and complex C4 phenotypes often proves difficult in conventional immunofixation electrophoresis. To improve the available technique of C4 typing a closed horizontal electrophoresis system was combined with poly- and monoclonal alkaline phosphatase immunoprobing on contact diffusion blots. The high resolution and sensitivity of this method not only facilitated C4 allotyping but also revealed additional polymorphic variation. Relative electrophoretic mobilities specific for each C4 allotype were established by computerized remission densitometry and provided the basis for a quantitative denomination of C4 variants. Typing by high resolution electrophoresis and the proposed relational C4 nomenclature could be valuable for further immunogenetical studies of the C4 protein polymorphism.

A monoclonal antibody which can distinguish between the two isotypes of human C4

A monoclonal antibody to Human C4 (L003) has been shown to bind to the polymorphic C4d fragment of the alpha-chain of C4. Another monoclonal antibody (L001) was shown to bind to the beta-chain. L003 reacts differently with the two isotypes of C4, C4-A and C4-B. The equilibrium constant of L003 for C4-B is approx. 7-fold higher than that for C4-A, while L001 has similar affinity for both C4 types. Also, L003 binding to C4-A is very much more pH dependent than is its binding to C4-B. These observations explain the very useful property of L003 in being able to separate the two isotypes by affinity chromatography.

The purification and properties of some less common allotypes of the fourth component of human complement

Human complement component C4 is coded by two genes situated between HLA-D and HLA-B. Both genes are highly polymorphic; C4-A gene products normally carry the blood group antigen Rodgers and C4-B proteins usually carry the Chido antigen. Using a monoclonal antibody which binds Rodgers-positive and Chido-positive proteins with different affinities, we have purified a number of less common C4 allotypes and compared their properties. All C4-B allotypes tested have similar specific hemolytic activities and binding efficiencies to small molecules. All C4-A proteins tested had similar binding to small molecules and hemolytic activities except for the C4-A6 proteins from two individuals with different extended haplotypes, both of which had identical hemolytic activities and much lower ones than other C4-A allotypes. Two allotypes, C4-A1, Rodgers-negative but Chido-positive, and C4-B5, Chido-negative but probably Rodgers-positive, were found to behave as typical C4-A and C4-B proteins, respectively, apart from the switch in their antigenic properties.

Membrane-bound C4b interacts endogenously with complement receptor CR1 of human red cells

Activation of the classical complement pathway on the membrane of autologous cells results in the deposition of C4b on their surface and in the assembly of the C3 convertase C4b2a, one of the amplifying enzymes of the cascade. Here we study the sequence of events leading to irreversible inactivation of the potentially harmful C4b bound to human red cells. We show that deposited C4b interacts endogenously with complement receptor type 1 (CR1) present on the membrane of the same red cell. Complexes containing CR1 and C4b are found in extracts of membranes of C4b-bearing red cells after treatment of the intact cells with a bifunctional crosslinking reagent. The amount of complexed CR1 increases with the number of deposited C4b molecules. Only small amounts of free CR1 are observed on red cells bearing as few as 1,900 molecules of C4b, suggesting that the binding avidity between C4b and endogenous CR1 is high. In agreement with this observation, we find that the deposited C4b inhibits the exogenous cofactor activity of the red cell CR1 for the factor I-mediated cleavage of target-bound clustered C3b. The C4b bound to the human red cells is cleaved by the serum enzyme C3b/C4b inactivator (factor I) and a large fragment (C4c) is released in the incubation medium. The cleavage is totally inhibited by mAbs against CR1, showing that the complement receptor is an essential cofactor for the activity of I. When the number of bound C4b per red cell is relatively small (less than 1,000 molecules) the substrate for the enzymatic activity of factor I is mostly or exclusively the C4b bound endogenously to CR1. Indeed, the kinetics or the extent of cleavage of C4b are not affected by greatly augmenting the concentration of exogenous CR1 or of C4b-bearing red cells in the incubation mixture, thereby increasing the frequency of collisions between CR1 on the surface of one cell with C4b deposited on the membrane of a different cell. On the basis of the present and prior observations, we speculate that both DAF and CR1 act endogenously to inactivate the function of autologous red cell-bound C4b and prevent the progression of the cascade. DAF binding prevents the formation of the C3 convertase, C4b2a. The cleavage and irreversible inactivation of C4b only occurs after the concerted activities of endogenous CR1 and serum factor I.(ABSTRACT TRUNCATED AT 400 WORDS)

A product of the C4B locus lacking hemolytic activity

An unusual C4B allotype, C4B 4I, was identified in a study of C4 polymorphism in Japanese. This variant was defined as C4B using a murine monoclonal antibody specific for the C4d region of C4B. Hemolytic assay, however, revealed that the C4 variant, C4 BI was hemolytically inactive in contrast to other well-defined C4B locus products.

HLA haplotypes with C4B5; evidence for further allelic heterogeneity

Twenty-three individuals from various disease groups and normal controls were identified by immunofixation with anti-C4, C4-dependent lysis, determination of Rg (Rodgers) and Ch (Chido) phenotypes, and immunoblotting with C4-specific mouse monoclonal antibody. We found that one haplotype predominates with the C4B*5 allele, HLA-A11, B22(55), Cw3, Bf*S, C4A*4B*5, which also carries the Ch1,-2, 3 haplotype. The B5 allotype was also found with HLA-B60, HLA-B35 in Caucasoids, and HLA-B18 in non-Caucasoids; these carried the Ch-1, -2, -3 haplotype. Our results are in accord with an earlier report of two B5 subtypes, B5Rg+ and B5Rg- (Roos et al. 1984). The specificity of the mouse monoclonal antibodies IC4 and 2B12 had been previously related to C4A and C4B, respectively, but our results suggest that they relate more closely to Rg and Ch determinants.

Complement in the pathophysiology and diagnosis of human diseases

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

The molecular genetics of components of complement

Rapid progress has been made in establishing linkages and in chromosome allocation of the genes of some 9 complement components. In the MHC, C2, Factor B, and two C4 or C4 related genes have been placed in some detail in both man and mouse. The gene coding for the cytochrome P-450 21-hydroxylase has been shown to be duplicated and immediately 3' to the two C4 genes, though it appears to be functionally and structurally unrelated to the complement components. Thus six genes have been mapped to this region where particular haplotypes are associated with increased susceptibility to a number of diseases, some of which are autoimmune in character. The complete gene structure of Factor B has been solved in man and rapid progress is being made with the C2 and C4 genes. The structural basis of the polymorphisms of these genes is being established. In C4, the polymorphism is exceptionally complex with varying numbers of loci and probably more than 50 allotypes occurring in man. A structural basis has also been found for the big differences in the biological activity of some of the C4 allotypes in man. Apart from the genes in the MHC, linkage has been found between the genes coding for C4bp, CR1, and Factor H. Remarkably there are sequence homologies between these proteins and C2 and Factor B, probably related to the ability to bind to one or other of the structurally similar proteins C3b and C4b. The complete cDNA sequences of C3 and C4 in mouse and man have given much information on the many posttranslational modifications of these proteins. A partial structure has been obtained for the C3 gene and the homology shown between C3, C4, C5, alpha 2-macroglobulin, and pregnancy zone protein. Although the amount of detailed information in the molecular genetics of complement components is accumulating rapidly, there appears to be a reasonable prospect that linkages and homologies will classify the data into a comprehensible form.

C4B3 allotype with a novel Ch phenotype

The fourth component of complement (C4) has two classes of protein, C4A and C4B, both of which have many allelic forms. The serological determinants Rodgers (Rg1, Rg2) and Chido (Ch1, Ch2, Ch3) are generally associated with C4A and C4B, respectively. The C4B3 allotype has been detected in a single Canadian family that expresses a novel Ch phenotype, Ch:-1, 2, -3. There was no information for the Rg determinants, as the C4A*2B*3 haplotype would normally express Rg on the C4A protein. Other C4B3 allotypes in informative families have different Ch phenotypes, and the relationships of these within extended major histocompatibility complex haplotypes are discussed in this paper.

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.