Structural and functional differences between the H-2 controlled Ss and Slp proteins

Sex-limited protein: in vitro and in vivo functions

Mouse complement component C4 exists in two isoforms, C4 and a protein with expression restricted to male animals called sex-limited protein (Slp). Although Slp is about 95% homologous to C4, it is generally believed to be non-functional, at least in conventional haemolytic complement assays. In a previous study, however, we showed that Slp is haemolytically active in a C1-inhibitor (C1INH)-regulated, EDTA-resistant mouse complement activation pathway. To study other possible implications of this finding, we generated constitutively expressing Slp-transgenic mice. The transgene was crossed into otherwise Slp-deficient C57Bl/6J and NZB mice. Members of the third backcross generation of C57Bl/6J mice were tested for functional Slp and classical and alternative complement pathway activities (CH50 and AP50 levels, respectively). Slp-transgenic C57Bl/6J mice showed enhanced CH50, but normal AP50 levels when compared with non-transgenic littermates. To discover a possible protective role for Slp in spontaneous systemic lupus erythematosus (SLE) in NZBxNZW (NZBxW) mice, the third backcross generation of Slp-transgenic NZB mice was mated with NZW mice and the development of SLE in the female offspring was followed. In these introductory experiments, Slp-transgenic NZBxW animals presented with a significantly extended life span. Our results imply that Slp is a mouse complement component with functions which partially resemble some of those of human C4A.

Rapid, activity-guided isolation of sex-limited protein (Slp) from mouse serum by fractionated precipitation and high-performance liquid chromatography

We have recently demonstrated that sex-limited protein (Slp) plays a key role in an EDTA-resistant mouse complement activation pathway. A rapid procedure, utilizing classical chromatography methods on an FPLC system, was developed for the isolation of functionally active Slp. The method is based on the fractionated precipitation of serum by polyethylene glycol 6000, followed by heparin Sepharose Cl-6B affinity chromatography, Mono Q anion exchange and Superose 12 gel filtration. The isolation of Slp was monitored by a hemolytic assay. The procedure resulted in the purification of Slp, which by SDS-PAGE gave a single band of M(r) 2000,000 under non-reducing conditions, and under reducing conditions three bands corresponding to M(rs) of 105,000, 76,000 and 37,000.

The androgen-dependent C4-Slp gene is driven by a constitutively competent promoter

The androgen-dependent liver protein Slp, together with its constitutively expressed closely related isoform C4, provides a model to address the question of which minimal alteration in DNA can shut off the expression of a gene in a manner reversible by testosterone or by trans-acting mutations. Previous work indicated that sequences located at -1.9, -0.45, and -0.25 kb from the transcription start site of the C4-Slp gene played a critical role in determining its unusual functional divergence from C4. Now, using quantitatively and qualitatively controlled transfection assays in HepG2 human hepatoma cells and mouse L fibroblasts, we have observed that the C4-Slp promoter is fully effective and unhindered by upstream sequences and that the C4 promoter has a consistent albeit modest superiority. The determinant of this nearly 2-fold difference does not coincide with the sites highlighted in previous studies but lies within the most cap-site-proximal nucleotides, at positions -189 to +48. We have also established conditions for cell-free transcription of C4 and C4-Slp from plasmid and cosmid templates by using nuclear extracts from rat and mouse liver of both sexes as well as from L cells. At variance with the rat alpha 2u-globulin gene, C4-Slp transcription in vitro does not require male factors, for it is expressed as efficiently as C4 by all nuclear extracts. Further, the minimal promoter sequences required to direct accurate initiation extend not farther than the most proximal 19 nucleotides. Because L cells efficiently express transfected cosmids covering the whole C4 gene or C4/C4-Slp recombinants, as well as plasmids carrying the C4-Slp promoter, but fail to express the full C4-Slp gene, we favor a model in which the expression of the gene is modulated intragenically.

The thioester and isotypic sites of complement component C4 in sheep and cattle

The region inclusive of the thioester and the isotype-determining sites of the sheep C4 genes from a single animal was amplified by polymerase chain reaction (PCR). Two bands, at 880 base pairs (bp) and 1000 bp, were resolved by agarose gel electrophoresis. Four different clones were obtained for the 880 bp (type 1) product and two from the 1000 bp (type 2) product. Two of the type 1 clones (type 1H) and both type 2 clones (type 2H) code for the PCPVIH sequence at the isotypic site whereas the other two type 1 clones (type 1D) code for the PFPVMD sequence. By restriction mapping and Southern blot analysis, there appears to be four C4 gene loci for the sheep: two type 1H, one type 1D, and one type 2H. The type 1H and type 2H genes are likely to code for proteins with C4B-like properties whereas the type 1D genes for proteins with C4A-like properties. The same region of the sheep C4 genes of nine other breeds of sheep are also amplified by PCR and analyzed by restriction mapping and Southern hybridization. Each of the sheep has type 1H, type 2H, and type 1D genes and appears to have four C4 gene loci except for the Orkney, which may have five. A single band of 880 bp was obtained from the PCR product from the genomic DNA of a single cow. Five different clones were identified, two of which code for the PFPVMD sequence and three for the PCPVIH sequence at the isotypic site, which is consistent with previous finding that C4 proteins with A- and B-like activities could be purified from the plasma of the same animal. Comparison of the nucleotide sequences of the isotype-determining region of the sheep and cattle C4 genes with those of the primates and mouse suggests that the C4A-like genes evolved independently in the primates and the ungulates.

Slp is an essential component of an EDTA-resistant activation pathway of mouse complement

Slp (sex-limited protein) is a mouse serum protein encoded by a major histocompatibility complex class III gene. It is considered to be a product of a duplicated complement component C4 gene, but without functional activity. Originally it has been found expressed only in adult males with the S region of the H-2d or H-2s haplotype. In this report we present evidence that Slp is involved in a form of mouse complement activation that occurs after fractionation of serum by polyethylene glycol precipitation. This activation pathway is EDTA-resistant (i.e., independent of classical and alternative pathway activation), is regulated by C1 inhibitor, and leads to the generation of hemolytically active membrane attack complexes. A positive correlation between this EDTA-resistant mouse complement activity and reported Slp levels was found. Direct evidence for a functional role of Slp came from substitution experiments in which purified Slp induced hemolytic activity in polyethylene glycol-fractionated, Slp-deficient mouse serum. Selective depletion of other complement components suggested a role for C1s-, C2, and C5, but not C3, in the Slp-dependent complement activation. A model for this type of mouse complement activation is presented.

Mutants of complement component C3 cleaved by the C4-specific C1-s protease

To identify some of the structural features determining specific protease recognition of complement components C3 and C4, we used site-specific mutagenesis to construct mutants of murine C3 that are cleaved by the C4-specific C1-s protease. Insertion of three amino acid residues corresponding to residues at the C1-s cleavage site of human C4 into murine C3 at the analogous C3 convertase cleavage site was adequate to render the mutant protein susceptible to C1-s cleavage. In addition, insertion of C3-specific residues at the same site or introduction of the C4-specific residues as substitutions rather than as an insertion also rendered the site susceptible to cleavage, but with 10- to 50-fold lower efficiencies, and insertion of even a single amino acid residue affected recognition by C1-s. Finally, insertion of amino acid residues into mC3 partially inhibited cleavage by the alternative-pathway C3 convertase, with insertion of C3- or C4-specific residues giving about the same level of inhibition. A simple interpretation of these data is that C1-s cleavage is dependent primarily on steric accessibility and on recognition of specific amino acid residues at the cleavage site, whereas C3 convertase cleavage is dependent primarily on specific interactions distal to the cleavage site, with only relatively weak, non-C3-specific interactions at the cleavage site itself.

Differential expression of the five C4-related genes of H-2w7 mice

Mice bearing the H-2w7 haplotype have five C4-related genes, one C4, one Slp, and three C4/Slp hybrid genes. The expression of these five genes in the liver of H-2w7 mice was estimated at the steady state level of their respective mRNA. We have amplified by the polymerase chain reaction (PCR) three regions of the C4/Slp mRNA where some of these five genes show nucleotide substitution. A relative amount of each gene product was estimated by single-strand conformation polymorphism (SSCP) analysis or by direct counting of the number of respective clones after subcloning into a plasmid vector. A steady state level of the C4 mRNA was most abundant among C4-related gene transcripts. The hybrid 1 and 3 genes were expressed at a similar level which is about 1/2-1/3 of the C4 level. The hybrid 2 gene was expressed at about 1/5 of the hybrid 1 or 3 level. Neither male nor female H-2w7 mice expressed the Slp gene. These results showed that the expression of the five C4-related genes of H-2w7 mice is differentially regulated in spite of the close similarity in the nucleotide sequences in both the 5' flanking and coding regions of these genes.

Three extra copies of a C4-related gene in H-2w7 mice are C4/Slp hybrid genes generated by multiple recombinational events

Mice bearing the H-2w7 haplotype have five C4-related genes and constitutively express the Slp antigen. To understand the structure and evolution of the five C4-related genes of the C3H.W7 mouse, we have determined nucleotide sequences of the 5' end region of these genes. A C4/Slp hybrid nature was confirmed for three of five C4-related genes as predicted previously by restriction enzyme analysis. The nucleotide sequences of the 5' flanking regions of these three hybrid genes showed close similarity to that of the C4 gene, while the 3' side of the ninth exon of the three hybrid genes showed close similarity to that of the Slp gene. In contrast, the regions between the first exon and the middle of the ninth exon of the three hybrid genes showed a mosaic structure of C4-like and Slp-like sequences. Moreover, the boundaries of the C4-like and Slp-like sequences were quite different among the three hybrid genes. The pattern of nucleotide sequence diversity in this region among the five C4-related sequences could be mainly explained not by point mutations but by gene conversions or unequal crossovers. These results suggest that multiple genetic recombinational events between two homologous sequences played an important role in the generation and diversification of the extra copies of the C4/Slp gene in the H-2w7 mouse.

C4 from C4-high and C4-low mouse strains have identical sequences in the region corresponding to the isotype-specific segment of human C4

The human complement component C4 isotypes, C4A and C4B, show a substantial and biologically important difference in chemical reactivity. Murine C4 from different mouse strains has recently been reported to have a comparable difference in reactivity. In human C4, the difference in reactivity has been attributed to the effect(s) of one or more of only four amino acid residues, within a six-residue-long segment of the alpha subunit, which distinguish the two isotypes. In the present study, we sought to assess the role of the corresponding four amino acids in mouse C4 in the strain-specific modulation of C4 reactivity. In order to compare the sequences of the corresponding region in murine C4 among different mouse strains, we used the polymerase chain reaction method to amplify an approximately 229-bp segment of the murine gene that includes the codons for these four amino acid residues. Because the difference in chemical reactivity of murine C4 has been reported to be between C4 from strains which express different levels of C4, we examined sequences from mouse strains C57BL/6, DBA/2, C3H/He, B10.BR and CBA/J; these represent two C4-high strains and three C4-low strains. The amplified segments were cloned into the pUC19 vector and 20 independent clones from each mouse strain were sequenced. Across the entire amplified segment, our results revealed expected isotypic differences between C4 and its nonfunctional isotype in the mouse, Slp, as well as allelic differences among the C4 and Slp genes. However, all of these differences were quite distant from the amino acid residues corresponding to the human isotype-specific residues and those corresponding residues were identical in all five mouse strains. This result indicates that any strain-specific difference in chemical reactivity of murine C4 must be due to a mechanism distinct from that operating in human C4.

Murine complement component C4 and sex-limited protein: identification of amino acid residues essential for C4 function

Murine sex-limited protein (Slp) is an isotype of murine complement component C4 that shares 95% sequence identity with C4 as well as the intramolecular thioester necessary for C4 function but has no complement activity. Slp is nonfunctional at least in part because it is not cleaved by the activated form of complement protease C1s (C1s), which proteolytically activates C4 in the classical complement pathway. Slp is also distinct from C4 in that its expression in some mouse strains is under testosterone control. In the present studies, we used site-directed mutagenesis of C4 and expression of the mutant proteins in cultured cells to identify the amino acid substitutions in Slp that are responsible for resistance to C1s cleavage. We focused on sequence changes immediately downstream of the cleavage site in C4 because the arginine at that site is conserved in Slp, but the downstream sequences diverge substantially, with six differences in the first 7 residues followed by a 3-residue deletion in Slp. We found that a C4 mutant carrying only the 3-residue deletion is not cleaved by C1s and has essentially no hemolytic activity, whereas a mutant carrying only the six replacement changes is cleaved by C1s and has normal hemolytic activity. Both mutants have intact thioesters. A third mutant in which two acidic residues in the segment deleted in Slp were replaced by aliphatic residues is also cleaved by C1s, has an intact thioester group, and has normal hemolytic activity. These results indicate that the downstream mutations are responsible for the resistance of Slp to C1s cleavage and suggest that the length rather than the specific sequence of this segment is critical in determining susceptibility to the protease.

Trans-regulatory genes affect Slpa and Slpo expression and act in a tissue-specific manner

The plasma protein C4 and its androgen-dependent homologue Slp are encoded by genes located in the mouse major histocompatibility complex, H-2. The C4 and Slp protein levels and liver mRNA levels are influenced by non-H-2-linked regulatory genes. The allele-specific regulation of C4 expression and the androgen regulation of Slp expression are manifest only in some of the tissues where these genes are expressed. Therefore, we studied tissues in which the effects of the non-H-2 regulatory genes are apparent. We show that these genes only affect the Slp expression in tissues where it is androgen-dependent. This indicates that the non-H-2 regulatory genes most likely act through interaction with the androgen regulation of Slp expression. We also show that liver cells of mice with the Slpo allele, which do not produce Slp protein, do express Slp mRNA; this expression is also androgen-induced and regulated by non-H-2 genes. Thus, both the Slpa and Slpo alleles appear to be regulated in the same way.

Use of octadecyl silica as an alternative non-conventional solid phase in immunoradiometric assays

The use of chemically modified silica (octadecyl silica or Si-C18), a liquid chromatography stationary reverse phase, is proposed as an alternative solid phase for immunoradiometric assays (IRMAs). Si-C18 and polyvinylchloride solid phases were compared in their efficiency to detect a murine plasma protein (sex-limited protein or Slp) in a two site-two antibody IRMA. Si-C18 is at least as sensitive as PVC since Slp could be detected in plasmas diluted 1/128,000 and 1/64,000 respectively. The fact that 1 mg of Si-C18 can adsorb 5-14 times more protein than a PVC well is in agreement with the larger relative surface of Si-C18 beads (approximately 50 cm2/mg), compared with the PVC (approximately 1 cm2/well). These results strongly suggest that the Si-C18 matrix, saturated with specific antibodies, could be an exceptional probe to rescue and detect antigenic moieties diluted in large volumes of complex biological fluids.

Purification and characterisation of ovine C4: evidence for two molecular forms in ovine plasma

A relatively rapid procedure is described for the isolation of the fourth component of complement (C4) from ovine plasma. The method, which recovers approximately 30% C4, is based upon DEAE Sephacel anion exchange chromatography of PEG precipitated plasminogen depleted plasma followed by cation exchange chromatography on CM Sepharose and finally gel filtration. SDS-PAGE of purified ovine C4 under reducing conditions revealed a complex pattern of bands which was interpreted on the basis of a three polypeptide chain structure for each of two distinct species, or isotypes, of C4 molecule herein termed C4A and C4B. Each isotype differs in the mol. wt of the alpha chain--108 and 95 K respectively. Nucleophilic substitution of immunoprecipitated ovine C4 with radiolabelled methylamine revealed that both C4 species contained a reactive thiol ester site and that each could be cleaved into an activated form (presumably C4b) characterised by a truncated alpha' chain some 8 K lower in mol. wt. A comparison of the isotype composition of purified C4 with that of immunoprecipitated C4 from the same animal indicated that the purification procedure favoured isolation of the C4B isotype. The mol. wts of both the alpha and beta chains were lowered following digestion of ovine C4 with neuraminidase.

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.

Multiple C4/Slp genes distinguished by expression after transfection

The S region of the murine major histocompatibility complex contains two closely related genes: C4, encoding the fourth component of complement, and Slp, encoding sex-limited protein. We cloned these genes from a cosmid library of the B10.W7R strain that does not show androgen regulation of the Slp protein. Restriction site polymorphisms revealed at least four C4-like genes within the Sw7 locus, indicating evolutionary amplification of this region. Transfection of these genes into L cells resulted in expression, processing, and secretion of immunologically correct C4 and Slp proteins. At least two different Slp genes and one C4 gene were capable, after transfection, of expressing C4 and Slp indistinguishable from macrophage-derived protein. A third Slp gene exists within this locus whose recombinant cognate did not express in L cells. Thus, the B10.W7R S region includes one C4 gene and at least three Slp-like genes.

Identification of the 5'-flanking regulatory region responsible for the difference in transcriptional control between mouse complement C4 and Slp genes

To elucidate the molecular basis underlying the difference in the mode of gene expression between mouse complement C4 (constitutive) and sex-limited protein (Slp) (testosterone-regulated), we compared nucleotide sequences and transcriptional regulatory activities of the 5'-flanking regions of these two genes. Although the two sequences showed a high degree of overall homology (95%) up to 1.9 kilobases (kb) upstream from the transcription initiation site, the Slp sequence lacked a 31-nucleotide segment containing ACACCC repeats and a 60-nucleotide segment containing ACAC repeats, which are present, respectively, 1.6 kb and 200 base pairs (bp) upstream from the transcription initiation site of the C4 gene. When assayed in human hepatoma-derived HepG2 cells, the 1.8-kb 5'-flanking DNA fragment of the C4 gene demonstrated strong transcriptional activity, whereas the corresponding DNA fragment of the Slp gene showed only negligible activity. By progressive deletion experiments, it was shown that the difference in the constitutive transcriptional activity of the C4 and Slp genes was accounted for by the presence or absence of the positive regulatory domain located between 1700 bp and 400 bp upstream of the transcription initiation site.

Sequence comparison of alleles of the fourth component of complement (C4) and sex-limited protein (Slp)

cDNA clones specific for the fourth component of complement (C4) and its androgen-regulated isotype, sex-limited protein (Slp), have been isolated from two mouse haplotypes (H-2d and H-2w7) that show differential C4 activity and differential regulation of Slp. Clones were first isolated using a cDNA probe enriched by subtractive hybridization. Subsequent screening has resulted in cDNAs spanning the entire C4d mRNA, as well as much of C4w7, Slpw7 and a short region of Slpd. The cDNAs for C4 and Slp show extensive sequence homology, but can be distinguished using oligonucleotide probes synthesized to regions of greatest sequence divergence. Sequence differences between C4 and Slp indicate structurally important features of C4 that have been altered in Slp such that Slp is unable to participate in the complement pathway. Of the few nucleotide differences between C4d and C4w7, a single base change resulting in one less glycosylation site in the C4w7 alpha chain could account for its 4-fold reduced hemolytic efficiency. Sequence comparison of multiple alleles of C4 and Slp indicates that possible gene conversion events occurred in the H-2w7 strain that has multiple Slp genes.

Multiple C4/Slp genes distinguished by expression after transfection

The S region of the murine major histocompatibility complex contains two closely related genes: C4, encoding the fourth component of complement, and Slp, encoding sex-limited protein. We cloned these genes from a cosmid library of the B10.W7R strain that does not show androgen regulation of the Slp protein. Restriction site polymorphisms revealed at least four C4-like genes within the Sw7 locus, indicating evolutionary amplification of this region. Transfection of these genes into L cells resulted in expression, processing, and secretion of immunologically correct C4 and Slp proteins. At least two different Slp genes and one C4 gene were capable, after transfection, of expressing C4 and Slp indistinguishable from macrophage-derived protein. A third Slp gene exists within this locus whose recombinant cognate did not express in L cells. Thus, the B10.W7R S region includes one C4 gene and at least three Slp-like genes.

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.

Structure and expression of murine fourth complement component (C4) and sex-limited protein (Slp)

During the past 4 years we have used recombinant DNA technology to build upon previous genetic and biochemical studies of the C4 and Slp genes and their products. We have isolated DNA probes specific for C4 and Slp, determined the complete sequences of C4 and Slp molecules, established the role of liver mRNA levels in determining serum C4 and Slp levels identified the C4 and Slp genes in Balb/c mice, and begun to probe the structures of the C4 and Slp genes in a variety of inbred mouse strains. This work has provided the tools and a framework for future studies aimed at understanding the multiple functions of the C4 protein and the regulatory mechanisms controlling the expression of the C4 and Slp genes.

Mouse serum amyloid P-component (SAP) levels controlled by a locus on chromosome 1

Recombinant inbred strains were used to demonstrate the existence of a major locus on chromosome 1, designated Sap, which controls the endogenous concentration of the mouse acute phase reactant, serum amyloid P-component (SAP). Levels of SAP were associated with alleles at the Ly-9 locus in two sets of RI strains: BXD (C57BL/6J X DBA/2) and BXH (C57BL/6J X C3H/HeJ). Low endogenous levels of SAP were present in the C57BL/6J progenitor strain and in most of the RI strains which inherited the Ly-9b allele. High levels of SAP were present in the DBA/2J and C3H/HeJ progenitors and in most of the RI strains which inherited the Ly-9a allele. In the BXD strains 91% of the genetic variation of SAP levels was accounted for by segregation at the Ly-9 locus while an additional 9% was attributed to genetic factors unlinked to Ly-9. In the BXH strains the percentage of genetic variation accounted for by Ly-9 segregation was reduced to 46%, while 54% was accounted for by other genetic factors. Because of background genetic variation it was not possible to detect any crossovers between Sap and Ly-9. However, in the BXD strains the linkage between Sap and Ly-9 appears to be quite close. The B6.C-H-25c congenic strain, which carries a segment of BALB/c chromosome 1 including the minor histocompatibility locus H-25 on a C57BL/6By background, had the same endogenous SAP level as the BALB/c donor strain.

Molecular genetics of the S region of the murine H-2 major histocompatibility complex

The mouse H-2S region has been conventionally identified by the functional, serological and electrophoretic variations of two plasma proteins which constitute its primary genetic markers, C4 (the fourth component of complement) and C4-Slp (sex-limited protein). Recently, recombinant DNA procedures applied in our laboratory as well as in several others have made available molecular clones corresponding to these two nonallelic genes. This paper focuses on the progress provided by the study of C4 and C4-Slp cDNA and genomic clones, as well as by the application of such clones to the probing of the S region structure in conventional and special H-2 haplotypes. At variance with the K, I and D regions of H-2, the S region contains genes which have no obvious structural interrelationship, although in some cases they display a functional interaction. The heterogeneity of S region products is emphasized by the recent detection of steroid 21-hydroxylase genes associated with each of the C4 gene copies.

Restriction fragment length polymorphism of C4 genes in mice with t chromosomes

Genomic DNA was isolated from 29 t strains and 4 congenic lines of mice, digested with restriction endonucleases, and hybridized with a probe representing the complement component 4 (C4) gene. All but one of the enzymes revealed restriction fragment length polymorphism in this sample of C4-related genes. Double digestion analysis suggested the presence of three C4 gene copies in some of the t chromosomes and two copies in others. The enzymes distinguished 16 different haplotypes among the 33 strains tested. Based on their restriction fragment length patterns, the t strains could be divided into four groups with strains in each group more closely related to each other with respect to their C4-region genes than strains belonging to different groups. At least three of these four groups represent different branches of the evolutionary tree constructed for the t chromosomes. The C4-related genes of the chromosomes are in strong linkage disequilibrium with the class II genes of the H-2 complex. Typing for the Ss and Slp allotypes of C4 has revealed the presence of the Ss1 phenotype in two t strains and of the Slpa phenotype in one strain.

Two genes encoding steroid 21-hydroxylase are located near the genes encoding the fourth component of complement in man

Two genes encoding steroid 21-hydroxylase [21-OHase; steroid 21-monooxygenase; steroid, hydrogen-donor: oxygen oxidoreductase (21-hydroxylating); EC 1.14.99.10], a cytochrome P-450 enzyme, have been located within the HLA major histocompatibility complex. Congenital adrenal hyperplasia due to 21-OHase deficiency is a common inherited disorder of cortisol biosynthesis which is in genetic linkage disequilibrium with certain extended HLA haplotypes. These haplotypes include characteristic serum complement allotypes. A series of cosmid clones was isolated from a human genomic library by using a probe encoding part of the fourth component of complement, C4. These clones also hybridized with a probe encoding most of human 21-OHase. Restriction mapping and hybridization analysis showed that there are two 21-OHase genes, each located near the 3' end of one of the two C4 genes. Hybridization with probes specific for the 5' and 3' ends of the 21-OHase gene showed that the 21-OHase and C4 genes all have the same orientation. The 21-OHase genes 3' to C4A and C4B carry T aq I fragments of 3.2 and 3.7 kilobases (kb), respectively. Both of these fragments are found in genomic DNA of most individuals. In DNA from an individual with the severe, "salt-wasting" form of 21-OHase deficiency who was homozygous for HLA-A3;Bw47;C4A*1;C4B*Q0(null); DR7, the 3.7-kb Taq I fragment is absent, whereas hormonally normal individuals homozygous for HLA-A1;B8;C4A*Q0;C4B*1;DR3 do not carry the 3.2-kb Taq I fragment. These data suggest that the 21-OHase "B" gene (3.7-kb Taq I fragment) is functional, but the 21-OHase "A" gene (3.2-kb Taq I fragment) is not.

Activation of the complement system in patients with porphyrias after irradiation in vivo

Irradiation of the forearms of two patients with erythropoietic protoporphyria and one patient with porphyria cutanea tarda resulted in an in vivo activation of the complement system, as assessed by diminution of the hemolytic titers of the third component of complement by 23-57%, and of the fifth component of complement (C5) by 19-47%. Such treatment also generated chemotactic activity for human polymorphonuclear cells; the chemotactic activity was stable at 56 degrees C and antigenically related to human C5. On Sephadex G-75 chromatography the chemotactic activity eluted with an apparent molecular weight of 15,000. These in vivo results extend our previous in vitro observation of photoactivation of complement in sera from patients with erythropoietic protoporphyria and porphyria cutanea tarda, and suggest that the complement system may participate in the pathogenesis of cutaneous phototoxicity in these patients.

Two steroid 21-hydroxylase genes are located in the murine S region

A common inherited disorder of steroidogenesis in man, 21-hydroxylase (21-OH) deficiency, is linked to the HLA major histocompatibility complex (MHC), and is associated in particular with certain allotypes of the HLA-linked complement proteins. Recently, this disorder was demonstrated to result from a defective structural gene for the 21-OH enzyme, also termed cytochrome P-450C21. The human (HLA) and murine (H-2) MHCs are homologous in overall organization and in the structures of their component genes. To determine whether 21-OH genes are located in the H-2 complex, we have now used a bovine adrenal complementary DNA clone encoding part of 21-OH to examine a cluster of overlapping cosmid clones derived from the S region of the BALB/c mouse. We found that there are two 21-OH genes in this region, located immediately 3' to the C4 and Slp genes.

Isolation of cDNA clones specifying the fourth component of mouse complement and its isotype, sex-limited protein

cDNA clones specific for the fourth component of mouse complement (C4) and its hormonally regulated isotype, sex-linked protein (Slp), were isolated using as a probe a 20-mer synthetic oligonucleotide corresponding to a known sequence of human C4 cDNA. Two types of clones, one specific for C4 (pFC4/10, with a 3.7 kilobase insert) and one specific for Slp (pFSlp/1, with a 4.7 kilobase insert), were isolated from liver cDNA libraries constructed from the Slp-producing FM mouse strain. The cDNA inserts of these clones shared 70% of the restriction sites determined. Only one type of clone was isolated from the Slp-negative DBA/1 strain; this type showed restriction maps indistinguishable from that of pFC4/10. pFC4/10 and pFSlp/1 displayed extensive homology: 94% nucleotide homology and 89% derived amino acid homology in the C4a region and 92% nucleotide homology and 89% derived amino acid homology in the thiol-ester region. An Arg-Gln-Lys-Arg sequence in the beta-alpha junction and a Cys-Ala-Glu-Gln sequence in the thiol-ester site were identified for both proteins. A remarkable divergency between C4 and Slp sequences was recognized in the region immediately following the C4a sequence.

Sexual dimorphism of the fifth component of mouse complement

Sexual dimorphism of mouse complement component 5 (C5) was detected by isoelectric focusing of desialated ethylenediamine tetraacetic acid (EDTA)-plasma on agarose gel, followed by immunofixation with anti-mouse C5. Male plasma displayed two C5 bands, basic and acidic, while female plasma had only a basic C5 band. In all mouse strains tested except C5-deficient strains, the identical patterns of the dimorphism were obtained. The basic and acidic C5 were antigenically and hemolytically indistinguishable from each other. In the neonatal mice of both sexes, the basic C5 was observed at the same levels, but the acidic C5 was only slightly detected. Injection of testosterone into mice resulted in an appearance of the acidic C5 in females and its increase in males. By contrast, injection of estradiol decreased the acidic C5. These results indicate that the expression of the acidic C5 is under the control of testosterone.

Purification of the fourth, second and fifth components of mouse complement

The fourth, second and fifth components of mouse complement were purified by a combination of polyethylene glycol precipitation, ion exchange chromatography and gel filtration. The final products were homogeneous on SDS-PAGE, and the activity yields were 8.5% for C4, 32% for C2 and 40% for C5. C4 was composed of three polypeptide chains with mol. wts of 90,000, 78,000 and 32,000. C2 was composed of a single polypeptide chain with a mol. wt. of 115,000 and cleaved by C1s into two fragments with mol. wts of 80,000 and 35,000. The half life of C4b2a was 7 min and was not prolonged by the iodination of C2. C2 activity could not be measured using EAC14hu or EAC14gp cells, but measurement was possible with the use of EAC14mo cells with purified C5 components of mouse complement. C5 was composed of two polypeptide chains with mol. wts of 135,000 and 84,000. This is the first report on the purification of functionally active mouse complement components C4, C2 and C5 from plasma.

The complement components of the major histocompatibility locus

Polymorphism of complement components, recognized by differences in either their antigenic specificity or their electrophoretic mobility, together with studies of inherited deficiencies, has enabled many of their structural genes to be mapped. In humans, three genes (for C2, C4, and factor B) have been placed between HLA-D and HLA-B on chromosome 6 and in mice, C4 between H2-I and H2-D, chromosome 17. Structural studies show that these components have exceptional features. C2 and factor B which contain the proteolytic active site of the C3 and C5 convertases are of the classical and alternative pathway respectively and are similar in structure and function. Both are novel types of serine proteases. C4 (as C3) contains an intrachain thioester bond essential for hemolytic activity. Molecular genetic investigations are determining the relative positions of these genes, and their precise structure, and should clarify their relation to the inherited diseases which are associated with defects in this section of the human genome.

Molecular map of the murine S region

Eighteen overlapping cosmid clones spanning 240 kilobases and encoding the gene for factor B and two genes related to the fourth component of complement (C4) were isolated from a murine H-2d genomic library. Cosmid clones were identified by hybridization to human cDNA probes for factor B and C4 and were linked by chromosomal walking procedures. The cluster of clones contains two regions with sequences homologous to the C4 cDNA probe, both in the same orientation, representing a direct duplication of at least 55 kilobases of chromosomal DNA, separated by a shorter (less than 25 kilobases) segment of nonduplicated DNA. Restriction fragment-length polymorphism seen by using C4 probes maps these sequences to the S region of the major histocompatibility complex. 5' to the two C4-like sequences is an approximately equal to 40-kilobase-long region of chromosomal DNA remarkable for its lack of restriction fragment-length polymorphism, containing sequences homologous to the human factor B cDNA probe. These experiments demonstrate that the structural gene for factor B is located in the S region of the murine major histocompatibility complex and that this region contains an extensive direct duplication that contains the structural gene for mouse C4 and, we presume, for the sex-limited protein variant, Slp. RNA transfer blot analysis of total liver RNA from high C4- and low C4-producing strains showed that steady-state levels of C4-hybridizing RNA were much greater in high C4-producing strains. Regulation of circulating C4 levels in high C4 and low C4 strains is at least partly at the level of mRNA transcription, processing, or degradation.

Role of complement and polymorphonuclear cells in demethylchlortetracycline-induced phototoxicity in guinea pigs. Inhibition by decomplementation in vivo

In this study, demethylchlortetracycline was used as a prototype of exogenous phototoxic substances. In vitro, exposure of serum containing demethylchlortetracycline to ultraviolet-A irradiation resulted in the diminution of total complement hemolytic activity and C4, C2, C3, and C5 activities. In addition, chemotactic activity for human polymorphonuclear cells was generated, which was thermostable and antigenically related to human C5 but not human C3. In vivo, phototoxic lesions were induced in guinea pigs upon intradermal injections of demethylchlortetracycline solution, followed by ultraviolet-A irradiation. On a scale of 0-3+, the animals developed a maximal response of 2.5 at 20 h. This clinical response was associated with cellular infiltrate in the dermis, consisting of 29 +/- 2% of neutrophils at 24 h. The participation of the polymorphonuclear cells was evaluated in guinea pigs rendered neutropenic by treatment with cyclophosphamide. In these guinea pigs, demethylchlortetracycline and ultraviolet-A induced a maximal response of 0.75 +/- 0.5, which was associated histologically with 1.2 +/- 0.5% neutrophils in the dermis. The role of complement in this process was studied in guinea pigs congenitally deficient in C4, and in guinea pigs decomplemented by treatment with cobra venom factor. In contrast to normal guinea pigs, C4-deficient animals exhibited a maximal reaction of 0.83 +/- 0.16 at 6 h, which subsided within 24 h. Cobra venom factor-treated guinea pigs developed a maximal response of 0.5 at 0.5 and at 6 h. These clinical changes were associated with the development of an increased vascular permeability, as demonstrated by studies using guinea pigs injected intravenously with Evans blue solution. In animals with a normal complement system, there was intense localized bluing at the sites of phototoxic lesion. In contrast, only minimal bluing was observed in decomplemented guinea pigs. These data indicate that a normal number of polymorphonuclear cells and an intact complement system are required for the full development of demethylchlortetracycline-induced phototoxic lesions.

cDNA clone spanning the alpha-gamma subunit junction in the precursor of the murine fourth complement component (C4)

cDNA clones carrying parts of murine fourth complement component (C4, serum substance protein) mRNA sequences have been identified by differential hybridization to mRNA from a high C4-producing strain, B10.WR, and a congeneic low C4 strain, B10.BR, followed by hybrid-selected translation and DNA sequence analysis. One clone, pMLC4/w7-2, encodes an open amino acid reading frame that includes four tandem arginine residues immediately preceding a sequence 85% homologous with the NH2-terminal sequence of the human C4 gamma-chain. The amino acid composition of the predicted sequence upstream of the tandem arginines matches quite closely with the composition of a similar sized peptide at the COOH terminus of the human C4 alpha chain. The latter result raises questions regarding the nature and extent of plasma-mediated postsynthetic processing of the C4 alpha-chain COOH terminus. The results also demonstrate that strain differences in plasma C4 levels (low C4 vs. high C4) reflect differences in steady-state levels of liver C4 mRNA in these strains.

Structural polymorphism of murine factor B controlled by a locus closely linked to the H-2 complex and demonstration of multiple alleles

New alleles of murine factor B (Bf) protein were demonstrated. When ethylenediaminetetraacetic acid (EDTA)-plasmas from inbred and wild mice were analyzed by isoelectrofocusing (IEF) and immunofixation, murine Bf proteins were visualized as distinct protein bands in all mice tested. Four variants of murine Bf could be demonstrated in a large number of tested mice: Bf 1 (isoelectrofocusing point (P.I.) range of 5.8-6.1) exemplified by B10 and B10.BR, Bf 2 (P.I. range of 5.8-6.0) exemplified by B10.MOL (OHM), Bf 3 (P.I. range of 5.6-5.9) exemplified by B10.MOL (TEN2) and Mus musculus (Mus m.) subspecies Chc, Bf 4 (P.I. range of 6.0-6.3) exemplified by Mus m. subspecies Shh. The genetic linkage between S locus and Bf locus was studied with two backcross progenies--[B10.BR X (B10.BR X Mus m. subspecies Chc)F1] and [B10.BR X (B10.BR X Mus m. subspecies Shh)F1]. Totally, 256 backcross progenies were typed for Bf type and for Ss type (plasma level of the fourth complement protein regulated by S locus). The results indicated that murine Bf was controlled by a single codominant locus located close to the H-2 complex because no mouse showing recombination between Bf locus and S locus was found.

Murine sex-limited protein (Slp) antigenic sites in human complement component C4

Human complement component C4 is encoded by two HLA-linked loci, A and B. In the mouse, the H-2 region contains structural genes for two serum proteins that react with antibodies to human C4, but one of these proteins (Slp) has no C4 hemolytic activity. Because the product of C4-A locus in man has low hemolytic activity, a previous report suggests it may be the homologue of murine Slp. We show here that Slp antigenic determinants are found in human C4. However, they are expressed in the products of both loci A and B, that is, C4A and C4B, since both proteins were specifically immunoprecipitated by the IgG fraction of alloantisera to mouse Slp. Therefore, Slp-associated structural features are preserved in evolution, although they do not seem to be relevant to the hemolytic properties of C4.

Characterization of the Mr difference between secreted murine fourth component of complement and the major plasma form: evidence for carboxyl-terminal cleavage of the alpha chain

The alpha-chain of murine fourth component of complement (C4) secreted by cells in vitro and in vivo has a Mr that is larger by approximately equal to 4,000 than that of the alpha-chain of the principal form of C4 in plasma. By using in vivo labeling of C4 with [35S]methionine, C4 was shown to be first synthesized with the higher Mr ("secreted") alpha-chain, which was then quickly processed (t1/2 approximately equal to 1 hr) extracellularly to the mature ("plasma") C4 possessing the lower Mr alpha-chain. Both forms of C4 were functional as assayed by the ability of their alpha-chains to be cleaved by the protease C1, to bind methylamine, and to undergo denaturation-dependent autolysis. When secreted C4 and plasma C4 were activated to C4b, the Mr difference of 4,000 was maintained in the alpha'-chains. The Mr difference was localized to the carboxyl-terminal autolytic fragment of the alpha-chain and was unaffected by the removal of carbohydrate. C4 from resident peritoneal macrophage cultures could be converted to the plasma form by incubation with heparin/plasma. This conversion could be blocked by EDTA or 1,10-phenanthroline. These data suggest that an enzyme, presumably a neutral proteinase present in mouse plasma, cleaves the carboxyl terminus of newly synthesized C4 alpha-chains, thereby creating the major form of C4 in plasma.

A molecular basis for the two locus model of human complement component C4

The major histocompatibility complex(MHC)-linked fourth component of complement (C4) shows a high degree of polymorphism in several animal species. In man C4 polymorphism was detected by distinct charge differences of the variants. O'Neill et al. showed that this C4 polymorphism was controlled by two closely linked genetic loci, F (C4A) and S (C4B) and these results were extended by Awdeh et al. with an improved typing method. Biochemical analysis of human C4 has revealed that it consists of three polypeptide chains, alpha, beta and gamma. In all reports so far on the molecular analysis of human C4, no molecular weight differences between the A and B locus-encoded molecules have been noticed. Here we demonstrate that the C4A and C4B locus-encoded alpha-chains have a molecular weight (MW) of 96,000 and 94,000, respectively, presenting for the first time a molecular basis for the difference between all C4A and C4B variants tested. Even rare variants that are difficult to allocate to the A or B locus on the basis of charge differences could be identified as C4A or C4B variants in this way, thereby providing new insights into the relationships between the C4A and C4B loci.

Activation of the guinea pig alternative complement pathway by mouse IgA immune complexes

Activation of the complement system by IgA was investigated with immune complexes containing a mouse IgA myeloma protein with specificity for phosphorylcholine linked to bovine serum albumin (PC-BSA). These IgA anti-PC-BSA immune complexes activated the alternative complement pathway in mouse and guinea pig serum, while human complement was not affected. The activation proceeded with consumption of C3 but little or no consumption of C5. C3 did not bind to the IgA immune complexes during complement activation although it did bind covalently to IgG immune complexes. It is suggested that IgA immune complexes do not supply a suitable surface for C3 binding and effective alternative pathway convertase assembly; therefore, cleavage is limited and occurs primarily in the fluid phase. Without C3 binding, C5 cleavage does not occur nor can the alternative pathway activation proceed to the amplification step.