Murine complement factor B (BF): sexual dimorphism and H-2-linked polymorphism

Sexual dimorphism in immune function: the role of prenatal exposure to androgens and estrogens

Perinatal exposure to androgens permanently transforms certain tissues, e.g., the brain, the genitalia, etc. This process involves both masculinization and defeminization. Immune function also is transformed by early steroid exposure; however, it is not yet known whether the response capabilities of the immunocytes themselves are directly modified or whether they are responding to signals from other masculinized tissues, e.g., the brain. Most evidence points to a direct effect since androgen and estrogen receptors are present in developing immunocytes. Both androgens and estrogens have a role in regulating adult immunity including Th1/Th2 balance. Adult susceptibility to autoimmune and other diseases is also related to steroid exposure. How immune cells respond to gonadal steroids in adulthood may depend on the pattern of androgenic and estrogenic stimulation during early development.

Human major histocompatibility complex contains several leukemia susceptibility genes

In mice, homozygosity for the Mhc haplotype H-2k is associated with increased susceptibility to spontaneous and virus-induced leukemia, lymphoma and other neoplasms in the predisposed host. The influence of the Mhc on malignant development in these models is to shorten the latency after virus inoculation. Here, we present evidence that a similar phenomenon results in early-onset of human leukemia. A molecular analysis of the MHC in 112 CML patients showed that those who developed the disease when aged less than 35 years (early-onset group) had higher homozygosity rates for the DOA1, HSP70 and C4 alleles of the DR53 group of ancestral haplotypes, for a subtype of HLA-A3, and a higher allele frequency of BfFb compared to the late-onset group. The oldest patient (n = 13) homozygous for DR53 was 52-years-old (p = 0.004), and all HLA-A3 homozygous patients (n = 4) were in the early-onset group (p = 0.01). The relative risk for early-onset CML yielded by HLA-A3 homozygosity was 17.6. The well-known serological HLA-Cw4 association was not confirmed at the DNA level and thought to be due to linkage disequilibrium with BfFb. The factor B association was sex-limited. The DR52 group haplotypes appeared to be protective. The HLA-identical sibling frequency was increased only in the early-onset group (p < 0.01). Our findings agree with the concept of an MHC influence on the development of malignancies. The similarity in the location of the susceptibility loci and the serological cross-reaction between H-2Ek and DR53 raise the possibility that the mouse and human MHC share the same leukemia susceptibility genes.

Two forms of guinea pig factor B of the alternative complement pathway with different molecular weights

Two forms of guinea pig factor B (B) of the alternative complement pathway with different mol. wts (Mr) have been isolated from plasma and characterized. The Mr of the two B species, tentatively termed B1 and B2, were estimated to be about 100,000 and 96,000, respectively, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Incubation of B with guinea pig C3 and human factor D (D) in the presence of Mg2+ generated two cleavage fragments of B, namely Ba and Bb. Although the Bb fragments showed the same migration corresponding to an Mr of 62,000, Ba fragments showed different mobilities corresponding to an Mr of 38,000 from B1 and 34,000 from B2. Digestion of B1-Ba, the Ba fragment derived from B1, and B2-Ba, the Ba fragment derived from B2, with endoglycosidase F resulted in a band at Mr 30,000 on an SDS-PAGE in both cases, indicating a difference in structure of the asparagine-linked oligosaccharide moiety in B1-Ba and B2-Ba. No difference in antigenicity was noted between B1 and B2 on immunodiffusion with anti-B sera. Immunoblotting analysis showed that all individual Hartley guinea pigs examined in this study possessed both B1 and B2 at similar levels, as determined by the intensity of staining of their sera. Furthermore, treatment of their serum with zymosan led to the generation of two Ba species corresponding to the Ba fragments from B1 and B2. The capacity to form C3/C5 convertase, as determined by hemolytic assay, was found to be similar between B1 and B2. Furthermore, kinetics of the decay of C3 convertase showed the same half-life of 3.0 min at 30 degrees C. The NH2-terminal amino acid sequences of B1 and B2 and their Bb fragments were determined and found to be identical.

DNA polymorphism of MHC III genes in inbred and wild mouse strains

Genes encoding the second component (C2), factor B, and complement protein C4 and Slp (sex-limited protein) are members of the major histocompatibility complex class III gene cluster. In this report we describe isolation of a mouse C2 cDNA clone and its use together with factor B and C4 cDNA clones to examine the S region in a panel of 42 haplotypes in laboratory and wild mice representing 5 species and subspecies of Mus. Conservation of the C2 factor B gene duplex was evidenced by relatively limited polymorphism associated with speciation and nucleotide sequence homology between mouse and human C2 and factor B. The C4-Slp gene duplex, on the other hand, showed extensive polymorphism by DNA blot analysis. This polymorphism correlated poorly with the C2/factor B restriction fragment length polymorphism, suggesting independent evolution of these two segments of the S region. Taken together, these data will be of particular importance in studies of mouse strains with abnormal regulation of immune effector systems since the class III gene products are essential for activation of the complement cascade.

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.

Molecular organization and in vitro expression of murine class III genes

These experiments demonstrate that at least two types of gene duplications have occurred during the evolution of the S region. The first type, which produced the C2 and factor B genes, involved a short segment of the chromosome encompassing a single gene. The related products have subsequently diverged yielding sequences which do not cross-hybridize. Further duplication of these genes has not been observed. The second type of duplication consisted of a much longer primordial sequence, spanning approximately 55 kb of genomic DNA and including at least two genes, C4/Slp and 21-hydroxylase. The duplicated sequences are separated by a segment of single copy sequence of as yet undefined length. These duplicated sequences have been relatively conserved. There is evidence that further duplication of this region is possible (as seen in the H-2w7 strain) although the exact nature of the increase in gene number has not been fully characterized. Detailed analysis of cosmid clones which span these two duplications has permitted the assignment of a new pair of loci to the S region, encoding 21-hydroxylase A and B. The advantage conferred by linkage of the gene encoding this adrenal steroid biosynthesis enzyme to the genes encoding complement components C2, factor B, and C4 is unclear, as is the advantage of the association of all of the class III genes with the remainder of the MHC. The availability of cloned sequences containing all of the class III genes permits further study of the factors which govern the tissue specificity of their expression and which confer androgen responsiveness on certain of the Slp alleles.

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.

The factor B and C2 genes

Factor B and C2, components of the complement system, are novel types of serine proteinase that are encoded by genes in the major histocompatibility complex. cDNA probes specific for these two proteins have been used to isolate cosmid clones of genomic DNA which contain the Factor B and C2 genes. Southern blot analysis of the cosmid clones and of uncloned genomic DNA has shown that there are single Factor B and C2 loci that are less than 1 kilobase apart. The Factor B gene has been further characterized by DNA sequence analysis. It is approximately 6 kilobases in length, and is split into 18 exons. The amino acid sequence of the Ba fragment contains three homologous regions which are encoded by separate exons, suggesting that they arose by DNA duplication events. In the serine proteinase domain each of the functionally important parts of the active site are encoded on separate exons. Comparison of this region of the gene with the exon organization of other serine proteinases shows a close correlation between them, but also reveals the presence of an exon in Factor B with no homologous counterpart in the other serine proteinases. The possible functional significance of the peptide encoded by this exon and the evolution of this novel serine proteinase are discussed.

Chromosome maps of man and mouse II

Chromosome displays and listings are presented showing loci whose position is known in both man and mouse, in similar manner to our previous report (Dalton et al. 1981). There is now evidence for at least 27 conserved autosomal segments with two or more loci in the two species. The human and mouse chromosome maps show the location of homologous genes. The mouse map also shows the positions of translocations used in gene location and of some other genes used in linkage studies on them.

Subdivision of the S region of the mouse major histocompatibility complex by identification of genomic polymorphisms of the class III genes

The S region of the mouse major histocompatibility complex (MHC) encodes the class III proteins, the second (C2) and fourth (C4) components of complement, and factor B. Previously, the assignment of S-region haplotypes was based on analysis of protein polymorphisms. The recent availability of C2, C4, and factor B cDNA probes prompted a search for restriction fragment length polymorphisms which would serve as additional genetic markers for these loci. DNA was isolated from livers of mice of all standard inbred H-2 haplotypes and of haplotypes pz and bs. These DNA samples were digested with restriction endonucleases and analyzed by Southern blot. By the pattern of restriction fragment length polymorphism observed, specific markers have been identified in factor B of haplotypes f, u, z, bs, r, and v, and in C4 of haplotypes b,q,f,j,p,s,pz,r, and v. These genetic markers were used in the analysis of S-region composition in strains B10.TFR5(H-2ap5) and C3H.LG(H-2dx), and a possible intra-S-region recombinant was revealed in the H-2dx haplotype. The genetic markers identified here subdivide the S region and will be of value in defining further the composition of the complement gene complex of the mouse MHC.

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.

Structural polymorphism of mouse complement C2 detected by microscale peptide mapping: linkage to H-2

Complement C2 was isolated from 17 mouse strains by immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis and examined for structural polymorphism by using micro-peptide mapping. By comparing the peptide maps of tryptic digest of C2 from various strains, two allotypic variations were detected. B10 and 14 other mouse strains demonstrated C2.1 type, while a wild mouse line (M.Mol-Ohm) and one B10 congenic strain, B10.MOL.OHM, which carries the H-2 derived from M.Mol-Ohm, demonstrated C2.2 type. (B10 X B10.MOL.OHM)F1 demonstrated codominantly expressed C2 type (C2.1.2). Desialation of mouse C2 did not abolish the observed variation of mouse C2. It is concluded that an H-2-linked codominant locus controls the structure of mouse complement C2, further confirming the extensive homology of the major histocompatibility complex among higher vertebrate species.

Inherited disorders of complement

Isolated complement component deficiencies are uncommon. Deficiencies of all eleven components and two inhibitors of the classical pathway have been described. Complete absence of the components of the alternative pathway has not been described. The consequences of a single defect in complement are often predictable from an understanding of the biologic activities associated with activation of the complement system. Deficiency of C1 esterase inhibitor gives rise to the disease, hereditary angioedema; deficiency of the early components of the classical pathway are associated with lupus erythematosus; C3 and C3 inactivator deficiencies with pyogenic infections; C5 dysfunction with Leiner's disease; deficiencies of the terminal components with recurrent Neisseria bacteremia; and C9 deficiency with normal health. The complement system and its associated biologic activities are reviewed. The present knowledge of the inherited complement deficiencies and associated diseases, with particular emphasis on the dermatologic manifestations, genetics, and diagnosis, is summarized.

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.

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.

H-2-linked murine factor B phenotypes

A hemolytic assay has been developed which is specific for Factor B (B) activity in murine EDTA-plasma. Three discrete levels of B activity were observed among B10-congenic strains. Mice with standard H-2 haplotypes, b, d, k, r, f, q, s, and u, all exhibited the same mean level of activity. However, plasma from H-2v (B10.SM) mice contained only 0.25 of that level, and those with standard haplotype H-2ja (B10.WB) or wild haplotype H-2wr7 (B10.WR) exhibited 2.5 times the H-2b (B10) basal level of activity. These differences among B10 congenic lines suggested that the activity is H-2 controlled; further tentative mapping with intra-H-2 recombinants indicated that the gene is located in the S region. A fourth phenotype was found among progeny of backcross generations between B10.BR (H-2k) and mice of subspecies Mus musculus molossinus and M. m. bactrianus. This ultra-high activity was found also to be governed by a gene very closely linked to Ss, the primary S region marker. F1 generations between disparate phenotypes yielded progeny with activity levels intermediate between the parents; progeny of parents of different strains with the same phenotype expressed B hemolytic titres equal to those of the parental strains. No differences in antigenic levels of the protein among the strains of different phenotypes could be detected by radial immunodiffusion. In mixing experiments, resultant activity levels were intermediate between the higher and the lower phenotype, ruling out independent inhibitors or activators of the reaction. These studies indicate that an H-2-linked S region-located single gene governs structural differences in allelic B molecules that lead to differences in specific activities.

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.