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

Shedding light on the role of complement C4 activation in cancer

Complement C4 is a key component in the activation of classical and lectin complement pathways, which are observed in both animal tumor models and cancer patients. While its role in autoimmune disorders has been extensively studied, the functions of complement C4 and its activation in cancer have received inadequate consideration. Recent studies have detected C4 activation in animal tumor models and cancer patients, with its fragment C4d found in cancer tissues and lymph nodes. Elevated C4d levels could be a useful biomarker for detecting various cancers. This review aims to summarize recent developments on the role of complement C4 activation in promoting an immunosuppressive tumor microenvironment, thereby supporting tumor progression and metastasis; C4d as a biomarker; and its potential as a target for cancer immunotherapy. We also conduct a critical evaluation of methods used to measure complement C4 and its activation products, highlighting possible pitfalls and areas for improvement in existing research.

Complement Receptor 2 Based Immunoassay Measuring Activation of the Complement System at C3-Level in Plasma Samples From Mice and Humans

We aimed at establishing a sensitive and robust assay for estimation of systemic complement activation at complement component C3 level in mouse and human plasma samples. In order to capture the activation products iC3b and C3dg in a specific and physiological relevant manner we utilized a construct consisting of the iC3b/C3dg-binding site of human complement receptor 2 (CR2) attached to an Fc-part of mouse IgG. This construct binds C3dg and iC3b from both mice and humans. We purified the CR2-IgG construct from mouse B myeloma cell line supernatants, J558L-CR2-IgG, by protein G affinity chromatography. The CR2-IgG construct was used for capturing C3 fragments in microtiter wells and an anti-mouse or an anti-human-C3 antibody was used for detection of bound C3 fragments. Initially we tested the specificity of the assays with the use of purified C3 fragments. Further, with the use of the CR2-based assay, we measured an up to three-fold higher signal in activated mouse serum as compared to non-activated mouse serum, whereas activated serum from a C3 knock-out mouse gave no signal. We tested in vivo generated samples from a mouse experiment; complement activation was induced by injecting cobra venom factor or heat aggregated IgG into C57bl6 mice, followed by withdrawal of EDTA blood samples at different time points and measurement of iC3b/C3dg. We observed a clear time-dependent distinction in signals between samples with expected high and low complement activation. Furthermore, with the use of the assay for human C3 fragments, we observed that patients with systemic lupus erythematosus (SLE) (n = 144) had significantly higher iC3b/C3dg levels as compared to healthy individuals (n = 144) (p < 0.0001). We present two functional immunoassays, that are able to measure systemic levels of the C3-activation products iC3b and C3dg in mice and humans. To our knowledge, these are the first assays for complement activation that use a physiological relevant capture construct such as CR2. These assays will be a relevant tool when investigating mouse models and human diseases involving the complement system.

Transposable elements donate lineage-specific regulatory sequences to host genomes

The evolutionary implications of transposable element (TE) influences on gene regulation are explored here. An historical perspective is presented to underscore the importance of TE influences on gene regulation with respect to both the discovery of TEs and the early conceptualization of their potential impact on host genome evolution. Evidence that points to a role for TEs in host gene regulation is reviewed, and comparisons between genome sequences are used to demonstrate the fact that TEs are particularly lineage-specific components of their host genomes. Consistent with these two properties of TEs, regulatory effects and evolutionary specificity, human-mouse genome wide sequence comparisons reveal that the regulatory sequences that are contributed by TEs are exceptionally lineage specific. This suggests a particular mechanism by which TEs may drive the diversification of gene regulation between evolutionary lineages.

Of mice and men: the relevance of the mouse to the study of human SLE

A number of mouse models have been utilized to study the pathophysiology of immune complex (IC) disease, and the hallmark IC disease systemic lupus erythematosus (SLE). Many of these studies have provided exciting new insights into IC-mediated inflammation and autoimmunity. However, numerous differences exist between mice and humans that suggest that mouse studies are not always applicable to human disease. These differences can be found in the biological systems that interact with circulating IC, in the specifics of disease presentation, and in the general physiology of the two species. Furthermore, although the mechanisms of SLE-like autoimmune disease in the mouse are being defined through analyses of the murine models of SLE, it remains to be proven that these mechanisms are relevant to human SLE. Thus, generalizing the results of the mouse studies to human SLE and other human IC diseases must be done with caution.

In vivo gene transfer in mouse skeletal muscle mediated by baculovirus vectors

Baculovirus vectors are efficient tools for gene transfer into mammalian cells in vitro. However, in vivo gene delivery by systemic administration is hindered by the vector inactivation mediated by the complement system. To characterize further the gene transfer efficacy of baculovirus we examined the vector transduction efficiency in skeletal muscle. Vectors expressing vesicular stomatitis virus glycoprotein (VSV-G) in the viral envelope were generated by inserting the VSV-G coding sequence downstream of the polyhedrin promoter. Two viruses were constructed to carry either the Escherichia coli beta-galactosidase (beta-Gal) gene or the mouse erythropoietin (EPO) cDNA cloned downstream of the cytomegalovirus immediate-early promoter and enhancer. The greater gene transduction efficiency of the Bac-G-betaGal vector was confirmed by comparing the beta-Gal expression level in a variety of human and mouse cell lines with that obtained on infection with Bac-betaGal, a vector that lacks VSV-G. Similarly, a 5- to 10-fold increase in beta-Gal expression between Bac-G-betaGal and Bac-betaGal was observed when mouse myoblasts and myotubes were infected. The same increase in beta-Gal expression was detected on injection of the Bac-G-betaGal vector in the quadriceps of BALB/c and C57BL/6 mice. In contrast, a 2-fold difference in transduction was observed between these two vectors in DBA/2J mouse strain. Last, expression of EPO cDNA was detected for at least 178 days in DBA/2J mice on Bac-G-EPO injection into the quadriceps whereas EPO expression declined to normal values by 35 days postinfection in BALB/c and C57BL/6 mice. Thus, these results indicate that baculovirus may be considered a useful vector for gene transfer in mouse skeletal muscle and that persistence of expression may depend on the mouse strain used.

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 human and mouse MHC class III region: a parade of 21 genes at the centromeric segment

The human major histocompatibility complex (MHC) class III region contains 57-60 structural genes spanning 654-759 kb of genomic DNA. Analysis of the sequence identities of the human and mouse genomic regions between NOTCH4 and complement C2 yields important information on the locations of the coding and regulatory sequences. It also provides insights into the relationship between protein function and level of sequence conservation, and on the clustering of genes with related functions.

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.

Regulatory capacity of an androgen-specific enhancer of the mouse Slp gene in transgenic mice

Different steroid hormone receptors can activate transcription from the same hormone response element (HRE) in vitro, but in vivo the effects of each hormone on gene activity are distinct. To determine sequences mediating androgen-specific response in a physiological setting, we placed the androgen-responsive mouse sex-limited protein gene (Slp) enhancer before a tkCAT reporter in transgenic mice. The enhancer contains a consensus HRE plus accessory factor binding sites that act in concert to direct transcription in response to androgen. A 160 bp fragment, C'delta2, is responsive to several steroids in transfection; in transgenic mice, this enhancer was active in several tissues of male and female mice, in four of six transgenic lines. In striking contrast, C'delta9, a 120 bp sub-fragment of C'delta2 that responds only to androgen in transfection, showed activity in testes, prostate and kidney, where it was strongly androgen-inducible in females. However, expression was obtained in only one transgenic line. Multimerization of the C'delta9 enhancer conferred expression in prostate, but again in only one line. The greater penetrance of C'delta2 expression was not driven by glucocorticoids, as adrenalectomy had little effect, but may be dependent on the NF-kappaB-like element absent from the C'delta9 fragment. That two transgenic lines showed expression in androgen target sites driven by enhancers that are androgen-specific in vitro suggested that activation of this enhancer, when it could occur, was in response to androgen. The dramatically different behavior of the two related enhancer sequences underscores the importance of chromosomal context to the activity and specificity of regulatory elements.

Steroid hormone responsiveness of a family of closely related mouse proviral elements

Regulation of the mouse sex-limited protein (Slp) gene in unusual in that hormone response is conferred by the 5' LTR of an upstream inserted provirus, dubbed the imposon (imp1). In a search for additional genes whose regulation has been affected by retrotransposition events, we isolated two partial proviral elements by stringent screening of a mouse genomic library. One clone (imp2) contained a portion of the envelope gene and a 3' LTR that was nearly identical to the 3' LTR of imp1; this similarity extended to insertion into a B1 repetitive element. The second proviral clone (imp3) contained a 5' LTR and associated coding sequences, but lacked its 3' LTR; the LTR of imp3 differed by 12% from the imp1 sequence. To assess potential hormone response, proviral enhancer regions cloned into reporter vectors were tested in transfection. The imp2 enhancer was similar in behavior to imp1, conferring both androgen and glucocorticoid induction in one fragment context and an androgen-specific response in another. In contrast, the imp3 enhancer allowed high expression in the absence of hormone and was less responsive to steroids in general and androgen in particular. These three proviral elements define a small family of steroid responsive proviruses in the mouse genome, and at least one member has had a lasting impact on an endogenous gene's regulation.

DNA sequence insertion and evolutionary variation in gene regulation

Current evidence on the long-term evolutionary effect of insertion of sequence elements into gene regions is reviewed, restricted to cases where a sequence derived from a past insertion participates in the regulation of expression of a useful gene. Ten such examples in eukaryotes demonstrate that segments of repetitive DNA or mobile elements have been inserted in the past in gene regions, have been preserved, sometimes modified by selection, and now affect control of transcription of the adjacent gene. Included are only examples in which transcription control was modified by the insert. Several cases in which merely transcription initiation occurred in the insert were set aside. Two of the examples involved the long terminal repeats of mammalian endogenous retroviruses. Another two examples were control of transcription by repeated sequence inserts in sea urchin genomes. There are now six published examples in which Alu sequences were inserted long ago into human gene regions, were modified, and now are central in control/enhancement of transcription. The number of published examples of Alu sequences affecting gene control has grown threefold in the last year and is likely to continue growing. Taken together, all of these examples show that the insertion of sequence elements in the genome has been a significant source of regulatory variation in evolution.

Localization of the mouse gene releasing sex-limited expression of Slp

To probe genetic variation in the regulation of sexual dimorphism, we have characterized the mouse protein Slp, coded by the gene sex-limited protein (Slp). Slp expression in many strains is limited to males and is androgen-dependent. However, female expression is also observed in rare strains, due to nonlinked gene(s) termed regulator of sex-limitation (rsl). In this report we demonstrate that female expression of Slp results from homozygous recessive allele(s) at a single autosomal locus that maps to a 2.2-centimorgan interval on chromosome 13. This conclusion was supported by extensive genetic analyses including the use of polymorphic microsatellites to type numerous backcross progeny and a recombinant inbred series and to identify the congenic interval in three independently derived congenic strains. Four attractive candidate genes were identified by the localization of rsl. Interestingly, rsl was found not only to enable expression in females but to also increase expression in males. The findings suggest that the expression of Slp and perhaps other sexually dimorphic proteins is regulated by two pathways, one that is dependent upon rsl but not androgens and another that is rsl-independent but requires androgens.

Avridine-induced arthritis in rats; a T cell-dependent chronic disease influenced both by MHC genes and by non-MHC genes

Avridine is a potent synthetic adjuvant that can induce arthritis is most rat strains. The clinical appearance and histopathology of avridine-induced arthritis show great similarity to other arthritis models such as collagen-induced arthritis. In LEW and DA rats the avridine-induced arthritis is severe and long lasting. To investigate a possible genetic influence on the disease we compared LEW, DA and E3 rats, which are of different genetic origins, for their ability to develop arthritis after injection of a low dose of avridine (1.5 mg/rat). The E3 rat was shown to be resistant, whereas all of the DA rats developed arthritis. Recombinant inbred strains derived from DA and E3 parentals varied in susceptibility to avridine. Only strains sharing RT1av1 with DA developed arthritis, indicating a role for the MHC genes. The MHC association was further analysed in a series of Lewis congenic strains using the 1.5 mg avridine dose. All strains developed arthritis. LEW.1C and LEW.1W developed only acute arthritis, whereas LEW.1A, LEW, LEW.1D, LEW.1N and LEW.1F developed chronic arthritis. In particular, the LEW.1F rats developed a chronic severe arthritis of high incidence. The chronic arthritis showed an active, erosive joint inflammation several months after induction. Nude rats are resistant to avridine-induced arthritis, indicating a T cell dependence of the disease which supports the importance of MHC. However, non-MHC genes are also crucial to arthritis development. Recombinants between DA and E3, sharing RT1av1 with DA, showed either a lower incidence or a lower severity of disease than the DA rats. The E3 rat and the recombinants with RT1u were completely resistant, whereas LEW.1W, also RT1u, were highly susceptible.

Sexually dimorphic DNA demethylation in the promoter of the Slp (sex-limited protein) gene in mouse liver

Mouse Slp, a duplicate of the fourth complement component (C4) gene, exhibits EDTA-independent complement activity with a hepatic expression that is male specific. To provide an underlying mechanism for the male-specific expression, we have analyzed the promoter activity of the various 5'-flanking sequences and CpG demethylation of the Slp gene. Transient transfections using HepG2 cells indicate that the element TTCCGGGC (nt -124 to -117) regulates the promoter activity. Moreover, CpG at position -121 of this regulatory element is demethylated to a much higher degree in males than in females. This sexually dimorphic DNA demethylation is consistent with the male-specific expression of the Slp gene in DBA/2 males. The regulatory element binds to the different TTCCGGGC-specific nuclear proteins depending on the methylation of the CpG site. In contrast, the corresponding CpG at position -119 of the C4 gene, which is expressed in both males and females, is demethylated at equal and high levels in both sexes. We therefore propose that the DNA demethylation and methylation-sensitive transcription factors may be a part of the regulatory mechanism for the male-specific expression of the Slp gene.

Inhibition of complement alternative pathway in mice with Fab antibody to recombinant adipsin/factor D

Mouse adipsin is a serine protease secreted mainly by adipocytes. Similarly to factor D of human complement, it cleaves factor B. That adipsin is the equivalent of human factor D in the mouse is further suggested by their structural homology. Specific antisera against recombinant mouse adipsin (r-adipsin) were produced in rabbits. Anti-r-adipsin IgG was shown to bind to radiolabeled r-adipsin and to inhibit its hemolytic activity. In vitro, these antibodies Ab and Fab fragments thereof inhibited the adipsin/factor D hemolytic activity of mouse serum. They also blocked C3 activation induced by cobra venom factor (CVF), but did not interfere with classical pathway function. After intravenous injection of anti-r-adipsin Fab into BALB/c mice, the adipsin/factor D hemolytic activity of serum was abolished during a 4-h period. The C3 depleting effect of CVF injected intravenously was significantly delayed in BALB/c mice which had been pretreated with anti-r-adipsin Fab. These experiments demonstrate that mouse adipsin is the only form of mouse factor D and that anti-r-adipsin antibody can be used to produce a specific inhibition of the alternative pathway in vivo.