The organization of the human complement factor I gene (IF): a member of the serine protease gene family

Cardiovascular disease and thrombosis: Intersections with the immune system, inflammation, and the coagulation system

The coagulation and immune system, both essential physiological systems in the human body, are intricately interconnected and play a critical role in determining the overall health of patients. These systems collaborate via various shared regulatory pathways, such as the Tissue Factor (TF) Pathway. Immunological cells that express TF and generate pro-inflammatory cytokines have the ability to affect coagulation. Conversely, coagulation factors and processes have a reciprocal effect on immunological responses by stimulating immune cells and regulating their functions. These interconnected pathways play a role in both preserving well-being and contributing to a range of pathological disorders. The close relationship between blood clotting and inflammation in the development of vascular disease has become a central focus of clinical study. This research specifically examines the crucial elements of this interaction within the contexts of cardiovascular disease and acute coronary syndrome. Tissue factor, the primary trigger of the extrinsic coagulation pathway, has a crucial function by inducing a proinflammatory reaction through the activation of coagulation factors. This, in turn, initiates coagulation and subsequent cellular signalling pathways. Protease-activated receptors establish the molecular connection between coagulation and inflammation by interacting with activated clotting factors II, X, and VII. Thrombosis, a condition characterised by the formation of blood clots, is the most dreaded consequence of cardiovascular disorders and a leading cause of death globally. Consequently, it poses a significant challenge to healthcare systems. Antithrombotic treatments efficiently target platelets and the coagulation cascade, but they come with the inherent danger of causing bleeding. Furthermore, antithrombotics are unable to fully eliminate thrombotic events, highlighting a treatment deficiency caused by a third mechanism that has not yet been sufficiently addressed, namely inflammation. Understanding these connections may aid in the development of novel approaches to mitigate the harmful mutual exacerbation of inflammation and coagulation. Gaining a comprehensive understanding of the intricate interaction among these systems is crucial for the management of diseases and the creation of efficacious remedies. Through the examination of these prevalent regulatory systems, we can discover novel therapeutic approaches that specifically target these complex illnesses. This paper provides a thorough examination of the reciprocal relationship between the coagulation and immune systems, emphasising its importance in maintaining health and understanding disease processes. This review examines the interplay between inflammation and thrombosis and its role in the development of thrombotic disorders.

Complement factor I: Regulatory nexus, driver of immunopathology, and therapeutic

Complement factor I (FI) is the nexus for classical, lectin and alternative pathway complement regulation. FI is an 88 kDa plasma protein that circulates in an inactive configuration until it forms a trimolecular complex with its cofactor and substrate whereupon a structural reorganization allows the catalytic triad to cleave its substrates, C3b and C4b. In keeping with its role as the master complement regulatory enzyme, deficiency has been linked to immunopathology. In the setting of complete FI deficiency, a consumptive C3 deficiency results in recurrent infections with encapsulated microorganisms. Aseptic cerebral inflammation and vasculitic presentations are also less commonly observed. Heterozygous mutations in the factor I gene (CFI) have been demonstrated to be enriched in atypical haemolytic uraemic syndrome, albeit with a very low penetrance. Haploinsufficiency of CFI has also been associated with decreased retinal thickness and is a strong risk factor for the development of age-related macular degeneration. Supplementation of FI using plasma purified or recombinant protein has long been postulated, however, technical difficulties prevented progression into clinical trials. It is only using gene therapy that CFI supplementation has reached the clinic with GT005 in phase I/II clinical trials for geographic atrophy.

Kidney diseases

Dysregulation and accelerated activation of the alternative pathway (AP) of complement is known to cause or accentuate several pathologic conditions in which kidney injury leads to the appearance of hematuria and proteinuria and ultimately to the development of chronic renal failure. Multiple genetic and acquired defects involving plasma- and membrane-associated proteins are probably necessary to impair the protection of host tissues and to confer a significant predisposition to AP-mediated kidney diseases. This review aims to explore how our current understanding will make it possible to identify the mechanisms that underlie AP-mediated kidney diseases and to discuss the available clinical evidence that supports complement-directed therapies. Although the value of limiting uncontrolled complement activation has long been recognized, incorporating complement-targeted treatments into clinical use has proved challenging. Availability of anti-complement therapy has dramatically transformed the outcome of atypical hemolytic uremic syndrome, one of the most severe kidney diseases. Innovative drugs that directly counteract AP dysregulation have also opened new perspectives for the management of other kidney diseases in which complement activation is involved. However, gained experience indicates that the choice of drug should be tailored to each patient's characteristics, including clinical, histologic, genetic, and biochemical parameters. Successfully treating patients requires further research in the field and close collaboration between clinicians and researchers who have special expertise in the complement system.

Haplotypes of HTRA1 rs1120638, TIMP3 rs9621532, VEGFA rs833068, CFI rs10033900, ERCC6 rs3793784, and KCTD10 rs56209061 Gene Polymorphisms in Age-Related Macular Degeneration

BACKGROUND

To determine the impact of HTRA1 rs1120638, TIMP3 rs9621532, VEGFA rs833068, CFI rs10033900, ERCC6 rs3793784, and KCTD10 rs56209061 genotypes on the development of age-related macular degeneration (AMD) in the Lithuanian population.

METHODS

A total of 916 subjects were examined: 309 patients with early AMD, 301 patients with exudative AMD, and 306 healthy controls. The genotyping of HTRA1 rs11200638, TIMP3 rs9621532, VEGFA rs833068, CFI rs10033900, ERCC6 rs3793784, and KCTD10 rs56209061 was carried out using the RT-PCR method.

RESULTS

Our study showed that single-nucleotide polymorphisms rs3793784 and rs11200638 were associated with increased odds of early and exudative AMD, and the variant in KCTD10 (rs56209061) was found to be associated with decreased odds of early and exudative AMD development after adjustments for age and gender in early AMD analysis and after adjustments only for age in exudative AMD. The haplotype containing two minor alleles C-A and the G-A haplotype in rs3793784-rs11200638 were statistically significantly associated with an increased risk of exudative AMD development after adjustment for age, while the G-G haplotype showed a protective role against early and exudative AMD and the haplotype C-G in rs3793784-rs11200638 was associated with a decreased risk only of exudative AMD development.

CONCLUSIONS

Our study identified two markers, rs11200638 and rs3793784, as risk factors for early and exudative AMD, and one marker, rs56209061, as a protective factor for early and exudative AMD development. The haplotypes constructed of rs3793784-rs11200638 were found to be associated with AMD development, as well.

The Challenges and Promise of Complement Therapeutics for Ocular Diseases

Ocular inflammation is a defining feature of sight threating diseases and its dysregulation can catalyze and or propagate ocular neurodegenerative maladies such as age-related macular degeneration (AMD). The complement system, an intrinsic component of the innate immunity, has an integral role in maintaining immune-surveillance and homeostasis in the ocular microenvironment; however, overstimulation can drive ocular inflammatory diseases. The mechanism for complement disease propagation in AMD is not fully understood, although there is accumulating evidence showing that targeted modulation of complement-specific proteins has the potential to become a viable therapeutic approach. To date, a major focus of complement therapeutics has been on targeting the alternative complement system in AMD. Recent studies have outlined potential complement cascade inhibitors that might mitigate AMD disease progression. First-in-class complement inhibitors target the modulation of complement proteins C3, C5, factor B, factor D, and properdin. Herein, we will summarize ocular inflammation in the context of AMD disease progression, current clinical outcomes and complications of complement-mediated therapeutics. Given the need for additional therapeutic approaches for ocular inflammatory diseases, targeted complement modulation has emerged as a leading candidate for eliminating inflammation-driven ocular maladies.

Haemolytic uraemic syndrome

Haemolytic uraemic syndrome (HUS) is defined by the simultaneous occurrence of nonimmune haemolytic anaemia, thrombocytopenia and acute renal failure. This leads to the pathological lesion termed thrombotic microangiopathy, which mainly affects the kidney, as well as other organs. HUS is associated with endothelial cell injury and platelet activation, although the underlying cause may differ. Most cases of HUS are associated with gastrointestinal infection with Shiga toxin-producing enterohaemorrhagic Escherichia coli (EHEC) strains. Atypical HUS (aHUS) is associated with complement dysregulation due to mutations or autoantibodies. In this review, we will describe the causes of HUS. In addition, we will review the clinical, pathological, haematological and biochemical features, epidemiology and pathogenetic mechanisms as well as the biochemical, microbiological, immunological and genetic investigations leading to diagnosis. Understanding the underlying mechanisms of the different subtypes of HUS enables tailoring of appropriate treatment and management. To date, there is no specific treatment for EHEC-associated HUS but patients benefit from supportive care, whereas patients with aHUS are effectively treated with anti-C5 antibody to prevent recurrences, both before and after renal transplantation.

Complement factor I from flatfish half-smooth tongue (Cynoglossus semilaevis) exhibited anti-microbial activities

Complement factor I (Cfi) is a soluble serine protease which plays a crucial role in the modulation of complement cascades. In the presence of substrate modulating cofactors (such as complement factor H, C4bp, CR1, etc), Cfi cleaves and inactivates C3b and C4b, thereby controlling the complement-mediated processes. In this study, we sequenced and characterized Cfi gene from Cynoglossus Semilaevis (designated as CsCfi) for the first time. The full-length cDNA of CsCfi was 2230 bp in length, including a 98 bp 5'-untranslated region (UTR), a 164 bp 3'-UTR and a 1968 bp open reading frame (ORF). It encoded a polypeptide of 656 amino acids, with a molecular mass of 72.28 kDa and an isoelectric point of 7.71. A signal peptide was defined at N-terminus, resulting in a 626-residue mature protein. Multiple sequence alignment revealed that Cfi proteins were well conserved with the typical modular architecture and identical active sites throughout the vertebrates, which suggested the conserved function of Cfi. Phylogenetic analysis indicated that CsCfi and the homologous Cfi sequences from teleosts clustered into a clade, separating from another clade from the cartilaginous fish and other vertebrates. Tissue expression profile analysis by quantitative real-time PCR (qRT-PCR) showed that CsCfi mRNA constitutively expressed in all tested tissues, with the predominant expression in liver and the lowest in stomach. Temporal expression levels of CsCfi after challenging with Vibrio anguillarum showed different expression patterns in intestine, spleen, skin, blood, head kidney and liver. The recombinant CsCfi (rCsCfi) protein showed broad-spectrum antimicrobial activities against the Gram-positive bacteria Staphylococcus aureus and the Gram-negative bacteria Escherichia coli, Pseudomonas aeruginosa and Shewanella putrefaciens. The research revealed that CsCfi plays an important role in C. Semilaevis immunity.

Molecular characterization and expression analysis of the complement factor I (CpFI) in the whitespotted bamboo shark (Chiloscyllium plagiosum)

Complement factor I (FI) is a plasma serine proteinase that plays an essential role in the modulation of the complement cascade. In the presence of substrate modulating cofactors (Factor H, C4bp, CR1, etc), FI cleaves the activation products of C3 (i.e. C3b) and C4 (i.e. C4b) to limit complement activity. In this study, the full length cDNA of factor I (CpFI) is isolated from the liver of the whitespotted bamboo shark (Chiloscyllium plagiosum). The CpFI cDNA is 2326 bp in length, encoding a protein of 671 amino acids, which shares 72-80% identity with FI molecules of other sharks, higher than the teleosts (37-40%) and mammals (44-47%). The sequence alignment and comparative analysis indicates the FI proteins are well conserved, with the typical modular architecture and identical active sites throughout vertebrate evolution, suggesting the conserved function. However, the additional sequence present between the leader peptide (LP) and the factor I membrane attack complex (FIMAC) domain in other fishes is also found in CpFI, which consists of two kind of tandem repeats. Phylogenetic analysis suggests that CpFI belongs to the elasmobranch clade, in parallel with the higher vertebrates, to form a sister taxa to teleosts. Expression analysis revealed that CpFI is ubiquitously distributed in a variety of tissues, with the constitutive expression in liver, which might reflect the species-specific distribution patterns of FI. Together with earlier reports, the presence of FI in various sharks might suggest the existence of a well-developed complement regulation mechanism in cartilaginous fish.

A new era in the diagnosis and treatment of atypical haemolytic uraemic syndrome

The haemolytic uraemic syndrome (HUS) is characterised by haemolytic anaemia, thrombocytopenia and acute renal failure. The majority of cases are seen in childhood and are preceded by an infection with Shiga-like toxin producing Escherichia coli (STEC-HUS; so-called typical HUS). Non-STEC or atypical HUS (aHUS) is seen in 5 to 10% of all cases and occurs at all ages. These patients have a poorer outcome and prognosis than patients with STEC-HUS. New insights into the pathogenesis of aHUS were revealed by the identification of mutations in genes encoding proteins of the alternative pathway of the complement system in aHUS patients. Specific information of the causative mutation is important for individualised patient care with respect to choice and efficacy of therapy, the outcome of renal transplantation, and the selection of living donors. This new knowledge about the aetiology of the disease has stimulated the development of more specific treatment modalities. Until now, plasma therapy was used with limited success in aHUS, but recent clinical trials have demonstrated that patients with aHUS can be effectively treated with complement inhibitors, such as the monoclonal anti-C5 inhibitor eculizumab.

Characterization of shark complement factor I gene(s): genomic analysis of a novel shark-specific sequence

Complement factor I is a crucial regulator of mammalian complement activity. Very little is known of complement regulators in non-mammalian species. We isolated and sequenced four highly similar complement factor I cDNAs from the liver of the nurse shark (Ginglymostoma cirratum), designated as GcIf-1, GcIf-2, GcIf-3 and GcIf-4 (previously referred to as nsFI-a, -b, -c and -d) which encode 689, 673, 673 and 657 amino acid residues, respectively. They share 95% (<or=) amino acid identities with each other, 35.4-39.6% and 62.8-65.9% with factor I of mammals and banded houndshark (Triakis scyllium), respectively. The modular structure of the GcIf is similar to that of mammals with one notable exception, the presence of a novel shark-specific sequence between the leader peptide (LP) and the factor I membrane attack complex (FIMAC) domain. The cDNA sequences differ only in the size and composition of the shark-specific region (SSR). Sequence analysis of each SSR has identified within the region two novel short sequences (SS1 and SS2) and three repeat sequences (RS1-3). Genomic analysis has revealed the existence of three introns between the leader peptide and the FIMAC domain, tentatively designated intron 1, intron 2, and intron 3 which span 4067, 2293 and 2082bp, respectively. Southern blot analysis suggests the presence of a single gene copy for each cDNA type. Phylogenetic analysis suggests that complement factor I of cartilaginous fish diverged prior to the emergence of mammals. All four GcIf cDNA species are expressed in four different tissues and the liver is the main tissue in which expression level of all four is high. This suggests that the expression of GcIf isotypes is tissue-dependent.

Variation near complement factor I is associated with risk of advanced AMD

A case-control association study for advanced age-related macular degeneration was conducted to explore several regions of interest identified by linkage. This analysis identified a single nucleotide polymorphism just 3' of complement factor I on chromosome 4 showing significant association (P<10(-7)). Sequencing was performed on coding exons in linkage disequilibrium with the detected association. No obvious functional variation was discovered that could be the proximate cause of the association, suggesting a noncoding regulatory mechanism.

A mutation in factor I that is associated with atypical hemolytic uremic syndrome does not affect the function of factor I in complement regulation

Factor I (FI) is the major complement inhibitor that degrades C3b and C4b in the presence of cofactors such as factor H (FH) and membrane cofactor protein (MCP). Recently, mutations and polymorphisms in complement regulator molecules FH and MCP but also in FI have been associated with atypical hemolytic uremic syndrome (aHUS). HUS is a disorder characterized by hemolytic anemia, thrombocytopenia and acute renal failure. In this study, we report three unrelated patients with an identical heterozygous mutation, G261D, in the FI heavy chain who developed severe aHUS at different time points in their lives. Two of the patients also have polymorphisms in FH previously associated with risk of developing aHUS. Testing in particular one patient and control serum samples we did not observe major differences in complement hemolytic activity, FI plasma levels or the capability to degrade C4b or C3b. A recombinant protein was produced in order to analyze the functional consequences of the mutation. Mutant FI had a slightly different migration pattern during electrophoresis under reducing conditions. An alteration due to alternative splicing or glycosylation was ruled out, thus the altered migration may be due to proximity of the mutation to a cysteine residue. The recombinant mutant FI degraded C3b and C4b in a manner comparable to wild-type protein. In conclusion, despite the association between the heterozygous mutation in FI and aHUS we did not observe any abnormalities in the function of FI regarding complement regulation.

Genetics of HUS: the impact of MCP, CFH, and IF mutations on clinical presentation, response to treatment, and outcome

Hemolytic uremic syndrome (HUS) is a thrombotic microangiopathy with manifestations of hemolytic anemia, thrombocytopenia, and renal impairment. Genetic studies have shown that mutations in complement regulatory proteins predispose to non-Shiga toxin-associated HUS (non-Stx-HUS). We undertook genetic analysis on membrane cofactor protein (MCP), complement factor H (CFH), and factor I (IF) in 156 patients with non-Stx-HUS. Fourteen, 11, and 5 new mutational events were found in MCP, CFH, and IF, respectively. Mutation frequencies were 12.8%, 30.1%, and 4.5% for MCP, CFH, and IF, respectively. MCP mutations resulted in either reduced protein expression or impaired C3b binding capability. MCP-mutated patients had a better prognosis than CFH-mutated and nonmutated patients. In MCP-mutated patients, plasma treatment did not impact the outcome significantly: remission was achieved in around 90% of both plasma-treated and plasma-untreated acute episodes. Kidney transplantation outcome was favorable in patients with MCP mutations, whereas the outcome was poor in patients with CFH and IF mutations due to disease recurrence. This study documents that the presentation, the response to therapy, and the outcome of the disease are influenced by the genotype. Hopefully this will translate into improved management and therapy of patients and will provide the way to design tailored treatments.

Mutations in complement factor I predispose to development of atypical hemolytic uremic syndrome

Mutations in the plasma complement regulator factor H (CFH) and the transmembrane complement regulator membrane co-factor protein (MCP) have been shown to predispose to atypical hemolytic uremic syndrome (HUS). Both of these proteins act as co-factors for complement factor I (IF). IF is a highly specific serine protease that cleaves the alpha-chains of C3b and C4b and thus downregulates activation of both the classical and the alternative complement pathways. This study looked for IF mutations in a panel of 76 patients with HUS. Mutations were detected in two patients, both of whom had reduced serum IF levels. A heterozygous bp change, c.463 G>A, which results in a premature stop codon (W127X), was found in one, and in the other, a heterozygous single base pair deletion in exon 7 (del 922C) was detected. Both patients had a history of recurrent HUS after transplantation. This is in accordance with the high rate of recurrence in patients with CFH mutations. Patients who are reported to have mutations in MCP, by contrast, do not have recurrence after transplantation. As with CFH- and MCP-associated HUS, there was incomplete penetrance in the family of one of the affected individuals. This study provides further evidence that atypical HUS is a disease of complement dysregulation.

Molecular cloning of the complement regulatory factor I isotypes from the common carp (Cyprinus carpio)

Factor I is a novel serine protease that regulates complement activation. Here we report the complete primary structure of two isotypic factor Is isolated from the common carp ( Cyprinus carpio), a pseudotetraploid teleost. A carp hepatopancreas cDNA library was screened using two RT-PCR-amplified cDNA fragments encoding part of the carp factor I-like serine protease domain. Two distinct cDNA clones, designated FI-A and FI-B, were isolated. Their deduced amino acid sequences share 75.2% identity with each other. FI-A has a typical factor I-like domain organization composed of two disulfide-linked polypeptides (H-chain and L-chain). On the other hand, FI-B contains a novel sequence of 115 amino acids inserted at the N-terminus of the H-chain. Genomic Southern hybridization suggests that FI-A and FI-B are encoded by distinct genes in the carp genome. Expression analysis by RT-PCR revealed that the major site of FI-A expression is the ovary, whereas FI-B expression is detected mainly in the hepatopancreas at a level higher than that of FI-A. The present data, taken together, suggest that carp have duplicated genes coding for factor I, and FI-B with the novel insertion plays a dominant role in the complement system. In addition, homology search of the fugu genome database using the carp FI-A and FI-B sequences identified a putative fugu factor I gene, which has an exon/intron organization different from that of the human orthologue.

Molecular characterization of homozygous hereditary factor I deficiency

We have studied the molecular basis of factor I (fI) deficiency in two Brazilian sisters from a consanguineous family. By reverse transcription-polymerase chain reaction we observed that all fI cDNA amplified products from one sister had the same size as those of normal cDNA, however, they were significantly less intense. Sequencing analysis of subcloned cDNA revealed a dinucleotide insertion (AT) between positions 1204 and 1205 in the 11th exon that creates a stop codon 13 bp downstream of the insertion site. Genomic DNA sequencing and heteroduplex analysis confirmed that both probands are homozygous for this mutation, whereas their parents are heterozygous. The stop codon and the diminished amounts of fI cDNA could indicate increased fI mRNA instability, perhaps due to a mechanism of nonsense-mediated decay. This hypothesis is consistent with our observation that treatment with the translation inhibitor cycloheximide stabilized fI mRNA expression in proband's fibroblasts.

Conservation of the modular structure of complement factor I through vertebrate evolution

Mammalian complement factor I plays pivotal roles in the regulation of complement activation and generation of important biological activities from C3. The evolutionary origin of factor I has been unclear except with regard to the molecular cloning of factor I from amphibian Xenopus. Here, we report the identification and characterization of factor I cDNA from the liver of the banded houndshark. The deduced amino acid sequence of shark factor I showed a modular organization that was completely identical to that of mammalian factor I, suggesting the functional conservation of factor I throughout vertebrate evolution. Functionally important amino acid residues such as the basic residues at the processing site and the residues at the active site of the serine protease domain are conserved. Repeated sequences composed of 16 amino acids were inserted at a site between the leader peptide and the factor I/membrane attacking complex module in the shark factor I. This repeat is missing from mammalian and amphibian factor I, and the biological significance of the sequence, if any, is not clear at the moment. There was only one copy of the shark factor I gene, and Northern blotting analysis showed that the shark factor I gene was expressed only in the liver among several organs tested. While the lack of functional data does not exclude the possibility that factor I could have a different function, all these facts, together with the earlier reported data suggest the existence of a well developed complement system in cartilaginous fish.

Complement factor I is upregulated in rat hepatocytes by interleukin-6 but not by interferon-gamma, interleukin-1beta, or tumor necrosis factor-alpha

Complement factor I (FI) is a regulatory serine protease of the complement system which cleaves three peptide bonds in the alpha-chain of C3b and two bonds in the alpha-chain of C4b and thus prevents the assembly of the C3 and C5 convertases. We have investigated the proinflammatory cytokines IL-6, IL-1beta, TNF-alpha and IFN-gamma for their potential role in the regulation of FI expression. Of the investigated cytokines, only IL-6 increased the FI-specific RT-PCR signal in isolated hepatocytes, in the two rat hepatoma-derived cell lines FAO and H4IIE or in HUVECs. Quantitative competitive RT-PCR showed an IL-6 induced upregulation of FI-specific mRNA by about ten-fold. These data are in accord with Northern blot analyses in which the FI-mRNA was upregulated by IL-6 between five- and seven-fold. IL-6, but not IL-1beta, TNF-alpha or IFN-gamma also increased FI-protein levels in cell culture supernatants by about five-fold as determined by a semiquantitative immunoblot using a novel monoclonal antibody specific for rat FI.

A modular chitin-binding protease associated with hemocytes and hemolymph in the mosquito Anopheles gambiae

Sp22D, a modular serine protease encompassing chitin binding, low density lipoprotein receptor, and scavenger receptor cysteine-rich domains, was identified by molecular cloning in the malaria vector, Anopheles gambiae. It is expressed in multiple body parts and during much of development, most intensely in hemocytes. The protein appears to be posttranslationally modified. Its integral, putatively glycosylated form is secreted in the hemolymph, whereas a smaller form potentially generated by proteolytic processing is associated with the tissues. Bacterial challenge or wounding result in low-level RNA induction, but the protein does not bind to bacteria, nor is its processing affected by infection. However, Sp22D binds to chitin with high affinity and undergoes transient changes in processing during pupal to adult metamorphosis; it may respond to exposure to naked chitin during tissue remodeling or damage.

Mechanistic studies of the effects of anti-factor H antibodies on complement-mediated lysis

We have recently reported that complement factor H, a negative regulator of complement-mediated cytotoxicity, is produced and secreted by most bladder cancers. This observation was exploited in the development of the BTA stat and BTA TRAK diagnostic assays, both of which make use of two factor H-specific monoclonal antibodies in sandwich format. Here we show that both antibodies exert interesting effects on the biochemistry of complement activation in in vitro systems. Antibody X13.2 competes with C3b for association with factor H and strongly inhibits factor H/factor I-mediated cleavage of C3b, thereby evidently inactivating a negative regulator of complement; yet, the antibody strongly inhibits complement-mediated lysis as well. Conversely, antibody X52. 1, which does not compete with C3b and has no effect on solution-phase cleavage of C3b, is capable of enhancing complement-mediated lysis of various cell types, including cancer cells, by over 10-fold. Our observations indicate that it is possible to deconvolute the biochemical roles of factor H in complement by means of appropriate inhibitors, a finding with potentially valuable implications for both basic research and cancer therapy.

Molecular cloning, genomic organization and cell-binding characteristics of mouse Spalpha

Several group B scavenger receptor cysteine-rich (SRCR) proteins have been shown to function as modulators in the immune response. Recently, we reported the cloning of a new member of this family, human Spalpha (hSpalpha). Herein we report the cloning and characterization of the mouse homologue of hSpalpha. Like its human counterpart, mouse Spalpha (mSpalpha), is a secreted protein containing three SRCR domains. Most lymphoid tissues express RNA transcripts encoding mSpalpha. Characterization of a genomic clone encoding the mature mSpalpha protein showed that each of the SRCR domains of mSpalpha is encoded by a single exon. Comparison of the sequence of mSPalpha with those of other published proteins indicates that it is the same as the recently reported protein named AIM (apoptosis inhibitor expressed by macrophages). Cell-binding studies with a mSpalpha immunoglobulin (mSpalpha-Rgamma) fusion protein indicated that mSpalpha is capable of binding to spleen-derived CD19+ B cells and minimally to peritoneal cavity-derived CD19+ B cells but not to peripheral blood-derived B cells. Spleen-derived CD3+ T cells also bound mSpalpha-Rgamma; however, no binding was observed to either peripheral blood mononuclear cells or peritoneal cavity-derived CD3+ T cells. The mSpalpha-Rgamma fusion protein was also shown to bind to the mouse cell lines WEHI3 (monocytic) and EL-4 (thymoma, T cell). The cloning of cDNA and genomic clones encoding mSpalpha and the identification of cells expressing a putative mSpalpha receptor(s) should facilitate in vivo studies designed to investigate the function of Spalpha in the immune compartment.

Transcriptional modulation of the human complement factor I gene in Hep G2 cells by protein kinase C activation

This study examined the role of the protein kinase C (PKC) signalling pathway in the regulation of expression of human complement factor I (CFI) gene. The production of CFI by Hep G2 cells was enhanced in a dose- and time-dependent fashion by 12-O-tetradecanoyl-1,2-phorbol 13-acetate (TPA), a potent PKC activator. 4Alpha-phorbol didecanoate, an inactive phorbol ester, had no effect on CFI synthesis. The TPA-dependent increase in CFI secretion was correlated with an increase in CFI mRNA levels. Forskolin, a cAMP-inducing agent, augmented the TPA response. W7, an inhibitor of protein kinase A and genistein, an inhibitor of protein tyrosine kinase(s) both did not prevent the increase in CFI expression mediated by TPA. However, calphostin C, a specific inhibitor of PKC, abolished the TPA-induced increase in CFI mRNA levels. Down regulation of intracellular PKC levels by prior exposure of Hep G2 cells to a high concentration of TPA also blocked the increase in CFI mRNA levels induced by TPA suggesting that the TPA effects were mediated via activation of PKC. mRNA decay studies indicated that the half-life of CFI mRNA in TPA-induced cells was not significantly different from control. Nuclear run-on transcriptional assays on the other hand demonstrated that whereas the CFI gene is transcribed under basal conditions in Hep G2 cells, TPA induced a 3-4 fold increase in the transcription rate of CFI gene in 24 h. The transcription rate of GAPDH gene did not change, indicating that the effects were not general on gene transcription. Transient transfections of Hep G2 cells with chloramphenicol acetyltransferase reporter gene (CAT) constructs containing a series of sequential 5' deletions of the CFI promoter and CAT assays showed that the sequence between -136 and -130, containing an AP-1 consensus sequence (TGAGTCA) was required for the TPA response. This observation was substantiated by the finding that mutation of this AP-1 site to TttaTCA or TtAtcCA abolished the TPA responsiveness. The enhancement of the activity of transfected chimeric CAT constructs by TPA was abrogated by calphostin C and by pyrrolidine dithiocarbamate (an inhibitor of NF-kappaB and AP-1 transactivation). These results indicate that TPA regulation of CFI gene requires PKC signalling and is mediated by via a TPA response element (TRE) in the CFI promoter region located at -136/-130 and involves the transactivation of AP-1 and NF-kappaB transcription factors. We suggest that PKC may be one of the intracellular pathways that control CFI gene expression and that cellular processes (involving growth factors, hormones, cytokines etc.) that activate PKC may upregulate the expression of the CFI gene.

Rat complement factor I: molecular cloning, sequencing and expression in tissues and isolated cells

Factor I (FI) is a regulatory serine protease of the complement system which cleaves three peptide bonds in the alpha-chain of C3b and two bonds in the alpha-chain of C4b thereby inactivating these proteins. The human protein and the recently characterized mouse factor I are heterodimers of about 88,000 MW which consist of a non-catalytic heavy chain of 50,000 MW which is linked to a catalytic light chain of 38,000 MW by a disulphide bond. For the screening of a rat liver cDNA library we used a hybridization probe produced by polymerase chain reaction (PCR) using degenerated primers which corresponded to conserved parts of the human and the murine factor I nucleotide sequences. One of the identified sequences, which had a length of 2243 base pairs (bp), contained the complete coding region and the whole 3' untranslated region. The length of the coding region in rat consisted of 1812 bp followed by a 3' untranslated region of 207 bp including the polyadenylation signal and the beginning of the poly A tail. Comparison of the rat cDNA-derived coding sequence revealed identities of 87% to the mouse and of 78% to the human FI nucleotide sequence. The translation product of rat FI mRNA was 604 amino acid residues (aa) in length with an identity of 85% to the mouse (603 aa) and 69% to the human protein (583 aa). The comparison of the molecular mass predicted by the primary structure and derived from rat FI isolated from rat serum as detected in immunoblot analyses suggested a glycosylation of more than 20% of the total mass of the FI protein. Expression studies using reverse transcription (RT)-PCR assays indicated that FI-specific mRNA could neither be identified in B cells, nor in T cells, monocytes or granulocytes from rat and human peripheral blood nor in rat peritoneal macrophages. These data were in agreement with the results of RT-PCR obtained with several human lymphoma cell lines (Jurkat, MOLT-4, HUT102, Wil 2-NS, Ramos, Raji, U937) all of which were devoid of FI-specific mRNA. In accord with our data from two rat hepatoma cell lines (FAO and H4IIE) and one from man (HepG2) only isolated rat hepatocytes (HC) but neither Kupffer cells (KC), hepatic stellate cells (HSC; Ito cells) nor sinusoidal endothelial cells (SEC) expressed FI-specific mRNA. FI mRNA was also detected in human umbilical vein endothelial cells (HUVEC) and in the uterus and small intestine of the rat. Spleen and lymph nodes did not contain any detectable FI-specific mRNA.

An initiator element and a proximal cis-acting sequence are essential for transcriptional activation of the complement factor I (CFI) gene

Human complement factor I (CFI) is a serine protease which regulates the complement system by inactivation of C3b and C4b in the presence of appropriate cofactors. In this study, we have analyzed the mechanism controlling the constitutive transcriptional activation of the CFI gene. Using deletion analysis and transient CAT expression assays, we have mapped the minimal promoter to the region located between -46 and +160 bp relative to the major transcription start point (tsp), and also shown that cis-acting elements both upstream and downstream of the tsp are important for promoter activity. A silencer element was also found between -71 and -46 bp. The sequence surrounding the tsp was related to the mouse terminal deoxynucleotidyltransferase initiator element (Inr) and point mutations in this sequence, from -3 to +4, drastically reduced CFI promoter activity. Mutations in a -9 to -5 bp CTGAA sequence immediately upstream of the tsp also reduced promoter activity. Gel mobility shift analysis demonstrated the binding of nuclear factors to a CTGAA repeat located at -9 to -5 and +101 to +105. Our results suggest that CFI promoter contains a functional Inr element that is essential for promoter activity, and the interactions of the CTGAA element located between -9 and +5 with nuclear factor(s) may be part of the machinery required for CFI Inr-dependent transcription.

Genomic organization and chromosomal localization of the human CD163 (M130) gene: a member of the scavenger receptor cysteine-rich superfamily

The human protein CD163 (M130) is a member of the scavenger receptor cysteine-rich (SRCR) superfamily, which is exclusively expressed by monocytes and macrophages. Here, we investigated the genomic organization and the chromosomal localization of the human CD163 gene. The CD163 gene is composed of 17 exons and 16 introns and spans over 35 kb. Each of its nine SRCR domains is encoded by a separate exon, which is similar to other members of the group B SRCR subfamily. Two cytoplasmic variants of CD163 arise from alternative splicing of intron 15, while a truncated and an extracellular variant results from alternative splicing of intron 5 or intron 7, respectively. Using fluorescence in situ hybridization we mapped this gene to the human chromosome 12p13. The transcription initiation sites of the CD163 gene were determined and the 5'-flanking region was sequenced. The nucleotide analysis revealed several putative binding sites for transcription factors, which have been shown to play an important role in myeloid specific gene expression. In addition, we identified a L1 element located 1.4 kb upstream of the major transcription initiation site.

Gene structure of the P100 serine-protease component of the human Ra-reactive factor

The Ra-reactive factor (RaRF) is a complement dependent anti-microbial factor that reacts with numerous microorganisms such as viruses, bacteria, fungi and protozoa. It is a complex of a mannan-binding lectin (MBL) and the serine protease, P100 (MASPI). P100 activates the C4 component of the complement system and its domain organization is similar to C1r and C1s. In this study, determination was made of the structure of the human P100 gene which was found longer than 67 kbp and to be comprised of 16 exons. Its non-protease region consisted of 10 exons, as in the case of C1r and C1s, and the introns were found present in the boundary separating two CUB domains, an EGF-like domain and two CCP domains and each CUB and CCP domain contained extra internal introns. The serine protease region was comprised of 6 exons in contrast to C1r and C1s, either of which consists of a single exon. The exon-intron structure was found to reflect the evolution of these molecules and P100 to have derived earlier in the stage of evolution than C1r or C1s.

Transcriptional and post-transcriptional regulation of complement factor I (CFI) gene expression in Hep G2 cells by interleukin-6

We have investigated the effects of IL-1 and IL-6 on human complement factor I (CFI) production by Hep G2 cells. IL-6 treatment caused a dose- and time-dependent increase in CFI secretion while IL-1 did not demonstrate such effects. The increase in CFI synthesis correlated with increase in CFI mRNA levels. The half-life of CFI mRNA in untreated cells was approx. 23 h and this was increased to 31 h (26% increase) following induction with IL-6. The IL-6 induced increase in CFI gene expression was inhibited by actinomycin D indicating regulatory effects at the level of transcription. Nuclear run-on experiments showed that IL-6 increased the rate of CFI gene transcription 4.2-fold. Transient transfection analysis of chloramphenicol acetyltransferase reporter gene constructs containing truncated segments of the 5'-flanking region of CFI gene showed that the cis-acting sequence(s) controlling the IL-6 inducible transcription resides in an 83 bp region located between -738 bp and -655 bp relative to the transcription start site. Our results indicate that the upregulation of CFI gene expression by IL-6 involves a coordinate effort at the level of transcription and mRNA stability, with the enhanced rate of transcription being the principal mechanism.

Cloning and characterization of the promoter for the human complement factor I (C3b/C4b inactivator) gene

Complement factor I is a serine proteinase that regulates the classical and alternative pathways of complement by cleaving C3b and C4b and preventing the assembly of C3 and C5 convertase enzymes. In order to understand the regulation of factor I gene expression in liver cells, 4kb of the 5' flanking region of the gene was cloned, and the 1474-bp 3'-end was sequenced and shown to contain a number of transcription factor consensus sequences. A major and two minor transcription start sites were identified, respectively, at 152, 178, and 198bp upstream of the translation start site by primer extension analysis. The transcriptional activity of the 1474-bp fragment was analyzed by fusion of 5' deletion constructs to a cat-encoding gene expression vector and transient transfections into Hep G2 cells. A 273-bp fragment located at -112 to +161 relative to the major transcription start site was sufficient for promoter activity. The 3' fragment spanning +3 to +161 and containing a TATA-like element did not demonstrate promoter activity, suggesting that the core promoter resides in a 115-bp sequence located between -112 and +3. This region contains an Inr-like element overlapping the major cap site and a CTF-NF1 element, two potential CCAAT boxes and an AP-2 element partially overlapping an Sp-1 site. Thus, factor I promoter may belong to the TATA-less Inr-driven class II promoters whose transcription is regulated by Sp-1. The transcriptional activity of the 1474-bp 5' flanking fragment was upregulated by PMA, IL-6 and TNF-alpha, suggesting that factor I may be an acute phase reactant.

CD6-ligand interactions: a paradigm for SRCR domain function?

The scavenger receptor cysteine-rich (SRCR) superfamily, which includes proteins expressed by leukocytes, can be subdivided into groups A and B. Group B contains the lymphocyte cell-surface receptor CD6. This article reviews recent progress in understanding the interaction between CD6 and its ligand, activated leukocyte cell adhesion molecule (ALCAM). Analysis of the CD6-ALCAM interaction may help to understand how other SRCR domains bind to their ligands.

Induction of a putative serine protease transcript in immune challenged Drosophila

In an effort to identify serine proteases involved in the insect's immune response, we used a degenerate PCR approach to amplify putative serine protease gene fragments in Drosophila. Sequencing of the cloned PCR products identified one serine protease previously isolated in D. melanogaster (SER1/SER2), as well as two novel putative serine protease gene fragments (SP2, SP3). The involvement of the corresponding genes in the immune response was examined by analyzing their expression in larval mRNA following both parasitic and bacterial exposures. The overexpression of one of the serine proteases-related mRNAs in immune challenged larvae suggests its involvement in the Drosophila immune response.

The molecular basis of hereditary complement factor I deficiency

The molecular basis of hereditary complement factor I deficiency is described in two pedigrees. In one pedigree, there were two factor I-deficient siblings, one of whom was asymptomatic and the other suffered from recurrent pyogenic infections. Their factor I mRNA was analyzed by reverse transcription of fibroblast RNA followed by amplification using the polymerase chain reaction. Both siblings were homozygous for the same transversion (adenine to thymine) at nucleotide 1282 in the cDNA. This mutation causes histidine-400 to be replaced by leucine. The altered histidine is a semi-conserved residue within the serine proteinase family, although no function has been ascribed to it. The proband of the second pedigree studied was found to be a compound heterozygote. One allele had the same mutation as the first family, the second allele had a donor splice site mutation that resulted in the deletion of the mRNA encoded in the fifth exon (a low-density lipoprotein receptor domain) from its transcript.

Control of the complement system

The complement system has developed a remarkably simple but elegant manner of regulating itself. It has faced and successfully dealt with how to facilitate activation on a microbe while preventing the same on host tissue. It solved this problem primarily by creating a series of secreted and membrane-regulatory proteins that prevent two highly undesirable events: activation in the fluid phase (no target) and on host tissue (inappropriate target). Also, if not checked, even on an appropriate target, the system would go to exhaustion and have nothing left for the next microbe. Therefore, the complement enzymes have an intrinsic instability and the fluid-phase control proteins play a major role in limiting activation in time. The symmetry of the regulatory process between fluid phase and membrane inhibitors at the C4/C3 step of amplification and convertase formation as well as at the MAC steps are particularly striking features of the self/nonself discrimination system. The use of glycolipid anchored proteins on membranes to decay enzymes and block membrane insertion events is unlikely to be by chance. Finally, it is economical for the cofactor regulatory activity to produce derivatives of C3b that now specifically engage additional receptors. Likewise, C1-Inh leads to C1q remaining on the immune complex to interact with the C1q receptor. Thus the complement system is designed to allow rapid, efficient, unimpeded activation on an appropriate foreign target while regulatory proteins intervene to prevent three undesirable consequences of complement activation: excessive activation on a single target, fluid phase activation, and activation on self.