Structure, organization, and regulation of the complement genes

Proprotein Convertases and the Complement System

Proteins destined for secretion - after removal of the signal sequence - often undergo further proteolytic processing by proprotein convertases (PCs). Prohormones are typically processed in the regulated secretory pathway, while most plasma proteins travel though the constitutive pathway. The complement system is a major proteolytic cascade in the blood, serving as a first line of defense against microbes and also contributing to the immune homeostasis. Several complement components, namely C3, C4, C5 and factor I (FI), are multi-chain proteins that are apparently processed by PCs intracellularly. Cleavage occurs at consecutive basic residues and probably also involves the action of carboxypeptidases. The most likely candidate for the intracellular processing of complement proteins is furin, however, because of the overlapping specificities of basic amino acid residue-specific proprotein convertases, other PCs might be involved. To our surprise, we have recently discovered that processing of another complement protein, mannan-binding lectin-associated serine protease-3 (MASP-3) occurs in the blood by PCSK6 (PACE4). A similar mechanism had been described for the membrane protease corin, which is also activated extracellularly by PCSK6. In this review we intend to point out that the proper functioning of the complement system intimately depends on the action of proprotein convertases. In addition to the non-enzymatic components (C3, C4, C5), two constitutively active complement proteases are directly activated by PCs either intracellularly (FI), or extracellularly (MASP-3), moreover indirectly, through the constitutive activation of pro-factor D by MASP-3, the activity of the alternative pathway also depends on a PC present in the blood.

Analysis of the Complement System in the Clinical Immunology Laboratory

The complement system is a critical component of both the innate and adaptive immune systems that augments the function of antibodies and phagocytes. Antigen-antibody immune complexes, lectin binding, and accelerated C3 tick-over can activate this well-coordinated and carefully regulated process. The importance of this system is highlighted by the disorders that arise when complement components or regulators are deficient or dysregulated. This article describes the pathways involved in complement activation and function, the regulation of these various pathways, and the interpretation of laboratory testing performed for the diagnosis of diseases of complement deficiency, exuberant complement activation, and complement dysregulation.

Antibacterial properties of plasma from the prairie rattlesnake (Crotalus viridis)

The innate immune system functions to quickly respond to pathogens and is likely the primary line of defense for ectothermic vertebrates. Snake populations appear to be in widespread decline globally, but despite the threats from emerging pathogens, very little work has been conducted to characterize their basic immune function. We used a wide-ranging snake species, the Prairie Rattlesnake (Crotalus viridis), to measure effects of snake plasma on the growth of eight bacterial species. Additionally, we quantified bacterial killing ability and kinetics of the immune response. Our results show that Prairie Rattlesnakes have robust innate immune systems, and concentrations of 10% snake plasma inhibit growth of 6 of 8 bacteria tested. Undiluted snake plasma inhibited nearly all bacterial growth. The immune response was fairly rapid, inhibiting 73% of bacterial growth within 20 min of exposure. These results are encouraging for conservation of wild populations, as snakes appear to exhibit a strong innate immune response. However, further work needs to be directed toward the evaluation of immune system capabilities in individual populations of conservation concern, and against pathogens known to cause mortality in wild snakes.

Identification and expression of complement component C8α, C8β and C8γ gene in half-smooth tongue sole (Cynoglossus semilaevis) and C8α recombinant protein antibacterial activity analysis

Complement component C8, which mediates membrane attack complex formation and bacterial lysis, plays important roles in the complement system. The cDNA sequences of the C8α, C8β and C8γ genes were cloned from half-smooth tongue sole (Cynoglossus semilaevis). Full-length cDNA of CsC8α (C8α of C. semilaevis), CsC8β and CsC8γ was 1990, 2219 and 886 bp, respectively, which contained open reading frames of 1797, 1749 and 666 bp, encoding 598, 582 and 221 amino acids, respectively. The deduced proteins of CsC8α, CsC8β and CsC8γ showed the closest amino acid similarity to C8α (73%) of Siniperca chuatsi, C8β (76%) of Oryzias latipes and C8γ (72%) of Takifugu rubripes, respectively. The highest expression level of CsC8α, CsC8β and CsC8γ among the 13 normal tissues was observed in liver tissue, followed by much lower levels in other tissues. After infection with Vibrio anguillarum, CsC8α, CsC8β and CsC8γ were significantly up-regulated in all of the detected tissues, including the intestine, liver, gill, head kidney, blood and spleen. Then, a recombinant expression plasmid was constructed, and the recombinant CsC8α protein was expressed in GS115 pichia pastoris yeast. Furthermore, to investigate the biological functions of recombinant CsC8α, an antibacterial assay was performed, and the results showed that recombinant CsC8α obviously inhibited growth of V. anguillarum, Edwardsiella tarda and Vibrio parahaemolyticus. Taken together, these results suggest that CsC8α, CsC8β and CsC8γ may play important roles in the immune defense of C. semilaevis.

Evolutionary analysis of two complement C4 genes: Ancient duplication and conservation during jawed vertebrate evolution

The complement C4 is a thioester-containing protein, and a histidine (H) residue catalyzes the cleavage of the thioester to allow covalent binding to carbohydrates on target cells. Some mammalian and teleost species possess an additional isotype where the catalytic H is replaced by an aspartic acid (D), which binds preferentially to proteins. We found the two C4 isotypes in many other jawed vertebrates, including sharks and birds/reptiles. Phylogenetic analysis suggested that C4 gene duplication occurred in the early days of the jawed vertebrate evolution. The D-type C4 of bony fish except for mammals formed a cluster, termed D-lineage. The D-lineage genes were located in a syntenic region outside MHC, and evolved conservatively. Mammals lost the D-lineage before speciation, but D-type C4 was regenerated by recent gene duplication in some mammalian species or groups. Dual C4 molecules with different substrate specificities would have contributed to development of the antibody-dependent classical pathway.

Expression of acute phase proteins by bone marrow stromal cells

The current work examines the expression of acute phase genes in a murine-derived bone marrow stromal cell model (BMS2) which exhibits adipocyte and osteoblast characteristics and supports lymphopoiesis in vitro. Each of these physiologic processes is responsive to inflammatory events such as endotoxemia. Exposure of BMS2 cells to pro-inflammatory cytokines induced the expression of the serum amyloid A and complement factor B. During adipocyte differentiation, expression of complement C3, complement factor D (adipsin), and angiotensinogen increased in a time dependent manner. The bone metabolic steroid, 1,25 dihydroxy vitamin D3, specifically induced complement C3 expression in a time- and dose-dependent manner. Based on gel retention analysis, BMS2 nuclear extract contained proteins recognizing specific response elements from the complement C3, angiotensinogen, and complement factor B promoters. These results suggest that the bone marrow's repertoire of acute phase proteins is dependent on the stromal cell's phenotype or activation state.

Molecular cloning of the alpha subunit of complement component C8 (CpC8α) of whitespotted bamboo shark (Chiloscyllium plagiosum)

Complement-mediated cytolysis is the important effect of immune response, which results from the assembly of terminal complement components (C5b-9). Among them, α subunit of C8 (C8α) is the first protein that traverses the lipid bilayer, and then initiates the recruitment of C9 molecules to form pore on target membranes. In this article, a full-length cDNA of C8α (CpC8α) is identified from the whitespotted bamboo shark (Chiloscyllium plagiosum) by RACE. The CpC8α cDNA is 2183 bp in length, encoding a protein of 591 amino acids. The deduced CpC8α exhibits 89%, 49% and 44% identity with nurse shark, frog and human orthologs, respectively. Sequence alignment indicates that the C8α is well conserved during the evolution process from sharks to mammals, with the same modular architecture as well as the identical cysteine composition in the mature protein. Phylogenetic analysis places CpC8α and nurse shark C8α in cartilaginous fish clade, in parallel with the teleost taxa, to form the C8α cluster with higher vertebrates. Hydrophobicity analysis also indicates a similar hydrophobicity of CpC8α to mammals. Finally, expression analysis revealed CpC8α transcripts were constitutively highly expressed in shark liver, with much less expression in other tissues. The well conserved structure and properties suggests an analogous function of CpC8α to mammalian C8α, though it remains to be confirmed by further study.

Curcumin alleviates immune-complex-mediated glomerulonephritis in factor-H-deficient mice

Complement factor H (Cfh) is a key regulator of the complement cascade and protects C57BL/6 mice from immune complex-mediated complement-dependent glomerulonephritis. In chronic serum sickness (CSS) there are increased deposits of immune complexes in the glomeruli with inflammation and a scarring phenotype. As cucurmin is an effective anti-inflammatory agent and reduces complement activation, we hypothesized that it should alleviate renal disease in this setting. To determine the effectiveness of curcumin, an apoferritin-induced CSS model in Cfh-deficient (Cfh(-/-)) mice was used. Curcumin treatment (30 mg/kg) given every day in parallel with apoferritin reduced glomerulonephritis and enhanced kidney function (blood urea nitrogen, 45·4 ± 7·5 versus 35·6 ± 5·1; albuminuria, 50·1 ± 7·1 versus 15·7 ± 7·1; glomerulonephritis, 2·62 + 0·25 versus 2 + 0·3, P < 0·05). In line with reduced IgG deposits in mice with CSS given curcumin, C9 deposits were reduced indicating reduced complement activation. Mice treated with curcumin had a significant reduction in the number of splenic CD19(+) B cells and the ratio of CD19 : CD3 cells (P < 0·05) with no change in the T-cell population. Myeloperoxidase assay showed reduced macrophages in the kidney. However, a significant reduction in the M2 subset of splenic macrophages by apoferritin was prevented by curcumin, suggesting a protective function. Curcumin treatment reduced mRNA expression of inflammatory proteins monocyte chemoattractant protein-1 and transforming growth factor-β and matrix proteins, fibronectin, laminin and collagen. Our results clearly illustrate that curcumin reduces glomerulosclerosis, improves kidney function and could serve as a therapeutic agent during serum sickness.

Importance of glial activation in neuropathic pain

Glia plays a crucial role in the maintenance of neuronal homeostasis in the central nervous system. The microglial production of immune factors is believed to play an important role in nociceptive transmission. Pain may now be considered a neuro-immune disorder, since it is known that the activation of immune and immune-like glial cells in the dorsal root ganglia and spinal cord results in the release of both pro- and anti-inflammatory cytokines, as well as algesic and analgesic mediators. In this review we presented an important role of cytokines (IL-1alfa, IL-1beta, IL-2, IL-4, IL-6, IL-10, IL-15, IL-18, TNFalpha, IFNgamma, TGF-beta 1, fractalkine and CCL2); complement components (C1q, C3, C5); metaloproteinases (MMP-2,-9) and many other factors, which become activated on spinal cord and DRG level under neuropathic pain. We discussed the role of the immune system in modulating chronic pain. At present, unsatisfactory treatment of neuropathic pain will seek alternative targets for new drugs and it is possible that anti-inflammatory factors like IL-10, IL-4, IL-1alpha, TGF-beta 1 would fulfill this role. Another novel approach for controlling neuropathic pain can be pharmacological attenuation of glial and immune cell activation. It has been found that propentofylline, pentoxifylline, minocycline and fluorocitrate suppress the development of neuropathic pain. The other way of pain control can be the decrease of pro-nociceptive agents like transcription factor synthesis (NF-kappaB, AP-1); kinase synthesis (MEK, p38MAPK, JNK) and protease activation (cathepsin S, MMP9, MMP2). Additionally, since it is known that the opioid-induced glial activation opposes opioid analgesia, some glial inhibitors, which are safe and clinically well tolerated, are proposed as potential useful ko-analgesic agents for opioid treatment of neuropathic pain. This review pointed to some important mechanisms underlying the development of neuropathic pain, which led to identify some possible new approaches to the treatment of neuropathic pain, based on the more comprehensive knowledge of the interaction between the nervous system and glial and immune cells.

Complement in health and disease

The complement system consists of about 35-40 proteins and glycoproteins present in blood plasma or on cell surfaces. Its main biological function is to recognise "foreign" particles and macromolecules, and to promote their elimination either by opsonisation or lysis. Although historically complement has been studied as a system for immune defence against bacteria, it has an important homeostatic role in which it recognises damaged or altered "self" components. Thus complement has major roles in both immune defence against microorganisms, and in clearance of damaged or "used" host components. Since complement proteins opsonise or lyse cells, complement can damage healthy host cells and tissues. The system is regulated by many endogenous regulatory proteins. Regulation is sometimes imperfect and both too much and too little complement activation is associated with many diseases. Excessive or inappropriate activation can cause tissue damage in diseases such as rheumatoid arthritis, age-related macular degeneration (AMD), multiple sclerosis, ischemia-reperfusion injury (e.g. ischemic stroke). Insufficient complement activity is associated with susceptibility to infection (mainly bacterial) and development of autoimmune disease, like SLE (systemic lupus erythematosus).

Cobra venom factor: Structure, function, and humanization for therapeutic complement depletion

Cobra venom factor (CVF) is the complement-activating protein in cobra venom. This manuscript reviews the structure and function of CVF, how it interacts with the complement system, the structural and functional homology to complement component C3, and the use of CVF as an experimental tool to decomplement laboratory animals to study the functions of complement in host defense and immune response as well as in the pathogenesis of diseases. This manuscript also reviews the recent progress in using the homology between CVF and C3 to study C3 structure and function, and to develop human C3 derivatives with the complement-depleting function of CVF. These human C3 derivatives represent humanized CVF, and are a conceptually different concept for pharmacological intervention of the complement system, therapeutic complement depletion. The use of humanized CVF for therapeutic complement depletion in several pre-clinical models of human diseases is also reviewed.

Fluorous-based Peptide Microarrays for Protease Screening

As ever more protease sequences are uncovered through genome sequencing projects, efficient parallel methods to discover the potential substrates of these proteases becomes crucial. Herein we describe the first use of fluorous-based microarrays to probe peptide sequences and begin to define the scope and limitations of fluorous microarray technologies for the screening of proteases. Comparison of a series of serine proteases showed that their ability to cleave peptide substrates in solution was maintained upon immobilization of these substrates onto fluorous-coated glass slides. The fluorous surface did not serve to significantly inactivate the enzymes. However, addition of hydrophilic components to the peptide sequences could induce lower rates of substrate cleavage with enzymes such as chymotrypsin with affinities to hydrophobic moieties. This work represents the first step to creating robust protease screening platforms using noncovalent microarray interface that can easily incorporate a range of compounds on the same slide.

Fc-receptors and immunity to malaria: from models to vaccines

The complexity and number of antigens (Ags) seen during an immune response has hampered the development of malaria vaccines. Antibodies (Abs) play an important role in immunity to malaria and their passive administration is effective at controlling the disease. Abs represent approximately 25% of all proteins undergoing clinical trials, and these 'smart biologicals' have undergone a major revival with the realization that Abs lie at the interface between innate and adaptive immunity. At least 18 Abs have FDA approval for clinical use and approximately 150 are in clinical trials, the majority for the treatment of cancer, allograft rejection or autoimmune disease. Despite these triumphs none are in development for malaria, principally because they are perceived as being too expensive for a disease mainly afflicting poor and marginalized populations. Although unlikely, at least in the foreseeable future, that Ab-based prophylaxis will be made available to the millions of people at risk from malaria, they may be incorporated into current vaccine approaches, since Abs act as correlates of protection in studies aimed at defining the best Ags to include in vaccines. Abs may also form the basis for novel vaccination strategies by targeting Ags to appropriate antigen presenting cells. Therefore, to develop the most efficacious vaccines it will be necessary to fully understand which Abs and Fc-receptors (FcRs) are best engaged for a positive outcome.

Identification and characterization of serum complement activity in the American crocodile (Crocodylus acutus)

Incubation of unsensitized sheep red blood cells with serum from the American crocodile (Crocodylus acutus) resulted in a concentration-dependent hemolysis. The hemolytic activity was heat-sensitive, and inhibited by EDTA in a concentration-dependent manner. The EDTA-inhibited SRBC hemolysis could be restored by the addition of excess Ca2+ or Mg2+, but not Ba2+ or Cu2+, revealing the specificity of this activity for these two divalent cations. The hemolytic activity of crocodile serum was titer-dependent, with 329 microL producing 50% of maximal SRBC hemolysis. The complement activity was also temperature-dependent, with decreased activity at lower temperatures (5-15 degrees C) and maximal activity occurred at 30-40 degrees C. The hemolysis occurred relatively slowly, with near zero activity after 10 min, 40% of activity observed within 15 min of exposure to SRBCs, and maximal activity at 30 min.

Early complement proteases: C1r, C1s and MASPs. A structural insight into activation and functions

C1r, C1s and the mannose-binding lectin-associated serine proteases (MASPs) are responsible for the initiation of the classical- and lectin pathway activation of the complement system. These enzymes do not act alone, but form supramolecular complexes with pattern recognition molecules such as C1q, MBL, and ficolins. They share the same domain organization but have different substrate specificities and fulfill different physiological functions. In the recent years the rapid progress of structural biology facilitated the understanding of the molecular mechanism of complement activation at atomic level. In this review we summarize our current knowledge about the structure and function of the early complement proteases, delineate the latest models of the multimolecular complexes and present the functional consequences inferred from the structural studies. We also discuss some open questions and debated issues that need to be resolved in the future.

The structure of OMCI, a novel lipocalin inhibitor of the complement system

The complement (C) system is a potent innate immune defence system against parasites. We have recently characterised and expressed OmCI, a 16 kDa protein derived from the soft tick Ornithodoros moubata that specifically binds C5, thereby preventing C activation. The structure of recombinant OmCI determined at 1.9 A resolution confirms a lipocalin fold and reveals that the protein binds a fatty acid derivative that we have identified by mass spectrometry as ricinoleic acid. We propose that OmCI could sequester one of the fatty acid-derived inflammatory modulators from the host plasma, thereby interfering with the host inflammatory response to the tick bite. Mapping of sequence differences between OmCI and other tick lipocalins with different functions, combined with biochemical investigations of OmCI activity, supports the hypothesis that OmCI acts by preventing interaction with the C5 convertase, rather than by blocking the C5a cleavage site.

Early changes in sperm motility, acrosome reaction, and gene expression of reproductive organs in rats treated with sulfasalazine

Previously, we reported that decreased epididymal expression of CD59 and decay accelerating factor (DAF) genes may affect sperm motility and the acrosome reaction in rats treated long-term (28 days) with sulfasalazine. To investigate the early effects of sulfasalazine on the male reproductive tract, we presently examined sperm motility, the acrosome reaction, and gene expression in the testes and epididymides of rats treated with sulfasalazine for 1, 7 or 14 days. Reduced sperm motility and acrosome reactions were noted on day 7, however, there were no remarkable changes in testicular gene expression. On the other hand, attenuated epididymal gene expression of CD59 and DAF was observed as early as day 1. As CD59 and DAF are secreted from the epididymis and play a role in sperm maturation, we hypothesize that sulfasalazine affects sperm maturation as an early effect and that CD59 and DAF genes are related to the negative effect.

Characterization of serum complement activity of saltwater (Crocodylus porosus) and freshwater (Crocodylus johnstoni) crocodiles

We employed a spectroscopic assay, based on the hemolysis of sheep red blood cells (SRBCs), to assess the innate immune function of saltwater and freshwater crocodiles in vitro. Incubation of serum from freshwater and saltwater crocodiles with SRBCs resulted in concentration-dependent increases in SRBC hemolysis. The hemolytic activity occurred rapidly, with detectable activity within 2 min and maximum activity at 20 min. These activities, in both crocodilian species, were heat sensitive, unaffected by 20 mM methylamine, and completely inhibited by low concentrations of EDTA, suggesting that the alternative serum complement cascade is responsible for the observed effects. The hemolytic activities of the sera were inhibited by other chelators of divalent metal ions, such as phosphate and citrate. The inhibition of SRBC hemolysis by EDTA could be completely restored by the addition of 10 mM Ca2+ or Mg2+, but not Ba2+, Cu2+ or Fe2+, indicating specificity for these metal ions. The serum complement activities of both crocodilians were temperature-dependent, with peak activities occurring at 25-30 degrees C and reduced activities below 25 degrees C and above 35 degrees C.

Identification of alternative pathway serum complement activity in the blood of the American alligator (Alligator mississippiensis)

Incubation of different dilutions of alligator serum with sheep red blood cells (SRBCs) that had not been sensitized with antibodies resulted in concentration-dependent hemolytic activity. This hemolytic activity was not affected by the presence of ammonium hydroxide and methylamine, known inactivators of the classical complement cascade. However, the hemolytic activities were inhibited by EDTA and salicylaldoxime, indicating that the alternate pathway is primarily responsible for these activities. Immunofixation of electrophoretically-resolved alligator serum proteins with antihuman C3 polyclonal antibodies resulted in detection of a protein antigenically similar to human C3 in alligator serum. SDS-PAGE, followed by Western blot analysis, revealed the presence of two alligator serum proteins with nearly identical molecular weights as human C3alpha and C3beta. SRBC hemolysis and antibacterial activity by alligator serum was significantly reduced in the presence of antihuman C3 antibodies. The hemolytic effect of alligator serum was shown to occur rapidly, with significant activity within 5 min and maximal activity occurring at 15 min. SRBC hemolysis was also temperature-dependent, with reduced activity below 15 degrees C and above 30 degrees C. These data suggest that the antibiotic properties of alligator serum are partially due to the presence of a complement-facilitated humoral immune response analogous to that described in mammalian systems.

Characterization of a C3-like cDNA in a coral: phylogenetic implications

C3, C4, and C5 are thiolester-containing proteins (TEPs) of vertebrate complement. The identification of the molecular origin of the TEP family, and more specifically the ancestor protein of complement components C3, C4, and C5, remains a fundamental question. The prevailing paradigm suggests that duplication and divergence of these proteins occurred after the deuterostome split in phylogeny. It is believed that the ancestor of thiolester-containing complement proteins was alpha-2-macroglobulin (A2M)-like, a noncomplement-related protein. Here we describe a C3-like cDNA from a gorgonian coral, Swiftia exserta. This study provides evidence for the origins of a complement-related C3-like gene in the Precambrian period, predating both protostomes and deuterostomes. Furthermore, one may speculate that complement-like opsonic reactions were evolving at the earliest stages of metazoan evolution. This calls for a reassessment of the present concepts concerning the origins and evolution of TEPs.

High Throughput Substrate Specificity Profiling of Serine and Cysteine Proteases Using Solution-phase Fluorogenic Peptide Microarrays

Proteases regulate numerous biological processes with a degree of specificity often dictated by the amino acid sequence of the substrate cleavage site. To map protease/substrate interactions, a 722-member library of fluorogenic protease substrates of the general format Ac-Ala-X-X-(Arg/Lys)-coumarin was synthesized (X=all natural amino acids except cysteine) and microarrayed with fluorescent calibration standards in glycerol nanodroplets on glass slides. Specificities of 13 serine proteases (activated protein C, plasma kallikrein, factor VIIa, factor IXabeta, factor XIa and factor alpha XIIa, activated complement C1s, C1r, and D, tryptase, trypsin, subtilisin Carlsberg, and cathepsin G) and 11 papain-like cysteine proteases (cathepsin B, H, K, L, S, and V, rhodesain, papain, chymopapain, ficin, and stem bromelain) were obtained from 103,968 separate microarray fluorogenic reactions (722 substrates x 24 different proteases x 6 replicates). This is the first comprehensive study to report the substrate specificity of rhodesain, a papain-like cysteine protease expressed by Trypanasoma brucei rhodesiense, a parasitic protozoa responsible for causing sleeping sickness. Rhodesain displayed a strong P2 preference for Leu, Val, Phe, and Tyr in both the P1=Lys and Arg libraries. Solution-phase microarrays facilitate protease/substrate specificity profiling in a rapid manner with minimal peptide library or enzyme usage.

Complement Factor H Limits Immune Complex Deposition and Prevents Inflammation and Scarring in Glomeruli of Mice with Chronic Serum Sickness

Factor H is the major complement regulator in plasma. Abnormalities in factor H have been implicated in membranoproliferative glomerulonephritis in both humans and experimental animals. It has been shown that factor H on rodent platelets functions analogously to human erythrocyte complement receptor 1 in its role to traffic immune complexes to the mononuclear phagocyte system. C57BL/6 factor H-deficient mice (Cfh(-/-)) and wild-type (wt) controls were immunized daily for 5 wk with heterologous apoferritin to study the chronic serum sickness GN model. Immunizations were started in 6- to 8-wk-old mice, which was before the development of spontaneous membranoproliferative glomerulonephritis in some Cfh(-/-) animals. Glomerular deposition of IgG immune complexes in glomeruli was qualitatively and quantitatively increased in Cfh(-/-) mice compared with wt mice. Consistent with the increase in glomerular immune complexes and possibly because of alternative pathway complement activation, Cfh(-/-) mice had increased glomerular C3 deposition. Wt mice developed no glomerular pathology. In contrast, Cfh(-/-) mice developed diffuse proliferative GN with focal crescents and glomerulosclerosis. In addition, there was significantly increased expression of collagen IV, fibronectin, and laminin mRNA in Cfh(-/-) glomeruli. These data show a role for platelet-associated factor H to process immune complexes and limit their accumulation in glomeruli. Once deposited in glomeruli, excessive complement activation can lead to glomerular inflammation and the rapid development of a scarring phenotype.

The sea urchin complement homologue, SpC3, functions as an opsonin

The purple sea urchin Strongylocentrotus purpuratus expresses a homologue of complement component C3 (SpC3), which acts as a humoral opsonin. Significantly increased phagocytic activity was evident when yeast target cells were opsonized after incubation with coelomic fluid containing SpC3. SpC3 could be detected on the surface of yeast, and phagocytic activity could be inhibited by an anti-SpC3 antibody. This indicates that SpC3 promotes phagocytosis by physically tagging target cells for ingestion. Confocal microscopy showed that opsonized yeast were phagocytosed by a single coelomocyte type (polygonal phagocytes), presumably because these cells express SpC3 receptors. Overall, these data indicate that SpC3 is a major humoral opsonin in S. purpuratus coelomic fluid.

Rat glomerular epithelial cells produce and bear factor H on their surface that is up-regulated under complement attack

BACKGROUND

Factor H is a potent complement inhibitory molecule that is primarily produced by the liver and appears in plasma as a soluble protein. Yet there is evidence that other cells, including those in the kidney, can produce factor H, and that it can be cell-associated as well as present as a plasma protein. Here we studied factor H in rat glomerular epithelial cells (GEC).

METHODS

A polyclonal antibody to factor H was used to identify factor H protein. A polymerase chain reaction (PCR)-based strategy was utilized to clone the full-length cDNA of GEC factor H. The relative quantity of factor H mRNA was measured by quantitative reverse transcription (RT)-PCR in cultured GEC exposed to complement activation and in the passive Heymann nephritis (PHN) model of membranous nephropathy.

RESULTS

By immunofluorescence microscopy, factor H protein was present on the plasma membranes of cultured GEC. Based upon Western blot studies, this appeared to be the full-length 150 kD factor H protein. Factor H cDNA cloned from GEC was identical to the newly deposited sequence for rat liver factor H cDNA. In cultured GEC in which complement was activated, factor H mRNA increased over time. Similarly, in the PHN model in which complement was activated on GEC in vivo, factor H mRNA and protein also increased over time.

CONCLUSION

Cultured GEC and glomeruli express factor H mRNA and protein. As modeled both in vitro and in vivo in the rat, factor H is up-regulated in membranous nephropathy. This is likely to be a direct response of GEC to complement attack and may represent a protective response of this cell.

Isolation and characterization of a novel rat factor H-related protein that is up-regulated in glomeruli under complement attack

The factor H family in humans is composed of seven distinct proteins, including factor H-related proteins (FHR) 1-5. All members contain tandemly arranged short consensus repeats (SCR) typical of the regulators of complement activation gene family. FHR-5 is unusual for this group of proteins, as it was initially identified as a component of immune deposits in glomerular diseases. During our cloning of the cDNA for rat factor H from glomerular epithelial cells (GEC), we identified an alternative 2729-bp cDNA transcript. The translated sequence encoded a protein containing 11 SCRs, most similar to SCRs 7-15 and 19-20 in native rat factor H, which is the same basic structure of human FHR-5. As such, this rat protein was termed FHR. Recombinant rat FHR produced in a eukaryotic expression system had a molecular mass of 78 kDa. In functional studies, recombinant FHR bound C3b and inhibited the complement alternative pathway in a dose-dependent fashion. Given the prominent expression of FHR-5 in human membranous nephropathy, a disease in which complement activation occurs in the vicinity of GEC, the expression of FHR in a rat model of this disease was evaluated. In both in vitro and in vivo models of complement activation on the GEC, FHR mRNA was up-regulated by a factor of 3-6-fold compared with controls in which complement could not be activated. Thus, we have identified a novel factor H family member in rats. This FHR protein is analogous to human FHR-5, both in structure and in potential involvement in glomerular immune complex diseases.

Decay-accelerating factor expression in the rat kidney is restricted to the apical surface of podocytes

BACKGROUND

Decay-accelerating factor (DAF) has inhibitory activity toward complement C3 and C5 convertases. DAF is present in human glomeruli and on cultured human glomerular visceral epithelial cells (GEC). We studied the distribution and function of rat DAF.

METHODS

Function-neutralizing antibodies (Abs) were raised against DAF. The distribution of DAF in vivo was determined by immunoelectron microscopy. Functional studies were performed in cultured GEC and following IV injection of anti-DAF Abs into rats.

RESULTS

DAF was present exclusively on the apical surfaces of GEC, and was not present on the basal surfaces of GEC, nor other glomerular or kidney cells. DAF was functionally active on cultured GEC, and served to limit complement activation in concert with CD59, an inhibitor of C5b-9 formation. Upon injection into normal rats, anti-DAF F(ab')2 Abs bound to GEC in vivo, yet there was no evidence for complement activation and animals did not develop abnormal albuminuria. Anti-megalin complement-activating IgG Abs were "planted" on GEC, which activated complement as evidenced by the presence of C3d on GEC. Attempts to inhibit DAF function with anti-DAF Abs did not affect the quantity of complement activation by these anti-megalin Abs, nor did it lead to development of abnormal albuminuria. In contrast, in the puromycin aminonucleoside model of GEC injury and proteinuria, anti-DAF Abs slowed the recovery from renal failure that occurs in this model.

CONCLUSION

In cultured rat GEC, DAF is an effective complement regulator. In vivo, DAF is present on GEC apical surfaces. Yet, it appears that DAF is not essential to prevent complement activation from occurring under normal circumstances and in those cases in which complement-activating Abs are present on the basal surfaces of GEC in vivo. However, in proteinuric conditions, DAF appears to be protective to GEC.

Bacterial expression and membrane targeting of the rat complement regulator Crry: a new model anticomplement therapeutic

Inappropriate or unregulated activation of complement can contribute to pathology in inflammatory diseases. Previous studies have shown that soluble recombinant regulators of complement are effective in animal models and some human diseases. However, limitations include cost, rapid clearance, and unwanted systemic effects. To avoid some of these problems, bacterial expression of regulators has been optimized and methods for the addition of a membrane-targeting moiety to the complement regulator developed. When administered directly to sites of inflammation, membrane-targeted human regulators are retained and inhibit complement-activation locally. To test the efficacy of membrane-targeted complement regulators in vivo, we have undertaken the expression and membrane targeting of the rat-complement regulator Crry. A soluble recombinant form of Crry, containing only the first four short consensus repeats, was expressed in a mammalian expression system and shown to be functional as a fluid phase regulator. To generate the quantities required for testing in vivo, Crry was expressed in bacteria and refolded successfully. Refolded protein had full-complement regulatory activity in vitro. Attachment of a membrane address tag conferred membrane-binding capacity and greatly increased complement regulatory function in vitro. This novel anticomplement agent can now be applied to rat models of arthritis and other inflammatory diseases.

Use of complement inhibitors in tissue injury

A great deal of information has accumulated implicating the complement system in several human disease processes. Although some of this information is circumstantial, protein inhibitors of the complement system have been developed and applied successfully to experimental disease models in animals. Two inhibitors, soluble complement receptor 1 (sCR1) and anti-C5 monoclonal antibody, are now being investigated in a variety of clinical conditions such as systemic lupus erythematosus and rheumatoid arthritis (RA), diseases for which current therapy has changed little and remains unsatisfactory. Preliminary successes in Phase II clinical trials of RA have provided optimism that complement inhibition might prove useful in these diseases and become part of standard medical therapy.

Transgenic expression of a soluble complement inhibitor protects against renal disease and promotes survival in MRL/lpr mice

To investigate the role of complement in lupus nephritis, we used MRL/lpr mice and a transgene overexpressing a soluble complement regulator, soluble CR1-related gene/protein y (sCrry), both systemically and in kidney. Production of sCrry in sera led to significant complement inhibition in Crry-transgenic mice relative to littermate transgene negative controls. This complement inhibition with sCrry conferred a survival advantage to MRL/lpr mice. In a total of 154 animals, 42.5% transgene-negative animals had impaired renal function (blood urea nitrogen > 50 mg/dl) compared with 16.4% mice with the sCrry-producing transgene (p < 0.001). In those animals that died spontaneously, MRL/lpr mice with the sCrry-producing transgene did not die of renal failure, while those without the transgene did (blood urea nitrogen values of 46.6 +/- 9 and 122 +/- 29 mg/dl in transgene-positive and transgene-negative animals, respectively; p < 0.001). Albuminuria was reduced in those transgenic animals in which sCrry expression was maximally stimulated (urinary albumin/creatinine = 12.4 +/- 4.3 and 36.9 +/- 7.7 in transgene-positive and transgene-negative animals, respectively; p < 0.001). As expected in the setting of chronic complement inhibition, there was less C3 deposition in glomeruli of sCrry-producing transgenic mice compared with transgene-negative animals. In contrast, there was no effect on glomerular IgG deposition, levels of anti-dsDNA Ab and rheumatoid factor, or spleen weights between the two groups. Thus, long-term complement inhibition reduces renal disease in MRL/lpr mice, which translates into improved survival. MRL/lpr mice in which complement is inhibited still have spontaneous mortality, yet this is not from renal disease.

A protein with characteristics of factor H is present on rodent platelets and functions as the immune adherence receptor

Complement-coated particles interact with specific immune adherence receptors (IAR). In primates, this function is served by complement receptor 1 (CR1) on erythrocytes. In contrast, rodent platelets bear IAR distinct from CR1, the identity of which was studied here. A 150-kDa C3b-binding protein was isolated from rat platelets, which had immunochemical and biochemical identity to plasma factor H. Immunofluorescence microscopy and flow cytometry demonstrated that factor H was present on the surface of rat and mouse platelets, which could be removed by treatment with neuraminidase. Sheep erythrocytes bearing C3b underwent immune adherence with rat and mouse platelets, which was blocked with anti-factor H F(ab')(2) antibodies, but not with antibodies binding to the complement regulator, Crry, on the platelet surface. By reverse transcription-polymerase chain reaction using rat platelet RNA and primers designed from mouse factor H, a 472-base pair product was generated that was identical in sequence to that produced from rat liver RNA. The translated protein product was 85% similar to mouse liver factor H. The 3'-nucleotide sequence from platelets predicted a soluble factor H protein. By Northern analysis, liver and platelets had identically sized factor H mRNA. Thus, rat and mouse platelets have a membrane protein with characteristics of factor H that is linked via sialic acid residues and functions as the IAR. Whether platelet factor H is acquired by passive adsorption from sera and/or is produced by platelets remains to be determined.

Multiple ligand binding sites on domain seven of human complement factor H

Foreign particles and damaged host cells can activate the complement system leading to their destruction by the host defense system. Factor H (fH) plays a vital role in restricting complement activation on host cells through interactions with polyanions such as heparin, while allowing activation to proceed on foreign surfaces. Complement activation by damaged host cells is also down regulated by fH, which is localized to injured areas through interactions with C-reactive protein (CRP). A number of pathogens have developed mechanisms by which they can also bind fH and thus exploit its protective properties. One such organism is Group A Streptococcus (GAS) which mediates fH binding via its surface expressed M-protein. fH consists of 20 conserved short consensus repeat (SCR) units and mutagenesis studies indicate that the seventh repeat is responsible for interactions with heparin, CRP and M-protein. We recently performed molecular modelling of fH SCR 7 and identified a cluster of positively charged residues on one face of the domain. By alanine replacement mutagenesis, we demonstrated that these residues are involved in heparin, CRP and M protein binding, which indicates that there is a common site within fH SCR 7 responsible for multiple ligand recognition.

Cooperation between decay-accelerating factor and membrane cofactor protein in protecting cells from autologous complement attack

Decay-accelerating factor (DAF or CD55) and membrane cofactor protein (MCP or CD46) function intrinsically in the membranes of self cells to prevent activation of autologous complement on their surfaces. How these two regulatory proteins cooperate on self-cell surfaces to inhibit autologous complement attack is unknown. In this study, a GPI-anchored form of MCP was generated. The ability of this recombinant protein and that of naturally GPI-anchored DAF to incorporate into cell membranes then was exploited to examine the combined functions of DAF and MCP in regulating complement intermediates assembled from purified alternative pathway components on rabbit erythrocytes. Quantitative studies with complement-coated rabbit erythrocyte intermediates constituted with each protein individually or the two proteins together demonstrated that DAF and MCP synergize the actions of each other in preventing C3b deposition on the cell surface. Further analyses showed that MCP's ability to catalyze the factor I-mediated cleavage of cell-bound C3b is inhibited in the presence of factors B and D and is restored when DAF is incorporated into the cells. Thus, the activities of DAF and MCP, when present together, are greater than the sum of the two proteins individually, and DAF is required for MCP to catalyze the cleavage of cell-bound C3b in the presence of excess factors B and D. These data are relevant to xenotransplantation, pharmacological inhibition of complement in inflammatory diseases, and evasion of tumor cells from humoral immune responses.

Two clusters of acidic amino acids near the NH2 terminus of complement component C4 alpha'-chain are important for C2 binding

Previous work has indicated a role for the NH2-terminal segment of the C3 alpha'-chain in the binding interactions of C3b with a number of its protein ligands. In particular, we have identified two clusters of acidic residues, namely, E736 and E737 and to a lesser extent D730 and E731, as being important in the binding of C3b to factor B and complement receptor 1 and the binding of iC3b to complement receptor 3. Whereas human C3 and C4 have an overall sequence identity of 29%, over a segment near the NH2 termini of their respective alpha'-chains the sequence identity is 56% (70% chemical similarity). Given the functional similarity between the C4b-C2 and C3b-B interactions in the respective formation of the classical and alternative pathway C3 convertases, as well as the sequence conservation of two acidic clusters, we hypothesized that residues 744EED and 749DEDD within the NH2-terminal segment of the C4 alpha'-chain would mediate in part the binding of C2 to C4b. We tested this hypothesis using three independent approaches. Site-directed mutagenesis experiments revealed that replacing subsets of the charged residues by their isosteric amides within either acidic cluster resulted in molecules having reduced C2 binding activity. Moreover, a synthetic peptide (C4 residues 740-756) encompassing the two acidic clusters was a specific inhibitor of the binding of C2 to red cell-associated C4b. Finally, Ab raised against the above peptide was able to block the interaction between red cell-associated C4b and fluid phase C2. Taken together, these results strongly suggest that the NH2-terminal acidic residue-rich segment of C4 alpha'-chain contributes importantly to the interaction of C4b with C2.

Production and functional analysis of rat CD59 and chimeric CD59-Crry as active soluble proteins in Pichia pastoris

Crry (CR1-related gene/protein) is a rodent complement regulator that inhibits C3 convertases. CD59 is a conserved protein inhibitor active towards C8 and C9. We have previously produced rat Crry as a recombinant soluble (rs) protein in Pichia pastoris. In this study we produced functionally active rat rsCD59 and a chimeric rsCD59-Crry protein in P. pastoris. The GPI anchor addition site of rat CD59 (Asn-79) was replaced either by a stop codon to produce rsCD59, or with the sequence of the first five short consensus repeats of Crry to produce rsCD59-Crry. Proteins were generated by fermentation and purified by affinity chromatography on an anti-CD59 column. In a standard classical pathway haemolysis assay, all three rs proteins had inhibitory activity, with 50% inhibition at 0.5 microM (rsCrry and rsCD59-Crry) and 4.4 microM (rsCD59). In an assay examining inhibition of C5b-9, in which C5b-7 was first formed, followed by purified C8 and C9, rsCD59 and rsCD59-Crry were active with 50% inhibition at 0.8 microM (rsCD59-Crry) and 1.3 microM (rsCD59). The degree of inhibition was independent of whether the C8 and C9 were of rat or human origin. Therefore, we have produced rsCD59 and rsCD59-Crry in P. pastoris. The rsCD59 retains its inhibitory activity towards C5b-9, while rsCD59-Crry appears to have the combined activities of Crry and CD59. In a haemolytic assay, the inclusion of CD59 to Crry is of no additional benefit to Crry, which may illustrate the overall importance of the C3 convertase step. Yet, inclusion of Crry to CD59 increases the potency of CD59 towards C5b-9.

Analysis of human C4A and C4B binding to an immune complex in serum

Previous studies using isolated complement proteins have shown that more C4A than C4B binds to certain types of immune complexes. However, the in vivo binding of the C4 isoforms to an immune complex has not been investigated in detail and may differ from events when measured with the isolated proteins. We report here the binding of C4A and C4B to an immune complex of bovine serum albumin (BSA) anti-BSA as it occurs in serum. We found that when using the isolated C4 proteins more C4A than C4B bound to the complex, but in serum similar amounts of C4A and C4B were found to bind. Furthermore, these results were not explainable by a difference in activity between isoforms. In an attempt to explain these results a number of unexpected observations were noted. First C4A, but not C4B, bound specifically to a yet unidentified 38-kD serum protein. Second, when both covalent and non-covalent binding was assessed, we found that as serum concentration increased there followed a concomitant decrease in covalent binding and C4B was more affected than C4A. The potential biological significance of these findings is discussed.

Echinoderm immunity and the evolution of the complement system

Our understanding of inflammatory responses in humans has its roots in the comparative approach to immunology. In the late 1900s, research on echinoderms provided the initial evidence for the importance of phagocytic cells in reactions to foreign material. Studies of allograft rejection kinetics have shown that echinoderms have a non-adaptive, activation type of immune response. Coelomocytes mediate the cellular responses to immune challenges through phagocytosis, encapsulation, cytotoxicity, and the production of antimicrobial agents. In addition, a variety of humoral factors found in the coelomic fluid, including lectins, agglutinins, and lysins, are important in host defense against pathogens and other foreign substances. Recently, a simple complement system has been identified in the purple sea urchin that is homologous to the alternative pathway in vertebrates. The sea urchin [corrected] homologue of C3, is inducible by challenge with lipopolysaccharide, which is known to activate coelomocytes. Complement components have been identified in all vertebrate classes, and now have been characterized in protochordates and echinoderms indicating the primordial nature of the complement system. Because it is thought that the complement system evolved from a few primordial genes by gene duplication and divergence, the origin of this system appears to have occurred within the common ancestor of the deuterostomes.

Coelomocytes Express SpBf, a Homologue of Factor B, the Second Component in the Sea Urchin Complement System

A homologue of factor B, SpBf, has been cloned and sequenced from an LPS-activated coelomocyte cDNA library from the purple sea urchin, Strongylocentrotus purpuratus. The deduced amino acid sequence and domain structure show significant similarity to the vertebrate Bf/C2 family proteins. SpBf is a mosaic protein, composed of five short consensus repeats, a von Willebrand Factor domain, and a serine protease domain. It has a deduced molecular mass of 91 kDa, with a conserved cleavage site for a putative factor D protease. It has ten consensus recognition sites for N-linked glycosylation. Amino acids involved in both Mg2+ binding and in serine protease activity in the vertebrate C2/Bf proteins are conserved in SpBf. Phylogenetic analysis of SpBf indicates that it is the most ancient member of the vertebrate Bf/C2 family. Additional phylogenetic analysis of the SCRs indicates that five SCRs in SpBf may be ancestral to three SCRs, which is the typical pattern in the vertebrate Bf/C2 proteins. RNA gel blots show that SpBf transcripts are 5.5 kb and are specifically expressed in coelomocytes. Genome blots suggest that the SpBf gene (Sp152) is single copy gene per haploid genome. This is the second complement component to be identified from the sea urchin, and, with the sea urchin C3 homologue, these two components may be part of a simple complement system that is homologous to the alternative pathway in higher vertebrates.

Expression of complement C4 and C9 genes by human astrocytes

Evidence exists that complement activation is involved in the pathogenesis of Alzheimer's disease (AD). It has been previously demonstrated that central nervous system (CNS) resident cells can synthesize complement proteins. Two key proteins in the complement pathway are the complement C4 and C9 proteins. Using reverse transcription-polymerase chain reaction, ELISA, immunocytochemical and immunoblot techniques, we showed that primary human astrocytes constitutively expressed complement C4 mRNA and protein, and that this was increased when cells were treated with interferon-gamma, but inhibited when cells were treated with interleukin-1beta (IL-1beta). C4 immunoreactivity could be localized to GFAP-positive astrocytes when protein secretion was inhibited. These results indicated that astrocytes could be a source of complement C4 in the human CNS. In addition it was shown that stimulated astrocytes could also express complement C9 mRNA, though C9 protein was not detectable in culture supernatants.

The C-terminus of factor H: monoclonal antibodies inhibit heparin binding and identify epitopes common to factor H and factor H-related proteins

We have generated monoclonal antibodies (mAbs) specific for the C-terminus of factor H that can be used as inhibitory antibodies for heparin binding and for the specific detection of factor H and factor H-related proteins (FHRs) in plasma and triacylglycerol-rich lipoproteins. Four distinct mAbs were established: IXF9 (IgG1), VD3 (IgG2a), VIG8 (IgG1) and IIC5 (IgG1). Each reacts specifically with FHR-1 and factor H (and also with FHR-2 in the case of VIG8), but none binds to the related FHR-3 and FHR-4 proteins nor to factor H-like protein 1. By the use of deletion mutants of factor H and by comparing the reactivity with FHR-1 and FHR-2, the binding epitopes of the mAbs were identified and localized to different short consensus repeats (SCRs): mAbs IXF9 and VD3 bind to related or even identical sites within SCR18 (factor H) and SCR3 (FHR-1) respectively. mAbs VIG8 and IIC5 bind to different epitopes located within SCRs 19 to 20 of factor H and SCRs 4 to 5 of FHR-1 respectively. Only mAb VIG8 reacts with the corresponding SCRs 3 to 4 of FHR-2. These antibodies are useful for the detection of the corresponding proteins in biological specimens such as fractions of lipoproteins. In addition, mAb VIG8 has the unique feature of inhibiting binding of factor H to heparin. Given the recent identification of a heparin- and a C3b-binding domain within the C-terminus of factor H, these mAbs should provide useful tools for functional analysis and for the precise localization of the domain(s) required for this interaction.

Structure, functions, and evolution of the third complement component and viral molecular mimicry

The third component of the complement system, C3, is a common denominator in the activation of the classical, alternative, and lectin pathways. The ability of C3 molecule to interact with at least 20 different proteins makes it the most versatile component of this system. Since these interactions are important for phagocytic, immunoregulatory, and immune evasion mechanisms, the analysis of its structure and functions has been a subject of intense research. Here we review our current work on the C3-ligand interactions, C3-related viral molecular mimicry, evolution of the complement system, and identification of C3-based complement inhibitors.

Coating of human decay accelerating factor (hDAF) onto medical devices to improve biocompatibility

In passing blood through an artificial circulatory system, the blood is exposed to surfaces that result in activation of the complement system. The consequences of the activation of complement can be extremely serious for the patient ranging from mild discomfort to respiratory distress and even anaphylaxis. An entirely novel approach was to express recombinant GPI anchored human decay accelerating factor (hDAF) using the baculovirus system and then coat the recombinant protein onto the surfaces of these materials to reduce complement activation. Expression of hDAF in Sf9 cells was shown by ELISA, FACS analysis, and Western blot. Functional activity was tested by CH50 assay. For the coating experiments a small scale model of a cardiovascular bypass circuit constructed from COBE tubing was used. hDAF was either coated onto the circuit using adsorption or covalently linked via the photoreactive crosslinker, p-azidobenzoyl hydrazide. After coating, heparinised human blood was pumped around the circuit and samples were collected into EDTA collection tubes at different time points. Complement activation was measured using a Quidel C3a-des-arg EIA. The photolinked circuits gave a reduction in C3a production of 20-50%, compared to 10-20% seen with an absorbed hDAF circuit. Furthermore, the inhibition of complement was seen over the whole time scale of the photolinked circuit, 60-90 min, whilst in the adsorbed circuit inhibition was not seen to a significant degree after 60 min. The time scale of a standard cardiac bypass is 45-90 min, therefore, the photolinked circuit results are encouraging, as significant inhibition of complement activation is seen within this time frame.

Molecular evolution of the vertebrate immune system

Adaptive immunity is unique to the vertebrates, and the molecules involved (including immunoglobulins, T cell receptors and the major histocompatibility complex molecules) seem to have diversified very rapidly early in vertebrate history. Reconstruction of gene phylogenies has yielded insights into the evolutionary origin of a number of molecular systems, including the complement system and the major histocompatibility complex (MHC). These analyses have indicated that the C5 component of complement arose by gene duplication prior to the divergence of C3 and C4, which suggests that the alternative complement pathway was the first to evolve. In the case of the MHC, phylogenetic analysis supports the hypothesis that MHC class II molecules evolved before class I molecules. The fact that the MHC-linked proteasome components that specifically produce peptides for presentation by class I MHC appear to have originated before the separation of jawed and jawless vertebrates suggests that the MHC itself may have been present at this time. Immune system gene families have evolved by gene duplication, interlocus recombination and (in some cases) positive Darwinian selection favoring diversity at the amino acid level.

Origin of genes encoding multi-enzymatic proteins in eukaryotes

In several biosynthetic pathways of eukaryotes, multiple steps are catalyzed by enzymes physically linked as domains of multi-enzymatic proteins. The same steps in prokaryotes are frequently carried out by mono-enzymatic proteins. If genes encoding mono-enzymatic proteins are the precursors to those genes encoding multi-enzymatic proteins, how these genes fused remains an open question. However, the recent discovery of a cleavage-polyadenylation signal within an intron of the GART gene provides clues to this process and might also have more general implications for the origin of genes that contain alternative RNA processing reactions at their 5' or 3' ends.

Production of the rat complement regulator, Crry, as an active soluble protein in Pichia pastoris

In this report, we describe the use of the methylotrophic yeast Pichia pastoris for the production of the rat complement regulator, Crry. Crry normally exists as an intrinsic membrane protein containing six to seven short consensus repeats (SCRs), a transmembrane region, and a cytoplasmic tail. To produce Crry as a soluble recombinant protein, nucleotides encoding the five N-terminal SCRs from the rat Crry cDNA were amplified by PCR, and cloned into the P. pastoris expression vector, pPIC9. This vector contains the yeast alpha-factor signal sequence, thereby leading to secretion of recombinant protein. This construct was subsequently integrated into P. pastoris strain GS115 genomic DNA. Secreted soluble Crry was produced by induction of the AOX1 promoter with methanol. Recombinant Crry protein was purified to homogeneity by sequential Mono Q and Mono P chromatography. The protein was highly active toward the alternative and classical pathways of complement, inhibiting the latter by approximately 90% at a concentration of 15 nM. The P. pastoris system offers an efficient method for the production of soluble recombinant Crry. Production of active rat Crry offers opportunities to study long-term models of disease in rats, which has not been possible with available heterologous complement inhibitors.

Characterization of C3 receptors on cultured rat glomerular endothelial cells

In this study we characterized C3 receptors on cultured rat glomerular endothelial cells (GEnC), using immunochemical and molecular techniques. GEnC membrane proteins were immunoprecipitated with a polyclonal antibody directed towards mouse complement receptor 2 (CR2). This anti-MCR2 immunoprecipitated GEnC proteins of 120 and 150 kDa. By immunohistochemistry, anti-MCR2 stained GEnC in rat glomeruli in vivo. Given the presence of CR2-like proteins on GEnC, subsequent studies were done to determine whether GEnC had C3-binding proteins. GEnC proteins of 80, 200, and 300 kDa specifically bound to columns of rat C3d-Sepharose and C3b-Sepharose, illustrating that these proteins were binding to the C3d portion of C3. The 80, 200, and 300 kDa C3d-binding proteins were distinct from the 120 and 150 kDa anti-MCR2 reactive proteins, as shown by immunoabsorption studies. Next, a specific cDNA probe for rat CR2 was generated by RT-PCR. Oligonucleotides were chosen from highly conserved regions in mouse and human CR2 spanning 224 bases, with the rationale that these would also be conserved in the rat. A 224 bp PCR product was generated from both rat GEnC and rat kidney cDNA, illustrating the presence of CR2 mRNA in these tissues. By Northern analysis, the CR2 PCR product hybridized to mRNA of 2 and 5 kb from GEnC. The 5 kb transcript was also identified in rat kidney mRNA. Therefore, proteins immunologically related to mouse CR2 are present in GEnC in vitro and in vivo. C3d-binding proteins of 80, 200, and 300 kDa are also present on rat GEnC, yet these appear to be immunologically distinct from the proteins identified by anti-MCR2. Whether the GEnC CR2 mRNA transcripts of 2 and 5 kb are translated into the 80 and 200 kDa C3d-binding proteins or the 120 and 150 kDa mouse CR2-like proteins remains to be defined.

A complement factor B-like cDNA clone from the zebrafish (Brachydanio rerio)

An important molecule in the activation of the complement system in vertebrates is factor B, a serine protease with a molecular mass of 95,000. Factor B and the complement component C2 are thought to have arisen by gene duplication. In mammals and in Xenopus the factor B gene is linked to the major histocompatibility complex (MHC), whereas in domestic fowl it segregates independently of the MHC. Here we describe the isolation of a cDNA clone coding for factor B in the zebrafish, Brachydanio rerio. The deduced protein sequence exhibits a characteristic mosaic structure consisting of the short consensus repeat (SCR), the von Willebrand factor, and the serine protease domains. The estimated time of factor B and C2 divergence (approximately 350 million years ago), combined with the fact that C2 has thus far been found only in mammals, suggest that the factor B-C2 gene duplication occurred after the divergence of mammal-like reptiles from other reptiles and hence also birds. After the duplication, the C2 component evolved significantly faster than factor B.