Functional and structural diversities of C-reactive proteins present in horseshoe crab hemolymph plasma

An evolutionarily conserved function of C-reactive protein is to prevent the formation of amyloid fibrils

C-reactive protein (CRP) binds to phosphocholine (PCh)-containing substances and subsequently activates the complement system to eliminate the ligand. The PCh-binding function of CRP has been conserved throughout evolution from arthropods to humans. Human CRP, in its structurally altered conformation at acidic pH, also binds to amyloid-β (Aβ) and prevents the formation of Aβ fibrils. It is unknown whether the Aβ-binding function of CRP has also been evolutionarily conserved. The aim of this study was to determine whether CRP isolated from American horseshoe crab Limulus polyphemus was also anti-amyloidogenic and whether this function required structural alteration of Limulus CRP (Li-CRP). Two CRP species Li-CRP-I and Li-CRP-II were purified from hemolymph by employing PCh-affinity chromatography and phosphoethanolamine-affinity chromatography, respectively. Both Li-CRP-I and Li-CRP-II bound to immobilized Aβ at physiological pH. Unlike human CRP, Li-CRP did not require any changes in its overall structure to bind to Aβ. Both Li-CRP-I and Li-CRP-II bound to Aβ in the fluid phase also and prevented the fibrillation of Aβ. Additionally, ion-exchange chromatography of purified Li-CRP indicated that a variety of Li-CRP molecules of different subunit compositions were present in Limulus hemolymph, raising the possibility that the presence of various Li-CRP species in hemolymph facilitates the recognition of a range of proteins with differing amyloidogenicity. We conclude that the binding of CRP to Aβ is an ancient function of CRP. In invertebrates, the Aβ-binding function of CRP can protect the host from toxicity caused by amyloidogenic and pathogenic proteins. In humans, the Aβ-binding function of CRP can protect against inflammatory diseases in which the host proteins are ectopically deposited on either host cells or foreign cells in an inflammatory milieu since immobilized proteins may expose Aβ-like structures after deposition at places where they are not supposed to be.

Pentraxins in invertebrates and vertebrates: From structure, function and evolution to clinical applications

The immune system is divided into two broad categories, consisting of innate and adaptive immunity. As recognition and effector factors of innate immunity and regulators of adaptive immune responses, lectins are considered to be important defense chemicals against microbial pathogens, cell trafficking, immune regulation, and prevention of autoimmunity. Pentraxins, important members of animal lectins, play a significant role in protecting the body from pathogen infection and regulating inflammatory reactions. They can recognize and bind to a variety of ligands, including carbohydrates, lipids, proteins, nucleic acids and their complexes, and protect the host from pathogen invasion by activating the complement cascade and Fcγ receptor pathways. Based on the primary structure of the subunit, pentraxins are divided into short and long pentraxins. The short pentraxins are comprised of C-reactive protein (CRP) and serum amyloid P (SAP), and the most important member of the long pentraxins is pentraxin 3 (PTX3). The CRP and SAP exist in both vertebrates and invertebrates, while the PTX3 may be present only in vertebrates. The major ligands and functions of CRP, SAP and PTX3 and three activation pathways involved in the complement system are summarized in this review. Their different characteristics in various animals including humans, and their evolutionary trees are analyzed. The clinical applications of CRP, SAP and PTX3 in human are reviewed. Some questions that remain to be understood are also highlighted.

Biomolecules of the Horseshoe Crab’s Hemolymph: Components of an Ancient Defensive Mechanism and Its Impact on the Pharmaceutical and Biomedical Industry

Without adaptive immunity, invertebrates have evolved innate immune systems that react to antigens on the surfaces of pathogens. These defense mechanisms are included in horseshoe crab hemocytes’ cellular responses to pathogens. Secretory granules, large (L) and small (S), are found on hemocytes. Once the invasion of pathogens is present, these granules release their contents through exocytosis. Recent data in biochemistry and immunology on the granular constituents of granule-specific proteins are stored in large and small granules which are involved in the cell-mediated immune response. L-granules contain most clotting proteins, which are necessary for hemolymph coagulation. They also include tachylectins; protease inhibitors, such as cystatin and serpins; and anti-lipopolysaccharide (LPS) factors, which bind to LPS and agglutinate bacteria. Big defensin, tachycitin, tachystatin, and tachyplesins are some of the essential cysteine-rich proteins in S-granules. These granules also contain tachycitin and tachystatins, which can agglutinate bacteria. These proteins in granules and hemolymph act synergistically to fight infections. These biomolecules are antimicrobial and antibacterial, enabling them to be drug resistant. This review is aimed at explaining the biomolecules identified in the horseshoe crab’s hemolymph and their application scopes in the pharmaceutical and biotechnology sectors.

New insights into the hemolymph coagulation cascade of horseshoe crabs initiated by autocatalytic activation of a lipopolysaccharide-sensitive zymogen

The concept of a chain reaction of proteolytic activation of multiple protease zymogens was first proposed to explain the blood clotting system in mammals as an enzyme cascade. In multicellular organisms, similar enzyme cascades are widely present in signal transduction and amplification systems. The initiation step of the blood coagulation cascade often consists of autocatalytic activation of the corresponding zymogens located on the surfaces of host- or foreign-derived substances at injured sites. However, the molecular mechanism underlying the concept of autocatalytic activation remains speculative. In this review, we will focus on the autocatalytic activation of prochelicerase C on the surface of lipopolysaccharide as a potential initiator of hemolymph coagulation in horseshoe crabs. Prochelicerase C is presumed to have evolved from a common complement factor in Chelicerata; thus, evolutionary insights into the hemolymph coagulation and complement systems in horseshoe crabs will also be discussed.

In vivo immunostimulatory effect of the amoebocyte lysate and plasma of Asian horseshoe crab, Tachypleus gigas in a piscine model

Antimicrobial proteins/peptides are becoming a new generation of immunostimulants for prevention and disease control in human and animals, including aquatic animals. As the haemolymph of horseshoe crabs (Tachypleus) contains broad ranges of bioactive compounds, we have explored the in vivo immunostimulating potential of amoebocyte lysate and plasma using a fish model. Indian major carp, Labeo rohita, yearlings were injected intraperitoneally with two doses of lysate and plasma at 50 and 100 µg protein per fish. No abnormalities and/or mortalities were recorded in any group. L. rohita injected with 50 µg lysate and 100 µg plasma protein showed significant enhancement (P < 0.01) of various haematological and immunological parameters. There was a significant rise in the total protein and globulin content, myeloperoxidase and respiratory burst activity following injection with 50 µg lysate and 100 µg plasma protein. The agglutinating and haemagglutinating activities were increased albeit not significantly (P > 0.01) in any groups. On the contrary, a significantly high hemolysin titre was recorded in fish that received 100 µg plasma protein. Following challenge with Aeromonas hydrophila, both lysate and plasma protein(s) cross protected the fish after 30 days. The highest survival (50%) was recorded in group injected with 50 µg lysate protein, followed by 45% in both 100 µg lysate and plasma protein injected groups.

Post-translational protein deimination signatures and extracellular vesicles (EVs) in the Atlantic horseshoe crab (Limulus polyphemus)

The horseshoe crab is a living fossil and a species of marine arthropod with unusual immune system properties which are also exploited commercially. Given its ancient status dating to the Ordovician period (450 million years ago), its standing in phylogeny and unusual immunological characteristics, the horseshoe crab may hold valuable information for comparative immunology studies. Peptidylarginine deiminases (PADs) are calcium dependent enzymes that are phylogenetically conserved and cause protein deimination via conversion of arginine to citrulline. This post-translational modification can lead to structural and functional protein changes contributing to protein moonlighting in health and disease. PAD-mediated regulation of extracellular vesicle (EV) release, a critical component of cellular communication, has furthermore been identified to be a phylogenetically conserved mechanism. PADs, protein deimination and EVs have hitherto not been studied in the horseshoe crab and were assessed in the current study. Horseshoe crab haemolymph serum-EVs were found to be a poly-dispersed population in the 20-400 nm size range, with the majority of EVs falling within 40-123 nm. Key immune proteins were identified to be post-translationally deiminated in horseshoe crab haemolymph serum, providing insights into protein moonlighting function of Limulus and phylogenetically conserved immune proteins. KEGG (Kyoto encyclopaedia of genes and genomes) and GO (gene ontology) enrichment analysis of deiminated proteins identified in Limulus revealed KEGG pathways relating to complement and coagulation pathways, Staphylococcus aureus infection, glycolysis/gluconeogenesis and carbon metabolism, while GO pathways of biological and molecular pathways related to a range of immune and metabolic functions, as well as developmental processes. The characterisation of EVs, and post-translational deimination signatures, revealed here in horseshoe crab, contributes to current understanding of protein moonlighting functions and EV-mediated communication in this ancient arthropod and throughout phylogeny.

C-reactive protein: a predominant LPS-binding acute phase protein responsive to Pseudomonas infection

As a structural component of the outer membrane of Gram-negative bacteria, endotoxin, also known as lipopolysaccharide (LPS) exhibits strong immunostimulatory properties, rendering it a pivotal role in the pathogenesis of Gram-negative septicaemia. Our attempt to identify LPS-binding proteins from the hemolymph of the horseshoe crab led to the isolation and identification of C-reactive protein (CRP) as the predominant LPS-recognition protein during Pseudomonas infection. CRP is an evolutionarily ancient member of a superfamily of `pentraxins'. It is a major protein in acute phase of infection in humans. Our investigation of CRP response to Pseudomonas aeruginosa unveiled a robust innate immune system in the horseshoe crab, which displays rapid suppression of a dosage of 106 CFU of bacteria in the first hour of infection and effected complete clearance of the pathogen by 3 days. Such a high dose would have been lethal to mice. Full-length CRP cDNA was cloned. Analysis of the untranslated regions suggests their crucial role in post-transcriptional regulation of CRP transcript levels. Northern blot analysis demonstrated an acute up-regulation of CRP by about 60-fold in 6—48 h of Pseudomonas infection. Taken together, our results provide new insights into the importance of CRP as a conserved molecule for pathogen recognition.

The antimicrobial properties of C-reactive protein (CRP)

C-reactive protein, CRP, is a predominant pattern-recognition receptor (PRR) in the plasma of the horseshoe crab, which recognizes lipopolysaccharide (LPS). Native CRP2 has previously been shown to exhibit agglutination activity against the polysialic capsule of Escherichia coli K1 but its role in bacterial clearance is not well characterized. In this work, the antimicrobial activity of a recombinant CRP2 isoform (rCRP2) was tested against E. coli, Pseudomonas aeruginosa and Staphylococcus aureus. rCRP2 agglutinates bacteria and exhibits bactericidal activity against Gram-negative bacteria. In addition, the antimicrobial activity of rCRP2 is calcium-independent. GST pulldown experiments suggest that in the naïve physiological state, CRP2 interacts with hemocyanin, native CRPs, a 35-kDa plasma lectin and an as yet unidentified 40-kDa protein. This interaction was enhanced upon Pseudomonas infection. We propose that rCRP2 is a PRR with potent antimicrobial activity and its interacting partners contribute to effective bacterial clearance.

Crystal structures for short-chain pentraxin from zebrafish demonstrate a cyclic trimer with new recognition and effector faces

Short-chain pentraxins (PTXs), including CRP and SAP, are innate pattern recognition receptors that play vital roles in the recognition and elimination of various pathogenic bacteria by triggering the classical complement pathway through C1q. Similar to antibodies, pentraxins can also activate opsonisation and phagocytosis by interacting with Fc receptors (FcRs). Various structural studies on human PTXs have been performed, but there are no reports about the crystal structure of bony fish pentraxins. Here, the crystal structures of zebrafish PTX (Dare-PTX-Ca and Dare-PTX) are presented. Both Dare-PTX-Ca and Dare-PTX are cyclic trimers, which are new forms of crystallised pentraxins. The structures reveal that the ligand-binding pocket (LBP) in the recognition face of Dare-PTX is deep and narrow. Homology modelling shows that LBPs from different Dare-PTX loci differ in shape, reflecting their specific recognition abilities. Furthermore, in comparison with the structure of hCPR, a new C1q binding mode was identified in Dare-PTX. In addition, the FcR-binding sites of hSAP are partially conserved in Dare-PTX. These results will shed light on the understanding of a primitive PTX in bony fish, which evolved approximately 450 million years ago.

Microbe-specific C3b deposition in the horseshoe crab complement system in a C2/factor B-dependent or -independent manner

Complement C3 plays an essential role in the opsonization of pathogens in the mammalian complement system, whereas the molecular mechanism underlying C3 activation in invertebrates remains unknown. To understand the molecular mechanism of C3b deposition on microbes, we characterized two types of C2/factor B homologs (designated TtC2/Bf-1 and TtC2/Bf-2) identified from the horseshoe crab Tachypleus tridentatus. Although the domain architectures of TtC2/Bf-1 and TtC2/Bf-2 were identical to those of mammalian homologs, they contained five-repeated and seven-repeated complement control protein domains at their N-terminal regions, respectively. TtC2/Bf-1 and TtC2/Bf-2 were synthesized and glycosylated in hemocytes and secreted to hemolymph plasma, which existed in a complex with C3 (TtC3), and their activation by microbes was absolutely Mg²⁺-dependent. Flow cytometric analysis revealed that TtC3b deposition was Mg²⁺-dependent on Gram-positive bacteria or fungi, but not on Gram-negative bacteria. Moreover, this analysis demonstrated that Ca²⁺-dependent lectins (C-reactive protein-1 and tachylectin-5A) were required for TtC3b deposition on Gram-positive bacteria, and that a Ca²⁺-independent lectin (Tachypleus plasma lectin-1) was definitely indispensable for TtC3b deposition on fungi. In contrast, a horseshoe crab lipopolysaccharide-sensitive protease factor C was necessary and sufficient to deposit TtC3b on Gram-negative bacteria. We conclude that plasma lectins and factor C play key roles in microbe-specific TtC3b deposition in a C2/factor B-dependent or -independent manner.

Molecular characterization and expression analysis of two new C-reactive protein genes from common carp (Cyprinus carpio)

C-Reactive protein (CRP) plays an important role in the acute phase response. Transcripts encoding two new CRP-like molecules (ccCRP1 and ccCRP2) from European common carp have been characterized which has enabled seven CRP-like genes to be identified in zebrafish. 79.3% (ccCRP1) and 74.5% (ccCRP2) identity to CRP from East-Asian common carp occurs and fish CRP genes form a distinct clade. ccCRP2 gene organization comprises four exons and three introns, in contrast to the two exons/one intron organization of mammalian CRP genes. Gene expression assays showed both ccCRP-like molecules are constitutively expressed in liver, skin, gill, gut, muscle, kidney, spleen and blood. Protein levels of ccCRP in serum and spleen were significantly different from other organs analyzed, and levels were greatest in the liver. It is proposed that the two carp CRP genes defined differ in their expression profiles which may suggest differences in their biological activities.

Expression, crystallization and preliminary crystallographic analysis of C-reactive protein from zebrafish

C-reactive protein (CRP) is an acute phase protein that is found in blood, the concentration of which in plasma rises rapidly in response to inflammation. It functions as a pattern-recognition molecule, recognizing dead cells and various pathogenic agents and eliminating them by utilizing the classical complement pathway and activating macrophages. CRP is phylogenetically highly conserved in invertebrates and mammals. To date, information on the CRP gene has been reported from numerous species of animals, but little is known about the structure of CRP from species other than humans. In order to solve the structure of CRP from bony fish, the CRP gene from zebrafiah (Danio rerio) was cloned and expressed in Escherichia coli. The zebrafish CRP (Dare-CRP) was then purified and crystallized. The crystal diffracted to 2.3 Å resolution and belonged to space group R3, with unit-cell parameters a = b = 114.7, c = 61.0 Å. The Matthews coefficient and solvent content were calculated to be 3.28 Å(3) Da(-1) and 62.55%, respectively. Determination of the zebrafish CRP structure should be helpful in investigating the evolution of CRPs in the innate immune system.

Crystal structures of Limulus SAP-like pentraxin reveal two molecular aggregations

The serum-amyloid-P-component-like pentraxin from Limulus polyphemus, a recently discovered pentraxin species and important effector protein of the hemolymph immune system, displays two distinct doubly stacked cyclic molecular aggregations, heptameric and octameric. The refined three-dimensional structures determined by X-ray crystallography, both based on the same cDNA sequence, show that each aggregate is constructed from a similar dimer of protomers, which is repeated to make up the ring structure. The native octameric form has been refined at a resolution of 3 A, the native heptameric form at 2.3 A, and the phosphoethanolamine (PE)-bound octameric form at 2.7 A. The existence of the hitherto undescribed heptameric form was confirmed by single-particle analysis using cryo-electron microscopy. In the native structures, the calcium-binding site is similar to that in human pentraxins, with two calcium ions bound in each subunit. Upon binding PE, however, each subunit binds a third calcium ion, with all three calcium ions contributing to the binding and orientation of the bound phosphate group within the ligand-binding pocket. While the phosphate is well-defined in the electron density, the ethanolamine group is poorly defined, suggesting structural and binding variabilities of this group. Although sequence homology with human serum amyloid P component is relatively low, structural homology is high, with very similar overall folds and a common affinity for PE. This is due, in part, to a "topological" equivalence of side-chain position. Identical side chains that are important in both function and fold, from different regions of the sequence in human and Limulus structures, occupy similar space within the overall subunit fold. Sequence and structure alignment, based on the refined three-dimensional structures presented here and the known horseshoe crab pentraxin sequences, suggest that adaptation and refinement of C-reactive-protein-mediated immune responses in these ancient creatures lacking antibody-based immunity are based on adaptation by gene duplication.

Isolation and partial characterisation of four novel plasma lectins from the American lobster Homarus americanus

Although numerous haemolymph-derived crustacean lectins are described, few have been reported for the American lobster Homarus americanus. In the present study, affinity chromatography was used to isolate and partially describe the carbohydrate affinity of four new lectins from H. americanus plasma. HaMBP and HaDNABP were homodimers of approximately 30 kDa subunits which bound to mannan- and DNA-agarose columns, respectively. These proteins had partially overlapping elution profiles, and both shared and unique amino acid sequences and fragmentation patterns after trypsin digestion. A third homodimer of approximately 29 kDa subunits eluted with HaMBP and HaDNABP under certain conditions. HaNBP occurred as a monomer and dimer of approximately 40 kDa subunits and was recovered in relatively large quantities from mannan-agarose with N-acetylated sugars. Transmission electron microscopy revealed HaNBP to be a linear protein composed of multiple globular subunits.

Factor C acts as a lipopolysaccharide-responsive C3 convertase in horseshoe crab complement activation

The complement system in vertebrates plays an important role in host defense against and clearance of invading microbes, in which complement component C3 plays an essential role in the opsonization of pathogens, whereas the molecular mechanism underlying C3 activation in invertebrates remains unknown. In an effort to understand the molecular activation mechanism of invertebrate C3, we isolated and characterized an ortholog of C3 (designated TtC3) from the horseshoe crab Tachypleus tridentatus. Flow cytometric analysis using an Ab against TtC3 revealed that the horseshoe crab complement system opsonizes both Gram-negative and Gram-positive bacteria. Evaluation of the ability of various pathogen-associated molecular patterns to promote the proteolytic conversion of TtC3 to TtC3b in hemocyanin-depleted plasma indicated that LPS, but not zymosan, peptidoglycan, or laminarin, strongly induces this conversion, highlighting the selective response of the complement system to LPS stimulation. Although originally characterized as an LPS-sensitive initiator of hemolymph coagulation stored within hemocytes, we identified factor C in hemolymph plasma. An anti-factor C Ab inhibited various LPS-induced phenomena, including plasma amidase activity, the proteolytic activation of TtC3, and the deposition of TtC3b on the surface of Gram-negative bacteria. Moreover, activated factor C present on the surface of Gram-negative bacteria directly catalyzed the proteolytic conversion of the purified TtC3, thereby promoting TtC3b deposition. We conclude that factor C acts as an LPS-responsive C3 convertase on the surface of invading Gram-negative bacteria in the initial phase of horseshoe crab complement activation.

Membrane pore formation by pentraxin proteins from Limulus, the American horseshoe crab

The pentraxins are a family of highly conserved plasma proteins of metazoans known to function in immune defence. The canonical members, C-reactive protein and serum amyloid P component, have been identified in arthropods and humans. Mammalian pentraxins are known to bind lipid bilayers, and a pentraxin representative from the American horseshoe crab, Limulus polyphemus, binds and permeabilizes mammalian erythrocytes. Both activities are Ca2+-dependent. Utilizing model liposomes and planar lipid bilayers, in the present study we have investigated the membrane-active properties of the three pentraxin representatives from Limulus and show that all of the Limulus pentraxins permeabilize lipid bilayers. Mechanistically, Limulus C-reactive protein forms transmembrane pores in asymmetric planar lipid bilayers that mimic the outer membrane of Gram-negative bacteria and exhibits a Ca2+-independent form of membrane binding that may be sufficient for pore formation.

Evolution of polydom-like molecules: identification and characterization of cnidarian polydom (Cnpolydom) in the basal metazoan Hydractinia

End sequencing of random BAC clones from a Hydractinia symbiolongicarpus (Cnidaria: Hydrozoa) genomic library revealed a gene across a approximately 37.5kb region of the H. symbiolongicarpus genome sharing highest sequence identity and domain architecture to mammalian polydom that we in turn named cnidarian polydom (CnPolydom). Sharing all eight domain types characteristic of polydom and organized in a similar 5'-3' manner, CnPolydom was predicted to contain three additional domain types: PAN, FA58C, and CUB that are characteristic of CnPolydom. Expression analysis of CnPolydom from H. symbiolongicarpus (Hysy-CnPolydom) showed upregulation in response to bacterial and primarily fungal challenges, with transcripts produced specifically by a subset of interstitial stem cells (i-cells) and/or neural cells throughout the ectodermal tissue layer of feeding polyps (gastrozooids). This is the first description of a polydom-like molecule outside of Mammalia and provides evolutionary perspective on the ancestral structure and role of this pentraxin family clade.

A female-specific pentraxin, CrOctin, bridges pattern recognition receptors to bacterial phosphoethanolamine

Pathogen recognition and binding are crucial functions of innate immunity. It has been observed that the short pentraxin superfamily including C-reactive protein (CRP) and serum amyloid P component are pathogen pattern recognition receptors (PRR) in the plasma. We isolated and characterized a novel and distinctive pentraxin from the plasma of horseshoe crab, Carcinoscorpius rotundicauda, henceforth named CrOctin, which binds to bacteria via phosphoethanolamine (PE), a chemical component present on lipid A and core polysaccharide moieties of bacterial lipopolysaccharide (LPS). Infection enhances the formation of the PRR interactome constituting CrOctin, CRP and galactose-binding protein. In particular, infection increases the affinity of CRP to CrOctin by 1000-fold. Furthermore, we observed that by binding to PE, CrOctin acts as a linker that bridges the PRR interactome to the inner core of LPS. On the other hand, under normal physiological conditions, binding of CrOctin to PE appears to obscure other PRR from interacting directly with PE. Interestingly, the cluster of "CrOctin-interactive PRR" is sex specific. We report, for the first time, the change in PRR protein profiles with a distinctive gender difference during Pseudomonas infection.

The macromolecular assembly of pathogen-recognition receptors is impelled by serine proteases, via their complement control protein modules

Although the innate immune response is triggered by the formation of a stable assembly of pathogen-recognition receptors (PRRs) onto the pathogens, the driving force that enables this PRR-PRR interaction is unknown. Here, we show that serine proteases, which are activated during infection, participate in associating with the PRRs. Inhibition of serine proteases gravely impairs the PRR assembly. Using yeast two-hybrid and pull-down methods, we found that two serine proteases in the horseshoe crab Carcinoscorpius rotundicauda are able to bind to the following three core members of PRRs: galactose-binding protein, Carcinolectin-5 and C-reactive protein. These two serine proteases are (1) Factor C, which activates the coagulation pathway, and (2) C2/Bf, a protein from the complement pathway. By systematic molecular dissection, we show that these serine proteases interact with the core "pathogen-recognition complex" via their complement control protein modules.

C-reactive protein collaborates with plasma lectins to boost immune response against bacteria

Although human C-reactive protein (CRP) becomes upregulated during septicemia, its role remains unclear, since purified CRP showed no binding to many common pathogens. Contrary to previous findings, we show that purified human CRP (hCRP) binds to Salmonella enterica, and that binding is enhanced in the presence of plasma factors. In the horseshoe crab, Carcinoscorpius rotundicauda, CRP is a major hemolymph protein. Incubation of hemolymph with a range of bacteria resulted in CRP binding to all the bacteria tested. Lipopolysaccharide-affinity chromatography of the hemolymph co-purified CRP, galactose-binding protein (GBP) and carcinolectin-5 (CL5). Yeast two-hybrid and pull-down assays suggested that these pattern recognition receptors (PRRs) form pathogen recognition complexes. We show the conservation of PRR crosstalk in humans, whereby hCRP interacts with ficolin (CL5 homologue). This interaction stabilizes CRP binding to bacteria and activates the lectin-mediated complement pathway. We propose that CRP does not act alone but collaborates with other plasma PRRs to form stable pathogen recognition complexes when targeting a wide range of bacteria for destruction.

The phylogeny of the complement system and the origins of the classical pathway

The origins of the complement system have now been traced to near to the beginnings of multi-cellular animal life. Most of the evidence points to the earliest activation mechanism having been more similar to the lectin pathway than to the alternative pathway. C1q, the immunoglobulin recognition molecule of the classical pathway of the vertebrates, has now been shown to predate the development of antibody as it has been found in the lamprey, a jawless fish that lacks an acquired immune system. In this species, C1q acts as a lectin that binds MASPs and activates the C3/C4-like thioester protein of the lamprey complement system. The classical pathway can, therefore, be regarded as a specialised arm of the lectin pathway in which the specificity of C1q for carbohydrate has been recruited to recognise the Fc region of immunoglobulin.

Recognition of pathogens and activation of immune responses in Drosophila and horseshoe crab innate immunity

In innate immunity, pattern recognition receptors discriminate between self- and infectious non-self-matter. Mammalian homologs of the Drosophila Toll protein, which are collectively referred to as Toll-like receptors (TLRs), recognize pathogen-associated molecular patterns (PAMPs), including lipopolysaccharides (LPS) and lipoproteins, whereas the Drosophila Toll protein does not act as a PAMP receptor, but rather binds to Spätzle, an endogenous peptide. In Drosophila, innate immune surveillance is mediated by members of the peptidoglycan recognition protein (PGRP) family, which recognize diverse bacteria-derived peptidoglycans and initiate appropriate immune reactions including the release of antimicrobial peptides and the activation of the prophenoloxidase cascade, the latter effecting localized wound healing, melanization, and microbial phagocytosis. In the horseshoe crab, LPS induces hemocyte exocytotic degranulation, resulting in the secretion of various defense molecules, such as coagulation factors, antimicrobial peptides, and lectins. Recent studies have demonstrated that the zymogen form of the serine protease factor C, a major granular component of hemocyte, also exists on the hemocyte surface and functions as a biosensor for LPS. The proteolytic activity of activated factor C initiates hemocyte exocytosis via a G protein mediated signal transduction pathway. Furthermore, it has become clear that an endogenous mechanism for the feedback amplification of the innate immune response exists and is dependent upon a granular component of the horseshoe crab hemocyte.

Ligand specificities and structural requirements of two Tachypleus plasma lectins for bacterial trapping

TPL (Tachypleus plasma lectin)-1 was purified by using a Sepharose column and TPL-2 was purified from an LPS-Sepharose (LPS coupled to Sepharose matrix) affinity column, as described previously [Chiou, Chen, Y.-W., Chen, S.-C., Chao and Liu (2000) J. Biol. Chem. 275, 1630-1634] and the corresponding genes were cloned [Chen, Yen, Yeh, Huang and Liu (2001) J. Biol. Chem. 276, 9631-9639]. In the present study, TPL-1 and -2 were produced in yeast, and the recombinant proteins secreted into the media were purified and characterized. The proteins show specific PGN (peptidoglycan)- and LPS-binding activity, suggesting a role in trapping Gram-positive and Gram-negative bacteria respectively in innate immunity. Using BIAcore assays, the dissociation constant for the TPL-1-PGN complex was measured as 8x10(-8) M. Replacement of Asn74, the N-glycosylation site of TPL-1, with Asp abolishes the PGN-binding affinity, whereas the unglycosylated TPL-2 N3D mutant retains LPS-binding activity. DTT (dithiothreitol) treatment to break disulphide linkages abrogates TPL-2 activity but does not interfere with TPL-1 function. Cys4 in TPL-2 may form an intermolecular disulphide bond, which is essential for activity. As a result, the TPL-2 C4S mutant is inactive and is eluted as a monomer on a non-reducing gel. TPL-2 C6S is active and forms a non-covalently linked dimer. A model describing TPL-2 binding with LPS is proposed. These two plasma lectins that have different ligand specificities can be used for the detection and discrimination of bacteria and removal of endotoxins.

Recent advances in the innate immunity of invertebrate animals

Invertebrate animals, which lack adaptive immune systems, have developed other systems of biological host defense, so called innate immunity, that respond to common antigens on the cell surfaces of potential pathogens. During the past two decades, the molecular structures and functions of various defense components that participated in innate immune systems have been established in Arthropoda, such as, insects, the horseshoe crab, freshwater crayfish, and the protochordata ascidian. These defense molecules include phenoloxidases, clotting factors, complement factors, lectins, protease inhibitors, antimicrobial peptides, Toll receptors, and other humoral factors found mainly in hemolymph plasma and hemocytes. These components, which together compose the innate immune system, defend invertebrate from invading bacterial, fungal, and viral pathogens. This review describes the present status of our knowledge concerning such defensive molecules in invertebrates.

Analysis of binding sites in human C-reactive protein for Fc{gamma}RI, Fc{gamma}RIIA, and C1q by site-directed mutagenesis

Human C-reactive protein (CRP) is a classical, acute phase serum protein synthesized by the liver in response to infection, inflammation, or trauma. CRP binds to microbial antigens and damaged cells, opsonizes particles for phagocytosis and regulates the inflammatory response by the induction of cytokine synthesis. These activities of CRP depend on its ability to activate complement and to bind to Fcgamma receptors (FcgammaR). The goal of this study was to elucidate amino acid residues important for the interaction of CRP with human FcgammaRI (CD64) and FcgammaRIIa (CD32). Several mutations of the CRP structure were studied based on the published crystal structure of CRP. Mutant and wild-type recombinant CRP molecules were expressed in the baculovirus system and their interactions with FcgammaR and C1q were determined. A previous study by our laboratory identified an amino acid position, Leu(176), critical for CRP binding to FcgammaRI and work by others (Agrawal, A., Shrive, A. K., Greenhough, T. J., and Volanakis, J. E. (2001) J. Immunol. 166, 3998-4004) determined several residues important for C1q binding. The amino acid residues important to CRP binding to FcgammaRIIa were previously unknown. This study newly identifies residues Thr(173) and Asn(186) as important for the binding of CRP to FcgammaRIIa and FcgammaRI. Lys(114), like Leu(176), was implicated in binding to FcgammaRI, but not FcgammaRIIa. Single mutations at amino acid positions Lys(114), Asp(169), Thr(173), Tyr(175), and Leu(176) affected C1q binding to CRP. These results further identify amino acids involved in the binding sites on CRP for FcgammaRI, FcgammaRIIa, and C1q and indicate that these sites are overlapping.

A Toll-like receptor in horseshoe crabs

Non-self-recognition of invading microbes relies on the pattern-recognition of pathogen-associated molecular patterns (PAMPs) derived from microbial cell-wall components. Insects and mammals conserve a signaling pathway of the innate immune system through cell-surface receptors called Tolls and Toll-like receptors (TLRs). Bacterial lipopolysaccharides (LPSs) are an important trigger of the horseshoe crab's innate immunity to infectious microorganisms. Horseshoe crabs' granular hemocytes respond specifically to LPS stimulation, inducing the secretion of various defense molecules from the granular hemocytes. Here, we show a cDNA which we named tToll, coding for a TLR identified from hemocytes of the horseshoe crab Tachypleus tridentatus. tToll is most closely related to Drosophila Toll in both domain architecture and overall length. Human TLRs have been suggested to contain numerous PAMP-binding insertions located in the leucine-rich repeats (LRRs) of their ectodomains. However, the LRRs of tToll contained no obvious PAMP-binding insertions. Furthermore, tToll was non-specifically expressed in horseshoe crab tissues. These observations suggest that tToll does not function as an LPS receptor on granular hemocytes.

Molecular basis of non-self recognition by the horseshoe crab tachylectins

The self/non-self discrimination by innate immunity through simple ligands universally expressed both on pathogens and hosts, such as monosaccharides and acetyl group, depends on the density or clustering patterns of the ligands. The specific recognition by the horseshoe crab tachylectins with a propeller-like fold or a propeller-like oligomeric arrangement is reinforced by the short distance between the individual binding sites that interact with pathogen-associated molecular patterns (PAMPs). There is virtually no conformational change in the main or side chains of tachylectins upon binding with the ligands. This low structural flexibility of the propeller structures must be very important for specific interaction with PAMPs. Mammalian lectins, such as mannose-binding lectin and ficolins, trigger complement activation through the lectin pathway in the form of opsonins. However, tachylectins have no effector collagenous domains and no lectin-associated serine proteases found in the mammalian lectins. Furthermore, no complement-like proteins have been found in horseshoe crabs, except for alpha(2)-macroglobulin. The mystery of the molecular mechanism of the scavenging pathway of pathogens in horseshoe crabs remains to be solved.

Complete cDNA sequence of SAP-like pentraxin from Limulus polyphemus: implications for pentraxin evolution

The serum amyloid P component (SAP)-like pentraxin Limulus polyphemus SAP is a recently discovered, distinct pentraxin species, of known structure, which does not bind phosphocholine and whose N-terminal sequence has been shown to differ markedly from the highly conserved N terminus of all other known horseshoe crab pentraxins. The complete cDNA sequence of Limulus SAP, and the derived amino acid sequence, the first invertebrate SAP-like pentraxin sequence, have been determined. Two sequences were identified that differed only in the length of the 3' untranslated region. Limulus SAP is synthesised as a precursor protein of 234 amino acid residues, the first 17 residues encoding a signal peptide that is absent from the mature protein. Phylogenetic analysis clusters Limulus SAP pentraxin with the horseshoe crab C-reactive proteins (CRPs) rather than the mammalian SAPs, which are clustered with mammalian CRPs. The deduced amino acid sequence shares 22% identity with both human SAP and CRP, which are 51% identical, and 31-35% with horseshoe crab CRPs. These analyses indicate that gene duplication of CRP (or SAP), followed by sequence divergence and the evolution of CRP and/or SAP function, occurred independently along the chordate and arthropod evolutionary lines rather than in a common ancestor. They further indicate that the CRP/SAP gene duplication event in Limulus occurred before both the emergence of the Limulus CRP variants and the mammalian CRP/SAP gene duplication. Limulus SAP, which does not exhibit the CRP characteristic of calcium-dependent binding to phosphocholine, is established as a pentraxin species distinct from all other known horseshoe crab pentraxins that exist in many variant forms sharing a high level of sequence homology.

The molecular basis of innate immunity in the horseshoe crab

During the past two decades, the molecular structures and functions have been established for various defense molecules, using horseshoe crab (Limulus) as a model animal. These defense molecules include clotting factors, proteinase inhibitors, lectins, antimicrobial peptides and other humoral factors found mainly in the hemolymph. These components of the cellular and humoral systems, which together comprise innate immunity, defend horseshoe crab effectively from invading microbes.

Acute phase response of C-reactive protein of Labeo rohita to aquatic pollutants is accompanied by the appearance of distinct molecular forms

Different forms of C-reactive proteins have been purified to electrophoretic homogeneity by calcium dependent affinity chromatography on a phosphorylcholine (PC)-Sepharose column from the sera of Labeo rohita confined in fresh water (CRP(N)) and water polluted with sublethal doses of cadmium (CRP(Cd)), mercury (CRP(Hg)), phenol (CRP(Ph)), and hexachlorocyclohexane (CRP(Hx)), which elevate serum CRP levels by three- to fivefold. On native PAGE, induced forms of CRP show remarkable differences in their electrophoteric mobility indicating differences in molecular mass, charge, and/or shape. Kinetic studies reveal the appearance of a pollutant specific molecular variant, which replaces the normal form at the peak of induction. Studies on amino acid and carbohydrate compositions, isoelectric focusing, binding to PC, C-polysaccharide (CPS) & lectins, and secondary structures of the purified CRPs, indicate, that, they differ significantly from each other, but grossly share the common properties of a CRP, including pentraxin, structure revealed by electron microscopy.

Evolution of effectors and receptors of innate immunity

The bony fishes are derived from one of the earliest divergent vertebrate lineages to have both innate and acquired immune systems. They are considered by some to be an ideal model to study the underpinnings of immune systems precisely because of their phylogenetic position and the fact that their adaptive immune systems have not been elaborated to the extent seen in mammals. By the same token, examination of innate immune systems in invertebrates and early chordates can provide insight into how homologous systems operate in fish and higher vertebrates. Herein, we provide an overview of the molecular evidence that we hope helps clarify the evolutionary relationships of innate immune molecules identified in bony fishes. The innate immune systems being considered include select chemokines (CC and CXC chemokines and their receptors), cytokines (IL-1, IL-8, interferons, TGF-beta, TNF-alpha), acute phase proteins (SAA, SAP, CRP, alpha2M, and the complement components--C3-C9, MASP, MBL, Bf), NK cell receptors, and molecules upstream and downstream of the Toll signaling pathways.

Biochemical properties and cDNa cloning of two new lectins from the plasma of Tachypleus tridentatus: Tachypleus plasma lectin 1 and 2+

A Sepharose CL-4B-binding protein, Tachypleus plasma lectin 1 (TPL-1), and a lipopolysaccharide (LPS)-binding protein, Tachypleus plasma lectin-2 (TPL-2), have been isolated from the plasma of Tachypleus tridentatus and biochemically characterized. Each protein is coded by a homologous family of multigenes. TPL-1 binds to Sepharose CL-4B and was eluted with buffer containing 0.4 m GlcNAc. The deduced amino acid sequence of TPL-1 consisted of 232 amino acids with an N-glycosylation site, Asn-Gly-Ser at residues 74-76. It shares a 65% sequence identity and similar internal repeats of about 20 amino acid motifs with tachylectin-1. Tachylectin-1 was identified as a lipopolysaccharide-agarose binding nonglycosylated protein from the amebocytes of T. tridentatus. TPL-2 was eluted from the LPS-Sepharose CL-4B affinity column in buffer containing 0.4 m GlcNAc and 2 m KCl. The deduced amino acid sequence of TPL-2 consisted of 128 amino acids with an N-glycosylation site, Asn-Cys-Thr, at positions 3-5. It shares an 80% sequence identity with tachylectin-3, isolated from the amebocytes of T. tridentatus. TPL-2 purified by LPS-affinity column from the plasma predominantly exists as a dimer of a glycoprotein with an apparent molecular mass of 36 kDa. Tachylectin-3 is an intracellular nonglycosylated protein that also exists as a dimer in solution with an apparent molecular mass of 29 kDa. It recognizes Gram-negative bacteria through the 0-antigen of LPS. Western blot analyses showed that, in the plasma, TPL-1 and TPL-2 exist predominantly as oligomers with molecular masses above 60 kDa. They both bind to Gram-positive and Gram-negative bacteria, and this binding is inhibited by GlcNAc. Possible binding site of TPL-1 and TPL-2 to the bacteria could be at the NAc moiety of GlcNAc-MurNAc of the peptidoglycan. The physiological function of TPL-1 and TPL-2 is most likely related to their ability to form a cluster of interlocking molecules to immobilize and entrap invading organisms.