A complement component C3a-like peptide stimulates chemotaxis by hemocytes from an invertebrate chordate-the tunicate, Pyura stolonifera

Domain-Dependent Evolution Explains Functional Homology of Protostome and Deuterostome Complement C3-Like Proteins

Complement proteins emerged early in evolution but outside the vertebrate clade they are poorly characterized. An evolutionary model of C3 family members revealed that in contrast to vertebrates the evolutionary trajectory of C3-like genes in cnidarian, protostomes and invertebrate deuterostomes was highly divergent due to independent lineage and species-specific duplications. The deduced C3-like and vertebrate C3, C4 and C5 proteins had low sequence conservation, but extraordinarily high structural conservation and 2-chain and 3-chain protein isoforms repeatedly emerged. Functional characterization of three C3-like isoforms in a bivalve representative revealed that in common with vertebrates complement proteins they were cleaved into two subunits, b and a, and the latter regulated inflammation-related genes, chemotaxis and phagocytosis. Changes within the thioester bond cleavage sites and the a-subunit protein (ANATO domain) explained the functional differentiation of bivalve C3-like. The emergence of domain-related functions early during evolution explains the overlapping functions of bivalve C3-like and vertebrate C3, C4 and C5, despite low sequence conservation and indicates that evolutionary pressure acted to conserve protein domain organization rather than the primary sequence.

Molecular Characterization of Complement Component 3 (C3) in the Pearl Oyster Pinctada fucata Improves Our Understanding of the Primitive Complement System in Bivalve

As the central component in the complement system, complement component 3 (C3) plays essential roles in both the innate and adaptive immune responses. Here, a C3 gene (designated as pf-C3) was obtained from the pearl oyster Pinctada fucata by RT-PCR and rapid amplification of cDNA ends (RACE). The pf-C3 cDNA consists of 5,634 bp with an open reading frame (ORF) of 5,193 bp encoding a protein of 1,730 amino acids with a 19 residue signal peptide. The deduced pf-C3 protein possessed the characteristic structural features present in its homologs and contained the A2M_N_2, ANATO, A2M, A2M_comp, A2M_recep, and C345C domains, as well as the C3 convertase cleavage site, thioester motif, and conserved Cys, His, and Glu residues. Phylogenetic analysis revealed that pf-C3 is closely related to the C3s from other mollusks. Pf-C3 mRNA was expressed in all examined tissues including gill, digestive gland, adductor muscle, mantle and foot, while the highest expression was found in the digestive gland. Following the challenge with Vibrio alginolyticus, pf-C3 expression was significantly induced in hemocytes. Luciferase reporter assays indicated that pf-C3a could activate the NF-κB signal pathway in HEK293T cells. Further knockdown of pf-C3 by specific siRNA could significantly reduce the phagocytosis of V. alginolyticus by hemocytes in vitro. These results would help increase understanding of the function of C3 in the invertebrate immune system and therefore provide new insights into the roles of the primitive complement system in invertebrates.

Transcriptomic analysis of Manila clam Ruditapes philippinarum under lipopolysaccharide challenge provides molecular insights into immune response

The Manila clam, Ruditapes philippinarum, is an economically important shellfish in marine aquaculture. A better understanding of the immune system in R. philippinarum will provide the basis for the development of strategies to mitigate the impact of infectious diseases affecting this species but can also be of relevance for other bivalves of commercial interest. In this study, the transcriptional response of the Manila clam under lipopolysaccharide (LPS) challenge was characterized using RNA sequencing. The transcriptomes of LPS challenged group of clams (LH1, LH2 and LH3), and the PBS control group (CH1, CH2 and CH3), were sequenced with the Illumina HiSeq platform. Compared with the unigene expression profile of the control group, 223 unigenes were up-regulated and 389 unigenes were down-regulated in the LPS challenged group. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses revealed that signal transduction, defense response, and immune-related pathways such as Chemokine signaling pathway, Complement and coagulation cascades, NOD-like receptor signaling pathway, and Inflammatory mediator regulation of TRP channels in sensory system were the most highly enriched pathways among the genes that were differentially expressed under LPS challenge. This study present understanding of the molecular basis underpinning response to LPS challenge and provides useful information for future work on the molecular mechanism of pathogen resistance and immunity in Manila clam.

Complement system and phagocytosis in a colonial protochordate

In the present work, we investigated, in the colonial ascidian Botryllus schlosseri, the role of complement C3 (BsC3) in phagocytosis. We studied the modulation of BsC3 transcription in the course of the colonial blastogenetic cycle, with particular reference to the takeover, when apoptotic cells in the tissues of old zooids are cleared by circulating phagocytes. In situ hybridisation with BsC3 riboprobes labelled only morula cells, the most abundant haemocytes. Anti-hC3 antibody recognised morula cells and also phagocytes when haemocytes were previously incubated with zymosan. The inhibition of C3 activation prevented the labelling of phagocytes. In phagocytosis assays with haemocytes from colonies injected with anti-hC3 antibody or bsc3 iRNA, the capability to ingest target cells was significantly (p < 0.001) reduced. Therefore, our results strongly support a key role of BsC3 in phagocytosis and open to new investigations on the nature of the receptors of the products of BsC3 activation.

Clam Genome Sequence Clarifies the Molecular Basis of Its Benthic Adaptation and Extraordinary Shell Color Diversity

Ruditapes philippinarum is an economically important bivalve with remarkable diversity in its shell coloration patterns. In this study, we sequenced the whole genome of the Manila clam and investigated the molecular basis of its adaptation to hypoxia, acidification, and parasite stress with transcriptome sequencing and an RNA sequence analysis of different tissues and developmental stages to clarify these major issues. A number of immune-related gene families are expanded in the R. philippinarum genome, such as TEP, C3, C1qDC, Hsp70, SABL, and lysozyme, which are potentially important for its stress resistance and adaptation to a coastal benthic life. The transcriptome analyses demonstrated the dynamic and orchestrated specific expression of numerous innate immune-related genes in response to experimental challenge with pathogens. These findings suggest that the expansion of immune- and stress-related genes may play vital roles in resistance to adverse environments and has a profound effect on the clam's adaptation to benthic life.

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We present a new genome assembly of the Manila clam Ruditapes philippinarum

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Analysis of gene family expansions and transcriptome characterization were conducted

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Tyr and mitf genes were potentially involved in shell color patterns of Manila clam

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Expansion of GPCRs and immune-related genes were found in R. philippinarum

Stressors Due to Handling Impair Gut Immunity in Meagre (Argyrosomus regius): The Compensatory Role of Dietary L-Tryptophan

In the context of intensive aquaculture, meagre (Argyrosomus regius) is one of the most important new aquaculture species in Southern Europe and several studies are focused on the optimization of its culture. Nevertheless, stressors such as handling during transport or culture maintenance may affect the immune system, thereby impairing some immune responses or provoking cellular damage. One strategy that has been used to avert this type of negative stress response is the supplementation of amino acids to improve resistance to stress. In this experiment, meagre (105.0 ± 2.6 g, mean ± standard deviation) juveniles were fed two diets for a period of 7 days, the first a commercial diet supplemented with 1% tryptophan (Trp) and second, the same commercial diet without tryptophan supplementation (control group). The effects of two types of handling stressors (air exposure and confinement/netting) on fish fed both diets was evaluated in terms of gene expression of the selected gut immunity markers, such as (1) innate immune response processes: c3 complement (c3), lysozyme (lys), and cyclooxygenase (cox2); (2) humoral immune response processes: interferon type 1 (ifn1), mx protein (mxp), interleukin 1b (il-1b), tumor necrosis factor 1a (tnf1a), and interleukin 10 (il-10); (3) antimicrobial peptides: defensin (def), hepcidin (hep), piscidin (pis), and a marker for mitochondrial respiration: glyceraldehyde 3-phosphate dehydrogenase (gapdh). Samples of the anterior intestine were collected at 1 and 6 h post-stress (hps). Results showed that in fish fed 1% Trp, the air exposure resulted in an upregulation of gene expression at 6 hps for c3, lys, cox2, ifn1, mxp, il-10 and gapdh, and il-1b and pis. The confinement/netting test for fish fed 1% Trp resulted in an upregulation of c3 and mxp and a downregulation of cox2, ifn1, il-1b, tnf1a, il-10, def, hep, and gapdh at both post-stress times (1 and 6 hps). According to the present study, dietary supplementation with 1% Trp may be considered as a proper nutritional strategy for improving tolerance and/or alleviating acute response to handling stressors.

Molecular characterization of complement component 3 (C3) in Mytilus coruscus improves our understanding of bivalve complement system

Complement component 3 (C3) plays a central role in the complement system whose activation is essential for all the important functions performed by this system. Here, a novel C3 gene, termed Mc-C3, was identified from thick shell mussel (Mytilus coruscus). The deduced Mc-C3 protein possessed the characteristic structure features present in its homologs and contained the A2M_N_2, ANATO, A2M, A2M_comp, A2M_recep, and C345C domains, as well as the C3 convertase cleavage site, thioester motif, and conserved Cys, His, and Glu residues. Mc-C3 gene constitutively expressed in all examined tissues and predominantly expressed in immune-related tissues such as gills, hemocytes and hepatopancreas. After stimulation with lipopolysaccharide or Cu²⁺, the expression of Mc-C3 was significantly induced in gills. Further luciferase reporter assays showed the ability for activation of NF-κB signaling transduction of Mc-C3a. Taken together, these results show that C3 may play an essential role in the immune defense of M. coruscus. The present data therefore provide a more detailed insight into the functional activities of the bivalve complement system.

A novel C1q-domain-containing protein from razor clam Sinonovacula constricta mediates G-bacterial agglutination as a pattern recognition receptor

Complement component 1q (C1q) with a characteristic C1q globular domain is an important pattern recognition molecule in the classical complement systems and plays a major role in the crosslinking between innate immunity and specific immunity in vertebrates. In this study, a homologous gene encoding typically C1q domains was obtained from the razor clam Sinonovacula constricta (designated ScC1qDC) by rapid amplification of the cDNA end. The full-length cDNA of ScC1qDC was 1225 bp in length with a 5'UTR of 258 bp, a 3'UTR of 223 bp, and an open reading frame of 744 bp encoding a polypeptide of 247 amino acids containing a typical C1q globular domain. The mRNA transcripts of ScC1qDC were constitutively transcribed in all examined tissues with higher expression in the hepatopancreas. Time-course expression analysis indicated that ScC1qDC was significantly up-regulated both in hepatopancreas and gills after Vibrio parahaemolyticus challenge. The recombinant ScC1qDC (rScC1qDC) displayed high binding activities to various pathogen-associated molecular patterns, including LPS, PGN, and MAN. Recombinant ScC1qDC showed no agglutinating activity to Gram-positive bacterium of Micrococcus luteus but showed obvious activities towards all the three examined Gram-negative bacteria. All our results indicated that ScC1qDC might be served as a pattern recognition receptor and promoted Gram-negative bacteria agglutination during the pathogen challenge.

Immunity in Protochordates: The Tunicate Perspective

Tunicates are the closest relatives of vertebrates, and their peculiar phylogenetic position explains the increasing interest toward tunicate immunobiology. They are filter-feeding organisms, and this greatly influences their defense strategies. The majority of the studies on tunicate immunity were carried out in ascidians. The tunic acts as a first barrier against pathogens and parasites. In addition, the oral siphon and the pharynx represent two major, highly vascularized, immune organs, where circulating hemocytes can sense non-self material and trigger immune responses that, usually, lead to inflammation and phagocytosis. Inflammation involves the recruitment of circulating cytotoxic, phenoloxidase (PO)-containing cells in the infected area, where they degranulate as a consequence of non-self recognition and release cytokines, complement factors, and the enzyme PO. The latter, acting on polyphenol substrata, produces cytotoxic quinones, which polymerize to melanin, and reactive oxygen species, which induce oxidative stress. Both the alternative and the lectin pathways of complement activation converge to activate C3: C3a and C3b are involved in the recruitment of hemocytes and in the opsonization of foreign materials, respectively. The interaction of circulating professional phagocytes with potentially pathogenic foreign material can be direct or mediated by opsonins, either complement dependent or complement independent. Together with cytotoxic cells, phagocytes are active in the encapsulation of large materials. Cells involved in immune responses, collectively called immunocytes, represent a large fraction of hemocytes, and the presence of a cross talk between cytotoxic cells and phagocytes, mediated by secreted humoral factors, was reported. Lectins play a pivotal role as pattern-recognition receptors and opsonizing agents. In addition, variable region-containing chitin-binding proteins, identified in the solitary ascidian Ciona intestinalis, control the settlement and colonization of bacteria in the gut.

Preliminary characterization of complement in a colonial tunicate: C3, Bf and inhibition of C3 opsonic activity by compstatin

The complement system is a fundamental effector mechanism of the innate immunity in both vertebrates and invertebrates. The comprehension of its roots in the evolution is a useful step to understand how the main complement-related proteins had changed in order to adapt to new environmental conditions and life-cycles or, in the case of vertebrates, to interact with the adaptive immunity. Data on organisms evolutionary close to vertebrates, such as tunicates, are of primary importance for a better understanding of the changes in immune responses associated with the invertebrate-vertebrate transition. Here we report on the characterization of C3 and Bf transcripts from the colonial ascidian Botryllus schlosseri (BsC3 and BsBf, respectively), a reliable model organism for immunobiological research, and present a comparative analysis of amino acid sequences of C3s and Bfs suggesting that, in deuterostomes, the structure of these proteins remained largely unchanged. We also present new data on the cells responsible of the expression of BsC3 and BsBf showing that cytotoxic immunocytes are the sole cells where the relative transcripts can be found. Finally, using the C3 specific inhibitor compstatin, we demonstrate the opsonic role of BsC3 in accordance with the idea that promotion of phagocytosis is one of the main function of C3 in metazoans.

Interplay between invertebrate C3a with vertebrate macrophages: functional characterization of immune activities of amphioxus C3a

Our current knowledge of the structure and function of C3a comes from the study of vertebrate C3a anaphylatoxins, virtually nothing is known about the structure and function of C3a molecules in invertebrates. Here we demonstrated that C3a from the invertebrate chordate Branchiostoma japonicum, BjC3a, was similar to vertebrate C3a possessing potential antibacterial activity, as revealed by sequence analysis and computational modeling. The antibacterial activity of BjC3a was definitely confirmed by both antibacterial assay and TEM observation showing that recombinant BjC3a was directly bactericidal. Additionally, recombinant BjC3a, like vertebrate C3a, was capable of inducing sea bass macrophage migration and enhancing macrophage phagocytosis and respiratory burst response. Moreover, recombinant BjC3a-desArg (generated by removal of the C-terminal arginine), like mammalian C3a-desArg, retained the immunological activities of BjC3a such as antibacterial and respiratory burst-stimulating activities, indicating that the immunological functions of C3a-desArg were conserved throughout chordate evolution. Altogether, our findings show that invertebrate (amphioxus) BjC3a is able to interact with vertebrate (sea bass) macrophages and mediate immune activities, suggesting the emergence of the inflammatory pathway of the complement system similar to that of vertebrates in the basal chordate amphioxus.

"Nested" cryptic diversity in a widespread marine ecosystem engineer: a challenge for detecting biological invasions

Background

Ecosystem engineers facilitate habitat formation and enhance biodiversity, but when they become invasive, they present a critical threat to native communities because they can drastically alter the receiving habitat. Management of such species thus needs to be a priority, but the poorly resolved taxonomy of many ecosystem engineers represents a major obstacle to correctly identifying them as being either native or introduced. We address this dilemma by studying the sea squirt Pyura stolonifera, an important ecosystem engineer that dominates coastal communities particularly in the southern hemisphere. Using DNA sequence data from four independently evolving loci, we aimed to determine levels of cryptic diversity, the invasive or native status of each regional population, and the most appropriate sampling design for identifying the geographic ranges of each evolutionary unit.

Results

Extensive sampling in Africa, Australasia and South America revealed the existence of "nested" levels of cryptic diversity, in which at least five distinct species can be further subdivided into smaller-scale genetic lineages. The ranges of several evolutionary units are limited by well-documented biogeographic disjunctions. Evidence for both cryptic native diversity and the existence of invasive populations allows us to considerably refine our view of the native versus introduced status of the evolutionary units within Pyura stolonifera in the different coastal communities they dominate.

Conclusions

This study illustrates the degree of taxonomic complexity that can exist within widespread species for which there is little taxonomic expertise, and it highlights the challenges involved in distinguishing between indigenous and introduced populations. The fact that multiple genetic lineages can be native to a single geographic region indicates that it is imperative to obtain samples from as many different habitat types and biotic zones as possible when attempting to identify the source region of a putative invader. "Nested" cryptic diversity, and the difficulties in correctly identifying invasive species that arise from it, represent a major challenge for managing biodiversity.

Molecular cloning and characterization of European seabass (Dicentrarchus labrax) and Gilthead seabream (Sparus aurata) complement component C3

We present the complete C3 cDNA sequence of Gilthead seabream (Sparus aurata) and European seabass (Dicentrarchus labrax) and its molecular characterization with a descriptive analysis of their structural elements. We obtained one sequence for Gilthead seabream (gsbC3) which encodes a predicted protein of 1656 amino acids, and two sequences for European seabass (esbC3_1 and esbC3_2) which encode two predicted proteins of 1654 and 1587 amino acids respectively. All sequences present the characteristic structural features of C3 but interestingly esbC3_2 lacks the anaphylotoxin domain and the cysteine residue responsible for thiolester bond formation. Moreover, we have detected and quantified (by real-time PCR-based absolute quantification) specific isoform expression in European seabass depending on pathogen and density conditions in vivo. In addition, we have analyzed the tissue distribution pattern of European seabass and Gilthead seabream C3 genes under crowding stress and under pathological challenges in vivo, and we have observed that crowding and infection status provoke changes in expression levels, tissue expression pattern and C3 isoform expression balance.

Analysis of evolutionarily conserved innate immune components in coral links immunity and symbiosis

Reef-building corals are representatives of one of the earliest diverging metazoan lineages and are experiencing increases in bleaching events (breakdown of the coral-Symbiodinium symbiosis) and disease outbreaks. The present study investigates the roles of two pattern recognition proteins, the mannose binding lectin Millectin and a complement factor C3-like protein (C3-Am), in the coral Acropora millepora. The results indicate that the innate immune functions of these molecules are conserved and arose early in evolution. C3-Am is expressed in response to injury, and may function as an opsonin. In contrast, Millectin expression is up-regulated in response to lipopolysaccharide and peptidoglycan. These observations, coupled with localization of Millectin in nematocysts in epidermal tissue, and reported binding of pathogens, are consistent with a key role for the lectin in innate immunity. Furthermore, Millectin was consistently detected binding to the symbiont Symbiodinium in vivo, indicating that the Millectin function of recognition and binding of non-self-entities may have been co-opted from an ancient innate immune system into a role in symbiosis.

Hedistin: A novel antimicrobial peptide containing bromotryptophan constitutively expressed in the NK cells-like of the marine annelid, Nereis diversicolor

A novel antimicrobial peptide, named hedistin was identified from the coelomocytes of Nereis diversicolor. Hedistin shows no obvious similarities with other known peptides and constitutes the first antimicrobial peptide containing bromotryptophans demonstrated in annelids. cDNA and mass spectrometry analysis revealed that, upon bacteria challenge, this peptide is secreted following processing of a precursor containing a signal peptide and prosequences. Hedistin was shown to possess an activity against a large spectrum of bacteria including the methicillin resistant Staphylococcus aureus and Vibrio alginolyticus. The gene was demonstrated to be constitutively and exclusively expressed in circulating NK cells like known to play an important role in the immunity of the sand worm. These data contrast with those observed in another annelid, the leech, in which genes coding for antimicrobial peptides are upregulated in a specific tissue and peptides are rapidly released into the hemolymph after septic injury.

Colony specificity and chemotaxis in the compound ascidian Botryllus schlosseri

We re-investigated the behavior of hemocytes during the non-fusion (rejection) reaction between genetically incompatible colonies of the ascidian Botryllus schlosseri. In the course of the reaction, hemocytes - mainly morula cells - crowd inside the blind ends of marginal vascular vessels (known as ampullae) of the colonial leading edge (LE) facing the foreign colony which suggests the occurrence of chemotactic attraction of circulating hemocytes towards the ampullar lumen. Then, cells migrate, through the ampullar tips, into the partially fused tunics and contribute to the formation of the necrotic spots along the contact borders which characterize the reaction. Studies on histological sections clearly indicate that, although morula cell concentration is always higher in ampullae of the LE than in those of the lateral (L) part of the colony, their frequency significantly increases in LE ampullae of rejecting colonies with respect to LE ampullae of both fusing and isolated colonies. In addition, in vitro chemotaxis experiments demonstrated that blood plasma from incompatible colonies can stimulate morula cell migration through polycarbonate filters and this passage is inhibited by antibodies raised against mammalian pro-inflammatory cytokines. The possible nature and role of molecules recognized by anti-cytokine antibodies in hemocyte migration are discussed.

First identification of a chemotactic receptor in an invertebrate species: structural and functional characterization of Ciona intestinalis C3a receptor

In mammals, the bioactive fragment C3a, released from C3 during complement activation, is a potent mediator of inflammatory reactions and exerts its functional activity through the specific binding to cell surface G protein-coupled seven-transmembrane receptors. Recently, we demonstrated a Ciona intestinalis C3a (CiC3a)-mediated chemotaxis of hemocytes in the deuterostome invertebrate Ciona intestinalis and suggested an important role for this molecule in inflammatory processes. In the present work, we have cloned and characterized the receptor molecule involved in the CiC3a-mediated chemotaxis and studied its expression profile. The sequence, encoding a 95,394 Da seven-transmembrane domain protein, shows the highest sequence homology with mammalian C3aRs. Northern blot analysis revealed that the CiC3aR is expressed abundantly in the heart and neural complex and to a lesser extent in the ovaries, hemocytes, and larvae. Three polyclonal Abs raised in rabbits against peptides corresponding to CiC3aR regions of the first and second extracellular loop and of the third intracellular loop react specifically in Western blotting with a single band of 98-102 kDa in hemocyte protein extracts. Immunostaining performed on circulating hemocytes with the three specific Abs revealed that CiC3aR is constitutively expressed only in hyaline and granular amoebocytes. In chemotaxis experiments, the Abs against the first and second extracellular loop inhibited directional migration of hemocytes toward the synthetic peptide reproducing the CiC3a C-terminal sequence, thus providing the compelling evidence that C. intestinalis expresses a functional C3aR homologous to the mammalian receptor. These findings further elucidate the evolutionary origin of the vertebrate complement-mediated proinflammatory process.

Characterization of a C3a Receptor in Rainbow Trout andXenopus: The First Identification of C3a Receptors in Nonmammalian Species

Virtually nothing is known about the structure, function, and evolutionary origins of the C3aR in nonmammalian species. Because C3aR and C5aR are thought to have arisen from the same common ancestor, the recent characterization of a C5aR in teleost fish implied the presence of a C3aR in this animal group. In this study we report the cloning of a trout cDNA encoding a 364-aa molecule (TC3aR) that shows a high degree of sequence homology and a strong phylogenetic relationship with mammalian C3aRs. Northern blotting demonstrated that TC3aR was expressed primarily in blood leukocytes. Flow cytometric analysis and immunofluorescence microscopy showed that Abs raised against TC3aR stained to a high degree all blood B lymphocytes and, to a lesser extent, all granulocytes. More importantly, these Abs inhibited trout C3a-mediated intracellular calcium mobilization in trout leukocytes. A fascinating structural feature of TC3aR is the lack of a significant portion of the second extracellular loop (ECL2). In all C3aR molecules characterized to date, the ECL2 is exceptionally large when compared with the same region of C5aR. However, the exact function of the extra portion of ECL2 is unknown. The lack of this segment in TC3aR suggests that the extra piece of ECL2 was not necessary for the interaction of the ancestral C3aR with its ligand. Our findings represent the first C3aR characterized in nonmammalian species and support the hypothesis that if C3aR and C5aR diverged from a common ancestor, this event occurred before the emergence of teleost fish.

Activation of the complement system generates antibacterial peptides

The complement system represents an evolutionary old and significant part of the innate immune system involved in protection against invading microorganisms. Here, we show that the anaphylatoxin C3a and its inactivated derivative C3a-desArg are antibacterial, demonstrating a previously unknown direct antimicrobial effect of complement activation. The C3a peptide, as well as functional epitopes in the sequence, efficiently killed the Gram-negative bacteria Escherichia coli, Pseudomonas aeruginosa, and the Gram-positive Enterococcus faecalis. In mice, a C3a-derived peptide suppressed infection by Gram-positive Streptococcus pyogenes bacteria. Fluorescence and electron microscopy demonstrated that C3a binds to and induces breaks in bacterial membranes. C3a was also found to induce membrane leakage of liposomes. These findings provide an interesting link between the complement system and antimicrobial peptides, which are two important branches of innate immunity.

Production of recombinant C5a from rainbow trout (Oncorhynchus mykiss): role in leucocyte chemotaxis and respiratory burst

Activation of the complement system can lead to the formation of the membrane attack complex, in which the component C5 is cleaved into C5a and C5b fragments. The C5a anaphylatoxin is a very potent pro-inflammatory molecule that induces chemotaxis and respiratory burst processes in a variety of mammalian leucocytes. While C5a has been well studied in mammals, little is known about the structure and function of C5a in teleost fish or other non-mammalian species. In the present study, we have produced and purified recombinant rainbow trout C5a (rtC5a), and we have shown that it plays an important role in inducing leucocyte migration as well as in triggering the respiratory burst of peripheral blood (PBLs) and head kidney leucocytes (HKLs). When the carboxy-terminal Arg was removed from rtC5a, its ability to induce cell migration and superoxide production remained intact. Interestingly, we show that leucocytes migrating towards rtC5a attached to the plate with a well-spread circular morphology, whereas those migrating towards activated trout serum displayed more irregular and dendritic-like shapes. Our data suggest that the basic mechanisms of action of the C5a anaphylotoxin have remained conserved for more than 300 million years.

Exocytosis of a complement component C3-like protein by tunicate hemocytes

This study investigates the exocytic responses of invertebrate hemocytes to pathogen-associated antigens. It demonstrates that a homologue of complement component C3, a key defensive protein of the innate immune system, is expressed by phagocytic hemocytes (non-refractile vacuolated cells) of the tunicate, Styela plicata. C3-like molecules are localized in sub-cellular vesicles and are rapidly exocytosed after stimulation with bacterial, fungal or algal cell surface molecules. Signal transduction analysis indicated that the induced secretion of C3-like molecules is mediated by a G-protein dependent signaling pathway, which modulates tubulin microtubules. All of this evidence indicates that hemocytes can contribute to host defense responses by rapidly exocytosing C3-like proteins at sites of infection.

CiC3-1a-mediated chemotaxis in the deuterostome invertebrate Ciona intestinalis (Urochordata)

Deuterostome invertebrates possess complement genes, and in limited instances complement-mediated functions have been reported in these organisms. However, the organization of the complement pathway(s), as well as the functions exerted by the cloned gene products, are largely unknown. To address the issue of the presence of an inflammatory pathway in ascidians, we expressed in Escherichia coli the fragment of Ciona intestinalis C3-1 corresponding to mammalian complement C3a (rCiC3-1a) and assessed its chemotactic activity on C. intestinalis hemocytes. We found that the migration of C. intestinalis hemocytes toward rCiC3-1a was dose dependent, peaking at 500 nM, and was specific for CiC3-1a, being inhibited by an anti-rCiC3-1a-specific Ab. As is true for mammalian C3a, the chemotactic activity of C. intestinalis C3-1a was localized to the C terminus, because a peptide representing the 18 C-terminal amino acids (CiC3-1a(59-76)) also promoted hemocyte chemotaxis. Furthermore, the CiC3-1a terminal Arg was not crucial for chemotactic activity, because the desArg peptide (CiC3-1a(59-75)) retained most of the directional hemocyte migration activity. The CiC3-1a-mediated chemotaxis was inhibited by pretreatment of cells with pertussis toxin, suggesting that the receptor molecule mediating the chemotactic effect is G(i) protein coupled. Immunohistochemical analysis with anti-rCiC3-1a-specific Ab and in situ hybridization experiments with a riboprobe corresponding to the 3'-terminal sequence of CiC3-1, performed on tunic sections of LPS-injected animals, showed that a majority of the infiltrating labeled hemocytes were granular amebocytes and compartment cells. Our findings indicate that CiC3-1a mediates chemotaxis of C. intestinalis hemocytes, thus suggesting an important role for this molecule in inflammatory processes.

Tunicate cytokine-like molecules and their involvement in host defense responses

Tunicates (ascidians or sea squirts) are a large group of invertebrate chordates that are closely related to vertebrates. Their critical phylogenetic position has stimulated substantial interest in their host defense ("immune") responses. Whilst this interest has generated a wealth of knowledge regarding the humoral and cellular mechanisms that undertake defensive responses, there is less known about the regulation of those reactions. This chapter focuses on three cellular responses (cell proliferation, phagocytosis and chemotaxis) that are known to be regulated by cytophilic humoral molecules. Some of the humoral factors that affect these responses have functional and physicochemical similarities to vertebrate cytokines, like interleukin-1. However, the only regulatory molecules that have been characterized at a molecular level bear far greater similarity to C-type lectins or complement components.