Primitive complement system--recognition and activation

Comparative and developmental study of the immune system in Xenopus

Xenopus laevis is the model of choice for evolutionary, comparative, and developmental studies of immunity, and invaluable research tools including MHC-defined clones, inbred strains, cell lines, and monoclonal antibodies are available for these studies. Recent efforts to use Silurana (Xenopus) tropicalis for genetic analyses have led to the sequencing of the whole genome. Ongoing genome mapping and mutagenesis studies will provide a new dimension to the study of immunity. Here we review what is known about the immune system of X. laevis integrated with available genomic information from S. tropicalis. This review provides compelling evidence for the high degree of similarity and evolutionary conservation between Xenopus and mammalian immune systems. We propose to build a powerful and innovative comparative biomedical model based on modern genetic technologies that takes take advantage of X. laevis and S. tropicalis, as well as the whole Xenopus genus. Developmental Dynamics 238:1249-1270, 2009. (c) 2009 Wiley-Liss, Inc.

An ancient and variable mannose-binding lectin from the coral Acropora millepora binds both pathogens and symbionts

Corals form the framework of the world's coral reefs and are under threat from increases in disease and bleaching (symbiotic dysfunction), yet the mechanisms of pathogen and symbiont recognition remain largely unknown. Here we describe the isolation and characterisation of an ancient mannose-binding lectin in the coral Acropora millepora, which is likely to be involved in both processes. The lectin ('Millectin') was isolated by affinity chromatography and was shown to bind to bacterial pathogens as well as coral symbionts, dinoflagellates of the genus Symbiodinium. cDNA analysis of Millectin indicate extensive sequence variation in the binding region, reflecting its ability to recognise various mannose-like carbohydrate structures on non-self cells, including symbionts and pathogens. This is the first mannose-binding lectin to show extensive sequence variability as observed for pattern recognition proteins in other invertebrate immune systems and, given that invertebrates rely on non-adaptive immunity, is a potential keystone component of coral defence mechanisms.

Preimplantation embryos cooperate with oviductal cells to produce embryotrophic inactivated complement-3b

Human oviductal epithelial (OE) cells produce complement protein 3 (C3) and its derivatives, C3b and inactivated complement-3b (iC3b). Among them, iC3b is the most potent embryotrophic molecule. We studied the production of iC3b in the oviductal cell/embryo culture system. In the immune system, C3 convertase converts C3 into C3b, and the conversion of C3b to iC3b requires factor I (fI) and its cofactors, such as factor H or membrane cofactor protein. Human oviductal epithelium and OE cells expressed mRNA and protein of the components of C3 convertase, including C2, C4, factor B, and factor D. The OE cell-conditioned medium contained active C3 convertase activity that was suppressed by C3 convertase inhibitor, H17 in a dose and time-dependent manner. Although the oviductal epithelium and OE cells produced fI, the production of its cofactor, factor H required for the conversion of C3b to iC3b, was weak. Thus, OE cell-conditioned medium was inefficient in producing iC3b from exogenous C3b. On the contrary, mouse embryos facilitated such conversion to iC3b, which was taken up by the embryos, resulting in the formation of more blastocysts of larger size. The facilitatory activity was mediated by complement receptor 1-related gene/protein Y (Crry) with known membrane cofactor protein activity on the trophectoderm of the embryos as anti-Crry antibody inhibited the conversion and embryotrophic activity of C3b in the presence of fI. In conclusion, human oviduct possesses C3 convertase activity converting C3 to C3b, and Crry of the preimplantation embryos may be involved in the production of embryotrophic iC3b on the surface of the embryos.

Comparative genomic analysis of the Tribolium immune system

The annotation, and comparison with homologous genes in other species, of immunity-related genes in the Tribolium castaneum genome allowed the identification of around 300 candidate defense proteins, and revealed a framework of information on Tribolium immunity.

Background

Tribolium castaneum is a species of Coleoptera, the largest and most diverse order of all eukaryotes. Components of the innate immune system are hardly known in this insect, which is in a key phylogenetic position to inform us about genetic innovations accompanying the evolution of holometabolous insects. We have annotated immunity-related genes and compared them with homologous molecules from other species.

Results

Around 300 candidate defense proteins are identified based on sequence similarity to homologs known to participate in immune responses. In most cases, paralog counts are lower than those of Drosophila melanogaster or Anopheles gambiae but are substantially higher than those of Apis mellifera. The genome contains probable orthologs for nearly all members of the Toll, IMD, and JAK/STAT pathways. While total numbers of the clip-domain serine proteinases are approximately equal in the fly (29), mosquito (32) and beetle (30), lineage-specific expansion of the family is discovered in all three species. Sixteen of the thirty-one serpin genes form a large cluster in a 50 kb region that resulted from extensive gene duplications. Among the nine Toll-like proteins, four are orthologous to Drosophila Toll. The presence of scavenger receptors and other related proteins indicates a role of cellular responses in the entire system. The structures of some antimicrobial peptides drastically differ from those in other orders of insects.

Conclusion

A framework of information on Tribolium immunity is established, which may serve as a stepping stone for future genetic analyses of defense responses in a nondrosophiline genetic model insect.

Complement system is activated in stenotic aortic valves

OBJECTIVE

To examine the role of the complement system, a source of powerful proinflammatory mediators, in aortic valve stenosis (AS).

METHODS AND RESULTS

Stenotic aortic valves (n=24) were obtained at valve replacement surgery, and non-stenotic (n=12) and early sclerotic (n=4) valves at cardiac transplantations. The terminal complement complex C5b-9 was stained by immunohistochemistry. Expression of the anaphylatoxin receptors C3aR and C5aR was studied in the valves by immunohistochemistry and RT-PCR, and in isolated valve myofibroblasts after stimulation with potential AS-accelerating factors (TNF-alpha and cigarette smoke) by RT-PCR. Cultured myofibroblasts were exposed to C3a, and their secretion of proinflammatory cytokines was assessed by ELISA. C5b-9 was found already in early aortic valve lesions, and its deposition was augmented in advanced stenotic valves. In stenotic valves, expression of C3aR mRNA was upregulated (p<0.05) and strong staining of C3aR and C5aR was detected. Myofibroblasts in stenotic, but not in control valves, expressed C3aR, and, in isolated myofibroblasts, TNF-alpha and cigarette smoke induced C3aR mRNA expression (p<0.05 for both). Stimulation of myofibroblasts with C3a resulted in enhanced secretion of MCP-1 (p<0.001), IL-6 (p=0.003), and IL-8 (p=0.03).

CONCLUSIONS

In stenotic aortic valves, complement is activated leading to generation of the anaphylatoxins C3a and C5a. Upregulation of C3aR in the valves as a result of inflammation and external risk factors, such as cigarette smoke, leads to an inflammatory response in aortic valve myofibroblasts. Complement activation in stenotic valves emerges as a novel pathogenic component of AS and may serve as a therapeutic target in this disease.

Characterization of highly concentrated serum lectins in spotted halibutVerasper variegatus

Mannose-binding lectins were purified from flatfish spotted halibut (Verasper variegatus) serum. These lectins, which we named VVL (Verasper variegatuslectin)-α (~33 kDa) and VVL-β (~30 kDa) (VVLs), under non-reducing SDS-PAGE, were surprisingly highly concentrated in serum (1·92±0·55 mg/ml;n=5), compared with other serum lectins. Both VVLs are heterodimers comprised of 2 types of subunit via inter-subunit disulfide bonds, and one subunit of VVL-α has a N-linked sugar chain. Based on N-terminal amino acid sequences, the nucleotide sequences of one subunit of VVL cDNAs were determined by 3′- and 5′-rapid amplification of the cDNA ends. The full-length VVL subunit cDNAs contained 489 bp, encoding an open reading frame of 163 amino acids. We found that VVLs bind to an ~8 kDa ciliary surface glycoprotein (a putative agglutination/immobilization antigen that we reported previously) of the fish parasiteNeobenedenia girellae, and agglutinate this parasitein vitro.

Urochordate whole body regeneration inaugurates a diverse innate immune signaling profile

The phenomenon of whole body regeneration (WBR) from minute soma fragments is a rare event in chordates, confined to the subfamily of botryllid ascidians and is poorly understood on the cellular and molecular levels. We assembled a list of 1326 ESTs from subtracted mRNA, at early stages of Botrylloides leachi WBR, and classified them into functional categories. Sixty-seven (15%) ESTs with roles in innate immunity signaling were classified into a broad functional group, a result supported by domain search and RT-PCR reactions. Gene ontology analysis for human homologous to the immune gene category, identified 22 significant entries, of which "peptidase activity" and "protease inhibitor activity", stood out as functioning during WBR. Analyzing expressions of serine protease Bl-TrSP, a representative candidate gene from the "peptidase activity" subgroup, revealed low transcript levels in naïve vasculature with upregulated expression during WBR. This was confirmed by in situ hybridization that further elucidated staining restricted to a circulating population of macrophage cells. Furthermore, Bl-TrSP was localized in regeneration niches within vasculature, in regenerating buds, and in buds, during blastogenesis. Functional inhibition of serine protease activity disrupts early remodeling processes of the vasculature microenvironment and hinders WBR. Comparison of genome-wide transcription of WBR with five other developmental processes in ascidians (including metamorphosis, budding and blastogenesis), revealed a broad conservation of immune signaling expressions, suggesting a ubiquitous route of harnessing immune-related genes within a broader range of tunicate developmental context. This, in turn, may have enabled the high diversity of life history traits represented by urochordate ascidians.

Trivalent recognition unit of innate immunity system: crystal structure of trimeric human M-ficolin fibrinogen-like domain

Ficolins are a kind of pathogen-recognition molecule in the innate immune systems. To investigate the discrimination mechanism between self and non-self by ficolins, we determined the crystal structure of the human M-ficolin fibrinogen-like domain (FD1), which is the ligand-binding domain, at 1.9A resolution. Although the FD1 monomer shares a common fold with the fibrinogen gamma fragment and tachylectin-5A, the Asp-282-Cys-283 peptide bond, which is the predicted ligand-binding site on the C-terminal P domain, is a normal trans bond, unlike the cases of the other two proteins. The trimeric formation of FD1 results in the separation of the three P domains, and the spatial arrangement of the three predicted ligand-binding sites on the trimer is very similar to that of the trimeric collectin, indicating that such an arrangement is generally required for pathogen-recognition. The ligand binding study of FD1 in solution indicated that the recombinant protein binds to N-acetyl-d-glucosamine and the peptide Gly-Pro-Arg-Pro and suggested that the ligand-binding region exhibits a conformational equilibrium involving cis-trans isomerization of the Asp-282-Cys-283 peptide bond. The crystal structure and the ligand binding study of FD1 provide an insight of the self- and non-self discrimination mechanism by ficolins.

Application of complement 1q for the site-selective recognition of immune complex in protein chip

Complement 1q (C1q) was applied for the specific recognition of antibody-antigen complex in antibody-based protein chip. The specific binding of C1q to antibody-antigen complex was investigated by surface plasmon resonance (SPR) with respect to Yersinia entericolitica, Salmonella typimurium, insulin, and bovine serum albumin. The protein chip was fabricated with two different kinds of antibodies a zigzag configuration. When one of antigens and fluorescein-isothiocyanate (FITC)-labeled C1q was applied on the protein chip, the specific binding event of C1q to immune complexes formed on protein chip was observed by fluorescence microscopy. These results implicate that the C1q can be used as an alternative to many antibodies that may be utilized individually on each spot of the protein chip.

Prohibitin binds to C3 and enhances complement activation

Prohibitin (PHB1) is a multifunction protein that is released in lipid droplets from adipocytes and possibly other cells and is detectable in the circulation. We used crosslinking, immunoprecipitation and proteomic analysis to investigate binding partners for circulating PHB1. Crosslinking of PHB1 to serum resulted in two complexes of approximately 150 and 100 kDa, which contained both PHB1 and fragments of C3. The binding of PHB1 to C3 was confirmed using a solid phase assay where the dissociation constant was approximately 90 fmol/l. PHB1, but not the closely related PHB2, was able to enhance complement activation and induce lysis of sensitized sheep erythrocytes when added with normal serum but not with C3-deficient serum. The ability of PHB1 to bind to, and activate C3 suggests that PHB1 may have a previously unrecognized role in innate immunity.

Gene expression profiling studies on Caenorhabditis elegans dystrophin mutants dys-1(cx-35) and dys-1(cx18)

The Caenorhabditis elegans genome contains a single dystrophin/utrophin orthologue, dys-1. Point mutations in this gene, dys-1(cx35) and dys-1(cx18), result in truncated proteins. Such mutants offer potentially valuable worm models of human Duchenne muscular dystrophy. We have used microarrays to examine genes expressed differentially between wild-type C. elegans and dys-1 mutants. We found 106 genes (115 probe sets) to be differentially expressed when the two mutants are compared to wild-type worms, 49 of which have been assigned to six functional categories. The main categories of regulated genes in C. elegans are genes encoding intracellular signalling, cell-cell communication, cell-surface, and extracellular matrix proteins; genes in these same categories have been shown by others to be differentially expressed in muscle biopsies of muscular dystrophy patients. The C. elegans model may serve as a convenient vehicle for future genetic and chemical screens to search for new drug targets.

Lectin complement system and pattern recognition

Living organisms have strong defense mechanisms against invading microorganisms as survival strategies. One of the defense mechanisms is the complement system, composed of more than 30 serum and cell surface components. This system collaborates in recognition and elimination of pathogens as a part of both the innate and acquired immune systems. The two collagenous lectins, mannose-binding lectin (MBL) and ficolins, are pattern recognition proteins acting in innate immunity and, upon recognition of the pathogens, they trigger the activation of the lectin complement pathway through attached serine proteases (MASPs). A similar lectin-based complement system, consisting of the lectin-protease complex and C3, is present in ascidians, our closest invertebrate relatives and in lamprey, the most primitive vertebrate. Furthermore, a lamprey N-acetylglucosamine (GlcNAc)-binding lectin was identified as the orthlogue of mammalian C1q, and lamprey MASP is suggested as the prototype of MASP-2/C1r/C1s, indicating that the classical complement pathway arose as a part of the innate immune system. Thus, the complement system is one of the most highly organized innate immune systems in invertebrates and jawless vertebrates, and this system has survived in vertebrates with its core components little changed for 600-700 million years.

Cloning and characterization of mannose-binding lectin from lamprey (Agnathans)

The recognition of pathogens is mediated by a set of pattern recognition molecules that recognize conserved pathogen-associated molecular patterns shared by broad classes of microorganisms. Mannose-binding lectin (MBL) is one of the pattern recognition molecules and activates complement in association with MBL-associated serine protease (MASP) via the lectin pathway. Recently, an MBL-like lectin was isolated from the plasma of a urochordate, the solitary ascidian. This ascidian lectin has a carbohydrate recognition domain, but the collagen-like domain was replaced by another sequence. To elucidate the origin of MBLs, the aim of this study is to determine the structure and function of the MBL homolog in lamprey, the most primitive vertebrate. Using an N-acetylglucosamine (GlcNAc)-agarose column, MBL-like lectin (p25) was isolated from lamprey serum and cDNA cloning was conducted. From the deduced amino acid sequence this lectin has a collagenous region and a typical carbohydrate recognition domain. This lectin also binds mannose, glucose, and GlcNAc, but not galactose, indicating that it is structurally and functionally similar to the mammalian MBLs. Furthermore, it associated with lamprey MASPs, and the MBL-MASP activated lamprey C3 in fluid-phase and on the surface of pathogens. In conjunction with the phylogenetic analysis, it seems likely that the lamprey MBL is an ortholog of the mammalian MBL. Because acquired immunity seems to have been established only from jawed vertebrates onward, the lectin complement pathway in lamprey, as one of the major contributors to innate immunity, plays a pivotal role in defending the body against microorganisms.

The role of the complement system in innate immunity

Complement is a major component of innate immune system involved in defending against all the foreign pathogens through complement fragments that participate in opsonization, chemotaxis, and activation of leukocytes and through cytolysis by C5b-9 membrane attack complex. Bacterias and viruses have adapted in various ways to escape the complement activation, and they take advantage of the complement system by using the host complement receptors to infect various cells. Complement activation also participates in clearance of apoptotic cells and immune complexes. Moreover, at sublytic dose, C5b-9 was shown to promote cell survival. Recently it was also recognized that complement plays a key role in adaptive immunity by modulating and modifying the T cell responses. All these data suggest that complement activation constitutes a critical link between the innate and acquired immune responses.

The evolution of immune mechanisms

From early on in evolution, organisms have had to protect themselves from pathogens. Mechanisms for discriminating "self" from "non-self" evolved to accomplish this task, launching a long history of host-pathogen co-evolution. Evolution of mechanisms of immune defense has resulted in a variety of strategies. Even unicellular organisms have rich arsenals of mechanisms for protection, such as restriction endonucleases, antimicrobial peptides, and RNA interference. In multicellular organisms, specialized immune cells have evolved, capable of recognition, phagocytosis, and killing of foreign cells as well as removing their own cells changed by damage, senescence, infection, or cancer. Additional humoral factors, such as the complement cascade, have developed that co-operate with cellular immunity in fighting infection and maintaining homeostasis. Defensive mechanisms based on germline-encoded receptors constitute a system known as innate immunity. In jaw vertebrates, this system is supplemented with a second system, adaptive immunity, which in contrast to innate immunity is based on diversification of immune receptors and on immunological memory in each individual.Usually, each newly evolved defense mechanism did not replace the previous one, but supplemented it, resulting in a layered structure of the immune system. The immune system is not one system but rather a sophisticated network of various defensive mechanisms operating on different levels, ranging from mechanisms common for every cell in the body to specialized immune cells and responses at the level of the whole organism. Adaptive changes in pathogens have shaped the evolution of the immune system at all levels.

Comparative genetics and innate immune functions of collagenous lectins in animals

Collagenous lectins such as mannan-binding lectins (MBLs), ficolins (FCNs), surfactant proteins A and D (SP-A, SP-D), conglutinin (CG), and related ruminant lectins are multimeric proteins with carbohydrate-binding domains aligned in a manner that facilitates binding to microbial surface polysaccharides. MBLs and FCNs are structurally related to C1q, but activate the lectin complement pathway via interaction with MBL-associated serine proteases (MASPs). MBLs, FCNs, and other collagenous lectins also bind to some host macromolecules and contribute to their removal. While there is evidence that some lectins and the lectin complement pathway are conserved in vertebrates, many differences in collagenous lectins have been observed among humans, rodents, and other vertebrates. For example, humans have only one MBL but three FCNs, whereas most other species express two FCNs and two MBLs. Bovidae express CG and other SP-D-related collectins that are not found in monogastric species. Some dysfunctions of human MBL are due to single nucleotide polymorphisms (SNPs) that affect its expression or structure and thereby increase susceptibility to some infections. Collagenous lectins have well-established roles in innate immunity to various microorganisms, so it is possible that some lectin genotypes or induced phenotypes influence resistance to some infectious or inflammatory diseases in animals.

Mannan-binding-lectin-associated serine proteases, characteristics and disease associations

Mannan-binding lectin (MBL)-associated serine proteases (MASPs) circulate in plasma as zymogens in complexes with MBL and with L- and H-ficolin. Upon binding of MBL or ficolin to pathogen-associated molecular patterns, the MASPs are activated. MASP-2 can now cleave C4 and C2 to generate the C3 convertase, C4bC2b. The functions of the other two MASPs, MASP-1 and MASP-3 have not been elucidated. MASP-1 can cleave C2, and with low efficiency also C3, and may serve a function through direct C3 activation. No natural substrate for MASP-3 has been identified. MBL deficiency, occurring at a frequency of about 10%, is the most common congenital immunodeficiency and is associated with susceptibility to infections and autoimmune disorders. Inherited MASP-2 deficiency has been described as the result of a mutation causing the exchange of aspartic acid with a glycine at position 105, a position in the first domain, CUB1, involved in calcium binding. This mutation abolishes the binding to MBL and ficolins, and deprives MASP-2 of functional activity. The index case suffered from recurrent severe infections and autoimmune reactions. The gene frequency of the mutation among Caucasians is 3.6%. It is not found in Chinese, who present a different mutation also associated with MASP-2 deficiency.

M-ficolin, an innate immune defence molecule, binds patterns of acetyl groups and activates complement

Ficolins play a role in the innate immune defence as pathogen-associated molecular pattern recognition molecules. Three ficolins are found in humans: H-ficolin, L-ficolin and M-ficolin. L-ficolin and H-ficolin circulate in blood in complexes with mannan-binding lectin-associated serine proteases (MASPs) and are capable of activating the complement system. L-ficolin shows affinity for acetylated compounds and binds to various capsulated strains of bacteria. H-ficolin has been shown to bind Aerococcus viridans. Less is known about M-ficolin, but it is thought to be present only on monocytes. We have synthesized recombinant M-ficolin and find that it, in a manner similar to L-ficolin, is able to bind to acetylated compounds and to associate with recombinant MASP-2. Upon binding to M-ficolin ligands, the associated MASP-2 zymogen is activated and cleaves C4, thus triggering the complement system. We developed a monoclonal rat anti-human-M/L-ficolin antibody and verified by flow cytometric analysis the presence of ficolin on the surface of peripheral blood monocytes.

Function and heterogeneity of fish lectins

Lectins are primordial molecules with multiple known functions. They have been known to exist in fish and other animals for decades and were initially identified as (hem)agglutinins. Demonstration of the importance of vertebrate lectins in innate immunity is a recent effort and is still largely unrealised for fish. This mini-review will tabulate those fish lectins identified since the last major review. In addition, particular lectins for which either functional relevance or functional or structural heterogeneity has been demonstrated are discussed in greater detail.

Critical roles of platelets in lipopolysaccharide-induced lethality: effects of glycyrrhizin and possible strategy for acute respiratory distress syndrome

Within a few minutes of an intravenous injection of a lipopolysaccharide (LPS) into mice, platelets accumulate, largely in the lung. At higher doses, LPS induces rapid shock (within 10 min), leading to death within 1 h. This type of shock differs from so-called endotoxin shock, in which shock signs and death occur several hours or more later. Here, we found that platelet depletion (by a monoclonal anti-platelet antibody) prevented LPS-induced rapid shock, but increased delayed lethality. In Japan, glycyrrhizin (GL), a compound isolated from licorice, is daily and slowly infused intravenously into chronic hepatitis C patients. A single bolus intravenous injection into mice of GL (200 mg/kg or less) shortly before (or simultaneously with) LPS injection reduced the pulmonary platelet accumulation and the severity of the rapid shock, and prevented death in both the early and later periods. GL itself, at 400 mg/kg, produced no detectable abnormalities in the appearance or activity of mice. Intraperitoneal injection of aspirin or dexamethasone had only marginal effects on LPS-induced platelet responses and lethality. These results suggest that platelets play important roles in the development of both the rapid and delayed types of shock induced by LPS. Although the mechanism by which GL suppresses platelet responses and delayed lethality remains to be clarified, GL might provide a strategy for alleviating the acute respiratory distress syndrome seen in sepsis. Our results may also support the proposal by Cinatl et al. [Cinatl J, Morgenstern B, Bauer G, Chandra P, Ravenau H, Doerr HW. Glycyrrhizin, an active component of liquorice roots, and replication of SARS-associated coronavirus. Lancet 2003; 361: 2045–6.] that GL may be an effective drug against severe acute respiratory syndrome.

Ciona intestinalis: chordate development made simple

Thanks to their transparent and rapidly developing mosaic embryos, ascidians (or sea squirts) have been a model system for embryological studies for over a century. Recently, ascidians have entered the postgenomic era, with the sequencing of the Ciona intestinalis genome and the accumulation of molecular resources that rival those available for fruit flies and mice. One strength of ascidians as a model system is their close similarity to vertebrates. Literature reporting molecular homologies between vertebrate and ascidian tissues has flourished over the past 15 years, since the first ascidian genes were cloned. However, it should not be forgotten that ascidians diverged from the lineage leading to vertebrates over 500 million years ago. Here, we review the main similarities and differences so far identified, at the molecular level, between ascidian and vertebrate tissues and discuss the evolution of the compact ascidian genome.

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.

Evolution of innate immune systems*

Innate immunity is the oldest form of defense and is found to some degree in all species. It predates the adaptive immune system, consisting of antibodies, B cells, T cells, and the major histocompatibility antigens. These are found only in higher vertebrates and have been the focus of the majority of immunological research, particularly in mice and humans, over the years. Knowledge of immunity in lower vertebrate and invertebrate species is now increasing rapidly, shedding light on the evolution of immunity and in many cases adding to our understanding of the mammalian system. Several recurring structural, genetic, and developmental mechanisms are common features in these processes in both the cellular and the molecular aspects of innate immunity.

Structural and functional diversity of lectin repertoires in invertebrates, protochordates and ectothermic vertebrates

During the past few years, substantial progress has been accomplished in the elucidation of the structural diversity of the lectin repertoires of invertebrates, protochordates and ectothermic vertebrates, providing particularly valuable information on those groups that constitute the invertebrate/vertebrate 'boundary'. Although representatives of lectin families typical of mammals, such as C-type lectins, galectins and pentraxins, have been described in these taxa, the detailed study of selected model species has yielded either novel variants of the structures described for the mammalian lectin representatives or novel lectin families with unique sequence motifs, multidomain arrangements and a new structural fold. Along with the high structural diversity of the lectin repertoires in these taxa, a wide spectrum of biological roles is starting to emerge, underscoring the value of invertebrate and lower vertebrate models for gaining insight into structural, functional and evolutionary aspects of lectins.