The complement system in sea urchins

Construction and immune efficacy of recombinant Lactobacillus casei expressing OmpAI of Aeromonas veronii C5-I as molecular adjuvant

Despite advancements in diagnosis and control, Aeromonas infections are considered the leading cause of economic aquaculture loss. In this study, to enhance DNA vaccine efficacy against Aeromonas infections, a fused DNA fragment (1504 bp) of the OmpAI gene from Aeromonas veronii (A. veronii) combined with the C5-I gene from the common carp was generated with splicing by overlapping PCR (SOE-PCR) and expressed in Lactobacillus casei strain CC16. Protein C5-I served as a molecular adjuvant for the antigen OmpAI. Two types of fusion antigens were developed (anchored and secretory). Generally, anchored-type antigens are more effective in inducing immune responses in fish than secretory antigens. Western blot analysis showed that the bands of both antigens were present at 58 kDa. After oral immunization, both DNA vaccines enhanced the serum levels of AKP, ACP, SOD and LZM in immunized carp; the genes IL-10, IL-1β, TNF-α, and IFN-γ in the heart, liver, spleen, head kidney, and intestinal tract were upregulated; and a stronger phagocytic response was triggered in immunized fish. In addition, common carp administered the fused antigens were more protected from Aeromonas challenge (60-73.3% protection). Recombinant Lactobacillus bacteria expressing the fused protein showed a greater propensity for colonization in the intestinal tract in immunized fish than in controls. Here, we provide a promising approach to improve DNA vaccine immunogenicity for protecting common carp from A. veronii infections.

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.

The Axial Organ and the Pharynx Are Sites of Hematopoiesis in the Sea Urchin

Background: The location of coelomocyte proliferation in adult sea urchins is unknown and speculations since the early 1800s have been based on microanatomy and tracer uptake studies. In adult sea urchins (Strongylocentrotus purpuratus) with down-regulated immune systems, coelomocyte numbers increase in response to immune challenge, and whether some or all of these cells are newly proliferated is not known. The gene regulatory network that encodes transcription factors that control hematopoiesis in embryonic and larval sea urchins has not been investigated in adults. Hence, to identify the hematopoietic tissue in adult sea urchins, cell proliferation, expression of phagocyte specific genes, and expression of genes encoding transcription factors that function in the conserved regulatory network that controls hematopoiesis in embryonic and larval sea urchins were investigated for several tissues. Results: Cell proliferation was induced in adult sea urchins either by immune challenge through injection of heat-killed Vibrio diazotrophicus or by cell depletion through aspiration of coelomic fluid. In response to either of these stimuli, newly proliferated coelomocytes constitute only about 10% of the cells in the coelomic fluid. In tissues, newly proliferated cells and cells that express SpTransformer proteins (formerly Sp185/333) that are markers for phagocytes are present in the axial organ, gonad, pharynx, esophagus, and gut with no differences among tissues. The expression level of genes encoding transcription factors that regulate hematopoiesis show that both the axial organ and the pharynx have elevated expression compared to coelomocytes, esophagus, gut, and gonad. Similarly, an RNAseq dataset shows similar results for the axial organ and pharynx, but also suggests that the axial organ may be a site for removal and recycling of cells in the coelomic cavity. Conclusions: Results presented here are consistent with previous speculations that the axial organ may be a site of coelomocyte proliferation and that it may also be a center for cellular removal and recycling. A second site, the pharynx, may also have hematopoietic activity, a tissue that has been assumed to function only as part of the intestinal tract.

SpTransformer proteins from the purple sea urchin opsonize bacteria, augment phagocytosis, and retard bacterial growth

The purple sea urchin, Strongylocentrotus purpuratus, has a complex and robust immune system that is mediated by a number of multi-gene families including the SpTransformer (SpTrf) gene family (formerly Sp185/333). In response to immune challenge from bacteria and various pathogen-associated molecular patterns, the SpTrf genes are up-regulated in sea urchin phagocytes and express a diverse array of SpTrf proteins. We show here that SpTrf proteins from coelomocytes and isolated by nickel affinity (cNi-SpTrf) bind to Gram-positive and Gram-negative bacteria and to Baker's yeast, Saccharomyces cerevisiae, with saturable kinetics and specificity. cNi-SpTrf opsonization of the marine bacteria, Vibrio diazotrophicus, augments phagocytosis, however, opsonization by the recombinant protein, rSpTrf-E1, does not. Binding by cNi-SpTrf proteins retards growth rates significantly for several species of bacteria. SpTrf proteins, previously thought to be strictly membrane-associated, are secreted from phagocytes in short term cultures and bind V. diazotrophicus that are located both outside of and within phagocytes. Our results demonstrate anti-microbial activities of native SpTrf proteins and suggest variable functions among different SpTrf isoforms. Multiple isoforms may act synergistically to detect a wide array of pathogens and provide flexible and efficient host immunity.

Molecular characterization of the immune system: emergence of proteins, processes, and domains

Many genes and proteins are required to carry out the processes of innate and adaptive immunity. For many studies, including systems biology, it is necessary to have a clear and comprehensive definition of the immune system, including the genes and proteins that take part in immunological processes. We have identified and cataloged a large portion of the human immunology-related genes, which we call the essential immunome. The 847 identified genes and proteins were annotated, and their chromosomal localizations were compared to the mouse genome. Relation to disease was also taken into account. We identified numerous pseudogenes, many of which are expressed, and found two putative new genes. We also carried out an evolutionary analysis of immune processes based on gene orthologs to gain an overview of the evolutionary past and molecular present of the human immune system. A list of genes and proteins were compiled. A comprehensive characterization of the member genes and proteins, including the corresponding pseudogenes is presented. Immunome genes were found to have three types of emergence in independent studies of their ontologies, domains, and functions.

Unexpected diversity displayed in cDNAs expressed by the immune cells of the purple sea urchin, Strongylocentrotus purpuratus

We recently identified a unique family of transcripts, the 185/333 family, that comprise approximately 60% of the mRNAs induced by coelomocytes from the purple sea urchin in response to immunological challenge from lipopolysaccharide. An analysis of 81 full-length cDNAs revealed 67 unique nucleotide sequences encoding 64 different proteins. Diversity of the transcripts was based on 25 sequence blocks, or "elements," which resulted in 22 different element patterns based on their presence or absence. Furthermore, there was a high level of nucleotide variation within elements, including single nucleotide polymorphisms and insertions/deletions, both of which resulted in amino acid sequence variability. The deduced 185/333 proteins contained an NH2-terminal leader, a glycine-rich region with an RGD motif, a histidine-rich region, and a COOH-terminal region. Two 185/333 genes, identified in the partially assembled Strongylocentrotus purpuratus genome, have two exons. The first encoded the leader, and the second encoded the remainder of the predicted protein. Estimates from quantitative PCR indicated that there were approximately 100 alleles in the diploid genome. These results suggested that the purple sea urchin may have mechanisms for generating high levels of diversity in response to immunological challenge that have not been considered previously.

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.

Constitutive expression and alternative splicing of the exons encoding SCRs in Sp152, the sea urchin homologue of complement factor B. Implications on the evolution of the Bf/C2 gene family

The purple sea urchin, Strongylocentrotus purpuratus, possesses a non-adaptive immune system including elements homologous to C3 and factor B (Bf) of the vertebrate complement system. SpBf is composed of motifs typical of the Bf/C2 protein family. Expression of Sp152 (encodes SpBf) was identified in the phagocyte type of coelomocyte in addition to gut, pharynx and esophagus, which may have been due to the presence of these coelomocytes in and on all tissues of the animal. Sp152 expression in coelomocytes was constitutive and non-inducible based on comparisons between pre- and post-injection with lipopolysaccharide or sterile seawater. The pattern of five short consensus repeats (SCRs) in SpBf has been considered ancestral compared to other deuterostome Bf/C2 proteins that contain either three or four SCRs. Three alternatively spliced messages were identified for Sp152 and designated Sp152Delta1, Sp152Delta4, and Sp152Delta1+Delta4, based on which of the five SCRs were deleted. Sp152Delta4 had an in-frame deletion of SCR4, which would encode a putative SpBfDelta4 protein with four SCRs rather than five. On the other hand, both Sp152Delta1 and Sp152Delta1+Delta4 had a frame-shift that introduced a stop codon six amino acids downstream of the splice site for SCR1, and would encode putative proteins composed only of the leader. Comparisons between the full-length SpBf and its several splice variants with other Bf/C2 proteins suggested that the early evolution of this gene family may have involved a combination of gene duplications and deletions of exons encoding SCRs.

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

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

Two cDNAs from the purple sea urchin, Strongylocentrotus purpuratus , encoding mosaic proteins with domains found in factor H, factor I, and complement components C6 and C7

The vertebrate complement system is composed of about 30 serum and cell surface proteins that make up three activation pathways, a lytic pathway, and a set of proteins that regulate complement. Regulatory proteins are required for host protection against autologous complement attack and to control the amplification feedback loop of the alternative pathway. Purple sea urchin, Strongylocentrotus purpuratus, homologues of complement C3 (SpC3) and factor B (SpBf) have been identified, suggesting the presence of an alternative complement pathway. This implies that echinoderms require a complement regulatory system for the same reasons that it is required in higher vertebrates. Two cDNAs, Sp5 and Sp5013, have been characterized from coelomocytes and the deduced structures of the encoded mosaic proteins, SpCRL ( S. p urpuratus complement related protein, long form) and SpCRS ( short form), have domains that are also found in regulatory proteins such as factor H and factor I and the terminal pathway components C6 and C7. These domains include multiple short consensus repeats, a fucolectin domain, Ser/Thr/Pro-rich regions, a Cys-rich region, and a factor I-membrane attack complex domain. The genes are constitutively expressed in all tissues of the sea urchin and are not induced in response to immune challenge. Multiple bands of varying intensity on both genome blots and RNA blots suggest that Sp5 and Sp5013 are members of a small gene family and that they might undergo alternative splicing. Based on the domains present in SpCRL and SpCRS, they might be either examples of complement regulatory proteins or members of the terminal pathway of complement.

The gene encoding the sea urchin complement protein, SpC3, is expressed in embryos and can be upregulated by bacteria

Sea urchins have an innate immune response that functions in the absence of adaptive capabilities. It is mediated, in part, by components of the complement system, an important subsystem of the innate response in mammals. A homologue of complement C3, SpC3, has been identified in adult Strongylocentrotus purpuratus and is expressed in coelomocytes. In this study, transcript levels from the gene, Sp064, which encodes SpC3, were examined in developing embryos and found to be present in unfertilized eggs and throughout embryogenesis with a peak in transcript levels just prior to and during gastrulation. In addition, continuous exposure of embryos, beginning with the hatched blastula stage, to heat killed Vibrio diazatrophicus, a marine pathogen of sea urchins, significantly increased Sp064 message content in plutei compared to unexposed controls. These results suggest that sea urchin embryos may use a complement-based immune system for defense against pathogens in their aquatic environment.

Thioester function is conserved in SpC3, the sea urchin homologue of the complement component C3

The amino acid sequence of the thioester site in the alpha chain of SpC3, the sea urchin homologue of C3, is conserved. This implies a conserved function of covalent bond formation with amine or hydroxyl groups on target molecules. When coelomic fluid (CF) was incubated with 14C-methylamine, a classic assay for thioester binding function, the alpha chain became labeled. When CF was treated to induce autolysis, peptide bond cleavage occurred at the thioester site. Autolysis could be blocked or reduced by pre-treating CF with either methylamine or yeast, both of which are known to bind to thioester sites C3 proteins from other organisms. The data suggest that SpC3 can bind to target cell surfaces, constituting indirect evidence that it can covalently bind to pathogen surfaces and function as an opsonin in vivo. This activity may be an important aspect of host defense in the sea urchin.