Lipopolysaccharide activates the sea urchin immune system

Single-cell RNA sequencing of the holothurian regenerating intestine reveals the pluripotency of the coelomic epithelium

In holothurians, the regenerative process following evisceration involves the development of a "rudiment" or "anlage" at the injured end of the mesentery. This regenerating anlage plays a pivotal role in the formation of a new intestine. Despite its significance, our understanding of the molecular characteristics inherent to the constituent cells of this structure has remained limited. To address this gap, we employed state-of-the-art scRNA-seq and HCR-FISH analyses to discern the distinct cellular populations associated with the regeneration anlage. Through this approach, we successfully identified thirteen distinct cell clusters. Among these, two clusters exhibit characteristics consistent with putative mesenchymal cells, while another four show features akin to coelomocyte cell populations. The remaining seven cell clusters collectively form a large group encompassing the coelomic epithelium of the regenerating anlage and mesentery. Within this large group of clusters, we recognized previously documented cell populations such as muscle precursors, neuroepithelial cells and actively proliferating cells. Strikingly, our analysis provides data for identifying at least four other cellular populations that we define as the precursor cells of the growing anlage. Consequently, our findings strengthen the hypothesis that the coelomic epithelium of the anlage is a pluripotent tissue that gives rise to diverse cell types of the regenerating intestinal organ. Moreover, our results provide the initial view into the transcriptomic analysis of cell populations responsible for the amazing regenerative capabilities of echinoderms.

Recombinant SpTransformer proteins are functionally diverse for binding and phagocytosis by three subtypes of sea urchin phagocytes

Introduction

The California purple sea urchin, Strongylocentrotus purpuratus, relies solely on an innate immune system to combat the many pathogens in the marine environment. One aspect of their molecular defenses is the SpTransformer (SpTrf) gene family that is upregulated in response to immune challenge. The gene sequences are highly variable both within and among animals and likely encode thousands of SpTrf isoforms within the sea urchin population. The native SpTrf proteins bind foreign targets and augment phagocytosis of a marine Vibrio. A recombinant (r)SpTrf-E1-Ec protein produced by E. coli also binds Vibrio but does not augment phagocytosis.

Methods

To address the question of whether other rSpTrf isoforms function as opsonins and augment phagocytosis, six rSpTrf proteins were expressed in insect cells.

Results

The rSpTrf proteins are larger than expected, are glycosylated, and one dimerized irreversibly. Each rSpTrf protein cross-linked to inert magnetic beads (rSpTrf::beads) results in different levels of surface binding and phagocytosis by phagocytes. Initial analysis shows that significantly more rSpTrf::beads associate with cells compared to control BSA::beads. Binding specificity was verified by pre-incubating the rSpTrf::beads with antibodies, which reduces the association with phagocytes. The different rSpTrf::beads show significant differences for cell surface binding and phagocytosis by phagocytes. Furthermore, there are differences among the three distinct types of phagocytes that show specific vs. constitutive binding and phagocytosis.

Conclusion

These findings illustrate the complexity and effectiveness of the sea urchin innate immune system driven by the natSpTrf proteins and the phagocyte cell populations that act to neutralize a wide range of foreign pathogens.

Coelomocyte populations in the sea urchin, Strongylocentrotus purpuratus, undergo dynamic changes in response to immune challenge

The sea urchin, Strongylocentrotus purpuratus has seven described populations of distinct coelomocytes in the coelomic fluid that are defined by morphology, size, and for some types, by known functions. Of these subtypes, the large phagocytes are thought to be key to the sea urchin cellular innate immune response. The concentration of total coelomocytes in the coelomic fluid increases in response to pathogen challenge. However, there is no quantitative analysis of how the respective coelomocyte populations change over time in response to immune challenge. Accordingly, coelomocytes collected from immunoquiescent, healthy sea urchins were evaluated by flow cytometry for responses to injury and to challenge with either heat-killed Vibrio diazotrophicus, zymosan A, or artificial coelomic fluid, which served as the vehicle control. Responses to the initial injury of coelomic fluid collection or to injection of V. diazotrophicus show significant increases in the concentration of large phagocytes, small phagocytes, and red spherule cells after one day. Responses to zymosan A show decreases in the concentration of large phagocytes and increases in the concentration of small phagocytes. In contrast, responses to injections of vehicle result in decreased concentration of large phagocytes. When these changes in coelomocytes are evaluated based on proportions rather than concentration, the respective coelomocyte proportions are generally maintained in response to injection with V. diazotrophicus and vehicle. However, this is not observed in response to zymosan A and this lack of correspondence between proportions and concentrations may be an outcome of clearing these large particles by the large phagocytes. Variations in coelomocyte populations are also noted for individual sea urchins evaluated at different times for their responses to immune challenge compared to the vehicle. Together, these results demonstrate that the cell populations in sea urchin immune cell populations undergo dynamic changes in vivo in response to distinct immune stimuli and to injury and that these changes are driven by the responses of the large phagocyte populations.

A flow cytometry based approach to identify distinct coelomocyte subsets of the purple sea urchin, Strongylocentrotus purpuratus

The sea urchin, Strongylocentrotus purpuratus, possesses at least seven distinguishable cell populations in the coelomic fluid, which vary in morphology, size, and function. Of these, the large phagocytes, small phagocytes, and red spherule cells are thought to be key to the echinoid immune response. Because there are currently no effective and rapid means of evaluating sea urchin coelomocytes, we developed a flow cytometry based approach to identify these subsets from unseparated, unstained, live cells. In particular our gating strategy distinguishes between the large phagocytes, small phagocytes, red spherule cells, and a mixed population of vibratile cells and colorless spherule cells. This flow cytometry based analysis increases the speed and improves the reliability of coelomocyte analysis compared to differential cell counts by microscopy.

Echinochrome A Treatment Alleviates Atopic Dermatitis-like Skin Lesions in NC/Nga Mice via IL-4 and IL-13 Suppression

Atopic dermatitis (AD) is a chronic inflammatory skin disease in which skin barrier dysfunction leads to dryness, pruritus, and erythematous lesions. AD is triggered by immune imbalance and oxidative stress. Echinochrome A (Ech A), a natural pigment isolated from sea urchins, exerts antioxidant and beneficial effects in various inflammatory disease models. In the present study, we tested whether Ech A treatment alleviated AD-like skin lesions. We examined the anti-inflammatory effect of Ech A on 2,4-dinitrochlorobenzene (DNCB)-induced AD-like lesions in an NC/Nga mouse model. AD-like skin symptoms were induced by treatment with 1% DNCB for 1 week and 0.4% DNCB for 5 weeks in NC/Nga mice. The results showed that Ech A alleviated AD clinical symptoms, such as edema, erythema, and dryness. Treatment with Ech A induced the recovery of epidermis skin lesions as observed histologically. Tewameter^® and Corneometer^® measurements indicated that Ech A treatment reduced transepidermal water loss and improved stratum corneum hydration, respectively. Ech A treatment also inhibited inflammatory-response-induced mast cell infiltration in AD-like skin lesions and suppressed the expression of proinflammatory cytokines, such as interferon-γ, interleukin-4, and interleukin-13. Collectively, these results suggest that Ech A may be beneficial for treating AD owing to its anti-inflammatory effects.

Colorless spherule cells and lysozyme contribute to innate immunological responses in the sea urchin Lytechinus variegatus, exposed to bacterial challenge

The sea urchin Lytechinus variegatus is considered a good candidate for aquaculture, but bacterial diseases are a major challenge in culture conditions. The innate immunological defenses of L. variegatus to bacterial challenges were assessed through hematology parameters, in vitro phagocytosis, lysozyme activity and total plasma protein concentrations in cell-free coelomic fluid. Adult sea urchins were inoculated with Microccocus lysodeikticus, Escherichia coli and Vibrio parahaemolyticus in the cavity coelomic. Filtrated and sterile seawater (FSW) injected and non-injected sea urchins were used as control groups. Righting time, external aspects and behavior of sea urchins were evaluated. Twenty-four hours post-inoculation, we found an increase in the population of colorless spherule cells (CLS), phagocytosis, and humoral responses in sea urchins challenged by bacterial inoculations. Righting time was not affected by the treatments and apparent external signs of disease were not observed at least during 96h post-inoculation. The immunological system of L. variegatus quickly eliminated pathogenic microorganisms. CLS and lysozyme activity cooperate in the immune defenses of L. variegatus, showing an extraordinary efficiency for adjusting the immune defenses under stress caused by microbes. We recommend that the cellular and humoral markers serve as routine tests to monitor health status in sea urchins.

The Diverse Transformer (Trf) Protein Family in the Sea Urchin Paracentrotus lividus Acts through a Collaboration between Cellular and Humoral Immune Effector Arms

Sea urchins are long-living marine invertebrates with a complex innate immune system, which includes expanded families of immune receptors. A central immune gene family in sea urchins encodes the Transformer (Trf) proteins. The Trf family has been studied mainly in the purple sea urchin Strongylocentrotus purpuratus. Here, we explore this protein family in the Mediterranean Sea urchin Paracentrotus lividus. The PlTrf genes and predicted proteins are highly diverse and show a typical Trf size range and structure. Coelomocytes and cell-free coelomic fluid from P. lividus contain different PlTrf protein repertoires with a shared subset, that bind specifically to E. coli. Using FACS, we identified five different P. lividus coelomocyte sub-populations with cell surface PlTrf protein expression. The relative abundance of the PlTrf-positive cells increases sharply following immune challenge with E. coli, but not following challenge with LPS or the sea urchin pathogen, Vibrio penaeicida. Phagocytosis of E. coli by P. lividus phagocytes is mediated through the cell-free coelomic fluid and is inhibited by blocking PlTrf activity with anti-SpTrf antibodies. Together, our results suggest a collaboration between cellular and humoral PlTrf-mediated effector arms in the P. lividus specific immune response to pathogens.

How sea urchins face microplastics: Uptake, tissue distribution and immune system response

Plastic pollution represents one of the major threats to the marine environment. A wide range of marine organisms has been shown to ingest microplastics due to their small dimensions (less than 1 mm). This negatively affects some biological processes, such as feeding, energy reserves and reproduction. Very few studies have been performed on the effect of microplastics on sea urchin development and virtually none on adults. The aim of this work was to evaluate the uptake and distribution of fluorescent labelled polystyrene microbeads (micro-PS) in the Mediterranean sea urchin Paracentrotus lividus and the potential impact on circulating immune cells. Differential uptake was observed in the digestive and water vascular systems as well as in the gonads based on microbeads size (10 and 45 μm in diameter). Treatment of sea urchins with particles of both sizes induced an increase of the total number of immune cells already after 24 h. No significant differences were observed among immune cell types. However, the ratio between red and white amoebocytes, indicative of sea urchin healthy status, increased with both particles. This effect was detectable already at 24 h upon exposure to smaller micro-PS (10 μm). An increase of intracellular levels of reactive oxygen and nitrogen species was observed at 24 h upon both micro-PS exposure, whereas at later time these levels became comparable to those of controls. A significant increase of total antioxidant capacity was observed after treatment with 10 μm micro-PS. Overall data provide the first evidence on polystyrene microbeads uptake and tissue distribution in sea urchins, indicating a stress-related impact on circulating immune cells.

Individual Sea Urchin Coelomocytes Undergo Somatic Immune Gene Diversification

The adaptive immune response in jawed vertebrates is marked by the ability to diversify somatically specific immune receptor genes. Somatic recombination and hypermutation of gene segments are used to generate extensive repertoires of T and B cell receptors. In contrast, jawless vertebrates utilize a distinct diversification system based on copy choice to assemble their variable lymphocyte receptors. To date, very little evidence for somatic immune gene diversification has been reported in invertebrate species. Here we show that the SpTransformer (SpTrf ; formerly Sp185/333) immune effector gene family members from individual coelomocytes from purple sea urchins undergo somatic diversification by means of gene deletions, duplications, and acquisitions of single nucleotide polymorphisms. While sperm cells from an individual sea urchin have identical SpTrf gene repertoires, single cells from two distinct coelomocyte subpopulations from the same sea urchin exhibit significant variation in the SpTrf gene repertoires. Moreover, the highly diverse gene sequences derived from single coelomocytes are all in-frame, suggesting that an unknown mechanism(s) driving these somatic changes involve stringent selection or correction processes for expression of productive SpTrf transcripts. Together, our findings infer somatic immune gene diversification strategy in an invertebrate.

Methods for collection, handling, and analysis of sea urchin coelomocytes

Sea urchin coelomocytes can be collected in large numbers from adult sea urchins of the species, Strongylocentrotus purpuratus, which typically has 12-40mL of coelomic fluid. Coelomocytes are used for analysis of immune reactions and immune gene expression in addition to basic functions of cells, in particular for understanding structure and modifications of the cytoskeleton in phagocytes. The methods described here include coelomocyte isolation, blocking the clotting reaction, establishing and maintaining primary cultures, separation of different types of coelomocytes into fractions, processing live coelomocytes for light microscopy, fixation and staining for light and electron microscopy, analysis of coelomocyte populations by flow cytometry, and sorting single cells for more detailed follow-up analyses including transcriptomics or genomic characteristics. These methods are provided to make working with coelomocytes accessible to researchers who are unfamiliar with these cells and perhaps to aid others who have worked extensively with invertebrate cells.

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.

Echinochrome A Release by Red Spherule Cells Is an Iron-Withholding Strategy of Sea Urchin Innate Immunity

Cellular immune defences in sea urchins are shared amongst the coelomocytes - a heterogeneous population of cells residing in the coelomic fluid (blood equivalent) and tissues. The most iconic coelomocyte morphotype is the red spherule cell (or amebocyte), so named due to the abundance of cytoplasmic vesicles containing the naphthoquinone pigment echinochrome A. Despite their identification over a century ago, and evidence of antiseptic properties, little progress has been made in characterising the immunocompetence of these cells. Upon exposure of red spherule cells from sea urchins, i.e., Paracentrotus lividus and Psammechinus miliaris, to microbial ligands, intact microbes, and damage signals, we observed cellular degranulation and increased detection of cell-free echinochrome in the coelomic fluid ex vivo. Treatment of the cells with ionomycin, a calcium-specific ionophore, confirmed that an increase in intracellular levels of Ca2+ is a trigger of echinochrome release. Incubating Gram-positive/negative bacteria as well as yeast with lysates of red spherule cells led to significant reductions in colony-forming units. Such antimicrobial properties were counteracted by the addition of ferric iron (Fe3+), suggesting that echinochrome acts as a primitive iron chelator in echinoid biological defences.

The Roles of Spinochromes in Four Shallow Water Tropical Sea Urchins and Their Potential as Bioactive Pharmacological Agents

Spinochromes are principally known to be involved in sea urchin pigmentation as well as for their potentially interesting pharmacological properties. To assess their biological role in sea urchin physiology, experiments are undertaken on crude extracts from four species and on four isolated spinochromes in order to test their antibacterial, antioxidant, inflammatory and cytotoxic activities. First, the antibacterial assays show that the use of crude extracts as representatives of antibacterial effects of spinochromes are inaccurate. The assays on purified spinochromes showed a decrease in the growth of four strains with an intensity depending on the spinochromes/bacteria system, revealing the participation of spinochromes in the defense system against microorganisms. Secondly, in the 2,2-diphenyl-1-picrylhydrazyl antioxidant assays, spinochromes show an enhanced activity compared to the positive control. This latter observation suggests their involvement in ultraviolet radiation protection. Third, spinochromes present a pro-inflammatory effect on lipopolysaccharide-stimulated macrophages, highlighting their possible implication in the sea urchin immune system. Finally, cytotoxicity assays based on Trypan blue exclusion, performed in view of their possible future applications as drugs, show a weak cytotoxicity of these compounds against human cells. In conclusion, all results confirm the implication of spinochromes in sea urchin defense mechanisms against their external environment and reveal their potential for pharmacological and agronomical industries.

Extraordinary Diversity of Immune Response Proteins among Sea Urchins: Nickel-Isolated Sp185/333 Proteins Show Broad Variations in Size and Charge

Effective protection against pathogens requires the host to produce a wide range of immune effector proteins. The Sp185/333 gene family, which is expressed by the California purple sea urchin Strongylocentrotus purpuratus in response to bacterial infection, encodes a highly diverse repertoire of anti-pathogen proteins. A subset of these proteins can be isolated by affinity to metal ions based on multiple histidines, resulting in one to four bands of unique molecular weight on standard Western blots, which vary depending on the individual sea urchin. Two dimensional gel electrophoresis (2DE) of nickel-isolated protein samples followed by Western blot was employed to detect nickel-isolated Sp185/333 (Ni-Sp185/333) proteins and to evaluate protein diversity in animals before and after immune challenge with marine bacteria. Ni-Sp185/333 proteins of the same molecular weight on standard Western blots appear as a broad complex of variants that differ in pI on 2DE Western blots. The Ni-Sp185/333 protein repertoire is variable among animals, and shows a variety of changes among individual sea urchins in response to immune challenges with both the same and different species of bacteria. The extraordinary diversity of the Ni-Sp185/333 proteins may provide significant anti-pathogen capabilities for sea urchins that survive solely on innate immunity.

Single sea urchin phagocytes express messages of a single sequence from the diverse Sp185/333 gene family in response to bacterial challenge

Immune systems in animals rely on fast and efficient responses to a wide variety of pathogens. The Sp185/333 gene family in the purple sea urchin, Strongylocentrotus purpuratus, consists of an estimated 50 (±10) members per genome that share a basic gene structure but show high sequence diversity, primarily due to the mosaic appearance of short blocks of sequence called elements. The genes show significantly elevated expression in three subpopulations of phagocytes responding to marine bacteria. The encoded Sp185/333 proteins are highly diverse and have central effector functions in the immune system. In this study we report the Sp185/333 gene expression in single sea urchin phagocytes. Sea urchins challenged with heat-killed marine bacteria resulted in a typical increase in coelomocyte concentration within 24 h, which included an increased proportion of phagocytes expressing Sp185/333 proteins. Phagocyte fractions enriched from coelomocytes were used in limiting dilutions to obtain samples of single cells that were evaluated for Sp185/333 gene expression by nested RT-PCR. Amplicon sequences showed identical or nearly identical Sp185/333 amplicon sequences in single phagocytes with matches to six known Sp185/333 element patterns, including both common and rare element patterns. This suggested that single phagocytes show restricted expression from the Sp185/333 gene family and infers a diverse, flexible, and efficient response to pathogens. This type of expression pattern from a family of immune response genes in single cells has not been identified previously in other invertebrates.

Expression of antimicrobial peptides in coelomocytes and embryos of the green sea urchin (Strongylocentrotus droebachiensis)

Antimicrobial peptides (AMPs) play a crucial role in innate immunity. We have previously reported the isolation and characterization of the AMPs, strongylocins 1 and 2, and centrocin 1, from coelomocyte extracts of Strongylocentrotus droebachiensis. Here we show that these AMPs were expressed in phagocytes. In addition, transcripts of strongylocin 1 were detected in vibratile cells and/or colorless spherule cells, while transcripts of strongylocin 2 were found in red spherule cells. Results from immunoblotting and immunocytochemistry studies showed that centrocin 1 was produced by phagocytes and stored in granular vesicles. Co-localization of centrocin 1 and phagocytosed bacteria suggests that the granular vesicles containing centrocin 1 may be involved in the formation of phagolysosomes. We also analyzed the temporal and spatial expression of AMPs throughout larval development. Strongylocins were expressed in the early pluteus stage, while centrocin 1 was expressed in the mid pluteus stage. The spatial expression pattern showed that centrocin 1 was mainly located in blastocoelar cells (BCs) around the stomach and the esophagus. In addition, a few patrolling BCs were detected in some larval arms. Together, these results suggest that AMPs are expressed in different types of coelomocytes and that centrocin 1 is involved in response against bacteria. Furthermore, the expression of AMPs in larval pluteus stage, especially in BCs, indicates that AMPs and BCs are engaged in the larval immune system.

Shotgun proteomics of coelomic fluid from the purple sea urchin, Strongylocentrotus purpuratus

The purple sea urchin has a complex immune system that is likely mediated by gene expression in coelomocytes (blood cells). A broad array of potential immune receptors and immune response proteins has been deduced from their gene models. Here we use shotgun mass spectrometry to describe 307 proteins with possible immune function in sea urchins including proteins involved in the complement pathway and numerous SRCRs. The relative abundance of dual oxidase 1, ceruloplasmin, ferritin and transferrin suggests the production of reactive oxygen species in coelomocytes and the sequestration of iron. Proteins such as selectin, cadherin, talin, galectin, amassin and the Von Willebrand factor may be involved in generating a strong clotting reaction. Cell signaling proteins include a guanine nucleotide binding protein, the Rho GDP dissociation factor, calcium storage molecules and a variety of lipoproteins. However, based on this dataset, the expression of TLRs, NLRs and fibrinogen domain containing proteins in coelomic fluid and coelomocytes could not be verified.

Aggregation of sea urchin phagocytes is augmented in vitro by lipopolysaccharide

Development of protocols and media for culturing immune cells from marine invertebrates has not kept pace with advancements in mammalian immune cell culture, the latter having been driven by the need to understand the causes of and develop therapies for human and animal diseases. However, expansion of the aquaculture industry and the diseases that threaten these systems creates the need to develop cell and tissue culture methods for marine invertebrates. Such methods will enable us to better understand the causes of disease outbreaks and to develop means to avoid and remedy epidemics. We report a method for the short-term culture of phagocytes from the purple sea urchin, Strongylocentrotus purpuratus, by modifying an approach previously used to culture cells from another sea urchin species. The viability of cultured phagocytes from the purple sea urchin decreases from 91.6% to 57% over six days and phagocyte morphology changes from single cells to aggregates leading to the formation of syncytia-like structures. This process is accelerated in the presence of lipopolysaccharide suggesting that phagocytes are capable of detecting this molecular pattern in culture conditions. Sea urchin immune response proteins, called Sp185/333, are expressed on the surface of a subset of phagocytes and have been associated with syncytia-like structures. We evaluated their expression in cultured phagocytes to determine their possible role in cell aggregation and in the formation of syncytia-like structures. Between 0 and 3 hr, syncytia-like structures were observed in cultures when only ∼10% of the cells were positive for Sp185/333 proteins. At 24 hr, ∼90% of the nuclei were Sp185/333-positive when all of the phagocytes had aggregated into syncytia-like structures. Consequently, we conclude that the Sp185/333 proteins do not have a major role in initiating the aggregation of cultured phagocytes, however the Sp185/333 proteins are associated with the clustered nuclei within the syncytia-like structures.

The Sp185/333 immune response genes and proteins are expressed in cells dispersed within all major organs of the adult purple sea urchin

Purple sea urchins (Strongylocentrotus purpuratus) express a highly variable set of immune genes called Sp185/333 by two subtypes of coelomocytes: the polygonal and small phagocytes. We report that the Sp185/333 genes and their encoded proteins are also expressed in all of the major organs in the adult sea urchin, including the axial organ, pharynx, esophagus, intestine and gonads. After immune challenge, there is an increase in the level of Sp185/333 mRNA in cells associated with the intestine and axial organ. The Sp185/333 proteins increase in the axial organ, pharynx, esophagus and intestine after challenge. However, the proportion of Sp185/333-positive cells only increases in the axial organ, while there is no change in that proportion in the other organs after challenge. The size range of the major Sp185/333 proteins expressed by organs is broader (5 kDa to > 250 kDa) compared with those in coelomocytes (∼40 kDa to < 250 kDa). Images of the different organs do not clarify whether coelomocytes or parenchymal cells express the Sp185/333 proteins. The increase in levels of Sp185/333 transcripts, protein expression and Sp185/333-positive cells in the axial organ in response to challenge suggests that this organ may have an important role in immunity for this species.

Time course proteomic profiling of cellular responses to immunological challenge in the sea urchin, Heliocidaris erythrogramma

Genome sequences and high diversity cDNA arrays have provided a detailed molecular understanding of immune responses in a number of invertebrates, including sea urchins. However, complementary analyses have not been undertaken at the level of proteins. Here, we use shotgun proteomics to describe changes in the abundance of proteins from coelomocytes of sea urchins after immunological challenge and wounding. The relative abundance of 345 reproducibly identified proteins were measured 6, 24 and 48 h after injection. Significant changes in the relative abundance of 188 proteins were detected. These included pathogen-binding proteins, such as the complement component C3 and scavenger receptor cysteine rich proteins, as well as proteins responsible for cytoskeletal remodeling, endocytosis and intracellular signaling. An initial systemic reaction to wounding was followed by a more specific response to immunological challenge involving proteins such as apolipophorin, dual oxidase, fibrocystin L, aminopeptidase N and α-2-macroglobulin.

Cloning of profilin (FcPFN) from the shrimp Fenneropenaeus chinensis, a highly expressed protein in white spot syndrome virus (WSSV)-infected shrimp

We isolated and characterized the profilin (FcPFN) cDNA from hemocytes of Fenneropenaeus chinensis, a unique shrimp species from the Yellow Sea. The FcPFN cDNA consists of 830 bp and encodes a polypeptide of 125 amino acids, having a predicted isoelectric point of 5.06. The deduced amino acid sequence of FcPFN shows 36% and 90% amino acid sequence identity to the profilin genes of Pacific white shrimp Litopenaeus vannamei and black tiger shrimp Penaeus monodon, respectively. The FcPFN mRNA was highly expressed in hemocytes and hepatopancreas and moderately in muscle of normal shrimp. The higher expression of FcPFN mRNA is observed in shrimp infected with the white spot syndrome virus (WSSV), which is a major concern in all shrimp-growing regions of the world. These results suggest a potential role for FcPFN in viral host defense mechanisms.

Polysaccharides from Astragalus membranaceus promote phagocytosis and superoxide anion (O2-) production by coelomocytes from sea cucumber Apostichopus japonicus in vitro

The potential immunostimulatory effects of Astralagus membranaceus polysaccharides (APS) on sea cucumber, Apostichopus japonicus (Selenka), were investigated in vitro. Phagocytosis and superoxide anion (O(2)(-)) production by phagocytic amoebocytes (PA) from A. japonicus coelomic fluid were measured during incubation at 18 degrees C, 22 degrees C, or 25 degrees C with APS at 0, 10, 20, or 40 microg mL(-1) (n=3). Phagocytic activity against yeast cells was quantified by direct visualization, and O(2)(-) production by nitroblue tetrazolium (NBT) reduction assay. Compared with controls, including APS at 20 microg mL(-1) significantly increased (P<0.05) the percentage of phagocytic capacity (PC) and phagocytic index (PI) at 18 degrees C and 22 degrees C, but no significant enhancement was observed at 25 degrees C. In contrast, the coelmocytes of A. japonicus can have an obvious generation of O(2)(-) after the stimulation. The concentration of 20 microg mL(-1) APS resulted in a significant increase in nitroblue tetrazolium (NBT) positive cells (P<0.05) at different temperature and even 10 microg mL(-1) APS could increase O(2)(-) generation significantly at 18 degrees C and 22 degrees C. Both phagocytosing and O(2)(-) production increased with the increase of APS concentration from 0 to 20 microg mL(-1) at different temperature, and when APS at 40 microg mL(-1), they were decreased. It suggested that immunocytes activity in A. japonicus decreased with the temperature increasing from 18 degrees C to 25 degrees C, and APS could be an effective immunostimulant to enhance phagocytic activity and O(2)(-) production.

Extracellular heat shock protein 70 has novel functional effects on sea urchin eggs and coelomocytes

Numerous reports document that the 70 kDa heat shock proteins are not only intracellular proteins but are also present in blood and other extracellular compartments. How they affect cell function from the extracellular space remains unclear. Using two well-characterized cell types from the sea urchin, we show that extracellular mixtures of the constitutive and inducible forms of the 70 kDa heat shock proteins (Hsc70 and Hsp70, respectively) have dramatic effects on initiation of cell division in fertilized eggs and on the clotting reaction of hypotonically stressed coelomocytes. In suspensions of fertilized eggs to which Hsc70 or a 2:3 mixture of Hsc and Hsp70 was added, progression to the first mitotic division was accelerated. Evidence is provided that the extracellular Hsc70 passes into the egg cells in an unconventional manner, being distributed through the cytoplasm, and that it may alter the intracellular signaling cascade initiated by sperm penetration. In coelomocytes that were stimulated by hypotonic shock to mimic injury, the spreading reaction of the clotting response was significantly inhibited when either Hsp70 or Hsc70 was in the medium. These results suggest that the presence of Hsc and/or Hsp70 in the extracellular fluid may promote mitosis of dividing cells and suppress the reactivity of immune system cells.

Distinctive expression patterns of 185/333 genes in the purple sea urchin, Strongylocentrotus purpuratus: an unexpectedly diverse family of transcripts in response to LPS, beta-1,3-glucan, and dsRNA

Background

A diverse set of transcripts called 185/333 is strongly expressed in sea urchins responding to immune challenge. Optimal alignments of full-length 185/333 cDNAs requires the insertion of large gaps that define 25 blocks of sequence called elements. The presence or absence of individual elements also defines a specific element pattern for each message. Individual sea urchins were challenged with pathogen associated molecular patterns (PAMPs) (lipopolysaccharide, β-1,3-glucan, or double stranded RNA), and changes in the 185/333 message repertoire were followed over time.

Results

Each animal expressed a diverse set of 185/333 messages prior to challenge and a 0.96 kb message was the predominant size after challenge. Sequence analysis of the cloned messages indicated that the major element pattern expressed in immunoquiescent sea urchins was either C1 or E2.1. In contrast, most animals responding to lipopolysaccharide, β-1,3-glucan or injury, predominantly expressed messages of the E2 pattern. In addition to the major patterns, extensive element pattern diversity was observed among the different animals before and after challenge. Nucleotide sequence diversity of the transcripts increased in response to β-1,3-glucan, double stranded RNA and injury, whereas diversity decreased in response to LPS.

Conclusion

These results illustrate that sea urchins appear to be able to differentiate among different PAMPs by inducing the transcription of different sets of 185/333 genes. Furthermore, animals may share a suite of 185/333 genes that are expressed in response to common pathogens, while also maintaining a large number of unique genes within the population.

Generation of monoclonal antibodies to coelomocytes of the purple sea urchin Arbacia punctulata: characterization and phenotyping

Cellular cytotoxicity is a key component of animal innate immune responses that is one of the first lines of defense against invaders. There is increasing interest in the study of the cellular immune response, particularly non-specific cytotoxic cells and natural killer cells and their receptors. Studies of non-specific cytotoxic cell and natural killer cell recognition and killing (and the receptors involved) will reveal new and important insights into cellular mechanisms of host defense. Here we describe mAbs specific for coelomocyte sub-populations of the purple sea urchin, Arbacia punctulata, using highly purified coelomocyte populations as the antigen source. Monoclonal antibodies were selected using flow cytometric screening methods. Several of the mAbs were shown to bind to two sub-types of coelomocytes when assayed by fluorescence microscopy. Furthermore, these mAbs inhibited coelomocyte cytotoxicity against vertebrate target cells in a functional assay. The mAbs have been used in immunoprecipitation studies.

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.

Macroarray analysis of coelomocyte gene expression in response to LPS in the sea urchin. Identification of unexpected immune diversity in an invertebrate

The purple sea urchin, Strongylocentrotus purpuratus, is a member of the phylum Echinodermata, which is basal to the phylum Chordata within the deuterostome lineage of the animal kingdom. This relationship makes the analysis of the sea urchin immune system relevant to understanding the evolution of the deuterostome immune system leading to the Vertebrata. Subtractive suppression hybridization was employed to generate cDNA probes for screening high-density arrayed, conventional cDNA libraries to identify genes that were upregulated in coelomocytes responding to lipopolysaccharide. Results from 1,247 expressed sequence tags (ESTs) were used to infer that coelomocytes upregulated genes involved in RNA splicing, protein processing and targeting, secretion, endosomal activities, cell signaling, and alterations to the cytoskeletal architecture including interactions with the extracellular matrix. Of particular note was a set of transcripts represented by 60% of the ESTs analyzed, which encoded a previously uncharacterized family of closely related proteins, provisionally designated as 185/333. These transcripts exhibited a significant level of variation in their nucleotide sequence and evidence of putative alternative splicing that could yield up to 15 translatable elements. On the basis of the striking increase in gene expression in response to lipopolysaccharide and the unexpected level of diversity of the 185/333 messages, we propose that this set of transcripts encodes a family of putative immune response proteins that may represent a major component of an immunological response to bacterial challenge.

Insights into the anti-microbial defense of marine invertebrates: the penaeid shrimps and the oyster Crassostrea gigas

Research on innate immunity of the penaeid shrimps and the oyster Crassostrea gigas is motivated greatly by economical necessities. Indeed, the aquaculture of these organisms is now limited by the development of infectious diseases. Studying anti-microbial peptides/proteins (AMPs), which are effector molecules of the host defense, is particularly attractive not only for progressing basic knowledge on immunity but also because they offer various possible applications for disease management in aquaculture. AMPs are explored with a global approach,considering their structure, properties, function, gene expression, and tissue distribution during the response to infections. In shrimp, investigations of the penaeidins, which are constitutively expressed peptides, have highlighted the importance of hemocytes and hematopoiesis as major elements of the immune response, providing both local and systemic reactions. The activation of hematopoiesis must be regarded as a regulatory way for the expression and distribution of constitutively expressed immune effectors. As complementary approaches, genomics and gene profiling are promising to deepen our understanding of the anti-microbial defense of the oyster and the shrimp. However, real progress will depend also on the characterization of hemocyte lineages and hematopoiesis of these marine invertebrates as well as on the ontogenesis of their immune systems.

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.

A perforin-like protein from a marine mollusk

Abalone (gastropod mollusks) express a protein, abMpeg1, which is a homolog of two mammalian proteins that share homology with mammalian perforin, a cytolytic and immune-regulatory protein of lymphocytes. One of the mammalian proteins, Mpeg1, is expressed in mature macrophage and prion-infected mouse brains, while the other, Epcs50, is expressed in ectoplacental cone cells of the invading placenta. Although the functions of these three proteins remain unknown, their structural similarity to mammalian perforin suggests that they may be involved in cell killing, the inflammatory response or tissue invasion. Consistent with these proposed functions, the Mpeg1 gene family shows the signature of positive Darwinian selection (adaptive evolution). The perforin-homology domain of abMpeg1 contains the cytolytic "helix-turn-helix" domain of perforin, supporting the idea that abMpeg1 is a cytolytic protein of the abalone innate immune system. The alpha-helices of abMpeg1 are amphipathic as are those of perforin. The conservation among abMpeg1, mammalian Mpeg1, and Epcs50 shows that Mpeg1 proteins represent a novel, ancient protein family of probable immunological function.

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.

Lipopolysaccharides induce intestinal serum amyloid A expression in the sea cucumber Holothuria glaberrima

We have previously characterized the first invertebrate homolog of serum amyloid A (SAA) proteins in the sea cucumber Holothuria glaberrima, where its expression is associated with intestinal regeneration, suggesting a possible involvement of SAA proteins in intestinal morphogenesis. Here we show that bacterial lipopolysaccharides (LPS) trigger a coelomocyte-mediated immune response in H. glaberrima, inducing an approximately threefold increase in coelomocyte phagocytic activity. Furthermore, LPS induces an approximately fourfold increase in SAA mRNA levels in non-regenerating intestines. These results show that in H. glaberrima, LPS act as an immune activator and that SAA expression can be modulated by immune-associated processes.

Differentiation of immune cells challenged by bacteria in the common European starfish, Asterias rubens (Echinodermata)

Amoebocytes are the main effector cells of the echinoderm immune system. In starfishes, a taxon in which bacterial diseases have been rarely reported, amoebocytes are considered to be the only circulating and immune cell type. The present paper addresses the question of amoebocyte differentiation in the starfish Asterias rubens when challenged by bacteria. Starfishes were injected with FITC-coupled bacteria (Micrococcus luteus). Amoebocytes were collected at regular time intervals for 24 h. The cytometric characteristics and the phagocytic activity were studied by flow cytometry. Three amoebocyte groups of different size were identified. The cell concentrations of the two largest and more numerous of these groups (G2 and G3) were modulated by immune stimulation while the group of smallest, less numerous, cells (G1) was unaffected. All of these cell groups were phagocytic but their kinetics of cell activation and bacteria ingestion differed. G1 cells showed the lowest phagocytic activity while G3 cells had the highest and fastest phagocytic activity. Starfish amoebocytes appear to be segregated in three groups, two of them (G2 and G3) being immunomodulated and one of them presenting a very fast reaction to bacteria. It is suggested that the high efficiency of the immune system in starfishes is related to this fast reaction.

Phylogeny of natural cytotoxicity: cytotoxic activity of coelomocytes of the purple sea urchin, Arbacia punctulata

Coelomocyte-mediated nonspecific cell cytotoxic activity against human and murine target cells by the purple sea urchin Arbacia punctulata was investigated in vitro. Cytotoxic activity toward target cells was shown to be mediated by different coelomocyte populations isolated by discontinuous density gradient centrifugation. The population of phagocytic amebocytes showed the strongest cytotoxic activity and the highest binding to human NK markers by cytometry analysis. Our immunophenotypic studies showed that A. punctulata phagocytic amebocytes are CD14(+), CD56(+), CD158b(+), CD3(-), CD4(-), CD8(-), and CD16(-). The cytotoxic activity was independent of experimental incubation temperatures, required viable effector cells, and required cell-cell contact between the effector and target cells. Sodium azide significantly decreased coelomocyte cytotoxicity, indicating that cytotoxicity is metabolically dependent, and EDTA reduction of cytotoxic activity is consistent with the involvement of divalent cations in the cytotoxic process. These data describe a population of sea urchin coelomocytes (the phagocytic amebocyte) that are CD14(+), CD56(+), and CD158b(+), with cytotoxic activities.

Comparative Study of in Vivo and in Vitro Phagocytosis Including Germicidal Capacity in Odontaster validus (Koehler, 1906) at 0°C

The phagocytosis and germicidal capacity of Saccharomyces cerevisiae by phagocytic amoebocytes (PA) of the Antarctic starfish Odontaster validus were studied in vivo (after incubation periods of 1, 2, and 4 h) and in vitro (after incubation periods of 1, 2, 4, 8, and 12 h) at 0 degree C. The total number of PA and the phagocytic capacity (PC), phagocytic index (PI), and germicidal capacity (GC) of the PA were calculated. Results showed significant variability of the total PA number in different animals. There was a significant increase in PC and no significant differences in PI and GC for different in vitro incubation times. In vivo, experiments showed no significant difference of PC and PI, but there was a significant increase in GC as incubation periods increased. Comparison between in vitro and in vivo results revealed that PI and PC were significantly higher in vitro and that GC was significantly higher in vivo. The present study shows for the first time the phagocytosis and GC of an Antarctic invertebrate in vivo at low temperature (0 degree C), and the results are comparing with the available literature for echinoderms.

Dynamic expression of multiple scavenger receptor cysteine-rich genes in coelomocytes of the purple sea urchin

Coelomocytes, the heterogeneous population of sea urchin putative immune cells, were found to express a complex set of transcripts featuring scavenger receptor cysteine-rich (SRCR) repeats. SRCR domains define a metazoan superfamily of proteins, many of which are implicated in development and regulation of the immune system of vertebrates. Coelomocytes transcribe multiple SRCR genes from among a multigene family encoding an estimated number of 1,200 SRCR domains in specific patterns particular to each individual. Transcription levels for given SRCR genes may range from pronounced to undetectable, yet all tested animals harbor the genomic loci encoding these genes. Analysis of several SRCR genes revealed multiple loci corresponding to each type. In the case of one SRCR type, a cluster of at least three genes was detected within a 133-kb bacterial artificial chromosome insert, and conserved as well as unique regions were identified in sequences of three genomic clones derived from a single animal. Array hybridizations with repeated samples of coelomocyte messages revealed substantial alterations in levels of expression of many SRCR genes, with fluctuations of up to 10-fold in 1 week and up to 30-fold over a period of 3 months. This report is the first demonstration of genomic and transcriptional complexity in molecules expressed by invertebrate coelomocytes. The mechanisms controlling SRCR gene expression and the functional significance of this dynamic system await elucidation.

Insect cellular reactions to the lipopolysaccharide component of the bacterium Serratia marcescens are mediated by eicosanoids

Nodulation, which begins with the formation of cellular microaggregates, is the predominant cellular defense reaction to bacterial infections in insects. We suggested that these reactions to bacterial infections are mediated by eicosanoids. The lipopolysaccharide (LPS) component of some bacterial cells stimulates defense reactions in mammals and insects. Here, we report on experiments designed to test the hypothesis that eicosanoids mediate microaggregation reactions to LPS. Injections of LPS (purified from the bacterium, Serratia marcescens) into larvae of the tenebrionid beetle, Zophobas atratus, stimulated microaggregation reactions in a dose-dependent manner. Treatments with eicosanoid-biosynthesis inhibitors immediately prior to LPS challenge sharply reduced the microaggregation responses. Separate treatments with specific inhibitors of phospholipase A(2), cyclooxygenase and lipoxygenase reduced microaggregation, supporting our view that microaggregate formation involves lipoxygenase and cyclooxygenase products. The inhibitory influence of dexamethasone was apparent within 30min after injection, and microaggregation was significantly reduced, relative to control insects, over the following 90min. The dexamethasone effects were reversed by treating LPS-injected insects with the eicosanoid precursor, arachidonic acid. These findings indicate that cellular defense reactions to a specific component of bacterial cells are mediated by eicosanoids, and open up new possibilities for dissecting detailed hemocytic actions in insect immune reactions to bacterial infections.

Echinoderm immunity and the evolution of the complement system

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

Complement systems in invertebrates. The ancient alternative and lectin pathways

The complement system in higher vertebrates is composed of about thirty proteins that function in three activation cascades and converge in a single terminal pathway. It is believed that these cascades, as they function in the higher vertebrates, evolved from a few ancestral genes through a combination of gene duplications and divergences plus pathway duplication (perhaps as a result of genome duplication). Evidence of this evolutionary history is based on sequence analysis of complement components from animals in the vertebrate lineage. There are fewer components and reduced or absent pathways in lower vertebrates compared to mammals. Modern examples of the putatively ancestral complement system have been identified in sea urchins and tunicates, members of the echinoderm phylum and the protochordate subphylum, which are sister groups to the vertebrates. Thus far, this simpler system is composed of homologues of C3, factor B, and mannose binding protein associated serine protease suggesting the presence of simpler alternative and lectin pathways. Additional components are predicted to be present. A complete analysis of this invertebrate defense system, which evolved before the invention of rearranging genes, will provide keys to the primitive beginnings of innate immunity in the deuterostome lineage of animals.

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

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

Sea Urchin Coelomocytes Specifically Express a Homologue of the Complement Component C3

A homologue of complement component C3 (SpC3) has been cloned and sequenced from the purple sea urchin, Strongylocentrotus purpuratus. The preprocessed, deduced protein size is estimated to be 186 kDa with a short leader and two chains, α and β. There are cysteines in conserved positions for interchain disulfide bonding, and there is a conserved thioester site in the α-chain with an associated histidine. There are five consensus N-linked glycosylation sites, and putative cleavage sites for factor I and C3 convertase. Partially purified SpC3 on protein gels shows a nonreduced size of 210 kDa and, under reducing conditions, reveals an α-chain of 130 kDa and a β-chain of 80 kDa. These sizes are larger than the deduced sizes, suggesting that the protein has carbohydrates added to most of the consensus N-linked glycosylation sites. Phylogenetic analysis of SpC3 compared with other members of the thioester protein family, which includes C3, C4, C5, and α2-macroglobulin, shows that SpC3 is the first divergent complement protein, falling at the base of the complement protein clade. Transcripts from the SpC3 gene (Sp064) are 9 kb, and the gene is expressed specifically in coelomocytes, which are the immunocytes in the sea urchin. Genome blots suggest that SpC3 is encoded by a single copy gene per haploid genome. This is the first identification of a complement component in an invertebrate, and suggests homology of the innate immune system within the deuterostome lineage of animals.