Allogeneic inhibition in a compound ascidian, Botryllus primigenus Oka. II. cellular and humoral responses in "nonfusion"reaction

Marine invertebrates cross phyla comparisons reveal highly conserved immune machinery

Naturally occurring histocompatibility responses, following tissue-to-tissue allogeneic contacts, are common among numerous colonial marine invertebrate taxa, including sponges, cnidarians, bryozoans and ascidians. These responses, often culminating in either tissue fusions or rejections, activate a wide array of innate immune components. By comparing two allorejection EST libraries, developed from alloincompatible challenged colonies of the stony coral Stylophora pistillata and the ascidian Botryllus schlosseri, we revealed a common basis for innate immunity in these two evolutionary distant species. Two prominent genes within this common basis were the immunophilins, Cyclophilin A (CypA) and FK506-binding protein (FKBP). In situ hybridizations revealed that mRNA expression of the coral and ascidian immunophilins was restricted to specific allorecognition effector cell populations (nematoblasts and nematocytes in the coral and morula cells in the ascidian). The expressions were limited to only some of the effector cells within a population, disclosing disparities in numbers and location between naïve colonies and their immune challenged counterparts. Administration of the immunosuppression drug Cyclosporine-A during ascidian's allogeneic assays inhibited both fusion and rejection reactions, probably through the inhibition of ascidian's immunocytes (morula cells) movement and activation. Our results, together with previous published data, depict an immunophilins-based immune mechanism, which is similarly activated in allogeneic responses of distantly related animals from sponges to humans.

Model systems of invertebrate allorecognition

Nearly all colonial marine invertebrates are capable of allorecognition--the ability to distinguish between self and genetically distinct members of the same species. When two or more colonies grow into contact, they either reject each other and compete for the contested space or fuse and form a single, chimeric colony. The specificity of this response is conferred by genetic systems that restrict fusion to self and close kin. Two selective pressures, intraspecific spatial competition between whole colonies and competition between stem cells for access to the germline in fused chimeras, are thought to drive the evolution of extensive polymorphism at invertebrate allorecognition loci. After decades of study, genes controlling allorecognition have been identified in two model systems, the protochordate Botryllus schlosseri and the cnidarian Hydractinia symbiolongicarpus. In both species, allorecognition specificity is determined by highly polymorphic cell-surface molecules, encoded by the fuhc and fester genes in Botryllus, and by the alr1 and alr2 genes in Hydractinia. Here we review allorecognition phenomena in both systems, summarizing recent molecular advances, comparing and contrasting the life history traits that shape the evolution of these distinct allorecognition systems, and highlighting questions that remain open in the field.

'Rejected' vs. 'rejecting' transcriptomes in allogeneic challenged colonial urochordates

In botryllid ascidians, allogeneic contacts between histoincompatible colonies lead to inflammatory rejection responses, which eventually separate the interacting colonies. In order to elucidate the molecular background of allogeneic rejection in the colonial ascidian Botryllus schlosseri, we performed microarray assays verified by qPCR, and employed bioinformatic analyses of the results, revealing disparate transcription profiles of the rejecting partners. While only minor expression changes were documented during rejection when both interacting genotypes were pooled together, analyses performed on each genotype separately portrayed disparate transcriptome responses. Allogeneic interacting genotypes that developed the morphological markers of rejection (points of rejection; PORs), termed 'rejected' genotypes, showed transcription inhibition of key functional gene groups, including protein biosynthesis, cell structure and motility and stress response genes. In contrast, the allogeneic partners that did not show PORs, termed 'rejecting' genotypes, showed minor expression changes that were different from those of the 'rejected' genotypes. This data demonstrates that the observed morphological changes in the 'rejected' genotypes are not due to active transcriptional response to the immune challenge but reflect transcription inhibition of response elements. Based on the morphological and molecular outcomes we suggest that the 'rejected' colony activates an injurious self-destructive mechanism in order to disconnect itself from its histoincompatible neighboring colony.

Sea squirts and immune tolerance

Transplantation specificity and protective immunity occur in both adaptive and innate branches of the vertebrate immune system. Understanding the mechanisms that underlie specificity and self-tolerance of immune function has major significance, from preventing a rejection reaction after transplantation to dissecting the causes of autoimmune disease. The core of vertebrate immunity is the ability to discriminate between highly polymorphic ligands, and this process is also found in allorecognition systems throughout the metazoa. Botryllus schlosseri is a tunicate, the modern-day descendents of the phylum that made the transition between invertebrates and vertebrates. In addition, B. schlosseri undergoes a natural transplantation reaction, which is controlled by a single, highly polymorphic locus called fuhc, reminiscent of major histocompatibility complex (MHC)-based allorecognition. The life-history characteristics of Botryllus make it an excellent model to dissect the functional and developmental mechanisms underlying allorecognition, and have the potential to reveal novel insights into issues from innate recognition strategies to the evolution of genetic polymorphism. In addition, we hypothesize that allorecognition in Botryllus must be based on conserved processes that are fundamental to all immune function: education and tolerance, or the ontogeny and maintenance of specificity.

Urochordate histoincompatible interactions activate vertebrate-like coagulation system components

The colonial ascidian Botryllus schlosseri expresses a unique allorecognition system. When two histoincompatible Botryllus colonies come into direct contact, they develop an inflammatory-like rejection response. A surprising high number of vertebrates' coagulation genes and coagulation-related domains were disclosed in a cDNA library of differentially expressed sequence tags (ESTs), prepared for this allorejection process. Serine proteases, especially from the trypsin family, were highly represented among Botryllus library ortholgues and its “molecular function” gene ontology analysis. These, together with the built-up clot-like lesions in the interaction area, led us to further test whether a vertebrate-like clotting system participates in Botryllus innate immunity. Three morphologically distinct clot types (points of rejection; POR) were followed. We demonstrated the specific expression of nine coagulation orthologue transcripts in Botryllus rejection processes and effects of the anti-coagulant heparin on POR formation and heartbeats. In situ hybridization of fibrinogen and von Willebrand factor orthologues elucidated enhanced expression patterns specific to histoincompatible reactions as well as common expressions not augmented by innate immunity. Immunohistochemistry for fibrinogen revealed, in naïve and immune challenged colonies alike, specific antibody binding to a small population of Botryllus compartment cells. Altogether, molecular, physiological and morphological outcomes suggest the involvement of vertebrates-like coagulation elements in urochordate immunity, not assigned with vasculature injury.

fester, A candidate allorecognition receptor from a primitive chordate

Histocompatibility in the primitive chordate, Botryllus schlosseri, is controlled by a single, highly polymorphic locus, the FuHC. By taking a forward genetic approach, we have identified a locus encoded near the FuHC, called fester, which is polymorphic, polygenic, and inherited in distinct haplotypes. Somatic diversification occurs through extensive alternative splicing, with each individual expressing a unique repertoire of splice forms, both membrane bound and potentially secreted, all expressed in tissues intimately associated with histocompatibility. Functional studies, via both siRNA-mediated knockdown and direct blocking by monoclonal antibodies raised against fester, were able to disrupt predicted histocompatibility outcomes. The genetic and somatic diversity, coupled to the expression and functional data, suggests that fester is a receptor involved in histocompatibility.

Rejection patterns in botryllid ascidian immunity: the first tier of allorecognition

Botryllid ascidians, a small but geographically widely distributed group of compound tunicates, are being used as a model system for the study of allorecognition. Botryllid ascidians possess a unique type of immunity. Pairs of colonies that meet through their extending ampullae either fuse to form a chimera or develop cytotoxic lesions at contact zones (rejection). This first tier of allorecognition is succeeded (in cases of fusion) by two additional tiers, not reviewed here (the colony resorption phenomenon and the phenomenon of somatic and germ cell parasitism). Fusion and rejection are controlled by a single highly polymorphic gene locus termed the fusibility/histocompatibility (Fu/HC) locus. One shared allele on the Fu/HC locus is enough for fusion. Rejecting colonies do not share any Fu/HC alleles. To date, 14 botryllid ascidians have been studied for their fusibility patterns; of these, the cosmopolitan species Botryllus schlosseri (Pallas, 1766) has emerged as the most studied taxon. This review summarizes studies revealing the various types of noncompatible responses that are expressed following the application of the "colony allorecognition assay" and the "cut surface assay". These include divergent alloresponses related to different populations of the same botryllid species, distinctive allorecognition sites, polymorphism and a repertoire of Fu/HC alleles, a state of low responsiveness as opposed to the expected immunological memory, the retreat growth phenomenon, and the irreversible nature of the rejection process. A detailed description of the accumulated knowledge on the effector cells (morula cells and macrophages), the humoral and cellular molecules (at the biochemical and molecular levels), and the prophenoloxidase system is given. Links between allogeneic responses and the evolutionary ecology of botryllid ascidians are revealed. Since tunicates occupy a key phylogenetic position in the origin of the vertebrates, the study of colony allorecognition in this group may shed light on self/nonself recognition elements in other multicellular organisms, including vertebrates.

Behavior of Hemocytes in the Allorejection Reaction in Two Compound Ascidians, Botryllus scalaris and Symplegma reptans

In botryllid ascidians, the type of allorejection reaction differs among species. Comparative studies of these different reactions contribute to our understanding of the allorecognition and nonself-rejection system. We studied the morphology of hemocyte behavior during allorejection reactions in two species, Botryllus scalaris and Symplegma reptans, which stand at important points in botryllid phylogeny. In B. scalaris, phagocytes mediated hemocyte aggregation, resulting in interruption of blood flow just after vascular fusion of incompatible colonies. Although previous studies indicate that morula cells (MCs) play a central role in the rejection reaction, the MCs of B. scalaris did not participate in the rejection reaction. Colonies of S. reptans showed two types of allorejection reaction that started at different points in the process of vascular fusion between two colonies. In both types of rejection reaction, the MCs played a central role and behaved similarly to those of all botryllids except B. scalaris and a botryllid from Israel. These observations suggest that the differences in hemocyte behavior and allorecognition site observed in this study reflect the variation in allorejection reactions among botryllids.

Phenoloxidase and cytotoxicity in the compound ascidian Botryllus schlosseri

The vacuoles of morula cells (MC) of the colonial ascidian Botryllus schlosseri contain phenoloxidase (PO). As the release of their vacuolar content at the border of incompatible contacting colonies is associated with the formation of necrotic masses which characterize the rejection reaction, the role of PO in Botryllus cytotoxicity was investigated. When hemocytes are incubated with blood plasma from incompatible (heterologous) colonies, MC degranulate and, after 60 min, the cytotoxicity index becomes significantly greater than that observed in controls incubated with autologous plasma. The rise in cell mortality is completely inhibited by the addition of PO inhibitors sodium benzoate, tropolone and phenylthiourea, and serine protease inhibitors phenylmethylsulfonyl fluoride, benzamidine, N-tosyl-L-phenylalanine chloromethyl ketone and N-tosyl-L-lysine chloromethyl ketone. The addition of either reducing agents L-cysteine and ascorbic acid or reactive oxygen species scavenger enzymes superoxide dismutase and catalase has a similar effect. Significant inhibition of cytotoxicity is also observed with the quinone scavenger, 3-methyl-2-benzothiazolinone hydrazone. In the presence of sodium benzoate and phenylthiourea, there is a significant reduction in the number, size and color intensity of necrotic masses along the contact border of incompatible colonies. A significant increase in superoxide anion production, completely inhibited by sodium benzoate, is observed when hemocytes are incubated with heterologous blood plasma. These results indicate that: (i) PO is the enzyme responsible for the cytotoxicity observed in both hemocyte cultures and rejection reactions; (ii) PO is present inside MC vacuoles as a proenzyme which is activated, upon release, by humoral proteases; (iii) cytotoxicity appears to be mainly due to oxidative stress generated by PO during oxidation of polyphenols to quinones without the involvement of other oxidases such as NADPH oxidase and peroxidase.

Purification and characterization of a 58,000-Da proteinase inhibitor from the hemolymph of a solitary ascidian, Halocynthia roretzi

A new endogenous proteinase inhibitor from the cell-free hemolymph of a solitary ascidian, Halocynthia roretzi, was purified by a combination of ammonium sulfate fractionation, hydrophobic interaction chromatography on Ether-Toyopearl and affinity chromatography on Heparin-Sepharose. The purity of the inhibitor was examined by SDS-PAGE, gel-permeation chromatography, reversed-phase chromatography, isoelectric focusing, immunological analysis and amino-terminal amino acid sequence analysis. The inhibitor is a single polypeptide chain whose molecular weight, isoelectric point and the first 10 amino-terminal amino acid sequences are 58 kDa, pI 9.2 and NH2-Thr-Lys-Lys-Asp-Gly-Glu-Glu-Lys-Val-Ala, respectively. The purified protein inhibits plasma enzyme(s) of H. roretzi, and the rate of inhibition to the plasma enzyme(s) activity was accelerated by incubation with dextran sulfate, but the effect was neutralized by further incubation with polycation, such as polybrene or protamine sulfate. The inhibitory activity was not affected appreciably by pH 7-10 but ceased completely below pH 5 or by heating at 50 degrees C for 30 min.

Incidents of rejection and indifference in Fu/HC incompatible protochordate colonies

We test here for the existence of specific alloimmune memory in the rejection responses of the colonial tunicate Botryllus schlosseri. Colony specificity in these organisms is controlled by the Fu/HC locus. Rejection occurs only between colonies that do not share any allelic determinant at this locus. Two sets of experiments were conducted: 1) Forty-nine pairs of nonfusible oozooids were interacted naturally along a period of several months. They rejected, disconnected, and in 20% of the cases, interacted again. 2) Repeated colony allorecognition assays were done on 15 pairs of interacting subclones (up to 5 consecutive tests/pair). Major results indicate: 1) Not all ampulla-ampulla interactions resulted in necrotic areas, points of rejection (PORs). 2) A full repertoire of PORs was attained within the first 10 days. Thereafter, no more PORs were added. 3) The outcome of indifference (cases where ampulla-ampulla contacts did not result in rejection) was repeatedly recorded in multiple tests, and its frequency increased in the secondary and tertiary tests along a set of 5 consecutive tests. It is concluded that allospecific memory, as measured by an accelerated production of PORs and amplification in their number, was not characteristic of the Botryllus rejection phenomenon, which, however, reveals the qualities of a low responder. These results are discussed in the light of some aspects of tolerance in the mammalian system.

Studies on colony specificity in the compound ascidian, Botryllus primigenus Oka. II. In vivo bioassay for analyzing the mechanism of "nonfusion" reaction

Fusion experiments with three colonies (AC-BC-BD) by Tanaka and Watanabe indicated that the mixture of blood from two nonfusible colonies induced a "nonfusion" reaction (NFR)-like responses. Thus, artificial mixture of the blood from two nonfusible colonies is expected to result in NFR-like reactions. The present study deals with an in vivo bioassay using syngeneic and allogeneic blood injections. Syngeneic injection of whole blood induced no response, while allogeneic injection of whole blood induced inflammatory responses indicating recognition of allogeneic tissue. Syngeneic injection of blood plasma or blood cells induced no response. On the other hand, allogeneic injection of blood cells resulted in a response similar to that of whole blood injections. Allogeneic blood plasma induced only weak responses. It is hypothesized that NFR is initiated by the mutual interaction between blood cells and blood humoral factor(s).

Studies on colony specificity in the compound ascidian, Botryllus primigenus Oka. I. Initiation of "nonfusion" reaction with special reference to blood cells infiltration

When two incompatible colonies of the compound ascidian, Botryllus primigenus Oka come into contact, a specific rejection, called "nonfusion" reaction (NFR), always occurs in the contact area between them. At the beginning of the NFR, concentrations of cells were always observed near the ampullae tips. In the present study, it was revealed that these cells appear to be blood cells infiltrating through the ampullae and were identified as morula cells (vanadocytes). When India ink was injected into vessels prior to the occurrence of NFR, it leaked from the tips of the ampullae into the test matrix, suggesting an increase in the permeability of the ampullar epidermis. Experimental studies to elucidate the mechanism of NFR were carried out. Contact between a colony piece and a zooid-free ampullae fragment or overlapping of one colony to another, respectively, resulted in NFR only at one side of a pair. Furthermore, in stage 3 of NFR, when an opaque area was pushed with a pair of forceps and the blood in this area was transferred to healthy part of vessels, the stoppage of blood flow and subsequent contraction of vessels always took place in the latter. These observations suggest that ampullae penetration into the opposite colony is essential to NFR and that NFR is directly initiated in the ampullar lumen.