Novel immunoglobulin-like transcripts in teleost fish encode polymorphic receptors with cytoplasmic ITAM or ITIM and a new structural Ig domain similar to the natural cytotoxicity receptor NKp44

Who is who within the universe of TREM-like transcripts (TREML)?

The Triggering Receptor Expressed on Myeloid Cells (TREM) family of receptors plays a crucial role in the immune response across various species. Particularly, TREM-1 and TREM-2 have been extensively studied, both in terms of their applications and their expression sites and signaling pathways. However, the same is not observed for the other family members collectively known as TREM-like-transcripts (TREML). The TREML family consists of eight receptors, with TREML1-5 identified in humans and mice, TREML-6 exclusive found in mice, TREML-7 in dogs and horses, and TREML-8 in rabbits and opossums. Despite the limited data available on the TREML members, they have been implicated in different immune and non-immune activities, which have been proposed to display both pro and anti-inflammatory activities, and to influence fundamental biological processes such as coagulation, bone and neurological development. In this review, we have compiled available information regarding the already discovered members of the family and provided foundational framework for understanding the function, localization, and therapeutic potential of all TREML members. Additionally, we hope that this review may shed light on this family of receptors, whose underlying mechanisms are still awaiting elucidation, while emphasizing the need for future studies to explore their functions and potential therapeutic application.

Costimulatory receptors in the channel catfish: CD28 family members and their ligands

The CD28-B7 interaction is required to deliver a second signal necessary for T-cell activation. Additional membrane receptors of the CD28 and B7 families are also involved in immune checkpoints that positively or negatively regulate leukocyte activation, in particular T lymphocytes. BTLA is an inhibitory receptor that belongs to a third receptor family. Fish orthologs exist only for some of these genes, and the potential interactions between the corresponding ligands remain mostly unclear. In this work, we focused on the channel catfish (Ictalurus punctatus), a long-standing model for fish immunology, to analyze these co-stimulatory and co-inhibitory receptors. We identified one copy of cd28, ctla4, cd80/86, b7h1/dc, b7h3, b7h4, b7h5, two btla, and four b7h7 genes. Catfish CD28 contains the highly conserved mammalian cytoplasmic motif for PI3K and GRB2 recruitment, however this motif is absent in cyprinids. Fish CTLA4 share a C-terminal putative GRB2-binding site but lacks the mammalian PI3K/GRB2-binding motif. While critical V-domain residues for human CD80 or CD86 binding to CD28/CTLA4 show low conservation in fish CD80/86, C-domain residues are highly conserved, underscoring their significance. Catfish B7H1/DC had a long intracytoplasmic domain with a P-loop-NTPase domain that is absent in mammalian sequences, while the lack of NLS motif in fish B7H4 suggests this protein may not regulate cell growth when expressed intracellularly. Finally, there is a notable expansion of fish B7H7s, which likely play diverse roles in leukocyte regulation. Overall, our work contributes to a better understanding of fish leukocyte co-stimulatory and co-inhibitory receptors.

The biology of TREM receptors

Triggering receptors expressed on myeloid cells (TREMs) encompass a family of cell-surface receptors chiefly expressed by granulocytes, monocytes and tissue macrophages. These receptors have been implicated in inflammation, neurodegenerative diseases, bone remodelling, metabolic syndrome, atherosclerosis and cancer. Here, I review the structure, ligands, signalling modes and functions of TREMs in humans and mice and discuss the challenges that remain in understanding TREM biology.

A highly diverse set of novel immunoglobulin-like transcript (NILT) genes in zebrafish indicates a wide range of functions with complex relationships to mammalian receptors

Multiple novel immunoglobulin-like transcripts (NILTs) have been identified from salmon, trout, and carp. NILTs typically encode activating or inhibitory transmembrane receptors with extracellular immunoglobulin (Ig) domains. Although predicted to provide immune recognition in ray-finned fish, we currently lack a definitive framework of NILT diversity, thereby limiting our predictions for their evolutionary origin and function. In order to better understand the diversity of NILTs and their possible roles in immune function, we identified five NILT loci in the Atlantic salmon (Salmo salar) genome, defined 86 NILT Ig domains within a 3-Mbp region of zebrafish (Danio rerio) chromosome 1, and described 41 NILT Ig domains as part of an alternative haplotype for this same genomic region. We then identified transcripts encoded by 43 different NILT genes which reflect an unprecedented diversity of Ig domain sequences and combinations for a family of non-recombining receptors within a single species. Zebrafish NILTs include a sole putative activating receptor but extensive inhibitory and secreted forms as well as membrane-bound forms with no known signaling motifs. These results reveal a higher level of genetic complexity, interindividual variation, and sequence diversity for NILTs than previously described, suggesting that this gene family likely plays multiple roles in host immunity.

A subset of leukocyte immune-type receptors (LITRs) regulates phagocytosis in channel catfish (Ictalurus punctatus) leukocytes

Channel catfish, Ictalurus punctatus, leukocyte immune-type receptors (LITRs) constitute a large family of paired, immunoregulatory receptors unique to teleosts. A role for LITRs in phagocytosis has been proposed based on studies in mammalian cell lines; however, LITR-mediated phagocytosis has not been examined in the catfish model. In this study, we use two anti-LITR monoclonal antibodies, CC41 and 125.2, to contrast the effects of crosslinking subsets of inhibitory and activating LITRs. Briefly, LITRs expressed by catfish γδ T cells, αβ T cells, and macrophage cell lines were crosslinked using mAb-conjugated fluorescent microbeads, and bead uptake was evaluated by flow cytometry and confirmed by confocal microscopy. A clear difference in the uptake of 125.2- and CC41-conjugated beads was observed. Crosslinking LITRs with mAb 125.2 resulted in efficient bead internalization, while mAb CC41 crosslinking of inhibitory LITRs resulted predominantly in a capturing phenotype. Pretreating catfish macrophages with mAb CC41 resulted in a marked decrease in LITR-mediated phagocytosis of 125.2-conjugated beads. Overall, these findings provide insight into fish immunobiology and validate LITRs as regulators of phagocytosis in catfish macrophages and γδ T cells.

Identification of the Fc-alpha/mu receptor in Xenopus provides insight into the emergence of the poly-Ig receptor (pIgR) and mucosal Ig transport

The polyimmunoglobulin receptor (pIgR) transcytoses J chain-containing antibodies through mucosal epithelia. In mammals, two cis-duplicates of PIGR, FCMR, and FCAMR, flank the PIGR gene. A PIGR duplication is first found in amphibians, previously annotated as PIGR2 (herein xlFCAMR), and is expressed by APCs. We demonstrate that xlFcamR is the equivalent of mammalian FcamR. It has been assumed that pIgR is the oldest member of this family, yet our data could not distinguish whether PIGR or FCAMR emerged first; however, FCMR was the last family member to emerge. Interestingly, bony fish "pIgR" is not an orthologue of tetrapod pIgR, and possibly acquired its function via convergent evolution. PIGR/FCAMR/FCMR are members of a larger superfamily, including TREM, CD300, and NKp44, which we name the "double-disulfide Ig superfamily" (ddIgSF). Domains related to each ddIgSF family were identified in cartilaginous fish (sharks, chimeras) and encoded in a single gene cluster syntenic to the human pIgR locus. Thus, the ddIgSF families date back to the earliest antibody-based adaptive immunity, but apparently not before. Finally, our data strongly suggest that the J chain arose in evolution only for Ig multimerization. This study provides a framework for further studies of pIgR and the ddIgSF in vertebrates.

Analysis of shark NCR3 family genes reveals primordial features of vertebrate NKp30

Natural killer (NK) cells play major roles in innate immunity against viruses and cancer. Natural killer receptors (NKR) expressed by NK cells recognize foreign- or self-ligands on infected and transformed cells as well as healthy cells. NKR genes are the most rapidly evolving loci in vertebrates, and it is generally difficult to detect orthologues in different taxa. The unique exception is NKp30, an activating NKR in mammals that binds to the self-ligand B7H6. The NKp30-encoding gene, NCR3, has been found in most vertebrates including sharks, the oldest vertebrates with human-type adaptive immunity. NCR3 has a special, non-rearranging VJ-type immunoglobulin superfamily (IgSF) domain that predates the emergence of the rearranging antigen receptors. Herein we show that NCR3 loci are linked to the shark major histocompatibility complex (MHC), proving NCR3's primordial association with the MHC. We identified eight subtypes of differentially expressed highly divergent shark NCR3 family genes. Using in situ hybridization, we detected one subtype, NS344823, to be expressed by predominantly single cells outside of splenic B cell zones. The expression by non-B cells was also confirmed by PCR in peripheral blood lymphocytes. Surprisingly, high expression of NS344823 was detected in the thymic cortex, demonstrating NS344823 expression in developing T cells. Finally, we show for the first time that shark T cells are found as single cells or in small clusters in the splenic red pulp, also unassociated with the large B cell follicles we previously identified.

Differential expression and functional diversification of diverse immunoglobulin domain-containing protein (DICP) family in three gynogenetic clones of gibel carp

Diverse immunoglobulin (Ig) domain-containing protein (DICP) family is a novel bony fish-specific multi-gene family encoding diversified immune receptors. However, their function and the implication of binding partners remain unknown. In this study, we first identified 28 DICPs from three gibel carp gynogenetic clones and revealed their high variability and clone-specific feature. After crucian carp herpesvirus (CaHV) infection, these DICPs were significantly upregulated in head kidney, kidney and spleen. The up-regulation folds in clone A⁺, F and H were related to the susceptibility to CaHV, progressively increasing from resistant clone to susceptible clone. Overexpression of gibel carp DICPs inhibited interferon (IFN) and viperin promoter-driven luciferase activity. The additions of E. coli extracts and lipid A significantly enhanced the inhibition effect. In addition, gibel carp DICPs can interact with SHP-1 and SHP-2. These findings suggest that gible carp DICPs, as inhibitory receptors, might specifically recognize lipid A, and then interact with SHP-1 and SHP-2 to inhibit the induction of IFN and ISGs.

Spotted Gar and the Evolution of Innate Immune Receptors

The resolution of the gar genome affords an opportunity to examine the diversification and functional specialization of immune effector molecules at a distant and potentially informative point in phylogenetic development. Although innate immunity is effected by a particularly large number of different families of molecules, the focus here is to provide detailed characterization of several families of innate receptors that are encoded in large multigene families, for which orthologous forms can be identified in other species of bony fish but not in other vertebrate groups as well as those for which orthologs are present in other vertebrate species. The results indicate that although teleost fish and the gar, as a holostean reference species, share gene families thought previously to be restricted to the teleost fish, the manner in which the members of the multigene families of innate immune receptors have undergone diversification is different in these two major phylogenetic radiations. It appears that both the total genome duplication and different patterns of genetic selection have influenced the derivation and stabilization of innate immune genes in a substantial manner during the course of vertebrate evolution.

Evolution of the angiopoietin-like gene family in teleosts and their role in skin regeneration

BACKGROUND

The skin in vertebrates is a protective barrier and damage is rapidly repaired to re-establish barrier function and maintain internal homeostasis. The angiopoietin-like (ANGPTL) proteins are a family of eight secreted glycoproteins with an important role in skin repair and angiogenesis in humans. In other vertebrates their existence and role in skin remains largely unstudied. The present study characterizes for the first time the homologues of human ANGPTLs in fish and identifies the candidates that share a conserved role in skin repair using a regenerating teleost skin model over a 4-day healing period.

RESULTS

Homologues of human ANGPTL1-7 were identified in fish, although ANGPTL8 was absent and a totally new family member designated angptl9 was identified in fish and other non-mammalian vertebrates. In the teleost fishes a gene family expansion occurred but all the deduced Angptl proteins retained conserved sequence and structure motifs with the human homologues. In sea bream skin angptl1b, angptl2b, angptl4a, angptl4b and angptl7 transcripts were successfully amplified and they were differentially expressed during skin regeneration. In the first 2 days of skin regeneration, re-establishment of the physical barrier and an increase in the number of blood vessels was observed. During the initial stages of skin regeneration angptl1b and angptl2b transcripts were significantly more abundant (p < 0.05) than in intact skin and angptl7 transcripts were down-regulated (p < 0.05) throughout the 4-days of skin regeneration that was studied. No difference in angptl4a and angptl4b transcript abundance was detected during regeneration or between regenerating and intact skin.

CONCLUSIONS

The angptl gene family has expanded in teleost genomes. In sea bream, changes in the expression of angptl1b, angptl2b and angptl7 were correlated with the main phases of skin regeneration, indicating the involvement of ANGPTL family members in skin regeneration has been conserved in the vertebrates. Exploration of the fish angptl family in skin sheds new light on the understanding of the molecular basis of skin regeneration an issue of importance for disease control in aquaculture.

The confounding complexity of innate immune receptors within and between teleost species

Teleost genomes encode multiple multigene families of immunoglobulin domain-containing innate immune receptors (IIIRs) with unknown function and no clear mammalian orthologs. However, the genomic organization of IIIR gene clusters and the structure and signaling motifs of the proteins they encode are similar to those of mammalian innate immune receptor families such as the killer cell immunoglobulin-like receptors (KIRs), leukocyte immunoglobulin-like receptors (LILRs), Fc receptors, triggering receptors expressed on myeloid cells (TREMs) and CD300s. Teleost IIIRs include novel immune-type receptors (NITRs); diverse immunoglobulin domain containing proteins (DICPs); polymeric immunoglobulin receptor-like proteins (PIGRLs); novel immunoglobulin-like transcripts (NILTs) and leukocyte immune-type receptors (LITRs). The accumulation of genomic sequence data has revealed that IIIR gene clusters in zebrafish display haplotypic and gene content variation. This intraspecific genetic variation, as well as significant interspecific variation, frequently confounds the identification of definitive orthologous IIIR sequences between teleost species. Nevertheless, by defining which teleost lineages encode (and do not encode) different IIIR families, predictions can be made about the presence (or absence) of specific IIIR families in each teleost lineage. It is anticipated that further investigations into available genomic resources and the sequencing of a variety of multiple teleost genomes will identify additional IIIR families and permit the modeling of the evolutionary origins of IIIRs.

Coevolution of MHC genes (LMP/TAP/class Ia, NKT-class Ib, NKp30-B7H6): lessons from cold-blooded vertebrates

Comparative immunology provides the long view of what is conserved across all vertebrate taxa versus what is specific to particular organisms or group of organisms. Regarding the major histocompatibility complex (MHC) and coevolution, three striking cases have been revealed in cold-blooded vertebrates: lineages of class Ia antigen-processing and -presenting genes, evolutionary conservation of NKT-class Ib recognition, and the ancient emergence of the natural cytotoxicity receptor NKp30 and its ligand B7H6. While coevolution of transporter associated with antigen processing (TAP) and class Ia has been documented in endothermic birds and two mammals, lineages of LMP7 are restricted to ectotherms. The unambiguous discovery of natural killer T (NKT) cells in Xenopus demonstrated that NKT cells are not restricted to mammals and are likely to have emerged at the same time in evolution as classical α/β and γ/δ T cells. NK cell receptors evolve at a rapid rate, and orthologues are nearly impossible to identify in different vertebrate classes. By contrast, we have detected NKp30 in all gnathostomes, except in species where it was lost. The recently discovered ligand of NKp30, B7H6, shows strong signs of coevolution with NKp30 throughout evolution, i.e. coincident loss or expansion of both genes in some species. NKp30 also offers an attractive IgSF candidate for the invasion of the RAG transposon, which is believed to have initiated T-cell receptor/immunoglobulin adaptive immunity. Besides reviewing these intriguing features of MHC evolution and coevolution, we offer suggestions for future studies and propose a model for the primordial or proto MHC.

The identification of additional zebrafish DICP genes reveals haplotype variation and linkage to MHC class I genes

Bony fish encode multiple multi-gene families of membrane receptors that are comprised of immunoglobulin (Ig) domains and are predicted to function in innate immunity. One of these families, the diverse immunoglobulin (Ig) domain-containing protein (DICP) genes, maps to three chromosomal loci in zebrafish. Most DICPs possess one or two Ig ectodomains and include membrane-bound and secreted forms. Membrane-bound DICPs include putative inhibitory and activating receptors. Recombinant DICP Ig domains bind lipids with varying specificity, a characteristic shared with mammalian CD300 and TREM family members. Numerous DICP transcripts amplified from different lines of zebrafish did not match the zebrafish reference genome sequence suggesting polymorphic and haplotypic variation. The expression of DICPs in three different lines of zebrafish has been characterized employing PCR-based strategies. Certain DICPs exhibit restricted expression in adult tissues whereas others are expressed ubiquitously. Transcripts of a subset of DICPs can be detected during embryonic development suggesting roles in embryonic immunity or other developmental processes. Transcripts representing 11 previously uncharacterized DICP sequences were identified. The assignment of two of these sequences to an unplaced genomic scaffold resulted in the identification of an alternative DICP haplotype that is linked to a MHC class I Z lineage haplotype on zebrafish chromosome 3. The linkage of DICP and MHC class I genes also is observable in the genomes of the related grass carp (Ctenopharyngodon idellus) and common carp (Cyprinus carpio) suggesting that this is a shared character with the last common Cyprinidae ancestor.

Multigene families of immunoglobulin domain-containing innate immune receptors in zebrafish: deciphering the differences

Five large multigene families encoding innate-type immune receptors that are comprised of immunoglobulin domains have been identified in bony fish, of which four do not possess definable mammalian orthologs. The members of some of the multigene families exhibit unusually extensive patterns of divergence and the individual family members demonstrate marked variation in interspecific comparisons. As a group, the gene families reveal striking differences in domain type and content, mechanisms of intracellular signaling, basic structural features, haplotype and allelic variation and ligand binding. The potential functional roles of these innate immune receptors, their relationships to immune genes in higher vertebrate species and the basis for their adaptive evolution are of broad interest. Ongoing investigations are expected to provide new insight into alternative mechanisms of immunity.

Differential expression and ligand binding indicate alternative functions for zebrafish polymeric immunoglobulin receptor (pIgR) and a family of pIgR-like (PIGRL) proteins

The polymeric immunoglobulin (Ig) receptor (pIgR) is an integral transmembrane glycoprotein that plays an important role in the mammalian immune response by transporting soluble polymeric Igs across mucosal epithelial cells. Single pIgR genes, which are expressed in lymphoid organs including mucosal tissues, have been identified in several teleost species. A single pigr gene has been identified on zebrafish chromosome 2 along with a large multigene family consisting of 29 pigr-like (PIGRL) genes. Full-length transcripts from ten different PIGRL genes that encode secreted and putative inhibitory membrane-bound receptors have been characterized. Although PIGRL and pigr transcripts are detected in immune tissues, only PIGRL transcripts can be detected in lymphoid and myeloid cells. In contrast to pIgR which binds Igs, certain PIGRL proteins bind phospholipids. PIGRL transcript levels are increased after infection with Streptococcus iniae, suggesting a role for PIGRL genes during bacterial challenge. Transcript levels of PIGRL genes are decreased after infection with Snakehead rhabdovirus, suggesting that viral infection may suppress PIGRL function.

Teleost T and NK cell immunity

The main function of the immune system is to maintain the organism's homeostasis when invaded by foreign material or organisms. Prior to successful elimination of the invader it is crucial to distinguish self from non-self. Most pathogens and altered cells can be recognized by immune cells through expressed pathogen- or danger-associated molecular patterns (PAMPS or DAMPS, respectively), through non-self (e.g. allogenic or xenogenic cells) or missing major histocompatibility (MHC) class I molecules (some virus-infected target cells), and by presenting foreign non-self peptides of intracellular (through MHC class I-e.g. virus-infected target cells) or extracellular (through MHC class II-e.g. from bacteria) origin. In order to eliminate invaders directly or by destroying their ability to replicate (e.g. virus-infected cells) specialized immune cells of the innate and adaptive responses appeared during evolution. The first line of defence is represented by the evolutionarily ancient macrophages and natural killer (NK) cells. These innate mechanisms are well developed in bony fish. Two types of NK cell homologues have been described in fish: non-specific cytotoxic cells and NK-like cells. Adaptive cell-mediated cytotoxicity (CMC) requires key molecules expressed on cytotoxic T lymphocytes (CTLs) and target cells. CTLs kill host cells harbouring intracellular pathogens by binding of their T cell receptor (TCR) and its co-receptor CD8 to a complex of MHC class I and bound peptide on the infected host cell. Alternatively, extracellular antigens are taken up by professional antigen presenting cells such as macrophages, dendritic cells and B cells to process those antigens and present the resulting peptides in association with MHC class II to CD4(+) T helper cells. During recent years, genes encoding MHC class I and II, TCR and its co-receptors CD8 and CD4 have been cloned in several fish species and antibodies have been developed to study protein expression in morphological and functional contexts. Functional assays for innate and adaptive lymphocyte responses have been developed in only a few fish species. This review summarizes and discusses recent results and developments in the field of T and NK cell responses with focus on economically important and experimental model fish species in the context of vaccination.

CD300 molecule regulation of human dendritic cell functions

Dendritic cells (DC) are a heterogeneous population of leucocytes which play a key role in initiating and modulating immune responses. The human CD300 family consists of six immunoregulatory leucocyte membrane molecules that regulate cellular activity including differentiation, viability, cytokine and chemokine secretion, phagocytosis and chemotaxis. Recent work has identified polar lipids as probable ligands for these molecules in keeping with the known evolutionary conservation of this family. CD300 molecules are all expressed by DC; CD300b, d, e and f are restricted to different subpopulations of the myeloid DC lineage. They have been shown to regulate DC function both in vitro and in vivo. In addition DC are able to regulate their CD300 expression in an autocrine manner. The potential to form different CD300 heterodimers adds further complexity to their role in fine tuning DC function. Expression of CD300 molecules is altered in a number of diseases including many where DC are implicated in the pathogenesis. CD300 antibodies have been demonstrated to have significant therapeutic effect in animal models. The mechanisms underlying the immunoregulatory effects of the CD300 family are complex. Deciphering their physiology will allow effective targeting of these molecules as novel therapies in a wide variety of inflammatory diseases.

A single immunoglobulin-domain IgSF protein from Sciaenops ocellatus regulates pathogen-induced immune response in a negative manner

The immunoglobulin superfamily (IgSF) is a large group of cell surface proteins that include various immunoregulatory receptors such as novel immune type receptors (NITRs), which are a family of diversified proteins found exclusively in bony fish. In this study, we identified and analyzed an IgSF protein, SoIgSF1, from red drum (Sciaenops ocellatus). SoIgSF1 is composed of 225 amino acid residues and moderately related to teleost NITRs. In silico analysis indicated that SoIgSF1 is a type I transmembrane glycoprotein and contains an N-terminal signal peptide sequence, a single extracellular immunoglobulin V domain, a transmembrane region, and a cytoplasmic region. However, unlike most NITRs, the cytoplasmic region of SoIgSF1 exhibits no consensus inhibitory or stimulatory signaling sequences but has two tyrosine-containing motifs that conform to the right-half sequence of the immunoreceptor tyrosine-based inhibitory motif (ITIM). Quantitative real time RT-PCR analysis showed that SoIgSF1 expression occurred mainly in immune organs and was drastically induced by viral and bacterial infection. Immunofluorescence microscopy indicated that viral infection of head kidney (HK) leukocytes induced surface expression of SoIgSF1, which was able to interact with antibodies against recombinant SoIgSF1. Antibody cross-linking of SoIgSF1 on HK leukocytes inhibited the expression of immune relevant genes and promoted viral and bacterial infection. Taken together, these results indicate that SoIgSF1, though lacking canonical intracellular signaling motifs, is involved in regulation of host immune response during pathogen infection possibly by functioning as a negative signaling receptor through a novel mechanism.

Evolution of the B7 family: co-evolution of B7H6 and NKp30, identification of a new B7 family member, B7H7, and of B7's historical relationship with the MHC

The B7 family of genes is essential in the regulation of the adaptive immune system. Most B7 family members contain both variable (V)- and constant (C)-type domains of the immunoglobulin superfamily (IgSF). Through in silico screening of the Xenopus genome and subsequent phylogenetic analysis, we found novel genes belonging to the B7 family, one of which is the recently discovered B7H6. Humans and rats have a single B7H6 gene; however, many B7H6 genes were detected in a single large cluster in the Xenopus genome. The B7H6 expression patterns also varied in a species-specific manner. Human B7H6 binds to the activating natural killer receptor, NKp30. While the NKp30 gene is single-copy and maps to the MHC in most vertebrates, many Xenopus NKp30 genes were found in a cluster on a separate chromosome that does not harbor the MHC. Indeed, in all species so far analyzed from sharks to mammals, the number of NKp30 and B7H6 genes correlates well, suggestive of receptor-ligand co-evolution. Furthermore, we identified a Xenopus-specific B7 homolog (B7HXen) and revealed its close linkage to B2M, which we have demonstrated previously to have been originally encoded in the MHC. Thus, our study provides further proof that the B7 precursor was included in the proto MHC. Additionally, the comparative analysis revealed a new B7 family member, B7H7, which was previously designated in the literature as an unknown gene, HHLA2.

Diversity of CD2 subfamily receptors in cyprinid fishes

CD2 family receptor (CD2f) is evolutionarily conserved and is widely expressed by various types of leukocytes. To elucidate the phylogenetic diversity of the CD2f, we characterized CD2f in teleosts using ginbuna crucian carp and zebrafish. The identified CD2f isoforms of the ginbuna carp (caauCD2f) exhibited high sequence similarity to the mammalian CD2 subsets CD48, CD244, and CD319, but it was difficult to classify them into their respective mammalian CD2f based on sequence similarity, the presence of an immunoreceptor tyrosine-based switch motif (ITSM), and phylogenetic tree analysis. Although the four caauCD2f isoforms share an extracellular domain with quite high identity (83-94% identity at the nucleic acid level), they differ in the number of ITSM motifs in their cytoplasmic tail. RT-PCR and in situ hybridization analyses showed that the caauCD2f isoforms are expressed by different cell populations, suggesting that they, like mammalian CD2f, have diverse roles. Interestingly, immunoglobulin (Ig) domain-like sequences with high identity to caauCD2fs are clustered close together within 0.6 Mbp on zebrafish chromosomes 1 and 2 (at least 8 and 35 sequences, respectively), and many pairs of the Ig domains share more than 90% identity at the amino acid level. Therefore, the teleost CD2fs with considerably high identity have been probably generated from a common ancestral Ig-domain gene by a very recent gene duplication event. These findings suggest that the identified CD2f acquired functional diversification through successive duplications together with the acquisition of ITSM.

Examination of the stimulatory signaling potential of a channel catfish leukocyte immune-type receptor and associated adaptor

Expressed by various subsets of myeloid and lymphoid immune cells, channel catfish (Ictalurus punctatus) leukocyte immune-type receptors (IpLITRs) are predicted to play a key role in the initiation and termination of teleost cellular effector responses. These type I transmembrane proteins belong to the immunoglobulin superfamily and display features of immunoregulatory receptors with inhibitory and/or stimulatory signaling potential. Expanding on our previous work, which demonstrated that putative stimulatory IpLITR-types associated with the catfish adaptor proteins IpFcRγ and FcRγ-L, this study focuses on the functional significance of this immune receptor-adaptor signaling complex. Specifically, we generated an epitope-tagged chimeric receptor construct by fusing the extracellular domain of IpLITR 2.6b with the transmembrane region and cytoplasmic tail of IpFcRγ-L. This chimera was stably expressed in a rat basophilic leukemia (RBL) cell line, RBL-2H3, and following cross-linking of the surface receptor with an anti-hemagglutinin monoclonal antibody or opsonized microspheres, the chimeric teleost receptor induced cellular degranulation and phagocytic responses, respectively. Site-directed mutagenesis of the immunoreceptor tyrosine-based activation motif encoded within the cytoplasmic tail of the chimera confirmed that these functional responses were dependent on the phosphorylated tyrosines within this motif. Using a combination of phospho-specific antibodies and pharmacological inhibitors, we also demonstrate that the IpLITR/IpFcRγ-L-induced degranulation response requires the activity of Src homology 2 domain containing protein tyrosine phosphatases, phosphatidylinositol 3-kinase, protein kinase C, and mitogen-activated protein kinases but appears independent of the c-Jun N-terminal kinase and p38 MAP kinase pathways. In addition to this first look at stimulatory IpLITR-mediated signaling and its influence on cellular effector responses, the advantage of generating RBL-2H3 cells stably expressing a functional IpLITR-adaptor chimera will be discussed.

Teleost IgSF immunoregulatory receptors

In all animals innate immunity is the first line of immune defense from invading pathogens. The prototypical innate cellular responses such as phagocytosis, degranulation, and cellular cytotoxicity are elicited by leukocytes in a diverse range of animals including fish, amphibians, birds and mammals reinforcing the importance of such primordial defense mechanisms. In mammals, these responses are intricately controlled and coordinated at the cellular level by distinct subsets of immunoregulatory receptors. Many of these surface proteins belong to the immunoglobulin superfamily and in mammals elaborate immunoregulatory receptor networks play a major role in the control of infectious diseases. Recent examination of teleost immunity has begun to further illustrate the complexities of these receptor networks in lower vertebrates. However, little is known about the mechanisms that control how immunoregulatory receptors influence cellular decision making in ectothermic vertebrates. This review focuses on several families of recently discovered immunoglobulin superfamily members in fish that share structural, phylogenetic and in some cases functional relationships with mammalian immunoregulatory receptors. Further characterization of these teleost innate immune receptor families will provide detailed information regarding the conservation and importance of innate immune defense strategies throughout vertebrate evolution.

A novel soluble immune-type receptor (SITR) in teleost fish: carp SITR is involved in the nitric oxide-mediated response to a protozoan parasite

Background

The innate immune system relies upon a wide range of germ-line encoded receptors including a large number of immunoglobulin superfamily (IgSF) receptors. Different Ig-like immune receptor families have been reported in mammals, birds, amphibians and fish. Most innate immune receptors of the IgSF are type I transmembrane proteins containing one or more extracellular Ig-like domains and their regulation of effector functions is mediated intracellularly by distinct stimulatory or inhibitory pathways.

Methodology/Principal Findings

Carp SITR was found in a substracted cDNA repertoire from carp macrophages, enriched for genes up-regulated in response to the protozoan parasite Trypanoplasma borreli. Carp SITR is a type I protein with two extracellular Ig domains in a unique organisation of a N-proximal V/C2 (or I-) type and a C-proximal V-type Ig domain, devoid of a transmembrane domain or any intracytoplasmic signalling motif. The carp SITR C-proximal V-type Ig domain, in particular, has a close sequence similarity and conserved structural characteristics to the mammalian CD300 molecules. By generating an anti-SITR antibody we could show that SITR protein expression was restricted to cells of the myeloid lineage. Carp SITR is abundantly expressed in macrophages and is secreted upon in vitro stimulation with the protozoan parasite T. borreli. Secretion of SITR protein during in vivo T. borreli infection suggests a role for this IgSF receptor in the host response to this protozoan parasite. Overexpression of carp SITR in mouse macrophages and knock-down of SITR protein expression in carp macrophages, using morpholino antisense technology, provided evidence for the involvement of carp SITR in the parasite-induced NO production.

Conclusion/Significance

We report the structural and functional characterization of a novel soluble immune-type receptor (SITR) in a teleost fish and propose a role for carp SITR in the NO-mediated response to a protozoan parasite.

The phylogenetic origins of natural killer receptors and recognition: relationships, possibilities, and realities

Natural killer (NK) cells affect a form of innate immunity that recognizes and eliminates cells that are infected with certain viruses or have undergone malignant transformation. In mammals, this recognition can be mediated through immunoglobulin- (Ig) and/or lectin-type NK receptors (NKRs). NKR genes in mammals range from minimally polymorphic single-copy genes to complex multigene families that exhibit high levels of haplotypic complexity and exhibit significant interspecific variation. Certain single-copy NKR genes that are present in one mammal are present as expanded multigene families in other mammals. These observations highlight NKRs as one of the most rapidly evolving eukaryotic gene families and likely reflect the influence of pathogens, especially viruses, on their evolution. Although well characterized in human and mice, cytotoxic cells that are functionally similar to NK cells have been identified in species ranging from birds to reptiles, amphibians and fish. Although numerous receptors have been identified in non-mammalian vertebrates that share structural relationships with mammalian NKRs, functionally defining these lower vertebrate molecules as NKRs is confounded by methodological and interpretive complexities. Nevertheless, several lines of evidence suggest that NK-type function or its equivalent has sustained a long evolutionary history throughout vertebrate species.

Genomic organisation analysis of novel immunoglobulin-like transcripts in Atlantic salmon (Salmo salar) reveals a tightly clustered and multigene family

Background

Several novel immunoglobulin-like transcripts (NILTs) which have previously been identified in the salmonid species rainbow trout (Oncorhynchus mykiss) contain either one or two extracellular Ig domains of the V-type. NILTs also possess either an immunoreceptor tyrosine-based activating motif (ITAM) or immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in the cytoplasmic region resulting in different signalling abilities. Here we report for the first time the genomic organisation and structure of the multigene family of NILTs in Atlantic salmon (Salmo salar) using a BAC sequencing approach.

Results

We have identified six novel Atlantic salmon NILT genes (Ssa-NILT1-6), two pseudogenes (Ssa-NILTp1 and Ssa-NILTp2) and seven genes encoding putative transposable elements in one BAC covering more than 200 kbp. Ssa-NILT1, 2, 4, 5 and 6 contain one Ig domain, all having a CX₃C motif, whereas Ssa-NILT3 contains two Ig domains, having a CX₆C motif in Ig1 and a CX₇C motif in Ig2. Atlantic salmon NILTs possess several ITIMs in the cytoplasmic region and the ITIM-bearing exons are in phase 0. A comparison of identity between the amino acid sequences of the CX₃C Ig domains from NILTs varies from 77% to 96%. Ssa-NILT1, 2, 3 and 4 were all confirmed to be expressed either by their presence in EST databases (Ssa-NILT1) or RT-PCR (Ssa-NILT2, 3, and 4) using cDNA as template. A survey of the repertoire of putative NILT genes in a single individual revealed three novel genes (Ssa-NILT7-9) represented by the Ig domain, which together with Ig domains from Ssa-NILT1-6 could be divided into different groups based on specific motifs.

Conclusions

This report reveals a tightly clustered, multigene NILT family in Atlantic salmon. By screening a highly redundant Atlantic salmon BAC library we have identified and characterised the genomic organisation of six genes encoding NILT receptors. The genes show similar characteristics to NILTs previously identified in rainbow trout, having highly conserved cysteines in the Ig domain and several inhibitory signalling motifs in the cytoplasmic region. In a single individual three unique NILT Ig domain sequences were discovered at the genomic DNA level, which were divided into two different groups based on a four residue motif after the third cysteine. Our results from the BAC screening and analysis on the repertoire of NILT genes in a single individual indicates that many genes of this expanding Ig containing NILT family are still to be discovered in fish.

Identification and characterisation of a novel immune-type receptor (NITR) gene cluster in the European sea bass, Dicentrarchus labrax, reveals recurrent gene expansion and diversification by positive selection

In the last decade, a new gene family encoding non-rearranging receptors, called novel immune-type receptors (NITRs), has been discovered in teleost fish. NITRs belong to the immunoglobulin superfamily and represent an extraordinarily divergent and rapidly evolving gene complex. Genomic analysis of a region spanning 270 kb led to the discovery of a NITR gene cluster in the European sea bass (Dicentrarchus labrax). In total, 27 NITR genes and three putative pseudogenes, organised in a tandemly arrayed cluster, were identified. Sea bass NITR genes maintain the three major genomic organisations that appear to be essentially conserved among fish species along with new features presumably involving processes of intron loss, exon deletion and acquisition of new exons. Comparative and evolutionary analyses suggest that these receptors have evolved following a “birth-and-death” model of gene evolution in which duplication events together with lineage-specific gain and loss of individual members contributed to the rapid diversification of individual gene families. In this study, we demonstrate that species-specific gene expansions provide the raw material for diversifying, positive Darwinian selection favouring the evolution of a highly diverse array of molecules.

Identification of natural killer cell receptor clusters in the platypus genome reveals an expansion of C-type lectin genes

Natural killer (NK) cell receptors belong to two unrelated, but functionally analogous gene families: the immunoglobulin superfamily, situated in the leukocyte receptor complex (LRC) and the C-type lectin superfamily, located in the natural killer complex (NKC). Here, we describe the largest NK receptor gene expansion seen to date. We identified 213 putative C-type lectin NK receptor homologs in the genome of the platypus. Many have arisen as the result of a lineage-specific expansion. Orthologs of OLR1, CD69, KLRE, CLEC12B, and CLEC16p genes were also identified. The NKC is split into at least two regions of the genome: 34 genes map to chromosome 7, two map to a small autosome, and the remainder are unanchored in the current genome assembly. No NK receptor genes from the LRC were identified. The massive C-type lectin expansion and lack of Ig-domain-containing NK receptors represents the most extreme polarization of NK receptors found to date. We have used this new data from platypus to trace the possible evolutionary history of the NK receptor clusters.

The CD300 molecules regulate monocyte and dendritic cell functions

The CD300 glycoproteins are a family of related leucocyte surface molecules that modulate a diverse array of cell processes via their paired triggering and inhibitory receptor functions. All family members have a single Ig-V like domain and they share a common evolutionary pathway. At least one member of the family has undergone significant positive selection (ranked second in the top 50) indicating a need to maintain some crucial function. Here we have reviewed the CD300 family members, and their expression on cells of the monocyte and dendritic cell lineages. The consequences of CD300 molecule expression by these leucocyte lineages are only now beginning to be understood. The ability to fine tune monocyte and dendritic cell function and immune responses highlights several potential options to exploit these molecules as therapeutic targets in chronic inflammatory diseases, allergy and other disease states.

The CD300 family of molecules are evolutionarily significant regulators of leukocyte functions

The CD300 glycoproteins are a family of cell surface molecules that modulate a diverse array of cell processes via their paired triggering and inhibitory receptor functions. Family members share a common evolutionary pathway and at least one member of the family has undergone significant positive selection, indicating their crucial value to the host. This review clarifies the occasionally confusing usage of nomenclature for the CD300 family and summarizes our current understanding of their genomics, expression and function. Their ability to fine tune leukocyte function and immune responses highlights several potential options to exploit the CD300 molecules as therapeutic targets in chronic inflammatory diseases, allergy and other disease states.

Cell surface expression of channel catfish leukocyte immune-type receptors (IpLITRs) and recruitment of both Src homology 2 domain-containing protein tyrosine phosphatase (SHP)-1 and SHP-2

Channel catfish leukocyte immune-type receptors (IpLITRs) are immunoglobulin superfamily (IgSF) members believed to play a role in the control and coordination of cellular immune responses in teleost. Putative stimulatory and inhibitory IpLITRs are co-expressed by different types of catfish immune cells (e.g. NK cells, T cells, B cells, and macrophages) but their signaling potential has not been determined. Following cationic polymer-mediated transfections into human cell lines we examined the surface expression, tyrosine phosphorylation, and phosphatase recruitment potential of two types of putative inhibitory IpLITRs using 'chimeric' expression constructs and an epitope-tagged 'native' IpLITR. We also cloned and expressed the teleost Src homology 2 domain-containing protein tyrosine phosphatases (SHP)-1 and SHP-2 and examined their expression in adult tissues and developing zebrafish embryos. Co-immunoprecipitation experiments support the inhibitory signaling potential of distinct IpLITR-types that bound both SHP-1 and SHP-2 following the phosphorylation of tyrosine residues within their cytoplasmic tail (CYT) regions. Phosphatase recruitment by IpLITRs represents an important first step in understanding their influence on immune cell effector functions and suggests that certain inhibitory signaling pathways are conserved among vertebrates.

Rainbow trout (Oncorhynchus mykiss) possess multiple novel immunoglobulin-like transcripts containing either an ITAM or ITIMs

The recognition of pathogens by the innate immune system relies on a wide range of inhibitory and activating receptors. Some of these non-rearranging receptors belong to the immunoglobulin superfamily (IgSF) and in teleost fish the novel immune-type receptor (NITR) and the novel immunoglobulin-like transcript (NILT) have been reported. Here we describe the identification and characterisation of three new NILTs from rainbow trout (Oncorhynchus mykiss), with one NILT alternatively spliced into a long isoform containing two Ig domains and a short isoform containing one Ig domain. The cytoplasmic regions contain either immunoreceptor tyrosine-based inhibitory motifs (ITIMs) or an immunoreceptor tyrosine-based activating motif (ITAM) for downstream signalling. Alignment of the various NILT Ig domains revealed a high similarity, especially between Ig domains from NILTs found in this study. Furthermore, a phylogenetic tree showed that NILTs are more closely related to the triggering receptor expressed on myeloid (TREM) cells and NKp44 than to NITRs. The expression of NILTs was studied in six different tissues and two different cell lines, with expression apparent in immunologically important tissues. Expression of NILTs was also shown to be an early event in development, with both eyed eggs and embryos expressing all four genes. The results obtained in this study and future experiments will contribute to our knowledge of the immune system in fish and provide useful information for the control of inflammatory processes in rainbow trout.

A large new subset of TRIM genes highly diversified by duplication and positive selection in teleost fish

Background

In mammals, the members of the tripartite motif (TRIM) protein family are involved in various cellular processes including innate immunity against viral infection. Viruses exert strong selective pressures on the defense system. Accordingly, antiviral TRIMs have diversified highly through gene expansion, positive selection and alternative splicing. Characterizing immune TRIMs in other vertebrates may enlighten their complex evolution.

Results

We describe here a large new subfamily of TRIMs in teleosts, called finTRIMs, identified in rainbow trout as virus-induced transcripts. FinTRIMs are formed of nearly identical RING/B-box regions and C-termini of variable length; the long variants include a B30.2 domain. The zebrafish genome harbors a striking diversity of finTRIMs, with 84 genes distributed in clusters on different chromosomes. A phylogenetic analysis revealed different subsets suggesting lineage-specific diversification events. Accordingly, the number of fintrim genes varies greatly among fish species. Conserved syntenies were observed only for the oldest fintrims. The closest mammalian relatives are trim16 and trim25, but they are not true orthologs. The B30.2 domain of zebrafish finTRIMs evolved under strong positive selection. The positions under positive selection are remarkably congruent in finTRIMs and in mammalian antiviral TRIM5α, concentrated within a viral recognition motif in mammals. The B30.2 domains most closely related to finTRIM are found among NOD-like receptors (NLR), indicating that the evolution of TRIMs and NLRs was intertwined by exon shuffling.

Conclusion

The diversity, evolution, and features of finTRIMs suggest an important role in fish innate immunity; this would make them the first TRIMs involved in immunity identified outside mammals.

Avian NK activities, cells and receptors

Natural killer (NK) activity has been examined in birds for over 30 years, but evidence that avian NK activity plays crucial roles in disease is only suggestive. In chickens, NK activity is mediated by TCR0 cells in the intestinal epithelium, but elsewhere subsets of alphabeta and gammadelta T cells (NKT cells) may be more important. There are few lectin-like NK receptor genes, located in the genomic region syntenic with the natural killer complex (NKC) as well as the major histocompatibility complex (MHC). In contrast, a huge number of Ig-like receptor genes are located in a region syntenic with the leukocyte receptor complex (LRC).

Cell-mediated immune responses in rainbow trout after DNA immunization against the viral hemorrhagic septicemia virus

To identify viral proteins that induce cell-mediated cytotoxicity (CMC) against viral hemorrhagic septicemia virus (VHSV)-infected cells, rainbow trout were immunized with DNA vectors encoding the glycoprotein G or the nucleocapsid protein N of VHSV. The G protein was a more potent trigger of cytotoxic cells than the N protein. Peripheral blood leukocytes (PBL) isolated from trout immunized against the G protein killed both VHSV-infected MHC class I matched (RTG-2) and VHSV-infected xenogeneic (EPC) target cells, suggesting the involvement of both cytotoxic T lymphocytes (CTL) and NK cells, respectively. In contrast, PBL from trout that were immunized against the N protein only killed VHSV-infected RTG-2 cells, indicating that this protein only elicits a CTL response. Further, a significant killing capacity of these PBL was only observed during summer months. PBL from fish that were immunized against the VHSV G protein significantly killed VHSV-infected but not infectious hematopoietic necrosis virus (IHNV)-infected targets indicating antigen specificity. Thus, this is the first report on cytotoxic immune responses after DNA vaccination in fish. Furthermore, cells isolated from the inflamed site of DNA injection were stained and transferred to isogeneic DNA-vaccinated recipients. Most of the stained donor leukocytes accumulated at the recipients' DNA injection site showing, for the first time, leukocyte homing in fish. Transferred donor leukocytes mainly migrated to the homologous vaccine injection site rather than to injection sites of heterologous vaccines, suggesting the antigen specificity of homing. By demonstrating CMC responses to distinct viral proteins and homing in rainbow trout, these results substantially contribute to the understanding of the teleost immune system.

The TREM-1/DAP12 pathway

DNAX activation protein of 12kDa (DAP12) is an immunoreceptor tyrosine-based activation motif (ITAM)-bearing adapter, which couples to multiple receptors expressed on natural killer (NK) cells, monocytes, and neutrophils. Initially, DAP12-mediated signaling was mainly investigated downstream of receptors expressed on NK cells. In myeloid cells, one of the receptors associating with DAP12 is the triggering receptor expressed on myeloid cells (TREM)-1. Since the real nature of TREM-1L(s) is still illusive, TREM-1 biology was so far only studied using agonistic monoclonal antibodies for receptor ligation. Triggering via TREM-1 results in the production of pro-inflammatory cytokines, chemokines, reactive oxygen species (ROS), and leads to rapid degranulation of neutrophilic granules, and phagocytosis. Furthermore, application of a TREM-1/Ig fusion protein in an animal model of experimentally induced sepsis increases survival. It is obvious that targeting components of the TREM-1/DAP12 pathway could be a promising therapeutic strategy for the treatment of inflammatory diseases. Therefore, it is of great importance to get further insight into the signaling cascade downstream of TREM-1. This review summarizes the current understanding of the TREM-1/DAP12 pathway in monocytes and neutrophils.

A novel immunoglobulin-like transcript from rainbow trout with two Ig-like domains and two isoforms

Within the innate immune response in primates the nonrearranging killer immunoglobulin-like receptors (KIR) enable natural killer cells to discern target cells exposing "missing self" signals. Recently the novel immune-type receptor (NITR) and the novel immunoglobulin-like transcript (NILT) gene families have been discovered in fish encoding nonrearranging receptors with a similar molecular structure to that of KIRs. Besides the structural similarity the high degree of haplotypic and allelic variation suggests these genes to be functional KIR homologs and involved in recognizing self-determinants in lower vertebrates. Whereas numerous NITR sequences have been detected in several fish species only two NILT genes have been published for carp yet. Here we report a first rainbow trout NILT sequence, Onmy-NILT2D, alternatively spliced into a long membrane-bound and a short, putatively secreted form, both with the same two immunoglobulin (Ig)-like domains. The second Ig-like domain comprises a consensus pattern present both in NILTs and NITRs. The cytoplasmic region of the long form simultaneously contains immunoreceptor tyrosine-based inhibitory motifs (ITIM) and an immunoreceptor tyrosine-based activating motif (ITAM).

Channel catfish leukocyte immune-type receptors contain a putative MHC class I binding site

The recent identification of a large and diverse family of leukocyte immune-type receptors (IpLITRs) in channel catfish (Ictalurus punctatus) indicates that immunoglobulin superfamily (IgSF) members related to both mammalian Fc receptors (FcRs) and leukocyte receptor complex (LRC)-encoded proteins exist in fish. In the present study, it was found that IpLITR messages were preferentially up regulated in catfish peripheral blood leukocytes (PBL) and clonal cytotoxic T cells (CTL) after alloantigen stimulation. Detailed sequence analyses of the expressed IpLITR cDNAs from two clonal CTL lines indicated an unexpectedly large array of putative activatory and inhibitory IpLITR-types containing variable numbers of extracellular immunoglobulin (Ig)-like domains. Importantly, all expressed IpLITRs shared similar membrane distal Ig domains (i.e., D1 and D2), suggesting that they may bind a common type of ligand. Sequence alignments and comparative homology modeling revealed that IpLITR domains, D1 and D2, have similar predicted 3-D structural properties with the corresponding domains of the human LRC-encoded leukocyte Ig-like receptor (LILR) family. Furthermore, conservation of key major histocompatibility class I (MHC I)-binding residues were located at similar positions within the membrane distal tip of D1 between representative IpLITRs and group 1 LILRs. Taken together, these results suggest that fish LITRs have an orthologous relationship to LRC-encoded receptors such as the human LILRs and could potentially function as a diverse family of MHC class I-binding receptors.

The TREM receptor family and signal integration

TREM proteins are a family of cell surface receptors that participate in diverse cell processes, including inflammation, bone homeostasis, neurological development and coagulation. TREM-1, the first to be identified, acts to amplify inflammation. Other TREM proteins regulate the differentiation and function of macrophages, microglia, dendritic cells, osteoclasts and platelets. Here we discuss the state of the field, putative ligands of TREM proteins and the challenges that remain in understanding TREM biology.

Head Kidney-Derived Macrophages of Common Carp (Cyprinus carpio L.) Show Plasticity and Functional Polarization upon Differential Stimulation

Cells from the myeloid lineage are pluripotent. To investigate the potential of myeloid cell polarization in a primitive vertebrate species, we phenotypically and functionally characterized myeloid cells of common carp (Cyprinus carpio L.) during culture. Flow cytometric analysis, Ab labeling of cell surface markers, and light microscopy showed the presence of a major population of heterogeneous macrophages after culture. These head kidney-derived macrophages can be considered the fish equivalent of bone marrow-derived macrophages and show the ability to phagocytose, produce radicals, and polarize into innate activated or alternatively activated macrophages. Macrophage polarization was based on differential activity of inducible NO synthase and arginase for innate and alternative activation, respectively. Correspondingly, gene expression profiling after stimulation with LPS or cAMP showed differential expression for most of the immune genes presently described for carp. The recently described novel Ig-like transcript 1 (NILT1) and the CXCR1 and CXCR2 chemokine receptors were up-regulated after stimulation with cAMP, an inducer of alternative activation in carp macrophages. Up-regulation of NILT1 was also seen during the later phase of a Trypanosoma carassii infection, where macrophages are primarily alternatively activated. However, NILT1 could not be up-regulated during a Trypanoplasma borreli infection, a model for innate activation. Our data suggest that NILT1, CXCR1, and CXCR2 could be considered markers for alternatively activated macrophages in fish.

Ancient divergence of a complex family of immune-type receptor genes

Multigene families of activating/inhibitory receptors belonging to the immunoglobulin superfamily (IgSF) regulate immunological and other cell-cell interactions. A new family of such genes, termed modular domain immune-type receptors (MDIRs), has been identified in the clearnose skate (Raja eglanteria), a phylogenetically ancient vertebrate. At least five different major forms of predicted MDIR proteins are comprised of four different subfamilies of IgSF ectodomains of the intermediate (I)- or C2-set. The predicted number of individual IgSF ectodomains in MDIRs varies from one to six. MDIR1 contains a positively charged transmembrane residue and MDIR2 and MDIR3 each possesses at least one immunoreceptor tyrosine-based inhibitory motif in their cytoplasmic regions. MDIR4 and MDIR5 lack characteristic activating/inhibitory signalling motifs. MDIRs are encoded in a particularly large and complex multigene family. MDIR domains exhibit distant sequence similarity to mammalian CMRF-35-like molecules, polymeric immunoglobulin receptors, triggering receptors expressed on myeloid cells (TREMs), TREM-like transcripts, NKp44 and FcR homologs, as well as to sequences identified in several different vertebrate genomes. Phylogenetic analyses suggest that MDIRs are representative members of an extended family of IgSF genes that diverged before or very early in evolution of the vertebrates and subsequently came to occupy multiple, fully independent distributions in the present day.

The chicken immunoregulatory receptor families SIRP, TREM, and CMRF35/CD300L

Mammalian immunoregulatory families of genes encoding activating and inhibitory Ig-like receptor pairs have been located on distinct chromosomes. In chicken, a single Ig-like receptor family with many members had been described so far. By looking at sequence similarity and synteny conservations in the chicken genome, the signal-regulatory protein (SIRP), triggering receptor expressed on myeloid cells (TREM), and CMRF35/CD300L Ig-like gene families were identified on chromosomes 20, 26, and 3, respectively. Further analysis of the three corresponding genomic regions and partial bacterial artificial chromosome sequencing were used to identify more members and to realign several contigs. All putative genomic sequences were monitored by investigating existing expressed sequence tag and cloning cDNA. This approach yielded a single pair of activating and inhibitory SIRP, two inhibitory, and one activating TREM as well as one inhibitory CMRF35/CD300L with a potentially soluble variant and an additional member lacking categorizing motifs. The CMRF35/CD300L and TREM receptors were composed of one or two V-set Ig domains, whereas in SIRP, either a single Ig V domain was present or a combination of a V and C1 domains. Like in many Ig superfamily members, separate exons encode individual Ig domains. However, in two CMRF35/CD300L genes, the signal peptide and the distal Ig domain were encoded by a single exon. In conclusion, the mammalian diversity of immunoregulatory molecules is present the chicken suggesting an important role for TREM, SIRP, and CMRF35/CD300L in a functionally conserved network.

Immune-related, lectin-like receptors are differentially expressed in the myeloid and lymphoid lineages of zebrafish

The identification of C-type lectin (Group V) natural killer (NK) cell receptors in bony fish has remained elusive. Analyses of the Fugu rubripes genome database failed to identify Group V C-type lectin domains (Zelensky and Gready, BMC Genomics 5:51, 2004) suggesting that bony fish, in general, may lack such receptors. Numerous Group II C-type lectin receptors, which are structurally similar to Group V (NK) receptors, have been characterized in bony fish. By searching the zebrafish genome database we have identified a multi-gene family of Group II immune-related, lectin-like receptors (illrs) whose members possess inhibiting and/or activating signaling motifs typical of Group V NK receptors. Illr genes are differentially expressed in the myeloid and lymphoid lineages, suggesting that they may play important roles in the immune functions of multiple hematopoietic cell lineages.