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

A leukocyte immune-type receptor specific polyclonal antibody recognizes goldfish kidney leukocytes and activates the MAPK pathway in isolated goldfish kidney neutrophil-like cells

Leukocyte immune-type receptors (LITRs) belong to a large family of teleost immunoregulatory receptors that share phylogenetic and syntenic relationships with mammalian Fc receptor-like molecules (FCRLs). Recently, several putative stimulatory Carassius auratus (Ca)-LITR transcripts, including CaLITR3, have been identified in goldfish. CaLITR3 has four extracellular immunoglobulin-like (Ig-like) domains, a transmembrane domain containing a positively charged histidine residue, and a short cytoplasmic tail region. Additionally, the calitr3 transcript is highly expressed by goldfish primary kidney neutrophils (PKNs) and macrophages (PKMs). To further investigate the immunoregulatory potential of CaLITR3 in goldfish myeloid cells, we developed and characterized a CaLITR3-epitope-specific polyclonal antibody (anti-CaL3.D1 pAb). We show that the anti-CaL3.D1 pAb stains various hematopoietic cell types within the goldfish kidney, as well as in PKNs and PKMs. Moreover, cross-linking of the anti-CaL3.D1-pAb on PKN membranes induces phosphorylation of p38 and ERK1/2, critical components of the MAPK pathway involved in controlling a wide variety of innate immune effector responses such as NETosis, respiratory burst, and cytokine release. These findings support the stimulatory potential of CaLITR3 proteins as activators of fish granulocytes and pave the way for a more in-depth examination of the immunoregulatory functions of CaLITRs in goldfish myeloid cells.

Transcripts of repetitive DNA elements signal to block phagocytosis of hematopoietic stem cells

Macrophages maintain hematopoietic stem cell (HSC) quality by assessing cell surface Calreticulin (Calr), an "eat-me" signal induced by reactive oxygen species (ROS). Using zebrafish genetics, we identified Beta-2-microglobulin (B2m) as a crucial "don't eat-me" signal on blood stem cells. A chemical screen revealed inducers of surface Calr that promoted HSC proliferation without triggering ROS or macrophage clearance. Whole-genome CRISPR-Cas9 screening showed that Toll-like receptor 3 (Tlr3) signaling regulated b2m expression. Targeting b2m or tlr3 reduced the HSC clonality. Elevated B2m levels correlated with high expression of repetitive element (RE) transcripts. Overall, our data suggest that RE-associated double-stranded RNA could interact with TLR3 to stimulate surface expression of B2m on hematopoietic stem and progenitor cells. These findings suggest that the balance of Calr and B2m regulates macrophage-HSC interactions and defines hematopoietic clonality.

Teleost leukocyte immune-type receptors

Leukocyte immune-type receptors (LITRs) are a large family of teleost immunoregulatory receptor-types belonging to the immunoglobulin superfamily. These immune genes are phylogenetically and syntenically related to Fc receptor-like protein genes (fcrls) present in other vertebrates, including amphibians, birds, mice, and man. In vitro-based functional analyses of LITRs, using transfection approaches, have shown that LITRs have diverse immunoregulatory potentials including the activation and inhibition of several innate immune effector responses such as cell-mediated killing responses, degranulation, cytokine secretion, and phagocytosis. The purpose of this mini review is to provide an overview of fish LITR-mediated immunoregulatory potentials obtained from various teleost model systems, including channel catfish, zebrafish, and goldfish. We will also describe preliminary characterization of a new goldish LITR-specific polyclonal antibody (pAb) and discuss the significance of this tool for further investigation of the functions of fish LITRs.

Expression analysis of Carassius auratus-leukocyte-immune-type receptors (CaLITRs) during goldfish kidney macrophage development and in activated kidney leukocyte cultures

Carassius auratus leukocyte immune-type receptors (CaLITRs) were recently discovered immunoregulatory receptors in goldfish that have diverse immunoglobulin-like (Ig-like) ectodomains and intracellular signaling motifs. Genomic analysis shows that CaLITR-types are also located as distinct gene clusters across multiple goldfish chromosomes. For example, CaLITR1 (unplaced) is a functionally ambiguous receptor having two Ig-like domains, a transmembrane domain (TM), and a short cytoplasmic tail (CYT) devoid of any recognizable signaling motifs. CaLITR2 (Chr47) is a putative inhibitory receptor containing four Ig-like domains, a TM, and a long CYT with an immunoreceptor tyrosine-based inhibition motif (ITIM) and immunoreceptor tyrosine-based switch motif (ITSM). A putative activating receptor-type, CaLITR3 (Chr3), has four Ig-like domains, a TM, and a short CYT containing a positively charged histidine residue and CaLITR4 (ChrLG28B) is a receptor with putative multifunctional signaling potential as well as five Ig-like domains, a TM, and a long tyrosine-motif containing CYT region. The variable genomic locations of the CaLITRs suggest that they are likely under the influence of different cis- and/or trans-regulatory elements. To better understand the transcriptional activities of select CaLITRs from variable genomic regions, we used an RT-qPCR-based approach to examine the expression of CaLITR1, CaLITR2, CaLITR3, and CaLITR4 during goldfish primary kidney macrophage (PKM) development and in mixed leukocyte reaction cultures (MLRs) of the goldfish. Our results showed that the select CaLITRs are differentially expressed during PKM development and in goldfish MLRs exposed to T-cell mitogens/immunosuppressive drugs, supporting that the transcription of these CaLITRs is likely regulated by distinct cis- and/or trans-regulatory elements.

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.

Immunoglobulin-like receptors and the generation of innate immune memory

Host immunity is classically divided into "innate" and "adaptive." While the former has always been regarded as the first, rapid, and antigen-nonspecific reaction to invading pathogens, the latter represents the more sophisticated and antigen-specific response that has the potential to persist and generate memory. Recent work however has challenged this dogma, where murine studies have successfully demonstrated the ability of innate immune cells (monocytes and macrophages) to acquire antigen-specific memory to allogeneic major histocompatibility complex (MHC) molecules. The immunoreceptors so far identified that mediate innate immune memory are the paired immunoglobulin-like receptors (PIRs) in mice, which are orthologous to human leukocyte immunoglobulin-like receptors (LILRs). These receptor families are mainly expressed by the myelomonocytic cell lineage, suggesting an important role in the innate immune response. In this review, we will discuss the role of immunoglobulin-like receptors in the development of innate immune memory across species.

Potential Applications of Fluorescence-Activated Cell Sorting (FACS) and Droplet-Based Microfluidics in Promoting the Discovery of Specific Antibodies for Characterizations of Fish Immune Cells

Akin to their mammalian counterparts, teleost fish possess a complex assortment of highly specialized immune cells that are capable of unleashing potent innate immune responses to eradicate or mitigate incoming pathogens, and also differentiate into memory lymphocytes to provide long-term protection. Investigations into specific roles and functions of fish immune cells depend on the precise separation of each cell type. Commonly used techniques, for example, density gradient centrifugation, rely on immune cells to have differing sizes or densities and thus fail to separate between similar cell types (e.g. T and B lymphocytes). Furthermore, a continuously growing database of teleost genomic information has revealed an inventory of cellular markers, indicating the possible presence of immune cell subsets in teleost fish. This further complicates the interpretation of results if subsets of immune cells are not properly separated. Consequently, monoclonal antibodies (mAbs) against specific cellular markers are required to precisely identify and separate novel subsets of immune cells in fish. In the field of fish immunology, mAbs are largely generated using the hybridoma technology, resulting in the development of mAbs against specific cellular markers in different fish species. Nevertheless, this technology suffers from being labour-intensive, time-consuming and most importantly, the inevitable loss of diversities of antibodies during the fusion of antibody-expressing B lymphocytes and myeloma cells. In light of this, the focus of this review is to discuss the potential applications of fluorescence-activated cell sorting and droplet-based microfluidics, two emerging technologies capable of screening and identifying antigen-specific B lymphocytes in a high-throughput manner, in promoting the development of valuable reagents for fish immunology studies. Our main goal is to encourage the incorporation of alternative technologies into the field of fish immunology to promote the production of specific antibodies in a high-throughput and cost-effective way, which could better allow for the precise separation of fish immune cells and also facilitate the identification of novel immune cell subsets in teleost fish.

The Genomic Organization of the LILR Region Remained Largely Conserved Throughout Primate Evolution: Implications for Health And Disease

The genes of the leukocyte immunoglobulin-like receptor (LILR) family map to the leukocyte receptor complex (LRC) on chromosome 19, and consist of both activating and inhibiting entities. These receptors are often involved in regulating immune responses, and are considered to play a role in health and disease. The human LILR region and evolutionary equivalents in some rodent and bird species have been thoroughly characterized. In non-human primates, the LILR region is annotated, but a thorough comparison between humans and non-human primates has not yet been documented. Therefore, it was decided to undertake a comprehensive comparison of the human and non-human primate LILR region at the genomic level. During primate evolution the organization of the LILR region remained largely conserved. One major exception, however, is provided by the common marmoset, a New World monkey species, which seems to feature a substantial contraction of the number of LILR genes in both the centromeric and the telomeric region. Furthermore, genomic analysis revealed that the killer-cell immunoglobulin-like receptor gene KIR3DX1, which maps in the LILR region, features one copy in humans and great ape species. A second copy, which might have been introduced by a duplication event, was observed in the lesser apes, and in Old and New World monkey species. The highly conserved gene organization allowed us to standardize the LILR gene nomenclature for non-human primate species, and implies that most of the receptors encoded by these genes likely fulfill highly preserved functions.

Identification of distinct LRC- and Fc receptor complex-like chromosomal regions in fish supports that teleost leukocyte immune-type receptors are distant relatives of mammalian Fc receptor-like molecules

Leukocyte immune-type receptors (LITRs) are a large family of immunoregulatory receptor-types originally identified in the channel catfish (Ictalurus punctatus (Ip)LITRs). Phylogenetic analyses of LITRs show that they share distant evolutionary relationships with important mammalian immunoregulatory receptors belonging to the Fc receptors family and the leukocyte receptor complex (LRC), but their syntenic relationships with these immunoglobulin superfamily members have not been investigated. To further examine the possible evolutionary connections between teleost LITRs and various mammalian immunoregulatory receptor-types, we surveyed the genomic databases of representative vertebrate taxa and our results show that teleost LITRs generally exist in large genomic clusters, which are linked to vangl2, arhgef11, and slam family genes, features that are also shared by amphibian and mammalian Fc receptor-like molecules (FCRLs). Moreover, detailed phylogenetic comparisons between the individual Ig-like domains of LITRs and mammalian FCRLs shows that these receptors share related Ig-like domains indicative of their common ancestry. However, contrary to our previous reports, no supportive evidence for phylogenetic relationships between the Ig-like domains of LITRs with the Ig-like domains of LRC-encoded mammalian immunoregulatory receptors was found. We also identified an LRC-like region in the zebrafish genome, but no expanded litr-related genes were located in this region. Similarly, no lilr-related genes were found in spotted gar, a representative basal ray-finned fish. Finally, two distantly related fcrls and an LRC-like gene were identified in the elephant shark genome, suggesting that the loss of an immunoregulatory receptor-containing LRC region may be unique to ray-finned fish.

A Fish Leukocyte Immune-Type Receptor Uses a Novel Intracytoplasmic Tail Networking Mechanism to Cross-Inhibit the Phagocytic Response

Channel catfish (Ictalurus punctatus) leukocyte immune-type receptors (IpLITRs) are a family of immunoregulatory proteins shown to regulate several innate immune cell effector responses, including phagocytosis. The precise mechanisms of IpLITR-mediated regulation of the phagocytic process are not entirely understood, but we have previously shown that different IpLITR-types use classical as well as novel pathways for controlling immune cell-mediated target engulfment. To date, all functional assessments of IpLITR-mediated regulatory actions have focused on the independent characterization of select IpLITR-types in transfected cells. As members of the immunoglobulin superfamily, many IpLITRs share similar extracellular Ig-like domains, thus it is possible that various IpLITR actions are influenced by cross-talk mechanisms between different IpLITR-types; analogous to the paired innate receptor paradigm in mammals. Here, we describe in detail the co-expression of different IpLITR-types in the human embryonic AD293 cell line and examination of their receptor cross-talk mechanisms during the regulation of the phagocytic response using imaging flow cytometry, confocal microscopy, and immunoprecipitation protocols. Overall, our data provides interesting new insights into the integrated control of phagocytosis via the antagonistic networking of independent IpLITR-types that requires the selective recruitment of inhibitory signaling molecules for the initiation and sustained cross-inhibition of phagocytosis.

Identification of goldfish (Carassius auratus L.) leukocyte immune-type receptors shows alternative splicing as a potential mechanism for receptor diversification

Leukocyte immune-type receptors (LITRs) are a multigene family of teleost immunoregulatory proteins that share structural, phylogenetic, and likely functional relationships with several innate immune receptor proteins in other vertebrates, including mammals. Originally discovered in channel catfish (Ictalurus punctatus), representative IpLITR-types have been shown to regulate diverse innate immune cell effector responses including phagocytosis, degranulation, and cytokine secretion. To date, IpLITRs have been primarily characterized using mammalian cell line expression systems, therefore many unanswered questions remain regarding their actual regulatory roles in fish immunity. In the present study, we report on the preliminary molecular characterization of five goldfish (Carassius auratus) CaLITR-types and the identification of several putative splice variants of these receptors cloned from various goldfish tissues and primary myeloid cell cultures. In general, CaLITR mRNA transcripts were detected in all goldfish tissues tested, and also in primary kidney macrophage and neutrophil cultures. Specifically, CaLITR1 is a functionally ambiguous receptor with no charged amino acids in its transmembrane (TM) segment and is devoid of tyrosine-based signaling motifs in its short cytoplasmic tail (CYT) region. CaLITR2 is a putative activating receptor-type that contains immunotyrosine-based activation motifs (ITAMs) within its long CYT region, and CaLITR3 has a positively charged TM segment, suggesting that it may recruit intracellular stimulatory adaptor signaling molecules. CaLITR4 and CaLITR5 appear to have diverse signaling capabilities since they contain various immunoregulatory signaling motifs within their CYT regions including putative Nck and STAT recruitment motifs as well as ITAM-like and ITIM sequences. We also identified putative CaLITR splice variants with altered extracellular Ig-like domain compositions and variable CYT regions. Interestingly, this suggests that alternative splicing-mediated diversification of CaLITRs can generate receptor forms with possible variable binding and/or intracellular signaling abilities. Overall, these findings reveal new information about the teleost LITRs and sets the stage for exploring how alternative splicing leads to the functional diversification of this complex multigene immunoregulatory receptor family.

Catfish lymphocytes expressing CC41-reactive leukocyte immune-type receptors (LITRs) proliferate in response to Edwardsiella ictaluri infection in vitro

Monoclonal antibodies (mAbs) CC34 and CC41 recognize overlapping subsets of leukocyte immune-type receptors (LITRs). The mAb CC34 was raised against the clonal TS32.15 cytotoxic T cell line and the mAb CC41 was raised against the clonal NK cell line TS10.1. In this study, an in vitro model was developed to monitor CC34- and CC41-reactive cells in response to Edwardsiella ictaluri infection. Briefly, head kidney leukocytes and peripheral blood lymphocytes (PBL) were isolated from individual catfish and labeled with CellTrace Violet and CellTrace FarRed dye, respectively. Head kidney-derived macrophages were infected with E. ictaluri and then cocultured with autologous PBL. The combined cell cultures were then analyzed using flow cytometry. A significant increase in CC41 staining was observed in the PBL population at 2, 5 and 7 days after culture, which suggest that LITRs are involved in cell-mediated immunity to E. ictaluri.

Microbial Danger Signals Control Transcriptional Induction of Distinct MHC Class I L Lineage Genes in Atlantic Salmon

Antigen processing and presentation by major histocompatibility complex (MHC) molecules is a cornerstone in vertebrate immunity. Like mammals, teleosts possess both classical MHC class I and multiple families of divergent MHC class I genes. However, while certain mammalian MHC class I-like molecules have proven to be integral in immune regulation against a broad array of pathogens, the biological relevance of the different MHC class I lineages in fish remains elusive. This work focuses on MHC class I L lineage genes and reveals unique regulatory patterns of six genes (Sasa-lia, Sasa-lda, Sasa-lca, Sasa-lga, Sasa-lha, and Sasa-lfa) in antimicrobial immunity of Atlantic salmon (Salmo salar L.). Using two separate in vivo challenge models with different kinetics and immune pathologies combined with in vitro stimulation using viral and bacterial TLR ligands, we show that de novo synthesis of different L lineage genes is distinctly regulated in response to various microbial stimuli. Prior to the onset of classical MHC class I gene expression, lia was rapidly and systemically induced in vivo by the single-stranded (ss) RNA virus salmonid alpha virus 3 (SAV3) but not in response to the intracellular bacterium Piscirickettsia salmonis. In contrast, lga expression was upregulated in response to both viral and bacterial stimuli. A role for distinct MHC class I L-lineage genes in anti-microbial immunity in salmon was further substantiated by a marked upregulation of lia and lga gene expression in response to type I IFNa stimulation in vitro. Comparably, lha showed no transcriptional induction in response to IFNa stimulation but was strongly induced in response to a variety of viral and bacterial TLR ligands. In sharp contrast, lda showed no response to viral or bacterial challenge. Similarly, induction of lca, which is predominantly expressed in primary and secondary lymphoid tissues, was marginal with the exception of a strong and transient upregulation in pancreas following SAV3 challenge Together, these findings suggest that certain Atlantic salmon MHC class I L lineage genes play important and divergent roles in early anti-microbial response and that their regulation, in response to different activation signals, represents a system for selectively promoting the expression of distinct non-classical MHC class I genes in response to different types of immune challenges.

Imaging flow cytometry and confocal microscopy-based examination of F-actin and phosphoinositide dynamics during leukocyte immune-type receptor-mediated phagocytic events

Cells of the innate immune system rapidly detect and eliminate invading microbes using surface-expressed immunoregulatory receptors that translate extracellular binding events into potent effector responses. Channel catfish (Ictalurus punctatus) leukocyte immune-type receptors (IpLITRs) are a family of immunoregulatory proteins that have been shown to regulate several innate immune cell effector responses including the phagocytic process. The mechanisms by which these receptors regulate phagocytosis are not entirely understood but we have previously shown that different IpLITR-types use ITAM-dependent as well as ITAM-independent pathways for controlling target engulfment. The main objective of this study was to develop and use imaging flow cytometry and confocal microscopy-based assays to further examine both F-actin and phosphoinositide dynamics that occur during the different IpLITR-mediated phagocytic pathways. Results show that the ITAM-dependent IpLITR-induced phagocytic response promotes canonical changes in F-actin polymerization and PI(4,5)P₂ redistributions. However, the ITAM-independent IpLITR phagocytic response induced unique patterns of F-actin and PI(4,5)P₂ redistributions, which are likely due to its ability to regulate alternative signaling pathways. Additionally, both IpLITR-induced phagocytic pathways induced target internalization into PI(3)P-enriched phagosomes indicative of a maturing phagosome compartment. Overall, this imaging-based platform can be further applied to monitor the recruitment and distribution of signaling molecules during IpLITR-mediated phagocytic processes and may serve as a useful strategy for functional examinations of other immunoregulatory receptor-types in fish.

Selective Regulation of Cytoskeletal Dynamics and Filopodia Formation by Teleost Leukocyte Immune-Type Receptors Differentially Contributes to Target Capture During the Phagocytic Process

Phagocytosis evolved from a fundamental nutrient acquisition mechanism in primitive unicellular amoeboids, into a dynamic and complex component of innate immunity in multicellular organisms. To better understand the cellular mechanisms contributing to phagocytic processes across vertebrates, our research has focused on characterizing the involvement of innate immune proteins originally identified in channel catfish (Ictalurus punctatus) called leukocyte immune-type receptors (IpLITRs). These unique teleost proteins share basic structural as well as distant phylogenetic relationships with several immunoregulatory proteins within the mammalian immunoglobulin superfamily. In the present study, we use a combination of live-cell confocal imaging and high-resolution scanning electron microscopy to further examine the classical immunoreceptor tyrosine-based activation motif (ITAM)-dependent phagocytic pathway mediated by the chimeric construct IpLITR 2.6b/IpFcRγ-L and the functionally diverse immunoreceptor tyrosine-based inhibitory motif-containing receptor IpLITR 1.1b. Results demonstrate that IpLITR 1.1b-expressing cells can uniquely generate actin-dense filopodia-like protrusions during the early stages of extracellular target interactions. In addition, we observed that these structures retract after contacting extracellular targets to secure captured microspheres on the cell surface. This activity was often followed by the generation of robust secondary waves of actin polymerization leading to the formation of stabilized phagocytic cups. At depressed temperatures of 27°C, IpLITR 2.6b/IpFcRγ-L-mediated phagocytosis was completely blocked, whereas IpLITR 1.1b-expressing cells continued to generate dynamic actin-dense filopodia at this lower temperature. Overall, these results provide new support for the hypothesis that IpLITR 1.1b, but not IpLITR 2.6b/IpFcRγ-L, directly triggers filopodia formation when expressed in representative myeloid cells. This also offers new information regarding the directed ability of immunoregulatory receptor-types to initiate dynamic membrane structures and provides insights into an alternative ITAM-independent target capture pathway that is functionally distinct from the classical phagocytic pathways.

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.

Imaging flow cytometry and GST pulldown assays provide new insights into channel catfish leukocyte immune-type receptor-mediated phagocytic pathways

Channel catfish (Ictalurus punctatus) leukocyte immune-type receptors (IpLITRs) control various innate immune cell effector responses including the phagocytic process. This large immunoregulatory receptor family also consists of multiple receptor-types with variable signaling abilities that is dependent on their inherent or acquired tyrosine-containing cytoplasmic tail (CYT) regions. For example, IpLITR 2.6b associates with the immunoreceptor tyrosine-based activation motif (ITAM)-containing adaptor molecule IpFcRγ-L, and when expressed in mammalian cells it activates phagocytosis using a similar profile of intracellular signaling mediators that also regulate the prototypical mammalian Fc receptor (FcR) phagocytic pathway. Alternatively, IpLITR 1.1b contains a long tyrosine-containing CYT with multifunctional capabilities including both inhibitory and stimulatory actions. Recently, we demonstrated that IpLITR 1.1b activates a unique phagocytic pathway involving the generation of multiple plasma membrane extensions that rapidly capture extracellular targets and secure them on the cell surface in phagocytic cup-like structures. Occasionally, these captured targets are completely engulfed albeit at a significantly lower rate than what was observed for IpLITR 2.6b. While this novel IpLITR 1.1b phagocytic activity is insensitive to classical blockers of phagocytosis, its distinct target capture and engulfment actions depend on the engagement of the actin polymerization machinery. However, it is not known how this protein translates target recognition into intracellular signaling events during this atypical mode of phagocytosis. Using imaging flow cytometry and GST pulldown assays, the aims of this study were to specifically examine what regions of the IpLITR 1.1b CYT trigger phagocytosis and to establish what profile of intracellular signaling molecules likely participate in its actions. Our results show that in stably transfected AD293 cells, the membrane proximal and distal CYT segments of IpLITR 1.1b independently regulate its phagocytic activities. These CYT regions were also shown to differentially recruit various SH2 domain-containing intracellular mediators, which provides new information about the dynamic immunoregulatory abilities of IpLITR 1.1b. Overall, this work further advances our understanding of how certain immunoregulatory receptor-types link extracellular target binding events to the actin polymerization machinery during a non-classical mode of phagocytosis.

Trypsin differentially modulates the surface expression and function of channel catfish leukocyte immune-type receptors

Channel catfish (Ictalurus punctatus) leukocyte immune-type receptors (IpLITRs) are immunoregulatory proteins that control innate immune cellular responses. Previously, we demonstrated that two representative IpLITR forms, IpLITR 2.6b and IpLITR 1.1b, engage distinct components of the phagocytic machinery resulting in unique target capture and engulfment phenotypes. IpLITR-induced phagocytic mechanisms were also differentially susceptible to temperature and pharmacological inhibitors of canonical signaling mediators. In the present study, we examined the sensitivity of IpLITR-mediated phagocytosis to the endogenous serine-protease trypsin, a well-known mediator of immunoregulatory receptor functions. Trypsin selectively reduced IpLITR 1.1b cell surface expression and phagocytic activity in a dose-dependent manner. We also observed a significant alteration of the IpLITR 1.1b phagocytic phenotype post-trypsin exposure; whereas, the IpLITR 2.6b-mediated target engulfment phenotype was unchanged. Recovery experiments suggested that trypsin-induced inhibition of IpLITR 1.1b-dependent phagocytosis was reversible and that the re-establishment of phagocytic function was associated with a recovery of receptor surface expression. Cell-surface biotinylation and immunoprecipitation studies demonstrated that IpLITR 1.1b normally exists as a mature (∼70 kDa) protein on the cell surface. However, trypsin treatment reduced expression of the mature receptor and processed IpLITR 1.1b into an ∼60 kDa form. The trypsin-generated and putative immature IpLITR 1.1b form was not present on the cell surface; suggesting that the cleaved receptor may have been internalized, post-processing, by regulated endocytosis. Taken together, these results reveal a unique role for trypsin as a selective modulator of IpLITR-mediated phagocytosis and highlight a conserved role for serine proteases as potent immunomodulatory factors.

Biochemical and Functional Insights into the Integrated Regulation of Innate Immune Cell Responses by Teleost Leukocyte Immune-Type Receptors

Across vertebrates, innate immunity consists of a complex assortment of highly specialized cells capable of unleashing potent effector responses designed to destroy or mitigate foreign pathogens. The execution of various innate cellular behaviors such as phagocytosis, degranulation, or cell-mediated cytotoxicity are functionally indistinguishable when being performed by immune cells isolated from humans or teleost fishes; vertebrates that diverged from one another more than 450 million years ago. This suggests that vital components of the vertebrate innate defense machinery are conserved and investigating such processes in a range of model systems provides an important opportunity to identify fundamental features of vertebrate immunity. One characteristic that is highly conserved across vertebrate systems is that cellular immune responses are dependent on specialized immunoregulatory receptors that sense environmental stimuli and initiate intracellular cascades that can elicit appropriate effector responses. A wide variety of immunoregulatory receptor families have been extensively studied in mammals, and many have been identified as cell- and function-specific regulators of a range of innate responses. Although much less is known in fish, the growing database of genomic information has recently allowed for the identification of several immunoregulatory receptor gene families in teleosts. Many of these putative immunoregulatory receptors have yet to be assigned any specific role(s), and much of what is known has been based solely on structural and/or phylogenetic relationships with mammalian receptor families. As an attempt to address some of these shortcomings, this review will focus on our growing understanding of the functional roles played by specific members of the channel catfish (Ictalurus punctatus) leukocyte immune-type receptors (IpLITRs), which appear to be important regulators of several innate cellular responses via classical as well as unique biochemical signaling networks.

A Leukocyte Immune-Type Receptor Subset Is a Marker of Antiviral Cytotoxic Cells in Channel Catfish, Ictalurus punctatus

Channel catfish, Ictalurus punctatus, leukocyte immune type receptors (LITRs) represent a multigene family that encodes Ig superfamily proteins that mediate activating or inhibitory signaling. In this study, we demonstrate the use of mAb CC41 to monitor viral cytotoxic responses in catfish and determine that CC41 binds to a subset of LITRs on the surface of catfish clonal CTLs. Homozygous gynogenetic catfish were immunized with channel catfish virus (CCV)-infected MHC-matched clonal T cells (G14D-CCV), and PBL were collected at various times after immunization for flow cytometric analyses. The percentage of CC41(+) cells was significantly increased 5 d after primary immunization with G14D-CCV and at 3 d after a booster immunization as compared with control fish only injected with G14D. Moreover, CC41(+) cells magnetically isolated from the PBL specifically killed CCV-infected targets as measured by (51)Cr release assays and expressed messages for CD3γδ, perforin, and at least one of the CD4-like receptors as analyzed by RNA flow cytometry. When MLC effector cells derived from a G14D-CCV-immunized fish were preincubated with CC41 mAb, killing of G14D-CCV targets was reduced by ∼40%, suggesting that at least some LITRs have a role in target cell recognition and/or cytotoxicity. The availability of a LITR-specific mAb has allowed, to our knowledge for the first time, functional characterization of LITRs in an autologous system. In addition, the identification of an LITR subset as a cytotoxic cell marker will allow for more effective monitoring of catfish immune responses to pathogens.

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.

Fc receptors for immunoglobulins and their appearance during vertebrate evolution

Receptors interacting with the constant domain of immunoglobulins (Igs) have a number of important functions in vertebrates. They facilitate phagocytosis by opsonization, are key components in antibody-dependent cellular cytotoxicity as well as activating cells to release granules. In mammals, four major types of classical Fc receptors (FcRs) for IgG have been identified, one high-affinity receptor for IgE, one for both IgM and IgA, one for IgM and one for IgA. All of these receptors are related in structure and all of them, except the IgA receptor, are found in primates on chromosome 1, indicating that they originate from a common ancestor by successive gene duplications. The number of Ig isotypes has increased gradually during vertebrate evolution and this increase has likely been accompanied by a similar increase in isotype-specific receptors. To test this hypothesis we have performed a detailed bioinformatics analysis of a panel of vertebrate genomes. The first components to appear are the poly-Ig receptors (PIGRs), receptors similar to the classic FcRs in mammals, so called FcRL receptors, and the FcR γ chain. These molecules are not found in cartilagous fish and may first appear within bony fishes, indicating a major step in Fc receptor evolution at the appearance of bony fish. In contrast, the receptor for IgA is only found in placental mammals, indicating a relatively late appearance. The IgM and IgA/M receptors are first observed in the monotremes, exemplified by the platypus, indicating an appearance during early mammalian evolution. Clearly identifiable classical receptors for IgG and IgE are found only in marsupials and placental mammals, but closely related receptors are found in the platypus, indicating a second major step in Fc receptor evolution during early mammalian evolution, involving the appearance of classical IgG and IgE receptors from FcRL molecules and IgM and IgA/M receptors from PIGR.

Induction of phagocytosis and intracellular signaling by an inhibitory channel catfish leukocyte immune-type receptor: evidence for immunoregulatory receptor functional plasticity in teleosts

Immunoregulatory receptors are categorized as stimulatory or inhibitory based on their engagement of unique intracellular signaling networks. These proteins also display functional plasticity, which adds versatility to the control of innate immunity. Here we demonstrate that an inhibitory catfish leukocyte immune-type receptor (IpLITR) also displays stimulatory capabilities in a representative myeloid cell model. Previously, the receptor IpLITR 1.1b was shown to inhibit natural killer cell-mediated cytotoxicity. Here we expressed IpLITR 1.1b in rat basophilic leukemia-2H3 cells and monitored intracellular signaling and functional responses. Although IpLITR 1.1b did not stimulate cytokine secretion, activation of this receptor unexpectedly induced phagocytosis as well as extracellular signal-related kinase 1/2- and protein kinase B (Akt)-dependent signal transduction. This novel IpLITR 1.1b-mediated response was independent of an association with the FcRγ chain and was likely due to phosphotyrosine-dependent adaptors associating with prototypical signaling motifs within the distal region of its cytoplasmic tail. Furthermore, compared to a stimulatory IpLITR, IpLITR 1.1b displayed temporal differences in the induction of intracellular signaling, and IpLITR 1.1b-mediated phagocytosis had reduced sensitivity to EDTA and cytochalasin D. Overall, this is the first demonstration of functional plasticity for teleost LITRs, a process likely important for the fine-tuning of conserved innate defenses.

The innate immune-related genes in catfish

Catfish is one of the most important aquaculture species in America (as well as in Asia and Africa). In recent years, the production of catfish has suffered massive financial losses due to pathogen spread and breakouts. Innate immunity plays a crucial role in increasing resistance to pathogenic organisms and has generated increasing interest in the past few years. This review summarizes the current understanding of innate immune-related genes in catfish, including pattern recognition receptors, antimicrobial peptides, complements, lectins, cytokines, transferrin and gene expression profiling using microarrays and next generation sequencing technologies. This review will benefit the understanding of innate immune system in catfish and further efforts in studying the innate immune-related genes in fish.

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.

Antisense approaches for elucidating ranavirus gene function in an infected fish cell line

Viral virulence/immune evasion strategies and host anti-viral responses represent different sides of the continuing struggle between virus and host survival. To identify virus-encoding molecules whose function is to subvert or blunt host immune responses, we have adapted anti-sense approaches to knock down the expression of specific viral gene products. Our intention is to correlate knock down with loss of function and thus infer the role of a given viral gene. As a starting point in this process we have targeted several structural and catalytic genes using antisense morpholino oligonucleotides (asMO) and small, interfering RNAs (siRNA). In proof of concept experiments we show the feasibility of this approach and describe recent work targeting five frog virus 3 genes. Our results indicate that both 46K and 32R, two immediate-early viral proteins, are essential for replication in vitro, and confirm earlier findings that the major capsid protein, the largest subunit of the viral homolog of RNA polymerase II, and the viral DNA methyltransferase are also essential for replication in cell culture.

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.

The red jungle fowl leukocyte receptor complex contains a large, highly diverse number of chicken immunoglobulin-like receptor (CHIR) genes

The chicken Ig-like receptor (CHIR) gene family is located on microchromosome 31, the orthologous region to the mammalian leukocyte receptor complex. CHIR are equally related to the mammalian killer Ig-like receptors and leukocyte Ig-like transcripts, but they occur in a much higher number and diversity. The chicken microchromosome 31 has been neglected in the genome sequence analysis. Here, we provide a first analysis of this region. For this purpose bacterial artificial chromosome (BAC) sequences originating from a single inbred red jungle fowl that served as basis for the chicken genome project were screened for the presence of CHIR sequences and eight BACs were identified as major CHIR containing regions. Since the sequences of these BACs that were available in the database were not complete, sequence gaps were further closed by novel data from the chicken genome project. The entire sequence was aligned into 26 contigs covering 875kbp that contained 84 functional CHIR and 46 CHIR pseudogenes that were hampered by different reasons such as premature stop codons. The 84 functional CHIR were further categorized into 35 activating (CHIRA), 26 inhibitory (CHIRB) and 23 bifunctional (CHIRAB) genes. A detailed comparison of the annotated sequence taking also into account the previously published CHIR BAC sequence originating from an Lohman selected leghorn chicken revealed that the CHIR locus seems to be a very active region with a high degree of gene reorganization that resembles a constant birth and death evolution. The present report provides a framework for the future completion of the CHIR locus. It further suggests that the entire microchromosome 31 may resemble a locus of extraordinary genomic diversity that is beneficial for the development of a large CHIR repertoire, but that has therefore lost all other genes, where such a diversification would be fatal.

The amphibians Xenopus laevis and Silurana tropicalis possess a family of activating KIR-related Immunoglobulin-like receptors

In this study, we searched the amphibian species Xenopus laevis and Silurana (Xenopus) tropicalis for the presence of genes homologous to mammalian KIRs and avian CHIRs (KRIR family). By experimental and computational procedures, we identified four related ILR (Ig-like Receptors) genes in S. tropicalis and three in X. laevis. ILRs encode type I transmembrane receptors with 3-4 Ig-like extracellular domains. All predicted ILR proteins appear to be activating receptors. ILRs have a broad expression pattern, the gene transcripts were found in both lymphoid and non-lymphoid tissues. Phylogenetic analysis shows that the amphibian KRIR family receptors evolved independently from their mammalian and avian counterparts. The only conserved structural element of tetrapod KRIRs is the NxxR motif-containing transmembrane domain that facilitates association with FcRgamma subunit. Our findings suggest that if KRIRs of various vertebrates have any common function at all, such a function is activating rather than inhibitory.

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.

Expression profiles of cloned channel catfish (Ictalurus punctatus) lymphoid cell lines and mixed lymphocyte cultures

Clonal channel catfish lymphoid cell lines and mixed lymphocyte cultures (MLCs) have proven extremely useful in examining immune responses at the cellular and molecular levels. To date clonal catfish cell lines and MLCs have been biologically and phenotypically characterized using a variety of techniques including reverse transcription polymerase chain reaction (RT-PCR), as well as Northern and Southern blotting. To expand the molecular characterization of these cultures, microarray analysis was employed. Clonal B (3B11), macrophage (42TA), and cytotoxic T cell (TS32.15 and TS32.17) lines and MLCs were examined using a cDNA array containing approximately 2500 probes derived from EST libraries prepared from the 42TA macrophage cell line, a MLC, and 5-14-day-old catfish fry. Analysis showed that each cell line displayed a unique RNA expression profile that included a variety of immune-related genes. Pearson correlation analysis indicated that one cytotoxic T cell line (TS32.15) clustered with the MLC, whereas a second cytotoxic T cell line (TS32.17) was more closely associated with a second cluster containing B cells and macrophages. This study illustrates the utility of microarray analyses in profiling RNA expression patterns in catfish lymphoid cell lines and will serve as a platform for examining catfish immune responses following virus infection or poly [I:C] treatment.

The Xenopus FcR family demonstrates continually high diversification of paired receptors in vertebrate evolution

BACKGROUND

Recent studies have revealed an unexpected diversity of domain architecture among FcR-like receptors that presumably fulfill regulatory functions in the immune system. Different species of mammals, as well as chicken and catfish have been found to possess strikingly different sets of these receptors. To better understand the evolutionary history of paired receptors, we extended the study of FcR-like genes in amphibian representatives Xenopus tropicalis and Xenopus laevis.

RESULTS

The diploid genome of X. tropicalis contains at least 75 genes encoding paired FcR-related receptors designated XFLs. The allotetraploid X. laevis displays many similar genes primarily expressed in lymphoid tissues. Up to 35 domain architectures generated by combinatorial joining of six Ig-domain subtypes and two subtypes of the transmembrane regions were found in XFLs. None of these variants are shared by FcR-related proteins from other studied species. Putative activating XFLs associate with the FcRgamma subunit, and their transmembrane domains are highly similar to those of activating mammalian KIR-related receptors. This argues in favor of a common origin for the FcR and the KIR families. Phylogenetic analysis shows that the entire repertoires of the Xenopus and mammalian FcR-related proteins have emerged after the amphibian-amniotes split.

CONCLUSION

FcR- and KIR-related receptors evolved through continual species-specific diversification, most likely by extensive domain shuffling and birth-and-death processes. This mode of evolution raises the possibility that the ancestral function of these paired receptors was a direct interaction with pathogens and that many physiological functions found in the mammalian receptors were secondary acquisitions or specializations.

Characterization of additional novel immune type receptors in channel catfish, Ictalurus punctatus

Mining of channel catfish (Ictalurus punctatus) expressed sequence tag databases identified seven new novel immune type receptors (IpNITRs). These differed in sequence, but not structure, from previously described IpNITR1-11. IpNITR12a, 12b, 13, and 14 encode proteins containing a single variable (V)-like immunoglobulin (Ig) domain. IpNITR12a and 13 encode a transmembrane (TM) region and cytoplasmic tail (CYT) containing immunoreceptor tyrosine inhibition motifs (ITIMs). IpNITR14 contains a TM and short CYT devoid of signaling motifs and is similar in structure to IpNITR7. IpNITR12b lacks a TM and may represent an IpNITR12a splice variant. In contrast, IpNITR15a, 15b, and 16 encode two Ig domains (V-like domain 1 and V/C2-like domain 2). IpNITR15a and 15b contain TM and CYT with ITIMs. IpNITR16 appears to be a secreted form. The first V-like domains of IpNITR12-16 (except a/b pairs) share 17-32% amino acid identity with each other and with V domains of IpNITR1-11. They therefore represent five additional IpNITR V families (defined as possessing 70% or more amino acid identity). The V/C2 domains of IpNITR15a, 15b and 16 have 94-98% amino acid identity, but share 37-50% amino acid identity with corresponding V/C2 domains found in IpNITR1-4. Phylogenetic analyses indicate IpNITR12-16 are more closely related to other teleost NITRs than to IpNITR1-11. Gene mapping indicates that IpNITRs are linked, and members of the ten known IpNITR families are interspersed. IpNITR12-16 are differentially expressed in various catfish immune-type cells and preferentially up regulated in peripheral blood leukocytes by allogeneic stimulation.