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

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.

Counteracting immunotyrosine-based signaling motifs augment zebrafish leukocyte immune-type receptor-mediated phagocytic activity

Leukocyte immune-type receptors (LITRs) represent a polymorphic and polygenic family of immunoregulatory proteins originally discovered in channel catfish (Ictalurus punctatus; IpLITRs). Belonging to the immunoglobulin superfamily (IgSF), IpLITRs are generally classified as stimulatory or inhibitory types based on their utilization of various intracellular tyrosine-based signaling motifs. While research has shown that IpLITRs can activate as well as abrogate different immune cell effector responses including phagocytosis, recent identification of LITRs within the zebrafish genome (Danio rerio; DrLITRs) revealed the existence of fish LITR-types uniquely containing counteracting stimulatory and inhibitory cytoplasmic tail (CYT) region motifs (i.e., an immunoreceptor tyrosine-based activation motif; ITAM, and immunoreceptor tyrosine-based inhibitory motif; ITIM) within the same receptor. This arrangement is unusual as these motifs typically exist on separate stimulatory (i.e., ITAM-containing) or inhibitory (i.e., ITIM-containing) immunoregulatory receptors that then co-engage to fine-tune cellular signaling and effector responses. Using a flow cytometric-based phagocytosis assay, we show here that engagement of DrLITR 1.2-expressing cells with antibody coated 4.5 μm beads causes a robust ITAM-dependent phagocytic response and reveal that its tandem ITIM motif surprisingly enhances the DrLITR 1.2-induced phagocytic activity while simultaneously decreasing the receptors ability to bind the beads. Confocal microscopy studies also revealed that the ITIM-associated inhibitory signaling molecule SHP-2 is localized to the phagocytic synapse during the phagocytic response. Overall, these results provide the first functional characterization of teleost immune receptors containing a tandem ITAM and ITIM and allow for the proposal of an intracytoplasmic tail signaling model for ITIM-mediated enhancement of ITAM-dependent cellular activation.

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.