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

Abstract

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.

Leukocyte Immunoglobulin-Like Receptors A2 and A6 are Expressed in Avian Macrophages and Modulate Cytokine Production by Activating Multiple Signaling Pathways

“…A deeper understanding of the intracellular signal transduction pathways initiated by LILRA2 or LILRA6 is required to understand how they activate immune-related gene expression. Moreover, LILRA genes (LILRA1, 3 and 5) has been shown to play an important role in innate immune responses through regulation of the ERK/MEK [ 15 ], TLR [ 3 , 39 ] and JNK/p38MAPK [ 16 ] signaling pathways, as well as the antigen-presenting phenotype and cytokine production [ 3 , 6 , 9 ]. Increased expression levels of phosphorylated STAT1, STAT3 and JAK2 and un-phosphorylated STAT1/3, JAK2 and TYK2 molecules in response to LILRA2 and LILRA6 transfected in HD11 cells were detected by western blot and FACS analyses.…”

“…Moreover, LILRAs have been demonstrated to play important roles in infection or autoimmune diseases such as HIV infection [ 11 ], multiple sclerosis [ 12 ], atopic dermatitis [ 13 ] and rheumatoid arthritis [ 14 ]. It has been reported that LILRA1, 3, 4 and 5 bind with HLA-G, HLA-C and classical HLAs [ 3 , 7 , 11 , 13 , 14 ] and regulate adaptive or innate immune pathways such as the ERK/MEK [ 15 ], TLR [ 3 ] and JNK/p38MAPK [ 16 ] signaling pathways. Additionally, they have been reported to upregulate the cytokines IL-1R, IL-4, IL-6, IL-10, IL-12, IL-17, TNFα and IFN-γ [ 3 , 7 , 11 , 14 ].…”

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

“…These results suggest that IpLITR 1.1b can uniquely network with intracellular components requisite for the production of F-actin containing filopodia-like structures. Previously, we hypothesized that IpLITR 1.1b-controlled signaling events induce formation of macromolecular complexes with its CYT that pre-assemble prior to receptor engagement; effectively priming the receptor for subsequent interactions with extracellular targets ( 28 – 30 ). Pre-associations of IpLITR 1.1b with intracellular effectors capable of modulating the cytoskeletal machinery would allow for dynamic membrane remodeling events prior to the formation of stable receptor–ligand interactions ( 28 – 30 ).…”

“…The following day cells were washed with phosphate buffered saline (PBS) and then incubated in phagocytosis buffer (1:1 mixture of 1× PBS containing 2 mg/mL bovine serum albumin, Sigma-Aldrich) and 1× Opti-MEM reduced serum medium (Fisher Scientific Company) containing 9 × 10 5 4.5-µm target microspheres (beads; Polybead ® Carboxylate YG microspheres; Polysciences, Warrington, PA, USA) opsonized with 10 µg/mL of αHA mAb (Cedarlane Laboratories Ltd., Burlington, ON, Canada) or 10 µg/mL of the isotype control mouse IgG3 (Beckman Coulter, Mississauga, ON, Canada). Antibody opsonization was performed by absorbing them onto protein A precoated microspheres (isolated from Staphylococcus aureus ; Sigma-Aldrich) as previously described ( 28 , 29 ). Plates containing cells and target beads were then centrifuged at 1,500 rpm for 1 min to synchronize cell–bead interactions and then incubated for 1 h at either 27 or 37°C.…”

“…To date, IpLITRs have only been reported in the channel catfish ( 23 , 24 ) and zebrafish ( Danio rerio) ( 24 , 26 , 27 ) but they are likely not exclusive to these species as we have identified IpLITR-related sequences in several other fish species from the non-redundant protein sequence databases ( unpublished data ). Overall, IpLITRs share basic structural as well as distant phylogenetic relationships with several immunoregulatory proteins within the mammalian immunoglobulin superfamily ( 23 ) and they appear to be important regulators of innate cellular responses via classical as well as unique biochemical signaling networks ( 28 – 30 ). Although our functional characterization of IpLITR-types has relied on heterologous expression of teleost proteins in mammalian cells, this strategy has allowed us to demonstrate important conserved aspects regarding IpLITR-mediated immunoregulatory signaling events and revealed some unanticipated aspects regarding the versatility of IpLITR-mediated transduction.…”