Processing of fish Ig heavy chain transcripts: diverse splicing patterns and unusual nonsense mediated decay

Tracing the origin of fish immunoglobulins

We have studied the origin of immunoglobulin genes in fish. There are two evolutionary lines of bony fish, Actinopterygii and Sarcopterygii. The former gave rise to most of the current fish and the latter to the animals that went to land. Non-teleost actinopterygians are significant evolutionary, sharing a common ancestor with sarcopterygians. There are three different immunoglob- ulin isotypes in ray-finned fish: IgM, IgD and IgT. We deduce that translocon formation in im- munoglobulins genes occurred already in non-teleost Actinopterygii. We establish a relationship between no teleosts and teleostean fish at the domain level of different immunoglobulins. We found two evolutionary lines of immunoglobulin. A line that starts from Immunoglobulin M and another from an ancestral Immunoglobulin W. The M line is stable, and the W line gives rise to the IgD of the fish. Immunoglobulin T emerges by recombination between both lines.

Immunoglobulin D and its encoding genes: An updated review

Since the identification of a functional Cδ gene in ostriches, immunoglobulin (Ig) D has been considered to be an extremely evolutionarily conserved Ig isotype besides the IgM found in all classes of jawed vertebrates. However, in contrast to IgM (which remains stable over evolutionary time), IgD shows considerable structural plasticity among vertebrate species and, moreover, its functions are far from elucidated even in humans and mice. Recently, several studies have shown that high expression of the IgD-B-cell receptor (IgD-BCR) may help physiologically autoreactive B cells survive in peripheral lymphoid tissues thanks to unresponsiveness to self-antigens and help their entry into germinal centers to "redeem" autoreactivity via somatic hypermutation. Other studies have demonstrated that secreted IgD may enhance mucosal homeostasis and immunity by linking B cells with basophils to optimize T-helper-2 cell-mediated responses and to constrain IgE-mediated basophil degranulation. Herein, we review the new discoveries on IgD-encoding genes in jawed vertebrates in the past decade. We also highlight advances in the functions of the IgD-BCR and secreted IgD in humans and mice.

Mucosal immunoglobulins of teleost fish: A decade of advances

Immunoglobulins (Igs) are complex glycoproteins that play critical functions in innate and adaptive immunity of all jawed vertebrates. Given the unique characteristics of mucosal barriers, secretory Igs (sIgs) have specialized to maintain homeostasis and keep pathogens at bay at mucosal tissues from fish to mammals. In teleost fish, the three main IgH isotypes, IgM, IgD and IgT/Z can be found in different proportions at the mucosal secretions of the skin, gills, gut, nasal, buccal, and pharyngeal mucosae. Similar to the role of mammalian IgA, IgT plays a predominant role in fish mucosal immunity. Recent studies in IgT have illuminated the primordial role of sIgs in both microbiota homeostasis and pathogen control at mucosal sites. Ten years ago, IgT was discovered to be an immunoglobulin class specialized in mucosal immunity. Aiming at this 10-year anniversary, the goal of this review is to summarize the current status of the field of fish Igs since that discovery, while identifying knowledge gaps and future avenues that will move the field forward in both basic and applied science areas.

Viral Resistance and IFN Signaling in STAT2 Knockout Fish Cells

IFN belong to a group of cytokines specialized in the immunity to viruses. Upon viral infection, type I IFN is produced and alters the transcriptome of responding cells through induction of a set of IFN stimulated genes (ISGs) with regulatory or antiviral function, resulting in a cellular antiviral state. Fish genomes have both type I IFN and type II IFN (IFN-γ), but no type III (λ) IFN has been identified. Their receptors are not simple counterparts of the mammalian type I/II IFN receptors, because alternative chains are used in type I IFN receptors. The mechanisms of the downstream signaling remain partly undefined. In mammals, members of the signal transducer and activator of family of transcription factors are responsible for the transmission of the signal from cytokine receptors, and STAT2 is required for type I but not type II IFN signaling. In fish, its role in IFN signaling in fish remains unclear. We isolated a Chinook salmon (Oncorhynchus tshawytscha) cell line, GS2, with a stat2 gene knocked out by CRISPR/Cas9 genome editing. In this cell line, the induction of ISGs by stimulation with a recombinant type I IFN is completely obliterated as evidenced by comparative RNA-seq analysis of the transcriptome of GS2 and its parental counterpart, EC. Despite a complete absence of ISGs induction, the GS2 cell line has a remarkable ability to resist to viral infections. Therefore, other STAT2-independent pathways may be induced by the viral infection, illustrating the robustness and redundancy of the innate antiviral defenses in fish.

Effects of Sex Steroids on Fish Leukocytes

In vertebrates, in addition to their classically reproductive functions, steroids regulate the immune system. This action is possible mainly due to the presence of steroid receptors in the different immune cell types. Much evidence suggests that the immune system of fish is vulnerable to xenosteroids, which are ubiquitous in the aquatic environment. In vivo and in vitro assays have amply demonstrated that oestrogens interfere with both the innate and the adaptive immune system of fish by regulating the main leukocyte activities and transcriptional genes. They activate nuclear oestrogen receptors and/or G-protein coupled oestrogen receptor. Less understood is the role of androgens in the immune system, mainly due to the complexity of the transcriptional regulation of androgen receptors in fish. The aim of this manuscript is to review our present knowledge concerning the effect of sex steroid hormones and the presence of their receptors on fish leukocytes, taking into consideration that the studies performed vary as regard the fish species, doses, exposure protocols and hormones used. Moreover, we also include evidence of the probable role of progestins in the regulation of the immune system of fish.

B cells and their role in the teleost gut

Mucosal surfaces are the main route of entry for pathogens in all living organisms. In the case of teleost fish, mucosal surfaces cover the vast majority of the animal. As these surfaces are in constant contact with the environment, fish are perpetually exposed to a vast number of pathogens. Despite the potential prevalence and variety of pathogens, mucosal surfaces are primarily populated by commensal non-pathogenic bacteria. Indeed, a fine balance between these two populations of microorganisms is crucial for animal survival. This equilibrium, controlled by the mucosal immune system, maintains homeostasis at mucosal tissues. Teleost fish possess a diffuse mucosa-associated immune system in the intestine, with B cells being one of the main responders. Immunoglobulins produced by these lymphocytes are a critical line of defense against pathogens and also prevent the entrance of commensal bacteria into the epithelium. In this review we will summarize recent literature regarding the role of B-lymphocytes and immunoglobulins in gut immunity in teleost fish, with specific focus on immunoglobulin isotypes and the microorganisms, pathogenic and non-pathogenic that interact with the immune system.

Antibody Repertoires in Fish

As in mammals, cartilaginous and teleost fishes possess adaptive immune systems based on antigen recognition by immunoglobulins (Ig), T cell receptors (TCR), and major histocompatibility complex molecules (MHC) I and MHC II molecules. Also it is well established that fish B cells and mammalian B cells share many similarities, including Ig gene rearrangements, and production of membrane Ig and secreted Ig forms. This chapter provides an overview of the IgH and IgL chains in cartilaginous and bony fish, including their gene organizations, expression, diversity of their isotypes, and development of the primary repertoire. Furthermore, when possible, we have included summaries of key studies on immune mechanisms such as allelic exclusion, somatic hypermutation, affinity maturation, class switching, and mucosal immune responses.

Adaptive immune responses at mucosal surfaces of teleost fish

This review describes the extant knowledge on the teleostean mucosal adaptive immune mechanisms, which is relevant for the development of oral or mucosal vaccines. In the last decade, a number of studies have shed light on the presence of new key components of mucosal immunity: a distinct immunoglobulin class (IgT or IgZ) and the polymeric Ig receptor (pIgR). In addition, intestinal T cells and their putative functions, antigen uptake mechanisms at mucosal surfaces and new mucosal vaccination strategies have been reported. New information on pIgR of Atlantic cod and common carp and comparison of natural and specific cell-mediated cytotoxicity in the gut of common carp and European seabass, is also included in this review. Based on the known facts about intestinal immunology and mucosal vaccination, suggestions are made for the advancement of fish vaccines.

Identification of sturgeon IgD bridges the evolutionary gap between elasmobranchs and teleosts

IgD has been found in almost all jawed vertebrates, including cartilaginous and teleost fish. However, IgD is missing in acipenseriformes, a branch that is evolutionarily positioned between elasmobranchs and teleost fish. Here, by analyzing transcriptome data, we identified a transcriptionally active IgD-encoding gene in the Siberian sturgeon (Acipenser baerii). Phylogenetic analysis indicated that it is orthologous to mammalian IgD and closely related to the IgD of other fish. The lengths of sturgeon membrane-bound IgD transcripts ranged from 1.2kb to 6.2kb, encoding 3-19 CH domains. As in teleosts, the first CH domain of the sturgeon IgD transcript is also derived from μCH1 by RNA splicing. However, the variable region of the expressed sturgeon IgD shows limited V(D)J usage. In addition to IgD, three IgM variants were also identified in this species, whereas no IgT/Z-encoding genes were observed. This study bridges the gap in Ig evolution between elasmobranchs and teleosts and provides significant insight into the early evolution of immunoglobulins.