Is Xenopus IgX an analog of IgA?

The Janus (dual) model of immunoglobulin isotype evolution: Conservation and plasticity are the defining paradigms

The study of antibodies in jawed vertebrates (gnathostomes) provides every immunologist with a bird's eye view of how human immunoglobulins (Igs) came into existence and subsequently evolved into their present forms. It is a fascinating Darwinian history of conservation on the one hand and flexibility on the other, exemplified by the Ig heavy chain (H) isotypes IgM and IgD/W, respectively. The cartilaginous fish (e.g., sharks) Igs provide a glimpse of "how everything got off the ground," while the amphibians (e.g., the model Xenopus) reveal how the adaptive immune system made an about face with the emergence of Ig isotype switching and IgG-like structure/function. The evolution of mucosal Igs is a captivating account of malleability, convergence, and conservation, and a call to arms for future study! In between there are spellbinding chronicles of antibody evolution in each class of vertebrates and rather incredible stories of how antibodies can adapt to occupy niches, for example, single-domain variable regions, cold-adapted Igs, convergent mechanisms to dampen antibody function, provision of mucosal defense, and many more. The purpose here is not to provide an encyclopedic examination of antibody evolution, but rather to hit the high points and entice readers to appreciate how things "came to be."

Possible transport routes of IgM to the gut of teleost fish

Fish rely on mucosal surfaces as their first defence barrier against pathogens. Maintaining mucosal homeostasis is therefore crucial for their overall well-being, and it is likely that secreted immunoglobulins (sIg) play a pivotal role in sustaining this balance. In mammals, the poly-Ig receptor (pIgR) is an essential component responsible for transporting polymeric Igs across mucosal epithelia. In teleost fish, a counterpart of pIgR has been identified and characterized, exhibiting structural differences and broader mRNA expression patterns compared to mammals. Despite supporting evidence for the binding of Igs to recombinant pIgR proteins, the absence of a joining chain (J-chain) in teleosts challenges the conventional understanding of Ig transport mechanisms. The transport of IgM to the intestine via the hepatobiliary route is observed in vertebrates and has been proposed in a few teleosts. Investigations on the stomachless fish, ballan wrasse, revealed a significant role of the hepatobiliary route and interesting possibilities for alternative IgM transport routes that might include pancreatic tissue. These findings highlight the importance of gaining a thorough understanding of the mechanisms behind Ig transport to the gut in various teleosts. This review aims to gather existing information on pIgR-mediated transport across epithelial cells and immunoglobulin transport pathways to the gut lumen in teleost fish. It provides comparative insights into the hepatobiliary transport of Igs to the gut, emphasizing the current understanding in teleost fish while exploring potential alternative pathways for Ig transport to the gut lumen. Despite significant progress in understanding various aspects, there is still much to uncover, especially concerning the diversity of mechanisms across different teleost species.

The adaptive microbiome hypothesis and immune interactions in amphibian mucus

The microbiome is known to provide benefits to hosts, including extension of immune function. Amphibians are a powerful immunological model for examining mucosal defenses because of an accessible epithelial mucosome throughout their developmental trajectory, their responsiveness to experimental treatments, and direct interactions with emerging infectious pathogens. We review amphibian skin mucus components and describe the adaptive microbiome as a novel process of disease resilience where competitive microbial interactions couple with host immune responses to select for functions beneficial to the host. We demonstrate microbiome diversity, specificity of function, and mechanisms for memory characteristic of an adaptive immune response. At a time when industrialization has been linked to losses in microbiota important for host health, applications of microbial therapies such as probiotics may contribute to immunotherapeutics and to conservation efforts for species currently threatened by emerging diseases.

The amphibian immune system

Amphibians are at the forefront of bridging the evolutionary gap between mammals and more ancient, jawed vertebrates. Currently, several diseases have targeted amphibians and understanding their immune system has importance beyond their use as a research model. The immune system of the African clawed frog, Xenopus laevis, and that of mammals is well conserved. We know that several features of the adaptive and innate immune system are very similar for both, including the existence of B cells, T cells and innate-like T cells. In particular, the study of the immune system at early stages of development is benefitted by studying X. laevis tadpoles. The tadpoles mainly rely on innate immune mechanisms including pre-set or innate-like T cells until after metamorphosis. In this review we lay out what is known about the innate and adaptive immune system of X. laevis including the lymphoid organs as well as how other amphibian immune systems are similar or different. Furthermore, we will describe how the amphibian immune system responds to some viral, bacterial and fungal insults. This article is part of the theme issue 'Amphibian immunity: stress, disease and ecoimmunology'.

Evolution of the Immunoglobulin Isotypes-Variations of Biophysical Properties among Animal Classes

The adaptive immune system arose around 500 million years ago in jawed fish, and, since then, it has mediated the immune defense against pathogens in all vertebrates. Antibodies play a central role in the immune reaction, recognizing and attacking external invaders. During the evolutionary process, several immunoglobulin isotypes emerged, each having a characteristic structural organization and dedicated function. In this work, we investigate the evolution of the immunoglobulin isotypes, in order to highlight the relevant features that were preserved over time and the parts that, instead, mutated. The residues that are coupled in the evolution process are often involved in intra- or interdomain interactions, meaning that they are fundamental to maintaining the immunoglobulin fold and to ensuring interactions with other domains. The explosive growth of available sequences allows us to point out the evolutionary conserved residues and compare the biophysical properties among different animal classes and isotypes. Our study offers a general overview of the evolution of immunoglobulin isotypes and advances the knowledge of their characteristic biophysical properties, as a first step in guiding protein design from evolution.

Roles of Secretory Immunoglobulin A in Host-Microbiota Interactions in the Gut Ecosystem

In the gastrointestinal tract (GIT), the immune system interacts with a variety of microorganisms, including pathogens as well as beneficial symbionts that perform important physiological functions for the host and are crucial to sustain intestinal homeostasis. In normal conditions, secretory immunoglobulin A (SIgA) is the principal antibody produced by B cells in the GIT mucosa. Polyreactivity provides certain SIgA molecules with the ability of binding different antigens in the bacterial surface, such as O-antigens and teichoic acids, while cross-species reactivity allows them to recognize and interact with different types of bacteria. These functions may be crucial in allowing SIgA to modulate the complex gut microbiota in an efficient manner. Several studies suggest that SIgA can help with the retention and proliferation of helpful members of the gut microbiota. Gut microbiota alterations in people with IgA deficiency include the lack of some species that are known to be normally coated by SIgA. Here, we discuss the different ways in which SIgA behaves in relation to pathogens and beneficial bacteria of the gut microbiota and how the immune system might protect and facilitate the establishment and maintenance of certain gut symbionts.

Secreted immunoglobulin domain effector molecules of invertebrates and management of gut microbial ecology

The origins of a "pass-through" gut in early bilaterians facilitated the exploration of new habitats, motivated the innovation of feeding styles and behaviors, and helped drive the evolution of more complex organisms. The gastrointestinal tract has evolved to consist of a series of interwoven exchanges between nutrients, host immunity, and an often microbe-rich environmental interface. Not surprisingly, animals have expanded their immune repertoires to include soluble effectors that can be secreted into luminal spaces, e.g., in the gut, facilitating interactions with microbes in ways that influence their settlement dynamics, virulence, and their interaction with other microbes. The immunoglobulin (Ig) domain, which is also found in some non-immune molecules, is recognized as one of the most versatile recognition domains lying at the interface of innate and adaptive immunity; among vertebrates, secreted Igs are known to play crucial roles in the management of gut microbial communities. In this mini-review, we will focus on secreted immune effectors possessing Ig-like domains in invertebrates, such as the fibrinogen-related effector proteins first described in the gastropod Biomphalaria glabrata, the Down syndrome cellular adhesion molecule first described in the arthropod, Drosophila melanogaster, and the variable region-containing chitin-binding proteins of the protochordates. We will highlight our current understanding of their function and their potential role, if not yet recognized, in the establishment and maintenance of host-microbial interfaces and argue that these Igs are likely also essential to microbiome management.

Study on the characterization of grouper (Epinephelus coioides) immunoglobulin T and its positive cells

Immunoglobulins (Igs) play a vital role in the adaptive immunity of gnathostomes. IgT, a particular Ig class in teleost fishes, receives much attention concerning the mucosal immunity. While, the characteristic and function of Epinephelus coioides IgT is still unknown. In our study, a polyclonal antibody was first prepared with grouper IgT heavy chain recombinant protein. IgT was revealed to be polymeric in serum and mucus. In normal groupers, IgT had high expression level in head kidney and spleen, while little amount in gills, thymus, gut and liver. The number of IgT-positive cells in different tissues was in line with their IgT expression. Furthermore, IgT could coat fractional bacteria in the mucus. In conclusion, this research revealed the protein characteristic, basal expression and bacterial coverage of grouper IgT. This is the first study to identify the characteristic of grouper IgT and demonstrate the capacity of coating microbes.

The Immune System and the Antiviral Responses in Chinese Giant Salamander, Andrias davidianus

The Chinese giant salamander, belonging to an ancient amphibian lineage, is the largest amphibian existing in the world, and is also an important animal for artificial cultivation in China. However, some aspects of the innate and adaptive immune system of the Chinese giant salamander are still unknown. The Chinese giant salamander iridovirus (GSIV), a member of the Ranavirus genus (family Iridoviridae), is a prominent pathogen causing high mortality and severe economic losses in Chinese giant salamander aquaculture. As a serious threat to amphibians worldwide, the etiology of ranaviruses has been mainly studied in model organisms, such as the Ambystoma tigrinum and Xenopus. Nevertheless, the immunity to ranavirus in Chinese giant salamander is distinct from other amphibians and less known. We review the unique immune system and antiviral responses of the Chinese giant salamander, in order to establish effective management of virus disease in Chinese giant salamander artificial cultivation.

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.

Four immunoglobulin isotypes and IgD splice variants in urodele amphibians

Until recently, different families of urodele amphibians were thought to express distinct subsets of immunoglobulin (Ig) isotypes. In this study, we explored cDNAs encoding Ig heavy-chains (H-chains) in three species of urodele amphibians. We found that Cynops pyrrhogaster, Pleurodeles waltl, and Ambystoma mexicanum each carry genes encoding four Ig H-chain isotypes, including IgM, IgY, IgD, and IgX, similar to those found in anuran amphibians. We also found that urodele IgDs have a long constant region similar to those found in anuran, reptiles, and bony fishes. We also found several putative IgD splice variants. Our findings indicated that P. waltl IgP is not a novel isotype but an IgD splice variant. Altogether, our findings indicate that IgD splice variants may be universally expressed among amphibian species.

Insights into the evolution of IG genes in Amphibians and reptiles

Immunoglobulins are essential proteins of the immune system to neutralize pathogens. Gene encoding B cell receptors and antibodies (Ig genes) first appeared with the emergence of early vertebrates having a jaw, and are now present in all extant jawed vertebrates, or Gnathostomata. The genes have undergone evolutionary changes. In particular, genomic structural changes corresponding to genes of the adaptive immune system were coincident or in parallel with the adaptation of vertebrates from the sea to land. In cartilaginous fish exist IgM, IgD/W, and IgNAR and in bony fish IgM, IgT, IgD. Amphibians and reptiles witnessed significant modifications both in the structure and orientation of IG genes. In particular, for these amphibians and Amniota that adapted to land, IgM and IgD genes were retained, but other isotypes arose, including genes for IgA(X)1, IgA(X)2, and IgY. Recent progress in high throughput genome sequencing is helping to uncover the IG gene structure of all jawed vertebrates. In this work, we review the work and present knowledge of immunoglobulin genes in genomes of amphibians and reptiles.

Immunological Aspects of Chytridiomycosis

Amphibians are currently the most threatened vertebrate class, with the disease chytridiomycosis being a major contributor to their global declines. Chytridiomycosis is a frequently fatal skin disease caused by the fungal pathogens Batrachochytrium dendrobatidis (Bd) and Batrachochytrium salamandrivorans (Bsal). The severity and extent of the impact of the infection caused by these pathogens across modern Amphibia are unprecedented in the history of vertebrate infectious diseases. The immune system of amphibians is thought to be largely similar to that of other jawed vertebrates, such as mammals. However, amphibian hosts are both ectothermic and water-dependent, which are characteristics favouring fungal proliferation. Although amphibians possess robust constitutive host defences, Bd/Bsal replicate within host cells once these defences have been breached. Intracellular fungal localisation may contribute to evasion of the induced innate immune response. Increasing evidence suggests that once the innate defences are surpassed, fungal virulence factors suppress the targeted adaptive immune responses whilst promoting an ineffectual inflammatory cascade, resulting in immunopathology and systemic metabolic disruption. Thus, although infections are contained within the integument, crucial homeostatic processes become compromised, leading to mortality. In this paper, we present an integrated synthesis of amphibian post-metamorphic immunological responses and the corresponding outcomes of infection with Bd, focusing on recent developments within the field and highlighting future directions.

The Immunoglobulins: New Insights, Implications, and Applications

Immunoglobulins (Igs), as one of the hallmarks of adaptive immunity, first arose approximately 500 million years ago with the emergence of jawed vertebrates. Two events stand out in the evolutionary history of Igs from cartilaginous fish to mammals: (a) the diversification of Ig heavy chain (IgH) genes, resulting in Ig isotypes or subclasses associated with novel functions, and (b) the diversification of genetic and structural strategies, leading to the creation of the antibody repertoire we know today. This review first gives an overview of the IgH isotypes identified in jawed vertebrates to date and then highlights the implications or applications of five new recent discoveries arising from comparative studies of Igs derived from different vertebrate species.

IgA Responses to Microbiota

Various immune mechanisms are deployed in the mucosa to confront the immense diversity of resident bacteria. A substantial fraction of the commensal microbiota is coated with immunoglobulin A (IgA) antibodies, and recent findings have established the identities of these bacteria under homeostatic and disease conditions. Here we review the current understanding of IgA biology, and present a framework wherein two distinct types of humoral immunity coexist in the gastrointestinal mucosa. Homeostatic IgA responses employ a polyreactive repertoire to bind a broad but taxonomically distinct subset of microbiota. In contrast, mucosal pathogens and vaccines elicit high-affinity, T cell-dependent antibody responses. This model raises fundamental questions including how polyreactive IgA specificities are generated, how these antibodies exert effector functions, and how they exist together with other immune responses during homeostasis and disease.

IgM and IgD heavy chains of yellow catfish (Pelteobagrus fulvidraco): Molecular cloning, characterization and expression analysis in response to bacterial infection

Three different immunoglobulin (Ig) isotypes, namely IgM, IgD, and IgT/IgZ have been described in most teleost, among which IgM and IgT are considered crucial in systematic and mucosal immunity, respectively. However, some teleost have no IgT/IgZ and it is unclear how other Ig isotypes interact to perform immune-protective roles in both systematic and mucosal sites. In this study, the complete cDNA sequences of IgM and IgD heavy chains were cloned and analyzed from yellow catfish (Pelteobagrus fulvidraco). The full-length cDNA of Pf-IgM and Pf-IgD heavy chains contained an open reading frame (ORF) of 1710 and 2991 bp encoding a predicted protein of 570 and 997 amino acids, respectively. Tissue-specific expression analysis indicated that both IgM and IgD were highly expressed in kidney and spleen, and higher expression levels were found at zygote and 13th day post hatching during early development. Multiple sequence alignment and phylogenetic analysis showed IgM and IgD of yellow catfish are closely related to other fish of Siluriformes. Moreover, we also constructed the infection model of yellow catfish with bacteria (Flavobacterium columnare G₄) for the first time to study the function of Pf-IgM and Pf-IgD heavy chain genes in immune response. Quantitative real-time PCR (qRT-PCR) showed that significantly up-regulated expression of Pf-IgM was not only detected in liver and spleen, but also in mucosal tissues including skin and intestine, while Pf-IgD was just significantly increased in liver and spleen, which might suggest the main immune-protecting roles of IgM in mucosal tissues of yellow catfish.

A cold-blooded view of adaptive immunity

The adaptive immune system arose 500 million years ago in ectothermic (cold-blooded) vertebrates. Classically, the adaptive immune system has been defined by the presence of lymphocytes expressing recombination-activating gene (RAG)-dependent antigen receptors and the MHC. These features are found in all jawed vertebrates, including cartilaginous and bony fish, amphibians and reptiles and are most likely also found in the oldest class of jawed vertebrates, the extinct placoderms. However, with the discovery of an adaptive immune system in jawless fish based on an entirely different set of antigen receptors - the variable lymphocyte receptors - the divergence of T and B cells, and perhaps innate-like lymphocytes, goes back to the origin of all vertebrates. This Review explores how recent developments in comparative immunology have furthered our understanding of the origins and function of the adaptive immune system.

Effects of imidacloprid on Rana catesbeiana immune and nervous system

Imidacloprid (IMD), a neonicotinoid, is generally considered to be of low toxicity in vertebrates. However, the inhibition of acetylcholine (ACh) receptors can have a profound effect on both the immune and nervous system due to the anti-inflammatory effects of ACh. Vertebrates, such as amphibians, might be affected by IMD because they breed in wetlands where the concentration of IMD is high. In our study, we experimentally exposed Rana catesbeiana tadpoles to environmentally relevant IMD and then quantified the ACh and antibody to non-replicating antigens. We hypothesized that IMD exposure would result in higher AChE and antibody levels. We completed a factorial experiment in which tadpoles were divided into four groups, two of which were exposed to 100 ng/L of IMD. After five weeks, two groups were injected with the novel antigen keyhole limpet hemocyanin (KLH) and two injected with a control. Three weeks later, tadpoles were euthanized and blood samples collected. At 100 ng/L, IMD exposure did not cause a significant difference in AChE levels or KLH-specific IgY antibodies. However, tadpoles injected with KLH had slightly higher levels of AChE. In addition, we saw a trend in total IgM with higher levels in tadpoles exposed to IMD. While we found no effect of IMD at 100 ng/L on antibody response to a novel, non-replicating antigen nor on ACh production, further research is needed to determine if higher concentrations of IMD or parasite infection can influence development of R. catesbeiana.

Natural polyreactive IgA antibodies coat the intestinal microbiota

Large quantities of immunoglobulin A (IgA) are constitutively secreted by intestinal plasma cells to coat and contain the commensal microbiota, yet the specificity of these antibodies remains elusive. Here we profiled the reactivities of single murine IgA plasma cells by cloning and characterizing large numbers of monoclonal antibodies. IgAs were not specific to individual bacterial taxa but rather polyreactive, with broad reactivity to a diverse, but defined, subset of microbiota. These antibodies arose at low frequencies among naïve B cells and were selected into the IgA repertoire upon recirculation in Peyer's patches. This selection process occurred independent of microbiota or dietary antigens. Furthermore, although some IgAs acquired somatic mutations, these did not substantially influence their reactivity. These findings reveal an endogenous mechanism driving homeostatic production of polyreactive IgAs with innate specificity to microbiota.

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.

Emergence and Evolution of Secondary Lymphoid Organs

For effective adaptive immunity to foreign antigens (Ag), secondary lymphoid organs (SLO) provide the confined environment in which Ag-restricted lymphocytes, with very low precursor frequencies, interact with Ag on Ag-presenting cells (APC). The spleen is the primordial SLO, arising in conjunction with adaptive immunity in early jawed vertebrates. The spleen, especially the spleen's lymphoid compartment, the white pulp (WP), has undergone numerous modifications over evolutionary time. We describe the progressive advancement of splenic WP complexity, which evolved in parallel with the increasing functionality of adaptive immunity. The Ag-presenting function of follicular dendritic cells (FDC) also likely emerged at the inception of adaptive immunity, and we propose that a single type of hematopoietically derived APC displayed Ag to both T and B cells. A dedicated FDC, derived from a vascular precursor, is a recent evolutionary innovation that likely permitted the robust affinity maturation found in mammals.

The urodele amphibian Pleurodeles waltl has a diverse repertoire of immunoglobulin heavy chains with polyreactive and species-specific features

Urodele amphibians are an interesting model because although they possess the cardinal elements of the vertebrate immune system, their immune response is apparently subdued. This phenomenon, sometimes regarded as a state of immunodeficiency, has been attributed by some authors to limited antibody diversity. We reinvestigated this issue in Pleurodeles waltl, a metamorphosing urodele, and noted that upsilon transcripts of its IgY repertoire were as diverse as alpha transcripts of the mammalian IgA repertoire. Mu transcripts encoding the IgM repertoire were less diverse, but could confer more plasticity. Both isotypes present potential polyreactive features that may confer urodele antibodies with the ability to bind to a variety of antigens. Finally, we observed additional cysteines in CDR1 and 2 of the IGHV5 and IGHV6 domains, some of which specific to urodeles, that could allow the establishment of a disulfide bond between these CDRs. Together, these data suggest that urodele antibody diversity is not as low as previously thought.

Evolutionary implication of B-1 lineage cells from innate to adaptive immunity

The paradigm that B cells mainly play a central role in adaptive immunity may have to be reevaluated because B-1 lineage cells have been found to exhibit innate-like functions, such as phagocytic and bactericidal activities. Therefore, the evolutionary connection of B-1 lineage cells between innate and adaptive immunities have received much attention. In this review, we summarized various innate-like characteristics of B-1 lineage cells, such as natural antibody production, antigen-presenting function in primary adaptive immunity, and T cell-independent immune responses. These characteristics seem highly conserved between fish B cells and mammalian B-1 cells during vertebrate evolution. We proposed an evolutionary outline of B cells by comparing biological features, including morphology, phenotype, ontogeny, and functional activity between B-1 lineage cells and macrophages or B-2 cells. The B-1 lineage may be a transitional cell type between phagocytic cells (e.g., macrophages) and B-2 cells that functionally connects innate and adaptive immunities. Our discussion would contribute to the understanding on the origination of B cells specialized in adaptive immunity from innate immunity. The results might provide further insight into the evolution of the immune system as a whole.

Alternative adaptive immunity strategies: coelacanth, cod and shark immunity

The advent of high throughput sequencing has permitted to investigate the genome and the transcriptome of novel non-model species with unprecedented depth. This technological advance provided a better understanding of the evolution of adaptive immune genes in gnathostomes, revealing several unexpected features in different fish species which are of particular interest. In the present paper, we review the current understanding of the adaptive immune system of the coelacanth, the elephant shark and the Atlantic cod. The study of coelacanth, the only living extant of the long thought to be extinct Sarcopterygian lineage, is fundamental to bring new insights on the evolution of the immune system in higher vertebrates. Surprisingly, coelacanths are the only known jawed vertebrates to lack IgM, whereas two IgD/W loci are present. Cartilaginous fish are of great interest due to their basal position in the vertebrate tree of life; the genome of the elephant shark revealed the lack of several important immune genes related to T cell functions, which suggest the existence of a primordial set of TH1-like cells. Finally, the Atlantic cod lacks a functional major histocompatibility II complex, but balances this evolutionary loss with the expansion of specific gene families, including MHC I, Toll-like receptors and antimicrobial peptides. Overall, these data point out that several fish species present an unconventional adaptive immune system, but the loss of important immune genes is balanced by adaptive evolutionary strategies which still guarantee the establishment of an efficient immune response against the pathogens they have to fight during their life.

Mucosal Immunity and B Cells in Teleosts: Effect of Vaccination and Stress

Fish are subjected to several insults from the environment, which may endanger animal survival. Mucosal surfaces are the first line of defense against these threats, acting as a physical barrier to protect the animal but also functioning as an active immune tissue. Thus, four mucosal-associated lymphoid tissues (MALTs), which lead the immune responses in gut, skin, gills, and nose, have been described in fish. Humoral and cellular immunity, as well as their regulation and the factors that influence the response in these mucosal lymphoid tissues, are still not well known in most fish species. Mucosal B-lymphocytes and immunoglobulins (Igs) are key players in the immune response that takes place in those MALTs. The existence of IgT as a mucosal specialized Ig gives us the opportunity of measuring specific responses after infection or vaccination, a fact that was not possible until recently in most fish species. The vaccination process is influenced by several factors, being stress one of the main stimuli determining the success of the vaccine. Thus, one of the major goals in a vaccination process is to avoid possible situations of stress, which might interfere with fish immune performance. However, interaction between immune and neuroendocrine systems at mucosal tissues is still unknown. In this review, we will summarize the latest findings about B-lymphocytes and Igs in mucosal immunity and the effect of stress and vaccination on B-cell response at mucosal sites. It is important to point out that a limited number of studies have been published regarding stress in mucosa and very few about the influence of stress over mucosal B-lymphocytes.

Evolution of B cell immunity

Two types of adaptive immune strategies are known to have evolved in vertebrates: the VLR-based system, which is present in jawless organisms and is mediated by VLRA and VLRB lymphocytes, and the BCR/TCR-based system, which is present in jawed species and is provided by B and T cell receptors expressed on B and T cells, respectively. Here we summarize features of B cells and their predecessors in the different animal phyla, focusing the review on B cells from jawed vertebrates. We point out the critical role of nonclassical species and comparative immunology studies in the understanding of B cell immunity. Because nonclassical models include species relevant to veterinary medicine, basic science research performed in these animals contributes to the knowledge required for the development of more efficacious vaccines against emerging pathogens.

Microbiota and mucosal immunity in amphibians

We know that animals live in a world dominated by bacteria. In the last 20 years, we have learned that microbes are essential regulators of mucosal immunity. Bacteria, archeas, and viruses influence different aspects of mucosal development and function. Yet, the literature mainly covers findings obtained in mammals. In this review, we focus on two major themes that emerge from the comparative analysis of mammals and amphibians. These themes concern: (i) the structure and functions of lymphoid organs and immune cells in amphibians, with a focus on the gut mucosal immune system; and (ii) the characteristics of the amphibian microbiota and its influence on mucosal immunity. Lastly, we propose to use Xenopus tadpoles as an alternative small-animal model to improve the fundamental knowledge on immunological functions of gut microbiota.

Antibody Isotype Switching in Vertebrates

The humoral or antibody-mediated immune response in vertebrates has evolved to respond to diverse antigenic challenges in various anatomical locations. Diversification of the immunoglobulin heavy chain (IgH) constant region via isotype switching allows for remarkable plasticity in the immune response, including versatile tissue distribution, Fc receptor binding, and complement fixation. This enables antibody molecules to exert various biological functions while maintaining antigen-binding specificity. Different immunoglobulin (Ig) classes include IgM, IgD, IgG, IgE, and IgA, which exist as surface-bound and secreted forms. High-affinity autoantibodies are associated with various autoimmune diseases such as lupus and arthritis, while defects in components of isotype switching are associated with infections. A major route of infection used by a large number of pathogens is invasion of mucosal surfaces within the respiratory, digestive, or urinary tract. Most infections of this nature are initially limited by effector mechanisms such as secretory IgA antibodies. Mucosal surfaces have been proposed as a major site for the genesis of adaptive immune responses, not just in fighting infections but also in tolerating commensals and constant dietary antigens. We will discuss the evolution of isotype switching in various species and provide an overview of the function of various isotypes with a focus on IgA, which is universally important in gut homeostasis as well as pathogen clearance. Finally, we will discuss the utility of antibodies as therapeutic modalities.

The Evolution and Structure of Atypical T Cell Receptors

The T cell receptor structure and genetic organization have been thought to have been stable in vertebrate evolution relative to the immunoglobulins. For the most part, this has been true and the content and organization of T cell receptor genes has been fairly conserved over the past 400 million years of gnathostome evolution. Analyses of TCRδ chains in a broad range of vertebrate lineages over the past decade have revealed a remarkable and previously unrealized degree of plasticity. This plasticity can generally be described in two forms. The first is broad use of antibody heavy chain variable genes in place of the conventional Vδ. The second form containing an unusual three extracellular domain structures has evolved independently in both cartilaginous fishes and mammals. Two well-studied vertebrate lineages, the eutherian mammals such as mice and humans and teleost fishes, lack any of these alternative TCR forms, contributing to why they went undiscovered for so long after the initial description of the conventional TCR chains three decades ago. This chapter describes the state of knowledge of these unusual TCR forms, both their structure and genetics, and current ideas on their function.

The immunoglobulins of cold-blooded vertebrates

Although lymphocyte-like cells secreting somatically-recombining receptors have been identified in the jawless fishes (hagfish and lamprey), the cartilaginous fishes (sharks, skates, rays and chimaera) are the most phylogenetically distant group relative to mammals in which bona fide immunoglobulins (Igs) have been found. Studies of the antibodies and humoral immune responses of cartilaginous fishes and other cold-blooded vertebrates (bony fishes, amphibians and reptiles) are not only revealing information about the emergence and roles of the different Ig heavy and light chain isotypes, but also the evolution of specialised adaptive features such as isotype switching, somatic hypermutation and affinity maturation. It is becoming increasingly apparent that while the adaptive immune response in these vertebrate lineages arose a long time ago, it is most definitely not primitive and has evolved to become complex and sophisticated. This review will summarise what is currently known about the immunoglobulins of cold-blooded vertebrates and highlight the differences, and commonalities, between these and more “conventional” mammalian species.

Putting J chain back on the map: how might its expression define plasma cell development?

Joining chain (J chain) is a small polypeptide that regulates multimerization of secretory IgM and IgA, the only two mammalian Igs capable of forming multimers. J chain also is required for poly-Ig receptor-mediated transport of these Ig classes across the mucosal epithelium. It is generally assumed that all plasma cells express J chain regardless of expressed isotype, despite the documented presence of J chain(-) plasma cells in mammals, specifically in all monomeric IgA-secreting cells and some IgG-secreting cells. Compared with most other immune molecules, J chain has not been studied extensively, in part because of technical limitations. Even the reported phenotype of the J chain-knockout mouse is often misunderstood or underappreciated. In this short review, we discuss J chain in light of the various proposed models of its expression and regulation, with an added focus on its evolutionary significance, as well as its expression in different B cell lineages/differentiation states.

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.

Fighting a losing battle: vigorous immune response countered by pathogen suppression of host defenses in the chytridiomycosis-susceptible frog Atelopus zeteki

The emergence of the disease chytridiomycosis caused by the chytrid fungus Batrachochytrium dendrobatidis (Bd) has been implicated in dramatic global amphibian declines. Although many species have undergone catastrophic declines and/or extinctions, others appear to be unaffected or persist at reduced frequencies after Bd outbreaks. The reasons behind this variance in disease outcomes are poorly understood: differences in host immune responses have been proposed, yet previous studies suggest a lack of robust immune responses to Bd in susceptible species. Here, we sequenced transcriptomes from clutch-mates of a highly susceptible amphibian, Atelopus zeteki, with different infection histories. We found significant changes in expression of numerous genes involved in innate and inflammatory responses in infected frogs despite high susceptibility to chytridiomycosis. We show evidence of acquired immune responses generated against Bd, including increased expression of immunoglobulins and major histocompatibility complex genes. In addition, fungal-killing genes had significantly greater expression in frogs previously exposed to Bd compared with Bd-naïve frogs, including chitinase and serine-type proteases. However, our results appear to confirm recent in vitro evidence of immune suppression by Bd, demonstrated by decreased expression of lymphocyte genes in the spleen of infected compared with control frogs. We propose susceptibility to chytridiomycosis is not due to lack of Bd-specific immune responses but instead is caused by failure of those responses to be effective. Ineffective immune pathway activation and timing of antibody production are discussed as potential mechanisms. However, in light of our findings, suppression of key immune responses by Bd is likely an important factor in the lethality of this fungus.

Coevolution of Mucosal Immunoglobulins and the Polymeric Immunoglobulin Receptor: Evidence That the Commensal Microbiota Provided the Driving Force

Immunoglobulins (Igs) in mucosal secretions contribute to immune homeostasis by limiting access of microbial and environmental antigens to the body proper, maintaining the integrity of the epithelial barrier and shaping the composition of the commensal microbiota. The emergence of IgM in cartilaginous fish represented the primordial mucosal Ig, which is expressed in all higher vertebrates. Expansion and diversification of the mucosal Ig repertoire led to the emergence of IgT in bony fishes, IgX in amphibians, and IgA in reptiles, birds, and mammals. Parallel evolution of cellular receptors for the constant (Fc) regions of Igs provided mechanisms for their transport and immune effector functions. The most ancient of these Fc receptors is the polymeric Ig receptor (pIgR), which first appeared in an ancestor of bony fishes. The pIgR transports polymeric IgM, IgT, IgX, and IgA across epithelial cells into external secretions. Diversification and refinement of the structure of mucosal Igs during tetrapod evolution were paralleled by structural changes in pIgR, culminating in the multifunctional secretory IgA complex in mammals. In this paper, evidence is presented that the mutualistic relationship between the commensal microbiota and the vertebrate host provided the driving force for coevolution of mucosal Igs and pIgR.

Restructuring of the amphibian gut microbiota through metamorphosis

Vertebrates maintain complex symbioses with a diverse community of microbes residing within their guts. The microbial players in these symbioses differ between major taxa of vertebrates, such that fish and amniotes maintain notably different communities. To date, there has not been a culture-independent inventory of an amphibian gut microbial community. Here, we compared gut microbial communities of tadpoles and frogs of the Northern leopard frog (Lithobates pipiens). We utilized Illumina sequencing, which allowed us to inventory more than 450 000 microbial sequences. We found that tadpoles and frogs differ markedly in the composition of their gut microbial communities, with tadpoles maintaining a community more similar to fish, whereas the frog community resembles that of amniotes. Additionally, frogs maintain a community with lower phylogenetic diversity compared with tadpoles. The significant restructuring of the microbiota is likely due to changes in diet as well as the large reorganization of the intestinal organ during metamorphosis. Overall, we propose that amphibians represent an important system in which to study regulation and selection of gut microbial communities.

Molecular cloning and expression analysis of polymeric immunoglobulin receptor in flounder (Paralichthys olivaceus)

The polymeric immunoglobulin receptor (pIgR) is one of the most important mucosal effectors mediating the transcytosis of polymeric immunoglobulins (pIgs) to protect the organisms. In this paper, a full-length cDNA of pIgR was firstly cloned from flounder (Paralichthys olivaceus) using rapid amplification of cDNA ends approaches, and it was of 1384 bp, containing an open reading frame (ORF) of 1005 bp encoding a polypeptide of 335 amino acids with the predicted molecular mass of 37.6 kDa. The flounder pIgR exhibited a unique structure containing only two immunoglobulin-like domains (ILD) corresponding to mammalian pIgR ILD1 and ILD5. The mRNA transcripts of pIgR were detected in all the tested tissues of flounder by semi-quantitative RT-PCR, and the pIgR was expressed at the highest level in liver and higher levels in intestine, gill, skin, spleen and head kidney than in stomach and muscle. The ORF was successfully expressed in Escherichia coli BL21 (DE3) and the recombinant protein displayed binding capability to the purified mucus IgM and serum IgM of flounder by ELISA. The polyclonal antibody against flounder recombinant pIgR was developed by immunization of Balb/C mice, which specifically reacted to the recombinant pIgR in Western blot. Moreover, a secretory component-like molecule was detected in the skin mucus but not in the serum of flounder, which molecular mass (about 37 kDa) was near the theoretical mass obtained from the sequence of flounder pIgR. All these results indicated that flounder pIgR probably involved in the pIgs transport and provided insights into the roles of fish pIgR in the mucosal immunity.

Noncoordinate expression of J-chain and Blimp-1 define nurse shark plasma cell populations during ontogeny

B-lymphocyte-induced maturation protein 1 (Blimp-1) is the master regulator of plasma cell development, controlling genes such as those encoding J-chain and secretory Ig heavy chain. However, some mammalian plasma cells do not express J-chain, and mammalian B1 cells secrete "natural" IgM antibodies without upregulating Blimp-1. While these results have been controversial in mammalian systems, here we describe subsets of normally occurring Blimp-1(-) antibody-secreting cells in nurse sharks, found in lymphoid tissues at all ontogenic stages. Sharks naturally produce large amounts of both pentameric (classically "19S") and monomeric (classically "7S") IgM, the latter an indicator of adaptive immunity. Consistent with the mammalian paradigm, shark Blimp-1 is expressed in splenic 7S IgM-secreting cells, though rarely detected in the J-chain(+) cells producing 19S IgM. Although IgM transcript levels are lower in J-chain(+) cells, these cells nevertheless secrete 19S IgM in the absence of Blimp-1, as demonstrated by ELISPOT and metabolic labeling. Additionally, cells in the shark BM equivalent (epigonal) are Blimp-1(-). Our data suggest that, in sharks, 19S-secreting cells and other secreting memory B cells in the epigonal are maintained for long periods without Blimp-1, but like in mammals, Blimp-1 is required for terminating the B-cell program following an adaptive immune response in the spleen.

Extensive diversification of IgH subclass-encoding genes and IgM subclass switching in crocodilians

Crocodilians are a group of reptiles that are closely related to birds and are thought to possess a strong immune system. Here we report that the IgH locus in the Siamese crocodile and the Chinese alligator contains multiple μ genes, in contrast to other tetrapods. Both the μ2 and μ3 genes are expressed through class-switch recombination involving the switch region and germline transcription. Both IgM1 and IgM2 are present in the serum as polymers, which implies that IgM class switching may have significant roles in humoural immunity. The crocodilian α genes are the first IgA-encoding genes identified in reptiles, and these genes show an inverted transcriptional orientation similar to that of birds. The identification of both α and δ genes in crocodilians suggests that the IgH loci of modern living mammals, reptiles and birds share a common ancestral organization.

IgH loci of American alligator and saltwater crocodile shed light on IgA evolution

Immunoglobulin loci of two representatives of the order Crocodylia were studied from full genome sequences. Both Alligator mississippiensis and Crocodylus porosus have 13 genes for the heavy chain constant regions of immunoglobulins. The IGHC locus contains genes encoding four immunoglobulins M (IgM), one immunoglobulin D (IgD), three immunoglobulins A (IgA), three immunoglobulins Y (IgY), and two immunoglobulins D2 (IgD2). IgA and IgD2 genes were found in reverse transcriptional orientation compared to the other Ig genes. The IGHD gene contains 11 exons, four of which containing stop codons or sequence alterations. As described in other reptiles, the IgD2 is a chimeric Ig with IgA- and IgD-related domains. This work clarifies the origin of bird IgA and its evolutionary relationship with amphibian immunoglobulin X (IgX) as well as their links with mammalian IgA.

Ancient T-independence of mucosal IgX/A: gut microbiota unaffected by larval thymectomy in Xenopus laevis

Many studies address the influence of the gut microbiome on the immune system, but few dissect the effect of T cells on gut microbiota and mucosal responses. We have employed larval thymectomy in Xenopus to study the gut microbiota with and without the influence of T lymphocytes. Pyrosequencing of 16S ribosomal RNA genes was used to assess the relative abundance of bacterial groups present in the stomach, small and large intestine. Clostridiaceae was the most abundant family throughout the gut, while Bacteroidaceae, Enterobacteriaceae, and Flavobacteriaceae also were well represented. Unifrac analysis revealed no differences in microbiota distribution between thymectomized and unoperated frogs. This is consistent with immunization data showing that levels of the mucosal immunoglobulin IgX are not altered significantly by thymectomy. This study in Xenopus represents the oldest organisms that exhibit class switch to a mucosal isotype and is relevant to mammalian immunology, as IgA appears to have evolved from IgX based upon phylogeny, genomic synteny, and function.

A comparative overview of immunoglobulin genes and the generation of their diversity in tetrapods

In the past several decades, immunoglobulin (Ig) genes have been extensively characterized in many tetrapod species. This review focuses on the expressed Ig isotypes and the diversity of Ig genes in mammals, birds, reptiles, and amphibians. With regard to heavy chains, five Ig isotypes - IgM, IgD, IgG, IgA, and IgE - have been reported in mammals. Among these isotypes, IgM, IgD, and IgA (or its analog, IgX) are also found in non-mammalian tetrapods. Birds, reptiles, and amphibians express IgY, which is considered the precursor of IgG and IgE. Some species have developed unique isotypes of Ig, such as IgO in the platypus, IgF in Xenopus, and IgY (ΔFc) in ducks and turtles. The κ and λ light chains are both utilized in tetrapods, but the usage frequencies of κ and λ chains differ greatly among species. The diversity of Ig genes depends on several factors, including the germline repertoire and recombinatorial and post-recombinatorial diversity, and different species have evolved distinct mechanisms to generate antibody diversity.

Evolutionary genomics of immunoglobulin-encoding Loci in vertebrates

Immunoglobulins (or antibodies) are an essential element of the jawed vertebrate adaptive immune response system. These molecules have evolved over the past 500 million years and generated highly specialized proteins that recognize an extraordinarily large number of diverse substances, collectively known as antigens. During vertebrate evolution the diversification of the immunoglobulin-encoding loci resulted in differences in the genomic organization, gene content, and ratio of functional genes and pseudogenes. The tinkering process in the immunoglobulin-encoding loci often gave rise to lineage-specific characteristics that were formed by selection to increase species adaptation and fitness. Immunoglobulin loci and their encoded antibodies have been shaped repeatedly by contrasting evolutionary forces, either to conserve the prototypic structure and mechanism of action or to generate alternative and diversified structures and modes of function. Moreover, evolution favored the development of multiple mechanisms of primary and secondary antibody diversification, which are used by different species to effectively generate an almost infinite collection of diverse antibody types. This review summarizes our current knowledge on the genomics and evolution of the immunoglobulin-encoding loci and their protein products in jawed vertebrates.

Extensive diversification of IgD-, IgY-, and truncated IgY(δFc)-encoding genes in the red-eared turtle (Trachemys scripta elegans)

IgY(ΔFc), containing only CH1 and CH2 domains, is expressed in the serum of some birds and reptiles, such as ducks and turtles. The duck IgY(ΔFc) is produced by the same υ gene that expresses the intact IgY form (CH1-4) using different transcriptional termination sites. In this study, we show that intact IgY and IgY(ΔFc) are encoded by distinct genes in the red-eared turtle (Trachemys scripta elegans). At least eight IgY and five IgY(ΔFc) transcripts were found in a single turtle. Together with Southern blotting, our data suggest that multiple genes encoding both IgY forms are present in the turtle genome. Both of the IgY forms were detected in the serum using rabbit polyclonal Abs. In addition, we show that multiple copies of the turtle δ gene are present in the genome and that alternative splicing is extensively involved in the generation of both the secretory and membrane-bound forms of the IgD H chain transcripts. Although a single μ gene was identified, the α gene was not identified in this species.

Evidence of IgY subclass diversification in snakes: evolutionary implications

Mammalian IgG and IgE are thought to have evolved from IgY of nonmammalian tetrapods; however, no diversification of IgY subclasses has been reported in reptiles or birds, which are phylogenetically close to mammals. To our knowledge, we report the first evidence of the presence of multiple IgY-encoding (υ) genes in snakes. Two υ genes were identified in the snake Elaphe taeniura, and three υ genes were identified in the Burmese python (Python molurus bivittatus). Although four of the υ genes displayed a conventional four-H chain C region exon structure, one of the υ genes in the Burmese python lacked the H chain C region 2 exon, thus exhibiting a structure similar to that of the mammalian γ genes. We developed mouse mAbs specific for the IgY1 and IgY2 of E. taeniura and showed that both were expressed in serum; each had two isoforms: one full-length and one truncated at the C terminus. The truncation was not caused by alternative splicing or transcriptional termination. We also identified the μ and δ genes, but no α gene, in both snakes. This study provides valuable clues for our understanding of Ig gene evolution in tetrapods.

Development and evolution of RORγt+ cells in a microbe's world

The nuclear hormone receptor retinoid-related orphan receptor γt (RORγt) induces a pro-inflammatory program in lymphoid cells, culminating in the expression of interleukin-6 (IL-6), IL-17, IL-22, granulocyte-macrophage colony-stimulating factor, and tumor necrosis factor. During ontogeny, the first type of cells expressing RORγt are lymphoid tissue inducer cells, a type of innate lymphoid cell (ILC) generated in mammalian fetuses to induce the development of lymph nodes and Peyer's patches. After birth, RORγt(+) ILCs and RORγt(+) T cells are involved in the defense of epithelial surfaces against extracellular microbes and play an important role in the intestinal homeostasis with symbiotic microbiota. The development and evolution of RORγt(+) cells is intimately associated with the construction of a stable host-microbe interface.

The habitat, double life, citizenship, and forgetfulness of IgA

Immunoglobulin A (IgA) is the main secretory immunoglobulin of mucous membranes and is powerfully induced by the presence of commensal microbes in the intestine. B cells undergo class switch recombination to IgA in the mucosa-associated lymphoid tissues, particularly mesenteric lymph nodes (MLNs) and Peyer's patches, through both T-dependent and T-independent pathways. IgA B cells primed in the mucosa traffic from the intestinal lymphoid structures, initially through the lymphatics and then join the bloodstream, to home back to the intestinal mucosa as IgA-secreting plasma cells. Once induced, anti-bacterial IgA can be extremely long-lived but is replaced if there is induction of additional IgA specificities by other microbes. The mucosal immune system is anatomically separated from the systemic immune system by the MLNs, which act as a firewall to prevent penetration of live intestinal bacteria to systemic sites. Dendritic cells sample intestinal bacteria and induce B cells to switch to IgA. In contrast, intestinal macrophages are adept at killing extracellular bacteria and are able to clear bacteria that have crossed the mucus and epithelial barriers. There is both a continuum between innate and adaptive immune mechanisms and compartmentalization of the mucosal immune system from systemic immunity that function to preserve host microbial mutualism.