Membrane exon sequences of the three Xenopus Ig classes explain the evolutionary origin of mammalian isotypes

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.

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.

Oral immunization of the African clawed frog (Xenopus laevis) upregulates the mucosal immunoglobulin IgX

The frog Xenopus laevis is a model species for developmental biology but is also of significant interest to comparative immunologists. Amphibians are the oldest group of organisms in which both the B lymphocytes of some species undergo immunoglobulin (Ig) class switch recombination and also have a dedicated mucosal Ig isotype. The purpose of this study was to test the hypothesis that frog IgX would be produced in response to oral immunization. In order to facilitate studies of humoral, and especially mucosal immunity, in this model species, we developed a gavage technique for oral immunization. The result of this oral administration of antigen to frogs was assayed by the induction of the mucosal antibody isotype, IgX, in plasma by enzyme linked immunosorbant assay (ELISA), and a significant IgX upregulation was detected compared to frogs receiving systemic immunization into the coelom. These data are consistent with the view that IgX is the functional analog of mammalian IgA and mandate further studies of the relationship between IgX and IgA. Additionally, the gavage technique should be adaptable for functional studies of gut-associated immunology in other small aquatic vertebrates.

The evolution of adaptive immunity in vertebrates

Approximately 500 million years ago, two types of recombinatorial adaptive immune systems (AISs) arose in vertebrates. The jawed vertebrates diversify their repertoire of immunoglobulin domain-based T and B cell antigen receptors mainly through the rearrangement of V(D)J gene segments and somatic hypermutation, but none of the fundamental AIS recognition elements in jawed vertebrates have been found in jawless vertebrates. Instead, the AIS of jawless vertebrates is based on variable lymphocyte receptors (VLRs) that are generated through recombinatorial usage of a large panel of highly diverse leucine-rich-repeat (LRR) sequences. Whereas the appearance of transposon-like, recombination-activating genes contributed uniquely to the origin of the AIS in jawed vertebrates, the use of activation-induced cytidine deaminase for receptor diversification is common to both the jawed and jawless vertebrates. Despite these differences in anticipatory receptor construction, the basic AIS design featuring two interactive T and B lymphocyte arms apparently evolved in an ancestor of jawed and jawless vertebrates within the context of preexisting innate immunity and has been maintained since as a consequence of powerful and enduring selection, most probably for pathogen defense purposes.

Comparative genomics and evolution of immunoglobulin-encoding loci in tetrapods

The immunoglobulins (Igs or antibodies) as an integral part of the tetrapod adaptive immune response system have evolved toward producing highly diversified molecules that recognize a remarkably large number of different antigens. Antibodies and their respective encoding loci have been shaped by different and often contrasting evolutionary forces, some of which aim to conserve an established pattern or mechanism and others to generate alternative and diversified structural and functional configurations. The genomic organization, gene content, ratio between functional genes and pseudogenes, number and position of recombining genetic elements, and the different levels of divergence present at the germline of the Ig-encoding loci have been evolutionarily shaped and optimized in a lineage- and, in some cases, species-specific mode aiming to increase organismal fitness. Further, evolution favored the development of multiple mechanisms of primary and secondary antibody diversification, such as V(D)J recombination, class switch recombination, isotype exclusion, somatic hypermutation, and gene conversion. Diverse tetrapod species, based on their specific germline configurations, use these mechanisms in several different combinations to effectively generate a vast array of distinct antibody types and structures. This chapter summarizes our current knowledge on the Ig-encoding loci in tetrapods and discusses the different evolutionary mechanisms that shaped their diversification.

Antibody dependent enhancement of frog virus 3 infection

Background

Viruses included in the family Iridoviridae are large, icosahedral, dsDNA viruses that are subdivided into 5 genera. Frog virus 3 (FV3) is the type species of the genus Ranavirus and the best studied iridovirus at the molecular level. Typically, antibodies directed against a virus act to neutralize the virus and limit infection. Antibody dependent enhancement occurs when viral antibodies enhance infectivity of the virus rather than neutralize it.

Results

Here we show that anti-FV3 serum present at the time of FV3 infection enhances infectivity of the virus in two non-immune teleost cell lines. We found that antibody dependent enhancement of FV3 was dependent on the Fc portion of anti-FV3 antibodies but not related to complement. Furthermore, the presence of anti-FV3 serum during an FV3 infection in a non-immune mammalian cell line resulted in neutralization of the virus. Our results suggest that a cell surface receptor specific to teleost cell lines is responsible for the enhancement.

Conclusions

This report represents the first evidence of antibody dependent enhancement in iridoviruses. The data suggests that anti-FV3 serum can either neutralize or enhance viral infection and that enhancement is related to a novel antibody dependent enhancement pathway found in teleosts that is Fc dependent.

The mast cell: an evolutionary perspective

This review article is an attempt to trace the evolution of mast cells (MCs). These immune cells have been identified in all vertebrate classes as single-lobed cells containing variable amounts of membrane-bound secretory granules which store a large series of mediators, namely histamine, proteases, cytokines and growth factors. Other MC features, at least in mammals, are the c-kit receptor for the stem cell factor and the high-affinity receptor, FcepsilonRI, for immunoglobulin E (IgE). The c-kit receptor also has been identified in fish MCs. The FcepsilonRI receptor seems to be a more recent acquisition in MC phylogenesis given that IgE originated in mammalian species. Tryptase and histamine have also been recognized in MCs of teleost fish. Thus, a cell population with the overall characteristics of higher vertebrate MCs is identifiable in the most evolutionarily advanced fish species. Two potential MC progenitors have been identified in ascidians (urochordates which appeared approximately 500 million years ago): the basophil/MC-like granular haemocyte and the test cell. Both contain histamine and heparin, and provide defensive functions. Some granular haemocytes in Arthropoda also closely approximate the ultrastructure of modern MCs. The origin of MCs is probably to be found in a leukocyte ancestor operating in the context of a primitive local innate immunity and involved in phagocytic and killing activity against pathogens. From this type of defensive cell, the MC phylogenetic progenitor evolved into a tissue regulatory and remodelling cell, which was incorporated into the networks of recombinase activating genes (RAG)-mediated adaptive immunity in the Cambrian era, some 550 million years ago. Early MCs probably appeared in the last common ancestor we shared with hagfish, lamprey and sharks about 450-500 million years ago.

Going adaptive: the saga of antibodies

Because of their extreme importance to human health, we probably know more about the structure and function of antibodies than practically any other molecule. Despite all the knowledge that has been accrued in the understanding of antibodies, modern approaches, especially comparative genomics, continue to yield novel findings regarding their underlying biology and evolution. In this review, we describe recent research that led to these revelations, and discuss the broad evolutionary implications of these findings. We have restricted our discussion to three vignettes. Considerable attention has been paid to the recent discovery that the teleost IgH locus is highly similar in organization to the Tcra-Tcrd locus, implicating an evolutionary common ancestor and parallels between the functions of B and T cells during development. Second, we discuss how a new type of antibody, recently discovered in jawless vertebrates, composed not of immunoglobulins but leucine-rich repeats, sheds new light on the overall forces driving evolution of all adaptive antigen receptors. Lastly, we discuss how accumulation of genomic sequences of various human subpopulations leads to better understanding of the directionality of antibody evolution. There is always more to learn from the unfolding saga of antibodies.

Diverse splicing pathways of the membrane IgHM pre-mRNA in a Chondrostean, the Siberian sturgeon

Teleosts and tetrapods have evolved different splice patterns to generate their membrane-bound IgM. In the tetrapod lineage, the first transmembrane exon is spliced to an internal cryptic site located close to the end of the fourth constant exon. Because teleosts lack this site they use the regular 3'-splice site of the CH3 exon instead. We characterized the mum splicing patterns in a Chondrostean, the Siberian sturgeon. We observed a surprising diversity of splice patterns, the TM1 exon being spliced to a cryptic site at the end of CH4, to a cryptic site in CH3 or to the 3'-end of CH1. These different pathways lead to mIGHM transcripts encoding four, two or one complete C-domain(s), respectively. The short variant CH1-TM1 was found only in VH2 positive transcripts, while the two other variants were observed for IgHM transcripts expressing all VH families. These results shed light on the evolution of IgM splicing pathways.

AID can restrict L1 retrotransposition suggesting a dual role in innate and adaptive immunity

Retrotransposons make up over 40% of the mammalian genome. Some copies are still capable of mobilizing and new insertions promote genetic variation. Several members of the APOBEC3 family of DNA cytosine deaminases function to limit the replication of a variety of retroelements, such as the long-terminal repeat (LTR)-containing MusD and Ty1 elements, and that of the non-LTR retrotransposons, L1 and Alu. However, the APOBEC3 genes are limited to mammalian lineages, whereas retrotransposons are far more widespread. This raises the question of what cellular factors control retroelement transposition in species that lack APOBEC3 genes. A strong phylogenetic case can be made that an ancestral activation-induced deaminase (AID)-like gene duplicated and diverged to root the APOBEC3 lineage in mammals. Therefore, we tested the hypothesis that present-day AID proteins possess anti-retroelement activity. We found that AID can inhibit the retrotransposition of L1 through a DNA deamination-independent mechanism. This mechanism may manifest in the cytoplasmic compartment co- or posttranslationally. Together with evidence for AID expression in the ovary, our data combined to suggest that AID has innate immune functions in addition to its integral roles in creating antibody diversity.

A novel IgA-like immunoglobulin in the reptile Eublepharis macularius

The appearance of antibody genes over evolution coincided with the origin of the vertebrates. Reptiles are of great interest in evolution since they are the link between the amphibians, birds, and mammals. This work describes the presence of a gene in the reptile leopard gecko (Eublepharis macularius) where phylogenetic studies suggest that it is the gene orthologue of immunoglobulin A (IgA) and immunoglobulin X (IgX) in Xenopus. Messenger RNA samples taken from different tissues showed expression of this antibody in intestinal tissue. Data on the structure deduced from the sequence of nucleotides showed an antibody with four domains in the constant region. There is a sequence of 20 amino acids in the C terminus similar to the secretory tail of immunoglobulin M (IgM) and IgA. A detailed analysis of the sequence of amino acids displayed a paradox, i.e., domains CH1 and CH2 showed a clear homology with domains CH1 and CH2 of immunoglobulin Y (IgY) while domains CH3 and CH4 were homologous with domains CH3 and CH4 of IgM. This homology pattern is also seen in Xenopus IgX and bird IgA. The most logical explanation for this phenomenon is that a recombination between the IgM and IgY gave rise to the IgA.

Fc receptor-like molecules

Discovery of a large family of Fc receptor-like (FCRL) molecules, homologous to the well-known receptors for the Fc portion of immunoglobulin (FCR), has uncovered an impressive abundance of immunoglobulin superfamily (IgSF) genes in the human 1q21-23 chromosomal region and revealed significant diversity for these genes between humans and mice. The observation that FCRL representatives are members of an ancient multigene family that share a common ancestor with the classical FCR is underscored by their linked genomic locations, gene structure, shared extracellular domain composition, and utilization of common cytoplasmic tyrosine-based signaling elements. In contrast to the conventional FCR, however, FCRL molecules possess diverse extracellular frameworks, autonomous or dual signaling properties, and preferential B lineage expression. Most importantly, there is no strong evidence thus far to support a role for them as Ig-binding receptors. These characteristics, in addition to their identification in malignancies and autoimmune disorders, predict a fundamental role for these receptors as immunomodulatory agents in normal and subverted B lineage cells.

Identification of IgF, a hinge-region-containing Ig class, and IgD in Xenopus tropicalis

Only three Ig isotypes, IgM, IgX, and IgY, were previously known in amphibians. Here, we describe a heavy-chain isotype in Xenopus tropicalis, IgF (encoded by C(phi)), with only two constant region domains. IgF is similar to amphibian IgY in sequence, but the gene contains a hinge exon, making it the earliest example, in evolution, of an Ig isotype with a separately encoded genetic hinge. We also characterized a gene for the heavy chain of IgD, located immediately 3' of C(mu), that shares features with the C(delta) gene in fish and mammals. The latter gene contains eight constant-region-encoding exons and, unlike the chimeric splicing of muC(H)1 onto the IgD heavy chain in teleost fish, it is expressed as a unique IgD heavy chain. The IgH locus of X. tropicalis shows a 5' V(H)-D(H)-J(H)-C(mu)-C(delta)-C(chi)-C(upsilon)-C(phi) 3' organization, suggesting that the mammalian and amphibian Ig heavy-chain loci share a common ancestor.

Structural Phylogenetic Analysis of Activation-Induced Deaminase Function

In mammals, activation-induced deaminase (AID) initiates somatic hypermutation (SHM) and class switch recombination (CSR) of Ig genes. SHM and CSR activities require separate regions within AID. A chromosome region maintenance 1 (CRM1)-dependent nuclear export signal (NES) at the AID C terminus is necessary for CSR, and has been suggested to associate with CSR-specific cofactors. CSR appeared late in AID evolution, during the emergence of land vertebrates from bony fish, which only display SHM. Here, we show that AID from African clawed frog (Xenopus laevis), but not pufferfish (Takifugu rubripes), can induce CSR in AID-deficient mouse B cells, although both are catalytically active in bacteria and mammalian cell systems, albeit at decreased level. Like mammalian AID, Takifugu AID is actively exported from the cell nucleus by CRM1, and the Takifugu NES can substitute for the equivalent region in human AID, indicating that all the CSR-essential NES motif functions evolutionarily predated CSR activity. We also show that fusion of the Takifugu AID catalytic domain to the entire human noncatalytic domain restores activity in mammalian cells, suggesting that AID features mapping within the noncatalytic domain, but outside the NES, influence its function.

Evolution of isotype switching

This review discusses evolution of the process of Ig heavy chain class switching, relating it to the first appearance of somatic hypermutation (SHM) of variable region genes. First, we discuss recent findings on the mechanism of class switch recombination (CSR) in mice and humans, and then review the mechanisms of expression of Ig heavy chain isotypes from fishes to mammals. Importantly, activation-induced cytidine deaminase (AID), which is essential for CSR and somatic hypermutation, is found in fishes. Although at least some fishes are likely to undergo SHM, CSR is highly unlikely to occur in this group. We discuss the first appearance of CSR in amphibians and how it differs in birds and mammals.

Cloning of IgE from the echidna (Tachyglossus aculeatus) and a comparative analysis of epsilon chains from all three extant mammalian lineages

In continuation of our evolutionary studies of immunoglobulin (Ig) expression, we present here the cloning of IgE from a monotreme, the short-beaked echidna (Tachyglossus aculeatus). Including echidna IgE, 15 epsilon chain sequences have been isolated and each of the three mammalian lineages (placentals, marsupials and monotremes) is now represented by at least two sequences. Phylogenetic analyses based on all available epsilon chains and a selection of other mammalian Ig isotypes (IgM, IgA and IgG) were generated using three different algorithms. The resulting trees strongly support the Theria hypothesis, which states that the monotreme lineage was the first of the three extant mammalian lineages to appear in evolution. Furthermore, to increase our understanding of IgE we have done a detailed comparative analysis, with focus on primary structure, potential N-glycosylation, charge distribution and conservation of residues in the putative receptor-binding site. The overall structure of IgE, i.e. four constant domains and the positions of putative disulfide-bridge formations, are conserved, as is an N-glycosylation site in the third constant domain. An increased homology was observed in the putative receptor-binding site, which suggests an important function for the IgE/Fc epsilon RI interaction. IgE has been found exclusively in mammals, but it is present in all extant mammalian lineages. This, together with the overall conservation of structure, indicates that IgE appeared as a separate isotype early in mammalian evolution and that structural maintenance may have a selective advantage.

Comparative analyses of immunoglobulin genes: surprises and portents

The study of immunoglobulin genes in non-mouse and non-human models has shown that different vertebrate groups have evolved distinct methods of generating antibody diversity. By contrast, the development of T cells in the thymus is quite similar in all of the species that have been examined. The three mechanisms by which B cells uniquely modify their immunoglobulin genes -- somatic hypermutation, gene conversion and class switching -- are increasingly believed to share some fundamental mechanisms, which studies in different vertebrate groups have helped (and will continue to help) to resolve. When these mechanisms are better understood, we should be able to look to the constitutive pathways from which they have evolved and perhaps determine whether the rearrangement of variable, diversity and joining antibody gene segments -- V(D)J recombination -- was superimposed on an existing adaptive immune system.

Major histocompatibility complex and immunoglobulin loci visualized by in situ hybridization on Xenopus chromosomes

A technique for fluorescent in situ hybridization (FISH) on chromosomes of the amphibian Xenopus laevis is described. Positive results were obtained with cDNA probes of about 1kb when at least three adjacent copies of the gene are present. The immunoglobulin heavy chain locus is in the centre of the long arm of chromosome 1. Previously, family studies showed that bona fide MHC class Ib genes segregated independently. Now we show that MHC class II alpha and beta genes and class Ib genes are on the same acrocentric chromosome, with MHC in the middle of the long arm, the class Ib complex (XNC) at the tip or the same arm. Each locus or complex is found on only one pair of chromosomes confirming the diploidization of these genes in the pseudotetraploid X. laevis.

Structural and functional properties of membrane and secreted IgD

More than 35 years ago, study of an unknown immunoglobulin (Ig) in the serum from a myeloma patient led to the discovery of IgD. Subsequently, the finding that it also exists as a membrane-bound Ig stimulated a large number of studies during the 70s. Then, the interest on IgD shrank, largely because of the lack of known function of secretory IgD (secIgD) and of a stagnating knowledge of the functions of surface IgD. In the recent years, very significant advances followed the tremendous accumulation of data on the physiology of the B cell receptor, of which IgD is the major component, on the role of secIgD in normal and diseased individuals. This review, which is focused on human IgD but integrates data in the mouse and other species when needed, summarizes present data on the structure, synthesis and functions of both membrane and secIgD, IgD receptors and the involvement of IgD in various diseases, especially the hyperIgD syndrome.

Class switch recombination of the chicken IgH chain genes: implications for the primordial switch region repeats

In mammals and the amphibian, Xenopus, isotypes of antibodies have been shown to be changed through class switch recombination within the IgH chain gene locus. Here, we identified switch (S) repetitive sequences in the 5' introns of the Ig C(mu) and C(gamma) genes of the chicken. The S(mu) region is composed of two homologous regions, S(mu)1 and S(mu)2. The S(mu)1 region is an upstream 3.7 kb sequence composed of 37 repeats of a consensus sequence containing tandem repeats of the decamer ACCAGTATGG. The S(mu)2 region is a downstream 1.4 kb sequence consisting of simple tandem repeats of a decamer CCCAGTACAG. The S(gamma) region contains repeats of the decamer TATGGGGCAG. Analysis of chicken IgG-producing hybridomas revealed that the C(mu) gene was deleted from the chromosome by the recombination occurring between the S(mu) and S(gamma) regions. Recombination breakpoints at the C(mu) gene of splenocytes from an immunized chicken were scattered around the S(mu) region and two such breakpoints, the precise position of which were determined, were located within possible hairpin loop structures at the palindromic sequence of S(mu)1. A primordial palindromic sequence from which the prevalent switch repeat motifs of mammals, chickens and amphibians may have diverged is presented.

Evolution of antigen binding receptors

This review addresses issues related to the evolution of the complex multigene families of antigen binding receptors that function in adaptive immunity. Advances in molecular genetic technology now permit the study of immunoglobulin (Ig) and T cell receptor (TCR) genes in many species that are not commonly studied yet represent critical branch points in vertebrate phylogeny. Both Ig and TCR genes have been defined in most of the major lineages of jawed vertebrates, including the cartilaginous fishes, which represent the most phylogenetically divergent jawed vertebrate group relative to the mammals. Ig genes in cartilaginous fish are encoded by multiple individual loci that each contain rearranging segmental elements and constant regions. In some loci, segmental elements are joined in the germline, i.e. they do not undergo genetic rearrangement. Other major differences in Ig gene organization and the mechanisms of somatic diversification have occurred throughout vertebrate evolution. However, relating these changes to adaptive immune function in lower vertebrates is challenging. TCR genes exhibit greater sequence diversity in individual segmental elements than is found in Ig genes but have undergone fewer changes in gene organization, isotype diversity, and mechanisms of diversification. As of yet, homologous forms of antigen binding receptors have not been identified in jawless vertebrates; however, acquisition of large amounts of structural data for the antigen binding receptors that are found in a variety of jawed vertebrates has defined shared characteristics that provide unique insight into the distant origins of the rearranging gene systems and their relationships to both adaptive and innate recognition processes.

Evolutionary variation of immunoglobulin mu heavy chain RNA processing pathways: origins, effects, and implications

Immunoglobulins (Ig) can occur in two physical forms, soluble (secreted) and membrane bound. The soluble form is secreted from B cells, and is present in the blood and other fluids where it plays a role as an immune effector molecule. The membrane-bound form of the Ig molecule is inserted into the B-cell membrane, where it serves as a receptor for antigen. The function of the membrane-bound Ig as a receptor for antigen requires additional accessory molecules, the membrane Ig plus accessory molecules are referred to, collectively, as the B-cell receptor (BCR) complex. The secreted and membrane-bound forms of an Ig result from alternative patterns of RNA processing of the primary transcript from the heavy chain gene. IgM is the only class of Ig known to be conserved in all vertebrate species (perhaps exclusive of the agnathan fish). While the structure of the IgM heavy (mu) chain gene has been highly conserved in vertebrate evolution, the patterns of alternative RNA processing of the mu transcript show surprising diversity. In particular, the bony fish (teleosts) produce membrane mu-chain message by a splicing pathway that is quite different from that seen in other vertebrates; it results in the production of membrane IgM that lacks the C mu 4 domain. How this unusual RNA splicing pattern could have evolved and its implications for the function of the BCR in the bony fishes are considered here.

Secretory immune system of the duck (Anas platyrhynchos). Identification and expression of the genes encoding IgA and IgM heavy chains

IgA has not previously been identified in waterfowl. Studies instead revealed physical and antigenic similarities between duck bile immunoglobulin (Ig) and serum IgM. Here, a differential screening approach was used to clone, from a duck spleen library, the cDNA encoding the heavy (H) chains of IgM and the Ig, identified here as IgA, occurring in duck secretions. Phylogenetic comparisons of inferred amino acid sequences of entire H chain constant (C) regions and of individual domains revealed that the duck mu chain was closest to chicken mu (54% overall identity), and duck alpha was closest to chicken alpha (50% identity). Comparison of the mu and alpha C regions revealed areas of up to 65% amino acid similarity within the C4 domains, accounting for the previously noted antigenic overlap of duck IgM and IgA. Messages for alpha and mu were detected in duck lymphoid organs but the alpha message was most abundant in the respiratory, alimentary and reproductive tracts. The alpha message first appeared around 14 days of age and reached adult levels of expression only at 35-50 days. The results indicate that the duck has a mucosal immune system which utilizes IgA; however, the delayed expression and secretion of duck IgA explains the susceptibility of ducklings to mucosal pathogens. Since the waterfowl are among the most primitive extant birds, the recognition of IgA in the duck supports the conclusion that IgA occurs throughout the class Aves and also existed in the common ancestors of birds and mammals.

Microsites for immunoglobulin switch recombination breakpoints from Xenopus to mammals

Immunoglobulin (Ig) heavy chain class switch recombination has been studied at the DNA level in a non-mammalian vertebrate, the amphibian Xenopus. A switch (S) region of about 5 kb has been identified in the JH-C mu intron of the Ig heavy chain locus in Xenopus. S mu contains 23 repeats approximately 150 bp long. Each repeat consists of internal shorter repeats and palindromic sequences, such as AGCT, which they share with mammalian switch regions. A deletion of the mu gene and the joining of the S regions of mu and chi occurs in B cells expressing IgX, one of the two non-mu isotypes in Xenopus. S chi shows no sequence homology with S mu and is characterized by 16 and 121 bp repeats and a high frequency of CATG, AGCA and TGCA palindromes. Both IgM and IgX S regions are AT rich and not GC rich like mammalian S regions. Recombination occurs, most of the time, at positions (microsites) where a single-stranded DNA folding program predicts the transition from a stem to a loop structure. This feature is conserved in most mammalian switch junctions which points to the general existence and involvement of microsites at one step of the determination of the recombination break-point. The recombinogenic nature of the switch regions is therefore linked to its structure rather than to its base composition, the repetitive occurrence of palindromes being essential at creating many microsites.

Structure and organization of the immunoglobulin M heavy chain genes in Atlantic salmon, Salmo salar

To determine the structure and organization of the germline immunoglobulin M heavy chain (IgH) genes in Atlantic salmon, Salmo salar, relevant clones from a genomic library (of one individual fish) have been characterized. Two closely related IgH constant region genes, CHA and CHB, have been sequenced completely. In addition, an allotypic variant of CHA was identified and partially sequenced. Five joining (JH) elements were found in a distance of 0.5-1.6 kb upstream of the first constant exon (CH1), in both CHA and CHB, substantiating the hypothesis that the entire gene complex is duplicated; possibly a remnant of a tetraploid event in the salmonid ancestor. An octamer motif (ATGTATTT, and its reverse complementary sequence) was found to be dispersed in the JH-CH1 region, but not elsewhere, signifying a role in these loci. Four closely related variable (VH) genes which were subcloned from three distinct lambda clones showed the classical structure of a two exon unit split by a 100 bp intron. The split-intron and a few hundred base pairs of the flanking sequences of the genes were highly similar. Three of the four genes were interrupted by stop codons and/or frame shifts, indicating a high proportion of VH-pseudogenes in this species. Based on the present results, and comparison with sequences of rainbow trout, Oncorhynchus mykiss, it is likely that the IgH loci have remained tetrasomicly inherited throughout the radiation of the genus Salmo and Oncorhynchus, and that the duplicated loci have gone into a disomic inheritance pattern in the comparatively recent past.

Is Xenopus IgX an analog of IgA?

Using class specific monoclonal antibodies we analyzed the tissue distribution of B cells expressing the three immunoglobulin (Ig) isotypes (IgM, IgX, IgY) in Xenopus. Large numbers of IgM- and IgX-, but not IgY-, positive B cells are located in the gut epithelium of the intestine. In this organ up to 60% of all B cells can be IgX positive, while in the spleen or liver they are hardly detectable. The majority of IgX-producing cells resemble plasma cells. IgY-producing cells are found in the liver and spleen but not in the intestine. In contrast to IgY, the expression of IgM and IgX is thymus independent. Upon systemic immunization, a several-fold increase of specific IgM and IgY, but not IgX, antibodies was detected in the sera. This and its association with the mucosae of the intestine resembles results reported for mammalian IgA; therefore, IgX of Xenopus might be considered an analog of IgA.