Nucleotide sequence of a cDNA encoding a third distinct Xenopus immunoglobulin heavy chain isotype

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.

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.

Three IgH isotypes, IgM, IgA and IgY are expressed in Gentoo penguin and zebra finch

Previous studies on a limited number of birds suggested that the IgD-encoding gene was absent in birds. However, one of our recent studies showed that the gene was definitely expressed in the ostrich and emu. Interestingly, we also identified subclass diversification of IgM and IgY in these two birds. To better understand immunoglobulin genes in birds, in this study, we analyzed the immunoglobulin heavy chain genes in the zebra finch (Taeniopygia guttata) and Gentoo penguin (Pygoscelis papua), belonging respectively to the order Passeriformes, the most successful bird order in terms of species diversity and numbers, and Sphenisciformes, a relatively primitive avian order. Similar to the results obtained in chickens and ducks, only three genes encoding immunoglobulin heavy chain isotypes, IgM, IgA and IgY, were identified in both species. Besides, we detected a transcript encoding a short membrane-bound IgA lacking the last two CH exons in the Gentoo penguin. We did not find any evidence supporting the presence of IgD gene or subclass diversification of IgM/IgY in penguin or zebra finch. The obtained data in our study provide more insights into the immunoglobulin heavy chain genes in birds and may help to better understand the evolution of immunoglobulin genes in tetrapods.

Multiple IgH Isotypes Including IgD, Subclasses of IgM, and IgY Are Expressed in the Common Ancestors of Modern Birds

Although evolutionarily just as ancient as IgM, it has been thought for many years that IgD is not present in birds. Based on the recently sequenced genomes of 48 bird species as well as high-throughput transcriptome sequencing of immune-related tissues, we demonstrate in this work that the ostrich (Struthio camelus) possesses a functional δ gene that encodes a membrane-bound IgD H chain with seven CH domains. Furthermore, δ sequences were clearly identified in many other bird species, demonstrating that the δ gene is widely distributed among birds and is only absent in certain bird species. We also show that the ostrich possesses two μ genes (μ1, μ2) and two υ genes (υ1, υ2), in addition to the δ and α genes. Phylogenetic analyses suggest that subclass diversification of both the μ and υ genes occurred during the early stages of bird evolution, after their divergence from nonavian reptiles. Although the positions of the two υ genes are unknown, physical mapping showed that the remaining genes are organized in the order μ1-δ-α-μ2, with the α gene being inverted relative to the others. Together with previous studies, our data suggest that birds and nonavian reptile species most likely shared a common ancestral IgH gene locus containing a δ gene and an inverted α gene. The δ gene was then evolutionarily lost in selected birds, whereas the α gene lost in selected nonavian reptiles. The data obtained in this study provide significant insights into the understanding of IgH gene evolution in tetrapods.

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.

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.

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.

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.

Avian IgY antibodies and their recombinant equivalents in research, diagnostics and therapy

The generation and use of avian antibodies is of increasing interest in a wide variety of applications within the life sciences. Due to their phylogenetic distance, mechanisms of immune diversification and the way in which they deposit IgY immunoglobulin in the egg yolk, chickens provide a number of advantages compared to mammals as hosts for immunization. These advantages include: the one-step purification of antibodies from egg yolk in large amounts facilitates having a virtually continuous supply; the epitope spectrum of avian antibodies potentially grants access to novel specificities; the broad absence of cross-reactivity with mammalian epitopes avoids assay interference and improves the performance of immunological techniques. The polyclonal nature of IgY antibodies has limited their use since avian hybridoma techniques are not well established. Recombinant IgY, however, can be generated from mammalian monoclonal antibodies which makes it possible to further exploit the advantageous properties of the IgY scaffold. Moreover, cloning and selecting the immune repertoire from avian organisms is highly efficient, yielding antigen-specific antibody fragments. The recombinant approach is well suited to circumvent any limitations of polyclonal antibodies. This review presents comprehensive information on the generation, purification, modification and applications of polyclonal and monoclonal IgY antibodies.

The immunoglobulin δ gene in jawed vertebrates: a comparative overview

Immunoglobulin D (IgD) was recently suggested to be an ancient Ig class, as old as IgM, arising approximately 500 million years ago. Its encoding gene has now been identified in nearly all classes of jawed vertebrates (except birds). Variance in the number of CH encoding exons and alternative RNA splicing confers this Ig class a marked structural plasticity, which differs substantially from IgM. Expression of the δ gene can be achieved through co-transcription with the μ gene or by class switching. Although a recent study has suggested that IgD functions as an immunomodulator in immunity and inflammation in humans, its functions are still far from clear. Further studies at the protein levels in additional species may help answer this question.

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.

Comparative and developmental study of the immune system in Xenopus

Xenopus laevis is the model of choice for evolutionary, comparative, and developmental studies of immunity, and invaluable research tools including MHC-defined clones, inbred strains, cell lines, and monoclonal antibodies are available for these studies. Recent efforts to use Silurana (Xenopus) tropicalis for genetic analyses have led to the sequencing of the whole genome. Ongoing genome mapping and mutagenesis studies will provide a new dimension to the study of immunity. Here we review what is known about the immune system of X. laevis integrated with available genomic information from S. tropicalis. This review provides compelling evidence for the high degree of similarity and evolutionary conservation between Xenopus and mammalian immune systems. We propose to build a powerful and innovative comparative biomedical model based on modern genetic technologies that takes take advantage of X. laevis and S. tropicalis, as well as the whole Xenopus genus. Developmental Dynamics 238:1249-1270, 2009. (c) 2009 Wiley-Liss, Inc.

Duck immunoglobulins: structure, functions and molecular genetics

Four types of immunoglobulin (Ig) have been identified in ducks: IgM, a secretory Ig resembling IgM, a 7.8S IgG, and a 5.7S IgG. Structurally and antigenically the 5.7S IgG resembles an F(ab')2 fragment of the 7.8S IgG. When ducks mount serum antibody responses, the sequence of Ig involvement is IgM --> 7.8S IgG --> 5.7S IgG. Although serum Ig levels increase, and antigen-binding Igs can be demonstrated, sera from repeatedly immunized ducks commonly lack secondary antibody activities such as agglutination, precipitation, complement fixation and tissue sensitization. These deficiencies are most likely attributable to the absence of functionally important components of the predominant Ig (5.7S IgG) and/or a possibly unusual steric structure of duck Igs. A related issue concerns production of the two antigenically related IgGs: what are the cellular and molecular events involved, and how are they controlled? Evidence from current molecular genetic studies has confirmed the similarity of the VH, CH1 and CH2 domains of the 7.8S and 5.7S IgGs and shown, by virtue of the existence of separate mature messages for the heavy (H) chains of these molecules, that they are biosynthesized independently. Models for the possibilities that the two H chains are products of one gene or of two genes are presented. Cloning and sequencing the duck H chain gene locus, which is in progress, is providing data supporting the one gene hypothesis. The results obtained from cDNA sequencing also confirm that the duck IgGs are unusual in terms of the anatomy of the hinge region and of the number and location of intra- and inter-chain disulphide bonds, observations which will be of importance for understanding structure/function relationships of these unusual and interesting molecules.

IgX antibodies in the urodele amphibian Ambystoma mexicanum

Until recently, it was believed that urodele amphibians are able to synthesize only two immunoglobulin isotypes, IgM and IgY. We reinvestigated this issue in the Iberian ribbed newt Pleurodeles waltl and reported recently that this urodele expresses at least three isotypes: IgM, IgP and IgY. In this study, we demonstrate that another urodele, Ambystoma mexicanum, has also a third isotype whose amino acid sequence presents the highest homology with the amino acid sequence of Xenopus IgX. This isotype has typical Ig H-chain characteristics, could form multimers and is mainly expressed in mucosal tissues thereby indicating that it is likely the physiological counterpart of Xenopus IgX and mammalian IgA. Interestingly, no IgP could be found in A. mexicanum, in contrast to P. waltl, in which IgX was not found in previous investigations. These data indicate, for the first time, that different families of urodeles can express different immunoglobulin isotypes.

Genomic organization of the immunoglobulin light chain gene loci in Xenopus tropicalis: evolutionary implications

Based on presently available genome data, we characterized the genomic organization of all three light chain gene (rho, sigma and type III) loci in Xenopus tropicalis. The rho gene locus in X. tropicalis, structurally similar to the kappa gene loci in mammals, was shown to contain a single C rho gene and nine J rho segments. The sigma locus also contains a single C gene, although two distinct C sigma genes have previously been found in Xenopus laevis (most likely due to chromosome polyploidy). Four J sigma gene segments were identified upstream of the C sigma. The type III light chain gene locus, spanning approximately 170 kb DNA, structurally resembles the topology of mammalian lambda gene loci, containing three C genes (C III 1-3). C III 2 and C III 3 are both preceded by single, unique, J genes, whereas C III 1 contains three J gene segments. Furthermore, two additional J gene segments, termed J III x1 and J III x2, were found in the intron separating V III 2 and pV III 1 (a pseudogene). Based on BLAST searches against the X. tropicalis EST database, all the C genes identified in this study were shown to be functional. On the basis of similarity of protein sequences, genomic organization and chromosomal location of the light chain genes among frogs and mammals, our data strongly support the previous suggestions that the rho genes belong to the kappa gene lineage, whereas the type III genes share a common origin with the lambda genes.

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.

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.

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.

Molecular cloning and phylogenetic analysis of the Atlantic salmon immunoglobulin D gene

A gene homologous to the IgD heavy chain (delta) gene in channel catfish (Ictalurus punctatus) was found 0.9 kb downstream of the IgM heavy chain (mu) gene in Atlantic salmon (Salmo salar). As in catfish, the first constant mu exon is spliced into the delta transcripts. In agreement with the tetraploid ancestry of the salmonid fish family there are two highly similar delta genes in Atlantic salmon. Characterization of these genes showed that they encode seven 'unique' Ig domains, three of which are tandem duplicated, i.e. like delta1-(delta2-delta3-delta4)*-(delta2- delta3-delta4)-delta5-delta6-d elta7. Sequence analysis indicates that delta1-delta7 arose from two duplication events. Accordingly, salmon delta can be reduced to a unit of three Ig domains corresponding to the three C-terminal domains of a prototypic Ig molecule. The ancestral three-domain unit is apparently best conserved in delta1-delta5-delta6. Phylograms indicate a relationship between teleost and mammalian IgD mainly because of the similarity between the teleost delta5 and human delta2. The corresponding domain in mouse IgD has been deleted during evolution. The teleost delta1 and delta6 sequences are most similar to domains of other non-IgM isotypes, including those in cartilaginous fishes.

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.

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

We have cloned and sequenced the genes corresponding to the membrane exons of the three immunoglobulin (Ig) heavy chain isotypes (mu, upsilon and chi) of Xenopus. Among membrane Ig (mIg) polypeptides, the transmembrane domain are the most highly conserved. The transmembrane and cytoplasmic domains of Xenopus mIgM are similar to the corresponding domains of all known vertebrate mIgM molecules, supporting the idea that amphibian mu gene is homologous, not just analogous, to the mu gene of higher vertebrates. The membrane forms of the two other Ig isotypes mIgX and mIgY exhibit the specific structure found in all Ig membrane exons, but are not homologous with any specific mammalian non-mu Ig isotype; they are most similar to Xenopus mIgM. Based on the conserved transmembrane domains of Xenopus mIgX, mIgY, we suggest that first the upsilon and later the chi genes arose by duplication from the original mu gene. The transmembrane and the 37-amino-acid-long cytoplasmic domains of Xenopus mIgY have conserved residues found in avian mIgY and mammalian mIgG and mIgE, suggesting that the modern isotypes might share a common ancestor with amphibian mIgY. However, while the sequence similarity between the membrane exons of avian mIgY and mammalian mIgG and IgE is striking, the overall similarity with Xenopus mIgY is very low. Thus, the genes giving rise to Xenopus mIgY and those eventually leading to avian mIgY and mammalian mIgG and mIgE must have diverged early in evolution, probably at the level of the primitive amphibians or before.

IgY: clues to the origins of modern antibodies

IgY is the functional equivalent of IgG in birds, reptiles and amphibia, but many aspects of its biology are poorly understood. Recent studies have increased awareness of the genetics and functions of this molecule, and have revealed its position as the ancestor of the uniquely mammalian antibodies IgG and IgE. Here, Greg Warr, Kathy Magor and David Higgins review current knowledge of IgY structure, function and expression in the context of the evolutionary role of this primitive immunoglobulin.

Characterization of serum immunoglobulins in a chondrostean fish, Acipenser baeri

The euglobulin fraction of sturgeon (Acipenser baeri) serum was analyzed using electrophoretic and immunoblotting techniques. The major protein of this fraction is an IgM-like molecule composed of equimolar 70-kDa glycosylated H chains and 26-30 kDa L chains. In the absence of a reducing agent, the L and H polypeptides may form (mu 2L2)n high molecular weight polymers, mu 2L2 170-kDa units or L2 dimers. These different bonding patterns suggest some structural heterogeneity in the distribution of cysteine residues along the sturgeon Ig chains. The H chain N-terminal sequence indicates significant homologies with the conserved VHIII subgroup. Heavy chains antigenically different from the 70-kDa H chain were not detected, suggesting that IgM is the only Ig class synthesized by this sturgeon species.

MHC restriction of T-cell proliferative responses in Xenopus

The MHC restriction of Xenopus allogeneic MHC- and antigen-specific T-cell proliferative responses was assessed. Xenopus MHC-specific monoclonal antibodies that recognize class I and class II molecules were tested for inhibitory effects on the generation of secondary T-cell proliferative responses. Antigen-specific T-cell lines were inhibited by anti-class II but not anti-class I monoclonal antibodies. Secondary alloantigen-specific proliferative responses also demonstrated MHC class II restriction. Allogeneic MHC- and antigen-specific T-cell lines demonstrated differential sensitivity to anti-class II monoclonal antibodies directed at discrete class II epitopes. These results indicate that Xenopus T cells interact with antigen-presenting cells similarly to mammals, and directly confirm previous data indicating that MHC class II restriction of proliferative responses is present in amphibians.

Sequence of ovine Ig gamma 2 constant region heavy chain cDNA and molecular modelling of ruminant IgG isotypes

Ovine mesenteric lymph node mRNA was used for PCR amplification of DNA coding for immunoglobulin gamma 1 and gamma 2 heavy chain constant regions. Primers complementary to regions of CH1 conserved between ruminants were used for upstream priming, with downstream priming on the poly-A segment. PCR products of the appropriate length were cloned and gamma positive clones selected with a CH1 conserved-region probe. Of these, gamma 1 clones were positively selected and gamma 2 clones negatively selected with a gamma 1 hinge-specific probe. Ovine gamma 2 cDNA has 93% identity of nucleotides with ovine gamma 1. Both ovine gamma 1 and gamma 2 CH1 domains encoded two consecutive cysteine residues (Cys-127, -128, Kabat numbering), an arrangement which is deduced to form a pair of disulphide bridges, one to the L chain and one as an intra-chain bridge to the uppermost Cys of the hinge, as in rabbit and goat IgG. The majority of the differences between the isotypes occur in the hinge region and an evolutionary pattern for ruminant IgG hinges can now be identified. IgG1 isotypes are typical, with hinges containing the C-terminal Cys-Pro motif, but deletion and replacement of nucleotides (in the ancestral gene) of ruminant gamma 2 has shortened the IgG2 hinge, removing the Cys-Pro motif and the consensus high affinity Fc gamma RI receptor motif at the start of CH2. An N-terminal glycosylation site and the peptide motif for complement C1q binding are present in CH2 of both isotypes. The hinge regions of gamma 1 and gamma 2 and predicted structures for ovine IgG1 and IgG2 have been modelled. Close apposition of Fab and Fc in IgG2 produces steric hindrance at the normally accessible Fab/hinge/Fc interface; the structural differences between the ruminant isotypes form a basis for understanding some of the differences in their effector properties.

Phylogeny of immunoglobulin heavy chain isotypes: structure of the constant region of Ambystoma mexicanum upsilon chain deduced from cDNA sequence

An RNA polymerase chain reaction strategy was used to amplify and clone a cDNA segment encoding for the complete constant part of the axolotl IgY heavy (C upsilon) chain. C upsilon is 433 amino acids long and organized into four domains (C upsilon 1-C upsilon 4); each has the typical internal disulfide bond and invariant tryptophane residues. Axolotl C upsilon is most closely related to Xenopus C upsilon (40% identical amino acid residues) and C upsilon 1 shares 46.4% amino acid residues among these species. The presence of additional cysteines in C upsilon 1 and C upsilon 2 domains is consistent with an additional intradomain S-S bond similar to that suggested for Xenopus C upsilon and C chi, and for the avian C upsilon and the human C epsilon. C upsilon 4 ends with the Gly-Lys dipeptide characteristic of secreted mammalian C gamma 3, human C epsilon 4, and avian and anuran C upsilon 4, and contains the consensus [G/GT(AA)] nucleotide splice signal sequence for joining C upsilon 4 to the transmembrane region. These results are consistent with the hypothesis of an ancestral structural relationship between amphibian, avian upsilon chains, and mammalian epsilon chains. However, these molecules have different biological properties: axolotl IgY is secretory Ig, anuran and avian IgY behave like mammalian IgG, and mammalian IgE is implicated in anaphylactic reactions.

The genomic organization of immunoglobulin VH genes in Xenopus laevis shows evidence for interspersion of families

The complete genomic sequences of Xenopus laevis immunoglobulin heavy chain variable region (VH) genes comprising families IV-XI are reported. Using VH family-specific probes, linkage relationships for Xenopus VHI-VHXI have been determined. With the possible exceptions of VHIII and VHVII, Xenopus VH genes appear to be interspersed. When from two to five VH segments are identified in individual clones, the elements are found to be in the same relative transcriptional orientation. Although the relationships of promoter sequences, including the regulatory octamer, resemble those seen in other vertebrate VH genes, several Xenopus VH families are associated with additional 5' octamer sequences and octamer-like motifs. The similarities between the genomic organization of VH genes in Xenopus and higher vertebrates, indicate an early phylogenetic emergence of the mammalian-type of gene organization and regulation.

Eleven distinct VH gene families and additional patterns of sequence variation suggest a high degree of immunoglobulin gene complexity in a lower vertebrate, Xenopus laevis

Lower vertebrate species, including Xenopus laevis, exhibit restricted antibody diversity relative to higher vertebrates. We have analyzed more than 180 VH gene-containing recombinant clones from an unamplified spleen cDNA library by selective sequencing of JH and CH positive clones following iterative hybridization screening with family-specific VH probes, 11 unique families of VH genes, each associated with a unique genomic Southern blot hybridization pattern, are described and compared. Considerable variation in the number of hybridizing components detected by each probe is evident. The nucleotide sequence difference between VH families is as great as, if not more than, that reported in other systems, including representatives of the mammalian, avian, and elasmobranch lineages. Some Xenopus Ig gene families encode alternative amino acids at positions that are otherwise invariant or very rarely substituted in known Igs. Furthermore, variations in complementarity determining region sequences among members of the same gene family and high degrees of DH and JH region complexity are described, suggesting that in at least this lower vertebrate species, the diversity of expressed Ig VH genes is not restricted.