Cosegregation of the polymorphic C4 with the MHC in the frog, Xenopus laevis

Susceptibility of amphibians to chytridiomycosis is associated with MHC class II conformation

The pathogenic chytrid fungus Batrachochytrium dendrobatidis (Bd) can cause precipitous population declines in its amphibian hosts. Responses of individuals to infection vary greatly with the capacity of their immune system to respond to the pathogen. We used a combination of comparative and experimental approaches to identify major histocompatibility complex class II (MHC-II) alleles encoding molecules that foster the survival of Bd-infected amphibians. We found that Bd-resistant amphibians across four continents share common amino acids in three binding pockets of the MHC-II antigen-binding groove. Moreover, strong signals of selection acting on these specific sites were evident among all species co-existing with the pathogen. In the laboratory, we experimentally inoculated Australian tree frogs with Bd to test how each binding pocket conformation influences disease resistance. Only the conformation of MHC-II pocket 9 of surviving subjects matched those of Bd-resistant species. This MHC-II conformation thus may determine amphibian resistance to Bd, although other MHC-II binding pockets also may contribute to resistance. Rescuing amphibian biodiversity will depend on our understanding of amphibian immune defence mechanisms against Bd. The identification of adaptive genetic markers for Bd resistance represents an important step forward towards that goal.

Xenopus class II A genes: studies of genetics, polymorphism, and expression

The amphibian Xenopus laevis has been a central model for the study of evolution of the major histocompatibility complex (MHC). Many of the counterparts of mammalian MHC genes have been identified in Xenopus, facilitating the understanding of MHC structure and function. Herein we characterize X. laevis MHC class II-alpha chain genes. There are three related class II A genes/haplotype in the four commonly used partially inbred strains, all of which linked to the functional MHC. At least two of these genes in the f haplotype encode full-length cDNA clones and a genomic fragment encoding the immunoglobulin-like domain of the third gene was also characterized. The protein structure and domain organization deduced from the two f/f cDNA clones are similar to mammalian MHC class II-alpha chains. Expression of class II A genes is highest in the spleen and intestine, similar to the previously examined tissue distribution of class II B genes. The two highly expressed genes display high sequence diversity among alleles, similar to what has been found in most other species. Surprisingly, transcript sizes of class II A alleles/isotypes are diverse, suggesting that Xenopus class II allelic lineages are very old.

Insight into the primordial MHC from studies in ectothermic vertebrates

MHC classical class I and class II genes have been identified in representative species from all major jawed vertebrate taxa, the oldest group being the cartilaginous fish, whereas no class I/II genes of any type have been detected in animals from older taxa. Among ectothermic vertebrate classes, studies of MHC architecture have been done in cartilaginous fish (sharks), bony fish (several teleost species), and amphibians (the frog Xenopus). The Xenopus MHC contains class I, class II, and class III genes, demonstrating that all of these genes were linked in the ancestor of the tetrapods, but the gene order is not the same as that in mouse/man. Studies of polyploid Xenopus suggest that MHC genes can be differentially silenced when multiple copies are present; i.e. MHC 'subregions' can be silenced. Surprisingly, in all teleosts examined to date class I and class II genes are not linked. Likewise, class III genes like the complement genes factor B (Bf) and C4 are scattered throughout the genome of teleosts. However, the presumed classical class I genes are closely linked to the 'immune' proteasome genes, LMP2 and LMP7, and to the peptide-transporter genes (TAP), implying that a true 'class I region' exists in this group. A similar type of linkage group is found in chickens and perhaps Xenopus, and thus it may reveal the ancestral organization of class I-associated genes. In cartilaginous fish, classical and non-classical class I genes have been isolated from three shark species, and class II A and B chain genes from nurse sharks. Studies of MHC linkage in sharks are being carried out to provide further understanding of the putative primordial organization of MHC Segregation studies in one shark family point to linkage of classical class I and class II genes, suggesting that the non-linkage of these genes in teleosts is a derived characteristic.

Major histocompatibility complex gene mapping in the amphibian Xenopus implies a primordial organization

One of the most provocative recent discoveries in immunology was the description of a genetic linkage in the major histocompatibility complex (MHC) between structurally unrelated genes whose products are involved in processing and presentation of antigens for recognition by T lymphocytes. Genes encoding MHC class I molecules, which bind and present at the cell surface proteolytic fragments of cytosolic proteins, are linked to nonhomologous genes whose products are involved in the production and subsequent transfer of such fragments into the endoplasmic reticulum. In mammals, the class I presentation and processing genes are found in different regions of the MHC. To examine the evolutionary origins of this genetic association, linkage studies were carried out with Xenopus, an amphibian last sharing an ancestor with mammals over 350 million years ago. In contrast to mammals, the single copy Xenopus class I gene is located between the class II and III regions, speculated to be in close linkage with the processing and transport genes. In addition to suggesting a primordial organization of genes involved in class I antigen presentation, these linkage studies further provide insight into the origins of the MHC class III region and the phenomenon of class I gene instability in the mammalian MHC.

Fourth component of Xenopus laevis complement: cDNA cloning and linkage analysis of the frog MHC

Complement C4 shows extensive structural and functional similarity to complement C3, hence these components are believed to have originated by gene duplication from a common ancestor. Although to date C3 cDNA clones have been isolated from all major classes of extant vertebrates including Xenopus, C4 cDNA clones have been isolated from mammalian species only. We describe here the molecular cloning and structural analysis of Xenopus C4 cDNA. The cDNA sequence encoding the thioester region of Xenopus C4 was amplified by reverse transcriptase-polymerase chain reaction using Xenopus liver mRNA as a template, and then used to screen a liver cDNA library. The amino acid sequence of Xenopus C4 deduced from a clone containing the entire protein-coding sequence showed 39%, 30%, 25%, and 20% overall identity with those of human C4, C3, C5, and alpha2-macroglobulin, respectively. The predicted amino acid sequence consisted of a 22-residue putative signal peptide, a 634-residue beta chain, a 732-residue alpha chain, and a 287-residue gamma chain. Of 30 cysteine residues, 27 were found in exactly the same positions as in human C4. Genomic Southern blotting analysis indicated that C4 is a single copy gene in Xenopus and is part of the frog MHC cluster. These results clearly demonstrate that C3/C4 gene duplication and linkage between the C4 gene and the major histocompatibility complex predate mammalian/amphibian divergence.

Fish major histocompatibility complex genes: an expansion

The advent of polymerase chain reaction technology has provoked a large amount of progress in the field of fish major histocompatibility complex (MHC) research. Many new teleost sequences have been reported in the last four years, including representatives of all classes of MHC genes. While the intron-exon structure of teleost MHC genes is now becoming clear, the organisation of the genes within the teleost MHC is still unclear. The sequences reported to date have been used for phylogenetic analysis and, due to their evolutionary position, are discussed in relation to hypotheses regarding the origin of the MHC. Teleost MHC gene sequences are also examined to see if conserved features of the both the nucleotide and amino acid sequences of higher vertebrate MHC genes are present. Differences in these features will reflect functional differences between teleost and mammalian MHC genes and may also have evolutionary implications.

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.

Origin and evolution of the vertebrate immune system

The immune system is a complex evolutionary unit and it would be simplistic to conclude that the immune systems of all primitive vertebrates are primitive. Because of the large number of elements involved, many evolutionary events must have taken place, some of them neutral, some of them selected, to constitute the systems that we are looking at towards the end of the 20th century. All these systems have perhaps evolved beyond the apparent evolutionary state of the species in which they are found. They have been modulated by factors linked not only to the internal evolution of their elementary genes, but also by coevolution with factors in the internal environment, such as cellular constraints, metabolism, mode of reproduction and progeny size. It seems that good inventions are long lasting, which is the reason why some elements of the invertebrate immune system can be found with similar functions in vertebrates (defensins). It is also the reason why Ig domains have been exploited in so many ways, whether for the immune system or not. Again, they had an evolution of their own. The comparative study of the immune systems carried out on the occasion of this phylogenetic survey shows a world particularly dynamic and diverse. The comparisons between the solutions chosen by the various phyla of the animal kingdom, or closer to us by the various classes of vertebrates, allow us to distinguish the essential features of the immune system. From this viewpoint, this approach is not only of phylogenetic interest, but also has an applied aspect. Increasing our knowledge in this area could help suggest solutions to clinicians when they are faced with deficiencies and abnormalities in the immune system of man.

Evolution of the MHC: antigenicity and unusual tissue distribution of Xenopus (frog) class II molecules

Antibodies that recognize Xenopus class II molecules have been developed. Mouse monoclonal antibodies were prepared by immunizing BALB/c mice with frog MHC antigens that had been partially purified with alloantisera, and by immunizing mouse spleen cells in vitro with activated Xenopus T lymphocytes. In addition, five mouse monoclonal antibodies specific for human class II antigens were found to cross-react with Xenopus class II antigens. A.TH mice, which do not express E class II molecules, always produce immunoprecipitating antibodies reactive with frog class II molecules after immunization with frog lymphocytes; other mouse strains rarely produce such antibodies. Two of the monoclonal antibodies raised against frog class II molecules recognize the denatured class II beta chain on Western blots, and the other three appear to recognize only the class II heterodimeric complex. The antibodies display differential reactivity with the allelic class II products of Xenopus. The monoclonal antibodies react with all adult lymphocytes in the spleen and peripheral blood, T cells and B cells having equivalent levels of class II antigens per cell. Class II molecules are "differentiation antigens" on adult thymocytes as the expression is greatest on the mature medullary population. The number of class II molecules/lymphocyte increases after culturing in medium containing fetal bovine serum. Sequential immunoprecipitation and isoelectric focusing experiments have shown that cell surface class II molecules immunoprecipitated with the monoclonal antibodies are the same as those immunoprecipitated with the cross-reactive antiserum specific for DR antigens which was previously used to identify frog class II molecules.

The MHC molecules of nonmammalian vertebrates

There is very little known about the long-term evolution of the MHC and MHC-like molecules. This is because both the theory (the evolutionary questions and models) and the practice (the animals systems, functional assays and reagents to identify and characterize these molecules) have been difficult to develop. There is no molecular evidence yet to decide whether vertebrate immune systems (and particularly the MHC molecules) are evolutionarily related to invertebrate allorecognition systems, and the functional evidence can be interpreted either way. Even among the vertebrates, there is great heterogeneity in the quality and quantity of the immune response. The functional evidence for T-lymphocyte function in jawless and cartilagenous fish is poor, while the bony fish seem to have many characteristics of a mammalian immune system. The organization and sequence of fish Ig genes also indicate that important events in the evolution of the immune system and the MHC occurred in the fish, but thus far there is no molecular evidence for recognizable MHC-like molecules in any fish. There is clearly an MHC in amphibians and birds with many characteristics like the MHC of mammals (a single genetic region encoding polymorphic class I and class II molecules) and evidence for polymorphic class I and class II molecules in reptiles. However, many details differ from the mammals, and it is not clear whether these reflect historical accident or selection for different lifestyles or environment. For example, the adult frog Xenopus has a vigorous immune system with many similarities to mammals, a ubiquitous class I molecule, but a much wider class II tissue distribution than human, mouse and chicken. The Xenopus tadpole has a much more restricted immune response, no cell surface class I molecules and a mammalian class II distribution. The axolotl has a very poor immune response (as though there are no helper T cells), a wide class II distribution and, for most animals, no cell surface class I molecule. It would be enlightening to understand both the mechanisms for the regulation of the MHC molecules during ontogeny and the consequences for the immune system and survival of the animals. These animals also differ markedly in the level of MHC polymorphism. Another difference from mammals is the presence of previously uncharacterized molecules. In Xenopus and reptiles, there are two populations of class I alpha chain on the surface of erythrocytes, those in association with beta 2m and those in association with a disulfide-linked homodimer.(ABSTRACT TRUNCATED AT 400 WORDS)

Conservation of structural and functional domains in complement component C3 of Xenopus and mammals

The cDNA sequence and the deduced amino acid sequence of the Mr 34,000 C-terminal fragment of Xenopus laevis complement component C3 are presented. The sequence of Xenopus C3 has 57% nucleotide identity to the corresponding sequence of human C3 and approximately 49% amino acid identity to C3 from human, mouse, and rabbit. The Xenopus C3 sequence shows clusters of high and of low similarity to the mammalian C3 sequences. One of these regions of high similarity represents the domain of mammalian C3b involved in the binding of properdin, a regulator of the alternative pathway of complement activation. It is not clear whether the other highly conserved regions are involved in binding to other C3 ligands. The Xenopus C3 sequence completely lacks the Arg-Gly-Asp sequence, which has been suggested to be the recognition site of the human complement receptor type 3 on the iC3b fragment of human C3. The Xenopus C3 gene is shown not to be linked to the Xenopus major histocompatibility complex, as is also the case in mammals. Since the gene of the related molecule C4 is MHC-linked in both mammals and Xenopus, the C3 and C4 genes may have separated before Xenopus and mammals speciated.