Immunoglobulin genes and B cell development in amphibians

Xenopus tropicalis: Joining the Armada in the Fight Against Blood Cancer

Aquatic vertebrate organisms such as zebrafish have been used for over a decade to model different types of human cancer, including hematologic malignancies. However, the introduction of gene editing techniques such as CRISPR/Cas9 and TALEN, have now opened the road for other organisms featuring large externally developing embryos that are easily accessible. Thanks to its unique diploid genome that shows a high degree of synteny to the human, combined with its relatively short live cycle, Xenopus tropicalis has now emerged as an additional powerful aquatic model for studying human disease genes. Genome editing techniques are very simple and extremely efficient, permitting the fast and cheap generation of genetic models for human disease. Mosaic disruption of tumor suppressor genes allows the generation of highly penetrant and low latency cancer models. While models for solid human tumors have been recently generated, genetic models for hematologic malignancies are currently lacking for Xenopus. Here we describe our experimental pipeline, based on mosaic genome editing by CRISPR/Cas9, to generate innovative and high-performing leukemia models in X. tropicalis. These add to the existing models in zebrafish and will extend the experimental platform available in aquatic vertebrate organisms to contribute to the field of hematologic malignancies. This will extend our knowledge in the etiology of this cancer and assist the identification of molecular targets for therapeutic intervention.

A prominent role for invariant T cells in the amphibian Xenopus laevis tadpoles

Invariant T (iT) cells expressing an invariant or semi-invariant T cell receptor (TCR) repertoire have gained attention in recent years because of their potential as specialized regulators of immune function. These iT cells are typically restricted by nonclassical MHC class I molecules (e.g., CD1d and MR1) and undergo differentiation pathways distinct from conventional T cells. While the benefit of a limited TCR repertoire may appear counterintuitive in regard to the advantage of the diversified repertoire of conventional T cells allowing for exquisite specificity to antigens, the full biological importance and evolutionary conservation of iT cells are just starting to emerge. It is generally considered that iT cells are specialized to recognize conserved antigens equivalent to pathogen-associated molecular pattern. Until recently, little was known about the evolution of iT cells. The identification of class Ib and class I-like genes in nonmammalian vertebrates, despite the heterogeneity and variable numbers of these genes among species, suggests that iT cells are also present in ectothermic vertebrates. Indeed, recent studies in the amphibian Xenopus have revealed a drastic overrepresentation of several invariant TCRs in tadpoles and identified a prominent nonclassical MHC class I-restricted iT cell subset critical for tadpole antiviral immunity. This suggests an important and perhaps even dominant role of multiple nonclassical MHC class I-restricted iT cell populations in tadpoles and, by extension, other aquatic vertebrates with rapid external development that are under pressure to produce a functional lymphocyte repertoire with small numbers of cells.

Phylogeny of B-cell development

All vertebrates with jaws (Gnathostomata) have B cells. With the exception of some B cells in cartilaginous fish that express germ-line joined Ig genes, all B cells, irrespective of the organization of their Ig genes (which varies among vertebrates), rearrange the Ig-gene segments somatically. Somatic diversification occurs in all species during rearrangement (junctional diversity) and later by somatic mutation of gene conversion. Somatic mutants are poorly selected in species that lack germinal centers, which may explain the differences in antibody repertoire among vertebrates. The early (larval or neonate) B-cell repertoire is restricted in all species so far studied because of a lack of N-region diversity and in some cases because of a special usage of the D segments of the heavy chain genes.

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.