Development of the early B cell population in Xenopus.

Like mammals, the amphibian Xenopus uses combinatorial joining of the immunoglobulin V, D and J elements and multiple rearrangements to generate its B cell repertoire. Xenopus larvae hatch 2 days after fertilization and individuals are under pressure to develop an immune repertoire when the number of available cells is small (approximately 5 and 200 IgM-positive cells on days 5 and 11 after fertilization, respectively). In the liver, in a first phase of differentiation spanning days 5-12 after fertilization before immunological competence, the heavy (H) chain locus starts rearranging followed by the light (L) chain locus 3 days later. By immunohistology the first B cells expressing H and L chain are detectable on day 10. Despite the small number of cells available and the lack of external antigen selection at these early stages, the repertoire is heterogeneous. The VH families are used stepwise, although their genes are interspersed in the genome. The earliest family used (VH1) is homologous to the VH3 family of human and to the VH7183 of the mouse which are also overrepresented in early mammalian development. In the second phase, from day 12-13 onwards, the spleen differentiates and the animal becomes immunologically competent. The V, D and J usage is similar to that of adults although VDJ junctions lack N nucleotides until metamorphosis. A preferential reading frame for D and one specific DJ junction are overrepresented during this second phase. The visible bias toward homology-based junction results in fact from selection after rearrangement.