Evolution of Mucosal Immunoglobulins: Xenopus Laevis IgX and Thunnus Orientalis IgZ/T
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Despite a large number of studies during the last decade that investigated mucosal immunity in humans, very few works have been done on this immune compartment in lower vertebrates. In the following two studies we focused on the mucosal immunoglobulins in two important species of two classes of ectothermic vertebrates: amphibians and bony fishes. 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, and the small and large intestine. Clostridiaceae were 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. It is now appreciated that in addition to the immunoglobulin (Ig)M and D isotypes fish also make the mucosal IgT. In this study we sequenced the full length of Ig tau as well as mu in the commercially important Thunnus orientalis (Pacific bluefin tuna), the first analysis of both of these Ig isotypes in a member of the order Perciformes. Tuna IgM and IgT are each composed of four constant (CH) domains. We cloned and sequenced 48 different variable (VH) domain rearrangements of tuna immunoglobulins and grouped the VH gene sequences to four VH gene segment families based on 70% nucleotide identity. Three VH gene families were used by both IgM and IgT but one group was only found to be used by IgM. Most interestingly, both Ig mu and Ig tau clones appear to use the same diversity (DH) segment, unlike what has been described in other species, although they have dedicated IgT and IgM joining (JH) gene segments. We complemented this repertoire study with phylogenetic and tissue expression analysis. In addition to supporting the development of humoral vaccines in this important aquaculture species, these data suggest that the DH-JH recombination rather than the VH-DH recombination may be instructive for IgT versus IgM/D bearing lymphocyte lineages in some fish.
Mashoof, Sara (2014). Evolution of Mucosal Immunoglobulins: Xenopus Laevis IgX and Thunnus Orientalis IgZ/T. Doctoral dissertation, Texas A & M University. Available electronically from