Ancient T-independence of mucosal IgX/A: gut microbiota unaffected by larval thymectomy in Xenopus laevis

Abstract

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 rRNA genes was used to assess the relative abundance of bacterial groups present in the stomach, small and large intestine. Clostridiaceae was 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.

Coevolution of Mucosal Immunoglobulins and the Polymeric Immunoglobulin Receptor: Evidence That the Commensal Microbiota Provided the Driving Force

“…Because the ostrich is one of the most primitive extant species of birds, it appears that the distinctive organization of the IGH locus evolved very early during the divergence of the avian lineage. Furthermore, a phylogenetic comparison of Ig heavy chain sequences from amphibians with those of higher vertebrates demonstrated with high statistical support that IgX from amphibians and IgA from birds share a recent common ancestor [50]. Taken together, the evidence is consistent with an IGH locus in a common ancestor of reptiles and birds that included gene segments encoding IgM, IgD, IgA, and IgY.…”

“…Interestingly, thymectomy abolished expression of IgY, but not IgM or IgX, consistent with a T-independent role for IgX in the mucosal immune system of amphibians. Genes encoding IgX and IgY were subsequently identified in 2 species of salamanders [ [50]. These investigators found that larval thymectomy did not affect levels of IgX and did not alter the composition of the gut microbiota, confirming the T-independence of the IgX response in Xenopus.…”

“…The IGH locus in all known mammals is organized as a translocon with gene segments encoding IgM, IgD, 2 or more subclasses of IgG, 1 or more subclasses of IgA, and IgE, an isotype unique to mammals ( Figure 1). A recent phylogenetic analysis of Ig heavy chain classes in tetrapods indicated with high statistical probability that the IgA gene(s) in mammals evolved from an IgX gene in a common ancestor of all tetrapods, while the IgG and IgE genes evolved more recently from IgY in a common ancestor of amniotes [50]. Continuing diversification in the IGH locus in mammals is evidenced by differences in the number of IgG and IgA subclasses and in the structure of Ig heavy chains.…”

“…Subsequent analyses using antibodies to chicken IgA failed to detect a cross-reactive Ig isotype in reptiles [54,55]. The recent availability of whole genome sequences for 2 species of turtles (Chrysemys picta and Pelodiscus sinensis) [36,56] [50,58] (designated Ig"A" for the purposes of this review). The recent availability of whole genome sequences for 4 crocodilians (Alligator sinensis, Alligator mississippiensis, Crocodylus siamensis, and Crocodylus porosus) revealed significant differences in the IGH loci compared to other reptiles, including multiple subclasses of IgM, IgD, and IgA (orthologous to the IgA genes of birds and mammals), but no IgY [59,60].…”

T lymphocytes control microbial composition by regulating the abundance of Vibrio in the zebrafish gut

“…36 In contrast, thymectomy of tadpoles (Xenopus laevis), a model in between zebrafish and higher vertebrates, did not result in differences in microbial composition. 37 Although, Rag-deficiency and thymectomy cannot directly be compared, further investigation into this difference might yield clues concerning the mechanism by which T cells regulate microbial composition independently of B cells in zebrafish. In corroboration with our data, Kawamoto and colleagues showed that not only Rag1-/-mice, but also mice lacking only B cells (Ighm-/-) or only T cells (Cd3e-/-) had reduced bacterial diversity and bacterial communities, indicating that T lymphocytes do have a B lymphocyte independent effect on the microbiota in mice.…”