T cell receptor delta gene repertoire and diversity of intestinal intraepithelial lymphocytes in athymic mice

Origins of intestinal intraepithelial lymphocytes: direct evidence for a thymus-derived gamma delta T cell component

The contribution of T cell precursors from the thymus and the bone marrow to the pool of intestinal intraepithelial lymphocvtes (IELs) has been studied in a system using donor cells from enhanced-green fluorescent protein (EGFP+ ) transgenic mice a doptively transferred into EGFP- recipient mice. Consistent with previous studies, regeneration of gamma delta and alpha beta T cell populations in the intestinal epithelium occurred within 2-3 weeks of bone marrow transfer into irradiatiated EGFP- animals and prior to T cell repopulation of the spleen, of interest, however, although transfer of whole adult EGFP+ thymocytes to non-irradiated EGFP- congenitally-athymic nude mice produced alpha beta T cells in both the spleen and intestine. Gamma delta T cells in significant number were detected only in the intestine of recipient mice. In contrast, transfer of CD3-, CD4-, CD8- immature thymocytes resulted in no detectable T cells in either the intestine or the spleen of nude mice up to twelve weeks post-cell transfer, suggesting that intestinal IELs generated from thymocytes arose from differentiated lineage-committed cells rather than from immature thymocytes. These findings provide direct evidence for both thymus-independent and thymus-dependent sources of intestinal gamma delta T cells, and they suggest that murine IELs consist of diverse groups of T cells with distinct developmental origins.

Conservation of a master hematopoietic switch gene during vertebrate evolution: isolation and characterization of Ikaros from teleost and amphibian species

The generation of T, B and NK lymphocyte lineages from pluripotent hematopoietic stem cells is dependent upon the early expression of the Ikaros locus which by means of alternative splicing produces a variety of zinc finger DNA binding transcription factors. We assessed the general biological importance of Ikaros by studying its conservation and expression in teleost fish and amphibians. Portions of Ikaros cDNA from rainbow trout and Xenopus were amplified by reverse transcription-polymerase chain reaction (RT-PCR). They show roughly 75% conservation of the amino acid sequence with mammalian Ikaros. The trout fragment was then used to isolate full-length Ikaros clones from a trout thymocyte cDNA library. In mice and humans, Ikaros produces six alternatively spliced isoforms, but in trout two additional novel splice variants designated Ik-7 and Ik-8 were also found. Ik-7 is expressed in a similar fashion to Ik-1 and Ik-2, the predominant isoforms expressed in mammalian lymphocytes. In trout and zebrafish, as in mammals, Ikaros is a single-copy gene, but in Xenopus segregation analysis demonstrates that Ikaros has been duplicated, most likely a result of polyploidization. We then examined the expression of Ikaros in trout and Xenopus tumor T cell lines via Northern blot, RT-PCR, immunofluorescence and Western blot analysis. Overall, Ikaros is expressed in a lymphoid-specific fashion similar to that found in mice and humans. In addition Ikaros is expressed early in trout ontogeny, beginning roughly at days 3-4 in the yolk sac and at day 5-6 in the embryo proper. The conservation of Ikaros structure and expression confirms it as a master switch of hematopoiesis.

Regional specialization of intraepithelial T cells in the murine small and large intestine

We investigated intraepithelial T cells from the small intestine, SI (jejunum, ileum) and the large intestine, LI (colon) of euthymic (BALB/c, H-2d; C.B-17+/+, H-2d; C57BL/6, H-2b) and athymic (C57BL/6 nu/nu; BNX bg/bg nu/nu xid/xid) mice. From individual euthymic and athymic mice, 7 x 10(6) intraepithelial lymphocytes (IEL) per mouse were isolated from the SI. Ten-fold lower numbers of IEL were obtained from the LI epithelium (4 x 10(5) IEL per mouse). Thymus-dependent and -independent T cells represented > 80% of SI-IEL but the fraction of T cells was reduced from 20% to 40% in LI-IEL. In euthymic mice, alpha beta T cells predominated in SI-IEL and in particular in LI-IEL populations, while SI-IEL and LI-IEL populations of athymic mice contained predominantly gamma delta T cells. The intraepithelial T cell subset distribution was different in SI versus LI: mainly CD8+ T cells were present in the SI, but a large CD4+ T cell subset was present in the LI. 'Double positive' CD4+ CD8 alpha+ T cells were present mainly in the SI epithelium but were rare in the LI epithelium. In euthymic as well as athymic mice, T cells expressing the homodimeric CD8 alpha alpha isoform predominated in the SI epithelium, while T cells expressing the heterodimeric CD8 alpha beta isoform predominated in the LI epithelium. LI-derived TCR alpha beta+ IEL displayed the CD2+ CD28+ LPAM-1/2- M290+ phenotype, and a fraction of them expressed the L-selectin LECAM-1. In contrast, a large fraction of TCR alpha beta+ SI-IEL was CD2- CD28- LPAM-1/2- M290+ and LECAM-1-. RAG-1/2 expression was detectable by RT-PCR in IEL from the SI but not the LI. Striking differences in phenotype were thus apparent between thymus-dependent and thymus-independent T cells in the epithelial layer of the jejunum/ileum and the colon of the mouse.

Thymus-neuroendocrine interactions in extrathymic T cell development

Studies of the development of murine intestinal intraepithelial lymphocytes (IELs) have yielded markedly different results depending on the experimental system used. In athymic radiation chimeras, IELs consist of all subsets found in euthymic mice; adult mice that were athymic at birth have only IELs that are positive for T cell receptor gamma delta and CD8 alpha alpha. These differences are resolved by the finding that administration of the neuropeptide thyrotropin-releasing hormone to adult mice thymectomized as neonates leads to the development of all IEL T cells. Thus, a neuroendocrine signal initiated by the thymus during fetal or neonatal life appears to be required for subsequent extrathymic maturation of gut alpha beta T cells.

The V delta 1 T cell receptor repertoire in human small intestine and colon

V delta 1 bearing T cells comprise the major population of gamma/delta T cells in the human intestinal tract. To gain insight into mechanisms involved in the generation of these cells and the diversity of their repertoire, we have characterized the junctional sequences of V delta 1 T cell receptor transcripts in the human small intestine and colon. Mucosal biopsies obtained from defined regions along the length of the small intestine or colon contained a high frequency of either one or a few identical in frame V delta 1 sequences. Less abundant sequences were also detected repeatedly throughout the length of small intestine or colon. Moreover, the intestinal V delta 1 repertoire in the small intestine and colon appeared compartmentalized and showed no overlap with the V delta 1 repertoire in peripheral blood. Dominant V delta 1 transcripts in each subject differed between the small intestine and colon, and the dominant transcripts within these sites differed among individuals. Analysis of small intestinal transcripts obtained at a 1-yr interval revealed that the V delta 1 repertoire was stable over time. The fact that the majority of V delta 1 transcripts, both dominant and rare, are distributed throughout a several meter length of the adult intestinal tract and are stable over time suggests they are not generated by an ongoing process of in situ VDJ gene rearrangement. Our results favor a model in which the repertoire of V delta 1 T cells in the intestinal tract is shaped by positive selection in response to a limited array of ligands before the migration of V delta 1 cells throughout the small intestine or colon.