Several unique waves of γδ T cells are generated solely in the fetal/neonatal thymus, whereas additional γδ T cell subsets are generated in adults. One intriguing feature of γδ T cell development is the coordination of differentiation and acquisition of effector function within the fetal thymus; however, it is less clear whether this paradigm holds true in adult animals. In this study, we investigated the relationship between maturation and thymic export of adult-derived γδ thymocytes in mice. In the Rag2pGFP model, immature (CD24+) γδ thymocytes expressed high levels of GFP whereas only a minority of mature (CD24−) γδ thymocytes were GFP+. Similarly, most peripheral GFP+ γδ T cells were immature. Analysis of γδ recent thymic emigrants (RTEs) indicated that most γδ T cell RTEs were CD24+ and GFP+, and adoptive transfer experiments demonstrated that immature γδ thymocytes can mature outside the thymus. Mature γδ T cells largely did not recirculate to the thymus from the periphery; rather, a population of mature γδ thymocytes that produced IFN-γ or IL-17 remained resident in the thymus for at least 60 d. These data support the existence of two populations of γδ T cell RTEs in adult mice: a majority subset that is immature and matures in the periphery after thymic emigration, and a minority subset that completes maturation within the thymus prior to emigration. Additionally, we identified a heterogeneous population of resident γδ thymocytes of unknown functional importance. Collectively, these data shed light on the generation of the γδ T cell compartment in adult mice.
The immune system is composed of a variety of different T cell lineages distributed through both secondary lymphoid tissue and non-lymphoid tissue. The intestinal epithelium is a critical barrier surface that contains numerous intraepithelial lymphocytes that aid in maintaining homeostasis at that barrier. This review focuses on T Cell Receptor αβ (TCRαβ) CD8αα intraepithelial lymphocytes, and how recent advances in the field clarify how this unique T cell subset is selected, matures, and functions in the intestines. We consider how the available evidence reveals a story of ontogeny starting from agonist selection of T cells in the thymus and finishing through the specific signaling environment of the intestinal epithelium. We conclude with how this story raises further key questions about the development of different ontogenic waves of TCRαβ CD8αα IEL and their importance for intestinal epithelial homeostasis.
The thymus is the site of both αβ and γδ-T cell development. After several unique waves of γδ-T cells are generated in, and exported from, the fetal/neonatal thymus, the adult thymus continues to produce a stream of γδ-T cells throughout life. One intriguing feature of γδ T cell development is the coordination of differentiation and acquisition of effector function within the fetal thymus, however, it is less clear whether this paradigm holds true in adult animals. To investigate the relationship between maturation and time since V(D)J recombination in adult-derived γδ-thymocytes, we used the Rag2pGFP model. Immature (CD24+) γδ-thymocytes expressed high levels of GFP while only a small minority of mature (CD24-) γδ-thymocytes were GFP+. Similarly, most GFP+ γδ-splenocytes were immature, while some were mature. Analysis of γδ-recent thymic emigrants (RTEs) indicated that most γδ-T cell RTEs were CD24+ and GFP+ and adoptive transfer experiments showed that immature γδ-thymocytes could be maintained in the periphery for at least 3 days over which time they matured. With respect to the mature γδ-thymocytes that were GFP-, parabiosis experiments demonstrated that mature γδ-T cells did not recirculate from the periphery. Instead, a population of mature γδ-thymocytes remained resident in the thymus for at least 60 days while mature γδ-thymocytes derived solely from adult hematopoiesis were mostly lost from the thymus within 60 days. Collectively, these data demonstrate two streams of actively developing γδ-T cells in adult mice: an immature subset that quickly leaves the thymus and matures in the periphery, and one that completes maturation within the thymus over a longer period of time. Furthermore, there is a fetal-derived and heterogeneous population of resident γδ-thymocytes of unknown functional importance.
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