Normal T cell selection occurs in CD205-deficient thymic microenvironments

Generation and Regeneration of Thymic Epithelial Cells

The thymus is unique in its ability to support the maturation of phenotypically and functionally distinct T cell sub-lineages. Through its combined production of MHC-restricted conventional CD4⁺ and CD8⁺, and Foxp3⁺ regulatory T cells, as well as non-conventional CD1d-restricted iNKT cells and invariant γδT cells, the thymus represents an important orchestrator of immune system development and control. It is now clear that thymus function is largely determined by the availability of stromal microenvironments. These specialized areas emerge during thymus organogenesis and are maintained throughout life. They are formed from both epithelial and mesenchymal components, and collectively they support a stepwise program of thymocyte development. Of these stromal cells, cortical, and medullary thymic epithelial cells represent functional components of thymic microenvironments in both the cortex and medulla. Importantly, a key feature of thymus function is that levels of T cell production are not constant throughout life. Here, multiple physiological factors including aging, stress and pregnancy can have either short- or long-term detrimental impact on rates of thymus function. Here, we summarize our current understanding of the development and function of thymic epithelial cells, and relate this to strategies to protect and/or restore thymic epithelial cell function for therapeutic benefit.

Single-Cell RNA Sequencing Resolves Spatiotemporal Development of Pre-thymic Lymphoid Progenitors and Thymus Organogenesis in Human Embryos

Generation of the first T lymphocytes in the human embryo involves the emergence, migration, and thymus seeding of lymphoid progenitors together with concomitant thymus organogenesis, which is the initial step to establish the entire adaptive immune system. However, the cellular and molecular programs regulating this process remain unclear. We constructed a single-cell transcriptional landscape of human early T lymphopoiesis by using cells from multiple hemogenic and hematopoietic sites spanning embryonic and fetal stages. Among heterogenous early thymic progenitors, one subtype shared common features with a subset of lymphoid progenitors in fetal liver that are known as thymus-seeding progenitors. Unbiased bioinformatics analysis identified a distinct type of pre-thymic lymphoid progenitors in the aorta-gonad-mesonephros (AGM) region. In parallel, we investigated thymic epithelial cell development and potential cell-cell interactions during thymus organogenesis. Together, our data provide insights into human early T lymphopoiesis that prospectively direct T lymphocyte regeneration, which might lead to development of clinical applications.

Thymic Epithelial Cells

Intrathymic T cell development is a complex process that depends upon continuous guidance from thymus stromal cell microenvironments. The thymic epithelium within the thymic stroma comprises highly specialized cells with a high degree of anatomic, phenotypic, and functional heterogeneity. These properties are collectively required to bias thymocyte development toward production of self-tolerant and functionally competent T cells. The importance of thymic epithelial cells (TECs) is evidenced by clear links between their dysfunction and multiple diseases where autoimmunity and immunodeficiency are major components. Consequently, TECs are an attractive target for cell therapies to restore effective immune system function. The pathways and molecular regulators that control TEC development are becoming clearer, as are their influences on particular stages of T cell development. Here, we review both historical and the most recent advances in our understanding of the cellular and molecular mechanisms controlling TEC development, function, dysfunction, and regeneration.

β-catenin activation drives thymoma initiation and progression in mice

Thymoma is the most commonly identified cancer in the anterior mediastinum. To date, the causal mechanism that drives thymoma progression is not clear. Here, we generated K5-ΔN64Ctnnb1/ER^T2 transgenic mice, which express an N-terminal deletion mutant of β-catenin fused to a mutated ligand-binding domain of estrogen receptor (ER^T2) under the control of the bovine cytokeratin 5 (K5) promoter. The transgenic mouse lines named Tg1 and Tg4 were characterized. Forced expression of ΔN64Ctnnb1/ER^T2 in the Tg1 and Tg4 mice developed small thymoma lesions in response to tamoxifen treatment. In the absence of tamoxifen, the Tg1 mice exhibited leaky activation of β-catenin, which activated the TOP-Gal transgene and Wnt/β-catenin-targeted genes. As the Tg1 mice aged in the absence of tamoxifen, manifest thymomas were found at 10-12 months. Interestingly, we detected loss of AIRE and increase of p63 in the thymomas of Tg1 mice, similar to that observed in human thymomas. Moreover, the β5t protease subunit, which was reported as a differential marker for human type B3 thymoma, was expressed in the Tg1 thymomas. Thus, the Tg1 mice generated in this study accurately mimic the characteristics of human thymomas and may serve as a model for understanding thymoma pathogenesis.

Serial progression of cortical and medullary thymic epithelial microenvironments

Thymic epithelial cells (TECs) provide key instructive signals for T-cell differentiation. Thymic cortical (cTECs) and medullary (mTECs) epithelial cells constitute two functionally distinct microenvironments for T-cell development, which derive from a common bipotent TEC progenitor. While seminal studies have partially elucidated events downstream of bipotent TECs in relation to the emergence of mTECs and their progenitors, the control and timing of the emergence of the cTEC lineage, particularly in relation to that of mTEC progenitors, has remained elusive. In this review, we describe distinct models that explain cTEC/mTEC lineage divergence from common bipotent progenitors. In particular, we summarize recent studies in mice providing evidence that mTECs, including the auto-immune regulator(+) subset, derive from progenitors initially endowed with phenotypic properties typically associated with the cTEC lineage. These observations support a novel "serial progression" model of TEC development, in which progenitors serially acquire cTEC lineage markers, prior to their commitment to the mTEC differentiation pathway. Gaining a better understanding of the phenotypic properties of early stages in TEC progenitor development should help in determining the mechanisms regulating cTEC/mTEC lineage development, and in strategies aimed at thymus reconstitution involving TEC therapy.