Non-Epithelial Thymic Stromal Cells: Unsung Heroes in Thymus Organogenesis and T Cell Development

The stromal microenvironment in the thymus is essential for generating a functional T cell repertoire. Thymic epithelial cells (TECs) are numerically and phenotypically one of the most prominent stromal cell types in the thymus, and have been recognized as one of most unusual cell types in the body by virtue of their unique functions in the course of the positive and negative selection of developing T cells. In addition to TECs, there are other stromal cell types of mesenchymal origin, such as fibroblasts and endothelial cells. These mesenchymal stromal cells are not only components of the parenchymal and vascular architecture, but also have a pivotal role in controlling TEC development, although their functions have been less extensively explored than TECs. Here, we review both the historical studies on and recent advances in our understanding of the contribution of such non-TEC stromal cells to thymic organogenesis and T cell development. In particular, we highlight the recently discovered functional effect of thymic fibroblasts on T cell repertoire selection.

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Inborn errors of thymic stromal cell development and function

Seminars in Immunopathology ◽

10.1007/s00281-020-00826-9 ◽

2020 ◽

Author(s):

Alexandra Y. Kreins ◽

Stefano Maio ◽

Fatima Dhalla

Keyword(s):

T Cell ◽

Stromal Cells ◽

Stromal Cell ◽

Cell Development ◽

Thymic Epithelial Cells ◽

Inborn Errors ◽

Vascular Elements ◽

Thymus Tissue ◽

Thymic Stromal Cells ◽

Thymic Stromal Cell

AbstractAs the primary site for T cell development, the thymus is responsible for the production and selection of a functional, yet self-tolerant T cell repertoire. This critically depends on thymic stromal cells, derived from the pharyngeal apparatus during embryogenesis. Thymic epithelial cells, mesenchymal and vascular elements together form the unique and highly specialised microenvironment required to support all aspects of thymopoiesis and T cell central tolerance induction. Although rare, inborn errors of thymic stromal cells constitute a clinically important group of conditions because their immunological consequences, which include autoimmune disease and T cell immunodeficiency, can be life-threatening if unrecognised and untreated. In this review, we describe the molecular and environmental aetiologies of the thymic stromal cell defects known to cause disease in humans, placing particular emphasis on those with a propensity to cause thymic hypoplasia or aplasia and consequently severe congenital immunodeficiency. We discuss the principles underpinning their diagnosis and management, including the use of novel tools to aid in their identification and strategies for curative treatment, principally transplantation of allogeneic thymus tissue.

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Non-Epithelial Stromal Cells in Thymus Development and Function

Frontiers in Immunology ◽

10.3389/fimmu.2021.634367 ◽

2021 ◽

Vol 12 ◽

Cited By ~ 1

Author(s):

Kieran D. James ◽

William E. Jenkinson ◽

Graham Anderson

Keyword(s):

T Cell ◽

Stromal Cells ◽

T Cell Development ◽

Cell Development ◽

Thymic Epithelial Cells ◽

Developmental Origins ◽

Functional Classification ◽

Focus Attention ◽

Thymus Development ◽

And Function

The thymus supports T-cell developmentviaspecialized microenvironments that ensure a diverse, functional and self-tolerant T-cell population. These microenvironments are classically defined as distinct cortex and medulla regions that each contain specialized subsets of stromal cells. Extensive research on thymic epithelial cells (TEC) within the cortex and medulla has defined their essential roles during T-cell development. Significantly, there are additional non-epithelial stromal cells (NES) that exist alongside TEC within thymic microenvironments, including multiple subsets of mesenchymal and endothelial cells. In contrast to our current understanding of TEC biology, the developmental origins, lineage relationships, and functional properties, of NES remain poorly understood. However, experimental evidence suggests these cells are important for thymus function by either directly influencing T-cell development, or by indirectly regulating TEC development and/or function. Here, we focus attention on the contribution of NES to thymic microenvironments, including their phenotypic identification and functional classification, and explore their impact on thymus function.

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Keratinocyte growth factor (KGF) enhances postnatal T-cell development via enhancements in proliferation and function of thymic epithelial cells

Blood ◽

10.1182/blood-2006-10-049767 ◽

2007 ◽

Vol 109 (9) ◽

pp. 3803-3811 ◽

Cited By ~ 124

Author(s):

Simona W. Rossi ◽

Lukas T. Jeker ◽

Tomoo Ueno ◽

Sachiyo Kuse ◽

Marcel P. Keller ◽

...

Keyword(s):

T Cell ◽

Growth Factor ◽

Epithelial Cells ◽

Stromal Cells ◽

Keratinocyte Growth Factor ◽

T Cell Development ◽

Cell Development ◽

Thymic Epithelial Cells ◽

Thymic Stromal Cells ◽

Architectural Organization

Abstract The systemic administration of keratinocyte growth factor (KGF) enhances T-cell lymphopoiesis in normal mice and mice that received a bone marrow transplant. KGF exerts protection to thymic stromal cells from cytoablative conditioning and graft-versus-host disease–induced injury. However, little is known regarding KGF's molecular and cellular mechanisms of action on thymic stromal cells. Here, we report that KGF induces in vivo a transient expansion of both mature and immature thymic epithelial cells (TECs) and promotes the differentiation of the latter type of cells. The increased TEC numbers return within 2 weeks to normal values and the microenvironment displays a normal architectural organization. Stromal changes initiate an expansion of immature thymocytes and permit regular T-cell development at an increased rate and for an extended period of time. KGF signaling in TECs activates both the p53 and NF-κB pathways and results in the transcription of several target genes necessary for TEC function and T-cell development, including bone morphogenetic protein 2 (BMP2), BMP4, Wnt5b, and Wnt10b. Signaling via the canonical BMP pathway is critical for the KGF effects. Taken together, these data provide new insights into the mechanism(s) of action of exogenous KGF on TEC function and thymopoiesis.

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Selection of the T-Cell Repertoire: Receptor-Controlled Checkpoints in T-Cell Development

Advances in Immunology ◽

10.1016/s0065-2776(04)84006-9 ◽

2004 ◽

pp. 201-238 ◽

Cited By ~ 99

Author(s):

Harald von Boehmer

Keyword(s):

T Cell ◽

T Cell Development ◽

Cell Development ◽

T Cell Repertoire ◽

Cell Repertoire ◽

Selection Of

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Injury to the Thymic Niche for DN2/DN3 Developing T Cells Impairs Reconstitution.

Blood ◽

10.1182/blood.v108.11.68.68 ◽

2006 ◽

Vol 108 (11) ◽

pp. 68-68

Author(s):

Susan Prockop ◽

Sotiris Nikolopoulos ◽

Aaron Myers ◽

Filipo Giancotti ◽

Howard T. Petrie ◽

...

Keyword(s):

T Cell ◽

Stromal Cells ◽

De Novo ◽

T Cell Development ◽

Cell Development ◽

Cytotoxic Agents ◽

Thymic Epithelial Cells ◽

Laminin 5 ◽

Thymic Stroma ◽

Thymic Stromal Cells

Abstract Despite recovery of most hematopoietic functions, prolonged defects in generating functional T lymphocytes is a common occurrence after T cell depleted bone marrow transplant. While the mechanisms of these defects have not all been elucidated, contributing factors include age, T cell depletion of the graft, therapy with radiation and cytotoxic agents, and graft versus host disease (GvHD). We have designed an in vivo functional assessment of the ability of thymic stroma to support de novo T lymphocyte development. Mice deficient for the alpha chain of the IL-7 receptor (IL7Rα−/−) support robust thymic reconstitution after transplant of limited numbers of congenic precursors. We demonstrated that this capacity for reconstitution of immunodeficient strains depends on the paucity of specific endogenous precursors (DN3) cells in the IL7Rα−/− thymus and reflects the presence of functionally normal but empty stromal niches in this immunodeficient strain. We have proceeded to demonstrate that a variety of chemotherapeutic agents as well as aging, impair the ability of IL7Rα−/− thymic stroma to support de novo T cell development. Some agents allow donor chimerism in the thymus, but not rescue of the hypocellularity (eg cyclophosphamide) while others do not affect reconstitution (eg fludarabine). Multi-agent regimens have been administered and in some instances demonstrate an additive detrimental impact on thymic reconstitution. For several agents, damage has been localized to specific stromal niches by isolating changes in lymphoid subsets and stromal keratin expression. Decreased availability of the stromal niche for DN3 progenitors is associated with a decreased frequency of DN3s and an absolute block in recipient IL7Rα−/− T cell development. In addition, RNA from treated and untreated thymic stromal cells has been used to evaluate the gene expression pattern in thymic stroma of IL7Rα−/− mice treated with cytotoxic agents. In one example to be presented we find decreased thymic reconstitution in mice treated with Busulfan with sustained changes in stromal elements. These findings are consistent with sustained damage to thymic stromal cells. Among other changes, busulfan leads to decreased expression of laminin 5 by cortical thymic epithelial cells. The integrin heterodimerα6β4 is a binding partner for laminin 5 and is expressed uniformly by DN2 thymocytes. The significance of laminin 5/α6β4 signaling during T cell development was assessed using mice with a targeted mutation in the integrin β4 signaling domain. In these experiments mutant fetal liver was used to create hematopoietic chimeras and evaluate T cell development. Our studies provide insight into the nature of damage to thymic stroma by cytotoxic regimens and an understanding of the effect of this damage on subsequent immune reconstitution.

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Development of a diverse human T-cell repertoire despite stringent restriction of hematopoietic clonality in the thymus

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1519118112 ◽

2015 ◽

Vol 112 (44) ◽

pp. E6020-E6027 ◽

Cited By ~ 22

Author(s):

Martijn H. Brugman ◽

Anna-Sophia Wiekmeijer ◽

Marja van Eggermond ◽

Ingrid Wolvers-Tettero ◽

Anton W. Langerak ◽

...

Keyword(s):

T Cell ◽

T Cell Development ◽

Xenograft Model ◽

Subsequent Development ◽

Cell Receptor ◽

Cell Development ◽

Cell Repertoire ◽

Hematopoietic Stem ◽

Human T Cell ◽

Hsc Transplantation

The fate and numbers of hematopoietic stem cells (HSC) and their progeny that seed the thymus constitute a fundamental question with important clinical implications. HSC transplantation is often complicated by limited T-cell reconstitution, especially when HSC from umbilical cord blood are used. Attempts to improve immune reconstitution have until now been unsuccessful, underscoring the need for better insight into thymic reconstitution. Here we made use of the NOD-SCID-IL-2Rγ−/− xenograft model and lentiviral cellular barcoding of human HSCs to study T-cell development in the thymus at a clonal level. Barcoded HSCs showed robust (>80% human chimerism) and reproducible myeloid and lymphoid engraftment, with T cells arising 12 wk after transplantation. A very limited number of HSC clones (<10) repopulated the xenografted thymus, with further restriction of the number of clones during subsequent development. Nevertheless, T-cell receptor rearrangements were polyclonal and showed a diverse repertoire, demonstrating that a multitude of T-lymphocyte clones can develop from a single HSC clone. Our data imply that intrathymic clonal fitness is important during T-cell development. As a consequence, immune incompetence after HSC transplantation is not related to the transplantation of limited numbers of HSC but to intrathymic events.

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The Aire family expands

Journal of Experimental Medicine ◽

10.1084/jem.20190246 ◽

2019 ◽

Vol 216 (5) ◽

pp. 1010-1011

Author(s):

Adrian Liston ◽

James Dooley

Keyword(s):

T Cell ◽

Epithelial Cells ◽

Lymph Nodes ◽

Thymic Epithelial Cells ◽

T Cell Tolerance ◽

T Cell Repertoire ◽

Cell Tolerance ◽

Cell Repertoire

T cell tolerance depends upon Aire-expressing cells to purge the T cell repertoire of autoreactive clones. Once thought to be the exclusive domain of thymic epithelial cells, a new study by Yamano et al. (https://doi.org/10.1084/jem.20181430) in this issue of JEM identifies ILC3-like cells in the lymph nodes with similar properties.

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IL-7 Effects on Thymocyte Progenitors.

Blood ◽

10.1182/blood.v106.11.3318.3318 ◽

2005 ◽

Vol 106 (11) ◽

pp. 3318-3318

Author(s):

Nahed El Kassar ◽

Baishakhi Choudhury ◽

Francis Flomerfelt ◽

Philip J. Lucas ◽

Veena Kapoor ◽

...

Keyword(s):

Bone Marrow ◽

T Cell ◽

B Cell ◽

Notch Signaling ◽

Stromal Cells ◽

T Cell Development ◽

Fold Increase ◽

Cell Development ◽

B Cell Development ◽

Over Expression

Abstract IL-7 is a non-redundant cytokine in T cell development. We studied the role of IL-7 in early T-cell development using a model of transgenic (Tg) mice with the murine IL-7 gene under control of the lck proximal promoter. At high IL-7 over-expression (x39 fold increase at day 1 in total thymic tissue), we observed a disruption of TCRαβ development along with increased B cell development in the thymus (7- to 13-fold increase) (El Kassar, Blood, 2004). In order to further explore abnormal T and B cell thymic development in these mice, we first confirmed that they both arise in parallel and were non-cell autonomous, by in vivo injection of neutralizing anti-IL-7 MAb and mixed bone marrow chimera experiments. Using a six color flow cytometry analysis, we found a dramatic decrease of the early thymocyte progenitors (ETPs, lin−CD44+CD25−c-kithiIL-7R−/lo) in the adult Tg mice (x4.7 fold decrease). Lin−CD44+CD25−c-kit+ thymocytes were sorted and cultured on OP9 and OP9 delta-like1 (OP9-DL1) stromal cells (kindly provided by Pr Zuniga Pflucker). At day 14, we observed an important decrease of T cell development (54% vs. 1% of DP cells) and an increase of NK cells (x5 fold increase) in the Tg-derived DN1 cell culture. DN2 (Lin−CD44+CD25−c-kit+) Tg thymocytes showed the same, but less dramatic abnormalities. While DN1 progenitors developed effectively into B220+CD19+ cells on OP9 stromal cells, no B cell development was observed on OP-DL stromal cells from DN1-Tg derived progenitors or by addition of increasingly high doses of IL-7 (x10, x40, x160) to normal B6-derived DN1 progenitors. Instead, a block of T-cell development was observed with increased IL-7. We hypothesized a down regulation of Notch signaling by IL-7 over-expression and analyzed by FACS Notch expression in the DN thymocytes. By staining the intra-cellular part of Notch cleaved after Notch 1/Notch ligand activation, Tg-derived DN2 cells showed decreased Notch signaling. More importantly, HES expression was decreased in the DN2, DN3 and DN4 fractions by semi-quantitative PCR. Sorted Pro/Pre B cells from Tg thymi showed TCR Dβ1-Jβ1 rearrangement indicating their T specific origin, in opposition to Pro/Pre B cells sorted from the bone marrow of the same mice. We suggest that more than one immature progenitor seeds the thymus from the bone marrow. While ETPs had T and NK proliferative capacity, another thymic progenitor with B potential may be responsible for thymic B cell development in normal and IL-7 Tg mice. Finally, IL-7 over-expression may induce a decreased Notch signaling in thymic progenitors, inducing a switch of T vs. B lineage development.

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