Thymic cysts originate from Foxn1 positive thymic medullary epithelium

Thymus Inception: Molecular Network in the Early Stages of Thymus Organogenesis

The thymus generates central immune tolerance by producing self-restricted and self-tolerant T-cells as a result of interactions between the developing thymocytes and the stromal microenvironment, mainly formed by the thymic epithelial cells. The thymic epithelium derives from the endoderm of the pharyngeal pouches, embryonic structures that rely on environmental cues from the surrounding mesenchyme for its development. Here, we review the most recent advances in our understanding of the molecular mechanisms involved in early thymic organogenesis at stages preceding the expression of the transcription factor Foxn1, the early marker of thymic epithelial cells identity. Foxn1-independent developmental stages, such as the specification of the pharyngeal endoderm, patterning of the pouches, and thymus fate commitment are discussed, with a special focus on epithelial–mesenchymal interactions.

Twenty Years of AIRE

About two decades ago, cloning of the autoimmune regulator (AIRE) gene materialized one of the most important actors on the scene of self-tolerance. Thymic transcription of genes encoding tissue-specific antigens (ts-ags) is activated by AIRE protein and embodies the essence of thymic self-representation. Pathogenic AIRE variants cause the autoimmune polyglandular syndrome type 1, which is a rare and complex disease that is gaining attention in research on autoimmunity. The animal models of disease, although not identically reproducing the human picture, supply fundamental information on mechanisms and extent of AIRE action: thanks to its multidomain structure, AIRE localizes to chromatin enclosing the target genes, binds to histones, and offers an anchorage to multimolecular complexes involved in initiation and post-initiation events of gene transcription. In addition, AIRE enhances mRNA diversity by favoring alternative mRNA splicing. Once synthesized, ts-ags are presented to, and cause deletion of the self-reactive thymocyte clones. However, AIRE function is not restricted to the activation of gene transcription. AIRE would control presentation and transfer of self-antigens for thymic cellular interplay: such mechanism is aimed at increasing the likelihood of engagement of the thymocytes that carry the corresponding T-cell receptors. Another fundamental role of AIRE in promoting self-tolerance is related to the development of thymocyte anergy, as thymic self-representation shapes at the same time the repertoire of regulatory T cells. Finally, AIRE seems to replicate its action in the secondary lymphoid organs, albeit the cell lineage detaining such property has not been fully characterized. Delineation of AIRE functions adds interesting data to the knowledge of the mechanisms of self-tolerance and introduces exciting perspectives of therapeutic interventions against the related diseases.

FOXN1 in thymus organogenesis and development

Development of the primary T-cell repertoire takes place in the thymus. The linked processes of T-cell differentiation and T-cell repertoire selection each depend on interactions between thymocytes and thymic stromal cells; in particular, with the epithelial cells of the cortical and medullary thymic compartments (cortical and medullary thymic epithelial cells; cTECs and mTECs, respectively). The importance of the thymic epithelial cell lineage in these processes was revealed in part through analysis of nude (nu/nu) mice, which are congenitally hairless and athymic. The nude phenotype results from null mutation of the forkhead transcription factor FOXN1, which has emerged as a pivotal regulator both of thymus development and homeostasis. FOXN1 has been shown to play critical roles in thymus development, function, maintenance, and even regeneration, which positions it as a master regulator of thymic epithelial cell (TEC) differentiation. In this review, we discuss current understanding of the regulation and functions of FOXN1 throughout thymus ontogeny, from the earliest stages of organogenesis through homeostasis to age-related involution, contextualising its significance through reference to other members of the wider Forkhead family.

Monocyte chemoattractant protein 1 (MCP-1/CCL2) contributes to thymus atrophy in acute myeloid leukemia

Recent studies on acute myelogenous leukemia (AML) patients have revealed the existence of T-cell immunodeficiencies, characterized by peripheral T lymphocytes that are unable to interact with blasts, reduced thymic emigrants and oligoclonal restricted repertoires. These observations suggest that there is a profound thymic dysregulation, which is difficult to study in AML patients. Using the C1498 AML mouse model, we demonstrated that leukemia development was associated with thymus atrophy, which was defined by abnormal organ weight and reduced cellularity. In addition, we observed a dramatic loss of peripheral CD4(+) and CD8(+) T-cell numbers with increased frequencies of CD4(+) FoxP3(+) regulatory and activated/memory T cells. Investigating the mechanisms leading to this atrophy, we observed a significant accumulation of the monocyte chemoattractant protein 1 (MCP-1/CCL2) in thymi of leukemic mice. Treatment of AML-bearing animals with a blocking anti-CCL2 antibody revealed a lower tumor burden, augmented antileukemic T-cell responses, and improved survival rate compared to nontreated mice. These results were not observed when neutralization of CCL2 was performed in thymectomized mice. Altogether, we show that the CCL2 protein participates in thymic atrophy in AML mice, and this could have important implications for future immunotherapeutic strategies.

Conditioned deletion of ephrinB1 and/or ephrinB2 in either thymocytes or thymic epithelial cells alters the organization of thymic medulla and favors the appearance of thymic epithelial cysts

Our understanding about medullary compartment, its niches composition and formation is still limited. Previous studies using EphB2 and/or EphB3 knockout mice showed an abnormal thymic development that affects mainly to the epithelial component, including the cortex/medulla distribution, thymic epithelial cell (TEC) morphology and different epithelial-specific marker expression. We have already demonstrated that the lack of ephrinB1 and/or ephrinB2, either on thymocytes or on TECs, alters the cell intermingling processes necessary for thymus organization and affect cortical TEC subpopulations. In the present work, we have used the Cre-LoxP model to selectively delete ephrinB1 and/or ephrinB2 in thymocytes (EfnB1(thy/thy), EfnB2(thy/thy), EfnB1(thy/thy)EfnB2(thy/thy) mice) or TECs (EfnB1(tec/tec), EfnB2(tec/tec), EfnB1(tec/tec)EfnB2(tec/tec) mice) and have analyzed their role on the medullary compartment. In all the studied mutants, medullary areas are smaller and more compact than in the wt thymuses. In most of them, we observe abundant big cysts and a higher proportion of UEA(hi)MTS10(-) cells than in wt mice, which are often forming small cysts. On EfnB1(tec/tec)EfnB2(tec/tec), changes affecting organ size and medullary compartment start at perinatal stage. Our data shed some light on knowledge about wt medulla histological structure and cysts meaning and formation process and on the role played by ephrinB in them.

The thymus and tail regenerative capacity in Xenopus laevis tadpoles

A morphofunctional analysis of the thymus from differently aged Xenopus laevis tadpoles during regeneration of the tail is reported. In stage 50 larvae, competent to regenerate, the appendage cut provoked thymic structural modifications that affected the medullary microenvironment cells and changes in TNF-α immunoreactivity. Mucocyte-like cells, multicellular epithelial cysts, myoid cells and cells immunoreactive to TNF-α increased in number. Increased numbers of lymphocytes were also found in regenerating areas and, at the end of regeneration, thymic structural and immunocytochemical patterns were restored to control levels. The observed cellular responses and the induction of molecules critical for thymus constitutive processes suggest a stimulation of thymic function after tail amputation. In older larvae, whose capacity to form a new complete and correctly patterned tail was reduced, thymic morphological changes were more severe and may persist throughout the regeneration process with a significant reduction in organ size. In these larvae the histological patterns and the marked thymic decrease may be related to the events occurring during regeneration, i.e. the higher inflammatory response and the reduced tail regenerative potential.

Morphogenesis and maintenance of the 3D thymic medulla and prevention of nude skin phenotype require FoxN1 in pre- and post-natal K14 epithelium

Expansion of thymic epithelial cysts represents disruption of an organized three-dimensional (3D) thymic epithelial cell (TEC) meshwork, which is crucial for T-lymphocyte development. Although the FoxN1-null mutant develops a rudimentary two-dimensional (2D) cystic thymus, 2D thymic cyst lining resulting from a dGUO culture was reported to be FoxN1-independent; thus, it is unclear whether loss of FoxN1 facilitates cyst formation and whether FoxN1 regulates the morphogenesis and maintenance of the 3D thymic microstructure. Using the loxP-floxed-FoxN1 mouse model, we demonstrated that specific deletion of FoxN1 in keratin (K)-14 promoter-driven TECs induced the loss of 3D thymic medullary structure by producing a large number of morphologic pulmonary alveolar-like 2D epithelial cysts, which increased with age. The cystic lining was positive for differential polarized keratins and had strong claudin-3,4, but reduced MHC-II, expression. However, an increased percentage of claudin-3,4(+) TECs, which are presumptive precursors of UEA-1(+) and Aire(+) mature medullary TECs, failed to promote the development of these mature descendants. Meanwhile, the K14Cre-mediated FoxN1 deletion alone was sufficient to induce a complete hair follicle defect, causing a nude phenotype in the skin, but was not sufficient to cause a complete loss of the thymus. All these changes to occur require deletion of FoxN1 in both prenatal (Cre-recombinase from parents during fertilization) and postnatal (Cre-recombinase from offspring themselves after fertilization) life. These findings provide new insights into FoxN1 regulation of 3D thymic epithelial morphogenesis and maintenance, the distinct impacts of FoxN1 in the K14 epithelial subset of the thymus and skin, and its postnatal requirement.