Foxn1 is required to maintain the postnatal thymic microenvironment in a dosage-sensitive manner

Overexpression of Foxn1 attenuates age-associated thymic involution and prevents the expansion of peripheral CD4 memory T cells

The forkhead box n1 (Foxn1) transcription factor is essential for thymic organogenesis during embryonic development; however, a functional role of Foxn1 in the postnatal thymus is less well understood. We developed Foxn1 transgenic mice (Foxn1Tg), in which overexpression of Foxn1 is driven by the human keratin-14 promoter. Expression of the Foxn1 transgene increased the endogenous Foxn1 levels. In aged mice, overexpression of Foxn1 in the thymus attenuated the decline in thymocyte numbers, prevented the decline in frequency of early thymic progenitors, and generated a higher number of signal joint TCR excised circle. Histologic studies revealed that structural alterations associated with thymic involution were diminished in aged Foxn1 Tg. Total numbers of EpCAM+ MHC II+ and MHC II(hi) thymic epithelial cells were higher in young and old Foxn1Tg and more EpCAM+ MHC II(hi) TEC expressed Ki-67 in aged Foxn1Tg compared with WT. Furthermore, Foxn1Tg displayed a significant reduction in the expansion of splenic CD4+ memory compartments and attenuated the decline in CD4+ and CD8+ naive compartments. Our data indicate that manipulation of Foxn1 expression in the thymus ameliorates thymopoiesis in aged mice and offer a strategy to combat the age-associated decline in naive T-cell production and CD4 naive/memory ratios in the elderly.

It's not all equal: a multiphasic theory of thymic involution

Regression of the thymus is a key feature of immunosenescence, which coincides with a decrease in T cell output and contributes to the restriction of the T cell repertoire in the elderly, leading to increased susceptibility to illness and disease. However, the mechanisms involved in thymic involution are still not fully known. Although, it is often believed that thymic involution occurs during the onset of puberty, increasing data suggests alterations to the thymus happen much earlier in life. Therefore, the changes in the thymus and subsequent thymic function may not just be an ageing phenomenon. In this article, we propose that there are several, non-linear, phases to thymic atrophy, which are regulated by different mechanisms, including the familiar age-dependent thymic involution and a much earlier growth-dependent thymic involution.

Greater than the sum of their parts: combination strategies for immune regeneration following allogeneic hematopoietic stem cell transplantation

Cytoreductive conditioning regimes designed to allow for successful allogeneic hematopoietic stem cell transplantation (allo-HSCT) paradoxically are also detrimental to recovery of the immune system in general but lymphopoiesis in particular. Post-transplant immune depletion is particularly striking within the T cell compartment which is exquisitely sensitive to negative regulation, evidenced by the profound decline in thymic function with age. As a consequence, regeneration of the immune system remains a significant unmet clinical need. Over the past decade studies have revealed several promising therapeutic strategies to address ineffective lymphopoiesis and post-transplant immune deficiency. These include the use of cytokines such as IL-7, IL-12 and IL-15; growth factors and hormones like keratinocyte growth factor (KGF), insulin-like growth factor (IGF)-1 and growth hormone (GH); adoptive transfer of ex vivo-generated precursor T cells (pre-T) and sex steroid ablation (SSA). Moreover, recently several novel approaches have been proposed to generate whole thymii ex vivo using stem cell technologies and bioscaffolds. Increasingly, however, when transferred to the clinic, these strategies alone are not sufficient to restore thymopoiesis in all patients leading to the potential of combination strategies as a way to reign in non-responders. Synergistic enhancement in combination may be due to differential targets may therefore be effective in improving clinical outcomes in the transplant settings as well as in other lymphopenic states induced by high dose chemotherapy/radiation therapy or HIV, and may also be useful in improving responses to vaccination and augmenting anti-tumor immunotherapy.

Wnt-4 protects thymic epithelial cells against dexamethasone-induced senescence

Glucocorticoids are widely used immunosuppressive drugs in treatment of autoimmune diseases and hematological malignancies. Glucocorticoids are particularly effective immune suppressants, because they induce rapid peripheral T cell and thymocyte apoptosis resulting in impaired T cell-dependent immune responses. Although glucocorticoids can induce apoptotic cell death directly in developing thymocytes, how exogenous glucocorticoids affect the thymic epithelial network that provides the microenvironment for T cell development is still largely unknown. In the present work, we show that primary thymic epithelial cells (TECs) express glucocorticoid receptors and that high-dosage dexamethasone induces degeneration of the thymic epithelium within 24 h of treatment. Changes in organ morphology are accompanied by a decrease in the TEC transcription factor FoxN1 and its regulator Wnt-4 parallel with upregulation of lamina-associated polypeptide 2α and peroxisome proliferator activator receptor γ, two characteristic molecular markers for adipose thymic involution. Overexpression of Wnt-4, however, can prevent upregulation of adipose differentiation-related aging markers, suggesting an important role of Wnt-4 in thymic senescence.

Enhanced isolation of adult thymic epithelial cell subsets for multiparameter flow cytometry and gene expression analysis

The epithelial cells (TECs) are microenvironmental niche cells which support T lymphocyte development in the thymus. Most studies of TEC biology have focused on TEC at the fetal stage of development, whereas the biology of adult-stage TECs is not as well-understood. Delineating the molecular mechanisms that control adult TEC differentiation has implications for the success of T-lymphocyte based therapies for autoimmune diseases and induction of immunological tolerance to stem cell-derived tissues. Detailed analysis of adult TECs is technically challenging due to their rarity, their diminishing numbers with age, and the limited number of markers to distinguish between unique TEC subpopulations. Here, we have devised an improved isolation protocol for adult mouse TECs and combined it with six-color multiparameter flow cytometry. Using these techniques, we have identified four distinct subsets of CD45- EpCAM+ TECs in adult mice: a) UEA1(low) CDR1(low) (UC(low)); b) UEA1(high) CDR1(high)(UC(high)); c) UEA1(low) CDR1(high) MHC(high) (cTEC); and d) UEA1(high)CDR1(low) MHC(int/high) (mTEC). PCR analysis verified that these TEC subsets differentially expressed known TEC genes. TEC subsets were further analyzed using high-throughput quantitative PCR arrays to reveal novel genes that could be important for TEC subset maintenance. Intracellular staining for keratin-5 and keratin-8 can also be added, but our results suggest that keratin expression alone cannot be used to distinguish adult TEC subsets. Our enhanced isolation allows for detailed analysis of rare TEC subpopulations in the adult mouse at the cellular and molecular levels.

Thymic fatness and approaches to enhance thymopoietic fitness in aging

With advancing age, the thymus undergoes striking fibrotic and fatty changes that culminate in its transformation into adipose tissue. As the thymus involutes, reduction in thymocytes and thymic epithelial cells precede the emergence of mature lipid-laden adipocytes. Dogma dictates that adipocytes are 'passive' cells that occupy non-epithelial thymic space or 'infiltrate' the non-cellular thymic niches. The provenance and purpose of ectopic thymic adipocytes during aging in an organ that is required for establishment and maintenance of T cell repertoire remains an unsolved puzzle. Nonetheless, tantalizing clues about elaborate reciprocal relationship between thymic fatness and thymopoietic fitness are emerging. Blocking or bypassing the route toward thymic adiposity may complement the approaches to rejuvenate thymopoiesis and immunity in elderly.

Impaired thymic selection and abnormal antigen-specific T cell responses in Foxn1(Δ/Δ) mutant mice

Background

Foxn1^Δ/Δ mutant mice have a specific defect in thymic development, characterized by a block in TEC differentiation at an intermediate progenitor stage, and blocks in thymocyte development at both the DN1 and DP cell stages, resulting in the production of abnormally functioning T cells that develop from an atypical progenitor population. In the current study, we tested the effects of these defects on thymic selection.

Methodology/Principal Findings

We used Foxn1^Δ/Δ; DO11 Tg and Foxn1^Δ/Δ; OT1 Tg mice as positive selection and Foxn1^Δ/Δ; MHCII I-E mice as negative selection models. We also used an in vivo system of antigen-specific reactivity to test the function of peripheral T cells. Our data show that the capacity for positive and negative selection of both CD4 and CD8 SP thymocytes was reduced in Foxn1^Δ/Δ mutants compared to Foxn1^+/Δ control mice. These defects were associated with reduction of both MHC Class I and Class II expression, although the resulting peripheral T cells have a broad TCR Vβ repertoire. In this deficient thymic environment, immature CD4 and CD8 SP thymocytes emigrate from the thymus into the periphery. These T cells had an incompletely activated profile under stimulation of the TCR signal in vitro, and were either hypersensitive or hyporesponsive to antigen-specific stimulation in vivo. These cell-autonomous defects were compounded by the hypocellular peripheral environment caused by low thymic output.

Conclusions/Significance

These data show that a primary defect in the thymic microenvironment can cause both direct defects in selection which can in turn cause indirect effects on the periphery, exacerbating functional defects in T cells.

Declining expression of a single epithelial cell-autonomous gene accelerates age-related thymic involution

Age-related thymic involution may be triggered by gene expression changes in lymphohematopoietic and/or nonhematopoietic thymic epithelial cells (TECs). The role of epithelial cell-autonomous gene FoxN1 may be involved in the process, but it is still a puzzle because of the shortage of evidence from gradual loss-of-function and exogenous gain-of-function studies. Using our recently generated loxP-floxed-FoxN1(fx) mouse carrying the ubiquitous CreER(T) (uCreER(T)) transgene with a low dose of spontaneous activation, which causes gradual FoxN1 deletion with age, we found that the uCreER(T)-fx/fx mice showed an accelerated age-related thymic involution owing to progressive loss of FoxN1(+) TECs. The thymic aging phenotypes were clearly observable as early as at 3-6 months of age, resembling the naturally aged (18-22-month-old) murine thymus. By intrathymically supplying aged wild-type mice with exogenous FoxN1-cDNA, thymic involution and defective peripheral CD4(+) T-cell function could be partially rescued. The results support the notion that decline of a single epithelial cell-autonomous gene FoxN1 levels with age causes primary deterioration in TECs followed by impairment of the total postnatal thymic microenvironment, and potentially triggers age-related thymic involution in mice.

Transcriptional regulation of thymus organogenesis and thymic epithelial cell differentiation

Transcriptional regulatory networks are the central regulatory mechanisms that control organ identity, patterning, and differentiation. In the case of the thymus, several key transcription factors have been identified that are critical for various aspects of thymus organogenesis and thymic epithelial cell (TEC) differentiation. The thymus forms from the third pharyngeal pouch endoderm during embryogenesis. Organ development progresses from initial thymus cell fate specification, through multiple stages of TEC differentiation and cortical (cTEC) and medullary (mTEC) formation. Transcription factors have been identified for each of these stages: a Hoxa3-dependent cascade at initial fate specification, Foxn1 for early (and later) TEC differentiation, and NF-kappaB for mTEC differentiation. As important as these factors are, their interrelationships are not understood, and many more transcription factors are likely required for complete thymus organogenesis to occur. In this chapter, we review the literature on these known genes, as well as identify gaps in our knowledge for future studies.

Postnatal tissue-specific disruption of transcription factor FoxN1 triggers acute thymic atrophy

The transcription factor FoxN1 is essential for differentiation of thymic epithelial cell (TEC) progenitors during thymic organogenesis. However, limited information is available on the postnatal contribution of FoxN1 to thymic maintenance. To address this question, we generated a loxP-floxed FoxN1 (fx) mouse with three different promoter-driven inducible CreER(T) transgenes. Postnatal ubiquitous deletion of FoxN1 caused dramatic thymic atrophy in 5 days and more severe deterioration in medullary TECs (mTECs) than in cortical TECs (cTECs). Induction of FoxN1 deletion selectively in K5 promoter-driven somatic epithelial cells (mostly mTECs and possibly some adult epithelial stem cells) was sufficient to cause significant thymic atrophy, whereas FoxN1 deletion in K18 promoter-driven somatic epithelial cells (mostly cTECs) was not. Thymic atrophy resulted from increased apoptosis and was associated with activation of the p53 gene in mature mTECs. Although FoxN1 is required for the development of both mTECs and cTECs in thymic organogenesis, it is most important for the maintenance of mTECs in the postnatal thymus, which are in turn necessary to prevent thymic atrophy.

Thymic epithelial cells: the multi-tasking framework of the T cell "cradle"

The thymus provides the anatomical "cradle" that fosters developing thymocytes. Thymic epithelial cells (TECs) are specialized cellular components that may be viewed as a multifunctional "frame" to nurture distinct stages of thymopoiesis. A symbiotic relationship between TECs and thymocytes exists because reciprocal interactions are required to achieve complete maturation of both cell types. Here, we propose that crucial instructive signals delivered by developing thymocytes negatively regulate functional attributes of immature TECs (including the expression of Delta-like 4 (DLL4) and interleukin-7 (IL-7)) that are required during early stages of thymopoiesis, while promoting the diversification of more mature TEC subsets. Thus, the division of labour among TECs may be coordinated directly by local cellular feedback mechanisms operating within distinct thymic niches.

An evolutionary perspective on the mechanisms of immunosenescence

There is an accumulating body of evidence that a decline in immune function with age is common to most if not all vertebrates. For instance, age-associated thymic involution seems to occur in all species that possess a thymus, indicating that this process is evolutionary ancient and conserved. The precise mechanisms regulating immunosenescence remain to be resolved, but much of what we do know is consistent with modern evolutionary theory. In this review, we assess our current knowledge from an evolutionary perspective on the occurrence of immunosenescence, we show that life history trade-offs play a key role and we highlight the possible advantages of the age-related decline in thymic function.