Number Of Thymic Epithelial Cells Research Articles

Obesity induces premature thymic involution and restricts T cell repertoire diversity (36.47)

Abstract Yun-Hee Youm, Hyunwon Yang, Bolormaa Vandanmagsar, Jennifer Rood, K. Ganesh Kumar, Andrew A. Butler and Vishwa Deep Dixit Obesity increases risk of infections and cancer, suggesting adverse effects on immune-surveillance. Here we report that obesity compromises the mechanisms regulating T cell generation via inducing premature thymic involution. Diet-induced obesity reduced the total number of thymocytes and significantly increased apoptosis in developing T cell populations. Obesity accelerated the age-related reduction of naïve T cells and decreased T cell receptor excision circle bearing peripheral T cells, an index of recently generated T cells from thymus. Defects in thymopoiesis in obese mice were due to a reduction in the lymphoid progenitor pool and deterioration in the thymic stromal microenvironment. Interestingly, obesity also increased the frequency of thymic fibroblasts and reduced the number of thymic epithelial cells (TECs). Furthermore, the TECs of DIO mice displayed reduced expression of IL-7 and auto immune regulator (Aire) mRNA. Obesity induced via melanocortin 4 receptor (Mc4r) deficiency also constricted the T cell receptor repertoire diversity, recapitulating the thymic defects observed with diet-induced obesity. In humans, progressive adiposity with or without type-2-diabetes also compromised thymic output. Collectively, these findings establish that obesity constricts T cell diversity by accelerating age-related thymic involution.

Single Cell Analysis of Complex Thymus Stromal Cell Populations: Rapid Thymic Epithelia Preparation Characterizes Radiation Injury

Thymic epithelial cells (TECs) and dendritic cells are essential for the maintenance of thymopoiesis. Because these stromal elements define the progenitor niche, provide critical survival signals and growth factors, and direct positive and negative selection, detailed study of these populations is necessary to understand important elements for thymic renewal after cytotoxic injury. Study of TEC is currently hindered by lengthy enzymatic separation techniques with decreased viability. We present a new rapid separation technique that yields consistent viable TEC numbers in a quarter of the prior preparation time. Using this new procedure, we identify changes in stroma populations following total body irradiation (TBI). By flow cytometry, we show that TBI significantly depletes UEA+ medullary TEC, while sparing Ly51+ CD45- cells. Further characterization of the Ly51+ subset reveals enrichment of fibroblasts (CD45- Ly51+ MHCII-), while cortical TECs (CD45- Ly51+ MHCII+) were markedly reduced. Dendritic cells (CD11lc+ CD45+) were also decreased following TBI. These data suggest that cytotoxic preparative regimens may impair thymic renewal by reducing critical populations of cortical and medullary TEC, and that such thymic damage can be assessed by this new rapid separation technique, thereby providing a means of assessing optimal conditioning pretransplantfor enhancing thymic-dependent immune reconstitution posttranspiant.

Age-Related Perturbation of Thymic Epithelial Cells (TEC) Homeostasis Is Caused by Increased TNFR/Fas-Mediated Apoptosis Coupled with Decreased Proliferation

Age-related thymic involution contributes to severe and clinically significant immune deficiency in the elderly. Proposed mechanisms of age-related thymic involution have focused on either intrinsic defects of lymphohematopoietic progenitors or primary defects of the thymic microenvironment with aging. Previously, we have demonstrated that keratinocyte growth factor (KGF) transiently reverses murine age-related thymic involution by regenerating the TEC compartment. We sought to understand the mechanisms of age-related loss of TECs and to evaluate a potential therapeutic strategy to durably regenerate the thymus. We found a significant increase in the frequency of apoptotic TECs (CD45- MHCII+) coupled with a decline in the frequency of S-phase TECs in young versus aged mice (11 vs 19% and 35% vs 17%, respectively). Telomere shortening was observed in both aged thymocytes and thymic stromal cells, indicating increased replicative senescence in both compartments. Previous studies have demonstrated lack of age-related thymic involution in Fas−/− mice. We found that the frequency of Fas+ TECs was 3-fold higher in aged mice compared to young mice. A distinct TEC subpopulation expressed Fas, but there was no increase in Fas+ CD45+ lymphoid cells. Although FasL was not expressed by thymocytes, expression of FasL by intrathymic mature T cells (ITMTs, CD4+CD8−CD44high or CD4−CD8+CD44high) increased 2-fold in aged mice. In contrast, there was no change in the frequency of FasL+ splenic memory T cells. To understand how Fas expression is induced in aged TEC, we examined the effects of two pro-inflammatory cytokines, TNF-α and IL-1β, which are known to induce Fas expression. Both TNF-α and IL-1β up-regulated Fas expression on K5+K8+ (possible precursor), K5−K8+ (cortical), or K5+K8− (medullary) TEC clones derived from young mice. RT-PCR and FACS analyses showed that the aged thymus had 4–5-fold higher levels of TNF-α, and TNF-α expression was increased in aged CD4+ and CD8+ ITMT, as well as all subsets of thymocytes. To elucidate the molecular mechanisms of apoptosis in the aged TEC, we analyzed levels of activated caspase-8, a key mediator of TNFR/Fas-induced apoptosis. Almost 50% of TECs in aged mice but only 25% in young mice had activated caspase-8. Furthermore, in vivo treatment (0.4mg/mouse, every 3rd day for 1 month) of a caspase-8-specific inhibitor (z-IETD-fmk) increased the number of TECs, decreased TEC apoptosis, and enhanced thymopoiesis in aged mice. The data suggest that thymic aging is mediated by decreased regenerative potential of TEC coupled with increased expression of inflammatory cytokines by both ITMT and thymocytes that make TECs more susceptible to apoptosis by either inducing Fas expression on TECs or directly triggering TNFR-mediated apoptosis. The inflammatory effects of ITMT on TEC death provides a feed-forward mechanism by which peripheral memory T-cell generation and immune senescence is linked to thymic involution.

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

AbstractThe 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.

Sustained thymopoiesis and improvement in functional immunity induced by exogenous KGF administration in murine models of aging

AbstractAge-related thymopoietic insufficiency has been proposed to be related to either defects in lymphohematopoietic progenitors or the thymic microenvironment. In this study, we examined whether keratinocyte growth factor (KGF), an epithelial cell–specific growth factor, could increase thymopoietic capacity in aged mice by restoration of the function of thymic epithelial cells (TECs). The thymic cellularity in KGF-treated aged mice increased about 4-fold compared to placebo-treated mice, resulting in an equivalent thymic cellularity to young mice. Enhanced thymopoiesis was maintained for about 2 months after a single course of KGF, and sustained improvement was achieved by administration of monthly courses of KGF. With the enhanced thymopoiesis after KGF treatment, the number of naive CD4 T cells in the periphery and T-cell–dependent antibody production improved in aged mice. KGF induced increased numbers of TECs and intrathymic interleukin-7 (IL-7) production and reorganization of cortical and medullary architecture. Furthermore, KGF enhanced thymopoiesis and normalized TEC organization in klotho (kl/kl) mice, a model of premature degeneration and aging, which displays thymopoietic defects. The result suggests that TEC damage is pathophysiologically important in thymic aging, and KGF therapy may be clinically useful in improving thymopoiesis and immune function in the elderly.

C- fos expression interferes with thymus development in transgenic mice

To study the function of the proto-oncogene c-fos in hematopoietic tissues, transgenic mice were generated that express c-fos from the H2-Kb promoter in several organs. These H2-c-fos mice have enlarged spleens and hyperplastic thymuses containing an increased number of thymic epithelial cells. The exogenous c-fos expression specifically affects T cell development in the thymus, thereby increasing the fraction of mature thymocytes. Results obtained with bone marrow radiation chimeras suggest that the altered distribution of T cell subsets is not a direct effect of c-fos expression within the T cell lineage. No changes in the proportion of hematopoietic cell lineages are seen in the spleen, and these mice do not develop lymphoid malignancies. B and T cell function, however, is impaired, and H2-c-fos mice are immune deficient. It appears that c-fos specifically stimulates the proliferation of thymic epithelial cells, and may thus indirectly affect T cell development.

Human thymic epithelial cells function as accessory cells for autologous mature thymocyte activation.

Using human thymocytes and autologous thymic epithelial (TE) cells grown in vitro in long-term culture, we have found TE cells can function as accessory cells for mitogen-induced mature thymocyte activation. Tritiated thymidine incorporation, blast formation, and protein synthesis were all induced in accessory cell-depleted thymocytes by autologous TE cells in the presence of suboptimal concentrations of PHA. After 3 days of mitogen stimulation of thymocyte-TE cell cocultures in vitro, thymocyte blasts bound to TE cells and 77 +/- 4% (mean +/- SEM) of TE cells acquired expression of major histocompatibility complex (MHC) class II (DR) antigen. TE accessory cell function for thymocyte activation was dependent on the number of TE cells added to thymocyte cultures, was not dependent on TE cell division, but did require TE cell protein synthesis. In thymocyte separation experiments, the predominant cell type responding to PHA in the presence of TE cells was T6- mature (stage III) thymocytes. Thus, human TE cells are capable of providing signals that lead to mature thymocyte activation.

Cytotoxic T cell response and thymic hormonal dysfunction in graft-vs-host mice.

As an approach to dissect complex mechanisms that lead to graft-vs-host (GvH)-associated immune disorders, we have compared the splenic cytotoxic T lymphocyte (CTL) response and thymic hormonal function in nonirradiated F1 hybrid mice injected with parental spleen cells. Thymic secretory function was studied by the determination of serum thymulin levels, and the number of thymic epithelial cells containing thymulin as assessed by indirect immunofluorescence with the use of an anti-thymulin monoclonal antibody. In addition, the epithelial cell network was analyzed with an anti-keratin serum, and the general histology pattern was studied by conventional histologic methods. An initial analysis was performed on day 15, which was characterized by CTL suppression mediated by parental suppressor T cells. No thymic abnormalities were detected at this time. By day 45 after GvH induction, active CTL suppression had decreased, and GvH was associated with a progressive decline in thymic hormonal function. Finally, by day 60 and thereafter, F1 GvH mice recovered normal in vitro CTL responsiveness, which contrasted with profound alterations of the epithelial cell network and severely reduced serum thymulin levels. This hormonal dysfunction was shown to be directly associated with a reduction in the number of thymulin-containing cells. Moreover, no anti-thymulin auto-antibodies could be detected. The results are discussed with respect to the role of thymic hormonal dysfunction in the modulation of F1 CTL responses observed during the course of a GvH reaction, and the additional analogy of this GvH model with human immunodeficiency.