Thymic Involution Research Articles

Quantification of dendritic cell subsets in human thymus tissues of various ages

BackgroundDendritic cells (DCs) in the thymus are involved in central tolerance formation, but they also have other functions in the thymus, such as pathogen recognition. The density changes of human thymic DCs have been hardly investigated. In this study, human thymus samples of various ages were collected for tissue sectioning and staining. The thymic cortex and medulla area as well as the densities of various subsets of thymic DCs were calculated.ResultsAll common DC subsets were found in the human thymus of various ages. Most DCs had accumulated in the human thymic epithelial space, especially the medulla. We also found that the human thymic cortex had atrophied relatively faster than the medulla, which led to a gradual increase of the area ratio of the medulla to cortex with the increase of age. The densities of DC subsets in the human thymus showed various changes with increasing age, which contributed to the composition changes of DC subsets. The density of plasmacytoid DCs (pDCs) in the human thymus had increased gradually with aging, which suggested that pDCs plays another essential role in the thymus in addition to central tolerance.ConclusionsInconsistent with the shrinking of the epithelial space in the thymus, the densities of DC subsets in the epithelial space of the thymus are maintained at a constant level with aging to preserve highly efficient autoreactive thymocyte screening. An increasing density of the thymic pDCs with aging implies an extra function of DCs in the thymus beyond central tolerance.

IMMU-19. EVALUATING EFFECTS OF REVERSING DISTINCT FACETS OF IMMUNOSUPPRESSION IN EXPERIMENTAL GBM

Abstract Glioblastoma is associated with severe and multifaceted immunosuppression affecting all immune organs. Immunosuppression in GBM is a critical barrier to the success of immunotherapies and patient survival. We demonstrated that immunosuppression in the GL261-model of experimental GBM presents with significant thymic and spleen atrophy, MHCII downregulation, presence of potent immunosuppressive factors in serum, and sequestration of T-cells in the bone marrow. Parabiosis studies determined that soluble factors mediate immunosuppression by inhibiting T-cell proliferation, thymic involution, and loss of peripheral T-cells. In contrast, bone marrow T-cell sequestration was not mediated through soluble factors. While the immunosuppression in GBM is severe, a causative link between each facet of immunosuppression and overall survival is lacking. We used two strategies to block T-cell sequestration into the bone marrow and evaluated the extent survival was impacted in experimental GBM. First, we evaluated the extent a novel and off-the-shelf combination immunotherapy that uses extended 1/2-life IL-2 and anti-PD-1 reverses bone marrow T-cell sequestration. Sham treatment or anti-PD1 monotherapy did not alter T-cell sequestration in the bone marrow and animals had no enhanced survival. Extended 1/2-life IL-2 monotherapy and combination strategy both prevented T-cell sequestration into the bone marrow. However, only combined therapy, which also prevented MHC class II downregulation, improved survival. Second, we determined that glioma-bearing adrenalectomized mice do not present with bone marrow T-cell sequestration. However, sera of glioma-bearing adrenalectomized mice is as immunosuppressive as glioma-bearing controls. Blocking bone marrow T-cell sequestration in the presences of serum immunosuppression led to no survival benefit in glioma-bearing adrenalectomized mice compared to controls. In short, bone marrow T-cell sequestration alone does not correspond with overall survival in experimental glioma. Importantly, a concerted effort to reverse MHC class II downregulation and define inhibitory circulating factors may have the highest impact in immunotherapeutic efficacy and improving patient survival.

Les hyperplasies thymiques

Thymic hyperplasia presents as an anterior mediastinal mass and poses important diagnostic and therapeutic challenge. Two types of thymic hyperplasia are described: true hyperplasia and follicular hyperplasie. Literature data are peculiar concerning both entities. We aimed to describe the clinical and microscopic characteristics of thymic hyperplasia through a single institution experience during an 11-year-period.Thymic hyperplasia diagnosed during the period between 2009 and 2020 were included.In all, 46 thymic hyperplasias were diagnosed. The 46 patients consisted in 33 women and 13 men with a mean age of 30 years. Microscopic diagnosis concluded to a follicular hyperplasia in 12 cases and a true thymic hyperplasia in 34 cases. The diagnosis of true thymic hyperplasia posed a diagnostic challenge with an involuted thymus in 1 case and a thymolipoma in 1 case. The confrontation with the clinical data allowed retaining the diagnosis.The diagnosis of thymic hyperplasia is based on microscopic features. The confrontation with clinical data and the measurements of the thymus according to the age allow to retain the diagnosis in most challenging cases.

Assessment of the combined effects of chromium and benzene on the rat neuroendocrine and immune systems

This study assessed the hypothalamic-pituitary-adrenocortical (HPA) axis and lymphoid organs (thymus, spleen, and bone marrow) of Wistar rats treated with a mixture of chromium and benzene. Animals were assessed at three time-points (45, 90 and 135 days) following oral mixture exposure. The hypothalamus-pituitary system was examined in light and electron microscopy. Lymphoid organs underwent a morphological assessment and the immunophenotype of splenocytes was characterized immunohistochemically using monoclonal antibodies. Splenocytes cytokine production of was determined by ELISA after Con-A stimulation. Combined exposure to chromium and benzene in average doses of 20 mg Cr (VI)/kg body weight/day and 0.6 ml benzene/kg body weight/day impaired the responsiveness of the central compartment of the HPA axis, as evidenced by functional activation of the secretory activity of the hypothalamus and pituitary gland, which was not followed by a sufficient extrusion of nonapeptides at the neurohypophysis and hypothalamic median eminence. Chromium and benzene exposure reduced the thymus mass, thymocytes count, and caused a number of structural and functional changes indicative of transient thymus involution. In the spleen, exposure to both chemicals resulted in lymphoreticular hyperplasia and plasma cell-macrophage transformation (also observed in lymph nodes). Apoptosis of thymocytes and lymphocytes was also observed in T-zones of the spleen. Notably, the effects were similar to those observed earlier for the single agents, under the same experimental conditions, without evidence of additivity.

Ectopic cervical thymi and no thymic involution until midlife in naked mole rats.

Immunosenescence is a hallmark of aging and manifests as increased susceptibility to infection, autoimmunity, and cancer in the elderly. One component of immunosenescence is thymic involution, age‐associated shrinkage of the thymus, observed in all vertebrates studied to date. The naked mole rat (Heterocephalus glaber) has become an attractive animal model in aging research due to its extreme longevity and resistance to disease. Here, we show that naked mole rats display no thymic involution up to 11 years of age. Furthermore, we found large ectopic cervical thymi in addition to the canonical thoracic thymus, both being identical in their cell composition. The developmental landscape in naked mole rat thymi revealed overt differences from the murine T‐cell compartment, most notably a decrease of CD4+/CD8+ double‐positive cells and lower abundance of cytotoxic effector T cells. Our observations suggest that naked mole rats display a delayed immunosenescence. Therapeutic interventions aimed at reversing thymic aging remain limited, underscoring the importance of understanding the cellular and molecular mechanisms behind a sustained immune function in the naked mole rat.

Brain-specific PAPP-A knock-out mice?

PAPP-A knock-out (KO) mice are a valuable model for investigating the effects of down-regulating localized insulin-like growth factor (IGF) action, which has been shown to extend lifespan and healthspan when the PAPP-A gene is globally deleted. Based on previous mouse models of brain-specific reduction in IGF signaling associated with longevity, we sought to generate brain-specific PAPP-A KO mice and determine effects on metabolism and lifespan. Mice with the PAPP-A gene floxed (fPAPP-A) were crossed with Nestin promoter-driven Cre recombinase transgenic mice. This cross-breeding of mice for Nestin-Cre and mice with other floxed target alleles has been used extensively to investigate brain-specific effects. Our cross-breeding generated four genotypes for study: fPAPP-A/Nestin positive (brain-specific PAPP-A KO); fPAPP-A/Nestin negative (Control for floxed PAPP-A); WT/Nestin positive (Control for Nestin-Cre); WT/Nestin negative (Wild-type Control).The basic genotype screen of neonatal tail snip DNA clearly indicated PAPP-A gene status and the presence (pos) or absence (neg) of Nestin-Cre. We then determined tissue specificity of PAPP-A gene excision. We had expected fPAPP-A/pos mice to be relatively brain-specific for PAPP-A gene deletion and the controls (fPAPP-A/neg, WT/neg and WT/pos mice) to show no effect on PAPP-A expression in brain or other tissues. However, in fPAPP-A/neg mice we found evidence of PAPP-A excision in all tissues examined, i.e., in the presumed absence of Nestin-Cre, indicating germline recombination. We further found that fPAPP-A/pos mice showed near complete excision of the PAPP-A gene in brain, but some also showed germline recombination affecting all tissues tested.To determine if the level of excision indicated by tissue genotyping approximated PAPP-A mRNA expression, we performed RT-qPCR. fPAPP-A/pos mice that showed markedly decreased whole brain PAPP-A mRNA expression (~80%), with little or no effect on expression in the other tissues tested, were designated as “brain-specific” PAPP-A KO. fPAPP-A/pos mice that showed germline recombination had similar decreases in PAPP-A expression in brain but also showed 40–65% decreased PAPP-A mRNA expression in other tissues as well, which was especially striking in kidney, tibia, thymus and spleen. These were designated as “non-specific” PAPP-A KO mice.With unknown and unpredictable specificity until harvest, we chose to assess a surrogate marker of lifespan i.e., thymic involution, in 15- to 18-month-old fPAPP-A/pos and WT/pos mice, the latter an important control for a possible effect of Nestin-Cre per se. Diminished thymic involution as indicated by increased thymic weight (135%, P = 0.035) and decreased histological disruption was seen in “non-specific” PAPP-A KO mice, similar to what was previously reported in 18-month-old global PAPP-A KO mice. There was no significant difference between “brain-specific” PAPP-A KO and control mice.This study highlights the importance of thorough characterization of assumed tissue-specific mouse models and awareness of potential germline recombination for proper data interpretation.

Circulating mature dendritic cells homing to the thymus promote thymic epithelial cells involution via the Jagged1/Notch3 axis

Multiple proinflammatory conditions, including chemotherapy, radiotherapy, transplant rejection, and microbial infections, have been identified to induce involution of the thymus. However, the underlying cellular and molecular mechanisms of these inflammatory conditions inducing apoptosis of thymic epithelial cells (TECs), the main components of the thymus, remain largely unknown. In the circulation, mature dendritic cells (mDCs), the predominant initiator of innate and adaptive immune response, can migrate into the thymus. Herein, we demonstrated that mDCs were able to directly inhibit TECs proliferation and induce their apoptosis by activating the Jagged1/Notch3 signaling pathway. Intrathymic injection of either mDCs or recombinant mouse Jagged1-human Fc fusion protein (rmJagged1-hFc) into mice resulted in acute atrophy of the thymus. Furthermore, DAPT, a γ-secretase inhibitor, reversed the effects induced by mDC or rmJagged1-hFc. These findings suggest that acute or aging-related thymus degeneration can be induced either by mass migration of circulating mDCs in a short period of time or by a few but constantly homing mDCs.

Thymic Involution and Altered Naive CD4 T Cell Homeostasis in Neuromyelitis Optica Spectrum Disorder.

Circulating T helper cells with a type 17-polarized phenotype (TH17) and expansion of aquaporin-4 (AQP4)-specific T cells are frequently observed in patients with neuromyelitis optica spectrum disorder (NMOSD). However, naive T cell populations, which give rise to T helper cells, and the primary site of T cell maturation, namely the thymus, have not been studied in these patients. Here, we report the alterations of naive CD4 T cell homeostasis and the changes in thymic characteristics in NMOSD patients. Flow cytometry was performed to investigate the naive CD4+ T cell subpopulations in 44 NMOSD patients and 21 healthy controls (HC). On immunological evaluation, NMOSD patients exhibited increased counts of CD31+thymic naive CD4+ T cells and CD31-cental naive CD4+ T cells along with significantly higher fraction and absolute counts of peripheral blood CD45RA+ CD62L+ naive CD4+ T cells. Chest computed tomography (CT) images of 60 NMOSD patients and 65 HCs were retrospectively reviewed to characterize the thymus in NMOSD. Thymus gland of NMOSD patients exhibited unique morphological characteristics with respect to size, shape, and density. NMOSD patients showed exacerbated age-dependent thymus involution than HC, which showed a significant association with disease duration. These findings broaden our understanding of the immunological mechanisms that drive severe disease in NMOSD.

Zinc supplementation prevents the development of thymic involution induced by tumor growth in mice

Aim. The aim of this study was to investigate the influence of zinc supplementation on the development of thymic involution during the growth of transplantable tumor in mice. Materials and methods. Inbred C3HA mice after subcutaneous syngeneic hepatoma 22a inoculation received zinc sulfate with drinking water. After 3 weeks of tumor growth animals were killed, and evaluated for zinc content in serum blood samples, thymus status and parameters of tumor growth. Results. At the 21 day of tumor 22a growth weight and cellular content of the thymus were decreased by 3 times and endogenous zinc content in samples of blood serum was decreased by 1,9 times. Oral zinc sulfate supplementation at concentration of 22 mkg/ml in drinking water during 3 weeks, started from the first day after tumor inoculation, increased thymus weight and cellularity as well as serum zinc content. At the same time, it did not influence tumor size and survival rate of mice with hepatoma 22a. As a possible mechanism of zinc action, we studied the influence of zinc supplementation on the activity of two anti-oxidative defense enzymes: Cu,Zn-superoxide dismutase and catalase in the thymus. Zinc supplementation had no influence on these parameters, both remained at a higher level in tumor-bearing mice. Conclusion. Oral zinc supplementation in hepatoma 22a bearing mice induced retards the development of thymic involution with no influence on the tumor growth per se. The study allows to suggest that oral zinc administration may be considered as a prospective strategy for thymus reconstitution in oncology patients.

Prostaglandin E2 Promotes Recovery of Thymus in Survivors of the Hematopoietic Acute Radiation Syndrome (H-ARS)

We have previously developed a murine model of the hematopoietic acute radiation syndrome [H-ARS, total body irradiated (TBI)] and delayed effects of acute radiation exposure (DEARE) for efficacy testing of potential medical countermeasures (MCM) against these two syndromes. Using this model, we have previously reported that prophylactic use of 16,16 dimethyl-prostaglandin E2 (dmPGE2), a long-acting formulation of PGE, before irradiation significantly increases 30-day survival rate. The current study investigated the effects of the prophylactic use of dmPGE2 with or without therapeutic use of dmPGE2 in survivors on long-term thymus recovery in H-ARS survivors at 6 and 12-months post-LD70/30 (872cGy, 137Cs, 95.2±4.8 cGy/min). Mice were irradiated at 12 weeks of age and then injected subcutaneously with dmPGE2 (1 dose of 35 ug dmPGE2/mouse) or vehicle (Veh; ethanol/PBS solution) at 30“ or -1hr pre-TBI. Survivors were injected monthly with dmPGE2 or Veh and then analyzed for evidence of thymic reconstitution at 6 and 12 months.post-TBI. The three test groups were: prophylactic dmPGE2 + monthly dmPGE2 (PP), prophylactic dmPGE2 + monthly Veh (PV), and prophylactic Veh + monthly Veh (VV), n = 3-6 mice/group. Thymus weight and cellularity of mice in the dmPGE groups, PV and PP, were increased compared to Veh both at 6-months and 12-months post-TBI (p <0.05). The difference was not related to body weight. No difference was observed between PP and PV group. The number of thymocyte subgroups (CD4 single positive, CD8 single positive, double positive and double negative) was also increased in PV and PP mice. Complete blood count (CBC) was also analyzed in the same mice. The VV group showed myeloid skewing, evidenced as an increase in neutrophils and decrease in lymphocytes. The PV and PP groups both exhibited reversal of myeloid skewing, and once again dmPGE maintenance did not provide extra benefit. Thymic progenitors in bone marrow, known as the lymphoid-primed multipotent progenitors (LMPP, Lin-Kit+Sca1+Flt3hiCD34+) and common lymphoid progenitor (CLP, Lin-IL7R+KitloSca1lo Flt3+) were also increased in PV and PP groups compared with VV group (p <0.05). No difference was observed in common myeloid progenitors (CMP), granulocyte-macrophage progenitors (GMP), and MEP (megakaryocyte-erythroid progenitor) between PP and PV group. Competitive bone marrow transplantation showed more than 10 times higher donor-derived chimerism in the thymus of PP and PV mice compared with the VV group. Again, no difference was observed between PP and PV group. In conclusion, these data show that dmPGE2, given before irradiation, protects thymic settling progenitors in bone marrow, resulting in reversal of thymic involution, increased peripheral lymphocytes, and partial alleviation of the myeloid skewing observed in radiation survivors. dmPGE maintenance does not bring extra benefit. F unded by Department of the Army PR140433P1 and NIAID 1U01AI107340-01. DisclosuresNo relevant conflicts of interest to declare.

Premature Senescence and Increased Oxidative Stress in the Thymus of Down Syndrome Patients.

Down syndrome (DS) patients prematurely show clinical manifestations usually associated with aging. Their immune system declines earlier than healthy individuals, leading to increased susceptibility to infections and higher incidence of autoimmune phenomena. Clinical features of accelerated aging indicate that trisomy 21 increases the biological age of tissues. Based on previous studies suggesting immune senescence in DS, we hypothesized that induction of cellular senescence may contribute to early thymic involution and immune dysregulation. Immunohistochemical analysis of thymic tissue showed signs of accelerated thymic aging in DS patients, normally seen in older healthy subjects. Moreover, our whole transcriptomic analysis on human Epcam-enriched thymic epithelial cells (hTEC), isolated from three DS children, which revealed disease-specific transcriptomic alterations. Gene set enrichment analysis (GSEA) of DS TEC revealed an enrichment in genes involved in cellular response to stress, epigenetic histone DNA modifications and senescence. Analysis of senescent markers and oxidative stress in hTEC and thymocytes confirmed these findings. We detected senescence features in DS TEC, thymocytes and peripheral T cells, such as increased β-galactosidase activity, increased levels of the cell cycle inhibitor p16, telomere length and integrity markers and increased levels of reactive oxygen species (ROS), all factors contributing to cellular damage. In conclusion, our findings support the key role of cellular senescence in the pathogenesis of immune defect in DS while adding new players, such as epigenetic regulation and increased oxidative stress, to the pathogenesis of immune dysregulation.

High-fat diet and dyslipidemia synergistically contribute to T cell senescence in gut associated lymphoid tissue.

Obesity can compromise immune response and immune surveillance. Recent studies have linked obesity with systemic T cell senescence and thymus involution. However, these studies failed to distinguish the influence of obesity from the influence of diet composition on premature T cell senescence. High-fat diet (HFD) can influence the immunity of gut-associated lymphoid tissue (GALT) preceding the onset of obesity. Despite GALT is sensitive to the changes in the dietary composition and metabolic status, it still remains elusive how HFD and obesity contribute to T cell senescence in the GALT. In this study, we illustrated the interplay of the HFD, obesity and intestinal immunity by comparing the immune features of diet-induced obese (DIO) to those of diet-resistant (DR) mice. As expected, DIO mice exhibited increased serum lipid levels and liver steatosis whereas dyslipidemia was absent in DR mice. DIO mice demonstrated a shift from naïve to effector-memory (EM) phenotype in the T cells derived from the spleens and PPs. Moreover, DIO mice showed up-regulation of PD1 and KLRG1 on the T cells. Similar but mild trends were observed in the naïve and EM T cells from DR mice. Furthermore, we proved that the senescent-like changes in splenic and PPs-derived T cells positively correlated with the serum lipid concentrations. Taken together, our results indicate that HFD components function synergistically with dyslipidemia to induce T cell senescence in the GALT.

Radiation inducible MafB gene is required for thymic regeneration

The thymus facilitates mature T cell production by providing a suitable stromal microenvironment. This microenvironment is impaired by radiation and aging which lead to immune system disturbances known as thymic involution. Young adult thymus shows thymic recovery after such involution. Although various genes have been reported for thymocytes and thymic epithelial cells in such processes, the roles of stromal transcription factors in these remain incompletely understood. MafB (v-maf musculoaponeurotic fibrosarcoma oncogene homolog B) is a transcription factor expressed in thymic stroma and its expression was induced a day after radiation exposure. Hence, the roles of mesenchymal MafB in the process of thymic regeneration offers an intriguing research topic also for radiation biology. The current study investigated whether MafB plays roles in the adult thymus. MafB/green fluorescent protein knock-in mutant (MafB+/GFP) mice showed impaired thymic regeneration after the sublethal irradiation, judged by reduced thymus size, total thymocyte number and medullary complexity. Furthermore, IL4 was induced after irradiation and such induction was reduced in mutant mice. The mutants also displayed signs of accelerated age-related thymic involution. Altogether, these results suggest possible functions of MafB in the processes of thymic recovery after irradiation, and maintenance during aging.

Hallmarks of T cell aging.

The aged adaptive immune system is characterized by progressive dysfunction as well as increased autoimmunity. This decline is responsible for elevated susceptibility to infection and cancer, as well as decreased vaccination efficacy. Recent evidence indicates that CD4+ T cell-intrinsic alteratins contribute to chronic inflammation and are sufficient to accelerate an organism-wide aging phenotype, supporting the idea that T cell aging plays a major role in body-wide deterioration. In this Review, we propose ten molecular hallmarks to represent common denominators of T cell aging. These hallmarks are grouped into four primary hallmarks (thymic involution, mitochondrial dysfunction, genetic and epigenetic alterations, and loss of proteostasis) and four secondary hallmarks (reduction of the TCR repertoire, naive-memory imbalance, T cell senescence, and lack of effector plasticity), and together they explain the manifestation of the two integrative hallmarks (immunodeficiency and inflammaging). A major challenge now is weighing the relative impact of these hallmarks on T cell aging and understanding their interconnections, with the final goal of defining molecular targets for interventions in the aging process.

Aging is accompanied by a decrease in the number of naive Tregs

Abstract Funding Acknowledgements Type of funding sources: None. Background. Age-related changes in the immune system are an important factor contributing to the maintenance of chronic inflammatory status. There are undoubted data on the decrease of the number of T-lymphocytes with age caused by thymus involution, but there are currently no unambiguous data on changes of minor T-cell subpopulations, in particular, regulatory T-cells (Treg). The aim of this study was to analyze the content of effector and regulatory CD4+ T cell subsets in patients with coronary and/or carotid atherosclerosis depending on age. Methods. 111 patients (men, median age 63 (55;69)) with coronary and/or carotid atherosclerosis, without smoking anamnesis, were enrolled. Mononuclear leuocytes were isolated from blood by gradient centrifugation, and CD4 + CD25high and CD4 + Foxp3+ Treg, CD4 + IL17+ T-helpers (Th) 17 and CD4 + INFγ Th1 were evaluated by direct immunofluorescence and flow cytometry. For intracellular cytokine detection cells were pre-activated in vitro in the presence of PMA/ionomycin/brefeldin A. In 74 patients cells were additionally stained with CD39, CD278, CD45RA Mabs to reveal naïve and primed T-cells. Results. According to age the patients formed three groups: I – &amp;lt;55 y.o. (n = 23), II – 55-64 y.o. (n = 42), III - ≥65 y.o. (n = 46). All patients were taking statins at baseline. The groups were comparable in traditional risk factors of CVD (BMI, arterial hypertension, diabetes mellitus, previous myocardial infarction anamnesis). The absolute content of CD4+ T cells was lower in group III (646.3 (516.0;806.4)) compared to groups I (903.0 (585.6;1113.8), p = 0.03) and II (745.4 (502.2;924.0), p = 0.06). The absolute content of CD4 + CD25high Treg was lower in group III (24.2 (18.4;35.2)) compared to groups I (35.0 (28.7;54.4), p = 0.01) and II (31.0 (21.1;43.6), p = 0.03). There were no differences in Th1, Th17, CD39 + CD45RA- and CD278+ Treg content between groups. A negative correlation was found between age and the content of CD4+ T cells (r= -0.28), CD4 + CD25high Treg (r= -0.27), p &amp;lt; 0.05. A negative correlation was found between age and CD4 + CD25highCD45RA+ Treg (r= -0.24) and CD4 + CD45RA+ T cells (r= -0.36), CD4 + CD45RA+/CD4 + CD45RA- T-cells ratio (r= -0.24), p &amp;lt; 0.05. Conclusion. Here we demonstrate an age-dependent decrease of total CD4+ T cell population and Treg subset in patients with atherosclerosis. The changes observed were primary due to the deficiency of CD45RA+ naïve T cells. The effector cell Th1 and Th17 quantities were at the same levels. Future research will show whether the identified immunological patterns can contribute to the progression of atherosclerosis and other chronic inflammatory diseases.

The role of Klf4 in thymic epithelial cells

Abstract Thymic epithelial cells (TECs) are essential for the development and the selection of naïve T cells that will be able to discriminate the non-self from self-peptides. The thymus is the organ that starts degenerating the earliest in the organism, leading to a decrease in naïve T cell production. Starting from 1 year old in humans, thymic involution accelerates at puberty, partially caused by an increase in circulating sex steroids (SS). SS induce thymic involution, mediated indirectly by nuclear receptors in TECs. Little is known about how and why the age-related thymic involution is induced. We identified Klf4 as a potential regulator of TEC function and differentiation. Klf4 is a transcription factor that induces differentiation and inhibits proliferation in epithelial cells from various tissues. We showed that Klf4 expression is negatively correlated with TEC proliferation in different contexts. Interestingly, Klf4 is known as a mediator of nuclear receptors response and is a sex hormone target in several tissues. We discovered that Klf4 was more expressed in male TECs than in female TECs. Importantly, TECs are less proliferative in males than in females. Moreover, Klf4 expression was reduced when males were castrated. These results suggest that Klf4 has an important role in TEC homeostasis and is a mediator of SS response. We are currently testing the role of Klf4 in TECs by phenotypic and transcriptomic analysis on transgenic mice in which Klf4 is specifically deleted in TECs. Using testosterone treatment, we are studying the effect of SS on TEC and thymic output with or without Klf4 expression. Finally, we will assess the direct KLF4 target genes in TECs using ChIP-Seq.

An early decline in circulating lymphoid progenitors contributes to the age-associated reduction in ETP numbers

Abstract Age-associated thymic involution is characterized by the decreased generation and export of T cells from the thymus, which in turn, is associated with increased susceptibility to infections. Elucidating the cause of thymic involution will inform strategies to restore thymopoiesis with age. T cells develop from thymus seeding progenitors that migrate from the bone marrow (BM) to the thymus. Early T-cell progenitors (ETPs) require signals from the thymic microenvironment to undergo T-lineage commitment and differentiation. In mice, we find that there is a significant decline in thymocyte cellularity as early as 3 months of age (3MO), which is mirrored by a striking reduction in ETPs. This early thymic involution could be driven by changes in hematopoietic progenitors and/or thymic stromal cells. Using a multicongenic BM transplantation approach, we determined that the number of ETP-supportive niches does not diminish with age. However, modulating the thymic stromal compartment sustains ETP cellularity, suggesting that age-associated defects in ETP niche quality may contribute to ETP decline. Notably, the number of lymphoid progenitors in the BM and blood declines dramatically by 3MO, although their cell-intrinsic capacity to seed and differentiate in the thymus is unimpaired. Together, our findings suggest that the early decline in ETP numbers is due to both hematopoietic and stromal age-associated changes. This work was supported by P01 AG052359 from the NIH-National Institute of Aging.

GUS05: Thymus specific nuclear progesterone receptor governs maternal thymic involution in pregnancy

GUS05: Thymus specific nuclear progesterone receptor governs maternal thymic involution in pregnancy

Defective dystrophic thymus determines degenerative changes in skeletal muscle

In Duchenne muscular dystrophy (DMD), sarcolemma fragility and myofiber necrosis produce cellular debris that attract inflammatory cells. Macrophages and T-lymphocytes infiltrate muscles in response to damage-associated molecular pattern signalling and the release of TNF-α, TGF-β and interleukins prevent skeletal muscle improvement from the inflammation. This immunological scenario was extended by the discovery of a specific response to muscle antigens and a role for regulatory T cells (Tregs) in muscle regeneration. Normally, autoimmunity is avoided by autoreactive T-lymphocyte deletion within thymus, while in the periphery Tregs monitor effector T-cells escaping from central regulatory control. Here, we report impairment of thymus architecture of mdx mice together with decreased expression of ghrelin, autophagy dysfunction and AIRE down-regulation. Transplantation of dystrophic thymus in recipient nude mice determine the up-regulation of inflammatory/fibrotic markers, marked metabolic breakdown that leads to muscle atrophy and loss of force. These results indicate that involution of dystrophic thymus exacerbates muscular dystrophy by altering central immune tolerance.

Prepubertal gonad investment modulates thymus function: evidence in a teleost fish.

Thymus plasticity following gonadectomy or sex hormone replacement has long since exemplified sex hormone effects on the immune system in mammals and, to a lesser extent, in 'lower vertebrates', including amphibians and fish. Nevertheless, the underlying physiological significances as well as the ontogenetic establishment of this crosstalk remain largely unknown. Here, we used a teleost fish, the European sea bass, Dicentrarchus labrax, to investigate: (1) whether the regulation of thymus plasticity relies on resource trade-off with somatic growth and reproductive investment and (2) if the gonad-thymus interaction takes place during gonadal differentiation and development. Because gonadal development and, supposedly, thymus function in sea bass depend on environmental changes associated with the winter season, we evaluated thymus changes (foxn1 expression, and thymocyte and T cell content) in juvenile D. labrax raised for 1year under either constant or fluctuating photoperiod and temperature. Importantly, in both conditions, intensive gonadal development following sex differentiation coincided with a halt of thymus growth, while somatic growth continued. To the best of our knowledge, this is the first study showing that gonadal development during prepuberty regulates thymus plasticity. This finding may provide an explanation for the initiation of the thymus involution related to ageing in mammals. Comparing fixed and variable environmental conditions, our work also demonstrates that the extent of the effects on the thymus, which are related to reproduction, depend on ecophysiological conditions, rather than being directly related to sexual maturity and sex hormone levels.