Genetic regulation of thymic involution

T cell aging and Alzheimer’s disease

The brain has long been considered an immune-privileged organ due to the presence of the blood-brain barrier (BBB). However, recent discoveries have revealed the underestimated role of T cells in the brain through the meningeal lymphatic system. Age is the primary risk factor for Alzheimer’s disease (AD), resulting in marked age-dependent changes in T cells. Manipulating peripheral T cell immune response has been shown to impact AD, but the relationship between T cell aging and AD remains poorly understood. Given the limited success of targeting amyloid beta (Aβ) and the growing evidence of T cells’ involvement in non-lymphoid organ aging, a deeper understanding of the relationship between T cells and AD in the context of aging is crucial for advancing therapeutic progress. In this review, we comprehensively examine existing studies on T cells and AD and offer an integrated perspective on their interconnections in the context of aging. This understanding can inform the development of new interventions to prevent or treat AD.

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.

Body weight and high-fat diet are associated with epigenetic aging in female members of the BXD murine family

DNA methylation (DNAm) is shaped by genetic and environmental factors and modulated by aging. Here, we examine interrelations between epigenetic aging, body weight (BW), and life span in 12 isogenic strains from the BXD family of mice that exhibit over twofold variation in longevity. Genome-wide DNAm was assayed in 70 liver specimens from predominantly female cases, 6-25 months old, that were maintained on normal chow or high-fat diet (HFD). We defined subsets of CpG regions associated with age, BW at young adulthood, and strain-by-diet-dependent life span. These age-associated differentially methylated CpG regions (age-DMRs) featured distinct genomic characteristics, with DNAm gains over time occurring in sites such as promoters and exons that have high CpG density and low average methylation. CpG regions associated with BW were enriched in introns, tended to have lower methylation in mice with higher BW, and were inversely correlated with gene expression (i.e., higher mRNA levels in mice with higher BW). CpG regions associated with life span were linked to genes involved in life span modulation, including the telomerase reverse transcriptase gene, Tert, which had both lower methylation and higher expression in long-lived strains. An epigenetic clock defined from age-DMRs revealed accelerated aging in mice belonging to strains with shorter life spans. Both higher BW and the HFD were associated with accelerated epigenetic aging. Our results highlight the age-accelerating effect of heavier BW. Furthermore, we demonstrate that the measure of epigenetic aging derived from age-DMRs can predict genotype and diet-induced differences in life span among female BXD members.

Bioprofiling TS/A Murine Mammary Cancer for a Functional Precision Experimental Model

The TS/A cell line was established in 1983 from a spontaneous mammary tumor arisen in an inbred BALB/c female mouse. Its features (heterogeneity, low immunogenicity and metastatic ability) rendered the TS/A cell line suitable as a preclinical model for studies on tumor-host interactions and for gene therapy approaches. The integrated biological profile of TS/A resulting from the review of the literature could be a path towards the description of a precision experimental model of mammary cancer.

Cytokine modulation, oxidative stress and thymic dysfunctions: Role of age-related changes in the experimental Trypanosoma cruzi infection: Age-related thymic dysfunctions and Trypanosoma cruzi infection

Aging is linked with a thymic oxidative damage and some infectious diseases such as Chagas' disease may aggravate this process. The aim of this study was to evaluate the production of distinct cytokines as well as the antioxidant/oxidant status of the thymus and thymocytes populations during Trypanosoma cruzi (T. cruzi) infection. Young (5 weeks old) and aged (18 weeks old) male Wistar rats were inoculated with blood trypomastigotes forms of the Y strain of T. cruzi. On the 16th day after T. cruzi infection, increased concentrations of transforming growth factor β (TGF-β), interleukin (IL)-12, IL-17 were detected in aged infected subjects as compared to young infected ones. Interestingly, a reduction in the production of tumor necrose factor (TNF)-α was observed in aged infected rats when compared to young infected subjects. Aged-infected rats presented increased O₂^- levels, compared to young counterparts. Significant raise in the generation of O₂^- in aged infected animals, as compared to uninfected counterparts was observed. Up-regulated expression of Nox2 in the thymus of young and aged infected animals was observed. An increased SOD2 expression was detected in the thymus of young animals infected with T. cruzi, when compared to uninfected young rats. Aged animals showed reduced thymus weight and the number of thymocytes. Decreased percentages of SPCD4⁺ and SPCD8⁺T cells were detected in aged and control groups when compared to young counterparts. In summary, this is the first data to directly examine the influence of aging on age-related dysfunctions during the acute phase of experimental Chagas disease. Concerning to oxidative stress, it is clear from our analysis that aged infected rats suffer a more intense oxidative damage when compared to young and infected ones. Age and infection triggered a dynamic interplay of cytokines, oxidative stress and thymic dysfunctions which led to impaired response from aged and infected rats. Such findings may have significant functional relevance in therapeutic strategies in order to reestablish the thymic immunological function which occurs in aged and T. cruzi infected subjects.

MicroRNA-195a-5p inhibits mouse medullary thymic epithelial cells proliferation by directly targeting Smad7

MiR-195 has been implicated in inhibiting cell proliferation in different types of tumors. Whether it contributes to the process of thymic epithelial cells (TECs) proliferation remains unclear. In this study, we found that miR-195a-5p was highly up-regulated in the TECs isolated from the aging mice. Further experiments showed that miR-195a-5p mimic transfection inhibited the proliferation of mouse medullary thymic epithelial cell line 1 (MTEC1), whereas the transfection of miR-195a-5p inhibitor in MTEC1 had the opposite effect. In addition, miR-195a-5p had no obvious effect on MTEC1 apoptosis. Furthermore, Smad7, a negative regulator of transforming growth factor β pathway, was confirmed as a direct target of miR-195a-5p by luciferase assays. Taken together, our results indicate that miR-195a-5p inhibits MTEC1 proliferation, at least in part, via down-regulation of Smad7.

Interpreting Stress Responses during Routine Toxicity Studies

Stress often occurs during toxicity studies. The perception of sensory stimuli as stressful primarily results in catecholamine release and activation of the hypothalamic–pituitary–adrenal (HPA) axis to increase serum glucocorticoid concentrations. Downstream effects of these neuroendocrine signals may include decreased total body weights or body weight gain; food consumption and activity; altered organ weights (e.g., thymus, spleen, adrenal); lymphocyte depletion in thymus and spleen; altered circulating leukocyte counts (e.g., increased neutrophils with decreased lymphocytes and eosinophils); and altered reproductive functions. Typically, only some of these findings occur in a given study. Stress responses should be interpreted as secondary (indirect) rather than primary (direct) test article–related findings. Determining whether effects are the result of stress requires a weight-of-evidence approach. The evaluation and interpretation of routinely collected data (standard in-life, clinical pathology, and anatomic pathology endpoints) are appropriate and generally sufficient to assess whether or not changes are secondary to stress. The impact of possible stress-induced effects on data interpretation can partially be mitigated by toxicity study designs that use appropriate control groups (e.g., cohorts treated with vehicle and subjected to the same procedures as those dosed with test article), housing that minimizes isolation and offers environmental enrichment, and experimental procedures that minimize stress and sampling and analytical bias.

This article is a comprehensive overview of the biological aspects of the stress response, beginning with a Summary (Section 1) and an Introduction (Section 2) that describes the historical and conventional methods used to characterize acute and chronic stress responses. These sections are followed by reviews of the primary systems and parameters that regulate and/or are influenced by stress, with an emphasis on parameters evaluated in toxicity studies: In-life Procedures (Section 3), Nervous System (Section 4), Endocrine System (Section 5), Reproductive System (Section 6), Clinical Pathology (Section 7), and Immune System (Section 8). The paper concludes (Section 9) with a brief discussion on Minimizing Stress-Related Effects (9.1.), and a final section explaining why Parameters routinely measured are appropriate for assessing the role of stress in toxicology studies (9.2.).

Tbata modulates thymic stromal cell proliferation and thymus function

Niche availability provided by stromal cells is critical to thymus function. Thymi with diminished function contain fewer stromal cells, whereas thymi with robust function contain proliferating stromal cell populations. Here, we show that the thymus, brain, and testes-associated gene (Tbata; also known as SPATIAL) regulates thymic epithelial cell (TEC) proliferation and thymus size. Tbata is expressed in thymic stromal cells and interacts with the enzyme Uba3, thereby inhibiting the Nedd8 pathway and cell proliferation. Thymi from aged Tbata-deficient mice are larger and contain more dividing TECs than wild-type littermate controls. In addition, thymic reconstitution after bone marrow transplantation occurred more rapidly in Rag2(-/-)Tbata(-/-) mice than in Rag2(-/-)Tbata(+/+) littermate controls. These findings suggest that Tbata modulates thymus function by regulating stromal cell proliferation via the Nedd8 pathway.

Early-life nutrition influences thymic growth in male mice that may be related to the regulation of longevity

Nutrition and growth rate during early life can influence later health and lifespan. We have demonstrated previously that low birthweight, resulting from maternal protein restriction during pregnancy followed by catch-up growth in rodents, was associated with shortened lifespan, whereas protein restriction and slow growth during lactation increased lifespan. The underlying mechanisms by which these differences arise are unknown. In the present study, we report that maternal protein restriction in mice influences thymic growth in early adult life. Offspring of dams fed a low-protein diet during lactation (PLP offspring) had significant thymic growth from 21 days to 12 weeks of age, whereas this was not observed in control mice or offspring of dams fed a low-protein diet during pregnancy (recuperated offspring). PCNA (proliferating-cell nuclear antigen) and SIRT1 (silent information regulator 1) protein levels at 21 days of age were significantly higher in the thymus from both PLP mice (P<0.001 and P<0.05 respectively) and recuperated mice (P<0.001 and P<0.01 respectively) compared with controls. At 12 weeks, PLP mice maintained a higher SIRT1 level, whereas PCNA was decreased in the thymus from recuperated offspring. This suggests that mitotic activity was initially enhanced in the thymus from both PLP and recuperated offspring, but remained sustained into adulthood only in PLP mice. The differential mitotic activity in the thymus from PLP and recuperated mice appeared to be influenced by changes in sex hormone concentrations and the expression of p53, p16, the androgen receptor, IL-7 (interleukin-7) and the IL-7 receptor. In conclusion, differential thymic growth may contribute to the regulation of longevity by maternal diet.

Maintenance of naïve CD8 T cells in nonagenarians by leptin, IGFBP3 and T3

Research into the age-associated decline in the immune system has focused on the factors that contribute to the accumulation of senescent CD8 T cells. Less attention has been paid to the non-immune factors that may maintain the pool of naïve CD8 T cells. Here, we analyzed the status of the naïve CD8 T-cell population in healthy nonagenarians (>or=90-year-old), old (60-79-year-old), and young (20-34-year-old) subjects. Naïve CD8 T cells were defined as CD28(+)CD95(-) as this phenotype showed a strong co-expression of the CD45RA(+), CD45RO(-), and CD127(+) phenotypes. Although there was an age-associated decline in the percentage of CD28(+)CD95(-) CD8 T cells, the healthy nonagenarians maintained a pool of naïve CD28(+)CD95(-) cells that contained T-cell receptor excision circles (TREC)(+) cells. The percentages of naïve CD28(+)CD95(-) CD8 T cells in the nonagenarians correlated with the sera levels of insulin-like growth factor binding protein 3 (IGFBP3) and leptin. Higher levels of triiodothyronine (T3) negatively correlated with the accumulation of TREC(-)CD28(-)CD95(+) CD8 T cells from nonagenarians. These results suggest a model in which IGFBP3, leptin and T3 act as non-immune factors to maintain a larger pool of naïve CD8 T cells in healthy nonagenarians.

Reactive oxygen intermediate-induced pathomechanisms contribute to immunosenescence, chronic inflammation and autoimmunity

Deregulation of reactive oxygen intermediates (ROI) resulting in either too high or too low concentrations are commonly recognized to be at least in part responsible for many changes associated with aging. This article reviews ROI-dependent mechanisms critically contributing to the decline of immune function during physiologic - or premature - aging. While ROI serve important effector functions in cellular metabolism, signalling and host defence, their fine-tuned generation declines over time, and ROI-mediated damage to several cellular components and/or signalling deviations become increasingly prevalent. Although distinct ROI-associated pathomechanisms contribute to immunosenescence of the innate and adaptive immune system, mutual amplification of dysfunctions may often result in hyporesponsiveness and immunodeficiency, or in chronic inflammation with hyperresponsiveness/deregulation, or both. In this context, we point out how imbalanced ROI contribute ambiguously to driving immunosenescence, chronic inflammation and autoimmunity. Although ROI may offer a distinct potential for therapeutic targeting along with the charming opportunity to rescue from deleterious processes of aging and chronic inflammatory diseases, such modifications, owing to the complexity of metabolic interactions, may carry a marked risk of unforeseen side effects.

The magnitude of thymic output is genetically determined through controlled intrathymic precursor T cell proliferation

The thymus plays a crucial role in providing the immune system with naive T cells showing a diverse TCR repertoire. Whereas the diversity of thymic production is mainly ensured by TCR rearrangement at both the TRA and TRB loci, the number of cells reaching the double-positive differentiation stage defines the extent of thymic output. A quantitative analysis of TCR excision circles (TREC; signal-joint TRECs and DJbetaTRECs) produced at different stages of thymopoiesis was performed in nine laboratory mouse strains. The results clearly demonstrate that the magnitude of thymic output is directly proportional to the extent of proliferation in the double-negative 4 thymocyte subset. Strikingly, intrathymic precursor T cell proliferation was found to be strain dependent, thus suggesting a genetic regulation of thymic output. The inherited character of thymic output was further confirmed by the transmission of the phenotype in a recessive fashion in F(1) progeny of the different parental strains. Our results provide the first demonstration of the genetic regulation of thymic output.

Wild-derived mouse stocks: an underappreciated tool for aging research

Virtually all biomedical research makes use of a relatively small pool of laboratory-adapted, inbred, isogenic stocks of mice. Although the advantages of these models are many, there are a number of disadvantages as well. When studying a multifaceted process such as aging, the problems associated with using laboratory stocks are greatly inflated. On the other hand, wild-derived mouse stocks, loosely defined here as either wild-caught individuals or the recent progeny of wild-caught individuals, have much to offer to biogerontology research. Hence, the aims of this review are threefold: (1) to (re)acquaint readers with the pros and cons of using a typical inbred laboratory mouse model for aging research; (2) to reintroduce the notion of using wild-derived mouse stocks in aging research as championed by Austad, Miller and others for more than a decade, and (3) to provide an overview of recent advances in biogerontology using wild-derived mouse stocks.

Maintenance of a normal thymic microenvironment and T-cell homeostasis require Smad4-mediated signaling in thymic epithelial cells

Signals mediated by the transforming growth factor-beta superfamily of growth factors have been implicated in thymic epithelial cell (TEC) differentiation, homeostasis, and function, but a direct reliance on these signals has not been established. Here we demonstrate that a block in canonical transforming growth factor-beta signaling by the loss of Smad4 expression in TECs leads to qualitative changes in TEC function and a progressively disorganized thymic microenvironment. Moreover, the number of thymus resident early T-lineage progenitors is severely reduced in the absence of Smad4 expression in TECs and directly correlates with extensive thymic and peripheral lymphopenia. Our observations hence place Smad4 within the signaling events in TECs that determine total thymus cellularity by controlling the number of early T-lineage progenitors.

Using hippocampal microRNA expression differences between mouse inbred strains to characterise miRNA function

Micro-RNAs (miRNAs) are short, single-stranded, noncoding RNAs that are involved in the regulation of protein-coding genes at the level of messenger RNA (mRNA). They are involved in the regulation of numerous traits, including developmental timing, apoptosis, immune function, and neuronal development. To better understand how the expression of the miRNAs themselves is regulated, we looked for miRNA expression differences among four mouse inbred strains, A/J, BALB/cJ, C57BL/6J, and DBA/2J, in one tissue, the hippocampus. A total of 166 miRNA RT-PCR assays were used to screen RNA pools for each strain. Twenty miRNA species that were markedly different between strains were further investigated using eight individual samples per strain, and 11 miRNAs showed significant differences across strains (p < 0.05). This is the first observation of miRNA expression differences across inbred mice strains. We conducted an in silico correlation analysis of the expression of these differentially expressed miRNAs with phenotype data and mRNA expression to better characterise the effects of these miRNAs on both phenotype and the regulation of mRNA expression. This approach has allowed us to nominate miRNAs that have potential roles in anxiety, exploration, and learning and memory.

TGF-beta signaling in thymic epithelial cells regulates thymic involution and postirradiation reconstitution

The thymus constitutes the primary lymphoid organ responsible for the generation of naive T cells. Its stromal compartment is largely composed of a scaffold of different subsets of epithelial cells that provide soluble and membrane-bound molecules essential for thymocyte maturation and selection. With senescence, a steady decline in the thymic output of T cells has been observed. Numeric and qualitative changes in the stromal compartment of the thymus resulting in reduced thymopoietic capacity have been suggested to account for this physiologic process. The precise cellular and molecular mechanisms underlying thymic senescence are, however, only incompletely understood. Here, we demonstrate that TGF-beta signaling in thymic epithelial cells exerts a direct influence on the cell's capacity to support thymopoiesis in the aged mouse as the physiologic process of thymic senescence is mitigated in mice deficient for the expression of TGF-beta RII on thymic epithelial cells. Moreover, TGF-beta signaling in these stromal cells transiently hinders the early phase of thymic reconstitution after myeloablative conditioning and hematopoietic stem cell transplantation. Hence, inhibition of TGF-beta signaling decelerates the process of age-related thymic involution and may hasten the reconstitution of regular thymopoiesis after hematopoietic stem cell transplantation.

Phenotype of genetically regulated thymic involution in young BXD RI strains of mice

Age-related thymic involution is a multifactorial process related to age-related changes in intrathymic T-cell development and cytokines. In contrast, early thymic involution, because of genetic differences that cause rapid or slow thymic involution at younger age, is less well characterized. Here, we analysed three representative rapid-involuting strains of mice, BXD 8, 18 and 32, compared with three representative slow-involuting strains, BXD 9, 19 and 29, all at 2 months of age. In rapid-involuting strains compared with slow involution strains, thymocyte production, as indicated by CD4+ and CD8+ T-cell receptor recombination excision circle (TREC), were decreased. Rapid-involution strains of mice exhibited a developmental block at the DN1 to DN2 and CD4-CD8- (DN) to CD4+CD8+ (double positive, DP) transition stages. There was also increased susceptibility to H2O2-induced apoptosis, decreased thymic expression of IL-7, decreased expression of an IL-7 downstream anti-apoptosis gene, Bcl-2, and increased expression of a pro-apoptotic gene, Bad. In contrast, IL-7R expression was higher on DN thymocytes of rapid-involution strains. The increased expression of IL-7R was associated with an increased thymocyte proliferation in response to anti-CD3 + IL-7 or anti-CD3 + IL-12 + IL-7. These findings indicate that, even at young age, genetic differences of IL-7/IL-7R regulation pathway in BXD strains of mice can lead to characteristic phenotypic changes that have been previously associated with age-related thymic involution.

Sex-specific regulation of aging and apoptosis

Genetic analysis of Drosophila, mice and humans indicates that gene alleles, mutations and transgenes that affect life span tend to do so differently depending on the sex of the organism. The likely reason for this is that the sexes are different genotypes (e.g., X/X vs. X/Y) and face quite different environments: e.g., to reproduce, males have to mate with females while females have to mate with males. Genes are subject to different genetic interactions and different gene-by-environment effects in male vs. female. The consequence is that through evolution certain genes are differently selected and optimized for each sex. Both the mitochondrial genome and the X chromosome are asymmetrically inherited in Drosophila and mammals; through evolution these genes spend relatively more time under selection in females and are therefore expected to be better optimized for function in the female than in the male. Consistent with this the Drosophila X chromosome has been found to be a hotspot for sexually antagonistic fitness variation. Old Drosophila and old mammals exhibit apoptosis-an observation consistent with the idea that the mitochondria are less functional during aging due to maternal-only inheritance. One feature of aging that is common to Drosophila and mammals is that females tend to live longer than males, and this may be due in part to sub-optimal mitochondrial function in males. The data support the conclusion that a significant part of the aging phenotype is due to antagonistic pleiotropy of gene function between the sexes. Liberal application of Occam's razor yields a molecular model for the co-regulation of sex, apoptosis and life span based on the on/off status of a single gene: Sxl in Drosophila melanogaster and Xist in humans. Aging may simply represent an ancient and conserved mechanism by which genes re-assort.

Age-associated changes in CD90 expression on thymocytes and in TCR-dependent stages of thymocyte maturation in male rats

To elucidate the effects of ageing on T-cell-maturation, in 3- and 18-month-old rats, we analysed the expression of: (i) CD4/CD8/TCRalphabeta and (ii) Thy-1, which is supposed to be a regulator of TCRalphabeta signalling, and thereby the thymocyte selection thresholds. Since an essential role for TCRalphabeta signalling in the development of CD4+25+T(reg)-cells was suggested, the frequency of these cells was also quantified. We demonstrated that, as for mice, early thymocyte differentiational steps within the CD4-8- double negative (DN) developmental stage are age-sensitive. Furthermore, we revealed that TCRalphabeta-dependent stages of T-cell development are affected by ageing, most likely due to an impaired expression of Thy-1 on TCRalphabeta(low) thymocytes entering selection processes. The diminished frequency of the post-selection CD4+8+ double positive (DP) cells in aged rats, together with an overrepresentation of mature single positive (SP) cells, most probably suggests more efficient differentiational transition from the DP TCRalphabeta(high) to the SP TCRalphabeta(high) developmental stage, which is followed by an increase in pre-migration proliferation of the mature SP cells. Moreover, the study indicated impaired intrathymic generation of CD4+25+T(reg)-cells in aged rats, thus providing a possible explanation for the increased frequency of autoimmune diseases in ageing.