Apparent induction of partial thymic regeneration in a normal human subject: a case report

Epigenetic clock: A promising biomarker and practical tool in aging

As a complicated process, aging is characterized by various changes at the cellular, subcellular and nuclear levels, one of which is epigenetic aging. With increasing awareness of the critical role that epigenetic alternations play in aging, DNA methylation patterns have been employed as a measure of biological age, currently referred to as the epigenetic clock. This review provides a comprehensive overview of the epigenetic clock as a biomarker of aging and a useful tool to manage healthy aging. In this burgeoning scientific field, various kinds of epigenetic clocks continue to emerge, including Horvath's clock, Hannum's clock, DNA PhenoAge, and DNA GrimAge. We hereby present the most classic epigenetic clocks, as well as their differences. Correlations of epigenetic age with morbidity, mortality and other factors suggest the potential of epigenetic clocks for risk prediction and identification in the context of aging. In particular, we summarize studies on promising age-reversing interventions, with epigenetic clocks employed as a practical tool in the efficacy evaluation. We also discuss how the lack of higher-quality information poses a major challenge, and offer some suggestions to address existing obstacles. Hopefully, our review will help provide an appropriate understanding of the epigenetic clocks, thereby enabling novel insights into the aging process and how it can be manipulated to promote healthy aging.

Reversal of epigenetic aging and immunosenescent trends in humans

Epigenetic “clocks” can now surpass chronological age in accuracy for estimating biological age. Here, we use four such age estimators to show that epigenetic aging can be reversed in humans. Using a protocol intended to regenerate the thymus, we observed protective immunological changes, improved risk indices for many age‐related diseases, and a mean epigenetic age approximately 1.5 years less than baseline after 1 year of treatment (−2.5‐year change compared to no treatment at the end of the study). The rate of epigenetic aging reversal relative to chronological age accelerated from −1.6 year/year from 0–9 month to −6.5 year/year from 9–12 month. The GrimAge predictor of human morbidity and mortality showed a 2‐year decrease in epigenetic vs. chronological age that persisted six months after discontinuing treatment. This is to our knowledge the first report of an increase, based on an epigenetic age estimator, in predicted human lifespan by means of a currently accessible aging intervention.

Aging of lymphoid organs: Can photobiomodulation reverse age-associated thymic involution via stimulation of extrapineal melatonin synthesis and bone marrow stem cells?

Thymic atrophy and the subsequent reduction in T-cell production are the most noticeable age-related changes affecting lymphoid organs in the immune system. In fact, thymic involution has been described as "programmed aging." New therapeutic approaches, such as photobiomodulation (PBM), may reduce or reverse these changes. PBM (also known as low-level laser therapy) involves the delivery of non-thermal levels of red or near-infrared light that are absorbed by mitochondrial chromophores, in order to prevent tissue death and stimulate healing and regeneration. PBM may reverse or prevent thymic involution due to its ability to induce extrapineal melatonin biosynthesis via cyclic adenosine monophosphate (AMP) or NF-kB activation, or alternatively by stimulating bone marrow stem cells that can regenerate the thymus. This perspective puts forward a hypothesis that PBM can alter thymic involution, improve immune functioning in aged people and even extend lifespan.

Towards segmentation of the thymus in fat and water parametric MR images

The thymus, an organ responsible for the development, selection, and maintenance of the peripheral T-cell population, is an important regulator of the immune system. Despite its physiological significance, it has received little attention in the medical image analysis literature. In practice, the anatomical location and variable shape of this gland pose challenges both in the image acquisition and analysis processes. We present an automated method for segmenting the thymus from water and fat parametric MR images that permits further analysis of volumetrics and tissue characterization. We compute fat ratio and water ratio parametric images and introduce the use of a stochastic edge detector that is embedded in a geometric variational segmentation model. Validation experiments of the proposed algorithm against manual delineations of the thymus indicate the applicability of our approach.

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.

Regulatory CD4+ CD25+ T cells prevent thymic dysfunction in experimental chronic colitis

Chronic colitis in T-cell deficient Tg epsilon26 mice develops due to a dysfunction of the thymus which generates colitogenic T cells after bone marrow (BM) transplantation. Regulatory CD4+ CD25+ T cells have been shown to prevent colitis in this model by normalizing the peripheral T-cell pool. We tested the hypothesis that T-cell normalization takes place in the thymus. Tg epsilon26 mice were transplanted with BM (BM-->Tg epsilon26 mice) and consequently received either CD4+ CD25+ or CD4+ CD25- cells from syngenic wild type mice. Furthermore, untransplanted Tg epsilon26 mice received CD4+ CD25+ or CD4+ CD25- cells or complete mesenteric lymph node cells. Transfer of regulatory. CD4+ CD25+ cells normalized the total number of thymocytes and the percentage and number of double positive CD4+ CD8+ cells in transplanted mice while percentage of single positive CD4+ and CD8+ thymocytes in BM-->Tg epsilon26 mice was reduced upon CD4+ CD25+ transfer. Timing of CD4+ CD25+ cell injection was important as transfer later than 7 days after BM transplantation failed to prevent abnormal thymic T-cell distribution in BM-->Tg epsilon26 mice. Isolated CD4+ CD25+ cell transfer without preceding BM transplantation failed to reconstitute thymic architecture. Differences of thymic cell composition could not be exclusively explained by presence or absence of colitis, respectively, because 19 days after BM transplantation when both groups showed no histological signs of colitis, animals transferred with CD4+ CD25+ T cells had a significantly higher percentage and number of CD4+ CD25+ thymocytes and CD4+ Foxp3+ cells than BM-->Tg epsilon26 mice. In conclusion, early CD4+ CD25+ cotransfer prevents thymic dysfunction which underlies immune-mediated bowel inflammation in BM-->Tg epsilon26 mice.

Thymic hyperplasia in a child treated with growth hormone

OBJECTIVE

To report a case of thymic hyperplasia diagnosed three months after initiation of recombinant human growth hormone (GH) for the treatment of GH deficiency.

DESIGN

Retrospective chart review was conducted to evaluate the temporal relationship between treatment with GH and thymic enlargement in a 7-year-old girl who had a history of embryonal rhabdomyosarcoma of the nasopharynx diagnosed at the age of 3 years.

RESULTS

The diagnosis of GH deficiency was made based on clinical and auxological criteria, an insufficient response to clonidine-arginine stimulation testing (peak GH level of 4.8 microg/L) and low insulin-like growth factor 1 (IGF-1) level (30 ng/mL, -2.7 SD). The patient was started on GH at a dose of 0.3mg/kg/week. At the initiation of treatment with GH, the baseline growth velocity was 0.8 cm/year (-6.0 SD) and height was 112.5 cm (-1.7 SD). After three months of treatment with GH, her height increased by 4.2 cm (from -1.7 to -1.2 SD), and the IGF-1 level from -2.7 SD to -1.1 SD. A chest CT performed at that time for recurrence surveillance showed 89% increase in thymic volume relative to previous scan before treatment with growth hormone. A thoracoscopic biopsy of the thymus was performed and revealed hyperplasia with normal thymic architecture without evidence of malignancy.

CONCLUSIONS

The timing of the development of thymic hyperplasia, along with data from in vitro and in vivo animal studies showing that GH and IGF-1 can directly stimulate growth of the thymus, suggests that GH contributed to the development of thymic hyperplasia in this patient.

Monitoring insulin-like growth factors in HIV infection and AIDS

There is a close association between the growth hormone (GH)-insulin-like growth factor I (IGF-I) axis, infection and immunity. Infection with the human immunodeficiency virus (HIV) is often associated with a decrease of the concentrations of IGF-I, IGF-II, IGF-binding protein 3 (IGFBP-3) and an increase of IGFBP-1 and -2. Many investigators have studied the relationship between the GH-IGF-I system and some of the most common characteristics of disease progression, such as decreased CD4 cell counts, weight loss and fat redistribution. Although conditions for restoration of thymic function and lymphopoiesis with GH or IGF-I are still not well defined, many studies led to the development of clinical trials on the therapeutic use of GH, IGF-I and GHRH for the treatment of weight loss or fat redistribution, two problems which persist despite the introduction of highly active antiretroviral therapy. Monitoring IGF-I concentrations during treatment with GH and GHRH is likely to become an essential component of their therapeutic use. IGF-I levels are the first indicator of treatment efficacy and can be used to monitor compliance. High levels of IGF-I are a warning sign for the increased risk of potential adverse effects, such as acromegalic-like symptoms or malignancy. This could lead to a reduction of the therapeutic dose or the temporary interruption of treatment until IGF levels reach a safe range. IGF-I levels are also likely to increase with other hormones used in HIV patients, such as erythropoietin for the treatment of anemia or anabolic androgens in HIV-infected women.