Thymic involution

Though the thymus is fully developed before birth,

Parham, P. 2005. The immune system: Second edition Garland Science.

newborns have an essentially empty peripheral immune compartment immediately after birth.

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Hence, T lymphocytes are not present in the peripheral lymphoid tissues, where naïve, mature lymphocytes are stimulated to respond to pathogens.

In order to populate the peripheral system, the thymus increases in size and upregulates its function during the early neonatal period.

Though some sources{{which|date=May 2020}} continue to cite puberty as the time of onset, studies have shown thymic involution to start much earlier.

The crucial distinction came from the observation that the thymus consists of two main components: the true thymic epithelial space (TES) and the perivascular space (PVS).

Thymopoiesis, or T-cell maturation, only occurs in the former. In humans, the TES starts decreasing from the first year of life at a rate of 3% until middle age (35–45 years of age), whereupon it decreases at a rate of 1% until death.

Hypothetically, the thymus should stop functioning at around 105 years of age;

{{cite journal

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but, studies with bone marrow transplant patients have shown that the thymi

of the majority of patients over forty were unable to build a naïve T cell compartment.

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With both qualitative and quantitative changes to thymus production occurring as age increases, thymic involution corresponds with the progressive deterioration of the stroma of the thymus and a significant loss of thymic epithelial cells (TECs). Thymic epithelial cells aid in Thymopoiesis and the development of new T-cells. {{cite journal | pmc=3375084 | date=2012 | last1=Gui | first1=J. | last2=Mustachio | first2=L. M. | last3=Su | first3=D. M. | last4=Craig | first4=R. W. | title=Thymus Size and Age-related Thymic Involution: Early Programming, Sexual Dimorphism, Progenitors and Stroma | journal=Aging and Disease | volume=3 | issue=3 | pages=280–290 | pmid=22724086 }}

The ability of the immune system to mount a strong protective response depends on the receptor diversity of naive T cells (TCR). Thymic involution results in a decreased output of naïve T lymphocytes – mature T cells that are tolerant to self antigens, responsive to foreign antigens, but have not yet been stimulated by a foreign substance. In adults, naïve T-cells are hypothesized to be primarily maintained through homeostatic proliferation, or cell division of existing naïve T cells. Though homeostatic proliferation helps sustain TCR even with minimal to nearly absent thymic activity, it does not increase the receptor diversity.

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For yet unknown reasons, TCR diversity drops drastically around age 65.

Loss of thymic function and TCR diversity is thought to contribute to weaker immune function in the elderly, including increasing instances of diseases such as cancers, autoimmunity, and opportunistic infections.

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There is growing evidence that thymic involution is plastic and can be therapeutically halted or reversed in order to help boost the immune system. Under certain circumstances, the thymus has been shown to undergo acute thymic involution (alternatively called transient involution).

For example, transient involution has been induced in humans and other animals by stresses

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such as infections,

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{{cite journal

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|author4=Silva-Barbosa SD

|year=2007

|title=The thymus is a common target in malnutrition and infection

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pregnancy,

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and malnutrition.

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The thymus has also been shown to decrease during hibernation and, in frogs, change in size depending on the season, growing smaller in the winter.

Wytycz, B., Mica, J., Jozkowir, A. & Bigaj J. 1996. Letters: Plasticity of thymuses of ectothermic vertebrates. Immunology Today (Comment). 442: No.9.

Studies on acute thymic involution may help in developing treatments for patients, who for example are unable to restore immune function after chemotherapy, ionizing radiation, or infections like HIV. Research has shown the rate of thymus involution to reduce when, for men the testes, or for women the ovaries, were removed; demonstrating that sex hormones, and especially testosterone, have a marked influence on the involution process. However, the manner in which the sex hormones moderate this process is not yet fully understood. In other research the results of the Greg Fahy TRIIM trial showed clinically significant reversal of thymus involution after the administration of human growth hormone (HGH), Dehydroepiandrosterone (DHEA) and metformin.{{Cite web|title=Reversing Thymic Involution – Intervene Immune|url=http://interveneimmune.com/?page_id=1131|access-date=2020-12-31|language=en-US}} The two results could mean that HGH and mTOR inhibition in autophagy reverses thymus involution with testosterone advancing thymus involution.{{Cite journal|last1=Sutherland|first1=Jayne S.|last2=Goldberg|first2=Gabrielle L.|last3=Hammett|first3=Maree V.|last4=Uldrich|first4=Adam P.|last5=Berzins|first5=Stuart P.|last6=Heng|first6=Tracy S.|last7=Blazar|first7=Bruce R.|last8=Millar|first8=Jeremy L.|last9=Malin|first9=Mark A.|last10=Chidgey|first10=Ann P.|last11=Boyd|first11=Richard L.|date=2005-08-15|title=Activation of Thymic Regeneration in Mice and Humans following Androgen Blockade|url=https://www.jimmunol.org/content/175/4/2741|journal=The Journal of Immunology|language=en|volume=175|issue=4|pages=2741–2753|doi=10.4049/jimmunol.175.4.2741|issn=0022-1767|pmid=16081852|doi-access=free}}

Thymic involution remains an evolutionary mystery since it occurs in most vertebrates despite its negative effects.

Since it is not induced by senescence, many scientists have hypothesized that there may have been evolutionary pressures for the organ to involute. A few hypotheses are as follows:

• Developing T cells that interact strongly with antigen being presented within the thymus are induced to undergo programmed cell death. The intended effect is deletion of self-reactive T cells. This works well when the antigen being presented within the thymus is truly of self origin, but antigen from pathogenic microbes that happens to infiltrate the thymus has the potential to subvert the entire process. Rather than deleting T cells that would cause autoimmunity, T cells capable of eliminating the infiltrating pathogen are deleted instead. It has been proposed that one way to minimize this problem is to produce as many long-lived T cells as possible during the time of life when the thymus is most likely to be pristine, which generally would be when organisms are very young and under the protection of a functional maternal immune system.

{{cite journal

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|year=1995

|title=Microbial parasites versus developing T cells: an evolutionary arms race with implications for the timing of thymic involution and HIV pathenogenesis

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|journal=Thymus

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|issue=1

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Thus, in mice and humans, for example, the best time to have a prodigiously functional thymus is prior to birth.

:In turn, it is well known from Williams'

{{cite journal

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theory of the evolution of senescence that strong selection for enhanced early function readily accommodates, through antagonistic pleiotropy, deleterious later occurring effects, thus potentially accounting for the especially early demise of the thymus.

• The disposable soma hypothesis and life history hypothesis say similarly that tradeoffs are involved in thymic involution. Since the immune system must compete with other bodily systems, notably reproduction, for limited physiological resources, the body must invest in the immune system differentially at different stages of life. There is high immunological investment in youth since immunological memory is low.
• There are also hypotheses that suggest that thymic involution is directly adaptive. For example, some hypotheses have proposed that thymic involution may help in avoidance of autoimmunity or other dangers,

{{cite journal

|author=Aronson M

|year=1991

|title=Hypothesis: involution of the thymus with aging–programmed and beneficial

|url=

|journal=Thymus

|volume=18

|issue=1

|pages=7–13

|pmid=1926291}}

prevention of infection, and production of an optimal repertoire of T-cells.

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• Zinc deficiency may also play a role.

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{{spoken Wikipedia|thymic involution.ogg|date= May 5th, 2020}}

category:thymus

category:ageing processes