Brandon Barth Nydick

The Role of Thymic Involution in Aging: Causes, Effects, and Potential Solutions

March 24, 2021

Abstract:

Despite being a relatively unknown and little-studied organ, the thymus constitutes a central mediator of the aging process. Found in nearly all vertebrates, the thymus serves as a key organ of the immune system, and it plays an essential role in the development and selection of the T cells, a white blood cell that helps the body recognize and destroy pathogens. Nevertheless, as vertebrates age, the thymus progressively shrinks in a process known as thymic involution, and it starts to lose its functional ability. Thymic involution leads to immunosenescence (the deterioration of the immune system) and inflammaging (chronic inflammation linked to aging). Immunosenescence and inflammaging constitute two key drivers of aging and its associated diseases; thus, if doctors could rejuvenate the thymus, humans may be able to live significantly healthier and longer lives. This paper seeks to provide a review of the most noteworthy and recent literature on the topic with a particular focus on the mechanisms of involution, its consequences, various types of rejuvenation strategies, and emerging areas of research. Ultimately, this essay strives to persuade the reader of the centrality of the thymus and immune system to aging, inspiring him to dive deeper into this fascinating subject.

Introduction:

The thymus, located in the upper section of the chest in humans, emerged about 500 million years ago and represents one of the major evolutionary developments that distinguish vertebrates from invertebrates (Swann et. al., 2020, p. 1). The organ’s key role in the immune system was first experimentally verified in 1961 when scientists observed that mice whose thymus had been removed right after birth had poor lymphatic tissue and significantly greater susceptibility to infections (Rezzani et al., 2014, p. 314). Now, researchers know that the thymus plays a vital role in the maintenance of the adaptive immune system since it is responsible for the maturation and selection of the T cells, which can recognize foreign antigens, destroy infected cells, aid B cells, and regulate the lymphatic response. In the process of thymopoiesis – the development of T cells in the thymus – the first mechanism that occurs is the negative selection of immature immune cells known as thymocytes, where the majority of the self-reactive T cells, which can mistake a body’s natural components for a foreign invader and wrongfully attack them, are destroyed via apoptosis (Thomas et al., 2020, p. 2). Second, regulatory T (Treg) cells are generated to suppress the potential self-reactivity of any T cells that pass through the first selection stage in error (Thomas et al., 2020, p. 3).

Beginning about a year after birth and accelerating during puberty, however, the thymus atrophies, and adipose tissue progressively replaces the thymic epithelium (Palmer et al., 2018, p. 1883). This involution leads to the paradoxically tightly connected processes of immunosenescence, a term denoting the deterioration of the body’s immune system to the point of insufficiency, and inflammaging, which refers to the systemic low-grade inflammation that is caused in part by the over-activation of the immune system and worsens with age. Since these two processes are linked to numerous age-related diseases, such as cancer, metabolic disorders, and cardiovascular disease (Thomas et al., 2020, pp. 2-3), it follows that any way to reverse them or at least halt their progression could significantly increase lifespan. Rejuvenating the thymus could prove a solution to these systemic problems and thus merits great focus. This paper will further analyze the mechanisms of involution and evaluate the current regeneration strategies, including cytokine therapies, hormone adjustments, ATP administration, and the upregulation of the transcription factor FOXN1, while also suggesting promising new areas of research, namely the thymus-pineal gland axis, bioengineering approach, and potential use of epigenetic reprogramming. Although it is uncertain which of these methods (if any) will prove successful, it is clear that thymic rejuvenation is a necessary condition for drastic life extension and thus mandates great focus from the research community.

Discussion:

Although the specific mechanisms of thymic involution are still unverified, two explanations dominate the scholarship on the reasons behind the age-linked decline of thymopoiesis, the process by which thymocytes transform into mature T cells in the thymus. The first explanation links this reduction to defective hematopoietic stems cells (HSCs), a term denoting the stem cells that can develop into all kinds of blood cells, as studies have shown that bone marrow produces less numbers of HSCs as organism grow older (Thomas et al., 2020, p. 3). Thus, fewer progenitors of T cells enter the thymus, which leads to a decline in the number of produced T cells. The second thesis, which has the greatest support from recent research, argues that age-related changes to the thymic stroma are chiefly responsible for the reduction in thymopoiesis. For reference, the thymic stroma refers to the part of the thymus lined with connective tissues and blood vessels. In particular, studies have shown that the decline in the expression of the gene for FOXN1 may be an important cause of involution (Thomas et al., 2020, p. 3). Among other functions, FOXN1 serves as a master regulator of thymopoiesis, functioning as a key transcription factor for the thymic epithelial cells (TECs), which constitute the majority of cells in the thymic stroma and play an essential role in T cell development (Wang et al., 2020, p. 3). FOXN1 controls the evolution, differentiation, and function of TECs. Without FOXN1, studies have shown organisms experiencing significant impairment of T cell development and homeostasis. The genes encoding the chemokines Cc125 and Cxcl12, which play a role in the development, movement, and survival of T cells are not expressed without FOXN1. Similarly, the absence of FOXN1 can inhibit T cell production as TECs lose the ability to mature and attract progenitor cells necessary for their maturation (Wang et al., 2020, p. 3). While the reasons for the decline in FOXN1 expression remain unknown, a 2015 experiment suggests the mechanism may be epigenetic. For reference, the epigenome regulates the expression of the genetic code through a variety of means, including methylation and acetylation. After analyzing samples from human donors, the scientists observed increased CpG methylation of the regulatory segments of the FOXN1 gene in older individuals (Reis et al., 2015, p. 9). Methylation – the addition of a methyl group to the genetic code – serves as an epigenetic silencer, and the increased presence of the marks may explain the age-related decline in FOXN1 expression and thereby thymopoiesis. Other studies have linked the decline in growth hormone and IGF-1 to thymic involution, but these connections remain unclear (Thomas et al., 2020, p. 11). The sex hormones may also play a role in inducing thymic involution. Some studies have shown that androgens (male sex hormones) can cause the atrophy of the thymus while others suggest estrogens but not progesterone halt T cell maturation (Rezzani et al., 2020, pp. 3-4). Nevertheless, conflicting data exist with regards to the role of the sex hormones, and the mechanisms of these changes are not well understood. Increased research on the pathways and factors responsible for involution will be necessary to give scientists the knowledge to design more precisely targeted anti-aging drugs.

While the underlying causes of thymic involution generate significant debate, its results – immunosenescence and inflammaging – are agreed upon by most researchers. Involution leads to a significant reduction in the thymic output of mature T cells, which in turn causes a radical decline in the diversity of the T cell receptor (TCR) repertoire (Lian et al., 2020, p. 7). Because of this change, the immune system can recognize a markedly less diverse array of pathogens in the body. As thymic involution progressively increases, the secondary lymphoid organs begin to receive less and less T cells from the thymus (Salam et al., 2013). In order to maintain T cell homeostasis, fewer old naïve T cells die, and many naïve T cells are converted into memory T cells, which are unable to adapt to recognize new, different types of pathogens, further exacerbating immunosenescence (Salam et al., 2013). In addition to making the organism much more susceptible to infections, the shrinking of the TCR repertoire impairs tumor surveillance and identification, making it easier for cancer to spread uncontrollably in the body.

The immune system is less able to clear away senescent cells – cells which lose the ability to divide and undergo a phenotypic switch that can be harmful to the body. The increased proliferation of senescent cells with age leads to the development of the senescence-associated secretory phenotype (SASP), which is linked to inflammaging and tumorigenesis. Senescent cells release numerous inflammatory cytokines, such as IL-6, causing inflammation and creating a favorable cytokine environment for tumor development (Thomas et al., 2020, p. 8). IL-6 has been shown to promote tumor growth in liver, breast, and kidney cancers. In response to inflammation, cells can also release TNF alpha, which can worsen DNA damage. Furthermore, the macrophage migration inhibitory factor, yet another inflammatory cytokine, can reduce the benefit of p53, an essential tumor suppressor (Thomas et al., 2020, p. 8). A 2018 paper by Palmer et al. supported this argument that a reduction in T cell production is strongly linked to increasing cancer and infectious disease incidence with a two-parameter model (Palmer et al., 2018, p. 1883). The aging of the thymus not only leads to a reduction in naïve T cell output but also catalyzes an impairment of central tolerance – the ability of the Treg cells to identify self-reactive T cells and induce their death (Thomas et al., 2020, p. 2). The age-associated decline in negative selection of the thymocytes leads to an increase in the number of autoreactive T cells in the body, which can attack the body’s own cells, tissues, and organs (Csaba, 2016, p. 147). This type of auto-immune response can cause inflammaging and damage to important structures. In the pancreas, these self-reactive T cells have a deleterious effect on the beta cells, causing them to no longer possess the capacity to produce insulin and potentially contributing to the development of Type 1 Diabetes (Rezzani et al., 2014, p. 343). Moreover, inflammaging has been linked to numerous age-related disorders – metabolic diseases, such as Type-2 Diabetes, cardiovascular issues, like atherosclerosis, and neurodegenerative diseases, such as Alzheimer’s while immunosenescence is implicated in cancer and greater vulnerability to infections (Thomas et al., 2020, pp. 2-3). Given its linkage to so many age-related diseases, thymic involution may be viewed as an important upstream cause of aging on the systems level.

Source: (Thomas et al., 2020, p. 2).

The earliest work in the field largely focused on sex hormone ablation through castration as the primary method of rejuvenation. The role of the androgens, the male sex hormones, has received special attention given the acceleration of thymic involution in males is greater than that of females following puberty (Rezzani et al., 2014, p. 339). While some experiments have demonstrated that castration can lead to an increase in the size of the thymus, the data is inconclusive, and it unclear whether the ablation of the sex hormones can have a lasting, meaningful benefit on thymopoiesis (Bredenkamp et al., 2014, p. 1627). The obvious trade-offs of castration make it an undesirable method for the vast majority of the population. Scientists have also known for a while that certain lifestyle and exercise habits seem to promote thymic function (Lian et al., 2020, p. 11). They have found that maintaining a health BMI, refraining from smoking, and engaging in frequent physical activity can all help slow the reduction in thymopoiesis and limit inflammaging (Thomas et al., 2020, p. 12). Physical activity has also been shown to reduce the speed of thymic atrophy by reducing the amount of Il-6 and increasing the amounts of IL-7 and Il-5 in the thymus (Lian et al., 2020, p. 11).

While the two aforementioned rejuvenation strategies should not inspire much optimism about full-scale thymic regeneration, newer ideas, namely methods to induce the upregulation of FOXN1, have already shown hints of promise in animal models. As previously discussed, FOXN1 plays a vital role in the development and differentiation of TECS, which in turn aid the T cell maturation process. FOXN1 promotes the vascularization of the thymus (Rezzani et al., 2014, p. 351). In mice, the expression of FOXN1 has been linked to the production of the vascular endothelial growth factor (VEGF), which stimulates the development of new blood vessels (Rezzani et al., 2014, p. 351). With age, however, FOXN1 expression declines, potentially representing the key factor in causing the atrophy of the thymus (Reis et al., 2015, p. 9). Scientists have thus tried to reverse this age-related reduction with the hope of regenerating the thymus. In 2014, Bredenkamp and his team demonstrated that they could rejuvenate the thymus to its youthful state through the upregulation of a single transcription factor, FOXN1 (Bredenkamp et al., 2014, p. 1627). The increase in FOXN1 led to greater proliferation of the progenitors of TECs and stimulated their differentiation into mature TECs, which play an essential role in thymopoiesis (Bredenkamp et al., 2014, p. 1631). In 2019, Su et al. demonstrated a supporting result: his team injected FOXN1-cDNA through a plasmid vector into the thymus of old mice, resulting in a significant (but not complete) increase in the number of thymocytes, size of the organ, and activity of CD4+ T cells in the periphery (Lian et al., 2020, p. 11). Despite the promising nature of FOXN1 interventions, more research is needed to understand the mechanisms of these changes, its applicability to humans, and whether its administration could ever lead to complete thymic rejuvenation.

Growth hormone administration could also prove to be a powerful rejuvenation method, especially if the results from Gregory Fahy’s preliminary study can be replicated on a larger scale. Scientists are quite sure that growth hormone declines with age along with IGF-1, and some research has linked its reduction to thymic involution (Fahy et al., 2019, pp. 7-8). The presence of growth hormone helps stimulate T cell differentiation (Hirokawa et al., 2015, p. 55). Furthermore, the administration of growth hormone into mice has been linked with some level of thymic rejuvenation and improved activation of the immune system. However, a dangerous trade-off exists since increasing levels of growth hormone have been linked to shorter life expectancy as it can augment an organism’s risk of cancer and diabetes (Hirokawa et al., 2015, p. 55). In a 2019 study, Gregory Fahy administered a cocktail of three drugs (growth hormone, metformin, and DHEA) to nine humans to see if he could promote thymic rejuvenation (Fahy, 2019, p. 8). The results were quite astounding. Fahy not only stimulated the regeneration of the thymus but also the patients in the study shed an average of 2.5 years of biological age according to results from four epigenetic clocks (Fahy, 2019, p. 6). While the sample size was quite small, and there was no control, the preliminary results are very promising and could lead to greater focus on the thymus as a central target for life-extension therapies. 

Interleukin-7 therapies may also prove helpful in maintaining immune system health but also pose risks and dangerous trade-offs (Nguyen, 2017, p. 7). Interleukin-7 is a cytokine that plays an important role in the development of the T and B cells, and studies have shown that mice without IL-7 possess significantly compromised immune systems: many have a drastically reduced population of T cells, exhibit the absence of certain B cells, and almost completely lack lymph nodes (Seddon, 2015, p. 337). By activating BCL-2 (a gene that can inhibit apoptosis), IL-7 can promote the survival of many types of immune cells (Seddon, 2015, p. 337). IL-7 also plays a particularly important role in stimulating T cell maturation in the thymus (Nguyen, 2017, p. 5). It is unclear, however, how to best manipulate IL-7 levels in the body to maximize lifespan. In the Leiden Longevity Study, scientists found that the expression of the IL-7 receptor decreased with age, yet the results also suggested a link between lower IL-7R levels and a decreased risk of mortality over a 10-year period (Nguyen, 2017, pp. 6-7) In a clinical trial for adults infected with HIV, treatment with IL-7 was shown to improve thymic function, increasing the population and diversity of T cells (Nguyen, 2017, p. 10). On the other hand, low levels of IL-7 have been linked to reduced activation of mTOR, which many studies have tied to increased incidence of age-related diseases (Nguyen, 2017, p. 8). This seemingly conflicting data makes IL-7 interventions a difficult yet perhaps promising area of research. Perhaps scientists in the future could design nanorobots that could monitor our IL-7 levels in real-time and constantly adjust them depending on other variables in order to provide the benefit of higher amounts of IL-7 when necessary and reduce them when possible to limit the disruption to other functions and pathways.

A particularly interesting area of emerging research is the potential link between the thymus and pineal gland in the regulation of the immunoendocrine system and the aging process as a whole. The pineal gland is a small organ located in the brain that releases the hormone melatonin, which helps promote a healthy sleep-wake cycle but declines with age. Growing evidence suggests that the immune system works with both the nervous and endocrine systems, constituting a functional network (Rezzani et al., 2020, p. 2). New studies have also suggested a connection between the co-involution of the thymus and pineal gland. In mice, the removal of the pineal gland catalyzes the total destruction of the thymus, which significantly compromises the immune system (Rezzani et al., 2020, p. 2). When scientists employed pharmacological interventions to block the pineal gland, they observed a decrease in thymulin, a hormone produced in the thymus that plays a role in modulating the immune system (Rezzani et al., 2020, p. 24). Moreover, research has shown that melatonin helps regulate thymic function and block involution. Scientists postulate that the mechanism for this result is melatonin’s role in modulating the secretion of the Thyrotropin Releasing Hormone (TRH) in the hypothalamus (Rezzani et al., 2020, p. 23). The infusion of melatonin into athymic mice has a significant restorative effect on the immune system (Rezzani et al., 2020, p. 23). Both TECs and thymocytes have melatonin receptors, suggesting the hormone’s potential role in T cell development (Csaba, 2016, p. 144). Perhaps the decline in melatonin with age represents an underappreciated yet central component of thymic involution. If this is true, melatonin administration could be a potential therapeutic to spur thymic regeneration. Increasing research into the intersections of the immune, neural, and endocrine systems could radically expand the range of potential solutions to the problem of immunosenescence.

Another group of researchers argue that the thymus possesses a natural regenerative capacity and have been focusing on developing treatments to stimulate this response (Richards, 2020). They came to this insight when they observed that radiation damage to the thymus resulted in a change in the way T cells died in mice. Instead of apoptosis occurring, the cells died in a highly inflammatory form of cell death known as pyroptosis. When these pyroptotic cells burst, they release a host of molecules into their environment, which function as damage signals to other cells. The researchers determined that one of these secreted molecules, ATP, plays a key role in stimulating the nearby cells to stimulate the rejuvenation of the thymus. In an experiment, they exposed mice to total body irradiation and found that the subset of mice dosed with extra ATP experienced markedly improved thymic regeneration compared to the other organisms. Future research will need to focus on translating this experimental insight into a therapeutic given ATP cannot be administered clinically and determining its potential applicability to humans through models (Richards, 2020).

The two most futuristic regeneration strategies are the bioengineering and epigenetic reprogramming approaches, both facing significant obstacles but possessing the potential to deliver drastic improvements in thymic rejuvenation. Immune incompatibility constitutes a significant barrier to the bioengineering approach (Tajima, 2016, p. 135). The special three-dimensional architecture of the thymic stroma is vital to ensuring the proper function of the TECs but extremely hard to replicate in an artificial organ system (Tajima, 2016, p. 131). Moreover, a very small number of TECs can be harvested from the adult thymus, and these cells are very difficult to expand for scientists (Tajima, 2016, p. 132). On the epigenetic side, scientists have showed that the induction of the Yamanaka factors (O, S, K, and M) can transform differentiated somatic cells into pluripotent stem cells. David Sinclair used three of these transcriptional factors (O, S, and K) to restore vision in mice (Lu et al., 2020, p. 124). While the eye is a very different organ than the thymus, especially given the latter does not contain stem cells, the theoretical idea of injecting the Yamanaka factors into the thymus is very exciting given the organ’s central role in the regulation of the aging process. An indirect approach could be the induction of the Yamanaka factors into the hematopoietic stem cells, the source of early T cell progenitors. This method could have a benefit on thymopoiesis, but it unclear whether it could lead to actual, lasting reversal of thymic involution. More research is needed on both the epigenetic and bioengineering approaches to determine their potential impacts on the thymus.

Conclusion:

Despite being an undervalued organ, especially by individuals outside the scientific community, the thymus undoubtedly plays a key, system-wide role in aging through its influence on the immune system. Thymic involution is directly connected to the processes of immunosenescence and inflammaging, both of which are linked to pathogenesis of numerous age-related diseases. By adding thymic involution to the official list of the hallmarks of aging, it is this author’s view that the thymus would receive much-needed, additional research attention. Further work should focus on analyzing the relative importance of hematopoietic stem cell exhaustion, changes to the TECs, and the atrophy of the pineal gland as the mechanisms of involution. By developing a greater understanding of the upstream causes of thymic involution, scientists can better allocate their resources to the rejuvenation strategies most likely to stimulate large-scale thymic regeneration, which could lead to drastic improvements in human health and extensions in lifespan.

References

Bredenkamp, N., Nowell, C. S., & Blackburn, C. C. (2014). Regeneration of the aged thymus by a single transcription factor. Development, 141(8), 1627-1637. https://doi.org/10.1242/dev.103614

Csaba, G. (2016). The immunoendocrine thymus as a pacemaker of lifespan. Acta Microbiologica Et Immunologica Hungarica, 63(2), 139-158. https://doi.org/10.1556/030.63.2016.2.1

Fahy, G. M., Brooke, R. T., Watson, J. P., Good, Z., Vasanawala, S. S., Maecker, H., Leipold, M. D., Lin, D. T. S., Kobor, M. S., & Horvath, S. (2019). Reversal of epigenetic aging and immunosenescent trends in humans. Aging Cell, 18(6). https://doi.org/10.1111/acel.13028

Hirokawa, K., Utsuyama, M., & Kikuchi, Y. (2015). Trade off situation between thymus and growth hormone: Age-related decline of growth hormone is a cause of thymic involution but favorable for elongation of lifespan. Biogerontology, 17(1), 55-59. https://doi.org/10.1007/s10522-015-9590-z

Lian, J., Yue, Y., Yu, W., & Zhang, Y. (2020). Immunosenescence: A key player in cancer development. Journal of Hematology and Oncology, 13(151), 1-18. https://doi.org/10.1186/s13045-020-00986-z

Lu, Y., Brommer, B., Tian, X., Krishnan, A., Meer, M., Wang, C., Vera, D. L., Zeng, Q., Yu, D., Bonkowski, M. S., Yang, J.-H., Zhou, S., Hoffmann, E. M., Karg, M. M., Schultz, M. B., Kane, A. E., Davidsohn, N., Korobkina, E., Chwalek, K., . . . Sinclair, D. A. (2020). Reprogramming to recover youthful epigenetic information and restore vision. Nature, 588(7836), 124-129. https://doi.org/10.1038/s41586-020-2975-4

Nguyen, V., Mendelsohn, A., & Larrick, J. W. (2017). Interleukin-7 and immunosenescence. Journal of Immunology Research, 1-17. https://doi.org/10.1155/2017/4807853

Palmer, S., Albergante, L., Blackburn, C. C., & Newman, T.J. (2018). Thymic involution and rising disease incidence with age. Immunology and Inflammation, 115(8), 1883-1888. https://doi.org/10.1073/pnas.1714478115

Reis, M. D. D. S., Csomos, K., Dias, L. P. B., Prodan, Z., Szerafin, T., Savino, W., & Takacs, L. (2015). Decline of FOXN1 gene expression in human thymus correlates with age: Possible epigenetic regulation. Immunity & Ageing, 12(1). https://doi.org/10.1186/s12979-015-0045-9

Rezzani, R., Franco, C., Hardeland, R., & Rodella, L. F. (2020). Thymus-Pineal gland axis: Revisiting its role in human life and ageing. International Journal of Molecular Sciences, 21(22), 8806. https://doi.org/10.3390/ijms21228806

Rezzani, R., Nardo, L., Favero, G., Peroni, M., & Rodella, L. F. (2014). Thymus and aging: Morphological, radiological, and functional overview. AGE, 36, 313-351. https://doi.org/10.1007/s11357-013-9564-5

Richards, S. (2020, December 7). Repair of key immune organ triggered by change in how immune cells die. Fred Hutch Cancer Research Center. https://www.fredhutch.org/en/news/center-news/2020/12/dudakov-ash-pyroptosis-thymus-regeneration.html

Salam, N., Rane, S., Das, R., Faulkner, M., Gund, R., Kandpal, U., Lewis, V., Mattoo, H., Prabhu, S., Ranganathan, V., Durdik, J., George, A., Rath, S., & Bal, V. (2013). T cell ageing: Effects of age on development, survival, and function. Indian Journal of Medical Research, 138(5), 595-608.

Seddon, B. (2015). Thymic IL-7 signaling goes beyond survival. Nature Immunology, 16, 337-338. https://doi.org/10.1038/ni.3128

Swann, J. B., Nusser, A., Morimoto, R., Nagakubo, D., & Boehm, T. (2020). Retracing the evolutionary emergence of thymopoiesis. Science Advances, 6(48). https://doi.org/10.1126/sciadv.abd9585

Tajima, A., Pradhan, I., Trucco, M., & Fan, Y. (2016). Restoration of thymus function with bioengineered thymus organoids. Current Stem Cell Reports, 2(2), 128-139. https://doi.org/10.1007/s40778-016-0040-x

Thomas, R., Wang, W., & Su, D.-M. (2020). Contributions of age-related thymic involution to immunosenescence and inflammaging. Immunity & Ageing, 17(2). https://doi.org/10.1186/s12979-020-0173-8

Wang, H.-X., Pan, W., Zheng, L., Zhong, X.-P., Tan, L., Liang, Z., He, J., Feng, P., Zhao, Y., & Qiu, Y.-R. (2020). Thymic epithelial cells contribute to thymopoiesis and T cell development. Frontiers in Immunology, 10(3099). https://doi.org/10.3389/fimmu.2019.03099