Models Suggest that Declining T Cell Production is the Primary Reason for Age-Related Increases in Cancer Risk

In the open access paper noted here, researchers use modeling to suggest that age-related decline of the thymus, and thus of the immune system, is more important than mutation as a determinant of cancer risk. Cancer is at root caused by mutational damage to DNA. While DNA repair and replication mechanisms are highly efficient, mutations nonetheless occur - and must occur at some rate in order for evolution to take place. It is a numbers game, in that the more time, the more cells, and the more cell activity, the greater the odds that a cancerous mutation will occur. Mutation rates are also affected by external factors such as radiation, toxic molecules in the cellular environment, and other forms of stress put upon cells. But this is just the primary cause, the trigger enables a cell to replicate without restraint.

After a mutation occurs, there are several classes of process that work to shut down or destroy potentially cancerous cells. We suffer countless potential cancers in our lives, but near all are suppressed before they start. The first line of defense is internal to cells: mechanisms such as those related to p53 that can respond to cancerous mutations and aberrant behavior by inducing immediate programmed cell death or inducing the state of cellular senescence. The latter shuts down replication, sets the cell on the path to self-destruction via apoptosis, and further issues signaling that calls in the immune system to destroy the errant cell. The immune system is the second, and perhaps more important line of defense. Immune cells of various types aggressively seek out and destroy cells that show signs of cancer or other undesirable behavior.

Unfortunately, the immune system declines in effectiveness with age. One of the reasons for this decline is a slowing of the rate at which new T cells are created. This is in part a question of the loss of stem cell activity that occurs throughout the body, reducing the generation of new cells of all sorts. Perhaps more important in the case of T cells is the age-related atrophy of the thymus, however. This organ is where T cells mature before taking up their assigned roles in the body. It is highly active in childhood, but the active tissue begins to be replaced by fat at the onset of maturity, a process called involution. This continues over a life span and into old age, and the pace at which new T cells mature falls along with it.

A slow rate of T cell replacement causes the existing specialized and active T cell populations to become ever more worn and ragged, lacking reinforcements that can respond effectively to new challenges. This affects most of the aspects of immune function, from the response to invading pathogens to the ability to catch and destroy cancerous cells before they start in earnest the process of generating a tumor. For this reason there is considerable interest in the research community in finding ways to rejuvenate the thymus, to restore the active tissue that acts as a nursery for T cell maturation. If successful, this should go some way towards regaining the lost capacity of the immune system.

Thymic involution and rising disease incidence with age

T cells develop from hematopoietic stem cells as part of the lymphoid lineage and have the ability to detect foreign antigens and neoantigens arising from cancer cells. In the thymus, lymphoid progenitors commit to a specific T cell receptor and undergo selection events that screen against self-reactivity. Cells that pass these selection gates then leave the thymus, clonally expanding to form the patrolling naive T cell pool.

The vast majority of vertebrates experience thymic involution (or atrophy) in which thymic epithelial tissue is replaced with adipose tissue, resulting in decreasing T cell export from the thymus. In humans, this is thought to begin as early as 1 year of age. The rate of thymic T cell production is estimated to decline exponentially over time with a half-life of ∼15.7 years. Declining production of new naive T cells is thought to be a significant component of immunosenescence, the age-related decline in immune system function. With the recent successes of T cell-based immunotherapies, it is timely to assess how thymic involution may affect cancer and infectious disease incidence.

It is clear from epidemiological data that incidence of infectious disease and cancer increases dramatically with age, and, specifically, that many cancer incidence curves follow an apparent power law. The simplest model to account for this assumes that cancer initiation is the result of a gradual accumulation of rare "driver" mutations in one single cell. Furthermore, the fitting of this power law model (PLM) can be used to estimate the number of such mutations. Exponential curves have also been used to fit cancer incidence data, resulting in worse fits than the PLM overall. Nevertheless, it is worth noting that exponential rates close to the declining curve for thymic T cell production can be seen to emerge from the incidence data, indicating the relevance of the thymic involution timescale. While the PLM fits well, it does not account for changes in the immune system with age. To better determine the processes underlying carcinogenesis, we asked whether an alternative model, based only on age-related changes in immune system function, might partly or entirely explain cancer incidence.

Our model outperforms the power law model with the same number of fitting parameters in describing cancer incidence data across a wide spectrum of different cancers, and provides excellent fits to infectious disease data. Our hypothesis and results add to the understanding of infectious disease and cancer incidence, suggesting in the latter case that immunosenescence, rather than gradual accumulation of mutations, serves as the predominant reason for an increase in cancer incidence with age for many cancers. For future therapies, including preventative therapies, strengthening the functionality of the aging immune system appears to be more feasible than limiting genetic mutations, which raises hope for effective new treatments.