Senescent T Cells Contribute to Neurodegenerative Conditions
T cell senescence is a noted feature of the aging immune system. T cells mature in the thymus, which atrophies with age. Absent a supply of new T cells, the existing populations are forced into greater cellular replication in order to (a) maintain a steady number of cells, and (b) continue to respond to infection with an expansion of the number of T cells equipped to attack the pathogen. Cellular senescence occurs when a cell reaches the Hayflick limit to replication. With each cell division, telomeres at the ends of chromosomes shorten. When telomeres become too short, a cell either self-destructs or becomes senescent. In some subsets of the immune cell population, almost half of all T cells are senescent in old people.
Senescent cells generate a pro-inflammatory, pro-growth mix of signals, the senescence-associated secretory phenotype (SASP). These cells serve a purpose when present in the short term, but when they linger the SASP becomes highly disruptive to tissue structure and function. As today's open access paper points out, even though T cells are not present in the brain in any great number, their state of senescence does matter. Inflammatory signaling moves throughout the body, and can and does link the distinct immune systems of the body and brain.
With the increasing proportion of the aging population, neurodegenerative diseases have become one of the major health issues in society. Neurodegenerative diseases (NDs), including multiple sclerosis (MS), Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS), are characterized by progressive neurodegeneration associated with aging, leading to a gradual decline in cognitive, emotional, and motor functions in patients. The process of aging is a normal physiological process in human life and is accompanied by the aging of the immune system, which is known as immunosenescence.
T-cells are an important part of the immune system, and their senescence is the main feature of immunosenescence. The appearance of senescent T-cells has been shown to potentially lead to chronic inflammation and tissue damage, with some studies indicating a direct link between T-cell senescence, inflammation, and neuronal damage. The role of these subsets with different functions in NDs is still under debate. A growing body of evidence suggests that in people with a ND, there is a prevalence of CD4+ T-cell subsets exhibiting characteristics that are linked to senescence. This underscores the significance of CD4+ T-cells in NDs. In this review, we summarize the classification and function of CD4+ T-cell subpopulations, the characteristics of CD4+ T-cell senescence, the potential roles of these cells in animal models and human studies of NDs, and therapeutic strategies targeting CD4+ T-cell senescence.