**Abstract**

Age-related thymic atrophy or involution, a hallmark of thymic aging, takes place both in humans and animals. In this chapter, we will discuss age-related thymic atrophy, outlining the underlying cellular and molecular mechanisms of its occurrence. We will also address the downstream influences on the aged T cell immune system, not only regarding insufficiency against pathogens, but also hyper-reactivity to self. Particularly, we will focus on how thymic atrophy disrupts efficient establishment of central T cell immune tolerance primarily via impairment of thymocyte negative selection, resulting in an increased number of self-reactive conventional T cells, and on thymic-derived regulatory T cell generation. Finally, we will provide a framework for understanding the significant role that the atrophied thymus plays in shaping inflammaging: a chronic, low-grade, systemic inflammatory phenotype observed in aged individuals in the absence of acute infection. The involvement of T cell adaptive immunity in mediating inflammaging plays a crucial role in the progression of many age-related neurological and cardiovascular diseases.

**Keywords:** thymic atrophy, aging, inflammaging, central tolerance, regulatory T (Treg) cells

### **1. Introduction**

The thymus gland is the primary central lymphoid organ involved in development and selection of T lymphocytes (T cells) [1]. It is also responsible for the establishment of central T cell immune tolerance, which includes two mechanisms: thymocyte negative selection, through which most self (auto)-reactive T cells are depleted [2], and the generation of CD4 single positive (CD4SP)FoxP3+ regulatory T (Treg) cells [3], which act to suppress self-reactive T cell-mediated reactions in the periphery [4]. It is thought that Treg cells provide some level of compensation for imperfections in negative selection that allow some self-reactive T cells to escape this protective process [5]. As part of the aging process, the thymus undergoes progressive involution or atrophy in most vertebrates, exhibiting not only morphological changes, but also a functional decline resulting in [6, 7] significantly lowered thymic output [8].

The theoretical causes of this age-related diminishment of thymopoiesis are two-fold. First, is the notion of a hematopoietic defect. This stems from the observations that there are reduced numbers of hematopoietic stem cell (HSC) progenitors produced by the bone marrow with age, [9] that could cause a reduction in early T-cell progenitors (ETP) entering the thymus [10]. Second, is the notion of a

non-hematopoietic defect, which suggests that the primary age-related atrophy of the thymus is derived from HSC niche cells [11, 12] and thymic stromal cells, or ETP niches [13, 14]. The myriad of changes that characterize thymic atrophy first occur within the thymic niche and then extend to the ETPs as a result of age. We believe that these substantial age-related alterations in thymic microstructure and microenvironment, which provide important thymic factors, contribute more heavily to the diminished thymopoiesis observed in the elderly [7, 13] The primary thymic stromal cells are thymic epithelial cells (TECs), including two subpopulations distinct in their localization, function, and molecular expression patterns, namely medullary TECs (mTECs) and cortical TECs (cTECs) [15]. Compelling evidence show that age-related thymic atrophy is tightly associated with postnatal TEC homeostasis, which is regulated by TEC autonomous transcription factors (TFs), such as Forkhead box N1 (FoxN1) [16].

Age-related changes to immune system function, often referred to as immunosenescence [17–20], are generally thought of as immune insufficiency, such as reduced anti-infection and vaccine immunity [21] and reduced tumor surveillance [22, 23]. However, self-reactive immune responses are elevated in the elderly, which is a result of inflammaging, a chronic, low-grade, systemic inflammatory phenotype in the absence of acute infection observed in aged individuals [24–31]. Immunosenescence and inflammaging are antagonistic phenotypes, but they actually comprise two sides of the same coin in terms of age-related immune dysregulation [19, 20, 32, 33]. It has been proposed that the basal inflammatory state defined by inflammaging greatly contributes to many age-related degenerative diseases, including neurodegenerative diseases, such as Alzheimer's disease, metabolic diseases, and cardiovascular diseases, among others [30, 34, 35].

Here, we will outline the cellular and molecular mechanisms underlying the occurrence of age-related thymic atrophy including some of the aforementioned hallmarks, and its effects on general T cell output. We will also describe its effects on the establishment of central T cell immune tolerance via a combination of both mechanistic arms of central tolerance: thymocyte negative selection and thymicderived CD4SPFoxP3+ T regulatory (tTreg) cell generation. We will discuss why we believe many aspects of the adaptive immune system's role in the development of inflammaging can be attributed to these thymic manifestations. Finally, in light of new trends in T cell immune system aging, we will expand on some future research goals in the field of thymic atrophy interventions and therapeutics as a potential conduit for normalizing aged T cell-mediated immunity. This is of clinical significance for combating age-related neurological and cardiovascular diseases.
