Age-Related Changes in Thymic Central Tolerance

Abstract

Thymic epithelial cells (TECs) and hematopoietic antigen presenting cells (HAPCs) in the thymus microenvironment provide essential signals to self-reactive thymocytes that induce either negative selection or generation of regulatory T cells (Treg), both of which are required to establish and maintain central tolerance throughout life. HAPCs and TECs are comprised of multiple subsets that play distinct and overlapping roles in central tolerance. Changes that occur in the composition and function of TEC and HAPC subsets across the lifespan have potential consequences for central tolerance. In keeping with this possibility, there are age-associated changes in the cellular composition and function of T cells and Treg. This review summarizes changes in T cell and Treg function during the perinatal to adult transition and in the course of normal aging, and relates these changes to age-associated alterations in thymic HAPC and TEC subsets.

Keywords: central tolerance; dendritic cells; life span; thymic epithelial cells; thymus.

Figure 1

Age-associated changes in T cell generation and function throughout the lifespan. The T cell landscape is in flux throughout life, shaped by age-associated changes in T-cell subset composition and function, which are influenced by cell-intrinsic factors as well as microenvironmental cues that support T cell development and differentiation. While the perinatal T cell pool is dominated by naive conventional T cells (Tconv) and recent thymic emigrants (RTEs), the aged T cell pool contains a higher proportion of memory T cells. Perinatal and aged T cells share several striking similarities in phenotypes and functions. The perinatal and aged CD8+ Tconv cells, including virtual memory T cells (Tvm), are shifted towards short-lived, innate-like, effector responses characterized by increased proliferative potential and rapid cytokine production, at the expense of long-lasting memory generation. Naive CD4+ T cells also display age-associated changes at both ends of the age spectrum, such as reduced T cell receptor (TCR) responsiveness and IL-2 production. In addition, T cells are more self-reactive both early and late in life, which may reflect age-associated changes in thymic selection and/or peripheral maintenance. Regulatory T cells (Treg) generation in the thymus peaks in the perinatal period, but Tregs at both ends of the age spectrum have superior suppressive capacity compared to adult Tregs. These age-associated changes implicate the thymic microenvironment in selecting Tconv cells and Tregs that cater to rapidly changing immune challenges throughout life, while at the same time curbing the risk of triggering autoimmunity. T cell output from the thymus is also lower in both fetal/neonatal periods as well as in the elderly. The uneven pattern of thymic output depicted in the histogram reflects variability throughout life due to numerous extrinsic stressors, such as infections and pregnancy, that alter thymic cellularity and output. In keeping with the above similarities between T cells in the perinatal and elderly stages, immune outcomes, such as overall responsiveness to vaccines and pathogens, as well susceptibility to new onset autoimmunity change in similar directions at both extremes of the lifespan. Phenotypes with question marks are yet to be defined clearly, and dotted lines indicate variable findings in the indicated attributes. All features have been reported in both humans and mice, except those denoted with an asterisk that indicates findings currently reported only in mice in the perinatal to adult and/or adult to aged transitions.

Figure 2

Thymic epithelial cells and hematopoietic antigen presenting cells provide essential signals to guide αβT cell maturation and the induction of central tolerance in the thymus. Cross-sectional view of a thymus lobe reveals cortical and medullary regions, through which thymocytes must travel in an orchestrated manner to encounter heterogeneous stromal cell subsets. Progenitor cells from the bone marrow migrate through the vasculature to seed the thymus at the cortico-medullary junction (CMJ). DN1-DN4 thymocytes require signals from cortical thymic epithelial cells (cTECs) to support their survival, proliferation, and T-lineage commitment. During the DN2-DN3 stages, TCRβ gene segments are recombined, and thymocytes that successfully express TCRβ and signal through the pre-TCR undergo proliferation and further differentiation through the process of β-selection. Subsequently, thymocytes upregulate CD4 and CD8 to become double-positive cells (DPs), which initiate TCRα gene rearrangements. DPs that successfully express a TCRαβ heterodimer are tested for reactivity with self-peptide MHC complexes presented by cTECs. Only those DPs that receive a TCR signal pass positive selection, enabling them to survive and further differentiate. Positively selected DPs transit from the cortex into the medulla. Along the way, some clones may be deleted in an early wave of negative selection in the cortex, driven by strong TCR reactivity to self-peptide MHC complexes displayed by dendritic cells (DCs). In the medulla, DPs downregulate either CD4 or CD8 to become single-positive thymocytes (CD8SP or CD4SP) and interact with medullary APCs to establish central tolerance to a broad array of self-antigens. Strong TCR signals, induced by self-antigens displayed by medullary thymic epithelial cells (mTECs), conventional DCs (cDCs), plasmacytoid DCs (pDCs), or B cells result in either negative selection (apoptosis) or Treg diversion of the autoreactive T cell clones, enforcing central tolerance. SPs that survive these collective thymic selection processes emigrate from the thymus to join the peripheral T cell pool.

Figure 3

Changes in thymic size, organization, and/or lipid content accompany age-associated thymic involution in humans and mice. (A) In humans, the percentage of the thymus comprised of functional thymic tissue progressively declines with age, and is replaced by adipose tissue, as shown in these hematoxylin and eosin-stained images. The percent of thymus area containing thymic epithelium, representing functional thymic tissue, was calculated via morphometric analysis of cytokeratin immunohistochemical slides. The results for the subjects shown are 91% at 5 days (5d), 55% at 28 years (28y), and 0.5% at 78 years (78y). (B) The mouse thymus grows substantially between postnatal day 1 (1d) (scale bar = 400 µm) and 4 weeks of age (4w) (scale bar = 2 mm), and then declines steadily and is highly involuted by 12 months of age (12m). The small islands of medullary tissue seen at 1d expand and coalesce to form the larger, more organized medullary regions characteristic of adult thymus (4w). Age-associated replacement by adipose tissue is not a prominent characteristic of involution in mice. The corresponding weights (mean ± SD) of murine thymus at the ages shown are 5 ± 0.5 mg at 1d (n = 3), 57 ± 8 mg at 4w (n = 8), and 38 ± 2 mg at 12 m (n = 3).