Successful and Maladaptive T Cell Aging

Abstract

Throughout life, the T cell system adapts to shifting resources and demands, resulting in a fundamentally restructured immune system in older individuals. Here we review the cellular and molecular features of an aged immune system and discuss the trade-offs inherent to these adaptive mechanisms. Processes include homeostatic proliferation that maintains compartment size at the expense of partial loss in stemness and incomplete differentiation and the activation of negative regulatory programs, which constrain effector T cell expansion and prevent increasing oligoclonality but also interfere with memory cell generation. We propose that immune failure occurs when adaptive strategies developed by the aging T cell system fail and also discuss how, in some settings, the programs associated with T cell aging culminates in a maladaptive response that directly contributes to chronic inflammatory disease.

Figure 1. Maintenance of Functional T Cell Compartments with Age

To be successful in aging, each T cell subset has to cope with unique challenges. In the absence of thymic activity, naive T cells are maintained by peripheral proliferation that with increasing age is associated with partial differentiation and contraction in TCR diversity. In the memory compartment, oligoclonal memory inflation as well as loss of memory T cells can occur. Contraction in TCR repertoire breadth to viral antigens can impair virus control as well as cross-protection. Treg cells change in frequencies, composition, and tissue compartmentalization.

Figure 2. Functional Consequences Deriving from Cumulative Homeostatic Proliferation

The decades-long replicative history of naive and memory T cells is associated with reduced telomeric protection and T cell differentiation initiation. Many of the age-associated functional changes including loss of stemness, signal strength calibration, and mitochondrial dysfunction can be traced back to these processes.

Figure 3. Dimensions of Maladaptive T Cell Aging

Adaptive processes of T cells to changing demands during aging can lead to maladaptations. Chronically stimulated T cells are exhausted with a decline in effector function and proliferative potential. A phenotype reminiscent of the senescence-associated secretory phenotype in senescent fibroblasts is seen in terminally differentiated CD45RA T cells (TEMRA) that are efficient cytokine producers while having lost proliferative potential due to a DNA damage-induced cell cycle block. Deficient reactive oxygen signaling and metabolic reprogramming in aged naive T cells accelerate proliferative responses and differentiation in Th1/Th17 effector cells. Defective DNA damage responses to telomere uncapping in aged naive T cells promote tissue infiltration and inflammation. Whether these different processes are sequential, co-existing, or interdependent needs to be explored.

Figure 4. Inflammation Resulting from Maladaptive DNA Damage Responses in T Cell Aging

Defects in DNA damage responses mediated by ATM (left) and MRE11 loss (right) in naive CD4⁺ T cells have been linked to increased tissue inflammation in patients with rheumatoid arthritis, who present with premature immune aging.