Sian M. Henson

Are senescence and exhaustion intertwined or unrelated processes that compromise immunity

Can the immune system be reactivated continuously throughout the lifetime of an organism or is there a finite point at which repeated antigenic challenge leads to the loss of lymphocyte function or the cells themselves or both? Replicative senescence and exhaustion are processes that control T cell proliferative activity and function; however, there is considerable confusion over the relationship between these two intrinsic cellular control mechanisms. In this Opinion article, we compare the molecular regulation of senescence and exhaustion in T cells. Available data suggest that both processes are regulated independently of each other and that it may be safer to block exhaustion than senescence to enhance immunity.

KLRG1 signaling induces defective Akt (ser473) phosphorylation and proliferative dysfunction of highly differentiated CD8+ T cells.

Highly differentiated CD8+CD28-CD27- T cells have short telomeres, defective telomerase activity, and reduced capacity for proliferation, indicating that they are close to replicative senescence. In addition, these cells express increased levels of the senescence-associated inhibitory receptor KLRG1 and have poor capacity for IL-2 synthesis and defective Akt (ser(473)) phosphorylation after activation. It is not known whether signaling via KLRG1 contributes to any of the attenuated differentiation-related functional changes in CD8+ T cells. To address this, we blocked KLRG1 signaling during T-cell receptor activation using antibodies against its major ligand, E-cadherin. This resulted in a significant enhancement of Akt (ser(473)) phosphorylation and T-cell receptor-induced proliferative activity of CD8+CD28-CD27- T cells. Furthermore, the increase of proliferation was directly linked to the Akt-mediated induction of cyclin D and E and reduction in the cyclin inhibitor p27 expression. In contrast, the reduced telomerase activity in highly differentiated CD8+CD28(-)CD27- T cells was not altered by KLRG1 blockade, indicating the involvement of other mechanisms. This is the first demonstration of a functional role for KLRG1 in primary human CD8+ T cells and highlights that certain functional defects that arise during progressive T-cell differentiation toward replicative senescence are maintained actively by inhibitory receptor signaling.

Memory T-Cell Homeostasis and Senescence during Aging

Human memory T cell pools proliferate and differentiate at varying rates that are determined by the frequency of lifelong antigenic re-encounter with different specific antigens. An important question concerning immunity is whether certain specific pools of memory T cells are driven to exhaustion in elderly subjects, a pertinent point in view of increasing human life expectancy. An emerging consensus is that cytomegalovirus (CMV), a beta-herpesvirus with a prevalence of 60-90% worldwide, is an agent that induces specific T cells to extreme differentiation. The question that begs to be answered is whether this can explain why CMV seropositivity and the presence of highly differentiated CMV-specific T cells are included in a cluster of immune parameters that have been shown recently to predict the early mortality of elderly humans.

Cytomegalovirus Infection Reduces Telomere Length of the Circulating T Cell Pool

Short telomeres of circulating leukocytes are a risk factor for age-related diseases, such as atherosclerosis, but the exact mechanisms generating variations in telomere length are unknown. We hypothesized that induction of differentiated T cells during chronic CMV infection would affect T cell telomere length. To test this, we measured the amount of differentiated T cells and telomere length of lymphocytes during primary CMV infection as well as CMV-seropositive and -seronegative healthy individuals. After primary CMV infection, we observed an increase in highly differentiated cells that coincided with a steep drop in telomere length. Moreover, we found in a cohort of 159 healthy individuals that telomere shortening was more rapid in CMV-seropositive individuals and correlated with the amount of differentiated T cells in both CD4+ T cells and CD8+ T cells. Finally, we found that telomere length measured in blood leukocytes is correlated with lymphocyte telomere length. Thus, CMV infection induces a strong decrease in T cell telomere length, which can be explained by changes in the composition of the circulating lymphocyte pool.

Reversible Senescence in Human CD4+CD45RA+CD27− Memory T Cells

Persistent viral infections and inflammatory syndromes induce the accumulation of T cells with characteristics of terminal differentiation or senescence. However, the mechanism that regulates the end-stage differentiation of these cells is unclear. Human CD4+ effector memory (EM) T cells (CD27−CD45RA−) and also EM T cells that re-express CD45RA (CD27−CD45RA+; EMRA) have many characteristics of end-stage differentiation. These include the expression of surface KLRG1 and CD57, reduced replicative capacity, decreased survival, and high expression of nuclear γH2AX after TCR activation. A paradoxical observation was that although CD4+ EMRA T cells exhibit defective telomerase activity after activation, they have significantly longer telomeres than central memory (CM)-like (CD27+CD45RA−) and EM (CD27−CD45RA−) CD4+ T cells. This suggested that telomerase activity was actively inhibited in this population. Because proinflammatory cytokines such as TNF-α inhibited telomerase activity in T cells via a p38 MAPK pathway, we investigated the involvement of p38 signaling in CD4+ EMRA T cells. We found that the expression of both total and phosphorylated p38 was highest in the EM and EMRA compared with that of other CD4+ T cell subsets. Furthermore, the inhibition of p38 signaling, especially in CD4+ EMRA T cells, significantly enhanced their telomerase activity and survival after TCR activation. Thus, activation of the p38 MAPK pathway is directly involved in certain senescence characteristics of highly differentiated CD4+ T cells. In particular, CD4+ EMRA T cells have features of telomere-independent senescence that are regulated by active cell signaling pathways that are reversible.

p38 signaling inhibits mTORC1-independent autophagy in senescent human CD8+ T cells

T cell senescence is thought to contribute to immune function decline, but the pathways that mediate senescence in these cells are not clear. Here, we evaluated T cell populations from healthy volunteers and determined that human CD8+ effector memory T cells that reexpress the naive T cell marker CD45RA have many characteristics of cellular senescence, including decreased proliferation, defective mitochondrial function, and elevated levels of both ROS and p38 MAPK. Despite their apparent senescent state, we determined that these cells secreted high levels of both TNF-α and IFN-γ and showed potent cytotoxic activity. We found that the senescent CD45RA-expressing population engaged anaerobic glycolysis to generate energy for effector functions. Furthermore, inhibition of p38 MAPK signaling in senescent CD8+ T cells increased their proliferation, telomerase activity, mitochondrial biogenesis, and fitness; however, the extra energy required for these processes did not arise from increased glucose uptake or oxidative phosphorylation. Instead, p38 MAPK blockade in these senescent cells induced an increase in autophagy through enhanced interactions between p38 interacting protein (p38IP) and autophagy protein 9 (ATG9) in an mTOR-independent manner. Together, our findings describe fundamental metabolic requirements of senescent primary human CD8+ T cells and demonstrate that p38 MAPK blockade reverses senescence via an mTOR-independent pathway.

The kinase p38 activated by the metabolic regulator AMPK and scaffold TAB1 drives the senescence of human T cells

In T lymphocytes, p38 MAP kinase (MAPK) regulates pleiotropic functions and is activated by canonical MAPK signaling or the alternative T cell receptor (TCR) activation pathway. Here we show that senescent human T cells lack the canonical and alternative pathways of p38 activation, but spontaneously engage the metabolic master regulator AMPK to trigger p38 recruitment to the scaffold TAB1 causing p38 auto-phosphorylation. Signaling via this pathway inhibits telomerase activity, T cell proliferation and expression of key components of the TCR signalosome. Our findings identify an unrecognized mode of p38 activation in T cells driven by intracellular changes such as low-nutrient and DNA-damage signaling (‘intra-sensory’ pathway). The proliferative defect of senescent T cells is reversed by blocking AMPK-TAB1-dependent p38 activation.

CD25-Expressing CD8+ T Cells Are Potent Memory Cells in Old Age

We have recently described an IL-2/IL-4-producing CD8+CD25+ nonregulatory memory T cell population that occurs in a subgroup of healthy elderly persons who characteristically still have a good humoral response after vaccination. The present study addresses this specific T cell subset and investigates its origin, clonal composition, Ag specificity, and replicative history. We demonstrate that CD8+CD25+ memory T cells frequently exhibit a CD4+CD8+ double-positive phenotype. The expression of the CD8 αβ molecule and the occurrence of signal-joint TCR rearrangement excision circles suggest a thymic origin of these cells. They also have longer telomeres than their CD8+CD25− memory counterparts, thus indicating a shorter replicative history. CD8+CD25+ memory T cells display a polyclonal TCR repertoire and respond to IL-2 as well as to a panel of different Ags, whereas the CD8+CD25− memory T cell population has a more restricted TCR diversity, responds to fewer Ags, and does not proliferate in response to stimulation with IL-2. Molecular tracking of specific clones with clonotypic primers reveals that the same clones occur in CD8+CD25+ and CD8+CD25− memory T cell populations, demonstrating a lineage relationship between CD25+ and CD25− memory CD8+ T cells. Our results suggest that CD25-expressing memory T cells represent an early stage in the differentiation of CD8+ cells. Accumulation of these cells in elderly persons appears to be a prerequisite of intact immune responsiveness in the absence of naive T cells in old age.

Variation of human natural killer cell phenotypes with age: identification of a unique KLRG1-negative subset.

Human natural killer (NK) cells subsets are phenotypically characterized by their lack of CD3 and low/high expression of CD56. This study revealed an age-associated increase in the ratio of CD3(-)CD56(dim) to CD3(-)CD56(bright) NK cells, whereas distinct expression patterns of CD2, CD16, CD57, and the C-type lectin family members killer cell lectin-like receptor -D1 (CD94) and -G1 (KLRG1), were noted on both these NK and the CD3(+)CD56(+) T cell subsets; moreover, CD94 and KLRG1 expression were significantly reduced with age. Although the proportion of CD3(-)CD56(bright) NK cells vs CD3(-)CD56(dim) cells decreased with age, the percentage of CD3(-)CD56(bright) cells expressing IFN-gamma after activation significantly increased, potentially representing compensatory augmentation of cytokine production to maintain the important immunoregulatory role of these cells in older individuals. Collectively, these results highlight new evidence for a continuum of change during immunologic aging and present unique data for variation of NK cell subsets with human aging.

The role of the T cell in age-related inflammation

Ageing is accompanied by alterations to T-cell immunity and also by a low-grade chronic inflammatory state termed inflammaging. The significance of these phenomena is highlighted by their being predictors of earlier mortality. We have recently published that the proinflammatory cytokine TNFα is a strong inducer of CD4+ T-cell senescence and T-cell differentiation, adding to the growing body of literature implicating proinflammatory molecules in mediating these critical age-related T-cell alterations. Moreover, the inflammatory process is also being increasingly implicated in the pathogenesis of many common and severe age-related diseases, including cancer, cardiovascular diseases and type 2 diabetes. Furthermore, major age-related risk factors for poor health, such as obesity, stress and smoking, are also associated with an upregulation in systemic inflammatory markers. We propose the idea that the ensuing inflammatory response to influenza infection propagates cardiovascular diseases and constitutes a major cause of influenza-related mortality. While inflammation is not a negative phenomenon per se, this age-related dysregulation of inflammatory responses may play crucial roles driving age-related pathologies, T-cell immunosenescence and CMV reactivation, thereby underpinning key features of the ageing process.