The ability to generate protective immunity against a wide array of pathogens depends on the diversity of T cells. T cell repertoire diversity remains remarkably constant throughout adulthood when new naïve T cells can be produced either from lymphocyte precursors in the thymus or by autoproliferation of existing naïve cells stimulated by IL-7 and other homeostatic cytokines in the periphery. However, with age, the thymus involutes, drastically reducing the generation of new T cells, while the naïve T cell pool continually converts to memory T cells by lifelong homeostatic proliferation and exposure to antigens [14
]. The diversity of the T cell pool is further compromised by the development of age-associated CD8+ T cell clonal expansions which in mice can comprise >80% of the entire CD8+ T cell compartment [15
]. Altogether, these changes have the potential to precipitate a massive loss of naïve T cells and an overrepresentation of memory T cells that significantly restricts T cell repertoire diversity in old individuals.
Making matters worse, life-long infections with persistent pathogens (e.g. HSV, CMV) result in bouts of viral reactivation, which repeatedly stimulate memory T cells specific for these pathogens and further contribute to the imbalance between naïve and memory T cells [20
]. Given that T cell stimulation is intermittent (only during reactivations), these T cells do not exhibit the cardinal signs of exhaustion seen in chronic infections (HCV, HIV), and are usually very efficient in providing immune control. However, over time the pool of memory T cells dedicated to controlling persistent pathogens may continue to expand [21
] and can comprise up to 50% of the human memory pool [22
]. Recently, it has been estimated that every individual is infected with ~8–12 lifelong viral infections [23
], suggesting these types of responses may be normal components of the aging process and should be considered in models of human immune aging.
Several studies have investigated to what extent the naïve T cell compartment is depleted in old age. Naylor et. al demonstrated that in humans CD4 T cell diversity is maintained up to 60 years of age, but is then followed by a dramatic 100 fold decline in diversity after the age of 70[24
]. These authors observed a compensatory two fold increase in homeostatic T cell proliferation as thymic output decreased, and the phenotypic distinctions between naïve and memory T cell populations became blurred when this threshold age was reached. Similarly, Ahmed et. al demonstrated a decrease in naïve T cell diversity in old mice (as shown by skewed spectratyping analysis) with evidence of clonal expansions [25
]. As these studies were performed in specific pathogen free mice, it would also be informative to repeat these studies in mice harboring persistent viral infections. Altogether, these reports indicate that the age-related decrease in T cell diversity is due to both reduced thymic output and exaggerated activity of homeostatic mechanisms invoked to correct this naive T-cell lymphopenia. Further support for that scenario was obtained in monkeys for CD8+ T-cells [26
]. In this study, naïve cells declined with aging, and surprisingly their turnover was increased in old animals, with a clear threshold effect - only when naïve CD8 T-cell pool in blood decreased below 5% of the total, was there pronounced naïve T-cell turnover. While these pioneering studies provide a broad perspective on how aging shapes our T cell compartments, deep sequencing analysis may be required to resolve precisely how the clonotypic structure of the T cell responses is altered within and among older individuals.
One important question is whether age-related changes impact the composition of the naïve T cell compartment in a stochastic manner, or whether there are preferential and predictable biases. For example, it is still not known whether CD8+ T cell epitopes have different lifespans or susceptibilities to aging. A recent study addressed this important question by examining the influence of aging on co-dominant influenza epitopes derived from the viral nucleoprotein (NP366-374) and acid polymerase (PA224-233) proteins [2
]. TCR sequencing established that the NP366-specific repertoire was dominated by a few public clonotypes shared among individuals; while the PA224-specific repertoire was much more diverse and individualized [27
]. Yet the PA224-specific repertoire consisted of ~10 times more naïve precursors [2
]. However, the key finding was that old mice (>18 months) preferentially lost the less abundant and more restricted NP366-specific clonotypes. This could indicate that with advancing age, we first shed epitope-specific CD8+ T cells with a low naïve precursor frequency, so that in old age we are only left with the pools of epitope-specific CD8+ T cells that were initially most diverse and abundant.
Recently, a tetramer-enrichment approach has been used to quantify and isolate endogenous antigen-specific T cells from naïve mice [32
]. Using this technique, the sizes of epitope specific CD8+ and CD4+ T cell precursor pools were estimated at 80–1200 and 20–200 cells per mouse, respectively [32
]. Interestingly, Kedl and Jameson examined the phenotype of naïve precursors in young adult mice and found that up to >30% of these naïve precursors already shows signs of extensive homeostatic proliferation and can masquerade as memory T cells [34
]. These studies suggest that a significant proportion of our memory compartment, at least in mice, may be derived from homeostatic proliferation rather than exposure to foreign antigens. Moreover, the precursors exhibiting prior signs of homeostatic proliferation were also found to be the more responsive upon stimulation with peptide. It will be important to reexamine the phenotype and functionality of these precursors pools as they become increasingly reliant on homeostatic mechanisms with advancing age. It is possible that epitopes undergoing the fastest rates of homeostatic proliferation may have divided many times and will be those that first become senescent in old age. Therefore, correlating precursor frequency, T cell diversity, rates of homeostatic proliferation, and immune functionality for various epitopes in old individuals should provide us with better guidelines in choosing the most robust targets for vaccination in the elderly,.