Clinical implications of immunosenescence
As mentioned above, complications from acute infectious are likely to be more severe in the elderly owing to impaired innate immunity. However, questions remain concerning ‘normal, healthy’ ageing and the important clinical issue of responses to vaccinations in old age. In a mouse model of the highly relevant human pathogen influenza, the virus is cleared from the lungs more slowly in old animals, correlating with a delayed and decreased peak of cytotoxic T‐cell production (D. Murasko, Philadelphia, PA, USA). Therefore, cellular responses are crucial for controlling the virus, but do not function adequately in old animals. Although there is an accumulation of memory cells (the clonal expansion referred to above), they are not solely responsible for this decrease in the virus‐specific response. Both memory and naive T cells in old, but not young, mice are resistant to apoptosis, and do not ‘make space’ for new responses. In the mouse model, cell‐transfer experiments showed that both the old environment and the old cells contributed to the problem—young cells did not deplete when transferred to an old environment and old cells did not deplete when transferred to a young environment. The factors inducing apoptosis resistance have not yet been identified; however, it is clearly important to do so and to search for them in humans.
Apoptosis‐resistant cells that accumulate in old mice and humans—and fill the ‘immunological space’—might be dysfunctional in several ways. In young mice, the number of T cells staining with soluble major histocompatibility complex (MHC)–peptide multimers carrying influenza epitopes was similar to the number of cells producing the antiviral and pro‐inflammatory cytokine interferon‐γ (IFNγ) on antigen stimulation. However, in old mice, the number of tetramer‐positive cells exceeded the number of IFNγ‐producers, indicating that some cells bearing antigen‐specific receptors failed to respond appropriately to receptor ligation (H. Ertl, Philadelphia, PA, USA). This is similar to the situation in elderly humans, who have been found to accumulate large clonal expansions, primarily—and for unknown reasons—of cytomegalovirus (CMV)‐specific CD8 cells (
Pawelec et al, 2005). In the mice, this lack of reactivity was not due to poor antigen presentation by dendritic cells (Ertl). The reason for poor reactivity remains unknown; however, responses could be restored, in part, by vaccination using an adenovirus vector AdC68 that naturally infects chimpanzees rather than mice, as a way of improving immunizations by modifying the vaccine product. This might also be possible in humans by using better adjuvants for vaccination (E. Nagy, Vienna, Austria). Deciphering the mechanisms by which adjuvants enhance responses in order to design ‘elderly‐specific’ vaccines will become increasingly important. This applies not only to infectious diseases but also possibly to vaccinating against cancer, as illustrated by differences in responses to anticancer immunizations in young and old mice. In a breast cancer model, preventive vaccination using DNA encoding certain cancer antigens was successful in protecting 90% of the young mice, but only 60% of the old mice, from developing metastases. This correlated with lower levels of IFNγ and IL‐2 in old mice (C. Gravekamp, San Francisco, CA, USA). The production of IL‐6, which is a potential inhibitor of vaccine‐induced T‐cell responses, was high in both young and old mice. Increasing IFNγ and IL‐2, and depressing IL‐6 production in the elderly, would therefore seem to be desirable.
The function of naïve and memory T cells in ageing
Clearly, vaccination can only be effective if cells able to respond are present (
Nikolich‐Zugich, 2005). These might be either naive cells that require stimulation with vaccine containing novel antigens, or memory cells that require increasing by previously encountered antigens. This raises the question of whether old individuals have naive cells towards the end of their lives and, if so, whether they are fully functional. In mice, naive T cells from old animals do seem to be impaired. CD4 cells show decreased helper activity—and, in this case, transfer into a young environment fails to restore immune competence; this is not because of altered trafficking of lymphocytes through the tissues (L. Haynes, Saranac Lake, NY, USA). Helper function could be restored by a mixture of pro‐inflammatory cytokines—IL‐1, IL‐6 and TNFα; therefore, judicious local use of these as adjuvants might be beneficial. However, elderly humans might have few naive cells that can be targeted in this way. Even CD8 cells with an apparently naive phenotype—CD45RA+ CD28+ —expressed fewer additional naive markers—CD62L and CCR7—than those from the young, had shorter telomeres and had restricted TCR repertoires (B. Grubeck‐Loebenstein, Innsbruck, Austria). In fact, the presence of cells with an unusual phenotype, more reminiscent of memory cells, seemed to correlate with better responses to influenza vaccination in the elderly. If the elderly mostly rely on their memory cells for pathogen responses in later life, it is crucial to know whether these are retained and function normally, and, if not, what can be done to improve this situation.
A diverse TCR repertoire must be maintained at all times to respond to all types of antigen. Repertoire attrition would be predicted to be a bad prognostic sign in the elderly. Given the high levels of peripheral cell turnover (Beverley), their potential for proliferation could be rapidly exhausted. In humans, telomere shortening with each cell division results in proliferative arrest—replicative senescence—unless cells express telomerase. Extensive proliferation results in CD28 downregulation and, although there are other coreceptors that can upregulate telomerase, for example, CD134, CD137 and inducible T‐cell co‐stimulator (ICOS), ‘end‐stage’ cells that have lost both CD27 and CD28 cannot do so (A. Akbar, London, UK). Repetitive stimulation therefore results in ever lower levels of telomerase, at which time the telomeres begin to shorten (R. Hodes, Bethesda, MD, USA). In some cases, transfection of the catalytic component of telomerase, hTERT, can result in stabilization of telomere lengths and extended growth of the cells in vitro; for example, human immunodeficiency virus (HIV)‐specific hTERT‐transfected CD8 cells have been cultured for more than three years without acquiring karyotypic abnormalities and retain their ability to control HIV infection in vitro (R. Effros, Los Angeles, CA, USA). In the absence of continuous telomerase expression, however, memory cells might become impaired as they turn over in vivo. Signals regulating this turnover have not been determined; however, peripheral levels are well‐maintained in the healthy aged—only the unhealthy might have less peripheral cells per unit blood, that is, be leukopaenic. Antigens do not seem to be required in infections caused by acute agents that do not persist. After the clearance of an acute viral infection in mice, memory cells persist for the lifetime of the animal, and undergo clonal expansions and accumulations of memory cells even in the absence of the original antigen. These persistent memory cells remain fully functional and even become more effective with time (D. Woodland, Saranac Lake, NY, USA). So, what maintains them over extended periods in the absence of antigenic stimulation? The answer is possibly cytokines such as IL‐7 and IL‐15. CD8 cells carrying low levels of the receptor IL‐7Rα already had a limited TCR repertoire in the young, but this was even more restricted in the elderly, with poor responses to TCR‐mediated stimulation. These memory cells might be maintained by IL‐15 but they are dysfunctional owing to replicative senescence, whereas high IL‐7Rα expressers are functional (I. Kang, New Haven, CT, USA). Distinguishing between functional and dysfunctional CD8 cells with the same TCR specificity, on the basis of markers such as IL‐7Rα, might be important when considering interventions that would benefit from deletion of the apoptosis‐resistant dysfunctional cells, the accumulation of which fills the immunological space and contributes to decreased immunity in the elderly.
Gene arrays of functional and dysfunctional cells might provide clues as to the reasons for their differences. Analysis of gene expression between CD28+ (functional) and CD28− (partly dysfunctional) CD8 T cells revealed that the latter upregulated several stimulatory NK receptors (N.‐P. Weng, Bethesda, MD, USA). Many of these differences decreased after culture in the ‘maintenance’ cytokine IL‐15. Both CD28+ and CD28− cells responded in a similar manner to IL‐15, and this was the same in both the young and the old. Intriguingly, IL‐15 also induced loss of CD28 expression in CD28+ CD8 cells, indicating that maintenance of memory cells by IL‐15 could cause the generation of CD28‐ cells with age.
These and other data suggest the accumulation of clonal expansions of CD8—and, to a lesser extent, CD4—cells in elderly humans and mice. In the absence of foreign antigen, these events still occur in mice; however, in human ageing, there are probably always foreign antigens driving clonal expansions. This might also apply to ‘premature’ immunosenescence, such as that caused by HIV infection (V. Appay, Paris, France). In humans, longitudinal studies of the elderly (aged more than 85 years) have revealed an overriding importance of persistent CMV and a lesser contribution of Epstein–Barr virus—but not herpes simplex virus—to this phenomenon. CMV‐specific CD8 cells begin to accumulate during middle age (G. Pawelec, Tübingen, Germany). The number of different clonal expansions initially increases with age and is a risk factor predicting mortality; however, at the terminal phase, repertoire contraction occurs, which is even more highly predictive of mortality. The CMV‐specific CD8 clones that accumulate contain functional cells—producing IFNγ on specific stimulation—and a larger fraction of dysfunctional (anergic) cells. It is the latter that cause inversion of the CD4:8 ratio in severely affected donors, which is a prime risk factor for incipient mortality. Age‐associated alterations of TCR diversity might occur with surprising suddenness (J. Goronzy, Atlanta, GA, USA). Diversity seems to be well‐maintained in both naive and memory CD4 cells up to 60–65 years of age, despite thymic output mostly ceasing by approximately 50 years of age. However, repertoire diversity in individuals aged 75–80 years was a mere 1% of that in the younger group, suggesting the rapid loss of clonal heterogeneity between late middle age and early old age. These were not longitudinal studies; however, the findings are consistent with the data from the longitudinal studies described above for CD8 cells, in which the same individuals were followed over time and displayed a highly significant negative association between the number of different clonal expansions present and the remaining survival time. Therefore, if these data are confirmed, an enormous future challenge will be to find out exactly what happens at the crucial time point of repertoire crash, and how to prevent it. One possibility could be caloric restriction, which, in monkeys, helps to maintain naive T‐cells, and to preserve TCR repertoire diversity and gene‐expression patterns at youthful levels (J. Nikolich‐Zugich, Beaverton, OR, USA).