Richard Aspinall

B-cell responses to vaccination at the extremes of age.

Infants and the elderly share a high vulnerability to infections and therefore have specific immunization requirements. Inducing potent and sustained B-cell responses is as challenging in infants as it is in older subjects. Several mechanisms to explain the decreased B-cell responses at the extremes of age apply to both infants and the elderly. These include intrinsic B-cell limitations as well as numerous microenvironmental factors in lymphoid organs and the bone marrow. This Review describes the mechanisms that shape B-cell responses at the extremes of age and how they could be taken into account to design more effective immunization strategies for these high-risk age groups.

Thymic involution in aging.

The size of the na¨ıve T-cell pool is governed by output from the thymus and not by replication. This pool contributes cells to the activated/memory T-cell pool whose size can be increased through cell multiplication; both pools together constitute the peripheral T-cell pool. Aging is associated with involution of the thymus leading to a reduction in its contribution to the na¨ıve T-cell pool; however, despite this diminished thymic output, there is no significant decline in the total number of T cells in the peripheral T-cell pool. There are, however, considerable shifts in the ratios of both pools of cells, with an increase in the number of activated/memory T cells and the accumulation in older individuals of cells that fail to respond to stimuli as efficiently as T cells from younger individuals. Aging is also associated with a greater susceptibility to some infections and some cancers. An understanding of the causal mechanism of thymic involution could lead to the design of a rational therapy to reverse the loss of thymic tissue, renew thymic function, increase thymic output, and potentially improve immune function in aged individuals.

Age-associated thymic atrophy is linked to a decline in IL-7 production

Age-associated thymic atrophy results in a decline in T lymphocyte output and has been identified as one of the key events that precede inefficient functioning of the immune system in later life. Thymic atrophy is thought to result from a failure of the thymic microenvironment to support thymopoiesis in old age and recent evidence suggests that a decline in interleukin-7 (IL-7) expression may limit thymocyte development by restricting combinations of survival, proliferation and rearrangement of the TCRbeta chain. Using RT-PCR and the RNase protection assay, we show that the expression of IL-7 declines with age. Analysis of Connexin 43 expression, a component molecule of gap junctions, whose function is to connect epithelial cells, does not markedly decline with age. These observations suggest that a decline in IL-7 expression is not matched by a similar loss of epithelial cells. These results in conjunction with other studies lead us to speculate that IL-7 producing MHC class II positive TECs are being replaced by cells that do not have this capacity.

Challenges for vaccination in the elderly

The increased susceptibility of the elderly to infection presents a major challenge to public health services. An aging immune system is well documented as the cause of increased infection rates in elderly people. Such immunosenescence is multi-factorial and incompletely understood. Immunosenescent changes include malfunctioning of innate immune system cellular receptors; involution of the thymus, with consequent reduction of the naïve T cell population; alteration of the T cell population composition; modified phenotypes of individual T cells; and replicative senescence of memory cells expressing naïve markers. Unfortunately, immunosenescence also renders vaccination less effective in the elderly. It is therefore important that the vaccines used against common but preventable diseases, such as influenza, are specifically enhanced to overcome the reduced immune responsiveness of this vulnerable population.

IL-7 and Not Stem Cell Factor Reverses Both the Increase in Apoptosis and the Decline in Thymopoiesis Seen in Aged Mice

Thymic atrophy is an age-associated decline in commitment to the T cell lineage considered to be associated with defective TCR β-chain rearrangement. Both IL-7 and stem cell factor (SCF) have dominant roles at this stage of triple negative (TN) thymocyte development. Because there is no age-associated decrease in the number of CD44+CD25−CD3−CD4−CD8− cells, this study investigated whether alterations in apoptosis within the TN pathway accounted for diminishing thymocyte numbers with age. Here we show significant age-associated increases in apoptotic TN thymocytes, specifically within CD44+CD25+ and CD44−CD25+ subpopulations, known to be the location of TCR β-chain rearrangement. IL-7 added to TN cultures established from old mice significantly both reduces apoptosis and increases the percentage of live cells within CD44+CD25+ and CD44−CD25+ subpopulations after 24 h, with prosurvival effects remaining after 5 days. SCF failed to demonstrate prosurvival effects in old or young cultures, and IL-7 and SCF together did not improve upon IL-7 alone. IL-7R expression did not decline with age, ruling out the possibility that the age-associated increase in apoptosis was attributed to reduced IL-7R expression. Compared with PBS, treatment of old mice with IL-7 produced significant increases in live TN cells. By comparison, treatment with SCF failed to increase live TN numbers, and IL-7 and SCF together failed to significantly improve thymopoiesis above that shown by IL-7 alone. Thus, treatment with IL-7 alone can reverse the age-associated defect in TN thymocyte development revealed by in vitro studies to be located at the stages of TCR β-chain rearrangement.

Both age and gender affect thymic output: more recent thymic migrants in females than males as they age

The thymus undergoes age‐associated involution, with studies showing thymic size decreasing from birth at a rate of approximately 3% per year until middle age, and at a rate of 1% per year thereafter. The aim of this study was to determine the effect of thymic atrophy on T‐lymphocyte production by the thymus, and to clarify the ongoing uncertainty regarding gender differences in thymic function. We quantified recent thymic emigrants (RTEs) in blood through the measurement of signal joint T‐cell receptor rearrangement excision circles (sjTRECs), and showed that the decline in the number of RTEs in the blood with increasing age is gender‐linked. Peripheral blood from females contained significantly higher levels of sjTRECs per CD3+ T cell than blood from males (P = 0·002), despite there being no significant gender difference in the absolute number of CD3+ T cells in the populations analysed (P > 0·10). Our findings suggest better thymic function in females compared with males, providing females with a higher number of recent thymic emigrants for longer periods of life. Such a finding provides a plausible explanation for the immunological gender differences observed in previous studies and possibly, for the general longer life expectancy in females compared with males.

Thymic atrophy in the mouse is a soluble problem of the thymic environment

Age related deterioration in the function of the immune system has been recognised in many species. The clinical presentations of such immune dysfunction are an age-related increased susceptibility to certain infections, and an increased incidence of autoimmune disease and certain cancers. Laboratory investigations reveal a reduced ability of the cells from older individuals, compared with younger individuals, to perform in functional in vitro assays. These manifestations are thought to be causally linked to an age associated involution of the thymus, which precedes the onset of immune dysfunction. Hypotheses to account for the age-related changes in the thymus include: (i) an age related decline in the supply of T cell progenitors from the bone marrow (ii) an intrinsic defect in the marrow progenitors, or (iii) problems with rearrangement of the TCR beta chain because of a defect in the environment provided by the thymus. We have analysed these possible options in normal mice and also in mice carrying a transgenic T cell receptor. The results from these studies reveal no age related decline either in the number of function of T cell progenitors in the thymus, but changes in the thymic environment in terms of the cytokines produced. We have shown that specific cytokine replacement therapy leads to an increase in thymopoiesis in old animals.

Influenza control in the 21st century: Optimizing protection of older adults.

Older adults (> or =65 years of age) are particularly vulnerable to influenza illness. This is due to a waning immune system that reduces their ability to respond to infection, which leads to more severe cases of disease. The majority ( approximately 90%) of influenza-related deaths occur in older adults and, in addition, catastrophic disability resulting from influenza-related hospitalization represents a significant burden in this vulnerable population. Current influenza vaccines provide benefits for older adults against influenza; however, vaccine effectiveness is lower than in younger adults. In addition, antigenic drift is also a concern, as it can impact on vaccine effectiveness due to a mismatch between the vaccine virus strain and the circulating virus strain. As such, vaccines that offer higher and broader protection against both homologous and heterologous virus strains are desirable. Approaches currently available in some countries to meet this medical need in older adults may include the use of adjuvanted vaccines. Future strategies under evaluation include the use of high-dose vaccines; novel or enhanced adjuvantation of current vaccines; use of live attenuated vaccines in combination with current vaccines; DNA vaccines; recombinant vaccines; as well as the use of different modes of delivery and alternative antigens. However, to truly evaluate the benefits that these solutions offer, further efficacy and effectiveness studies, and better correlates of protection, including a precise measurement of the T cell responses that are markers for protection, are needed. While it is clear that vaccines with greater immunogenicity are required for older adults, and that adjuvanted vaccines may offer a short-term solution, further research is required to exploit the many other new technologies.

Longevity and the immune response

The domino hypothesis of the onset of age associatedimmune insufficiency suggests that it is theconsequence of a cascade of events beginning withinvolution of the thymus. Involution is associatedwith a reduced thymic output leading to fewernaïve T cells contributing to the peripheral T-cell pool. Homeostatic mechanisms, which maintain thenumber of T cells in the peripheral pool withinprecise limits, induce the proliferation and prolongthe survival of resident T cells to fill the nichesleft vacant by the absent naïve T cells. In thishypothesis, falling thymic output would be matched byresident T-cell proliferation and with age theseproliferating cells will reach their replicativelimit. Their prolonged survival will lead to theaccumulation of cells unable to replicate, producinga decline in immune function and a susceptibility toinfection, or certain cancers.Comparison of gender differences in life-span andrates of death in each age group due to infectious orparasitic disease suggests that the immune system infemales works more efficiently and effectively forlonger than the immune system in males. This leads tothe suggestion that involution of the thymus, andhence thymic output, occurs more rapidly in males thanin females.

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.