Involution of the thymus revisited: immunological trade-offs as an adaptation to aging

Thymus development in the zebrafish (Danio rerio) from an ecoimmunology perspective

The thymus is present in all gnathostome vertebrates and is an essential organ for the adaptive immune system via the generation of functional mature T-cells. Over the life span of mammals, the thymus undergoes morphological and functional alterations, including an age-related involution, which in humans starts in early life. Life history tradeoffs have been suggested as possible reasons for thymus involution. While in teleost fish, only a few studies have investigated alterations of thymus structure and function over different life stages, resulting in a fragmented database. Here, we investigated the thymus growth of zebrafish (Danio rerio) from early life, throughout puberty and reproductive stage, up to 1-year-old. We assessed thymus growth by histological and morphometric analyses and thymocyte numbers. Thymus function was assessed by measuring the transcripts of the thymocyte marker genes, ikaros, tcrα, and tcrδ. Additionally, we analyzed gonad maturity and tail homogenate vitellogenin concentrations to align thymus status with the status of the reproductive system. Our results showed that the zebrafish thymus, in contrast to the human thymus, grew strongly during early life and puberty but started to undergo involution when the fish reached the reproductive age. The involution was characterized by reduced thymus area and thymocyte number, altered histoarchitecture, and decreasing thymocyte marker gene transcript levels. Our findings suggest that age-related changes of the zebrafish thymus do exist and could be partly explained in terms of resource tradeoffs, but also in terms of the ontogenetically late development of a functional adaptive immune system in teleosts.

Aging of the vertebrate immune system

We have summarized current knowledge on the aging of the immune system in three vertebrate groups: fish, amphibians and birds. Available data are few due to difficulties in studying ageing in natural populations and in accurately determining age. In all vertebrates, the most obvious evidence of the senescence of lymphoid tissue is the involution of thymus, which courses with decreased numbers of thymocytes, and loss of the histological organization of gland. On the other hand, there is little information on aged secondary lymphoid organs. Possible influence of the endocrine system in the changes observed in aged lymphoid organs is also discussed.

Myoid cell density in the thymus is reduced during mdx dystrophy and after muscle crush

Thymic myoid cells share structural and behavioural features with cells of the skeletal muscle lineage: they express regulatory genes and contractile proteins, and they can form myofibers in culture. Historically, those features suggested that myoid cells could be precursors for muscle repair in addition to the satellite cells in muscle that are typically designated as the only muscle precursors. Muscles of the mutant mdx dystrophic mouse strain have a large demand for precursors, which is greatest at a young age. In the present study, immunostaining for troponin T was used to localize myoid cells. We tested the hypothesis that the myoid cell population changes when there is a demand for muscle precursors and that these changes would be anticipated if myoid cells have a role as myogenic precursors or stem cells in muscle. Chronic demands for muscle precursors in mdx dystrophic mice were accompanied by lower myoid cell density in comparison with density in two normal strains (C57BL10/ScSn and Swiss Webster). Acute demand for precursors was accompanied by a sharp decline in thymic myoid cell density within 2 days after a crush injury to one tibialis anterior muscle in normal but not dystrophic animals. To standardize the developmental age of the thymus, density was determined in all animals at 28 days of age. Given the current interest in nonmuscle sources of myogenic stem cells, these data suggest that changes in the density of thymic myoid cells may accompany acute and chronic demands for muscle precursors. Further experiments are required to determine whether thymic myoid cells are participants in distant muscle cell proliferation, new fiber formation, or the establishment of new stem cells in regenerated muscle.Key words: thymus, myoid cells, troponin T, MyoD, tissue repair, myoblasts, mdx dystrophy.

Zinc, T-cell pathways, aging: role of metallothioneins

Zinc is an essential trace element for many biological functions, including immune functions. Indeed zinc is required for the biological activity of a thymic hormone, called thymulin in its zinc-bound form, important for the maturation and differentiation of T-cells. With advancing age zinc, thymic functions and peripheral immune efficiency show a progressive decline. Supplementing zinc in old age restores them. Zinc is also relevant for liver extrathymic T-cell pathway, being preeminent in old age. Since zinc is also required for metallothioneins (MTs) biological functions, binding zinc with high affinity, aim of the present article is to summarize findings from our laboratory regarding the role of zinc on T-cell pathways, investigating also the possible cause of thymic involution and impaired liver extrathymic T-cell pathway in aging. Partial hepatectomy and liver regeneration are good models for this aim because of the likeness with aging for many immune functions, including thymic functions. MTs levels are increased, other than into the liver, also into the thymus during aging and in young hepatectomized (pHx) mice as compared to young sham controls. MTs may be one of the possible causes of reduced thymic efficiency and impaired liver extrathymic T-cell pathway in old age because of their higher zinc binding affinity rather than thymulin with consequent reduction of the free quota of zinc available for normal cell-mediated immunity. Following that, MTs may contribute to thymic involution and impaired peripheral immune efficiency in aging and in young pHx mice with different roles during the whole life of an organism: protective in young-adult age which may became, at least, dangerous for immune responses in aging. In order to limit or avoid this latter MTs possible role in aging, supplementing physiological zinc may be useful to improve immune responses in old age because of no interference of endogenous zinc on already high thymus MTs levels, but with caution for competition phenomena with copper, as documented in old mice and in syndrome of accelerate aging.

Clinical features associated with correction of T-cell senescence: increased acute-phase response, amyloidosis and arthritis

Two prominent features associated with immunosenescence are thymic involution and altered T-cell phenotype and responsiveness. We have shown previously that in CD2-fas transgenic mice, in which the Fas apoptosis molecule is constituatively expressed on T cells, T-cell senescence is greatly reduced. Using a different experimental approach, the relationship between T-cell senescence and apoptosis was analyzed on human PBMCs. The results indicate that there was increased apoptosis of CD45RO- (CD45RA+) T cells upon activation. We propose that this could account for the increase in CD45RO+ 'memory' T cells with aging in humans. Together these results are consistent with the notion that T-cell senescence is associated with altered apoptosis and decreased T-cell responsiveness. T-cell responsiveness remained high in CD2-fas transgenic aged mice, but there was no increase in overall life span of the mice. Increased T-cell responsiveness was associated with an increased acute-phase response and amyloid A deposition in the glomerulus of these mice. These data suggest that restoration of the T-cell immune function in aged individuals must be carried out in concert with correction of other immune factors that down modulate the acute-phase response to prevent undesirable side-effects.

Zinc and metallothioneins on cellular immune effectiveness during liver regeneration in young and old mice

Partial hepatectomy in young mice (pHx) induces thymic atrophy, disregulation of thymocytes subsets and a strong accumulation of zinc in thymic tissue after 1-2 days of liver regeneration. Zinc is relevant for good immune functioning. Restoration of zinc into both the thymus and thymocytes subsets in the late period of liver regeneration is observed in young pHx mice. These findings have suggested a link between the thymus and the liver influencing T-cell functions and involving zinc. This kind of link could be relevant in aging because thymic involution, negative crude zinc balance and crippled immune functions are constant events. The preminence of a liver extrathymic T-cell pathway after pHx or during aging has been suggested. Thus the study of pHx in young and old mice may offer a good model to better understand the role played both by thymic involution and by liver extrathymic T-cell pathway and the role of zinc in these physiological processes during aging. Young pHx mice after 1-2 days of liver regeneration show: reduced thymic endocrine activity, increment of double negative (DN) thymocytes subsets, impairment of peripheral immune efficiency (PHA, NK activity and IL-2) and negative crude zinc balance, which are all restored in the late period of liver regeneration. By contrast the thymic and peripheral immune defects and the negative crude zinc balance, already present in old sham mice, are not modified during liver regeneration in old pHx mice. Circulating leukocytes and lymphocytes are not significantly modified both in young and old pHx mice as compared to respective sham controls. Zinc may also be crucial for extrathymic T-cells pathway, being preminent in aging, rather than in young age, due to its metallothioneins (MT) binding capacity. MT are significantly increased in young pHx and in aging inducing a low zinc-free quota for thymic and peripheral immune efficiency in young pHx mice, and for extrathymic T-cell pathway, in old age. Thus low zinc bioavailability, due to MT, may play a pivotal role, not only for thymocytes but also for liver extrathymic T-cell pathway.

Increased apoptosis of CD45RO- T cells with aging

Increasing susceptibility to infectious and autoimmune phenomena have long been recognized to accompany advancing age in otherwise healthy individuals. Recently animal models of aging have suggested that age-associated immune dysfunction may correlate with defects in T cell apoptosis. We have examined activation-induced apoptosis defects in human peripheral T cells from young individuals (mean age = 31 +/- 3 years old) compared to aged individuals (mean age = 67 +/- 8 years old). Following in vitro activation of T cells with PHA and IL-2, apoptosis was measured in T cell subsets using 7-amino actinomycin D (7-AAD) staining and analysis via three colour flow cytometry. There was no significant difference in apoptosis of the total CD3+ T cell population at early and late time points. Interestingly, increased apoptosis in the CD3+ CD45RO- T cell population of older adults was observed by culture day 6. While the total number of CD3+ CD45RO- cells was not different between young (< 33 years) and old (> 65 years) individuals, 32% of these cells did not undergo apoptosis in younger individuals while only 10% of these cells avoided this fate in older individuals. These results suggest that accumulation of CD45RO+ T cells may occur in aged subjects due in part to preferential elimination of CD45RO- cells with activation. Furthermore, as new or continued immune response requires differentiation of CD45RO+ T cells to CD45RO+ T cells after activation, increased apoptosis instead of survival in aged individuals could lead to observed T cell immune deficiency.

Age-dependent changes in thymuses in the European common frog, Rana temporaria

The thymus of the adult frog Rana temporaria is generally small each winter and grows through the spring to reach a large size each summer. The summer thymus has a cortex full of small thymocytes and a medulla in the centre, whereas the winter atrophy is manifested by a loss of distinction between cortex and medulla, an abundance of cells filled with secretory granules, and the formation of intercellular cysts. These seasonal changes are superimposed on age changes. The thymus grows rapidly in froglets. The differences in weight and cell number between winter and summer organs are strongest in middle-aged animals (3-6 years old) and decrease in old specimens. The thymus slowly involutes with age, this being connected with increasing winter atrophy, leading to the formation of huge cysts that fill almost the whole organ in the oldest individuals. In senescent frogs (around 10 years old) seasonal differences still concern corticomedullary division but without pronounced fluctuations in thymic size. The skeletochronological technique applied here for age estimation underestimated rather than overestimated the real age of old animals.