Aging of the vertebrate immune system

Abstract

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.

Architectural changes in the thymus of aging mice

“…In contrast to the decline in thymic epithelium there is an increase in fibroblasts (Fig. 3B), which is also observed in fish (Torroba & Zapata, 2003) and humans (Bertho et al ., 1997). The exact role of fibroblasts in the thymus is unclear, and the increase in number, particularly in the medulla, might be part of the cause or the affect of thymic involution, although it appears to be another conserved aspect of this process.…”

“…Thymic involution has become a hallmark of the aging immune system and almost all vertebrates with a thymus undergo age-associated thymic atrophy, making it an evolutionary ancient and conserved process (Torroba & Zapata, 2003). Several hypotheses have been proposed (Taub & Longo, 2005;Aw et al ., 2007), including extrinsic influences such as growth hormones and sex steroids Sutherland et al ., 2005) or defects and/or decline in T-cell progenitors (Min et al ., 2004).…”

An evolutionary perspective on the mechanisms of immunosenescence

“…Thus, it is unclear whether these types of acute thymic atrophy actually occur by the same cellular and molecular mechanisms that underlie age-related involution. Substantial evidence suggests that the age-associated regression of the thymus occurs in perhaps all vertebrates that have a thymus, including avians [48], amphibians [49] and teleosts [50,51], indicating that this is an evolutionary ancient and conserved event [52]. Despite chronic thymic involution being one of the major hallmarks of immunosenescence, there is much confusion over the initiation of the process.…”

It’s not all equal: a multiphasic theory of thymic involution

“…However, with age there is a decline in the thymus size and function (George and Ritter 1996;Taub and Longo 2005;Lynch et al 2009), an event that is evolutionary ancient and conserved (Torroba and Zapata 2003). Regression of the thymus leads to a decline in naïve T cell output (George and Ritter 1996;Taub and Longo 2005;Lynch et al 2009), and subsequently affects the composition of the peripheral T cell pool, including constriction of T cell diversity, alterations in their phenotype and function and corrosion of telomeres due to replicative senescence (Nikolich-Zugich 2005; Goronzy and Weyand 2005;Henson and Akbar 2010).…”