Rejuvenation of the aging thymus: growth hormone-mediated and ghrelin-mediated signaling pathways

Abstract

One of the major fundamental causes for the aging of the immune system is the structural and functional involution of the thymus, and the associated decline in de novo naïve T-lymphocyte output. This loss of naïve T-cell production weakens the ability of the adaptive immune system to respond to new antigenic stimuli and eventually leads to a peripheral T-cell bias to the memory phenotype. While the precise mechanisms responsible for age-associated thymic involution remain unknown, a variety of theories have been forwarded including the loss of expression of various growth factors and hormones that influence the lymphoid compartment and promote thymic function. Extensive studies examining two hormones, namely growth hormone (GH) and ghrelin (GRL), have demonstrated their contributions to thymus biology. In the current review, we discuss the literature supporting a role for these hormones in thymic physiology and age-associated thymic involution and their potential use in the restoration of thymic function in aged and immunocompromised individuals.

Figure 1. GH enhances thymic cellularity and thymopoiesis in aged mice

(A) GH infusion for 2 weeks via s/c osmotic pumps causes a significant increase in thymic cellularity in 14m old BALB/c mice. (B) GH infusion in old mice led to a >2 fold increase in ETPs in thymus (p<0.05) with no significant change in DN2 (lin^lowC-kit⁺CD25⁺) DN3 (lin^lowC-kit^-CD25⁺) or DN4 populations (data not shown). In addition, the bone marrow cells were analyzed for lin^lowsca1⁺ckit⁺ (LSK) populations where GH infusion also significantly increased (p<0.05) the bone marrow LSK cells in old but not young mice. Data is represented at mean ± SEM (n = 6) of 14m old mice. (C) GH infusions also increased the number of recent thymic emigrants in aged mice. (D) GH induced significant and reproducible alterations in the TCR diversity as assessed by CDR3 length PCR analysis. In comparison to ghrelin and leptin, GH was found to induce much more dramatic changes in the Gaussian profiles of infused mice compared to sham controls. The CDR3 sizes are shown in x-axis and the peak fluorescence intensity is shown on the y-axis. While not shown, similar results were observed using 6m and 18-20m old mice but not using 2m old mice suggesting that this hormone mediates it optimal effects when immunological space is available and age-associated thymic atrophy or involution has initiated. Moreover, an increase in spleen cellularity was also observed in the aged mice infused with GH vs. vehicle (data not shown). The GH doses utilized in these studies did not result in significant weight gain or loss demonstrating that the effects on immune parameters occur using doses of GH that do not induce appreciable anabolic effects. It should also be noted that it is quite possible that the mechanisms by which GH exerts its pro-thymopoietic effects may be via the production of other hormonal/cytokine survival factors such as IGF1, KGF or possible even GRL.

Figure 2. Intrathymically-expressed GH and acylated GRL proteins appear to co-localize in the murine thymus and this co-expression appears to diminish with age

(Upper Panel) GH (green) and acylated GRL (red) immunopositivity appears to be quite strong in both the cortical and medullary regions of the 2- and 4-month old thymi shown here. The majority of the thymic GH staining appears to co-localize with the acylated GRL protein (yellow). Over time, the thymic GH and acylated GRL levels start to diminish. In 18- and 24-month old mice (Lower Panel), the total intrathymic GH and acylated GRL levels are almost entirely absent in these sections. These results have since been confirmed using real time RT-PCR analysis of whole thymi and thymocytes derived from mice at different ages (data not shown). These sections are representative of 2 or more thymi in each group,

Figure 3. Acylated GRL increases thymic size and cellularity in aging mice

(A) Acylated GRL is infused for 2 weeks via s/c osmotic pumps after which thymic size, weight and cellularity is examined. A significant increase in thymic size in 20m mice infused with acylated GRL compared to vehicle controls. (B) GRL infusion is also associated with increased cellularity and much clearer delineation of cortex from medulla as well as a well defined CMJ, the site where progenitors enter the thymus. (C) Moreover, acylated GRL infusion into aged (20m) mice increases the total thymocyte numbers and thymic weight (not shown) compared to vehicle (Sham) control animals. More detailed studies on the prothymic effects of GRL can be found in Ref. 97.