In humans, both aging and GH deficiency are associated with reduced protein synthesis, decreased lean body and bone mass, and increased percent body fat. In healthy individuals, spontaneous and stimulated GH secretion, as well as circulating IGF-I and IGFBP-3 levels, are significantly decreased with advancing age. The extent to which these age-related changes in GH and IGF-I contribute to alterations in body composition and function remains to be elucidated. GH treatment of GH-deficient adults or old men with reduced IGF-I levels with exogenous GH increases plasma IGF-I, nitrogen retention, and lean body mass, decreases percent body fat, and exerts little effect on bone mineral density. Short-term adverse effects of GH therapy have been minimized by using low-dose regimens, but it is still uncertain whether long-term GH supplementation in adult life increases the risk of metabolic abnormalities or malignancy. Administration of GHRH, which has been shown to maintain the pattern of pulsatile GH secretion in old men, may represent another possible physiological approach to therapy. It may be justifiable initially to limit use of GH to certain elderly patients such as those suffering from catabolic illnesses, malnourishment, burns, cachexia, etc. A great deal more research will be necessary to determine whether normalization of GH and IGF-I levels in healthy older persons will lead to improvements in their physical and psychological functional capacity and quality of life.

Human Growth Hormone and Human Aging

Aging is associated with increased inflammatory activity reflected by increased circulating levels of TNF-α, IL-6, cytokine antagonists and acute phase proteins in vivo. Epidemiologic studies suggest that chronic low-grade inflammation in aging promotes an atherogenic profile and is related to age-a

Aging and proinflammatory cytokines.

Age-related inflammatory cytokines and disease

We evaluated TLR function in primary human dendritic cells (DCs) from 104 young (age 21-30 y) and older (> or =65 y) individuals. We used multicolor flow cytometry and intracellular cytokine staining of myeloid DCs (mDCs) and plasmacytoid DCs (pDCs) and found substantial decreases in older compared with young individuals in TNF-alpha, IL-6, and/or IL-12 (p40) production in mDCs and in TNF-alpha and IFN-alpha production in pDCs in response to TLR1/2, TLR2/6, TLR3, TLR5, and TLR8 engagement in mDCs and TLR7 and TLR9 in pDCs. These differences were highly significant after adjustment for heterogeneity between young and older groups (e.g., gender, race, body mass index, number of comorbid medical conditions) using mixed-effect statistical modeling. Studies of surface and intracellular expression of TLR proteins and of TLR gene expression in purified mDCs and pDCs revealed potential contributions for both transcriptional and posttranscriptional mechanisms in these age-associated effects. Moreover, intracellular cytokine production in the absence of TLR ligand stimulation was elevated in cells from older compared with young individuals, suggesting a dysregulation of cytokine production that may limit further activation by TLR engagement. Our results provide evidence for immunosenescence in DCs; notably, defects in cytokine production were strongly associated with poor Ab response to influenza immunization, a functional consequence of impaired TLR function in the aging innate immune response.

Age-Associated Decrease in TLR Function in Primary Human Dendritic Cells Predicts Influenza Vaccine Response

Since it is too difficult to study aging of the organism as a whole, most investigators try to focus on a specific physiological system that exhibits age-dependent functional changes, in the hopes that elucidation (in biochemical and developmental terms) of the mechanism of senescent change will provide insight into the aging process itself. The immune system is among the most maleable of such models, in that well-defined cell types will produce well-defined molecules with predictable functions in vitro and in vivo. The increasing power of basic immunological science should, in the next decade, permit an increasingly fine appreciation of how aging leads to immune decline. This expanding conceptual framework will then suggest new ideas about the role of immunosenescence in degenerative, infectious, and neoplastic illnesses and may also generate increasingly rational strategies for therapeutic intervention.

Aging and immune function

Growth hormone (GH) appears to play a major role in a reciprocal axis that has been postulated between the thymus and pituitary glands. Our previous studies showed that thymic structure, as well as T-cell proliferation and IL-2 synthesis, could be restored in aged female Wistar-Furth rats by the implantation of GH3 pituitary adenoma cells. These cells secrete GH and some prolactin. We have now used three different approaches to determine whether GH affects a variety of immune events in vivo in both old and young rodents, and whether GH3 cells can directly affect progenitor T-cells in nude rats that congenitally lack a thymus gland. To test the effects of GH in aged rats, 750 μg of pituitary-derived ovine GH was injected 2× daily into 26-month-old Fischer 344 rats for 5 weeks. This approach demonstrated that GH augments splenocyte proliferation to T-cell lectins as well as natural killer (NK) activity at low effector: target ratios even though morphologic characteristics of the thymus were not altered. To assess the effect of GH in young rodents, mice were studied that were transgenic for the rat metallothionein-GH gene. Histologic evaluation of thymus glands revealed that the amount of adipose tissue and the number of epithelial cells and Hassall's corpuscles are augmented in transgenic mice. Splenocyte proliferation at suboptimal mitogen doses is greater in transgenic than in control littermate mice, but neither IL-2 synthesis nor antibody synthesis to sheep erythrocytes is affected. The role of pituitary hormones on progenitor T-cells was then explored by implanting GH3 cells into Rowett nude rats. GH3 cells are unable to augment proliferation to T-cell lectins, IL-2 synthesis, antibody production to sheep erythrocytes, or NK activity in nude rats. Collectively, these experiments show that T-cells and NK cells from aged rats are not inherently defective because their proliferative responses to lectins and cytolytic activity can be augmented by exogenous GH. If GH affects progenitor T-cells, these effects are not apparent in nude rats. Finally, GH can also augment T-cell proliferative responses in young rodents, but the immunoenhancing effects are not as apparent as thos observed in immunocompromised aged animals.

Role of growth hormone in regulating T-dependent immune events in aged, nude, and transgenic rodents.

Macrophage responses to recombinant IFN-gamma decline during aging, as measured by two criteria of macrophage activation, O2- and TNF-alpha secretion. The production of O2- by macrophages in response to opsonized-zymosan and recombinant rat IFN-gamma is 75% lower in 23-month-old rats than in 3-month-old rats. Furthermore, the secretion of TNF-alpha in response to IFN-gamma and LPS is almost absent in macrophages from aged rats. Production of both O2- and TNF-alpha by resident peritoneal macrophages from specific pathogen-free aged rats in response to priming and triggering stimuli was partially or fully restored by implantation of syngeneic pituitary grafts from young rats. These data demonstrate that macrophages from aged rats are defective in their response to a priming signal induced by IFN-gamma, and they suggest that impaired macrophage responses during aging may be reversible.

Interferon-gamma-induced priming for secretion of superoxide anion and tumor necrosis factor-alpha declines in macrophages from aged rats.

Sex differences in lectin-induced interleukin-2 synthesis in aging rats.

Thymic involution is a normal consequence of aging. It has often been speculated that if this age-associated atrophy of the thymus gland could be prevented, the natural decline that occurs in a number of T-cell-mediated immune responses could be reversed. It has recently become clear that thymic involution can indeed be reversed by altering the hormonal environment of aged animals. These data support the concept of an active and functional pituitary-thymus axis. Since thymic reconstitution can result in restoration of some T-cell responses, it would appear that intrinsic defects which exist in T cells of aged animals can be at least partially reversed. This suggests that the aged environment plays a greater role in the decline of T-cell functions than has been previously recognized. Furthermore, phorbol esters and calcium ionophores can restore suppressed proliferative responses of T cells from aged rodents, so we speculate that intrinsic defects in T cells of aged subjects lie between the recognition system for antigen/lectin and intracellular transmission of this signal.

A pituitary-thymus connection during aging.

Splenic mononuclear cells (SMC) proliferate well in response to phytomitogens in serum-free medium. Low concentrations of corticosterone significantly inhibit both spontaneous (1 ng/ml) and phytohemagglutinin-induced proliferation (12 ng/ml) of chicken SMC. Addition of as little as 3% normal heat-inactivated chicken serum to both autologous and heterologous suspensions of chicken SMC causes a 50% inhibition in lectin-induced proliferation. This suppression by normal chicken serum increases in a dose-dependent manner. Inhibition of DNA synthesis by chicken serum is not due to endogenous corticosterone because charcoal stripping of the serum to deplete glucocorticoids does not remove the immunosuppression. Chicken serum is not cytotoxic to lymphoid cells. These data demonstrate that low, physiologic concentrations of corticosterone inhibit proliferation of chicken SMC. Utilization of a serum-free culture system also permitted the identification of a serum factor that appears to be even more immunosuppressive than corticosterone. Since this factor inhibits the lectin-induced proliferation of both autologous and heterologous avian lymphoid cells, it may be a very important naturally occurring immunoregulatory compound.

Donna R. Davila

Papers

Role of growth hormone in regulating T-dependent immune events in aged, nude, and transgenic rodents.

Interferon-gamma-induced priming for secretion of superoxide anion and tumor necrosis factor-alpha declines in macrophages from aged rats.

Sex differences in lectin-induced interleukin-2 synthesis in aging rats.

A pituitary-thymus connection during aging.

Chicken serum inhibits lectin-induced proliferation of autologous splenic mononuclear cells.