Heterogeneous aging of macrophage-lineage cells in the capacity for TNF production and self renewal in C57BL/6 mice

Toll-like Receptor function of murine macrophages, probed by cytokine induction, is biphasic and is not impaired globally with age

Aging is associated with a waning of normal immune function. This "immunosenescence" is characterized by a diverse repertoire of seemingly discreet and unbalanced immune alterations. A number of studies have suggested that aging-associated alterations in innate immune responsiveness, especially responsiveness dependent on Toll-like Receptor (TLR) engagement, are causally involved. We find, however, that the magnitude and dose-dependency of responsiveness to TLR engagement (assessed with respect to cytokine production) in distinct populations of murine macrophages are not altered generally with animal age or as a consequence of immunosenescence. Responses elicited with a wide array of TLR agonists were examined by extensive functional analyses, principally on the level of the individual cell. These studies reveal an intriguing "all-or-nothing" response behavior of macrophages, independent of animal age. Although reports to the contrary have been cited widely, aging-associated immune decline cannot be attributed to widespread alterations in the extents of TLR-dependent innate immune macrophage responses.

Glucocorticoids produced during exercise may be necessary for optimal virus-induced IL-2 and cell proliferation whereas both catecholamines and glucocorticoids may be required for adequate immune defense to viral infection

Prolonged, exhaustive exercise has been associated with impaired immune responsiveness and increased susceptibility to infection. We have shown that one bout of exercise to fatigue followed by viral challenge increases mortality. Stress hormones such as corticosteroids and catecholamines have been suggested as potential mediators of exhaustive exercise-induced immunosuppression. The purpose of this study was to determine whether the administration of pharmacological agents to block the effect of catecholamines or corticosteroids would minimize the immunosuppression associated with this type of exercise. Mice either exercised to fatigue or were exposed to control conditions, and mice received an i.p. injection of either nadolol (beta-adrenergic receptor antagonist), RU486 (glucocorticoid type II receptor antagonist), or vehicle. Fifteen minutes post-exercise, mice were exposed to viral infection (Herpes simplex virus; HSV) via an intranasal route, and cells were collected 3 days post-infection. The results showed that exercise suppressed HSV-specific cell proliferation, HSV-specific IL-2, and IFN-gamma, but did not alter these same immune parameters when the mitogen ConA was used to stimulate cells. In addition, exercise reduced NK cell cytotoxicity, alveolar cell TNFalpha, and peritoneal IL-1beta, but did not affect IL-10. The pharmacological blockade did not attenuate the exercise-associated immunosuppression. In fact, RU486 treatment exacerbated the exercise-induced decline in HSV-induced IL-2 production and cell proliferation. RU486 and nadolol treatment also tended to decrease IL-10, IFN-gamma, TNFalpha (nadolol only), and IL-1beta (RU486 only) in both exercise and control mice, suggesting that stress hormones may be necessary during infection for optimal responsiveness. These findings suggest that suppression of immune defenses during viral infection persists for at least 3 days post-exercise, and stress hormones may be essential for optimal immune defense to viral challenge, rather than detrimental.

Immunosenescence and macrophage functional plasticity: dysregulation of macrophage function by age-associated microenvironmental changes

The macrophage lineage displays extreme functional and phenotypic heterogeneity, which appears to be because, in large part, of the ability of macrophages to functionally adapt to changes in their tissue microenvironment. This functional plasticity of macrophages plays a critical role in their ability to respond to tissue damage and/or infection and to contribute to clearance of damaged tissue and invading microorganisms, to recruitment of the adaptive immune system, and to resolution of the wound and of the immune response. Evidence has accumulated that environmental influences, such as stromal function and imbalances in hormones and cytokines, contribute significantly to the dysfunction of the adaptive immune system. The innate immune system also appears to be dysfunctional in aged animals and humans. In this review, the hypothesis is presented and discussed that the observed age-associated 'dysfunction' of macrophages is the result of their functional adaptation to the age-associated changes in tissue environments. The resultant loss of orchestration of the manifold functional capabilities of macrophages would undermine the efficacy of both the innate and adaptive immune systems. The macrophages appear to maintain functional plasticity during this dysregulation, making them a prime target of cytokine therapy that could enhance both innate and adaptive immune systems.

Age effects on macrophage function vary by tissue site, nature of stimulant, and exercise behavior

We explored the effects of aging on macrophage function in male BALB/c mice from three age groups: young (2 months), middle-aged (12 months), and old (21 months). Macrophages were collected from alveoli, peritonea, and spleens of each age group. Cells were cultured in vitro with LPS or LPS+IFN-gamma and assayed for production of IL-1, IL-12, NO, and TNF-alpha. Using herpes simplex virus-1, age-related changes in intrinsic antiviral resistance (plaque assay) and extrinsic antiviral resistance (NO and TNF-alpha production) were determined in alveolar and/or peritoneal macrophages. Effects of chronic exercise on age-related macrophage changes were examined. In vitro, macrophages from the alveoli and spleen of older mice generally produced more cytokine and NO compared to younger counterparts. Conversely, macrophages from the peritonea of older mice generally produced less cytokine and NO in vitro compared to younger counterparts. Alveolar macrophages from both old and young mice showed higher intrinsic antiviral resistance to HSV-1 compared to middle-aged mice, while peritoneal macrophages from young mice showed reduced intrinsic resistance compared to those from both middle-aged and old mice. When challenged with HSV-1, a trend towards decreased peritoneal macrophage production of TNF-alpha and decreased alveolar macrophage production of IL-12 with advancing age was found. Chronic moderate exercise tended to reverse age-associated changes in macrophage function in old mice.

Immune dysfunction following trauma-haemorrhage: influence of gender and age

Recent studies indicate that young female proestrus mice show an enhanced immune response following trauma-haemorrhage, as opposed to the immunodepression observed in males of comparable age. Testosterone is suggested as the cause of immunodepression in males, whereas oestradiol seems to be responsible for the enhanced immune response in females, however, sex hormone levels decrease with age. To determine if the sexual dimorphism in immune responses observed in young mice following trauma-haemorrhage changes with age, young (2-3 months) and aged (18-19 months) male and female CBA/J NIA mice were subjected to soft-tissue trauma (laparatomy) and haemorrhage (35+5 mmHg for 90 min and fluid resuscitation) or sham operation. Mice were killed 24 h later, and whole blood, as well as splenic and peritoneal macrophages (Mstraight phi) obtained. Plasma 17beta-oestradiol and free testosterone decreased in aged females and males, respectively. Mstraight phi from young females had enhanced IL-1beta and suppressed IL-10 production following trauma-haemorrhage, while aged females had unchanged production IL-1beta and IL-6 production and enhanced IL-10 release. In contrast, IL-1beta and IL-6 production by Mbeta from young males was suppressed and IL-10 production enhanced following trauma-haemorrhage, whereas Mstraight phi from aged males produced elevated levels of IL-1beta and IL-6 and suppressed levels of IL-10 following trauma-haemorrhage. Thus, the gender-related changes in the immune response to trauma-haemorrhage were reversed in aged mice.