Is mechanical loading essential for exercise to preserve the aging immune system?

Excerpt

Exercise improves all aspects of health, including immunity. It is well known that exercise at a level consistent with most national physical activity guidelines can improve immune responses to vaccination, lower the incidence of infection and latent viral reactivation, lower chronic low-grade inflammation, and is associated with a reduction in many hallmark features of an aging immune system, especially in older adults [1]. The lymphoid compartment is particularly susceptible to age-related changes and also appears to be most responsive to exercise. For instance, the blood T-cell compartment in physically active compared to sedentary older adults express fewer markers of differentiation and exhaustion and have higher numbers and proportions of naïve T-cells. Moreover, T-cells (particularly CD8+ T-cells) from those who are physically active have an enhanced ability to secrete IL-2 and proliferate in response to antigenic stimulation, while NK-cells in those who are physically active have an increased ability to secrete effector cytokines and perform anti-tumor cytotoxic functions [1]. While the beneficial effects of exercise on immunity are apparent, details of the mechanisms responsible have remained opaque. We do know that exercise evokes the release of several cytokines from contracting skeletal muscle (i.e. ‘myokines’) that can have a direct impact on the peripheral lymphoid compartment. For instance, IL-7 is released from muscle during exercise which can, in turn, help preserve thymic mass and T-cell production to potentially help sustain the naïve T-cell compartment with advancing age. Similarly, IL-15 released by muscle can maintain peripheral T-cell and NK-cell numbers and enhance NK-cell cytotoxicity and cytokine secretion. While these links between exercise-released myokines and immune maintenance are intuitive and have been demonstrated in cross-sectional studies, they have yet to be tested mechanistically [1]. Furthermore, the optimal exercise prescription/dose as a function of the FITT (frequency, intensity, time and type) criteria and principles of exercise training (e.g. overload, progression) required to evoke positive changes in immunity remain elusive. …