Chronic exercise appeared to have the greatest benefit in terms of the reduction of illness symptoms (eg, decreased appetite, weight loss). Acute and chronic exercise resulted in reduced infection-associated weight loss and improved food intake. This effect appeared to persist throughout the course of infection in C-EX mice, whereas the benefit appeared to be limited to the early phase of infection in the A-EX group. The immunopathologic findings correlate with the symptoms of illness and lend further support to the finding that illness is reduced by moderate exercise training.
At 2 days after infection, there was a trend toward less virus in the lungs of exercised mice; this difference approached statistical significance only in the C-EX mice. By 5 days after infection, exercised mice had lower lung virus titers, and the magnitude of the reduction in virus was comparable for mice in the 2 types of exercise groups. These findings suggest that the early innate antiviral defenses appear to be improved by identified. Type I interferons have long been known to exhibit antiviral action and are produced by multiple cell types in the respiratory tract [12
]. However, we did not observe enhanced production of IFN-α in the BAL fluid of exercised mice measured 48 h after infection, although exercise may have had an effect at earlier time points (data not shown). Others have shown that peak IFN-β levels are found 48 h after infection [13
]. A trend toward an exercise-associated increase in IFN-β 24 h after infection was observed when mice were exercised on days 0–3 after infection [23
]. IFN-β was not measured in our study owing to limited BAL fluid. NK cells also contribute to the innate defense against influenza virus infection, and depletion of this cell population results in greater mortality [25
]. We did not assess lung NK function owing to limited cell numbers in the BAL, and we are not aware of other exercise studies that have evaluated lung NK response during infection.
Other immune-mediated defenses that could contribute to an early reduction in viral titer include the β-defensins, collectins, and lung surfactant proteins A and D. These proteins have been shown to play a role in protection from influenza infection through actions such as neutralization of influenza virus and opsonization [26
]. It has been shown that an acute session of intense exercise enhances surfactant-mediated phagocytosis by alveolar macrophages [30
]. To our knowledge, the effect of regular exercise training on the function of any of these innate antiviral defenses (β-defensins, collectins, or surfactant proteins) during infection has not been evaluated. Studies of airway function and inflammation in elite athletes show a higher prevalence of respiratory symptoms and airway inflammation [31
]. Although higher numbers of neutrophils, eosinophils, and lymphocytes have been observed in elite athletes, greater cell activation may not be present [32
]. Elite athletes may have prolonged periods of hyperpnea with intense exercise, but moderate exercise, like that used in our study, would not be expected to cause hyperpnea. Therefore, further research on lung adaptations to moderate exercise is necessary to provide a better understanding of protective mechanisms.
Exercise also reduced inflammatory factors in the lungs. Levels of IL-6, TNF-α, MCP-1, MIP-1β, KC, and RANTES were reduced at day 2 after infection in both groups of exercised mice. This effect appeared to be transient in the A-EX mice but persisted in the C-EX mice. The decrease in TNF-α may be relevant to the reduced morbidity found in both exercise groups. Elevated TNF-α levels have been linked to increased mortality, supported by the finding that TNF-α knockout mice have improved survival after influenza infection [35
The early changes in chemokines (MCP-1, MIP-1β, RANTES) observed in both exercise treatments may affect inflammatory cell recruitment. Mice deficient in CCR2+
(MCP-1 receptor) show reduced infiltration of inflammatory cells leading to reduced mortality and immunopathologic response [18
]. Perhaps the reduction of MCP-1 that we observed at days 2 and 5 after infection in C-EX mice contributed to the reduced morbidity. We did not assess CCR2 in our study; although the data on lung cell populations did not reach statistical significance, they followed a pattern that would be predicted by the change in chemokines (reduced KC levels would predict the trend toward lower numbers of neutrophils in A-EX and C-EX mice). Neutrophil accumulation in the lungs may play a role in virus elimination, but massive accumulation of neutrophils associated with a secondary infection has been correlated with increased mortality [36
]. On the basis of symptoms and lung viral load, one can conclude that the reduction in the chemokine KC along with the tendency toward reduced neutrophil accumulation found in our study did not impair the response to infection. Explanations include the possibility that neutrophils are not critical to recovery from infection or that very early innate defenses reduced viral titer enough that a large neutrophil influx was not necessary.
Our findings can be compared with those of another study in which exercised mice were challenged with influenza virus [23
]. The results from both studies showed decreased KC and a tendency to reduced leukocyte infiltration. However, the other researchers concluded that exercise may shift toward a Th2 profile, whereas our results showed a decrease in IL-12. It is not clear whether an enhancement of Th2 responses would be of benefit, because some have suggested that IL-4 may lead to impaired clearance of primary infection or delayed clearance of secondary influenza infection [38
]. The studies used different techniques to measure cytokines and chemokines, as well as different exercise models, which may account for the discrepancies in findings.
In summary, the findings presented here first demonstrate that repeated moderate exercise before infection can positively affect infection outcome. A single session of exercise also confers some benefit, although this appears to be present only in the first days after infection. Coupled with the improvement in morbidity and reduction of viral load is a pattern of changes in chemokines and cytokines that provides some insight into potential mechanisms of action, but further research is necessary to identify these mechanisms precisely. Our findings are also the first to show that regular moderate exercise before infection has anti-inflammatory effects at a local site of infection, expanding the current literature on the anti-inflammatory benefits of exercise.