Due to the difficulties in experimentally establishing the effects of exercise on specific viral pathogens in humans, studies in mice, including work performed in our laboratory, have been undertaken. One limitation of all these animal studies is that they have all examined primary, and not secondary (after immunological memory has been obtained), responses to infection. Clearly, the effects of exercise on responses to secondary respiratory pathogen exposure need to be performed.
Davis et al.
) inoculated mice with herpes simplex virus-1 (HSV-1) 15 minutes after either a single bout of 30 minutes of moderate-intensity (18 m·min−1
) or high-intensity (gradually increasing from 18 m·min−1
to 36 m·min−1
) treadmill running to volitional fatigue. Sedentary mice served as controls and all mice remained in their home cages for 21 days post-infection. At the 21 day time point, 41% of the mice which exercised to fatigue had died; significantly higher than the 16% mortality rate experienced by the control group. There was no statistical difference in mortality rate between the control and moderately exercised group (9% mortality at 21 days). These results demonstrated that a single bout of intense exercise can cause serious deficiencies in immune defense when a potentially lethal viral infection is contracted. Follow-up studies have confirmed this finding (2
), although one study has also indicated that short-term moderate exercise prior to infection in mice can be protective against viral infection-associated mortality (6
). Unfortunately, HSV-1 is not a true respiratory pathogen, thus this model is limited in the amount of information it can provide regarding the effects of exercise on common respiratory tract infection pathogenesis in people.
As a result of this limitation, our lab conducted a study (15
) which tested the effects of exercise on influenza virus infection. We utilized the common laboratory influenza strain A/Puerto Rico/8/34 at a dose designed to induce approximately 50% mortality in sedentary control Balb/cByJ mice. Four hours after infection, mice began a 4 day exercise program of either 30 (moderate) or 150 (prolonged) minutes of exercise daily. We reasoned that, by applying exercise after infection but before symptom onset, we would impart exercise effects on the developing immune response better than if exercise was applied at some point prior to infection. Moreover, because no one knows exactly when they become infected, we thought this paradigm was more realistic than a single bout of exercise followed by infection. The exercise program was terminated after Day 4 when mice began exhibiting symptoms (e.g.
, lethargy, reduced nest building) of infection. We found that mice assigned to moderate exercise had significantly lower mortality rates than did the control, non-exercising group (18% vs. 56% mortality, respectively) after 25 days (). Mice in the prolonged exercise group did not have a statistically different mortality rate than controls; although prolonged exercise did result in a greater percentage of mortality than controls (70% vs. 56% mortality, respectively). Indeed, the detrimental effect of intense, prolonged exercise on mortality due to influenza virus has recently been established (21
). Mice in the prolonged exercise group also had greater influenza-associated composite morbidity score (as measured by observed physical activity, response to handling, and physical appearance) prior to death than did their control and moderate exercise counterparts; although morbidity scores were not different at any time point between the latter two groups. This study was the first to indicate that moderate exercise initiated after infection but before symptom onset can have a beneficial effect on susceptibility to a true human respiratory pathogen. It is important to note that the dose at which the influenza virus was given induced a lower-respiratory tract infection, thus the results of this study may have implications on viral infections that are considerably more serious than normal infections to which athletes and exercisers are routinely exposed.
Figure 3 Influence of exercise on mortality due to influenza (H1N1 Puerto Rico A/8/34) in male 20–24 wk old Balb/c mice. Mean survival was 14 ± 1, 17 ± 2, and 16 ± 3 days for control, moderate and prolonged exercise, respectively. (more ...)
We conducted a further experiment to define potential mechanisms through which exercise improves survival in this influenza virus model (16
). Using the identical exercise and influenza infection model described above, we analyzed cellular infiltration in lungs, spleen, and draining mediastinal lymph nodes and cytokine gene and protein expression at several time points post-infection. We hypothesized exercise training would promote an anti-inflammatory shift from a Th1 dominated phenotype towards a Th2-type immune response; while not totally abrogating Th1-mediated immunity. Th1 inflammatory immune responses induce upregulation of pro-inflammatory cytokines, particularly IFN-γ, which initiates improved Mϕ antigen presentation and enhanced phagocytotic/cytotoxic activity. A sufficient Th1 response is essential for early anti-viral activity and promotes elevated immune surveillance, enhanced viral clearance, and memory responses. Evidence suggests, however, that a prolonged or exaggerated Th1 inflammatory response triggers tissue damage causing lung pathology ultimately reducing survival rates (31
We found that the same moderate exercise that improved survival also resulted in significantly lower cell infiltration into the lungs and draining lymph nodes and reduced (but not absent) IFN-γ mRNA and protein expression 3 and 5 days post influenza infection. Qualitative protein expression analysis (e.g., antibody array) revealed a two-fold reduction in Th1 type cytokines and chemokines including IFN-γ, IL-17, IL-13, interferon-inducible T-cell alpha chemoattractant (ITAC), leptin, stromal cell-derived factor-1 (SDF-1), and lipopolysaccharide-inducible CXC chemokine (LIX). Contradicting our hypothesis, however, was an observed increase in IL-12, and no change in IL-2, both hallmark cytokines of a Th1 response. In regard to IL-12, our data revealed that the protein was expressed at extremely low levels in the lung tissue. IL-2 plays a critical role in the differentiation and maturation of T regulatory cells (CD4+CD25+), which are anti-inflammatory immune cells which play a crucial role in controlling Th1-type inflammatory responses. A possible explanation is that exercise may not reduce IL-2, but instead improves Treg maturation, promoting an anti-inflammatory environment in the lungs. This hypothesis has yet to be tested.
In parallel with the reduction in many (but not all) Th1 cytokines, we observed a shift toward Th2-type anti-inflammatory cytokine profile. IL-4 lung protein levels were 2-fold higher for the exercising mice compared to sedentary mice 3 days post-infection. IL-4 promotes the differentiation of naïve Th cells to a Th2 phenotype, which then co-stimulate B cells, initiating the production of virus neutralizing antibodies. Viral antibodies reduce viral load by inhibiting infection of uninfected cells and opsonizing virus-infected cells for ADCC. Further supporting the anti-inflammatory hypothesis, exercise increased soluble TNF-α receptor (sTNFrII), which binds circulating TNF-α, preventing membrane binding and the subsequent activation of NF-κβ signaling pathways. In addition to elevated expression of Th2-type cytokines, exercise increased expression of eosinophil chemoattractants, which induce extravasation of eosinophils into viral infected tissue, where their ribonucleases can degrade viral single-stranded RNA, inhibiting virus replication. In summary, we found that moderate intensity exercise following influenza infection reduced lung inflammation by inducing a shift from Th1-type inflammatory response to an anti-inflammatory Th2-type response and this was associated with a reduction in morbidity and mortality. Our findings are consistent with a study by Kohut et al.
who demonstrated that intense, prolonged exercise resulted in reduced IFN-γ and pro-inflammatory cytokines in splenocytes stimulated ex vivo
with HSV-1 (13
). In contrast, they also reported reduced IL-2 production. Comprehensive quantitative analysis of the Th1 and Th2 response in the lungs and respiratory tract of infected mice is needed to definitively determine whether exercise skews cytokine balance.
Explanation of the hormetic effect (e.g.
, favorable biological responses at low doses, unfavorable responses at higher doses) of exercise on survival in response to a primary viral infection is a difficult task. Although speculative, it may be that intense, prolonged exercise affects the cytokine balance and immune cell function more dramatically than moderate exercise. Based upon our data, our current hypothesis () maintains that moderate exercise causes a subtle shift away from Th1 and toward a Th2 response, enhancing recovery and improving survival rates in cases where viral load and morbidity/mortality risk is high. Under these conditions, an exaggerated inflammatory response to the high viral load contributes to the pathology seen in the lung and, ultimately, increased morbidity and mortality. There appears to be a point of diminishing return however; as intense, prolonged exercise leads to a suppression of inflammation and reduction in critical anti-viral effector functions, including those of alveolar Mϕ's (5
) and perhaps NK cells (22
) resulting in increased morbidity and mortality. Indeed, a role for exercise-induced modulation of alveolar Mϕ function in response to HSV-1 infection has been elegantly described by Murphy et al.
). In that study, intranasal treatment with clodronate liposomes (which depleted alveolar Mϕ's) completely inhibited the protective effect of exercise on HSV-1 mortality and morbidity, suggesting a critical role of lung Mϕ's in the initial recognition and clearance of that virus. This contrast between moderate intensity exercise and prolonged or high-intensity exercise is supported by numerous studies (1
) which demonstrate a highly polarized Th2 response, as observed during prolonged intense exercise (29
), may be detrimental to influenza recovery. Together, these findings suggest influenza outcomes are primarily mediated by the Th1/Th2 balance, and a moderate exercise-induced counter-regulatory shift toward Th2 response, without profound suppression of Th1 response, may reduce mortality and morbidity in high risk individuals or in response to inoculums that are quite large (as were the one's used in the above animal studies). It is important to note, however, that our hypothesized model of exercise and respiratory viral infection may not hold true for lower inoculums of virus or secondary exposures.
Figure 4 Hypothetical model describing the exercise dose-response effect on Th1 and Th2 immune responses to respiratory viral infection. Moderate exercise transiently increases glucocorticoids, catecholamines and IL-6 to moderate levels resulting in a subtle skewing (more ...)
As for the direct modulators responsible for a skewing of the immune response, exercise and other physical/physiological stressors promote upregulation of stress hormones, particularly catecholamines and glucocorticoids, which are capable of binding immune cells and influencing anti-viral immune functions. Dhabhar et al.
suggests stress hormones exert a bi-directional effect on immune function, with the slightly elevated concentrations of glucocorticoids and catecholamines observed during acute stress providing crucial immunoenhancing and anti-inflammatory effects during pro-inflammatory reactions (7
). In contrast, chronic stress, which affects circadian rhythms and significantly elevates stress hormone concentration for prolonged periods, exerts immunosuppressive effects and increases susceptibility to infection. Indeed, adrenalectomy and glucocorticoid/catecholamine blockade exacerbates inflammatory diseases and eliminates stress-induced enhancement of skin delayed-type hypersensitivity (DTH) reactions (7
). In addition to stress hormones, exercise increases IL-6 locally in muscle and systemically in blood (26
) which subsequently induces IL1-ra, sTNF receptor and IL-10 that may limit excessive inflammation induced by respiratory virus infection (). It appears that the balance between inadequate and excessive stress responses is the result of evolutionary selective pressure. Acute stressors of limited duration, such as moderate intensity exercise or being chased by a predator, stimulate “fight or flight” responses priming the immune system for potential challenges imposed by the stressor. Chronic stressors, on the other hand, may be evolutionarily adaptive in that immunosuppression conserves energy potentially utilized for coping with the stressor; albeit at the cost of increased risk for infection (7
). Moderate intensity exercise may provide an appropriate stress response that leads to immunopotentiation and anti-inflammatory actions resulting in improved recovery and survival following respiratory viral infection.