Abstract
Background. Decreases in immune responsiveness with age contribute to the increased incidence and severity of infectious disease among elderly adults. The immune response to immunization also decreases with advancing age. Lifestyle factors (exercise, diet) have been established to play an important role in immunosenescence, and the practice of “healthy” behavior may minimize the age-associated decline of immune function. The objective of this study was to determine whether exercise, diet, and psychosocial factors were associated with altered immune response to influenza vaccine.
Methods. Adults aged 62 years and older were categorized into one of three groups: active (≥20 min vigorous exercise three or more times per week), moderately active (regular exercise but with less intensity, frequency, and/or duration), or sedentary (no exercise). Two weeks postimmunization, serum was frozen for antibody analysis, and peripheral blood mononuclear cells (PBMC) were cultured in vitro with influenza vaccine to elicit antigen-specific responses (proliferation and cytokine [IL-2, IFN-γ, IL-10] production). Cytokines and antibody were measured by enzyme-linked immunosorbent assay.
Results. The results demonstrated that anti-influenza IgG and IgM were greater in active as compared with moderately active or sedentary participants. PBMC proliferation was lowest in sedentary subjects. Perceived stress was a significant predictor of IL-2. Greater optimism and social activity were associated with greater IL-10. Daily multivitamin intake was significantly correlated with IL-2.
Conclusions. These results suggest that lifestyle factors including exercise may influence immune response to influenza immunization. The practice of regular, vigorous exercise was associated with enhanced immune response following influenza vaccination in older adults.
NUMEROUS changes in immune responsiveness occur with age including diminished T-cell proliferation, reduced IL-2 production, and decreased antibody production (1)(2)(3)(4). The age-associated decline of immune function may contribute to an increased susceptibility to infectious disease. For example, in 1997, pneumonia/influenza was the fifth leading cause of mortality among those 65 years of age and older, but the 10th leading cause of death for adults aged 25–44 (5). Influenza vaccine efficacy estimates for elderly adults range from 31% to 65% in preventing influenza (6), whereas vaccine efficacy ranges from 68% to 88% among younger adults (7)(8). A significant number of older adults may not be fully protected against influenza even though they have been immunized. Upon infection with influenza virus, cell-mediated immune responses are important in clearing the infection; however, cell-mediated responses are also impaired among elderly people (9)(10)(11)(12).
The degree of immune decline varies widely among older adults and may be related to health behaviors. Nutrient intake and psychosocial factors can affect resistance to infection as well as the immune response to influenza immunization (13)(14)(15)(16). Exercise may also alter immune function in older adults (17)(18)(19). Nutritional and psychosocial factors can modulate antigen-specific responses among older adults (13)(16)(20); however, very little data exist regarding the potential effect of exercise on influenza-specific immunity.
Influenza is a significant cause of mortality, and vaccination does not always provide adequate protection in older adults. Therefore, it is worthwhile to evaluate the potential role of exercise as a method of enhancing the immune response to influenza immunization. The purpose of this study was to test the hypothesis that exercise is associated with enhanced influenza-specific immune responses following immunization. Antibody (IgG, IgM) and cell-mediated immune responses (influenza-specific lymphocyte proliferation and IL-2, IL-10, IFNγ production) were evaluated in older adults of differing physical activity levels.
Methods
Subjects
Fifty-six adults aged 62 years and older participated in the study. Individuals suffering from untreated chronic disease, autoimmune disease, cancer, or any other disease known to alter immunity were excluded. All subjects completed a written informed consent, and the project was approved by the Iowa State University Human Subjects review board.
Exercise Group Assignment
A phone interview was used to assess the level of physical activity. Subjects reported the type(s) of exercise they participate in, described how often and how many minutes they typically exercise, and rated the intensity of exercise as moderate or vigorous. Vigorous was defined as “exercise that is intense enough to cause large increases in heart rate, breathing, sweating, at a level that makes it somewhat difficult to carry on a normal conversation.” Moderate was defined as “exercise that is not intense enough to cause large increases in heart rate, breathing, sweating, at a level that makes it possible to carry on a normal conversation.” Participants were then classified as sedentary, moderately active, or active. Active people (n = 16; nine women and seven men) participated in aerobic exercise at a “vigorous” intensity 20 minutes or longer three or more times per week for the previous year. Moderately active people (n = 25; 17 women, and 8 men) participated in aerobic exercise at a “moderate” intensity one or more times per week over the previous year, but did not meet criteria for inclusion in the active group. Sedentary people (n = 15; nine women, six men) did not exercise or participated in aerobic exercise less than once per week.
Diet and Psychosocial Questionnaires
Diet was assessed by the Block 1998 food frequency questionnaire. The Perceived Stress Scale (PSS) (20) and Life Orientation Test (LOT), a measure of optimism (21), were administered. Subjects also reported the number of different social activities they participated in on a monthly basis.
Immunization
All participants were immunized during the first 2 weeks of October, 1999. The trivalent Influenza Type A and B vaccine (Flushield, Wyeth-Ayerst, Marietta, PA) was administered to all subjects and contained 15 μg HA of A/Beijing/262/95 (H1N1), 15 μg HA of A/Sydney/5/97 (H3N2), and 15 μg HA of B/Yamanashi/166/98 (B/Beijing 184/93-like).
Peripheral Blood Mononuclear Cell (PBMC) Isolation
Blood was collected on day 14 postimmunization. PBMC were isolated by centrifugation over Ficoll-Paque plus (Amershan Pharmacia Biotech, Piscataway, NJ) and were adjusted to 4 × 106 cells/ml in RPMI media (Life Technologies, Grand Island, NY) plus 5% fetal bovine serum (Hyclone Laboratories, Logan, UT) and 100 U/ml penicillin, 100 μg/ml streptomycin sulfate (Sigma Chemicals, St. Louis, MO). Cells were incubated in vitro with influenza virus (Flushield) at 0.18 μg/ml to stimulate antigen-specific responses.
Proliferation Assay
To assess virus-specific lymphocyte proliferation, 100 μl of cells were plated in triplicate with or without inactivated influenza virus for 5 days at 37°C in 5% CO2. Ten microliters of 5 mg/ml of MTT (Sigma Chemical Co., St. Louis, MO) was added for the last 4 hours of incubation, followed by the addition of 0.04 N HCl in isopropanolol. Absorbance was read at a dual wavelength of 570 and 630 nm.
Assay for Influenza-Specific Cytokine Production
Lymphocyte influenza-specific cytokine production (IL2, IL-10, IFN-γ) was measured in cell supernatants. PBMC were incubated with 0.18 μg/ml of inactivated influenza virus in vitro at 37°C, and 5% CO2 and cell supernatants were collected. Enzyme-linked immunosorbent assay (ELISA) kits were used to measure cytokine (PharMingen, San Diego, CA).
Anti-influenza Antibody ELISA
Influenza-specific antibody (IgG, IgM) in serum was determined by ELISA. Plates were coated overnight with 0.18 μg/ml of influenza virus. Sera was diluted in phosphate-buffered saline–Tween containing 1M NaCl, added to plates and incubated for several hours at 37°C. Alkaline-phosphatase-conjugated mouse, antihuman IgM, and IgG were added, and the plates were incubated overnight at 4°C. The substrate was added (p-nitrophenyl phosphate). Absorbance at 405 nm was measured on a microplate reader.
Flow Cytometry
PBMC were incubated with fluorescein-conjugated anti-CD3 and PE-conjugated anti-CD4 or PE-conjugated anti-CD8. Fluorescein- or PE-conjugated mouse antihuman IgG1 (isotype controls) were diluted in phosphate-buffered saline–1% bovine serum albumin. Cells were washed and fixed in 1% paraformaldehyde. Cells were analyzed for fluorescent intensity on a Coulter XL flow cytometer (Coulter Instruments, Hialeah, FL).
Influenza Symptom Incidence
All subjects reported influenza-like symptoms (high fever, aches, and fatigue, followed by respiratory symptoms such as sore throat, stuffy/runny nose, cough) that occurred from October through April. Subjects were contacted every 2 weeks to check the occurrence of any illness symptoms.
Analysis and Interpretation
Statistical analysis was performed using SPSS software (SPSS Inc., Chicago, IL). A three-way analysis of covariance (activity, gender, day of blood draw) was used for each of the dependent variables. If there was not a significant main effect of gender or day of blood draw, the data were combined for the analysis of activity. Dietary factors and psychosocial variables were assessed as covariates. Linear regressions were used to determine whether specific dietary factors or psychosocial factors were predictive of the immune response to influenza immunization. A regression analysis was also performed to assess the significance of activity level net of psychosocial factors and nutrient intake. A test for interactions and a test for correlation were done between activity level and each psychosocial and diet factor.
Results
Nutrient Intake
Nutrient intake did not differ between the three activity groups for any of the nutrients measured with the exception of carbohydrate intake. Carbohydrate intake was lower (p = .007) in the moderately active group than either the sedentary or the active group. A significantly greater kcal intake and protein intake was found in men than women (p < .05).
Psychosocial Survey Results
Social activity scores and LOT scores did not differ significantly between activity groups (Table 1 ). However, active and moderately active subjects reported lower levels of perceived stress than the sedentary group (p = .043, Table 1 ).
Influenza-Specific Proliferation
Exercise was associated with improved in vitro proliferation to the influenza vaccine (Fig. 1). PBMC from moderately active and active groups had greater proliferation than the sedentary group (p < .05).
Anti-influenza IgM and IgG
IgG titer in serum was higher in active participants (p = .014) compared to moderately active and sedentary participants (Fig. 2). PSS score and zinc supplementation were associated with IgG titer, and these factors were included in the model as covariates. IgM anti-influenza titer was also greater in active than in moderately active or sedentary participants (p = .047, Fig. 3).
Anti-influenza IL-2, IL-10, IFN-γ
Influenza-specific cytokine production was measured at the time of peak level in culture, and this time varied according to cytokine (see figure legend). Cytokine production was not altered by physical activity (Fig. 4).
CD3+CD4+ and CD3+CD8+ Cell Percentage
There was not a significant effect of activity on the percentage of CD3+CD4+ cells or CD3+CD8+ cells (Fig. 5). However, women had a greater percentage of CD3+ CD4+ cells than men (women = 56.8%, men = 41.9%, p = .001), and, conversely, men had a significantly greater percentage of CD3+CD8+ cells (women = 12.3%, men = 19.2%, p = .003).
Influenza Symptom Incidence
In the sedentary group, one of 15 subjects reported flu symptoms; three of 25 subjects in the moderately active group reported flu symptoms; and 0 of 16 subjects in the active group reported flu symptoms. There was not a significant difference between the groups regarding symptom incidence.
Nutrient Intake, Psychosocial Factors, and Influenza-Specific Immune Response
Subjects consuming a daily multivitamin produced greater amounts of IL-2 (see Fig. 6). There was also a trend (p = .08) toward increased influenza-specific antigen-proliferation in subjects consuming a daily multivitamin.
Greater zinc intake and lower perceived stress were both significant predictors of serum anti-influenza IgG titer and, along with activity, accounted for 27.8% of the variance. Although perceived stress score was associated with physical activity (see Table 1 ), including stress as a factor in the model did not decrease the amount of variance explained by activity, suggesting that activity and stress may independently influence anti-influenza IgG. The perceived stress score was also a statistically significant predictor (p = .042) of influenza-specific IL-2, accounting for 7.8% of the variance (lower perceived stress was associated with greater IL-2 production). LOT and social activity together accounted for 13.7% of the variance in influenza-specific IL-10. Social activity was a statistically significant predictor (p = .031) whereas the LOT score approached statistical significance (p = .061). Greater social activity and higher optimism scores were associated with greater IL-10 production.
Discussion
The findings from this study suggest that there is an association between physical activity, diet, psychosocial factors, and the immune response to influenza immunization in older adults. This is the first study, to our knowledge, that observed an association between physical activity and immune response to influenza immunization. Older adults classified as active had greater anti-influenza IgG, anti-influenza IgM, and greater influenza-specific lymphocyte proliferation than sedentary individuals. Older adults classified as moderately active had greater influenza-specific lymphocyte proliferation than sedentary subjects, but did not demonstrate greater antibody titer.
Serum IgG is important in protection against influenza infection and is considered to be a good predictor of resistance to infection (22). In our study, the subjects who exercised vigorously had a higher concentration of both IgG and IgM, suggesting a greater degree of protection, and our data on symptom incidence are consistent with this possibility.
Cell-mediated responses to influenza virus are essential for viral clearance and may also provide cross-reactive protection from strains of influenza that may not have been included in the annual influenza vaccine (23)(24). With respect to cell-mediated response, we observed greater influenza-specific proliferation among the active and moderately active groups compared with the sedentary group.
We did not find any association between activity level and cytokine production. This was surprising considering that IL-2 promotes cell proliferation, and cell proliferation was higher in the active and moderately active groups. However, we measured cytokine concentration only at one time (peak level of production in vitro). Our most recent data suggest that vigorous activity is associated with a more rapid production of IL-2. Therefore, the greater PBMC proliferation observed among active and moderately active individuals may reflect a shift in the kinetics of IL-2 production rather than higher peak levels of IL-2. Also, it is possible that cytokine production varies by individual viral strain (25), and, in future studies, we will evaluate cytokine response to each individual viral strain.
Overall, our findings are consistent with other investigators who have reported improved immune function in well-conditioned elders (17)(18)(19)(26). However, it is important to note that physical activity may not be strongly associated with enhanced immune function among older adults (27). Instead, it is possible that overall health status impacts immunosenescence to a greater degree. For example, when only “healthy” elderly subjects were included, there was no difference in natural killer cell function or response to exercise between young and old subjects (27).
In this investigation, our findings are new in that we evaluated the effect of exercise on the immune response to an antigen (influenza virus), whereas other human studies have examined nonspecific immune defenses and/or mitogen-stimulated immune response (17)(18)(19)(26). With an animal model (28), the role of exercise training in modulating antigen-specific immunity was examined, but no effect of exercise on IgM or IgG production in response to challenge with the antigen Keyhole Limpet Hemocyanin was found. Exercise may impact primary antibody and recall antibody responses in a different manner. In our study, we examined a recall response, whereas the animal study evaluated the primary antibody response.
The role of exercise in modulating antibody response to influenza immunization has been examined in college-aged students, but no effect of exercise was found (29). It is possible that the immunomodulatory effects of exercise may be greater in aged populations than in younger populations (30)(31) or that the use of differing techniques yielded different results. In our study, ELISA, which exhibits greater sensitivity (32)(33) was used to detect anti-influenza IgG and IgM, whereas hemagglutination inhibition was used in the other study.
Psychosocial factors have been shown to influence immune response and alter susceptibility to infection (34). Chronic stress was related to reduced antibody titer and influenza-specific IL-2 following influenza vaccination in older adults (16)(35). We also found that high stress was a significant predictor of reduced anti-influenza IgG and influenza-specific IL-2. In our study, greater optimism and a greater number of social interactions were associated with higher levels of influenza-specific IL-10. Individuals with a greater number of social ties may have decreased susceptibility to infection (36), and optimism has been linked to improved immune status (37).
We observed a higher IL-2 and a trend toward enhanced proliferation in subjects consuming a daily multivitamin. Also, higher anti-influenza IgG titers were associated with greater zinc intake. Others have found a positive correlation between zinc + selenium intake, or vitamin E intake and antibody titer following influenza immunization (38)(39), but these findings are not consistent (40). Given the small number of subjects in our study, it is premature to make any dietary supplement recommendations, rather, it is important to consider nutrient intake as a factor when analyzing immune function in older adults given the possibility that malnutrition exists in this population.
In summary, many older adults receive the influenza vaccine annually, yet vaccine efficacy is reduced among this population. Therefore it is important to identify specific lifestyle factors that may enhance vaccine efficacy. This is the first study to suggest that the practice of regular, vigorous exercise for at least 1 year may contribute to an enhanced immune response to influenza immunization in older adults. A follow-up randomized controlled trial is now being planned to replicate these findings and explore mechanisms that may account for exercise-induced alterations of immunity.
Subject Age and Psychosocial Characteristics
| Group | Age (SD) | Social (SD) | LOT (SD) | PSS (SD) |
| Sedentary | 71.5 ± 7.1 | 3.9 ± 1.6 | 49.7 ± 9.0 | 20.5 ± 8.7* |
| Moderately Active | 0.7 ± 6.3 | 4.1 ± 1.7 | 50.5 ± 7.6 | 15.7 ± 5.2 |
| Active | 71.9 ± 5.2 | 4.6 ± 2.2 | 50.4 ± 7.7 | 14.9 ± 6.1 |
| Group | Age (SD) | Social (SD) | LOT (SD) | PSS (SD) |
| Sedentary | 71.5 ± 7.1 | 3.9 ± 1.6 | 49.7 ± 9.0 | 20.5 ± 8.7* |
| Moderately Active | 0.7 ± 6.3 | 4.1 ± 1.7 | 50.5 ± 7.6 | 15.7 ± 5.2 |
| Active | 71.9 ± 5.2 | 4.6 ± 2.2 | 50.4 ± 7.7 | 14.9 ± 6.1 |
Note: LOT = Life Orientation Test; PSS = Perceived Stress Scale.
p < .05, Sedentary > Moderately Active and Active.
Subject Age and Psychosocial Characteristics
| Group | Age (SD) | Social (SD) | LOT (SD) | PSS (SD) |
| Sedentary | 71.5 ± 7.1 | 3.9 ± 1.6 | 49.7 ± 9.0 | 20.5 ± 8.7* |
| Moderately Active | 0.7 ± 6.3 | 4.1 ± 1.7 | 50.5 ± 7.6 | 15.7 ± 5.2 |
| Active | 71.9 ± 5.2 | 4.6 ± 2.2 | 50.4 ± 7.7 | 14.9 ± 6.1 |
| Group | Age (SD) | Social (SD) | LOT (SD) | PSS (SD) |
| Sedentary | 71.5 ± 7.1 | 3.9 ± 1.6 | 49.7 ± 9.0 | 20.5 ± 8.7* |
| Moderately Active | 0.7 ± 6.3 | 4.1 ± 1.7 | 50.5 ± 7.6 | 15.7 ± 5.2 |
| Active | 71.9 ± 5.2 | 4.6 ± 2.2 | 50.4 ± 7.7 | 14.9 ± 6.1 |
Note: LOT = Life Orientation Test; PSS = Perceived Stress Scale.
p < .05, Sedentary > Moderately Active and Active.
Influenza-stimulated proliferation in peripheral blood mononuclear cells (PBMC) from sedentary, moderately active, and active. PBMC were collected 2 weeks postimmunization and were cultured with influenza vaccine for 5 days. Results are expressed as mean ± SEM. Asterisks indicate significant differences (p < .05) compared with all other treatment groups. Background proliferation was determined from the wells containing cells and media without virus. The background was subtracted from the average reading obtained in the wells incubated with virus.
Serum IgG anti-influenza antibody response in sedentary, moderately active, and active at 1:25 and 1:125 dilution. Values are means ± SEM. Asterisks indicate significant differences (p < .05) compared with all other treatment groups. + indicates active significantly greater than moderately active.
Serum IgM anti-influenza antibody response in sedentary, moderately active, and active at 1:25 and 1:125 dilution. Values are means ± SEM. Asterisks indicate significant differences (p < .05) compared with all other treatment groups. + indicates active significantly greater than moderately active.
In vitro influenza-stimulated IL-2 at 48 hours in vitro (A); and at 96 hours in vitro IFN-γ (B) and IL-10 (C) production by peripheral blood mononuclear cells (PBMC) from sedentary, moderately active, or active subjects. PBMC were collected 2 weeks postimmunization and were restimulated in vitro with influenza vaccine.
Percentage of CD3+ CD4+ and CD3+ CD8+ cells in peripheral blood mononuclear cells of sedentary, moderately active, and active subjects. Cells were collected 2 weeks postimmunization.
In vitro influenza-stimulated IL-2 production by peripheral blood mononuclear cells from subjects consuming a daily multivitamin compared to subjects not consuming a daily multivitamin. The asterisk indicates significant differences (p < .05) between treatment groups.
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