3.7 Correlation of unstimulated and stimulated cytokines in
preschoolers with low physical activity and high TV attendance
Asthmatic children with no or only occasional PA and / or TVA ≥ 3 hours
per day showed high correlations of proinflammatory cytokines (Fig. 6A
and 6C), whereas barely any clusters of correlation could be identified
in healthy children of the same PA and TVA groups (Fig. 6B and 6D).
Cytokines that showed particularly high correlations in asthmatics
include IL-1β, IL-5, IL-6, IL-7, IL-9, IL-10, IL-12p70, IL-13, IL-23A,
IL-27, TNF-α, IFNα2, IFNγ, CCL3, CCL4 and CXCL10. Mostly, these highly
correlating cytokines were either measured in unstimulated conditions or
after PBMC stimulation with PHA, poly I:C and R848.
Discussion
In the present study, we investigated the immunological effects of PA
and asthma control. Children with controlled asthma engaged in vigorous
PA considerably more often compared to those with partially
controlled or uncontrolled asthma. In addition, asthmatic
preschoolers reported more daily TV hours compared to their healthy
peers. As a general finding, physically active asthmatics expressed
higher levels of various cytokines in PBMC cultures under both
unstimulated conditions and stimulation with different polyclonal
stimulants, while long daily TVA was associated with an overall decrease
in cytokine levels.
Asthmatics are commonly perceived as being more physically inactive in
comparison to healthy individuals.3 Significantly less
TV hours in healthy children shown in the present study, which we
interpret as less sedentary behavior, supports this notion. One
explanation for this behavior can be described as a vicious circle: in
fear of experiencing exercise-induced dyspnea, the parents or the child
might consciously or unconsciously restrict his or her practice of
PA.33 Of course, such avoidance behavior particularly
affects subjects with severe and/or uncontrolled
asthma.3,13 Our study supports the importance of PA in
asthma control, showing that children with controlled asthma are engaged
in PA significantly more often than their peers with uncontrolled
asthma.
Although there was a significant difference between controlled and
uncontrolled asthma, we did not find any significant difference
regarding the weekly amount of PA between asthmatic and healthy children
in the present study. The impact of asthma diagnosis on sports in
childhood and adolescence is generally accepted. The AIRE study
demonstrated that 30% of asthmatic children felt limited in their
physical activities.34 It was also reported that a
majority of asthmatic children perceived the inability to participate in
sports as the worst thing about their asthma.35 So
far, studies comparing the level of PA in asthmatic and non-asthmatic
children have shown controversial results.3 Whereas
Firrincieli et al. found that asthmatic children are less physically
active,33 others suggested their level of PA does not
differ from healthy children,5,13,35 or that they are
even more active.36 Factors that could make for this
inconsistency could be the asthma diagnosis criteria used in this age
group, awareness of the doctors for suggesting PA in the cohort,
regional cultural differences as well as the instruments used to
quantify PA.
Exercise is known to enhance the health-related quality of life in
asthmatics not only by improving aerobic capacity, but also by reducing
dyspnea, the intensity of exercise-induced bronchoconstriction, the dose
of inhaled corticosteroids and exacerbation of their
asthma.13,37 Therefore, the American College of Sports
Medicine and the American Thoracic Society endorse prescription of PA
for all asthmatic subjects.6 Guidelines focused on
physical activity for pediatric asthma patients are lacking. A
recommendation to exercise on a regular basis for children with
controlled asthma is made by the Global Initiative for Asthma
(GINA).38 In addition, there has been substantial
research on the use of exercise to treat asthma, which also proved to be
safe and beneficial for pediatric asthmatic
subjects.39 Bonini et al. conducted a study with
Italian Olympic athletes that may give young asthmatics every reason to
support their PA. The authors reported that adequately diagnosed and
treated asthmatic athletes can compete at the highest
level.7,40
The present study also investigated in vitro immune response of
asthmatic and healthy children by analyzing cytokines produced from PBMC
that may contribute to asthmatic inflammation in allergic and
nonallergic asthma.15 Baseline cytokine levels of
unstimulated PBMC did not differ between the two groups. For our
investigation of immune responses in PBMC cultures, we chose four
stimulants: phytohemagglutinin (PHA) acts as a mitogen that leads to a
polyclonal immune activation, poly I:C and R848 both mimic respiratory
viral infections and zymosan (zymo) mimics immune response to a fungal
infection. Compared to healthy subjects, asthmatic preschoolers show
higher cytokine levels particularly after stimulation poly I:C and R848,
indicating a strong response in case of respiratory virus contraction.
Regular exercise is further known to have anti-inflammatory effects,
which most likely play an important role in its ability to reduce the
risk of chronic metabolic and cardiorespiratory
diseases.23,41 The three main mechanisms that are
thought to lead to the anti-inflammatory effects of regular exercise are
a reduction in visceral fat mass (leading to a decrease in
pro-inflammatory adipokines, e.g. TNF-α), an increased production of
anti-inflammatory cytokines from contracting skeletal muscle (myokines;
e.g. IL-6 leading to a subsequent rise in anti-inflammatory IL-10 and
IL-1-RA) and a reduction of Toll-like receptor expression on monocytes
and macrophages.41
Our results show that a high amount of weekly vigorous PA is associated
with a great number of elevated cytokine levels in response to all four
stimulants, indicating an immune system prepared for responding strongly
in case of infection. PA is known to affect both innate and acquired
immune response in various ways. As such, an increase in NK cell numbers
and NK cell cytotoxicity, as well as a decrease in T cell functionality
have been described in response to exercise.22,24 An
imbalance between Th1/Th2/Th17 cells and their control by Treg cells can
play a crucial role in asthma development, while different phenotypes
show distinct immunological patterns.15,18-20 Type 2
inflammation is linked to the most common asthma phenotype, allergic
asthma.15,19 Type 2 cytokines (e.g. IL-4, IL-5, IL-9
and IL-13), which are activated by allergen exposure, can cause airway
hyperresponsiveness by contraction of smooth muscles, mucus production,
eosinophil activation and an induction of allergen-specific IgE by
B-lymphocytes.19,42 Those type 2 cytokines can also be
secreted by innate lymphoid cells (ILC2), after stimulation by
epithelial IL-25 and IL-33, which are generated by impaired airway
epithelial cells in asthmatic subjects.15,19,43
Due to ethical limitations, immune response to PA in children has been
poorly investigated.24 In atopic individuals, PA may
cause further Th2 polarization, leading to more severe allergic symptoms
or exercise-related symptoms.24 However, it has also
been shown that exercising on a regular basis induced beneficial changes
in allergic subjects, such as a reduction in pro-inflammatory cytokines
(e.g. IL-444) and a switch to a type 1 profile, which
in turn may reduce allergic inflammation.24 Studies in
murine asthma models reported an enhancement of Treg responses to
aerobic exercise.25
In the present study, we found some type 2 cytokines (IL-5, IL-9, IL-13
and CCL5) to be upregulated in asthmatic preschoolers with a high level
of weekly PA, whereas IL-25 levels were significantly lower in more
physically active individuals. The increase is not only found in type 2
cytokines, because regarding type 1- and type 17-related cytokines,
IL-12B, IL-17, IL-27, TNF-α and CXCL10 were found to be upregulated in
asthmatic preschoolers with high weekly PA, supporting the readiness to
produce both type 1, type 2 and type 17 cytokines in response to various
stimuli. These findings can be perceived as immunological fitness
without any skew to a certain subtype observed by extensive PA.
Apart from the Th1/Th2 imbalance predominantly found in allergic
asthmatics, a disequilibrium in Th17 and Treg cells has been noted in
nonallergic asthmatic subjects with neutrophilic airway
inflammation.15,18 IL-17, produced by Th17 cells, may
be upregulated in these patients, while Treg functions are inhibited in
children with asthma.45,46 This Th17/Treg imbalance
was shown to be closely associated with asthma severity and
steroid-resistance.15,47 The present study
demonstrated IL-17A is upregulated in asthmatic preschoolers with high
PA. As Th1 and Th17 cells, which produce IL-17A, are dominant in
neutrophilic asthma,15 it could be hypothesized that
PA might not have a beneficial effect in every asthma phenotype.
However, we also observed a decrease of IL-6 and IL-1β levels in highly
physically active asthmatic preschoolers. IL-6 is required for Th17
differentiation and IL-1β promotes Th17 cell-dependent
inflammation,15 both leading to a disequilibrium of
Th17/Treg cells (towards Th17). A fall in IL-6 and IL-1β levels may
therefore positively impact the disbalance in such patients.
Longer TVA represent a longer indoor stay and less outdoor allergen and
air pollutant exposure and a less physically active condition. Our
results demonstrate an overall decrease in cytokine levels in asthmatic
preschoolers with high daily TVA, indicating weaker immune responses to
various stimuli (bacterial, viral or fungal) compared to subjects with
less daily TV hours. However, the Th2-related cytokines IL-25, IL-33 and
IL-13, all playing a major role in allergic asthma, were found to be
downregulated in asthmatic children with high TVA. Since they are
epithelial cytokines, longer stay indoors and less epithelial cell
activation can be one of the reasons for this. It could therefore be
argued that high TVA might have a positive impact on the epithelial cell
alarmins that may be the initiators of type 2 inflammation. Furthermore,
IFNγ, which has previously been found to be associated with
non-eosinophilic asthma and steroid-resistant
asthma,15 was significantly downregulated in asthmatic
preschoolers with high TVA.
While some of these results indicate a potential positive influence of
TVA on immunological reactions in asthmatic preschoolers, our data also
show that asthmatic subjects with low weekly PA and / or high daily TVA
exhibited highly positively correlating proinflammatory cytokines under
stimulated, but also unstimulated conditions, suggesting an overall
proinflammatory state in those individuals. Furthermore, the association
between a sedentary lifestyle and obesity is the be kept in mind. The
worldwide increase in asthma prevalence occurred together with an
increase in obesity and a sedentary lifestyle.39 A
number of studies reported an association between obesity and childhood
asthma, however, the causality is not clear.5,48
Our study has a number of limitations. Firstly, the PreDicta cohort is
moderate in size, however performing cell cultures in a standard way and
measurement of many cytokines in all these patients and controls should
be appreciated. The lack of objective criteria makes the diagnosis of
asthma more difficult in preschoolers. In addition, parameters such as
weekly PA or daily TVA were collected from questionnaires based on
parental reports. Even though short-term parental reports were shown to
be accurate,49 recall bias might be a source of error.
For example, strictly interpreting daily TV hours as sedentary behavior
can lead to the wrong assumption that a subject with high daily TVA
cannot simultaneously be vigorously physically active more than 3 times
a week. It is unclear whether the differences observed in cytokine
levels actually result from the physical activity status of the
subjects. Many factors, such as genetics, type of asthma, asthma
control, current medication, infections, immunizations, various
exposures and diet can influence cytokine levels. For example, it can be
well argued that the decrease in IL-25 levels in asthmatic children with
high weekly PA might be due to their well-controlled asthma and the
regular use of asthma medication. It has to be additionally considered
and needs further studies whether children staying indoors with
increased TVA are having less exposure to environmental pollutants and
outdoor allergens. Furthermore, low PA and high TVA are more likely to
be results of uncontrolled asthma, which will be addressed elsewhere.
Lastly, it would have added great value to this study if BMI values had
been assessed.
In conclusion, our results show that limited PA is likely the result of
poor asthma control and that both PA and TVA possibly impact systemic
immune response and immune and inflammatory thresholds in asthmatic
preschoolers. Based on our findings, we recommend PA to be encouraged in
asthmatic preschoolers, while good asthma control is essential. The
fitness and readiness of the immune system to secrete cytokines is
becoming more and more important, with the recent knowledge in COVID-19,
for example, in timely release of anti-viral
interferons.50,51 It is also of great importance not
to forget about the association of physical inactivity, poor physical
cardiovascular fitness and obesity – all of which threatens a child’s
health and well-being.