4. Discussion
This study shows for the first time that asthmatic children presenting
increased airway resistance together with the increased levels of
pro-inflammatory and pro-fibrotic factors, beyond increased levels of
exhaled nitric oxide.
Systemic eosinophilc inflammation has been considered as an important
cellular biomarker to classify different asthma phenotypes and to
predict response to treatments [13]. In summary, blood eosinophils
above 300 cells/µl is an important
cellular biomarker to predict severe asthma and/or corticosteroid
resistant asthma [13]. In the present study, it was observed that
the group of asthmatic children enrolled in the study have values
ranging 200 cells/µl until nearly 500 cells/µl, corresponding to values
above 6% of eosinophils. In this context, it has been established that
such profile of patients presents more severe hyperresponsiveness,
bronchospasm and airflow limitation [14]. However, in the present
study, while we only measure the hyperresponsiveness indirectly using
pre- and post-bronchodilator for spirometry, without a confirmation of
hyperresponsiveness, the susceptibility to bronchospasm and airflow
limitation were additionally accessed by pre- and post-bronchodilator
response to impulse oscillometry, which has been done for the first
time. In this context, the present study found that asthmatic children
presented not only impaired FEV1%pred and FEV1/FVC response, as
classically found, but also impaired lung mechanics, as denoted by
impaired impedance of respiratory systems (Z5Hz), respiratory reactance
(X5Hz); proximal airways resistance (R20Hz), total resistance of
respiratory system (R5Hz) and resistance of peripheral/distal airways
(R5Hz-R20Hz). In addition, a slight but novel mechanistic investigation
was performed, as demonstrated by increased levels of systemic and
pulmonary pro-inflammatory humoral mediators (GM-CSF, IL-4, IL-5) and
pro-fibrotic mediators (TGF-beta).
Increases in the levels of Th2 pro-inflammatory (GM-CSF, IL-4, IL-5) and
pro-fibrotic mediators (TGF-beta) are key players in the process of
airway remodeling, stimulating hypertrophy and hyperplasia of airway
smooth muscle and of airway epithelial cells, leading to exaggerated
mucus and extracellular matrix proteins production and accumulation
[15]. In this view, the present study shows for the first time that
increased airway resistance in asthmatic children, which reflect
functionally the airway remodeling, is followed by increased levels of
Th2 pro-inflammatory mediators as well as the growth factor TGF-beta,
not only in the systemic circulation, but also into the lungs, as
demonstrated in the breath condensate.
Beside Th2 mediators and growth factors, nitric oxide (NO), has also
been linked to asthma development, severity and exacerbations [6, 7,
8, 9]. NO is an instable gas, considered a highly reactive nitrogen
specie, which present a very short half-life. NO can be measured
noninvasively in the exhaled air by chemiluminescence, by using bench
[16] or portable [12] NO detecting devices. In fact, increased
levels of NO have been associated not only with several aspects of
asthma [6, 7, 8, 9], but also with exercise-induced
bronchoconstriction, which is a highly prevalent characteristic of
asthmatic patients [16, 17]. In the present study, it was found that
asthmatic children presented high levels of exhaled NO, ranging among
25-30 ppb, which clearly indicates an inflammatory process of the
airways. However, although these levels of NO already indicate airway
inflammation, it was not enough to predict asthma exacerbation or even
hyperresponsiveness [18], as observed in the present study (no
response ≥ 10% in the FEV1%pred after bronchodilator). On the other
side, the increased levels of NO were associated with increased levels
of systemic and pulmonary Th2 cytokines (GM-CSF, IL-4 and IL-5) as well
as with growth factor TGF-beta. In fact, pre-clinical studies have
classically reported that high levels of NO is involved in exacerbated
Th2 cell related airway inflammation, remodeling and
hyperresponsiveness, while the blocking of inducible nitric oxide
synthase (iNOS), responsible for very high levels of NO synthesis, can
prevent or even revert this asthmatic phenotype (Th2 airway
inflammation, remodeling and hyperresponsiveness) [19, 20].