Persistent airway inflammation may require the use of different markers for monitoring airway inflammation. In this study, the authors investigated whether adenosine, which may be produced in allergic inflammatory conditions, could be measured with good reproducibility in exhaled breath condensate (EBC), and whether its concentration was elevated in patients with asthma.EBC adenosine and exhaled nitric oxide (eNO), a noninvasive marker of asthmatic airway inflammation, were measured in 40 healthy volunteers and 43 patients with allergic bronchial asthma. Repeatability of adenosine measurement was checked in 20 pairs of samples collected from healthy control subjects.Adenosine was detectable in all EBC samples by the applied high-performance liquid chromatographic method. The mean difference between repeated measurements of adenosine was -0.1 nM and all differences were within the coefficient of repeatability. Adenosine concentration was higher in steroid-naive patients (n=23) compared with healthy control subjects and steroid-treated patients (n=20). In patients with worsening symptoms of asthma (n=23), adenosine concentration was elevated compared with those in a stable condition (n=20). Furthermore, adenosine concentrations were related to eNO levels in asthmatic patients.These results, showing good reproducibility of adenosine measurements and increased adenosine concentrations in steroid-naive patients and in patients with worsening of asthmatic symptoms, indicate that adenosine measurement in exhaled breath condensate might be an acceptable novel method to investigate the role of local production of adenosine in the airways.

The innate immune molecule surfactant protein-D (SP-D) plays an important regulatory role in the allergic airway response. In this study, we demonstrate that mice sensitized and challenged with either Aspergillus fumigatus (Af) or OVA have increased SP-D levels in their lung. SP-D mRNA and protein levels in the lung also increased in response to either rIL-4 or rIL-13 treatment. Type II alveolar epithelial cell expression of IL-4Rs in mice sensitized and challenged with Af, and in vitro induction of SP-D mRNA and protein by IL-4 and IL-13, but not IFN-γ, suggested a direct role of IL-4R-mediated events. The regulatory function of IL-4 and IL-13 was further supported in STAT-6-deficient mice as well as in IL-4/IL-13 double knockout mice that failed to increase SP-D production upon allergen challenge. Interestingly, addition of rSP-D significantly inhibited Af-driven Th2 cell activation in vitro whereas mice lacking SP-D had increased numbers of CD4+ cells with elevated IL-13 and thymus- and activation-regulated chemokine levels in the lung and showed exaggerated production of IgE and IgG1 following allergic sensitization. We propose that allergen exposure induces elevation in SP-D protein levels in an IL-4/IL-13-dependent manner, which in turn, prevents further activation of sensitized T cells. This negative feedback regulatory circuit could be essential in protecting the airways from inflammatory damage after allergen inhalation.

Analysis of exhaled breath condensate is a method for noninvasive assessment of the lung. Condensate can be collected with a nose clip (subjects inhale and exhale via the mouth) or without it (subjects inhale via the nose and exhale via the mouth), but the mode of inhalation may influence condensate volume and mediator levels. We compared condensate volume and adenosine, ammonia, and thromboxane B2 levels in young healthy volunteers (n = 25) in samples collected for 10 minutes from subjects with or without a nose clip. Patients with allergic rhinitis (n = 8) were also studied to assess the effect of upper airway inflammation on mediator levels. Adenosine, ammonia, and thromboxane B2 levels were determined by HPLC, spectrophotometry, and radioimmunoassay, respectively. Volume of condensate was significantly higher without nose clip than that with nose clip (mean +/- SD, 2321 +/- 736 microl and 1746 +/- 400 microl, respectively; p = 0.0001). We found no significant difference in any mediator levels between these two collection modes in healthy volunteers, but adenosine showed a tendency to differ between oral and nasal inhalation in patients with allergic rhinitis. Our data indicate that whereas a greater volume of condensate can be obtained when subjects inhale through their noses, the mode of inhalation does not influence mediator levels in young healthy volunteers, but may affect these levels in patients with allergic rhinitis.

In asthmatic patients, airway obstruction provoked by exercise challenge is accompanied by an increase in plasma adenosine level. In this study, the current authors investigated if exercise-induced bronchoconstriction was associated with local changes of adenosine concentration in the airways.Oral exhaled breath condensate (EBC) collection (5-min duration) and forced expiratory volume in one second (FEV1) measurements were performed at rest (baseline) and 4-8 times after treadmill exercise challenge in healthy and asthmatic subjects. Adenosine concentration in EBC was determined by HPLC.Observations indicated that physical exercise results in bronchoconstriction together with a significant increase of adenosine level in EBC in asthmatic patients (mean¡SD maximal fall in FEV1 27¡13%; associated increase in adenosine 110¡76% as compared to baseline), but not in healthy control subjects. Exercise-induced changes in adenosine concentration correlated significantly with the fall in FEV1 values in asthmatic patients.In conclusion, the observed increase in adenosine concentration of oral exhaled breath condensate most probably reflects changes in the airways during exercise-induced bronchoconstriction. Due to its known bronchoconstrictor property in asthma, adenosine may contribute to the development of bronchospasm.

It has been known for a long time that inhaled adenosine-monophosphate (AMP) induces airway obstruction in asthmatic patients, but not in healthy subjects. The mechanism of AMP is indirect and occurs via its decay product, adenosine. It stimulates mast cells through its low-affinity receptor A2B to release histamine, which ultimately leads to smooth muscle contraction. This feature of adenosine reveals its pro-inflammatory function, which may play important role in asthma. Indeed, mice lacking adenosine deaminase (ADA), an enzyme which decomposes adenosine, develop asthma-like disorder with elevated IgE, eosinophilia and airway hyperresponsiveness. Human studies showed elevated adenosine levels in bronchoalveolar lavage and exhaled breath condensate of asthmatics as compared to healthy people. Furthermore, certain human ADA phenotypes are associated with prevalence of asthma. These data suggest a protective role for ADA and a pro-inflammatory function for adenosine in asthma. The role of adenosine in inflammatory processes, however, is not unequivocal. Some in vitro studies showed that adenosine binding to its high-affinity receptor A2A results in inhibition of leukotriene synthesis or function of adhesion molecules. It is possible that the concentration of adenosine in lung tissues determines whether it promotes or reduces inflammation. Adenosine has also been associated with other respiratory diseases such as fibrosis, sarcoidosis, cystic fibrosis or tuberculosis. Identification of adenosine receptor subtypes and their role in the pathomechanism of respiratory diseases may provide new therapeutical targets. This review aims to summarize the role of adenosine and adenosine receptors in asthma and other pulmonary disorders.