Inflammatory subtypes in asthma are related to airway hyperresponsiveness to mannitol and exhaled NO

Asthma may be defined as eosinophilic or non-eosinophilic based on the presence of eosinophils in sputum. Recently a further classification into four inflammatory subtypes has been suggested. The aim of the present study was to describe the association between these inflammatory subtypes and markers of airway inflammation and hyperresponsiveness. In 62 adult non-smoking asthmatics, (18-65 yr) not taking inhaled steroids, sputum induction, bronchial challenge with mannitol and measurement of exhaled NO (eNO) were performed. Based on the eosinophil and neutrophil proportions in sputum, subjects were categorised into four inflammatory subtypes: Eosinophilic asthma: i.e., sputum eosinophils > 1.0%. Neutrophilic asthma: i.e., sputum neutrophils > 61%. Mixed granulocytic asthma: both increased eosinophils and neutrophils. Paucigranulocytic asthma: i.e., normal levels of both eosinophils and neutrophils. Among subjects with non-eosinophilic asthma, neutrophilic asthma was associated with low levels of eNO (Median (IQR): 12 ppb (8-27 ppb), whereas subjects with non-eosinophilic asthma of the paucigranulocytic subtype had levels of eNO (48 ppb (29-65 ppb)) that were comparable to subjects with eosinophilic asthma of the mixed granulocytic type (47 ppb (33-112 ppb). Purely eosinophilic asthma was associated with higher levels of eNO (77 ppb (37-122 ppb)). Furthermore, a low degree of airway hyperresponsiveness to mannitol was observed in neutrophilic asthma (PD(15): (Median (IQR) 512 mg (291-610 mg))), whereas it was moderate in paucigranulocytic asthma (238 mg (77-467 mg)) and comparable to eosinophilic asthma of the mixed granulocytic subtype (186 mg (35-355 mg)). The highest degree of AHR to mannitol was observed in purely eosinophilic asthma (107 mg (68-245 mg)). In conclusion, further subclassification of eosinophilic and non-eosinophilic asthma showed significant differences in airway hyperresponsiveness to mannitol and exhaled NO levels among the four inflammatory subtypes.