Inclusion Criteria:  Diagnosis of Severe Eosinophilic Asthma (as per Southampton Severe Asthma MDT and based on ATS/ERS consensus criteria)  Approved for treatment with Mepolizumab by Southampton Severe Asthma MDT (in accordance with NICE TA 431)  Blood eosinophil count is ≥0.3 x109cells/L in preceding 12 months ≥4 asthma exacerbations needing systemic steroids in the preceding 12 months Age ≥18 years Able to provide written informed consent  Exclusion Criteria:  On maintenance daily oral steroids, asthma immune modulators On long-term systemic antibiotics or antifungal treatment Current smoker or ex-smoker with >10 pack year smoking history Known clinically significant respiratory disorder such as bronchiectasis, interstitial lung disease, allergic bronchopulmonary aspergillosis Currently pregnant or planning pregnancy within the treatment period or breast-feeding History of psychiatric, medical or surgical disorders that in the opinion of the chief investigator may interfere with undergoing a bronchoscopy or may compromise study completion or data collection Current malignancy Alcohol or recreational drug abuse Diagnosis of immunodeficiency requiring treatment Previous bronchial thermoplasty Previous treatment with biological therapy Previously unable to tolerate a bronchoscopy Current or recent (within the last 12 weeks) infection with SARS-CoV-2 Unable to provide written informed consent