Rationale: Anti-eosinophil therapies improve clinical measurements of asthma in parallel with full depletion of eosinophils in circulation but only partial depletion in airways.  We examined eosinophil phenotypes across these compartments and after airway triggers.

Methods: Blood and sputum were collected at baseline, and at 7hr and 24hrs after inhaled allergen and diluent challenges. Sputum cells and whole blood was stained for eosinophils (CD45+ CD16- CD15+). Resident (rEOS: CD62L+ CD123-) and inflammatory (iEOS: CD62L- CD123+) eosinophils were expressed as % of total eosinophils.

Results: Blood and sputum eosinophils increased at 7hr and 24hrs post-allergen compared to diluent challenge (p<0.05).  Frequency of eosinophil sub-groups in blood was CD62L+ CD123-(rEOS) > CD62L+ CD123+ > CD62L-CD123- > CD62L- CD123+ (iEOS), and in sputum was CD62L-CD123- > CD62L- CD123+ (iEOS) > CD62L+ CD123- (rEOS) > CD62L+CD123+. Blood had significantly higher iEOS and rEOS compared to sputum at all time points (p<0.001). Post-allergen proportions of rEOS drifted downwards in blood and upwards in sputum, with opposite trends in iEOS populations.

Conclusion: Higher CD62L-CD123+ (iEOS) and lowering of CD62L+CD123- (rEOS) in blood post-allergen reflects a priming and efflux of eosinophils into airways. The high proportion of CD62L-CD123- eosinophils in sputum could simply represent the post-migration and post-activated airway phenotype.