Differences in the direct effects of various type 2 cytokines on functions of blood eosinophils from healthy subjects

Eosinophil inflammation often persists in the airways of severe asthmatics, even under treatment with high-dose inhaled corticosteroids. Biologics for various type 2 cytokines have been recently developed for corticosteroid-resistant, eosinophil-dominant, severe asthma. However, it is unclear whether these biologics act directly on eosinophils.

In this study, we examined whether various type 2 cytokines targeted by biologics can directly modify the functions of eosinophils obtained from the peripheral blood of healthy individuals.

Peripheral eosinophils of healthy subjects were purified by conventional negative-depletion methods using anti-CD16 beads to avoid the priming effect (i.e., stimulation in vitro) to the maximum extent possible. Eosinophils were stimulated with interleukin (IL)-4, IL-5, IL-13, or thymic stromal lymphopoietin (TSLP), and eosinophil adhesiveness to recombinant human-intercellular adhesion molecule (ICAM)-1 was evaluated by eosinophil peroxidase assays. The effect of these cytokines on eosinophil superoxide anion (O₂ ^-) generation was evaluated by the superoxide dismutase-inhibitable reduction of cytochrome C. To determine whether eosinophil degranulation was induced, the concentration of eosinophil-derived neurotoxin (EDN) in the supernatant was measured using enzyme-linked immuno sorbent assay.

As reported previously, at 100 pM, IL-5 increased eosinophil adhesiveness to ICAM-1, O₂ ^- generation, and EDN release. Conversely, at concentrations up to 10 nM, IL-4, IL-13, and TSLP did not induce eosinophil adhesiveness, O₂ ^- generation, or EDN release.

Type 2 cytokines other than IL-5 do not directly affect the functions of eosinophils from healthy individuals when used at clinical concentrations. These findings suggest that eosinophils play little, or no, direct role in the effects of anti-IL-4 receptor α or anti-TSLP antibody on severe asthma.