Background: Eosinophilic inflammation of the airways is a key characteristic of asthma. A defect in apoptosis might contribute to the chronic tissue eosinophilia associated with asthma.
Objective: Our purpose was to examine whether the rate of apoptosis differs between peripheral blood eosinophils from asthmatic patients and healthy volunteers.
Methods: Peripheral blood was obtained from volunteers with asthma and from control volunteers. Eosinophils were isolated by CD16-negative selection to >99% purity and were cultured for 48 hours. The number of apoptotic eosinophils in the culture was assessed by flow cytometric analysis of relative DNA content in propidium iodide-stained cells. Eosinophil apoptosis is expressed as apoptosis index (number of apoptotic cells/total number of cells).
Results: Eosinophils from asthmatic patients not taking steroid medication survived longer (apoptosis index 0.25) than those of healthy control subjects (apoptosis index 0.40, P <.05). In contrast, the rate of apoptosis in eosinophils from patients concurrently taking steroids (apoptosis index 0.46) is higher than that of those not using steroids (P <.01) and not different from that of healthy subjects. To assess whether endogenous IL-3, IL-5, and GM-CSF production contributes to the delayed eosinophil apoptosis, the effects of the corresponding neutralizing antibodies were studied on eosinophil longevity. Neutralization of GM-CSF, but not of IL-3 or IL-5, increased slightly but significantly (P <.01) the rate of apoptosis in eosinophils obtained from patients with asthma. To assess whether beta(2)-agonist medication could contribute to the observed differences, we determined the in vitro effects of albuterol, fenoterol, and salmeterol on eosinophil apoptosis. All beta(2)-agonists inhibited eosinophil apoptosis by 12% to 19%. A possibility existed that a prior in vivo exposure to IL-5, GM-CSF, or beta(2)-agonists would explain the observed difference. To study this, eosinophils were incubated with GM-CSF, IL-5, and albuterol for 2 to 3 hours, followed by washout of the added compounds, and were subsequently cultured for 48 hours. However, an exposure to GM-CSF (7 pmol/L) or IL-5 (10 pmol/L) for 15 to 180 minutes was not a sufficient signal to prevent eosinophil apoptosis. In contrast, exposure to albuterol (100 nmol/L) for 120 minutes was sufficient to induce a significant (P <.05) decrease in eosinophil apoptosis.
Conclusions: The results suggest that eosinophil apoptosis is delayed in asthma. This delay may be partly explained by production of GM-CSF. The in vitro effects of beta(2)-agonists suggest that beta(2)-agonist use might contribute to the prolonged eosinophil survival through inhibition of apoptosis and thus may worsen eosinophilia in asthmatic patients. Use of inhaled glucocorticoids seems to totally reverse the delayed eosinophil apoptosis in asthma.