Cultured human airway smooth muscle cells stimulated by interleukin-1β enhance eosinophil survival

Airway smooth muscle may be an important cellular source of proinflammatory mediators and cytokines and may participate directly in airway inflammation. In this study we have examined whether airway smooth muscle cells could contribute to mechanisms of eosinophil accumulation by prolonging their survival. To investigate this possibility, conditioned medium from human airway smooth muscle cells stimulated with interleukin (IL)-1 beta was examined on the in vitro survival of highly purified human peripheral blood eosinophils. After 7 d, when cultured in control medium, less than 1 +/- 0.2% of the initial eosinophil population remained viable. In contrast, culture in medium conditioned for 96 h by human airway smooth muscle cells stimulated with IL-1 beta (1 pg-100 ng/ml) resulted in a concentration-dependent increase in eosinophil survival. (The concentration that produced 50% of this effect was 0.03 ng/ml IL-1 beta.) Maximum eosinophil, survival occurred at 1 to 3 ng/ml IL-1 beta. This effect was also time-dependent and was readily detected in airway smooth muscle cell-conditioned medium after just 3 h of stimulation with IL-1 beta (1 ng/ml). It continued to increase before reaching a plateau around 24 h, with no decrease in activity for up to 120 h of stimulation. Conditioned medium from unstimulated airway smooth muscle cells did not enhance eosinophil survival. The survival-enhancing activity was completely inhibited (the concentration that inhibited 50% [IC50] was 6.9 mu g/ml) by a polyclonal goat antihuman antibody to granulocyte-macrophage colony stimulating factor (GM-CSF) (0.3-100 mu g/ml), but antibodies (10-100 mu g/ml) to IL-3 and IL-5, and a normal goat immunoglobulin G control had no effect on the eosinophil survival-enhancing activity. GM-CSF levels in culture medium from smooth muscle cells were markedly increased by IL-1 beta and were maximum at 30 ng/ml (0.037 ng/ml/10(6) cells versus 3.561 ng/ml/10(6) cells, unstimulated versus 30 ng/ml IL-1 beta). The IL-1 receptor antagonist inhibited both the production of GM-CSF (IC50 19.1 ng/ml) and the eosinophil survival-enhancing (IC50 53.7 ng/ml) activity stimulated by IL-1 beta. Release of GM-CSF elicited by IL-1 beta was inhibited by dexamethasone but not by indomethacin. These data indicate that cultured human airway smooth muscle cells stimulated with IL-1 beta support eosinophil survival through production of GM-CSF and thus may contribute to the local control of inflammatory cell accumulation in the airways.