Airway inflammation is a consistent finding in asthma, and increased amounts of eosinophil-derived cationic proteins are present in bronchoalveolar lavage fluid from asthmatic subjects. Tracheal instillation of a variety of naturally occurring and synthetic cationic proteins has been shown to induce airway hyperresponsiveness in animal models. Cationic proteins may alter the barrier function of airway epithelium, allowing increased access of agonists to underlying nerves and airway smooth muscle. To examine the effect of cationic proteins on airway epithelial cell function, rabbit tracheal epithelial cells were isolated and cultured on collagen-coated filter membranes. Both apical and basolateral exposure of the cell cultures to poly-L-lysine and poly-L-arginine decreased transepithelial electrical resistance (Rt) over 60 min. There were no discernable light microscopic changes in the morphology of the cultures at 60 min after poly-L-lysine exposure, but permeability to mannitol was increased compared to controls. Evidence for the critical role of cationic charge included the following observations: (1) Poly-L-aspartate, an anionic polyamino acid, had no significant effect on Rt, and (2) the addition of heparin prior to the addition of poly-L-lysine blocked the reduction in Rt. Furthermore, when applied after poly-L-lysine addition, heparin reversed the decrease in Rt in a time-dependent fashion. Increasing the [Ca2+] in the medium from 1 to 10 mM resulted in significant attenuation of the response to polycation addition. These findings suggest that cationic proteins significantly alter the barrier properties of airway epithelium and that cationic charge is a crucial factor. This alteration is not an "all or none" phenomenon, since subsequent addition of heparin resulted in a reversal of the effect. While the precise mechanisms responsible for these observations remain to be elucidated, cationic proteins may be modifying the interaction of extracellular calcium with tight junctions thereby resulting in increased permeability. The barrier function of the epithelium may be perturbed in asthma and a variety of other airway diseases through the presence of cationic proteins derived from inflammatory cells within the airway lumen and/or the subepithelium.

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