To elucidate the role of IgE-dependent mechanisms in inducing altered airway responsiveness in the atopic asthmatic state, the expression and actions of Fc receptor activation were examined in isolated rabbit tracheal smooth muscle (TSM) tissue and cultured cells passively sensitized with sera from atopic asthmatic patients or nonatopic/nonasthmatic (control) subjects. Relative to control tissues, the atopic asthmatic-sensitized TSM exhibited significantly increased maximal isometric contractility to acetylcholine (P < 0.01) and attenuated maximal relaxation responses and sensitivity (i.e., −log ED₅₀) to isoproterenol (P < 0.005). These changes in agonist responsiveness in atopic sensitized TSM were ablated by pretreating the tissues with a blocking mAb to the low affinity receptor for IgE, FcɛRII (i.e., CD23) or by depleting the sensitizing serum of its immune complexes. Moreover, in complimentary experiments, exogenous administration of IgE immune complexes to naive TSM produced changes in agonist responsiveness that were qualitatively similar to those obtained in the atopic asthmatic-sensitized state. Extended studies further demonstrated that, in contrast to their respective controls, atopic asthmatic serum-sensitized human and rabbit TSM tissue and cultured cells exhibited markedly induced mRNA and cell surface expression of FcɛRII, whereas constitutive expression of the IgG receptor subtype, FcγRIII, was unaltered. Finally, the up-regulated mRNA expression of FcɛRII observed following exposure of TSM to atopic asthmatic serum or to exogenously administered IgE immune complexes was significantly inhibited by pretreating the tissues or cells with anti-CD23 mAb. Collectively, these observations provide evidence demonstrating that the altered agonist responsiveness in atopic asthmatic sensitized airway smooth muscle is largely attributed to IgE-mediated induction of the autologous expression and activation of FcɛRII receptors in the airway smooth muscle itself.