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CD4+ T helper (TH)1-and TH2-type cytokines reportedly play an important role in the pathobiology of asthma. Recent evidence suggests that proasthmatic changes in airway smooth muscle (ASM) responsiveness may be induced by the autocrine release of certain proinflammatory cytokines by the ASM itself. We examined whether TH1- and TH2-type cytokines are expressed by atopic asthmatic sensitized ASM and serve to autologously regulate the proasthmatic phenotype in the sensitized ASM. Expression of these cytokines and their receptors was examined in isolated rabbit and human ASM tissues and cultured cells passively sensitized with sera from atopic asthmatic patients or control subjects. Relative to controls, atopic sensitized ASM cells exhibited an early increased mRNA expression of the TH2-type cytokines, interleukin-5 (IL-5) and granulocyte-macrophage colony-stimulating factor (GM-CSF), and their receptors. This was later followed by enhanced mRNA expression of the TH1-type cytokines, IL-2, IL-12, and interferon-[small gamma, Greek] (IFN-[small gamma, Greek]), as well as their respective receptors. In experiments on isolated ASM tissue segments (a) exogenous administration of IL-2 and IFN-[small gamma, Greek] to atopic asthmatic serum-sensitized ASM ablated both their enhanced constrictor responsiveness to acetylcholine (ACh) and their attenuated relaxation responsiveness to [small beta, Greek]-adrenoceptor stimulation with isoproterenol, and (b) administration of IL-5 and GM-CSF to naive ASM induced significant increases in their contractility to ACh and impaired their relaxant responsiveness to isoproterenol. Collectively, these observations provide new evidence demonstrating that human ASM endogenously expresses both TH1- and TH2-type cytokines and their receptors, that these molecules are sequentially upregulated in the atopic asthmatic sensitized state, and that they act to downregulate and upregulate proasthmatic perturbations in ASM responsiveness, respectively.J.Clin. Invest. 103:1077-1087 (1999).