The bronchoconstrictive muscarinic agonist methacholine in the OVA-sensitized mouse, as a model for RSV-induced acute asthma exacerbations. Although we had hypothesized that RSV infection would further increase airway hyperresponsiveness to methacholine in OVA-sensitized animals, we did not find this to be the case. Instead, we found that infection with RSV paradoxically reversed airway hyperresponsiveness to methacholine in a keratinocyte cytokine -dependent, pertussis toxin-sensitive fashion. This suggests that acute RSV infection modulates muscarinic receptor function in ovalbumin-sensitized mice in a paracrine fashion. Likewise, since ligands for TLR-3, protein kinase R, and RIG-I are only generated during viral replication our data indicate that induction of methacholine hyporesponsiveness is replication-dependent. Nevertheless, given the semi-permissive nature of the mouse for RSV replication, induction of airway hyporesponsiveness in OVAsensitized animals may not fully reflect the effects of RSV in human asthmatics. Hence, both the inherent limitations of the RSV mouse model and the paradoxical effects of this virus on airway function in previously-sensitized mice indicate that the OVA-sensitized, RSV-infected mouse may not be appropriate for investigating the BKM120 citations pathogenesis of viral asthma exacerbations. Although less widely-used, other paramyxoviruses such as Sendai virus and pneumonia virus of mice cause more severe disease in this species. It is therefore possible that infection of OVAsensitized mice with these pathogens may better model human viral asthma exacerbations. Both viruses have been shown to promote airway hyperresponsiveness when mice are infected either prior to or during OVA sensitization. Unfortunately, however, effects on airway function of post-sensitization infection with either Sendai virus or pneumonia virus have not been reported to our knowledge. In conclusion, we found that RSV infection of OVA-sensitized mice reversed airway hyperresponsiveness to the bronchoconstrictor methacholine. Reversal of airway hyperresponsiveness was induced by the chemokine KC, and could be replicated by direct activation of pertussis toxin-sensitive Gai. This suggests that reversal results from Gai-mediated cross-inhibition of phospholipase C, which is normally activated by Gaq in response to binding of methacholine to M3-subtype muscarinic receptors. Our data indicate that KC released in response to RSV infection triggers a previously unrecognized increase in Gai activity in OVAsensitized mice, which results in significant derangement of airway responses to muscarinic agonists. The effect of RSV on methacholine responsiveness in the OVA-sensitized mouse is rather paradoxical, which suggests that this model may be of limited value for studies of viral asthma exacerbations. Nevertheless, when viewed in the context of our previous findings, these studies reinforce the potential importance of IL-8 as a therapeutic target following RSV infection. Epithelial-mesenchymal transition denotes a process in which cells change their phenotype between epithelial and mesenchymal states. This phenotypic change involves complex molecular and cellular programs by which epithelial cells can dispose of their differentiated characteristics, including cell-cell adhesion, planar and apical-basal polarity, lack of motility and gain instead mesenchymal features such as motility, invasiveness and increased apoptotic resistance.