In vivo animal models can offer valuable information on several aspects of asthma pathogenesis and treatment. The mouse is increasingly used in these models, mainly because this species allows for the application in vivo of a broad range of immunological tools, including gene deletion technology. Mice, therefore, seem particularly useful to further elucidate factors influencing the response to inhaled allergens. Examples include: the role of immunoregulatory mechanisms that protect against T-helper cell type 2 cell development; the trafficking of T-cells; and the contribution of the innate immunity. However, as for other animal species, murine models also have limitations. Mice do not spontaneously develop asthma and no model mimics the entire asthma phenotype. Instead, mice should be used to model specific traits of the human disease. The present task force report draws attention to specific aspects of lung structure and function that need to be borne in mind when developing such models and interpreting the results. In particular, efforts should be made to develop models that mimic the lung function changes characteristic of asthma as closely as possible. A large section of this report is therefore devoted to an overview of airway function and its measurement in mice.

1 Non-selective inhibitors of cyclic nucleotide phosphodiesterase (PDE) block allergen-induced contraction of passively sensitized human airways in vitro by a dual mechanism involving a direct relaxant eect on smooth muscle and inhibition of histamine and cysteinyl leukotriene (LT) release from airways. We investigated the eects of non-selective PDE inhibitors and selective inhibitors of PDE3 and PDE4 in order to determine the involvement of PDE isoenzymes in the suppression of allergic bronchoconstriction. 2 Macroscopically normal airways from 76 patients were sensitized with IgE-rich sera (4250 u ml 71) containing speci®c antibodies against allergen (Dermatophagoides farinae). Contractile responses of bronchial rings were assessed using standard organ bath techniques. 3 Passive sensitization caused increased contractile responses to allergen, histamine and LTC 4 . Non-selective PDE inhibitors (theophylline, 3-isobutyl-1-methylxanthine [IBMX]), a PDE3-selective inhibitor (motapizone), PDE4-selective inhibitors (RP73401, rolipram, AWD 12-281) and a mixed PDE3/4 inhibitor (zardaverine) all signi®cantly relaxed inherent bronchial tone at resting tension and to a similar degree. Theophylline, IBMX, zardaverine and the combination of motapizone and RP73401 inhibited the contractile responses to allergen and LTC 4 . Pre-treatment with motapizone, RP73401, rolipram or the methylxanthine adenosine receptor antagonist, 8-phenyltheophylline, did not signi®cantly decrease responses to either allergen or LTC 4 . 4 We conclude that combined inhibition of PDE3 and PDE4, but not selective inhibition of either isoenzyme or antagonism of adenosine receptors, is eective in suppressing allergen-induced contractions of passively sensitized human airways. The relationship between allergen-and LTC 4 -induced responses suggests that PDE inhibitors with PDE3 and PDE4 selectivity are likely to act in part through inhibition of mediator release and not simply through direct relaxant actions on airway smooth muscle.

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Passive sensitization of human airways in vitro causes increased responsiveness to histamine and induces specific immunoglobulin (Ig)E-dependent contractile responsiveness to allergen. Leukotrienes (LTs) and, to a lesser extent, histamine are the major mediators of allergen-induced contraction. Since it is unclear whether passively sensitized airways are also hyperresponsive to cysteinyl leukotrienes, this study investigated the effect of passive sensitization on LTC 4 -, in addition to histamine-and allergen-induced contractions in vitro.Bronchial rings from nine nonatopic patients were sensitized overnight with serum containing high levels of total IgE (>250 U . mL -1 ) and allergen-specific IgE against Dermatophagoides farinae (fluorescence allergosorbent test) (FAST class $3).The potency (-log 10 of the mediator concentration causing a half maximal response (pEC50) of histamine was significantly increased in serum-sensitized tissues compared to nonsensitized controls ((mean SEM) pEC50 5.20 0.27 versus 5.64 0.18; p=0.02) and maximal contractions were enhanced (877 47 versus 543 51 mg; p<0.0001). Similarly, the potency of LTC 4 was significantly increased in sensitized compared to nonsensitized bronchial rings (pEC50 9.37 0.20 versus 8.66 0.26; p=0.004); maximal contractions were also enhanced (811 57 versus 361 86 mg; p<0.0001).These data demonstrate that passive sensitization of human airways induces an increase not only in histamine but also in leukotriene responsiveness. Therefore, it might be speculated that allergen responses in sensitized airways are effected through a combination of increased mediator release from inflammatory cells and increased responsiveness of airway smooth muscle.

Cysteinyl-leukotrienes and histamine are the major determinants of inherent tone in isolated human bronchi, which is mainly the result of a balance of continual production and release of contractile mediator, in particular cysteinyl-leukotrienes and to a lesser extent histamine, and on the other side bronchodilating prostanoids. Cysteinyl-leukotrienes are also powerful constrictors of isolated human airways through direct interaction with Cys-LT1 receptors on airway smooth muscle, and with a potency 1,000-fold higher than histamine. On stimulation inflammatory cells such as eosinophils and mst cells produce and release significantly increased amounts of leukotrienes leading to smooth muscle contration in vitro. In isolated human airways, leukotrienes are the most important mediators of allergen and adenosine-induced contractile responses. The induction of allergen responses in passively sensitized airways is not only related to an increased release of leukotrienes and histamines, but also to an enhanced responsiveness of the airway smooth muscle, particularly to LTC4. Studies in isolated human airways in vitro have demonstrated that understanding the regulation of human airway tone and airway reactivity are closely linked to the understanding of baseline and stimulated production of and smooth muscle responsiveness to leukotrienes in vitro and in vivo.