Effects of immunomodulators on airways hyperresponsiveness to adenosine induced in actively sensitised Brown Norway rats by exposure to allergen

Asthma: The Use of Animal Models and Their Translational Utility

Asthma is characterized by chronic lower airway inflammation that results in airway remodeling, which can lead to a permanent decrease in lung function. The pathophysiology driving the development of asthma is complex and heterogenous. Animal models have been and continue to be essential for the discovery of molecular pathways driving the pathophysiology of asthma and novel therapeutic approaches. Animal models of asthma may be induced or naturally occurring. Species used to study asthma include mouse, rat, guinea pig, cat, dog, sheep, horse, and nonhuman primate. Some of the aspects to consider when evaluating any of these asthma models are cost, labor, reagent availability, regulatory burden, relevance to natural disease in humans, type of lower airway inflammation, biological samples available for testing, and ultimately whether the model can answer the research question(s). This review aims to discuss the animal models most available for asthma investigation, with an emphasis on describing the inciting antigen/allergen, inflammatory response induced, and its translation to human asthma.

Activation of the mTOR signaling pathway is required for asthma onset

The mTOR pathway has been implicated in immune functions; however, its role in asthma is not well understood. We found that patients experiencing an asthma attack, when compared with patients in asthma remission, showed significantly elevated serum mTOR pathway activation, increased Th17 cells and IL-4, and decreased Treg cells and IFN-γ. In patients experiencing asthma, mTOR activation was positively correlated with the loss of Th17/Treg and Th1/Th2 balance. The role of mTOR in asthma was further confirmed using an ovalbumin-induced asthmatic mouse model. The mTOR pathway was activated in asthmatic mice, demonstrated by elevated levels of p-PI3K, p-Akt, p-mTOR, and p-p70S6k, and this activation was significantly reduced by treatment with budenoside or mTOR pathway inhibitors. Moreover, mTOR pathway inhibitor treatment reduced asthmatic markers and reversed the Th17/Treg and Th1/Th2 imbalances in asthmatic mice. Finally, different mTOR pathway inhibitor treatments have different inhibitory effects on signaling molecules in asthmatic mice. In summary, mTOR is activated during asthma onset and suppressed during asthma remission, and inhibiting the mTOR pathway in asthmatic mice alleviates asthmatic markers and restores the balances of Th17/Treg and Th1/Th2 cytokines. These data strongly suggest a critical requirement for mTOR pathway activation in asthma onset, suggesting potential targets for asthma treatments.

Rheb1 deletion in myeloid cells aggravates OVA-induced allergic inflammation in mice

The small GTPase ras homolog enriched in brain (Rheb) is a downstream target of tuberous sclerosis complex 1/2 (TSC1/2) and an upstream activator of the mechanistic target of rapamycin complex 1 (mTORC1), the emerging essential modulator of M1/M2 balance in macrophages. However, the role and regulatory mechanisms of Rheb in macrophage polarization and allergic asthma are not known. In the present study, we utilized a mouse model with myeloid cell-specific deletion of the Rheb1 gene and an ovalbumin (OVA)-induced allergic asthma model to investigate the role of Rheb1 in allergic asthma and macrophage polarization. Increased activity of Rheb1 and mTORC1 was observed in myeloid cells of C57BL/6 mice with OVA-induced asthma. In an OVA-induced asthma model, Rheb1-KO mice demonstrated a more serious inflammatory response, more mucus production, enhanced airway hyper-responsiveness, and greater eosinophil numbers in bronchoalveolar lavage fluid (BALF). They also showed increased numbers of bone marrow macrophages and BALF myeloid cells, elevated M2 polarization and reduced M1 polarization of macrophages. Thus, we have established that Rheb1 is critical for the polarization of macrophages and inhibition of allergic asthma. Deletion of Rheb1 enhances M2 polarization but decreases M1 polarization in alveolar macrophages, leading to the aggravation of OVA-induced allergic asthma.

Rapamycin inhibition of eosinophil differentiation attenuates allergic airway inflammation in mice

BACKGROUND AND OBJECTIVE

The mammalian target of rapamycin (mTOR) signalling pathway regulates immune responses, and promotes cell growth and differentiation. Inhibition of mTOR with rapamycin modulates allergic asthma, while the underlying molecular mechanisms remain elusive. Here, we demonstrate that rapamycin, effectively inhibits eosinophil differentiation, contributing to its overall protective role in allergic airway inflammation.

METHODS

Rapamycin was administered in a mouse model of ovalbumin-induced allergic airway inflammation, and the eosinophil differentiation was analysed in vivo and in vitro.

RESULTS

Rapamycin significantly attenuated allergic airway inflammation and markedly decreased the amount of eosinophils in local airways, peripheral blood and bone marrow, independently of levels of interleukin-5 (IL-5). In vitro colony forming unit assay and liquid culture demonstrated that rapamycin directly inhibited IL-5-induced eosinophil differentiation. In addition, rapamycin reduced the production of IL-6 and IL-13 by eosinophils. Rapamycin was also capable of reducing the eosinophil levels in IL-5 transgenic NJ.1638 mice, again regardless of the constitutive high levels of IL-5. Interestingly, rapamycin inhibition of eosinophil differentiation in turn resulted in an accumulation of eosinophil lineage-committed progenitors in bone marrow.

CONCLUSIONS

Altogether these results clearly demonstrate a direct inhibitory role of rapamycin in eosinophil differentiation and function, and reemphasize the importance of rapamycin and possibly, mTOR, in allergic airway disease.

Biphasic airway-lung response to anaphylactic shock in Brown Norway rats

Bronchospasm may be part of the response to systemic anaphylaxis in humans. The anaphylactic shock has been characterized in allergic rats, but little data are available on the concurrent changes in airway-lung mechanics. The aim was to describe the respiratory resistance (Rrs) and reactance (Xrs) response to ovalbumin (OVA) induced systemic anaphylaxis in allergic rats. Thirty five anesthetized and mechanically ventilated Brown Norway rats were randomly allocated to OVA (n=20) or vehicle (n=15) sensitization and provocation. Rrs and Xrs were obtained by the forced oscillation technique at 20 Hz. Allergic rats showed dramatic and reproducible concurrent Rrs peak and Xrs through within 4 min of OVA injection (p<0.0001). Thereafter, Rrs returned to baseline while Xrs remained significantly more negative (p<0.0001). It is concluded that systemic anaphylaxis in allergic rats is associated with severe early acute inhomogeneous bronchoconstriction followed by pulmonary interstitial/small airspace edema. The model may be of interest to assess treatments targeting the associated bronchoconstriction and/or airway vascular leakage.

Dose-dependent inhibition of allergic inflammation and bronchial hyperresponsiveness by budesonide in ovalbumin-sensitised Brown-Norway rats

Corticosteroids are known to inhibit bronchial hyperresponsiveness (BHR) and allergic inflammation but there is little information on its dose-dependence. We examined the effect of different doses of the glucocorticosteroid budesonide in an allergic model. Brown-Norway rats were sensitised to ovalbumin (OVA) and pretreated with an intra-gastric dose of budesonide (0.1, 1.0, or 10 mgkg(-1)). Exposure to OVA induced BHR, accumulation of eosinophils in the bronchoalveolar lavage (BAL) fluid and in the airways submucosa. Budesonide dose-dependently inhibited BAL fluid influx of lymphocytes, eosinophils and neutrophils, tissue eosinophils and lymphocytes and BHR. At 0.1 mgkg(-1), budesonide did not inhibit these parameters but at 1 mgkg(-1), BAL fluid eosinophils and T-cells, and submucosal T-cells were significantly reduced. At 10 mgkg(-1), budesonide suppressed BHR, BAL fluid inflammatory cells numbers and tissue eosinophilia. T-cell numbers were more related to BHR than eosinophil numbers. Budesonide inhibited both airway inflammation and BHR, but BAL fluid eosinophil cell counts may be dissociated from BHR.

Airway hyperresponsiveness to bradykinin induced by allergen challenge in actively sensitised Brown Norway rats

The mechanism(s) of bradykinin-induced bronchoconstriction was investigated in the Brown Norway (BN) rat model of allergic asthma. Bronchoconstrictor responses to i.v. bradykinin in BN rats were maximally augmented 24 h following challenge with allergen and declined at later time points. Histological evaluation of the inflammatory status of the lungs after ovalbumin (OA) challenge showed a marked inflammatory response, which was maximal at 24 h and declined thereafter. However, pretreatment with budesonide did not inhibit the augmented bronchoconstrictor response to bradykinin 24 h after allergen challenge. The selective B1 receptor agonist, Lys-[desArg9]-BK had no bronchoconstrictor effects, whereas the selective B2 receptor antagonist, HOE 140, abolished the response to bradykinin in OA-challenged animals. The augmented response to bradykinin was not affected by methysergide, indomethacin, disodium cromoglycate, iralukast, the 5-lipoxygenase inhibitor, CGS8515, or the NK2 receptor antagonist, SR48968. It was, however, partially inhibited by atropine both in saline- and OA-challenged animals. Pretreatment with captopril and thiorphan markedly potentiated responses to bradykinin both in saline- and OA-challenged animals. Thus, augmentation of the bronchoconstrictor response to bradykinin occurs in actively sensitised BN rats 24 h after challenge with OA and is associated with marked pulmonary inflammation. The response is entirely B2 receptor mediated and approximately 50% of the response is cholinergic. However, mast cell activation, the products of the cyclooxygenase or 5-lipoxygenase pathways and tachykinins are not involved. Peptidase inhibition mimics the effect of allergen challenge on the bronchoconstrictor response to bradykinin and it remains possible that the mechanism of the augmented response to bradykinin following allergen challenge involves downregulation of peptidase activity as a consequence of the inflammatory response.