Effect of an inhaled corticosteroid on airway eosinophils and allergen-induced airway hyperresponsiveness in dogs

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.

Regulation of arachidonate remodeling enzymes impacts eosinophil survival during allergic asthma

Although the role of arachidonic acid (AA) metabolism to eicosanoids has been well established in allergy and asthma, recent studies in neoplastic cells have revealed that AA remodeling through phospholipids impacts cell survival. This study tests the hypothesis that regulation of AA/phospholipid-remodeling enzymes, cytosolic phospholipase A(2) alpha(cPLA(2)-alpha, gIValphaPLA(2)) and CoA-independent transacylase (CoA-IT), provides a mechanism for altered eosinophil survival during allergic asthma. In vitro incubation of human eosinophils (from donors without asthma) with IL-5 markedly increased cell survival, induced gIValphaPLA(2) phosphorylation, and increased both gIValphaPLA(2) and CoA-IT activity. Furthermore, treatment of eosinophils with nonselective (ET18-O-CH(3)) and selective (SK&F 98625) inhibitors of CoA-IT triggered apoptosis, measured by changes in morphology, membrane phosphatidylserine exposure, and caspase activation, completely reversing IL-5-induced eosinophil survival. To determine if similar activation occurs in vivo, human blood eosinophils were isolated from either normal individuals at baseline or from subjects with mild asthma, at both baseline and 24 hours after inhaled allergen challenge. Allergen challenge of subjects with allergic asthma induced a marked increase in cPLA(2) phosphorylation, augmented gIValphaPLA(2) activity, and increased CoA-IT activity. These findings indicate that both in vitro and in vivo challenge of eosinophils activated gIValphaPLA(2) and CoA-IT, which may play a key role in enhanced eosinophil survival.

Canine models of asthma and COPD

Dogs have been extensively used to model the important components of asthma and COPD. Many of the key features of human asthma such as reversible airflow obstruction, pulmonary inflammation, airway hyperresponsiveness and cough are demonstrated in dogs after provocation with antigen, following a period of hyperventilation with dry air or after inhalation of ozone. Furthermore, standard anti-asthma drugs such as beta-adrenergic agonists, corticosteroids and leukotriene inhibitors are effective in these models. The pathology and pathophysiology of chronic bronchitis and emphysema can also be demonstrated in dogs after exposure to cigarette smoke, following inhalation of sulfur dioxide and by intra-tracheal or aerosol administration of proteolytic enzymes such as papain. These canine models of COPD have been used to evaluate a variety of new methodologies and treatments before they are tested in humans. This review highlights some of the important features of these canine models and how they have increased our understanding of the pathology, pathophysiology and control of human asthma and COPD.

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.

Dexamethasone alleviates meconium-induced airway hyperresponsiveness and lung inflammation in rabbits

The effects of dexamethasone on in vitro airway reactivity associated with lung inflammation were investigated in rabbits with meconium aspiration. Oxygen-ventilated adult rabbits received an intratracheal bolus of 4 ml/kg body weight of saline (Sal, n = 4) or human meconium (25 mg/ml). Thirty minutes later, meconium-instilled animals intravenously received 0.5 mg/kg of dexamethasone (Dexa, n = 6), or were left without treatment (Meco, n = 5). The animals were ventilated for a further 5 hr and then sacrificed. The left lungs were lavaged with saline, and the white blood cell (WBC) count was estimated. Tracheal and right-lung tissue strips were placed into organ chambers with Krebs-Henseleit solution. Cumulative doses of histamine (10(-8)-10(-3) mol/l) and acetylcholine (10(-8)-10(-3) mol/l) were added to the chambers, and recordings of contractions were made after a 30-min loading phase with a tension of 4 grams, and another 30-min adaptation phase with a tension of 2 g. Tracheal smooth muscle in vitro reactivity to histamine was higher in the Meco than in the Sal group, and dexamethasone decreased the reactivity compared to the Meco group (P < 0.05). Lung tissue in vitro reactivity to histamine was slightly higher in the Meco than in the Sal group (P > 0.05), and dexamethasone decreased the reactivity compared to both the Meco and Sal groups (P < 0.05). No between-group differences were observed in tracheal or lung in vitro reactivity to acetylcholine (P > 0.05). In the Meco group, blood WBC (P > 0.05) and neutrophil (P < 0.05) counts were lower than in the Sal and Dexa groups. Lung neutrophils and eosinophils were higher in both the Meco and Dexa groups than in the Sal group (P < 0.01). Dexamethasone decreased neutrophils (P < 0.05) compared to the Meco group. Meconium-induced airway hyperreactivity to histamine and lung inflammation were alleviated by dexamethasone.

Effects of dexamethasone on antigen-induced airway eosinophilia and M(2) receptor dysfunction

In antigen-challenged guinea pigs, airway hyperreactivity is due to recruitment of eosinophils to the airway nerves and dysfunction of M(2) muscarinic receptors. M(2) receptor dysfunction is caused by eosinophil major basic protein, which is an allosteric antagonist at the receptor. Because glucocorticoids inhibit airway hyperreactivity in humans and in animal models of asthma, we tested whether dexamethasone treatment (6 microg. kg(-)(1). d(-)(1) for 3 d, intraperitoneal) before antigen challenge prevents M(2) receptor dysfunction and airway hyperreactivity. Guinea pigs were sensitized to ovalbumin via intraperitoneal injections, and were challenged with ovalbumin via inhalation. Twenty-four hours later, hyperreactivity and M(2) receptor function were tested. Antigen-challenged animals were hyperreactive to vagal stimulation, and demonstrated loss of M(2) receptor function. Dexamethasone pretreatment prevented hyperreactivity and M(2) receptor dysfunction in antigen-challenged guinea pigs. Antigen challenge resulted in recruitment of eosinophils to the airways and to the airway nerves. Dexamethasone prevented recruitment of eosinophils to the airway nerves but did not affect total eosinophil influx into the airways. These results demonstrate that dexamethasone prevents antigen-induced hyperreactivity by protecting neuronal M(2) muscarinic receptors from antagonism by eosinophil major basic protein, and this protective mechanism appears to be by specifically inhibiting eosinophil recruitment to the airway nerves.

Mitochondria in eosinophils: functional role in apoptosis but not respiration

In most eukaryotic cells, mitochondria use the respiratory chain to produce a proton gradient, which is then harnessed for the synthesis of ATP. Recently, mitochondrial roles in regulation of apoptosis have been discovered in many cell types. Eosinophils (Eos) die by apoptosis, but the presence and function of mitochondria in Eos are unknown. This study found that Eos contain mitochondria in small numbers, as shown by labeling with membrane potential-sensitive dyes and in situ PCR for a mitochondrial gene. Eos generate mitochondrial membrane potential from hydrolysis of ATP rather than from respiration, as shown by mitochondrial respiratory inhibitors and mitochondrial uncouplers. The mitochondria provide insignificant respiration but can induce apoptosis, as shown by using the mitochondrial F(1)F(0)-ATPase inhibitor oligomycin and translocation of cytochrome c. Thus during differentiation of Eos, although respiration is lost, the other central role of mitochondria, the induction of apoptosis, is retained.

Release of epithelium-derived PGE2 from canine trachea after antigen inhalation

To investigate the role of prostaglandin (PG) E2 in allergen-induced hyperresponsiveness, dogs inhaled either the allergen Ascaris suum or vehicle (Sham). Twenty-four hours after inhalation, some animals exposed to allergen demonstrated an increased responsiveness to acetylcholine challenge in vivo (Hyp-Resp), whereas others did not (Non-Resp). Strips of tracheal smooth muscle, either epithelium intact or epithelium denuded, were suspended on stimulating electrodes, and a concentration-response curve to carbachol (10(-9) to 10(-5) M) was generated. Tissues received electrical field stimulation, and organ bath fluid was collected to determine PGE2 content. With the epithelium present, all three groups contracted similarly to 10(-5) M carbachol, whereas epithelium-denuded tissues from animals that inhaled allergen contracted more than tissues from Sham dogs. In response to electrical field stimulation, Hyp-Resp tissues contracted less than Sham tissues in the presence of epithelium and more than Sham tissues in the absence of epithelium. PGE2 release in the muscle bath was greater in Non-Resp tissues than in Sham or Hyp-Resp tissues when the epithelium was present. Removal of the epithelium greatly inhibited PGE2 release. We conclude that tracheal smooth muscle is hyperresponsive in vitro after in vivo allergen exposure only when the modulatory effect of the epithelium, largely through PGE2 release, is removed.

Retinoic acid modulation of induced basophil differentiation

Current models of the regulation of hematopoiesis postulate a combination of both factor-directed cell differentiation/survival acting through intracellular signaling pathways, and factor-independent differentiation along intrinsically set, default pathways. Recent reports have indicated that eosinophil/basophil (Eo-Baso) differentiation may represent one default pathway. Because of the strong modulating effect of all trans-retinoic acid (ATRA) on hematopoietic differentiation, we have investigated the role of ATRA in regulating Eo-Baso differentiation from pluripotent progenitors. Our results indicate that ATRA inhibits Eo-Baso maturation at early stages of lineage commitment. Because allergic responses may depend on the continued recruitment and differentiation of such inflammatory mediators, the ability to modulate this pathway may eventually prove to have a therapeutic role in allergic inflammation.

Effects of local and systemic budesonide on allergen-induced airway reactions in the pig

1. In this study, an attempt was made to distinguish between local and systemic effects of low doses of the topical glucocorticoid, budesonide. The effect of aerosolized budesonide administered to the lower airways versus intravenously administered budesonide on the acute and late response to nebulized Ascaris suum extract in the lung, was evaluated in the minipig after active sensitization with purified A. suum antigen. Budesonide was administered once, 1 h prior to A. suum challenge and airway reactions and mediator release were observed for 8 h after allergen challenge. 2. In the budesonide aerosol group (n = 6), 10.2 +/- 1.2 micrograms kg-1 budesonide was given locally and in the budesonide infusion group (n = 5), 5 micrograms kg-1 was given intravenously. The area under the plasma concentration curve for budesonide during the experiment was 11.4 +/- 1.2 and 10.3 +/- 1.2 nM h in the budesonide aerosol and budesonide infusion group, respectively (no significant difference). The lung tissue content of budesonide in the two groups was 45.2 +/- 4.9 and 18.4 +/- 3.5 nmol kg-1 dry tissue, respectively, 8 h after allergen challenge (P < 0.05). For comparison, 6 pigs were given budesonide vehicle as an infusion prior to A. suum challenge. 3. Total lung resistance (RL) increased acutely (maximal response within 15 min) in the budesonide aerosol, budesonide infusion and budesonide vehicle groups (by 91 +/- 40, 150 +/- 86 and 80 +/- 27%, respectively). The acute reaction partially resolved at about 1 h and was followed by a late increase in RL in the budesonide infusion and budesonide vehicle groups (by 251 +/- 148 and 281 +/- 136% at 8 h, respectively). However, no late change in RL was seen in the budesonide aerosol group (7 +/- 24%). 4. Aerosolized budesonide had a protective effect in that it attenuated the late changes in arterial blood gas and pH as well as the late elevation of plasma catecholamines. Budesonide given as an infusion did not protect against the late changes in these parameters. However, budesonide aerosol or infusion did not inhibit the late vasodilation in the bronchial circulation. 5. Histamine and cysteinyl-leukotrienes were released during the acute reaction as measured by urinary concentration of methylhistamine and leukotriene E4 respectively. There was no release of histamine during the late reaction. A late increase in leukotriene E4 was observed in 2 of the budesonide infusion and 3 of the budesonide vehicle pigs, whereas no such increase was seen in any of the budesonide aerosol pigs. 6. Budesonide concentration in lung tissue, but not in plasma at 8 h correlated negatively with the late increase in RL (P < 0.05, r = -0.53, n = 10), whereas budesonide concentration in plasma but not in lung tissue correlated negatively with the late decrease in dynamic compliance (P < 0.05, r = -0.67, n = 12). 7. This study has shown that a single low dose of locally administered budesonide can inhibit the late allergic reaction in the pig lower airways. If budesonide was given as an intravenous infusion in a dose yielding a plasma concentration similar to that seen after the aerosol treatment, the protective effect of budesonide was poor. It may be suggested that the tissue-bound portion of budesonide affects local mechanisms involved in the development of late changes in the airways (RL), although it does not affect the late increase in bronchial blood flow. We conclude that the inhibitory effect of budesonide on the allergen-induced late reaction in the pig airways relates to tissue-bound steroid, and that the systemic component is of less importance.

The effects of an inhaled corticosteroid on oxygen radical production by bronchoalveolar cells after allergen or ozone in dogs

Both ozone and allergen inhalation increase the capacity to produce oxygen radicals by bronchoalveolar lavage cells in dogs. The purpose of these studies was to determine whether inhaled corticosteroids inhibits these increases in oxygen radical production from bronchoalveolar lavage cells. Six random source dogs were studied after dry air or ozone inhalation (3 ppm, 30 min). Seven random source dogs were studied after diluent or allergen inhalation. The dogs inhaled budesonide (2.74 mg/day) or lactose powder, twice daily for 7 days before ozone and allergen. 90 min after ozone or dry air, and 24 h after Ascaris suum or diluent a bronchoalveolar lavage was carried out. Spontaneous luminol-enhanced chemiluminescence was measured from bronchoalveolar lavage cells (4 x 10(6) cells) for 10 min, followed by a measurement of phorbol myristate acetate (PMA 2.4 micromol/l) stimulated chemiluminescence for 10 min. Both ozone and allergen inhalation caused an increase in PMA stimulated chemiluminescence (P<0.05). Budesonide pretreatment inhibited ozone-induced (P<0.008), but not allergen-induced PMA stimulated chemiluminescence (P>0.90). Both ozone and allergen inhalation caused an increase in the bronchoalveolar lavage neutrophils. Budesonide pretreatment significantly inhibited the ozone-induced (P=0.007), but not the ascaris-induced neutrophil influx (P=0.93). These results demonstrate that ozone, but not allergen, stimulated oxygen radical release and neutrophil influx are attenuated by inhaled corticosteroids. This suggests that luminol-enhanced chemiluminescence from bronchoalveolar lavage cells measures oxygen radicals derived from neutrophils, and that ozone-and allergen-induced bronchoalveolar lavage neutrophilia are caused by different mechanisms.